MOKE and Unconventional Optical Rotation in the charge ordered state of Kagome metal CsV₃Sb₅

Kagome metals AV₃Sb₅ (A = K, Cs, Rb) serve as a fertile ground for investigating intertwined orders, where evidence for time-reversal symmetry breaking (TRSB), likely due to the long-sought loop currents, has emerged in STM and muon spin relaxation experiments. Several optical measurements on CsV₃Sb₅ have yielded contrasting results regarding observed optical rotations coinciding with the CDW onset, and whether or not they originate from TRSB. We perform comprehensive optical studies of possible TRSB in high quality CsV₃Sb₅ crystals obtained from multiple sources, utilizing both high-resolution polar Kerr and polarization rotation experiments at the same wavelength, to distinguish a true TRSB Kerr effect from optical activity. Sagnac interferometry measurements confirm the absence of spontaneous Kerr rotation in zero field [1,3], and a sharp drop in Kerr rotation that is proportional to an applied magnetic field, attributed to Pauli paramagnetism and a drop in the density of states below the CDW temperature[1,3]. However, polarization rotation measurements reveal large anisotropic and isotropic components of optical rotation that are unaffected by magnetic fields, can vary in sign and magnitude, and cannot be explained by TRSB. Our results point to the elusive specular optical rotation-rather [2] than TRSB-as the cause of the large optical rotation observed in CsV₃Sb₅, suggesting a new intertwined order.



*Work at UC Irvine was supported by the Gordon and Betty Moore Foundation through Emergent Phenomena in Quantum Systems (EPiQS) Initiative Grant GBMF10276. Work at Stanford University was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering, under Contract DE-AC02-76SF00515. S.D.W. and B.R.O. acknowledge support via the UC Santa Barbara NSF Quantum Foundry funded via the Q-AMASE-i program under award DMR-1906325 (S.D.W.).