Fermi Surface Nesting and the Origin of the Charge Density Wave in NbSe$_2$

TL;DR

This study uses density functional calculations to analyze the Fermi surface and susceptibility of NbSe2, concluding that Fermi surface nesting is not responsible for its charge density wave formation.

Contribution

It provides a detailed first-principles analysis showing weak Fermi surface nesting features, challenging previous theories about CDW origin in NbSe2.

Findings

Weak peaks in Re chi_0(q) near CDW wave vector

No peaks in Im chi_0(q), ruling out FS nesting

Broad and shallow Re chi_0(q) peaks unlikely to cause CDW

Abstract

We use highly accurate density functional calculations to study the band structure and Fermi surfaces of NbSe2. We calculate the real part of the non-interacting susceptibility, Re chi_0(q), which is the relevant quantity for a charge density wave (CDW) instability and the imaginary part, Im chi_0(q), which directly shows Fermi surface (FS) nesting. We show that there are very weak peaks in Re chi_0(q) near the CDW wave vector, but that no such peaks are visible in Im chi_0(q), definitively eliminating FS nesting as a factor in CDW formation. Because the peak in Re chi_0(q) is broad and shallow, it is unlikely to be the direct cause of the CDW instability. We briefly address the possibility that electron-electron interactions (local field effects) produce additional structure in the total (renormalized) susceptibility, and we discuss the role of electron-ion matrix elements.