Andreev reflection and tunneling results on YBa2Cu3O7 and Nd2−xCexCuO4−y single crystals
We will describe some results from recent experiments on Andreev reflection and tunneling in the superconductors YBa2Cu3O7 (“123”) and Nd2−xCexCuO4−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” superconductor1,2 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.
KeywordsContact Resistance Incident Electron Contact Impedance Interface Resistance Andreev Reflection
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