Thermal detection of flux-flow noise in type-II superconductors

In this paper we present an original method for the observation of flux-flow noise. A superconducting slab, placed in a normal magnetic field, and immersed in He II, constitutes one of the walls of a rectangular second-sound resonator. As is well known, the voltage observed in the flux-flow regime is noisy, due to irregularities in the vortex motion. Therefore, noise in Joule dissipation is expected to occur, making the slab act as a noisy second-sound transmitter. Second-sound noise is then amplified selectively on the different modes of the resonator, with amplitudes depending, in general, on the spatial distribution and coherence of the fluctuating heat sources on the sample. Thus careful and detailed second-sound analysis should provide valuable information on vortex motion and pinning, that is not necessarily given by the usual voltage noise measurements. Preliminary experiments reported in this paper are merely intended to demonstrate that we are able to detect and identify the thermal flux-flow noise, in spite of its minuteness (∼${10}^{\mathrm{-}15}$ ${\mathrm{W}}^2$/Hz), while obtaining reliable quantitative results.