Possible Unconventional High-Temperature Iron-Based Superconductors in Hexagonal Structure

Since the discovery of iron-based superconductors, many rich physics in these materials have been discovered and well understood, however the superconductivity mechanism remains a controversial subject. The essential difficulty lies on how to identify indispensable features that are directly tied to high temperature superconductivity among complex electronic structure and physical phenomena. A successful identification should also lead us to predict new families of high temperature superconductors. In this talk, we propose that hexagonal iron-based transition metal pnictides could be high temperature superconductors. We investigate the electronic and magnetic structures of the layered hexagonal transition metal pnictides with A=(Sr, Ca), M=(Cr, Mn, Fe, Co, Ni) and B=(As, P, Sb). We find that the family of materials shares critical similarities with those of tetragonal structures that include the famous iron-based superconductors. In both families, the next nearest neighbor effective antiferromagnetic exchange couplings reach the maximum value in the iron-based materials. We also find that the superconducting state is a time reversal symmetry broken d+id state upon doping in iron-base hexagonal materials.