ENHANCEMENT OF H-C1 AND ITS INFLUENCE ON THE IRREVERSIBILITY LINES INMULTILAYER YBA2CU3O7 PRBA2CU3O7 THIN-FILMS/

The irreversibility line H-irr cannot cross H-cl, and the temperature dependence of H-irr(T) differs from H-cl(T). Therefore there exists an intersection between them, and it is expected that the simple power-l aw behaviour of the irreversibility line H-irr(T) similar to (1 - T/T- c)(alpha) will break down over a wide range of temperature. We have ob served this interesting phenomenon in our three YBa2Cu3O7/PrBa2Cu3O7 ( YBCO/PrBCO) multilayer thin films with different YBCO superconducting layers for the first time due to the enhancement of H-cl in a parallel (parallel to CuO plane) magnetic field. The temperature dependence of H-cl(T) in these three films is in good agreement with calculations o f the enhanced H-cl derived from the London approximation for the case of a slab of thickness tau which is thinner than the London penetrati on depth lambda when the applied field is parallel to the sample surfa ce, and they mimic the results observed by Civale and co-workers in th inner pure YBCO ultra-thin films (similar to 25 nm).