Proximity coupling in high-T-c Josephson junctions produced by focused electron beam irradiation

Using the particular benefits of focused electron beam irradiation (FEBI) junctions, such as on chip modification of the barrier resistivity through controlled variation of the electron fluence and annealing, we show that the conventional model for superconductor-normal-metal-superconductor (SNS) junctions as derived by De Gennes can explain their behavior in great detail. We find that the damage distribution produced by the electron beam has a full width at half maximum of the order of 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 nonsuperconducting and has a much higher normal-stare resistivity than undamaged YBa2Cu3O7-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 junctions 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 scaling of I-c with T-c-T close to T-c and the reduced IcRn values of the junctions indicate that the SN interface has a soft boundary nature. From the low-temperature scaling of IcRn with the ratio of the barrier length and the coherence length we find that the suppressed superconducting gap at the SN interface is approximately 4.5 meV.