Observation of a Majorana zero mode in a topologically protected edge state
Superconducting pairing in the helical edge state of topological insulators is predicted to provide a unique platform for realizing Majorana zero modes (MZMs). We use (spin-polarized) scanning tunneling microscopy measurements to probe the influence of proximity induced superconductivity and local magnetism on the helical edge states of bismuth(111) thin films, which are grown on a superconducting niobium substrate and decorated with iron clusters. Consistent with model calculations, our measurements reveal the emergence of a localized MZM at the interface between the superconducting helical edge state and the ferromagnetic iron clusters with strong magnetization component along the edge (1). Our experiments also resolve the MZM’s unique spin signature by which it can be distinguished from trivial in-gap states that may accidently occur at zero energy in a superconductor. High-resolution spectroscopic mapping of quasiparticle interference further demonstrates quasiparticle backscattering inside the one-dimensional helical edge state, which is induced by the ferromagnetic iron clusters that locally break time-reversal symmetry (2).
(1) B. Jäck, Y. Xie, J. Li, S. Jeon, B.A. Bernevig, A. Yazdani, Science 364, 1255-1259 (2019)
(2) B. Jäck, Y. Xie, B.A. Bernevig, A. Yazdani, PNAS, DOI:10.1073/pnas.2005071117 (2020)