Anomalous phonon selection rules and Raman optical activity in RuO(2).

Breakdown of selection rules and emergence of optical activity in Raman spectra of single-crystal RuO2 indicate a previously overlooked symmetry-breaking event involving either spatial or time-reversal symmetry.

• Why it matters: Understanding symmetry-breaking in RuO2 is crucial because it relates to the material's potential altermagnetism and its position near quantum phase transition boundaries, which are key for exploring novel quantum phases.
• What they did: Using rotational anisotropy Raman spectroscopy, the study analyzed zone-center phonons in RuO2, revealing a breakdown of selection rules and splitting in polarimetry plots for opposite helicity-nonconserving channels, suggesting hidden order parameters.
• The result: These findings provide definitive evidence of symmetry-breaking involving multipole moments of Ruthenium(IV), highlighting the importance of lattice dynamics in phase transitions and prompting further experimental and theoretical investigations.