Angle-resolved photoemission study of the mixed valence oxide (formula presented) Quasi-one-dimensional electronic structure and its change across the metal-insulator transition

We have performed angle-resolved photoemission spectroscopy of mixed valence oxide (formula presented) that shows a metal-insulator transition (MIT) at (formula presented) In the metallic phase, we observe two bands near the Fermi level (formula presented) One is a prominent band located around 0.8 eV and the other is a weak structure around 0.2 eV that shows dispersion toward (formula presented) only along the b axis. Furthermore, though the momentum distribution curve at (formula presented) shows a peak at (formula presented) indicative of a (formula presented) crossing, the intensity of the band near (formula presented) is strongly suppressed in the region of (formula presented) These observations indicate quasi-one-dimensional electronic states of (formula presented) consistent with a highly anisotropic behavior observed from resistivity and optical conductivity measurements. Across the MIT, the band near (formula presented) shifts to higher binding energy and becomes less dispersive, resulting in opening of an energy gap of 0.2 eV. We discuss some implications of the experimental results.