Ultrafast electron dynamics in the charge density wave material TbTe₃

Gaining insights into the mechanisms of how order and broken symmetry emerges from many-particle interactions is a major challenge in solid state physics. Most experimental techniques—such as angle-resolved photoemission spectroscopy (ARPES)—probe the single-particle excitation spectrum and extract information about the ordering mechanism and collective effects, often indirectly through theory. Time-resolved ARPES (tr-ARPES) makes collective dynamics of a system after optical excitation directly visible through their influence on the quasi-particle band structure. Using this technique, we present a systematic study of TbTe3, a metal that exhibits a charge-density wave (CDW) transition. We discuss time-resolved data taken at different positions in the Brillouin zone (BZ) and at different temperatures. The transient change in the band structure due to the excitation is qualitatively different between the region gapped by the CDW order vector and an ungapped but otherwise equivalent region. Also, we discovered two distinct collective modes at roughly 3.5 and 2.5 THz, the latter of which only occurs in the CDW band near the gapped region, demonstrating the strength of tr-ARPES in discerning the origin of the modes from the way in which they couple to the quasi-particle bands. In addition, a systematic pump fluence dependence in the gapped region documents the crossover from a weakly perturbed to a strongly perturbed regime, which can be related to a crossover from a regime where mainly the amplitude mode gets excited to a regime where the CDW gap closes at least partially