Characterization of oxide superconductors by means of fluctuation-conductivity analysis

在这项研究中，我们通过分析电导率σ '在Tc以上的波动来研究铜超导体的物理性质。首先，我们分析了含有杂质的YBa_2Cu_3O_7 （YBCO）的sigma′(T)特性。分析结果表明，含锌和含镍的YBCO体系的波动幅度均随杂质浓度的增加而减小，且含氧量较低的YBCO体系波动幅度更大。这表明非磁性杂质（锌）和磁性杂质（镍）降低了Tc，抑制了超导涨落。观察到的杂质效应与我们在BCS理论中所期望的不同，支持了负u模型作为高tc超导机制的有效性。接下来，我们对mocvd沉积的YBCO薄膜采用上述方法来表征薄膜质量。膜的Tc随热循环次数的增加而减小，因此波动幅度趋于较小。这表明薄膜的质量反映在电导率波动的行为上。最后，我们进行了x射线衍射和交流磁化率测量来表征薄膜。结果，我们发现通过重复热循环，衍射图的FWHM和chi的虚部都增加了，从而表明了这些参数之间的相关性。我们认为气的测量也为表征薄膜质量提供了一种有用的方法。根据建议，我们将继续研究交流磁化率表征和波动电导率分析。

In this study, we have investigated physical properties of cuprate superconductors by means of analyzing fluctuation conductivity sigma' above Tc. First, we have analyzed sigma' (T) characteristics in YBa_2Cu_3O_7 (YBCO) containing impurities. From the analysis, we have obtained that the fluctuation amplitude in both YBCO systems with zinc and nickel decreases with increasing impurity concentration, and that the result is more pronounced for the YBCO systems having less oxygen contents. This indicates that non-magnetic impurity (zinc) as well as magnetic impurity (nickle) decrease Tc and suppress the superconducting fluctuation. The observed impurity-effect is different from what we expect in the BCS theory, supporting the validity of negative-U model as a mechanism of high-Tc superconductivity.Next we have adopted the above-metioned method to MOCVD-deposited YBCO thin films in order to characterize the film quality. The Tc of the films decreases with increasing the number of thermal cycling, and consequently the fluctuation amplitude tends to be lower. This indicates that the film quality is reflected on the behavior of the fluctuation conductivity.Finally, we have carried out x-ray diffraction and ac susceptibility chi measurements to characterize the films. As a result, we have found that both the FWHM of the diffraction patters and the imaginary part of the chi increases by repeating the thermal cycling, thus indicating the correlation between these parameters. We suggest that the measurement of chi also gives a useful way of characterizing the film quality. In line with the suggestion, we will continue to study the ac susceptibility characterization as well as the fluctuation conductivity analysis.