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

摘要

Charge-density-wave (CDW) order has long been interpreted as arising from aFermi-surface instability in the parent metallic phase. While phonon-electroncoupling has been suggested to influence the formation of CDW order inquasi-two-dimensional (quasi-2D) systems, the presumed dominant importance ofFermi-surface nesting remains largely unquestioned in quasi-1D systems. Here weshow that phonon-electron coupling is also important for the CDW formation in amodel quasi-1D system ZrTe$_3$. Our joint experimental and computational studyreveals that particular lattice vibrational patterns possess exceedingly strongcoupling to the conduction electrons, and are directly linked to the latticedistortions associated with the CDW order. The dependence of the couplingmatrix elements on electron momentum further dictates the opening of (partial)electronic gaps in the CDW phase. Since lattice distortions and electronic gapsare the defining signatures of CDW order, our result demonstrates that theconventional wisdom based on Fermi-surface geometry needs to be substantiallysupplemented by phonon-electron coupling even in the simplest quasi-1D case. Asprerequisites for the CDW formation, the highly anisotropic electronicstructure and strong phonon-electron coupling in ZrTe$_3$ give rise to adistinct Raman scattering effect, namely, measured phonon linewidths depend onthe direction of momentum transfer in the scattering process.