Supercurrents in magnesium diboride/metal composite wire

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 (J(C)) of the MgB2/ Ga(20%) wire is in excess of 5 x 10(4) A/cm(2) at 10 K in self-field, nearly 50 times that of the MgB2/Ag(20%) wire. The temperature dependent J(C) 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 J(C) at zero temperature. Eliminating the differences in the filament microstructure as the primary cause of the enhanced J(C), we suggest that J(C) 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. (c) 2005 Elsevier B.V. All rights reserved.