Highly anisotropic transient optical response of charge density wave order in ZrTe$_3$

TL;DR

This study reveals highly anisotropic transient optical responses in the charge density wave state of ZrTe3, showing polarization-dependent differences in quasiparticle relaxation, amplitude mode oscillations, and their sensitivities to the CDW gap.

Contribution

It provides the first detailed polarization-resolved measurements of the anisotropic optical response in ZrTe3's CDW state, highlighting the directional dependence of amplitude mode dynamics.

Findings

Significant in-plane anisotropy in optical signals below T_CDW.

Different polarization directions show contrasting amplitude mode lifetimes.

Persistence of amplitude mode oscillations at high fluence for one polarization.

Abstract

Low dimensionality in CDW systems leads to anisotropic optical properties, in both equilibrium and non-equilibrium conditions. Here we perform polarized two-color pump probe measurements on a quasi-1D material ZrTe$_3$, in order to study the anisotropic transient optical response in the CDW state. Profound in-plane anisotropy is observed with respect to polarization of probe photons. Below $T_\mathrm{CDW}$ both the quasi-particle relaxation signal and amplitude mode (AM) oscillation signal are much larger with $\mathbf{E}_\mathrm{pr}$ nearly parallel to $a$ axis ($\mathbf{E}_\mathrm{pr} \parallel a$) than for $\mathbf{E}_\mathrm{pr}$ parallel to $b$ axis ($\mathbf{E}_\mathrm{pr} \parallel b$). This reveals that $\mathbf{E}_\mathrm{pr} \parallel a$ signal is much more sensitive to the variation of the CDW gap. Interestingly, the lifetime of the AM oscillations observed with $\mathbf{E}_\mathrm{pr} \parallel b$ is longer than $\mathbf{E}_\mathrm{pr} \parallel a$. Moreover, at high pump fluence where the electronic order melts and the AM oscillations vanish for $\mathbf{E}_\mathrm{pr} \parallel a$ , the AM oscillatory response still persists for $\mathbf{E}_\mathrm{pr} \parallel b$. We discuss possible origins that lead to such unusual discrepancy between the two polarizations.