Optical spectroscopy and ultrafast pump-probe study of a quasi-one-dimensional charge density wave in CuTe

TL;DR

This study investigates the anisotropic charge density wave in CuTe using optical spectroscopy and ultrafast pump-probe techniques, revealing a temperature-dependent energy gap and collective excitations characteristic of CDW order.

Contribution

It provides the first detailed anisotropic optical and ultrafast spectroscopy analysis of CuTe's quasi-1D CDW, highlighting its unique anisotropic properties and collective modes.

Findings

Energy gap forms along the a-axis upon cooling

Optical evidence of the gap is absent along the b-axis

The 1.65-THz mode is identified as the CDW amplitude mode

Abstract

CuTe is a two-dimensional (2D) layered material, yet forming a quasi-one-dimensional (quasi-1D) charge-density-wave (CDW) along the a-axis in the ab-plane at high transition temperature $T_{CDW}=335$ K. However, the anisotropic properties of CuTe remain to be explored. Here we performed combined transport, polarized infrared reflectivity, and ultrafast pump-probe spectroscopy to investigate the underlying CDW physics of CuTe. Polarized optical measurement clearly revealed that an energy gap gradually forms along the a-axis upon cooling, while optical evidence of gap signature is absent along the b-axis, suggesting pronounced electronic anisotropy in this quasi-2D material. Time-resolved optical reflectivity measurement revealed that the amplitude and relaxation time of photo-excited quasiparticles change dramatically across the CDW phase transition. Taking fast Fourier transformation of the oscillation signals arising from collective excitations, we identify the 1.65-THz mode as the CDW amplitude mode, whose energy softens gradually at elevated temperatures. Consequently, we provide further evidence for the formation of completely anisotropic CDW order in CuTe, which is quite rare in quasi-2D materials.