Site dependence of large oxygen isotope effect in Y{sub 0.7}Pr{sub 0.3}Ba{sub 2}Cu{sub 3}O{sub 6.97}

We report site-selective oxygen isotope effects on {ital T}{sub {ital c}} and penetration depth in Y{sub 0.7}Pr{sub 0.3}Ba{sub 2}Cu{sub 3}O{sub 6.97} and in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. In Pr-substituted (underdoped) samples with {sup 18}O in only the CuO{sub 2} plane-sites, the {ital T}{sub {ital c}} shift was {minus}1.46 K vs {minus}1.7 K for {sup 18}O at all sites; the {alpha}{sub O} values are 0.2 and 0.24. The corresponding {ital T}{sub {ital c}} shifts in pure YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} were {minus}0.24 K and {minus}0.3 K. The shift is dominated by CuO{sub 2} planar oxygen mass in both compounds, and not by the apical sites. Thus, apical site isotope induced charge transfer to CuO{sub 2} planes does not seem a viable explanation for the isotope effects in YBCO. Our results indicate that the oxygen related phonons in the CuO{sub 2} planes play a significant role in the pairing mechanism. {copyright} {ital 1996 The American Physical Society.}