Quantum tunneling of the interfaces between normal-metal and superconducting regions of a type-I Pb superconductor

Evidence of a nonthermal magnetic relaxation in the intermediate state of a type-I superconductor is presented. It is attributed to quantum tunneling of interfaces separating normal and superconducting regions. Tunneling barriers are estimated, and the temperature of the crossover from the thermal to the quantum regime is obtained from the Caldeira-Leggett theory. Comparison between theory and experiment points to tunneling of interface segments of a size comparable to the coherence length, by steps of the order of 1 nm.