Evidence for Majorana bound states in an iron-based superconductor

TLDR

The search for Majorana bound states, predicted as zero‑energy vortex core modes in two‑dimensional topological superconductors, is hampered by competing low‑lying bound states that obscure clear observation. Using scanning tunneling microscopy on the Dirac surface state of the iron‑based superconductor FeTe₀.₅₅Se₀.₄₅, the authors detected a sharp, unsplit zero‑bias peak within a vortex core. Systematic variation of magnetic field, temperature, and tunneling barrier shows the peak behaves as a nearly pure Majorana mode, isolated from non‑topological states by the material’s large superconducting gap to Fermi energy ratio, providing a robust high‑temperature platform for manipulation.

Abstract

The search for Majorana bound state (MBS) has recently emerged as one of the most active research areas in condensed matter physics, fueled by the prospect of using its non-Abelian statistics for robust quantum computation. A highly sought-after platform for MBS is two-dimensional topological superconductors, where MBS is predicted to exist as a zero-energy mode in the core of a vortex. A clear observation of MBS, however, is often hindered by the presence of additional low-lying bound states inside the vortex core. By using scanning tunneling microscope on the newly discovered superconducting Dirac surface state of iron-based superconductor FeTe1-xSex (x = 0.45, superconducting transition temperature Tc = 14.5 K), we clearly observe a sharp and non-split zero-bias peak inside a vortex core. Systematic studies of its evolution under different magnetic fields, temperatures, and tunneling barriers strongly suggest that this is the case of tunneling to a nearly pure MBS, separated from non-topological bound states which is moved away from the zero energy due to the high ratio between the superconducting gap and the Fermi energy in this material. This observation offers a new, robust platform for realizing and manipulating MBSs at a relatively high temperature.