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 inter-pocket hopping term with momentum (pi,pi,pi). We derive the hybridization term and relate it to the absence of inversion symmetry in the Fe-plane because of two non-equivalent 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 k_z =pi/2, and vertical line nodes split into two vertical loops centered at k_z =0 and k_z = pi. 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 highly non-trivial manner and eventually form horizontal loops in (k_x, k_y) plane, surrounding the nodes. Both vertical line nodes and horizontal "no-shift" loops surrounding them should be detectable in photoemission experiments.