Amplified Circular Photogalvanic Effect in a Strongly Electron-Phonon-Coupled Topological Material

Berry curvature physics is responsible for the anomalous electromagnetic responses in solids. One such response is the circular photogalvanic effect (CPGE), typically observed in systems with pronounced Berry curvature─such as flat-band systems or topological semimetals featuring band crossings near the Fermi level, where Berry curvature exhibits sharp discontinuities. To maximize CPGE, one must develop the ability to tune their electronic band dispersion without introducing disorder, which is a challenging endeavor. Here, we demonstrate that it is possible to maximize the CPGE response in a topological material by a fundamentally different approach: controlling the proximity of a given system to a symmetry-breaking phase transition that induces a reconstruction of the electronic band structure. Through measurements of the longitudinal circular photogalvanic effect in the Weyl semimetal (TaSe₄)₂I, we show that the circular photogalvanic effect can be amplified by a dramatic factor of 2 by tuning the proximity of this compound to charge density wave order. The first-principles calculations we present here show that this enhancement arises from the development of the CDW order parameter and the divergence of the associated relaxation time near the critical temperature. Therefore, this work provides a paradigm for boosting CPGE responses in solids─not by engineering band structure alone but by exploiting critical fluctuations near phase transitions in topological materials.