Observation of metal to nonmagnetic insulator transition in polycrystalline RuP by photoemission spectroscopy

We investigate the metal to nonmagnetic insulator (MI) transition of MnP-type Ru pnictide RuP using hard x-ray and ultraviolet photoemission spectroscopies. The spectral weight at ${E}_{\mathrm{F}}$ is suppressed below the MI transition temperature ${T}_{\mathrm{MI}}$, while there is no appreciable change across the pseudogap temperature ${T}_{\mathrm{PG}}$. The estimated energy scale of the gap is $\ensuremath{\sim}110$ meV, which is in good correspondence to the spin gap opening observed in the previous NMR study. According to the band structure calculation, the density of states at ${E}_{\mathrm{F}}$ mainly originates from narrow bands of the Ru $4{d}_{xy}$ orbitals. Our results suggest that the fourfold-degenerate Ru $4{d}_{xy}$ orbitals are deeply related to the MI transition. Based on the photoemission results and the band structure calculation, we argue the possible origin of the MI transition for polycrystalline RuP.