A dielectric layer at the interface between metals and superconducting ceramics

An attempt was made to develop a low-resistivity metal/superconductor contact which is stable to mechanical stress and durable against temperature variation. YBa₂Cu₃ O_7-x ceramics were produced by mixed oxide preparation and a sintering process in O₂ for 16 h at 950°C with a final cooling rate of 50 K/h. Surfaces were polished with silicon carbide and then Al, Cu, Au or Ag were evaporated onto the ceramic at a pressure of 10^-5 mbar. For measurement of the voltage-current relation of the contact a current I_dc was fed through the contact and in a four-point probe arrangement the voltage was measured. In order to derive the differential resistance δU/δl(U), a small AC current (28 Hz or 580 Hz) was superimposed on the DC current. The contact capacitance was measured for f⩽10 MHz with a standard capacitance bridge and for f⩾1 MHz by a reflection technique where the sample was placed at the end of a coaxial line. Results on contact resistance and capacitance are presented