Oxygen-isotope effect on the in-plane penetration depth in underdoped Y_(1-x)Pr_xBa₂Cu₃O_(7-delta) as revealed by muon-spin rotation

The oxygen-isotope (^16O/^18O) effect (OIE) on the in-plane penetration depth $\lambda_{ab} (0)$ in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} was studied by muon-spin rotation. A pronounced OIE on $\lambda_{ab}^{-2}(0)$ was observed with a relative isotope shift of $\Delta\lambda^{-2}_{ab}/\lambda^{-2}_{ab}$=-5(2)% for x =0.3 and -9(2)% for x=0.4. It arises mainly from the oxygen-mass dependence of the in-plane effective mass $m_{ab}^{\ast}$. The OIE exponents of T_{c} and of $\lambda_{ab}^{-2}(0)$ exhibit a relation that appears to be generic for cuprate superconductors.