Iron Pnictides




The discovery of iron pnictide superconductors in 2008 drives the history of high-TC superconductors from the “copper age”to the “iron age”. Since the first report of superconductivity at 26 K in F-doped LaFeAsO, the iron pnictide superconductor family has been fast expanded to more than six different structures (FeSe:“11”, LiFeAs:”111”, REFeAsO:“1111”, AEFe2As2:“122”, Sr2VO3FeAs:“21311”, and so on), and the superconducting transition temperature (TC) has been rapidly raised to approximately 57 K.

Why the iron pnicitdes are so significant? First, they promise interesting physics that stems from the relationship of superconductivity and magnetism. Second, with the multiband structure, they give the hope for uncovering the mechanism of superconductivity and exploring higher TC superconductors. Third, they are quite suitable in the future application for having higher HC than cuprates and high isotropic critical current.
As an important member in iron pnictide family, “122” iron pnictide consists of variety of different compounds with wide chemical-tuning of hole doping (e.g. Ba1-xKxFe2As2), electron doping (e.g. Ba(Fe1-xCox)2As2) or isovalent substitution (e.g. EuFe2(As1-xPx)2). Because of good crystal quality and the variety of compounds, the “122” iron pnictides are most studied by optical conductivity, neutron experiment, ARPES, and STM.
The THz and Infrared Lab of 1. Physics Institute was involved in the research of iron pnictide since 2008. With the concentration on “122” single crystals and thin films, we have plenty of fruitful and influential results in the field of iron pnictides research. We investigated the normal state and superconducting state on the electron doping superconductors: Ba(Fe1-xCox)2As2 (TC,MAX = 23 K) and Ba(Fe1-xNix)2As2 (TC,MAX = 21 K), isovalently substituted superconductors: EuFe(As1-xPx)2 (TC,MAX = 28 K), and antiferromagnetic compound: BaMn2As2 (TN = 625 K).


Figure 2: Magnetic structure of EuFe2As2 determined by single-crystal neutron diffraction and for EuFe2As2.

The superconducting gap symmetry, the interband interaction, the SDW gap, the coupling between superconductivity and magnetism, and charge dynamics were carefully investigated by THz, infrared and magnetism measurements.
Now we are interested in the research of the anisotropy and magnetism in “122” iron pnictides. Via improving our traditional optics methods, two main points will be addressed: the nematic phase and related anisotropy in the normal and superconducting state; the interplay of local magnetism and superconductivity with particular emphasis of effect of the localized magnetic moments. 





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