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.

Interplay of chemical disorder and electronic inhomogeneity in unconventional superconductors

“…For example, in FeSe, both Fe adatoms and Se vacancies show strong rotational symmetry breaking, exhibiting dramatic two-fold symmetric electronic signatures, although their structural environment is four-fold symmetric to within B0.5%. This observation is consistent with the elongated vortices seen in the same material 45 and suggests two-fold symmetry of the gap function, possibly arising from orbital-ordering. 48,49 In the same FeSe thin films, excess Se atoms substituting at the Fe sites ( Fig.…”

“…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.…”

“…Song et al investigated single-atom impurities in FeSe films grown by molecular beam epitaxy (MBE), with T c B 8 K. 45 Both an Fe adatom evaporated onto the surface (Fig. 5a) and a single 40 (b) Typical dI/dV spectra at and far away from a presumed Cu site vacancy in Bi-2212 (200 pA, À200 mV, 4.2 K).…”

Interface induced high temperature superconductivity in single unit-cell FeSe on SrTiO3(110)

“…In contrast to irregular bumps observed in Fig. 1(b), the 1-UC FeSe films on insulating STO exhibit nearly ordered stripes with periods consistent with the 6 × 1 and 5 × 1 reconstructions of STO (110) [1][2][3][4][5][6][7][8][9][10] direction, whereas an one-to-one correspondence between FeSe and STO exists along the [001] direction. It is worth noting that either irregular bumps ( Fig.…”

“…A superconducting gap of 20 meV 1 , almost one order of magnitude larger than that of bulk FeSe 9 , was observed by scanning tunneling microscopy (STM). Angle resolved photoemission spectroscopy (ARPES) revealed a similar gap that closed at ~65 K [2][3][4] , while transport measurement on this system covered by FeTe capping layers demonstrated a superconducting transition temperature TC > 40 K 5 .…”