Impurity-induced bound states inside the superconducting gap of FeSe

TL;DR

This study examines how non-magnetic Fe vacancies in FeSe create in-gap bound states, supporting the idea of a sign-changing pairing symmetry in its superconducting state.

Contribution

It provides experimental tunneling spectra and DFT calculations showing non-magnetic defects induce bound states, indicating a sign-changing pairing in FeSe.

Findings

Bound states observed inside the superconducting gap near Fe vacancies

DFT confirms the defect does not induce local magnetism

Results support sign-changing pairing symmetry in FeSe

Abstract

We investigate the local density of states in the vicinity of a native dumbbell defect arising from an Fe vacancy in FeSe single crystals. The tunneling spectra close to the impurity display two bound states inside the superconducting gap, equally spaced with respect to zero energy but asymmetric in amplitude. Using spin-polarized density functional theory (DFT) calculations on realistic slab models with Fe vacancy, we show that such a defect does not induce a local magnetic moment. Therefore, the dumbbell defect is considered as non-magnetic. Thus, the in-gap bound states emerging from a non-magnetic defect-induced pair-breaking suggest a sign changing pairing state in this material.