Andreev reflection and tunneling results on YBa₂Cu₃O₇ and Nd_2−xCe_xCuO_4−y single crystals

Abstract

We will describe some results from recent experiments on Andreev reflection and tunneling in the superconductors YBa₂Cu₃O₇ (“123”) and Nd_2−xCe_xCuO_4−y (“NCCO”)^1,2. The interface resistance between gold and 123 crystals has been greatly reduced using a technique³ which does not involve high temperature annealing. Gold is evaporated in situ onto a crystal cleaved in high vacuum. Using the specific contact resistivity p_[] as a gauge of the interface quality, we find that p_[] in these junctions approaches the ideal value p ⁰_[] derived by counting the number of conduction channels in 123. (p_[] is the product of the contact resistance R _c and the contact area.) With these junctions, we have studied the decrease of the interface resistance near T _c . We have also observed⁴ Andreev reflection of injected carriers at low temperatures T in these junctions. The T depedence of the Andreev signal is studied from 4.2 to 80 K. In the “T’ phase” superconductor^1,2 NCCO we have used the same cleaving technique to make Giaever tunneling junctions with Pb as the counter electrode. We observe rather clearly the gap in the tunneling spectrum⁵. The dependence of the spectrum on temperature and magnetic field has been studied. A number of anomalous features are observed. These are described below.