Anomalous phonon selection rules and Raman optical activity in RuO₂

Breakdown of selection rules and emergence of optical activity in Raman spectroscopy typically signal symmetry-breaking events essential for understanding novel phases of solids. Here, we apply our recently developed rotational anisotropy Raman spectroscopy technique to single-crystal ruthenium dioxide-a compound of recent interest as a candidate workhorse material for altermagnetism yet whose magnetic ground state remains under intense debate. Our data reveal a salient breakdown of selection rules for zone-center phonons, accompanied by clear splitting in the angle-dependent polarimetry plots for the two opposite helicity-nonconserving circularly polarized scattering channels. These observations provide definitive evidence of a previously overlooked symmetry-breaking event, involving either spatial or time-reversal symmetry. In either scenario, the underlying mechanism is likely a bulk hidden order parameter, potentially involving multipole moments of Ruthenium(IV) set by the local crystal field environment. Our work demonstrates how subtle symmetry-breaking events can manifest through lattice dynamics and calls for renewed experimental and theoretical efforts to map the phase diagram of ruthenium dioxide as a system poised near quantum phase transition boundaries.