Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

Superconductivity in iron compounds

The surprising discovery of high-temperature superconductivity in a material containing a strong magnet—iron—has led to thousands of publications. By placing all the data in context, it becomes clear what we know and where we are headed.

/pdf/high-temperature-superconductivity-in-iron-based-materials-t2ydrlz60b.pdf

High-temperature superconductivity in iron-based materials

The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

The recently discovered Fe-pnictide and chalcogenide superconductors display low-temperature properties suggesting superconducting gap structures which appear to vary substantially from family to family, and even within families as a function of doping or pressure. We propose that this apparent nonuniversality can actually be understood by considering the predictions of spin fluctuation theory and accounting for the peculiar electronic structure of these systems, coupled with the likely 'sign-changing s-wave' (s\pm) symmetry. We review theoretical aspects, materials properties and experimental evidence relevant to this suggestion, and discuss which further measurements would be useful to settle these issues.

Gap symmetry and structure of Fe-based superconductors

We analyze antiferromagnetism and superconductivity in novel Fe-based superconductors within the itinerant model of small electron and hole pockets near 0,0 and ,. We argue that the effective interactions in both channels logarithmically flow toward the same values at low energies; i.e., antiferromagnetism and superconductivity must be treated on equal footing. The magnetic instability comes first for equal sizes of the two pockets, but loses to superconductivity upon doping. The superconducting gap has no nodes, but changes sign between the two Fermi surfaces extended s-wave symmetry. We argue that the T dependencies of the spin susceptibility and NMR relaxation rate for such a state are exponential only at very low T, and can be well fitted by power laws over a wide T range below Tc.

https://www.pks.mpg.de/~ieremin/Pub/PhysRevB_78_134512.pdf

Magnetism, superconductivity, and pairing symmetry in iron-based superconductors

/pdf/momentum-dependence-of-the-superconducting-gap-in-ndfeaso-1-1opq40thbq.pdf

Momentum dependence of the superconducting gap in NdFeAsO$_{1-x}$F$_x$ single crystals measured by angle resolved photoemission spectroscopy

The superconducting penetration depth $\ensuremath{\lambda}(T)$ has been measured in $R{\mathrm{FeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ ($R=\mathrm{La}$, Nd) single crystals ($R\mathrm{\text{\ensuremath{-}}}1111$). In Nd-1111, we find an upturn in $\ensuremath{\lambda}(T)$ upon cooling and attribute it to the paramagnetism of the Nd ions, similar to the case of the electron-doped cuprate Nd-Ce-Cu-O. After the correction for paramagnetism, the London penetration depth variation is found to follow a power-law behavior, $\ensuremath{\Delta}{\ensuremath{\lambda}}_{L}(T)\ensuremath{\propto}{T}^{2}$ at low temperatures. The same ${T}^{2}$ variation of $\ensuremath{\lambda}(T)$ was found in nonmagnetic La-1111 crystals. Analysis of the superfluid density and of penetration depth anisotropy over the full temperature range is consistent with two-gap superconductivity. Based on this and on our previous work, we conclude that both the RFeAsO (1111) and ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ (122) families of pnictide superconductors exhibit unconventional two-gap superconductivity.

Nonexponential London penetration depth of FeAs-based superconducting RFeAsO(0.9)F(0.1) (R = La, Nd) single crystals.

Magneto-optical imaging was used to study the local magnetization in polycrystalline NdFeAsO0.9F0.1 (NFAOF). Individual crystallites up to ~200?100?30??m3 in size could be mapped at various temperatures. The in-grain, persistent current density is about j~105A?cm?2 and the magnetic relaxation rate in a remanent state peaks at about Tm~38?K. By comparison with the total magnetization measured in a bar-shaped, dense, polycrystalline sample, we suggest that NdFeAsO0.9F0.1 is similar to high-Tc cuprates, in between Bi2Sr2CaCu2O8+x and YBa2Cu3O7- x in terms of the vortex response in the mixed state. We find an apparent crossover in the pinning strength at ~38?K that could be associated either with vortex decoupling or melting. Below this temperature the static and dynamic vortex behavior is consistent with the collective pinning and creep.

Intrinsic magnetic properties of the superconductor NdFeAsO 0.9 F 0.1 from local and global measurements

Magneto-optical imaging was used to study the local magnetization in polycrystalline NdFeAsO$_{0.9}$F$_{0.1}$ (NFAOF). Individual crystallites up to $\sim200\times100\times30$ $\mu m^{3}$ in size could be mapped at various temperatures. The in-grain, persistent current density is about $j\sim10^{5}$ A/cm$^{2}$ and the magnetic relaxation rate in a remanent state peaks at about $T_{m}\sim38$ K. By comparison with with the total magnetization measured in a bar-shaped, dense, polycrystalline sample, we suggest that NdFeAsO$_{0.9}$F$_{0.1}$ is similar to a layered high-$T_{c}$, compound such as Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ and exhibits a $3D\to2D$ crossover in the vortex structure. The 2D Ginzburg parameter is about $Gi^{2D}% \simeq10^{-2}$ implying electromagnetic anisotropy as large as $\epsilon \sim1/30$. Below $T_{m}$, the static and dynamic behaviors are consistent with collective pinning and creep.

Intrinsic magnetic properties of NdFeAsO$_{0.9}$F$_{0.1}$ superconductor from local and global measurements

Systematic studies of the NdFeAsOF superconducting energy gap using point-contact Andreev-reflection (PCAR) spectroscopy are presented. At low temperatures the PCAR conductance spectra show a pair of gap-like peaks at about ± (4–7) mV and in most cases also a pair of humps at around ± 10 mV. Fits to the s-wave two-gap model of the PCAR conductance allowed to determine two superconducting energy gaps in the system. However, the energy-gap features disappear at T* = 15–20 K, much below the particular Tc of the junction under study. At T* a zero-bias conductance (ZBC) peak emerges, which at higher temperatures usually overwhelms the spectrum with an intensity significantly higher than the conductance signal at lower temperatures. Possible causes of this unexpected temperature effect are discussed. In some cases the conductance spectra show just a reduced conductance around the zero-bias voltage, the effect persisting well above the bulk transition temperature. This indicates the presence of a pseudogap in the system.

https://iopscience.iop.org/article/10.1088/0953-2048/22/1/014003/pdf

Possible two-gap superconductivity in NdFeAsO0.9F0.1 probed by point-contact Andreev-reflection spectroscopy

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