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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Classical interpretation of probability oscillations in low-energy atomic collisions

We perform close-coupling molecular and classical trajectory Monte Carlo (CTMC) calculations of charge-exchange probabilities in low-energy H{sup +}, He{sup 2+}+H collisions. We consider head-on collisions with a zero impact parameter and study the dependence of the charge-exchange probability on the impact velocity. This probability exhibits oscillations generally understood as signatures of quantum phase effects. CTMC calculations with dimensionality reduced to one fairly reproduce the oscillations and show that these latter stem from the back and forth motion of the electron between the nuclear centers in the internuclear region. In three dimensions, the probability oscillations are generally washed out in the statistical CTMC framework because of the electron transverse degrees of freedom. However three-dimensional Bohmian trajectories ascertain the classical interpretation of the oscillations in a quantum framework.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Excitation two-center interference and the orbital geometry in laser-induced nonsequential double ionization of diatomic molecules

We address the influence of the molecular orbital geometry and of the molecular alignment with respect to the laser-field polarization on laser-induced nonsequential double ionization of diatomic molecules for different molecular species, namely N{sub 2} and Li{sub 2}. We focus on the recollision excitation with subsequent tunneling ionization (RESI) mechanism, in which the first electron, upon return, promotes the second electron to an excited state, from where it subsequently tunnels. We assume that both electrons are initially in the highest occupied molecular orbital (HOMO) and that the second electron is excited to the lowest unoccupied molecular orbital (LUMO). We show that the electron-momentum distributions exhibit interference maxima and minima due to the electron emission at spatially separated centers. We provide generalized analytical expressions for such maxima or minima, which take into account s-p mixing and the orbital geometry. The patterns caused by the two-center interference are sharpest for vanishing alignment angle and get washed out as this parameter increases. Apart from that, there exist features due to the geometry of the LUMO, which may be observed for a wide range of alignment angles. Such features manifest themselves as the suppression of probability density in specific momentum regions due to the shape of the LUMO wave function, or as an overall decrease in the RESI yield due to the presence of nodal planes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Doppler cooling with coherent trains of laser pulses and a tunable velocity comb

We explore the possibility of decelerating and Doppler cooling an ensemble of two-level atoms by a coherent train of short, nonoverlapping laser pulses. We derive analytical expressions for mechanical force exerted by the train. In frequency space the force pattern reflects the underlying frequency comb structure. The pattern depends strongly on the ratio of the atomic lifetime to the repetition time between the pulses and pulse area. For example, in the limit of short lifetimes, the frequency-space peaks of the optical force wash out. We propose to tune the carrier-envelope offset frequency to follow the Doppler-shifted detuning as atoms decelerate; this leads to compression of atomic velocity distribution about comb teeth and results in a ''velocity comb''--a series of narrow equidistant peaks in the velocity space.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Superfluid density and Berezinskii-Kosterlitz-Thouless transition of a spin-orbit-coupled Fulde-Ferrell superfluid

We theoretically investigate the superfluid density and Berezinskii-Kosterlitz-Thouless (BKT) transition of a two-dimensional Rashba spin-orbit-coupled atomic Fermi gas with both in-plane and out-of-plane Zeeman fields. It was recently predicted that, by tuning the two Zeeman fields, the system may exhibit different exotic Fulde-Ferrell (FF) superfluid phases, including the gapped FF, gapless FF, gapless topological FF, and gapped topological FF states. Due to the FF paring, we show that the superfluid density (tensor) of the system becomes anisotropic. When an in-plane Zeeman field is applied along the x direction, the tensor component along the y direction n s ,yy is generally larger than n s ,xx in most parameter space. At zero temperature, there is always a discontinuity jump in n s ,xx as the system evolves from a gapped FF into a gapless FF state. With increasing temperature, such a jump is gradually washed out. The critical BKT temperature has been calculated as functions of the spin-orbit-coupling strength, interatomic interaction strength, and in-plane and out-of-plane Zeeman fields. We predict that the novel FF superfluid phases have a significant critical BKT temperature, typically at the order of 0.1T F , where T F is the Fermi degenerate temperature. Furthermore, their observation is within the reach of current experimental techniques in cold-atom laboratories.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Microbunch rotation in an x-ray free-electron laser using a first-order achromatic bend

Electrons in an x-ray free electron laser (XFEL) develop periodic density fluctuations, known as microbunches, which enable the exponential gain of x-ray power in an XFEL. When an electron beam microbunched at a hard x-ray wavelength is kicked, microbunches are often washed out due to the dispersion and R 56 of the bend. An achromatic (dispersion-free) bend with a small R 56 , however, can preserve microbunches, which rotate to follow the new trajectory of the electron bunch. Rotated microbunches can subsequently interact in a repointed undulator to produce a new beam of off-axis x rays. In this work, we demonstrate hard x-ray multiplexing in the Linac Coherent Light Source hard x-ray undulator line using microbunch rotation through a 10 μ rad first-order-achromatic bend created by transversely offsetting quadrupole magnets in the FODO lattice. Quadrupole offsets are determined analytically from beam-matrix theory. We also discuss the application of microbunch rotation to out-coupling a cavity-based XFEL. Published by the American Physical Society 2024

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Effect of high-{kappa} gate dielectrics on charge transport in graphene-based field effect transistors

The effect of various dielectrics on charge mobility in single-layer graphene is investigated. By calculating the remote optical phonon scattering arising from the polar substrates, and combining it with their effect on Coulombic impurity scattering, a comprehensive picture of the effect of dielectrics on charge transport in graphene emerges. It is found that though high-{kappa} dielectrics can strongly reduce Coulombic scattering by dielectric screening, scattering from surface phonon modes arising from them wash out this advantage. Calculation shows that within the available choice of dielectrics, there is not much room for improving carrier mobility in actual devices at room temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Charge transport in DNA nanowires connected to carbon nanotubes

DNA is perhaps the worlds most controllable nanowire, with potential applications in nanoelectronics and sensing. However, understanding of its charge transport (CT) properties remains elusive, with experiments reporting a wide range of behaviors from insulating to superconductive. Here, we report extensive first-principle simulations that account for DNA's high flexibility and its native solvent environment. The results show that the CT along the DNA's long axis is strongly dependent on DNA's instantaneous conformation varying over many orders of magnitude. In high CT conformations, delocalized conductive states extending over up to 10 base pairs are found. Their low exponential decay constants further indicate that coherent CT, which is assumed to be active only over 2–3 base pairs in the commonly accepted DNA CT models, can act over much longer length scales. We also identify a simple geometrical rule that predicts CT properties of a given conformation with high accuracy. The effect of mismatched base pairs is also considered: while they decrease conductivities of specific DNA conformations, thermally induced conformational fluctuations wash out this effect. Overall, our results indicate that an immobilized partially dried poly(G)-poly(C) B-DNA is preferable for nanowire applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Dispersion, damping, and intensity of spin excitations in the monolayer ( Bi,Pb ) 2 ( Sr,La ) 2 CuO 6 + δ cuprate superconductor family

Using Cu-${L}_{3}$ edge resonant inelastic x-ray scattering (RIXS) we measured here the dispersion and damping of spin excitations (magnons and paramagnons) in the high-${T}_{\mathrm{c}}$ superconductor ${(\text{Bi,Pb})}_{2}{(\text{Sr,La})}_{2}{\mathrm{CuO}}_{6+{\delta}}$ (Bi2201), for a large doping range across the phase diagram $(0.03{\lesssim}p{\lesssim}0.21)$. Selected measurements with full polarization analysis unambiguously demonstrate the spin-flip character of these excitations, even in the overdoped sample. We find that the undamped frequencies increase slightly with doping for all accessible momenta, while the damping grows rapidly, faster in the $(0,0){\rightarrow}(0.5,0.5)$ nodal direction than in the $(0,0){\rightarrow}(0.5,0)$ antinodal direction. We compare the experimental results to numerically exact determinant quantum Monte Carlo (DQMC) calculations that provide the spin dynamical structure factor $S(\mathbf{Q},{\omega})$ of the three-band Hubbard model. The theory reproduces well the momentum and doping dependence of the dispersions and spectral weights of magnetic excitations. These results provide compelling evidence that paramagnons, although increasingly damped, persist across the superconducting dome of the cuprate phase diagram; this implies that long-range antiferromagnetic correlations are quickly washed away, while short-range magnetic interactions are little affected by doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Equation of state and phase fluctuations near the chiral critical point

The thermodynamics and critical exponents and amplitudes of high temperature and dense matter near the chiral critical point are studied. The parameterized equation of state matches that calculated with lattice QCD at zero chemical potential and to the known properties of nuclear matter at zero temperature. The extent to which finite size effects wash out the phase separation near the critical point is determined.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Finite temperature quark matter under strong magnetic fields

In this paper, we use the mean-field approximation to investigate quark matter described by both SU(2) and SU(3) versions of the Nambu-Jona-Lasinio model at temperatures below 150 MeV and subject to a strong magnetic field. This kind of matter is possibly present in the early stages of heavy-ion collisions and in the interior of protoneutron stars. We have studied symmetric and asymmetric quark matter. The effect of the magnetic field on the effective quark masses and chemical potentials is only felt for quite strong magnetic fields, above 5x10{sup 18} G, with larger effects for the lower densities. Spin polarizations are more sensitive to weaker magnetic fields and are larger for lower temperatures and lower densities. Temperature tends to wash out the magnetic field effects.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Dynamical model of surrogate reactions

A new dynamical model is developed to describe the whole process of surrogate reactions: Transfer of several nucleons at an initial stage, thermal equilibration of residues leading to washing out of shell effects, and decay of populated compound nuclei are treated in a unified framework. Multidimensional Langevin equations are employed to describe time evolution of collective coordinates with a time-dependent potential energy surface corresponding to different stages of surrogate reactions. The new model is capable of calculating spin distributions of the compound nuclei, one of the most important quantities in the surrogate technique. Furthermore, various observables of surrogate reactions can be calculated, for example, energy and angular distribution of ejectile and mass distributions of fission fragments. These features are important to assess validity of the proposed model itself, to understand mechanisms of the surrogate reactions, and to determine unknown parameters of the model. It is found that spin distributions of compound nuclei produced in {sup 18}O+{sup 238}U{yields}{sup 16}O+{sup 240}*U and {sup 18}O+{sup 236}U{yields}{sup 16}O+{sup 238}*U reactions are equivalent and much less than 10({h_bar}/2{pi}) and therefore satisfy conditions proposed by Chiba and Iwamoto [Phys. Rev. C 81, 044604 (2010)] if they are used as a pair in the surrogate ratio method.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Axion dark matter detection with CMB polarization

Herein we point out two ways to search for low-mass axion dark matter using cosmic microwave background (CMB) polarization measurements. These appear, in particular, to be some of the most promising ways to directly detect fuzzy darkmatter. Axion dark matter causes rotation of the polarization of light passing through it. This gives rise to two novel phenomena in the CMB. First, the late-time oscillations of the axion field today cause theCMB polarization to oscillate in phase across the entire sky. Second, the early-time oscillations of the axion field wash out the polarization produced at last scattering, reducing the polarized fraction (TE and EE power spectra) compared to the standard prediction. Since the axion field is oscillating, the common (static) “cosmic birefringence” search is not appropriate for axion dark matter. These two phenomena can be used to search for axion dark matter at the lighter end of the mass range, with a reach several orders of magnitude beyond current constraints. We set a limit from the washout effect using existing Planck results, and find significant future discovery potential for CMB detectors searching in particular for the oscillating effect.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Tunneling away the relic neutrino asymmetry

The Earth acts as a matter potential for relic neutrinos which modifies their index of refraction from vacuum by δ ∼ 10 − 8 . It has been argued that the refractive effects from this potential should lead to a large O ( δ ) neutrino-antineutrino asymmetry at the surface of the Earth. This result was computed by treating the Earth as flat. In this work, we revisit this calculation in the context of a perfectly spherical Earth. We demonstrate, both numerically and through analytic arguments, that the flat-Earth result is only recovered under the condition δ 3 / 2 k R ≫ 1 , where k is the typical momentum of the relic neutrinos and R is the radius of the Earth. This condition is required to prevent antineutrinos from tunneling into classically inaccessible trajectories below the Earth’s surface and washing away the large asymmetry. As the physical parameters of the Earth do not satisfy this condition, we find that the asymmetry at the surface should only be O ( δ ) . While the asphericity of the Earth may serve as a loophole to our conclusions, we argue that it is still difficult to generate a large asymmetry even in the presence of local terrain. Published by the American Physical Society 2024

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

On the 1/c expansion of f(R) gravity

We derive for applications to isolated systems--on the scale of the Solar System--the first relativistic terms in the 1/c expansion of the space time metric g{sub {mu}{nu}}for metric f(R) gravity theories, where f is assumed to be analytic at R=0. For our purpose it suffices to take into account up to quadratic terms in the expansion of f(R), thus we can approximate f(R)=R+aR{sup 2} with a positive dimensional parameter a. In the nonrelativistic limit, we get an additional Yukawa correction with coupling strength G/3 and Compton wave length {radical}(6a) to the Newtonian potential, which is a known result in the literature. As an application, we derive to the same order the correction to the geodetic precession of a gyroscope in a gravitational field and the precession of binary pulsars. The result of the Gravity Probe B experiment yields the limit a < or approx. 5x10{sup 11} m{sup 2}, whereas for the pulsar B in the PSR J0737-3039 system we get a bound which is about 10{sup 4} times larger. On the other hand the Eoet-Wash experiment provides the best laboratory bound a < or approx. 10{sup -10} m{sup 2}. Although the former bounds from geodesic precession are much larger than the laboratory ones, they are still meaningful in the case some type of chameleon effect is present and thus the effective values could be different at different length scales.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Oscillation of neutrinos produced by the annihilation of dark matter inside the Sun

The annihilation of dark matter particles captured by the Sun can lead to a neutrino flux observable in neutrino detectors. Considering the fact that these dark matter particles are nonrelativistic, if a pair of dark matter annihilates to a neutrino pair, the spectrum of neutrinos will be monochromatic. We show that in this case, even after averaging over the production point inside the Sun, the oscillatory terms of the oscillation probability do not average to zero. This leads to interesting observable features in the annual variation of the number of muon track events. We show that smearing of the spectrum due to thermal distribution of dark matter inside the Sun is too small to wash out this variation. We point out the possibility of studying the initial flavor composition of neutrinos produced by the annihilation of dark matter particles via measuring the annual variation of the number of {mu}-track events in neutrino telescopes.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Holography and the Coleman-Mermin-Wagner theorem

In 2+1 dimensions at finite temperature, spontaneous symmetry breaking of global symmetries is precluded by large thermal fluctuations of the order parameter. The holographic correspondence implies that analogous effects must also occur in 3+1 dimensional theories with gauged symmetries in certain curved spacetimes with horizon. By performing a one loop computation in the background of a holographic superconductor, we show that bulk quantum fluctuations wash out the classical order parameter at sufficiently large distance scales. The low temperature phase is seen to exhibit algebraic long-range order. Beyond the specific example we study, holography suggests that IR singular quantum fluctuations of the fields and geometry will play an interesting role for many 3+1 dimensional asymptotically anti-de Sitter spacetimes with planar horizon.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Chameleons with field-dependent couplings

Certain scalar-tensor theories exhibit the so-called chameleon mechanism, whereby observational signatures of scalar fields are hidden by a combination of self-interactions and interactions with ambient matter. Not all scalar-tensor theories exhibit such a chameleon mechanism, which has been originally found in models with inverse power runaway potentials and field-independent couplings to matter. In this paper we investigate field theories with field-dependent couplings and a power-law potential for the scalar field. We show that the theory indeed is a chameleon field theory. We find the thin-shell solution for a spherical body and investigate the consequences for Eoet-Wash experiments, fifth-force searches and Casimir-force experiments. Requiring that the scalar field evades gravitational tests, we find that the coupling is sensitive to a mass scale which is of order of the Hubble scale today.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗