Chemistry and electronic structure of iron-based superconductors
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Discovery
Chemistry and properties
With the 2008 discovery1 of high-temperature superconductivity in fluorine-doped LaFeAsO—an iron-based material—the hunt for similar materials and the race toward understanding their properties began. This race has been tremendously fruitful, yielding both a wide variety of new superconducting compounds and unveiling unanticipated aspects of superconductivity. After the discovery, it was immediately observed1 that these materials have similarities to the other well-known family of very high-temperature superconductors, the cuprates. LaFeAsO and all the other known Fe-based superconductors feature a square planar transition-element sheet analogous to the square planar CuO2 sheets in the cuprates, and furthermore these compounds are near antiferromagnetism. Also, similar to the cuprates, a wide variety of Fe-based compounds both with the original structure and related structure types were discovered following the initial report of high-temperature superconductivity. Initial efforts focused on substitutions based on the prototypical material, LaFeAsO, which is superconducting when doped with F. The rare-earth substituted materials RFeAsO (R = rare earth) showed a clear and much-discussed systematic trend2: Reported critical temperatures Tc across the lanthanides are 26 K, 41 K, 52 K, 52 K, 55 K, 36 K, 46 K, 45 K, and 36 K for R = La,1 Ce,3 Pr,4 Nd,5 Sm,6,7 Gd,8 Tb,9 Dy,9 and Ho,10 respectively. Therefore, maximum Tc increases from R = La to R = Sm compounds and then decreases, though not uniformly.
So far, five additional families of iron-based superconductors have been found. They are all based on tetragonal structure types, with tetrahedrally coordinated, nominally divalent Fe on a square lattice. Beyond the ZrCuSiAs-type, RFeAsO (R = rare earth),1 these are the ThCr2Si2-type BFe2As2 (B = alkaline earth)11,12 and AFe2Se2 (A = Alkali metal or Tl),13 the Cu2Sb-type AFeAs,14–16 the PbO-type FeCh (Ch = chalcogen: Se or alloys with S and Te),17,18 the Sr3Fe2O5Cu2S2-type Sr3T2O5Fe2As2 (T = Sc),19 and the Sr4T2O6Fe2Pn2 compounds (T = Sc or V; Pn = pnictogen: As or P).20,21 The main structural families are shown in Figure 1. The six families are known as the 1111, 122, 111, 11, 32522, and 42622 compounds, respectively. Besides these Fe-based superconductors, there are families of Ni-based superconductors with similar structures.22 However, these materials have low Tc and are apparently standard electron-phonon materials,23,24 while the Fe-based materials are not.25,26 On the other hand, LaFePO, which was previously discovered to be a low-Tc superconductor,27 appears to be closely related to the high-Tc Fe-based superconductors. Remarkably, and in contrast to cuprates, the parent compounds of Fe-based superconductors can be doped by alloying on the active Fe-site, thereby inducing high-temperature superconductivity.28 The Fe atoms making up the square planes are tetrahedrally coordinated by pnictogen or chalcogen atoms in these structures. This is universal to the Fe-based superconductors.
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