Reproduction of the charge density wave phase diagram in $1T$-$\mathrm{TiSe}_2$ exposes its excitonic character

TL;DR

This paper demonstrates that excitonic interactions are crucial for accurately modeling the charge density wave phase in 1T-TiSe2, with many-body calculations aligning well with experimental phase diagrams and highlighting the role of electron-hole coupling.

Contribution

It introduces a self-consistent many-body approach that captures the CDW phase diagram of 1T-TiSe2, emphasizing the importance of excitonic effects over density functional perturbation theory.

Findings

Many-body calculations match experimental phase diagram

DFT-based methods overestimate critical doping

Electron-hole interactions drive the CDW transition

Abstract

Recent experiments suggest that excitonic degrees of freedom play an important role in precipitating the charge density wave (CDW) transition in $1T$-$\mathrm{TiSe}_2$. Through systematic calculations of the electronic and phonon spectrum based on density functional perturbation theory, we show that the predicted critical doping of the CDW phase overshoots the experimental value by 1 order of magnitude. In contrast, an independent self-consistent many-body calculation of the excitonic order parameter and renormalized band structure is able to capture the experimental phase diagram in extremely good qualitative and quantitative agreement. This demonstrates that electron-electron interactions and the excitonic instability arising from direct electron-hole coupling are pivotal to accurately describe the nature of the CDW in this system. This has important implications to understand the emergence of superconductivity within the CDW phase of this and related systems.