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

Abstract

We report the dependence of the oxygen isotope shift on transition temperature ${\mathit{T}}_{\mathit{c}}$ in the system ${\mathrm{YBa}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{La}}_{\mathit{x}}$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{z}}$ with 0≤x≤0.5. We find a significant oxygen isotope shift at low temperatures (${\mathrm{\alpha }}_0$=0.38 at ${\mathit{T}}_{\mathit{c}}$=38.3 K), which decreases gradually with increasing ${\mathit{T}}_{\mathit{c}}$ and finally falls rapidly above 73 K to ${\mathrm{\alpha }}_0$=0.025 for ${\mathit{T}}_{\mathit{c}}$=92.3 K. Our data are well fitted by the expression ${\mathrm{\alpha }}_0$(${\mathit{T}}_{\mathit{c}}$)=0.5(1-${\mathit{T}}_{\mathit{c}}$/${\mathit{T}}_{\mathit{c}0}$ ${)}^{0.59}$, with ${\mathit{T}}_{\mathit{c}0}$=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 ${\mathit{T}}_{\mathit{c}}$ approaches its maximum value in this system.