Flux noise in MgB 2 films

Abstract

We have performed flux noise and AC-susceptibility measurements on two 400 nm thick MgB 2 films. Both measurement techniques give information about the vortex dynamics in the sample, and hence the superconducting transition, and can be linked to each other through the fluctuation-dissipation-theorem. The transition widths for the two films are 0.3 and 0.8 K, respectively, and the transitions show a multi step-like behavior in the AC-susceptibility measurements. The same phenomenon is observed in the flux noise measurements through a change in the frequency dependence of the spectral density at each step in the transition. The results are discussed and interpreted in terms of vortices carrying an arbitrary fraction of a flux quantum as well as in terms of different macroscopic regions in the films having slightly different compositions, and hence, different critical temperatures.

Vortex fluctuations in high-Tcthin films close to the resistive transition

“…17,18 The solenoid used in the mutual inductance measurements has a diameter of 3 mm. Inside this solenoid is a concentrically mounted pick-up coil, wound as a first order gradiometer and having a diameter of 1.2 mm.…”

Current noise in stationary conditions in MgB<inf>2</inf> thin films

“…In many applications superconductors are used to carry high current density, then the transition has been particularly studied when fluxoids motion is the dominant process. The noise has been detected by measuring the induced voltage noise at the ends of the specimen connected to a constant current generator [2,3] or as magnetic noise with the use of a SQUID [4][5][6][7]. A percolation-like model can be used to explain the current noise observed in granular YBCO type materials, characterized by weak-links, in presence of temperature fluctuations [8][9][10].…”