Critical-current measurements in epitaxial films ofYBa2Cu3

Abstract

We have grown epitaxial films of the YBa2Cu307 -,compound on SrTi03 substrates. The superconducting critical current in these films at 77 K is in excess of 10 A/cm and at 4.2 K in excess of 10 A/cm'. PACS numbers: 74.70.Ya, 74.60.Jg Over the last fifteen months the superconducting transition temperature has increased by a factor of 4 to approximately 100 K. This development was initiated by the remarkable observations of Bednorz and Muller, ' who showed that in La2, Ba (Sr,Ca)Cu409, with the K2NiF4 structure, the superconducting transition temperature was in the range of 30-40 K. Shortly after these results were reproduced in a number of laboratories, Chu and his associates showed that the application of pressure increased the transition temperature to 52 K and, furthermore, in the Y-Ba-Cu-0 systems, transition temperatures as high as 100 K could be obtained. Over the last few months a very large number of publications have appeared addressing various aspects of superconductivity in these materials.One of the weak points that has often been cited about this class of superconducting materials is their small critical current. Although a number of publications have appeared on this subject, the value of the critical current is still small relative to the more familiar metallic alloys. For example, the highest published value at 77 K known to these authors is approximately 10 A/cm .The range of potential utility of these materials is determined by the critical current. We have prepared epitaxial films of these materials and find that the critical current at 4.2 K is in excess of 10 A/cm and at 77 K is in excess 10 A/cm . The value of 4.2 K compares favorably with some of the best metallic alloys and we believe it can be increased even further. Epitaxial films with a nominal composition of YBa2CU307, -were prepared by the evaporating of Y, Ba, and Cu from three separate electron-beam sources in a vacuum of approximately 10 to 10 Torr comprised primarily of 02. The substrate was SrTi03 with a [100] direction perpendicular to the plane of deposition. During deposition the substrate was kept atapproximately 400 C. The procedure for our making these films is similar to that described earlier. ' As deposited, the films showed a broad peak in the x ray diAraction pattern characteristic of a highly disordered or an amorphous atomic arrangement. These films were annealed at approximately 900 C in 02 to obtain epitaxial films. This was confirmed both by x-ray diA'raction and electron-microscopy examination. The c axis of the orthorhombic structure was primarily perpendicular to the plane of the film. However, a fraction of the film had crystalline orientation with the c axis in the plane of the film. In addition to the orthorhombic structure there were trace amounts of a second phase present. The details of the film structure will be described in later publications. Figure I shows the typical change in resistance with temperature of our samples. Perhaps the most notable feature of these data is the relatively sharp transiti...

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