Obstruction to superconducting pairing from Dirac monopoles in iron-based superconductors*

Iron based superconductors (FeSCs) are known to exhibit intriguing topological properties such as surface states, hinge modes and Majorana zero-modes. For clues to the origin of these properties, recent experiments [1, 2] have suggested the role of the parent state being a doped Dirac semimetal.

Here [3], we analyze topological obstruction to superconductivity in FeSCs stemming from Dirac nodes. We find that the diagonal elements of the superconducting gap matrix acquire a non-trivial vorticity and cannot be defined uniquely across the Brillouin zone.

Unlike Weyl semimetals, the structure of the vortex defect allows for the quasiparticle spectrum to be fully gapped, consistent with experiments. These conclusions are robust to the inclusion of other trivial spectator bands. We further discuss experimental signatures of such obstructed pairing phases, and demonstrate the emergence of Majorana bound-states on certain crystal terminations. Our work presents a new route for elucidating the topological properties of FeSCs.

[1] Zhang, P., et al., Nat. Phys., 15, 41 (2019).

[2] Kong, L., et al., Nat. Comm., 12, 1 (2021).

[3] Setty, C., Sur, S., Si. Q., (in preparation)