Nonthermal dynamics of the CDW in 1T-TiSe₂ revealed by time-resolved neutron scattering

Time-resolved elastic neutron scattering with nanosecond laser heating was used to study the charge density wave (CDW) dynamics in 1T-TiSe₂. The CDW order parameter, set by excitonic correlations or a pseudo Jahn-Teller distortion leads to periodic lattice distortions (PLD). PLD was probed by monitoring the superlattice Bragg peak at (-1.5, -1.5, 1.5) during pulsed 515 nm laser excitation. After the laser is turned on, equilibrium temperature state of the sample occurred in around 100 milliseconds, but the Bragg peak intensity dropped within 5 seconds indicating PLD is disappearing in much slower rate. This delay relative to the thermal response indicates a nonthermal melting pathway, likely associated with phonon bottlenecks and anharmonic scattering. Upon cooling, PLD recovered slowly over 30 - 40 s and followed processes like nucleation and growth consistent with Avrami’s law. It appears that, excited coherent phonons can destabilize the PLD. However, the material’s strong EPC will renormalize the phonon dispersion and suppress the phonon - mediated relaxation, likely causing the observed slow dynamics. These findings highlight the presence of distinct timescales in phase transition dynamics and open new avenues of research with potential relevance for future applications.