Figure 16: (a) Real part and (b) imaginary part of the complex conductivity versus frequency for 4.2 K (dots) and 7 K (diamonds) well below  K and 20 K (squares) in the normal state for both main axes of a NbN film. Solid black and blue lines are predictions calculated directly from raw data fits for both axes individually. The kink in signaling the energy gap is resolved clearly. The error bar shown is representative for all data (after [98]).