Abstract : This work aims at investigating the vortex lattice dynamics in a superconductor by means of noise measurements. This technique enable an accurate information on how the moving vortex lattice interacts with its pinning centers. A low-noise experimental set-up has be specially achieved for this purpose.
First, the noise regime is studied in bulk Niobium foils under different conditions: the sample sizes have been carefully tuned, and on the other hand, surface have been roughened using low energy ion irradiation, and monitored by atomic force microscopy. Low-frequency noise, as well as the critical current, are found to be mainly due to a surface pinning mechanism. Furthermore, the fluctuation range turns out to be correlated with the surface roughness spatial distribution.
In surface sheath superconductivity, with no vortex in the bulk, it is experimentally shown that pure surface current fluctuations yields the same noise regime as in the mixed state, with bulk vortices. This result is confirmed by complementary experiments in Pb-In alloy bulk samples, where the voltage fluctuations have been collected in different directions.
The noise process statistics have been investigated in micro-bridges made on a Niobium thick film: both surface roughness and edges state turn out to affect the depinning noise regime, during which non-Gaussian and non-stationary effects appear, similarly to Lévy flights.