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Phys Rev Lett. 2009 Jul 10;103(2):026402. Epub 2009 Jul 10.

Excitonic insulator state in Ta2NiSe5 probed by photoemission spectroscopy.

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Department of Physics, University of Tokyo, Kashiwa, Chiba 277-8561, Japan.


We report on a photoemission study of Ta2NiSe5 that has a quasi-one-dimensional structure and an insulating ground state. Ni 2p core-level spectra show that the Ni 3d subshell is partially occupied and the Ni 3d states are heavily hybridized with the Se 4p states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni 3d-Se 4p hole and the Ta 5d electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.

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