Work Area: High-Temperature Superconductivity related to Low-Current Applications
Keywords microwave properties, HTSC, microwave devices
Start Date: 1 September 92 / Duration: 36 months / Status: running
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Abstract SUPERMICA aims to gain a better understanding of microwave dissipation in epitaxially grown high-Tc superconducting films. Several promising deposition techniques and structural and electromagnetic characterisation methods will be used to improve the microstructure and reduce the microwave losses of YBa2Cu3O7-X films. Both standard and novel devices made from such optimised films will demonstrate the usefulness of this material for future microwave and electronic applications.
Passive microwave components were among the first practical devices fabricated from high-Tc superconductors. Compared to classical superconductors, however, high-Tc superconductors still yield several orders of magnitude higher residual losses, especially at elevated microwave field levels. SUPERMICA aims to identify the sources of these anomalously high microwave losses, minimise them using different fabrication techniques, and demonstrate an improved performance of standard and novel devices.
At present, the most advanced techniques for the deposition of high quality epitaxial YBa2Cu3O7-X thin films onto small planar substrates are laser ablation and sputtering from stoichiometric targets, and chemical or physical co-evaporation from organometallic or metallic sources. Comparative studies on films produced by these techniques with systematically varied preparation conditions will be pursued to reveal trends between the observed structural and electromagnetic properties, with the aim of establishing causal relationships and learning to make an optimised material. Emphasis will be placed on reproducible deposition of high-quality films on substrates suitable for microwave devices. The crystallographic quality of the films will be characterised by X-ray diffraction, ion channelling, and for exceptionally good films, high-resolution transmission electron microscopy. The composition and topography of the film surface will be monitored by Rutherford backscattering, Auger electron spectroscopy and SE/ST microscopy. Electric and magneto-transport properties will also be studied, mainly in the superconducting but also in the normal conducting state. Emphasis will be given to DC magnetic studies to clarifying the correlation between high critical current densities due to impurity or defect-induced pinning, and low residual microwave losses requiring more homogeneous material. Both linear and non-linear microwave dissipation will be studied using calibrated surface impedance measurements and low magnetic field modulated microwave absorption. Patterned passive microwave devices will be produced from the best films. While coplanar resonators will serve for testing of film deposition and patterning techniques, more complex novel structures will be investigated for advanced signal processing.
YBa2Cu3O7-X films of high structural quality have been epitaxially grown on single crystalline MgO and/or LaAlO3 substrates by all four deposition techniques. Transition temperatures between 85 K and 92 K, transition width of less than 1K, and critical current densities above 106 A/cm2 at 77K have been measured inductively.
The surface impedance of these films has been measured at 87 GHz and 19 GHz. While there is only little scatter of the data for the surface resistance above 60 K and the penetration depth, the residual losses depend strongly on the oxygen content and order in the CuO chains and weakly on the cation stoichiometry.
The magnetic properties and the harmonic generation in field modulated microwave absorption of selected films have been studied comparatively. The magnetoresistance, the irreversibility line and the harmonic generation depend on the film quality and thickness.
Coplanar resonators have been patterned by photolithography from some of these films. Quality factors in excess of 40000 at 15 K and 8 GHz up to microwave fields of 104 A/m have been achieved in the best case. However, we observe at high fields a much stronger dependence on the film quality than at low fields.
SUPERMICA's work will lead to an improved understanding of the residual microwave losses of high-Tc superconducting films and their correlation with structural and surface imperfections, as well as of their current-carrying capability and magnetic properties. This will help develop the technology of making fully optimised YBa2Cu3O7-X films for microwave applications, eg in communication systems.
The results of SUPERMICA are being published in regular Journals and/or reported as invited or contributed papers to the following international conferences: ASC'92 (Chicago), E-MRS'92 (Strasbourg), MOS-satellite and LT20 (Eugene), EUCAS (Göttingen), MRS'93 (Boston).
Bergische Universität Gesamthochschule Wuppertal - D
Postfach 10 01 27
D - 42119 WUPPERTAL
Universität Augsburg - D
Institut Polytechnique de Grenoble - F
CNRS - LEPES - F
LNETI - P
University of Birmingham - UK
Defense Research Agency - UK
Dr. G. Müller
SUPERMICA - 6113, August 1994
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html version of synopsis by Nick Cook