Direct observation of the oxygen isotope effect on the in-plane magnetic field penetration depth in optimally doped YBa$_2$Cu$_3$O$_{7-\delta}$

TL;DR

This study directly measures how replacing oxygen isotopes affects the magnetic penetration depth in a high-temperature superconductor, revealing isotope-dependent changes in supercarrier mass using a novel muon-spin rotation technique.

Contribution

It provides the first direct measurement of the oxygen isotope effect on the in-plane magnetic penetration depth in YBa2Cu3O7−δ using low-energy muon-spin rotation.

Findings

Observed a 2.8% isotope shift in λ_ab at 4 K.

Demonstrated a 5.5% isotope dependence of the supercarrier effective mass.

Introduced a novel method for direct measurement of magnetic field profiles in superconducting films.

Abstract

We report the first direct observation of the oxygen-isotope ($^{16}$O/$^{18}$O) effect on the in-plane penetration depth $\lambda_{ab}$ in a nearly optimally doped YBa$_2$Cu$_3$O$_{7-\delta}$ film using the novel low-energy muon-spin rotation technique. Spin polarized low energy muons are implanted in the film at a known depth $z$ beneath the surface and precess in the local magnetic field $B(z)$. This feature allows us to measure directly the profile $B(z)$ of the magnetic field inside the superconducting film in the Meissner state and to make a model independent determination of $\lambda_{ab}$. A substantial isotope shift $\Delta\lambda_{ab}/\lambda_{ab}=2.8(7)$% at 4 K is observed, implying that the in-plane effective supercarrier mass $m_{ab}^\ast$ is oxygen-isotope dependent with $\Delta m_{ab}^\ast/m_{ab}^\ast = 5.5(1.4)%$.