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

Leuenberger D, Sobota JA, Yang S, Kemper A, Giraldo-Gallo P, Moore RG, Fisher IR, Kirchmann PS, Devereaux TP, Shen Z
Phys. Rev. B 91, 201106(R) (2015)
doi
10.1103/PhysRevB.91.201106

Abstract

We present time-and angle-resolved photoemission spectroscopy (trARPES) measurements on the charge density wave system CeTe3. 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, Omega(2) = 2.7, 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 density-functional theory frozen phonon calculations. We conclude that 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.