| The symmetry and the structure of the superconducting gap in Fe-based superconductors have been subjects of numerous experimental and theoretical papers in recent years. There is a growing consensus among researchers that the gap has an extended s-wave symmetry-it belongs to a symmetric A1g representation of the D4h symmetry group of a square lattice and its average values along electron and hole Fermi surfaces FSs are of opposite signs. A more subtle and currently hotly debated issue is whether the gap has nodes. This is not a symmetry issue as, quite generally, extended s-wave gap can be approximated by a constant only along the hole FS, while along the two electron FSs it has angle independent and cos 2φ components. The issue whether or not the gap in Fe pnictides has nodes is crucial for the understanding of low-energy properties of these materials and deserves a careful study. Here we will analyze the transition from nodeless to nodal s+--wave superconducting gap as a function of the Couplomb interaction and its consequence for the magnetic response, specific heat, and Raman scattering. In particular, we discuss the role of the electronic correlations in the formation of the collective modes in Raman scattering and and its consistency with INS results. |
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