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

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.

arXiv comment5 pages, 4 figures

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