Study on the metal-insulator transition of Ti oxide single crystals caused from the band filling by means of photoemission and inverse-photoemission spectroscopies

点击翻译按钮获取中文摘要

Distorted perovskite-type Y_<1-x>Ca_xTiO_3 is a system which moves from the Mott insulator YTiO_3(x=0) to the band insulator CaTiO_3(x=1) across the metallic phase, by controlling the Ti 3d band filling with x. In particular, Y_<1-x>Ca_xTiO_3 with x=0.39 exhibits a sharp metal-insulator transition (MIT) around 150 K (high-temperature phase : insulator, low-temperature phase : metal).We have carried out for x=0.39 the Ti 3p-3d resonant photoemission and inverse-photoemission spectroscopies (PES abd IPES) and observed the occupied and unoccupied Ti 3d densities of states, that is, the remnant of the lower and upper Hubbard bands split by the electron correlation. The experimental results revealed that the U_<eff>-value, determined from their energy separation, increases with decreasing temperature. Since the Coulomb interaction energy of the bare Ti 3d electrons is considered to be almost intraatomic value, the U_<eff> reduction reflects that the Ti 3d bands become wider at low temperature.In order to analyze the IPES spectra of x=0.39, we have measured the O 1s soft x-ray absorption spectra in comparison with the IPES spectra. The subtle structure, possibly corresponding to the coherent part in the PES spectra, seems to appear in the lower energy side of the unoccupied Ti 3d structure. This component is considered to be closely related to MIT. We have to reconfirm the existence by the IPES with higher energy resolution.We have also PES for x=0.39 using hard x-ray (several keV), which is more bulk-sensitive. The lower Hubbard band shifts to the shallower binding-energy side compared with the PES spectra with several ten eV. This suggests that the correlation effect is strong near the surface. In addition, for the metallic phase, a new component has been found in the Ti 2p PES, probably being attributed to the well screened state as observed in the Mn 2p PES of La_<1-x>Sr_xMnO_3 in the metallic phase [1].[1]K.Horiba et al., Phys.Rev.Lett. 93,236401 (2004).