Abstract
Oxygen isotope shift experiments were performed on several series of high temperature superconductors doped with various ions (isovalent and non-isovalent) at different sites. Among these was Bi-2223 which contains three CuO 2 sheets and has T c ≈ 108 K. Bi-2223 samples were substituted with 18O to the extent of ≈90%. The oxygen isotope T cs were measured with high precision (resolution 2.5 mK, reproducibility 10 mK). We find Δ T c= -0.14±0.02K. Extrapolating to 100% exchange this yields α o=-0.013. In other high temperature superconducting compounds which we have also studied, substantial oxygen isotope shifts even above 70 K are found, but α o does not rise with decreasing T c in all cases. Changes in T c and α o can be correlated with the total hole concentration in several systems. Comparing different cuprate superconductors there appears to be an overall monotonic trend to lower values of α o with increasing T c and number of copper layers. This trend continues even above 100 K in Bi-2223 where α o becomes negative. Our results strongly suggest that phonon mediated electron pairing plays a vital role in high temperature superconductivity with near cancellation of α o by opposing effects when T c is high.
Published Version
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