How are cuprates and iron-based high temperature superconductors correlated? What is the common mechanism behind two different families of iron-based superconductors, iron-pnicitides and iron-chalcogenides? These two questions are two major challenges in the today’s field of high temperature superconductors.
In the past five years, it has been suggested that there is no universal pairing symmetry for different families of iron-based superconductors , as well as for the same materials with different doping level. A sign changed s-wave, s±, which belongs to A1g representation, was the leading candidate for the optimally-doped iron-pnictides. This proposal was commonly accepted because no universal nodes were observed along high symmetry lines.
In this talk, we suggest that a fundamental mistake may have been made in all previous studies of iron-based superconductors based on models with 1-Fe unit cell because of ignoring a pairing channel with odd parity which stems from the intrinsic 2-Fe unit cell. We argue that iron-based superconductors are special systems with odd parity ground states. Their superconducting states are η-pairing spin singlet states characterized by odd parity but without symmetry-protected gapless node points on Fermi surfaces. The new states have a fingerprint with a real space sign inversion between the top and bottom As/Se layers. This state is naturally obtained in a recently constructed S4 symmetry effective model.
We believe that the confirmation of the new states can not only establish a new state of matter, but also unifies two high tc superconductors to pave the way to settle the superconducting mechanism for high temperature superconductors.