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Start of funding 01.01.2001

Intrinsic limits in the application of high-temperature superconductors as compact low-power microwave devices

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Thin film high-Tc superconducting circuits offer the promise of providing compact microwave resonators, filters and delay lines. The low surface resistance of these materials at liquid nitrogen temperatures allows the design of compact, high “quality” (described by a quality factor Q) circuits, increasing the performance and reducing the size, compared to conventional devices. In recent years, with improved materials and designs, there has been a steady improvement in device performance, notably an increase in resonator Q and a decrease in the nonlinear intermodulation. It is important to understand how much improvement can be expected. In the cooperation the intrinsic limiting behavior that one might achieve with a perfect film and what the limiting behaviour implies for HTSC filters, will be derived.

Final report:
Thin film high-T_c superconducting circuits offer the promise of providing compact microwave resonators, filters and delay lines. The low surface resistance of these materials at liquid nitrogen temperatures, i.e. around the superconducting transition temperatures, allows the design of compact, “high quality” circuits, increasing the performance and reducing the size, compared to conventional devices.

In the co-operation between Prof. Dr. D. J. Scalapino, University of California, Santa Barbara and the German contact person Prof. Dr. W. Hanke, Institute for Theoretical Physics and Astrophysics, University of Würzburg the intrinsic limiting behavior that one might achieve with a perfect film and what the limiting behaviour implies for HTSC filters, have been explored and studied.

In order to replace the presently more or less entirely empirical search for significantly improved low-power rf –filters etc., by a systematic scheme, the physics of the HTSC at and around superconducting transition temperatures has been studied.

In particular, we have investigated the influence of the so-called pair-phase fluctuations on the electronic and superconducting properties of the Copper oxide high-temperature materials. These phase fluctuations play no role in the conventional low temperature superconductors, but acquire a dominant influence on the electronic and optical properties between the superconducting transition temperature T_c (typically 100° Kelvin) and a much higher temperature, called mean-field transition temperature (around several hundred degree Kelvin). This regime is called pseudo gap regime, since it exhibits a superconducting-like gap in the electronic excitations even above T_c. This pseudo gap behavior determines crucially the intrinsic limits, which one might achieve with a perfect film and also the limiting behavior of the HTS filters.

I would like to use the opportunity to thank the Bavaria California Technology Center for their generous travel support.