Evidence for nodeless <i>d</i>-wave superconductivity in single layer FeSe/SrTiO₃by quasiparticle scattering off step edges

The de Gennes extrapolation length is a direction-dependent measure of the pairing gap near the boundary of a superconductor, and thus provides a viable means to probe its pairing symmetry. The extrapolation length is infinite and isotropic for plain s-wave pairing, and finite and anisotropic for d-wave pairing. Here, we synthesize single layer FeSe films on SrTiO₃(001) substrates by molecular beam epitaxy, and determine the de Gennes extrapolation length by scanning tunneling microscopy/spectroscopy. We find a 40% reduction of the superconducting gap near specular [110]_Fe edges, yielding an extrapolation length of 8.0 nm. Near specular [100]_Fe edges, however, the extrapolation length is nearly infinite. These findings are consistent with a phase changing pairing with 2-fold symmetry, indicating nodeless d-wave superconductivity. This work provides direct experimental evidence for nodeless d-wave superconductivity in single layer FeSe/SrTiO₃, and demonstrates that quasiparticle scattering at boundaries can be a viable phase sensitive probe of pairing symmetry in Fe-based superconductors.<audio controls="controls" style="display: none;"> </audio>