Charge-density wave induced by combined electron-electron and electron-phonon interactions in1T−TiSe2: A variational Monte Carlo study

Abstract

To clarify the origin of a charge-density wave (CDW) phase in $1T\ensuremath{-}{\mathrm{TiSe}}_{2}$, we study the ground-state property of a half-filled two-band Hubbard model in a triangular lattice including electron-phonon interaction. By using the variational Monte Carlo method, the electronic and lattice degrees of freedom are both treated quantum mechanically on an equal footing beyond the mean-field approximation. We find that the cooperation between Coulomb interaction and electron-phonon interaction is essential to induce the CDW phase. We show that the ``pure'' exciton condensation without lattice distortion is difficult to realize under the poor nesting condition of the underlying Fermi surface. Furthermore, by systematically calculating the momentum-resolved hybridization between the two bands, we examine the character of electron-hole pairing from the viewpoint of BCS-BEC crossover within the CDW phase and find that the strong-coupling BEC-like pairing dominates. We therefore propose that the CDW phase observed in $1T\ensuremath{-}{\mathrm{TiSe}}_{2}$ originates from a BEC-like electron-hole pairing.