DOPING DEPENDENCE OF THE OXYGEN-ISOTOPE EFFECT IN YBA2CU3OX

Measurements of the total oxygen isotope shift lo and the condensate d ensity n of YBa2Cu3Ox (6.53 < x < 6.99) are presented. The T-c(n) depe ndence exhibits a two-plateau behavior which is associated with the or tho-I and ortho-II phase, respectively. Around optimum doping (x simil ar or equal to 6.9) and at the onset of the 60 K plateau where T-c onl y weakly depends on n the small alpha(o) similar or equal to 0.02.5 is associated to changes in the phonon spectrum. A further decrease of T -c leads to a rapid increase of alpha(o) proportional to partial deriv ative T-c/partial derivative(n) suggesting that alpha(o) is related to a change of the condensate density n upon isotope exchange.