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 ($\mu \mathrm{SR}$) 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 $\mu \mathrm{SR}$ 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.

• Received 26 April 2018
• Revised 12 September 2018

DOI:https://doi.org/10.1103/PhysRevB.98.180501