Robustness of s-wave pairing symmetry in iron-based superconductors and its implications to fundamentals on magnetically-driven high temperature superconductivity

Under the assumption that the superconducting state belongs to a single irreducible representation of lattice symmetry, we argue that the pairing symmetry in all measured iron-based superconductors is universally consistent with the A_{1g} s-wave. The robust s-wave pairing throughout the different families of iron-based superconductors at different doping regions signals two fundamental principles behind high T_c superconducting mechanisms: (1) the correspondence principle: the short range magnetic exchange interactions and the Fermi surfaces act collaboratively to achieve high T_c superconductivity and determine pairing symmetries; (2) the magnetic selection pairing rule: the superconductivity is only induced by the magnetic exchange couplings from the superexchange mechanism through cation-anion-cation chemical bondings. These principles explain why the unconventional high T_c superconductivity appears to be such a rare but robust phenomena with its strict requirement on electronic environment. The robust s-wave pairing also reveals that the current standard effective models with only onsite interactions are not sufficient and a minimum microscopic model must include strong nearest neighbor repulsive interactions resulted from the d-d direct bondings to serves as a s-wave symmetry stabilizer. Finally, the sign distribution of the superconducting order parameters in the reciprocal space is simply a consequence of the form factors given by the leading short-range pairings. The sign change of superconducting order parameters on Fermi surfaces is not a necessary requirement in repulsive-interaction-driven high T_c mechanism. The results will guide us to search for new electronic structure that supports high T_c superconductivity.