Abstract
Raman spectroscopic study indicates that a Raman line at 646 cm−1 disappears at temperatures above 234 K, manifesting a phase transition in high-Tc YBaCuO superconductors. The symmetry change from Pmmm to Pmm2 could be qualitatively interpreted as follows: The change in the coordination number of the Cu ions in YBaCuO alters the local symmetry so some copper ions form symmetric complexes and the others acquire reduced symmetry Many other measurements confirm this transition near 230 K: Aλ anomaly of the specific heat was observed near 230 K in YBaCuO, and it was found that the 643-cm1 Raman line in GdBaCuO reproducibly disappears on heating and reappears on cooling, manifesting a diffusive phase transition. The thermal hysteresis of Raman intensity of the 646-cm−1 line between Tc = 89 K and TO = 234 K in YBaCuO was measured and pinned at Tc and To, which is consistent with the results obtained by the measurements of the bulk modulus and elastic constant.
© 1991 Optical Society of America
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