Theory of magnetism and superconductivity for iron-chalcogenides

We discuss the difference on both magnetic and superconducting properties between iron-pnictides and iron-chalcogenides high temperature superconductors and construct a microscopic model based on S4 symmetry that faithfully represents the intrinsic unit cell doubling of the materials. Within this model, we can show that there are two different types of extended S-wave pairing states. One is in the A representation of S4 and the other is in the B representation of S4. The A-phase is well studied S± phase when there are both electron pockets at M points and hole pockets at Γ point. However, the B phase is the new phase which has been ignored by models with one-iron per unit cell. The B phase is characterized by the sign reverse of the superconducting order parameter in real space between top and bottom layers in the building block of the trilayer Fe-As(Se) structure. We show that the B-phase is most likely realized in the electron-doped 122' or strained single layer iron-chalcogenides.