Abstract

Starting with the zero-resistance property it is important to recognize that superconductors have truly zero resistance only for direct currents. The co-existence of both superconducting electron pairs and normal state ‘quasi-particles’ in real superconductors at finite temperatures, coupled with the finite mass (and hence inertia) of the pairs, implies that these pairs cannot instantly and completely screen out any applied electric field. The unscreened field can accelerate the quasi-particles and thus induce ohmic losses. The screening by pairs gets worse as the frequency is increased. Nevertheless, the surface resistance of superconductors is lower than that of a typical high conductivity normal metal at the equivalent cryogenic temperature at all frequencies below Δ/h where Δ is the superconducting energy gap and h is Planck's constant.