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Inevitable gloss and glitter in welcoming literature for a new family of superconductors has been followed with girth and gravitas in numerous studies. Despite which, a mechanism of superconductivity in iron pnictides and iron chalcogenides is not settled1. Interplay between magnetic and structural degrees of freedom is a central problem, for knowledge of the normal state from which superconductivity emerges is crucial to uncovering its true nature.
A seemingly subtle facet of the electron structure of Ba1 – x Nax Fe2 As2 with x ≈ 0.24 – 0.28 might prove exceptionally insightful. At a temperature ≈ 38 K, well above the temperature where superconductivity emerges (Tc ≈ 25 K), the orthorhombic magnetic material undergoes a structural transition that restores tetragonal symmetry of the parent (paramagnetic) structure. However, the re-entrant tetragonal phase actually hosts two orbital motifs depicted in Fig. 1 and neither one is labelled by a set of ion displacements, relative to the parent structure, that could be picked up by standard Bragg diffraction experiment2.
Figure 1: Ferrous ions - Fe2+ (5D) in tetrahedral coordination with As ions - possess degenerate xz and yz orbitals as shown in the picture in the tetragonal parent structure with the I4/mmm1’ symmetry. In the low temperature re-entrant tetragonal phase with either P4/mnc1’ or I4221’ symmetry orbital degeneracy is removed, but the symmetry of the orbital ordering does not allow any coupling to atomic displacements or symmetry-breaking strain components.
A similar, purely electronic, transition has been discovered in neptunium dioxide (NpO2) by exploiting Templeton & Templeton X-ray scattering3, 4. In the case of Ba1 – x Nax Fe2 As2 it has been shown that Templeton & Templeton scattering does indeed distinguish between the three tetragonal structures of interest; the parent (I4/mmm1’) and the two depicted orbital motifs (P4/mnc1’ and I4221’).
Observation of the orbital motifs offers more insight to electronic properties of Ba1 – x Nax Fe2 As2 than meets the naked eye, because they are distinct signatures of the mechanism by which the orthorhombic magnetic phase is created. The two motifs arise from orbital ordering of the iron 3d-electrons that renormalises exchange parameters in the material and triggers magnetic order. Other folk argue that orthorhombic magnetic order is instead created by magnetic fluctuations, but this mechanism does not lead to tetragonal orbital motifs in the re-entrant phase2.
Figure 2: Ba ions in Ba1 – x Nax Fe2 As2 are depicted as large green spheres, and Fe ions coordinated by As ions are red dots with their 3d-orbitals coloured blue.
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