Inelastic Neutron Research Articles

Evolution of magnetic surfboards and spin glass behavior in (Fe1−p Ga p )2TiO5

The unusual anisotropy of the spin glass (SG) transition in the pseudobrookite system Fe2TiO5 has been interpreted as arising from an induced, van der Waals-like, interaction among magnetic clusters. Here we present susceptibility () and specific heat data () for Fe2TiO5 diluted with non-magnetic Ga, (Fe1−p Ga p )2TiO5, for disorder parameter = 0, 0.11, and 0.42, and elastic neutron scattering data for = 0.20. A uniform suppression of is observed upon increasing , along with a value of that increases as decreases, i.e. We also observe in the low temperature limit. The observed behavior places (Fe1−p Ga p )2TiO5 in the category of a strongly geometrically frustrated SG.

Extraction of neutron density distributions from high-statistics coherent elastic neutrino-nucleus scattering data

Forthcoming fixed-target coherent elastic neutrino-nucleus scattering experiments aim at measurements with O(tonne)-scale detectors and substantially reduced systematic and statistical uncertainties. With such high quality data, the extraction of point-neutron distributions mean-square radii requires a better understanding of possible theoretical uncertainties. We quantify the impact of single-nucleon electromagnetic mean-square radii on the weak-charge form factor and compare results from weak-charge form factor parametrizations and weak-charge form factor decompositions in terms of elastic vector proton and neutron form factors, including nucleon form factors Q-dependent terms up to order Q2. We assess as well the differences arising from results derived using weak-charge form factor decompositions in terms of elastic vector proton and neutron form factors and a model-independent approach based solely on the assumption of spherically symmetric nuclear ground state. We demonstrate the impact of the main effects by assuming pseudo-data from a one-tonne LAr detector and find that, among the effects and under the assumptions considered in this paper, weak-charge form factor parametrizations and weak-charge form factor decompositions in terms of elastic vector proton and neutron form factors enable the extraction of the Ar40 point-neutron distribution mean-square radius with a ∼15% accuracy. With a substantial reduction of the beam-related neutron and steady-state backgrounds a ∼1% precision extraction seems feasible, using either of the two approaches.

In operando inelastic neutron scattering and pair distribution function analysis of water and carbon dioxide adsorption by calcium-based layered double hydroxide

<i>In operando</i> inelastic neutron scattering and pair distribution function analysis of water and carbon dioxide adsorption by calcium-based layered double hydroxide

Q-dependent electron-phonon coupling induced phonon softening and non-conventional critical behavior in the CDW superconductor LaPt2Si2

This paper reports the first experimental observation of phonons and their softening on single crystalline LaPt2Si2 via inelastic neutron scattering. From the temperature dependence of the phonon frequency in close proximity to the charge density wave (CDW) q-vector, we obtain a CDW transition temperature of TCDW = 230 K and a critical exponent β = 0.28 ± 0.03. This value is suggestive of a non-conventional critical behavior for the CDW phase transition in LaPt2Si2, compatible with a scenario of CDW discommensuration (DC). The DC would be caused by the existence of two CDWs in this material, propagating separately in the non equivalent (Si1–Pt2–Si1) and (Pt1–Si2–Pt1) layers, respectively, with transition temperatures TCDW−1 = 230 K and TCDW−2 = 110 K. A strong q-dependence of the electron-phonon coupling has been identified as the driving mechanism for the CDW transition at TCDW−1 = 230 K while a CDW with 3-dimensional character, and Fermi surface quasi-nesting as a driving mechanism, is suggested for the transition at TCDW−2 = 110 K. Our results clarify some aspects of the CDW transition in LaPt2Si2 which have been so far misinterpreted by both theoretical predictions and experimental observations and give direct insight into its actual temperature dependence.

Soft Phonon Mode Triggering Fast Ag Diffusion in Superionic Argyrodite Ag8 GeSe6.

The structural coexistence of dual rigid and mobile sublattices in superionic Argyrodites yields ultralow lattice thermal conductivity along with decent electrical and ionic conductivities and therefore attracts intense interest for batteries, fuel cells, and thermoelectric applications. However, a comprehensive understanding of their underlying lattice and diffusive dynamics in terms of the interplay between phonons and mobile ions is missing. Herein, inelastic neutron scattering is employed to unravel that phonon softening on heating to Tc ≈ 350 K triggers fast Ag diffusion in the canonical superionic Argyrodite Ag8 GeSe6 . Ab initio molecular dynamics simulations reproduce the experimental neutron scattering signals and identify the partially ultrafast Ag diffusion with a large diffusion coefficient of 10-4 cm-2 s-1 . The study illustrates the microscopic interconnection between soft phonons and mobile ions and provides a paradigm for an intertwined interaction of the lattice and diffusive dynamics in superionic materials.

Neutron spin echo spectroscopy with a moving sample

Neutron spin echo spectroscopy is a high resolution inelastic neutron scattering method probing nanosecond dynamics. It is well suited to study the atomistic motion in polymer systems and contributes to our understanding of viscoelasticity. However, for samples under shear, or moving samples in general, Doppler scattering has to be considered. We compare the measured phase shift and depolarisation due to Doppler scattering from a rotating graphite disk to numerical and analytical calculations and find excellent agreement. This allows to take into account Doppler scattering during the data processing and makes longer Fourier times as well as higher shear rates and Q ranges possible with neutron spin echo spectroscopy, enabling for example the study of polymers under high shear.

Author Correction: Optimizing the dynamic pair distribution function method for inelastic neutron spectrometry

Author Correction: Optimizing the dynamic pair distribution function method for inelastic neutron spectrometry

Magnetic super-structure and active surface role in the onset of magnetic excitons revealed in TbCu2 nanoparticles

Antiferromagnetic materials are receiving renewed interest on behalf of their potential for information technologies. Recent reports have also revealed how the physics governing such magnetic arrangements and their excitations become more complex compared to traditional ferromagnetic materials, especially at the nanoscale. Here, we address two main issues that are of prime interest to their technological transfer. First, using small-angle neutron scattering, we show the existence of a magnetic helix-like super-structure in a polycrystalline TbCu2 alloy, preserved at both bulk and nanoparticle ensembles of 8 nm. Second, using inelastic neutron scattering, we elucidate the magnetic excitons and the crystalline electric field energy level schemes of TbCu2 in bulk and nanoparticle ensembles. This allows to understad the effect of the surface broken symmetry on the quantum energy levels at the nanoscale, so as the key role of interfacial effects on the propagation of magnetic excitations. Our research provides insights for the realization of magnetic moment dynamics models based on complex nanometric super-structures, and for nanoparticles to be integrated in spintronics and information technology applications.

Control of H-Related Defects in γ-MnO2 in a Hydrothermal Synthesis.

Manganese dioxide is a good candidate for effective energy storage and conversion as it possesses rich electrochemistry. The compound also shows a wide polymorphism. The γ-variety, an intergrowth of β- and R-MnO2, has been extensively studied in several types of batteries (e.g., Zn/MnO2, Li-ion) and is a common electrode material for commercial batteries. It is well known that the insertion of protons thermodynamically stabilizes γ-MnO2 with respect to β-MnO2. Protons can enter the structure either by forming groups of 4 hydroxyls around a Mn4+ vacancy, called a Ruetschi defect, or by forming a hydroxyl group near a Mn3+ ion, called a Coleman defect. These defects differently affect the electrochemistry of manganese oxide, and tailoring their amount in the structure can be used to tune the material properties. Previous studies have addressed the proton insertion process, but the role of the synthesis pathway on the amount of defects created is not well understood. We here investigate how the parameters in a hydrothermal synthesis of γ-MnO2 nanoparticles influence the amount and type of H-related defects. Structural investigations are carried out using Pair Distribution Function analysis, X-ray absorption spectroscopy, thermogravimetric analysis, and inelastic neutron scattering. We demonstrate the possibility to control the amount and type of defects introduced during the synthesis. While the amount of Ruetschi defects increases with synthesis temperature, it decreases with extended synthesis time, along with the amount of Coleman defects. Moreover, we discuss the arrangement of the defects in the γ-MnO2 nanoparticles.

An overview of the spin dynamics of antiferromagnetic Mn5Si3

The metallic compound Mn5Si3 hosts a series of antiferromagnetic phases that can be controlled by external stimuli, such as temperature and magnetic field. In this work, we investigate the spin-excitation spectrum of bulk Mn5Si3 by combining inelastic neutron scattering measurements and density functional theory calculations. We study the evolution of the dynamical response under external parameters and demonstrate that the spin dynamics of each phase is robust against any combination of temperature and magnetic field. In particular, the high-energy spin dynamics is very characteristic of the different phases consisting of either spin waves or broad fluctuation patterns.

Alterations promoted by acid straightening and/or bleaching in hair microstructures.

Human hair is a biopolymer constituted mainly of keratin intermediate filaments, lipids, pigments and water. Cosmetic treatments usually interact with the hair at the molecular level, inducing changes in its components and modifying the physicochemical and mechanical properties of the fibers. Here, the effect of acid straightening on the morphology and ultrastructure of Caucasian hair was investigated by a group of complementary experimental methods: wide-, small- and ultra-small-angle X-ray scattering; high-resolution 3D X-ray microscopy; quasi-elastic neutron scattering and inelastic neutron scattering; thermogravimetry-mass spectrometry; and differential scanning calorimetry (DSC). X-ray diffraction patterns showed that acid straightening associated with a flat iron (∼180°C) changed the cortex of the fiber, shown by denaturation of the intermediate filaments (measured by DSC). The increase in the spacing of the lipid layers and the observation of the dehydration behavior of the fiber provided indications that water may be confined between these layers, while neutron spectroscopy showed alterations in the vibration mode of the CH2 groups of the lipids and an increase of the proton (H+) mobility in the hair structure. The latter may be associated with the extremely low pH of the formulation (pH ≃ 1). Additionally, this investigation showed that bleached hair (one-time bleached) is more damaged by the action of acid straightening than virgin hair, which was shown by a threefold increase in the percentage of total porosity of the tresses. The obtained results demonstrate that the investigation approach proposed here can provide very important thermodynamic and structural information on induced changes of hair structure, and certainly can be applied for the evaluation of the action mode and efficiency of cosmetic treatments.

Photochemistry sample sticks for inelastic neutron scattering.

Every material experiences atomic and molecular motions that are generally termed vibrations in gases and liquids or phonons in solid state materials. Optical spectroscopy techniques, such as Raman, infrared absorption spectroscopy, or inelastic neutron scattering (INS), can be used to measure the vibrational/phonon spectrum of ground state materials properties. A variety of optical pump probe spectroscopies enable the measurement of excited states or elucidate photochemical reaction pathways and kinetics. So far, it has not been possible to study photoactive materials or processes in situ using INS due to the mismatch between neutron and photon penetration depths, differences between the flux density of photons and neutrons, cryogenic temperatures for INS measurements, vacuum conditions, and a lack of optical access to the sample space. These experimental hurdles have resulted in very limited photochemistry studies using INS. Here we report on the design of two different photochemistry sample sticks that overcome these experimental hurdles to enable in situ photochemical studies using INS, specifically at the VISION instrument at Oak Ridge National Laboratory. We demonstrate the use of these new measurement capabilities through (1) the in situ photodimerization of anthracene and (2) the in situ photopolymerization of a 405nm photoresin using 405nm excitation as simple test cases. These new measurement apparatus broaden the science enabled by INS to include photoactive materials, optically excited states, and photoinitiated reactions.

Structure and magnetism of the triangular lattice material YbBO3

YbBO3 is a member of the orthoborate family of materials which contains a triangular arrangement of Yb3+ ions. Here we study the physical properties of YbBO3 with neutron diffraction, inelastic neutron scattering, specific heat, and ac susceptibility measurements. The neutron diffraction measurements confirm that our samples of YbBO3 crystallize in the monoclinic space group (#15) which contains two crystallographically distinct Yb3+ sites decorating a slightly distorted triangular lattice. Heat capacity and ac susceptibility measurements indicate a potential transition to magnetic order at 0.4 K. In agreement with these observations, neutron diffraction measurements at 0.044 K observe magnetic Bragg peaks which can be indexed by a propagation vector of (0 0 1). Although determining a unique spin configuration corresponding to the observed magnetic Bragg peaks is not possible, model refinements indicate that the ordered moments are likely in the range of 0.4–0.9 and, notably, require moments on both Yb sites. In addition to the magnetic Bragg peaks, diffuse scattering at low Q is observed indicating that the transition does not correspond to complete long range magnetic order. The two-site picture for YbBO3 is further evidenced by the number of crystal field excitations observed by inelastic neutron scattering measurements. Together these results show that YbBO3 is a two-site triangular lattice material with signatures of long-range order as well as shorter ranged spin correlations.

Update 2.5 to “Takin: An open-source software for experiment planning, visualisation, and data analysis”, (PII: S2352711016300152)

The new version 2.5 of the inelastic neutron scattering software suite Takin introduces a magnon calculator for use with its instrumental resolution convolution programme as well as further extension modules and improvements.

Study of the effective temperatures of two neutron moderating materials: Ethane and triphenylmethane

We studied the effective temperatures of hydrogen and carbon in solid ethane at 80 K and triphenylmethane at 22 K at the VESUVIO spectrometer (ISIS Neutron and Muon Source, Rutherford Appleton Laboratory, UK), using the Deep Inelastic Neutron Scattering (DINS) and Neutron Transmission techniques. The systems are of potential application as moderators for the design of cold neutron sources. We discuss in detail the process of data analysis and parameter fitting, in which we introduced improved algorithms. The results were compared with density functional theory calculations, which yield compatible results. The importance of these determinations for their applications to Nuclear Engineering calculations is highlighted. The results of the effective temperatures obtained are supported by comparisons with the total cross sections measured simultaneously with DINS in this experiment.

Interlayer coupling in the superconducting state of iron-based superconductors

High-temperature superconducting paring is generally believed to be mediated by the magnetic fluctuations that mostly occur within the two-dimensional conducting layers (Cu-O, Fe-As, or Fe-Se) in copper oxides, iron pnictides, iron chalcogenides, etc. Here, on the basis of inelastic neutron scattering measurements on a superconducting iron pnictide ${\mathrm{Ca}}_{0.33}{\mathrm{Na}}_{0.67}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, we have discovered a highly three-dimensional spin resonance mode with upward V-shape dispersions both in the $ab$ plane and along the $c$ axis. The superconducting gaps exhibit strong ${k}_{z}$ modulations involved with significant changes on the size of one hole pocket and the density state of the ${d}_{{z}^{2}}$ orbital of Fe. The resonance dispersions don't always seem confined by the total gaps summed on those imperfectly nesting Fermi sheets. It is demonstrated that the $c$-axis dependence of both the resonance energy and its intensity in iron pnictides can be universally scaled with the distance between two adjacent Fe-As layers ($d$), and the ${k}_{z}$ modulation of the gap is also anticorrelated with $d$. These results highlight the role of interlayer coupling in iron-based superconductivity, suggesting that the interlayer pairing may also be driven by magnetic fluctuations under certain spin-orbit couplings.

Magnetic ground state and perturbations of the distorted kagome Ising metal TmAgGe

Magnetic order and excitations of the distorted kagome intermetallic magnet TmAgGe are explored using a variety of methods, including single-crystal neutron diffraction, inelastic neutron scattering, Monte Carlo simulations, and the random phase approximation. The magnetic properties are governed by strong, local single-ion anisotropy and geometrically frustrated magnetic exchanges. The authors establish multi-k structures at several different magnetic field configurations and model effective exchange parameters using field-dependencies of the magnetic neutron diffraction reflections and the weakly dispersive crystal electric field mode.

Shedding Light on Cuprorivaite, the Egyptian Blue Pigment: Joining Neutrons and Photons for a Computational Spectroscopy Study

The unique properties of Egyptian blue, a cuprorivaite pigment, were herein studied through a holistic computational and experimental approach. A reliable model of the crystal of cuprorivaite was obtained through periodic density functional theory calculations, allowing for the elucidation of its lattice dynamics, including assessment of structural, electronic, and vibrational properties. From this model, a sound assignment of the inelastic neutron scattering spectrum was obtained, along with estimated values of heat capacity and Debye temperature, band-gap values, and magnetic properties of the crystal. Inelastic neutron scattering and diffuse reflectance infrared Fourier transform spectroscopies provided enlightenment on the chemical surfaces of the pigment Egyptian blue─which was missing hitherto and is critical for potential applications of the pigment, such as those involving host–matrix interactions or requiring surface derivatization─while confirming the simulation results. On that account, it was found that the intensity of the dangling νSiOd mode is ca. 8–13% of the total νSiO modes. Moreover, and regarding the electronic properties, the band structure confirmed that CaCuSi4O10 is a direct band-gap semiconductor, with the valence band maximum and the conduction band minimum located at the Γ-point, in both the alpha (spin-up) and beta (spin-down) density bands.

Ne, Ar, and Kr oscillators in the molecular cavity of fullerene C60.

We used THz (terahertz) and INS (inelastic neutron scattering) spectroscopies to study the interaction between an endohedral noble gas atom and the C60 molecular cage. The THz absorption spectra of powdered A@C60 samples (A = Ar, Ne, Kr) were measured in the energy range from 0.6 to 75meV for a series of temperatures between 5 and 300K. The INS measurements were carried out at liquid helium temperature in the energy transfer range from 0.78 to 54.6meV. The THz spectra are dominated by one line, between 7 and 12meV, at low temperatures for three noble gas atoms studied. The line shifts to higher energy and broadens as the temperature is increased. Using a spherical oscillator model, with a temperature-independent parameterized potential function and an atom-displacement-induced dipole moment, we show that the change of the THz spectrum shape with temperature is caused by the anharmonicity of the potential function. We find good agreement between experimentally determined potential energy functions and functions calculated with Lennard-Jones additive pair-wise potentials with parameters taken from the work of Pang and Brisse, J. Chem. Phys. 97, 8562 (1993).

Magnetic structure of α-U3O8: a DFT and experimental study

The magnetic ordering of α-U3O8 remains a puzzling question for a long time. Recent inelastic neutron scattering study exhibited magnetic peaks at (0.5,1,1) and (0.5,2,2) but without intensity at (0.5,0,0). The magnetic propagation vector could still be k = (0.5,0,0), as the probability that the magnetic moments are perpendicular to the reflection plane (0.5,0,0) leading to zero intensity should be not ruled out. Magnetic symmetry analysis leads to 4 distinct magnetic structures, and the first-principles calculations show that the possible ground state of U3O8 could be collinear antiferromagnetic (AFM) ordering (m∣∣a-axis). Only one paramagnetic-AFM phase transition at T 1 = 24 K in magnetization curves has been confirmed, which shifts to lower temperatures as the magnetic field increases. At T = 1.8 K, no hysteresis loop around zero field has been observed which could exclude ferromagnetic ground state in this compound. Neutron powder diffraction data have been collected at several temperatures (300, 40, 15 and 5 K), and the absence of visible magnetic reflections makes the unusual magnetic properties even more mysterious.