Evidence of nodal gap structure in the basal plane of the FeSe superconductor

TL;DR

This study uses muSR to reveal that FeSe superconductors have a nodal gap structure in the basal plane, providing insights into their pairing symmetry and anisotropic properties, which are crucial for understanding high-temperature superconductivity.

Contribution

The paper demonstrates that FeSe exhibits a nodal gap in the ab-plane and a nodeless gap along the c-axis, revealing anisotropic pairing symmetry through muSR measurements.

Findings

Nodal gap in the ab-plane of FeSe

Nodeless gap along the c-axis

Less anisotropic superconducting properties than expected

Abstract

Identifying the symmetry of the wave function describing the Cooper pairs is pivotal in understanding the origin of high-temperature superconductivity in iron-based superconductors. Despite nearly a decade of intense investigation, the answer to this question remains elusive. Here we use the muon spin rotation/relaxation (muSR) technique to investigate the underlying symmetry of the pairing state of the FeSe superconductor, the basic building block of all iron-chalcogenide superconductors. Contrary to earlier muSR studies on powders and crystals, we show that while the superconducting gap is most probably anisotropic but nodeless along the crystallographic c-axis, it is nodal in the ab-plane, as indicated by the linear increase of the superfluid density at low temperature. We further show that the superconducting properties of FeSe display a less pronounced anisotropy than expected.