OBSERVATION OF A POSSIBLE OXYGEN-ISOTOPE EFFECT ON THE EFFECTIVE-MASSOF CARRIERS IN YBA2CU3O6.94 - REPLY

In our recent paper [Zhao and Morris, Phys. Rev. B 51, 16 487 (1995)], we reported experiments which showed that there is a substantial oxyg en isotope effect on the effective mass of supercarriers m, but negli gible effect on the supercarrier density n(5) in the optimally doped Y Ba2Cu3O6.94. This is in contradiction with a model for the isotope eff ect proposed by Kresin and Wolf [Phys. Rev. B 49, 2652 (1994)], in whi ch n(s) (and thereby the penetration depth) would depend on the apical oxygen mass. In any event, their model has been ruled out by our subs equent measurements of the site-selective oxygen isotope effect in the underdoped Y0.7Pr0.3Ba2Cu3O7-y [Guo-meng Zhao er al., Phys. Rev. B 54 , 14 982 (1996)], which showed that the large isotope shift is caused by CuO2 plane oxygen mass and is independent of apical oxygen mass. Fu rther evidence also comes from our recent observation of a large Cu is otope effect in oxygen-depleted (underdoped) YBa2Cu3O6.6 [Guo-meng Zha o et al., Phys. Rev. B 54, 14 956 (1996)].