Superconducting electrodynamics of underdoped yttrium barium cuprate

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(Thesis) Ph.D.
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The cuprate high-temperature (high-Tc) superconductors remain an important open problem in physics, with currently no microscopic theory that allows calculation of their doping-temperature phase diagram from first principles. The difficulty stems from the importance of electronic correlations, which means that interactions between charge carriers cannot be treated in an average way. However, a good phenomenological understanding of the phase diagram has been developed, with the two most prominent phases being the antiferromagnetic Mott insulating state (AFM) of the undoped parent compound, and the d-wave high-Tc superconducting state. Microwave spectroscopy has been used to study the physics of YBa2Cu3O6+x in a range of dopings near the boundary between the AFM and the superconducting state. A special technique for continuously tuning hole doping in underdoped YBa2Cu3O6+x was developed. This takes advantage of the connection between CuO2-plane doping and the oxygen coordination number of chain Cu atoms. The experiments were performed on a high quality YBa2Cu3O6:333 single crystal, prepared by the U.B.C. superconductivity group, in which cation disorder is estimated to be at the 10^-5 to 10^-4 level. Microwave spectroscopy was performed using cavity perturbation of a dielectric resonator at 2.64 GHz, at temperatures ranging from 1 K to 150 K. Measurements of surface impedance were made at approximately 40 dopings for in-plane orientation and 13 dopings for c-axis orientation. Measurements have been used to obtain the doping dependant microwave conductivity and superfluid density. A model was developed to allow an accurate determination of intrinsic c-axis surface impedance in a finite-size spherical sample. The electrodynamic data have been used in a number of separate analyses, including: placing limits on the magnitude of various electronic orders that might be competing with pure d-wave superconductivity; a two-fluid analysis of the microwave conductivity to determine the quasiparticle relaxation rate; a scaling analysis of the doping-dependent superfluid density in the vicinity of the Tc -> 0 quantum critical point; a fluctuation analysis of the normal state paraconductivity; and a determination of the doping dependence of the d-wave gap magnitude, nodal gap slope, and charge-current renormalization factor of the d-wave quasiparticles.
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Thesis advisor: Broun, David
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