∼8 keV photoemission of the metal-insulator transition system VO ₂

Hard and soft x-ray photoelectron spectroscopies (called HAXPES and SXPES) were performed at ∼8 and ∼1.25keV for the metal-insulator transitions (MIT) system VO₂. From the sharp difference between HAXPES and SXPES in the O Is spectra, it was found that the clean surface of the metal VO ₂ is covered with a less itinerant surface layer. Clear changes of the spectral shapes were observed in HAXPES on MIT for the V Is, 2p, 3s, 3p and O 1s core levels. The enhanced intensity of these core levels in the low-binding-energy (E_B) region of the main peak in the metal phase is due to the non-local or long range screening effects. The V 3d valence band in the metal phase is composed of coherent and incoherent parts. The prominent peak near 0.9 eV in the insulator phase has an incoherent tail in the high-E _B region and the peak and its lower E_B components are understood to have a noticeable coherent component. It is experimentally confirmed that both electron correlation and band changes induced by lattice distortion are responsible for the spectral changes on MIT in VO₂.