Imaging orbitals and defects in superconducting FeSe/SrTiO₃

Monolayer FeSe on SrTiO₃ superconducts with T_c as high as 100 K, an order of magnitude enhancement over bulk FeSe. This dramatic enhancement motivates intense efforts to understand the superconducting pairing mechanism, and the crucial role of the SrTiO₃ surface. Nematicity, the breaking of 4-fold rotational symmetry, has been proposed as an important factor in the phase diagram of FeSe. Atomic defects can be used to pin nematicity, and to probe the local electronic structure of the superconductor.
Here we use scanning tunneling microscopy (STM) and other experimental tools to characterize SrTiO₃, search for orbital nematicity in monolayer FeSe/SrTiO₃, and investigate atomic-scale defects that locally influence superconductivity. From quasiparticle interference (QPI) images, we disentangle scattering intensities from the orthogonal Fe 3d_xz and 3d_yz bands and place an upper bound of δr ~ 20 nm on nematic domain size. Furthermore, we identify “dumbbell”-shaped atomic-scale defects as Fe vacancies.