title: Suppression of superconductivity by twin boundaries in FeSe.
abstract: Low-temperature scanning tunneling microscopy and spectroscopy are employed to investigate twin boundaries in stoichiometric FeSe films grown by molecular beam epitaxy. Twin boundaries can be unambiguously identified by imaging the 90° change in the orientation of local electronic dimers from Fe site impurities on either side. Twin boundaries run at approximately 45° to the Fe-Fe bond directions, and noticeably suppress the superconducting gap, in contrast with the recent experimental and theoretical findings in other iron pnictides. Furthermore, vortices appear to accumulate on twin boundaries, consistent with the degraded superconductivity there. The variation in superconductivity is likely caused by the increased Se height in the vicinity of twin boundaries, providing the first local evidence for the importance of this height to the mechanism of superconductivity.
authors.affiliation: State Key Laboratory for Surface Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Can-Li
authors.initials: CL
authors.lastname: Song
authors.forename: Yi-Lin
authors.initials: YL
authors.lastname: Wang
authors.forename: Ye-Ping
authors.initials: YP
authors.lastname: Jiang
authors.forename: Lili
authors.initials: L
authors.lastname: Wang
authors.forename: Ke
authors.initials: K
authors.lastname: He
authors.forename: Xi
authors.initials: X
authors.lastname: Chen
authors.forename: Jennifer E
authors.initials: JE
authors.lastname: Hoffman
authors.forename: Xu-Cun
authors.initials: XC
authors.lastname: Ma
authors.forename: Qi-Kun
authors.initials: QK
authors.lastname: Xue