Lattice fluctuations, not excitonic correlations, mediated electronic localization in TiSe$_2$

TL;DR

This study shows that TiSe2's insulating behavior is driven by lattice symmetry-breaking due to thermal fluctuations, not excitonic effects, challenging the idea of it being an excitonic insulator.

Contribution

The paper demonstrates that lattice fluctuations, rather than excitonic correlations, are responsible for electronic localization in TiSe2, using advanced many-body theory and molecular dynamics simulations.

Findings

TiSe2 is an insulator in both high- and low-symmetry phases.

Thermal fluctuations induce dynamical symmetry breaking leading to insulating behavior.

Excitonic effects are minor and do not cause the insulating state.

Abstract

TiSe$_2$ is thought to be an insulator with a bandgap of ~0.1eV. It has attracted a much interest because, among of a rich array of unique properties, many have thought TiSe$_2$ is a rare realisation of an excitonic insulator. Below 200 K, TiSe$_2$ undergoes a transition from a high-symmetry ({P-3m1}) phase to a low-symmetry ({P-3c1}) phase. Here we establish that TiSe$_2$ is indeed an insulator in both {P-3m1} and {P-3c1} phases. However, the insulating state is driven not by excitonic effects but by symmetry-breaking of the {P-3m1} phase. In the CDW phase the symmetry breaking is static. At high temperature, thermally driven instantaneous deviations from {P-3m1} break the symmetry on the characteristic time scale of a phonon. Even while the time-averaged \emph{lattice} structure assumes {P-3m1} symmetry, the time-averaged \emph{energy band} structure is closer to the CDW phase -- a rare instance of a metal-insulator transition induced by dynamical symmetry breaking. We establish these conclusions from a high-fidelity, self-consistent form of many body perturbation theory, in combination with molecular dynamics simulations to capture the effects of thermal disorder. The many-body theory includes explicitly ladder diagrams in the polarizability, which incorporates excitonic effects in an \emph{ab initio} manner. The excitonic modification to the potential is slight, ruling out the possibility that TiSe$_2$ is an excitonic insulator. Charge self-consistency is essential distinguish the metallic from insulating state.