Electronic properties of the narrow-band material α-RuCl3

Abstract

X-ray angle-integrated and ultraviolet angle-resolved photoemission spectra of the low-spin compound ${\mathit{t}}_{2\mathit{g}}^5$ α-${\mathrm{RuCl}}_3$ show that Ru 4d and Cl 3p states contribute to the valence-band structure of this magnetic material. The energy distribution curves measured along the azimuthal directions Γ-${\mathit{M}}^{\prime }$-Γ and Γ-K-M using He I radiation indicate an uppermost nearly dispersionless structure of Ru 4d origin, and two dispersive features obtained from Cl 3p-derived bands. The photoemission results, together with the optical and magnetic properties described by ligand-field theory, support the view of localized 4d states forming a very narrow Ru 4d band in the vicinity of the Fermi energy. The main 4d emission structure has been thus assigned to 4${\mathit{d}}^4$ unscreened hole states, where the band gap corresponds to intersite d-d transitions, and α-${\mathrm{RuCl}}_3$ can be classified as a Mott-Hubbard compound in consideration of its electronic and magnetic characteristics. The inconsistency between the photoemission results and the transport properties, describing this material as a conventional band-gap semiconductor, is finally discussed. © 1996 The American Physical Society.