DIRECT OBSERVATION OF ATOMIC IMAGE OF HIGH Tc SUPERCONDUCTORS BY FIELD ION MICROSCOPY Q.J. GAO, Z.X. ZHANG, S.Y. XU, S.Q. FENG and S.Z. WANG J. Phys. Colloques, 50 C8 (1989) C8-487-C8-488 DOI: 10.1051/jphyscol:1989883