Method for making low-resistivity contacts to high T c superconductors

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Abstract
A method for making low‐resistivity contacts to high Tc superconductors has been developed, which has achieved contact surface resistivities less than 10 μΩ cm2 at 76 K and does not require sample heating above ∼150 °C. This is an upper limit for the contact resistivity obtained at high current densities up to 102–103 A/cm2 across the contact interface. At lower measuring current densities the contact resistivities were lower and the voltage‐current curve was nonlinear, having a superconducting transition character. On cooling from 295 to 76 K, the contact resistivity decreased several times, in contrast to indium solder contacts where the resistivity increased on cooling. The contacts showed consistently low resistivity and little degradation when exposed to dry air over a four‐month period and when repeatedly cycled between room temperature and 76 K. The contacts are formed by sputter depositing a layer of a noble metal−silver and gold were used−on a clean superconductor surface to protect the surface and serve as a contact pad. External connections to the contact pads have been made using both solder and wire‐bonding techniques.