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The microwave conductivity of an ultra-pure single crystal of YBa2Cu3O6.50 has been measured deep in the superconducting state as a continuous function of frequency from 0.5 → 20 GHz. Conductivity spectra were first measured at four temperatures below 10 K after having prepared the crystal in the so called ortho-II phase in which the CuO chain oxygen are ordered into alternating full and empty chains. These spectra exhibit features expected for quasiparticle scattering from dilute weak impurities (small scattering phaseshift) in an otherwise clean d-wave superconductor. The measurements were repeated on the same crystal after heating and then rapidly quenching the sample to reduce the degree of oxygen order in the CuO chains. With the increased disorder, the conductivity spectra retain the distinctive weak-limit scattering features, but have increased widths reflecting an increase in quasiparticle scattering. These measurements unambiguously establish that CuO chain oxygen disorder is the dominant source of in-plane quasiparticle scattering in high purity YBCO.

The fictive temperature of vitreous silica containing approximately 900 wt ppm of hydroxyl groups was monitored with small angle x-ray scattering. The measurements were carried out during annealing and while scanning the temperature, with annealing temperatures ranging between 930 and 1330 K. Fitting the data to the Adam-Gibbs-Fulcher equation by using the Tool-Narayanaswamy method yields a particularly simple thermorheological behavior for type-III vitreous silica. Unlike the general case for glass kinetics, including vitreous silica with low hydroxyl content, the relaxation time constant is nearly decoupled from the fictive temperature. This high degree of decoupling of the state of the glass and the relaxation rate agrees with the results of viscosity measurements. By improving the data analysis procedure, we have significantly increased the precision of the results, and it was possible to resolve changes of the activation energy of the relaxation processes to within 0.5%. This has made sample aging effects that had previously been undetectable visible