Oxygen Chain Disorder as the Weak Scattering Source in YBa$_2$Cu$_3$O$_{6.50}$

TL;DR

This study measures microwave conductivity in ultra-pure YBa₂Cu₃O₆.₅ to identify oxygen chain disorder as the primary source of quasiparticle scattering in the superconducting state.

Contribution

It demonstrates that oxygen chain disorder is the dominant weak scattering source affecting quasiparticle behavior in high-purity YBCO.

Findings

Oxygen disorder increases quasiparticle scattering widths.

Weak-limit scattering features persist despite increased disorder.

Oxygen chain disorder is confirmed as the main scattering source.

Abstract

The microwave conductivity of an ultra-pure single crystal of YBa$_2$Cu$_3$O$_{6.50}$ has been measured deep in the superconducting state as a continuous function of frequency from $0.5\to20$ 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.