Switchable chiral pair density wave in pure CsV₃Sb₅

We investigate electron pairing in a super clean kagome superconductor CsV₃Sb₅ with a residual resistivity ratio (RRR) of 290. Using the dilution-refrigerator-based scanning tunneling microscopy (STM) at the Synergetic Extreme Condition User Facility (SECUF), we find that the pairing gap exhibits chiral 2 × 2 modulations, and their chirality can be controlled by magnetic field training. We introduce nonmagnetic impurities to observe the complete suppression of 2 × 2 pairing modulations in presence of persistent 2 × 2 charge order. This nonmagnetic pair-breaking effect provides phase-sensitive evidence for pair-density-wave (PDW)-induced pairing modulations. Our results support switchable chiral 2 × 2 PDW in this super clean kagome superconductor.