Phenomenology of ESR in heavy fermion systems: Fermi liquid and non-Fermi liquid regime

We extend and apply a recent theory of the dynamical spin response of Anderson lattice systems to interpret ESR data on YbRh2Si2. Starting within a semiphenomenological Fermi liquid description at low temperatures T < Tx (a crossover temperature) and low magnetic fields B << Bx, we extend the description to the non-Fermi liquid regime by adopting a quasiparticle picture with effective mass and spin susceptibility varying logarithmically with energy/temperature, as observed in experiment. We find a sharp ESR resonance line slightly shifted from the local f-level resonance and broadened by quasiparticle scattering (taking unequal g-factors of conduction and f electrons) and by spin-lattice relaxation, both significantly reduced by the effect of ferromagnetic fluctuations. A detailed comparison of our theory with the data shows excellent agreement in the Fermi liquid regime. In the non-Fermi liquid regime we find a close relation of the T-dependence of the specific heat/spin susceptibility with the observed T-dependence of line shift and linewidth.