Excitonic Theory of High Temperature Oxide Superconductors and the Lack of an Isotope Shift
The high temperature superconductivity1 observed in Y1Ba2Cu3O 7 2 is analyzed in terms of the “phonon + exciton” theory of Allender, Bray and Bardeen3 (ABB) applied to an atomically microscopic interfacial model of this anisotropic layered material. An approximate equation is presented that is conceptually transparent and allows one to visualize the relative interplay between the exciton and phonon contributions. Recent data 4,5 on the lack of an isotope shift on the transition temperature of Y1Ba2Cu3O7 is analyzed. The equation gives conditions for a lack of an isotope shift when the exciton mechanism dominates over the phonon mechanism. From ABB theory, numerical calculations are presented that demonstrate the plausibility of a dominant exciton mechanism in Y1Ba2Cu3O7. It will be very interesting to perform the isotope experiment on other oxide superconductors to further explore the possibility of excitonic superconductivity indicated by our calculations. Superconducting transition temperatures near room temperature appear attainable at reasonable carrier densities and within ABB’s excitonic theory and our data extrapolations.
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