ISOTOPE EFFECT IN YBA2-XLAXCU3OZ - EVIDENCE FOR PHONON-MEDIATED HIGH-TEMPERATURE SUPERCONDUCTIVITY

We report the dependence of the oxygen isotope shift on transition temperature T(c) in the system YBa(2-x)La(x)Cu3Oz with 0 less-than-or-equal-to x less-than-or-equal-to 0.5. We find a significant oxygen isotope shift at low temperatures (alpha-0 = 0.38 at T(c) = 38.3 K), which decreases gradually with increasing T(c) and finally falls rapidly above 73 K to alpha-0 = 0.025 for T(c) = 92.3 K. Our data are well fitted by the expression alpha-0(T(c)) = 0.5(1-T(c)/T(c0))0.59, with T(c0) = 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 T(c) approaches its maximum value in this system.