Advent of type II superconductors

This chapter describes how the phenomenological understanding of superconductors was vastly enhanced by the Ginzburg–Landau theory. Its success rested on the fact that it could quantitatively yield the two characteristic lengths λ and ξ and address the question of boundary energy between superconducting and normal phases in terms of the Ginzburg–Landau parameter κ. for κ <1/√2, the sign of the boundary is positive and these are type I superconductors (including pure elements) where superconductivity is abruptly lost at Hc. When κ ≥1/√2, the boundary energy is negative and such materials (including impure metals and alloys) are type II superconductors. Field penetration now begins at a lower critical field Hc1 (Hc), where the bulk of the superconductivity is lost. Between the two fields, the sample is in a mixed state. Basic features of type II superconductors are discussed.