Abrupt change in hybridization gap at the valence transition of YbInCu_{4}

Abstract

The valence transition of 4f electron states in rare-earth compounds is accompanied by a discontinuous change in the Kondo temperature, an energy scale tied to the interaction of the 4f states with the conduction-band states (c-f hybridization). The c-f hybridization causes the formation of heavy quasiparticle bands and produces a hybridization gap. Whether the hybridization gap develops at the valence-transition temperature T_{V} is one of the key questions in the 4f-electron systems. In our angle-resolved photoemission spectroscopy performed using synchrotron radiation, we observed such a hybridization gap of the valence-transition compound YbInCu_{4}. We found that the gap increases below T_{V} and has a momentum dependence. Notably, the gap enhancement occurs along with a substantial energy shift of the conduction bands. We suggest that the c-f hybridization and the enhancement of the density of states at E_{F} should be incorporated in the scenarios of the valence transition in YbInCu_{4}.