Nonlinear electron-phonon coupling drives light-induced symmetry switching in charge-density waves

TL;DR

This paper develops a first-principles theoretical framework to understand how nonlinear electron-phonon interactions induce light-driven symmetry changes and melting of charge-density waves, exemplified by monolayer TiSe2.

Contribution

It introduces a novel first-principles approach that explicitly includes quartic anharmonicities and nonlinear electron-phonon couplings to model ultrafast phase transitions in CDW materials.

Findings

Simulations match experimental observations of CDW melting in TiSe2.

Identifies nonlinear electron-phonon interactions as key to symmetry switching.

Captures transient structural dynamics and recovery timescales.

Abstract

Ultrafast optical excitation in charge-density wave (CDW) crystals can transiently suppress long-range order, driving the lattice toward higher symmetry on femtosecond timescales. Here, we formulate and implement a first-principles theory of light-induced melting of CDW order. The approach is based on the structural dynamics in the Heisenberg picture, and it explicitly accounts for quartic lattice anharmonicities, nonlinear electron-phonon interactions, and photoexcitation-induced modifications of the potential energy surface. We illustrate these concepts through first-principles calculations of the ultrafast melting of CDW order in monolayer TiSe$_2$ - a prototypical CDW crystal with a 2$\times$2 structural reconstruction. The simulations are in good agreement with existing experiments, and they capture the defining features of CDW melting, such as the damped coherent structural motion, the transient renormalization of the soft mode, and the restoration of CDW order over timescales of a few picoseconds. Besides identifying nonlinear electron-phonon interactions as the primary mechanism driving symmetry switching in CDW systems, our work establishes a generally applicable theoretical framework to treat quartic anharmonicities and light-induced phase transitions in first-principles ultrafast dynamics simulations.