Improper s-wave symmetry of the electronic pairing in iron-based superconductors by first-principles calculations

By means of space-group symmetry arguments, we argue that the electronic pairing in iron-based high-temperature superconductors shows a structure which is a linear combination of planar s-wave and d-wave symmetry channels, both preserving the three-dimensional A1g irreducible representation of the corresponding crystal point group. We demonstrate that the s- and d-wave channels are determined by the parity under reflection of the electronic orbitals through the iron planes and by improper rotations around the iron sites. We provide evidence of these general properties by performing accurate quantum Monte Carlo (QMC) ab initio calculations of the pairing function, for a FeSe lattice with tetragonal experimental geometry at ambient pressure. We find that this picture survives even in the FeSe under pressure and at low temperatures, when the tetragonal point-group symmetry is slightly broken. In order to achieve a higher resolution in momentum space we introduce a BCS model that faithfully describes our QMC variational pairing function on the simulated 4x4 FeSe unit cell. This allows us to provide a k-resolved image of the pairing function and show that nonisotropic contributions in the BCS gap function are related to the improper s-wave symmetry. Our theory can rationalize and explain a series of contradictory experimental findings, such as the observation of twofold symmetry in the FeSe superconducting phase, the anomalous drop of T c with Co impurity in LaFeAsO(1-x)Fx, the s- to d-wave gap transition in BaFe2As2 under K doping, and the nodes appearing in the LiFeAs superconducting gap upon P isovalent substitution. © 2013 American Physical Society.