Imaging Spatial Ordering of the Oxygen Chains in YBa2Cu3O6+y at the Insulator-to-Metal Transition

It is known that the mobile oxygen ions, y, in the basal plane of YBa2Cu3O6+y (0.33<y<0.67) form oxygen chains needed to create the metallic phase in the CuO2 layers. Here we visualize the spatial organization of oxygen chains in a crystal of YBa2Cu3O6+y very close to the insulator-to-superconductor transition with y=0.33 (T c =7 K). The distribution of oxygen defects chains has been obtained by performing scanning micro X-ray diffraction measurements. This experiment provides mixed real and reciprocal space information. We found a granular spatial pattern due to the oxygen chains being segregated in nanoscale puddles with ortho-II crystallographic structure embedded in an insulating matrix of disordered oxygen ions.