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 EF is suppressed below the MI transition temperature TMI, while there is no appreciable change across the pseudogap temperature TPG. The estimated energy scale of the gap is ∼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 EF mainly originates from narrow bands of the Ru 4dxy orbitals. Our results suggest that the fourfold-degenerate Ru 4dxy 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.
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