We extend and apply a recent theory of the dynamical spin response of Anderson lattice systems to interpret electron-spin resonance (ESR) data on ${\text{YbRh}}_2{\text{Si}}_2$. Starting within a semiphenomenological Fermi-liquid description at low temperatures $T<T_x$ (a crossover temperature) and low magnetic fields $B⪡B_x$, 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.

• Received 18 September 2009

DOI:https://doi.org/10.1103/PhysRevB.80.235112