Isotope effect in YBa2-xLaxCu3Oz: Evidence for phonon-mediated high-temperature superconductivity

We report the dependence of the oxygen isotope shift on transition temperature Tc in the system YBa2-xLaxCu3Oz with 0≤x≤0.5. We find a significant oxygen isotope shift at low temperatures (α0=0.38 at Tc=38.3 K), which decreases gradually with increasing Tc and finally falls rapidly above 73 K to α0=0.025 for Tc=92.3 K. Our data are well fitted by the expression α0(Tc)=0.5(1-Tc/Tc0)0.59, with Tc0=92.8 K. Our results suggest a dominant role for conventional electron-phonon coupling in the high-temperature cuprate superconductors, with strong suppression of the isotope effect as Tc approaches its maximum value in this system