Pairing strength and gap functions in multiband superconductors: 3D effects*

We examine the superconducting pairing symmetry in Fe-based superconductors using spin-fluctuation pairing theory. It has been shown in multi-orbital models that the different matrix elements of the pairing vertex are essential in determining the symmetry. In our approach we perform a 10-orbital spin-fluctuation calculation to account for the full matrix elements and the 3 dimensional character of the bandstructure which is most important in the systems under consideration (LiFeAs and K$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_2$). Our approach contains both, the deviations from tube-like Fermi surface that also allows different pairing strengths in the z-direction, and the hybridization of the Fermi surface. Starting from the tight-binding Hamiltonian corresponding to the real crystal cell, we find several competing 3D gap structures and compare with ARPES experiments.