A new Majorana platform in an Fe-As bilayer superconductor

TL;DR

This paper reports the discovery of Majorana zero modes in a bilayer Fe-As superconductor, CaKFe4As4, combining topological surface states and superconductivity, with detailed experimental and theoretical analysis confirming their Majorana nature.

Contribution

First identification of Majorana zero modes in an Fe-As superconductor with topological surface states and bilayer structure, expanding the platform for Majorana physics.

Findings

Majorana zero modes observed in CaKFe4As4 vortex cores

Topological surface states confirmed via spectroscopy

Theoretical model reproduces MZM energy and spatial profiles

Abstract

Recently, iron-chalcogenide superconductors have emerged as a new and promising platform for studying and manipulating Majorana zero mode (MZM). By combining topological band structure and superconductivity in a multiband material, they provide significant advantages such as higher superconducting transition temperature (Tc) and isolated Majorana mode. However, iron-chalcogenide superconductors, especially Fe(Te,Se), suffer from strong inhomogeneity which may hamper their practical application. On the other hand, some iron-pnictide (Fe-As) superconductors, such as LiFeAs, have been demonstrated to have a similar topological band structure, yet no MZM has been observed in its vortex cores, raising a question of universality of MZM presence in iron-based superconductors. In this work, by using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy, we identify the first Fe-As superconductor CaKFe4As4 (Tc = 35 K) which has both the superconducting Dirac surface states and the MZMs inside its vortex cores. The topological band inversion is largely due to the down-shift of the pz band caused by the bilayer band folding in this material. More strikingly, the energies and spatial line profiles of MZM and multiple quantized Caroli-de Gennes-Matricon bound states observed inside the topological vortex can be accurately reproduced by a simple theoretical model derived from a surface Dirac cone, firmly establishing Majorana nature of the zero mode.