A nanometre-scale non-periodic structural variation in high temperature superconducting ceramics and the implications for properties

Non-periodic structural variation has been found in the high T-c cuprates, YBa2Cu3O7-x and Hg0.67Pb0.33Ba2Ca2Cu3O8+delta, by image analysis of high re solution transmission electron microscope (HRTEM) images. We use two method s for analysis of the HRTEM images. The first method is a means for measuri ng the bending of lattice fringes at twin planes. The second method is a lo w-pass filter technique which enhances information contained by diffuse-sca ttered electrons and reveals what appears to be an interference effect betw een domains of differing lattice parameter in the top and bottom of the thi n foil. We believe that these methods of image analysis could be usefully a pplied to the many thousands of HRTEM images that have been collected by ot her workers in the high temperature superconductor field. This work provide s direct structural evidence for phase separation in high T-c cuprates, and gives support to recent stripes models that have been proposed to explain various angle resolved photoelectron spectroscopy and nuclear magnetic reso nance data. We believe that the structural variation is a response to an op ening of an electronic solubility gap where holes are not uniformly distrib uted in the material but are confined to metallic stripes. Optimum doping m ay occur as a consequence of the diffuse boundaries between stripes which a rise from spinodal decomposition. Theoretical ideas about the high T-c cupr ates which treat the cuprates as homogeneous may need to be modified in ord er to take account of this type of structural variation.