Upper Critical Field of Grain Oriented Cuprate Superconductors
One of the striking structural features of the Y1Ba2Cu307−x family of superconductors is the occurrence of twin planes aligned parallel to the c-axis. These materials generally form the proper phase at a temperature near 960°C with an oxygen deficiency with x approaching one. If the sample is cooled in flowing oxygen, gradually oxygen intercalates into the copper layer between the two BaO layers. At a temperature in the 600 to 700°C range, these oxygens order into chains along the b-axis with an accompanying tetragonal to orthorhombic phase transition. The strain associated with this orthorhombic distortion in turn leads to  twin planes with the a and b axis interchanged at each twin. Depending on the cooling rate, the grain size and possibly other factors, the twin spacing typically ranges from 20 to 200 nm. Sometimes the spacing is in the pm range or greater. Evidence of these twins is easily seen in a transmission electron microscope or by looking at a polished sample under polarized light.
KeywordsSintered Pellet Twin Plane Orthorhombic Distortion Thin Film Superconductor Twin Spacing
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