Site-selective oxygen isotope effect on the magnetic-field penetration depth in underdoped Y0.6Pr0.4Ba2Cu3O7-δ

We report site-selective oxygen isotope ( 1 6 O/ 1 8 O) effect (OIE) measurements on the in-plane penetration depth λ a b in underdoped Y 0 . 6 Pr 0 . 4 Ba 2 Cu 3 O 7 - δ , using the muon-spin rotation technique. A pronounced OIE on the transition temperature T c as well as on λ a b 2(0) was observed, which mainly arises from the oxygen sites within the superconducting CuO 2 planes (100% within error bar). The values of the corresponding relative isotope shifts were found to be ΔT c /T c =-3.7(4)% and Δλ - 2 ab(0)/λ - 2 ab(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.