Spin dynamics of $YbR{h}_{2}S{i}_2$ observed by Electron Spin Resonance

Below the Kondo temperature $T_{\mathrm{K}}$ electron spin resonance (ESR) usually is not observable from the Kondo-ion itself because the characteristic spin fluctuation energy results in a huge width of the ESR line. The heavy fermion metal YbRh${}_2$Si${}_2$ seems to be an exceptional case where definite ESR spectra show characteristic properties of the Kondo-ion Yb${}^{3+}$ well $\mathit{\text{below}}$ $T_{\mathrm{K}}$. We found that the spin dynamics of YbRh${}_2$Si${}_2$, as determined by its ESR relaxation, is spatially characterized by an anisotropy of the zero temperature residual relaxation only.