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.

Visualizing superconductivity in FeSe nanoflakes onSrTiO3by scanning tunneling microscopy

“…In Sn, ξ ∼ 240 nm is considerably larger than the size of the studied nanoparticles, 33 from which ∆ should change little with positions in a specific particle. However, ξ was found to be only 5.5 nm in thick FeSe films 34 and even smaller for single UC FeSe films due to the dimensionality effect. This appears smaller than the average lateral nanoflake size A 0.5 in Figs.…”

Superconducting Energy Gap Features of FeSe Investigated by Tunneling Spectroscopy on Planar Junctions

“…In-situ scanning tunneling spectroscopy (STS) on MBE grown FeSe thin films was interpreted as evidence for gap nodes by Song et al [13]. These authors observed one very clear single Vshaped energy gap with ∆ B ≈ 2.2 meV [13][14][15][16], which corresponds to ratios of 2∆ B /k B T c = 5.5 − 6.4. An additional conductivity feature at 2∆ C ≈ 9.4k B T c , was interpreted as strong coupling to a spin fluctuation mode [17].…”

Interplay of chemical disorder and electronic inhomogeneity in unconventional superconductors

“…The Fe site defect is surrounded by a larger electronic dimer structure: a C 2v -symmetric depression in dI/dV oriented along the Fe-Fe direction (100 pA, 10 mV). 50 (b) Average dI/dV centered at a Co substitution at a Fe site in Ca(Fe 1Àx Co x ) 2 As 2 showing a similar large electronic dimer structure. 76 (c) Topograph of an impurity in nominally stoichiometric LiFeAs showing the smaller dumbbell shape, but lacking the larger electronic dimer (10 pA, 20 mV).…”

“…6a). 45,50 The smaller dumbbellshaped signature is present in topographs taken up to 3.5 V imaging bias, but the larger, B16a Fe-Fe sized dimers only appear at around AE20 mV bias, pointing towards their electronic origin. Furthermore, the larger features are always oriented along the orthorhombic a (longer, antiferromagnetic) Fe-Fe axis, as they change their orientation by 901 across an orthorhombic twin boundary, regardless of the stochastic orientation of the smaller dumbbell-shaped signature.…”