VERY HIGH CRITICAL CURRENT IN A LONG YBA2CU3O7-DELTA SINGLE CRYSTALLINE ROD

We studied by magnetic and microscopic techniques a disk (2.0 mm diam and 1.0 mm thick) sliced from a quasi single-crystalline rod prepared by very slow solidification in a small temperature gradient. The sample presents features very similar to a typical millimetric single crystal obtained by the standard flux methods. We deduced a high critical current density J(c) from magnetic measurements: 2 x 10(6) A/cm2 at 4.2 K in a 3T field parallel to the c axis. The transport critical current density along the rod axis (i.e., approximately 45-degrees off the c axis) was found to be about 7000 A/cm2 at 77 K in zero applied field. It is comparable to that deduced from the hysteresis cycle at the same temperature for the same current direction.