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A tale of two dopings : the microwave charge conductivity of YBa2Cu3O6.5 (underdoped) and YBa2Cu3O6.993 (overdoped) in the superconducting state Harris, Richard Graydon


The microwave charge conductivity of the high temperature superconducting cuprate YBa2Cu306+x for currents flowing parallel to the ab oriented (CuO)2- planes has been investigated experimentally at two particular oxygen concentrations x: Ortho- II ordered YBa2Cu306.5 (underdoped, Tc = 60 K) and YBa2Cu306.993 (overdoped, Tc = 88 K). Measurements were performed by several members of the University of British Columbia Superconductivity Laboratory using a nonresonant broadband (0.3 —> 22.5 GHz) bolometry technique and six different microwave cavity perturbation experiments operating at discrete frequencies (1.14, 2.25, 2.99, 13.4, 22.7 and 75.4 GHz). The data were then analyzed within a phenomenological electronic band-structure approach. The observation of sharp spectral features at low temperatures and the successful tracking of spectral weight as it is transferred between superfluid and normal fluid both indicate that the materials studied herein are clean (relatively defect-free) d-wave superconductors with well defined quasiparticles. However, an anomalous amount of quasiparticle oscillator strength survives to low temperatures for both of the oxygen concentrations studied. It is demonstrated that this so called 'residual oscillator strength' cannot be explained by the presence of dilute Coulomb defects in the crystalline lattice.

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