Observation of half-integer level shift of vortex bound states in an iron-based superconductor

TL;DR

This study reveals two types of vortex bound states in FeTe0.55Se0.45, with a half-integer energy level shift linked to topological superconductivity, supporting the presence of Majorana zero modes.

Contribution

It demonstrates the coexistence of topological and conventional superconducting regions in FeTe0.55Se0.45 through spectroscopic evidence of half-integer level shifts.

Findings

Identification of two vortex classes with distinct energy spectra.

Observation of a half-integer level shift in vortex bound states.

Model calculations confirm topological superconductivity in the material.

Abstract

Vortices in topological superconductors host Majorana zero modes (MZMs), which are proposed to be building blocks of fault-tolerant topological quantum computers. Recently, a new single-material platform for realizing MZM has been discovered in iron-based superconductors, without involving hybrid semiconductor-superconductor structures. Here we report on a detailed scanning tunneling spectroscopy study of a FeTe0.55Se0.45 single crystal, revealing two distinct classes of vortices present in this system which differ by a half-integer level shift in the energy spectra of the vortex bound states. This level shift is directly tied with the presence or absence of zero-bias peak and also alters the ratios of higher energy levels from integer to half-odd-integer. Our model calculations fully reproduce the spectra of these two types of vortex bound states, suggesting the presence of topological and conventional superconducting regions that coexist within the same crystal. Our findings provide strong evidence for the topological nature of superconductivity in FeTe0.55Se0.45 and establish it as an excellent platform for further studies on MZMs.