Plaquette-centered rotation symmetry and octet-nodal superconductivity in $\text{KFe}_{2}\text{As}_{2}$

Abstract

A plaquette-centered rotation symmetry C p 4 is identified to play a significant role in determining and stabilizing the Fermi-surface structure of Fe-based superconductors. Together with the S4 symmetry previously found, we are able to sort out the tangling orbitals and solve the puzzle of pairing symmetry of superconductivity in KFe2As2 in a simple but comprehensive way. By modeling the material with a strong coupling t − J1 − J2 model, we find phase transitions of pairing symmetry driven by the competition between the local spin antiferromagnetic couplings from nodal d x 2 −y 2 × s x 2 +y 2 -wave to nodeless s x 2 y 2 -wave through the intermediate s + id × s mixed pairing phase, which is consistent with the observation of pressure experiments. The emergent d-wave form factor inevitably arises from the projection of inter-orbital Cooper pairing onto the Fermi surface and is inherited from the electronic structure in the representation of C p 4 symmetry. Moreover, the S4 symmetry dictates 2 copies of d-wave pairing condensates, counting 8 nodes in total. We further show that weakly breaking C p 4 naturally leads to the octet nodal gap as precisely observed in laser angle resolved photoemission spectroscopy. The octet nodes reflect the collaboration of the C p 4 and S4 symmetries, which sheds new light on the enigma of the pairing symmetry in KFe2As2.