Space group symmetry, spin-orbit coupling, and the low-energy effective Hamiltonian for iron-based superconductors

We construct the symmetry adapted low-energy effective Hamiltonian for the electronic states in the vicinity of the Fermi level in iron-based superconductors. We use Luttinger's method of invariants, expanding about Gamma and M points in the Brillouin zone corresponding to a two-iron unit cell, and then matching the coefficients of the expansion to the five-and eight-band models. We then use the method of invariants to study the effects of the spin-density wave order parameters on the electronic spectrum, with and without spin-orbit coupling included. Among the results of this analysis is the finding that the nodal spin-density wave is unstable once spin-orbit coupling is included. Similar analysis is performed for the A(1g) spin singlet superconducting state. Without spin-orbit coupling there is one pairing invariant near the Gamma point but two near the M point. This leads to an isotropic spectral gap at the hole Fermi surface near Gamma, but anisotropic near M. The relative values of these three parameters determine whether the superconducting state is s(++), s(+-), or nodal. Inclusion of spin-orbit coupling leads to additional mixing of spin triplet pairing, with one additional pairing parameter near Gamma and one near M. This leads to an anisotropic spectral gap near both hole and electron Fermi surfaces, the latter no longer cross but rather split.