Verlag des Forschungszentrums Jülich
JUEL-3774
Einfeld, Jan
Verbesserung der mechanischen und hochfrequenten Eigenschaften von YBCO-Schichten mittels unterschiedlicher Defektarten
123 S., 2000
The impact of defects upon the mechanical and microwave properties of HTS films is examined. It is demonstrated that adequate microscopic defects (Y2O3 precipitates) can increase the mechanical stability and, thus, the critical thickness and, at the same time, substantially reduce the microwave surface resistance of HTS films. Finally, the impact of artificial defects upon the surface resistance is examined. All experimental results are explained in terms of classical theories.
YBa2Cu3O7-[delta] films with different size and density of Y2O3 precipitates were grown on LaAlO3 and sapphire by variation of the energy of the ions during sputter deposition. It is demonstrated that the temperature dependence of the microwave surface resistance Rs does not depend on the type of substrate material but on the density of the defects. Films grown at low ion energy (resulting in a low density of microscopic defects) show a characteristic shoulder in the Rs(T) curve which shifts to higher temperature and decreases in size with increasing energy of the ions (i.e. increasing density of microscopic defects). Temperature dependence and reduction of the surface resistance with increasing density of defects are explained in terms of the two-fluid model with thermally excited quasiparticles characterised by a Drude-shaped conductivity spectrum. Values for the scattering rates can be derived from the measurements of the surface resistance, which agree with the classical Matthiessen rule. The impurity scattering rate increases with increasing defect density. The experimental data and the theoretical model demonstrate that the surface resistance can be reduced by up to a factor of two over a wide temperature range. The reduction of the surface resistance is accompanied by an improvement of the mechanical properties of the HTS films which leads to an increased critical film thickness. Both properties, namely the increase of the critical thickness and the reduction of the microwave surface resistance, demonstrate the potential of microscopic defects for the improvement of HTS films for applications.
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