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The 122 iron arsenide unconventional superconductors are part of a new class of iron-based superconductors. They form in the tetragonal I4/mmm, ThCr2Si2 type, crystal structure. The shorthand name "122" comes from their stoichiometry; the 122s have the chemical formula AEFe2Pn2, where AE stands for alkaline earth metal (Ca, Ba, Sr or Eu) and Pn is pnictide (As, P, etc.).〔 These materials become superconducting under pressure and also upon doping.〔 〕 The maximum superconducting transition temperature found to date is 38 K in the Ba0.6K0.4Fe2As2. The microscopic description of superconductivity in the 122s is yet unclear. ==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. The 122s contain the same iron-arsenide planes as the oxypnictides, but are much easier to synthesize in the form of large single crystals. 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.〔〔 The second is the introduction of other elements (dopants) into the crystal structure in very specific ratios. There are two doping schemes: The first type of doping involves the introduction of holes into the barium or strontium varieties; hole doping refers to the substitution of one ion for another with fewer electrons. Superconducting transition temperatures as high as 38 K have been reported upon substitution of the 40% of the Ba2+ or Sr2+ ions with K+.〔 The second doping method is to directly dope the iron-arsenide layer by replacing iron with cobalt. Superconducting transition temperatures up to ~20 K have been observed in this case. Unlike the oxypnictides, large single crystals of the 122s can be easily synthesized by using the flux method. The behavior of these materials is interesting by that superconductivity exists alongside antiferromagnetism.〔 Various studies including electrical resistivity, magnetic susceptibility, specific heat,〔 NMR, neutron scattering,〔〔 X-ray diffraction, Mössbauer spectroscopy, and quantum oscillations have been performed for the undoped parent compounds, as well as the superconducting versions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「122 iron arsenide」の詳細全文を読む スポンサード リンク
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