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.

Automated summary

This study reports the direct observation of the electronic structure of the phosphide superconductor ZrP2-xSex using high-resolution bulk-sensitive hard x-ray photoemission spectroscopy. It reveals that the valence charges of P atoms at different crystallographic sites are different and their occupation ratio varies with x. The findings also suggest that the electronic states at the Fermi level are mainly attributed to the Zr 4d band and replacing Se2- with P3- serves as hole doping.