A class of high-Tc YBa2Cu3O7?x grain boundary junctions with high-IcRn products

The properties of symmetric 24o and asymmetric 45o [100] tilt YBa2Cu3O7-x bicrystal grain boundary junctions (GBJs) have been extensively investigated. A large number of junctions, with dimensions ranging from 2 to 20 ?m, have been fabricated and characterized at 4.2 K. Experimental data have been compared with [001] tilt bicrystal and [100] tilt biepitaxial YBa2Cu3O7-x junctions. [100] GBJs show high IcRn products in the range 5-10 mV at T = 4.2 K, these being slightly sensitive to the critical current density values. A direct tunnelling mechanism for both Cooper pair and quasiparticles has been evidenced. The reduced IcRn values of asymmetric 45o with respect to symmetric 24o [100] tilt GBJs have been accounted for by considering a d-wave symmetry of the order parameter using the Sigrist-Rice equation.