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.
The superconducting coherence length is one of two parameters in the Ginzburg–Landau theory of superconductivity.
is a parameter in the Ginzburg–Landau equation for
In Landau mean-field theory, at temperatures
near the superconducting critical temperature
, it is equivalent to the characteristic exponent describing a recovery of the order parameter away from a perturbation in the theory of the second order phase transitions.
In some special limiting cases, for example in the weak-coupling BCS theory of isotropic s-wave superconductor it is related to characteristic Cooper pair size:[2] where
is the reduced Planck constant,
is the mass of a Cooper pair (twice the electron mass),
is the superconducting energy gap.
The superconducting coherence length is a measure of the size of a Cooper pair (distance between the two electrons) and is of the order of
The electron near or at the Fermi surface moving through the lattice of a metal produces behind itself an attractive potential of range of the order of
cm, the lattice distance being of order
For a very authoritative explanation based on physical intuition see the CERN article by V.F.
κ = λ
is the London penetration depth, is known as the Ginzburg–Landau parameter.
, and type-II superconductors are those with
In strong-coupling, anisotropic and multi-component theories these expressions are modified.