Transport and Thermal Properties of Some Selected Heavy-Fermion Materials: Probing the Electronic Instability

Abstract

In the past decade it has been recognized that the deloacalization of 4f or 5f electrons in cerium or uranium intermetallics occasionally gives rise to the low-temperature formation of a strongly correlated electron band close to the Fermi level. The unprecedented strong renormalization is reflected in a Fermi-liquid quasiparticle mass of the order of 100 times the free electron mass. Concurrently, the Fermi-liquid temperature is strongly renormalized: T F ~ 100 K. The enormous effective mass is built up by a variety of competing electron-electron interactions, of which the on-site Kondo effect and the inter-site Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions are thought to play the leading parts. The stabilization of the heavy-fermion state is attended by striking anomalies in the thermal and transport properties. As to the thermal properties, spin degrees of freedom turn up predominantly in the low-temperature entropy and enhance the electronic specific heat accordingly, while the strains are tightly coupled to the heavy-electron bands via anomalously large Gruneisen parameters giving rise to large coefficients of thermal expansion. As far as the transport properties are concerned, a rapid variation of the scattering processes takes place at low temperatures, as a consequence of the competing on-site and inter-site interactions. This is manifested in the electrical resistivity as a crossover from the Kondo-esque increase to the Fermi-liquid-like decrease in the coherent state, at lowering the temperature. At studying the physics of correlated electrons in heavy-fermion materials, the influence of clean external parameters, e.g. pressure and magnetic field, has received a wide attention. The pressure effects on the heavy-fermion bands are unusually large because of the strong hybridization. However, the influence of a magnetic field is relatively small, so that very strong magnetic fields are needed in order to investigate the heavy-fermion state.