Current transport across grain boundaries in TlBa/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ thick polycrystalline films and YBa/sub 2/Cu/sub 3/O/sub 7/ step edges

The authors have performed transport measurements in magnetic fields (H//c) up to 10 T on a series of YBa/sub 2/Cu/sub 3/O/sub 7/ grain boundaries induced by epitaxial growth on a substrate containing a series of step edges. An activation energy which goes as U(T) approximately (1-t) is found. Electric field versus current density curves taken at a constant temperature while varying the magnetic field allows a determination of the magnetic field dependence of the activation energy using the Ambagaokar-Halperin model. These values agree with results extracted from the resistive transitions. Similar characterizations of a sample of TlBa/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ are also presented. The temperature dependence of the activation energy for this system may be represented as U(T) approximately (1-t)/sup 2/, suggesting that the grain boundaries behave as SNS (superconducting-normal-superconducting) junctions.<
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