PROXIMITY COUPLING IN HIGH-T-C JOSEPHSON-JUNCTIONS PRODUCED BY FOCUSED ELECTRON-BEAM IRRADIATION

Using the particular benefits of focused electron beam irradiation (FE BI) junctions, such as on chip modification of the barrier resistivity through controlled variation of the electron fluence and annealing, w e show that the conventional model for superconductor-normal-metal-sup erconductor (SNS) junctions as derived by De Gennes can explain their behavior in great detail. We find that the damage distribution produce d by the electron beam has a full width at half maximum of the order o f 15 nm and is largely determined by the profile of the electron beam used in the fabrication process. Due to the high defect concentration produced by the electron beam, the barrier material is nonsuperconduct ing and has a much higher normal-stare resistivity than undamaged YBa2 Cu3O7-delta From the exponential scaling of the critical current (I-c) with the square root of the resistance (R-n) it is shown that FEBI ju nctions have a dirty limit SNS character and that the carrier mass in the irradiated material is of the order of m(e). Both the quadratic sc aling of I-c with T-c-T close to T-c and the reduced IcRn values of th e junctions indicate that the SN interface has a soft boundary nature. From the low-temperature scaling of IcRn with the ratio of the barrie r length and the coherence length we find that the suppressed supercon ducting gap at the SN interface is approximately 4.5 meV.