Exciton-phonon interactions and superconductivity bordering charge order in TiSe2

The strong coupling between lattice modes and charges which leads to the formation of charge density waves in materials such as the transition-metal dichalcogenides may also give rise to superconductivity in the same materials, mediated by the same exciton or phonon modes that dominate the charge-ordered state. Such a superconducting phase has recently been observed, for example, in TiSe2, both upon intercalation with copper and in the pristine material under pressure. Here we investigate the interplay of exciton formation and electron-phonon coupling within a simplified model description. We find that the combined exciton-phonon modes previously suggested to drive the charge density wave instability in TiSe2 are also responsible for the pairing of electrons in its superconducting regions. Based on these results, it is suggested that both of the observed domes form part of a single superconducting phase. We also study the effect of the quantum critical fluctuations emerging from the suppressed charge order on the transport properties directly above the superconducting region, and compare our finding with reported experimental results.