Magnetic field and voltage noise in type-II superconductors.

Abstract

The puzzling problem of flux-flow noise is investigated both theoretically and experimentally. Combined measurements of the voltage noise and magnetic flux noise in various search coils have been performed, using the classical geometry of soft long strips in a perpendicular field. Our samples are immersed in superfluid helium (T\ensuremath{\sim}1.8 K), allowing one to widely explore the flux-flow regime. Auto and cross spectra, in the 1--${10}^{4}$ Hz range, provide decisive information about the possible sources of noise. In particular, experimental results cannot be reconciled with a two-dimensional (2D) picture of the vortex motion, especially with the classical shot-noise model. A recently presented 3D mechanism of noise [Phys. Rev. Lett. 70, 1521 (1993)] is discussed at greater length in this paper. According to a continuum theory of transport in type-II superconductors by two of us (P.M. and Y.S.), the transport current J is separated into a nondissipative surface current ${\mathbf{J}}_{1}$ and a bulk dissipative current ${\mathbf{J}}_{2}$. The former is associated with the instantaneous distorted configuration of the vortex array, whereas the latter is connected with its motion. A model is developed, that describes surface currents as a 2D homogeneous turbulent flow. This is characterized locally by its normalized spectrum \ensuremath{\Sigma}(f), the intensity of the turbulence---denoted as ${\mathit{u}}^{\mathrm{*}}$(\ensuremath{\sim}${10}^{2}$ A/m) in the text, and directly related to the critical current density---and a correlation length c\ensuremath{\sim}1--3 \ensuremath{\mu}m. Auto or cross spectra of both voltage and magnetic field noises can be entirely predicted as a function of \ensuremath{\Sigma}(f), ${\mathit{u}}^{\mathrm{*}}$, and c, in full agreement with experiment.