Heavy fermions in Kondo lattice compounds (invited)

Abstract

Even in the absence of both magnetic order and superconductivity, Ce‐based Kondo lattice systems show distinct anomalies in the low‐temperature specific heat and thermopower. These features can be understood, in the frame of a microscopic treatment of the asymmetric Anderson model, by taking into account the periodicity of the Ce sites. A substantial variation in the observed phenomena between CeAl₃ and normal (n) state CeCu₂Si₂ on the one hand as well as CeRu₂Si₂ and CeCu₆ on the other highlight the importance of realistic band structure calculations for the quasiparticles (‘‘heavy fermions’’) in these different compounds. The temperature dependence of the upper critical magnetic field B_c2(T) for the heavy‐fermion superconductor CeCu₂Si₂ (T_c≂0.6 K) shows a flat maximum at ∼0.2 T_c, which is ascribed to coherence‐derived structure in the n‐state quasiparticle density of states. A dc Josephson effect with a critical pair current of ordinary size is observed for the first time on a weak link between CeCu₂Si₂ and Al. This proves that CeCu₂Si₂ is a superconductor with dominant spin‐singlet pairing. No Josephson effect can be found, however, between heavy‐fermion superconducting UPt₃ and Al, Nb, or UPt₃ as counterelectrodes, in accord with a possible anisotropic (L≥1, S=0 or 1) pairing in UPt₃. For this material, the thermal conductivity in the superconducting state approaches an asymptotic κ_S=αT² law as T→0, with α=32 mW/cm K³, and the thermopower above T_C shows a temperature dependence similar to that of the ‘‘spin fluctuation’’ system UAl₂.