Vertical loop nodes in iron-based superconductors

Abstract

We consider Fe-based superconductors with s+− gap with accidental nodes on electron pockets. We analyze how the gap structure changes if we include into the consideration the hybridization between the two electron pockets [the interpocket hopping term with momentum (π,π,π)]. We derive the hybridization term and relate it to the absence of inversion symmetry in the Fe plane because of two nonequivalent locations of pnictogen (chalcogen) above and below the plane. We find that the hybridization tends to eliminate the nodes—as it increases, the pairs of neighboring nodes approach each other, merge, and disappear once the hybridization exceeds a certain threshold. The nodes disappear first around kz=π/2, and vertical line nodes split into two vertical loops centered at kz=0 and kz=π. We also show that the hybridization moves the nodes along the loops away from the normal-state Fermi surfaces. This creates a subset of k points at which the peak in the spectral function does not shift as the system enters into a superconducting state (“no-shift” lines). These no-shift lines evolve with increasing hybridization in a highly nontrivial manner and eventually form horizontal loops in the (kx,ky) plane, surrounding the nodes. Both vertical line nodes and horizontal no-shift loops surrounding them should be detectable in photoemission experiments.

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