Tunneling interstitial impurity in iron-chalcogenide based superconductors

TL;DR

This paper investigates the local zero-energy bound states caused by interstitial Fe impurities in iron-chalcogenide superconductors using a tunneling impurity model and Bogoliubov-de Gennes equations, aligning with experimental observations.

Contribution

It introduces a new tunneling impurity mechanism to explain impurity-induced bound states in iron-based superconductors, providing theoretical support for recent STM findings.

Findings

ZBS peak remains stable under magnetic field with small tunneling parameter

Modulation of SDW or CDW suppresses ZBS intensity

Results agree with experimental STM observations

Abstract

A pronounced local in-gap zero-energy bound state (ZBS) has been observed by recent scanning tunneling microscopy (STM) experiments on the interstitial Fe impurity (IFI) and its nearest-neighboring (nn) sites in $\mathrm{FeTe_{0.5}Se_{0.5}}$ superconducting (SC) compound. By introducing a new impurity mechanism, the so-called tunneling impurity, and based on the Bogoliubove-de Gennes (BDG) equations we investigated the low-lying energy states of the IFI and the underlying Fe-plane. We found the peak of ZBS does not shift or split in a magnetic field as long as the tunneling parameter between IFI and the Fe-plane is sufficiently small and the Fe-plane is deep in the SC state. Our results are in good agreement with the experiments. We also predicted that modulation of spin density wave (SDW), or charge density wave (CDW) will suppress the intensity of the ZBS.