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-F is suppressed below the MI transition temperature T-MI, while there is no appreciable change across the pseudogap temperature T-PG. The estimated energy scale of the gap is similar to 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-F mainly originates from narrow bands of the Ru 4d(xy), orbitals. Our results suggest that the fourfold-degenerate Ru 4d(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.