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Studies of the flux flow resistivity in YBa₂Cu₃O₆ 95 by microwave techniques Morgan, David Craig


Measurements of the microwave surface resistance at 5.4, 27 and 35 GHz on a high quality single crystal of YBa₂Cu₃O₆.₉₅ in magnetic fields up to 8T (applied parallel to the c-axis) and at temperatures from 20K to ]00K are presented. The Coffey-Clem expression for the surface impedance of a superconductor in the mixed state is used to fit the data in terms of a pinning frequency and a free flux flow resistivity (thermal hopping of vortices is not included since it is not important for YBa₂Cu₃O₆.₉₅ at microwave frequencies). A temperature dependent pinning frequency is found that varies from ~20 GHz at 20K to effectively zero by 80K. The flux flow resistivity is strongly temperature dependent and consistent, down to at least 50K, with the Bardeen-Stephen expression pff/pn (T) = H/Hc₂(T) if an extrapolation of the linear normal state resistivity from above Tc is used for pn (T). This suggests that the traditional picture of a vortex core as a cylinder of normal material may be valid in these materials. In contrast, the normal fluid quasiparticle scattering rate, as determined by zero field microwave measurements on the same single crystals¹, drops precipitously below Tc and, thus, is probably not the relevant scattering rate for the charge carriers in the vortex core.

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