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

Abstract

Abstract We have studied metal-insulator transition (MIT) of Y 1− x Ca x TiO 3 ( x =0.37, 0.39 and 0.41) single crystals and a Kondo semiconductor Ce 3 Bi 4 Pt 3 single crystal by temperature-dependent high-resolution photoemission spectroscopy. At the phase boundary between insulating and metallic states of Y 1− x Ca x TiO 3 ( x ∼0.4), an unusual temperature-dependent spectral-weight redistribution has been observed. The x =0.39 compound showed a clear MIT at ∼150 K: a finite spectral weight at E F appears below ∼150 K. A narrow V-shaped pseudogap is formed in the conduction band of Ce 3 Bi 4 Pt 3 below a characteristic temperature T coh ∼100 K. The spectral weight 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 evidence for the crossover from an incoherent state above T coh to a coherent state below T coh . In these materials, electron correlation and intersite-hybridization interactions (or a bandwidth) are both important for the temperature-dependent MIT.