Charge density waves and phonon-electron coupling inZrTe3

Charge-density-wave (CDW) order has long been interpreted as arising from a Fermi-surface instability in an initiating metallic phase. While phonon-electron coupling has been recently 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. A key step toward this quest requires a close-knit synthesis of spectroscopic evidence and microscopic knowledge about the electronic structure and the lattice dynamics in a prototypical system. Here we take this approach to show that phonon-electron coupling is also important for the CDW formation in a model quasi-1D system ${\mathrm{ZrTe}}_3$, with joint experimental and computational investigation. It is revealed that singularly strong coupling between particular lattice-distortion patterns and conduction electrons gives rise to anomalously broad Raman phonon peaks, which exhibit a distinct anisotropy in both the measured and the computed linewidths. The dependence of the coupling strength on electron momentum further dictates the opening of (partial) electronic gaps in the CDW phase. Since lattice distortion and electronic gaps are defining signatures of CDW order, our results demonstrate that while Fermi-surface nesting determines the CDW periodicity in this quasi-1D system, the conventional wisdom needs to be substantially supplemented by phonon-electron coupling for a quantitative understanding of the CDW order.