title: Direct observation of nodes and twofold symmetry in FeSe superconductor.
abstract: 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.
authors.affiliation: State Key Laboratory for Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, the People's Republic of China.
authors.forename: Can-Li
authors.initials: CL
authors.lastname: Song
authors.forename: Yi-Lin
authors.initials: YL
authors.lastname: Wang
authors.forename: Peng
authors.initials: P
authors.lastname: Cheng
authors.forename: Ye-Ping
authors.initials: YP
authors.lastname: Jiang
authors.forename: Wei
authors.initials: W
authors.lastname: Li
authors.forename: Tong
authors.initials: T
authors.lastname: Zhang
authors.forename: Zhi
authors.initials: Z
authors.lastname: Li
authors.forename: Ke
authors.initials: K
authors.lastname: He
authors.forename: Lili
authors.initials: L
authors.lastname: Wang
authors.forename: Jin-Feng
authors.initials: JF
authors.lastname: Jia
authors.forename: Hsiang-Hsuan
authors.initials: HH
authors.lastname: Hung
authors.forename: Congjun
authors.initials: C
authors.lastname: Wu
authors.forename: Xucun
authors.initials: X
authors.lastname: Ma
authors.forename: Xi
authors.initials: X
authors.lastname: Chen
authors.forename: Qi-Kun
authors.initials: QK
authors.lastname: Xue