Microwave flux-ow impedance measurements of Type-II superconductors

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The electrodynamic properties of type-II superconductors have been studied in strong magnetic elds using microwave spectroscopy to access the ux- ow regime of vortex motion. The measurements give insight into two important physical quantities: ux- ow resistivity, which is intimately related to dissipation from electronic states near the vortex cores; and pinning forces, from the collective interactions of the vortex lattice with material defects. A notable aspect of this work is the ability to accurately separate contributions from viscous and elastic forces. This has been made possible by the development of microwave apparatus that allows the separate measurement of in-phase and out-of- phase components of surface impedance, and through the use of well-controlled, high quality samples. Measurements on conventional superconducting systems Nb, NbSe2 and V3Si have been used to test both the experimental technique and the subsequent data analysis, with the results in good accord with established data on these materials. In addi- tion, some interesting new features have been observed, likely associated with the multiband nature of these systems. The primary focus of this work is on the ux- ow dynamics of cuprate high temperature superconductors. Two material systems, YBa2Cu3O6+x and Tl2Ba2CuO6+x, have been used to carry out measurements that span the entire superconducting region of the cuprate phase diagram. For each sample, the pinning force constant and ux- ow resistivity have been extracted across the superconducting temperature range. As has been previously reported by our group, there is an anomalous logarithmic up- turn in ux- ow resistivity at low temperatures, previously attributed to localization physics. The main discovery of this work is that the logarithmic behaviour persists across the phase diagram, even in highly overdoped Tl2Ba2CuO6+x samples in which the normal state is known to be a metallic, conventional Fermi liquid. This suggests,for the frst time, that the resistivity upturns in cuprates are in fact intimately con- nected to the presence of vortices, which in turn has strong implications for the nature of the underdoped cuprate normal state. i
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