Low-density Phase Research Articles

The Structural Characteristics and Phase Transformation in Al2O3 Glass. A Molecular Dynamics Simulation

In this work, we have performed a simulation to study the structural characteristics and phase transformation in Al2O3 glass under compression. The structural characteristics of Al2O3 glass were examined via AlOx units, OAly linkages, the average bond distance distributions, order parameters, and visualization of simulation data. The result showed that the network structure of Al2O3 glass is built mainly by AlOx (x = 3, 4, 5, 6, 7) units that are linked to each other via common O atoms. We found that the distribution of AlOx units in network structure is not uniform but tends to form clusters contained AlOx units. In addition, during a moderately long time, the glass has a two-phase that consists of separate low-density (LD) and high-density (HD) phases. The size of these phases significantly depends on the compression.

Low-density phase diagram of the three-dimensional electron gas

Variational and diffusion quantum Monte Carlo methods are employed to investigate the zero-temperature phase diagram of the three-dimensional homogeneous electron gas at very low density. Fermi fluid and body-centered cubic Wigner crystal ground state energies are determined using Slater-Jastrow-backflow and Slater-Jastrow many-body wave functions at different densities and spin polarizations in finite simulation cells. Finite-size errors are removed using twist-averaged boundary conditions and extrapolation of the energy per particle to the thermodynamic limit of infinite system size. Unlike previous studies, our results show that the electron gas undergoes a first-order quantum phase transition directly from a paramagnetic fluid to a body-centered cubic crystal at density parameter $r_\text{s} = 86.6(7)$, with no region of stability for an itinerant ferromagnetic fluid. However there is a possible magnetic phase transition from an antiferromagnetic crystal to a ferromagnetic crystal at $r_\text{s}=93(3)$.

Cellulose Nanocrystal Aqueous Colloidal Suspensions: Evidence of Density Inversion at the Isotropic-Liquid Crystal Phase Transition.

The colloidal suspensions of aqueous cellulose nanocrystals (CNCs) are known to form liquid crystalline (LC) systems above certain critical concentrations. From an isotropic phase, tactoid formation, growth, and sedimentation have been determined as the genesis of a high-density cholesteric phase, which, after drying, originates solid iridescent films. Herein, the coexistence of a liquid crystal upper phase and an isotropic bottom phase in CNC aqueous suspensions at the isotropic-nematic phase separation is reported. Furthermore, isotropic spindle-like domains are observed in the low-density LC phase and high-density LC phases are also prepared. The CNCs isolated from the low- and high-density LC phases are found to have similar average lengths, diameters, and surface charges. The existence of an LC low-density phase is explained by the presence of air dissolved in the water present within the CNCs. The air dissolves out when the water solidifies into ice and remains within the CNCs. The self-adjustment of the cellulose chain conformation enables the entrapment of air within the CNCs and CNC buoyancy in aqueous suspensions.

Coexistence of soft and hard magnetic phases in single layer amorphous Tb–Co thin films

Amorphous, ferrimagnetic Tb–Co thin films prepared with a thin Ta underlayer and either a Ta or a Pt overlayer show evidence of both soft and hard magnetic phases. At room temperature, the films exhibit conventional ferromagnetism, but low temperature magnetometry measurements reveal the decoupling of the two magnetic phases with decreasing temperature due to increased anisotropy energy of the hard layer at lower temperatures. Decreasing the film thickness to 2 nm, slightly above the superparamagnetic limit found at 1 nm, a soft, low-density phase was isolated and found to be present in all the films as confirmed with x-ray reflectivity and Rutherford backscattering spectrometry measurements. For greater thicknesses, the bottom layer retains its soft magnetic nature, while the remainder of the film is denser and has strong perpendicular magnetic anisotropy, leading to the exchange-spring behavior when the anisotropy becomes large, either at low temperatures or via a Pt overlayer that adds a strong interfacial anisotropy to the layer. Micromagnetic simulations of a soft/hard bilayer model with the experimentally determined anisotropy and magnetization parameters into a soft/hard bilayer model reproduced the experimental hysteretic behavior very well. These findings demonstrate how the magnetic state and the response of a-Tb–Co films to external fields can be controlled, providing a high degree of tunability that is promising for high-performance nanoscale devices.

Sunyaev Zel’dovich high resolution view of filamentary structures between galaxy clusters pairs

Strong observational and theoretical evidences suggest that about half of the baryons in the Universe should lie outside galaxy clusters in a lowdensity and hot phase in filaments connecting galaxy clusters. Due to the low density, most of this filamentary plasma can not be detected by X-ray observatories. In particular cases of low redshift cluster pairs in the pre-merging phase, the Sunyaev Zel’dovich (SZ) effect can be used to observe the intercluster regions and detect the imprint of missing baryons. The Abell 399-401 (A399-401) system is the perfect laboratory to test our ability to detect filamentary structures via the SZ effect with <~ 1′ angular resolution. This pair has been well studied at several frequencies: it exhibits double radio-halos, an excess of X-ray emission in the intercluster region and a synchrotron radio ‘ridge’ connecting the two clusters. Moreover the Planck satellite provided the first SZ detection of the gas between A399-401 despite the poor angular resolution (~ 10′) of its SZ map. We have used an Atacama Cosmology Telescope (ACT) and Planck satellite Compton-y map (1:65′ angular resolution) that combines ACT data from 2008 to 2019 with Planck maps to study the A399-401 system in detail. We present the data analysis and results.

Hard core lattice gas with third next-nearest neighbor exclusion on triangular lattice: One or two phase transitions?

We obtain the phase diagram of the hard core lattice gas with third nearest neighbor exclusion on the triangular lattice using Monte Carlo simulations that are based on a rejection-free flat histogram algorithm. In a recent paper [Darjani et al., J. Chem. Phys. 151, 104702 (2019)], it was claimed that the lattice gas with third nearest neighbor exclusion undergoes two phase transitions with increasing density with the phase at intermediate densities exhibiting hexatic order with continuously varying exponents. Although a hexatic phase is expected when the exclusion range is large, it has not been seen earlier in hard core lattice gases with short range exclusion. In this paper, by numerically determining the entropies for all densities, we show that there is only a single phase transition in the system between a low-density fluid phase and a high density ordered sublattice phase and that a hexatic phase is absent. The transition is shown to be first order in nature, and the critical parameters are determined accurately.

Structural transformation and dynamical heterogeneity in Germania melt under compression: molecular dynamic simulation

The structural transformation and dynamical heterogeneity in Germania (GeO2) are investigated via molecular dynamics (MD) simulation. The MD model with 5499 atoms was constructed under pressure up to 150 GPa and at a temperature of 3500 K. The structural transformation mechanism has been studied by observing domain structures and boundary oxygen atoms. The simulation result reveals that GeO2 consists of separate domains and boundaries in its melt structure. Under compression, the structure of GeO2 changes gradually and represents many types of structures. The melt structure exhibits many structural domains Dx, and polymorphism appears at pressures of 12 and 20 GPa. The change of tetrahedral structure to octahedral structure in germanium coordination occurred in parallel with the process of merging and splitting of domain structure. Moreover, the existence of high- and low-density phases in GeO2 melt is indicated. The high-density phase is D6 domain and boundary oxygen while the low-density phase is D4 and D5 domain. The compression mechanism in GeO2 melt mainly is a reduction of average Voronoi volume of oxygen and Voronoi volume of D6, boundary atoms oxygen. Furthermore, we find the dynamical heterogeneity at ambient pressure. The separate “fast” regions and “slow” regions in GeO2 are detected via link-cluster function.

Cosmic-Ray Transport in Simulations of Star-forming Galactic Disks

Abstract Cosmic-ray transport on galactic scales depends on the detailed properties of the magnetized, multiphase interstellar medium (ISM). In this work, we postprocess a high-resolution TIGRESS magnetohydrodynamic simulation modeling a local galactic disk patch with a two-moment fluid algorithm for cosmic-ray transport. We consider a variety of prescriptions for the cosmic rays, from a simple, purely diffusive formalism with constant scattering coefficient, to a physically motivated model in which the scattering coefficient is set by the critical balance between streaming-driven Alfvén wave excitation and damping mediated by local gas properties. We separately focus on cosmic rays with kinetic energies of ∼1 GeV (high-energy) and ∼30 MeV (low energy), respectively important for ISM dynamics and chemistry. We find that simultaneously accounting for advection, streaming, and diffusion of cosmic rays is crucial for properly modeling their transport. Advection dominates in the high-velocity, low-density hot phase, while diffusion and streaming are more important in higher-density, cooler phases. Our physically motivated model shows that there is no single diffusivity for cosmic-ray transport: the scattering coefficient varies by four or more orders of magnitude, maximal at density n H ∼ 0.01 cm−3. The ion-neutral damping of Alfvén waves results in strong diffusion and nearly uniform cosmic-ray pressure within most of the mass of the ISM. However, cosmic rays are trapped near the disk midplane by the higher scattering rate in the surrounding lower-density, higher-ionization gas. The transport of high-energy cosmic rays differs from that of low-energy cosmic rays, with less effective diffusion and greater energy losses for the latter.

Emulsifying properties of lentil protein preparations obtained by dry fractionation

Dry fractionated legume protein ingredients are gaining attention as alternatives to conventional solvent extracted legume proteins, being more resource efficient and often exhibiting novel functional properties. However, lack of knowledge about the relationship between composition and functionality limit a more wide-spread use of dry-fractionated legume protein in applications. In this study, lentil fractions of different degrees of refinement were prepared using air classification having protein and starch contents of 16–59% and 4–64%, respectively. The dry fractionated lentil fractions could emulsify and stabilize 10 wt% oil-in-water emulsions, while a conventional lentil protein isolate used for comparison was not able to form stable emulsions. The latter had significantly larger mean droplet diameters (around 20 µm) due to droplet flocculation than emulsions made with the different lentil fractions ranging between 0.3 and 5.5 µm. Similar surface charges (between −22 and −31 mV) indicated that the discrepancy could be ascribed to differences in steric repulsion and mechanical strength of the interfacial layers between conventionally and dry fractionated lentil. Storage stability tests of emulsions stabilized with dry fractionated samples resulted in separation into a low and higher density phase with the individual droplets being stable against coalescence in both phases. The phase separation was attributed to gravimetrical sedimentation of larger insoluble components accumulating in the denser phase, which was impacted by the degree of refinement by air classification. The results highlight the potential of dry fractionation for the production of sustainable ingredients with unique composition and functionality.

Dynein-Inspired Multilane Exclusion Process with Open Boundary Conditions.

Motivated by the sidewise motions of dynein motors shown in experiments, we use a variant of the exclusion process to model the multistep dynamics of dyneins on a cylinder with open ends. Due to the varied step sizes of the particles in a quasi-two-dimensional topology, we observe the emergence of a novel phase diagram depending on the various load conditions. Under high-load conditions, our numerical findings yield results similar to the TASEP model with the presence of all three standard TASEP phases, namely the low-density (LD), high-density (HD), and maximal-current (MC) phases. However, for medium- to low-load conditions, for all chosen influx and outflux rates, we only observe the LD and HD phases, and the maximal-current phase disappears. Further, we also measure the dynamics for a single dynein particle which is logarithmically slower than a TASEP particle with a shorter waiting time. Our results also confirm experimental observations of the dwell time distribution: The dwell time distribution for dyneins is exponential in less crowded conditions, whereas a double exponential emerges under overcrowded conditions.

Asymmetric exclusion processes with fixed resources: Reservoir crowding and steady states.

We study the reservoir crowding effect by considering the nonequilibrium steady states of an asymmetric exclusion process (TASEP) coupled to a reservoir with fixed available resources and dynamically coupled entry and exit rate. We elucidate how the steady states are controlled by the interplay between the coupled entry and exit rates, both being dynamically controlled by the reservoir population, and the fixed total particle number in the system. The TASEP can be in the low-density, high-density, maximal current, and shock phases. We show that such a TASEP is different from an open TASEP for all values of available resources: here the TASEP can support only localized domain walls for any (finite) amount of resources that do not tend to delocalize even for large resources, a feature attributed to the form of the dynamic coupling between the entry and exit rates. Furthermore, in the limit of infinite resources, in contrast to an open TASEP, the TASEP can be found in its high-density phase only for any finite values of the control parameters, again as a consequence of the coupling between the entry and exit rates.

Active bath-induced localization and collapse of passive semiflexible polymers.

The conformational and dynamical properties of a passive polymer embedded in a bath of active Brownian particles (ABPs) are studied by Langevin dynamics simulations. Various activities and ABP concentrations below and above the critical values for motility-induced phase separation (MIPS) are considered. In a homogeneous ABP fluid, the embedded polymer swells with increasing bath activity, with stronger swelling for larger densities. The polymer dynamics is enhanced, with the diffusion coefficient increasing by a power-law with increasing activity, where the exponent depends on the ABP concentration. For ABP concentrations in the MIPS regime, we observe a localization of the polymer in the low-density ABP phase associated with polymer collapse for moderate activities and a reswelling for high activities accompanied by a preferred localization in the high-density ABP phase. Localization and reswelling are independent of the polymer stiffness, with stiff polymers behaving similarly to flexible polymers. The polymer collapse is associated with a slowdown of its dynamics and a significantly smaller center-of-mass diffusion coefficient. In general, the polymer dynamics can only partially be described by an effective (bath) temperature. Moreover, the properties of a polymer embedded in a homogeneous active bath deviate quantitatively from those of a polymer composed of active monomers, i.e., linear chains of ABPs; however, such a polymer exhibits qualitatively similar activity-dependent features.

Accepted manuscript of "High-pressure syntheses and crystal structure analyses of a new low-density CaFe2O4-related and CaTi2O4-type MgAl2O4 phases" by Ishii et al. [Am. Min., 106, 1105-1112, 2021

Accepted manuscript of "High-pressure syntheses and crystal structure analyses of a new low-density CaFe2O4-related and CaTi2O4-type MgAl2O4 phases" by Ishii et al. [Am. Min., 106, 1105-1112, 2021

High-pressure syntheses and crystal structure analyses of a new low-density CaFe2O4-related and CaTi2O4-type MgAl2O4 phases

AbstractThree single crystals of CaTi2O4 (CT)-type, CaFe2O4 (CF)-type, and new low-density CaFe2O4 (LD-CF) related MgAl2O4 were synthesized at 27 GPa and 2500 °C and also CT-type MgAl2O4 at 45 GPa and 1727 °C using conventional and advanced multi-anvil technologies, respectively. The structures of CT-type and LD-CF related MgAl2O4 were analyzed by single-crystal X-ray diffraction. The lattice parameters of the CT-type phases synthesized at 27 and 45 GPa were a = 2.7903(4), b = 9.2132(10), and c = 9.3968(12) Å, and a = 2.7982(6), b = 9.2532(15), and c = 9.4461(16) Å, respectively, (Z = 4, space group: Cmcm) at ambient conditions. This phase has an AlO6 octahedral site and an MgO8 bicapped trigonal prism with two longer cation-oxygen bonds. The LD-CF related phase has a novel structure with orthorhombic symmetry (space group: Pnma), and lattice parameters of a = 9.207(2), b = 3.0118(6), and c = 9.739(2) Å (Z = 4). The structural framework comprises tunnel-shaped spaces constructed by edge- and corner-sharing of AlO6 and a 4+1 AlO5 trigonal bipyramid, in which MgO5 trigonal bipyramids are accommodated. The CF-type MgAl2O4 also has the same space group of Pnma but a slightly different atomic arrangement, with Mg and Al coordination numbers of 8 and 6, respectively. The LD-CF related phase has the lowest density of 3.50 g/cm3 among MgAl2O4 polymorphs, despite its high-pressure synthesis from the spinel-type phase (3.58 g/cm3), indicating that the LD-CF related phase formed via back-transformation from a high-pressure phase during the recovery. Combined with the previously determined phase relations, the phase transition between CF-and CT-type MgAl2O4 is expected to have a steep Clapeyron slope. Therefore, CT-type phase may be stable in basaltic- and continental-crust compositions at higher temperatures than the average mantle geotherm in the wide pressure range of the lower mantle. The LD-CF related phase could be found in shocked meteorites and used for estimating shock conditions.

Metal–insulator transition and low-density phases in a strongly-interacting two-dimensional electron system

We review recent experimental results on the metal–insulator transition and low-density phases in strongly-interacting, low-disordered silicon-based two-dimensional electron systems. Special attention is given to the metallic state in ultra-clean SiGe quantum wells and to the evidence for a flat band at the Fermi level and a quantum electron solid.

Defect-induced anticorrelations in molecular motor traffic

We revisit the nonequilibrium phase transition between a spatially homogeneous low-density phase and a phase-separated high-density state in the deterministic sublattice totally asymmetric simple exclusion process with stochastic defect. We discuss this phase transition in a grandcanonical ensemble for which we obtain exact results for the stationary current-density correlations and for the average collective velocity. We identify defect-induced anticorrelations that are absent in similar boundary-induced phase transitions. The average collective velocity vanishes at the phase transition and in the phase-separated state due to its macroscopic spatial inhomogeneity.

(Invited) Crystal Phase Engineering of Cesium Lead Iodide By Nanoconfinement

Nanoscale dimensional reduction leads to perturbed electronic structure, a foundational concept in the study of quantum dots. In this work, we extend this concept to consider how nanoscale dimensionality perturbs the crystal phase diagram of a solid. We apply this to the phase behavior of cesium lead iodide, a material that possesses favorable optoelectronic properties in its perovskite crystal polymorph at high temperature, and exhibits an undesirable crystal phase at room temperature. We develop and test the hypothesis that for solids such as cesium lead iodide that simultaneously exhibit both predominantly covalent bonding (Pb—I) and predominantly ionic bonding (Cs—I), low-density, high-symmetry phases can be favored by restricting crystal dimensions below 100 nm. This is based on a model in which short-range, orbital-orbital repulsive forces, which favor low-density and high-symmetry phases, are in competition with long-range electrostatic forces, which favor high-density. The electrostatic forces, being long-range in nature, are more diminished by truncating the crystal. Experimentally, we show that the phase sequence can be inverted and the equilibrium shifted by as much as 250 ºC through various nanostructuring approaches, including colloidal nanocrystal co-assembly and crystallization within nanoporous scaffolds. Ultimately, we seek to develop a phase diagram in which temperature and size are the independent variables.

Structure of levitated Si–Ge melts studied by high-energy x-ray diffraction in combination with reverse Monte Carlo simulations

The short-range order in liquid Si, Ge and binary Si x –Ge1−x alloys (x = 0.25, 0.50, 0.75) was studied by x-ray diffraction and reverse Monte Carlo simulations. Experiments were performed in the normal and supercooled liquid states by using the containerless technique of aerodynamic levitation with CO2 laser heating, enabling deeper supercooling of liquid Si and Si–Ge alloys than previously reported. The local atomic structure of liquid Si and Ge resembles the β-tin structure. The first coordination numbers of about 6 for all compositions are found to be independent of temperature indicating the supercooled liquids studied retain this high-density liquid (HDL) structure. However, there is evidence of developing local tetrahedral ordering, as manifested by a shoulder on the right side of the first peak in S(Q) which becomes more prominent with increasing supercooling. This result is potentially indicative of a continuous transition from the stable HDL β-tin (high pressure) phase, towards a metastable low-density liquid phase, reminiscent of the diamond (ambient pressure) structure.

Separation performance of hydrocyclones with medium rearrangement internals

The inlet section of hydrocyclone has a significant effect on separation efficiency. However, in most existing studies the inlet of conventional hydrocyclones (CH) only serves as a channel for the mixed liquid to enter the hydrocyclone body. Based on CH's tangential inlet with a rectangular section, a novel hydrocyclone with medium rearrangement internals (HMRI) was designed and the rearrangement internals (RIs) were installed inside the inlets. The RIs include one separating block and 3 division plates following the block. Thus, the rearrangement of immiscible media can be achieved after passing through this inlet, i.e., media with different densities flow into different layers. Most of the rearranged low-density phase flows directly into the central area of HMRI. Meanwhile, most of high-density phase flows to the wall zone, which is beneficial for the subsequent centrifugal separation process in HMRI’s body. Taking the oil-water mixture as the example, three kinds of separating blocks, semi-cylindrical block (SCB), triangular prism block (TPB), and trapezoidal body block (TBB) were designed and compared to no-block cases (conventional inlet) through simulations. Results showed that RIs with SCB displayed excellent rearrangement effect as well as a low pressure drop. Additionally, the comparison between CH and HMRI was conducted via oil-phase distribution, velocity distribution, and separation efficiency. Our results show that the separation efficiency of the novel HMRI was 6.4–8.0% higher than the traditional CH, under various operating conditions. The application potential of HMRI in biological processes for wastewater treatment was also analysed and discussed.

Optimization-oriented modelling of neutral beam injection for EU pulsed DEMO

Neutral beam injection (NBI) is one of the auxiliary power systems considered for the EU DEMO pulsed plasma (‘DEMO1’). In this paper, we discuss the characteristics of optimized NBI in terms of the DEMO1 requirements, relating physics and engineering contexts in a novel parameter range compared to current NBI systems and in a larger plasma volume than ITER. Different injection options are investigated to account for various concepts discussed in the literature. The investigation is carried out using a wide range of sensitivity studies by means of the METIS 0.5D transport code and ASCOT Monte Carlo simulations of injected neutral-beam particles. This investigation has generated a series of recommendations for the NBI design, contributing to the system optimization process. We show how tangential injection aimed at the plasma core, with an energy 800 keV, is recommended to maintain high fusion power performance and fulfill the requirements for bulk heating. Compliance with engineering constraints on the NBI design, e.g., the least interference with breeding blanket modules or compatibility with solutions for the beamline components, imposes some restrictions when discussing the beam geometry. The minimum density at which an NBI can be safely operated without harmful shine-through losses is investigated for different injection energies and compared to the ITER case. The NBI operational window for DEMO is shown to be significantly extended to transient, low-density phases, also highlighting the importance of NBI systems designed for modular energy and power output. NBI can therefore sustain the plasma during a considerable portion of the transient phases, i.e., the current ramp-up and ramp-down phases. The final decision on the DEMO heating mix will be made on the basis of system operability and performance: the optimized NBI is shown to be a suitable and effective option for the EU DEMO inductive scenario.