Classification of Collective Modes in a Charge Density Wave by Momentum-Dependent Modulation of the Electronic Band Structure

TL;DR

This study uses time- and angle-resolved photoemission spectroscopy to identify and analyze collective modes in a charge density wave system, revealing their origins and how they modulate the electronic structure.

Contribution

It demonstrates how trARPES can distinguish different collective modes and determine their spatial polarization in a charge density wave material.

Findings

Modes $$ and $$ are from in-plane ionic lattice motions affecting charge order.

Mode $$ is an out-of-plane $A_{1g}$ phonon.

Charge order is selectively affected by specific phonon modes.

Abstract

We present time- and angle-resolved photoemission spectroscopy measurements on the charge density wave system CeTe$_{3}$. Optical excitation transiently populates the unoccupied band structure and reveals a gap size of 2$\Delta$ = 0.59 eV. The occupied Te-5p band dispersion is coherently modified by three modes at $\Omega_{1}$ = 2.2 THz, $\Omega_{2}$ = 2.7 THz and $\Omega_{3}$ = 3 THz. All three modes lead to small rigid energy shifts whereas $\Delta$ is only affected by $\Omega_{1}$ and $\Omega_{2}$. Their spatial polarization is analyzed by fits of a transient model dispersion and DFT frozen phonon calculations. We conclude that the modes $\Omega_{1}$ and $\Omega_{2}$ result from in-plane ionic lattice motions, which modulate the charge order, and that $\Omega_{3}$ originates from a generic out-of-plane $A_{1g}$ phonon. We thereby demonstrate how the rich information from trARPES allows identification of collective modes and their spatial polarization, which explains the mode-dependent coupling to charge order.