Imaging orbitals and defects in superconducting FeSe/SrTiO₃

Single-layer FeSe grown epitaxially on SrTiO₃ has been shown to superconduct with T_c as high as 100 K, more than a factor of 10 higher than bulk FeSe. This dramatic enhancement motivates intense efforts to understand the superconducting mechanism and to design and fabricate devices. Nematic order, breaking the 4-fold rotational symmetry of the crystal, has been proposed as an important factor in the superconducting phase diagram. Meanwhile, atomic defects, which may pin nematic fluctuations or otherwise perturb superconductivity, can provide important clues into the superconducting mechanism as well as practical routes to superconducting devices. Here we use scanning tunneling microscopy (STM) to search for orbital nematicity in single-layer FeSe/SrTiO₃, and to investigate atomic-scale defects which locally influence superconductivity. From quasiparticle interference (QPI) images, we disentangle scattering intensities from the orthogonal Fe 3d_xz and 3d_yz bands, and quantitatively exclude pinned nematic orbital order with domain size larger than δ r ∝ 20 nm. Furthermore, we identify a prevalent ''dumbbell''-shaped atomic-scale defect whose placement could be harnessed to define two-dimensional superconducting devices.