Limits on the Superconducting Order Parameter in NdFeAsO$_{1-x}$F$_y$ from Scanning SQUID Microscopy

TL;DR

This study uses scanning SQUID microscopy on NdFeAsO$_{1-x}$F$_{y}$ to investigate the superconducting order parameter's symmetry, finding no spontaneous currents that would suggest certain phase-shifted states, thus constraining theoretical models.

Contribution

The paper provides experimental evidence ruling out several proposed order parameters with internal phase shifts in NdFeAsO$_{1-x}$F$_{y}$ superconductors.

Findings

No spontaneous orbital currents detected in the sample.

Constraints placed on the symmetry of the superconducting order parameter.

Supports s-wave or non-phase-shifted pairing symmetry.

Abstract

Identifying the symmetry of the superconducting order parameter in the recently-discovered ferro-oxypnictide family of superconductors, RFeAsO$_{1-x}$F$_{y}$, where $R$ is a rare earth, is a high priority. Many of the proposed order parameters have internal $\pi$ phase shifts, like the d-wave order found in the cuprates, which would result in direction-dependent phase shifts in tunnelling. In dense polycrystalline samples, these phase shifts in turn would result in spontaneous orbital currents and magnetization in the superconducting state. We perform scanning SQUID microscopy on a dense polycrystalline sample of \NdFeAsO$_{0.94}$F$_{0.06}$ with $T_c=48$ K and find no such spontaneous currents, ruling out many of the proposed order parameters.