Oxygenation and deoxygenation methods for producing sharp 90 K and 60 K superconducting transitions in single crystal YBa2Cu3O6+x

Several oxygenation methods were developed for converting 90 K YBa2Cu3O6+x single crystals to 60 K single crystals. The temperature dependence of the dc magnetic susceptibility shows sharp superconducting transitions in the 90 K crystals and in 60 K crystals produced by the best method, with optimized parameters. Meissner fractions in the 60 K crystals are consistently lower, indicating, they believe, more flux pinning in them. Optical microscopy indicates that untwinned regions in the 90 K crystals remain generally untwinned upon conversion to 60 K crystals. Colors produced by crossed polarizers enable them to identify the a axis and b axis directions in the 90 K and the 60 K materials. Each untwinned region has the a axis oriented in the same direction on the opposite (001) faces of the crystal