| Summary: | 博士 === 國立臺灣大學 === 物理學研究所 === 88 === A systematical study of the relation between the transport and structure property of YBa2Cu3Oy (YBCO) grain boundaries (GBs) has been performed.
Using the atomic force microscope (AFM), the island growth mechanism of the YBCO film is found to be responsible for the common meandering characteristic observed in the GB. Besides, the strikingly wandering of the GB is related to the defects embedded in the bicrystal fusion bicrystal substrate detected by the optical microscope. The spatial properties of the GB are correlated to the film properties, film growth mechanism and the quality of the bicrystal substrate.
Because of the nondestructivity of the AFM investigation, the electrical property of junction devices fabricated across the film GB can be correlated to the structure property directly. The half integer Shapiro steps, Ic-versus-B curves and the poor performance of the SQUID show evidence of the inhomogeneous current distribution along the grain boundary junction due to the meandering behavior. The IcRn-versus-T curve exhibits the character of superconductor-normal-material- superconductor (SNS) junction.
It is observed that the formation of the groove is substantially developing along the boundary interface with a width as the magnitude of the defects embedded in the bicrystal fusion. Grooved bicrystal substrates are used to study the correlation between the characteristics of the grooved fusion line and the properties of the film GB. It is found that the density of spirals growth along the boundary and the meandering configuration change, depending on the depth and steepness of the groove. Experimental characteristics of the junction devices show strikingly evidence that the grooved substrates affect the behavior of GB junction essentially. Besides, from the measured signals of the SQUIDs, it is concluded that, to a certain extent, the homogeneity of the GB can be improved by the underlying grooved substrate.
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