Superconducting YBCO single crystals with macroscopically tetragonal phase

We have carried out a series of investigations in an attempt to demonstrate the existence of tweed tetragonal phase in oxygen-rich rich pure YBCO_7-δ (δ<0.6). A technique for producing such a phase was developed which involved heating the fully oxygenated orthorhombic crystal to just above its orthorhombic-tetragonal phase transition temperature for a short period in a high-oxygen-partial-pressure environment, followed by rapid quenching to room temperature. X-ray rocking curves and electron microscopy studies provided the evidence of the tweed tetragonal phase, and AC susceptibility measurements confirmed these crystals to be superconducting with a sharp transition.