Charge density waves and phonon-electron coupling in ZrTe$_3$ investigated by Raman spectroscopy and first-principles calculations

TL;DR

This study combines Raman spectroscopy and first-principles calculations to demonstrate that phonon-electron coupling plays a crucial role in charge density wave formation in ZrTe$_3$, a quasi-1D system, challenging the traditional Fermi-surface nesting perspective.

Contribution

It provides evidence that phonon-electron coupling is significant for CDW formation in quasi-1D materials, supported by experimental and computational analysis of ZrTe$_3$.

Findings

Strong phonon-electron coupling linked to lattice distortions in ZrTe$_3$

Dependence of coupling on electron momentum influences electronic gaps

Raman scattering linewidths vary with momentum transfer direction

Abstract

Charge-density-wave (CDW) order has long been interpreted as arising from a Fermi-surface instability in the parent metallic phase. While phonon-electron coupling has been suggested to influence the formation of CDW order in quasi-two-dimensional (quasi-2D) systems, the presumed dominant importance of Fermi-surface nesting remains largely unquestioned in quasi-1D systems. Here we show that phonon-electron coupling is also important for the CDW formation in a model quasi-1D system ZrTe$_3$. Our joint experimental and computational study reveals that particular lattice vibrational patterns possess exceedingly strong coupling to the conduction electrons, and are directly linked to the lattice distortions associated with the CDW order. The dependence of the coupling matrix elements on electron momentum further dictates the opening of (partial) electronic gaps in the CDW phase. Since lattice distortions and electronic gaps are the defining signatures of CDW order, our result demonstrates that the conventional wisdom based on Fermi-surface geometry needs to be substantially supplemented by phonon-electron coupling even in the simplest quasi-1D case. As prerequisites for the CDW formation, the highly anisotropic electronic structure and strong phonon-electron coupling in ZrTe$_3$ give rise to a distinct Raman scattering effect, namely, measured phonon linewidths depend on the direction of momentum transfer in the scattering process.