Superconducting screening on different length scales in high-quality bulk MgB2 superconductor

TL;DR

This study investigates the multiscale superconducting screening currents in high-quality bulk MgB2, revealing distinct contributions at different length-scales and correcting previous critical current estimations affected by sample size.

Contribution

It introduces a method to separate and analyze superconducting currents at multiple length-scales in MgB2, improving the accuracy of critical current measurements.

Findings

Superconducting currents circulate on three length-scales: ~1 μm, ~10 μm, and entire sample.

Sample size affects the apparent critical current and irreversibility field measurements.

Currents around the whole sample dominate in intermediate magnetic fields and allow accurate Jc estimation.

Abstract

High quality bulk MgB2 exhibit a structure of voids and agglomeration of crystals on different length-scales. Because of this, the superconducting currents percolate between the voids in the ensuing structure. Magnetic measurements reveal that the superconducting currents circulate on at least three different length-scales, of ~1 micrometre, ~10 micrometre and whole of the sample (~millimetre). Each of these screenings contributes to the measured irreversible magnetic moment (Dm). The analysis of the field dependence of Dm for samples of subsequently decreasing size showed that the critical current obtained using the simple critical state model is erroneous. This leads to the artefact of the sample size-dependent critical current and irreversibility field. Our data analysis enables the separation of the contribution of each of the screening currents to Dm. The field dependence of each of the currents follows a stretched exponential form. The currents flowing around whole of the sample give a dominant contribution to Dm in the intermediate fields (1T < H < 4T at 20K) and they can be used to obtain the value of Jc from critical state model, which corresponds to the transport Jc.