Fluctuation-driven multi-step charge density wave melting in monolayer TiSe${}_2$

Abstract

The exact microscopic origin, symmetry, and thermal melting mechanism of the charge density wave (CDW) phase in TiSe${}_2$ remain intensely debated, particularly regarding chiral structural order and multi-step phase transitions. Using large-scale molecular dynamics driven by an accurate machine-learning interatomic potential, we resolve the finite-temperature structural dynamics of the monolayer TiSe${}_2$. We demonstrate that CDW melting deviates from conventional mean-field second-order behavior, undergoing a two-step process characterized by an extended fluctuation region ($200-250$ K) with proliferating domain walls, phase decoherence, component-selective melting, and an overdamped soft optical phonon. Furthermore, anisotropic long-wavelength thermal fluctuations spontaneously stabilize an asymmetric $3Q$ CDW order with $P1$ symmetry. These findings provide a unified microscopic framework for fluctuation-driven transitions in 2D quantum materials, demonstrating that the complex CDW physics of TiSe${}_2$ is captured entirely via non-perturbative electron-phonon and phonon-phonon interactions.

Revised version