Surface and bulk electronic structure of La_1-xCa_xVO₃

Abstract

We report on a photoemission study of the electronic structure of metallic La_1-xCa_xVO₃, with x=0.4 and 0.5, using monochromatized He I, He II, and Al Kα radiations. He II spectra obtained using monochromatic source exhibit large intensity for the coherent feature in contrast to the absence of this feature in the satellite subtracted spectra and establish that the minimum in the photoelectron escape depth appears at a kinetic energy ≤ 16 eV in these metallic systems. As a function of photon energy, strong modifications of the spectral function in the valence-band region were observed, reflecting a difference in the electronic structure at the surface and in the bulk. Detailed analyses of the observed spectra reveal that the studied materials are good metals in the bulk, however, the surface layers are insulating as also reported earlier based on the spectra collected with lower resolutions [ Maiti et al. Phys. Rev. Lett. 80 2885 (1998)]. The insulating nature of the surface electronic structure can be attributed to the different local symmetry and enhanced electron correlation at the surface as compared to the bulk. The analyses of the bulk spectra and the temperature-dependent studies at high resolution exhibit a distinct dip in the density of states at the Fermi level that suggests a strong influence of disorder on the electronic structure.