Detection of oxygen ordering in superconducting cuprates

The recent discovery of superconductivity in ceramic cuprates¹ continues to lead to spectacular increases in the critical temperature. Results from several neutron diffraction experiments have shown that the system YBa ₂Cu₃O₇ displays long-range order in the disposition of its oxygen vacancies, giving rise to two- and one-dimensional features in the structure^2,3, which may have important electronic implications^4,5. Although diffraction techniques have proved invaluable in determining the overall structure, the local structure should also be investigated on a microscopic level. Because of the relatively small scattering power of the light oxygen atoms, lattice images obtained from very thin films of the new superconductors are insensitive to the precise arrangement of the oxygen vacancies. It is known however, that due to dynamical effects, the thickness dependence of lattice images can be extremely sensitive to small effects, such as ionicity⁶. This can be exploited to detect the presence of long-range order in the oxygen vacancies⁷. Other investigators have produced experimental⁸ or simulated⁹ images, which also purport to provide evidence for the presence or detectability of order. Indications of superconductivity at very high temperatures in multiphase materials¹⁰, and the potential importance of determining the oxygen arrangement in small regions of a multiphase sample, mean that it is important to establish in detail the conditions needed for a reliable microscopic investigation of the presence of long-range order in these materials. Here we present theoretical and experimental evidence that, under a carefully chosen and controlled set of experimental parameters, it is possible to detect the presence of long-range order in the disposition of the oxygen vacancies by high-resolution transmission electron microscopy.