In order to shed light on the puzzling thickness (t) dependence of the critical current density (J(c)) in YBa2Cu3O7-x (YBCO) films at 77 K and self-field, we have investigated J(c)-t behavior as a function of temperature (T) and applied magnetic field (H) in the thickness range that is within the modified Larkin length L-c. It has been found that the J(c)-t behavior can be well described by the collective pinning (CP) model as monotonically decreasing J(c)similar to t(-1/2) when thermal assisted flux motion (TAFM) is suppressed at low T close to zero. TAFM becomes increasingly important at elevated temperatures and causes deviation of the J(c)-t from the CP model in the thickness range below the magnetic penetration depth. The deviation is more pronounced at smaller thickness and increases with T and H. Consequently, a completely reversed J(c)-t from monotonically decreasing to monotonically increasing occurs when TAFM dominates in high T and H ranges. This result suggests that the monotonic decreasing J(c)-t behavior is an intrinsic property of uniform YBCO films, while extrinsic effects such as material nonuniformity can greatly complicate it.
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