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FeSe< Superconductivity.DopedCuprate | Index | Superconductivity.Topology > Alternative between nodal and nodeless {$s^\pm$} symmetry, as described in Can Li Song et al. Science 332 1410 (2011). The five band model by S. Graser, T. A. Maier, P. J. Hirschfeld, D. J. Scalapino, N. J. Phys. 11, 025016 (2009) dictates that the two electron pockets at {$M$} are formed by {$d_{xz},d_{xy}\,(\pi,0)$} and {$d_{yz},d_{xy}\,(0,\pi)$} bands, while the hole pocket at {$\Gamma$} i.e. at {$(0,0)$} are formed by {$d_{xz},d_{yz}$} bands. The two {$s^\pm$} gaps belong to the same irreducible representation of the crystal symmetry group, so both, or even a mixture of the two are possible. The more general gap function is {$\Delta_{s\pm}=\Delta_1\cos k_x \cos k_y + \Delta_2(\cos k_x + \cos k_y)$} Actually both parts have lines of nodes. Those of the first part, of amplitude {$\Delta_1$}, cannot intersect the small-radius electron Fermi pockets, whereas the lines of nodes of the second, of amplitude {$\Delta_2$}, cross these pockets. So, in practice, {$\Delta_1$} is nodeless and {$\Delta_2$} is nodal. Nodeless {$\Delta_1$} gaps are due to next-nearest-neighbor (NNN) electron pairing, dominated in real space by Se-mediated Fe-Fe J2 exchange interaction. This corresponds to the formation of the spin density wave (SDW) and, in k space, to a phase shift of {$\pi$} in going from the hole to the electron pockets. Nodal {$\Delta_2$} gaps are due to the competition with next-neighbor (NN) electron pairing, dominated in real space by direct exchange J1 between adjacent Fe ions. If J1 is larger but comparable to J2, energy may be minimized by developing a node on the electron pockets. < Superconductivity.DopedCuprate | Index | Superconductivity.Topology > |