Superconducting Proximity Effects in Epitaxial Cr/Nb Bi-Layers: A Novel Approach Using a Three-Terminal Device Architecture*

We have studied the proximity effects between a superconductor (Nb) and a SDW antiferromagnet (Cr) using epitaxial thin films and a novel three-terminal device structure. Single-crystal Cr/Nb bi-layers are grown on sapphire by means of molecular beam epitaxy. Interface quality is crucial -- oxidation of the Cr surface before Nb deposition can render the proximity effects unseen. Our three-terminal device architecture allows independent measurement of the bi-layer sheet-resistance and the Nb-Cr contact resistance. We find that the contact resistance goes to zero a few tenths of a degree below the Nb critical temperature. Over this temperature range the junction conductance increases at a rate well beyond that predicted by Andreev reflection alone. The bi-layer sheet resistance also shows a slight increase with decreasing temperature. As the devices are cooled further we find both linear and non-linear current-voltage regimes.