Resonant elastic soft x-ray scattering in oxygen-ordered YBa2Cu3O6+δ.

Static charge-density-wave (CDW) and spin-density-wave (SDW) order has been convincingly observed in La-based cuprates for some time. However, more recently it has been suggested by quantum oscillation, transport, and thermodynamic measurements that density-wave order is generic to underdoped cuprates and plays a significant role in YBa2Cu3O6+δ (YBCO). We use resonant soft x-ray scattering at the Cu L and O K edges to search for evidence of density-wave order in ortho-II and ortho-VIII oxygen-ordered YBCO. We report a null result—no evidence for static CDW order—in both ortho-II and ortho-VIII ordered YBCO. While this does not rule out static CDW order in the CuO2 planes of YBCO, these measurements place limits on the parameter space (temperature, magnetic field, scattering vector) in which static CDW order may exist. In addition, we present a detailed analysis of the energy and polarization dependence of the ortho-II superstructure Bragg reflection [0.5 0 0] at the Cu L edge. The intensity of this peak, which is due to the valence modulations of Cu in the chain layer, is compared with calculations using atomic scattering form factors deduced from x-ray absorption measurements. The calculated energy and polarization dependence of the scattering intensity is shown to agree very well with the measurement, validating the approach and providing a framework for analyzing future resonant soft x-ray scattering measurements. [ABSTRACT FROM AUTHOR]