Evidence for d-wave superconductivity in single layer FeSe/SrTiO3 probed by quasiparticle scattering off step edges

TL;DR

This study provides direct experimental evidence for d-wave superconductivity in single-layer FeSe on SrTiO3 by analyzing quasiparticle scattering at step edges, revealing phase-sensitive pairing symmetry.

Contribution

It demonstrates the use of de Gennes extrapolation length measurements via STM/STS as a phase-sensitive probe for pairing symmetry in Fe-based superconductors.

Findings

Reduced superconducting gap near [110] edges indicating anisotropic pairing

Nearly infinite extrapolation length near [010] edges consistent with d-wave symmetry

Presence of in-gap states near [110] edges supports d-wave pairing

Abstract

The de Gennes extrapolation length is a direction dependent measure of the spatial evolution of the pairing gap near the boundary of a superconductor, and thus provides a viable means to probe its symmetry. It is expected to be infinite and isotropic for plain s-wave pairing, and finite and anisotropic for d-wave. Here, we synthesize single layer FeSe films on SrTiO3(001) (STO) substrates by molecular beam epitaxy, and measure 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. However, near specular [010]Fe edges the extrapolation length is nearly infinite. These findings are consistent with a phase changing pairing with 2-fold symmetry, indicating d-wave superconductivity. This is further supported by the presence of in-gap states near the specular [110]Fe edges, but not the [010]Fe edges. This work provides direct experimental evidence for d-wave superconductivity in single layer FeSe/STO, and demonstrates quasiparticle scattering at boundaries to be a viable phase sensitive probe of pairing symmetry in Fe-based superconductors.