  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.
