Why could Electron Spin Resonance be observed in a heavy fermion Kondo lattice?

TL;DR

This paper presents a theoretical framework explaining the observation of electron spin resonance in heavy fermion Kondo lattice systems, highlighting the collective spin dynamics and relaxation processes.

Contribution

It introduces a model that accounts for the ESR in Kondo lattices, showing how collective spin modes are stabilized despite divergent contributions.

Findings

Relaxation rate of collective spin mode is significantly reduced.

Motional narrowing explains ESR linewidth behavior.

Model successfully matches experimental ESR data for YbRh2Si2.

Abstract

We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancelation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model successfully explains the ESR data of YbRh2Si2 in terms of their dependence on temperature and magnetic field.