electrical conductivity of metals

The response of a metal to electromagnetic radiation is determined by the frequency dependent conductivity. This in turn depends on the available mechanisms for energy absorption by the conduction electrons at the given frequency. The electronic excitation spectrum in the superconducting state is characterised by an energy gap 2Eg. So we expect the AC conductivity to differ substantially from its normal state form at frequencies small compared with 2Eg Ifi, and to be essentially the same in the superconducting and normal states at frequencies large compared with 2Eg / h. The value of 2Eg /h, is typically in the range between microwave and infrared frequencies. In the superconducting state, an AC behaviour is observed which is indistinguishable from that in the normal state at optical frequencies. Deviations from normal state behaviour first appear in the infrared. At microwave frequencies AC behaviour fully displaying the lack of electronic absorption characteristic of an energy gap becomes completely developed.