# Superconducting coherence length

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

In some special limiting cases, for example in the weak-coupling BCS theory it is related to characteristic Cooper pair size.

The superconducting coherence length is one of two parameters in the Ginzburg-Landau theory of superconductivity. It is given by:[1]

while in BCS theory:[2]

where is the reduced Planck constant, is the mass of a Cooper pair (twice the electron mass), is the Fermi velocity, and is the superconducting energy gap.

The ratio , where is the London penetration depth, is known as the Ginzburg–Landau parameter. Type-I superconductors are those with , and type-II superconductors are those with .

For temperatures T near the superconducting critical temperature Tc , ξ(T) (1-T/Tc)−1.