We investigated the electron-pairing mechanism in an iron-based superconductor, iron selenide
(FeSe), using scanning tunneling microscopy and spectroscopy. Tunneling conductance spectra of
stoichiometric FeSe crystalline films in their superconducting state revealed evidence for a gap
function with nodal lines. Electron pairing with twofold symmetry was demonstrated by direct
imaging of quasiparticle excitations in the vicinity of magnetic vortex cores, Fe adatoms, and Se
vacancies. The twofold pairing symmetry was further supported by the observation of striped
electronic nanostructures in the slightly Se-doped samples. The anisotropy can be explained in
terms of the orbital-dependent reconstruction of electronic structure in FeSe.