Abstract

Measurements of the electromagnetic response of superconductors in the microwave spectrum (for the purposes of this chapter defined to be in the range from 100 MHz to 500 GHz) and in the infrared spectrum (> 500 GHz) allow the simultaneous investigation of the dynamics and energy states of both paired carriers and unpaired carriers (quasiparticles) in a superconductor. This allows information to be deduced regarding the nature of the superconducting pairing state, the energy gap, the dynamics of quasiparticles and the effects of defects and impurities [1–3]. High-frequency measurements of conventional (i.e. low T _c) superconductors [4] provided the springboard for the BCS theory, and later confirmation of many of the theory's predictions [1–3]. Measurements in static applied magnetic fields [5, 6] probe the dynamics of flux lines, yielding flux line viscosities, pinning strengths and information regarding quasiparticle states within the flux line cores.