Sign changing pairing in single layer FeSe/SrTiO3 revealed by nonmagnetic impurity bound states

Abstract

The discovery of high-temperature superconductivity in single layer FeSe epitaxially grown on SrTiO3(001) substrates has instigated extensive debate over whether its pairing symmetry is conventional sign-preserving s-wave or unconventional sign-changing. Here we probe the pairing state for single layer FeSe/SrTiO3 grown by molecular beam epitaxy using scanning tunneling microscopy/spectroscopy. We observe robust in-gap bound states induced by non-magnetic Fe-vacancy defects within the Fe-plane, which exhibit strong spatial electron-hole asymmetry with two-fold symmetry in hole states and four-fold in electron states. The bound states exhibit no energy shift or splitting under an applied magnetic field, consistent with a sign-changing order parameter.

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• Multichannel superconductivity of monolayer FeSe on SrTiO${}_3$: Interplay of spin fluctuations and electron-phonon interaction
• Charge order driven by multiple-Q spin fluctuations in heavily electron-doped iron selenide superconductors
• Enhanced superconductivity in FeSe/SrTiO${}_3$ from the combination of forward scattering phonons and spin fluctuations
• Tomonaga Luttinger liquid in the topological edge channel of multilayer FeSe
• Checkerboard order state in superconducting FeSe/SrTiO3(001) monolayer
• Orbital-Selective High-Temperature Cooper Pairing Developed in the Two-Dimensional Limit
• Interplay between the extended s-wave symmetry of the gap and the spin-orbit coupling in the low-electron concentration regime of quasi-two-dimensional superconductors
• High-Resolution Spectroscopy of the Intermediate Impurity States near a Quantum Phase Transition
• Anisotropic response of defect bound states to magnetic field in epitaxial FeSn films
• Dynamical Coulomb Blockade as a Signature of the Sign-Reversing Cooper Pairing Potential