Simultaneous formation of two-fold rotation symmetry with charge order in the kagome superconductor CsV3Sb5 by optical polarization rotation measurement

Abstract

We report the observation of twofold rotation symmetry in charge-density wave (CDW) state in the newly discovered kagome superconductor CsV3Sb5. Just below its CDW transition temperature (TCDW), the polarization rotation of the reflected laser beam promptly emerges and increases close to about 1 mrad, and the rotation angle shows twofold rotation symmetry. This observation is very different from many other experimental methods in which nematicity or C2 rotation symmetry appears at low temperatures far below TCDW. There are two components included, where the d-wave-like component originates from electronic nematic order and constant term points to the possible existence of time-reversal-symmetry breaking (TRSB), respectively. With femtosecond laser pulse pumping, the likely TRSB and nematicity can be easily suppressed, and the latter is further modulated by coherent phonon. Our investigation provides clear optical evidence for the formation of nematic order with twofold rotation symmetry and possible existence of TRSB just below TCDW and, moreover, reveals the controlling method with femtosecond laser and coherent phonon.