supercurrent

A supercurrent is a current that flows without electrical resistance in a superconducting state. In conventional superconductors below their critical temperature, electrons form Cooper pairs that condense into a macroscopic quantum state with a common phase. The resulting superconducting order parameter supports a dissipationless flow of charge carried by Cooper pairs, producing essentially zero voltage drop under steady conditions.

The supercurrent is carried by Cooper pairs and is governed by quantum phase coherence. The supercurrent density is closely related to the spatial variation of the superconducting phase, and is described in part by the London equations and, more generally, by Ginzburg-Landau theory or BCS theory. In practice, the current is limited by a material-specific critical current, above which superconductivity breaks down due to depairing or phase-slip processes.

A central phenomenon involving supercurrents is the Josephson effect. In a weak link between superconductors, a phase difference φ across the link yields a direct current I = Ic sin φ (the dc Josephson effect). If a constant voltage V is applied, the phase evolves in time and the current becomes AC with a frequency f = (2e/h) V (equivalently, V = (h/2e) f). Supercurrents can also circulate in closed superconducting loops, leading to persistent currents and magnetic-flux quantization in units of Φ0 = h/2e.

Limitations include finite-temperature effects and the existence of a critical current; near Ic or at nonzero temperature, phase slips or quasiparticle excitations can generate a small voltage. Supercurrents underlie many applications, including SQUID magnetometers, superconducting quantum bits, and other superconducting electronic devices.