Experimental investigations of the isotope effect in high T-c superconductors

We present data on the copper isotope effect alpha(Cu) = -Delta In T-c/Delta In m(Cu) for oxygen depleted YBa2Cu3O7-delta as function of delta. alpha(Cu) is negative for all delta. In the 60 K plateau, alpha(Cu) reaches values of -0.14 or -0.34 for two comparison pairs. Outside the 60 K plateau, alpha(Cu) increases in absolute value. Similarities with results an T-c/dP in this system suggest that alpha(Cu) is related to mass-dependent ordering in the chain system.