Influence of the Current on Electron Tunnelling into a Thin Superconducting Film

An anomaly has been found in the current-voltage characteristics of the electron tunnelling in a cross type Sn-SnOx-Pb junction with a small tunnelling resistance and a very thin tin film, which was proved to correspond to the superconducting-to-normal transition in the tin film. Some properties of the anomaly were also examined. It is pointed out that as the electron tunnelling method requires relatively large current through the junction with a small tunnel resistance in order to measure the energy gap directly, the superconducting state of a very thin film is disturbed by this tunnel current.