Laser Melting and Loss of Interlayer Coherence of the Charge Density Wave Order in TiSe${}_2$ observed by time-resolved neutron diffraction

Abstract

Time-resolved elastic neutron scattering combined with laser-induced heating was used to probe the evolution of the periodic lattice distortion (PLD) associated with charge-density-wave (CDW) order in 1T-TiSe${}_2$. Upon pulsed laser heating, the crystal undergoes a phase transition, where the transient response of the average lattice is isotropic. In contrast, the PLD exhibits an anisotropic behavior. The superlattice intensity is rapidly suppressed in-plane and out-of-plane upon heating, consistent with melting of the ordered state, but recovers more quickly in-plane than along the c-axis as the sample cools below the CDW transition temperature. The out-of-plane component shows pronounced broadening due to reduction of the interlayer correlation length while the in-plane correlations recover. The disparity between the nearly isotropic lattice response and the anisotropic recovery of the superlattice demonstrates that the CDW dynamics cannot be described solely by the evolution of the average lattice temperature. Instead, the results indicate that interlayer phase coherence represents the slowest-relaxing component of the ordered state. These findings support a picture in which the CDW instability is established primarily within the 1T-TiSe${}_2$ layers, while three-dimensional long-range order depends on weaker coupling between layers. More broadly, they demonstrate how direction-resolved time-resolved neutron diffraction can disentangle thermal and correlation-driven contributions to CDW dynamics in low-dimensional materials.

arXiv comment8 main text pages, 10 supplementary pages, 18 figures