Growth of YBCO single crystals by the self-flux technique

Abstract

Preparation of high purity, highly perfect and homogeneous YBa2Cu3O6+ δ (YBCO) single crystals is a lengthy procedure that consists of five major steps. They are (a) fabrication of BaZrO3 ceramic crucibles, (b) self-flux growth of YBCO crystals using BaZrO3 crucibles, (c) setting of the oxygen content in the crystals, (d) removal of twins and homogenisation of oxygen content, and (e) formation of oxygen vacancy ordered superstructures by low temperature annealing. To obtain BaZrO3 ceramic impervious to the BaO–CuO melt, the volume of the grain boundary glass phase must be reduced to a very low level through the use of high purity starting materials and precise BaO:ZrO mole ratio control. The best quality YBCO crystals are obtained by slow cooling of YO1.5–BaO–CuO melt in the primary crystallisation region of YBCO. Oxygen content in heavily twinned orthorhombic YBCO is inhomogeneous due to the stress caused by twins. Therefore, homogenisation annealing must be carried out after removal of twin boundaries or, alternatively, under conditions where YBCO is tetragonal. In high purity YBCO, randomly distributed oxygen vacancies are the main source of charge carrier scattering. However, ordered superstructures of oxygen vacancies with significantly lower scattering rates can be generated by careful annealing at low temperatures.