Spectroscopy of high temperature superconductors at microwave and low far infrared frequencies

A study of the surface resistance and reactance of high-quality YBa₂Cu₃O₇ superconducting films in a transmission line geometry with electrical pulses of approximately 1-ps duration is reported. The spectrum of these extremely short electrical transients extends from DC to approximately 1 THz and allows measurement of the complex surface impedance of the superconductor over the same bandwidth. Compared to reflectivity measurements, the transmission line geometry offers the advantage of long interaction length, so that the surface impedance can be measured with high accuracy even for low temperatures. The surface resistance at 77 K determined this way, plotted versus frequency, is shown and compared with the theoretically calculated surface resistance for the Mattis-Bardeen theory as well as for the full BCS (Bardeen-Cooper-Schrieffer) calculation. The Mattis-Bardeen theory agrees very well with the experimental results, and the full BCS theory actually predicts a higher surface resistance than observed. These findings are attributed to the 2-D character of the conduction in YBa₂Cu₃O₇ superconductors.