Angle-resolved photoemission spectroscopy on mixed-valent Sm1−xYxS

We report the results of angle-resolved photoemission spectroscopy (ARPES) on the Sm1−xYxS system, which exhibits a black-to-golden phase transition induced by yttrium substitution and by an applied external pressure, in order to investigate the mechanism of this transition and the electronic structure of both phases. The obtained ARPES spectrum of pure SmS (x = 0) indicates a band insulator, in which the shallowest electronic state is the Sm2+ 4f state at a binding energy of about 0.4 eV. With increasing x, the top of the Sm2+ 4f band shifts to the lower binding energy side and touches the Fermi level for x = 0.33. Because of the discontinuous reduction of the lattice constant in the vicinity of the black-to-golden phase transition (x ∼ 0.17), the golden phase induced by yttrium substitution is concluded to be an intermediate valence state.