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.
Categories: cond-mat.str-el