Charge Order in NbSe$_2$

TL;DR

This paper presents a detailed model of charge order in NbSe$_2$, incorporating momentum-dependent electron-phonon interactions to explain experimental observations and the effects of strain and fluctuations.

Contribution

It introduces a comprehensive model that accounts for momentum and orbital dependencies in electron-phonon coupling, explaining charge order phenomena in NbSe$_2$.

Findings

Small uniaxial strain stabilizes unidirectional order.

Lattice fluctuations suppress charge order onset temperature.

The model reproduces experimental charge order geometries.

Abstract

We develop in detail a model of the charge order in NbSe$_2$ deriving from a strong electron-phonon coupling dependent on the ingoing and outgoing electron momenta as well as the electronic orbitals scattered between. Including both dependencies allows us to reproduce the full range of available experimental observations on this material. The stability of both experimentally-observed charge-ordered geometries (1Q and 3Q) is studied within this model as a function of temperature and uniaxial strain. It is found that a small amount of bulk strain suffices to stabilize the unidirectional order, and that in both ordering geometries, lattice fluctuations arising from the strong electron-phonon coupling act to suppress the onset temperature of charge order, giving a pseudogap regime characterized by local order and strong phase fluctuations.