OPTICAL STUDY OF THE TEMPERATURE-INDUCED VALENCE TRANSITION IN YBCU4IN

In the context of investigations on mixed valence and Kondo systems the comPound YbCu4In is interesting due to a temperature-induced valence transition Occurring at T(v)=40 K. Without distorsion in the C15(6) Crystal structure, the Yb state in the system changes from a nearly 2+ state below T(v) to a magnetic (3+) state above T(v). We investigated the effects-of such a change on the optical reflectivity which has been measured over a large spectral range (I meV-6 eV) and at different temperatures above and below T(v), both on a polycrystalline sample of YbCu4In, and on a similar sample of the reference compound LuCu4In. A detailed analysis of the spectra in the far infrared is complicated by the presence both in Yb and Lu-based compounds of an optical structure at about 5 meV, which could be due to a phonon mode. Regardless, some differences in the spectra and changes in the plasma frequency have been observed, allowing for considerations on the connections between the valence of Yb and the density and effective mass of the carriers.