Kondo Effect Due to Impurities with (5f)^n Configuration: Fermi Liquid Versus Non-Fermi Liquid

Abstract

The Kondo effect due to U-type impurities is studied in connectionwith the stability of the Fermi liquid and the non-Fermiliquid. First, an effective exchange Hamiltonian in terms ofirreducible tensor operators is derived for conduction electrons, which are coupled with plural 5f electrons. It is based on theassumption of strong spin-orbit and Hund couplings. Second, a modelfor f² configuration placed in a tetragonal or hexagonal crystalfield is studied by simplifying the internal structure as thenon-Kramers doublet ground state and the first singlet excitedstate. The numerical renormalization group analysis shows that thesinglet excited state plays an important role to stabilize the Fermiliquid over the non-Fermi liquid. Third, the difference of the Kondoeffect is examined between non-Kramers doublet for f² and Kramersdoublet for f³ in the presence of the cubic crystal field. It isshown that for f³ the Hund coupling reduces the channel number toone, while the situation is different for f².