**
I will discuss a 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 the Γ 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. This can be used 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 that, strictly speaking, the nodal
spin-density wave is unstable once spin-orbit coupling is included.
Similar analysis is performed for the A1g spin singlet superconducting
state. Without spin-orbit coupling there is one k-independent pairing
invariant near the Γ point but two near the M point. This leads to an
isotropic spectral gap at the hole Fermi surface near ., 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 Γ 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. (Vladimir Cvetkovic and Oskar Vafek, PRB 88, 134510
(2013))
**

**
**

To begin viewing slides, click on the first slide below. (Or, view as pdf.)