In contrast to conventional BCS superconductors, the observation that superconductivity in unconventional high-temperature materials appears in close proximity to a static antiferromagnetic phase ...suggests that magnetism plays a fundamental role in the microscopic origins of superconductivity. This review provides an overview of how elastic and inelastic neutron scattering is used to determine the magnetic structures and the doping evolution of spin excitations in iron-based superconductors. The interplay between magnetism and superconductivity is contrasted with related behavior in the copper oxide and heavy fermion superconductors and is important to future theoretical efforts. High-transition temperature (high-T sub()c superconductivity in the iron pnictides or chalcogenides emerges from the suppression of the static antiferromagnetic order in their parent compounds, similar to copper oxide superconductors. This raises a fundamental question concerning the role of magnetism in the superconductivity of these materials. Neutron scattering, a powerful probe to study the magnetic order and spin dynamics, plays an essential role in determining the relationship between magnetism and superconductivity in high-T sub()csuperconductors. The rapid development of modern neutron time-of-flight spectrometers allows a direct determination of the spin dynamical properties of iron-based superconductors throughout the entire Brillouin zone. In this paper, an overview is presented of the neutron scattering results on iron-based superconductors, focusing on the evolution of spin-excitation spectra as a function of electron and hole doping and isoelectronic substitution. Spin dynamical properties of iron-based superconductors are compared with those of copper oxide and heavy fermion superconductors and the common features of spin excitations in these three families of unconventional superconductors and their relationship with superconductivity are discussed.
High-temperature superconductivity in the iron-based materials emerges from, or sometimes coexists with, their metallic or insulating parent compound states. This is surprising, as these undoped ...states exhibit dramatically different antiferromagnetic spin arrangements and Néel temperatures. Although there is a general consensus that magnetic interactions are important for superconductivity, much remains unknown concerning the microscopic origin of the magnetic states. In this review, we summarize the progress in this area, focusing on recent experimental and theoretical results, and their microscopic implications. We conclude that the parent compounds are in a state that is more complex than that implied by a simple Fermi surface nesting scenario, and a dual description including both itinerant and localized degrees of freedom is needed to properly describe these fascinating materials. PUBLICATION ABSTRACT
The design of highly efficient, stable, and noble‐metal‐free bifunctional electrocatalysts for overall water splitting is critical but challenging. Herein, a facile and controllable synthesis ...strategy for nickel–cobalt bimetal phosphide nanotubes as highly efficient electrocatalysts for overall water splitting via low‐temperature phosphorization from a bimetallic metal‐organic framework (MOF‐74) precursor is reported. By optimizing the molar ratio of Co/Ni atoms in MOF‐74, a series of CoxNiyP catalysts are synthesized, and the obtained Co4Ni1P has a rare form of nanotubes that possess similar morphology to the MOF precursor and exhibit perfect dispersal of the active sites. The nanotubes show remarkable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic performance in an alkaline electrolyte, affording a current density of 10 mA cm−2 at overpotentials of 129 mV for HER and 245 mV for OER, respectively. An electrolyzer with Co4Ni1P nanotubes as both the cathode and anode catalyst in alkaline solutions achieves a current density of 10 mA cm−2 at a voltage of 1.59 V, which is comparable to the integrated Pt/C and RuO2 counterparts and ranks among the best of the metal‐phosphide electrocatalysts reported to date.
An unusual nanotube of nickel–cobalt bimetal phosphides is produced via a facile and controllable low‐temperature phosphorization from a bimetallic metal‐organic framework (MOF‐74) by optimizing the molar ratio of Co/Ni atoms in MOF precursor. It shows remarkable hydrogen evolution reaction and oxygen evolution reaction catalytic performance in an alkaline electrolyte and can be used as a highly efficient electrocatalyst for overall water splitting.
Superconductivity in FeSe emerges from a nematic phase that breaks four-fold rotational symmetry in the iron plane. This phase may arise from orbital ordering, spin fluctuations or hidden magnetic ...quadrupolar order. Here we use inelastic neutron scattering on a mosaic of single crystals of FeSe, detwinned by mounting on a BaFe
As
substrate to demonstrate that spin excitations are most intense at the antiferromagnetic wave vectors Q
= (±1, 0) at low energies E = 6-11 meV in the normal state. This two-fold (C
) anisotropy is reduced at lower energies, 3-5 meV, indicating a gapped four-fold (C
) mode. In the superconducting state, however, the strong nematic anisotropy is again reflected in the spin resonance (E = 3.6 meV) at Q
with incommensurate scattering around 5-6 meV. Our results highlight the extreme electronic anisotropy of the nematic phase of FeSe and are consistent with a highly anisotropic superconducting gap driven by spin fluctuations.
Due to their significance in energy-conversion applications, the development of efficient non-noble electrocatalysts for the oxygen reduction reaction (ORR) to accelerate the sluggish cathodic ...reaction for fuel cells and metal-air batteries has attracted extensive attention. Metal-organic frameworks (MOF), a family of crystalline organic-inorganic porous materials constituted by ligands and metal struts, have been identified as a promising platform for preparing efficient ORR catalysts. In this review, the progress and current developments of MOF derivatives as non-noble ORR catalysts are summarized. Beginning with an introduction of the general principles in fabricating efficient non-noble ORR catalysts, the MOF-directed synthesis in the fabrication of non-noble catalysts with finely controlled local electronic structures, extrinsic structures and interface properties is reviewed and discussed. The construction of special local electronic structures and morphologies under the guidelines of general principles is presented in detail. Particularly, the superiority of MOFs, as compared to other precursors, in the controlled immobilization of favourable factors into resulting catalysts is highlighted. Finally, prospects and future perspectives of MOF utilization in fabricating non-noble ORR catalysts are proposed.
This review provides an overview of crucial roles that MOFs have played in constructing non-noble ORR catalysts.
The separation of acetylene and carbon dioxide is an essential but challenging process owing to the similar molecular sizes and physical properties of the two gas molecules. Notably, these molecules ...usually exhibit different orientations in the pore channel. We report an adsorption site selective occupation strategy by taking advantage of differences in orientation to sieve the C2H2 from CO2 in a judiciously designed amine‐functionalized metal–organic framework, termed CPL‐1‐NH2. In this material, the incorporation of amino groups not only occupies the adsorption sites of CO2 molecules and shields the interaction of uncoordinated oxygen atom and CO2 molecules resulting in a negligible adsorption amount and a decrease in enthalpy of adsorption but also strengthened the binding affinity toward C2H2 molecules. This material thus shows an extremely high amount of C2H2 at low pressure and a remarkably high C2H2/CO2 IAST selectivity (119) at 1 bar and 298 K.
Metal–organic framework (CPL‐1‐NH2) with keyhole‐like pore apertures offers selective adsorption site properties for gas separation. Amine groups limit interactions with CO2 molecules by decreasing the enthalpy of CO2 adsorption, whereas the binding affinity of C2H2 is increased (see picture, distances in Å). CPL‐1‐NH2 demonstrates a remarkably high C2H2 selectivity with C2H2/CO2 (50/50) mixtures at 1 bar and 298 K.
Understanding the microscopic origins of electronic phases in high-transition temperature (high-Tc) superconductors is important for elucidating the mechanism of superconductivity. In the ...paramagnetic tetragonal phase of BaFe2–xTxAs2 (where T is Co or Ni) iron pnictides, an in-plane resistivity anisotropy has been observed. Here, we use inelastic neutron scattering to show that low-energy spin excitations in these materials change from fourfold symmetric to twofold symmetric at temperatures corresponding to the onset of the in-plane resistivity anisotropy. Because resistivity and spin excitation anisotropies both vanish near optimal superconductivity, we conclude that they are likely intimately connected.
In a superconductor electrons form pairs and electric transport becomes dissipation-less at low temperatures. Recently discovered iron-based superconductors have the highest superconducting ...transition temperature next to copper oxides. In this article, we review material aspects and physical properties of iron-based superconductors. We discuss the dependence of transition temperature on the crystal structure,the interplay between antiferromagnetism and superconductivity by examining neutron scatering experiments, and the electronic properties of these compounds obtained by angle-resolved photoemission spectroscopy in link with some results from scanning tunneling microscopy/spectroscopy measurements.Possible microscopic model for this class of compounds is discussed from a strong coupling point of view.
Superconductivity originates from the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature Tc (ref.). ...Electron Cooper pairs in most superconductors form anti-parallel spin singlets with total spin S = 0 (ref.), although they can also form parallel spin-triplet Cooper pairs with S = 1 and an odd parity wavefunction3. Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for quantum computation. Because spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations, uranium-based materials near an FM instability are considered to be ideal candidates for realizing spin-triplet superconductivity. Indeed, UTe2, which has a Tc ≈ 1.6 K (refs.), has been identified as a candidate for a chiral spin-triplet topological superconductor near an FM instability, although it also has antiferromagnetic (AF) spin fluctuations. Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe2 is coupled to a sharp magnetic excitation, termed resonance, at the Brillouin zone boundary near AF order. Because the resonance has only been found in spin-singlet unconventional superconductors near an AF instability, its observation in UTe2 suggests that AF spin fluctuations may also induce spin-triplet pairing or that electron pairing in UTe2 has a spin-singlet component.
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