THE LONG-RANGE PROXIMITY EFFECT - PAIR ECHOES

Recent experiments on N'-N-S junctions with high-temperature cuprate s uperconductors have shown that superconductivity can be induced in the adjacent, normal metal at distances many times greater than the few h undred Angstroms range of the conventional proximity effect. We show t hat this arises in the particular geometry used in these experiments t hrough the constructive interference of the tails of the de Gennes-Sai nt-James bound and quasi-bound states of the normal metal N that penet rate into the third metal, N'. This constructive interference results in the re-appearance of a pair amplitude in this metal which is analog ous to the ''spin echo'' of NMR and which we name a ''pair echo.''