New synchrotron X-ray diffraction results on YBCO have led to a reexamination of the role of oxygen ordering in this, the most famous high-Tc superconductor. Satellite reflections observed in both under-doped and optimally-doped detwinned YBCO crystals were originally interpreted as due to charge density waves, but a simpler and more realistic explanation can be given in terms of quasi-periodic atomic displacements resulting from oxygen ordering in the Cu-O planes. To that end, we performed first principles fully relaxed total energy calculations in oxygen deficient YBCO supercells. Calculated displacements of Y, Ba, Cu, and O atoms were then inserted into structure factor calculations which agreed very well with experimentally determined diffraction scans along the H direction in diffraction space. Temperature effects were studied by Monte Carlo simulation of an Ising model of oxygen hopping on the Cu-O planede.
E-mail: ddf@uclink.berkeley.edu