Local oxygen ordering in superconducting YBa₂Cu₃0_6.4 observed by neutron diffraction

Abstract

THE superconducting transition temperature, T_c, in the YBa₂Cu₃O_6+x system is known to depend not only on the oxygen stoichiometry¹, but also on the degree of oxygen order^2,3. Observations of oxygen ordering structures by electron microscopy^4,5 have led to the suggestion that the '60-K plateau' near x = 0.5 in the curve of T_c versus x arises from an ordered structure (the 'ortho-II phase') in which Cu–O and Cu chains alternate in the basal plane⁶. Until now, however, the ortho-II phase has been seen only by electron diffraction, except for one observation at an oxygen stoichiometry of x = 6.7 by X-ray diffraction⁷. Compared with true bulk methods such as X-ray and neutron diffraction, electron diffraction suffers from the disadvantage that only a very small portion of the sample is probed. Here we report the observation, by neutron diffraction, of local ortho-II ordering in an YBa₂Cu₃O_6.4 crystal (T_c = 38 K). The observed correlation lengths along the principal axes are 24b (along the chains), 10a and 2c. Our observation of the existence of the ortho-II phase for an oxygen stoichiometry close to the minimum for which superconductivity is observed (x≈0.4) supports existing theoretical models^8,9, in particular one⁸ that relates T_c to the presence of two types of ordered domain.