Proximity-effect-induced superconductivity in very dilute Pb-Zn composites

Very dilute bulk dispersions of micrometer-sized particles of Pb in Zn show resistive superconducting transitions at up to 3 K, far above that of Zn itself (0.8 K), although the Pb volume fraction is only 0.006. The $I-V$ characteristics contain reproducible structure that is not understood. At lower temperatures the samples expel a small, but significant, amount of flux. The superconducting transition can be explained in terms of proximity-effect coupling of the Pb grains through the Zn matrix.