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|Title:||Anisotropy in the electrical, magnetic and optical properties of lead(2) strontium(2) R copper(3) oxygen(8)|
|Abstract:||<p>The layered structure of the cuprate superconductors leads to a significant anisotropy between ab-plane and c-axis properties which is significantly greater in Bi₂Sr₂CaCu₂O₈ (BSCCO) than in YBa₂Cu₃O_(7-x) (YBCO); likely due to differences in intercalary structure since both systems contain double CuO₂ layers. Pb₂Sr₂(Y/Ca)Cu₃O₈ (PSYCCO) is another system containing double CuO₂ planes which are separated by a PbO-CuO_δ-PbO sequence, the PbO bi-layer analogues to the double BiO sheets in BSCCO and the CuO_δ layer comparable to an oxygen depleted chain layer such as in YBa₂Cu₃O₆. The structural similarities render this material a good candidate for further study. Measurements of anisotropy in the dc-resistivity, magnetization and optical properties of PSYCCO are presented. The magnetization measurements are analyzed to extract both the in-plane and c-axis fundamental superconducting parameters of this material: Tc, the magnetic penetration depth, the coherence length, the upper and lower critical fields, and the upper critical field slope near Tc. The dc-resistivity is an order of magnitude larger than in both YBCO and BSCCO while the optically-derived plasma frequency is considerably smaller than in the other systems. Together with a large penetration depth this suggests that the carrier concentration is unusually small given the high transition temperature (≈ 80 K), and represents a significant deviation from a proposed universal relationship between level of doping and Tc. The extent of the anisotropy in this system is found to be similar to that in YBCO. The low carrier density has facilitated observation of unusual behaviour in the optical properties such as a large finite frequency-centered peak which develops in the optical conductivity upon entry into the superconducting state. The doping dependence of the ab-plane optical properties is investigated via the Ca-free Pb₂Sr₂RCu₃O₈ series, which exhibits a metal/insulator transition as a function of increasing ionic radius of the rare earth. A transfer of spectral weight from the CuO charge transfer band to lower frequencies accompanies the progression to metallic character. As this electronic background develops phonons in the far infrared reflectance of the insulating members become weaker while concurrently there is an appearance of new phonon modes in the 400-600 cm⁻¹ region of the spectrum. These phonons develop an antiresonant-like lineshape at higher doping levels. Examination of several high Tc superconductors with similar structure leads to the hypothesis that these features are due to the coupling of longitudinal c-axis phonons to the ab-plane mid infrared continuum. A surprising consequence, confirmed by experiment, is that due to momentum conservation the ab-properties are different when measured on the ab-plane face and along the a or b directions of a face containing the c-axis.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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