Effect of equilibrium fluctuations on superfluid density in layered superconductors

Using a phenomenological approach based on the fluctuation-dissipation theorem we calculate suppression of superconducting currents due to phase fluctuations and find that, in contrast to a recent prediction, the effect of thermal fluctuations cannot account for linear temperature dependence of the superfluid density in high-T(c) superconductors at low temperatures. Quantum fluctuations are found to dominate over thermal fluctuations at low temperatures. Near T(c) sizable thermal fluctuations are found to suppress the critical current in the stack direction stronger than in the direction along the layers. The spectral density of voltage fluctuations at small frequencies is nonzero, in contrast to what may be expected from a naive interpretation of the Nyquist formula.