Student Laboratory Demonstration of Flux Quantization and the Josephson Effect in Superconductors

(Received 11 March 1968) A simple apparatus is described which permits observation of magnetic flux quantization and the alternating current Josephson effect in superconductors. The apparatus, which has been used successfully by students in the Advanced Undergraduate Physics Laboratory at Berkeley, consits of point contacts between two niobium wires which are attached to a probe so that the contact pressure can be adjusted while the contacts are in a storage Dewar of liquid helium. The current-voltage characteristic of the point contact junction is displayed on an oscilloscope. Flux quantization causes the rem-voltage current carried by the junction to oscillate as a magnetic field is applied by moving a permanent magnet close to the storage vessel. The ac Josephson effect causes constant voltage steps to appear on the current-voltage : characteristic when a small amount of power from a K-band (~24-GHz) klystron is coupled to the points.