Paraconductivity in YBa2Cu3O7 and YBa2Cu3O7 thin film composites with cerium oxide and gold

The paraconductivity for thin films of YBa2Cu3O7 and composites of YBCO with CeO2 and gold was investigated and analyzed using the Aslamazov and Larkin theory. Plain YBCO thin films sputter deposited on MgO, SrTiO3, and yttrium stabilized zirconium typically showed no dimensional crossover, were described by three dimensional (3D) fluctuation conductivity, and had a critical slope just above the critical region in agreement with the prediction of theory. With few exceptions, the CeO2 and gold composite films showed a dimensional crossover and a reduced critical exponent. The change in dimensionality was typically from 3D to two dimensional (2D) fluctuation conductivity. These composite films all had reduced values for the critical exponent which we attribute to an enlarged or extended critical region. This is in agreement with the critical current response of these composites to small magnetic fields. Additionally, the composites showed a two step transition in the paraconductivity and a minimum below the critical region. The subsequent fall- off to the zero resistance state was characterized by two different slopes. Lastly, the paraconductivity was related to film degradation resulting from photoresist processing and time storage. Dimensionality changes were observed due to this degradation. (C) 1998 American Institute of Physics. [S0021-8979(98)09723-0].