Fluctuation-driven multi-step charge density wave transition 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 a subject of intense debate, particularly regarding the presence of chiral structural order and a multi-step phase transition. Here, we resolve the finite-temperature structural dynamics of the monolayer TiSe$_{2}$ using large-scale molecular dynamics simulations driven by an accurate, first-principles-trained machine-learning interatomic potential. We demonstrate that the CDW melting deviates from a conventional second-order phase transition, while it undergoes a two-step melting process characterised by an extended fluctuation regime between $T^{\ast}\approx200$ K and $T_{\mathrm{CDW}}\approx250$ K, with proliferation of topological defects and domain walls, and accompanied by a completely overdamped soft optical phonon. Furthermore, we reveal that anisotropic long-wavelength thermal fluctuations spontaneously stabilise an asymmetric $3Q$ chiral CDW order with $C2$ symmetry. These findings provide a unified microscopic framework for understanding complex fluctuation-driven phase transitions in 2D quantum materials, demonstrating that the intricate CDW physics of TiSe$_{2}$ can be largely captured without invoking excitonic correlations.