Doping dependence of the oxygen isotope effect in YBa2Cu3Ox

Measurements of the total oxygen isotope shift lo and the condensate density n of YBa2Cu3Ox (6.53 < x < 6.99) are presented. The T-c(n) dependence exhibits a two-plateau behavior which is associated with the ortho-I and ortho-II phase, respectively. Around optimum doping (x similar or equal to 6.9) and at the onset of the 60 K plateau where T-c only 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 derivative T-c/partial derivative(n) suggesting that alpha(o) is related to a change of the condensate density n upon isotope exchange.