Figure 3: The density of electronic states in the close vicinity of the Fermi energy . (a) For a normal metal, the density of states is basically constant. The dark colored area indicates the occupied states according to the Fermi-Dirac statistic at finite temperature. (b) In the case of a superconductor, an energy gap opens around ; it grows continuously as the temperature is reduced below . The dotted arrow indicates possible excitations of the occupied states above the gap [first term in (1)], leading to a quasiparticle peak at . For the electronic excitations shown by the solid arrow, a minimum energy of is required; their contribution is captured by the second term in (1). The dark shaded area up to indicates states that can contribute to the conductivity by absorption of photons of arbitrary energy . (c) The full size of the superconducting energy gap is given by for . No quasiparticle peak is present, leading to absorption only above . The states removed from the gap area are pilled up below and above the gap, leading to a divergency.