Suppression of Superconductivity by Twin Boundaries in FeSe

TL;DR

This study uses low-temperature STM to show that twin boundaries in FeSe suppress superconductivity, with vortices accumulating there, likely due to increased Se height affecting the superconducting mechanism.

Contribution

First local evidence linking twin boundary-induced Se height increase to suppression of superconductivity in FeSe.

Findings

Twin boundaries identified by 90° change in electronic dimers.

Superconducting gap is noticeably suppressed at twin boundaries.

Vortices tend to accumulate on twin boundaries.

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{\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.