Anomalous Shiba states in topological iron-based superconductors

We demonstrate the formation of robust zero-energy modes close to magnetic impurities in the iron-based superconductor FeSe1-z Tez. We find that the Zeeman field generated by the impurity favors a spin-triplet interorbital pairing as opposed to the spin-singlet intraorbital pairing prevalent in the bulk. The preferred spin-triplet pairing preserves time-reversal symmetry and is topological, as robust, topologically protected zero modes emerge at the boundary between regions with different pairing states. Moreover, the zero modes form Kramers doublets that are insensitive to the direction of the spin polarization or to the separation between impurities. We argue that our theoretical results are consistent with recent experimental measurements on FeSe1-z Tez.