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

Huimin Zhang,Michael Weinert,Zhuozhi Ge,Lian Li

doi:10.1038/s42005-020-0351-1

Huimin Zhang, Michael Weinert + Show 2 more

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https://doi.org/10.1038/s42005-020-0351-1

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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. This is further confirmed by defect bound state quasiparticle interference that shows a sign-changing behavior with a pair of corresponding peaks at the positive and negative energies near the impurity bound states. Our findings provide unambiguous evidence for a sign-changing pairing symmetry for single layer FeSe/SrTiO3.

Highlights

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
A plain s-wave has been suggested in an earlier STM work[13], based on quasiparticle interference off adsorbates on the surface of FeSe, where no impurity-induced bound states were observed for nonmagnetic Zn, Ag, and K adatoms
The pairing symmetry for single-layer FeSe/STO remains controversial, and unambiguous verification of induced in-gap bound states and quasiparticle scattering off nonmagnetic impurities within the Fe plane is crucial

Summary

Introduction

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. The recent discovery of high-temperature superconductivity in single-layer FeSe epitaxially grown on SrTiO3(001) substrate[6,7] provides a model system to probe the pairing states of Fe-based superconductors. The lack of hole pockets challenges the spin fluctuation picture that involves a sign reversal order parameter between the electron and hole pockets[11,12] This has instigated extensive debate on the symmetry of pairing state in single-layer FeSe, with leading contender being sign-preserving conventional s-wave pairing[13], and unconventional sign-changing extended s-wave[14,15] and nodeless d-wave pairing[16,17,18,19], experimental validation are sorely needed[20]. The pairing symmetry for single-layer FeSe/STO remains controversial, and unambiguous verification of induced in-gap bound states and quasiparticle scattering off nonmagnetic impurities within the Fe plane is crucial

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