 Mattis–Bardeen theory

The Mattis–Bardeen theory is a possible explanation of a phenomenon observed in superconductors.
Contents
Introduction
The Mattis–Bardeen theory ^{[1]} was derived to explain the anomalous skin effect of superconductors. Originally, the anomalous skin effect indicates the nonclassical response of metals to high frequency electromagnetic field in low temperature, which was solved by R. G. Chambers. ^{[2]} In sufficiently low temperatures and high frequencies, the classically predicted skin depth 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.
The response to the electromagnetic wave
The most clear fact the BCS theory gives is the presence of the pairing of two electrons (Cooper pair). After the transition to superconducting state, the superconducting gap 2Δ arises, and the dispersion relation can be described like semiconductor with band gap 2Δ where the Fermi energy lies. From the Fermi golden rule, the transition probabilities can be written as
where N_{s} is the density of states. And M_{s} is the matrix element of an interaction Hamiltonian H_{1} where
In the superconducting state, each term of the Hamiltonian is dependent, because of the superconducting state consists of a phasecoherent superposition of occupied oneelectron states, whereas it is independent in the normal state. Therefore there appear interference terms in the absolute square of the matrix element. The result of the coherence changes the matrix element M_{s} into the matrix element M of single electron and the coherence factors F(Δ,E,E').
Then, the transition rate is
where the transition rate can be translated to real part of the complex conductivity, σ_{1}, because the electrodynamic energy absorption is proportional to the σ_{1}E^{2}.
In finite temperature condition, the response of electrons due to the incident electromagnetic wave can be regarded as two parts, the “superconducting” and “normal” electrons. The first one corresponds to the superconducting ground state and the next to the thermally excited electrons from the ground state. This picture is the socalled "twofluid" model. If we consider the “normal” electrons, the ratio of the optical conductivity to the one of the normal state is
The first term of the upper equation is the contribution of "normal" electrons, and the second term is due to the superconducting electrons.
Use in Optical Study
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. Many experimental data supports the prediction. This story on electrondynamics of superconductivity is the starting point of optical study. Because any superconducting T_{c} never exceeds 200K and the superconducting gap value is about the 3.5 k_{B}T, microwave or farinfrared spectroscopy is suitable technique applying this theory. With the Mattis–Bardeen theory, we can derive fruitful properties of superconducting gap, like gap symmetry.
References
 Michael Tinkham, Introduction to Superconductivity. Second edition.
 Mark Fox, Optical Properties of Solids. Oxford University Press.
 ShuAngZhou, Electrodynamics of Solids and Microwave Superconductivity.
Notes
Categories:
Wikimedia Foundation. 2010.