Type II Superconductors

In this chapter, the properties of Type II superconductors are described. Up to the lower critical field, magnetic field is excluded from the interior of a Type II superconductor as in the case of a Type I superconductor. However, between the lower and upper critical fields, there exists a mixed state in which magnetic flux can penetrate into the interior of the superconductor and normal and superconducting regions can coexist within the material. The Ginzburg–Landau theory, to which a brief introduction is presented, perfectly describes the behavior of Type II superconductors. Relations among the coherence length, the penetration depth, the thermodynamic critical field, and the lower and upper critical fields, are presented for isotropic and anisotropic superconductors. The factors limiting the critical current densities in Type I and Type II superconductors are discussed.