Photoinduced chiral charge density wave in TiSe2

Abstract

$1T\text{\ensuremath{-}}{\mathrm{TiSe}}_{2}$ has been found to host a chiral charge density wave (CDW). Some studies suggest the microscopic origin of this phase is due to electron-phonon coupling while other studies suggest it is due to an excitonic insulator phase transition based on nonthermal melting of the charge density wave. First, we propose these interpretations can be reconciled if one analyzes the available experimental and theoretical data within a formal definition of what constitutes an excitonic insulator as initially proposed by Keldysh and Kopaev. Next, we present pump-probe measurements of circularly polarized optical transitions and first-principles calculations to highlight the role of elevated electronic temperatures on structural distortions to understand the nonthermal melting of the CDW phase. We also uncover a noncentrosymmetric CDW structure that explains the finite chirality of the optical transitions observed in the CDW phase of ${\mathrm{TiSe}}_{2}$.