Grain boundary properties of TlBa₂Ca₂Cu₃Oxand Tl₂Ba₂CaCu₂Oxthin films

We have investigated the transport properties of the grain boundaries of TlBa 2 Ca 2 Cu 3 O x (Tl-1223) and Tl 2 Ba 2 CaCu 2 O x (Tl-2212) epitaxial thin films. Both Tl-1223 and Tl-2212 thin films were grown on SrTiO 3 bicrystal substrates with various tilt angles (5 degrees , 10 degrees , 15 degrees , 24 degrees and 36.8 degrees ) of (001) direction by a laser ablation and post-annealing method. For both Tl-1223 and Tl-2212 phases, the grain boundary J c for large tilt angles ( theta >or=15 degrees ) at 5 K dropped by almost one order of magnitude, compared with the intragrain J c . We also observed hysteretic magnetic-field dependence of grain boundary J c for large tilt angles in both phases. On the other hand, the grain boundaries for low tilt angles ( theta <or=10 degrees ) retained high J c in low magnetic fields. The critical tilt angle was found to have the same value (10 degrees ) in both phases and greater than that ( theta approximately=5 degrees ) for the YBa 2 Cu 3 O 7- delta system. These results suggest that the grain boundaries for high tilt angle larger than 15 degrees limit transport critical current density as weak links for Tl-Ba-Ca-Cu-O systems.