Distinguishing impurity-induced bound states from Majorana-like zero-energy peaks in strained by scanning tunneling microscopy
Phys. Rev. B 113, 224504 – Published 1 June, 2026
DOI: https://doi.org/10.1103/b5c9-xx1b
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 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 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) [Yin et al., 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 impurity-induced bound states rather than MZMs. Our study not only provides more insights into the superconducting pairing symmetry of but also establishes systematic experimental methods for identifying weak impurity state signals and discerning the physical origins of ZECPs.
