Quantum fluctuations and the c-axis optical conductivity of high-T-c superconductors

A theory of the frequency dependence of the interplane conductivity of a st rongly anisotropic superconductor is presented. The form of the conductivit y is shown to be a sensitive probe of the strength of quantum and thermal f luctuations of the phase of the superconducting order parameter. The temper ature dependence of the superfluid stiffness and of the form of the absorbt ion at frequencies of the order of twice the superconducting gap is shown t o depend on the interplay between superconducting pairing, phase coherence, and the mechanism by which electrons are scattered. Measurements of the c- axis conductivity of high-ir, superconductors are interpreted in terms of t he theory.