Enhancement of Hc1 and its influence on the irreversibility lines in multilayer YBa2Cu3O7/PrBa2Cu3O7 thin films

The irreversibility line Hirr, cannot cross Hc1, and the temperature dependence of Hirr(T) differs from Hc1(T). Therefore there exists an intersection between them, and it is expected that the simple power-law behaviour of the irreversibility line Hirr(T)∼(1-T/Tc)α will break down over a wide range of temperature. We have observed 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 Hc1 in a parallel (parallel to CuO plane) magnetic field. The temperature dependence of Hc1(T) in these three films is in good agreement with calculations of the enhanced Hc1 derived from the London approximation for the case of a slab of thickness tau which is thinner than the London penetration depth lambda , when the applied field is parallel to the sample surface, and they mimic the results observed by Civale and co-workers in thinner pure YBCO ultra-thin films (∼25 nm). (authors)