Jiangping Hu and Ningning Hao at Beijing Natio- nal Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, and Department of Physics, Purdue University, West Lafayette, USA, present a symmetry-based effective microscopic theory of simplicity that makes an extraordinary stride, in a new direction, toward the ultimate answers to these challenging questions. The current favorite effective microscopic theory for iron-based superconductors employs all five orbitals associated with the d electrons of an iron atom. While this theory offers flexibility in terms of allowing for fair agreement with fully microscopic (first principles) electronic structure calculations, it is unable, given its vast parameter space, to explain the robustness of the electron-pairing symmetry observed across many different members of the family. In a remarkable contrast, their theory involves only two iron orbitals, but the orbitals are deeply connected through the S4 lattice symmetry characteristic of this family of new superconductors. Despite its simplicity (and very few free parameters), the two-orbital model can explain the big changes seen in the band structures across different materials in the family.

Equally, if not more, remarkably, the apparently different electron-pairing symmetries seen in the new superconductors and the older cuprates turn out to be, within the framework of their model, connected by a mathematical (“gauge”) transformation, which is not unlike a transformation connecting two different reference frames.

The model strongly suggests, therefore, a unifying way to understand both the new superconductors and the older cuprates. The problem of studying the pairing symmetry in the iron-based superconductors becomes then the well-studied problem of pairing symmetry in the cuprates. Their introduction of the S4-symmetry-related two orbitals provides a new basic platform from which many exciting possibilities of predicting, discovering, and understanding novel properties of iron-based superconductors can be launched.

Jiangping Hu and Ningning Hao: S4 Symmetric Microscopic Model for Iron-Based Superconductors, In: Physical Review X, Volume 2, Issue 2, April - June 2012, Article 021009 [13 pages]:

http://prx.aps.org/abstract/PRX/v2/i2/e021009

Figure: The basic Fe-As or Fe-Se trilayer structure in iron-based superconductors and a sketch of the s-wave pairing symmetry to a d-wave pairing symmetry under gauge transformation. © Jiangping Hu et al.