Magnetic field dependence of the critical current in YBa₂Cu ₃O_7-δ bicrystal grain boundary junctions

We have performed a detailed study of the magnetic field dependence of the critical current, I_c(B), of YBa₂Cu₃O _7-δ bicrystal grain boundary junctions (GBJs). GBJs with width W much larger than the Josephson penetration depth λ_J show I_c(B) dependencies that are close to those of ideal large Josephson junctions with overlap geometry. The I_c(B) dependencies are symmetrical with respect to B=0 and I_c decreases linearly with increasing applied magnetic field for B≤B_J1, where B_J1 is the lower critical field of the GBJ. Furthermore, I_c(B=0) increases linearly with increasing width of the GBJs as expected for Josephson junctions with overlap geometry. From the measured I_c(B) dependencies the temperature dependence of B_J1 and the London penetration depth could be derived.