Oxide high-temperature superconductors are presently being explored in order to extend superconducting electronics to the liquid-nitrogen temperature range. In this technology the Josephson junction is the key element. The project's aim is to develop a reliable and reproducible fabrication process for Josephson junctions of high-temperature superconductors. A further aim is to investigate the properties of such junctions by measurements and model simulations with the goal of finding applications in the area of superconducting electronics.
High temperature superconducting Josephson elements based on artificially generated, engineered grain boundaries are being fabricated and evaluated. Important issues are the controllability, reproducibility and feasibility of the fabrication process, as well as the reliability, application potential, performance limits and physics of the fabricated devices.
Engineered GBJs of the bicrystal and step edge type have been prepared using the lithographic, ion beam etching and film deposition facilities of the consortium. Furthermore, parallel and series arrays of bicrystal GBJs have been fabricated in a reproducible and controllable way. The GBJs were characterized with respect to their low frequency, high frequency, and noise properties. In addition, the spatial homogeneity of the electrical transport properties of GBJs has been analysed by low temperature scanning electron microscopy (LTSEM). Furthermore, a new method for the determination of the supercurrent correlation function has been developed.
In improving the fabrication process of bicrystal GBJs a spread of the junction parameter of less than 30% on the same substrate could be achieved. The noise temperature of bicrystal and step edge GBJs was found to be consistent with the sample temperature and junction resistance. Under certain conditions, large excess noise was found and was attributed to fluxon fluctuations. Using LTSEM flux states in bicrystal GBJs could be imaged. Based on parallel arrays of bicrystal GBJs magnetic field effect 3-terminal devices showing a current amplification of up to 5 have been fabricated.
APPROACH AND METHODS
Based on the highly promising results obtained very recently in the USA and Europe, the project's work concentrates on engineered grain boundaries in thin-film high-Tc superconductors as the elementary Josephson weak link. The complementary skills of the consortium members provide a unique opportunity to simultaneously pursue all four main fabrication principles presently conceivable for engineered grain boundary junctions. At the same time, the combined instrumentation available to all consortium members ensures a complete and identical characterisation of the electrical and microstructural junction properties.
Following the optimisation of the fabrication process for the grain boundary Josephson junctions, the consortium will design a Josephson mixer and a flux flow amplifier. The performance of both devices will be quantitatively evaluated.
The reliable and reproducible fabrication of high-Tc grain boundary Josephson junctions operating in the liquid-nitrogen temperature range will have a strong impact on extending superconducting electronics to the higher temperature regime. Here the successful demonstration of the Josephson mixer and flux-flow amplifier opens the door to important high-frequency applications such as radio astronomy, satellite communication, and possibly ground communication as well.
G1 IXQ Glasgow
84084 Fisciano Salerno