Flux Noise in MgB2 Thin Films

TL;DR

This study investigates flux noise and vortex dynamics in MgB2 thin films using flux noise and AC-susceptibility measurements, revealing multi-step transitions and flux quantum variations linked to inhomogeneities.

Contribution

It provides new insights into vortex behavior and flux noise characteristics in MgB2 films, connecting flux noise spectra to inhomogeneous superconducting regions.

Findings

Multi-step transition behavior observed in both measurements

Flux noise spectral density changes at transition steps

Vortices may carry fractional flux quanta or originate from inhomogeneities

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.