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Among the fascinating phenomena observed in two-dimensional (2D) magnets, the magneto-exciton effect stands out as a pivotal link between optics and magnetism. Although the excitonic effect has been revealed and exhibits a considerable correlation with the spin structures in certain 2D magnets, the underlying mechanism of the magneto-exciton effect remains underexplored, especially under high magnetic fields. Here we perform a systematic investigation of the spin-exciton coupling in 2D antiferromagnetic NiPS3 under high magnetic fields. When an in-plane magnetic field is applied, the exceptional sharp excitonic emission at ~1.4756 eV exhibits a Zeeman-like splitting with g ≈ 2.0, experimentally identifying the exciton as an excitation of dominant triplet-singlet character. By examining the polarization of excitonic emission and simulating the spin evolution, we further verify the correlation between excitonic emission and Néel vector in NiPS3. Our work elucidates the mechanism behind the spin-exciton coupling in NiPS3 and establishes a strategy for optically probing the spin evolutions in 2D magnets. NiPS3, a van der Waals antiferromagnet exhibits exciton emission with a very sharp linewidth. The exact origin of this is has been a subject of active debate. Here, Wang et al study the behavior of this sharp exciton peak under applied magnetic fields, and find a Zeeman-like splitting, indicating the exciton has triplet-singlet character. [ABSTRACT FROM AUTHOR]

The field of research on magnetic van der Waals compounds—a special subclass of quasi-two-dimensional materials—is currently rapidly expanding due to the relevance of these compounds to fundamental research where they serve as a playground for the investigation of different models of quantum magnetism and also in view of their unique magneto-electronic and magneto-optical properties pertinent to novel technological applications. The electron spin resonance (ESR) spectroscopy plays an important role in the exploration of the rich magnetic behavior of van der Waals compounds due to its high sensitivity to magnetic anisotropies and unprecedentedly high energy resolution that altogether enable one to obtain thorough insights into the details of the spin structure in the magnetically ordered state and the low-energy spin dynamics in the ordered and paramagnetic phases. This article provides an overview of the recent achievements in this field made by the ESR spectroscopic techniques encompassing representatives of antiferro- and ferromagnetic van der Waals compounds of different crystal structures and chemical composition as well as of a special category of these materials termed magnetic topological insulators. [ABSTRACT FROM AUTHOR]

Understanding symmetry-breaking states of materials is a major challenge in the modern physical sciences. Quantum atmosphere proposed recently sheds light on the hidden world of these symmetry broken patterns. Yet, no experiment has been performed to demonstrate its potential. In our experiment, we prepare time-reversal-symmetry conserved and broken quantum atmosphere of a single nuclear spin and successfully observe their symmetry properties. Our work proves in principle that finding symmetry patterns from quantum atmosphere is conceptually viable. It also opens up entirely new possibilities in the potential application of quantum sensing in material diagnosis. [ABSTRACT FROM AUTHOR]

The triangular lattice antiferromagnet (TLAF) has been the standard paradigm of frustrated magnetism for several decades. The most common magnetic ordering in insulating TLAFs is the 120° structure. However, a new triple-Q chiral ordering can emerge in metallic TLAFs, representing the short wavelength limit of magnetic skyrmion crystals. We report the metallic TLAF Co1/3TaS2 as the first example of tetrahedral triple-Q magnetic ordering with the associated topological Hall effect (non-zero σxy(H = 0)). We also present a theoretical framework that describes the emergence of this magnetic ground state, which is further supported by the electronic structure measured by angle-resolved photoemission spectroscopy. Additionally, our measurements of the inelastic neutron scattering cross section are consistent with the calculated dynamical structure factor of the tetrahedral triple-Q state. Skyrmion crystals, where skyrmions are arranged close packed in a triangular lattice arise due to the superposition of three magnetic spin spirals, each with a distinct wave vector, Q. Such skrymion crystals have been found in a diverse array of materials. Here, Park et al find a short wavelength (or dense skyrmion) limit of this skyrmion crystal structure in Co1/3TaS2, a metallic triangular lattice antiferromagnet, in the form of a triple Q magnetic ordering, with four magnetic sublattices.' [ABSTRACT FROM AUTHOR]

Electrons at the border of localization generate exotic states of matter across all classes of strongly correlated electron materials and many other quantum materials with emergent functionality. Heavy electron metals are a model example, in which magnetic interactions arise from the opposing limits of localized and itinerant electrons. This remarkable duality is intimately related to the emergence of a plethora of novel quantum matter states such as unconventional superconductivity, electronic-nematic states, hidden order and most recently topological states of matter such as topological Kondo insulators and Kondo semimetals and putative chiral superconductors. The outstanding challenge is that the archetypal Kondo lattice model that captures the underlying electronic dichotomy is notoriously difficult to solve for real materials. Here we show, using the prototypical strongly-correlated antiferromagnet CeIn3, that a multi-orbital periodic Anderson model embedded with input from ab initio bandstructure calculations can be reduced to a simple Kondo-Heisenberg model, which captures the magnetic interactions quantitatively. We validate this tractable Hamiltonian via high-resolution neutron spectroscopy that reproduces accurately the magnetic soft modes in CeIn3, which are believed to mediate unconventional superconductivity. Our study paves the way for a quantitative understanding of metallic quantum states such as unconventional superconductivity. Kondo materials exhibit extremely rich physics, from unconventional superconductivity to topological phases. Unfortunately, for a real material, direct solution of the Kondo lattice is practically impossible. Here, Simeth et al. present a tractable approach to this problem, showing how a multi-orbital periodic Anderson model can be reduced to a Kondo lattice model, and be applied to relevant materials and quantitatively validated with neutron spectroscopy. [ABSTRACT FROM AUTHOR]

This work presents a new methodology for fabrication of membrane-assisted elongated YIG nanostructures through the sol-gel method using low-cost materials, and the estimation of the average size of the geometries formed within the pores through ferromagnetic resonance measurements. A precursor solution was deposited on the internal surface of porous aluminum oxide membrane, using an assembly consisting of a vacuum system coupled to an apparatus, which assists in the entry of the used precursor solution, breaking the barriers related to the existing surface tension, with subsequent heat treatment to form the single phase of yttrium iron garnet (YIG). The data collected by X-ray diffraction and Raman spectroscopy indicated this result. The study of the contact angle of the precursor solution and the alumina template indicated great compatibility, an angle of 35.4°. SEM analysis showed that elongated structures were deposited inside the pore's walls, with different lengths up to 3 micrometers and the EDS analysis showed the presence of the main elements of the YIG phase. The Kittel's equation and FMR spectral data were used to estimate the mean size of the elongated nanostructures. We found six different size categories with 1856, 1768, 1661, 1588, 1570, 1498 nm. The methodology proved to be efficient for the characterization of elongated YIG nanostructures, becoming a perspective of future applications. Highlights: Setup for the infiltration of the alumina membrane with the YIG precursor was proposed. Elongated YIG structures were obtained with diverse potential applications. The FMR was used to estimate the mean size of elongated YIG structures. [ABSTRACT FROM AUTHOR]

Fifty years after Anderson's resonating valence-bond proposal, the spin-1/2 triangular-lattice Heisenberg antiferromagnet (TLHAF) remains the ultimate platform to explore highly entangled quantum spin states in proximity to magnetic order. Yb-based delafossites are ideal candidate TLHAF materials, which allow experimental access to the full range of applied in-plane magnetic fields. We perform a systematic neutron scattering study of CsYbSe2, first proving the Heisenberg character of the interactions and quantifying the second-neighbor coupling. We then measure the complex evolution of the excitation spectrum, finding extensive continuum features near the 120°-ordered state, throughout the 1/3-magnetization plateau and beyond this up to saturation. We perform cylinder matrix-product-state (MPS) calculations to obtain an unbiased numerical benchmark for the TLHAF and spectacular agreement with the experimental spectra. The measured and calculated longitudinal spectral functions reflect the role of multi-magnon bound and scattering states. These results provide valuable insight into unconventional field-induced spin excitations in frustrated quantum materials. [ABSTRACT FROM AUTHOR]

Fifty years after Anderson's resonating valence-bond proposal, the spin-1/2 triangular-lattice Heisenberg antiferromagnet (TLHAF) remains the ultimate platform to explore highly entangled quantum spin states in proximity to magnetic order. Yb-based delafossites are ideal candidate TLHAF materials, which allow experimental access to the full range of applied in-plane magnetic fields. We perform a systematic neutron scattering study of CsYbSe2, first proving the Heisenberg character of the interactions and quantifying the second-neighbor coupling. We then measure the complex evolution of the excitation spectrum, finding extensive continuum features near the 120°-ordered state, throughout the 1/3-magnetization plateau and beyond this up to saturation. We perform cylinder matrix-product-state (MPS) calculations to obtain an unbiased numerical benchmark for the TLHAF and spectacular agreement with the experimental spectra. The measured and calculated longitudinal spectral functions reflect the role of multi-magnon bound and scattering states. These results provide valuable insight into unconventional field-induced spin excitations in frustrated quantum materials. [ABSTRACT FROM AUTHOR]

Magnon polarons are novel elementary excitations possessing hybrid magnonic and phononic signatures, and are responsible for many exotic spintronic and magnonic phenomena. Despite long-term sustained experimental efforts in chasing for magnon polarons, direct spectroscopic evidence of their existence is hardly observed. Here, we report the direct observation of magnon polarons using neutron spectroscopy on a multiferroic Fe2Mo3O8 possessing strong magnon-phonon coupling. Specifically, below the magnetic ordering temperature, a gap opens at the nominal intersection of the original magnon and phonon bands, leading to two separated magnon-polaron bands. Each of the bands undergoes mixing, interconverting and reversing between its magnonic and phononic components. We attribute the formation of magnon polarons to the strong magnon-phonon coupling induced by Dzyaloshinskii-Moriya interaction. Intriguingly, we find that the band-inverted magnon polarons are topologically nontrivial. These results uncover exotic elementary excitations arising from the magnon-phonon coupling, and offer a new route to topological states by considering hybridizations between different types of fundamental excitations.A magnetic crystal hosts both magnons, the quanta of spin waves, and phonons, the quanta of lattice vibrations. In some materials with strong coupling between spins and lattices, a magnon-polaron can form. Here, using neutron scattering on a multiferroic, Fe2Mo3O8, Bao et al. observe magnon-polaron, and show that it is topologically non-trivial. [ABSTRACT FROM AUTHOR]

Spin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at the nominal intersections of magnon and phonon modes. Here we use neutron scattering to show that in the two-dimensional (2D) van der Waals honeycomb lattice ferromagnetic CrGeTe3, spin waves propagating within the 2D plane exhibit an anomalous dispersion, damping, and breakdown of quasiparticle conservation, while magnons along the c axis behave as expected for a local moment ferromagnet. These results indicate the presence of dynamical SLC arising from the zero-temperature quantum fluctuations in CrGeTe3, suggesting that the observed in-plane spin waves are mixed spin and lattice quasiparticles fundamentally different from pure magnons and phonons. CrGeTe3 is a van der Waals honeycomb ferromagnet, known for exhibiting strong coupling between lattice and spin degrees of freedom. Here, Chen et al perform neutron scattering on CrGeTe3, find a broadened spin-wave excitation resulting from zero-temperature motion of the atoms in the lattice. [ABSTRACT FROM AUTHOR]

Spin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at the nominal intersections of magnon and phonon modes. Here we use neutron scattering to show that in the two-dimensional (2D) van der Waals honeycomb lattice ferromagnetic CrGeTe3, spin waves propagating within the 2D plane exhibit an anomalous dispersion, damping, and breakdown of quasiparticle conservation, while magnons along the c axis behave as expected for a local moment ferromagnet. These results indicate the presence of dynamical SLC arising from the zero-temperature quantum fluctuations in CrGeTe3, suggesting that the observed in-plane spin waves are mixed spin and lattice quasiparticles fundamentally different from pure magnons and phonons. CrGeTe3 is a van der Waals honeycomb ferromagnet, known for exhibiting strong coupling between lattice and spin degrees of freedom. Here, Chen et al perform neutron scattering on CrGeTe3, find a broadened spin-wave excitation resulting from zero-temperature motion of the atoms in the lattice. [ABSTRACT FROM AUTHOR]

The properties of prereduced (10%Co + 0.5%Pd)/Al2O3 (TiO2) systems in the CO hydrogenation reaction at atmospheric and high pressure were studied. At atmospheric pressure, alumina-supported catalysts were more selective toward methane but those using titania were more active. Alumina containing samples demonstrated high temperature H2 desorption, firmly held surface carbonate species, high tendency to agglomeration. During the reaction metal surface reconstruction and increased formation of CH2 groups occurred being more pronounced with titania-supported catalysts. Stability tests at 250 °C showed opposite behaviour of both systems. Monodentate carbonate intermediates adsorbed on sites of moderate strength prevailed on titania samples, while formate species predominated on high strength sites of alumina-supported catalysts. High pressure catalytic tests revealed dependence of activity on Tred, synthesis of C2+ hydrocarbons, decreased CO2 production, a higher CH4/CO2 ratio for alumina containing system. Due to SMSI, increased CO2 production on titania samples was preserved. Titania-supported catalysts revealed a stronger decrease of CO conversion rising Tred while alumina catalysts had almost unchanged activity. CO conversion decreased with time due to difficulties in surface diffusion of reagents/intermediates/products and metal particle agglomeration. Concerning Tred comparison of product distribution showed a steady trend. Because of stable CO and CHx surface species, titania containing catalysts produced lower content of C5+ compounds. Alumina-supported samples showed a higher selectivity to C5+ compounds at the expense of methane. A higher selectivity ratio for CH4 and CO2 determined in catalytic CO hydrogenation over a certain catalyst at atmospheric pressure could indicate that a given sample is predisposed to form C2+ hydrocarbons at a higher pressure. [ABSTRACT FROM AUTHOR]

The "Hypogeum of the Garlands" is a sepulchral site, recently found in Grottaferrata (Lazio, Italy), dating back to the first-second century AD. Two sarcophagi were discovered inside, hosting the human remains of Aebutia Quarta, a rich Roman woman, and her son Carvilius Gemellus. While the body of Carvilius is exceptionally well-preserved, following its embalming and perfect sealing of the sarcophagus, in the case of Aebutia only the bones were preserved because of the sarcophagus's seal breaking down, although she was covered with perfectly preserved flower garlands. Embalming of the body was a rare ritual in the Imperial Roman times when corpses were more often cremated. The remains of Aebutia showed possible traces of heating. Burned bones from a third individual were discovered on the chamber's floor and preliminary anthropological survey showed that this individual was a male of 40–50 years old. Here, a combination of spectroscopic techniques, including non-destructive inelastic neutron scattering and Raman spectroscopy, and minimally destructive Fourier transform infrared spectroscopy, were applied to the analysis of these bone samples to give information about ancient Roman funerary practices. The temperature and burning conditions were thus determined, showing that Aebutia Quarta was exposed to mild temperatures (200 °C) only in the upper part of the body, while the third individual was likely cremated as its bones were exposed to temperatures up to 900 °C in quasi-anaerobic conditions. [ABSTRACT FROM AUTHOR]

Availability of quantum materials as sufficiently large and high-quality single crystals holds the key to understanding their physical properties, which is crucial for making future progress in this exciting area of research. Here we review the crystal growth of a few representative quantum materials of topical interest grown in our laboratory using various crystal growth techniques, including optical floating zone, traveling solvent floating zone, chemical vapour transport and high-temperature flux methods. The chosen materials classes include: (a) low-dimensional quantum magnets, (b) superconductors of the pnictide family, (c) layered materials with triangular and honeycomb lattices and (d) 2D transition-metal based chalcogenides. Their quintessential physical properties demonstrating quantum behaviour are also shown in some cases. [ABSTRACT FROM AUTHOR]

Collective excitations of bound electron-hole pairs—known as excitons—are ubiquitous in condensed matter, emerging in systems as diverse as band semiconductors, molecular crystals, and proteins. Recently, their existence in strongly correlated electron materials has attracted increasing interest due to the excitons' unique coupling to spin and orbital degrees of freedom. The non-equilibrium driving of such dressed quasiparticles offers a promising platform for realizing unconventional many-body phenomena and phases beyond thermodynamic equilibrium. Here, we achieve this in the van der Waals correlated insulator NiPS3 by photoexciting its newly discovered spin–orbit-entangled excitons that arise from Zhang-Rice states. By monitoring the time evolution of the terahertz conductivity, we observe the coexistence of itinerant carriers produced by exciton dissociation and a long-wavelength antiferromagnetic magnon that coherently precesses in time. These results demonstrate the emergence of a transient metallic state that preserves long-range antiferromagnetism, a phase that cannot be reached by simply tuning the temperature. More broadly, our findings open an avenue toward the exciton-mediated optical manipulation of magnetism. Previous work has shown the existence of spin-orbit-entangled excitons and their coupling to antiferromagnetism in the correlated insulator NiPS3. Here the authors show that non-equilibrium driving of these excitons produces a transient metallic antiferromagnetic state that cannot be achieved by tuning the temperature in equilibrium. [ABSTRACT FROM AUTHOR]

A dual sodium and sulfur promoted haematite, representative of a candidate Fischer-Tropsch to olefins (FTO) catalyst, is prepared and contrasted with the performance of an unpromoted hematite sample in the ambient pressure CO hydrogenation reaction at 623 K as a function of time-on-stream (0–24 h). In-situ post-reaction temperature-programmed oxidation measurements show the carbon evolutionary phase of the catalyst conditioning process to be retarded for the FTO catalyst. Ex-situ inelastic neutron scattering measurements show the promoters perturb the formation of a previously described hydrocarbonaceous overlayer. Specifically, whilst the sp3 hybridised C–H modes of the hydrocarbonaceous overlayer are almost unaffected by the additives, the formation of the overlayer's sp2 hybridised C–H modes are noticeably impeded. The results are discussed in terms of the Na/S promoters disturbing the formation of an ordered hydrocarbonaceous overlayer that is thought to constrain the supply of adsorbed hydrogen atoms, which favours the formation of unsaturated hydrocarbons associated with the FTO process. [ABSTRACT FROM AUTHOR]

The status of surface species on solid catalysts during heterogeneous catalysis is often mysterious. Investigations of these surface species are crucial to deconvolute the reaction network and design more efficient catalysts. Vibrational spectroscopy is a powerful technique to study the interactions between surface species and the catalysts and infrared (IR) and Raman spectroscopies have been widely applied to study reaction mechanisms in heterogeneous catalysis. However, IR/Raman spectra are difficult to model computationally and important vibrational modes may be IR-, Raman- (or both) inactive due to restrictions by optical selection rules. Inelastic neutron scattering (INS) is another form of vibrational spectroscopy and relies on the scattering of neutrons by the atomic nucleus. A consequence of this is that INS is not subject to any optical selection rules and all vibrations are measurable in principle. INS spectroscopy has been used to investigate surface species on catalysts in a wide range of heterogeneous catalytic reactions. In this mini-review, we focus on applications of INS in two important fields: petrochemical reactions and C1 chemistry. We introduce the basic principles of the INS technique, followed by a discussion of its application in investigating two key catalytic systems: (i) the behaviour of hydrocarbons on metal-oxide and zeolite catalysts and (ii) the formation of hydrocarbonaceous species on methane reforming and Fischer–Tropsch catalysts. The power of INS in studying these important catalytic systems is demonstrated. [ABSTRACT FROM AUTHOR]

The CE phase is an extraordinary phase exhibiting the simultaneous spin, charge, and orbital ordering due to strong electron correlation. It is an ideal platform to investigate the role of the multiple orderings in the phase transitions and discover emergent properties. Here, we use a cryogenic high-field magnetic force microscope to image the phase transitions and properties of the CE phase in a Pr0.5Ca0.5MnO3 thin film. In a high magnetic field, we observed a clear suppression of magnetic susceptibility at the charge-ordering insulator transition temperature (TCOI), whereas, at the Néel temperature (TN), no significant change is observed. This observation favors the scenario of strong antiferromagnetic correlation developed below TCOI but raises questions about the Zener polaron paramagnetic phase picture. Besides, we discoverd a phase-separated surface state in the CE phase regime. Ferromagnetic phase domains residing at the surface already exist in zero magnetic field and show ultra-high magnetic anisotropy. Our results provide microscopic insights into the unconventional spin- and charge-ordering transitions and revealed essential attributes of the CE phase, highlighting unusual behaviors when multiple electronic orderings are involved. [ABSTRACT FROM AUTHOR]

Optimising the balance between propene selectivity, propene/ethene ratio and catalytic stability and unravelling the explicit mechanism on formation of the first carbon–carbon bond are challenging goals of great importance in state-of-the-art methanol-to-olefin (MTO) research. We report a strategy to finely control the nature of active sites within the pores of commercial MFI-zeolites by incorporating tantalum(V) and aluminium(III) centres into the framework. The resultant TaAlS-1 zeolite exhibits simultaneously remarkable propene selectivity (51%), propene/ethene ratio (8.3) and catalytic stability (>50 h) at full methanol conversion. In situ synchrotron X-ray powder diffraction, X-ray absorption spectroscopy and inelastic neutron scattering coupled with DFT calculations reveal that the first carbon–carbon bond is formed between an activated methanol molecule and a trimethyloxonium intermediate. The unprecedented cooperativity between tantalum(V) and Brønsted acid sites creates an optimal microenvironment for efficient conversion of methanol and thus greatly promotes the application of zeolites in the sustainable manufacturing of light olefins. Lower olefins are mainly produced from fossil resources and the methanol-to-olefins process offers a new sustainable pathway. Here, the authors show a new zeolite containing tantalum and aluminium centres which shows simultaneously high propene selectivity, catalytic activity, and stability for the synthesis of propene. [ABSTRACT FROM AUTHOR]

In quantum magnets, magnetic moments fluctuate heavily and are strongly entangled with each other, a fundamental distinction from classical magnetism. Here, with inelastic neutron scattering measurements, we probe the spin correlations of the honeycomb lattice quantum magnet YbCl3. A linear spin wave theory with a single Heisenberg interaction on the honeycomb lattice, including both transverse and longitudinal channels of the neutron response, reproduces all of the key features in the spectrum. In particular, we identify a Van Hove singularity, a clearly observable sharp feature within a continuum response. The demonstration of such a Van Hove singularity in a two-magnon continuum is important as a confirmation of broadly held notions of continua in quantum magnetism and additionally because analogous features in two-spinon continua could be used to distinguish quantum spin liquids from merely disordered systems. These results establish YbCl3 as a benchmark material for quantum magnetism on the honeycomb lattice. Honeycomb lattices with interacting spins can host rich magnetic behaviour; however, typically features are complicated by additional interactions. Here, the authors perform neutron scattering on YbCl3, which exhibits near perfect two-dimensional magnetism, providing a benchmark for other materials. [ABSTRACT FROM AUTHOR]

FeSe is arguably the simplest, yet the most enigmatic, iron-based superconductor. Its nematic but non-magnetic ground state is unprecedented in this class of materials and stands out as a current puzzle. Here, our nuclear magnetic resonance measurements in the nematic state of mechanically detwinned FeSe reveal that both the Knight-shift and the spin–lattice relaxation rate 1/T1 possess an in-plane anisotropy opposite to that of the iron pnictides LaFeAsO and BaFe2As2. Using a microscopic electron model that includes spin–orbit coupling, our calculations show that an opposite quasiparticle weight ratio between the dxz and dyz orbitals leads to an opposite anisotropy of the orbital magnetic susceptibility, which explains our Knight-shift results. We attribute this property to a different nature of nematic order in the two compounds, predominantly bond type in FeSe and onsite ferro-orbital in pnictides. The T1 anisotropy is found to be inconsistent with existing neutron scattering data in FeSe, showing that the spin fluctuation spectrum reveals surprises at low energy, possibly from fluctuations that do not break C4 symmetry. Therefore, our results reveal that important information is hidden in these anisotropies and they place stringent constraints on the low-energy spin correlations as well as on the nature of nematicity in FeSe. [ABSTRACT FROM AUTHOR]

Iron-based Fischer–Tropsch synthesis (FTS) catalysts evolve in situ on exposure to synthesis gas (CO & H2) forming a mixture of iron oxides, iron carbides and carbonaceous deposits. Recently, the application of inelastic neutron scattering has shown the progressive formation of a hydrocarbonaceous overlayer during this catalyst conditioning period. The evolving nature of the catalyst alters the proportion of phases present within the catalyst, which may influence the transport of hydrogen within the reaction system. Preliminary quasi-elastic neutron scattering (QENS) measurements are used to investigate hydrogen diffusion within an un-promoted iron FTS catalyst that has experienced varying levels of time-on-stream (0, 12 and 24 h) of ambient pressure CO hydrogenation at 623 K. Measurements on the catalyst samples in the absence of hydrogen show the unreacted sample (t = 0 h) to exhibit little increase in motion over the temperature range studied, whereas the t = 12 and 24 h samples exhibit a pronounced change in motion with temperature. The contrast is attributed to the presence of the afore-mentioned hydrocarbonaceous overlayer. Measurements on the samples in the presence of liquid hydrogen show hydrogen diffusional characteristics to be modified as a function of the catalyst conditioning process but, due to the complexity of the evolving catalyst matrix, the hydrogen motion cannot be attributed to a particular phase or component of the catalyst. Problems in the use of hydrogen as a probe molecule in this instance are briefly considered. Coincident neutron diffraction studies undertaken alongside the QENS measurements confirm the transition from hematite pre-catalyst to that of Hägg carbide during the course of extended times-on-stream. [ABSTRACT FROM AUTHOR]

Phonons are the main source of relaxation in molecular nanomagnets, and different mechanisms have been proposed in order to explain the wealth of experimental findings. However, very limited experimental investigations on phonons in these systems have been performed so far, yielding no information about their dispersions. Here we exploit state-of-the-art single-crystal inelastic neutron scattering to directly measure for the first time phonon dispersions in a prototypical molecular qubit. Both acoustic and optical branches are detected in crystals of [VO(acac) 2 ] along different directions in the reciprocal space. Using energies and polarisation vectors calculated with state-of-the-art Density Functional Theory, we reproduce important qualitative features of [VO(acac) 2 ] phonon modes, such as the presence of low-lying optical branches. Moreover, we evidence phonon anti-crossings involving acoustic and optical branches, yielding significant transfers of the spin-phonon coupling strength between the different modes. Molecular nanomagnets have potential applications for storing both classical and quantum information, with benefit of the high scalability of chemical synthesis. Here the authors use state-of-the-art experimental and theoretical methods to investigate phonons in a molecular qubit candidate. [ABSTRACT FROM AUTHOR]

The relevance of magnetic, structural, orbital, and charge degrees of freedom in the iron-based superconductors (FeSCs) and related materials occupies a central focus in condensed matter physics. While the majority of iron-based materials exhibit the same two-dimensional iron square lattice structural motif, a family of AFe2X3 (X = Se,S) compounds introduces a quasi-one-dimensional (1D) ladder motif, which resembles the two-legged spin ladder copper oxide materials. Furthermore, unlike most parent compounds of FeSCs, the members of this spin ladder family are insulators. Recently, a superconducting transition has been observed under pressure with Tc up to 24 K, similar to the pressure-induced superconductivity in the copper oxide ladder Sr14−xCaxCu24O41 material, stimulating much interest. Here, we review the magnetic, structural, and electronic properties in this family, particularly in the BaFe2X3 series tuned by pressure and by chemical substitution. The established pressure-temperature (P-T) and carrier concentration-temperature (x-T) phase diagrams in related materials provide useful information to extend the variety of high-temperature superconductors and compare with other FeSCs. We also review some essential information about analogous square lattice FeSCs. [ABSTRACT FROM AUTHOR]

The parent compound of high-Tc superconducting cuprates is a unique Mott insulator consisting of layers of spin-1/2 ions forming a square lattice and with a record high in-plane antiferromagnetic coupling. Compounds with similar characteristics have long been searched for without success. Here, we use a combination of experimental and theoretical tools to show that commercial AgF2 is an excellent cuprate analog with remarkably similar electronic parameters to La2CuO4 but larger buckling of planes. Two-magnon Raman scattering and inelastic neutron scattering reveal a superexchange constant reaching 70% of that of a typical cuprate. We argue that structures that reduce or eliminate the buckling of the AgF2 planes could have an antiferromagnetic coupling that matches or surpasses the cuprates. [ABSTRACT FROM AUTHOR]

Despite the widespread use of silicon in modern technology, its peculiar thermal expansion is not well understood. Adapting harmonic phonons to the specific volume at temperature, the quasiharmonic approximation, has become accepted for simulating the thermal expansion, but has given ambiguous interpretations for microscopic mechanisms. To test atomistic mechanisms, we performed inelastic neutron scattering experiments from 100 K to 1,500 K on a single crystal of silicon to measure the changes in phonon frequencies. Our state-of-the-art ab initio calculations, which fully account for phonon anharmonicity and nuclear quantum effects, reproduced the measured shifts of individual phonons with temperature, whereas quasiharmonic shifts were mostly of the wrong sign. Surprisingly, the accepted quasiharmonic model was found to predict the thermal expansion owing to a large cancellation of contributions from individual phonons. [ABSTRACT FROM AUTHOR]

We show that the crystal rotation method for measuring the difference between splay and bend flexoelectric coefficients (e1–e3) [Outram and Elston, Liq. Cryst.39, 149 (2012)] can be improved by using AC instead of DC fields. The field frequency is small (~0.05 Hz), yet sufficient to get rid of the difficulties connected with the migration of charge carriers. The method is sensitive enough so as to use fields with amplitudes as small as 1 V/mm. This has the advantage of avoiding the consideration of the interaction due to the dielectric anisotropy, and greatly simplifies the analysis of data. The results are extracted from a simple fitting scheme, which involves two parameters. In particular (e1–e3) is obtained from a scale factor in a one-parameter fit. Measurements of the temperature dependence of (e1–e3) are presented for two classical nematic materials. [ABSTRACT FROM AUTHOR]

Inelastic neutron scattering (INS) is used to investigate a ZSM-5 catalyst that has been exposed to methanol vapour at elevated temperature. In-line mass spectrometric analysis of the catalyst exit stream confirms methanol-to-gasoline chemistry, whilst ex situ INS measurements detect hydrocarbon species formed in/on the catalyst during methanol conversion. These preliminary studies demonstrate the capability of INS to complement infrared spectroscopic characterisation of the hydrocarbon pool present in/on ZSM-5 during the MTG reaction. Graphical Abstract: [ABSTRACT FROM AUTHOR]

We investigated the flexoelectric effect of a fringe field switching liquid crystal (LC) cell and characterised the resultant image flicker with different LC mixtures at different frame rates. Incorporating with human eye perception of 10 observers, we found that LC mixtures with a dielectric anisotropy smaller than ~7 lead to unnoticeable image flicker at 60 frames per second. The obtained flicker sensitivity line serves as important guidelines for optimising LC materials and display devices. [ABSTRACT FROM PUBLISHER]

A solid with larger sound speeds usually exhibits higher lattice thermal conductivity. Here, we report an exception that CuP2 has a quite large mean sound speed of 4155 m s−1, comparable to GaAs, but single crystals show very low lattice thermal conductivity of about 4 W m−1 K−1 at room temperature, one order of magnitude smaller than GaAs. To understand such a puzzling thermal transport behavior, we have thoroughly investigated the atomic structures and lattice dynamics by combining neutron scattering techniques with first-principles simulations. This compound crystallizes in a layered structure where Cu atoms forming dimers are sandwiched in between P atomic networks. In this work, we reveal that Cu atomic dimers vibrate as a rattling mode with frequency around 11 meV, which is manifested to be remarkably anharmonic and strongly scatters acoustic phonons to achieve the low lattice thermal conductivity. CuP2 has a puzzling thermal transport behavior, with low thermal conductivity but quite large mean sound speeds. Here, the authors conduct a systematical study of the atomic structure and lattice dynamics of CuP2 to reveal the origin, finding a dimer rattling behavior. [ABSTRACT FROM AUTHOR]

A phase transition is often accompanied by the appearance of an order parameter and symmetry breaking. Certain magnetic materials exhibit exotic hidden-order phases, in which the order parameters are not directly accessible to conventional magnetic measurements. Thus, experimental identification and theoretical understanding of a hidden order are difficult. Here we combine neutron scattering and thermodynamic probes to study the newly discovered rare-earth triangular-lattice magnet TmMgGaO4. Clear magnetic Bragg peaks at K points are observed in the elastic neutron diffraction measurements. More interesting, however, is the observation of sharp and highly dispersive spin excitations that cannot be explained by a magnetic dipolar order, but instead is the direct consequence of the underlying multipolar order that is "hidden" in the neutron diffraction experiments. We demonstrate that the observed unusual spin correlations and thermodynamics can be accurately described by a transverse field Ising model on the triangular lattice with an intertwined dipolar and ferro-multipolar order. Classically ordered phases normally coupled directly to external probes but exotic multipolar phases are not straightforwardly accessible. Here the authors show that TmMgGaO4 has multipolar order that can be inferred by inelastic neutron scattering and modeled by transverse field Ising model. [ABSTRACT FROM AUTHOR]

Lead chalcogenides have exceptional thermoelectric properties and intriguing anharmonic lattice dynamics underlying their low thermal conductivities. An ideal material for thermoelectric efficiency is the phonon glass–electron crystal, which drives research on strategies to scatter or localize phonons while minimally disrupting electronic-transport. Anharmonicity can potentially do both, even in perfect crystals, and simulations suggest that PbSe is anharmonic enough to support intrinsic localized modes that halt transport. Here, we experimentally observe high-temperature localization in PbSe using neutron scattering but find that localization is not limited to isolated modes – zero group velocity develops for a significant section of the transverse optic phonon on heating above a transition in the anharmonic dynamics. Arrest of the optic phonon propagation coincides with unusual sharpening of the longitudinal acoustic mode due to a loss of phase space for scattering. Our study shows how nonlinear physics beyond conventional anharmonic perturbations can fundamentally alter vibrational transport properties. To optimize the performance of lead chalcogenides for thermoelectric applications, strategies to further reduce the crystal's thermal conductivity is required. Here, the authors discover anharmonic localized vibrations in PbSe crystals for optimizing the crystal's vibrational transport properties. [ABSTRACT FROM AUTHOR]

Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state. Exploring the role of structural defect is essential to understand the exotic quantum spin phenoma in rare earth pyrochlores. Here the authors show oxygen vacancies can stabilise the spin liquid phase and reveal the ferromagnetic ground state when oxygen vacancies are eliminated in Yb2Ti2O7. [ABSTRACT FROM AUTHOR]

The charge and spin dynamics of the structurally simplest iron-based superconductor, FeSe, may hold the key to understanding the physics of high temperature superconductors in general. Unlike the iron pnictides, FeSe lacks long range magnetic order in spite of a similar structural transition around 90 K. Here, we report results of Raman scattering experiments as a function of temperature and polarization and simulations based on exact diagonalization of a frustrated spin model. Both experiment and theory find a persistent low energy peak close to 500 cm−1 in B1g symmetry, which softens slightly around 100 K, that we assign to spin excitations. By comparing with results from neutron scattering, this study provides evidence for nearly frustrated stripe order in FeSe. The origin of the superconductivity in iron-based superconductors remains elusive and whether a mechanism which describes all members can be found is under constant study. Using Raman spectroscopy the authors investigate magnetic ordering in FeSe, and further demonstrate that its properties are distinct among the iron-based superconductors. [ABSTRACT FROM AUTHOR]

Mn3Sn has recently attracted considerable attention as a magnetic Weyl semimetal exhibiting concomitant transport anomalies at room temperature. The topology of the electronic bands, their relation to the magnetic ground state and their nonzero Berry curvature lie at the heart of the problem. The examination of the full magnetic Hamiltonian reveals otherwise hidden aspects of these unusual physical properties. Here, we report the full spin wave spectra of Mn3Sn measured over a wide momentum—energy range by the inelastic neutron scattering technique. Using a linear spin wave theory, we determine a suitable magnetic Hamiltonian which not only explains the experimental results but also stabilizes the low-temperature helical phase, consistent with our DFT calculations. The effect of this helical ordering on topological band structures is further examined using a tight binding method, which confirms the elimination of Weyl points in the helical phase. Our work provides a rare example of the intimate coupling between the electronic and spin degrees of freedom for a magnetic Weyl semimetal system. Weyl semimetals: magnetic excitations in Mn3Sn The interplay between the electronic and spin degrees of freedom determines the removal of Weyl points in magnetic semimetal systems. A team led by Je-Geun Park at the Institute for Basic Science and Seoul National University used linear spin wave theory to obtain the full spin wave spectra of antiferromagnetic A-type Mn3Sn across a broad energy-momentum range. The magnetic Hamiltonian obtained via a local moment model was capable of explaining the experimental inelastic neutron scattering data. Additional density functional theory calculations indicated that the Mn3Sn low-temperature magnetic structure involves a helical ordering that eliminates the Weyl points, and consequently reduces Mn3Sn experimental anomalous Hall conductivity. These results indicate how topological Weyl points can be removed by introducing a minor change to the magnetic ground state. [ABSTRACT FROM AUTHOR]

FeSe is an intriguing iron-based superconductor. It presents an unusual nematic state without magnetism and can be tuned to increase the critical superconducting temperature. Recently it has been observed a noteworthy anisotropy of the superconducting gaps. Its explanation is intimately related to the understanding of the nematic transition itself. Here, we show that the spin-nematic scenario driven by orbital-selective spin fluctuations provides a simple scheme to understand both phenomena. The pairing mediated by anisotropic spin modes is not only orbital selective but also nematic, leading to stronger pair scattering across the hole and X electron pocket. The delicate balance between orbital ordering and nematic pairing points also to a marked kz dependence of the hole–gap anisotropy. Fe-based superconductors | Understanding the pairing mechanism for FeSe: FeSe is a high-temperature superconductor displaying a complex interplay between structure, magnetism and superconductivity; new theoretical results might now shed light on its pairing mechanism. FeSe is widely studied because its superconducting critical temperature can be tuned over a wide range by chemical intercalation or pressure. Superconductivity appears in a phase that is called nematic, which breaks rotational symmetry but preserves translational symmetry. Experimental measurements show that the superconducting gaps are anisotropic: this is probably related to the nematic transition, in which spin fluctuations seem to play a part. Laura Fanfarillo from the University of Rome and colleagues present a model suggesting that orbital-selective spin fluctuations provide the pairing mechanism, which is at the same time orbital selective and nematic. This explains both the anisotropy of the gaps and its connection to the nematic transition. [ABSTRACT FROM AUTHOR]