Optical properties, spectroscopy threshold

Spectroscopic measurements of the energy gap are based on the fundamental fact that absorption of radiation with frequency less than the magnitude of the gap is prohibited in a superconductor, and therefore the dissipative (real) part of the conductivity vanishes for 0) < 2A. A clear threshold in the conductivity spectra at tu = 2A is found only in dirty superconductors where 1/r > 2A. In clean materials where 1/r normal state, and unlike other spectroscopies where a sharp feature can be seen at the gap energy, there is little change in the overall optical properties at the gap frequency in a clean superconductor. A detailed discussion of this issue can be found in a review article by Timusk and Tanner (1989). 

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