abstract

We review pronounced non-Fermi-liquid (NFL) effects in the low-temperature normal state of the heavy-fermion superconductor UBe\(_{13}\) (\(T_{\rm c}\approx 0.9\,\textrm {K}\)). We argue that these NFL effects may presumably be related to short-range antiferromagnetic (AF) correlations which are manifested in the superconducting (SC) state by a “line of thermodynamic anomalies”, \(B^{\ast }(T)\), between \(T\approx 0.7\,\textrm {K}\; (B=0)\) and \(B\approx 4\,\textrm {T}\ (T\rightarrow 0)\). These anomalies are shown to mark the precursor of the lower of the two phase transitions (at \(T_{\rm c1}\) and \(T_{\rm c2}\)) in U\(_{1-x}\)Th\(_x\)Be\(_{13}\), \(x_{\rm c1}\approx 0.019\lt x\lt x_{\rm c2}\approx 0.0455\). For \(x_{\rm c2}\lt x\lt x_{\rm c} \lt 0.07\), a single SC transition is stated which due to thermal expansion, \(\alpha (T)\), and specific heat, \(C(T)\), measurements, coincides with this lower transition at \(T_{\rm c2}\). We discuss two possible scenarios both of which imply an intimate interrelation of superconductivity with the symmetry-broken state that forms below \(T_{\rm c2}\). Finally, we address two other lines of thermal expansion anomalies in the \(T\)–\(x\) phase diagram of U\(_{1-x}\)Th\(_x\)Be\(_{13}\) which show an only weak dependence on magnetic field: (i) A positive \(\alpha (T)\) peak, along with a \(C(T)\) peak, is found in pure UBe\(_{13}\) at \(T_{\rm max}\approx 2\,\textrm {K}\). Upon Th doping, \(T_{\rm max}(x)\) is depressed linearly and vanishes close to the upper critical Th concentration \(x_{\rm c2}\) at which the two phase transitions merge to one. (ii) A negative anomaly in \(\alpha (T)\) develops for \(x\gt x_{\rm c2}\) at \(T_{\rm min}\). \(T_{\rm min}(x)\) increases by more than a factor of two when raising the Th content to \(x=0.1\). Similar to previous results by Aliev et al. [F.G. Aliev et al., J. Phys.: Condens. Matter 8, 9807 (1995)], an almost temperature-independent non-linear susceptibility, \(\chi ^{(3)}(T)\), is found for U\(_{0.9}\)Th\(_{0.1}\)Be\(_{13}\), at striking variance to \(\chi ^{(3)}(T)\) for pure UBe\(_{13}\). The implications of this observation for the assignment of the valence state of Uranium at \(x=0\) and \(x=0.1\) are also addressed.

abstract

The experimental and theoretical status on the appearance of superconductivity in strongly correlated electronic system is reviewed with emphasis on ferromagnetism. The discovery of superconductivity in UGe\(_2\), URhGe and ZrZn\(_2\) has led to a boost of the thema. Focus is mainly given on UGe\(_2\), this system has been already studied by different technics. Even if the main trends go in the support of triplet pairing among the majority spin band, the new \(s\) wave mechanism by the polarization given by localized magnetism is attractive. Experimentally there is no doubt that the driven force is now the improvement of the materials and further discoveries of new cases. On the theoretical side, the activities cover classification of the order parameter by group theory, microscopic and phenomenological approaches and emphasis on the superconducting order parameter and the ferromagnetic domain.

abstract

We summarize the present status of the theories of spin fluctuations in dealing with the anomalous or non-Fermi liquid behavior and unconventional superconductivity in strongly correlated electron systems around their magnetic instabilities or quantum critical points. Arguments are given to indicate that the spin fluctuation mechanisms is the common origin of superconductivity in heavy electron systems, 2-dimensional organic conductors and high \(T_{\rm c}\) cuprates.

abstract

Late experimental investigations on the filling-control Mott transition are reviewed by taking examples of the perovskite Ti, V, and Mn oxides with controlled one-electron bandwidth. The complex interplay among the spin, charge, and orbital degrees of freedom is laid emphasis to elucidate anomalous features of the barely metallic phase close to the Mott transition point.

abstract

We report low temperature specific heat, \(C\), magnetization, \(M\), susceptibility, \(\chi \), and electrical resistivity, \(\rho \), measurements on high-quality single crystals of the heavy-fermion system YbRh\(_2\)(Si\(_{1-x}\)Ge\(_x\))\(_2\) (\({x=0}\) and 0.05). The undoped compound shows weak antiferromagnetic (AF) order at \({T_{\rm N}=70}\) mK which is suppressed to below 20 mK by a tiny volume expansion in the \({x=0.05}\) system. In the latter pronounced deviations from Landau Fermi liquid (LFL) behavior occur, e.g. \({{\mit \Delta }\rho \sim T}\) over three decades in \(T\). Both thermodynamic and magnetic properties show a crossover at about 0.3 K: At 0.3 K \({\leq T \leq }\) 10 K we observe \({C\,/\,T \sim \log (T_0\,/\,T)}\) and a “non-Curie” behavior \({\chi ^{-1} \sim T^{\,\alpha }}\) with \({\alpha \lt 1}\) similar to what was found for the prototypical system CeCu\(_{5.9}\)Au\(_{0.1}\). Below 0.3 K, \(\chi \) turns into a Curie–Weiss dependence \({\chi ^{-1}\sim (T-{\mit \Theta })}\) indicating large unscreened Yb\(^{3+}\) moments whereas in \({C(T)\,/\,T}\) a pronounced upturn occurs. In the undoped compound the AF order is suppressed continuously by critical fields \({B_{\rm c0}\simeq 0.06}\) T and 0.7 T applied perpendicular and parallel to the \(c\)-axis, respectively. For \({B\gt B_{\rm c0}}\) a LFL state with \({{\mit \Delta }\rho =A(B)T^{\,2}}\) and \({C(T)\,/\,T=\gamma _0(B)}\) is induced, that fulfills the Kadowaki–Woods scaling \({A\sim \gamma _0^2}\). Upon reducing the magnetic field to \({B_{\rm c0}}\) a \({1/(B-B_{\rm c0})}\) dependence of \({A(B)}\) and \({\gamma _0^2(B)}\) indicates singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass.

abstract

The theoretical investigation of spectral functions and pseudogap in systems with strongly correlated electrons is discussed, with the emphasis on the single-band \(t\)–\(J\) model as relevant for superconducting cuprates. The evidence for the pseudogap features from numerical studies of the model is presented. One of the promising methods to study spectral functions is the method of equations of motion. The latter can deal systematically with the local constraints and projected fermion operators inherent for strongly correlated electrons. In the evaluation of the self energy the decoupling of spin and single-particle fluctuations is performed. In an undoped antiferromagnet the method reproduces the selfconsistent Born approximation (SCBA). For finite doping the approximation evolves into a paramagnon contribution which retains large incoherent contribution in the hole part. On the other hand, the contribution of longer-range spin fluctuations is essential for the emergence of the pseudogap. The latter shows up at low doping in the effective truncation of the large Fermi surface, reduced electron density of states and at the same time reduced quasiparticle density of states at the Fermi level.

abstract

We present the Bethe ansatz solution for the generalized Anderson impurity model, in which localized electrons carrying spin and orbital degrees of freedom, interact in a shell via the Hubbard-like repulsion and Hund’s rule exchange interaction. Depending on the relative position of the impurity’s level with respect to the Fermi energy and strengths of Hubbard-like and Hund’s couplings, a magnetic impurity can reveal either the Fermi-liquid like behavior or the non-Fermi-liquid behavior.

abstract

We study the effect of strong disorder on the one-dimensional Kondo necklace model using a perturbative real-space renormalization group approach. The phase diagram of the model presents a random quantum critical point separating two phases; the random singlet phase of a quantum disordered XY chain and the random Kondo phase. We also consider an anisotropic version of the model which for strong disorder maps on the random transverse field Ising model. These results provide a microscopic basis for non-Fermi liquid behavior in disordered heavy fermions associated with the existence of Griffiths phases.

abstract

YbRh\(_2\)Si\(_2\) is a weak antiferromagnet with \(T_{\rm N} \approx 70\) mK, situated very close to an antiferromagnetic quantum critical point (QCP). Here, we present measurements of the initial Hall coefficient \(R_{\rm H}(T)\) on high-quality single-crystalline YbRh\(_2\)Si\(_2\) in the temperature range 16 mK to 300 K. Above 120 K, \(R_{\rm H}(T)\) is, as the magnetic susceptibility, of Curie–Weiss type. This allows for the separation of \(R_{\rm H}\) into a normal (\(R_0\)) and an anomalous contribution. Interestingly, the value obtained for \(R_0\) is very close to the value \(R_{\rm H}\) reaches at the lowest temperatures. This indicates that, at the lowest temperatures, \(R_{\rm H}\) is dominated by \(R_0\) and thus probes the charge-carrier concentration.

abstract

A recent reinvestigation of the alloy system CeIn\(_{3-x}\)Sn\(_x\) between the antiferromagnetic (AF) Kondo-lattice CeIn\(_3\) and the intermediate valent CeSn\(_3\) has shown that the AF order disappears at a quantum critical point (QCP) at \(x_{\rm c}\approx 0.65\). Preliminary evidences for a first order transition below \(T_{\rm N}\) at \(x\lt x_{\rm c}\), indicating a complex magnetic phase diagram, have motivated the present detailed investigation of specific heat and resistivity on fine tuned samples in the \(0.25 \leq x \leq x_{\rm c}\) range. These results confirm the first order transition at \(T_{\rm I} \lt T_{\rm N}\) in a reduced concentration range (\(0.25 \lt x \lt 0.47\)) and suggest that the phase boundary \(T_{\rm I}(x)\) merge with the \(T_{\rm N}(x)\) near \(x\approx 0.37\), leading to a tetracritical point. A similar first order transition was previously observed in the CeCu\(_2\)(Si,Ge)\(_2\) system at low Ge-concentration. Since in this alloy the first order transition also appears in the vicinity of a QCP, the question arises concerning how general this property is at AF critical points in Ce-based alloy systems.

abstract

Yb in ternary \(\rm Yb_2Pd_2In\) exhibits a valency \(\nu \approx 2.9\) and consequently, no clear evidence of long range magnetic order down to 40 mK. Low temperature resistivity and specific heat are characterised by significant deviations from a Fermi-liquid (FL) scenario. While the application of magnetic fields recovers a FL state, pressure drives the system closer to a magnetic instability at \(T=0\).

abstract

We have studied the effect of slight changes in the chemical composition on the low-temperature specific heat of Ce\(_{0.98}\)Ni\(_{2+x}\)Ge\(_{2-x}\) with \(0~ \leq x \leq ~0.015\). Above 0.3 K, the specific heat obeys a \(C\)(\(T\))/\(T\)=\(\gamma _{0}\)-\(\alpha \)\(T\)\(^{~1/2}\) dependence for all the Ni-Ge compositions investigated. Below 0.3 K, however, the \(C\)(\(T\))/\(T\) data are strongly sample dependent. While for \(x\)=0 the quantum-critical \(T\)\(^{~1/2}\) behavior is masked by a low-\(T\) upturn, which follows a \(C\)(\(T\))=\(AT^{~~-2}\) dependence with a large \(A\) parameter (62 \(\mu \)JK/mol), for \(x\)=0.015 a leveling off in \(C\)(\(T\))/\(T\) at \(\simeq \) 380 mJ/K\(^2\)mol is observed. The sample with \(x\)=0.005 and small low-\(T\) upturn below 0.14 K appears to be in the vicinity to the QCP.

abstract

Neutron scattering shows that non-Fermi-liquid behavior of the heavy-Fermion compound CeNi\(_2\)Ge\(_2\) is brought about by development of low-energy spin fluctuations with an energy scale of 0.6 meV. They appear around antiferromagnetic wave vectors \((\frac {1}{2} \frac {1}{2} 0)\) and \((0 0 \frac {3}{4})\) at low temperatures, and coexist with high-energy spin fluctuations with an energy scale of 4 meV and a modulation vector \((0.23, 0.23, \frac {1}{2})\). The energy dependence of the spin fluctuations is a peculiar character of CeNi\(_2\)Ge\(_2\) which differs from typical heavy-Fermion compounds, and suggests importance of low-energy structures of quasiparticle bands.

abstract

Resistivity and specific heat measurements at low temperatures on CeIn\(_{3-x}\)Sn\(_{x}\) samples with concentrations \(x=0.6, 0.65\) and \(0.7\) demonstrate that with increasing Sn-alloying, the antiferromagnetic ordering temperature decreases continuously down to \(T=0\) K and disappears at a quantum critical point. We observe non-Fermi liquid (NFL) behavior in the resistivity and the specific heat, however the temperature dependencies are different from the predictions for a spin density wave (SDW) scenario. The resistivity at the critical point shows  e.g. , a linear temperature dependence, as expected for two dimensional fluctuations, which however can be excluded in the cubic structure of CeIn\(_{3-x}\)Sn\(_{x}\).

abstract

We evaluate the Ginzburg–Landau functional for the case of a superconductor with spin–charge separation. We have obtained analytical results for this functional when \(T \leq T_{\rm c}\), in the limit of a spin–charge separation. For this case and, in the presence of the mass renormalization, we derived the form of the coherence length, \(\varepsilon (T)\), the penetration depth, \(\lambda (T)\), specific head jump, \(\Delta C(T_{\rm c})~/~T_{\rm c}\), at the critical point, and the magnetic upper critical field, \(H_{\rm c}2(T)\). The analytical results found here reduce to the BCS limit for a two–dimensional \(s\)-wave symmetry superconductor. We compare our results with recent works. In particular, we have performed a qualitative comparison with experimental results trying to fix the validity range of our spin–charge separation parameter, \(\eta \). The \(d\)-wave order parameter symmetry does not change drastically the results presented here.

abstract

Real time femtosecond pump-probe spectroscopy is used to study collective and single particle excitations in the charge density wave state of the quasi-1D metal, blue bronze. Along with the previously observed collective amplitudon excitation, the spectra show several additional coherent features. These additional resonances can be excited selectively by applying a sequence of pump pulses with intervals tuned to the period of the particular coherent excitation. A study of the pump power dependence shows a non-linear response of the amplitudon mode, in contrast to the linear power dependence of the single particle, phonon, and phason excitations, which is ascribed to the electron–amplitudon elastic scattering.

abstract

Itinerant antiferromagnetic order can gradually be suppressed by mismatching the nesting of the Fermi surfaces and a quantum critical point is obtained as \(T_{\rm N} \to 0\). Within a renormalization group approach we study the instabilities to spin- and charge-density waves and superconductivity, the low-\(T\) specific heat and the magnetic susceptibility. All quantities increase on a logarithmic scale when \(T\) is lowered, similar to the non-Fermi-liquid behavior observed in some heavy fermion compounds.

abstract

We report an investigation of the antiferromagnetic Heusler metal Mn\(_3\)Si and semi-Heusler metal CuMnSb. The Néel-temperatures \(T_{\rm N} \approx 23\rm K\) for Mn\(_3\)Si and \(T_{\rm N} \approx 50\rm K\) for CuMnSb as seen by a pronounced anomaly in the specific heat are not affected by a magnetic field up to 14 T. The magnetisation is unsaturated, but does not show any signs of metamagnetic transitions in the ordered state. The resistivity drops at the onset of antiferromagnetism, but is not affected by magnetic field apart from the residual value for \(T \to 0\), which increases slightly. This invariance of the antiferromagnetic order to high magnetic field is incompatible with present day models for magnetism in metals, and suggests an unconventional form of antiferromagnetic order.

abstract

We include atomic disorder in correlated narrow-band systems by assuming that the atomic level position fluctuates according to the Lorentzian distribution. We use the Lloyd exact form of the averaged single-particle Green function when the electronic correlations are absent or treated in the saddle-point slave-boson (or Gutzwiller) approximation. The weak disorder reduces drastically the threshold for the Mott–Hubbard localization.

abstract

A pair distribution function (PDF) analysis of temperature-dependent X-ray diffraction measurements from UCu\(_4\)Pd is presented. Fits to the displacement parameters (\(u^2\)’s) with a Debye model show better agreement with a model that includes 25% of the Pd atoms on 16\(e\) (Cu) sites. In addition, significant non-thermal disorder is observed in the Cu environment, in contrast to previous measurements of local order in the U–Cu pairs.

abstract

A single crystal of the orthorhombic UGe\(_2\) has been studied by magnetic (up to 5 T) and magnetotransport (up to 8 T) measurements. It was found that at low fields there exists a domain structure which is reconstructed to a monodomain one in fields higher than 0.1 T. The electrical resistivity has appeared to be highly anisotropic with \(\rho _b \gg \rho _c \gt \rho _a\). For all three crystallographic directions the Fermi liquid equation is followed below about 8 K. Also the transverse magnetoresistivity (TMR) is anisotropic. At low temperatures the TMR for all three principal directions of the unit cell is positive and the values at 4.2 K and 8 T are in the following proportions 1:4:2 for TMR measured along the \(a\)-, \(b\)- and \(c\)-axes, respectively. At temperatures above about 15 (\(b\)-axis) and 25 K (\(a\)- and \(c\)-axes) the TMR becomes negative with a distinct minimum at \(T_{\rm C}\) for the latter two directions. However, the most spectacular behaviour is observed when \(j||b\) and \(B||a\). The TMR goes through very broad negative minimum achieving as high value as –40% at \(T\)=27 K, thus close to so-called characteristic temperature \(T^*\) (= 30 K), that is established from the maximum in the temperature derivative of the resistivity. So far the effect in TMR can be considered as the most distinct manifestation of the existence of magnetic fluctuations in UGe\(_2\) taking place at temperatures at around 1/2\(T_{\rm C}\), without traces of a similar manifestation at \(T_{\rm C}\). The observed giant effect in magnetoresistivity can support the idea of the proximity of the system to a transition into the coupled CDW/SDW type ordering which probably takes place when applying pressure.

abstract

We report the pressure (\(P\))- induced evolution of the antiferromagnetism in CeRhIn\(_5\) which undergoes a superconducting transition at \(T_{\rm c}\)\(\sim \) 2.1 K at pressurs exceeding \(P_{\rm c}\)\(\sim \) 1.6 GPa. From measurements of \(^{115}\)In nuclear-spin-lattice-relaxation time (\(T_1\)) under \(P\), we found that Néel temperature \(T_{\rm N}\) is reduced above \(P\) = 1.23 GPa, which is accompanied by an emergent pseudogap behavior.

abstract

A possible mechanism of the charge stripe due to the four-spin cyclic exchange interaction in the high-\(T_{\rm c}\) cuprate is proposed. The realization of the mechanism is demonstrated by the numerical diagonalization of an extended \(t\)–\(J\) model.

abstract

We report de Haas–van Alphen effect measurements on UGe\(_2\) at pressures P up to \(\sim \)18 kbar, which exceeds the critical pressure P\(_{\rm c}\) \(\sim \)16 kbar for the suppression of ferromagnetism. Particular attention is given to the complicated pressure dependence of the Fermi surface and effective mass in an intermediate pressure region from \(\sim \)11 kbar to \(P_{\rm c}\).

abstract

Both perturbation approach in the Hubbard operator representation and the exact Lehmann representation proves that the electron quasiparticle in SCES has spin \(S=1/2\), electric charge \(e\), QP energy and QP spectral weight split over various QP bands. General theory and its application to the hole doped CuO\(_2\) layer are considered.

abstract

The \(^{115}\)In nuclear spin–lattice relaxation rate (\(1~/~T_1\)) measurements are reported for the heavy fermion (HF) compounds Ce(Ir,Rh)In\(_5\) along with their La analogs La(Ir,Rh)In\(_5\). \(1~/~T_1\) for Ce(Ir,Rh)In\(_5\) is enhanced by one order of magnitude over that in La(Ir,Rh)In\(_5\), indicating strong magnetic fluctuations in these compounds. It is evidenced that CeIrIn\(_5\) is located near a quantum critical point, with quasi-2D spin fluctuations. Also in CeIrIn\(_5\), \(1~/~T_1\) follows a \(T^{~3}\) variation below \(T_{\rm c}=0.40\) K, indicating unconventional superconductivity with line-node gap. These aspects are reminiscent of the high-\(T_{\rm c}\) copper oxides and suggest the importance of the magnetic fluctuations for the occurrence of the unconventional superconductivity in these HF compounds.

abstract

We have studied the pressure-induced superconductivity in an antiferromagnet CeRh\(_2\)Si\(_2\) by measuring the electrical resistivity for a high-quality single crystal. The superconducting resistivity drop was observed around \(P_{\rm c} \simeq 1.06\) GPa, at which the Néel temperature becomes zero, namely in a pressure region from 0.97 to 1.20 GPa. The zero resistivity appears below 0.4 K between 1.03 to 1.08 GPa. The resistivity at low temperatures follows the Fermi liquid \(AT^{~2}\) relation in the whole pressure region, even at \(P_{\rm c}\). The \(A\) value becomes a maximum around \(P_{\rm c}\).

abstract

By paying due attention to \(f\)-orbital symmetry, we propose a two-orbital Hubbard Hamiltonian with \(f\)-electron hopping and Coulomb interactions as an effective model for \(f\) electron systems. We analyze the ground state properties of this model by the exact diagonalization technique and discuss a possible mechanism of unconventional superconductivity.

abstract

A theoretical investigation of the rare earth (RE\(^{3+}\)) localized \(4f\) energy spectrum at regular sites of RE Ba\(_2\)Cu\(_3\)O\(_{6+x}\) (\(x = 0, 1\)), and RE\(_2\)CuO\(_4\) cuprates was performed. To predict the \(k=4\) and \(6\) crystal field (CF) parameters \(B_{kq}\), we employed the semiempirical superposition model (SM) used earlier for quantitative estimates of the CF interaction acting on \(4f\) states in RE cuprates. The SM model does not apply for the \(k=2\) CF parameters where the long range electrostatic contribution dominates. Therefore, to calculate the \(k=2\) CF parameters, we used the parameter-free first-principles method based on the density functional theory.

abstract

Nuclear magnetic resonance (NMR) under high pressure up to 3GPa was performed by using a single crystal of Sr\(_2\)Ca\(_{12}\)Cu\(_{24}\)O\(_{41}\) to investigate the spin dynamics on the critical regime neighboring the superconducting phase. The temperature dependence of the relaxation rate \(1~/~T_1\) shows an activated behavior at high temperatures above 50K and a power law behavior at low temperatures below 50K. The phenomenon is intrinsic to this system and reflects fluctuations originating from motion of holes.

abstract

We have investigated pressure dependence of the superconductivity of HoNi\(_2\)B\(_2\)C using high quality single crystals. Upon applying pressures, \(T_{\rm c}\) was slightly suppressed unlike \(T_{\rm N}\) taken from \(M(T)\) data that increases gradually with pressures. At the same time, reentrance behavior found in \(H_{\rm c2}\) gets subdued and disappears altogether above 8.3 kbar. Our experimental findings can be understood in terms of the Ginzburg–Landau type analysis.

abstract

Taking advantage of the additional uniaxial stress present in the non-ideal pressure conditions of a Bridgman anvil cell, we demonstrate the high sensitivity of the physical properties in CePd\(_2\)Si\(_2\) to anisotropic strain. Stress applied along the \(c\)-axis extends the phase diagram to higher pressures and enhances the superconducting phase emerging around the magnetic instability, with a 40% increase in the maximum superconducting temperature and a doubled pressure range. We discuss first the possible effect of anisotropic strain on the physics described only by spin fluctuations. However, the pressure dependence of the resistivity suggests a more complex ground state around the quantum critical point, where the Kondo and excited crystal field energies interact.

abstract

We discuss the origin of the unconventional superconductivity in a Hubbard model. The origin of the unconventional superconductivity is considered to be a wave number dependence of a quasi particle interaction, which is induced by the Coulomb interaction \(U\). Using the third order perturbation theory with respect to \(U\), we discuss the wave number dependence induced by spin fluctuations and vertex corrections. We investigate the pairing states for the various lattice structures in the Hubbard model and we point out the important factors in the origin of the singlet and triplet superconductivities.

abstract

DC magnetization process of a high-quality single crystal of CeCoIn\(_5\) has been measured at low temperatures down to 50 mK. A sharp magnetization jump with a small hysteresis is observed at the upper critical field \(H_{\rm c2}\) for both \(a\) and \(c\) directions, indicating the transition to the normal state to be of first-order. Although the results might suggest a strong Pauli paramagnetic effect, no Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) phase is observed.

abstract

Magnetism in the vortex core state has been studied by spatially-resolved NMR. The nuclear spin lattice relaxation rate \(T_1^{-1}\) of \(^{205}\)Tl in nearly optimal-doped Tl\(_2\)Ba\(_2\)CuO\(_{6+\delta }\) (\(T_{\rm c}=85\) K) is significantly enhanced in the vortex core region. The NMR results suggest that the suppression of the \(d\)-wave superconducting order parameter in the vortex core leads to the nucleation of islands with local antiferromagnetic (AF) order.

abstract

Electrical resistance and magnetization of RuSr\(_2\)GdCu\(_2\)O\(_8\) have been measured at high pressure in order to clarify the interplay between the magnetic ordering and superconductivity. It is found that the magnetic ordering temperature (\(T_{\rm m}\)) and the superconducting transition temperature (\(T _{\rm C}\)) increase with increasing pressure. These results imply that the superconductivity coexists with magnetic ordering at least up to 2.1 GPa.

abstract

We study the impurity effect on the superconducting \(d\)-wave state accounting for the momentum-dependent impurity potential. We discuss the impurity-induced critical temperature suppression, local density of states in the vicinity of a single impurity, and the density of states in a superconductor with a spatial impurity distribution.

abstract

We have measured isothermal magnetization curves of the ferromagnetic superconductor UGe\(_2\) at pressures up to 18.3 kbar and at temperatures down to 0.45 K. The pressure dependence of the Curie temperature \(T_{\rm Curie}\) evaluated from a linear part of the Arrott plot is relatively in agreement with the literatures. The paramagnetic Curie temperature, however, deviates from \(T_{\rm Curie}\) with increasing pressure, especially above about 10 kbar. In addition, we have found a staircase-like magnetization hysteresis loop with regular interval of magnetic fields only below about 1 K. We ascribe it to the macroscopic quantum tunneling, and the analysis suggests tiny magnetic domain formation whose size is smaller than the superconducting coherence length.

abstract

The pressure study of superconductivity on a pyrochlore oxide Cd\(_2\)Re\(_2\)O\(_7\) has been done by using a hybrid type piston cylinder cell up to 2.7 GPa. The superconductivity transition temperatures \(T_{\rm c}\) increase linearly from 1 K to 2.6 K with increasing pressure up to  2.4 GPa. The residual resistivity \({\rho }\)\(_0\) gradually increases with pressure and then suddenly increases around 2.4 GPa. We found that two step superconductivity transitions appear between 0.6 and 1.36 GPa and then disappear with increasing pressure.

abstract

We investigate a system composed of the itinerant electrons coexisting with the localized pairs (hard-core bosons) whose energies are assumed to be site dependent. The randomness of boson energies induces in turn fluctuations of the effective pairing potential between fermions. We analyze an effect of such randomness on the anisotropic \(d\)-wave superconducting phase of this model. In particular, we determine transition temperature \(T_{\rm c}\) and the amplitude of the order parameter parameter \(\chi _{ij}=\langle c_{i\downarrow }c_{j\uparrow } \rangle \) as functions of the boson energies fluctuations.

abstract

We present a stability analysis of the 2D \(t\)–\(t'\) Hubbard model on a square lattice for \(t'=-t/6\). We find possible phases of the model (\(d\)-wave Pomeranchuk and superconducting states, band splitting, singlet and triplet flux phases), and study the interplay of them.

abstract

We studied the dependence of the superconducting transition temperature \(T_{\rm SC}\) on the residual resistivity in the ferromagnetic superconductor URhGe. The strong suppression of \(T_{\rm SC}\) caused by defects was observed. Superconductivity is completely destroyed when the mean free path falls below the superconducting coherence length, suggesting unconventional superconductivity.

abstract

One of unusual features of high-\(T_{\rm c}\) superconductors, that we discuss in the present report, is related to inhomogeneous distribution of holes. It results in a stripe-phase which consists of antiferromagnetic domains separated by hole-rich domain walls. We study how the upper critical field is affected by this specific distribution of carriers. We consider a two-dimensional square lattice immersed in a perpendicular uniform magnetic field. In order to simulate the presence of a stripe-phase we carry out the calculations for a system with modulated hopping integral. Namely, the magnitude of the hopping integral is constant along the stripe, whereas it oscillates in the opposite direction.

abstract

We present a model of changing Fermi surface geometry in a ferromagnetic, spin-split environment, where the control parameter is the Stoner exchange energy. A two-peak density of states, here obtained from a quasi-one-dimensional bandstructure allows two jumps in magnetisation. The jump at finite magnetisation can be first order, and may occur near a maximum in the transition temperature for a triplet superconducting instability. Our motivation is the ferromagnetic superconductor, UGe\(_{2}\).

abstract

We performed the heat capacity, \(C(T)\), measurement under high pressure for a ferromagnetic superconductor UGe\(_2\). At 1.15 GPa, we found a peak in \(C/T\) at a characteristic temperature, \(T^{~*}= 6\) K where the magnetization showed anomalous increase. At 1.28 GPa, above the critical pressure \(P_{\rm C}^{~*}\) where \(T^{~*}\) becomes 0 K, \(C/T\) does not show notable anomaly at zero magnetic field, but the application of the external field above 2.5 T induces the peak again at \(T^{~*} \gt 0\) K. It corresponds to the induction of \(T^{~*}\) by the magnetic field. Our experimental result suggests that a second order phase transition takes place at \(T^{~*}\) and thus a disappearance of the phase transition at \(T^{~*}\) is of second order.

abstract

An effective Hamiltonian representing propagation and interaction of spin polarons forming in weakly doped antiferromgnets is derived. The systems becomes superconducting upon doping. Paring may be attributed to kinetic energy saving due to a particular type of motion of a spin bipolaron, which corresponds to creep of a line consisting of defects in the antiferromagnetic spin background which connects two holes. That line moves by shrinking and expanding at opposite ends. The kinetic energy term in the Hamiltonian is effective at all stages of this process, and the term related to the magnetic exchange does not have to intervene, not like in the case of a single propagating hole, which lowers the kinetic contribution to the energy. The effective attraction is strongest in the undoped system where the antiferromagnetic order is most robust, but the superconducting order parameter vanishes when the doping parameter decreases which should be attributed to emptying the spin polaron band and approaching the Mott insulator phase. Since the normal phase representing a gas of single spin polarons is unstable toward formation of bound hole pairs a pseudogap forms in the excitation spectrum.

abstract

Effects of magnetic field on the multiple-gap structure in the superconductor MgB\(_2\) have been studied by break junctions. With increasing the field, the gap value decreases with filling up of the states inside of the gap. The gap-closing field \({B_{\rm c}}\) correlates with the gap size. The extrapolated \({B_{\rm c}}\) value for the larger gap is almost consistent with the upper critical field of this compound.

abstract

We discuss the local spin-triplet pairing among correlated fermions that is induced by the Hund’s rule coupling in orbitally degenerate systems. The appearance of the spin-polarized superconducting phase makes the Stoner threshold a hidden critical point, since the pairing creates a small but detectable uniform magnetization even below this critical point.

abstract

The existence of long range magnetic order and superconductivity in single crystals of CeCu\(_{2}\)(Si\(_{0.9}\)Ge\(_{0.1}\))\(_{2}\) is investigated by measuring the electrical resistivity under pressure. At ambient pressure, antiferromagnetic order coexists with superconductivity. Upon applying pressure, the magnetic order vanishes continuously. Interestingly, we observe two superconducting domains in the \(T\)–\(P\) phase diagram, where superconductivity is lost inbetween. In the first domain (0\(\leq \) \(p\lt 3\) GPa), \(T_{\rm c}\) initially increases with pressure, reaching a maximum value of \(T_{\rm c}^{{\rm max}_{1}}\approx 0.39\) K at \(p \approx p_{\rm c1}\). Above \(p_{\rm c1}\), \(T_{\rm c}\) decreases with increasing pressure. In the second domain, the maximum \(T_{\rm c}\) (\(T_{\rm c}^{{\rm max}_{2}}\approx 0.96\) K at \(p_{\rm c2}\approx 5.4\) GPa) is much higher than that in the first domain. Non-Fermi-liquid behavior with resistivity exponent 1\(\leq \) \(n\) \(\leq \)1.5 (\(\rho =\rho _{0}+AT^{n}\)) is observed at pressures \(p_{\rm c1}\)\(\leq \) \(p\)\(\leq \) \(p_{\rm c2}\).

abstract

Magnetic and charge ordering in the stripe phases in the Hubbard model are investigated in the regime of large on-site Coulomb repulsion, using correlated wave functions. We have found that the appearance of stripes is a robust feature, while vertical and diagonal stripe phases compete with each other. The Hartree–Fock approximation yields qualitative information on the hole and magnetization density in the stripes, but their stability can be decided only by including the electron correlations.

abstract

We show that the two-channel Anderson lattice model develops unconventional superconductivity out of a metallic non-Fermi liquid phase. It is characterized by a composite order parameter comprising of a local spin or orbital degree of freedom bound to triplet Cooper pairs with an isotropic and a nearest neighbor form factor. The gap function is non analytic and odd in frequency, and a pseudo-gap develops in the conduction electron density of states which vanishes as \(|\omega |\) close to \(\omega =0\).

abstract

Vortex imaging method by the site selective NMR is explained by self-consistently solving the Bogoliubov–de Gennes equation for both \(s\)-wave and \(d\)-wave pairings. We analyze the temperature dependence of the relation time \(T_1\) using the resonance frequency at the intensity maximum, namely the saddle point \(T_1\) experiments. The site selective \(T_1\) data are shown to yield valuable information on low-lying electronic excitations around a vortex core.

abstract

The recently discovered superconductivity in MgB\(_2\) has created the world sensation. In spite of the relatively high superconducting transition temperature \(T_{\rm c}=39\) K, the superconductivity is understood in terms of rare two gap superconductor with energy gaps attached to the \(\sigma \) and \(\pi \)-band. However, this simple model cannot describe the temperature dependent anisotropy in \(H_{c2}\) or the temperature dependence of the anisotropic magnetic penetration depth. Here we propose a model with two anisotropic energy gaps with different shapes. Indeed the present model describes a number of peculiarities of MgB\(_2\) which have been revealed only recently through single crystal MgB\(_2\).

abstract

High-resolution bulk-sensitive photoemission has been measured for pyrochlore oxide Cd\(_{2}\)Re\(_{2}\)O\(_{7}\). Temperature variations of the spectral shapes between 20 and 250 K are obviously observed for the Re 4\(f\) inner core and the valence band spectra. The Re 5d states are dominant for the DOS near the Fermi level.

abstract

We have recently proposed the \(s+g\) wave model for superconducting borocarbides. In spite of a substantial \(s\)-wave component, this order parameter exhibits the \(\sqrt {H}\) dependent specific heat and a thermal conductivity linear in \(H\) in the vortex state. This is characteristic for nodal superconductors when \(T,{\mit \Gamma } \ll {\mit \Delta }\) where \({\mit \Gamma }\) is the quasiparticle scattering rate and \({\mit \Delta }\) the maximum superconducting gap. Here we investigate the thermal conductivity parallel to the \(c\)- and \(a\)-axis in a magnetic field tilted by \(\theta \) from the \(c\)-axis and rotating within the \(a\)–\(b\) plane.

abstract

Several recent experiments have revealed that the charge density \(\rho \) in a given compound (mostly underdoped) is intrinsic inhomogeneous with large spatial variations. Therefore, it is appropriate to define a local charge density \(\rho (r)\). These differences in the local charge concentration yield insulator and metallic regions, either in an intrinsic granular or in a stripe morphology. In the metallic region, the inhomogeneous charge density produces spatial or local distributions of superconducting critical temperatures \(T_{\rm c}(r)\) and zero temperature gap \(\Delta _0(r)\). We propose that the superconducting phase in high \(T_{\rm c}\) oxides is reached when the temperature reach a value which superconduction regions with different critical temperatures percolates. We show also that this novel approach is able to reproduce the phase diagram for a family of cuprates and provides new insights on several experimental features of high \(T_{\rm c}\) oxides.

abstract

In the ideal pressure conditions of a helium-filled diamond anvil cell, we have been able to probe the resistivity and, for the first time, the specific heat of the heavy fermion superconductor CeCu\(_2\)Si\(_2\) at pressures over 6 GPa, down to temperatures below 100 mK, and in a magnetic field up to 8 T. We clearly observed the superconducting jump using the AC calorimetry technique, which provides a semi-quantitative measure of the sample specific heat. The evolution of the superconducting transition with pressure was observed quasi-simultaneously in a single sample in both the resistivity and heat capacity. The jump in \(C_p\) hints at changes in the coupling regime. When \(T_{\rm c}\) is a rapidly varying function of pressure, the resistive transition is broadened and strongly dependent on the measuring current. When \(T_{\rm c}\) has a maximum at 2\(\:\)K, the residual resistivity shows a peak, and the resistivity is linear in temperature above \(T_{\rm c}\).

abstract

We propose that the pseudogap phase in high \(T_{\rm c}\) cuprates may well be \(d\)-wave spin density wave. We show that both the micro-magnetism observed by neutron scattering in the pseudogap regime of Bi2212 and the optical dichroism seen by ARPES follow naturally from USDW. Also we predict that the magneto-resistance in the pseudogap regime should exhibit a peculiar angular dependence, which should be accessible experimentally.

abstract

We report on investigations of CeCoIn\(_5\) in the superconducting state by point-contact spectroscopy. Andreev reflection of quasiparticles at a normal metal/superconductor interface leads to characteristic features in the differential resistance \(dV/dI\) as a function of applied bias \(V\). We measured spectra which show either a reduced resistance for bias \(|V|\lt {\mit \Delta }/e\) with a double minimum at \(V\approx \pm {\mit \Delta } /e\) or a single minimum of \(dV/dI\) for \(V=0\), i.e. , a zero-bias anomaly. Both features are weakened with increasing temperature and vanish close to \(T_{\rm c}\). The observation of a zero-bias conductance anomaly is expected only if the order parameter exhibits a sign change as a function of \(\vec k\). Andreev reflection of the quasiparticles at the surface leads to a surface bound state which is detected in the \(dV/dI\) vs \(V\) spectra. Therefore the data support unconventional superconductivity in CeCoIn\(_5\).

abstract

The superconducting gap structure, especially the direction of the nodes, is an unresolved issue in most of unconventional superconductors. Recently it has been demonstrated that the thermal conductivity \(\kappa \) is a powerful tool for probing the nodal structure. Here measuring \(\kappa \) in \(\mathbf {\mit H}\) rotating within the basal plane, we discuss the nodal structure of the unconventional superconductors, spin-triplet Sr\(_2\)RuO\(_4\), heavy fermion CeCoIn\(_{5}\), organic \(\kappa \)-(BEDT-TTF)\(_2\)Cu(NCS)\(_2\), and borocarbide YNi\(_2\)B\(_2\)C.

abstract

We study the impurity bound states in \(d\)-wave charge and spin density wave (CDW & SDW) phases, which are candidate models for the pseudogap regime in the high-\(T_{\rm c}\) cuprates. The Bogoliubov–de Gennes equations for a single impurity are solved. When the impurity is nonmagnetic, there is no distinction between CDW and SDW. The bound state wave function exhibits a fourfold symmetry pattern analogous to the \(d\)-wave superconducting phase. In addition, the wave function exhibits a checkerboard-like pattern, previously observed around the vortex bound states in the underdoped region of Bi2212. These predictions should be readily accessible to scanning tunneling microscope experiments.

abstract

We have performed studies of the out-of-plane reflectivity in the normal conducting state of pure and Ti-doped Sr\(_2\)RuO\(_4\) single crystals. The scattering rate in Sr\(_2\)RuO\(_4\) can be explained by a gap-like behavior with an exponential increase of the scattering rate which convincingly describes the temperature dependence of the out-of-plane dc resistance. On doping with non-magnetic Ti the gap becomes reduced and is fully closed for \(x=0.05\).

abstract

We consider a possibility of two-boson pairing in a dilute 2D Bose-gas on a lattice with strong hard-core repulsion \(U\) and a Van der Waals attractive tail \(V\). The phase diagram of Bose gas consisting of one sort of structureless bosons contains only two regions: the usual one particle Bose–Einstein condensation (BEC) and the region of total phase separation on the Mott–Hubbard Bose solid and dilute Bose gas. But in the system with two sorts of structureless bosons the creation of the two-particle condensate (\(\langle bb \rangle \neq 0\)) is possible. We show that the full set of equations for stability of homogeneous two-particle condensate is satisfied.

abstract

Filled skutterudites \(\rm RETM_4X_{12}\) with RE = rare earth, TM = transition metal and X = pnictogen represent a class of complex materials exhibiting a broad variety of ground state properties. These model systems also allow to tailor the charge carrier density in order to optimise the thermoelectric performance with respect to its applicability in both, energy conversion and cooling processes. The present review focuses mainly on strong electron correlations in such compounds and their effect on thermoelectric properties.

abstract

We report on sample preparation, annealing effects, electron microprobe analysis in the series Ce\(_{y}\)Fe\(_{4-x}\)Co\(_{x}\)X\(_{12}\) which shows that a phase separation occurs for substituted samples annealed at 650\(^\circ \) C. The valence state of Ce in homogeneous single phase samples has been studied using X-ray absorption spectroscopy (XAS). Ce ions are trivalent in the series.

abstract

We report optical investigations of the electronic properties of the clath- rate compound Ba\(_6\)Ge\(_{25}\) in which at room temperature Ba atoms “rattle” in Ge-network cavities. When lowering the temperature across \(T_{\rm S}\simeq 200\) K a lock-in of the Ba-atoms to split-site positions in the cages is observed. The low energy Drude type of reflectivity is characterized by a low charge carrier density which smoothly varies with temperature. However, the Drude relaxation time of the charge carriers is found to be almost temperature independent, especially when cooling below \(T_{\rm S}\) where, according to thermopower data, the effective mass is enhanced. This behavior could indicate a formation of polaronic quasi particles below \(T_{\rm S}\) which is also supported by previous measurements of magnetic susceptibility of Ba\(_6\)Ge\(_{25}\).

abstract

The mother material CuIr\(_2\)S\(_4\) of the thiospinel system Cu\(_{1-x}\)Zn\(_x\)Ir\(_2\)S\(_4\) undergoes a temperature-induced metal–insulator (MI) transition. We report the temperature dependence of the optical reflection spectra of Cu\(_{1-x}\)Zn\(_x\)Ir\(_2\)S\(_4\) (x\(\le \)0.5) at the temperatures of 8–300 K in the energy regions of 0.005–30 eV in order to study the change in the electronic structure due to the Zn substitution for Cu. Zn substitution induced mainly the splitting of the hybridization band between the Ir-\(5d(t_{2g})\) and S-\(3p\) states crossing the \(E_{\rm F}\). Obtained optical conductivity (\(\sigma \)) spectrum is discussed in relation to the change in the electronic structure close to the \(E_{\rm F}\).

abstract

The low temperature (\(T\lt 70\) K) specific heat obtained for the UCu\(_{4+x}\)Al\(_{8-x}\) derivatives in which the \(3d\) elements (Cr, Mn and Ni) have been substituted for Cu, and Ga for Al is presented. The parent alloys have exhibited an alloying induced transition from simple antiferromagnetic ordering for low Cu–concentration (for \(x \leq 1.25\)) to heavy-fermion-like properties (for \(x \geq 1.5\)). The crystallographic disorder introduced by a change of stoichiometry could be the reason for enhanced \(\gamma \). These components slightly change both the crystallographic relations and electronic structure, and the determined thermodynamic data strongly suggest other reasons for an enhanced \(\gamma \) than crystallographic disorder. The occurrence of the \(T^{~3}\ln T\) term in specific heat for all alloys except that of Ni provides the conclusion about spin fluctuation state. The Ni compound is antiferromagnetic below \(T_{\rm N}=27.5\) K.

abstract

We report on full 3-directional neutron polarization analysis studies of the magnetic correlations in LiV\(_2\)O\(_4\). The data show the evolution of anti-ferromagnetic spin fluctuations at low temperatures (\({T \le 40}\) K) with a characteristic wave vector of \(Q = 0.7\,\)Å \(^{-1}\). The measured magnetic cross section reveals a temperature induced cross-over from predominantly ferromagnetic (\({T \ge 40}\) K) to RKKY-like (anti-ferromagnetic + ferromagnetic) spin correlations (\({T \le 40}\) K), as evidenced by reverse Monte Carlo simulations. These results are in agreement with the results of the magnetic relaxation rate on LiV\(_2\)O\(_4\).

abstract

Conductance through a system consisting of a wire with two side-attached quantum dots is calculated. Both the quantum dots take part in destructive interference with ballistic channel through the wire. Such geometry of the device allows to control the strength of the quantum interference and suppression of the conductance through the system. The minimum present in the gate voltage characteristics of the conductance can be turned into plateau. We propose an experimental setup where the strength of the quantum interference can be smoothly controlled by changing the level positions inside quantum dots by appropriate gate voltages.

abstract

We calculate the conductance through a single quantum dot coupled to metallic leads, modeled by the spin 1/2 Anderson model. We adopt the finite-\(U\) extension of the noncrossing approximation method. Our results are in good agreement with exact numerical renormalization group results both in the high temperature and in the Kondo (low temperature) regime. Thanks to this approach, we were able to fit fairly well recently reported measurements [1]. in a quantum dot device. We show that, contrarily to what previously suggested, the conductance of this particular device can be understood within the spin 1/2 Anderson model, in which the effects of the multilevel structure of the dot are neglected.

abstract

A theoretical study of correlation has been performed for an electron pair confined in a vertically coupled double quantum dot by a method, which transforms the three-dimensional two-electron problem into the effective one-dimensional two-electron problem. We have found that the electron correlation cannot be neglected at any interdot distance and becomes strong at large interdot distances.

abstract

The coherent electronic transport through a quantum dot coupled to ferromagnetic electrodes is calculated for the parallel and antiparallel orientations of polarizations. The influence on transport of both the charge fluctuations and the Kondo resonance are considered by using the equation of motion method for the non-equilibrium Green functions. The largest spin accumulation is observed in mixed valence range. In Kondo regime the state of a particle is a singlet for any polarization of the leads.

abstract

We have proposed a novel method (EDABI) of approach to nanoscopic correlated systems that combines an exact diagonalization method with an ab initio readjustment of the single-particle orbitals. With the help of this method we study correlated electronic states of atoms and small clusters. In the case of atomic systems with \(Z\lt 10\) we can perform the analysis of the ground- and excited- states systematically improving the accuracy of the calculation. For the two- and three- dimensional clusters containing up to \(N=4\) atoms the electronic and lattice properties are analyzed as a function of interatomic distance. Three- and four- site interactions are included for simple s-like orbitals. With an increasing interatomic distance the Hubbard gap appears already for the cluster systems.

abstract

Exact ground state properties are presented by combining the exact diagonalization in the Fock space (including the long-range Coulomb interaction) with an ab initio optimization of the single-particle (Wannier) functions. The quasiparticle mass is almost divergent at the localization threshold, where the particle distribution of the Fermi-Dirac type gets smeared out. The analysis is performed using 1\(s\)-like Gaussian-type orbitals.

abstract

We have applied our novel numerical scheme combining Lanczos diagonalization in the Fock space with an ab initio renormalization of the single-particle (Wannier) functions, to study the ground state properties of the Extended Hubbard Model. Through the finite-size scaling we determine the discontinuity of the momentum distribution Fermi surface. Our results imply Fermi-liquid behavior for lattice parameter \(a\lesssim 3a_0\) (\(a_0\) is the Bohr radius) and zero-temperature transition to the localized spin system for larger \(a\). Possible further applications of the method are listed at the end.

abstract

Motivated by experiment, we argue that the enigmatic hidden order in URu\(_2\)Si\(_2\) demands a dual description that encompasses both its itinerant and its local aspects. A combination of symmetry considerations and observation allow us to rule out a number of possibilities. The two remaining scenarios, the quadrupolar charge density wave and the orbital antiferromagnet, are discussed and experiments are suggested to select between these proposals.

abstract

When pressure is applied to the cerium and uranium compounds, their magnetic ordering temperatures are suppressed and become zero at a critical pressure \(P_{\rm c}\). Around \(P_{\rm c}\), non-Fermi liquid and/or superconductivity are observed. We clarified a change of the electronic state via the de Haas–van Alphen (dHvA) experiment when pressure crosses \(P_{\rm c}\). The dHvA experiment under pressure was done for antiferromagnets CeRh\(_2\)Si\(_2\), CeRhIn\(_5\) and URu\(_2\)Si\(_2\), and a ferromagnet UGe\(_2\). We find an abrupt change of the Fermi surface for CeRh\(_2\)Si\(_2\) and UGe\(_2\) when crossing \(P_{\rm c}\), indicating a first-order like phase transition. For CeRhIn\(_5\) and URu\(_2\)Si\(_2\), a change of the cyclotron mass is clearly observed.

abstract

The Laves phase ZrZn\(_{2}\) exhibits weak ferromagnetism at low temperatures that differs in many ways from the predictions of a weakly spin polarised Fermi liquid. Surprisingly, ZrZn\(_{2}\) recently was even found to become superconducting in the milli-Kelvin temperature range. Ferromagnetism and superconductivity vanish above the same critical pressure \(p_{\rm c} \approx 21~\rm kbar\), suggesting that itinerant ferromagnetism may be a precondition for the superconductivity.

abstract

Although the concept of a quantum phase transition has been known since the nineteen seventies, their importance as a source of radical transformation in metallic properties has only recently been appreciated. A quantum critical point forms an essential singularity in the phase diagram of correlated matter. We discus new insights into the nature of this phenomenon recently gained from experiments in heavy electron materials.

abstract

Recent experiments on the quantum phase transition between nonmagnetic (\(x \leq x_{\rm c}\)) and magnetically ordered (\(x \gt x_{\rm c}\)) CeCu\(_{6-x}\)Au\(_x\) alloys, occurring at a critical concentration \(x_{\rm c} \approx 0.1\) are reviewed. In particular, we investigate how the \(q\)-dependence of the critical fluctuations observed for \(x = 0.1\) by inelastic neutron scattering evolve when moving away from the critical concentration. We also explore the fluctuations when a magnetically ordered alloy (\(x = 0.2\)) is exposed to a magnetic field suppressing the magnetic order. Finally, we address the question whether at the quantum phase transition the Kondo temperature vanishes, i.e. the heavy quasiparticles loose their identity, as proposed by recent theories.

abstract

Neutron diffraction experiments performed on single crystal of the antiferromagnetic compound CePd\(_{2}\)Si\(_{2}\) under applied pressure up to 18 kbar are reported. The magnetic structure and the temperature line-shape of the order parameter do not change with pressure. Anomalous suppression of the magnetic moment is found at high pressure.

abstract

We have studied the electronic state in UGe\(_2\) via the de Haas–van Alphen effect under pressure. The cyclotron masses are determined in the strongly polarized phase and in the paramagnetic phase for magnetic field along the \(c\)-axis and the results are compared with those along \(a\)- and \(b\)-axes.

abstract

From high frequency (up to 450 GHz) ESR and low temperature specific heat measurements we find that insertion of 1% Fe and 2% Co in CuGeO\(_{3}\) matrix smears both spin-Peierls and Néel transitions. For \(T\lt 30\) K we discovered an onset of the power asymptotics of magnetic susceptibility and specific heat characteristic to a quantum critical behaviour.

abstract

We present the simplest model to which one can apply the supersymmetric Hubbard operators recently introduced P. Coleman, C. Pépin, J. Hopkinson, Phys. Rev. B62, 3852 (2000). For the atomic model, \(H=-E_d X_{00}\), where \(X_{00} = |0\rangle \langle 0|\) is a Hubbard operator and \(E_d\) is the energy of the localized spin level, we show how one can develop exact solutions for the entropy and heat capacity as a function of temperature. With this gold standard we are able to develop a controlled approximation scheme to field theoretically treat the SUSY approximation at the level of mean field + Gaussian corrections and test its accuracy against the widely used slave boson and slave fermion approximations. We find that in addition to slave boson and slave fermion solutions, a new class of solutions exists in the physical case \(Q=1\), \(N=24\) which can be properly treated by neither previously existing approach. The phase diagram generated by the mean field saddle-point bears a superficial resemblance to the V-shaped phase diagram common to systems close to a quantum critical point and may provide a natural starting point for investigations of strongly correlated models capturing this physics.

abstract

Electronic Raman scattering has been employed to examine a number of different correlated insulators, including the high-temperature superconductors, Kondo insulators (like FeSi), and intermediate-valence compounds (like SmB\(_6\)). The experimental data all share a number of common features: in the B\(_{1g}\) channel (crossed polarizers) one finds (i) a sudden onset of low energy spectral weight transfered from a higher charge-transfer peak, which rapidly increases as \(T\) increases; (ii) the appearance of an isosbestic point (where the Raman response is independent of \(T\)) separating the regions where the spectral weight shifts; and (iii) a large ratio of the spectral range over which spectral weight increases as \(T\) increases (representative of the charge gap) to the onset temperature, where the gap appears to first open. We solve for the Raman response exactly using dynamical mean field theory for the Falicov–Kimball model and the Hubbard model. Our solutions illustrate all three of these experimental features. In addition, we calculate the inelastic light scattering from X-rays, which allows the photon to transfer both energy and momentum to the electronic charge excitations. We find that the charge transfer peak and the low energy peak both broaden and disperse through the Brillouin zone similar to what is seen in experiments in materials like Ca\(_2\)CuO\(_2\)Cl\(_2\).

abstract

Exact localised ground-states are presented for the one- and two-dimensional periodic Anderson model at finite \(U \gt 0\) in restricted regions of the parameter space, which extends from the low \(U\) to the high \(U\) regions as well. The physical properties of this phase are analysed in detail.

abstract

Recent X-ray and neutron diffraction experiments revealed that the metal–insulator transition of BaVS\(_3\) at \(T_{\rm MI} \simeq 70\) K is accompanied by crystallographic superlattice formation along the \(c\) axis and nearly uniform reduction of the magnetic moment to approximately half of \(S = 1/2\). The origin of the transition is discussed assuming anisotropic conduction bands.

abstract

The research of magnetic properties of transition metal borates FeBO\(_3\), VBO\(_3\), CrBO\(_3\) was carried out. New experimental data concerning magnetic structure of CrBO\(_3\) is obtained. The exchange and anisotropy field values are determined.

abstract

The extended Hubbard Hamiltonian is solved analytically for the case of two-particles in an infinite one-dimensional lattice, using a real-space mapping method and the Green function technique. The results obtained are in agreement with the numerical solution previously reported by O. Navarro and C. Wang Solid State Commun. 83, 473 (1992).

abstract

The pseudospin-electron model (PEM) is considered in the case of the weak pseudospin-electron coupling. It is shown that the transition to uniform and chess-board phases occurs when the chemical potential is situated near the electron band edges and near the band centre, respectively. The incommensurate phase is realized at the intermediate values of the chemical potential.

abstract

We investigate the Holstein model, which describes the coupling of a local phonon mode to a band of conduction electrons, in \(d=\infty \). In the limit of large phonon frequency \(\omega _0\), this model can be mapped onto a Hubbard model with effective attractive electron–electron interaction. The latter model is known to exhibit a metal–insulator transition at half filling for large enough coupling strengths. We show that the system with small phonon frequencies also develops a gap as function of the electron–phonon coupling. The physics of the gap formation differs for small and large phonon frequencies, e.g. the “hysteresis” seen for large \(\omega _0\) disappears for smaller values.

abstract

In the last three years, various theoretical interpretations have been proposed on the nature of the ground state in the antiferromagnetic (AFI) phase of V\(_2\)O\(_3\). There are two main results on this subject: the idea of a correlated vertical molecule and that of a complex ground state. In spite of the results achieved, we show, with the present work, that the pictures proposed are still lacking and more work is needed for a unified description of the AFI phase of V\(_2\)O\(_3\).

abstract

Ground-state properties of the periodic Anderson model with a dispersion of \(f\)-electrons are investigated at half-filling in infinite dimensions. We determine the magnetic phase diagram by using dynamical mean field theory combined with a perturbative treatment of the \(f\)-\(f\) Coulomb interaction. Nonmonotonic behavior is found in the phase boundary when a dispersion of \(f\)-electrons is changed, the origin of which is discussed in the light of formation of renormalized quasi-particles.

abstract

The electronic structures of the pyrochlore-type Ru oxides Sm\(_{2-x}\)Ca\(_x\)Ru\(_2\)O\(_7\) and Sm\(_{2-x}\)Bi\(_x\)Ru\(_2\)O\(_7\), which show metal–insulator transition with increasing Ca or Bi concentration, have been studied by ultraviolet photoemission spectroscopy. Spectral changes near the Fermi level are different but reflect the tendency of their transport properties in both systems. The Sm\(_{2-x}\)Ca\(_x\)Ru\(_2\)O\(_7\) system shows an energy shift, which is expected from the increase of hole in the Ru \(4d\) \(t_{2g}\) band and the Sm\(_{2-x}\)Bi\(_x\)Ru\(_2\)O\(_7\) system shows spectral weight transfer within the Ru \(4d\) \(t_{2g}\) band, which is expected to be observed in bandwidth-control Mott-Hubbard system.

abstract

Following to the comprehensive study of a steady magnetic field dc- and ac-transport and magnetization in the almost magnetic narrow-gap semiconductor FeSi the galvanomagnetic measurements have been carried out in pulsed magnetic fields up to 50T. It was shown from the analysis of the experimental data obtained on high quality single crystals of iron monosilicide that the Mott–Hubbard scenario of metal–insulator transition with on-site Coulomb interaction \(U\)\(\approx \)3D (D-is the band half-width) provides the most adequate description of the low temperature anomalies in this model system. From this point of view the pulsed field magnetoresistance and Hall coefficient anomalies may be also interpreted in terms of a MIT in a strong magnetic field.

abstract

In this paper both the metal–insulator transition and ferromagnetic ordering stabilization have been studied in the framework of the Hubbard model with correlated hopping and inter-atomic exchange interaction. The energy spectrum of the model has been calculated, expressions for the energy gap in the paramagnetic state, the ground state energy, the criterion of the ferromagnetism stabilization, the magnetization of system have been obtained and analyzed.

abstract

Longitudinal sound velocity has been measured for organic superconductors \(\kappa \)-(ET)\(_2\)Cu[N(CN)\(_2\)]Br as a function of temperature. We measured the velocity of the sound propagating along both parallel and perpendicular to the two-dimensional conduction plane. The sound velocity of both directions shows a remarkable anisotropic behavior below \(T_M\), in addition to the anomaly at \(T_M = 37\) K associated with Insulator-to-Metal crossover.

abstract

In hole-doped high-\(T_{\rm c}\) cuprates, the Nernst coefficient (\(\nu \)) as well as the magnetoresistance (\(\Delta \rho /\rho \)) increase drastically below the pseudo-gap temperature, \(T^\ast \). This unexpected result attracts much attention in that it reflects the fundamental feature of the electronic state in the pseudo-gap region, which has been a central issue on high-\(T_{\rm c}\) cuprates. In this article, we study these transport phenomena in terms of the fluctuation-exchange (FLEX)+\(T\)-matrix approximation. We focus on the role of the vertex corrections (VC’s) which are necessary to keep the conservation laws, and find that both \(\nu \) and \(\Delta \rho /\rho \) are strongly enhanced by the VC’s due to the antiferromagnetic (AC) and the superconducting (SC) fluctuations. In conclusion, the pseudo-gap region in high-\(T_{\rm c}\) cuprates is well described by the AF and SC fluctuation scenario based on the Fermi liquid theory.

abstract

The general Dynamical Mean-Field Theory approach based on the expansion over electron hopping and local nature of the irreducible parts (cumulants) of the Green’s functions in infinite dimensions is developed for the description of correlated hopping. In order to calculate the thermodynamical functions the new grand canonical potential functional is proposed that allows to build the \({\mit \Phi }\)-derivatible theory without introducing self-energy.

abstract

A sequence of transitions from an antiferro-type order to a phase separate state under doping away from half filling is studied within the 1D Falicov–Kimball model. Using the method of restricted phase diagrams the system is analyzed exactly in the thermodynamic limit. Various kinds of ordering, including periodic n-molecular phases and their mixtures are found for a set of values of the interaction constant \(U\).

abstract

The half-filled two-dimensional Hubbard model in presence of a uniform and static parallel magnetic field has been studied by means of the Composite Operator Method. A fully self-consistent solution, fulfilling all the constrains coming from the Pauli principle, has been found. The relevant features of a metal-insulator transition in presence of a magnetic field have been analyzed. The results qualitatively agree with the ones recently obtained by means of experimental investigations.

abstract

X-ray powder diffraction studies were performed on \({\rm Fe}_{3-x}{\rm Zn}_x{\rm O}_4\) (\(x=0\), \(0.0046\), \(0.0072\), \(0.015\), \(0.0185\), \(0.0249\), \(0.036\)) in the temperature range 70 K-300 K. We intended to check if structural characteristics distinguish between the samples exhibiting the Verwey transition of discontinuous and continuous character, as found e.g. for resistivity and heat capacity results. We have found that within experimental error range no clear distinction between those two regimes of the Verwey transition can be deduced from the compositional dependence of lattice constants and monoclinic angle.

abstract

The pressure dependence of the phase transitions in the quasi one-dimensional vanadium oxide \(\beta \)-Na\(_{1/3}\)V\(_2\)O\(_5\) has been studied by magnetic susceptibility and electrical resistivity measurements. Under pressure the metal–insulator (MI) transition at \(T_{\rm MI}=130\) K shifts to lower, the magnetic transition at \(T_{\rm N}=24\) K to higher temperatures. This behavior is discussed in terms of increasing interchain coupling.

abstract

The thermal conductivity \(\kappa \)(T) of the La\(_{1-X}\)AE\(_X\)MnO\(_3\) manganites (AE: divalent ions; Ca\(^{2+}\), Sr\(^{2+}\), Pb\(^{2+}\), Ba\(^{2+}\)) has been measured. For small AE ions (Ca\(^{2+}\) and Sr\(^{2+}\)), \(\kappa \)(T), which is dominated by the phonon contribution, shows an enhancement below the ferromagnetic metal transition temperature T\(_{\rm c}\). In contrast, the enhancement is completely wiped out for the largest Ba\(^{2+}\) ion. The increase of the ionic radius of AE ions reduces the average static lattice distortion with the tolerance factor \({\mit \Gamma }\) approaching to 1, while it increases the randomness of ionic radii in the (La\(_{1-X}\)AE\(_X\))-site cations defined by the cation radius variance \(\sigma ^{2}\). In the Ba\(^{2+}\) system, the \(\kappa \)(T) reduction caused by the largest \(\sigma \)\(^{2}\) masks out the \(\kappa \)(T) enhancement characteristic of the ferromagnetic-metal manganite systems.

abstract

The effect of the \(A\)-site cation radius variation \(\sigma \)\(^{2}\) on the ferromagnetic (FM) and metal–insulator (M–I) transition in R\(^{3+}\)\(_{0.75}\)M\(^{2+}\)\(_{0.25}\)MnO\(_3\) mixed crystals has been studied by measuring the magnetization M(T), electrical resistivity \(\rho \)(T) and thermal conductivity \(\kappa \)(T). With increasing \(\sigma \)\(^{2}\), the first-order-like character of FM and M–I transition observed in La\(_{0.75}\)Ca\(_{0.25}\)MnO\(_3\) is promptly suppressed in mixed crystals in spite of the same tolerance factor \({\mit \Gamma }\) value kept constant to that of La\(_{0.75}\)Ca\(_{0.25}\)MnO\(_3\).

abstract

Elastic properties of the Pr-doped bilayer manganite: (La\(_{1-z }\)Pr\(_{z }\)) \(_{1.2}\)Sr\(_{1.8}\)Mn\(_{2}\)O\(_{7}\) for \(z\)=0.6 was investigated by means of the ultrasonic measurement. No remarkable anomaly was observed around the transition temperature in the temperature dependence of C\(_{33}\) in zero field. A pronounced elastic anomaly, however, has been observed around the magnetic phase transition field Ht in the longitudinal elastic constants C\(_{11}\), indicating the phase can be induced in magnetic fields. The transition accompanies a large hysteresis, implying the ordered state to be so-called “orbital-glass state”. The origin of observed elastic anomalies are discussed in terms of the coupling between elastic strains and magnetic moments of Mn ions, and a change of carrier numbers.

abstract

New perovskites containing low concentration of Mn\(^{3+}\) (\(e_g^1\)) Jahn–Teller cations in a Mn\(^{4+}\) (\(e_g^0\)) cation matrix are synthesized. Theses manganites are metallic at room temperature (\(\rho \sim 10^{-2} \Omega \)cm) for small concentration of dopants (\(y \leq 8 \%\)). Remarkably the samples exhibit large negative thermopower values at room temperature (\(-S \gt 100 \mu {\rm V} \cdot {\rm K}^{-1}\)), which increases with increasing temperature. Therefore these materials are good candidates for \(n\)-leg parts of thermoelectric devices for the conversion of heat to electricity. In order to explain this interesting behavior, a new theoretical framework taking correlation effects and realistic electronic configurations into account, is introduced. It allows for determining the temperature and charge-carrier density dependences of the thermopower. The results are compared to the observed experimental values of CaMn\(_{1-x}\)M\(_x\)O\(_3\), with M=Ru, Mo, which exhibit two different background states.

abstract

We investigate the combined influence of electron-phonon and orbital polarization interactions on hole propagation in a ferromagnetic plane of orbitally ordered LaMnO\(_3\). The quasiparticle band found at the bottom of the hole spectrum is accompanied by broad structures representing vibrational side-bands resulting from the hole-lattice coupling.

abstract

The orbital ground state of LaTiO\(_3\) is still under debate. Recent letters [B. Keimer, et al., Phys. Rev. Lett. 85, 3946 (2000) and G. Khaliullin, S. Maekawa, Phys. Rev. Lett. 85, 3950 (2000)] discuss a scenario of a locally disordered orbital liquid and provide theoretical predictions about orbital contributions to the specific heat. We present magnetic measurements together with results for the heat capacity. In the magnetic susceptibility a distinct anisotropy can be found, not only in the magnetically ordered regime but also well above. The paramagnetic susceptibility cannot be evaluated in terms of a Curie-Weiss type behavior and seems to be determined by a small crystal-field splitting and spin-orbit coupling. In addition the heat capacity of LaTiO\(_3\) is compared with that of orbitally ordered LaMnO\(_3\) for temperatures around and below \(T_N\). No indications for additional orbital contributions could be detected.

abstract

We report on Ferromagnetic-Resonance experiments in a single crystal of La\(_{0.8}\)Sr\(_{0.2}\)MnO\(_3\) in the temperature range from 4 to 300 K. The observed anisotropy of the resonance line changes on crossing the transition from the orthorhombic \(O\)-phase to the rhombohedral \(R\)-phase at \(T\approx 100\) K and indicates a reorientation of the spins at about 130 K.

abstract

Combining the self-consistent theory of localization and the dynamical mean-field theory, we present a theoretical approach capable of describing both self-trapping of charge carriers during the process of polaron formation and disorder-induced Anderson localization. By constructing random samples for the local density of states (LDOS) we analyze the distribution function for this quantity and demonstrate that the typical rather than the mean LDOS is a natural measure to distinguish between itinerant and localized states. Significant polaron effects on the mobility edge are found.

abstract

We investigate the ground state of the spin-orbital superexchange model for cubic vanadates. For small Hund’s coupling \(J_{\rm H}\) strong fluctuations of \(t_{2g}\) orbitals stabilize a dimerized state with orbital valence bond correlations, while for realistic values of \(J_{\rm H}\) either a \(C\)-type or \(G\)-type antiferromagnetic phase is found, depending on the strength of the Jahn-Teller interaction.

abstract

We use the temperature of the structural phase transition to determine the Jahn-Teller (JT) coupling constant in the model derived for LaMnO\(_3\) which includes both the superexchange between \(S=2\) spins and the JT effect. We also investigate the dependence of the exchange constants on the value of the on-site Coulomb element \(U\), and on the orbital ordering.

abstract

We report a detailed Monte Carlo study of the magnetic properties of double exchange model in the highly anisotropic hopping integral \(t_c~/~t_{ab}\) regime. We find that the resistivity upturns at low temperature is related to the suppression of ferromagnetic arrangement, accompanied by strong spin fluctuations. This anomalous magnetic behavior comes from the competition between the reduced interlayer double exchange coupling and the thermal frustration of the ordered two dimensional ferromagnetic layer, which leads to a mixture of ferromagnetic and antiferromagnetic clusters.

abstract

A condensed excitonic phase has been realized in intermediate valent TmSe\(_{0.45}\)Te\(_{0.55}\) at low temperatures and high pressures. In this paper we report about some thermodynamic properties such as the heat conductivity and the thermal diffusivity which have been obtained between 300 K and 1 K and between ambient pressure and 17 kbar (1.7 GPa). Pressure and temperature are used to navigate in the intermediate valent semiconducting phase, in the condensed excitonic phase and in the intermediate valent metallic phase of the complete phase diagram. In the condensed excitonic phase the heat conductivity \(\lambda \) of the excitonic phase increases exponentially below about 20 K suggesting a superthermal current for \(T \rightarrow 0\). In a solid this has never been seen before. Also the thermal diffusivity \(a\) strongly increases below 20 K, giving evidence for second sound.

abstract

We report studies of the pressure-dependent superconducting and Néel temperatures of the heavy fermion compound Ce\(_2\)RhIn\(_8\) and the doping series CeRh\(_{1-x}\)Ir\(_x\)In\(_5\). Systematic evolution of their groundstates with pressure emphasizes the importance of spin fluctuations for superconductivity.

abstract

The filled skutterudite compound PrOs\(_4\)Sb\(_{12}\) exhibits superconductivity below a critical temperature \(T_{\mathrm {c}} = 1.85\) K that develops out of a nonmagnetic heavy Fermi liquid with an effective mass \(m^{*} \approx 50~m_{\mathrm {e}}\), where \(m_{\mathrm {e}}\) is the free electron mass. Analysis of magnetic susceptibility, specific heat, electrical resistivity and inelastic neutron scattering measurements within the context of a cubic crystalline electric field yields a Pr\(^{3+}\) energy level scheme that consists of a \({\mit \Gamma }_{3}\) nonmagnetic doublet ground state that carries an electric quadrupole moment, a low lying \({\mit \Gamma }_{5}\) triplet excited state at \(\sim 10\) K, and \({\mit \Gamma }_{4}\) triplet and \({\mit \Gamma }_{1}\) singlet excited states at much higher temperatures. The superconducting state appears to be unconventional and to consist of two distinct superconducting phases. An ordered phase of magnetic or quadrupolar origin occurs at high fields and low temperatures, suggesting that the superconductivity may occur in the vicinity of a magnetic or electric quadrupolar quantum critical point.

abstract

Dynamical mean field theory is used to solve the Falicov–Kimball model in the case where the \(f\)-ions have an internal structure. The solution provides a qualitative description of the properties of Eu-based intermetallic compounds which show anomalies due to a valence-change transition.

abstract

We have investigated a single crystal of CeAgSb\(_{2}\) using low field ac-susceptibility, thermal expansion and magnetostriction measurements. The thermal expansion coefficient \(\alpha \), exhibits highly anisotropic behaviour between 3 K and 80 K: \(\alpha \) (for \(\Delta L\)/\(L)~\perp c\) exhibits a sharp peak at \(T_{\rm N}\) followed by a broad maximum at 20 K, while a sharp negative peak at \(T_{\rm N}\) followed by a minimum at 20 K has been observed for (\(\Delta L\)/\(L \parallel \)) the \(c\) direction. The observed maximum and minimum in \(\alpha \)(T) at 20 K have been attributed to the crystalline field effect (CEF). The magnetostriction (MS) also exhibits anisotropic behaviour with a large MS along the \(c\)-axis.

abstract

We present high resolution photoemission investigations on the heavy fermion compound CeCu\(_6\). By application of a normalization procedure we can resolve the Kondo resonance and its crystal field structures in both the He II\(_{\alpha }\) and He I\(_{\alpha }\) data. A comparison with NCA calculations allows a quantitative determination of the Kondo temperature and the crystal field energies.

abstract

We have studied the phase formation, thermodynamic and transport properties of CeNi\(_9\)Si\(_4.\) The Rietveld refinement of the X-ray pattern confirms a fully ordered tetragonal crystal structure (space group \(I4\)/mcm) that is derived from the cubic NaZn\(_{13}\) structure type. Resistivity, magnetic susceptibility and specific heat measurements reveal Kondo-lattice behaviour with an enhanced Pauli susceptibility \(\chi _0=0.005\) emu/mol and a large Sommerfeld value \(\gamma = 155\) (5) mJ/molK\(^2\). The magnetic contributions to the specific heat and the magnetic susceptibility are well described by the Coqblin–Schrieffer model with a fully degenerate \(J = 5/2\) ground state and a characteristic temperature \(T_0\simeq 180\) K.

abstract

The recently discovered Pr-based skutterudite superconductor PrOs\(_{4}\)Sb\(_{12}\) was studied by means of low-temperature measurements of the thermal expansion coefficient \(\alpha \) along the cubic \((100)\) direction of a single crystal sample in magnetic fields up to 8 T. Two superconducting phase transitions were found with critical temperatures \(T_{\rm c1}=1.84\,\textrm {K}\) and \(T_{\rm c2}=1.71\,\textrm {K}\). Their hydrostatic pressure dependencies calculated using the Ehrenfest relation are \(-250\pm 50\,\textrm {mK/GPa}\) and \(-450\pm 70\,\textrm {mK/GPa}\), respectively. For \(B\geq 5\,\textrm {T}\), we observe a thermodynamic phase transition which is presumably due to the onset of quadrupolar order. The transition temperature shifts with increasing \(B\) towards higher temperatures. The strongly enhanced value of the Grüneisen parameter \({\mit \Gamma }\) provides clear evidence for the occurrence of a heavy-fermion normal state. The pronounced change of \({\mit \Gamma }\) at \(T_{\rm c}\) provides evidence for the formation of Cooper pairs out of the heavy quasiparticles.

abstract

We have studied single-crystalline CeCu\(_2\)(Si\(_{1-x}\)Ge\(_x\))\(_2\) using neutron diffraction to determine the antiferromagnetic order in the regime with low Ge content, \(x \le 0.5\). The system orders in an incommensurate magnetic structure with a propagation vector close to \(\tau = (0.25~0.25~0.5)\). Up to two additional phase transitions were observed below the Néel temperature characterized by a spin reorientation and a lock-in to a probably commensurate structure.

abstract

Neutron diffraction experiments have been used to investigate the structural and magnetic ordering in the moderately disordered heavy fermion compound UPd\(_{1.85}\)Sn. The material crystallizes in the fully ordered cubic Heusler lattice of \(Fm3m\) symmetry. Below the antiferromagnetic transition temperature \(T_N=25.7\) K magnetic Bragg scattering has been observed. The magnetic spectrum can be accounted for by an antiferromagnetic wave vector \(\vec {k}\) = [0 0 0], i.e., the magnetic and crystallographic unit cells are identical. The magnetic moment is evaluated to \(\mu _{\rm ord}\) = 0.97(9) \(\mu _{\rm B}\).

abstract

The periodic Anderson model (PAM) is mapped on an effective single-impurity model (SIAM) within the dynamical mean-field theory (DMFT). The SIAM is treated in modified perturbation theory (MPT), which is exact up to second order in the Coulomb correlation and reproduces the atomic limit and the lowest moments. We present results for the specific heat, the magnetic susceptibility, the resistivity, the thermopower and the frequency dependent (dynamical) conductivity.

abstract

The change of the electronic structure as well as the hybridization between the localized \(4f\) state and the conduction band (\(cf\) hybridization) of Ce\(_{1-x}\)La\(_x\)Pd\(_3\) (\(x\) = 0, 0.03) due to the La-substitution has been studied by the optical conductivity spectra in the infrared region. The width of the optical transition of Pd \(4d \rightarrow \) Ce \(4f\) states that was mainly observed in the energy region shrinks by the La-substitution. This means that the \(cf\) hybridization is strongly suppressed by the absence of the periodicity of the Ce-ion.

abstract

The magnetic excitation in a \(5f\) antiferromagnet UPt\(_{2}\)Si\(_{2}\) was studied by means of neutron inelastic scattering. A remarkable low energy quasi-elastic component has been observed around the antiferromagnetic zone center (100). We concluded that the low energy quasi-elastic response would be the excitation of the quasi-particles due to hybridization between \(5f\) and conduction electrons.

abstract

Electrical resistivity \(\rho (T)\), thermal expansion and magnetization \(M(T)\) measurements in high magnetic fields have been performed on filled skutterudite SmRu\(_{4}\)P\(_{12}\) in order to investigate the field dependence of the metal–insulator (M-I) transition (\(T_{\rm MI}\sim 16\) K). The M-I transition occurs in fact in two successive steps. The specific heat shows a double peak in field. The thermal expansion coefficient, the temperature derivative of the resistivity \(d\rho (T)\)/\(dT\) and of the magnetization \(dM(T)\)/\(dT\) also show two anomalies at the same position as the specific heat peak. The field dependence up to 30 T of the two anomalies suggests that the two successive transitions are antiferro-quadrupolar ordering and antiferro-magnetic ordering.

abstract

The magnetic phase diagram of UPdSn for fields applied along the \(c\) axis has been determined by means of magnetoresistance and neutron-diffraction studies. We established that the 13 T \(c\)-axis transition is connected with the 25 K zero-field transition, below which additional \(x\)-components to the magnetic moments are found.

abstract

Temperature dependence of specific heat as well as magnetization reveals that Dy\(_{0.8}\)Gd\(_{0.2}\)B\(_2\)C\(_2\) undergoes an antiferroquadrupolar transition at \(T_{\rm Q}=17.5\) K below an antiferromagnetic one at \(T_{\rm N}=20.0\) K. Gd substitution in DyB\(_2\)C\(_2\) decreases \(T_{\rm Q}\) and increases \(T_{\rm N}\), and results in a transition sequence reverse to that in DyB\(_2\)C\(_2\). The magnetization increases below \(T_{\rm N}\) and decreases below \(T_{\rm Q}\). The results are discussed as a manifestation of anomalous properties of the magnetic phase adjacent to the antiferroquadrupolar ordered phase.

abstract

Magnetic and superconducting properties have been investigated on a single crystal of the filled skutterudite superconductor PrOs\(_{4}\)Sb\(_{12}\) by means of static magnetization measurements at very low temperatures. There is no trace of the paramagnetic suppression of the superconductivity, indicating that the normal state paramagnetism is due to the Van Vleck contributions. An anomalous field-induced phase transition whose onset field gradually increases with temperature is found above 4.5 T and below 1.1 K. The nonlinear susceptibility increases from a negative value with decreasing temperature and attains a maximum at around 1 K. The origin of these anomalous magnetization behaviors is discussed with respect to the crystal-electric field excitations in PrOs\(_{4}\)Sb\(_{12}\).

abstract

We report the results of ac susceptibility (\(\chi \)) measurements for various compositions of the series, Ce\(_2\)Pd\(_{1-x}\)Co\(_x\)Si\(_3\), in the polycrystalline form, which has been previously reported to undergo a transformation from magnetic ordering to non-magnetic Kondo lattice behaviour as \(x\) is varied from 0 to 1. A finding of emphasis here is that the features in ac \(\chi \) data for the compositions, \(x=0.6\) and 0.7, in the close vicinity of quantum critical point (QCP), are typical of long range magnetic ordering systems, whereas the compositions far away from QCP at the Pd-end including Ce\(_2\)PdSi\(_3\) exhibit spin–glass characteristics. This trend is different from hitherto known behaviour among Kondo alloys.

abstract

We report Sb-NQR results, which evidence unusual superconducting (SC) property in a filled-skutterudite compound PrOs\(_4\)Sb\(_{12}\) with a SC transition temperature, \(T_{\rm c}=1.8\) K. The nuclear-spin–lattice relaxation rate, \(1\)/\(T_1\) have revealed that Pr-derived \(4f\) moments behave as if they are localized in a high temperature (\(T\)) above \(T_0\sim 10\) K. The observed NQR-line shift below \(T_0\) suggests the local redistribution of the charges associated with quadrupolar moments of Pr\(^{3+}\). The \(1\)/\(T_1\) in the SC state has revealed an isotropic energy gap of \({\mit \Delta }\)/\(k_{\rm B}T_{\rm c}\sim 2.7\). The absence of the coherence peak, but an exponential decrease in \(1\)/\(T_1T\) below \(T_{\rm c}=1.8\) K cannot be accounted for by either a conventional \(s\)-wave model or an anisotropic SC model with point- or line-node gap, pointing to a new class of unusual superconductivity.

abstract

Specific heat measurements of antiferromagnet TbB\(_2\)C\(_2\) with \(T_{\rm N}=21.7\) K were carried out under magnetic fields up to 8 T applied along the [1 0 0] and [1 1 0] directions. The application of magnetic fields in TbB\(_2\)C\(_2\) leads to increase of the transition temperature in both directions. In case for \(H{\parallel }[1\,1\,0]\), the transition temperature reaches 31.8 K under 8 T which indicates that magnetic fields anomalously stabilize the antiferromagnetic ordered state. The obtained field dependence of the transition temperature is quite anisotropic between the [1 0 0] and [1 1 0] directions, which is similar to the antiferroquadrupolar compounds DyB\(_2\)C\(_2\) and HoB\(_2\)C\(_2\).

abstract

We have measured infrared absorption spectra under pressure and reflectivity spectra of YbS in the wide photon energy range from 7 meV to 30 eV. The absorption edge shifts linearly toward lower energy with pressure, and above 11 GPa it disappeared in the infrared energy region. The results are considered to correspond to the development of a \(f\)–\(d\) mixing above this pressure, which lead to an occurrence of the semiconductor-to-metal transition.

abstract

HoB\(_{2}\)C\(_{2}\) and TbB\(_{2}\)C\(_{2}\) show anomalous magnetic phases which are adjacent to antiferroquadrupolar ordered phases and supposedly affected by multipolar interactions. We carried out specific heat and magnetization measurements to complete magnetic phase diagrams of pseudo-binary Ho\(_{1-x}\)Tb\(_{x}\)B\(_{2}\)C\(_{2}\). It is clarified that the interactions in the anomalous magnetic phases of HoB\(_{2}\)C\(_{2}\) and TbB\({2}\)C\(_{2}\) are quite similar and Tb\(^{3+}\) ions are cooperative to the antiferroquadrupolar order in HoB\(_{2}\)C\(_{2}\).

abstract

The electronic structure of the YbCu\(_{5-x}\)Ag\(_{x}\) system, which belongs to a series of dense Kondo compounds, is studied by high-resolution photoemission spectroscopy. A sharp Yb 4f-related peak, which is regarded as the Kondo peak, was observed just below the Fermi level for each compounds. As \(x\) decreases, the intensity of the Kondo peak decreases and its energy position is shifted towards the Fermi level.

abstract

We discuss electronic properties of CeIrIn\(_{5}\) based on the relativistic band-structure calculation. The obtained energy band-structure is reanalyzed by using a tight-binding \(f\)–\(p\) model, which is found to be a realistic and practical Hamiltonian for CeTIn\(_{5}\) (T = Ir, Rh, and Co).

abstract

We have studied the magnetic phase diagram and the de Hass-van Alphen (dHvA) effect of \({\rm PrPb_{3}}\) under pressure. The antiferroquadrupolar (AFQ) transition temperature increases only slightly with pressure, whereas the transition field from AFQ phase to paraquadrupolar phase increases considerably with pressure. The effective masses of the \(\alpha \) and \(\gamma \) oscillations increase largely with pressure implying that there is a significant change in the interaction or the hybridization between the \(f\) and conduction electrons. With the aid of the mean field analysis by Tayama et al. , we argue that the pressure strengthen the antiferromagnetic interaction, but does not affect the quadrupolar interaction appreciably.

abstract

We have performed a detailed analysis of the dHvA oscillations in the para(PQ)- and antiferro-quadrupolar(AFQ) phases of \({\rm PrPb_{3}}\). It is found that (i) each dHvA oscillation consists of up and down spin oscillations whose effective masses are very different, although the difference of the frequencies are small, (ii) the effective masses are largely enhanced compared to those of reference compound \({\rm LaPb_{3}}\), and (iii) there is a strong anisotropy of the effective masses. The transition behavior of the dHvA oscillations across the phase boundary from AFQ to PQ phase is found to be also anisotropic. Particularly, a drastic change of the dHvA wave form and mass enhancement were observed for [001], suggesting a fluctuation of quadrupole and accompanying magnetic fluctuation.

abstract

Pr \(4f\) electronic structures in Pr-based filled skutterudites PrT\(_4\)X\(_{12}\) (T=Fe and Ru; X=P and Sb) have been studied by the high-resolution bulk-sensitive Pr \(3d\to 4f\) resonance photoemission spectroscopy. A very strong Pr \(4f\) spectral intensity is observed just below the Fermi level in the heavy-Fermion PrFe\(_4\)P\(_{12}\). This is the first observation of the Kondo resonance due to the quadrupolar Kondo effect, the origin of which is attributed to the strong hybridization between the Pr \(4f\) and the conduction electrons.

abstract

We report on magnetic susceptibility measurements on a CePtSn single crystal under hydrostatic pressures up to 0.82 GPa. Application of hydrostatic pressure on CePtSn leaves both the Néel temperature \(T_{\mathrm {N}}\) and the AF1–AF2 transition temperature \(T_{\mathrm {M}}\) nearly intact. This is rather unexpected because the volume contraction was believed to be the main reason for the transformation from the antiferromagnetic Kondo metal with \(T_{\mathrm {N}}=\) 7.5 CePtSn to a non-magnetic Kondo semi-metal CeNiSn G.M. Kalvius et al., Physica B 289–290, 256 (2000). The only pressure effect observed is a 25% reduction of low-temperature magnetization by the pressure of 0.82 GPa. A possible scenario for physics of CePtSn is discussed accounting for the observed pressure effects.

abstract

We report on the magnetic and superconducting properties of single crystals of the alloys CeRh\(_{1-x}\)Co\(_{x}\)In\(_5\) (\(x\) = 0.0, 0.25, 0.5, 1.0). For \(x \leq \) 0.5 we do not find any signature of superconductivity down to 1.6 K. The previously observed metamagnetic transition in CeRhIn\(_5\) at 20 kOe is also present for small Co doping (\(x\) = 0.25), but disappears for \(x\) = 0.5. In the superconducting state, CeCoIn\(_5\) shows hysteresis in the magnetisation versus field measurements which is typical of a type II superconductor, with the pinning predominantly controlled by surface and geometrical factors.

abstract

Magnetoresistance measurements performed at 4.2 K up to \(B\) = 22 T applied along the tetragonal \(c\)-axis on antiferromagnetic single crystalline Ce(Rh\(_{1-x}\)Ru\(_x\))\(_2\)Si\(_2\) solid solutions (\(x\) = 0.05, 0.1 and 0.2) are reported. The 4.2 K resistivity, i.e. the residual resistivity, \(\rho _0\), is strongly reduced in 22 T, showing that the large increase of \(\rho _0\) with \(x\) can not be attributed to disorder alone. The results evidence two transitions at fields \(B_{{\rm c}_1}\) and \(B_{{\rm c}_2}\) which correspond to metamagnetic transitions, similar to the two step transition occuring in CeRh\(_{2}\)Si\(_{2}\) at 25.8 and 26.3 T, respectively. \(B_{{\rm c}_1}\) and \(B_{{\rm c}_2}\) decrease rapidly with increasing \(x\), and their splitting increases (showing some analogy with measurements reported for Ge doped CeRh\(_2\)Si\(_2\)). These variations and the rapid decrease of \(T_{{\rm N}_1}\) (which has vanished for \(x\) = 0.35), are discussed.

abstract

Photoelectron spectra of various Pu-based systems are discussed from the point of view of variations of the 5f localization.

abstract

The magnetic properties of PrInNi\(_4\) with the field-induced ferromagnetic transition have been investigated by the measurements of the magnetic susceptibility and the high-field magnetization up to 30 T. The experimental results have been described in terms of the single-site effect of the crystalline-electric-field and the inter-atomic exchange interaction with a ferromagnetic coupling.

abstract

We searched for an experimental condition of crystal growth for a ferromagnetic superconductor URhGe. Samples were prepared for both polycrystalline and single crystals, which were arc-melted and grown by the Czochralski-pulling method in a tetra-arc furnace, respectively. Annealing is an important process in order to increase sample-quality, namely the residual resistivity ratio. We tried to anneal the samples under various temperatures, which were wrapped by the Ta-foil and vacuum-sealed in the quartz ample, together with the solid state electrotransport (SSE) method in ultra-high vacuum.

abstract

Ga NQR measurements have been carried out for 5\(f\)-antiferromagnet, UPtGa\(_{5}\).The zero-applied-field NQR spectrum in the antiferromagnetic state can be explained as the resonance lines of a Ga nuclei, which is affected by the Zeeman interaction with the internal field from the uranium magnetic moments, as well as the quadrupole interaction. The hyperfine coupling constant in the antiferromagnetic state is evaluated.

abstract

We have investigated the nature of the antiferromagnetic (AF) phase induced by uniaxial stress \(\sigma \) in URu\(_2\)Si\(_2\), by performing elastic neutron scattering measurements up to 0.4 GPa. We have found that the AF Bragg-peak intensity shows a clear hysteresis loop with \(\sigma \) under the zero-stress cooling condition. The result strongly suggests that the \(\sigma \)-induced AF phase is metastable and separated from the coexisting “hidden ordered” phase by a first-order phase transition. We also present the analyses of the crystalline strain effects, and suggest that the \(c/a\) ratio plays an important role in the competition between these two phases.

abstract

We have successfully grown a single crystal of uranium intermetallic compound URhGe\(_2\). The temperature dependence of electrical resistivity shows highly anisotropic behavior. Two anomalies, which correspond to the magnetic ordering temperatures, were observed in the resistivity, magnetic susceptibility and specific heat measurements at \(T_{\rm M1}\)= 30 K and \(T_{\rm M2}\)=25 K. The magnetic susceptibility also shows a large uniaxial anisotropy with a magnetic easy-axis along the [010] direction.

abstract

We have measured the optical conductivity \(\sigma (\omega )\) of the mixed-valent compound YbAl\(_3\). \(\sigma (\omega )\) exhibits a mid-infrared peak centered at 0.15–0.2 eV, which becomes more pronounced with decreasing temperature (\(T\)). In addition, a strong depletion of the spectral weight, i.e., a pseudogap formation, is observed in \(\sigma (\omega )\) below \(\sim \) 0.1 eV. A comparison of \(\sigma (\omega )\) with the dc conductivity indicates the existence of an extremely narrow Drude peak at very low energy. Energy-dependent effective mass and scattering rate of the carriers are evaluated from the optical data, which indicates the formation of a heavy-mass Fermi liquid state within \(\sim \) 40 meV from the Fermi level. These observations are discussed in terms of the hybridization of a conduction band and a narrow 4\(f\) band.

abstract

The elastic constant (\(C_{11}-C_{12}\))/2 of skutterudite compound PrFe\(_{4}\)P\(_{12}\) has been studied in a magnetic field. A remarkable softening towards low temperatures appeared only in applied magnetic field. Its magnitude reaches a value of about 10 % in magnetic field of 8 T. The results confirm that the non-Kramers doublet ground state of the Pr ion is realized in PrFe\(_{4}\)P\(_{12}\) and indicate that the magnetic field helps to stabilize the non-Kramers doublet state.

abstract

In CePd\(_2\)Ga\(_3\), one of the few ferromagnetically ordered heavy fermion materials, the transition temperature \(T_{\rm mag}\) (\(\sim 6.4\) K) is lowered by pressure. From a comparison of resistance data with DC susceptibility data there is evidence, however, that the FM magnetization disappears before \(T_{\rm mag}\) is suppressed to zero. The isostructural compound CePd\(_2\)Al\(_2\)Ga also shows FM order, but with a much lower \(T_{\rm mag}\) \(\sim 1.8\) K. It could be expected, therefore, to reach \(T_{\rm mag} = 0\) K within a pressure range where thermodynamic properties like \(\chi \) and C\(_p\) can be determined. Earlier data of the ac susceptibility have shown that after an initial steep decline \(T_{\rm mag}\) saturates with a tendency to increase again above 0.4 GPa. Here we present specific heat data extending up to 2.5 GPa which confirm this dependence as a bulk property of CePd\(_2\)Al\(_2\)Ga. Moreover, they show a maximum in \(T_{\rm mag}\) around 1.8 GPa followed by a steep decline extrapolating to 0 K below 2.8 GPa. This decline is accompanied by a flattening of the transition anomaly above 1.9 GPa. Notwithstanding the hints on a change of the type of magnetic order from ac susceptibility results the magnetic structure under pressure is not known. So the interesting question if there exists a quantum critical point between a FM phase and a Kondo Fermi liquid is still open.

abstract

The effect of multipolar fluctuations on the quadrupolar phase transition in CeB\(_6\) is investigated theoretically. It is shown that the fluctuations become strong and field-dependent, reflecting the competition of coupled multipolar interactions. Some unusual phenomena around the transition in CeB\(_6\) are shown to be reasonably explained within the RKKY model.

abstract

A wide temperature range (1.8–300 K) Hall coefficient \(R_{\rm H}\) measurements have been carried out on a high quality polycrystalline samples of a magnetic Kondo-lattice CeAl\(_{2}\). The investigation of angular and magnetic field (up to 80 kOe) dependencies of the Hall coefficient and magnetoresistance in paramagnetic and modulated antiferromagnetic (AFM) phases of CeAl\(_{2}\) allows to distinguish between skew scattering and anomalous magnetic contributions. A complicated activation type behavior of the skew scattering component is found in this intermetallic compound for the first time. The anomalous magnetic Hall effect is caused by several magnetic phases on \(H\)–\(T\) diagram of CeAl\(_{2}\).

abstract

Following to the comprehensive study of the low temperature transport properties of SmB\(_{6}\) the noise characteristics of high quality single crystals have been investigated in this archetypal intermediate valence compound. The measurements along different crystallographic directions revealed the anisotropic strong enhancement of the low-frequency resistance noise at temperatures below 15K. The anomalies observed in this cubic compound are discussed in terms of a many-body states formation resulting from fast valence fluctuations on Sm-sites.

abstract

Thermal conductivity \(\lambda \) and electrical resistivity \(\rho \) of monocrystalline LaSn\(_3\) were measured in temperature range 6–300 K. A high and sharp maximum was observed for \(\lambda (T)\) dependence at temperature \(T_{\rm m}\) = 14 K. The dependence has revealed the linear behavior of \(\lambda \) with temperature below \(T_{\rm m}\) and the exponential one for temperatures higher than \(T_{\rm m}\). The electronic component to the thermal conductivity was estimated assuming validity of the Wiedeman–Franz law. It was found that the electrons play an essential role in the heat transport of LaSn\(_3\). Temperature dependence of the Lorenz function for LaSn\(_3\) is given as well.

abstract

We have performed the high-resolution Ce \(3d\)–\(4f\) resonance photoemission study of a considerably hybridized CeNi with the Kondo temperature of \(\sim 150\) K. The tail of the Kondo-resonance peak is predominantly observed in the bulk Ce \(4f\) photoemission spectra, where its spin–orbit partner is suppressed compared with the so far reported surface-sensitive Ce \(4d\)–\(4f\) resonance spectrum. Our results show that the bulk \(4f\) electronic states are essentially understood by the single impurity Anderson model.

abstract

X-ray diffraction experiment has been performed on the semiconductive valence fluctuation compound \({\rm CeRhAs}\). Strong superlattice reflections characterized by the wave vector of \(q=(0,1\)/\(2,1\)/\(2)\) was observed below \(T_{\rm 1} = 370\) K with intensity anomalies at \(T_{\rm 2} = 235\) K and \(T_{\rm 3} = 165\) K. Below \(T_{\rm 2}\), additional superlattice reflections were seen and some of them disappear below \(T_{\rm 3}\). The observed successive superlattice formation may reflect the electronic gap formation due to the complicated Fermi-surface structure.

abstract

Magnetic field induced antiferromagnetic (AF) Bragg reflections and superlattice reflections due to a crystal-structure modulation have been observed simultaneously by neutron diffraction in the low temperature phase of the filled-skutterudite \(\rm PrFe_4P_{12}\). The present result demonstrates that 4f electrons of Pr ions undergo antiferro-quadrupolar (AFQ) ordering accompanied by the structural phase transition. The field induced AF magnetic moment has been evaluated as 0.065\(\rm \mu _B\) at \(H =\) 5.0 T.

abstract

PrFe\(_4\)P\(_{12}\) exhibits a heavy-electron-like behavior and a phase transition to an antiferro-quadrupolar ordering state at the lowest temperature. Inelastic neutron scattering experiments for a polycrystalline sample revealed a quasielastic response and no crystal-field excitation in the heavy-electron state. It is in contrast to the observed sharp crystal-field excitations in PrInAg\(_2\) considered as a candidate of a quadrupolar Kondo system. In the ordered phase, sharp peaks at 1.4 and 3.0 meV are superimposed on the broad spectrum. These sharp excitations are consistent with the localized behavior of the \(4f\) electrons in the antiferro-quadrupolar ordered phase.

abstract

The magnetic structure in a localized \(5f\) system U\(_{3}\)Pd\(_{20}\)Si\(_{6}\) was studied by means of neutron diffraction. We revealed a remarkable collinear structure and found a new type of spin–flop transition due to the collinear coupling. Spin wave excitation was observed in a whole Brillouin zone. We concluded that the low energy quasi-elastic response is the excitation of the quasi-particles due to hybridization between \(5f\) and conduction electrons.

abstract

We have succeeded in observing the de Haas–van Alphen (dHvA) effect in PrOs\(_4\)Sb\(_{12}\). The Fermi surface topology is similar to the reference compound LaOs\(_4\)Sb\(_{12}\), indicating the localized character of 4\(f\)-electrons. The cyclotron effective mass, enhanced by about four times compared with LaOs\(_4\)Sb\(_{12}\), is a direct evidence of the strong electron correlation in this compound.

abstract

We report the effect of magnetic field on the transport properties in the exotic heavy fermion PrFe\(_4\)P\(_{12}\). The temperature \(T_{\rm P}\) of the resistivity maximum, ca. 13 K at zero field, shifts almost linearly with increasing field, correlated with the reduction of the effective mass determined from the de Haas–van Alphen (dHvA) measurement. The resistivity in 8 T shows a huge anisotropy with respect to the magnetic field direction.

abstract

We have studied the specific heat \(C_p(T)\), thermal expansion \(\alpha (T)\), thermal conductivity \(\kappa \), thermoelectric power S and magnetoresistance \(\Delta \rho /\rho \) of the heavy-fermion ferrimagnet UCu\(_5\)Sn. The anomalies observed in the \(C_p(T)\)-, \(\alpha (T)\)- and \(\Delta \rho /\rho (T)\)-dependencies confirm the existence of long-range magnetic order. \(\Delta \rho /\rho \) is negative in the investigated temperature and magnetic field ranges. The fitting of the magnetic specific heat in the temperature range 10–40 K gives \(\gamma _{\rm ord}\) = 78 mJ/moleK\(^2\), \(\beta \)= 0.5 mJ/moleK\(^{4}\) and \(\Delta /k_{\rm B}\) = 21 K. The effective Grüneisen parameter reaches a value of 4.7 at low temperatures, supporting the strongly correlated electron nature of this material. The thermoelectric power shows a maximum around 24 K, which might be related to either the Kondo effect or the phonon drag process. Below 100 K, the phonon contribution \(\kappa _{\rm ph}\) dominates the thermal conductivity. The reduced Lorenz number \(\kappa \rho /L_0T\) shows a broad peak at \(\sim \)10 K. This behavior might be explained by a dominant phonon scattering process off the U magnetic moments.

abstract

We report the first successful Shubnikov–de Haas (SdH) experiment under uniaxial pressure in the anomalous heavy-fermion semimetal CeRu\(_4\)Sb\(_{12}\). The nature of the quantum oscillations in the magnetoresistance is found to be significantly sensitive to uniaxial pressure. The results reveal that the nearly spherical Fermi surface elongates along the direction of the uniaxial pressure.

abstract

We have succeeded in growing high-quality single crystals of CeAgSb\(_2\), and carried out the de Haas-van Alphen (dHvA) experiment. Cylindrical Fermi surfaces were observed, together with closed Fermi suraces. The effective cyclotron mass is in the range from 0.9 to 32 m\(_0\). Large cylindrical Fermi surfaces occupying half of the Brillouin zone were detected, which are highly different from small Fermi surfaces in the reference compounds LaAgSb\(_2\) and YAgSb\(_2\) with a semimetalic character. CeAgSb\(_2\) is the first example in which the 4\(f\) electron becomes itinerant and possesses the magnetic moment.

abstract

Rare \(4f^{~2}\)-based heavy-fermion behaviors have been revealed recently in Pr-based filled skutterudites PrFe\(_4\)P\(_{12}\) and PrOs\(_4\)Sb\(_{12}\). Recent studies on the thermal properties on both compounds are reported, putting emphasis on the field-induced ordered phase found in PrOs\(_4\)Sb\(_{12}\).

abstract

We present the temperature (\(T\)) dependence of magnetic susceptibility (\(\chi \)) and electrical resistivity (\(\rho \)) of EuPt\(_2\)Si\(_2\) under high pressure. The Néel temperature (\(T_{\mathrm {N}}\)) is lowered with applying pressure. Above 2.5 GPa, the \(T_{\mathrm {N}}\) is collapsed and the residual resistivity drops dramatically with pressure. However, there exists no evidence of the valence transition, which is observed as a clear peak in the \(\rho \)–\(T\) curve in EuNi\(_2\)Ge\(_2\) under high pressure.

abstract

We present a study of the susceptibility and electronic transport properties of the alloying series URh\(_2\)Ge\(_{2-x}\)Sn\(_x\), \(x\) \(\leq \) 0.1. The magnetic behavior varies as function of \(x\) from an antiferromagnetic to a spin glass ground state. In contrast, the electronic transport does not correspondingly reflect the modification of the magnetic ground state properties. Instead, we argue that it indicates diffusive transport for all samples, possibly related to disorder induced electronic localization.

abstract

Yb\(_4\)Sb\(_3\) is known to show a valence fluctuation state, which is considered to relate with the high temperature phase of Yb\(_4\)As\(_3\). To clarify an electronic state of Yb\(_4\)Sb\(_3\), we tried to grow its high-quality single crystal and measure the de Haas–van Alphen effect. The residual resistivity ratio of the obtained single crystal was 500 and five dHvA branches were detected. Their cyclotron effective masses were in the range from 1.8\(m_0\) to 10\(m_0\).

abstract

Preliminary results of specific heat measurement in applied magnetic fields on the filled skutterudite CeOs\(_4\)Sb\(_{12}\) are presented. Clear anomaly in the specific heat data suggests the existence of an intrinsic phase transition at 0.9 K, accompanied by a gap opening in moderately mass-enhanced quasiparticle bands. Unusual magnetic field effect on the transition temperature, which shifts toward higher temperatures with increasing field, is revealed.

abstract

We have measured the specific heat and magnetocaloric effect of URu\(_2\)Si\(_2\) at magnetic fields up to 45 T. The large specific heat anomaly at \(T_{0}(H=0) = 17\) K shifts to lower temperatures when the magnetic field is increased and is suppressed at 36 T. Between 36 T and 38 T a new anomaly in the specific heat vs temperature indicates a magnetic phase never previously reported. At \(H \geq 40\) T no transition is observed and crystal electric field effects dominate the specific heat. Measurements of the magnetocaloric effect are used to stablish a new phase diagram for URu\(_{2}\)Si\(_{2}\).

abstract

We studied the proximity effect in order to probe the response of the heavy-fermion metal CeCu\(_{6}\) to induced superconductivity. High-quality CeCu\(_{6}\) films were grown by sputter deposition. Resistivity measurements show that the correlated electron state is formed. Bilayers were prepared of CeCu\(_{6}\)(75 nm)/Nb(\(d_{\rm Nb}\)), with the Nb thickness \(d_{\rm Nb}\) between 10 nm and 50 nm. Measured were the superconducting transition temperature \(T_{\rm c}\) and the parallel critical fields \(H_{\rm c2}^{||}\). We find that the interface transparency is high, indicating that Fermi-velocity mismatch effects are not significant, but the coherence length of the induced superconductivity is small, which is probably due to the very low Fermi-velocity in the heavy fermion metal.

abstract

The antiferroquadrupole orderings of the ternary rare earth compounds HoB\(_2\)C\(_2\) and DyB\(_2\)C\(_2\) have been investigated by means of ultrasonic measurements. The transverse \((C_{11}-C_{12})\)/\(2\), \(C_{44}\), \(C_{66}\) modes in HoB\(_2\)C\(_2\) exhibit characteristic softening above \(T_{\rm c2} = 5.0\) K, which consist with an \(E\)-doublet and a singlet for the ground state. Considerable elastic softening and ultrasonic attenuation in phase IV below \(T_{\rm c1} = 5.9\) K of HoB\(_2\)C\(_2\) indicates an enhancement of the quadrupole fluctuation with a relaxation rate \(\tau =7\times 10^{-9}\) sec. A softening of the transverse elastic constant \(C_{44}\) in DyB\(_2\)C\(_2\) above \(T_{\rm Q} = 24.7\) K indicates accidentally degenerated Kramers doublets of \(E_{1/2}\) and \(E_{3/2}\) for the ground state.

abstract

We performed measurements of magnetic susceptibility \((\chi )\) and L\({}_{\rm III}\)-edge X-ray absorption spectroscopy (XAS) in EuCu\({}_2\)(Si\({}_x\)Ge\({}_{1-x}\))\({}_2\). For \(0.70\leq x\leq 0.80\), with decreasing temperature, the \(\chi \) deviates from Curie–Weiss (CW) law with a Eu\({}^{2+}\) state and exhibits almost temperature-independent behaviour at lower temperatures. The behaviour, which is roughly similar to that accompanied by the valence transition in EuNi\({}_2\)(Si\({}_{1-x}\)Ge\({}_{x}\))\({}_2\), cannot be interpreted only in terms of the valence change. The hybridization between \(4f\) electrons and a conduction band should also be taken into consideration.

abstract

We report the pressure dependence of lattice parameter at room temperature up to 13 GPa and the electrical resistivity of CeRh\(_2\)Si\(_2\) in the temperature range from 2.5 K to 300 K up to 8 GPa. The compression curve of unit-cell volume is well fitted to Murnaghan equation of state. The resistivity below 10 K is described as \(\rho (T)= \rho _0 + AT^{~2}\), where \(\rho _0\) is the residual resistivity and \(A\) the constant. \(A(P)\) shows a peak around the critical pressure \(P_{\rm C}\sim 1.0\) GPa. The Grüneisen parameter of the Kondo temperature \(T_{\rm K}\) is estimated above \(P_{\rm C}\) and compared with those of the heavy fermion compounds.

abstract

We report measurements of the magnetic field dependence of the low temperature specific heat of single crystal samples of the heavy-fermion superconductor PrOs\(_4\)Sb\(_{12}\). The specific heat anomaly at \(T_{c}\) resides on a large Schottky anomaly and exhibits a double peak structure similar to the behaviour observed in UPt\(_3\). For \(B \gt \) 2 T, the Schottky anomaly becomes increasingly suppressed. At low \(T\), a maximum develops for \(B\gt 4.5\rm ~T\) which sharpens considerably and shifts to higher \(T\) with increasing \(B\) corresponding to a feature in the electrical resistivity. This suggests a field tuned \(T=0\) phase transition to a high field phase just above the suppression of superconductivity.

abstract

PrRu\(_{\rm 4}\)P\(_{\rm 12}\) shows a metal-insulator (M-I) transition with a structural phase transition and without magnetic anomaly. To investigate the origin of the M-I transition, we have calculated the band structures by using the FLAPW-LDA+U method with many types of lattice distortions. The result shows an insulator when P ions are slightly distorted with \({\mit \Gamma }_1\) mode, suggesting the M-I transition is caused by the perfect 3 dimensional nesting of the Fermi surface.

abstract

We have measured the electrical resistivity of single crystal YbInCu\(_4\) under hydrostatic pressure up to 3.76 GPa and at temperature down to 33 mK. At ambient pressure, YbInCu\(_4\) undergoes a valence transition at about 40 K. The transition temperature \(T_{\rm v}\) decreases linearly with increasing pressure: \(dT_{\rm v}\)/\(dP=-19.5\) K/GPa below 1 GPa. Above 3 GPa, a hysteresis due to valence transition disappears and the transition could not be confirmed clearly. The resistivity of YbInCu\(_4\) at low temperature varies as \(\rho (T) = \rho _0 + A T^{~2}\). The resistivity coefficient \(A\) and the residual resistivity \(\rho _0\) increase gradually with the increasing pressure. These values increase rapidly just before a transition disappears and take maximum around 3.5 GPa. We observed a ’filamentary’-superconductivity above 0.74 GPa and below \(\sim \) 1.2 K.

abstract

Results of electrical-resistivity measurements for a UPdSn single crystal (current along the \(c\)-axis of orthorhombic structure) at various temperatures, magnetic fields and hydrostatic pressures are presented. Large magnetoresistance effects are observed in antiferromagnetic (AF) state, but also at temperatures far above \(T_{{\rm N}}\). The latter result is attributed to the existence of AF correlations or short-range AF ordering in paramagnetic range. The value of \(T_{{\rm N}}\) is found increasing with increasing applied hydrostatic pressure whereas \(T_{{\rm 1}}\), the temperature of the AF-1 \(\iff \) AF-2 transition, simultaneously decreases. As a consequence, the stability range of AF-1 phase becomes extended with applied pressure.

abstract

A first-principle study of Fermi surface and magnetism for antiferromagnet UPtGa\(_5\) is done by all-electron band calculations using fully- relativistic spin-polarized LAPW method in a local spin-density approximation. The Fermi surfaces, mainly having cylinder-like shapes with open structure along the [001] direction, explain well the angular-dependence of de Haas-van Alphen frequencies. In a picture of \(5f\)-band antiferromagnet, the local magnetic moment at U site is in good agreement with the observed moment.

abstract

Specific heat measurements of (Gd\(_{1-x}\)Y\(_{x})_{3}\)T compounds (T= Co and Ni) revealed a strong concentration dependence of the coefficient \(\gamma \) of the \(T\)-linear specific heat. In particular, the electronic specific heat coefficient \(\gamma \) of Y\(_{3}\)Co was found to be significantly lower (15 mJ/molK\(^{2}\)) than that of isostructural Gd\(_{3}\)Co (110 mJ/molK\(^{2}\)) and (Gd\(_{0.2}\)Y\(_{0.8}\))\(_{3}\)Co (380 mJ/molK\(^{2}\)). Such a behaviour can be attributed to the presence of a huge contribution of spin fluctuations in the \(d\)-electron subsystem induced by the \(f\)-\(d\) exchange interaction.

abstract

Magnetic properties and electronic structure of CeTIn (T=Au, Ni) compounds with the hexagonal ZrNiAl-type structure have been investigated. CeAuIn is an antiferromagnet (\(T_{\rm N}=6.2\) K) while CeNiIn was found to exhibit mixed valence and no magnetic ordering above 1.8 K. XPS measurements show that in CeNiIn the Ni \(3d\) and Ce \(4f\) states are both near the Fermi level. In CeAuIn only the Ce \(4f\) state is near the Fermi level while the Au \(5d\) band is represented by two peaks in the region between 3 and 7 eV below the Fermi level. The interpretation of Ce \(3d\) XPS spectra in terms of the Gunnarsson–Schönhammer theory indicate that the hybridization energy \({\mit \Delta }\) between the Ce \(4f\) and conduction band is equal to 144 meV for CeAuIn and 205 meV for CeNiIn.

abstract

Magnetic, electrical and thermodynamic properties of the pseudobinary alloy system UGa\(_{3-x}\)Ge\(_{x}\), \(0\leq x\leq 3\), have been studied by means of magnetic susceptibility, electrical resistivity, thermoelectric power and specific heat measurements. The results indicate a gradual transformation from itinerant antiferromagnetism in Ga-rich alloys to Pauli paramagnetism in Ge-rich materials. Total suppression of the magnetic order occurs near the composition \(x\approx 0.6\). For UGa\(_{2.4}\)Ge\(_{0.6}\) curious behavior of the low-temperature bulk properties has been found that develops presumably because of the closeness of this alloy to a magnetic quantum critical point.

abstract

Inelastic neutron scattering experiments under magnetic field were performed on the one-dimensional quantum antiferromagnet Yb\(_4\)As\(_3\). When a magnetic field is applied perpendicular to the Yb\(^{3+}\) chain, the material exhibits a gap-opening phenomenon due to the induced staggered field originated from the Dzyaloshinsky–Moriya interaction. It was confirmed that the spectrum remains gapless when a magnetic field is applied parallel to the Yb\(^{3+}\) chain. However, the gapless mode at the incommensurate wave vector which is predicted by the theories was not found.

abstract

The \(T\)-dependence of the resistivity \(\rho \) of a start-of-the-art polycrystalline sample of CeFe\(_{4}\)Sb\(_{12}\) was measured between 5 and 300 K. The cerium contribution to the resistivity, \(\rho \)(Ce) was obtained by substracting the resistivity of LaFe\(_{4}\)Sb\(_{12}\). As a function of \(T\), \(\rho \)(Ce) exhibits a maximum near 150 K. This maximum is attributed to a combination of a Crystal Electric Field (CEF) and the Kondo effect. This interpretation is consistent with Hall data in the same temperature range. Our heat capacity data confirm that the electronic specific heat coefficient \(\gamma \) for LaFe\(_{4}\)Sb\(_{12}\) is larger than that for CeFe\(_{4}\)Sb\(_{12}\). The Debye temperatures of both materials are near 250 K.

abstract

X ray diffraction studies have been performed on a CeRhSn single crystal and its anomalous unit-cell volume was confirmed. This, together with temperature dependence of magnetic susceptibility indicate valence-fluctuating behaviour of Ce ions. Band structure calculations support such a behaviour. Anomalous value of the quadrupole interaction constant derived from \(^{119}\)Sn Mössbauer spectroscopy is observed. Ac and dc magnetic susceptibility investigations as well as preliminary resistivity measurements evidence that this compound does not order magnetically down to 2 K, but one of our samples is superconducting with a transition temperature of 6.5 K.

abstract

We performed the inelastic neutron scattering experiment on a powder sample of YbSb in order to clarify the nature of the unusual non-magnetic phase transition at 5 K. The two broad magnetic peaks at \({\sim }\)13 meV and \({\sim }\)18 meV were observed, as reported by the previous inelastic neutron experiment. The clear difference of these magnetic excitation spectra between the paramagnetic phase and the ordered phase may indicate the change of the \(4f\) electronic states due to the phase transition at 5 K. Furthermore a new magnetic excitation below \({\sim }\)3 meV, which is common feature in Yb-monopnictudes, was found.

abstract

The specific heat of CeNiGe\(_3\), Ce\(_2\)Ni\(_3\)Ge\(_5\), Ce\(_3\)NiGe\(_2\) and Ce\(_3\)Ni\(_2\)Ge\(_7\) and their isostructural analogues with La or Y was studied in the temperature range 2.5–70 K. For all the Ce-based compounds \(C_{\rm p}(T)\) exhibits pronounced \(\lambda \)-shaped peaks at the magnetic phase transitions. In the paramagnetic range Kondo and Schottky terms notably contribute to the total specific heat. In the ordered region, \(C_{\rm p}(T)\) is dominated by a spin-wave contribution. The characteristic Kondo and RKKY energy scales in all the compounds are estimated to be of similar magnitude.

abstract

We investigated the pressure dependence of the antiferromagnetic transition temperature \(T_{\rm N}\) of UPb\(_3\) by the electrical resistivity measurement. We found that at low pressures \(T_{\rm N}\) increases with increasing pressure. It has a maximum value 38 K at 5 GPa, then decreases with increasing pressure. In addition we observed another resistive anomaly followed by a steep decrease in resistivity below \(T_{\rm N}\) under pressure. The maximum pressure of 8 GPa in the present study was not enough to suppress the antiferromagnetic ordering. The critical pressure is estimated to be around 10 GPa.

abstract

We have grown single crystals of R\(_2\)MIn\(_8\) compounds (R=La, Ce, Pr; M = Rh, Ir) and measured magnetic and transport properties of these crystals in the temperature range 1.8–300 K. We have found that Ce\(_2\)RhIn\(_8\) is an antiferromagnet with a Néel temperature T\(_{\rm N}\)=2.8 K and Ce\(_2\)IrIn\(_8\) is in a paramagnetic state down to 1.8 K. The Ce-based compounds are dense Kondo materials with the Kondo temperatures of several tens of Kelvins and nearly a hundred Kelvin for Ce\(_2\)RhIn\(_8\) and Ce\(_2\)IrIn\(_8\), respectively. The Pr ions in the Pr-based compounds are in the singlet ground states.

abstract

Electrical resistivity measurements under hydrostatic pressure up to 2.2 GPa was carried out for a ferromagnetic ternary cerium compound CeCu\(_9\)Sn\(_4\). The ferromagnetic transition temperature increases with increasing pressure up to 0.8 GPa and then decreases with increasing pressure above 1 GPa. Origins of this pressure dependence may be the competition between magnetic interaction in the \(c\)-plane and along the \(c\)-direction.

abstract

Magnetic, electrical transport and thermodynamic properties of a novel Heusler compound YbPd\(_{2}\)Sb have been studied down to 40 mK in applied magnetic fields up to 14 T. The results hint at a heavy-particle nature of the electronic ground state, with some features characteristic of systems being close to Doniach’s magnetic–nonmagnetic instability.

abstract

The non-Fermi liquid behaviour of the Heusler-type Fe\(_{\rm 2+x}\)V\(_{\rm 1-x}\)Al alloys was investigated with the use of ab initio and many-body methods. Calculations have shown that the narrow \(d\) band originating from the impurity Fe atoms is responsible for the unusual temperature dependence of different physical properties of these materials.

abstract

Magnetic and electronic properties of NdAl\(_{2}\) have been found to be well described within the crystal-field approach. A \(\lambda \)-type peak observed at the magnetic ordering temperature in the temperature dependence of the heat capacity is related to the time-reversal symmetry breaking in the atomic scale. The good description proves the existence of the discrete atomic-like states, in the meV energy scale, in this intermetallic compound.

abstract

We have made extensive reciprocal space maps in the heavy-fermion superconductor URu\(_2\)Si\(_2\) using high-resolution time-of-flight single-crystal neutron diffraction to search for signs of a hidden order parameter related to the 17.5 K phase transition. Within the present sensitivity of the experiment (0.007 \(\mu _B\)/U-ion for sharp peaks), no additional features such as incommensurate structures or short-range order have been found in the \((h0l)\) or \((hhl)\) scattering planes. The only additional low-temperature scattering observed was the well-known tiny antiferromagnetic moment of 0.03 \(\mu _B\)/U-ion.

abstract

We present ultrasonic attenuation and elastic constant results on the heavy-fermion state of URu\(_{2}\)Si\(_{2}\) both as a function of temperature and magnetic fields up to 50 T. We find distinct anomalies as a function of field and temperature for the longitudinal modes c\(_{11}\) and c\(_{33}\). We construct a \(B\)–\(T\) phase diagram including \(T_{\rm N} (B)\), the so-called metamagnetic transition and the spin-flop transition into the paramagnetic phase.

abstract

Many cerium or ytterbium alloys and compounds are characterized by heavy-fermion behavior due to the Kondo effect. In the case of a lattice, there is a strong competition between the Kondo effect and magnetic ordering. The magnetic order close to the quantum critical point(QCP) is generally an antiferromagnetic one, but could be also a ferromagnetic one or a spin-glass one in disordered systems. We review here the main features of the Kondo lattice model within a mean-field treatment of the Hamiltonian, including both the \(s\)–\(f\) exchange intra-site interaction and the nearest neighbor inter-site \(f\)–\(f\) interaction. First, we study the non-magnetic case and discuss the effects of conduction band filling and of the inter-site exchange parameter on the occurrence of the Kondo effect and short-range magnetic correlations. Second, we treat the spin glass-Kondo competition by considering a random inter-site interaction and we study the competition between Kondo, spin-glass and ferromagnetic phases. The nature of the transition at the QCP is also discussed and comparison with experimental data in heavy fermions systems is finally presented.

abstract

The correlated Kondo lattice model simultaneously describes the interaction of itinerant conduction electrons with localized magnetic moments as well as with other electrons in the conduction band. As a limiting case a two-site cluster with \(S=1\)/\(2\) is studied. By analytical calculations we were able to find the exact expression for the energy poles and spectral weights of the one-particle Green’s function and all contributing correlation functions.

abstract

We have recently shown that CeRhSn exhibits non-Fermi liquid temperature dependences in its low-temperature physical properties. Here we suggest that the non-Fermi liquid behavior observed in CeRhSn may be due to the existence of a Griffiths phase in the vicinity of a quantum critical point, based on electrical resistivity, magnetic susceptibility, and specific heat measurements. For CeRhSn, the low-temperature scaling of bulk properties (\(C/T \propto \chi \propto T^{-1+\lambda }\), where \(\lambda \lt 1\)) is masked by an anomaly at about 6 K, which is of magnetic origin.

abstract

YbAl\(_{3}\) is an intermediate valent compound with a large Kondo temperature \(T_{\rm K}\) and moderately low conduction electron density. Because of this, YbAl\(_{3}\) is a prime candidate for the observation of effects caused by low conduction electron density, where coherence sets in below \(T_{\rm coh}\) rather than \(T_{\rm K}\) (\(T_{\rm coh}\ll T_{\rm K}\)). For the first time, we have directly observed the cross over between the energy scales by the application of a magnetic field above \(B^{\ast }\approx 40\) T (\(\approx k_{\rm B}T_{\rm coh}/\mu _{\rm B} \)). We also observe a reduction in the effective masses above \(B^{\ast }\) that is consistent with the energy scale crossover.

abstract

We report thermoelectric, magnetic and structural properties of the so-called Kondo semiconductor CeRhAs. The resistivity \(\rho (T)\) along the three principal axes exhibits step-like anomalies at \(T_1=370,~T_2=235\) and \(T_3=165\) K, respectively, and increases by two orders of magnitude on cooling to 1.5 K. Below \(T_1\) an abrupt drop in the magnetic susceptibility along all the axes is associated with the cell doubling along the \(b\)- and \(c\)-axes, suggesting a charge-density-wave transition. Below \(T_3\) the increase in the thermopower is strongest along the \(a\)-axis with a maximum of 186 \(\mu \)V/K at 35 K. The thermoelectric figure of merit becomes largest along the \(c\)-axis with the maximum value of \(1.0\times 10^{-3}\)/K, being comparable with that of CePd\(_3\).

abstract

(Ce\(_{1-x}\)La\(_x\))Cu\(_5\)In alloys have been investigated by X-ray diffraction, specific heat, electrical transport and magnetic susceptibility measurements. Resistivity \(\rho (T)\) measurements enabled the extraction of the Kondo temperature \(T_{\rm K}(x)\) for the different alloys. The monotonic decrease of \(T_K\) with increase in \(x\) has been interpreted in terms of the compressible Kondo model giving \(|JN(E_{\rm F})| = 0.063 \pm 0.005\) (\(J\) is the strength of the on-site Kondo interaction and \(N(E_{\rm F})\) the density of states at the Fermi level). Magnetoresistivity measurements interpreted within the single-ion Bethe ansatz description corroborate the magnitude of \(T_{\rm K}\) found in the \(\rho (T)\) measurements.

abstract

The Kondo lattice compound CeCu\(_2\) shows an antiferromagnetic ordering below \(T_{\rm N}=3.5\) K. The two moments in the primitive crystallographic unit cell are aligned antiparallel to each other and oriented along the orthorhombic \(c\)-axis even though the \(a\)-axis is the easy axis of magnetization. This can be explained by a strongly anisotropic antiferromagnetic exchange in the \(ac\)-plane which cancels out the crystal field anisotropy. The measurements of the spin wave dispersion at 1.5 K that we present in this paper for zero field and for 5 T in \(a\)-direction verify this assumption. Preliminary model calculations using the program package McPhase reproduce qualitatively the measured dispersion curves.

abstract

We have achieved high-pressure resistivity measurements on NpTe single crystal up to 21 GPa. We observe the phase transition from NaCl to CsCl phase at 13 GPa and the decrease of antiferromagnetic transition temperature \(T_{\rm N}\) from 41 K to 2 K. In the NaCl phase, insulator behaviour develops: it could be described by a simple activation energy law. At the phase transition, a metallic behaviour replaces insulator behaviour with a large decrease of resistivity. Simultaneously, a sharp collapse of resistance appears at very low temperature. The origin of this collapse in the CsCl phase at T\(\sim \)5 K is still unknown.

abstract

The \(a\)-axis thermoelectric power, \(S(T)\) of the two-level-system Kondo ferromagnets UPS, UAs\(_{1-x}\)Se\(_{1+x}\) and USbTe have been examined. Two peaks of \(S(T)\) dependence, related to two characteristic temperatures of electronic scattering, \(T^{*}_{1}\) and \(T^{*}_{2}\), are observed below the Curie temperature. The temperature \(T^{*}_{1}\), which we specify as the Kondo temperature is independent of \(x\) and equals \(29.9\pm 1.7\,\)K. An overall similarity of \(S(T,x)\) behaviour for UAs\(_{1-x}\)Se\(_{1+x}\) and that for Ce\(_{x}\)Y\(_{1-x}\)Cu\(_{2.05}\)Si\(_{2}\) heavy-fermion alloy system is observed and its origin is discussed.

abstract

Binary amorphous alloy system Mn\(_{100-x}\)Ce\(_x\) has been fabricated by a dc-sputtering deposition with various Ce compositions ranging from \(x=20\) to 80. Temperature dependence of electrical resistivity for Ce-rich side of amorphous Mn\(_{100-x}\)Ce\(_x\) alloy, especially in Mn\(_{21}\)Ce\(_{79}\), exhibits a large initial increase with \(T^{~2}\)-like behavior at low temperature side up to about 7 K followed by a quite rapid logarithmic-like decrease with the increase of temperature. In contrast, amorphous Mn\(_{75}\)Ce\(_{25}\) alloy shows only monotonic decrease in the resistivity with increasing temperature. These results reveal a clear evidence of a dense Kondo behavior for Ce rich side.

abstract

The model of two-level Kondo effect is studied by the Wilson numerical renormalization group method. It is shown that there exist a new type of weak-coupling fixed point other than the strong-coupling fixed point found by Vladar and Zawadowski two decades ago by means of the one-loop and two-loop renormalization group methods.

abstract

UNiSn undergoes an anomalous phase transition at \(T_{\rm N}=47\) K, at which temperature it transforms from an antiferromagnetic metal to a paramagnetic semiconductor with an energy gap \(\simeq 70\) meV. In order to investigate how the electronic structure of UNiSn changes as it crosses the transition temperature, we have used the X ray photoemission spectroscopy (XPS) technique from 20 to 70 K. According to the XPS studies, the U 4\(f\) core levels are almost temperature independent while the Ni 2\(p\) core levels and the satellite structure display a weak anomaly at \(T_{\rm N}\).

abstract

UPS is the second uranium-based pnictochalcogenide where electron-assisted tunneling presumably is realized. Although the unusual properties of UPS and UAsSe originate from the structural disorder, a clear-cut uniformity in their electrical resistivity is observed. The additional resistivity due to non-magnetic Kondo-like scattering was found as large as 140 \(\mu {\mit \Omega }\) cm at low temperatures. The estimated concentration of the fast TLS centers varies between 1\(0^{19}\) and \(10^{20}\)cm\(^{-3}\).

abstract

An energy-scale-dependent approximation, that allows to resolve low energy features embedded in a high energy background, is reviewed. The Kondo and Anderson models are studied and fully self-consistently resolved as significative examples. A Kondo-like peak is obtained at low enough temperatures. The method is shown to be capable to reproduce the exact results with very low numerical effort and it is applicable for any value of the external parameters.

abstract

The problem of a spin-1/2 impurity placed at the center of a small metallic sphere and contact exchange is reduced to the Bethe Ansatz solution of the Kondo model in a finite box. The average level spacing is an additional energy scale competing with \(T_{\rm K}\). We obtain the energy levels as a function of \(T_{\rm K}\) for two \(s\)-states, as well as the \(T\)-dependence of the entropy and the susceptibility. The results depend on the parity of the number of \(s\)-states.

abstract

Magnetic and transport properties of (Y\(_x\)Ce\(_{1-x}\))\(_7\)Rh\(_3\) have been studied by measuring magnetization, magnetic susceptibility and electrical resistivity. Ce\(_7\)Rh\(_3\) is a metal ferromagnet and Y\(_7\)Rh\(_3\) is a semimetallic Pauli paramagnet. In (Y\(_x\)Ce\(_{1-x}\))\(_7\)Rh\(_3\), ferromagnetic phase disappears at about \(x = 0.\)4. Intermediate valence state can exist in high Y concentration range above \(x = 0.5\). In this range, electrical properties can be realized by the Kondo scattering in semimetallic band structure.

abstract

Ce 4\(f\) states of the Kondo semiconductor CeRhAs, semimetal CeRhSb, and metal CePtSn single crystals were observed directly by high-resolution resonant photoemission spectroscopy. A large gap and a pseudogap at \(E_{\rm F}\) were found in CeRhAs and CeRhSb, respectively.

abstract

The Kondo lattice model has been analyzed in the presence of a random inter-site interaction among localized spins with non zero mean \(J_{0}\) and standard deviation J. Following the same framework previously introduced by us, the problem is formulated in the path integral formalism where the spin operators are expressed as bilinear combinations of Grassmann fields. The static approximation and the replica symmetry ansatz have allowed us to solve the problem at a mean field level. The resulting phase diagram displays several phase transitions among a ferromagnetically ordered region, a spin glass one, a mixed phase and a Kondo state depending on \(J_{0}\), \(J\) and its relation with the Kondo interaction coupling \(J_{\rm K}\).

abstract

An asymmetric shift in the position of the magnetic Bragg peak with respect to the fiducial lattice has been observed by resonant X-ray scattering in a diverse series of antiferromagnetic compounds. This apparent violation of Bragg’s law is interpreted in terms of a dynamically phased order parameter. We demonstrate the use of this effect as a novel probe of fragile or dynamic thermodynamic order in strongly correlated electronic systems. In particular, fresh light is shed on the paradoxical situation encountered in URu\(_{2}\)Si\(_{2} \) where the measured entropy gain on passing through \(T_{\mathrm {Néel}}\) is incompatible with the ground state moment estimated by neutron diffraction. The intrinsic space-time averaging of the probe used to characterise the thermodynamic macroscopic state may play a crucial and previously neglected role. In turn, this suggests the further use of resonant X-ray scattering in investigations of systems dominated by quantum fluctuations.

abstract

We review experiments devoted to the pressure influence on magnetism in uranium intermetallics, represented by several hexagonal UTX compounds (T = transition metal, X = \(p\)-metal) and by the tetragonal compound UNi\(_{2}\)Si\(_{2}\). The typical pressure induced phenomena involving the stability of U magnetic moments and their exchange coupling are discussed in conjunction with expected variations of the 5\(f\)-ligand hybridization.

abstract

Using rigorous analytical analysis and exact numerical data for the spin-1/2 transverse Ising chain we discuss the effects of regular alternation of the Hamiltonian parameters on the quantum phase transition inherent in the model.

abstract

The investigation on Ce–Ni–Ge phase diagram indicates a relationship between the physical behavior of these ternary germanides and their chemical composition. Those with more than 50 % Ge-atomic percentage order antiferromagnetically, with Néel temperatures \(T_{\rm N}\) decreasing with Ge-content. The Ce-magnetic moment at 1.4 K, determined by neutron powder diffraction, also decreases with Ge-content. As for the ternary germanides rich in Ni (\(\geq \)33 \(\%\)), they can be classified as intermediate valence compounds (CeNiGe, Ce\(_{3}\)Ni\(_{4}\)Ge\(_{4}\) and CeNi\(_{4.25}\)Ge\(_{0.75}\)) or heavy-fermions systems (CeNi\(_{2}\)Ge\(_{2}\) and CeNi\(_{9}\)Ge\(_{4}\)).

abstract

Recently several compounds revealed properties of a one-dimensional (1D) spin 1/2 antiferromagnetic Heisenberg chain (AHC) with gapless low-lying spin excitations in the absence of an external magnetic field \(H\). A weak external magnetic field leads an appearance of a spin gap. The gap onset is related to the staggered Dzyaloshinskii–Moriya (DM) interaction or effective \(g\)-factors of magnetic ions. Experimentally measured thermodynamic properties cannot distinguish between those two origins for the spin–gap formation. We propose the experiment, by which one can distinguish between those two possibilities: an electronic paramagnetic resonance (EPR) in the parallel pumping geometry.

abstract

We report on X-ray resonant magnetic reflectivity (XRMR) at the Ce L\(_{2}\) and M\(_{4,5}\) edges in an ex-situ grown Fe/Ce multilayer. We show that the measurement of the magnetic contribution to the intensities reflected at low angles allows us to investigate the profile of the Ce \(5d\) and \(4f\) magnetization. The calculated XRMR signals indicate that the Ce moments have a non-collinear structure.

abstract

NMR and EPR measurements in the two-dimensional frustrated antiferromagnets on a square lattice Li\(_2\)VOSiO\(_4\) and VOMoO\(_4\) are presented. It is found that a structural distortion, possibly driven by the frustration, takes place in both compounds. The modifications induced by the distortion in the NMR and EPR spectra on cooling can be accounted for by a progressive growth of the size of the domains where the lattice is distorted.

abstract

We discuss the local magnetic moments and magnetic hyperfine fields on actinide impurities diluted in Fe and Ni hosts. One adopts a Anderson–Moriya model in which a localized \(5f\) level is hybridized with a spin polarized and charge perturbed \(d\)-conduction band. Our self-consistent numerical calculations for the hyperfine fields on the impurity sites are in good agreement with the available experimental data.

abstract

\(^{139}\)La NQR spectra and relaxation and \(\mu \)SR precessional frequencies in La\(_2\)Cu\(_{1-x}\)M\(_x\)O\(_4\) (for M= Zn and Mg) are reported in order to study the effect of spin dilution in the planar quantum Heisenberg antiferromagnet (2DQHAF) La\(_2\)CuO\(_4\). The behavior of the spin stiffness \(\rho _{\rm s}(x)\) and of the in-plane correlation length \(\xi _{\rm 2D}(x,T)\), of the sublattice magnetization and of the Néel temperature, for a dilution approaching the percolation threshold depart sizeably from the ones expected in dilution-like models. In spite of the marked reduction of \(\rho _{\rm s}\) the transition to the ordered state occurs at a temperature, where \(\xi _{\rm 2D}(x,T_{\rm N})\) reaches a value close to the one in undoped 2DQHAF.

abstract

We report on the magnetic and electronic properties of AlB\(_{2}\)-type structure NpSi\(_{2 - x}\) and NpGe\(_{2 - x}\) as inferred from SQUID magnetisation and \(^{237}\)Np Mössbauer spectroscopy. NpGe\(_{2 - x}\) orders ferromagnetically below \(T_{\rm C}\) \( \approx \) 157.4 K with an ordered moment \(\mu _{\rm Np}\) \( \approx \) 2.01 \(\mu _{\rm B}\), whereas NpSi\(_{2 - x}\) exhibits three different ordered magnetic phases respectively below 110 K, 105 K and 90 K. The average ordered moment amounts to \(\mu _{\rm Np}\) \( \approx \) 1.51 \(\mu _{\rm B}\) at 4.2 K.

abstract

We present a study of uranium magnetic form factors in the UCoAl, UPtAl, UNiGa and UNiAl intermetallic compounds, all crystallizing in the hexagonal ZrNiAl-type structure. Our study is based on polarized neutron diffraction experiments. The \(-\mu _{\rm L}\)/\(\mu _{\rm S}\) ratio determined from our data is reduced compared to the U\(^{3+}\) free-ion value for all the studied compounds, indicating delocalization of the 5f-electron states.

abstract

We have measured temperature dependent magnetization of pyrochlore molybdates (Gd\(_{1-x}\)Dy\(_x\))\(_2\)Mo\(_2\)O\(_7\) (x=0, 0.1, 0.2, 0.4) under pressures up to 9.5 kbar. With increasing pressures, the magnetization of all four samples decreases substantially and the ferromagnetic transition seen in Gd\(_2\)Mo\(_2\)O\(_7\) is suppressed and not found above 9 kbar; which is very similar to the effects of Dy substitutions. Interestingly enough, we observed a clear difference in magnetization measured after zero field cooling (ZFC) and field cooling (FC). From these results we have confirmed that the lattice constant of R\(_2\)Mo\(_2\)O\(_7\) is crucial in determining the magnetic state and demonstrated that the ferromagnetic ordering coexists with metastable behavior of a spin glass type.

abstract

Measurements of magnetization and magnetic susceptibility have been carried out on a tetragonal ternary PrCu\(_2\)Ge\(_2\) single crystal compound. It orders antiferromagnetically below \(T_{\rm N}\)=14.4 K which is an anomalously high transition temperature. The magnetic behavior in the virgin state is different from one in the state after magnetization saturation process along the \(c\)-axis in low temperatures. The susceptibility of the virgin state is enhanced below 3.5 K while one after saturation becomes very small. An irreversible magnetization process appears in the virgin state while it becomes reversible after saturation, which has been never seen yet.

abstract

The ferromagnetic compound URhSi is a close analogue of the well-known “ferromagnetic superconductor” URhGe. We have measured, for fist time on a single crystal, the magnetization and susceptibility as functions of magnetic field and temperature. Arrott-plot analysis points to the Curie temperature \(T_{{\rm C}}\) = 10.5 K. At temperatures below \(T_{{\rm C}}\) we observe a strong magnetic anisotropy with the dominant spontaneous magnetic moment (0.47 \(\mu _{{\rm B}}\)) along the \(c\)-axis, similar to URhGe. Along the \(a\)- and \(b\)-axis, much smaller, nevertheless non-negligible, magnetic moments are indicated. A strong anisotropy is observed also in paramagnetic range.

abstract

We have measured the resistivity and magnetisation of several samples of the Eu\(_{1-x}\)Ca\(_{x}\)B\(_6\) series under pressure in order to probe the evolution of itinerant electron ferromagnetism in CaB\(_{6}\) from a localised moment ordered state in EuB\(_{6}\). Starting with pure EuB\(_{6}\), the Curie temperature \(T_{\rm C}\) is reduced upon increasing \(x\). In particular, we have investigated the magnetic field and pressure dependence of \(T_{\rm C}\), which is enhanced by both of these externally tuneable control parameters. At \(T_{\rm C}\), there is a large peak in the resistivity, which is eliminated by moderately strong magnetic fields.

abstract

We present a theoretical investigation of electronic properties and \(4f\)-excitation spectra on several rare earth RX\(_2\) compounds. Both compounds with a transition metal (X = Co), carrying an induced magnetic moments, and with essentially non-magnetic simple metals (X = Al) are considered. The calculations are based on a combination of density functional (DF) and crystal field model theories. The Kohn–Sham equations of DF theory were solved using local spin density approximation (LSDA) but we also tested the influence of the generalized gradient approximation (GGA).

abstract

A detailed study of Dy\(_3\)Co single crystal in moderate magnetic fields is presented. It forms the orthorhombic structure (Fe\(_3\)C-type, space group Pnma) and orders antiferromagnetically (AF) at \(T_{\rm N} = 72\) K. At least three additional AF phases exist below \(T_{\rm N}\) with corresponding transition temperatures \(T_1 = 43\) K, \(T_2 = 33\) K and \(T_3 = 24\) K, respectively. Step-like metamagnetic transitions were observed along all three crystallographic axes. The respective complex magnetic phase diagrams are presented.

abstract

The temperature dependence of specific heat of selected RCu\(_{2}\) (R = Sm and Tm) was studied in detail in comparison with that of the nonmagnetic analogues (YCu\(_{2}\) and LuCu\(_{2}\)). The electron and the phonon part of the specific heat was determined for all the investigated compounds as well as the magnetic entropy for the magnetically ordered ones. The density functional calculations were performed for YCu\(_{2}\) and LuCu\(_{2}\) compounds to obtain an additional information about the electronic structure of the RCu\(_{2}\) system.

abstract

We have grown a single crystal of UNiSi and measured the magnetization and susceptibility as functions of temperature and magnetic field applied along the principal crystallographic axes. Magnetic ordering with a relatively weak easy-plane anisotropy is indicated below 90 K. In addition, we present results of investigation of hydrogen absorption on the crystal structure and magnetic properties of UNiSi.

abstract

Yb\(_5\)Si\(_4\) and Yb\(_3\)Si\(_4\) are two new compounds with three and two inequivalent lattice sites for Yb ions, respectively. Using susceptibility, heat capacity and \(^{170}\)Yb Mössbauer spectroscopy, we find that only about 1/3rd of Yb ions order magnetically below 1.65 K in Yb\(_5\)Si\(_4\), with a substantial saturated moment of 1.9 \(\mu _{\rm B}\)/Yb at 80 mK. The remaining Yb ions are paramagnetic down to 80 mK, but with a valency close to 3, presumably due to appreciable hybridisation. In Yb\(_3\)Si\(_4\) the hybridisation is stronger and half of Yb ions orders magnetically below 3 K, with a very small saturated moment of just 0.28 \(\mu _{\rm B}\)/Yb at 80 mK.

abstract

The boron interstitial solid solutions UFe\(_{10}\)Si\(_{2}\)B\(_{x}\) (\(x\)= 0.1, 0.25, 0.5, 0.75, 1, 1.25, 1.5, 2) were synthesized with the arc furnace and the structural analysis was performed by X-ray diffraction. The magnetic hysteresis in room temperature and the temperature dependence of the magnetization from room temperature to 800\(^{\circ }\)C were measured. From these measurements, the saturated magnetization and the Curie point were clarified, respectively.

abstract

Neutron diffraction experiments on CeSb and CeBi under high pressure up to 4.6 GPa revealed the new features of the magnetic \(P\)–\(T\) phase diagrams. The development of each magnetic phase corresponds to the unusual enhancement of the electrical resistivity or the shrinking of the crystal lattice. The phase diagram of CeSb above about 2 GPa is very similar to that of CeBi above ambient pressure.

abstract

The magnetic form factor of ferromagnetic superconductor URhGe has been measured with the use of polarized neutrons. In spite of rather peculiar state of the \(5f\) electrons in this compound reflected in their itinerant behavior, strongly reduced uranium magnetic moments of 0.4 \(\mu _{{\rm B}}\) and in some cases also superconductivity at low temperatures, we found a magnetic form factor that does not deviate substantially from that one of the free U\(^{3+}\) ion calculated in a dipole approximation.

abstract

We have investigated the modifications induced by thermal treatments in the structural and magnetic properties of the CeNi\(_{0.8}\)Cu\(_{0.2}\) sample which presents a spin-glass-like behaviour below 6 K. No remarkable changes in the microstructure have been detected by X ray diffraction and scanning electron microscopy. However, the magnetic properties depend on the thermal treatment of the sample. This fact suggests that modifications in the size and number of the magnetic clusters are being induced. In addition, long-range magnetic order is evidenced below 0.6 K.

abstract

We present a novel extrapolation scheme for high order series expansions. The idea is to express the series, obtained in orders of an external variable, in terms of an internal parameter of the system. Here we apply this method to the 1-triplet dispersion in an antiferromagnetic \(S=1\)/\(2\) Heisenberg ladder. By the use of the internal parameter the accuracy of the truncated series is enhanced tremendously.

abstract

The pseudo-ternary series ErNi\(_{1 - x}\)Cu\(_{x}\)Al has been studied by means of X-rays and magnetization. Structure parameters exhibit a sudden “jump” between \(x\) = 0.5 and 0.6. The samples in the initial region with \(x\) \( \le \) 0.2 undergo two phase transitions and co-existence of F and AF ordering is possible. The samples with \(x\) \( \ge \) 0.4 are already rather ferromagnets.

abstract

We have found an efficient algorithm for high temperature expansion of the SU(\(n\)) Heisenberg model, using properties of permutation. We also comment on the \(n\rightarrow \infty \) limit.

abstract

We report the magnetic properties of Pr\(_{1-x}\)Sr\(_{1+x}\)MnO\(_4\) single crystals in the composition range \(0.3 \le x \le 0.8\). All the compositions studied show highly anisotropic spin-glass type magnetic ordering at low temperatures. Some of the compositions (\(x \gt 0.6\)) exhibit a broad maximum in the magnetic susceptibility above 100 K, highlighting the 2D magnetic interactions in these compounds. A possible charge ordering transition is observed around 265 K for the \(x = 0.8\) compound.

abstract

I study the thermodynamic properties of \(S=1\)/\(2\) two-dimensional quantum antiferromagnets with easy-plane anisotropy, both in the case of intrinsic anisotropy (XXZ model in zero field) and in the case of field-induced anisotropy (Heisenberg antiferromagnet in a uniform magnetic field), making use of the continuous-time Quantum Monte Carlo method. For the model with intrinsic anisotropy I single out the characteristic features of the Heisenberg-to-XY crossover anticipating the Berezhinskii– Kosterlitz–Thouless (BKT) transition. Then I show how these features are reproduced in the case of a field-induced anisotropy and BKT transition. Implications for the experimental realization and detection of a disordered two-dimensional XY-like phase in real antiferromagnets are discussed.

abstract

We have investigated a pyrochlore Mott insulator Y\(_{2}\)Ir\(_{2}\)O\(_{7}\) with an expectation that \(5d^{5}\) electrons of Ir ions in the \(t_{2g}\) orbitals give rise to an \(S = 1\)/\(2\) Heisenberg system and form a quantum spin-liquid state. However, it exhibits spin–glass ordering below about 170 K. Such spin-glass ordering is also observed in its isomorphs Y\(_{2}\)Mo\(_{2}\)O\(_{7}\) and Y\(_{2}\)Ru\(_{2}\)O\(_{7}\) with \(S = 1\).

abstract

Exchange couplings are calculated for the isostructural compounds Li\(_2\)VOSiO\(_4\) and Li\(_2\)VOGeO\(_4\) using LDA, suggesting the realization of a large \(J_2\)/\(J_1\) quasi-2D spin-half Heisenberg model in both compounds. High temperature expansions for the uniform susceptibility and specific heat are used to fit the experimental data. The range and quality of unbiased fits as a function of the parameters \(J_1\), \(J_2\) and \(g\) are reported, confirming \(J_2\) as the largest exchange constant for these materials.

abstract

Using a Majorana fermion representation, we discuss the influence of Dzyaloshinski–Moriya interaction on the magnetic properties of a spin-1/2 ladder. We calculate the spin-echo decay rate and analyze the modifications with respect to the isotropic ladder. Implications of our calculations for experiments on ladder systems are discussed.

abstract

We present selected results of systematic studies of AC susceptibility in the series of single crystalline \({\rm Fe}_{3-x}{\rm Zn}_x{\rm O}_4\) (\(x\lt 0.04\)) and \({\rm Fe}_{3(1-\delta )}{\rm O}_4\) samples. Two sets of anomalies were found: the first one connected with the Verwey transition and the second consisting of two different effects, one at 26 K, visible only for stoichiometric samples, and the other at 50 K. While the temperature position of the first anomaly (26 K) does not depend on \(f\), the effect at 50 K shifts to higher temperatures with increasing frequency. Both effects gradually move to lower temperatures and finally disappear with increasing \(x\) and \(\delta \). Qualitatively similar results were reported from Magnetic After Effect (MAE) technique, that records effects with the relaxation time \(10^4\) higher than that characteristic for our technique.

abstract

The experimental results of AC susceptibility, magnetization, and heat capacity of polycrystalline \({\rm La}_{0.67}{\rm Ca}_{0.33}{\rm Mn}_{1-x}{\rm Fe}_x{\rm O}_3\) (\(x = 0\), \(0.01\), \(0.06\), \(0.06\,^{57}{\rm Fe}\), \(0.10\,^{57}{\rm Fe}\) and \(0.015\,^{57}{\rm Fe}\)) are presented. A nonlinear reduction of the ordering temperature \(T_{\rm C}\) and a diminishing anomaly in heat capacity with increasing Fe contents were found. A similar lowering of the \(\chi _{\rm AC}\) signal by external magnetic field was observed for all samples but frequency dependent effects are visible for \(x\ge 0.06\) and \(M(H)\) for those compositions do not show saturation at \(T\) close to critical temperatures. The results are discussed based on formation of microscale magnetic clusters.

abstract

The lattice dynamics of the La\(_{2/3}\)Ca\(_{1/3}\)Mn\(_{1-x}\)Fe\(_x\)O\(_3\) system was studied with Mössbauer Spectroscopy in the temperature interval from 4 K to RT. Relative Lamb–Mössbauer factor vs \(T\) was determined for \(x = 0.01\), \(0.03\) and \(0.05\). The \(-\ln (f\)/\(f_0)\) i.e. the mean square-displacement of the Mössbauer atom from its equilibrium lattice position shows anomalous behaviour in the vicinity of \(T_{\rm C}\) or metal–insulator transition temperature \(T_{\mathrm {M-I}}\). In the Debye approximation the \(-\ln (f\)/\(f_0)\) results give higher Debye temperatures \({\mit \Theta }_{\rm D}\) for the paramagnetic-insulating phase (PI) than for the ferromagnetic-metallic (FM) phase.

abstract

The photoemission spectra (XPS/UPS) for iron oxides, stoichiometric magnetite and for selected Ti and Zn doped magnetite single crystals are presented. From the Fe-\(3s\) split lines the exchange energies for FeO, Fe\(_2\)O\(_3\) and magnetite based samples were estimated. It was shown that Ti and Zn ions are of 4+ and 2+ valency, respectively. The correlation energies were estimated from the Fe \(2p_{3/2}\) core-level spectra and from the \(L_3\)-\(M_{4,5}\),\(M_{4,5}\) Auger lines. The type of insulating gap in these compounds was discussed.

abstract

The AC and DC susceptibilities, \(^{57}{\rm Fe}\) and \(^{155}{\rm Gd}\) Mössbauer and XRD studies of \({\rm GdMnO}_3\) and \({\rm Gd}_{0.67}{\rm Ca}_{0.33}{\rm MnO}_3\) manganites are presented. It was found that Mn moments order antiferromagnetically at 40 K for \({\rm GdMnO}_3\) and at 60–70 K for \({\rm Gd}_{0.67}{\rm Ca}_{0.33}{\rm MnO}_3\) (although with ferromagnetic component). The Gd moments order antiferromagnetically at 20 K for \({\rm GdMnO}_3\) and at 25 K for \({\rm Gd}_{0.67}{\rm Ca}_{0.33}{\rm MnO}_3\) but in this case some polarisation of Gd moments by Mn sublattice exists up to 60–70 K.

abstract

BaB\(_{6}\) is a weakly ferromagnetic material with a Curie temperature \(T_{\rm C}\) well above room temperature. From the results of d.c. magnetization measurements on single crystalline BaB\(_{6}\), the saturation magnetization at low temperatures is 8\(\times 10^{-4}(\mu _{\rm B}\)/f.u.), in line with other weak ferromagnets of the hexaboride series. The \(^{11}\)B-NMR spectra measured on a collection of single crystals of BaB\(_{6}\) yield a quadrupolar frequency of 472 KHz, in good agreement with calculated field gradients for this type of materials. The central \(^{11}\)B-NMR transition consists of two partially resolved signals, where the frequency displacement between them is of the order of 10 KHz. One of the signals exhibits a positive, the other a negative frequency shift, both of the order of 50 ppm. Between 7 K and room temperature these shifts do not vary with temperature. The temperature dependence of the spin-lattice relaxation rate \(T_{1}^{-1}\)(\(T\)) at the B sites is similar to that of other alkaline-earth hexaborides.

abstract

Pressure effect on magnetic properties of YbMn\(_2\)Ge\(_2\) was investigated by magnetization measurements under hydrostatic pressures up to 2.0 GPa in temperature range from 2 to 400 K using single crystalline sample. The results indicated that the antiferromagnetism in YbMn\(_2\)Ge\(_2\) unusually and strongly depends on pressure. The anomalous pressure dependence may mainly be originated in the fact that Yb ion is in a mixed valence state between Yb\(^{2+}\) and Yb\(^{3+}\).

abstract

We obtain the phonon spectrum for a simplified structure (La,Ca)\(_{2}\)Cu\(_{2}\)O\(_{3}\) using a shell model calculation. The external photon–electron–phonon vertex is expanded up to fifth order in the copper–oxygen hopping (\(t_{pd}\)) and oxygen–oxygen hopping (\(t_{pp}\)) and the resulting form factors for the phonon-assisted magnetic contribution to the optical conductivity \(\sigma (\omega )\) of (La,Ca)\(_{14}\)Cu\(_{24}\)O\(_{41}\) are analyzed. In this way we can understand the shift of approx. 60 cm\(^{-1}\) observed experimentally between the upper two-triplet bound states observed in \(\sigma _{\rm rung}(\omega )\) and \(\sigma _{\rm leg}(\omega )\).

abstract

Structural and magnetic properties of new compounds RMg\(_{2}\)Cu\(_{9}\) were examined. The results indicated that RMg\(_{2}\)Cu\(_{9}\) crystallize in the CeNi\(_{3}\) type hexagonal structure for R = La, Ce, Pr, Nd, Sm and show magnetic order for R = Ce and Sm, that is in contrast with RMg\(_{2}\)Ni\(_{9}\) system. Some contrastive physical properties in RMg\(_{2}\)T\(_{9}\) (T = Ni, Cu) might be originated in both the two-dimensionality of R atoms and the number of carrier.

abstract

Quadruple perovskites with the \(bcc\) CaCu\(_3\)Ti\(_4\)O\(_{12}\) crystal structure are becoming interesting for unusual magnetic, electronic, dielectric, and structural properties. It is shown here that the magnetic Cu \(d_{xy}\) states have unusual interrelationships (in terms of a tight binding representation) that have important implications for magnetic coupling.

abstract

Using polarized inelastic neutron scattering the antisymmetric part of the dynamical susceptibility in non-centrosymmetric MnSi is determined. The paramagnetic fluctuations are found to be incommensurate with the chemical lattice and to have a chiral character. We show that antisymmetric interactions must be taken into account to properly describe the critical dynamics in MnSi above the Curie temperature. The inelastic neutron data is interpreted within the framework of the SCR-theory, taking into account the Dzyaloshinskii–Moriya interaction.

abstract

Electronic structure of the YbMn\(_{2}\)X\(_{2}\) (X=Si, Ge) compounds with the ThCr\(_{2}\)Si\(_{2}\)-type structure have been investigated. The valence band and the core levels states are analyzed. The XPS valence band spectra are compared with the results of ab initio electronic structure calculations using the Tight-Binding Linear Muffin-Tin Orbital method (TB LMTO). The obtained results indicate that in YbMn\(_{2}\)Ge\(_{2}\) the Yb ion is 2+ while in YbMn\(_{2}\)Si\(_{2}\) is 3+. For both compounds the Mn\(3d\) states are close to the Fermi level. The calculated Mn moments are 1.93 \(\mu _{\rm B}\) for YbMn\(_{2}\)Si\(_{2}\) and 2.30 \(\mu _{\rm B}\) for YbMn\(_{2}\)Ge\(_{2}\).

abstract

Experimental observations and theoretical analysis allow for understanding of the extremely anisotropic magnetic properties of compounds ErRu\(_2\)Si\(_2\) and PrRu\(_2\)Si\(_2\). The presented here calculations for TmRu\(_2\)Si\(_2\) and YbRu\(_2\)Si\(_2\) reveal that their properties are quite different from those of the Er and Pr compounds. We have found that a low-temperature ordering of YbRu\(_2\)Si\(_2\) and a weak ferromagnetism (\(T_{\rm C}\approx \)1K) of TmRu\(_2\)Si\(_2\) can be obtained by using crystal-field parameters directly recalculated from ErRu\(_2\)Si\(_2\) and PrRu\(_2\)Si\(_2\). Moreover, an effect of the temperature-dependent rotation of the easy magnetic axis in YbRu\(_2\)Si\(_2\) is predicted.

abstract

In order to gain insight into suppression of the ferromagnetic ordering temperature, local studies utilizing \(^{99,101}\)Ru zero-field spin-echo NMR, along with complimentary magnetization, high-angle X ray diffraction, and Ru K-edge XAFS measurements, have been carried out on samples of SrRuO\(_{3}\) annealed at both “ambient” (atmospheric) pressure and “high-pressure” oxygen (600 atm). For the “high-pressure” sample, the results indicate structural disorder and the existence of some vacancies on the Ru sites, along with a reduced magnetic ordering temperature.

abstract

Magnetic phase transitions in FeBr\(_{2}\), ErNi\(_{5}\) and UO\(_{2}\) have been found to be well described within the same quasi-atomic crystal-field approach with mean-field approximation for the exchange. The \(\lambda -\)type peak observed at the magnetic ordering temperature in the heat capacity is related to the time-reversal symmetry breaking at the atomic scale. The good description proves the existence of discrete atomic-like states, on the meV energy scale, both in 3\(d\), 4\(f\) and 5\(f\) systems confirming a posteriori the basic assumption of our theory. Our studies indicate that the intraatomic spin-orbit coupling and the orbital magnetism are indispensable for a physically adequate description of both electronic and magnetic properties.

abstract

Focus is given on low temperature properties, with some remarks on materials, instrumentation, salient features of phase diagrams, quantum critical point and superconductivity.

abstract

We briefly overview the highlights of some of the experimental papers presented at SCES’02.