A fluctuation-dissipation relation and its applications to thermodynamic fluctuations in superconducting cylinders

Modifying slightly Kubo's formulation of perturbation theory to take care of the fact that generalized susceptibility may not be zero at infinite frequency, we establish, in the classical limit, a general relation between covariance and generalized susceptibility. The relation is then applied to evaluate the covariances of the magnetic flux, currents, and magnetization of a superconducting cylinder. Expressions for the spectra of magnetic flux and magnetization are also obtained.