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

Negative thermal expansion (NTE) is an intriguing physical property of solids, which is a consequence of a complex interplay among the lattice, phonons, and electrons. Interestingly, a large number of NTE materials have been found in various types of functional materials. In the last two decades good progress has been achieved to discover new phenomena and mechanisms of NTE. In the present review article, NTE is reviewed in functional materials of ferroelectrics, magnetics, multiferroics, superconductors, temperature-induced electron configuration change and so on. Zero thermal expansion (ZTE) of functional materials is emphasized due to the importance for practical applications. The NTE functional materials present a general physical picture to reveal a strong coupling role between physical properties and NTE. There is a general nature of NTE for both ferroelectrics and magnetics, in which NTE is determined by either ferroelectric order or magnetic one. In NTE functional materials, a multi-way to control thermal expansion can be established through the coupling roles of ferroelectricity-NTE, magnetism-NTE, change of electron configuration-NTE, open-framework-NTE, and so on. Chemical modification has been proved to be an effective method to control thermal expansion. Finally, challenges and questions are discussed for the development of NTE materials. There remains a challenge to discover a “perfect” NTE material for each specific application for chemists. The future studies on NTE functional materials will definitely promote the development of NTE materials.

Negative thermal expansion in functional materials: controllable thermal expansion by chemical modifications

The recently discovered FeAs-based superconductors are a new, promising set of materials for technological and basic research. They offer transition temperatures as high as 55 K as well as essentially isotropic, and extremely large, upper, superconducting critical fields, in excess of 40 T at 20 K. In addition, they may well provide insight into exotic superconductivity that extends beyond just FeAs-based superconductivity, perhaps even shedding light on the still-perplexing CuO-based high-Tc materials. Whereas superconductivity can be induced in the RFeAsO (R = rare earth) and AEFe2As2 (AE = Ba, Sr, Ca) families by several means, transition metal (TM) doping of BaFe2As2 [e.g., Ba(Fe1–xTMx)2As2] offers the easiest experimental access to a wide set of materials and states. In this review, we present an overview and summary of the effect of TM-doping (TM = Co, Ni, Cu, Pd, and Rh) on BaFe2As2. The resulting phase diagrams reveal the nature of the interaction between the structural, magnetic, and superconduct...

FeAs-Based Superconductivity: A Case Study of the Effects of Transition Metal Doping on BaFe2As2

In this review, we present a summary of experimental studies of magnetism in Fe-based superconductors. The doping dependent phase diagram shows strong similarities to the generic phase diagram of the cuprates. Parent compounds exhibit magnetic order together with a structural phase transition, both of which are progressively suppressed with doping, allowing superconductivity to emerge. The stripe-like spin arrangement of Fe moments in the magnetically ordered state shows identical in-plane structure for the RFeAsO (R = rare earth) and AFe2As2 (A = Sr, Ca, Ba, Eu and K) parent compounds, notably different than the spin configuration of the cuprates. Interestingly, Fe1 + yTe orders with a different spin order despite having very similar Fermi surface topology. Studies of the spin dynamics of the parent compounds show that the interactions are best characterized as anisotropic three-dimensional interactions. Despite the room temperature tetragonal structure, analysis of the low temperature spin waves under the assumption of a Heisenberg Hamiltonian indicates strong in-plane anisotropy with a significant next-nearest-neighbor interaction. For the superconducting state, a resonance, localized in both wavevector and energy, is observed in the spin excitation spectrum as for the cuprates. This resonance is observed at a wavevector compatible with a Fermi surface nesting instability independent of the magnetic ordering of the relevant parent compound. The resonance energy (Er) scales with the superconducting transition temperature (TC) as Er ~ 4.9kBTC, which is consistent with the canonical value of ~ 5kBTC observed for the cuprates. Moreover, the relationship between the resonance energy and the superconducting gap, Δ, is similar to that observed for many unconventional superconductors (Er/2Δ ~ 0.64).

https://iopscience.iop.org/article/10.1088/0953-8984/22/20/203203/pdf

Magnetism in Fe-based superconductors.

Among various parent compounds of iron pnictide superconductors, EuFe2As2 stands out due to the presence of both spin density wave of Fe and antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal neutron diffraction studies have been carried out to determine the magnetic structure of this compound and to investigate the coupling of two magnetic sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190 K and 19 K, respectively. The ordering of Fe2+ moments is associated with the wave vector k = (1,0,1) and it takes place at the same temperature as the tetragonal to orthorhombic structural phase transition, which indicates the strong coupling between structural and magnetic components. The ordering of Eu moment is associated with the wave vector k = (0,0,1). While both Fe and Eu spins are aligned along the long a axis as experimentally determined, our studies suggest a weak coupling between the Fe and Eu magnetism.

Magnetic structure of EuFe 2 As 2 determined by single-crystal neutron diffraction

Magnetic structure of EuFe2As2 determined by single crystal neutron diffractionmanagement

Antiferromagnetic ordering and structural phase transition have been investigated via comprehensive neutron and x-ray diffraction on Sn-flux-grown ${\text{BaFe}}_{2}{\text{As}}_{2}$ single crystals, the $A\text{\ensuremath{-}}122$ family of FeAs-based high-${T}_{C}$ superconductor compounds. The incorporation of Sn in the lattice resulted to an average composition of ${\text{Ba}}_{0.95}{\text{Sn}}_{0.05}{\text{Fe}}_{2}{\text{As}}_{2}$. A tetragonal-to-orthorhombic structural phase transition and a three-dimensional long-range antiferromagnetic ordering of the iron magnetic moment, with a unique magnetic propagation wave vector $\mathbf{k}=(1,0,1)$ have been found to take place at $\ensuremath{\sim}90\text{ }\text{K}$. The magnetic moments of iron are aligned along the longer $\mathbf{a}$ axis in the low-temperature orthorhombic phase ($Fmmm$ with $\mathbf{b}l\mathbf{a}l\mathbf{c}$). Our results thus demonstrate that the magnetic structure of the Sn-flux-grown ${\text{BaFe}}_{2}{\text{As}}_{2}$ single crystal is the same as those in the polycrystalline samples and in other $A\text{\ensuremath{-}}122$ iron pnictides compounds. We argue that the Sn incorporation in the lattice is responsible for a smaller orthorhombic splitting and lower N\'eel temperature ${T}_{N}$ observed in the experiments.

M. Meven

Papers

Magnetic structure of EuFe 2 As 2 determined by single-crystal neutron diffraction

Magnetic structure of EuFe2As2 determined by single crystal neutron diffractionmanagement

Antiferromagnetic ordering and structural phase transition in Ba2Fe2As2 with Sn incorporated from the growth flux