Figure 6: Temperature dependence of the conductivity of a superconductor at different frequencies with respect to the gap frequency . (a) For low frequencies the conductivity first rises as decreases below and exhibits a pronounced maximum before it drops to zero for . This so-called coherence peak becomes smaller as the frequency increases; eventually drops monotonously to zero. (b) The imaginary part increases continuously with decreasing temperature. In the static limit, it resembles the development of the superconducting gap .