Winding numbers of nodal points in Fe-based superconductors

We analyze the nodal points in multi-orbital Fe-based superconductors from a topological perspective. We consider the $s^{+-}$ gap structure with accidental nodes, and the $d$-wave gap with nodes along the symmetry directions. In both cases, the nodal points can be moved by varying an external parameter, e.g., a degree of inter-pocket pairing. Eventually, the nodes merge and annihilate via a Lifshitz-type transition. We discuss the Lifshitz transition in Fe-based superconductors from a topological point of view. We show, both analytically and numerically, that the merging nodal points have winding numbers of opposite sign. This is consistent with the general reasoning that the total winding number is a conserved quantity in the Lifshitz transition.