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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Electronic structure, magnetic properties, spin orientation, and doping effect in Mn 3 Si 2 Te 6

The layered material Mn 3 Si 2 Te 6 , with alternating stacking honeycomb and triangular layers, is attracting considerable attention due to its rich physical properties. Here, in this study, using density functional theory and classical Monte Carlo (MC) methods, we systematically study this system with the 3d 5 electronic configuration. Near the Fermi level, the states are mainly contributed by Te 5p orbitals hybridized with Mn 3d orbitals, resembling a charge-transfer system. Furthermore, the spin orientations of the ferrimagnetic (FiM) ground state display different conductive behaviors when along the ab plane or out-of-plane directions: insulating vs metallic states. The energy difference between the FiM [110] insulating and FiM [001] metallic phases is very small (~0.71 meV/Mn) . Changing the angle θ of spin orientation from in-plane to out-of-plane directions, the band gaps of this system are gradually reduced, leading to an insulator-metal transition, resulting in an enhanced electrical conductivity, related to the colossal angular magnetoresistance (MR) effect. Although the three main magnetic couplings were found to be antiferromagnetic, overall the ground state is FiM. In addition, we also constructed the magnetic phase diagram using the classical XY spin model studied with the MC method. Three magnetic phases were obtained, including antiferromagnetic order, noncollinear spin patterns, and FiM order. Moreover, we also investigated the Se and Ge doping into the Mn 3 Si 2 Te 6 system: the FiM state has the lowest energy among the magnetic candidates for both Se- and Ge-doped cases. The magnetic anisotropy energy (MAE) decreases in the Se-doped case because the Mn orbital moment is reduced as the doping x increases. Due to the small spin-orbit-coupling effect of Se, the insulator-metal transition caused by the spin orientation disappears in the Se-doped case, resulting in an insulating phase in the FiM [001] phase. This causes a reduced colossal angular MR. However, both the MAE and the band gap of the Ge-doped case do not change much with increasing doping x . Our results for Mn 3 Si 2 Te 6 could provide guidance to experimentalists and theorists working on this system or related materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Stable and unstable tiling patterns formed by ABC miktoarm star triblock terpolymers of symmetric interactions

Several discrepancies exist among previous self-consistent field calculations of symmetrically interacting ABC miktoarm star triblock terpolymer melts. Here we address this issue by including all known tiling patterns, as well as several lamellar-type phases known to bound the regions in the parameter space occupied by these patterns. After carefully studying the effect of numerical parameters on the free-energy accuracy, the central region of the phase diagram is constructed in detail; both the (3.4.6.4) pattern and the three-dimensional phase of hierarchical-hexagonal lamellae are found to be stable for the first time. The energetic and entropic contributions to the free-energy density of several phases are analyzed and compared in detail to reveal their stability mechanisms, including the important ( 3 2 .4.3.4) pattern. Comparisons to previous studies of the same model system are present throughout, with our results resolving the aforementioned discrepancies and providing a sound basis for future work to build upon. Published by the American Physical Society 2024

Materials Science↗
DOE OSTI2021

Electrochemistry of the NaI-AlBr 3 Molten Salt System: A Redox-Active, Low-Temperature Molten Salt Electrolyte

NaI-AlBr 3 is a very appealing low melting temperature (<100 °C), salt system for use as an electrochemically-active electrolyte. This system was investigated for its electrochemical and physical properties with focus to energy storage considerations. A simple phase diagram was generated; at >100 °C, lower NaI concentrations had two partially miscible liquid phases, while higher NaI concentrations had solid particles. Considering the fully molten regime, electrical conductivities were evaluated over 5–25 mol% NaI and 110 °C–140 °C. Conductivities of 6.8–38.9 mS cm −1 were observed, increasing with temperature and NaI concentration. Effective diffusion coefficients of the I − /I 3 − redox species were found to decrease with both increasing NaI concentration and increasing applied potential. Regardless, oxidation current density at 3.6 V vs Na/Na + was observed to increase with increasing NaI concentration over 5–25 mol%. Finally, the critical interface between the molten salt electrolyte and electrode materials was found to significantly affect reaction kinetics. When carbon was used instead of tungsten, an adsorbed species, most likely I 2 , blocked surface sites and significantly decreased current densities at high potentials. This study shows the NaI-AlBr 3 system offers an attractive, low-temperature molten salt electrolyte that could be useful to many applied systems, though composition and electrode material must be considered.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Anelasticity and Phase Transition During Ramp-Release in Tin

This study examines the qualitative features of an anelasticity model associated with the bowing of dislocations in the presence of phase transition. A simple physically plausible mechanism is introduced to describe the interaction of anelasticity and the transformation. Varying the anelastic parameters results in strong differences in the deviatoric stress response. The model is applied to study the behavior of tin (Sn) and compared to data from ramp driven compression-release experiments. Tin exhibits a complex phase diagram within a relatively accessible range of temperature and pressures and the characterization of its phases is considered an open problem with significant scientific merit. The coupling between anelasticity, plasticity, and phase transformation contributes to release wave features traditionally associated with the phase transition effect alone suggesting the importance of accounting for the effects jointly. Posterior distributions of the plastic and anelastic parameters are computed using Bayesian-inference-based methods, further highlighting the importance of anelasticity in this regime.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Understanding the Polymorphism of Cobalt Nanoparticles Formed in Electrodeposition–An In Situ XRD Study

An advanced synchrotron-based in situ X-ray diffraction (XRD) technique is successfully developed and employed to track and monitor the formation and phase selection of cobalt (Co) in electrodeposition in real time and verify DFT computational results. The impacts of a number of controlling factors including the pH of the electrolyte and deposition overpotential are systematically studied. The results show that the yielded phase of the electrodeposited Co is controlled by both thermodynamics and kinetics. The low pH low overpotential condition favors the formation of the thermodynamically stable fcc phase. While the high pH high overpotential condition promotes the formation of the metastable hcp phase. The experimental results agree well with the nanometric phase diagram computed with DFT. Layer-by-layer alternative stacking of fcc-hcp polymorphic phases can be facilely fabricated by just varying the overpotential. This work not only offers an effective means to control the phase of electroplating of Co but also presents a new approach to reveal the fundamental insights of the formation of metals under electrochemical reduction driving force.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Charge density wave order on a $\textit{π}$-flux square lattice

The effect of electron-phonon coupling on Dirac fermions was recently explored numerically on a honeycomb lattice, leading to precise quantitative values for the finite temperature and quantum critical points. In this paper, we use the unbiased determinant quantum Monte Carlo method to study the Holstein model on a half-filled staggered-flux square lattice and compare the results with the honeycomb lattice geometry, presenting results for a range of phonon frequencies 0.1 ≤ $\omega$ ≤ 2.0. In this work, we find that the interactions give rise to charge density wave order, but only above a finite coupling strength $\gamma_{\text{crit}}$. The transition temperature is evaluated and presented in a $T_{c^{–λ}}$ phase diagram. An accompanying mean-field theory calculation also predicts the existence of a quantum phase transition, but at a substantially smaller coupling strength.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2009

Austere Human Missions to Mars

This slide presentation reviews a possible mission architecture for a more austere Mars mission, than the one developed by NASA outlined in the Design Reference Architecture 5 (DRA 5). "Austere" architecture is scaled back from DRA 5 and might offer lower development cost, lower flight cost, and lower development risk. This approach will not meet all the DRA 5 mission requirements. Included in the presentation are the elements of an Austere mission, diagrams of the trans-Mars injection (TMI), cruise, and Mars Orbital Insertion for various phases of the mission, the entry descent landing (EDL) concept. The key features of the Transit Habitat (TransHab), the Earth Departure Stage (EDS), the landers, are reviewed. A chart shows the Mass in tons, of the conceptual types of Mars Landers. The EDL concept, EDL Phase diagrams for the Mars Lander are reviewed. New technologies that would be required are also reviewed. Flight test programs for the various parts of the architecture and a flight schedule are reviewed.

transit habitat (TransHab)↗
DOE OSTI2022

Imaging Phase Segregation in Nanoscale Li x CoO 2 Single Particles

Li x CoO 2 (LCO) is a common battery cathode material that has recently emerged as a promising material for other applications including electrocatalysis and as electrochemical random access memory (ECRAM). During charge– discharge cycling LCO exhibits phase transformations that are significantly complicated by electron correlation. While the bulk phase diagram for an ensemble of battery particles has been studied extensively, it remains unclear how these phases scale to nanometer dimensions and the effects of strain and diffusional anisotropy at the single-particle scale. Understanding these effects is critical to modeling battery performance and for predicting the scalability and performance of electrocatalysts and ECRAM. Here we investigate isolated, epitaxial LiCoO 2 islands grown by pulsed laser deposition. After electrochemical cycling of the islands, conductive atomic force microscopy (c-AFM) is used to image the spatial distribution of conductive and insulating phases. Above 20 nm island thicknesses, we observe a kinetically arrested state in which the phase boundary is perpendicular to the Li-planes; we propose a model and present image analysis results that show smaller LCO islands have a higher conductive fraction than larger area islands, and the overall conductive fraction is consistent with the lithiation state. Thinner islands (14 nm), with a larger surface to volume ratio, are found to exhibit a striping pattern, which suggests surface energy can dominate below a critical dimension. When increasing force is applied through the AFM tip to strain the LCO islands, significant shifts in current flow are observed, and underlying mechanisms for this behavior are discussed. The c-AFM images are compared with photoemission electron microscopy images, which are used to acquire statistics across hundreds of particles. Finally, the results indicate that strain and morphology become more critical to electrochemical performance as particles approach nanometer dimensions.

25 ENERGY STORAGE↗
DOE OSTI2022

Ferromagnetism and doublon localization in a Wannier-Hubbard chain

We derive a “Wannier-Hubbard” model consisting of an array of overlapping atomic orbitals interacting via a local Coulomb interaction. Transforming to an orthogonal Wannier basis set, the resulting Hamiltonian displays long-range hopping and interactions, with new terms such as correlated hopping and ferromagnetic direct exchange, among others. We numerically study the one-dimensional version of the model at half-filling using the density matrix renormalization group method, unveiling a rich phase diagram as a function of the interaction U and the overlap s with metallic and ferromagnetic phases separated by a ferrimagnetic region. Our results indicate a path toward understanding emergent phases under pressure and beyond standard model Hamiltonians.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Dynamic scaling of order parameter fluctuations in model B

Brookhaven National Laboratory has performed a dedicated set of experiments, called the Beam Energy Scan (BES), with the goal of location a possible critical point in the phase diagram of nuclear matter, This point is analogous to the endpoint of the liquid-gas phase transition in water, where the liquid phase corresponds to a phase of hadrons, and the gas phase to a plasma of quarks and gluons. In order to describe the data generated by these experiments we have studied numerical simulations of the stochastic diffusion equation with a conserved charge. Here, we focus on the dynamics in the vicinity of a critical point in the Ising universality class. The model we consider is expected to describe the critical dynamics near a possible QCD critical point if the coupling of the order parameter to the momentum density of the fluid can be neglected. The simulations are performed on a spatial lattice, and the time evolution is performed using a Metropolis algorithm. We determine the dynamical critical exponent z ≃ 3.972(2), which agrees with predictions of the epsilon expansion. We also study non-equilibrium sweeps of the reduced temperature and observe approximate Kibble-Zurek scaling.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1975

The electrical and magnetic properties of /TTF//I/-sub 0.71

The synthesis and transport properties of the salts (TTF) (iodine)-sub x have been investigated. These salts are quasi-one-dimensional compounds containing cation radicals only, in contrast to the cation radical-anion radical systems such as (TTF)(TCNQ). The phase diagram of (TTF)(I)-sub x is discussed, and found to consist of two nonconducting, stoichiometric phases, x = 2.0 and 3.0, and two conducting, nonstoichiometric phases, x = 0.71 and 2.3. The transport properties of (TTF)(I)-sub 0.71 are discussed in detail. This phase is highly conducting, undergoes a sharp transition to a less conducting state at 210 K, and exhibits an unusual hysteresis in some of the transport properties upon thermal cycling. The nature of the high-temperature ground state is examined.

Somoano, R. B.↗
NASA NTRS2007

Liquidus Phases of the Richardson H5 Chondrite at High Pressures and Temperatures

Part of early mantle evolution may include a magma ocean, where core formation began before the proto-Earth reached half of its present radius. Temperatures were high and bombardment and accretion were still occurring, suggesting that the proto-Earth consisted of a core and an at least partially liquid mantle, the magma ocean. As the Earth accreted, pressure near the core increased and the magma ocean decreased in volume and became shallower as it began to cool and solidify. As crystals settled, or floated, the composition of the magma ocean could change significantly and begin to crystallize different minerals from the residual liquid. Therefore, the mantle may be stratified following the P-T phase diagram for the bulk silicate Earth. To understand mantle evolution, it is necessary to know liquidus phase relations at high pressures and temperatures. In order to model the evolution of the magma ocean, high pressure and temperature experiments have been conducted to simulate the crystallization process using a range of materials that most likely resemble the bulk composition of the early Earth.

Channon, M.↗
NASA NTRS2018

Mars Ecopoiesis Testbed

Mars surface conditions where liquid water is absent were simulated for the purposes of laboratory research. A pressure-temperature (P-T) profile was maintained in which no combination of pressure or temperature corresponds to the liquid region of the water phase diagram. The triple point of pure water occurs at T = 0.1oC and P(H2O) = 6.01 mbar; therefore all temperatures and pressures must be kept below these values, respectively. A 35-day test was performed in a commercial planetary simulation system (Techshot, Inc., Greenville, IN) in which the minimum night-time temperature was -80oC, the maximum daytime temperature was +26oC, the simulated day-night light cycle in earth hours was 12-on and 12-off, and the total pressure of the pure CO2 atmosphere was maintained below 11 mbar. Any water present was allowed to equilibrate with the changing temperature and pressure. The gas phase was sampled into a CR1-A condensation-mirror low-pressure hygrometer, which uses liquid nitrogen (down to 77oK) to determine the dew point (Buck Technologies, Boulder, CO). Dew point was measured once every hour and recorded on a data logger, along with the varying temperature in the chamber, from which the partial pressure of water was calculated. The resulting calculated daily cycles were tracked on the water P-T diagram, and no points were found to fall within the liquid-phase region of the diagram. It is concluded that there was no liquid water present throughout the test except during the initial pump-down period when aqueous specimens were introduced on the first day (less than 1 hour). Mars regolith simulant was present during this test, and further investigation is needed to determine whether liquid water could have been present or absent in the regolith in the form of brine. Biological samples consisting of Cyanobacteria: Anabena sp., Chroococcidiopsis CCMEE171, Plectonema boryanum; Eubacteria: Bacillus subtilis, Pseudomonas aeruginosa, and Eukaryota: Chlorella ellipsoidia were maintained in the simulator under the above-described conditions. The exposed specimens were tested for intracellular esterase activity, chlorophyll content (where appropriate) and reproductive survival. All tests yielded low-level positive results in all cases. In parallel to these terrestrial studies a planned design study was undertaken for the proposed test bed. Design requirements include compact assembly for transport and installation on the planetary surface (multiple units per mission would be expected), protective internal package for the release of organisms, a means of atmosphere exchange, access to sunlight, a means of penetrating the planetary surface, and most importantly a means of acquiring regolith while meeting the requirements of planetary protection. In consultation with advisers a design was created, and a large-scale mock-up of this design was fabricated by additive manufacturing at Techshot, Inc. with moving parts that simulated the components of the design. The mock-up assembly has been demonstrated to interested parties. A means of detecting live metabolism will also be included in the test bed. Several options were reviewed, and it is concluded that, by the time the ecopoiesis test bed is ready for testing the optimum instrument will be the equivalent of a hand-held mass spectrometer for metabolic gas analysis. This will maximize versatility and reveal much more information than could a detector of a single product (such as molecular oxygen), and the

Ecopoiesis↗
DOE OSTI2024

Observation of Thermally Induced Piezomagnetic Switching in Cu 2 OSeO 3 Polymorph Synthesized under High-Pressure

A polymorph of Cu 2 OSeO 3 with the distorted kagome lattice is successfully obtained using the high-pressure synthesis technique (Cu 2 OSeO 3 -HP). The structural analysis using X-ray and neutron powder diffraction suggests that the tetrahedral Cu 2+ clusters [similar to those in Cu 2 OSeO 3 ambient-pressure phase (Cu 2 OSeO 3 -AP)] exist in Cu 2 OSeO 3 -HP but with three symmetry inequivalent sites. No structural change is observed between 1.5 K and the room temperature. The complex magnetic H-T phase diagram is established based on the temperature- and field-dependent magnetization data, indicating two distinct antiferromagnetic phases at low and intermediate temperatures, in addition to the higher-temperature spin-glass-like phase. The low temperature phase is identified by neutron powder diffraction refinements as a canted noncollinear antiferromagnetic order with a weak ferromagnetic component along the b-axis. Size of the refined ordered moment is ≈1.00(4) µ B in Cu 2 OSeO 3 -HP, indicating a large enhancement compared to that of Cu 2 OSeO 3 -AP (≈0.61 µ B ). By applying a uniaxial stress, finite enhancement of weak ferromagnetic component in the noncollinear antiferromagnetic phase in Cu 2 OSeO 3 -HP is observed, which is the clear evidence of the piezomagnetic effect. Interestingly, the sign of the induced magnetization changes on heating from the low-temperature to the intermediate-temperature phases, indicating a novel piezomagnetic switching effect in this compound.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Correlation between Spin and Orbital Dynamics during Laser-Induced Femtosecond Demagnetization

Spin and orbital angular momenta are two intrinsic properties of an electron and are responsible for the physics of a solid. How the spin and orbital evolve with respect to each other on several hundred femtoseconds is largely unknown, but it is at the center of laser-induced ultrafast demagnetization. In this paper, we introduce a concept of the spin–orbital correlation diagram, where spin angular momentum is plotted against orbital angular momentum, much like the position-velocity phase diagram in classical mechanics. We use four sets of highly accurate time-resolved X-ray magnetic circular dichroism data to construct four correlation diagrams for iron and cobalt. To our surprise, a pattern emerges. The trace on the correlation diagram for iron is an arc, and at the end of demagnetization, it has a pronounced cusp. The correlation diagram for cobalt is different and appears more linear but with kinks. We carry out first-principles calculations with two different methods: time-dependent density functional theory (TDDFT) and time-dependent Liouville density functional theory. These two methods agree that the experimental findings for both Fe and Co are not due to experimental errors. It is the spin–orbit coupling that correlates the spin dynamics to the orbital dynamics. Microscopically, Fe and Co have different orbital occupations, which leads to distinctive correlation diagrams. We believe that this correlation diagram presents a useful tool to better understand spin and orbital dynamics on an ultrafast time scale. A brief discussion on the magnetic anisotropy energy is also provided.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Exploring nonequilibrium phases of photo-doped Mott insulators with generalized Gibbs ensembles

Many experiments show that strong excitations of correlated quantum materials can cause non-thermal phases without equilibrium analogues. Understanding the origin and properties of these nonequilibrium states has been challenging due to the limitations of theoretical methods for nonequilibrium strongly correlated systems. In this work, we introduce a generalized Gibbs ensemble description that enables a systematic analysis of the long-time behavior of photo-doped states in Mott insulators based on equilibrium methods. We demonstrate the power of the method by mapping out the nonequilibrium phase diagram of the one-dimensional extended Hubbard model, which features η -pairing and charge density wave phases in a wide photo-doping range. We furthermore clarify that the peculiar kinematics of photo-doped carriers, and the interaction between them, play an essential role in the formation of these non-thermal phases. Our results establish a new path for the systematic analysis of nonequilibrium strongly correlated systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Dry Deactivation of Sodium Metal in a Molten LiCl-KCl-CsCl-NaCl System

An experimental study was performed with the objective of investigating and characterizing a dry technique for deactivation of sodium metal in a molten salt system. The study was performed in three parts. First, proof-of-principle testing of the technique was performed at bench scale. It involved loading and melting tens of grams of clean sodium metal atop a pool of LiCl-KCl-CsCl eutectic at ∼300°C and then adding ammonium chloride particles while mixing. The ammonium chloride reacted with sodium to form sodium chloride and nitrogen/hydrogen off-gases. The sodium chloride assimilated into the salt pool, forming a quaternary salt mixture of LiCl-KCl-CsCl-NaCl. The proof-of-principle testing repeatedly exhibited complete deactivation of the loaded sodium metal. Second, characterization of a LiCl-KCl-CsCl-NaCl system was performed using differential scanning calorimetry to produce a partial phase diagram of LiCl-KCl-CsCl eutectic versus NaCl, which identified the liquidus, solidus, and two-phase regions of the quaternary system. Third, the dry deactivation technique was demonstrated at kg-scale in an inert atmosphere radiological glovebox with sodium metal that was previously separated in the same glovebox from uranium metal in unirradiated blanket elements for the Fermi-1 nuclear reactor. The quaternary salt product at the end of the demonstration was sampled and showed complete deactivation of the sodium metal. Here, in short, this study qualified a technique to completely deactivate batches of sodium metal in the absence of air or water. While this study focused on the deactivation of sodium metal, including bond sodium from unirradiated blanket elements, the technique is applicable to other sources of sodium metal, such as sodium metal from batteries. Furthermore, the technique could also be extended to other alkali metal systems, including lithium, sodium, potassium, rubidium, cesium, and mixtures thereof.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Reconstruction and uncertainty quantification of lattice Hamiltonian model parameters from observations of microscopic degrees of freedom

The emergence of scanning probe and electron beam imaging techniques has allowed quantitative studies of atomic structure and minute details of electronic and vibrational structure on the level of individual atomic units. These microscopic descriptors, in turn, can be associated with local symmetry breaking phenomena, representing the stochastic manifestation of the underpinning generative physical model. In this work, we explore the reconstruction of exchange integrals in the Hamiltonian for a lattice model with two competing interactions from observations of microscopic degrees of freedom and establish the uncertainties and reliability of such analysis in a broad parameter-temperature space. In contrast to other approaches, we specifically specify a loss function inherent to thermodynamic systems and utilize it to estimate uncertainty in simulated realizations of different models. As an ancillary task, we develop a machine learning approach based on histogram clustering to predict phase diagrams efficiently using a reduced descriptor space. We further demonstrate that reconstruction is possible well above the phase transition and in the regions of parameter space when the macroscopic ground state of the system is poorly defined due to frustrated interactions. This suggests that this approach can be applied to the traditionally complex problems of condensed matter physics such as ferroelectric relaxors and morphotropic phase boundary systems, spin and cluster glasses, and quantum systems once the local descriptors linked to the relevant physical behaviors are known.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗