Nature of electron paramagnetic resonance in a heavy fermion Kondo lattice

A theoretical model of spin relaxation is proposed to explain the existence of electron paramagnetic resonance (EPR) signal in Kondo lattice systems with heavy fermions. We show that the collective spin motion of quasi-localized $f$-electrons and wide-band conduction electrons is the key ingredient for understanding this phenomenon. A simple scaling theory describes the evolution of Kondo coupling constants with temperature providing for a common energy scale that regulates a logarithmic divergence of different kinetic coefficients. Our model successfully explains the temperature and magnetic field dependence of EPR linewidth and resonance frequency as experimentally observed in $\mathrm{YbRh_2Si_2}$.