Creation and Growth of Superconducting Nuclei

A SUPERCONDUCTOR in its normal state in a magnetic field can be `supercooled', in the sense that the field must be decreased some way below its critical value H_c before the transition into the superconducting state occurs. The explanation of this, originally given by London¹, is that a boundary between the two phases has a surface energy, which makes the creation of the first nucleus of the superconducting phase energetically difficult.