Dual-Circular Raman Optical Activity of Axial Multipolar Order.

Multipolar order, such as octupolar order, is a key concept in condensed matter physics, particularly in light of elusive hidden orders. However, its experimental identification remains challenging due to the absence of direct coupling to conventional external stimuli. In this Letter, we propose that dual-circular Raman scattering serves as a probe of multipolar anisotropies. By combining symmetry analysis with microscopic calculations, we identify that both time-reversal-even (θ -even) and time-reversal-odd (θ -odd) axial multipolar phases exhibit the sizable Raman optical activity as a direct consequence of multipolar symmetry breaking. The quantitative significance of the proposed response is demonstrated by the first-principles study of pyrite, a prototypical axial octupolar material. Furthermore, we reveal that a multipolar phonon, a three-dimensional and alternating displacement resembling the chiral phonon, plays a vital role in the proposed optical phenomena. Our findings open a pathway for identifying multipolar orders in various materials through dual-circular Raman spectroscopy as a sensitive and versatile probe.