Critical currents, proximity effect, and Josephson penetration depth in planar high-T-c Josephson junctions

We have studied the temperature dependence of the critical current in planar high-T-c Josephson junctions fabricated in YBa2Cu3O7 thin films by focused electron irradiation. We show that in the range of critical current densities spanning more than five orders of magnitude and temperature range 0.1 < T/T-c less than or equal to 1, the critical current density j(c) varies as (1-T/T-c)(2). The T-dependence of the critical current I-c, however, is affected By the transition from the narrow-function to the wide-junction limit as j(c) increases. An expression far the Josephson penetration depth in thin-film planar structures has been derived, and the value of magnetic field penetration depth hi junction hanks has been extracted from the I-c(T) dependences. Same issues of the proximity effect associated with unconventional pairing symmetry in high-T-c superconductors are also discussed.