ESR study of the Kondo effect inAu171Yb andAu174Yb

Recently we have reported on Kondo anomalies in the $g$-value shift and the relaxation rate of ESR in $\mathrm{Au}\mathrm{Yb}$. This work was extended (1) to a low Yb concentration of 70 ppm to avoid interaction effects and (2) to 1-GHz experiments, in addition to those at 3 and 9 GHz, to study frequency and magnetic field effects. Experiments were performed down to 80 mK ($\hslash \omega \gtrsim \mathrm{kT}$). We present a theoretical analysis of the longitudinal and transverse dynamic susceptibilities, taking into consideration the finite field and frequency. This analysis is based on the work of Götze and Wölfle, and agrees well with the experiments and shows in particular that in the $g$-value shift the frequency and field dependence is less important than for the relaxation. The only fit parameter for all relaxation data (different isotopes, concentrations, frequencies, and fields) is the Kondo temperature $T_K$. The theoretical fit explains the relaxation data quite well; however, the fit of the $g$-value shift yields a different $T_K$.