YBa2Cu3O7-δ Biepitaxial and Step Edge Josephson Junctions on MgO Substrates. SQUID Amplifier

Abstract: This thesis concerns the preparation of Josephson junctions and their applications in Superconducting Quantum Interference Devices (SQUIDs). High temperature superconductors were used for operation at 77 K, the boiling temperature of liquid nitrogen. To perform the study, high quality superconducting films had to be deposited and low noise junctions had to be prepared. This thesis treats the optimization process for the preparation of superconducting films, Josephson junctions and dc SQUIDs. The preparation of YBa2Cu3O7 (YBaCuO), high-TC superconducting films has been investigated. The optimization of the deposition parameters was performed by using the pulsed laser deposition technique. The superconducting films were prepared on (100)MgO and r-cut sapphire substrates. It has been shown that an annealing process of the MgO substrates at 950 °C, prior to the deposition of YBaCuO, improved substantially the reproducibility of the experiments. The deposition of a thin (10 nm thick) homoepitaxial MgO buffer layer improved the epitaxy of the YBaCuO thin films. Under certain deposition parameters, the application of CeO2 buffer layers improved the epitaxy of the YBaCuO layers on r-cut sapphire. Critical current density values of 1-2x106 A/cm2 have been measured at 77 K in microbridges prepared on MgO and CeO2-buffered sapphire substrates. MgO substrates have been covered with an structured photoresist mask and shortly (