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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

(2 + δ)-dimensional theory of the electromechanics of lipid membranes: Electrostatics

The coupling of electric fields to the mechanics of lipid membranes gives rise to intriguing electromechanical behavior, as, for example, evidenced by the deformation of lipid vesicles in external electric fields. Electromechanical effects are relevant for many biological processes, such as the propagation of action potentials in axons and the activation of mechanically gated ion channels. Currently, a theoretical framework describing the electromechanical behavior of arbitrarily curved and deforming lipid membranes does not exist. Purely mechanical models commonly treat lipid membranes as two-dimensional surfaces, ignoring their finite thickness. While holding analytical and numerical merit, this approach cannot describe the coupling of lipid membranes to electric fields and is thus unsuitable for electromechanical models. In a sequence of articles, we derive an effective surface theory of the electromechanics of lipid membranes, called the (2 + δ)-dimensional theory, which has the advantages of surface descriptions while accounting for finite thickness effects. The present article proposes a generic dimension reduction procedure relying on low-order spectral expansions. This procedure is applied to the electrostatics of lipid membranes to obtain the (2 + δ)-dimensional theory that captures potential differences across and electric fields within lipid membranes. Finally, this model is tested on different geometries relevant for lipid membranes, showing good agreement with the corresponding three-dimensional electrostatics theory.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1994

The detectability of extrasolar terrestrial and giant planets during their luminous final accretion

One of the outstanding scientific questions in astronomy is the frequency at which solar systems form. Answering this question is an observational challenge because extrasolar planets are intrinsically difficult to directly detect. The direct detectability of planets is examined during the short but unique epoch of giant impacts that is a hallmark of the standard theory of planetary formation. Sufficiently large impacts during this era are capable of creating a luminous, 1500-2500 K photosphere, which can persist for timescales exceeding 103 years in some cases. The detectability of such events and the number of young stars one would need to examine to expect to find a luminous terrestrial class planet after a giant impact are examined. With emerging IR interferometric technology, thermally-luminous earth-sized objects can be detected in nearby star forming regions in 1-2 nights observing time. Unfortunately, predictions indicate that approximately 250 young stars would have to be searched to expect to find one hot, terrestrial-sized planet. By comparison, the detection of Saturn and Uranus/Neptune-sized planets after a giant impact requires only 1-2 hours of observing time. A single Keck-class telescope should be able to determine whether such planets are common in the nearest star forming regions by examining about less than 100 young stars over a few tens of nights. The results obtained herein suggest a new strategy for the detection of solar systems with the potential for the observational confirmation of the standard theory of late-stage planetary accretion.

Stern, S. Alan↗
DOE OSTI2021

Heat transport in liquid water from first-principles and deep neural network simulations

In this work, we compute the thermal conductivity of water within linear response theory from equilibrium molecular dynamics simulations, by adopting two different approaches. In one, the potential energy surface (PES) is derived on the fly from the electronic ground state of density functional theory (DFT) and the corresponding analytical expression is used for the energy flux. In the other, the PES is represented by a deep neural network (DNN) trained on DFT data, whereby the PES has an explicit local decomposition and the energy flux takes a particularly simple expression. By virtue of a gauge invariance principle, established by Marcolongo, Umari, and Baroni, the two approaches should be equivalent if the PES were reproduced accurately by the DNN model. We test this hypothesis by calculating the thermal conductivity, at the GGA (PBE) level of theory, using the direct formulation and its DNN proxy, finding that both approaches yield the same conductivity, in excess of the experimental value by approximately 60%. Besides being numerically much more efficient than its direct DFT counterpart, the DNN scheme has the advantage of being easily applicable to more sophisticated DFT approximations, such as meta-GGA and hybrid functionals, for which it would be hard to derive analytically the expression of the energy flux. We find in this way that a DNN model, trained on meta-GGA (SCAN) data, reduces the deviation from experiment of the predicted thermal conductivity by about 50%, leaving the question open as to whether the residual error is due to deficiencies of the functional, to a neglect of nuclear quantum effects in the atomic dynamics, or, likely, to a combination of the two.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1994

The detectability of extrasolar terrestrial and giant planets during their luminous final accretion

One of the outstanding scienfific questions in astronomy is the frequency at which solar systems form. Answering this question is an observational challenge because extrasolar planets are intrinsically difficult to directly detect. Here I examine the direct detectability of planets during the short but unique epoch of giant impacts that is a hallmark of the standard theory of planetary formation. Sufficiently large impacts during this era are capable of creating a luminous, 1500-2500 K photosphere, which can persist for time scales exceeding 10(exp 3) yr in some cases. I examine the detectability of such events and the number of young stars one would need to examine to expect to find a luminous terrestrial-class planet after a giant impact. With emerging IR interferometric technology, thermally luminous Earth-sized objects can be detected in nearby star forming regions in 1-2 nights of observing time. Unfortunately, predictions indicated that approximately 250 young stars would have to be searched to expect to find one hot, terrestrial-sized planet. By comparison, the detection of Saturn and Uranus-Neptune-sized planets after a giant impact requires only 1-2 h of observing time. A single Keck-class telescope should be able to determine whether such planets are common in the nearest star forming regions by examining less than or approximately equal to 100 young stars over a few tens of nights. The results obtained here suggest a new strategy for the detection of solar systems with the potential for the observational confirmation of the standard theory of late-stage planetary accretion.

Stern, S. Alan↗
DOE OSTI2023

Theoretical Analysis of Resonant Tunneling Enhanced Field Emission

In this paper, we develop an exact analytical quantum theory for field emission from surfaces with a nearby quantum well, by solving the one-dimensional time-independent Schrödinger equation. The quantum well, which may be introduced by ions, atoms, nanoparticles, etc., is simplified as a square potential well with depth H, width d, and distance to the surface L. The theory is used to analyze the effects of the quantum well (d, H, and L), the cathode properties (work function W and Fermi energy E F ), and dc field F. It is found that the quantum well can lead to resonant tunneling enhanced field emission up to several orders of magnitude larger than that from bare cathode surfaces. In the meantime, the electron-emission-energy spectrum is significantly narrowed. The strong enhancement region is bounded by the conditions eFL + H ≥ W + C and eFL ≤ W, with e being the elementary charge (positive) and C a constant dependent on dc field F. It is also found that the linear shift of resonance peaks in the electron-emission-energy spectrum with dc field F follows ε p =ε p⁢0 –e⁢FL, with ε p⁢0 being approximately the eigenenergies for electrons confined in a square potential well without a dc field. Finally, the theory provides insights for the design of high-efficiency field emitters, which can produce a high current and highly collimated electron beams.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

A new interatomic potential for mixed Mg-Al-Ga-In spinels

While density functional theory (DFT) has become the de facto approach for accurate simulation of materials at the atomic scale, there are many aspects of materials that are simply out of reach of DFT methods. In particular, finite temperature properties such as diffusivities, the structure and properties of grain boundaries and interfaces, and the study of defect properties in complex alloys are computationally challenging for DFT methods. Recently, a new class of spinels in which three cations order over two sublattices was discovered. In order to predict the properties of these types of structures, classical potentials are a must. Here, in this work, we derive a new classical potential for Mg-bearing spinels in which the B cations are Al, Ga, and/or In. The potential does well in describing the DFT energetics of various spinel structures as a function of chemistry and inversion. In particular, it reproduces the thermodynamically favorable MgAlGaO 4 structure while correctly predicting that neither MgAlInO 4 nor MgGaInO 4 are stable. Further, it reproduces physical trends in elastic properties as compared against experiment.

36 MATERIALS SCIENCE↗
NASA NTRS1925

Potential flow in engine valves

The extensive applicability of the hydrodynamic theory to the problems of engine construction is clearly shown in the following attempt to determine by exact methods the nature of the flow in valves under variously restricted conditions. Observation shows that two principal kinds of flow occur in simple flat-seated valves. For small valve lifts, the flow is along the horizontal wall and is therefore deflected 90 degrees, but for greater valve lifts the flow separates and forms a free stream, whose angle of deflection naturally increases with increasing lift. Both these kinds of flow can, in fact, be theoretically explained

THEORIES - FLOW - KIRCHOFF AND HELMHOLTZ↗
DOE OSTI2022

The Importance of a Moving Boundary Approach for Modeling the SEI Layer Growth to Predict Capacity Fade

One of the contributing factors to the aging of lithium-ion batteries is the growth of the solid-electrolyte interphase (SEI) layer. The growth of the SEI layer leads to the irreversible loss of lithium available for cycling and increases the resistance of the battery. Physics-based models in literature model the kinetically limited or solvent diffusion-limited growth. In such models, the interface resistance is a constant, and the contribution to the overpotential of the intercalation reaction from the SEI layer is considered to be ohmic. In this study, we propose a model that describes the growth of the SEI layer on the electrode surface as a moving interface. The transport of lithium ions and the solvent in the electrolyte are affected by this moving interface. The equations that govern the species transport and the potential drop across the SEI layer are derived from dilute solution theory and solved by transforming the coordinates of the moving boundary. The ion transport induces changes in the conductivity across the SEI layer, which affects the potential drop that arises due to its growth. The effects of this potential on capacity fade are studied over cycling the battery.

25 ENERGY STORAGE↗
DOE OSTI2025

Stable, Efficient Iron Electrodeposition via Anion-Directed Control of Fe(II) Coordination

Traditional steelmaking processes consume about 7% of the world’s energy supply, with reduction of iron oxides into iron via blast furnaces representing the most energy-demanding and capital-intensive step. To economize and modularize iron reduction processes, we aim to develop an electrodeposition technique to reduce aqueous iron ions to metallic iron. However, the hydrogen reduction reaction (HER) occurs at a more positive standard reduction potential than the iron reduction reaction. In addition, aqueous Fe(II) cations easily precipitate at mildly acidic conditions (pH ≥ 3), which limits the deposition efficiency and degrades deposit quality. To address these challenges, we first search for anions that have intermediate coordination strength with Fe(II) based on the hard-soft acid-base theory, trading a slightly more negative Fe(II) reduction potential for a considerably broader pH stability range. We select citrate with predicted intermediate coordination strength, in combination with more weakly coordinating anions (e.g., SO 4 2- , Cl - ) to control the coordination structure of Fe 2+ for improved electrolyte stability and electrodeposition behavior. We find that citrate coordination stabilizes Fe 2+ -based electrolytes at higher pH conditions (4.8–5.5), significantly extending their shelf life while also suppressing HER during Fe electrodeposition by orders of magnitude. To measure Faradaic efficiencies (FE), we developed a straightforward, titration-based methodology to quantify the amount of deposited iron regardless of the rate of concurrent HER. Although coordination between citrate and Fe 2+ decreases the reduction potential of Fe(II), high FE (≥98%) was achieved at 10 mA cm -2 . FE and achievable deposition rates are tunable by both concentration and the ratio of Fe 2+ to citrate. In all cases, Raman spectroscopy and X-ray diffraction (XRD) reveal that iron deposition in citrate-containing electrolytes suppresses iron oxide/hydroxide precipitation, in contrast to deposits generated in citrate-free electrolytes. Altogether, this work demonstrates that citrate-mediated anion coordination enables high-purity iron electrodeposition with increased FE, high current density, and improved electrolyte stability. This multi-anion coordination strategy provides a versatile framework for designing stable electrolyte and efficient metal electrodeposition.

coordination↗
DOE OSTI2023

Exact and Model Exchange-Correlation Potentials for Open-Shell Systems

The conventional approaches to the inverse density functional theory problem typically assume nondegeneracy of the Kohn–Sham (KS) eigenvalues, greatly hindering their use in open-shell systems. Here, we present a generalization of the inverse density functional theory problem that can seamlessly admit degenerate KS eigenvalues. Additionally, we allow for fractional occupancy of the Kohn–Sham orbitals to also handle noninteracting ensemble-v-representable densities, as opposed to just noninteracting pure-v-representable densities. We present the exact exchange-correlation (XC) potentials for six open-shell systems–four atoms (Li, C, N, and O) and two molecules (CN and CH 2 )–using accurate ground-state densities from configuration interaction calculations. We compare these exact XC potentials with model XC potentials obtained using nonlocal (B3LYP, SCAN0) and local/semilocal (SCAN, PBE, PW92) XC functionals. Although the relative errors in the densities obtained from these DFT functionals are of $O$(10 –3 to 10 –2 ), the relative errors in the model XC potentials remain substantially large–$O$(10 –1 to 10 0 ).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1974

Comment on determination of the interaction potential between Ar and HCl

To test the accuracy of the Gordon-Kim theory of intermolecular forces, predicted and experimental values are compared for Ar-HCl. The method appears to accurately predict the short-range repulsive forces and also the position (but possibly not the depth) of the potential well.

Green, S.↗
DOE OSTI2022

Nonconventional NMR Spin-Coupling Constants in Oligosaccharide Conformational Modeling: Structural Dependencies Determined from Density Functional Theory Calculations

Nonconventional NMR spin-coupling constants were investigated to determine their potential as conformational constraints in MA’AT modeling of the O-glycosidic linkages of oligosaccharides. Four ( 1 J C1',H1' , 1 J C1',C2' , 2 J C1',H2' , and 2 J C2',H1' ) and eight ( 1 J C4,H4 , 1 J C3,C4 , 1 J C4,C5 , 2 J C3,H4 , 2 J C4,H3 , 2 J C5,H4 , 2 J C4,H5 , and 2 J C3,C5 ) spin-couplings in methyl β-d-galactopyranosyl-(1→4)-β-d-glucopyranoside (methyl β-lactoside) were calculated using density functional theory (DFT) to determine their dependencies on O-glycosidic linkage C–O torsion angles, $\phi$ and ψ, respectively. Long-range 4 J H1',H4 was also examined as a potential conformational constraint of either $\phi$ or ψ. Secondary effects of exocyclic (hydroxyl) C–O bond rotation within or proximal to these coupling pathways were investigated. Based on the findings of methyl β-lactoside, analogous J-couplings were studied in five additional two-bond O-glycosidic linkages [βGlcNAc-(1→4)-βMan, 2-deoxy-βGlc-(1→4)-βGlc, αMan-(1→3)-βMan, αMan-(1→2)-αMan, and βGlcNAc(1→2)-αMan] to determine whether the coupling behaviors observed in methyl β-lactoside were more broadly observed. Of the 13 nonconventional J-couplings studied, 7 exhibit properties that may be useful in future MA’AT modeling of O-glycosidic linkages, none of which involve coupling pathways that include the linkage C–O bonds. The findings also provide new insights into the general effects of exocyclic C–O bond conformation on the magnitude of experimental spin-couplings in saccharides and other hydroxyl-containing molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2003

Anharmonic Vibrational Spectroscopy of the F-(H20)n, complexes, n=1,2

We report anharmonic vibrational spectra (fundamentals, first overtones) for the F-(H(sub 2)O) and F-(H(sub 2)O)2 clusters computed at the MP2 and CCSD(T) levels of theory with basis sets of triple zeta quality. Anharmonic corrections were estimated via the correlation-corrected vibrational self-consistent field (CC-VSCF) method. The CC-VSCF anharmonic spectra obtained on the potential energy surfaces evaluated at the CCSD(T) level of theory are the first ones reported at a correlated level beyond MP2. We have found that the average basis set effect (TZP vs. aug-cc-pVTZ) is on the order of 30-40 cm(exp -1), whereas the effects of different levels of electron correlation [MP2 vs. CCSD(T)] are smaller, 20-30 cm(exp -1). However, the basis set effect is much larger in the case of the H-bonded O-H stretch of the F-(H(sub 2)O) cluster amounting to 100 cm(exp -1) for the fundamentals and 200 cm (exp -1) for the first overtones. Our calculations are in agreement with the limited available set of experimental data for the F-(H(sub 2)O) and F-(H(sub 2)O)2 systems and provide additional information that can guide further experimental studies.

Chaban, Galina M.↗
DOE OSTI2020

Laser spectroscopy of indium Rydberg atom bunches by electric field ionization

This work reports on the application of a novel electric field-ionization setup for high-resolution laser spectroscopy measurements on bunched fast atomic beams in a collinear geometry. In combination with multi-step resonant excitation to Rydberg states using pulsed lasers, the field ionization technique demonstrates increased sensitivity for isotope separation and measurement of atomic parameters over previous non-resonant laser ionization methods. The setup was tested at the Collinear Resonance Ionization Spectroscopy experiment at ISOLDE-CERN to perform high-resolution measurements of transitions in the indium atom from the 5s 2 5d 2 D 5/2 and 5s 2 5d 2 D 3/2 states to 5s 2 np 2 P and 5s 2 nf 2 F Rydberg states, up to a principal quantum number of n=72 . The extracted Rydberg level energies were used to re-evaluate the ionization potential of the indium atom to be 46,670.107(4)cm -1 . The nuclear magnetic dipole and nuclear electric quadrupole hyperfine structure constants and level isotope shifts of the 5s 2 5d 2 D 5/2 and 5s 2 5d 2 D 3/2 states were determined for 113,115 In. The results are compared to calculations using relativistic coupled-cluster theory. A good agreement is found with the ionization potential and isotope shifts, while disagreement of hyperfine structure constants indicates an increased importance of electron correlations in these excited atomic states. With the aim of further increasing the detection sensitivity for measurements on exotic isotopes, a systematic study of the field-ionization arrangement implemented in the work was performed at the same time and an improved design was simulated and is presented. The improved design offers increased background suppression independent of the distance from field ionization to ion detection.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

A tail of eternal inflation

Non-trivial inflaton self-interactions can yield calculable signatures of primordial non-Gaussianity that are measurable in cosmic surveys. We calculate the non-Gaussian corrections to Stochastic Inflation within the framework of Soft de Sitter Effective Theory, from which we derive the associated probability distribution for the scalar fluctuations. As a consequence of this new result, we show that the phase transition to slow-roll eternal inflation is often incalculable in these models. Instead, this transition is sensitive to the non-Gaussian tail of the distribution of scalar fluctuations, which probes physics inside the horizon, potentially beyond the cutoff scale of the Effective Field Theory of Inflation. We delineate the parameter space consistent with current observations and weak coupling at horizon crossing in which the large fluctuations relevant for eternal inflation can only be determined by appealing to a UV completion. We also argue that this breakdown of the perturbative description is required for the de Sitter entropy to reflect the number of de Sitter microstates.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1982

On the geometric nature of high energy nucleus-nucleus reaction cross sections

Within the context of a high energy double-folding optical potential approximation to the exact nucleus-nucleus multiple-scattering series, eikonal scattering theory is used to investigate the validity of geometric reaction cross sections in relativistic heavy ion collisions. The potential used includes a finite range interaction and nuclear single-particle densities extracted from nuclear charge distributions by unfolding the finite proton charge distribution. Pauli correlation effects are also included in an approximate way. The sensitivity of the predictions to the assumed interaction, Pauli correlation approximation, and nuclear density distributions is investigated. These results are in agreement with early predictions concerning the geometric nature of relativistic heavy ion collisions and in disagreement with a recent analysis, utilizing the zero range approximation, which suggested otherwise. Reasons for the lack of agreement between the analyses are also presented. Finally, approximate applicability limits for geometric reaction cross sections are determined.

Townsend, L. W.↗
DOE OSTI2026

Glauber-theory analysis of nuclear reactions on a 12 C target with variational Monte Carlo wave functions

The application of Glauber theory has been playing an increasingly important role with the study of unstable or exotic nuclei. Its adaptation to medium and high-energy nucleus-nucleus collisions is severely limited because one has to evaluate the matrix elements of multiple-scattering operators. The extraction of physical observables has been done using ‘approximate’ Glauber theory whose validity is hard to evaluate. Here, we perform a full calculation of the matrix elements using Monte Carlo integration and analyze the elastic differential cross sections and the total reaction cross sections for p+¹²C, ⁴,⁶He+¹²C, and ¹²C+¹²C collisions. We use the variational Monte Carlo wave functions for ⁴,⁶He and ¹²C obtained by using realistic two- and three-nucleon potentials. We demonstrate the performance of the Glauber-theory calculations by comparing with available experimental data. We further discuss the accuracy of the conventional approximate methods in the light of the cumulant expansion for Glauber’s phase-shift function.

Horiuchi, W. [Osaka Metropolitan University (Japan↗
NASA NTRS1957

On Slender-Body Theory at Transonic Speeds

The basic ideas of the slender-body approximation have been applied to the nonlinear transonic-flow equation for the velocity potential in order to obtain some of the essential features of slender-body theory at transonic speeds. The results of the investigation are presented from a unified point of view which demonstrates the similarity of slender-body solutions in the various Mach number ranges. The transonic area rule and some conditions concerning its validity follow from the analysis. (author)

Harder, Keith C↗