Type-I superconductor

Depending on the demagnetization factor, one may obtain an intermediate state. This state, first described by Lev Landau, is a phase separation into macroscopic non-superconducting and superconducting domains forming a Husimi Q representation.

This behavior is different from type-II superconductors which exhibit two critical magnetic fields. The first, lower critical field occurs when magnetic flux vortices penetrate the material but the material remains superconducting outside of these microscopic vortices. When the vortex density becomes too large, the entire material becomes non-superconducting; this corresponds to the second, higher critical field.

The ratio of the London penetration depth λ to the superconducting coherence length ξ determines whether a superconductor is type-I or type-II. Type-I superconductors are those with ${\displaystyle 0<\frac{\lambda }{\xi }<\frac{1}{\sqrt{2}}}$, and type-II superconductors are those with ${\displaystyle \frac{\lambda }{\xi }>\frac{1}{\sqrt{2}}}$.