High-transition temperature (high-$T_c$) superconductivity in the iron pnictides/chalcogenides emerges from the suppression of the static antiferromagnetic order in their parent compounds, similar to copper oxides 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-$T_c$ superconductors. 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 review, we present an overview of the neutron scattering results on iron-based superconductors, focusing on the evolution of spin excitation spectra as a function of electron/hole-doping and isoelectronic substitution. We compare spin dynamical properties of iron-based superconductors with those of copper oxide and heavy fermion superconductors, and discuss the common features of spin excitations in these three families of unconventional superconductors and their relationship with superconductivity.

Antiferromagnetic order and spin dynamics in iron-based superconductors

The experimental search for new thermoelectric materials remains largely confined to a limited set of successful chemical and structural families, such as chalcogenides, skutterudites, and Zintl phases. In principle, computational tools such as density functional theory (DFT) offer the possibility of rationally guiding experimental synthesis efforts toward very different chemistries. However, in practice, predicting thermoelectric properties from first principles remains a challenging endeavor [J. Carrete et al., Phys. Rev. X 4, 011019 (2014)], and experimental researchers generally do not directly use computation to drive their own synthesis efforts. To bridge this practical gap between experimental needs and computational tools, we report an open machine learning-based recommendation engine (http://thermoelectrics.citrination.com) for materials researchers that suggests promising new thermoelectric compositions based on pre-screening about 25 000 known materials and also evaluates the feasibility of use...

Perspective: Web-based machine learning models for real-time screening of thermoelectric materials properties

Superconductivity is a remarkably widespread phenomenon observed in most metals cooled down to very low temperatures. The ubiquity of such conventional superconductors, and the wide range of associated critical temperatures, is readily understood in terms of the celebrated Bardeen-Cooper-Schrieffer (BCS) theory. Occasionally, however, unconventional superconductors are found, such as the iron-based materials, which extend and defy this understanding in new and unexpected ways. In the case of the iron-based superconductors, this includes a new appreciation of the ways in which the presence of multiple atomic orbitals can manifest in unconventional superconductivity, giving rise to a rich landscape of gap structures that share the same dominant pairing mechanism. Besides superconductivity, these materials have also led to new insights into the unusual metallic state governed by the Hund's interaction, the control and mechanisms of electronic nematicity, the impact of magnetic fluctuations and quantum criticality, and the significance of topology in correlated states. Over the thirteen years since their discovery, they have proven to be an incredibly fruitful testing ground for the development of new experimental tools and theoretical approaches, both of which have extensively influenced the wider field of quantum materials. 

Ironing out the details of unconventional superconductivity

Iron pnictides and chalcogenides: a new paradigm for superconductivity

The experimental search for new thermoelectric materials remains largely confined to a limited set of successful chemical and structural families, such as chalcogenides, skutterudites, and Zintl phases. In principle, computational tools such as density functional theory (DFT) offer the possibility of rationally guiding experimental synthesis efforts toward very different chemistries. However, in practice, predicting thermoelectric properties from first principles remains a challenging endeavor, and experimental researchers generally do not directly use computation to drive their own synthesis efforts. To bridge this practical gap between experimental needs and computational tools, we report an open machine learning-based recommendation engine (this http URL) for materials researchers that suggests promising new thermoelectric compositions, and evaluates the feasibility of user-designed compounds. We show that this engine can identify interesting chemistries very different from known thermoelectrics. Specifically, we describe the experimental characterization of one example set of compounds derived from our engine, RE12Co5Bi (RE = Gd, Er), which exhibits surprising thermoelectric performance given its unprecedentedly high loading with metallic d and f block elements, and warrants further investigation as a new thermoelectric material platform.

/pdf/a-recommendation-engine-for-suggesting-unexpected-4pg90ldhx7.pdf

A Recommendation Engine for Suggesting Unexpected Thermoelectric Chemistries

High-resolution and high-flux neutron as well as X-ray powder-diffraction experiments were performed on the oxypnictide series LaO(1-x)FxFeAs with 0<x<0.15 in order to study the crystal and magnetic structure. The magnetic symmetry of the undoped compound corresponds to those reported for ReOFeAs (with Re a rare earth) and for AFe2As2 (A=Ba, Sr) materials. We find an ordered magnetic moment of 0.63(1)muB at 2 K in LaOFeAs, which is significantly larger than the values previously reported for this compound. A sizable ordered magnetic moment is observed up to a F-doping of 4.5% whereas there is no magnetic order for a sample with a F concentration of x=0.06. In the undoped sample, several interatomic distances and FeAs4 tetrahedra angles exhibit pronounced anomalies connected with the broad structural transition and with the onset of magnetism supporting the idea of strong magneto-elastic coupling in this material.

Crystal and magnetic structure of the oxypnictide superconductor LaFeAsO 1-x F x : A neutron-diffraction study

Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron-scattering experiments. There is no indication for static magnetic ordering, but inelastic correlations appear at the incommensurate wave vector ($0.5\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta},0.5\ensuremath{\mp}\ensuremath{\delta},0$) with $\ensuremath{\delta}\ensuremath{\sim}0.07$ slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.

Inelastic Neutron-Scattering Measurements of Incommensurate Magnetic Excitations on Superconducting LiFeAs Single Crystals

Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron scattering experiments. There is no indication for static magnetic ordering but inelastic correlations appear at the incommensurate wave vector (0:5+-d 0:-+d 0) with d=0.07 slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.

Incommensurate magnetic excitations in superconducting LiFeAs

The crystal structure and magnetic structure of the chromate series $A$CrO${}_{3}$ ($A$ $=$ Ca, Sr, Pb) where Cr exhibits the unusual Cr${}^{4+}$ oxidation state have been studied by comprehensive diffraction experiments. In addition we present a detailed analysis of the infrared optical properties of polycrystalline CaCrO${}_{3}$ which is found to be metallic along all crystallographic directions. CaCrO${}_{3}$ forms the rare case of a three-dimensional antiferromagnetic and metallic transition-metal oxide in which the occurrence of magnetic ordering is closely coupled to a flattening of the lattice. The same $C$-type magnetic order is also found in the Sr material and again is coupled to a structural distortion associated with lattice flattening. However, in SrCrO${}_{3}$ only a part of the sample transforms into the antiferromagnetic distorted phase. In contrast to CaCrO${}_{3}$ and SrCrO${}_{3}$, PbCrO${}_{3}$ clearly is insulating with a significantly larger unit-cell volume and exhibits $G$-type magnetic order.

Magnetic order, transport and infrared optical properties in the A CrO 3 system ( A = Ca, Sr, and Pb)

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Papers

Crystal and magnetic structure of the oxypnictide superconductor LaFeAsO 1-x F x : A neutron-diffraction study

Inelastic Neutron-Scattering Measurements of Incommensurate Magnetic Excitations on Superconducting LiFeAs Single Crystals

Incommensurate magnetic excitations in superconducting LiFeAs

Magnetic order, transport and infrared optical properties in the A CrO 3 system ( A = Ca, Sr, and Pb)