Evolution of optical response in the series YbCu5-xAux

Optical measurements from the far infrared to ultraviolet and at low temperature are used to study the evolution of Kondo lattice behaviors within the series YbCu5-xAux. Heavy fermion (Kondo lattice) features have been observed in the low energy part of the spectrum of YbCu5, resembling the data of the isostructural YbCu4Ag, but for a shift towards the lowest frequencies of the free carrier behavior (consistent with a lower TK value). By moving to samples with an increasing amount of Au atoms substituting the Cu atoms, three effects can be observed: the closing of the hybridization gap signed by the red shift of a typical structure in the optical conductivity; the increasing amount of broadening effects, possibly related to stoichiometric disorder; the disappearing of a vibrational structure at about 3.5 meV, which could be due or to disordering effects, or to a change in the Yb–Cu (Yb–Au) bonds.