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{\deg} change in the orientation of local electronic dimers from Fe site impurities on either side. Twin boundaries run at approximately 45{\deg} 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.
@article{arxiv.1203.2629,
title = {Suppression of Superconductivity by Twin Boundaries in FeSe},
author = {Can-Li Song and Yi-Lin Wang and Ye-Ping Jiang and Lili Wang and Ke He and Xi Chen and Jennifer E. Hoffman and Xu-Cun Ma and Qi-Kun Xue},
journal= {arXiv preprint arXiv:1203.2629},
year = {2015}
}