We have found that oxygen deficient regions in YBa2Cu3Ox superconductors exhibit different colors from that of the matrix under polarized light. When oxygen annealing was insufficient, large melt-grown YBaCuO crystals occasionally contained narrow bands exhibiting different colors where the twin structure was absent. Upon annealing in oxygen atmosphere, these banded regions disappeared, indicating that these regions are oxygen deficient. Raman spectroscopy measurements also confirmed that such regions are oxygen deficient.
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