Magnetic, superconducting, and topological surface states on FeTeSe*

The idea of employing non-Abelian statistics for error-free quantum computing ignited interest in reports of topological surface superconductivity. Recently, topological surface state and Majorana zero modes (MZMs) were detected on "11" Fe-based superconductor FeTe_0.55Se_0.45. However, these properties are not observed uniformly across the sample surface. Scanning tunneling spectroscopy experiments detect coexisting vortices with and without MZM, as well as an inhomogeneous bulk superconductivity. The understanding and practical control of these electronic inhomogeneities present a prominent challenge for potential applications of topological superconductivity. 

Here, we present neutron scattering, scanning angle-resolved photoemission spectroscopy, and microprobe composition and resistivity measurements that characterize the electronic state of Fe_1+yTe_1−xSe_x. Our results establish  a phase diagram in which the superconductivity is observed only at sufficiently low Fe concentration, in association with distinct antiferromagnetic correlations, whereas the coexisting topological surface state occurs only at sufficiently high Te concentration. We find that FeTe_0.55Se_0.45 is located very close to both phase boundaries, which explains the inhomogeneity of superconducting and topological states. We thus demonstrate the degree of compositional control that is required for use of topological MZMs in practical applications.