Nonreciprocal superconducting critical currents with normal state field trainability in kagome superconductor CsV3Sb5

Determining time-reversal symmetry (TRS) and chirality in the superconducting state and its relation to normal-state symmetry and topology are important issues in condensed matter physics. Here, we report nonreciprocal superconducting critical currents (Ic) at zero magnetic field in kagome superconductor CsV3Sb5 nanodevices: Ic differs for opposite directions, indicating spontaneous TRS and inversion symmetry breakings. The polarity of Ic asymmetry changes randomly in repeated thermal cycling to 300 K, consistent with spontaneous TRS breaking. Crucially, on applying a perpendicular magnetic field above the charge density wave (CDW) transition temperature and then removing it to zero above the superconducting onset temperature (Tc), the polarity of Ic asymmetry follows the field direction, ascertaining that the CDW state has a macroscopic and trainable TRS-breaking directionality. The symmetry breaking continues into the superconducting state and generates the nonreciprocal critical currents. These results provide evidence for the loop-current CDW normal state with TRS breaking in CsV3Sb5. The authors report superconducting diode effect (SDE) at zero magnetic field in flakes and micro bridges of kagome CsV3Sb5, consistent with spontaneous time-reversal symmetry (TRS) breaking. They also demonstrate magnetic-field training, implying that the charge-density-wave state itself breaks TRS.