Improved low contact resistance in high-T_c Y–Ba–Cu–O ceramic superconductors

Abstract

One of the important problems for electrical applications of high-temperature superconductors^1–3 is the contact resistance between the superconductor and a normal metallic wire. Recently, AT&T Bell researchers⁴ reported that the contact technique limited measurements of a critical current in their Y–Ba–Cu–O samples: even after necking the cross-section down to 2 x 10⁻³ cm², the sample stayed superconducting, the current through the contacts being limited to ∼2 A. Apparently contacts are a real problem, because the very high critical-current densities as reported recently by IBM laboratories⁵ have been measured on films with contact resistances of ∼20 Ω, and a pulsed-current method had to be used. Here we report a simple method of contact preparation for high-T_c copper-oxide materials (Y–Ba–Cu–O). Based on high-temperature metallization it reduces the contact resistance from typically 1 Ω mm⁻² down to ≲lOμΩmm⁻² (77 K), and enables us to measure the critical current of a bulk sample, with current densities in the contact area in excess of 2,000 A cm⁻².