Temperature-dependent metal-insulator transition in d- and f-electron systems studied by high-resolution photoemission spectroscopy

We have studied metal-insulator transition (MIT) of Y1-xCaxTiO3 (x=0.37, 0. 39 and 0.41) single crystals and a Kondo semiconductor Ce3Bi4Pt3 single cry stal by temperature-dependent high-resolution photoemission spectroscopy. A t the phase boundary between insulating and metallic states of Y1-xCaxTiO3 (x similar to0.4), an unusual temperature-dependent spectral-weight redistr ibution has been observed. The x=0.39 compound showed a clear MIT at simila r to 150 K: a finite spectral weight at E-F appears below similar to 150 K. A narrow V-shaped pseudogap is formed in the conduction band of Ce,Bi,Pt, below a characteristic temperature T(coh)similar to 100 K. The spectral wei ght of the Ce 4f state at E-F increases above T-coh on cooling, but reduces significantly below T-coh forming a V-shaped pseudogap. It is a clear evid ence for the crossover from an incoherent state above T-coh to a coherent s tate below T-coh. In these materials, electron correlation and intersite-hy bridization interactions (or a bandwidth) are both important for the temper ature-dependent MIT. (C) 2001 Elsevier Science B.V. All rights reserved.