Iron-based superconductor

Iron-based superconductors (sometimes misleadingly called iron superconductors) are chemical compounds (containing iron) with superconducting properties. In 2008, led by recently discovered iron pnictide compounds (originally known as oxypnictides), they were in the first stages of experimentation and implementation. (Previously most high-temperature superconductors were cuprates and being based on layers of copper and oxygen sandwiched between other (typically non-metal?) substances.[1] )

This new type of superconductors is based instead on conducting layers of iron and a pnictide (typically arsenic) and seems to show promise as the next generation of high temperature superconductors.[2]

Much of the interest is because the new compounds are very different from the cuprates and may help lead to a theory of non-BCS-theory superconductivity.

More recently these have been called the ferropnictides. The first ones found belong to the group of oxypnictides. Some of the compounds have been known since 1995 [3] and their semiconductive properties have been known and patented since 2006.[4] [5]

It has also been found that some iron chalcogens superconduct;[6] for example, doped FeSe can have a critical temperature (Tc) of 8 K at normal pressure, and 27 K under high pressure.

A subset of iron-based superconductors with properties similar to the oxypnictides, known as the 122 Iron Arsenides, attracted attention in 2008 due to their relative ease of synthesis.

The oxypnictides such as LaOFeAs are often referred to as the '1111' pnictides.

The crystalline material, known chemically as LaOFeAs, stacks iron and arsenic layers, where the electrons flow, between planes of lanthanum and oxygen. Replacing up to 11 percent of the oxygen with fluorine improved the compound — it became superconductive at 26 kelvins, the team reports in the March 19, 2008 Journal of the American Chemical Society. Subsequent research from other groups suggests that replacing the lanthanum in LaOFeAs with other rare earth elements such as cerium, samarium, neodymium and praseodymium leads to superconductors that work at 52 kelvins.[2]

Compounds such as Sr2ScFePO3 discovered in 2009 are referred to as the '42622' family, as FePSr2ScO3.[7] Noteworthy is the synthesis of (Ca4Al2O6-y)(Fe2Pn2) (or Al-42622(Pn); Pn = As and P) using high-pressure synthesis technique. Al-42622(Pn) exhibit superconductivity for both Pn = As and P with the transition temperatures of 28.3 K and 17.1 K, respectively. The a-lattice parameters of Al-42622(Pn) (a = 3.713 Å and 3.692 Å for Pn = As and P, respectively) are smallest among the iron-pnictide superconductors. Correspondingly, Al-42622(As) has the smallest As-Fe-As bond angle (102.1°) and the largest As distance from the Fe planes (1.5 Å).[8] High-pressure technique also yields (Ca3Al2O5-y)(Fe2Pn2) (Pn = As and P), the first reported iron-based superconductors with the perovskite-based '32522' structure. The transition temperature (Tc) is 30.2 K for Pn = As and 16.6 K for Pn = P. The emergence of superconductivity is ascribed to the small tetragonal a-axis lattice constant of these materials. From these results, an empirical relationship was established between the a-axis lattice constant and Tc in iron-based superconductors.[9]

In 2009, it was shown that undoped iron pnictides had a magnetic quantum critical point deriving from competition between electronic localization and itinerancy.

Contents

Superconductivity

Superconducting transition temperatures are listed in the tables (some at high pressure). BaFe1.8Co0.2As2 is predicted to have an upper critical field of 43 teslas from the measured coherence length of 2.8 nm.[10]

oxypnictide Tc (K)
LaO0.89F0.11FeAs 26[11]
LaO0.9F0.2FeAs 28.5[12]
CeFeAsO0.84F0.16 41[11]
SmFeAsO0.9F0.1 43[11]
La0.5Y0.5FeAsO0.6 43.1[13]
NdFeAsO0.89F0.11 52[11]
PrFeAsO0.89F0.11 52[14]
ErFeAsO1-y 45[15]
Al-32522 (CaAlOFeAs) 30(As), 16.6 (P)[9]
Al-42622 (CaAlOFeAs) 28.3(As), 17.2 (P)[8]
GdFeAsO0.85 53.5[16]
BaFe1.8Co0.2As2 25.3[10]
SmFeAsO~0.85 55[17]
non-oxypnictide Tc (K)
Ba0.6K0.4Fe2As2 38[18]
Ca0.6Na0.4Fe2As2 26[19]
CaFe0.9Co0.1AsF 22[20]
Sr0.5Sm0.5FeAsF 56[21]
LiFeAs <18[22][23]
NaFeAs 9–25[24][25]
FeSe <27[26][27]

References

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Further reading

See also