Site-selective oxygen isotope effect on the magnetic field penetration depth in underdoped Y$_{0.6}$Pr$_{0.4}$Ba$_2$Cu$_3$O$_{7-δ}$

Abstract

We report site-selective oxygen isotope ($^{16}$O/$^{18}$O) effect (OIE) measurements on the in-plane penetration depth $λ_{ab} $ in underdoped Y$_{0.6}$Pr$_{0.4}$Ba$_2$Cu$_3$O$_{7-δ}$, using the muon-spin rotation ($μ$SR) technique. A pronounced OIE on the transition temperature $T_c$ as well as on $λ_{ab}^{-2}(0)$ 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 $T_c/T_c= -3.7(4)%$ 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.