Modified role of oxygen in stressed yttrium barium copper oxide superconducting systems

This dissertation describes the modified role of oxygen content and order in stressed YBa2Cu3O7-delta superconducting systems. Materials or multilayer structures not comprised of optimally oxygenated, single crystal YBa2Cu3O7-delta are considered as stressed YBa2Cu3O7-delta systems. Cation doped thin films and thin film microstructures comprised of YSr2Cu2.75Mo0.25Oz and YBa 2(Cu1-xCOx)3Oz are the first category of stressed YBa2Cu3O7-delta systems examined. Three types of Josephson tunnel junctions were investigated; superconducting-normal layer-superconducting ramp edge junctions with cation doped YBa2Cu3O7-delta normal layers, grain boundary junctions, and interface engineered Josephson junctions where the interface layer between two superconducting electrodes is modified to yield a tunnel barrier. In all cases enhanced oxidation techniques including electromigration and ozone annealing were employed to produce previously unattained levels of oxygenation in each system. For cation doped materials this enhancement is demonstrated by an increase in the superconducting transition temperature, Tc, from 21 to 75 K for YBa2(Cu0.9Co 0.1)3Oz. Other electrical transport measurements, microRaman spectroscopy, and X-ray diffraction analysis also demonstrate the optimal level of oxygenation achieved by these two techniques. The enhancement of Tc for bulk film cation doped materials following ozone anneals at 500°C, or room temperature electromigration, translates into an elevation in operating temperature for ramp edge junctions utilizing doped materials as a normal metal layer. This is demonstrated by similar resistively shunted junction behavior observed at 35 K before ozone annealing, and at 80 K afterwards. In contrast, the changes induced in grain boundary and interface engineered Josephson junctions by enhanced oxidation are consistent with a simple increase in the effective area of the junction. This is attributed to the crystal stress in the vicinity of the grain boundary or engineered interface. Oxygen loss induced by this stress is overcome by the enhanced partial pressure of atomic oxygen provided by ozone and electromigration, thus increasing the volume of well oxygenated YBa2Cu3O 7-delta. In conclusion, we find that the optimal oxygenation of stressed YBa2Cu3O7-delta systems requires enhanced oxidation techniques beyond the standard 500°C O 2 anneals successfully employed for the parent material.