Supercurrents in Magnesium Diboride/Metal Composite Wire

TL;DR

This study demonstrates that adding specific metals like Ga to MgB2 wires significantly enhances their critical current density, with the improvement attributed to proximity effect-induced superconductivity and microstructural connectivity.

Contribution

It introduces a novel approach of metal additions to MgB2 wires to improve superconducting performance, supported by experimental and theoretical analysis.

Findings

MgB2/Ga wires achieve Jc > 5x10^4 A/cm^2 at 10K

Critical current enhancement linked to proximity effect and electron-electron interactions

Microstructural connectivity correlates with increased superconducting grains

Abstract

We have fabricated a series of ex situ copper sheathed powder-in-tube MgB2 wires with 20% by volume Ag, Pb, In, and Ga metal added to the MgB2 powder. We find the transport critical current of these wires increases significantly with the addition of specific metals to the core filament. In particular, the critical current density (Jc) of the MgB2/Ga(20%) wire is in excess of 5x10^4 A/cm^2 at 10K in self field, nearly 50 times that of the MgB2/Ag(20%) wire. The temperature dependent Jc of all wires is well described as an ensemble of clean S/N/S junctions in which the relevant parameters are the average thickness of the N layer, the critical temperature of the S layer, and a scaling term related to Jc at zero temperature. Eliminating the differences in the filament microstructure as the primary cause of the enhanced Jc, we suggest that Jc is determined by the magnitude of the proximity effect induced superconductivity in the normal metal layer, which is known to be proportional to the electron-electron interaction in N. We present one-dimensional material specific calculations that support this, and zero-field cooled DC magnetic susceptibility data that confirm an increased number of well-connected superconducting grains exist in the composite wires that contain metal additions with large electron-electron interactions and long electron mean free paths.