Time Reversal Breaking Raman Optical Activity

In this work we demonstrate that Raman optical activity (ROA) can emerge in centrosymmetric materials when time reversal symmetry (𝒯) is broken in the electronic ground state. Finite orbital moments such as ⟨L_z⟩ or ⟨L_z³⟩ are sufficient to generate ROA, with this mechanism being symmetry allowed as a first order effect in spatially invariant point groups of the form C_3n ⊗ (i/σₕ). We develop a general symmetry-based pairing between Stokes and anti-Stokes processes governed by conserved antiunitary operations U = 𝒯 or U = 𝒯O, where O is a spatial symmetry of the magnetic point group. Two archetypes emerge: state preserving pairing, where Stokes and anti-Stokes remain in the same cross-circular geometry, and state conjugating pairing, where they appear in opposite cross-circular geometries. We apply this analysis to 1T-TaS₂ across a series of charge density wave phase transitions, demonstrating that phonon excitations can acquire a finite moment that reveals the underlying symmetries of the magnetic point group.