Anomalous Aharonov-Bohm effect in percolating superconducting films

We have studied the superconducting transition in thin films of Pb and Al as a function of temperature and magnetic field. Examination of the films in a transmission electron microscope shows a percolative structure with typical small loop sizes of a few thousand angstroms in Pb and about a thousand in Al. An ac current passing through the sample is found to cause an anomalous dc voltage across the sample in the vicinity of the superconducting transition. The measured voltage consists of a symmetric and an antisymmetric part as a function of an external magnetic field. The antisymmetric signal shows a sharp oscillation pattern identified with an Aharonov-Bohm-like flux quantization effect in submicron loops of the sample.