Inelastic Neutron Research Articles

Confirming the high pressure phase diagram of the Shastry-Sutherland model

A Muon Spin Rotation (µ + SR) study was conducted to investigate the magnetic properties of SrCu2(BO3)2 (SCBO) as a function of temperature/pressure. Measurements in zero field and transverse field confirm the absence of long range magnetic order at high pressures and low temperatures. These measurements suggest changes in the Cu spin fluctuations characteristics above 21 kbar, consistent with the formation of a plaquette phase as previously suggested by inelastic neutron scattering measurements. SCBO is the only known realisation of the Shatry-Sutherland model, thus the ground state mediating the dimer and antiferromagnetic phase is likekly to be a plaquette state.

Spin excitation in the coupled honeycomb lattice compound Ni2InSbO6

We performed an inelastic neutron scattering experiment on a polycrystalline sample of a helimagnet Ni$_2$InSbO$_6$ to construct the spin Hamiltonian. Well-defined spin-wave excitation with a band energy of 20 meV was observed below $T_{N} = 76$ K. Using the linear spin-wave theory, the spectrum was reasonably reproduced with honeycomb spin layers coupled along the stacking axis (the $c$ axis). The proposed spin model reproduces the soliton lattice induced by a magnetic field applied perpendicular to the $c$ axis.

Nonharmonic contributions to the high-temperature phonon thermodynamics of Cr

Phonon densities of states (DOSs) of body-centered cubic chromium were measured by time-of-flight inelastic neutron scattering at temperatures up to 1493 K. Density functional theory calculations with both quasiharmonic (QH) and anharmonic (AH) methods were performed at temperatures above the N\'eel temperature. Features in the phonon DOSs decrease in energy (soften) substantially with temperature. A Born--von K\'arm\'an analysis using fits to the experimental DOSs reveals a softening of almost 17% of the high-transverse phonon branch between 330 and 1493 K. The low-transverse branch changes by approximately half this amount. The AH calculations capture the observed behavior of the two transverse phonon branches, but the QH calculations give some inverted trends. Vibrational entropies from phonons and electrons are obtained, and their sum is in excellent agreement with the entropy of chromium obtained by calorimetry, indicating that above 330 K, no explicit temperature-dependent magnetic contributions are necessary.

Spin wave spectra of single crystal CoPS3

The spin waves in single crystals of the layered van der Waals antiferromagnet CoPS$_3$ have been measured using inelastic neutron scattering. The data show four distinct spin wave branches with large ($\gtrsim 14$ meV) energy gaps at the Brillouin zone center indicating significant anisotropy. The data were modelled using linear spin wave theory derived from a Heisenberg Hamiltonian. Exchange interactions up to the third nearest-neighbour in the layered planes were required to fit the data with ferromagnetic $J_1 = -1.37$ meV between first neighbours, antiferromagnetic $J_3 = 3.0$ meV between third neighbours, and a very small $J_2 = 0.09$ meV between second neighbours. A biaxial single-ion anisotropy was required, with a collinear term $D^x = -0.77$ meV for the axis parallel to the aligned moment direction and a coplanar term $D^z=6.07$ meV for an axis approximately normal to the layered crystal planes.

Cross section for inelastic neutron scattering from 193Ir at 6 MeV

Two samples, one of natural iridium and the other of enriched 193Ir, were irradiated with a monoenergetic neutron beam of energy 6.0 MeV at the Triangle Universities Nuclear Laboratory. The product of the 193Ir(n,n′)193mIr reaction was determined by means of measuring X-rays following electron conversion of the isomeric state at 80.2 keV in 193Ir. The cross section for inelastic neutron scattering is reported disagreeing with the literature data.

Spin gap in the weakly interacting quantum spin chain antiferromagnet KCuPO4·H2O

The $S=1/2$ Heisenberg linear chain antiferromagnet is the simplest spin model; nevertheless it serves as a platform for various quantum many-body phenomena. Here, we report the magnetic behavior of a quasi-one-dimensional antiferromagnet ${\mathrm{KCuPO}}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}_{2}\mathrm{O}$. A long-range commensurate antiferromagnetic order with ordered moment 0.31(1) ${\ensuremath{\mu}}_{\mathrm{B}}$ per spin occurs at ${T}_{\mathrm{N}}=11.7(1)$ K. Above ${T}_{\mathrm{N}}$, the inelastic neutron excitation is characterized by a two spinon continuum. The intrachain interaction $J$ and interchain interaction $|{J}^{\ensuremath{'}}|$ are estimated to be 172 and 4.25(4) K, respectively; thus the ratio of $|{J}^{\ensuremath{'}}|/J=0.0247(3)$. At lower energies, below ${T}_{\mathrm{N}}$, a spin gap is observed in the dispersive excitations. These results are consistent with characteristics observed in a weakly interacting $S=1/2$ Heisenberg chain system.

Polymorphism and Ligand Binding Modulate Fast Dynamics of Human Telomeric G-Quadruplexes

Telomeric G-quadruplexes (G4s) are promising targets in the design and development of anticancer drugs. Their actual topology depends on several factors, resulting in structural polymorphism. In this study, we investigate how the fast dynamics of the telomeric sequence AG3(TTAG3)3 (Tel22) depends on the conformation. By using Fourier transform Infrared spectroscopy, we show that, in the hydrated powder state, Tel22 adopts parallel and mixed antiparallel/parallel topologies in the presence of K+ and Na+ ions, respectively. These conformational differences are reflected in the reduced mobility of Tel22 in Na+ environment in the sub-nanosecond timescale, as probed by elastic incoherent neutron scattering. These findings are consistent with the G4 antiparallel conformation being more stable than the parallel one, possibly due to the presence of ordered hydration water networks. In addition, we study the effect of Tel22 complexation with BRACO19 ligand. Despite the quite similar conformation in the complexed and uncomplexed state, the fast dynamics of Tel22-BRACO19 is enhanced compared to that of Tel22 alone, independently of the ions. We ascribe this effect to the preferential binding of water molecules to Tel22 against the ligand. The present results suggest that the effect of polymorphism and complexation on the G4 fast dynamics is mediated by hydration water.

Spin Reorientation Transition and Negative Magnetoresistance in Ferromagnetic NdCrSb3 Single Crystals.

High-quality NdCrSb3 single crystals are grown using a Sn-flux method, for electronic transport and magnetic structure study. Ferromagnetic ordering of the Nd3+ and Cr3+ magnetic sublattices are observed at different temperatures and along different crystallographic axes. Due to the Dzyaloshinskii-Moriya interaction between the two magnetic sublattices, the Cr moments rotate from the b axis to the a axis upon cooling, resulting in a spin reorientation (SR) transition. The SR transition is reflected by the temperature-dependent magnetization curves, e.g., the Cr moments rotate from the b axis to the a axis with cooling from 20 to 9 K, leading to a decrease in the b-axis magnetization f and an increase in the a-axis magnetization. Our elastic neutron scattering along the a axis shows decreasing intensity of magnetic (300) peak upon cooling from 20 K, supporting the SR transition. Although the magnetization of two magnetic sublattices favours different crystallographic axes and shows significant anisotropy in magnetic and transport behaviours, their moments are all aligned to the field direction at sufficiently large fields (30 T). Moreover, the magnetic structure within the SR transition region is relatively fragile, which results in negative magnetoresistance by applying magnetic fields along either a or b axis. The metallic NdCrSb3 single crystal with two ferromagnetic sublattices is an ideal system to study the magnetic interactions, as well as their influences on the electronic transport properties.

Crystal electric field level scheme leading to giant magnetocaloric effect for hydrogen liquefaction

In recent years, magnetic refrigeration has attracted considerable attention for hydrogen liquefaction. Most materials used for magnetic refrigeration contain heavy rare earth ions with complex crystalline electric field energy splittings, whose effect on the magnetic entropy change ΔSM has not been systematically studied. In particular, the theoretical upper limits of ∣ΔSM∣ for general heavy earth cases are unknown. Here, we show that the crystalline electric field level schemes result in a large ΔSM for general heavy rare earth cases. We provide a specific example of the magnetic refrigeration material HoB2 using inelastic neutron scattering experiments combined with mean-field calculations with crystal field splitting and exchange interactions. The relationship between ΔSM and crystal field parameters presented in this study can be useful for developing compounds with a large ∣ΔSM∣ and advancing the design of magnetic refrigeration materials.

Dynamical Axions in U(1) Quantum Spin Liquids.

Since their proposal nearly half a century ago, physicists have sought axions in both high energy and condensed matter settings. Despite intense and growing efforts, to date, experimental success has been limited, with the most prominent results arising in the context of topological insulators. Here, we propose a novel mechanism whereby axions can be realized in quantum spin liquids. We discuss the necessary symmetry requirements and identify possible experimental realizations in candidate pyrochlore materials. In this context, the axions couple to both the external and the emergent electromagnetic fields. We show that the interaction between the axion and the emergent photon leads to a characteristic dynamical response, which can be measured experimentally in inelastic neutron scattering. This Letter sets the stage for studying axion electrodynamics in the highly tunable setting of frustrated magnets.

Impact of Dzyaloshinskii-Moriya and anisotropic exchange interactions on the cubic kagome antiferromagnets Mn3X and Mn3AB

We perform a symmetry analysis of the ABC-stacked kagome planes of Mn atoms common to the $L1_2$ Mn$_3$X and antiperovskite Mn$_3$AB alloys. In addition to the single-ion-anisotropy and Kitaev-type anisotropic exchange known to stabilize 120{\deg} spin structures in these materials, our analysis results in a staggered Dzyaloshinskii-Moriya interaction and further gamma-type anisotropic exchange between nearest-neighbor spins. The presence of these new terms is shown not to affect the energetics of the 120{\deg} ground-states which explains their absence in prior minimal magnetic models. We go on to show, however, that their influence becomes apparent when spin-wave excitations are considered. We highlight these effects by calculating inelastic neutron scattering cross-sections to illustrate experimental means of probing the existence and relative strengths of these cloaked interactions.

Vibrational dynamics in the undercooled liquid of ultra-fragile metallic glasses

The lattice vibrational dynamics in ultra-fragile Pt57Cu23P20 and Pt60Cu20P20 glasses were investigated using in situ inelastic neutron scattering to measure the evolution of the phonon density of states (DOS) through the glass transition, into the undercooled liquid phase, and through crystallization. The phonon DOS curves are remarkably similar for the undercooled liquids and the amorphous solids. The vibrational entropy was obtained with these phonon DOS curves, and changes in vibrational entropy caused by the glass transition are less than 0.05kB/atom. For these highly fragile metal/metalloid glasses, the excess vibrational entropy of the undercooled liquid is about 0.5–1.5 J mol−1 K−1, compared to 0.1 J mol−1 K−1 reported earlier for stronger Cu–Zr glasses. Our work suggests that, consistent with our previous report for less fragile metallic glasses, the fraction of the total excess entropy that is vibrational is small, and the discontinuity in excess entropy at the glass transition is mainly due to configurational entropy.

Inelastic neutron scattering study of crystalline electric field excitations in the caged compounds NdT2Zn20 (T=Co, Rh, and Ir)

We have measured crystalline electric field (CEF) excitations of Nd3+ ions in the two-channel Kondo lattice candidates NdT2Zn20 (T = Co, Rh, and Ir) by means of inelastic neutron scattering (INS). In the INS measurements at 5 K, dispersionless excitations were observed at 3.8 and 7.2 meV for T = Co, 3.1 and 5.8 meV for T = Rh, and 3.0 and 5.3 meV for T = Ir. Analyses of the temperature dependence of the INS spectra confirm that the CEF ground states are the Gamma 6 doublet, that is a requisite for manifestation of the magnetic two-channel Kondo effect. For T = Co, a shoulder was observed at 7.7 meV close to the CEF excitation peak centered at 7.2 meV. The shoulder is attributed to a bound state of the CEF and low-lying optical phonon excitations.

Symmetry fractionalization in the gauge mean-field theory of quantum spin ice

Symmetry fractionalization is a ubiquitous feature of topologically ordered states that can be used to classify different symmetry-enriched topological phases and reveal some of their unique experimental signatures. Despite its vast popularity, there is currently no available framework to study symmetry fractionalization of quantum spin ice (QSI) -- a $U(1)$ quantum spin liquid (QSL) on the pyrochlore lattice supporting emergent photons -- within the most widely used theoretical framework to describe it, gauge mean-field theory (GMFT). In this work, we provide an extension of GMFT that allows for the classification of space-time symmetry fractionalization. The construction classifies all GMFT Ans\"atze that yield physical wavefunctions invariant under given symmetries and a specific low-energy gauge structure. As an application of the framework, we first show that the only two Ans\"atze with emergent $U(1)$ gauge fields that respect all space-group symmetries are the well-known 0- and $\pi$-flux states. We then showcase how the framework may describe QSLs beyond the currently known ones by classifying chiral $U(1)$ QSI. We find two new states described by $\pi/2$- and $3\pi/2$-fluxes of the emergent gauge field threading the hexagonal plaquettes of the pyrochlore lattice. We finally discuss how the different ways translation symmetries fractionalize for all these states lead to unique experimentally relevant signatures and compute their respective inelastic neutron scattering cross-section to illustrate the argument.

Interacting triplons in frustrated spin ladders: Binding and decay in BiCu2PO6

Establishing a comprehensive model of the rich spin dynamics in BiCu$_2$PO$_6$ has been a challenge over the last decade. Inelastic neutron scattering experiments revealed that its elementary triplons are non-degenerate showing the existence of significant anisotropic spin couplings. Evidence for triplon decay into two triplons has been found, but two prominent downturns in the dispersions eluded an explanation. Level repulsion due to hybridization of single triplons with the continuum of two-triplon scattering states has been proposed as explanation. We show that this concept may explain the weak downturn at higher energies, but fails for the most pronounced downturn at lower energy. In turn, we provide evidence that this downturn is the signature of a two-triplon bound state of essentially singlet character pointing to triplon-triplon interaction as the second crucial ingredient of the spin dynamics in this exemplary system.

Direct prediction of inelastic neutron scattering spectra from the crystal structure*

Inelastic neutron scattering (INS) is a powerful technique to study vibrational dynamics of materials with several unique advantages. However, analysis and interpretation of INS spectra often require advanced modeling that needs specialized computing resources and relevant expertise. This difficulty is compounded by the limited experimental resources available to perform INS measurements. In this work, we develop a machine-learning based predictive framework which is capable of directly predicting both one-dimensional INS spectra and two-dimensional INS spectra with additional momentum resolution. By integrating symmetry-aware neural networks with autoencoders, and using a large scale synthetic INS database, high-dimensional spectral data are compressed into a latent-space representation, and a high-quality spectra prediction is achieved by using only atomic coordinates as input. Our work offers an efficient approach to predict complex multi-dimensional neutron spectra directly from simple input; it allows for improved efficiency in using the limited INS measurement resources, and sheds light on building structure-property relationships in a variety of on-the-fly experimental data analysis scenarios.

Flexing of a Metal–Organic Framework upon Hydrocarbon Adsorption: Atomic Level Insights from Neutron Scattering

Metal–organic frameworks (MOFs) offer considerable opportunities for gas uptake, storage, and separation due to their porosity, chemical tunability, and flexibility. Flexible MOFs undergo reversible structural transformations triggered by external stimuli such as adsorption of specific guest molecules. The MUF-16 family of materials has exceptional gas adsorption properties including selective uptake of carbon dioxide over other gases. We observed one member of this family, MUF-16(Mn), to be flexible upon the adsorption of hydrocarbon gases. We used a combination of in situ synchrotron X-ray and neutron diffraction to identify the framework–gas interactions that underlie the structural flexibility. Inelastic neutron scattering, along with calculations, also enables an understanding of the dynamics of the flexibility. In essence, C3 hydrocarbons effectively bridge across hydrogen-bonded carboxyl dimers in the framework, triggering pore expansion and inhibiting certain types of motion in the framework.

Charge order and emergent symmetries in cuprate superconductors

In this paper, we present our studies of the phase diagram of the cuprate superconductors performed in recent years. We describe how a few field-theoretical concepts can be used to account for the puzzling properties of these compounds. Starting with a short exposition of recent experimental developments, we then introduce the concept of an emergent SU(2) symmetry, which rotates between the superconducting and charge order parameters. We describe a solvable model for which this symmetry turns out to be exact and derive the corresponding effective field theory in terms of a non-linear σ model. We then turn to the experimental consequences of our model with the presence of skyrmions inside the vortex core and an elegant account of the B−T phase diagram. Next, we move on to the second concept, which is the one of an emergent U(1) gauge field in the theory. Within this more general framework, the pseudogap transition can be viewed as a confining/deconfining transition. Experimental consequences for a few spectroscopic probes like inelastic neutron scattering, Raman spectroscopy, phonon line in x-ray scattering and angle-resolved photoemission experiments are studied. This theory suggests the presence of incoherent bosons at moderate temperatures in the phase diagram of the cuprates, which would undergo a jamming transition at optimal doping (Urbani, 2022). Consequences of the presence of such bosons on the transport properties of these materials are examined.

A database of synthetic inelastic neutron scattering spectra from molecules and crystals

Inelastic neutron scattering (INS) is a powerful tool to study the vibrational dynamics in a material. The analysis and interpretation of the INS spectra, however, are often nontrivial. Unlike diffraction, for which one can quickly calculate the scattering pattern from the structure, the calculation of INS spectra from the structure involves multiple steps requiring significant experience and computational resources. To overcome this barrier, a database of INS spectra consisting of commonly seen materials will be a valuable reference, and it will also lay the foundation of advanced data-driven analysis and interpretation of INS spectra. Here we report such a database compiled for over 20,000 organic molecules and over 10,000 inorganic crystals. The INS spectra are obtained from a streamlined workflow, and the synthetic INS spectra are also verified by available experimental data. The database is expected to greatly facilitate INS data analysis, and it can also enable the utilization of advanced analytics such as data mining and machine learning.Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Inelastic neutron scattering study of the H2 interaction with carbon-supported Pt and Pd catalysts

Carbon-supported Pt and Pd catalysts are largely employed as heterogeneous catalysts for hydrogenation reactions. Albeit their routine use in industry, many details about their interaction with H2 are still not completely understood. In this article, we propose a detailed investigation of the behavior in the presence of H2 of an activated carbon and of two catalysts obtained by depositing on it Pt and Pd nanoparticles by means of Inelastic Neutron Scattering (INS) spectroscopy. The high-quality INS spectra of the three samples were measured in absence and in presence of different amounts of H2, providing very detailed information about H2 physisorption on each system and about the nature of the metal hydrides formed upon hydrogenation of the catalysts. In addition, we were also able to obtain a direct observation of the occurrence of hydrogen spillover from the metal nanoparticles onto the activated carbon, as revealed by the increase of the number of C-H species at the borders of the graphenic domains. Overall, this work provides important insights for better understanding the complex nature of the interaction between H2 and Pt and Pd-loaded activated carbon catalysts.