OXYGEN-CONTROLLED TRANSITION FROM TUNNEL-LIKE TO A WEAK-LINK BEHAVIORIN YBA2CU3O7-DELTA YBA2COCU2OY/YBA(2)CU3O(7-DELTA) WEDGE-EDGE JUNCTIONS/

Differential conductance measurements in YBa2Cu3O7-delta/YBa2CoCu2Oy/Y Ba2CU3O7-delta (YBCO/YBCoCO/YBCO) wedge-edge junctions are reported on . The properties of the junctions change dramatically with the oxygen concentration in the films. Junctions prepared in an oxygen-deficient atmosphere show differential conductance which increases with the bias voltage. This behavior is typical of high-T-c tunnel junctions. In co ntrast, junctions prepared with fully oxygenated films show weak-link behavior where the differential conductance decreases with the bias vo ltage. In the tunnel-like type junctions a series of peaks is observed in the differential conductance versus voltage. We identify these pea ks as due to Tomasch oscillations in the cover electrode along the c d irection, yielding Delta(c) = 2.1 mV and a renormalized Fermi velocity upsilon(Fc)' = 2.4 x 10(7) cm/s.