Supersymmetric approach to heavy-fermion systems

We present a new supersymmetric approach to the Kondo lattice model in order to describe simultaneously the quasiparticle excitations and the low-energy magnetic fluctuations in heavy-Fermion systems. This approach mixes the fermionic and the bosonic representation of the spin following the standard rules of superalgebra. Our results show the formation of a bosonic band within the hybridization gap reflecting the spin collective modes. The density of states at the Fermi level is strongly renormalized while the Fermi surface sum rule includes n _c + 1 states. The dynamical susceptibility is made of a Fermi liquid superimposed on a localized magnetism contribution.     

Metamagnetism

This is a review of the physical properties of metamagnets. These are antiferromagnets which, upon the application of a magnetic field, can undergo first-order magnetic phase transitions to a state with a relatively large magnetic moment. The treatments of mean field theory describing these materials are reviewed, as are the treatments of more modern theories. The experimental properties of the known metamagnets are discussed, with emphasis on the variety of means by which the metamagnetic transitions have been observed and studied. For some materials, there have been studies of the tricritical behaviour, and a discussion of the experimental results of these studies is given, along with a comparison of the results with the present theory.     

Electronic structure of cerium heavy-fermion systems

A number of cerium-based systems are investigated using x-ray photoelectron spectroscopy. It is revealed that there is a correlation between the parameters of the x-ray photoelectron spectra and the heavy-fermion state. This correlation makes it possible to investigate the relaxation processes, anomalously strong reorganization of orbitals, profound collective effects induced by a vacancy field, and hybridization of localized unoccupied f states with delocalized occupied states.     

Generalized Fulde-Ferrell-Larkin-Ovchinnikov state in heavy-fermion and intermediate-valence systems

Anomalous properties of the magnetization, the crystal dilatation, and other physical quantities observed in the mixed state of compounds such as UPd₂Al₃ and CeRu₂ suggest the occurrence of a new state near H _c2. In such compounds with large spin susceptibility and high H _c2, a new inhomogeneous superconducting state is theoretically predicted to occur in high magnetic fields. In this inhomogeneous state which is shown to be a generalized Fulde-Ferrell-Larkin-Ovchinnikov state, the order parameter is spatially modulated, and planar nodes of the order parameter are periodically aligned perpendicular to the vortices. Various theoretical predictions are compared with experimental results on UPd₂Al₃ and CeRu₂ related to this new superconducting high-field state.     

Momentum dependence of magnetic response in heavy-fermion systems

The dynamical magnetic response of a heavy-fermion system CeCu₆ is analyzed with the concept of nearly localized quasi-particles. It is explained why the product χ(q)Г(q) of the susceptibility χ(q) and the magnetic relaxation rate Г(q) is independent of the momentum q although both χ(q) and Г(q) depend on q. A generalization of the Korringa relation between χ(q) and Г(q) is formulated.     

Metamagnetism in Jahn-Teller magnets

Metamagnetism is possible in Jahn-Teller magnetic systems, which include giant-magnetoresistance manganites, as a result of both spin-phonon interaction and the influence on spin exchange due to Jahn-Teller ordering of the electron orbits. Fiz. Tverd. Tela (St. Petersburg) 39, 1586–1588 (September 1997)     

The formation of heavy-fermion states in non-Fermi-liquid impurity systems

A mechanism for the occurrence of heavy-fermion states in non-Fermi-liquid (NFL) metals with f-shell impurities is proposed. The impurity with an unstable valence is suggested to have an energy spectrum consisting of a deep f-level and quasicontinuum states (narrow band) in resonance with the Fermi energy. Depending on the impurity concentration, the single-site NFL states are generated by the two-channel Kondo scattering for the low concentration (the Kondo regime) or by the screening interaction for a relatively high concentration (the X-ray-edge regime). It is shown that the NFL states are unstable against the scattering of the NFL excitations by electron states of the narrow band. This scattering generates additional narrow Fermi-liquid (FL) resonances at/near the Fermi level in the Kondo regime and in the X-ray-edge regime. The mixed-valence states are shown to be induced by new FL resonances. The mixed valence mechanism is local and is related to the instability of single-site NFL states. The FL resonances lead to the existence of additional energy scales and of pseudogaps near the Fermi level in the mixed-valence states. They also considerably narrow the region with a nearly integer valence.     

Metamagnetism of hexagonal iron

Summary We present the results of the fixed-spin-moment, self-consistent LMTO electronic structure calculations for hexagonal phase of iron. The relative stability of the nonmagnetic and ferromagnetic phases is discussed along with the magnetic and mechanical equations of state. It is shown that hexagonal Fe is metamagnetic with the nonmagnetic ground state, but at a sufficiently large atomic volume the system prefers a ferromagnetic solution at rather low internal magnetic pressure. These findings seem to be well correlated with the properties of the hexagonal Fe that has recently been grown epitaxially on the Ru substrate.     

Quasielastic magnetic scattering of neutrons by heavy-fermion systems

A theory of quasielastic scattering by a spin liquid with resonating valence bonds is constructed. It is demonstrated that the dependence of the scattering cross section of the spin liquid on the energy transfer has the same shape as the quasielastic peak observed experimentally in heavy-fermion systems. It is shown that a consequence of fact that the excitations of the spin-liquid state obey Fermi statistics is that the total quasielastic scattering cross section oscillates as a function of the momentum transfer. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 480–485 (25 September 1998)     

Model calculations for electronic densities of states in heavy-fermion systems

The influence of the lattice on the density of states for conduction-band andf-electrons in heavy fermion and mixed valent systems has been calculated from an extension of the non-crossing approximation to the lattice. It is shown that the main features of such a calculation can be obtained by a simple numerical simulation.     

Quantum effects on the competition between antiferromagnetism and superconductivity in heavy-fermion systems

We study a Ginzburg-Landau theory of two coupled fields describing superconductivity and antiferromagnetism in a metal. A coupling between the two-components superconductor and the antiferromagnetic (AF) field is included in the classical action. The classical results are improved calculating the quantum corrections to one-loop order with the method of the effective potential near the AF phase, but in the paramagnetic side. We discuss the influence of these corrections, including the possibility of fluctuation induced first order transitions. A scaling approach is used to obtain the critical and shift exponents at a quantum bicritical point.     

Metamagnetism in Ce(Ga,Al)2

Effect of Al substitution on the magnetic properties of Ce(Ga_1−x Al _x )₂ (x=0, 0.1 and 0.5) system has been studied. The magnetic state of CeGa₂ is found to be FM with a T _C of 8 K, whereas the compounds with x=0.1 and 0.5 are AFM and possess T _N of about 9 K. These two compounds undergo metamagnetic transition and the critical fields are about 1.2 T and 0.5 T. respectively at 2 K. These variations are explained on the basis of helical spin structure in these compounds.     

Advances in the study of intermediate-valence and heavy-fermion effects in rare-earth and uranium systems

Summary Three intermetallic systems, Yb−Al, U−Au and Ce−Co, are studied as most typical examples of intermediate-valence, heavy-fermion and coexistence of both intermediate-valence and heavy-fermion effects, respectively. YbAl₂ and YbAl₃, the two phases stable at low temperatures in the equilibrium phase diagram of Yb−Al, are both intermediate-valence systems with Yb valence values of 2.33 and 2.79 at room temperature. The modulation of these values by variations of external parameters like temperature, pressure and chemical substitutions with proper atoms was studied by structural, magnetic and spectroscopical techniques. Two phases are given also by U with Au at low temperatures with compositions UAu₂ and U₁₄Au₅₁: both systems exhibit heavy-fermion properties although with different ordering mechanisms. U₁₄Au₅₁ is an antiferromagnetic heavy-fermion system with a Nèel temperature around 23 K, UAu₂ shows spin-fluctuating behaviour with a saturation in theT→0 K magnetic susceptibility. Effects of substitutions with thorium were investigated only in U₁₄Au₅₁: competition between strong antiferromagnetic coupling and Kondo-like behaviours is observed. The Ce-rich side of the Ce−Co phase diagram shows a single Ce₂₄Co₁₁ phase stable at room temperature interest in the present work but in CeCo₂, which was demonstrated to be intermediate valent by resonant photoemission and XAS experiments and to exibit low magnetic moment and superconducting transition. The coexistence of intermediate-valence and heavy-fermion behaviours observed in Ce₂₄Co₁₁ and of intermediate valence and superconductivity in CeCo₂ is shown by the analyses of XPS, XAS and magnetic experiments with suitable phenomenological models. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.