122 iron arsenide

Overview

Ever since the discovery of high-temperature (High Tc) superconductivity in the cuprate materials, scientists have worked tirelessly to understand the microscopic mechanisms responsible for its emergence. To this day, no theory can fully explain the high-temperature superconductivity and unconventional (non-s-wave) pairing state found in these materials. However, the interest of the scientific community in understanding the pairing glue for unconventional superconductors—those in which the glue is electronic, i.e. cannot be attributed to the phonon-induced interactions between electrons responsible for conventional BCS theory s-wave superconductivity—has recently been expanded by the discovery of high temperature superconductivity (up to Tc = 55 K) in the doped oxypnictide (1111) superconductors with the chemical composition XOFeAs, where X = La, Ce, Pr, Nd, Sm, Gd, Tb, or Dy.{{Cite journal | doi = 10.1103/PhysRevLett.101.257003|pmid = 19113744 |bibcode=2008PhRvL.101y7003H| title = Neutron-Diffraction Measurements of Magnetic Order and a Structural Transition in the Parent BaFe2As2 Compound of FeAs-Based High-Temperature Superconductors| journal = Physical Review Letters| volume = 101| issue = 25|article-number = 257003 | year = 2008| last1 = Huang | first1 = Q.| last2 = Qiu | first2 = Y.| last3 = Bao | first3 = W. | last4 = Green | first4 = M. A.| last5 = Lynn | first5 = J. W.| last6 = Gasparovic | first6 = Y. C.| last7 = Wu | first7 = T.| last8 = Wu | first8 = G.| last9 = Chen | first9 = X. H.

There are two different ways in which superconductivity was achieved in the 122s. One method is the application of pressure to the undoped parent compounds.

Unlike the oxypnictides, large single crystals of the 122s can be easily synthesized by using the flux method.{{Cite journal|arxiv = 0901.4672

Synthesis

One of the important qualities of the 122s is their ease of synthesis; it is possible to grow large single crystals, up to ~5×5×0.5 mm, using the flux method. In a nutshell, the flux method uses some solvent in which the starting materials for a chemical reaction are able to dissolve and eventually crystallize into the desired compound. Two standard methods show up in the literature, each using a different flux. The first method employs tin,

Structural and magnetic phase transition

The 122s form in the I4/mmm tetragonal structure. For example, the tetragonal unit cell of SrFe2As2, at room temperature, has lattice parameters a = b = 3.9243 Å and c = 12.3644 Å. The planar geometry is reminiscent of the cuprate high-Tc superconductors in which the Cu-O layers are believed to support superconductivity.{{cite journal

These materials undergo a first-order structural phase transition into the Fmmm orthorhombic structure below some characteristic temperature T0 that is compound specific. NMR experiments on the CaFe2As2 show that there is a first-order antiferromagnetic magnetic phase transition at the same temperature; in contrast, the antiferromagnetic transition occurs at a lower temperature in the 1111s. The high temperature magnetic state is paramagnetic, while the low temperature state is an antiferromagnetic state known as a spin-density-wave.

Superconductivity

Superconductivity has been observed in the 122s up to a current maximum Tc of 38 K in Ba1−xKxFe2As2 with x ≈ 0.4. Resistivity and magnetic susceptibility measurements have confirmed the bulk nature of the observed superconducting transition. The onset of superconductivity is correlated with the loss of the spin-density-wave state.

The Tc of 38 K in Ba1−xKxFe2As2 (x ≈ 0.4) superconductor shows the inverse iron isotope effect.{{Cite journal | doi = 10.1103/PhysRevLett.103.257003|pmid = 20366277|bibcode=2009PhRvL.103y7003S|arxiv = 0903.3515| title = Inverse Iron Isotope Effect on the Transition Temperature of the (Ba,K)Fe2As2 Superconductor| journal = Physical Review Letters| volume = 103| issue = 25|article-number = 257003| year = 2009| last1 = Shirage | first1 = P. M. | last2 = Kihou | first2 = K. | last3 = Miyazawa | first3 = K. | last4 = Lee | first4 = C. H. | last5 = Kito | first5 = H. | last6 = Eisaki | first6 = H. | last7 = Yanagisawa | first7 = T. | last8 = Tanaka | first8 = Y. | last9 = Iyo | first9 = A.

Other compounds with 122 structure

In addition to the iron arsenides, the 122 crystal structure plays an important role for other material systems as well. Three famous examples from the field of heavy fermions are CeCu2Si2 (the "first unconventional superconductor" discovered 1978),{{cite journal |hdl-access=free}} URu2Si2 (which is also a heavy fermion superconductor but is the focus of active present research due to the so-called "hidden-order phase" below 17.5 K),{{cite journal and YbRh2Si2 (one of the prime examples of quantum criticality).{{cite journal|doi=10.1038/nphys892

References

References

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