Oxygen ordering and the 60 K plateau in the ortho-II phase of YBa2Cu3O6+x (0.48 <= x <= 0.62)

A simple method for sample preparation based on copper oxide Cu2O acting as an oxygen getter is presented, which results in very well-ordered ortho-II phases in both polycrystalline and single-crystal samples of the high T-c superconducting compound YBa2Cu3O6+x (0.48 less than or equal to x less tha n or equal to 0.62). It comes out from our measurements of the superstructu re correlation lengths and the critical temperature in samples with various degrees of oxygen ordering that the 60 K plateau behaviour of the T-c vs. x dependence is clearly related to the degree and quality of short-range ox ygen ordering into the ortho-ii phase. The best sample exhibits correlation lengths up to 70(2) unit cells along the a-axis and 5(1) unit cells along the c-axis, to be compared to approximate to 10 and approximate to 2 unit c ells, respectively, for the best ones published to date. The superstructure peaks are resolution-limited in the k direction. (C) 2000 Elsevier Science B.V. All rights reserved.