Flux Quantization

Abstract

The quantization of magnetic flux is a direct manifestation of the macroscopic quantum description of the superconducting state. Flux quantization is a defining property of any superconductor, as is the closely related Meissner effect. Although London assumed that the quantization of flux was of purely academic interest, worthy only of a footnote, this property now underpins many of the most important device applications of superconductors. In 1955, Abrikosov showed that the magnetic state of type II superconductors involved singly quantized flux lines penetrating the bulk of the material. At the centre of the flux line, the superconducting order parameter is reduced to zero, so that states involving a circulating vortex of current can exist within the bulk, the flux line core essentially acting as line singularity around which the quantized current can flow. Flux quantization is a direct consequence of a macroscopic wave-mechanical description of the superconducting state.