The interior of a bulk superconductor cannot be penetrated by a weak

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

, a phenomenon known as the

Meissner effect The Meissner effect (or Meissner–Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state when it is cooled below the critical temperature. This expulsion will repel a ne ...

. When the applied magnetic field becomes too large, superconductivity breaks down. Superconductors can be divided into two types according to how this breakdown occurs. In type-I superconductors, superconductivity is abruptly destroyed via a

first order phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states o ...

when the strength of the applied field rises above a critical value ''H''_c. This type of superconductivity is normally exhibited by pure metals, e.g. aluminium, lead, and mercury. The only alloy known up to now which exhibits type I superconductivity is TaSi₂. The

covalent superconductor Covalent superconductors are superconducting materials where the atoms are linked by covalent bonds. The first such material was boron-doped synthetic diamond grown by the high-pressure high-temperature (HPHT) method.L. Boeri, J. Kortus and O. K. ...

SiC:B,

silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal s ...

heavily doped with boron, is also type-I. Depending on the demagnetization factor, one may obtain an intermediate state. This state, first described by

Lev Landau Lev Davidovich Landau (russian: Лев Дави́дович Ланда́у; 22 January 1908 – 1 April 1968) was a Soviet-Azerbaijani physicist of Jewish descent who made fundamental contributions to many areas of theoretical physics. His ac ...

, is a phase separation into macroscopic non-superconducting and superconducting domains forming a

Husimi Q representation The Husimi Q representation, introduced by Kôdi Husimi in 1940, is a quasiprobability distribution commonly used in quantum mechanics to represent the phase space distribution of a quantum state such as light in the phase space formulation. ...

. This behavior is different from

type-II superconductors In superconductivity, a type-II superconductor is a superconductor that exhibits an intermediate phase of mixed ordinary and superconducting properties at intermediate temperature and fields above the superconducting phases. It also features the ...

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 In superconductors, the London penetration depth (usually denoted as \lambda or \lambda_L) characterizes the distance to which a magnetic field penetrates into a superconductor and becomes equal to e^ times that of the magnetic field at the surface ...

''λ'' to the

superconducting coherence length In superconductivity, the superconducting coherence length, usually denoted as \xi (Greek lowercase ''xi''), is the characteristic exponent of the variations of the density of superconducting component. The superconducting coherence length is one o ...

''ξ'' determines whether a superconductor is type-I or type-II. Type-I superconductors are those with 0 < ''λ/ξ'' < 1/√2, and type-II superconductors are those with ''λ/ξ'' > 1/√2.

References

See also