Discovery of Intrinsic Electronic Anisotropy of Iron Pnictides and Nematic Electronic Order in BaFe2As2

Electronic Anisotropy

For the first time, conduction electrons in iron arsenide superconductors were determined to reveal unusual anisotropy in the crystallographically-isotropic state—nematic liquid crystalline phase— similar to crystallographic organization of materials used in liquid-crystalline displays. A close relationship between the occurrence of the high-temperature superconductivity and the existence of this unusual nematic electronic phase has been discovered. On cooling, crystals of the recently found iron arsenide superconductors undergo structural transition, which distorts the lattice and reduces its symmetry. Because of the existence of several equivalent directions of structural distortion in the orthorhombic phase, the crystal splits into structural domains, which mask intrinsic anisotropy of the materials in the orthorhombic plane.

Application of mechanical strain drives the crystals into a single domain state. Intrinsic anisotropy of the orthorhombic phase is measured by electrical resistivity. Unexpectedly, anisotropy was discovered also in the structurally-isotropic tetragonal phase of BaFe2As2, but not CaFe2As2. This intrinsic electronic anisotropy suggests conduction electrons in iron pnictides follow a different organization than in usual metals, described in both weakly interacting Fermi-gas concepts and Landau Fermi-liquid models of metals with strong inert-electron interactions. The same organization is found in the only other family of high temperature superconductors—the superconducting cuprates. Importance of this finding for superconductivity is emphasized by the fact, that nematic order is observed only in the parent compounds of would be doping-induced superconductors.

 

Contact: M. A. Tanatar, tanatar@ameslab.gov

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