ESR in a gapped Anderson model

We applied the numerical renormalization group method to study the electron spin resonance (ESR) of a single-impurity Anderson model with a gap ΔΔ in the conduction electron density of states, centered at the Fermi level. We analyzed the relaxation rate 1/T11/T1 of a magnetic probe located at a position R→ around the Anderson impurity. It presents different behaviors for the symmetric and the asymmetric case. For the symmetric case and any Δ>0Δ>0, 1/T11/T1 goes to a constant for T≪ΓkT≪Γk (Kondo resonance). 1/T11/T1 decreases monotonically to zero only for Δ=0Δ=0. For the asymmetric case, there is a ΔΔ under which 1/T11/T1 decreases monotonically to zero as T→0T→0, and above which 1/T11/T1 saturates, as occurs in the symmetric case for Δ>0Δ>0. This behavior indicates a quantum phase transition from the quenched to the unquenched magnetic moment in the ground state of the Anderson ion.

► We calculated the longitudinal relaxation rate 1/T11/T1 of a gapped single impurity Anderson model (SIAM).
► There is a gap for which the asymmetric SIAM presents quantum phase transition from a singlet to a doublet ground state.
► For the symmetric SIAM the quantum phase transition occurs only at gap zero.
► The ratio 1/(T1T)1/(T1T) presents a pronounced peak inside the gap, in qualitative agreement with the experimental results.