Spin dynamics of YbRh2Si2 observed by electron spin resonance

Below the Kondo temperature TK, 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 YbRh2Si2 seems to be an exceptional case in which definite ESR spectra show characteristic properties of the Kondo ion Yb3+ well below TK. We found that the spin dynamics of YbRh2Si2, as determined by its ESR relaxation, is spatially characterized by an anisotropy of the zero-temperature residual relaxation only.