  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.
