An effect that occurs when a sufficiently thin layer of insulating material is introduced into a superconducting material (see superconductivity). A superconducting current can flow across the junction, known as a Josephson junction, in the absence of an applied voltage. This is the direct-current Josephson effect. If the value of the current exceeds a critical value, I_c, determined by the properties of the insulating barrier, current can only flow when a finite voltage is applied. The current-voltage characteristic is shown in the diagram, in which the dashed curve is the current-voltage characteristic in the nonsuperconducting state.

The alternating-current Josephson effect occurs when a small direct voltage, V, is applied across a Josephson junction. The superconducting current across the junction becomes an alternating current given by

where ω = 2πf = 2e/hV: h is the Planck constant, f the frequency, and e the electron charge.

The direct-current Josephson effect is utilized in several devices, particularly the Josephson memory. The alternating-current Josephson effect is utilized for radiofrequency detection, the determination of h/e, for accurate measurement of frequency, and as a monitor of voltage changes in standard cells or for the comparison of cells at different Standards laboratories.

I-V characteristic of Josephson junction

https://www.youtube.com/watch?v=M6mCsQSohac A short video explaining the Josephson effect

https://www.scientificamerican.com/article/what-are-josephson-juncti/ An explanation of Josephson junctions, from Scientific American