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A variety of experiments have been carried out to establish the origin of thechiral charge-density wave transition in 1T-TiSe$_2$, which in turn has led tocontradictory conclusions on the origin of this transition. Some studiessuggest the transition is a phonon-driven structural distortion while otherstudies suggest it is an excitonic insulator phase transition that isaccompanied by a lattice distortion. First, we propose these interpretationscan be reconciled if one analyzes the available experimental and theoreticaldata within a formal definition of what constitutes an excitonic insulator asinitially proposed by Keldysh and Kopaev. Next, we present pump-probemeasurements of circularly polarized optical transitions and first-principlescalculations where we highlight the importance of accounting for structuraldistortions to explain the finite chirality of optical transitions in the CDWphase. We show that at the elevated electronic temperature that occurs uponphotoexcitation, there is a non-centrosymmetric structure that isnear-degenerate in energy with the centrosymmetric charge density wavestructure, which explains the finite chirality of the optical transitionsobserved in the CDW phase of TiSe$_2$.

Chiral charge-density wave in TiSe$_2$ due to photo-induced structural distortions