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At least 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Hexane Cracking on ZSM-5 and Faujasite Zeolites: A QM/MM/QCT Study

Quantum mechanics/molecular mechanics (QM/MM) models are applied to investigate the adsorption and cracking of n -hexane on ZSM-5 and Faujasite zeolite structures. These simulations account for the long-range electrostatic and midrange van-der-Waals interactions in the zeolite and provide energy barriers that are close to experimental data. The active acidic site was modeled by dispersion corrected density functional theory (DFT, ω B97X-D6-311/G*). The long-range interactions were calculated by molecular mechanics (MM). The adsorbed molecules under investigation are characterized by their thermodynamic properties (adsorption energy and enthalpy). The influence of the zeolite type on the thermodynamic properties is also pointed out. The results reveal that the kinetics of cracking is insensitive to differences in acid strengths. The thermodynamic data obtained are mainly influenced by the adsorption energy of n -hexane on ZSM-5 and/or Faujasite (Y) structures. Here, the pore sizes of the zeolite types can lead to a stronger or weaker adsorption energy. Except for the thermodynamic property investigations in this article, the quasi-classical trajectory method (QCT) is used for investigating the pathways along metastable intermediates toward various cracking products. Not only the reaction barriers but also the reaction dynamics determine the reaction selectivity.

Tranca, D. C.↗
DOE OSTI2025

Two-Dimensional Binary Superlattice of BNNT-Surfactant Vesicle Complex Induced by Electrostatic Interaction

For a wide range of practical applications of boron nitride nanotubes (BNNTs), it is essential to achieve their highly ordered self-assembled structures. This study reports on a two-dimensional (2D) binary superlattice of individually exfoliated BNNTs with a negative surface charge (p-BNNT25) and cationic surfactant vesicles (CTAT/SDBS vesicles, prepared by mixing cetyltrimethylammonium tosylate (CTAT) and sodium dodecylbenzenesulfonate (SDBS)) complexes through electrostatic interactions. Depending on the surface charge density of the CTAT/SDBS vesicles and the mass ratio between the CTAT/SDBS vesicle and p-BNNT25, the CTAT/SDBS-BNNT complexes formed highly ordered superstructures. These structures include an intercalated lamellar phase with a centered rectangular structure (ICLP), in which a 2D array of p-BNNT25 is inserted into the multilamellar structure, and an AB 3 structure, in which the BNNTs are surrounded by surfactant micelles in a triangular arrangement. To the best of our knowledge, this is the first demonstration of the fabrication of highly ordered superstructures of individually exfoliated and negatively charged BNNTs with positively charged surfactant vesicles through electrostatic interactions. This approach for the 2D binary superlattices of CTAT/SDBS-BNNT complexes induced by electrostatic interactions is expected to be beneficial for a wide range of one-dimensional (1D) nanoparticle applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Importance of van der Waals effects on the hydration of metal ions from the Hofmeister series

The van der Waals (vdW) interaction plays a crucial role in the description of liquid water. Based on ab initio molecular dynamics simulations, including the non-local and fully self-consistent density-dependent implementation of the Tkatchenko-Scheffler dispersion correction, we systematically studied the aqueous solutions of metal ions (K + , Na + , and Ca 2+ ) from the Hofmeister series. Similar to liquid water, the vdW interactions strengthen the attractions among water molecules in the long-range, leading to the hydrogen bond networks softened in all the ion solutions. However, the degree that the hydration structure is revised by the vdW interactions is distinct for different ions, depending on the strength of short-range interactions between the hydrated ion and surrounding water molecules. Such revisions by the vdW interactions are important for the understanding of biological functionalities of ion channels.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1985

Sputtered W-N diffusion barriers

The thermal stability of reactively sputtered tungsten-nitrogen alloy thin films is investigated for the application as diffusion barriers in silicon contact metallizations. The composition of W-N barriers is varied over a wide range including pure W. Aluminum, gold, and silver are used as low resistivity overlayers. Metallurgical interactions at temperatures ranging from 500 to 900 C are studied. Incorporating nitrogen into tungsten advantageously stabilizes all three systems. The overall failure takes place rapidly above critical temperatures that depend on both the metal overlayer and the microstructure of the barrier. In some cases, W-N alloys can effectively prevent interdiffusion at temperatures as high as 800 C for 30 min.

Kattelus, H. P.↗
DOE OSTI2011

pp{yields}p{Lambda}K{sup +} reaction in search for the K{sup -}pp state - quest for a kaonic nuclei

The dibaryonic kaonic nuclear bound state, K{sup -}pp is searched by studying an exclusive p+p{yields}p+{Lambda}+K{sup +} process at several beam energies. A signature of the K{sup -}pp is explored in a p+p{yields}X({identical_to}K{sup -}pp)+K{sup +} reaction that follows a decay of the X into p+{Lambda}. We found in a missing-mass {Delta}M(K{sup +}) spectrum and a {Lambda}p invariant-mass M({Lambda}p) spectrum of DISTO data at 2.85 GeV a resonance with M = 2267 MeV/c{sup 2} and {Gamma} = 118 MeV. Those events are found to be associated with a mono energetic kaon. We investigate this resonance as a candidate of the K{sup -}pp further also with a different beam energies.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Dependence of dark matter - electron scattering on the galactic dark matter velocity distribution

The rate of dark matter-electron scattering depends on the underlying velocity distribution of the dark matter halo. Importantly, dark matter-electron scattering is particularly sensitive to the high-velocity tail, which differs significantly amongst the various dark matter halo models. In this work, we summarize the leading halo models and discuss the various parameters which enter them. We recommend updated values for these parameters based on recent studies and measurements. Furthermore, we quantify the dependence of the dark matter-electron scattering rate on the choice of halo model and parameters, and demonstrate how these choices propagate to the predicted cross-section limits. The rate is most sensitive to changes in the circular velocity v 0 ; in silicon targets, we find that the changes in the rate predictions can range from Script $\mathcal{O}$(0.01%) to Script $\mathcal{O}$(100%) for contact interactions and Script $\mathcal{O}$(10%) to Script $\mathcal{O}$(100%) for long-range interactions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Beam-beam simulation code BBSIM for particle accelerators

A highly efficient, fully parallelized, six-dimensional tracking model for simulating interactions of colliding hadron beams in high energy ring colliders and simulating schemes for mitigating their effects is described. The model uses the weak-strong approximation for calculating the head-on interactions when the test beam has lower intensity than the other beam, a look-up table for the efficient calculation of long-range beam-beam forces, and a self-consistent Poisson solver when both beams have comparable intensities. A performance test of the model in a parallel environment is presented. The code is used to calculate beam emittance and beam loss in the Tevatron at Fermilab and compared with measurements. They also present results from the studies of stwo schemes proposed to compensate the beam-beam interactions: (a) the compensation of long-range interactions in the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and the Large Hadron Collider (LHC) at CERN with a current carrying wire, (b) the use of a low energy electron beam to compensate the head-on interactions in RHIC.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Range extender mediates long-distance enhancer activity

Although most mammalian transcriptional enhancers regulate their cognate promoters over distances of tens of kilobases, some enhancers act over distances in the megabase range1. The sequence features that enable such long-distance enhancer–promoter interactions remain unclear. Here we used in vivo enhancer-replacement experiments at the mouse Shh locus to show that short- and medium-range limb enhancers cannot initiate gene expression at long-distance range. We identify a cis-acting element, range extender (REX), that confers long-distance regulatory activity and is located next to a long-range limb enhancer of Sall1. The REX element has no endogenous enhancer activity. However, addition of the REX to other short- and mid-range limb enhancers substantially increases their genomic interaction range. In the most extreme example observed, addition of REX increased the range of an enhancer by an order of magnitude from its native 73 kb to 848 kb. The REX element contains highly conserved [C/T]AATTA homeodomain motifs that are critical for its activity. These motifs are enriched in long-range limb enhancers genome-wide, including the ZRS (zone of polarizing activity (ZPA) regulatory sequence), a benchmark long-range limb enhancer of Shh2. The ZRS enhancer with mutated [C/T]AATTA motifs maintains limb activity at short range, but loses its long-range activity, resulting in severe limb reduction in knock-in mice. In summary, we identify a sequence signature associated with long-range enhancer–promoter interactions and describe a prototypical REX element that is necessary and sufficient to confer long-distance activation by remote enhancers.

Bower, Grace↗
DOE OSTI2015

Basis convergence of range-separated density-functional theory

Range-separated density-functional theory (DFT) is an alternative approach to Kohn-Sham density-functional theory. The strategy of range-separated density-functional theory consists in separating the Coulomb electron-electron interaction into long-range and short-range components and treating the long-range part by an explicit many-body wave-function method and the short-range part by a density-functional approximation. Among the advantages of using many-body methods for the long-range part of the electron-electron interaction is that they are much less sensitive to the one-electron atomic basis compared to the case of the standard Coulomb interaction. Here, we provide a detailed study of the basis convergence of range-separated density-functional theory. We study the convergence of the partial-wave expansion of the long-range wave function near the electron-electron coalescence. We show that the rate of convergence is exponential with respect to the maximal angular momentum L for the long-range wave function, whereas it is polynomial for the case of the Coulomb interaction. We also study the convergence of the long-range second-order Møller-Plesset correlation energy of four systems (He, Ne, N{sub 2}, and H{sub 2}O) with cardinal number X of the Dunning basis sets cc − p(C)V XZ and find that the error in the correlation energy is best fitted by an exponential in X. This leads us to propose a three-point complete-basis-set extrapolation scheme for range-separated density-functional theory based on an exponential formula.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Range-separated double-hybrid density-functional theory applied to periodic systems

Quantum chemistry methods exploiting density-functional approximations for short-range electron-electron interactions and second-order Møller-Plesset (MP2) perturbation theory for long-range electron-electron interactions have been implemented for periodic systems using Gaussian-type basis functions and the local correlation framework. The performance of these range-separated double hybrids has been benchmarked on a significant set of systems including rare-gas, molecular, ionic, and covalent crystals. The use of spin-component-scaled MP2 for the long-range part has been tested as well. The results show that the value of μ = 0.5 bohr{sup −1} for the range-separation parameter usually used for molecular systems is also a reasonable choice for solids. Overall, these range-separated double hybrids provide a good accuracy for binding energies using basis sets of moderate sizes such as cc-pVDZ and aug-cc-pVDZ.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Intersystem crossing in the exit channel

We report that intersystem crossing (ISC), plays an important role in photochemistry. It is understood to be efficient when heavy atoms are present due to strong spin–orbit coupling, or when strongly bound long-lived complexes are formed that increase the chance of finding the singlet–triplet intersection seam. Here we present evidence for a different ISC mechanism in the bimolecular reaction of O( 3 P) with alkylamines. In crossed-beam experiments, product velocity-flux maps are measured for aminoalkyl radicals produced from H-abstraction from the methyl group, which also gives OH radicals as co-fragments. The low translational-energy release and isotropic angular distributions of the products indicate that such reactions undergo the formation of a complex before OH and aminoalkyl are produced. However, there is no well on the triplet potential-energy-surface that could support such a complex. Multireference ab initio calculations suggest, instead, that ISC occurs in the exit-channel region due to the long-range dipole–dipole interaction between the nascent radical product pair coupled with the vanishing singlet–triplet splitting at long range. Finally, ISC then leads to a deep hydroxylamine well before OH elimination.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Assembly of three-dimensional binary superlattices from multi-flavored particles

Binary superlattices constructed from nano- or micron-sized colloidal particles have a wide variety of applications, including the design of advanced materials. Self-assembly of such crystals from their constituent colloids can be achieved in practice by, among other means, the functionalization of colloid surfaces with single-stranded DNA sequences. However, when driven by DNA, this assembly is traditionally premised on the pairwise interaction between a single DNA sequence and its complement, and often relies on particle size asymmetry to entropically control the crystalline arrangement of its constituents. The recently proposed “multi-flavoring” motif for DNA functionalization, wherein multiple distinct strands of DNA are grafted in different ratios to different colloids, can be used to experimentally realize a binary mixture in which all pairwise interactions are independently controllable. Here we use various computational methods, including molecular dynamics and Wang–Landau Monte Carlo simulations, to study a multi-flavored binary system of micron-sized DNA-functionalized particles modeled implicitly by Fermi–Jagla pairwise interactions. We show how self-assembly of such systems can be controlled in a purely enthalpic manner, and by tuning only the interactions between like particles, demonstrate assembly into various morphologies. Although polymorphism is present over a wide range of pairwise interaction strengths, we show that careful selection of interactions can lead to the generation of pure compositionally ordered crystals. Additionally, we show how the crystal composition changes with the like-pair interaction strengths, and how the solution stoichiometry affects the assembled structures.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Coupling strength in Coulomb and Yukawa one-component plasmas

In a non-ideal classical Coulomb one-component plasma (OCP), all thermodynamic properties are known to depend only on a single parameter—the coupling parameter Γ. In contrast, if the pair interaction is screened by background charges (Yukawa OCP) the thermodynamic state depends, in addition, on the range of the interaction via the screening parameter κ. How to determine in this case an effective coupling parameter has been a matter of intensive debate. Here we propose a consistent approach for defining and measuring the coupling strength in Coulomb and Yukawa OCPs based on a fundamental structural quantity, the radial pair distribution function (RPDF). The RPDF is often accessible in experiments by direct observation or indirectly through the static structure factor. Alternatively, it is directly computed in theoretical models or simulations. Our approach is based on the observation that the build-up of correlation from a weakly coupled system proceeds in two steps: First, a monotonically increasing volume around each particle becomes devoid of other particles (correlation hole), and second (upon further increase of the coupling), a shell structure emerges around each particle giving rise to growing peaks of the RPDF. Using molecular dynamics simulation, we present a systematic study for the dependence of these features of the RPDF on Γ and κ and derive a simple expression for the effective coupling parameter.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1988

Nitrogen uptake by wheat seedlings, interactive effects of four nitrogen sources: NO3-, NO2-, NH4+, and urea

The net influx (uptake) rates of NO3-, NH4+, NO2-, and urea into roots of wheat (Triticum aestivum cv Yecora Rojo) seedlings from complete nutrient solutions containing all four compounds were monitored simultaneously. Although urea uptake was too slow to monitor, its presence had major inhibitory effects on the uptake of each of the other compounds. Rates of NO3-, NH4+, and NO2- uptake depended in a complex fashion on the concentration of all four N compounds. Equations were developed which describe the uptake rates of each of the compounds, and of total N, as functions of concentrations of all N sources. Contour plots of the results show the interactions over the range of concentrations employed. The coefficients of these equations provide quantitative values for evaluating primary and interactive effects of each compound on N uptake.

NASA Discipline Number 61-10↗
DOE OSTI2020

Frustration and Atomic Ordering in a Monolayer Semiconductor Alloy

Frustrated interactions can lead to short-range ordering arising from incompatible interactions of fundamental physical quantities with the underlying lattice. The simplest example is the triangular lattice of spins with antiferromagnetic interactions, where the nearest-neighbor spin-spin interactions cannot simultaneously be energy minimized. In this work, we show that engineering frustrated interactions is a possible route for controlling structural and electronic phenomena in semiconductor alloys. Using aberration-corrected scanning transmission electron microscopy in conjunction with density functional theory calculations, we demonstrate atomic ordering in a two-dimensional semiconductor alloy as a result of the competition between geometrical constraints and nearest-neighbor interactions. Statistical analyses uncover the presence of short-range ordering in the lattice. In addition, we show how the induced ordering can be used as another degree of freedom to considerably modify the band gap of monolayer semiconductor alloys.

2-dimensional systems↗
DOE OSTI2025

Nanomechanical squeezing: Toward quantum imaging with atomic force microscopy

Leveraging quantum effects in mechanical oscillators promises major advancements in nanometrology, including sensing and imaging. Microscopic cantilevers serve as versatile force sensors with broad applications in nanoscience, nanotechnology, and, increasingly, quantum sensing. Here, we theoretically model and investigate the quantum mechanics of cantilever probes in atomic force microscopy under conditions of reduced thermal noise. Specifically, our model captures the cantilever interaction with a surface via long-range attractive van der Waals forces and short-range repulsive-adhesive interactions described by the Derjaguin-Muller-Toporov model. We find that when the probe resides within the attractive interaction regime, a coherent state emerges, whereas, in the repulsive regime, a squeezed state forms for the cantilever's deformation state. Our calculations explain the functional role of interaction potentials in the quantum dynamics of macroscopic mechanical systems, which are proving useful in quantum sensing and information processing. The presented calculations can be extended to investigate other interaction forces relevant to atomic force microscopy.

Al-Sawai, Wael M. [Lamar Univ., Beaumont, TX (Unit↗
DOE OSTI2010

Dipolar Fermi gases in anisotropic traps

The quest for quantum degenerate Fermi gases interacting through the anisotropic and long-range dipole-dipole interaction is an exciting and fast developing branch within the cold-atom research program. Recent experimental progress in trapping, cooling, and controlling polar molecules with large electric dipole moments has, therefore, motivated much theoretical effort. In a recent letter, we have briefly discussed the application of a variational time-dependent Hartree-Fock approach to study theoretically both the static and the dynamic properties of such a system in a cylinder-symmetric harmonic trap. We focused on the hydrodynamic regime, where collisions ensure the equilibrium locally. Here we present a detailed theory extended to encompass the general case of a harmonic trap geometry without any symmetry. After deriving the equations of motion for the gas, we explore their static solutions to investigate key properties like the aspect ratios in both real and momentum space as well as the stability diagram. We find that, despite the lack of symmetry of the trap, the momentum distribution remains cylinder symmetric. The equations of motion are then used to study the low-lying hydrodynamic excitations, where, apart from the quadrupole and monopole modes, the radial quadrupole mode is also investigated. Furthermore, we study the time-of-flight dynamics as it represents an important diagnostic tool for quantum gases. We find that the real-space aspect ratios are inverted during the expansion, while that in momentum space becomes asymptotically unity. In addition, anisotropic features of the dipole-dipole interaction are discussed in detail. These results could be particularly useful for future investigations of strongly dipolar heteronuclear polar molecules deep in the quantum degenerate regime.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS1975

High resolution Cerenkov and range detectors for balloon-borne cosmic-ray experiment

A combination of an active Cerenkov detector and passive range detectors is proposed for the high resolution measurement of isotopic composition in the neighborhood of iron in the galactic cosmic rays. A large area (4,300 sq cm) Cerenkov counter and passive range detectors were tested. Tests with heavy ions (2.1 GeV/amu C-12, 289 MeV/amu Ar-40, and 594 MeV/amu Ne-20) revealed the spatial uniformity of response of the Cerenkov counter to be better than 1% peak-to-peak. Light collection efficiency is independent of projectile energy and incidence angle to within at least 0.5%. Passive Lexan track recorders to measure range in the presence of the nuclear interaction background which results from stopping particles through 0.9 interaction lengths of matter were also tested. It was found that nuclear interactions produce an effective range straggling distribution only approximately 75% wider than that expected from range straggling alone. The combination of these tested techniques makes possible high mass resolution in the neighborhood of iron.

Ahlen, S. P.↗