Spin dynamics of $Yb Rh_2 Si_2$ observed by Electron Spin Resonance
  Below the Kondo temperature $T_{\rm K}$ 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 \textit{below} $T_{\rm K}$. 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. 
J. Sichelschmidt, J. Wykhoff, H.-A. Krug von Nidda, J. Ferstl, C.   Geibel, and F. Steglich