Inelastic Neutron Scattering Research Articles

Nematic Fluctuations in the Non-Superconducting Iron Pnictide BaFe1.9−xNi0.1CrxAs2

The main driven force of the electronic nematic phase in iron-based superconductors is still under debate. Here, we report a comprehensive study on the nematic fluctuations in a non-superconducting iron pnictide system BaFe1.9−xNi0.1CrxAs2 by electronic transport, angle-resolved photoemission spectroscopy (ARPES), and inelastic neutron scattering (INS) measurements. Previous neutron diffraction and transport measurements suggested that the collinear antiferromagnetism persists to x = 0.8, with similar Néel temperature TN and structural transition temperature Ts around 32 K, but the charge carriers change from electron type to hole type around x = 0.5. In this study, we have found that the in-plane resistivity anisotropy also highly depends on the Cr dopings and the type of charge carriers. While ARPES measurements suggest possibly weak orbital anisotropy onset near Ts for both x = 0.05 and x = 0.5 compounds, INS experiments reveal clearly different onset temperatures of low-energy spin excitation anisotropy, which is likely related to the energy scale of spin nematicity. These results suggest that the interplay between the local spins on Fe atoms and the itinerant electrons on Fermi surfaces is crucial to the nematic fluctuations of iron pnictides, where the orbital degree of freedom may behave differently from the spin degree of freedom, and the transport properties are intimately related to the spin dynamics.

Quantitative Hole Mobility Simulation and Validation in Substituted Acenes.

Knowledge of the full phonon spectrum is essential to accurately calculate the dynamic disorder (σ) and hole mobility (μh) in organic semiconductors (OSCs). However, most vibrational spectroscopy techniques under-measure the phonons, thus limiting the phonon validation. Here, we measure and model the full phonon spectrum using multiple spectroscopic techniques and predict μh using σ from only the Γ-point and the full Brillouin zone (FBZ). We find that only inelastic neutron scattering (INS) provides validation of all phonon modes, and that σ in a set of small molecule semiconductors can be miscalculated by up to 28% when comparing Γ-point against FBZ calculations. A subsequent mode analysis shows that many modes contribute to σ and that no single mode dominates. Our results demonstrate the importance of a thoroughly validated phonon calculation, and a need to develop design rules considering the full spectrum of phonon modes.

Cold neutron triple-axis spectrometer (Cold TAS) at HANARO

A cold neutron triple-axis spectrometer (Cold TAS) has been installed in HANARO at the Korea Atomic Energy Research Institute, South Korea. It has been designed for high-resolution inelastic-neutron-scattering (INS) measurements of spin and lattice dynamics using a neutron velocity selector (NVS) for incident cold neutrons. We evaluated the wavelength resolution and harmonics filtration of the NVS and verified the vertical focusing effects of the pyrolytic-graphite crystal monochromator. The beam flux at the sample position and the spectrometer resolution, which are key factors in INS experiments, attained the typical TAS specification. Finally, we exemplified INS spectra to show the current performance of the high-resolution mode and magnetic-excitation measurements.

Exchange interactions and spin dynamics in the layered honeycomb ferromagnet CrI3

We derive the microscopic spin Hamiltonian for rhombohedral CrI$_3$ using extensive first-principles density functional theory (DFT) calculations which incorporate spin-orbit coupling and Hubbard U. Our calculations indicate a dominant nearest-neighbor ferromagnetic Heisenberg exchange with weaker further-neighbor Heisenberg terms. In addition, we find a Dzyaloshinskii-Moriya interaction which primarily drives a topological gap in the spin-wave spectrum at the Dirac point, and uncover a non-negligible antiferromagnetic Kitaev coupling between the S=3/2 Cr moments. The out-of-plane magnetic moment is stabilized by weak symmetric bond-dependent terms and a local single-ion anisotropy. Using linear spin wave theory, we find that our exchange parameters are in reasonably good agreement with inelastic neutron scattering (INS) experiments. Employing classical Monte Carlo simulations, we study the magnetic phase transition temperature $T_c$ and its evolution with an applied in-plane magnetic field. We further demonstrate how future high-resolution INS experiments on the magnon dispersion of single crystals in an in-plane magnetic field may be used to quantitatively extract the strength of the antiferromagnetic Kitaev exchange coupling.

Optimization and inference of bin widths for histogramming inelastic neutron scattering spectra.

A data-driven bin-width optimization for the histograms of measured data sets based on inhomogeneous Poisson processes was developed in a neurophysiology study [Shimazaki & Shinomoto (2007). Neural Comput. 19, 1503-1527], and a subsequent study [Muto, Sakamoto, Matsuura, Arima & Okada (2019). J. Phys. Soc. Jpn, 88, 044002] proposed its application to inelastic neutron scattering (INS) data. In the present study, the results of the method on experimental INS time-of-flight data collected under different measurement conditions from a copper single crystal are validated. The extrapolation of the statistics on a given data set to other data sets with different total counts precisely infers the optimal bin widths on the latter. The histograms with the optimized bin widths statistically verify two fine-spectral-feature examples in the energy and momentum transfer cross sections: (i) the existence of phonon band gaps; and (ii) the number of plural phonon branches located close to each other. This indicates that the applied method helps in the efficient and rigorous observation of spectral structures important in physics and materials science like novel forms of magnetic excitation and phonon states correlated to thermal conductivities.

Assignment of the Vibrational Spectra of Diiron Nonacarbonyl, Fe2(CO)9

Diiron nonacarbonyl, Fe2(CO)9, was discovered in 1905 and was the third metal carbonyl to be found. It was the first to be synthesized by a photochemical route. This is a challenging material to study: it is insoluble in virtually all solvents and decomposes at 373 K before melting. This means that only solid-state spectroscopic data are available. New infrared, Raman and inelastic neutron scattering (INS) spectra have been measured and used to generate a complete assignment of the vibrational spectra of Fe2(CO)9. Density functional theory (DFT) calculations are used to support the assignments; however, for this material, they are much less useful than expected, although the calculated intensities provide crucial information.

H2O inside the fullerene C60: Inelastic neutron scattering spectrum from rigorous quantum calculations.

We present a methodology that, for the first time, allows rigorous quantum calculation of the inelastic neutron scattering (INS) spectra of a triatomic molecule in a nanoscale cavity, in this case, H2O inside the fullerene C60. Both moieties are taken to be rigid. Our treatment incorporates the quantum six-dimensional translation-rotation (TR) wave functions of the encapsulated H2O, which serve as the spatial parts of the initial and final states of the INS transitions. As a result, the simulated INS spectra reflect the coupled TR dynamics of the nanoconfined guest molecule. They also exhibit the features arising from symmetry breaking observed for solid H2O@C60 at low temperatures. Utilizing this methodology, we compute the INS spectra of H2O@C60 for two incident neutron wavelengths and compare them with the corresponding experimental spectra. Good overall agreement is found, and the calculated spectra provide valuable additional insights.

Metamagnetism and crystal-field splitting in pseudohexagonal CeRh3Si2

CeRh$_3$Si$_2$ has been reported to exhibit metamagnetic transitions below 5~K, a giant crystal field splitting, and anisotropic magnetic properties from single crystal magnetization and heat capacity measurements. Here we report results of neutron and x-ray scattering studies of the magnetic structure and crystal-field excitations to further understand the magnetism of this compound. Inelastic neutron scattering (INS) and resonant inelastic x-ray scattering (RIXS) reveal a $J_z$\,=\,1/2 groundstate for Ce when considering the crystallographic $a$ direction as quantization axis, thus explaining the anisotropy of the static susceptibility. Furthermore, we find a total splitting of 78\,meV for the $J$\,=\,5/2 multiplet. The neutron diffraction study in zero field reveals that on cooling from the paramagnetic state, the system first orders at $T_{\text{N}_1}=4.7$\,K in a longitudinal spin density wave with ordered Ce moments along the $b$-axis (i.e. the [0 1 0] crystal direction) and an incommensurate propagation vector $\textbf{k}=(0,0.43,0$). Below the lower-temperature transition $T_{\text{N}_2}=4.48$\,K, the propagation vector locks to the commensurate value $\textbf{k}=(0,0.5,0)$, with a so-called lock-in transition. Our neutron diffraction study in applied magnetic field $H\parallel b$-axis shows a change in the commensurate propagation vector and development of a ferromagnetic component at $H=3$\,kOe, followed by a series of transitions before the fully field-induced ferromagnetic phase is reached at $H = 7$\,kOe. This explains the nature of the steps previously reported in field-dependent magnetization measurements. A very similar behaviour is also observed for the $H\parallel$ [0 1 1] crystal direction.

Lattice Dynamics in Organic Ferroelectrics Using Neutron Spectroscopy and Ab-Initio Modeling

Lattice dynamics in organic ferroelectric PhMDA (2-phenylmalondialdehyde C9H8O2) has been investigated using inelastic neutron scattering (INS) spectroscopy and first principles based calculations. Most of the prominent features of the INS spectrum originated from the normal modes of hydrogen bonded malondialdehyde (MDA) units rather than from phenyl rings. It is also found that carbon-hydrogen bonds in the MDA unit are tighter than these in phenyl rings. From the calculated splitting of LO (longitudinal optical) and TO (transverse optical) normal modes, it is predicted that hydrogen bonds in MDA units, which hold the solid state structure, contribute more to the ferroelectric polarisation of PhMDA. Any distortion of H-ions in these hydrogen bonds thus affects ferroelectric properties of this material.

Bin-Width Optimization for Multidimensional Histogram and Its Application to Inelastic Neutron Scattering Experiments

Bin-Width Optimization for Multidimensional Histogram and Its Application to Inelastic Neutron Scattering Experiments

Signature of CEF-phonon coupling in Kondo lattice system CeCuGa3

We present the results of inelastic neutron scattering (INS) measurements on Kondo lattice heavy fermion CeCuGa3. The low-temperature magnetic susceptibility exhibits an anomaly near 2.6 K associated with an antiferromagnetic transition which is further confirmed by the heat capacity measurement. The analysis of magnetic heat capacity C mag (T) based on the crystal electric field (CEF) model reveals an overall CEF splitting of ∼ 21 meV. The INS data reveal two strong magnetic excitations near 4.5 and 6.9 meV, which, however failed to reproduce the observed C mag (T). We therefore analyze the INS data by a model based on magneto-elastic (i.e. CEF-phonon) coupling which suggests that the excitations observed near 4.5 and 6.9 meV originate from the CEF-phonon coupling as observed in CeCuAl3 and CeAuAl3.

Strong renormalization of Ba vibrations in thermoelectric type-IX clathrate Ba24Ge100

We report a combined experimental and theoretical study of the lattice dynamics of the type IX clathrate ${\mathrm{Ba}}_{24}{\mathrm{Ge}}_{100}$ by inelastic neutron scattering (INS) experiments and density functional theory (DFT) calculations. We observe low-energy optical modes at $\ensuremath{\sim}2--3\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ due to the motion of the heavy Ba atoms along the high-symmetry axis and the largest dimension of the open Ge@20 cages present in the compound. Even though the phonon participation ratio indicates that these low-energy modes are localized, their $Q$ dependence shows that the dynamics of the Ba guests are correlated. We observe a strong change in the spectral weight of these modes when the compound undergoes a temperature-induced structural transformation in the temperature range 190--230 K. In the high-temperature phase, the low-energy optical modes show high intensities in the INS data and frequencies rather insensitive to temperature changes up to \ensuremath{\sim}550 K. In the low-temperature structural modification, the low-energy mode intensities are strongly depleted and apparently shifted to higher energies; this behavior is in line with an off-centering of the Ba atoms at low temperatures. Our DFT calculations successfully approximate the essential features in the dynamics of the high-temperature ${\mathrm{Ba}}_{24}{\mathrm{Ge}}_{100}$ structure.

Investigating H2 Adsorption in Isostructural Metal-Organic Frameworks M-CUK-1 (M = Co and Mg) through Experimental and Theoretical Studies.

A combined experimental and theoretical study of H2 adsorption was carried out in Co-CUK-1 and Mg-CUK-1, two isostructural metal-organic frameworks (MOFs) that consist of M2+ ions (M = Co and Mg) coordinated to pyridine-2,4-dicarboxylate (pdc2-) and OH- ligands. These MOFs possess saturated metal centers in distorted octahedral environments and narrow pore sizes and display high chemical and thermal stability. Previous experimental studies revealed that Co-CUK-1 exhibits a H2 uptake of 183 cm3 g-1 at 77 K/1.0 atm [ Angew. Chem., Int. Ed. 2007, 46, 272-275, DOI: 10.1002/anie.200601627], while that for Mg-CUK-1 under the same conditions is 240 cm3 g-1 on the basis of the experimental measurements carried out herein. The theoretical H2 adsorption isotherms are in close agreement with the corresponding experimental measurements for simulations using electrostatic and polarizable potentials of the adsorbate. Through simulated annealing calculations, it was found that the primary binding site for H2 in both isostructural analogues is localized proximal to the center of the aromatic rings belonging to the pdc2- linkers. Inelastic neutron scattering (INS) spectroscopic studies of H2 adsorbed in both MOFs revealed a rotational tunnelling transition occurring at around 8 meV in the corresponding spectra; this peak represents H2 adsorbed at the primary binding site. Two-dimensional quantum rotation calculations for H2 localized at the primary and secondary binding sites in both MOFs yielded rotational energy levels that are in agreement with the transitions observed in the INS spectra. Even though both M-CUK-1 analogues possess different metal ions, they exhibit similar electrostatic environments, modeled structures at H2 saturation, and rotational potentials for H2 adsorbed at the most favorable adsorption site. Overall, this study demonstrates how important molecular-level details of the H2 adsorption mechanism inside MOF micropores can be derived from a combination of experimental measurements and theoretical calculations using two stable and isostructural MOFs with saturated metal centers and small pore windows as model systems.

Water in Deep Eutectic Solvents: New Insights From Inelastic Neutron Scattering Spectroscopy

The effect of water on the physicochemical properties of deep eutectic solvents (DES) is a trending research topic. In this work, inelastic neutron scattering (INS) spectroscopy, was used to probe intermolecular interactions in the water-deep eutectic solvent mixtures for the cases of choline chloride (the hydrogen bond acceptor) and three different hydrogen bond donors, with different degrees of acidity: urea, glycerol and lactic acid. It was found that quenching samples in liquid nitrogen is a procedure that may retain the liquid phase morphology of DES at the low temperatures required by INS spectroscopy. The three studied systems share the preference of water molecules to bind to chloride anion, as predicted by numerous molecular dynamics simulations. Despite this similarity, the three systems present several distinct INS features upon water addition that are related to their unique properties and structure at the molecular level. In the choline chloride:urea system, water molecules promote a strengthening of hydrogen bonds with the NH and OH donors, while for the choline chloride:lactic acid system INS probed the existence of solvated DES clusters instead of specifically interfering water molecules. This study takes advantage from the unique capabilities of INS and paves the way for future studies in these systems.

Comparison of the Possibilities of Inelastic Scattering of Synchrotron Radiation and Neutrons for Studying Atomic, Molecular, and Magnetic Dynamics in Condensed Matter

Comparison of the Possibilities of Inelastic Scattering of Synchrotron Radiation and Neutrons for Studying Atomic, Molecular, and Magnetic Dynamics in Condensed Matter

Unified Picture for the Spin Resonance Mode in Cr-Based Spinel Oxides Using Inelastic Neutron Scattering

Unified Picture for the Spin Resonance Mode in Cr-Based Spinel Oxides Using Inelastic Neutron Scattering

Enhanced covalency and nanostructured-phonon scattering lead to high thermoelectric performance in n-type PbS

High thermoelectric performance can be achieved either by tuning the electronic structure or by enhancement in scattering the heat-carrying phonons, which often affect each other. Thereby, a leap in the performance can be achieved by simultaneous modulation of electronic structure and lowering of thermal conductivity. Herein, we demonstrate a high thermoelectric figure of merit (zT) of 1.45 at 900 K for Ge doped (4–10 mol%) n-type PbS, which is the one of the highest values among all n-type PbS-based thermoelectric materials. This high performance is achieved by simultaneous (a) enhancement of covalency in chemical bonding which increases the electrical conductivity, and (b) reduction of lattice thermal conductivity (κlat) to an ultra-low value of 0.56 W m−1K−1 at 900 K by the introduction of nanometer-sized (5–10 nm) precipitates of Pb2GeS4 in PbS matrix which strongly scatter the heat-carrying phonons. The presence of low-lying transverse acoustic (TA) and longitudinal acoustic (LA) phonon modes at 48.24 cm−1 and 91.83 cm−1, respectively are experimentally revealed from inelastic neutron scattering (INS) experiments. The softening of low-frequency modes at a higher temperature and ultra-short phonon lifetime (1–4.5 ps) further explain the ultra-low κlat. Electron localization function (ELF) analysis confirms chemical bonding hierarchy and increased covalent bonding due to the presence of Ge in PbS. The increase in bond covalency upon Ge doping in n-type PbS weakens electron–phonon coupling, thereby increasing the electrical transport.

Spin excitations coupled with lattice and charge dynamics in La2−xSrxCuO4

Spin excitations of layered copper oxide show various characteristic features, depending on the carrier concentration. In this study, we conducted inelastic neutron scattering (INS) measurements on La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO), with $x$ = 0, 0.075, 0.18, and 0.30 and La$_{2-x}$Sr$_{x}$NiO$_4$ (LSNO) with 1/3, to clarify the origin of the intensity enhancement in the excitation spectrum of LSCO at the energy ($\omega$) of 16--19 meV [Phys. Rev. B {\bf 91}, 224404 (2015), {\it ibid}. {\bf 93}, 094416 (2016)]. We confirmed the presence of a peak-structure in the $\omega$-dependence of the local spin susceptibility $\chi''(\omega)$ of superconducting (SC) LSCO with a peak energy of 16--19 meV, where the spin excitations intersect optical phonon branches. A comparable peak-structure is not observed in the insulating La$_2$CuO$_4$, LSNO, and heavily overdoped LSCO with $x$ = 0.30. A dome-shaped $x$-dependence of the integrated intensity around the peak energies is revealed for SC phase by summarizing the present and previously reported results. Furthermore, our phonon calculation on LCO shows the existence of two optical branches at $\sim$19 meV that could stabilize stripe-alignment of carriers due to out-of-plane vibrations of Cu or O of the CuO$_2$ planes. These results indicate the interplay among spin, charge, and lattice dynamics and suggest that the intensity enhancement is associated with their composite excitations.

Magnetic properties and neutron spectroscopy of lanthanoid-{tetrabromocatecholate/18-crown-6} single-molecule magnets

Lanthanoid single-molecule magnets (Ln-SMMs) exhibit slow magnetic relaxation at low temperatures. This arises from an energy barrier to magnetisation reversal associated with the crystal field (CF) splitting of the Ln(III) ion. The magnetic relaxation is impacted by the interaction of the molecule with the crystal lattice, so factors including particle size and crystal packing can play an important role. In this work, a family of compounds of general formula [Ln(18-c-6)(NO3)(Br4Cat)]·X (Ln = La, Tb, Dy; 18-c-6 = 18-crown-6; Br4Cat2− = tetrabromocatecholate) has been studied by inelastic neutron scattering (INS) and magnetometry to elucidate the effects of crystal packing on the slow magnetic relaxation of the Tb(III) and Dy(III) compounds. The deuterated analogues [Ln(18-c-6-d24)(NO3)(Br4Cat)]·CH3CN-d3 (1-LnD; Ln = La, Tb, Dy) have been synthesised, with 1-TbD and the diamagnetic analogue 1-LaD measured by INS. The dynamic magnetic properties of 1-TbD and 1-DyD have also been measured and compared for two samples with different particle sizes. To probe packing effects on the slow magnetic relaxation, two new solvatomorphs of the hydrogenous compounds [Ln(18-c-6)(NO3)(Br4Cat)]·X (2-Ln: X = CH2Cl2; 3-Ln: X = 0.5 toluene) have been obtained for Ln = Tb and Dy. The CF splitting between the ground and first excited CF pseudo-doublets has been experimentally determined for 1-TbD by INS, and strongly rare earth dependent and anharmonic lattice vibrational modes have also been observed in the INS spectra, with implications for slow magnetic relaxation. Dynamic magnetic measurements reveal significant particle-size dependence for the slow magnetic relaxation for 1-TbD, while a previously reported anomalous phonon bottleneck effect in the 1-DyD analogue does not change with particle size. Further dynamic magnetic measurements of 2-Ln and 3-Ln show that the slow magnetic relaxation in these Ln-SMMs is strongly dependent on lattice effects and crystal packing, which has implications for the future use of Ln-SMMs in devices.

Structural and Dynamic Effects of Paraoxon Binding to Human Acetylcholinesterase by X-Ray Crystallography and Inelastic Neutron Scattering

Structural and Dynamic Effects of Paraoxon Binding to Human Acetylcholinesterase by X-Ray Crystallography and Inelastic Neutron Scattering