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

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.

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High-temperature superconductivity in iron-based materials

A possible sighting of Majorana states Nearly 80 years ago, the Italian physicist Ettore Majorana proposed the existence of an unusual type of particle that is its own antiparticle, the so-called Majorana fermion. The search for a free Majorana fermion has so far been unsuccessful, but bound Majorana-like collective excitations may exist in certain exotic superconductors. Nadj-Perge et al. created such a topological superconductor by depositing iron atoms onto the surface of superconducting lead, forming atomic chains (see the Perspective by Lee). They then used a scanning tunneling microscope to observe enhanced conductance at the ends of these chains at zero energy, where theory predicts Majorana states should appear. Science, this issue p. 602; see also p. 547 Scanning tunneling microscopy is used to observe signatures of Majorana states at the ends of iron atom chains. [Also see Perspective by Lee] Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.

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Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor

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?) 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.Satisfactoriness has to be measured by a multitude of standards, of which some, for aught we know, may fail in any given case; and what is more satisfactory than any alternative in sight, may to the end be a sum of pluses and minuses, concerning which we can only trust that by ulterior corrections and improvements a maximum of the one and a minimum of the other may some day be approached.??????????????????????William James, Meaning of Truth

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Gap symmetry and structure of Fe-based superconductors

We report 75As and 19F NMR studies on the superconducting properties of the newly discovered compound PrFeAsO0.89F0.11 (Tc=45 K). The Knight shift in the direction of H∥ab-plane decreases below Tc to almost zero, which indicates spin-singlet pairing. The temperature dependencies of both the Knight shift and the spin lattice relaxation rate indicate that there are two gaps opening below Tc, with nodes in the gap function.

https://iopscience.iop.org/article/10.1209/0295-5075/83/57001/pdf

Spin-singlet superconductivity with multiple gaps in PrFeAsO0.89F0.11

Since the discovery of high transition-temperature (Tc) superconductivity in copper oxides two decades ago, continuous efforts have been devoted to searching for similar phenomenon in other compounds. With the exception of MgB2 (Tc =39 K), however, Tc is generally far lower than desired. Recently, breakthrough has been made in a new class of oxypnictide compounds. Following the initial discovery of superconductivity in LaO1-x FxFeAs (Tc =26 K), Tc onset has been raised to 55 K in ReO1-xFxFeAs (Re: Ce, Pr, Nd, Sm). Meanwhile, unravelling the nature of the energy associated with the formation of current-carrying pairs (Cooper pairs), referred to as the superconducting energy gap, is the first and vital step towards understanding why the superconductivity occurs at such high temperature and is also important for finding superconductors with still higher Tc. Here we show that, on the basis of the nuclear magnetic resonance (NMR) measurements in PrO0.89F0.11FeAs (Tc =45 K), the Cooper pair is in the spin-singlet state (two spins are anti-paralleled), with two energy gaps opening below Tc. The results strongly suggest the existence of nodes (zeros) in the gap. None of superconductors known to date has such unique gap features, although copper-oxides and MgB2 share part of them.

Spin-singlet superconductivity with multiple gaps in PrO0.89F0.11FeAs

We report a $^{59}\mathrm{Co}$ Knight-shift measurement in a single crystal of the cobalt oxide superconductor ${\mathrm{Na}}_{x}\mathrm{Co}{\mathrm{O}}_{2}∙13{\mathrm{H}}_{2}\mathrm{O}$ $({T}_{c}=425\phantom{\rule{03em}{0ex}}\mathrm{K})$ We find that the shift due to the spin susceptibility, ${K}^{s}$, is substantially large and anisotropic, with the spin shift along the $a$-axis ${K}_{a}^{s}$ being two times that along the $c$-axis ${K}_{c}^{s}$ The shift decreases with decreasing temperature (T) down to $T\ensuremath{\approx}100$ K, then becomes a constant until superconductivity sets in Both ${K}_{a}^{s}$ and ${K}_{c}^{s}$ decrease below ${T}_{c}$ Our results indicate unambiguously that the electron pairing in the superconducting state is in the spin singlet form

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Spin singlet pairing in the superconducting state of Na x Co O 2 ∙ 1.3 H 2 O : Evidence from a Co 59 Knight shift in a single crystal

We report the 75As-NMR study on a single crystal of the hole-doped iron-pnictide superconductor Ba0.72K0.28Fe2As2 (Tc=31.5 K). We find that the Fe antiferromagnetic spin fluctuations are anisotropic and are weaker compared to underdoped copper-oxides or cobalt-oxide superconductors. The spin lattice relaxation rate 1/T1 decreases below Tc with no coherence peak and shows a stepwise variation at low temperatures, which is indicative of multiple superconducting gaps, as in the electron-doped Pr(La)FeAsO1- xFx. Furthermore, no evidence was obtained for a microscopic coexistence of a long-range magnetic order and superconductivity.

http://iopscience.iop.org/article/10.1209/0295-5075/87/27012/pdf

Anisotropic spin fluctuations and multiple superconducting gaps in hole-doped Ba0.72K0.28Fe2As2: NMR in a single crystal

We report the nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements for non-centrosymmetric superconductors Re7B3, LaBiPt, and BiPd containing heavy elements. For all three compounds, the spin–lattice relaxation rate $1/T_{1}$ shows a coherence peak just below $T_{\text{c}}$ and decreases exponentially at low temperatures, which indicates that an isotropic superconducting gap is dominant in these compounds. In BiPd, the height of the coherence peak just below $T_{\text{c}}$ is much suppressed, which suggests that there exists a substantial component of gap with nodes in this compound. Our results indicate that heavy element is not the only factor, but the extent of inversion symmetry breaking is also important to induce a large spin–orbit coupling and an unconventional superconducting state.

Kazuaki Matano

Papers

Spin-singlet superconductivity with multiple gaps in PrFeAsO0.89F0.11

Spin-singlet superconductivity with multiple gaps in PrO0.89F0.11FeAs

Spin singlet pairing in the superconducting state of Na x Co O 2 ∙ 1.3 H 2 O : Evidence from a Co 59 Knight shift in a single crystal

Anisotropic spin fluctuations and multiple superconducting gaps in hole-doped Ba0.72K0.28Fe2As2: NMR in a single crystal

NMR and NQR Studies on Non-centrosymmetric Superconductors Re7B3, LaBiPt, and BiPd