Quasiparticle interactions due to charge and spin fluctuations in the lattice heavy-fermion states

Abstract

Abstract We investigate quasiparticle interactions, which cause superconducting pairing and antiferromagnetic instability in the heavy-fermion superconductors, using the simple-cubic-lattice SU (N) Coqblin-Schrieffer model. Perturbative treatments of fluctuations from the mean-field Kondo-lattice states lead to both quasiparticle exchange interaction due to the RKKY coupling of conduction spin fluctuations and the boson-mediated renormalized Kondo interaction due to the hybridized density fluctuations. These interactions are calculated at T = 0 as a function of quasiparticle occupation. The renormalized Kondo interaction is strongly repulsive close to the insulating state of the occupied lower band, and becomes less repulsive with a decrease in quasiparticle filling, showing characteristic q dependences, while the exchange interaction has a constant q dependence with an antiferromagnetic RKKY coupling for a nearly half-filled conduction-band situation. An antiferromagnetic SDW-like instability is shown to be possible after the incoherent Kondo states are realized below the Kondo temperature TK but before the coherent Kondo states are attained at T0. It is argued that the SDW instability of heavy-fermion systems may be that of the conduction-band Fermi surface induced in the incoherence temperature range and not that of the quasiparticle Fermi surface obtained in the coherent states. For a heavy-fermion system with a high quasiparticle filling the RKKY-type exchange contribution to the pairing interaction, which does not scale with TK, is more severely restricted due to the small quasiparticle Fermi surface, and the filling-dependent strong boson-mediated interaction becomes dominant.