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

Abstract

We report site-selective oxygen isotope (${}^{16}$O/${}^{18}$O) effect (OIE) measurements on the in-plane penetration depth ${\lambda }_{ab}$ in underdoped Y${}_{0.6}$Pr${}_{0.4}$Ba${}_2$Cu${}_3$O${}_{7-\delta }$, using the muon-spin rotation ($\mu$SR) 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 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 $\Delta {\lambda }_{ab}^{-2}(0)/{\lambda }_{ab}^{-2}(0)=-6.2(1.0)$. Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons.

4pages, 4 figures