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Abstract

The rich phenomenology displayed by the high temperature (high-Tc) cuprate superconductors has attracted intense experimental and theoretical attention for nearly thirty years. Despite steady and continued progress, a complete and consistent microscopic theory of the cuprates continues to elude researchers. However, recent work appears to be converging on a picture of separate spin and charge order phase transitions – well below and slightly above optimal doping, respectively – along with associated Fermi surface reconstruction. As sensitive probes of the low-energy electrodynamics, microwave conductivity techniques are well-suited for characterizing the effects of such changes in electronic structure. For YBa₂Cu₃0₆₊x (YBCO), one of the cleanest and best-studied high-Tc cuprate superconductors, previous measurements had focused on a relatively sparse set of dopings. In this thesis, detailed measurements of the doping dependence of both the microwave penetration depth and surface resistance of YBCO are combined with low-energy muon spin rotation measurements of the absolute magnetic penetration depth to produce a comprehensive doping dependence survey of the microwave conductivity, spanning a wide range of oxygen contents between 6+x = 6.49 and 6.998. The temperature derivatives of the magnetic penetration depth continue to decrease at the highest dopings, challenging previous predictions of a peak in penetration depth near the proposed quantum critical point in the overdoped regime. Additionally, a sudden increase in the a–b anisotropy of low-temperature slopes was observed near optimal doping, suggesting the possibility of additional changes in electronic behaviour in this region. Microwave surface resistance measurements revealed a sharp increase in the scattering rate – along the a axis only – for samples near 1/8th hole doping. Surprisingly, the a-axis scattering rate was discovered to decrease – not increase – after disordering the CuO chain layer oxygen configuration. The short a-axis correlation lengths in this doping range, combined with the strong scattering potential produced by chain order domain boundaries, are proposed as an explanation for this counterintuitive behaviour.