Correlation between oxygen isotope shift and T<SUB>c</SUB> in 2-and 3-layer high temperature superconductors

Oxygen isotope shift experiments were performed on several series of high temperature superconductors doped with various ions (isovalent and non-isovalent) at different sites. Substantial oxygen isotope shifts are found even above 70 K, but &#945; 0 does not rise with decreasing Tc in all cases. Changes in Tc and &#945; 0 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 &#945; 0 with increasing Tc and number of copper layers. This trend continues even above 100 K where &#945; 0 becomes negative in Bi-2223. Our results strongly suggest that all oxide high-temperature superconductors are phonon-coupled with near cancellation of &#945; 0 by opposing effects when Tc is high