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Published online by Cambridge University Press: 21 March 2011
We show evidence from transmission electron microscopy of an unusual change in columnar defect morphology in YBCO upon annealing to 600°C. The disappearance of the nanotwinned, but not the larger scaled thermal twinned, structure is found. A removal of the large fluctuations in the diameter of the amorphous column, preserving a narrow continuous column, is surprisingly discovered. Correlations with magnetization measurements demonstrate the greater efficiency of vortex pinning at 77 K by the annealed defect structure.