Evolution of the Kondo State of YbRh2Si2 Probed by High-Field ESR

An electron spin resonance (ESR) study of the heavy fermion compound YbRh2Si2 for fields up to similar to 8 T reveals a strongly anisotropic signal in the Kondo state below similar to 25 K. A similarity between the T dependence of the ESR parameters and that of the specific heat and the Si-29 nuclear magnetic resonance data gives evidence that the ESR response is given by heavy fermions. Tuning the Kondo effect on the 4f states with magnetic fields similar to 2-8 T and temperature 2-25 K yields a gradual change of the ESR g factor and linewidth which reflects the evolution of the Kondo state in this Kondo lattice system.