FFLO Superconductivity Mediated by Excitonic Fluctuation in Semimetallic Ta2NiSe5

We investigate the excitonic fluctuation and its mediated superconductivity in the quasi one-dimensional three-chain Hubbard model for Ta₂NiSe₅ known as a candidate material for the excitonic insulator. In the semiconducting case and the semimetallic case with a small band-overlapping where one conduction (c) band and one valence (f) band cross the Fermi level, the excitonic fluctuation with q = 0 is enhanced due to the c-f Coulomb interaction and diverges towards the uniform excitonic order corresponding to the excitonic insulator. On the other hands, in the semimetallic case with a large band-overlapping where two c bands and one f band cross the Fermi level, the non-uniform excitonic fluctuation with q ≠ 0 corresponding to the nesting vector between the c and f Fermi-surfaces (FSs) becomes dominant and results in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) excitonic order characterized by the condensation of excitons with finite center-of-mass momentum q. Near the instability, the largely enhanced excitonic fluctuations mediate the c-f interband Cooper pairs with finite center-of-mass momentum resulting in the FFLO superconductivity, which is expected to be realized in the semimetallic Ta₂NiSe₅ under high pressure.