Heavy fermions in the periodic Anderson model with singlet–triplet crystal-field levels

Abstract

We investigate the two-orbital periodic Anderson model with f 2 -configuration by using the dynamical mean-field theory in which the effective impurity Anderson model is solved using the exact diagonalization method. The renormalization factor Z and the local moment 〈 S 2 〉 are obtained as functions of the Coulomb interaction U, the exchange (Hund's rule) coupling J and the crystal-field splitting Δ . When the ionic ground state is triplet for Δ < Δ c = 3 J , the heavy fermion state with the mass enhancement m * / m = Z - 1 ≳ 100 together with 〈 S 2 〉 ∼ 2 is realized in the strong correlation regime for U ≳ W , where W is the conduction bandwidth. When the ionic ground state is singlet for Δ ⪢ Δ c , m * / m ∼ 1 and 〈 S 2 〉 ∼ 0 for any value of U. When the singlet ground state and the triplet excited state form a quasi-quartet for Δ ≳ Δ c , the system shows a moderate mass enhancement m * / m ∼ 10 – 100 .