High-density Limit Research Articles

High Energy Density Battery Electrodes with Low and Anisotropic Tortuosity

The high inactive materials content (e.g., separators, current collectors, conductive additives, binder and packaging) of current nonaqueous battery designs contributes directly to high battery cost and reduces specific energy and energy density. Area specific capacity, which is perhaps the most objective measure of design performance, cannot be increased arbitrarily by increasing electrode thickness and/or density, due to kinetic limitations. In the limit of high density and large thickness, salt depletion within the electrolyte-filled porosity typically becomes rate-limiting. In order to reach higher usable energy at practical C-rates from thick, dense electrodes, it is essential to tailor the topology of the pore structure to reduce tortuosity anisotropy. Here we report on the fabrication, characterization of tortuosity, and electrochemical testing of thick, dense electrodes made by the freeze-casting of aqueous suspensions followed by drying/sintering. This shaping technique allows design of oriented and interconnected structures that have tortuosity anisotropy, low absolute tortuosity, and high ionic transport. We report on the characterization of tortuosity of directionally fabricated electrodes with similar porosity while varying the solidification rate that resulted in different microstructural features. The electrochemical performance of the fabricated electrodes in half cells exhibits high areal capacity. Hybrid pulse power characterization (HPPC) and dynamic stress tests (DST) were also performed.

Neutrino mass without cosmic variance

Measuring the absolute scale of the neutrino masses is one of the most exciting opportunities available with near-term cosmological datasets. Two quantities that are sensitive to neutrino mass, scale-dependent halo bias $b(k)$ and the linear growth parameter $f(k)$ inferred from redshift-space distortions, can be measured without cosmic variance. Unlike the amplitude of the matter power spectrum, which always has a finite error, the error on $b(k)$ and $f(k)$ continues to decrease as the number density of tracers increases. This paper presents forecasts for statistics of galaxy and lensing fields that are sensitive to neutrino mass via $b(k)$ and $f(k)$. The constraints on neutrino mass from the auto- and cross-power spectra of spectroscopic and photometric galaxy samples are weakened by scale-dependent bias unless a very high density of tracers is available. In the high density limit, using multiple tracers allows cosmic-variance to be beaten and the forecasted errors on neutrino mass shrink dramatically. In practice, beating the cosmic variance errors on neutrino mass with $b(k)$ will be a challenge, but this signal is nevertheless a new probe of neutrino effects on structure formation that is interesting in its own right.

Current driven vortex-antivortex pair breaking and vortex explosion in the Bi2Te3/FeTe interfacial superconductor

We investigated the dissipative regime of the Bi2Te3/FeTe topological insulator-chalcogenide interface superconductor at temperatures well below the Berezinski-Kosterlitz-Thouless transition. We observe a transition in the current-resistance and temperature-resistance curves that quantitatively agrees with the Likharev vortex-explosion phenomenon. In the limit of low temperatures and high current densities, we were able to demonstrate the regime of complete vortex-antivortex dissociation arising from current driven vortex-antivortex pair breaking.

High-density limit of quasi-two-dimensional dipolar Bose gas

We consider a simple model of the quasi-two-dimensional dipolar Bose gas confined in the one-dimensional square well potential. All dipoles are assumed to be oriented along the confining axis. By means of hydrodynamic approach it is shown that the general structure of the low-lying excitations can be analyzed exactly. We demonstrate that the problem significantly simplifies in the high-density limit for which the density profile in the confined direction as well as the leading-order contribution to the ground-state energy and spectrum of elementary excitations are calculated. The low-temperature result for the damping rate of the phonon mode is also presented.

Investigation of the hydrogen adsorbed density inside the pores of MOF-5 from path integral grand canonical Monte Carlo at supercritical and subcritical temperature

We examine the high density limit of the adsorption isotherms of hydrogen on MOF-5. The isotherms are calculated using quantum GCMC simulations over the pressure range: 0–150atm (1atm = 1.01325 × 105Pa) in the subcritical and supercritical state at 30, 50, 77, 113, 196 and 296K. The fluid phase density in the pores for each temperature is calculated and shown to reach values higher than normal liquid density. The fluid phase density obtained at 30K is observed to correspond to a highly compressed liquid. The radial distribution function of the adsorbed phase at 30 and 50K are calculated. The adsorption isotherms are compared with available experimental data at 30, 50, 77 and 298K.

Cavity polariton condensate in a disordered environment

We report on the influence of disorder on an exciton-polariton condensate in a ZnO based bulk planar microcavity and compare experimental results with a theoretical model for a non-equilibrium condensate. Experimentally, we detect intensity fluctuations within the far-field emission pattern even at high condensate densities which indicates a significant impact of disorder. We show that these effects rely on the driven dissipative nature of the condensate and argue that they can be accounted for by spatial phase inhomogeneities induced by disorder, which occur even for increasing condensate densities realized in the regime of high excitation power. Thus, non-equilibrium effects strongly suppress the stabilization of the condensate against disorder, contrarily to what is expected for equilibrium condensates in the high density limit. Numerical simulations based on our theoretical model reproduce the experimental data.

Equation of state for a trapped quantum gas: remnant of zero-point energy effects

The study of the thermodynamic properties of trapped gases has attracted great attention during the last few years and can be used as a tool to characterize such clouds in the presence of other phenomena. Here, we obtain an equation of state for a harmonically trapped Bose–Einstein condensate taking the limit of by means of global themodynamic variables. These variables allow us to explore limits in which the standard thermodynamics are not defined. Our results are taken in the high density limit, and the extrapolation for is done later. Even in this situation, we qualitatively observe the well known existence of a zero-point energy for harmonic potentials in which the determination of conjugated variables is limited by the quantum nature of the system.

Nonlocal energy-optimized kernel: Recovering second-order exchange in the homogeneous electron gas

In order to remedy some of the shortcomings of the random phase approximation (RPA) within adiabatic connection fluctuation-dissipation (ACFD) density functional theory, we introduce a short-ranged, exchange-like kernel that is one-electron self-correlation free and exact for two-electron systems in the high-density limit. By tuning a free parameter in our model to recover an exact limit of the homogeneous electron gas correlation energy, we obtain a nonlocal, energy-optimized kernel that reduces the errors of RPA for both homogeneous and inhomogeneous solids. Using wave-vector symmetrization for the kernel, we also implement RPA renormalized perturbation theory for extended systems, and demonstrate its capability to describe the dominant correlation effects with a low-order expansion in both metallic and nonmetallic systems. The comparison of ACFD structural properties with experiment is also shown to be limited by the choice of norm-conserving pseudopotential.

General theory of asymmetric steric interactions in electrostatic double layers.

We study the mean-field Poisson-Boltzmann equation in the context of dense ionic liquids where steric effects become important. We generalise lattice gas theory by introducing a Flory-Huggins entropy for ions of differing volumes and then compare the effective free energy density to other existing lattice gas approximations, not based on the Flory-Huggins Ansatz. Within the methodology presented we also invoke more realistic equations of state, such as the Carnahan-Starling approximation, that are not based on the lattice gas approximation and lead to thermodynamic functions and properties that differ strongly from the lattice gas case. We solve the Carnahan-Starling model in the high density limit, and demonstrate a slow, power-law convergence at high potentials. We elucidate how equivalent convex free energy functions can be constructed that describe steric effects in a manner which is more convenient for numerical minimisation.

Monte Carlo simulations of ionization potential depression in dense plasmas

A particle-particle grand canonical Monte Carlo model with Coulomb pair potential interaction was used to simulate modification of ionization potentials by electrostatic microfields. The Barnes-Hut tree algorithm [J. Barnes and P. Hut, Nature 324, 446 (1986)] was used to speed up calculations of electric potential. Atomic levels were approximated to be independent of the microfields as was assumed in the original paper by Ecker and Kröll [Phys. Fluids 6, 62 (1963)]; however, the available levels were limited by the corresponding mean inter-particle distance. The code was tested on hydrogen and dense aluminum plasmas. The amount of depression was up to 50% higher in the Debye-Hückel regime for hydrogen plasmas, in the high density limit, reasonable agreement was found with the Ecker-Kröll model for hydrogen plasmas and with the Stewart-Pyatt model [J. Stewart and K. Pyatt, Jr., Astrophys. J. 144, 1203 (1966)] for aluminum plasmas. Our 3D code is an improvement over the spherically symmetric simplifications of the Ecker-Kröll and Stewart-Pyatt models and is also not limited to high atomic numbers as is the underlying Thomas-Fermi model used in the Stewart-Pyatt model.

The Lennard-Jones melting line and isomorphism.

The location of the melting line (ML) of the Lennard-Jones (LJ) system and its associated physical properties are investigated using molecular dynamics computer simulation. The radial distribution function and the behavior of the repulsive and attractive parts of the potential energy indicate that the ML is not a single isomorph, but the isomorphic state evolves gradually with temperature, i.e., it is only "locally isomorphic." The state point dependence of the unitless isomorphic number, X̃, for a range of static and dynamical properties of the LJ system in the solid and fluid states, and for fluid argon, are also reported. The quantity X̃ typically varies most with state point in the vicinity of the triple point and approaches a plateau in the high density (temperature) limit along the ML.

Research on the effect of resonant magnetic perturbations on disruption limit in J-TEXT tokamak

The effect of resonant magnetic perturbations (RMPs) on high density limit and low-q limit discharges is studied in J-TEXT tokamak. It is found that a moderate amplitude of applied RMPs increases the density limit and delays the disruption, and the disruption precursor is suppressed with a slight reduction in toroidal plasma rotation. A too large amplitude of RMPs, however, leads to a lower density limit and an earlier disruption. For the optimal RMPs amplitude, the disruption is delayed by about 150 ms. For low-q discharges, the applied static RMP can either lower the limit of the edge safety factor qa from 2.15 to nearly 2.0 or sustain stable plasma discharge with qa = 2.1, and the disruptive precursor is also suppressed.

Fractional quantum Hall states versus Wigner crystals in wide quantum wells in the half-filled lowest and second Landau levels

We investigate numerically different phases that can occur at half filling in the lowest and the first excited Landau levels in wide-well twodimensional electron systems exposed to a perpendicular magnetic field. Within a twocomponent model that takes into account only the two lowest electronic subbands of the quantum well, we derive a phase diagram that compares favorably with an experimental one by Shabani et al. [Phys. Rev. B 88, 245413 (2013)]. In addition to the compressible composite-fermion Fermi liquid in narrow wells with a substantial subband gap and the incompressible twocomponent (331) Halperin state, we identify in the lowest Landau level a rectangular Wigner crystal occupying the second subband. This crystal may be the origin of the experimentally observed insulating phase in the limit of wide wells and high electronic densities. In the second Landau level, the incompressible Pfaffian state, which occurs in narrow wells and large subband gaps, is also separated by an intermediate region from a large-well limit in which a similar rectangular Wigner crystal in the excited subband is the ground state, as for the lowest Landau level. However, the intermediate region is characterized by an incompressible state that consists of two four-flux Pfaffians in each of the components.

Diffusion and Subdiffusion of Interacting Particles on Comblike Structures.

We study the dynamics of a tracer particle (TP) on a comb lattice populated by randomly moving hard-core particles in the dense limit. We first consider the case where the TP is constrained to move on the backbone of the comb only. In the limit of high density of the particles, we present exact analytical results for the cumulants of the TP position, showing a subdiffusive behavior ∼t^{3/4}. At longer times, a second regime is observed where standard diffusion is recovered, with a surprising nonanalytical dependence of the diffusion coefficient on the particle density. When the TP is allowed to visit the teeth of the comb, based on a mean-field-like continuous time random walk description, we unveil a rich and complex scenario with several successive subdiffusive regimes, resulting from the coupling between the geometrical constraints of the comb lattice and particle interactions. In this case, remarkably, the presence of hard-core interactions asymptotically speeds up the TP motion along the backbone of the structure.

A critical analysis of the feasibility of pure strain-actuated giant magnetostrictive nanoscale memories

Concepts for memories based on the manipulation of giant magnetostrictive nanomagnets by stress pulses have garnered recent attention due to their potential for ultra-low energy operation in the high storage density limit. Here, we discuss the feasibility of making such memories in light of the fact that the Gilbert damping of such materials is typically quite high. We report the results of numerical simulations for several classes of toggle precessional and non-toggle dissipative magnetoelastic switching modes. Material candidates for each of the several classes are analyzed and forms for the anisotropy energy density and ranges of material parameters appropriate for each material class are employed. Our study indicates that the Gilbert damping as well as the anisotropy and demagnetization energies are all crucial for determining the feasibility of magnetoelastic toggle-mode precessional switching schemes. The roles of thermal stability and thermal fluctuations for stress-pulse switching of giant magnetostrictive nanomagnets are also discussed in detail and are shown to be important in the viability, design, and footprint of magnetostrictive switching schemes.

Superconducting gap inBaFe2(As1−xPx)2from temperature-dependent transient optical reflectivity

Temperature and fluence dependence of the 1.55-eV optical transient reflectivity in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ was measured and analysed in the low and high excitation density limit. The effective magnitude of the superconducting gap of $\sim 5$ meV obtained from the low-fluence-data bottleneck model fit is consistent with the ARPES results for the $\gamma$-hole Fermi surface. The superconducting-state nonthermal optical destruction energy was determined from the fluence dependent data. The in-plane optical destruction energy scales well with T$_{\mathrm{c}}^{2}$ and is found to be similar in a number of different layered superconductors.

Strategies and tactics of tuna fishers in the payao (anchored FAD) fishery from general Santos city, Philippines

Payaos or anchored FADs are used extensively in the Philippine tuna fishery. Currently, spatial regulations on FAD use are implemented with limited observance or understanding of their potential impacts. A combination of semi-structured interviews (n=150) and a total of six focus group discussions (n=61) from purse seine, ringnet and handline fishers of General Santos City provided a coarse indication of fishers' operational patterns and decisions on fishing tactics and strategies. Over a period of about 25 years, Filipino FAD fishers moved from inshore areas to offshore areas to catch oceanic tunas and FAD associated small pelagics and neritic tunas. Presently their catch rates are reported to be significantly higher at more distant sites. Due to the high density and spatial limitations of FADs deployment in nearshore areas, offshore deployment of FADs in a spatial network that is owned by a company has become a strategy to maintain hold of a fishing ground. Purse seine and ringnet fishers, as area specialists, operate from 70 to 1000km from the fishing port and have an average reported catch rate of 14t per set. Handline fishers which roam freely between FAD areas claimed by purse seine and ringnet fishing companies which operate from 90 to 1500km from the fishing port with an average reported catch rate of 1.6t per fishing trip. Strategic decisions on the deployment of FADs in fishing grounds were strongly modulated by availability of fishing space, environmental considerations and both experience and information on areas with good catches. Day to day tactical decisions were mainly explained by socially mediated information flows, weather patterns and economic factors, and to a lesser extent by fish behavior and regulations.

Ground states of stealthy hyperuniform potentials: I. Entropically favored configurations.

Systems of particles interacting with "stealthy" pair potentials have been shown to possess infinitely degenerate disordered hyperuniform classical ground states with novel physical properties. Previous attempts to sample the infinitely degenerate ground states used energy minimization techniques, introducing algorithmic dependence that is artificial in nature. Recently, an ensemble theory of stealthy hyperuniform ground states was formulated to predict the structure and thermodynamics that was shown to be in excellent agreement with corresponding computer simulation results in the canonical ensemble (in the zero-temperature limit). In this paper, we provide details and justifications of the simulation procedure, which involves performing molecular dynamics simulations at sufficiently low temperatures and minimizing the energy of the snapshots for both the high-density disordered regime, where the theory applies, as well as lower densities. We also use numerical simulations to extend our study to the lower-density regime. We report results for the pair correlation functions, structure factors, and Voronoi cell statistics. In the high-density regime, we verify the theoretical ansatz that stealthy disordered ground states behave like "pseudo" disordered equilibrium hard-sphere systems in Fourier space. The pair statistics obey certain exact integral conditions with very high accuracy. These results show that as the density decreases from the high-density limit, the disordered ground states in the canonical ensemble are characterized by an increasing degree of short-range order and eventually the system undergoes a phase transition to crystalline ground states. In the crystalline regime (low densities), there exist aperiodic structures that are part of the ground-state manifold but yet are not entropically favored. We also provide numerical evidence suggesting that different forms of stealthy pair potentials produce the same ground-state ensemble in the zero-temperature limit. Our techniques may be applied to sample the zero-temperature limit of the canonical ensemble of other potentials with highly degenerate ground states.

Overview of the RFX-mod contribution to the international Fusion Science Program

The RFX-mod device is operated both as a reversed field pinch (RFP), where advanced regimes featuring helical shape develop, and as a tokamak. Due to its flexibility, RFX-mod is contributing to the solution of key issues in the roadmap to ITER and DEMO, including MHD instability control, internal transport barriers, edge transport and turbulence, isotopic effect, high density limit and three-dimensional (3D) non-linear MHD modelling. This paper reports recent advancements in the understanding of the self-organized helical states, featuring a strong electron transport barrier, in the RFP configuration; the physical mechanism driving the residual transport at the barrier has been investigated. Following the first experiments with deuterium as the filling gas, new results concerning the isotope effect in the RFP are discussed. Studies on the high density limit show that in the RFP it is related to a toroidal particle accumulation due to the onset of a convective cell. In the tokamak configuration, q(a) regimes down to q(a) = 1.2 have been pioneered, with (2,1) tearing mode (TM) mitigated and (2,1) resistive wall mode (RWM) stabilized: the control of such modes can be obtained both by poloidal and radial sensors. Progress has been made in the avoidance of disruptions due to the (2,1) TM by applying q(a) control, and on the general issue of error field control. The effect of externally applied 3D fields on plasma flow and edge turbulence, sawtooth control and runaway electron decorrelation has been analysed. The experimental program is supported by substantial theoretical activity: 3D non-linear visco-resistive MHD and non-local transport modelling have been advanced; RWMs have been studied by a toroidal MHD kinetic hybrid stability code.

STOUT: CLOUDY’S ATOMIC AND MOLECULAR DATABASE

We describe a new atomic and molecular database we developed for use in the spectral synthesis code Cloudy. The design of Stout is driven by the data needs of Cloudy, which simulates molecular, atomic, and ionized gas with kinetic temperatures 2.8 K < T < 1e10 K and densities spanning the low to high-density limits. The radiation field between photon energies $10^{-8}$ Ry and 100 MeV is considered, along with all atoms and ions of the lightest 30 elements, and ~100 molecules. For ease of maintenance, the data are stored in a format as close as possible to the original data sources. Few data sources include the full range of data we need. We describe how we fill in the gaps in the data or extrapolate rates beyond their tabulated range. We tabulate data sources both for the atomic spectroscopic parameters and for collision data for the next release of Cloudy. This is not intended as a review of the current status of atomic data, but rather a description of the features of the database which we will build upon.