Mixed phase of iron based Dirac superconductors*

Recently, bulk 3D Dirac semimetallic touching points were observed via ARPES in Li(Fe_1-xCo_x)As.[1] Separately, zero-bias vortex-core Majorana subgap states were reported using STM on the surface of the topological iron based superconductor FeTe_0.55Se_0.45.[2] Crucially, close to the topologically relevant Γ-Z line, this second material has a very similar band structure as Li(Fe_1-xCo_x)As. The Majorana modes are attributed to emergent topological 2D Dirac surface states with proximity induced superconductivity, i.e. via a Fu-Kane like mechanism which is independent from the 3D Dirac bulk states. Yet, we show that, in certain parameter regimes, the 3D Dirac semimetallic touching points also provide Majorana subgap states which disperse along the vortex tube. Here, we investigate experimental signatures of the latter and thereby demonstrate a route to unveil the rich topological structure of iron based superconductors.

[1] P. Zhang et al, Nature Physics, doi:10.1038/s41567-018-0280-z (2018),
[2] D. Wang et al, Science, eaao1797 (2018).