Single crystals

This paper concentrates on YBa2Cu3O- x. This has been grown from melts that contain Y2O3, BaO or Ba(OH)2 and CuO. Most often, the mixtures are melted in gold---although Pt, ZrO2 and Al2O3 have been used successfully under one atmosphere of oxygen and slowly cooled from around 970 C, with a fast-cooling step from 400 C. When this procedure is carried out in standard crucibles, the resulting product consists of nearly equiaxed crystals about 1--2 mm on a side. An annealing step in oxygen at 400--500 C yields crystals with Tc's around 94 K. Some physical or chemical separation of the matrix is generally necessary, although growers reduce the bulk of the molten flux by clever physical arrangements within the crucibles. Many large two dimensional crystals have been grown on sheets of gold, most often with compositions near the eutectic