Low frequency noise in freely suspended tin films at the superconducting transition

The spectral density, S/sub v/(f), of the voltage noise across 1-D current-biased tin films has been measured at the superconducting transition. Each film was freely suspended between two thermal clamps a distance L apart in a vacuum can. A thin layer of lead was evaporated on the outer portion of the films to leave an uncoated middle region of length l. S/sub v/(f) was flat at frequencies below f/sub L/ approximately D/L2, where D is the thermal diffusivity. At frequencies between f/sub L/ and f/sub l/ approximately (L/l)2f/sub L/ the slope was typically -0.8, while at frequencies above f/sub l/ the slope was somewhat less steep than -1.5. The shape and magnitude of S/sub v/(f) were in good agreement with an equilibrium temperature fluctuation model in which the temperature fluctuations are spatially uncorrelated. Measurements of the autocorrelation function also strongly supported this model. These results are in contrast with those obtained for normal films and films at the superconducting transition supported by substrates, for which a model was required with spatially correlated fluctuations. It is concluded that the 1/f noise for films on substrates is mediated by an interaction between the substrate and the film