Site-selective oxygen isotope effect on the magnetic-field penetration depth in underdoped Y0.6Pr0.4Ba2Cu3O7-δ -: art. no. 220506

We report site-selective oxygen isotope (O-16/O-18) effect (OIE) measurements on the in-plane penetration depth lambda(ab) in underdoped Y0.6Pr0.4Ba2Cu3O7-delta, using the muon-spin rotation technique. A pronounced OIE on the transition temperature T-c as well as on lambda(ab)(-2)(0) was observed, which mainly arises from the oxygen sites within the superconducting CuO2 planes (100% within error bar). The values of the corresponding relative isotope shifts were found to be DeltaT(c)/T-c=-3.7(4)% and Deltalambda(ab)(-2)(0)/lambda(ab)(-2)(0)=-6.2(1.0)%. Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons.