Correlation lengths, disorder, and dynamics of vortex matter as heard through the noise

Abstract

I have conducted a series of experiments measuring flux-flow voltage noise in the type II superconductor 2H-NbSe₂ concentrating on the dynamics and possible phase transitions of vortex matter associated with the peak-effect. I emphasize five conclusions: (1) Traditional models of vortex dynamics relying on purely dynamical correlations, rigid bundles, or simple two-state channel flow are invalid in the peak regime of2H-NbSe₂. (2) The noise reflects vortex configurations present in the pinned state, and our methods probe the static correlations among the vortices in the pinned state. (3) The broad-band noise is sensitive to extremely small AC magnetic field perturbations (at ~ 100 Hz); δH/H~ 10⁻⁵ can reduce the noise power at 1 Hz by a factor of2. This sensitivity of the broad-band noise reflects a loss of configurational memory, which is directly related to a correlation length (Ln) in the static vortex solid. Near the onset of the peak (at Hpl), Ln rapidly reaches an apparent sample-size limit as H decreases, suggesting a continuous phase transition or length-scale crossover in the vortex solid associated with the onset of the peak effect. (4) In the narrow range of fields around the noise maximizing field, the fluctuation kinetics are completely different from those at lower and higher H, indicating the inhomogeneous nucleation of a new phase. (5) The remarkable behavior under various types of AC driving implies low-dimensional, deterministic, periodic dynamics during periods when the Lorentz force is zero. For this work, I used the automated, digital synthesis of periodic cancellation signals based on the synchronized sampling of the periodic residual voltage. I invented this technique to automate the exploration of parameter space associated with AC noise measurement.