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

TL;DR

This study investigates how oxygen isotope substitution affects the magnetic field penetration depth and transition temperature in underdoped cuprate superconductors, revealing a strong coupling between planar oxygen phonons and electrons.

Contribution

It provides the first site-selective measurements showing the dominant role of planar oxygen phonons in the superconducting properties of underdoped cuprates.

Findings

Pronounced isotope effect on $T_c$ and $\lambda_{ab}^{-2}(0)$

Oxygen sites within CuO$_2$ planes mainly contribute to isotope effects

Planar oxygen phonons strongly coupled to electrons in the material

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 $\Delta\lambda^{-2}_{ab}(0)/\lambda^{-2}_{ab}(0)= -6.2(1.0)%$. Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons.