Charge-Density-Wave Order in 2H-NbSe$_{2}$

TL;DR

This paper investigates the charge-density-wave order in 2H-NbSe₂, proposing an electron-electron interaction mechanism involving incoherent excitonic liquid condensation, challenging the traditional Fermi surface nesting explanation.

Contribution

It introduces a novel understanding of charge order in 2H-NbSe₂ based on excitonic liquid condensation rather than electron-phonon Fermi surface nesting.

Findings

Normal state is a poor, incoherent metal

Coherence increases in the broken symmetry state

Charge order explained by excitonic liquid condensation

Abstract

Competition between collective states like charge density wave and superconductivity is played out in some of the transition metal dichalcogenides unencumbered by the spin degrees of freedom. Although 2H-NbSe$_2$ has received much less attention than some of the other members of the family (like 1T-TiSe$_2$ and 2H-TaSe$_2$), it shows superconductivity at 7.2 K and incommensurate charge ordering at 33 K. Recent experiments, notably Angle Resolved Photoemission spectroscopy, have cast serious doubts on the mechanism of Fermi surface nesting via electron-phonon interaction. The normal state has been found to be a poor, incoherent metal and remarkably, the coherence increases in the broken symmetry state. From a preformed excitonic liquid scenario, we show that there exists a natural understanding of the experimental data on 2H-NbSe$_2$ based on electron-electron interaction. The collective instabilities, in this scenario, are viewed as a condensation of an incoherent excitonic liquid already present at high temperature.