Evidence of Time-Reversal and Rotational Symmetry Breaking in the Charge Density Wave States in Kagome Superconductors

The kagome lattice provides a fascinating playground to study geometrical frustration, topology and strong correlations. The newly discovered kagome metals AV₃Sb₅ (A = K, Rb, Cs) exhibit exotic phases including charge density waves (CDWs) and superconductivity, with debated properties such as time reversal symmetry breaking (TRSB) and six-fold rotational symmetry breaking in the CDW phase. In our study, we perform scanning birefringence and circular dichroism (CD) microscopy on CsV₃Sb₅. The emergence of opposite CD domains within the same birefringence domain, along with field-induced CD switching, indicate broken time-reversal symmetry. Furthermore, ultrafast pump-probe reflectivity measurements reveal a splitting of charge density wave induced phonon modes in all three birefringence domains. Such a breaking in degeneracy corroborates the six-fold rotation symmetry breaking in the charge ordered phase.