Pavel Diko

STM study of YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+° cleaved surfaces

Abstract Room-temperature scanning tunneling microscopy has been used to study the cleavage process in YBa 2 Cu 3 O 7-° and Bi 2 Sr 2 CaCu 2 O 8+δ single crystals. Many stepped structures were observed on their cleaved surfaces, and close examination showed that some steps were smaller than the unit cell in the case of YBa 2 Cu 3 O 7-δ and then one half of the unit cell for Bi 2 Sr 2 CaCu 2 O 8+δ . We believe that the steps are related to collisions of cleavage fronts with intergrowth defects or stacking faults which are typical in layered structures.

Effect of zinc and iron on the (micro-)structure and copper charge excess of the YBaCuO superconductor

Samples of YBa2(Cu1−yZny)3O7−y and YBa2(Cu1−yFey)3O7+x, withy in the range 0.0–0.16 for zinc and 0.0–0.30 for iron were synthesized by solid-state reaction. The solubility limit has been found to be equal to 7% and 19 at % for zinc and iron, respectively. Zinc has little effect on the structure which remains orthorhombic throughout all the zinc concentration range, while iron induces an important structural modification (tweed structure formation). Both impurities induce grain-growth inhibition but densification appears to be independent of the impurity content with respect to the undoped material. However, iodometry shows that the oxygen stoichiometry decreases in zinc-doped samples while it increases in iron-doped samples. Charge balance resulting from the dopant charge and the evolution of the copper charge with doping have been invoked. In zinc-doped samples, the copper charge excess (copper charge fraction>2) decreases sharply for 0.0⩽y<0.04, then it shows a plateau-like behaviour for 0.04⩽y<0.06, while in iron-doped samples, it decreases almost monotonically. Confirming some of our previous results there is a correlatedTc decrease in the case of zinc-doped samples (occurrence of aTc plateau) and in the case of iron-doped samples (quasi monotonical decrease). This difference has been interpreted in terms of structural changes related to the different substitution behaviour of zinc and iron.

Microstructure and cracking in pure and substituted YBa2Cu3O7−δ

Abstract The cracking phenomena in pure and iron-doped YBa 2 Cu 3 O 7−δ ceramics and single crystals were studied in relation to microstructure using optical microscopy with normal and polarized light. The cracking in single crystals and bulk ceramics owing to the volume change arising from the tetragonal-orthorhombic transformation and from the anisotropy of the thermal expansion coefficient, is shown to be very sensitive to the oxygenation conditions, the grain size and the doping. Possible stress relaxation by the creation of new twins and twin boundary motion is discussed.