Iron-Based Superconductors: S₄ Symmetry Superconductors

I elaborate the construction of the recent two-orbital model for iron-based superconductors\cite{hu-hao} and discuss the properties of superconducting(SC) phases based on the $S_4$ symmetry. I demonstrate further that the underlining electronic structure is an almost decoupled two-orbital model obeying the $S_4$ symmetry. I discuss the classification of the SC states according to the $S_4$ symmetry, which leads to a natural prediction that there are two different phases even in the proposed $A_{1g}$ s-wave SC state of iron-based superconductors and the sign change in the SC state should be hidden between the upper and lower As(Se) layer in a single Fe-(As)Se layer, an analogy of the sign change under the $90^\circ$ degree rotation in the d-wave SC state of cuprates. The $S_4$ symmetry makes iron-based superconductors a unique class of superconductors with promising device applications.