Upper critical field

The upper critical field is the magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 (usually expressed in teslas

Tesla (unit)

The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...

 (T)) which completely suppresses superconductivity in a Type II superconductor at 0K (absolute zero

Absolute zero

Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....

).

More properly, the UCF is a function of temperature (and pressure) and if these are not specified absolute zero and standard pressure are implied.

Werthamer–Helfand–Hohenberg theory

Werthamer–Helfand–Hohenberg theory

This theory was proposed in 1966 to go beyond BCS theory of superconductivity and it provides predictions of upper critical field in type-II superconductors.The theory predicts the upper critical field at 0 K from Tc and the slope of Hc2 at Tc....

 predicts the upper critical field (H_c2) at 0 K from T_c and the slope of H_c2 at T_c.

The UCF (at 0 K) can also be estimated from the coherence length(ξ) using the Ginzburg-Landau expression : H_c2 = 2.07 x 10^-15 Tm²/(2 Pi ξ²)

Articles on superconductivity use H_c2 or B_c2 interchangeably since the materials are rarely magnetic.

Lower critical field

The magnetic field at which the magnetic flux starts to penetrate a type-2 superconductor at 0 K (absolute zero).