Observation of Spontaneous Thermal Hall Effect in a Kagome Superconductor CsV3Sb5

Broken time-reversal symmetry (TRS) in superconductors can induce not only spontaneous magnetization by the finite angular momentum of Cooper pairs, but also spontaneous chiral thermal edge current, whose detection has been extremely challenging. Here we report the successful observation of a spontaneous thermal Hall effect developing below the superconducting transition temperature at zero magnetic field in the kagome-lattice superconductor CsV₃Sb₅. This finding is verified by the absence of a signal in a conventional type-II superconductor using the same setup and by ruling out the trapped-vortex effects through micro-Hall array measurements. Remarkably, the observed spontaneous thermal Hall conductivity deviates strongly from theoretical predictions for chiral superconductors, implying a novel mechanism driving the spontaneous thermal flow in the superconducting state under an electronic order with broken TRS. Our study of the spontaneous thermal Hall effect offers a new approach to probe TRS breaking in the superconducting states.