It is commonly applied in the research field of optical spectroscopy on superconductors.
Originally, the anomalous skin effect indicates the non-classical response of metals to high frequency electromagnetic field in low temperature, which was solved by Robert G.
[3] At sufficiently low temperatures and high frequencies, the classically predicted skin depth (normal skin effect) fails because of the enhancement of the mean free path of the electrons in a good metal.
Not only the normal metals, but superconductors also show the anomalous skin effect which has to be considered with the theory of Bardeen, Cooper and Schrieffer (BCS).
After the transition to the superconducting state, the superconducting gap 2Δ in the single-particle density of states arises, and the dispersion relation can be described like the one of a semiconductor with band gap 2Δ around the Fermi energy.
From the Fermi golden rule, the transition probabilities can be written as where
Therefore, there appear interference terms in the absolute square of the matrix element.
The calculated optical conductivity breaks the sum rule that the spectral weight should be conserved through the transition.
This result implies that the missing area of the spectral weight is concentrated in the zero frequency limit, corresponding to the dirac delta function (which covers the conduction of the superconducting condensate, i.e. the Cooper pairs).
This story on electrodynamics of superconductivity is the starting point of optical study.