UBC Theses and Dissertations
New directions in microwave spectroscopy of high temperature superconductors Musselman, Kevin Philip Duncan
Two new variations on traditional microwave resonant cavity surface resistance measurements of high temperature superconductors are presented. A new cavity perturbation technique is developed for the measurement of T1₂Ba₂CuO₆±[delta] (Tl-2201) single crystals, and preliminary measurements are presented for uniquely prepared YBa₂Cu₃OD₆₊̮̮y single crystals in which a non-local normal fluid response is expected. The resonant cavity probe designed for measurement of T1-2201 single crystals operates in the TMolo mode such that the sample is located at a magnetic field node and electric field antinode. A dual-sapphire hot-finger technique was developed for supporting the sample in the electric field of the cavity, and non-perturbative issues associated with this technique were addressed. A dielectric layer resulting from the degradation of sample outer surfaces was identified as a significant source of dielectric loss. The dimensional dependence of this loss was treated theoretically and etching was shown to drastically reduce this unwanted absorption. The factor currently limiting use of this probe to measure T1-2201 single crystals is absorption by the optical-grade sapphire used to support the sample in the cavity. The loss due to the sapphire is currently on the same order of magnitude as the sample loss to be measured. Employment of premium-grade sapphire plates should significantly reduce this background level and permit accurate measurement of the surface resistance of single crystal T1-2201. YBaD₂Cu₃O₆+y, single crystals have been meticulously prepared with large ;-axis dimensions. This has permitted microwave surface resistance measurements where a magnetic field is applied perpendicular to the CuOz planes, while minimizing demagnetization effects. In this geometry, the electrodynamic response of the normal fluid is expected to be non-local, as the mean free path of the quasiparticles exceeds the penetration depth. R₃(T) measurements were obtained for the crystals using a resonant cavity operating in the TE₀₁₁ mode at 13.4 GHz. No increase in the surface resistance, which is expected for a non-local quasiparticle response, was observed at low temperatures. Possible explanations for this result, and recommendations for further study of these samples are discussed.
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