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Thermal evolution of spin excitations in honeycomb Ising antiferromagnetic FePSe 3
We use elastic and inelastic neutron scattering (INS) to study the antiferromagnetic (AF) phase transitions and spin excitations in the two-dimensional (2D) zig-zag antiferromagnet FePSe 3 . By determining the magnetic order parameter across the AF phase transition, we conclude that the AF phase transition in FePSe 3 is first-order in nature. In addition, our INS measurements reveal that the spin waves in the AF ordered state have a large easy-axis magnetic anisotropy gap, consistent with an Ising Hamiltonian, and possible biquadratic magnetic exchange interactions. On warming across T N , we find that dispersive spin excitations associated with three-fold rotational symmetric AF fluctuations change into FM spin fluctuations above T N . These results suggest that the first-order AF phase transition in FePSe 3 may arise from the competition between C 3 symmetric AF and C 1 symmetric FM spin fluctuations around T N , in place of a conventional second-order AF phase transition.
Crystal structures and rotational dynamics of a two-dimensional metal halide perovskite (OA) 2 PbI 4
The extended charge carrier lifetime in metal halide perovskites is responsible for their excellent optoelectronic properties. Recent studies indicate that the superb device performance in these materials is intimately related to the organic cation dynamics. Here, we focus on the investigation of the two-dimensional hybrid perovskite, (C 8 H 17 NH 3 ) 2 PbI 4 (henceforth, OA + = C 8 H 17 NH 3 + ). Using elastic and quasielastic neutron scattering techniques and group theoretical analysis, we studied the structural phase transitions and rotational modes of the C 8 H 17 NH 3 + cation in (OA) 2 PbI 4 . Here, our results show that, in the high-temperature orthorhombic ( T > 310 K) phase, the OA + cation exhibits a combination of a twofold rotation of the NH 3 –CH 2 head group about the crystal c -axis with a characteristic relaxation time of ~6.2 ps, threefold rotations (C 3 ) of NH 3 and CH 3 terminal groups, and slow librations of the other atoms. Contrastingly, only the C 3 rotation is present in the intermediate-temperature orthorhombic (238 K < T < 310 K) and low-temperature monoclinic ( T < 238 K) phases.
Recent advances in the quantification of uncertainties in reaction theory
Uncertainty quantification has become increasingly more prominent in nuclear physics over the past several years. In few-body reaction theory, there are four main sources that contribute to the uncertainties in the calculated observables: the effective potentials, approximations made to the few-body problem, structure functions, and degrees of freedom left out of the model space. In this work, we illustrate some of the features that can be obtained when modern statistical tools are applied in the context of nuclear reactions. This work consists of a summary of the progress that has been made in quantifying theoretical uncertainties in this domain, focusing primarily on those uncertainties coming from the effective optical potential as well as their propagation within various reaction theories. We use, as the central example, reactions on the doubly-magic stable nucleus 40 Ca, namely neutron and proton elastic scattering and single-nucleon transfer 40 Ca(d,p) 41 Ca. First, we show different optimization schemes used to constrain the optical potential from differential cross sections and other experimental constraints; we then discuss how these uncertainties propagate to the transfer cross section, comparing two reaction theories. Finally, we finish by laying out our future plans.
Non-magnetic ion site disorder effects on the quantum magnetism of a spin-1/2 equilateral triangular lattice antiferromagnet
Here, with the motivation to study how non-magnetic ion site disorder affects the quantum magnetism of Ba 3 CoSb 2 O 9 , a spin-1/2 equilateral triangular lattice antiferromagnet, we performed DC and AC susceptibility, specific heat, elastic and inelastic neutron scattering measurements on single crystalline samples of Ba 2.87 Sr 0.13 CoSb 2 O 9 with Sr doping on non-magnetic Ba 2+ ion sites. The results show that Ba 2.87 Sr 0.13 CoSb 2 O 9 exhibits (i) a two-step magnetic transition at 2.7 K and 3.3 K, respectively; (ii) a possible canted 120 degree spin structure at zero field with reduced ordered moment as 1.24 μ B /Co; (iii) a series of spin state transitions for both H∥ab-plane and H∥c-axis. For H∥ab-plane, the magnetization plateau feature related to the up–up–down phase is significantly suppressed; (iv) an inelastic neutron scattering spectrum with only one gapped mode at zero field, which splits to one gapless and one gapped mode at 9 T. All these features are distinctly different from those observed for the parent compound Ba 3 CoSb 2 O 9 , which demonstrates that the non-magnetic ion site disorder (the Sr doping) plays a complex role on the magnetic properties beyond the conventionally expected randomization of the exchange interactions. We propose the additional effects including the enhancement of quantum spin fluctuations and introduction of a possible spatial anisotropy through the local structural distortions.
Static and dynamic spin properties in the quantum triangular lattice antiferromagnet Ag 2 CoO 2
In Ag 2 CoO 2 , Co forms triangular lattice layers, which are separated by the metallic (Ag 2 ) block. The magnetic susceptibility and heat capacity measurements show that this material exhibits an antiferromagnetic transition at T N =17.5 K and the Weiss temperature (T Θ ) and the effective moment are -274 K and 1.62μ B , respectively, indicating that the Co ion carries spin (S) 1/2 and has a strongly frustrated state with T Θ /T N =15.7. A density functional theory calculation confirmed that the valence state of the Co ions is 2+ and the low-spin state with S=1/2 is realized at reduced on-site Coulomb interaction on Co. We performed elastic and inelastic neutron scattering experiments in a powder sample of Ag 2 CoO 2 . Although no noticeable magnetic Bragg peaks were observed below T N , distinct magnetic excitations were observed in the inelastic neutron scattering experiments. The excitations are consistent with those expected for the S=1/2 Heisenberg triangular lattice antiferromagnet. These results suggest that the ordered moment is reduced due to the quantum effect, which explains the absence of the magnetic Bragg peaks. Our results thus suggest that Ag 2 CoO 2 is a good candidate to realize a quantum Heisenberg triangular lattice antiferromagnet.
Magnetic exchange interactions in the van der Waals layered antiferromagnet MnPSe 3
Two-dimensional van der Waals compounds with magnetic ions on a honeycomb lattice are hosts to a variety of exotic behavior. The magnetic interactions in one such compound, MnPSe 3 , are investigated with elastic and inelastic neutron scattering. Magnetic excitations are observed in the magnetically ordered regime and persist to temperatures well above the ordering temperature, T N =74K, consistent with low dimensional magnetic interactions. The inelastic neutron scattering results allow a model spin Hamiltonian to be presented that includes dominant intralayer interactions of J 1ab =0.45 meV, J 2ab =0.03 meV, J 3ab =0.19 meV, consistent with theoretical predictions. Furthermore, despite the quasi-2D behavior, appreciable interlayer interactions of J c =0.031(5) meV are required to model the data. No evidence for anisotropy in the form of a spin gap is observed in the data collected. The measurements on MnPSe 3 are contrasted with those on MnPS 3 and reveal a large increase in the interlayer exchange interaction in MnPSe 3 that may stabilize the similar ordering temperatures in the bulk compounds.
Static and dynamic magnetic properties of honeycomb lattice antiferromagnets Na 2 M 2 TeO 6 , M = Co and Ni
The magnetic structures and spin dynamics of Na 2 CO 2 TeO 6 and Na 2 Ni 2 TeO 6 are investigated by means of elastic and inelastic neutron scattering measurements and the results are discussed in the context of a generalized Kitaev-Heisenberg model on a honeycomb lattice with strong spin-orbit coupling. The large number of parameters involved in the Hamiltonian model is evaluated by using an iterative optimization algorithm capable of extracting model solutions and simultaneously estimating their uncertainty. The analyses indicate that both Co 2+ (d 7 ) and Ni 2+ (d 8 ) antiferromagnets realize bond-dependent anisotropic nearest-neighbor interactions and support the theoretical predictions for the realization of Kitaev physics in 3 d electron systems with effective spins S = 1/2 and 1. By studying the Na-doped system Na 2.4 Ni 2 TeO 6 , we show that the control of Na content can provide an effective route for fine-tuning the magnetic lattice dimensionality, as well as to controlling the bond-dependent anisotropic interactions.
Antiferromagnetic fluctuations and orbital-selective Mott transition in the van der Waals ferromagnet Fe 3–x GeTe 2
Fe 3–x GeTe 2 is a layered magnetic van der Waals material of interest for both fundamental and applied research. Despite the observation of intriguing physical properties, open questions exist even on the basic features related to magnetism: is it a simple ferromagnet or are there antiferromagnetic regimes, and are the moments local or itinerant. In this work, we demonstrate that antiferromagnetic spin fluctuations coexist with the ferromagnetism through comprehensive elastic and inelastic neutron scattering and thermodynamic measurements. Our realistic dynamical mean-field theory calculations reveal that the competing magnetic fluctuations are driven by an orbital selective Mott transition (OSMT), where only the plane-perpendicular a 1g orbital of the Fe(3d) manifold remains itinerant. Our results highlight the multi-orbital character in Fe 3–x GeTe 2 that supports a rare coexistence of local and itinerant physics within this material.
Short-range magnetic correlations in quasicrystalline i -Tb-Cd
Here, we report on elastic and inelastic neutron scattering from single-grain isotopically enriched samples to elucidate the local magnetic correlations between Tb 3+ moments in quasicrystalline i -Tb-Cd. The inelastic neutron scattering measurements of the crystalline electric field excitations demonstrated that the Tb 3+ moments are directed primarily along the local fivefold axes of the Tsai-type cluster as was found for the TbCd 6 approximant phase. Based on the inelastic measurements, we consider a simple Ising-type model for the moment configurations on a single Tb 3+ icosahedron and enumerate the lowest energy moment configurations. We then calculate the diffuse scattering from these configurations and compare with the experimental magnetic diffuse scattering measurements to identify the most likely single cluster moment configurations and find reasonable agreement between the broad features observed in our scattering simulations. We further use a heuristic glass model as well as large-scale Monte-Carlo simulations of a multicluster model to consider the role of higher-order (longer-range) intercluster correlations for magnetic frustration and the magnetic scattering.
Spin dynamics in natural multiferroic pyroxene NaFeSi 2 O 6
Spin dynamics in the natural mineral aegirine, NaFeSi 2 O 6 , a member of the pyroxene family, was studied by elastic and inelastic neutron scattering. Magnetization and specific-heat measurements as well as single-crystal neutron diffraction maps, taken in the temperature range 2–20 K, confirm two successive magnetic transitions at 8.8 and 5.8 K, consistent with previous studies. The observed spin-wave excitations emerge from the incommensurate magnetic Bragg peaks corresponding to the propagation vector 𝑘 ICM = (0,0.77,0), and extend up to energies of about 1.5 meV. In the low-temperature helical phase, the spin dynamics of the Fe3+ ions is well described by a simple linear spin-wave model. The observed excitations can be modeled using a spin Hamiltonian that includes three primary exchange interactions—intrachain coupling 𝐽 = 0.142(2) meV, and interchain couplings 𝐽 1 = 0.083(1) meV and 𝐽 2 = 0.186(1) meV—and an easy-plane anisotropy 𝐷 = 0.020(6) meV. Furthermore, our results show that no single exchange interaction dominates the spin dynamics. The similar strengths of the intrachain and interchain couplings point to the fact that the magnetic interactions in aegirine are three dimensional rather than confined along one direction. As a result, the system cannot be considered quasi one dimensional, as previously suggested, and calls for further investigations.
Scoping Study: Scintillator Array for Fission Correlation Studies
This scoping study uses a simulation based on the PROSPECT (1) detector to assess the conceptual design of a modular system that could efficiently and simultaneously measure the Prompt Fast Neutron Spectrum (PFNS), the prompt fission neutron multiplicity, the prompt fission gamma-ray multiplicity and total energy, as well as temporal and angular correlations between the detected particles. The proposed experiment would require a mono-energetic pulsed neutron beam incident on an actinide target in the center of the detector. Beam induced backgrounds due to elastic and inelastic neutron scattering on the target are considered, while other backgrounds are ignored for this study.
Neutron Skin Measurement of208Pb Using Parity-Violating Electron Scattering
Nuclear stability requires large nuclei to have more neutrons than protons. While the central core of the nucleus is composed of symmetric nuclear matter, the extra neutrons in heavy nuclei are pushed out to the surface of the nucleus, forming a pure outer neutron skin layer. While the proton distribution inside a nucleus has been accurately measured using electromagnetic probes, previous measurements of neutron distributions in complex nuclei have suffered a lack of systematic precision. PREX-2 and CREX use Parity-Violating electron Scattering (PVeS), as an electroweak interaction probe, to measure the neutron distributions in208Pb and48Ca, respectively. Making use of elastic scattering of longitudinally polarized electrons from unpolarized (isotopically pure) targets, PREX-2 and CREX measure the asymmetry in scattering rates between opposite beam polarization states. Since both experiments share the same exact measurement concept and apparatus, much of this dissertation applies to both PREX-2 and CREX. However, the presented asymmetry analysis and neutron skin extraction are only for PREX-2. PREX-2 measures the parity-violating asymmetry, APV, for 953 MeV electrons scattered elastically from208Pb at?5°in the lab. The final result is APV= 550.00±16.09(stat.)±8.16(syst.)ppb. From this measurement, we report the neutron skin thickness in208Pb nuclei to be, Rskin= 0.278±0.078(exp.)±0.012(model) fm. This measurement has broad implications throughout the physics community, increasing our knowledge in neutron star structure, the equation of state of nuclear matter, nuclear baryon density, nuclear electric dipole polarizability, and more. Key Words: Neutron skin, electroweak interaction, PREX-2, CREX, parity-violating asymmetry, neutron star, quartz detector, weak charge radius, symmetry energy, Hall A, Jefferson Lab
Magnetic properties of a staggered S = 1 chain with an alternating single-ion anisotropy direction
Materials composed of spin-1 antiferromagnetic (AFM) chains are known to adopt complex ground states that are sensitive to the single-ion-anisotropy (SIA) energy ( D ), and intrachain ( J 0 ) and interchain ( J 1 , 2 ′ ) exchange energy scales. While theoretical and experimental studies have extended this model to include various other energy scales, the effect of the lack of a common SIA axis is not well explored. Here we investigate the magnetic properties of Ni ( pyrimidine ) ( H 2 O ) 2 ( NO 3 ) 2 , a chain compound where the tilting of Ni octahedra leads to a twofold alternation of the easy-axis directions along the chain. Muon-spin relaxation measurements indicate a transition to long-range order at T N = 2.3 K and the magnetic structure is initially determined to be antiferromagnetic and collinear using elastic neutron diffraction experiments. Inelastic neutron scattering measurements were used to find J 0 = 5.107 ( 7 ) K , D = 2.79 ( 1 ) K , J 1 ′ = 0.00 ( 5 ) K , J 2 ′ = 0.18 ( 3 ) K , and a rhombic anisotropy energy E = 0.19 ( 9 ) K . Mean-field modeling reveals that the ground state structure hosts spin canting of ϕ ≈ 6 . 5 ∘ , which is not detectable above the noise floor of the elastic neutron diffraction data. Monte Carlo simulation of the powder-averaged magnetization, M ( H ) , is then used to confirm these Hamiltonian parameters, while single-crystal M ( H ) simulations provide insight into features observed in the data. Published by the American Physical Society 2025
Nonlinear light-output calibration of the oxygenated xylene scintillators used in OMEGA neutron time-of-flight spectrometers
Neutron time-of-flight (nTOF) spectrometers are essential instruments for measuring and evaluating the performance of inertial confinement fusion implosions. The neutron spectrometers utilized for the OMEGA laser include two liquid-based scintillators, each consisting of a large volume filled with xylene that is coupled to four photomultiplier tubes. Analysis of the signal from these detectors requires detailed knowledge of the scintillator’s light output, which is needed to fit the nTOF spectrum, from which the neutron energy spectrum is informed. The light output is nonlinearly proportional to the neutron energy, which, in turn, affects the interpretation of the neutron energy spectrum from a TOF signal. A recent campaign on OMEGA was performed to calibrate the xylene detectors and infer the shape of the light-output curve. The campaign utilized materials with increasing Z placed in the OMEGA target chamber to initiate scattering events with the 14 MeV fusion neutrons. This process leads to the production of backscatter neutrons of varying energies that appear as peaks in the nTOF data. Simulations using a neutron transport code were combined with the measured deuterium–tritium neutron yields to calculate the expected backscattered neutron yields from the well-known scattering cross sections of each material. Furthermore, the neutron-energy dependent light output of the scintillator inferred from the experiment is compared to the light-output curve simulated with a neutron transport code for the following neutron energies: 1.5, 2.5, 6, and 14 MeV.
Generation of Enrichment-Dependent Thermal Neutron Scattering Data
This work details the generation of enrichment-dependent thermal neutron scattering cross sections for several crucial uranium fuel compounds. The evaluations of the thermal scattering law (TSL) and associated cross sections for uranium dioxide (UO 2 ), uranium carbide (UC), and uranium nitride (UN) were performed using standard ab initio lattice dynamics (AILD) methods. The data for uranium metal was produced using a novel hybrid approach of molecular dynamics combined with lattice dynamics methods. 235 U enrichments of 5%, 10% (LEU+), 19.75% (HALEU), 93% (HEU), and 100% were considered, in addition to natural uranium. The enrichment-dependent masses and free atom cross sections were used in the generation of elastic and inelastic thermal neutron scattering cross sections, while the calculation of the phonon density of states (DOS) and resulting TSL considered only the natural isotopic composition of uranium. The use of an identical DOS for all enrichments is expected to have minimal impact on the final data, as the small change in uranium mass should not significantly affect lattice vibrations. The cross sections are shown to exhibit significant dependence on 235 U enrichment. The submission of this data to the National Nuclear Data Center (NNDC) for release in the ENDF/B-VIII.1 database should support the design of advanced reactor concepts.