Schedule Sep 09, 2014
Space group symmetry, spin-orbit coupling and the low energy effective Hamiltonian for iron based superconductor
Oskar Vafek, Florida State & KITP

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))

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