Oxygen chain disorder as the weak scattering source in YBa2Cu3O6.50

The microwave conductivity of ultrapure single crystals of YBa2Cu3O6.5 has been measured deep in the superconducting state as a continuous function of frequency using a broadband spectrometer. 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 Cu-O chain oxygens are ordered into alternating full and empty chains. These spectra exhibit features expected for quasiparticle scattering from dilute weak impurities (small scattering phase shift) in a 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 Cu-O chains. The disordered conductivity spectra retained the weak-limit scattering features. However, the spectra were broadened due to increased quasiparticle scattering which we have identified as increased scattering from disorder in the Cu-O chains. The quasiparticle conductivity spectral widths of a new generation of ultrahigh-purity ortho-II ordered YBa2Cu3O6.5 samples, with an order of magnitude lower cation impurity concentration, were also measured and found to be unchanged from those in the older generation samples. It is concluded for both generations of ortho-II YBa2Cu3O6.5 crystals that the widths of the quasiparticle conductivity spectra are largely determined by residual disorder in the Cu-O chains.