Extended Phonon Collapse and the Origin of the Charge-Density-Wave in NbSe$_2$

TL;DR

This study reveals that in NbSe2, an extended phonon collapse driven by wavevector-dependent electron-phonon coupling, rather than Fermi surface nesting, causes charge-density-wave formation, challenging traditional theories.

Contribution

It demonstrates that electron-phonon coupling wavevector dependence, not Fermi surface nesting, drives the CDW in NbSe2, supported by inelastic x-ray scattering and ab initio calculations.

Findings

Phonons soften to zero frequency over an extended region around the CDW wavevector.

Electron-phonon coupling wavevector dependence explains the CDW periodicity.

The mechanism differs from the traditional Fermi surface nesting explanation.

Abstract

We report inelastic x-ray scattering measurements of the temperature dependence of phonon dispersion in the prototypical charge-density-wave (CDW) compound NbSe2. Surprisingly, acoustic phonons soften to zero frequency and become overdamped over an extended region around the CDW wavevector. This extended phonon collapse is dramatically different from the sharp cusp in the phonon dispersion expected from Fermi surface nesting. Instead, our experiments combined with ab initio calculations, show that it is the wavevector dependence of the electron-phonon coupling that drives the CDW formation in NbSe$_2$ and determines its periodicity. This mechanism explains the so far enigmatic behavior of CDW in NbSe$_2$ and may provide a new approach to other strongly correlated systems where electron-phonon coupling is important.