Fluctuation-driven multi-step charge density wave transition in monolayer TiSe$_2$

TL;DR

This study uses advanced simulations to reveal a two-step, fluctuation-driven charge density wave melting process in monolayer TiSe₂, highlighting the role of thermal fluctuations and topological defects in its phase transition.

Contribution

It provides a microscopic understanding of the complex CDW phase transition in TiSe₂, emphasizing fluctuation effects and spontaneous chiral order without relying on excitonic mechanisms.

Findings

CDW melting is a two-step process with an extended fluctuation regime.

Proliferation of topological defects occurs during the transition.

Anisotropic thermal fluctuations stabilize a chiral $3Q$ CDW order.

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.