Optical response of the Kondo lattices YbCu4Ag, YbCu4Au and YbCu4Pd

We investigated the three compounds YbCu4Ag, YbCu4Au and YbCu4Pd with reflectivity measurements from 2 meV to 6 eV at 300 and 77 K and with spectroscopic ellipsometry (1.4-5 eV). A heavy carrier plasma edge, scaled on the Kondo temperature, occurs at about 20 meV in the low temperature spectrum of YbCu4Ag, whereas no similar behaviour is observed for Au and Pd compound. At the lowest energies, crystal field excitations among f-levels are also detected in YbCu4Ag. This is allowed by the strong hybridization and points to a Kondo temperature T-K <= 90 K. The observed differences in the compounds are due to the different configuration of Yb f-electrons, mainly related to the volume differences of the unit cell.