Direct observation of the electronic structure of the layered phosphide superconductor ZrP2-xSex

A layered phosphide superconductor, AP(2-x)X(x) (A = Zr and Hf; X = Se and S), is expected to be a foothold for the quest of new superconductors. Here, we report direct experimental observation of the electronic structure of ZrP2-xSex (x = 0.55, 0.60, 0.75, and 0.85), using high-resolution bulk-sensitive hard x-ray photoemission spectroscopy. Core-level spectra of P show two distinct components separated by an energy of 1.2 eV. These reveal that the P atoms at two crystallographic sites have different valence charges, -1 and -3, playing different chemical roles, and furthermore that the occupation ratio between them varies monotonically with x. We observed valence-band features at -6.5, -4.0, and -1.4 eV together with a clear Fermi edge. The consistency with first-principles calculation indicates that the electronic states at Fermi level are mainly ascribed to the Zr 4d band. Notably, a hump feature near the Fermi level shows an upward energy shift by 0.49 eV with decreasing x from 0.85 to 0.55. The energy shifts of spectral features substantiate that replacing Se2- with P3- serves as hole doping, shedding light on the tunability of ZrP2-xSex. These findings on the electronic structure establish an essential basis for understanding the superconductivity in the AP(2-x)X(x) system.