Quasi one-dimensional chains and exciton–phonon interactions in TiSe2

In charge ordered materials such as the transition metal dichalcogenides, in which the lattice modes are strongly coupled to the charges, the critical fluctuations surrounding a quantum critical point will consist of slow charge modulations as well as soft phonons. The presence of both these types of fluctuations offers an excellent opportunity for novel phases and unconventional forms of supercondictivity to arise. One such material, TiSe2, has been found to superconduct upon intercalation of copper [Morosan et al., Nature Phys. 2, 544 (2006)], with a concomitant suppression of CDW order. The transition is driven by an imbalance of charge carriers, leading to conventional superconductivity. Very recently however, Kusmartseva et al. [Phys. Rev. Lett. 103, 236401 (2009)] also discovered superconductivity in the pure system under pressure. This finding cannot be explained solely by the an imbalance of charge carriers and hints at the possibility of exciton-mediated superconductivity made manifest in a clean experimental system. Here we analyze the electronic structure of TiSe2 using a tight binding model to fit its band structure. The resulting values found for the various overlap integrals in this material strongly suggest that TiSe2 is best viewed as a system of weakly linked one-dimensional chains. We argue that in these chains excitons and Jahn–Teller phonons will strongly interact, and that it is therefore likely that both the charge density wave transition and the superconductivity which arises upon suppression of the CDW phase are driven by excitonic as well as phononic modes.