Distinguishing impurity-induced bound states from Majorana-like zero-energy peaks in strained CsCa2Fe4As4F2 by scanning tunneling microscopy

TL;DR

This study uses scanning tunneling microscopy to analyze impurity states in strained CsCa2Fe4As4F2, distinguishing between Yu-Shiba-Rusinov states and Majorana zero modes, and explores strain effects on superconductivity.

Contribution

It provides a systematic method to differentiate impurity-induced states from Majorana zero modes in iron-based superconductors under local strain.

Findings

Strain significantly modulates the superconducting gap.

Zero-energy conductance peaks are due to Yu-Shiba-Rusinov states, not MZMs.

Methodology enhances identification of impurity states in superconductors.

Abstract

Iron-based superconductors offer a versatile platform for exploring topological superconductivity and Majorana zero modes (MZMs), with experimental confirmations in Fe(Te,Se), (Li,Fe)OHFeSe and CaKFe4As4 at ambient pressure, as well as in LiFeAs under local strain. The related properties in other iron-based superconductors still need to be explored, especially under the application of local strain. In this study, we conduct scanning tunneling microscopy/spectroscopy measurements on CsCa2Fe4As4F2 crystals under unidirectional local strain. A fully developed superconducting gap with multiple pairs of coherence peaks are observed, and the gap sizes can be significantly modulated by local strain. Spectroscopic measurements on various types of defects including the nonmagnetic Cs-site vacancies consistently reveal pair-breaking effects. These phenomena support a fully gapped multiband superconductivity scenario with sign-changing. Notably, a sharp zero-energy conductance peak (ZECP) is universally observed on a particular type of defects by using a metallic tip, resembling the MZMs observed at interstitial Fe atoms in Fe(Te,Se) [Nat. Phys. 11, 543 (2015)]. However, by using a superconducting tip to enhance energy resolution as well as by studying the ZECP evolution as functions of magnetic field and tunneling transmissivity, we demonstrate that the ZECP originates from nearly degenerate Yu-Shiba-Rusinov states rather than MZMs. Our study not only provides more insights into the superconducting pairing symmetry of CsCa2Fe4As4F2, but also establishes systematic experimental methods for identifying weak impurity state signals and discerning the physical origins of ZECPs.