Resonant chiral dressing by amplitude fluctuations in a ferroaxial electronic crystal

Symmetry provides a unifying framework for understanding the ordering and dynamics of matter, and imposes constraints on the interactions between distinct collective excitations. In crystalline solids, these constraints are expressed through selection rules that prohibit harmonic coupling between zone-centre modes that transform under inequivalent irreducible representations. Although anharmonic processes can mediate coupling across distinct symmetry sectors, direct evidence for such mechanisms under thermodynamic equilibrium has not yet been shown. Here we demonstrate a dynamical interaction between modes of different symmetry in a ferroaxial charge density wave, a complex ordered state in which electronic modulations intertwine with rotational lattice distortions to break multiple mirror symmetries. Our helicity-resolved Raman micro-spectroscopy can access individual ferroaxial domains and reveals a temperature-dependent interplay between phonons and charge order amplitude fluctuations with a distinct symmetry character. We refer to this phenomenon as resonant chiral dressing. We propose a microscopic model that attributes this phenomenon to the dynamical dressing of phonons with collective charge order fluctuations, a process that involves the planar chirality of the underlying electronic state. These results establish order parameter dynamics as a route to mediate otherwise symmetry-forbidden interactions.