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  • Featured in Physics
  • Open Access

S4 Symmetric Microscopic Model for Iron-Based Superconductors

Jiangping Hu and Ningning Hao

  • Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China
  • Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA

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Phys. Rev. X 2, 021009 – Published 30 May, 2012

DOI: https://doi.org/10.1103/PhysRevX.2.021009

Abstract

Although iron-based superconductors are multiorbital systems with complicated band structures, we demonstrate that the low-energy physics which is responsible for their high-Tc superconductivity is essentially governed by an effective two-orbital Hamiltonian near half filling. This underlying electronic structure is protected by the S4 symmetry. With repulsive or strong next-nearest-neighbor antiferromagnetic exchange interactions, the model results in a robust A1g s-wave pairing which can be mapped exactly to the d-wave pairing observed in cuprates. The classification of the superconducting (SC) states according to the S4 symmetry leads to a natural prediction of the existence of two different phases, named the A and B phases. In the B phase, the superconducting order has an overall sign change along the c axis between the top and bottom As (or Se) planes in a single Fe-As (or Fe-Se) trilayer structure, the common building block of iron-based superconductors. The sign change is analogous to the sign change in the d-wave superconducting state of cuprates upon 90° rotation. Our derivation provides a unified understanding of iron pnictides and iron chalcogenides, and suggests that cuprates and iron-based superconductors share an identical high-Tc superconducting mechanism.

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Untangling the Orbitals in Iron-Based Superconductors

Published 30 May, 2012

Symmetry considerations point to a universal mechanism responsible for superconductivity in the iron pnictides and iron chalcogenides.

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Popular Summary

The discoveries since 2008 of a large and chemically rather diverse class of high-temperature iron-based superconductors with maximal superconducting temperatures reaching above 50 K have renewed the excitement in superconductivity. In addition to offering the promise of new high-temperature superconductors, these materials have posed some great fundamental scientific challenges: Is there a microscopic theory that unifies our understanding of the superconductivity of these varied materials? What may this theory be? Are there deep connections between the new superconductors and the older, copper-based high-temperature superconductors (cuprates)? In this paper, we 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, our 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 our model, connected by a mathematical (“gauge”) transformation, which is not unlike a transformation connecting two different reference frames. Our 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.

We believe that our 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.

Article Text

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Phys. Rev. X 2, 021009– Published 30 May, 2012
  • Received 8 March 2012
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