Figure 8: (a) Real component (referring to the logarithmic scale on the left axis) and imaginary component (right axis) of the surface impedance of niobium, normalized to the normal state surface resistance measured at 60 GHz as a function of temperature. (b) Temperature dependence of the components of the conductivity and in niobium calculated from the results of the surface impedance shown in (a). Note the enhancement of just below  K. The full lines are calculated using the Mattis–Bardeen formalism [(1) and (2)] in the local limit, and the dashed lines follow from the Eliashberg theory of a strong coupling superconductor (after [37, 46]).