Charge order from structured coupling in VSe$_2$

TL;DR

This paper reveals that structured, momentum-dependent electron-phonon coupling explains charge order phenomena in VSe₂, resolving discrepancies between traditional theories and experimental observations, and highlighting the importance of coupling structure in charge-ordered materials.

Contribution

It introduces the concept that structured electron-phonon coupling is essential for accurately describing charge order in VSe₂, challenging the conventional purely electronic instability perspective.

Findings

Thermal evolution of CDW vectors is explained by structured coupling.

The elusive CDW gap appears as a spectral weight suppression above the Fermi level.

Incorporating structured coupling improves agreement with experimental data.

Abstract

Charge order--ubiquitous among correlated materials--is customarily described purely as an instability of the electronic structure. However, the resulting theoretical predictions often do not match high-resolution experimental data. A pertinent case is $1T$-VSe$_2$, whose single-band Fermi surface and weak-coupling nature make it qualitatively similar to the Peierls model underlying the traditional approach. Despite this, its Fermi surface is poorly nested, the thermal evolution of its charge density wave (CDW) ordering vectors displays an unexpected jump, and the CDW gap itself evades detection in direct probes of the electronic structure. We demonstrate that the thermal variation of the CDW vectors is naturally reproduced by the electronic susceptibility when incorporating a structured, momentum-dependent electron-phonon coupling, while the evasive CDW gap presents itself as a localized suppression of spectral weight centered above the Fermi level. Our results showcase the general utility of incorporating a structured coupling in the description of charge ordered materials, including those that appear unconventional.