  We investigate the electron-spin resonance (ESR) phenomenon in the
heavy-fermion metal YbRh2Si2 by applying hydrostatic pressure up to 3 GPa and
by inducing internal pressure on the Yb site chemically in Yb(Rh1-xCox)2Si2
samples. We found that the increase in pressure, reducing the hybridization
between 4f and conduction electrons, leads to a remarkable change in the
temperature dependence of the g factor and broadens the ESR line. We relate the
differences between the effect of internal and external pressure on the low
temperature ESR parameters to the disorder induced by Co doping. The pressure
effects on the Yb3+ related resonance in YbRh2Si2 again manifest its local
character when compared with the resonance of diluted Gd in heavy-fermion
metals.

Strongly Correlated Electrons