FFLO Excitonic State in the Three-Chain Hubbard Model for Ta_2NiSe_5

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Abstract

The three-chain Hubbard model for Ta_2NiSe_5, known as a candidate material for an excitonic insulator, is investigated over the wide range of the energy gap D between the twofold degenerate conduction bands and the nondegenerate valence band including both semiconducting (D > 0) and semimetallic (D < 0) cases. In the semimetallic case, the difference in the band degeneracy inevitably causes the imbalance of each Fermi wavenumber, resulting in a remarkable excitonic state characterized by the condensation of excitons with finite center-of-mass momentum q, the so-called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) excitonic state. With decreasing D corresponding to increasing pressure, the obtained excitonic phase diagram shows a crossover from BEC (D >__～ 0) to BCS (D <__～ 0) regime, and then shows a distinct phase transition at a certain critical value D_c(< 0) from the uniform (q = 0) to the FFLO (q ≠ 0) excitonic state, as expected to be observed in Ta_2NiSe_5 under high pressure.

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