Chemical activity in YBa₂Cu₃O_{7 ?#xa0;¦Ä} across the normal to superconducting phase transition

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• 作者：J.V.  ; Acrivos ;   ; ^a ;   ; ^{jacrivos@athens.sjsu.edu}
• 关键词：Layer-superconductors ; Phase-transition ; Enhanced-absorption&ndash ; diffraction
• 刊名：Microchemical Journal
• 出版年：2011
• 出版时间：November 2011
• 年：2011
• 卷：99
• 期：2
• 页码：239-245
• 全文大小：2049 K

文摘

The Gibbs free enthalpy, chemical activity across the transition temperature to superconductivity, T_c in YBa₂Cu₃O_{7 ?¦Ä} is obtained from reciprocally enhanced X-Ray absorbance, XAS and diffraction, XRD data near the Ba L_3,2 edges' energy E_a, and orientations in the X-ray beam for preferred Miller indexed [HKL] planes' scattering, that are enhanced near T_c. The standard enthalpy and entropy for the formation of mixed normal metal/superconducting domains above T_c, determined individually across the two Ba L_3,2 edges, to better than a percent accuracy: ¦¤H^{?/sup>_{?/sub> = ?220 meV, and ¦¤S?/sup>?/sub> = ?2 meV/K when 121 ?T ?92 K ?Tc1, indicate that energy is gained, with an increased order by forming a mixture with reduced entropy. Below Tc the standard enthalpy and entropy to form at least two mixed superconducting phases are halved to ¦¤H?/sup>?/sub> = ?86 meV, ¦¤S?/sup>?/sub> = ?1 meV/K when 92 K ?T ?Tc2 ?72 K. Thus reciprocal XAS/XRD enhancement at orientations in the X-ray field, of preferred 2D planes, induced by the transition to superconductivity in the layer solid, indicates one, the importance of the 2D-plane electron density scattering near Tc, and two, identifies the 2D-plane chemical activity, by the electron density leading to symmetry allowed excitations, a role similar to that of the electron density in linear bonds for molecular activity.}}