Renormalization group study of NMR in a gapped Anderson model

We study the temperature dependence of the impurity nuclear spin-lattice relaxation rate for the single-impurity spin degenerate Anderson model, with a gap Δ in the density of states of the conduction band, using the numerical renormalization group method. In the symmetric regime, where the particle-hole symmetry is conserved, 1/T1 presents Kondo behavior only if Δ=0, with 1/T1→0 as T→0, which is a characteristic of the impurity spin in a quenched state. In the asymmetric regime, where that symmetry is not conserved, 1/T1 presents different behavior for Δ⪯¡ΓK. In this case, even for Δ≠0, 1/T1 decreases monotonically to zero as T→0, presenting Kondo behavior, with the impurity spin in a quenched state. Increasing Δ, 1/T1 shows a minimum at T<ΓK (width of the Kondo resonance) and saturates as T→0. In this case there is no Kondo behavior, with the impurity spin in an unquenched state.