Thermal propagation of a normal region in a thin superconducting film and its application to a new type of bistable element

The movement of the interphase boundary due to Joule heating in a partially superconducting film carrying a current is analysed. It is shown that there is a value of the current at which the boundary remains stationary; above this value the normal region grows, and below it, the normal region collapses. As the current approaches the critical current of the film, the speed of propagation becomes very large and of the right order to explain the authors' previous observations on the rate of return of resistance to thin superconducting strips driven into the normal state by rectangular current pulses. The current required to maintain the boundary stationary is much less than that necessary to generate a normal region initially. This enables a new type of bistable element to be constructed, consisting of a strip of superconducting film carrying a continuous current equal to that necessary to maintain the boundary stationary. A short pulse of current in the same sense as the standing current generates a normal region in the film which is subsequently held constant by the standing current, while a short pulse in the opposite sense cancels the standing current long enough for the film to become completely superconducting again. This device is extremely simple in construction and can be switched from one state to the other in 10 mμs.