Long-lived dynamics of the charge density wave in TiSe$_2$ observed by neutron scattering

Abstract

Time-resolved elastic neutron scattering combined with rapid laser heating was used to probe the charge density wave (CDW) state in 1T-TiSe$_2$, capturing both the melting and reformation of the CDW on long timescales and providing clues on the roles of phonons and excitons. With the laser source on, superlattice Bragg peaks such as (-1.5, -1.5, 1.5) observed below the CDW transition due to the new lattice periodicity, dissipate within 5 seconds, at a rate that is much slower than the sample's thermal response to the heat wave propagation. Whereas the electronic ordering associated with the CDW phase is disrupted rapidly by the laser-induced heating, the periodic lattice distortion (PLD) exhibits a markedly slower evolution during the melting process. This delayed suppression of the PLD relative to the thermal response indicates that CDW melting proceeds through a nonthermal pathway, likely linked to the loss of superlattice phonons such as the soft mode at q = (0.5 ,0, 0.5 ).