∼8 keV photoemission of the metal–insulator transition system VO2

Hard and soft x-ray photoelectron spectroscopies (called HAXPES and SXPES) were performed at ∼8 and ∼1.25 keV for the metal–insulator transitions (MIT) system VO 2 . From the sharp difference between HAXPES and SXPES in the O 1s 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 1s, 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 VO 2 .