Electron spin resonance in Kondo systems

We calculate the dynamical spin response of Kondo impurity and Kondo lattice systems within a semiphenomenological Fermi-liquid description, at low temperatures $T<T_K$, the Kondo temperature, and low magnetic fields $B⪡k_B{T}_K/g{\mu }_B$. Fermi-liquid parameters are determined by comparison (i) with microscopic theory (numerical renormalization group) for the impurity model and (ii) with experiment for the lattice model. We find in the impurity case that the true impurity spin resonance has a width of the order of $T_K$ and disappears altogether if the $g$ factors of impurity spin and conduction-electron spin are equal. However, there is an impurity-induced resonance contribution at the conduction-electron resonance. The latter is broadened by spin-lattice relaxation and is usually unobservable. In contrast, for the Anderson lattice in the Kondo regime we find a sharp electron-spin resonance (ESR) line only slightly shifted from the local resonance and broadened by spin-lattice relaxation, the latter significantly reduced by both the effects of heavy-fermion physics and ferromagnetic fluctuations. We conjecture that our findings explain the sharp ESR lines recently observed in several heavy-fermion compounds.