A study of the metal-nonmetal transition in lanthanum strontium vanadate by high-resolution electron spectroscopy

Lanthanum strontium vanadate, La 1-x Sr x VO 3 , has been studied by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and high-resolution electron energy-loss spectroscopy (HREELS) over a composition range 0<x<0.3 embracing the metal-nonmetal transition (MNMT) at x approximately=0.22. XPS confirmed that the surface stoichiometry of samples cleaned by prolonged annealing in ultra-high vacuum mirrored that of the bulk. Owing to Mott-Hubbard-splitting of the V:3d band, LaVO 3 is nonmetallic with a localised t 2g 2 configuration. However, the observed width of the V:3d band in UPS reflects both electronic (band-structure) effects and final-state phonon broadening. Strontium doping leads to broadening of both primary and secondary electron-bands in UPS, introduction of holes into the V:3d band, and to the appearance of a Fermi edge in photoemission above x=0.2. The observed value of the density of states at the Fermi energy is consistent with the value predicted from Mott's estimate of the minimum metallic conductivity in a disordered system. However, the MNMT does not appear to take place in an impurity band separated from the V:3d band. The low frequency of the surface plasmon loss in HREELS emphasises the importance of electron correlation even in the metallic regime.