Supercurrents

Supercurrents are electrical currents that flow without electrical resistance in superconductors. They are carried by Cooper pairs—bound pairs of electrons with opposite spins and momenta—that move coherently as a single quantum state described by a macroscopic wavefunction. The current arises from the phase gradient of this wavefunction and can persist indefinitely in a closed superconducting loop, forming a persistent current.

Properties: In the absence of thermal agitation and magnetic flux beyond certain thresholds, the supercurrent experiences zero DC resistance. The maximum current before breakdown is called the critical current I_c. Exceeding it, or raising the temperature toward Tc, destroys superconductivity. In practice, phase slips or flux vortex motion can introduce dissipation, especially in thin wires or near Tc. In type-II superconductors, magnetic vortices can move under current, causing finite resistance until pinning occurs.

Josephson effect: When a supercurrent is transmitted through a weak link between two superconductors (a Josephson junction), the current across the link is determined by the phase difference phi between the superconducting condensates: I = I_c sin(phi). The phase evolves with time under applied voltage V as dphi/dt = 2eV/ħ, giving the AC Josephson effect when a voltage is present.

Applications and occurrences: Supercurrents are essential in SQUID magnetometers, superconducting quantum bits (qubits), and various high-speed digital logic based on Josephson junctions. They also enable persistent currents in superconducting rings and are studied in mesoscopic systems to explore quantum coherence and vortex dynamics.