Photoemission evidence for a Mott-Hubbard metal-insulator transition in VO2

The temperature (T) dependent metal-insulator transition (MIT) in VO2 is investigated using bulk sensitive hard x-ray (� 8 keV) valence band, core level, and V 2p-3d resonant photoemission spectroscopy (PES). The valence band and core level spectra are compared with full-multiplet cluster model calculations including a coherent screening channel. Across the MIT, V 3d spectral weight transfer from the coherent (d 1 C final) states at Fermi level to the incoherent (d 0 +d 1 L final) states, corresponding to the lower Hubbard band, lead to gap-formation. The spectral shape changes in V 1s and V 2p core levels as well as the valence band are nicely reproduced from a cluster model calculations, providing electronic structure parameters. Resonant-PES finds that the d 1 L states resonate across the V 2p-3d threshold in addition to the d 0 and d 1 C states. The results support a Mott-Hubbard transition picture for the first order MIT in VO2. PACS numbers: 79.60.-i, 71.30.+h VO2, a d 1 electron system, exhibits a sharp first-order metal-insulator transition (MIT) as a function of temperature (T), at TMI = 340 K. 1 The high-T metal phase has a rutile (R) structure, while the low-T insulating phase has a monoclinic (M1) structure with zig-zag type pairing of V atoms along the c-axis. 2 Magnetically, the metallic R phase shows enhanced susceptibility (�) with an effective mass m ∗ /m � 6, while the insulating M1 phase is non