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Microwave Study of the Mixed State and Superconducting Fluctuations in YBa_2Cu_3O_7 Single Crystals

In this Chapter we focus on some recent progress made in the study of field dependent microwave absorption in high-T_c superconductors. The prerequisite to this study was the formulation of an effective microwave conductivity in the mixed state. We present an outline of the theoretical models. The derived expression for the effective microwave conductivity has correct limits in zero field, and in the transition to the normal state. We give an example of temperature and field dependent microwave absorption signals in YBCO single crystals. In earlier studies, the microwave absorption in the mixed state could be understood only qualitatively. Now, it has become possible to use it as a method to determine some quantities which characterize the superconducting state of the sample. We describe how the upper critical field B_c2, and the mean field T_c can be determined. The fluctuation conductivity is evaluated, and its scaling laws are examined.

superconductivity, upper critical field, microwave absorption, fluctuations

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