Electron spin resonance of YbRh2Si2 under pressure

We investigated the electron spin resonance (ESR) in the heavy-fermion metal YbRh2Si2 by applying hydrostatic pressure up to 3 GPa. We found that pressure increases the temperature dependence of the g factor and broadens the ESR line. These effects are similar to those observed in Yb(Rh1-xCox)2Si2 where Co substitution for Rh induces chemical pressure. However, the effect of chemical and external pressure on the ESR is not identical indicating the relevance of Co induced disorder on the spin dynamics. We compare our pressure ESR results with the behavior of the Gd ESR in CeAl3. This reveals a similar behavior pointing on one hand to a local character Yb3+-ESR, on the other hand on the properties of a heavy quasiparticle spin resonance upon changing the hybridization strength between 4f and conduction electrons. Both findings are consistent with the properties of a collective 4f-conduction electron spin mode which is supported by the Kondo effect.