122 iron arsenide

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 Ba_0.6K_0.4Fe₂As₂. The microscopic description of superconductivity in the 122s is yet unclear.

Ever since the discovery of high-temperature (High T_c) 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 T_c = 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.

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