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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Predictions for axion couplings from ALP cogenesis

Adding an axion-like particle (ALP) to the Standard Model, with a field velocity in the early universe, simultaneously explains the observed baryon and dark matter densities. This requires one or more couplings between the ALP and photons, nucleons, and/or electrons that are predicted as functions of the ALP mass. These predictions arise because the ratio of dark matter to baryon densities is independent of the ALP field velocity, allowing a correlation between the ALP mass, ma, and decay constant, f a . The predicted couplings are orders of magnitude larger than those for the QCD axion and for dark matter from the conventional ALP misalignment mechanism. As a result, this scheme, ALP cogenesis, is within reach of future experimental ALP searches from the lab and stellar objects, and for dark matter.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1986

On the fine-structure constant in a plasma model of the fluctuating vacuum substratum

The existence of an intimate connection between the quivering motion of electrons and positrons (Zitterbewegung), predicted by the Dirac equation, and the zero-point fluctuations of the vacuum is suggested. The nature of the proposed connection is discussed quantitatively, and an approximate self-consistency relation is derived, supplying a purely mathematical expression that relates the dimensionless coupling strengths (fine-structure constants) alpha sub e and alpha sub g of electromagnetism and gravity. These considerations provide a tentative explanation for the heretofore puzzling number 1/alpha sub e of about 137.036 and suggest that attempts to unify gravity with the electroweak and strong interactions will ultimately prove successful.

Cragin, B. L.↗
DOE OSTI2021

In-plane plasmon coupling in topological insulator Bi 2 Se 3 thin films

The surface states of the 3D topological insulator (TI), Bi 2 Se 3 , are known to host two-dimensional Dirac plasmon polaritons (DPPs) in the terahertz spectral range. In TI thin films, the DPPs excited on the top and bottom surfaces couple, leading to an acoustic mode and an optical plasmon mode. Vertical coupling in these materials is, therefore, reasonably well-understood, but in-plane coupling among localized TI DPPs has yet to be investigated. In this paper, we demonstrate in-plane DPP coupling in TI stripe arrays and show that they exhibit dipole–dipole type coupling. Here, the coupling becomes negligible when the lattice constant is greater than approximately 2.8 times the stripe width, which is comparable to results for in-plane coupling of localized plasmons excited on metallic nanoparticles or graphene plasmon polaritons. This understanding could be leveraged for the creation of TI-based metasurfaces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Quantum Phase Engineering of Two-Dimensional Post-Transition Metals by Substrates: Toward a Room-Temperature Quantum Anomalous Hall Insulator

In this work, we propose a new strategy to engineer topological and magnetic properties of two-dimensional (2D) hexagonal lattices consisting of post-transition metals. Our first-principles calculations demonstrate that substrates serve as templates to form 2D lattices with high thermodynamic stability, where their topological properties as well as magnetic properties sensitively change as a function of lattice constants, i.e., the system undergoes a first-order phase transition from nonmagnetic to ferromagnetic state above a critical lattice constant. Consequently, substrates can be used to explore versatile magnetic, electronic, and quantum topological properties. We establish phase diagrams of versatile quantum phases in terms of exchange coupling and spin–orbit coupling effectively tuned by the lattice constants. We further reveal the first room-temperature quantum anomalous Hall (QAH) effect, i.e., Sn on 2√3 × 2√3 graphane is a QAH insulator with a large spin–orbit coupling gap of ~0.2 eV and a Curie temperature of ~380 K by using the 2D anisotropic Heisenberg model.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Kinetic Analysis of Proton-Coupled Electron Transfer at an Electrode-Immobilized Complex

An alkyne-terminated cobalt complex, [Co(Cp)(dppe ≡H )(Cl)] + (Cp = cyclopentadienyl; dppe ≡H = 1,2-bis-(di-(4-ethynyl-phenyl)phosphino)ethane), (Co ≡H ) was immobilized onto a glassy carbon electrode using two attachment strategies: Cu(I) catalyzed azide–alkyne click chemistry and reductive electropolymerization. The modified electrodes prepared through reductive electropolymerization exhibit current densities and peak resolutions for the electrochemical reduction of the cobalt species, which are amenable to electroanalytical quantification of coupled chemical reactions. Through peak shift analysis of cyclic voltammograms recorded in the presence of 4-chloroanilinium tetrafluoroborate, we quantified the proton transfer rate constant for the stepwise proton-coupled electron transfer reaction that reduces the electrode-immobilized [Co(Cp)(dppe ≡H )(Cl)] + to [H–Co(Cp)(dppe ≡H )] + (k PT app = (9.3 ± 1.8) × 10 5 M –1 s –1 ). In conclusion, the extraction of kinetic parameters for an elementary proton-coupled electron transfer reaction of an electrode-immobilized complex represents the first experimental measurement of its type and lays crucial groundwork for kinetic analyses of hybrid catalyst–electrode architectures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1999

Directional abnormalities of vestibular and optokinetic responses in cerebellar disease

Directional abnormalities of vestibular and optokinetic responses in patients with cerebellar degeneration are reported. Three-axis magnetic search-coil recordings of the eye and head were performed in eight cerebellar patients. Among these patients, examples of directional cross-coupling were found during (1) high-frequency, high-acceleration head thrusts; (2) constant-velocity chair rotations with the head fixed; (3) constant-velocity optokinetic stimulation; and (4) following repetitive head shaking. Cross-coupling during horizontal head thrusts consisted of an inappropriate upward eye-velocity component. In some patients, sustained constant-velocity yaw-axis chair rotations produced a mixed horizontal-torsional nystagmus and/or an increase in the baseline vertical slow-phase velocity. Following horizontal head shaking, some patients showed an increase in the slow-phase velocity of their downbeat nystagmus. These various forms of cross-coupling did not necessarily occur to the same degree in a given patient; this suggests that different mechanisms may be responsible. It is suggested that cross-coupling during head thrusts may reflect a loss of calibration of brainstem connections involved in the direct vestibular pathways, perhaps due to dysfunction of the flocculus. Cross-coupling during constant-velocity rotations and following head shaking may result from a misorientation of the angular eye-velocity vector in the velocity-storage system. Finally, responses to horizontal optokinetic stimulation included an inappropriate torsional component in some patients. This suggests that the underlying organization of horizontal optokinetic tracking is in labyrinthine coordinates. The findings are also consistent with prior animal-lesion studies that have shown a role for the vestibulocerebellum in the control of the direction of the VOR.

NASA Discipline Neuroscience↗
DOE OSTI2022

How to calculate the rate constants for nonradiative transitions between the M S components of spin multiplets?

Predicting the rates of spin-dependent processes characterized by nonradiative transitions between electronic states with different spin multiplicities is important for understanding the mechanisms of many photochemical and catalytic reactions. To calculate these rates, it is necessary to define the spin state representation and the couplings between these states that drives the interstate transitions. In this work, we describe three different approaches to calculating the spin-orbit coupling (SOC), transition probabilities, and rate constants between the MS components of the electronic states with different spin multiplicities. We implemented these approaches in our nonadiabatic statistical theory (NAST) software package, which predicts the transition probabilities and rate constants of spin-dependent processes using information obtained from electronic structure calculations. Here, we discuss the advantage and drawbacks of each approach and, as an example, calculate the rate constants for transitions between the spin states in the active site model of the protein rubredoxin.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

An Ab Initio Electronic Structure Investigation of the Ground and Excited States of ScH + , YH + , and LaH +

Multireference configuration interaction (MRCI), Davidson-corrected MRCI (MRCI+Q), coupled-cluster singles, doubles, and perturbative triples [CCSD(T)], and frozen-core full configuration interaction (fcFCI) calculations were carried out using large, correlation-consistent basis sets to investigate the excited states of the Sc atom and the spin–free and spin–orbit coupled potential energy profiles, energetics, spectroscopic constants, and electron populations of low-lying states of MH + (M = Sc, Y, La). The core electron correlation effects, complete basis set effects, and spin–orbit coupling effects were also evaluated. The first four electronic states of all MH + are 1 2 Δ, 1 2 Σ + , 1 2 Π, and 2 2 Σ + with 1σ 2 1δ 1 , 1σ 2 2σ 1 , 1σ 2 1π 1 , and 1σ 2 3σ 1 single-reference electron configurations, respectively. These states of MH + can be represented by the M 2+ H – ionic structure. The ground states of ScH + , YH + , and LaH + are 1 2 Δ 3/2 , 1 2 Σ + 1/2 , and 1 2 Δ 3/2 with 55.45, 60.54, and 62.34 kcal/mol bond energies, respectively. The core electron correlation was found to be vital for gaining accurate predictions on the ground and excited state properties of MH + . The spin–orbit coupling effects are minor for ScH + but become substantial moving to YH + and LaH + . Overall, the results of this work are in good agreement with the limited set of experimental findings of MH + available in the literature and will be of use for future investigations. Furthermore, the theoretical approaches, findings, and trends reported here are expected to aid studies of similar species.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Structural determinants for pH-dependent activation of a plant metacaspase

Arabidopsis thaliana metacaspase 9 (AtMC9) plays roles in clearing dead cells, forming xylem vessels, and regulating immunity and programmed cell death in plants. The protease's activation is controlled by pH levels, but the exact structural mechanism behind this has not been elucidated. In this work, high-resolution crystal structures for AtMC9 at active (pH 5.5 and pH 4.2) and inactive (pH 7.5) conditions are reported. The three structures are similar except for local conformations where their hydrogen bonding interactions with solvents are mediated through the protonation of specific titratable amino acid residues' side chains. By combining structural analysis, molecular dynamics simulations under constant pHs, and biochemical assays coupled with site-directed mutagenesis, we show that the regulation of AtMC9 activation involves multiple titratable glutamate and histidine residues across the three domains of p20, linker, and p10. Specifically, deprotonated Glu112, His193, and His208 can suppress AtMC9 proteolytic activity, while protonation of Glu255 and His307 at acidic pH may promote it. This study provides valuable insights into the pH-dependent activation of AtMC9 and could potentially lead to improving crops with enhanced immunity and controlled cell death, ultimately increasing agricultural productivity.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Enhanced Thermal Hall Effect in Nearly Ferroelectric Insulators

In the context of recent experimental observations of an unexpectedly large thermal Hall conductivity, $κ_H$, in insulating $La_2CuO_4$ (LCO) and $SrTiO_3$ (STO), in this paper we theoretically explore conditions under which acoustic phonons can give rise to such a large $κ_H$. Both the intrinsic and extrinsic contributions to $κ_H$ are large in proportion to the dielectric constant, $ε$, and the “flexoelectric” coupling, $F$. While the intrinsic contribution is still orders of magnitude smaller than the observed effect, an extrinsic contribution proportional to the phonon mean-free path appears likely to account for the observations, at least in STO. We predict a larger intrinsic $κ_H$ in certain insulating perovskites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1970

Adjustable support spring

Restraining spring mechanism acting against a force has adjustable spring constant, and its performance is not affected by changes in ambient pressure. Natural frequency of device is varied by changing absolute pressures within bellows and may be tuned to prevent coupling while maintaining a given spring constant.

Hadland, W. O.↗
NASA NTRS1984

The photochemical time constants of minor constituents and their families in the middle atmosphere

A comprehensive formula is worked out for the photochemical time constant of minor constituents in the middle atmosphere. The formula is particularly useful for evaluating the time constants for families of some chemical species that are strongly coupled by rapid exchange reactions. Height profiles of photochemical time constants are calculated for individual species and their families based on the chemical reaction rate constants recommended in the recent WMO and JPL reports. Potential exchange reactions among various family members are discussed, and the effects of the choice of family membership on the time constant are evaluated.

Shimazaki, T.↗
NASA NTRS1993

Performance results of a 300-degree linear phase modulator for spaceborne communications applications

A phase modulator capable of large linear phase deviation, low loss, and wide band operation with good thermal stability was developed for deep space spacecraft transponder (DST) applications at X-band (8.415 GHz) and Ka-band (32 GHz) downlinks. The design uses a two-stage circulator-coupled reflection phase shifter with constant gamma hyperabrupt varactors and an efficient modulator driver circuit to obtain a phase deviation of +/-2.5 rad with better than 8 percent linearity. The measured insertion loss is 6.6 dB +/- 0.35 dB at 8415 MHz. Measured carrier and relative sideband amplitudes resulting from phase modulation by sine wave and square modulating functions agree well with the predicted results.

Mysoor, N. R.↗
NASA NTRS1993

Performance results of a 300-deg linear phase modulator for spaceborne communications applications

A phase modulator capable of large linear phase deviation, low loss, and wide band operation with good thermal stability was developed for deep space spacecraft transponder (DST) applications at X-band (8.415 GHz) and Ka-band (32 GHz) downlinks. The design uses a two-stage circulator-coupled reflection phase shifter with constant gamma hyperabrupt varactors and an efficient modulator driver circuit to obtain a phase deviation of +/- 2.5 rad with better than 8 percent linearity. The measured insertion loss is 6.6 dB +/- 0.35 dB at 8415 MHz. Measured carrier and relative sideband amplitudes resulting from phase modulation by sine wave and square modulating functions agree well with the predicted results.

Mysoor, Narayan R.↗
NASA NTRS1994

Review of Slow-Wave Structures

The majority of recent theoretical and experimental reports published in the literature dealing with helical slow-wave structures focus on the dispersion characteristics and their effects due to the finite helix wire thickness and attenuation, dielectric loading, metal loading, and the introduction of plasma. In many papers, an effective dielectric constant is used to take into account helix wire dimensions and conductivity losses, while the propagation constant of the signal and the interaction impedance of the structure are found to depend on the surface resistivity of the helix. Also, various dielectric supporting rods are simulated by one or several uniform cylinders having an effective dielectric constant, while metal vane loading and plasma effects are incorporated in the effective dielectric constant. The papers dealing with coupled cavities and folded or loaded wave guides describe equivalent circuit models, efficiency enhancement, and the prediction of instabilities for these structures. Equivalent circuit models of various structures are found using computer software programs SUPERFISH and TOUCHSTONE. Efficiency enhancement in tubes is achieved through dynamic velocity and phase adjusted tapers using computer techniques. The stability threshold of unwanted antisymmetric and higher order modes is predicted using SOS and MAGIC codes and the dependence of higher order modes on beam conductance, section length, and effective Q of a cavity is shown.

Wallett, Thomas M.↗
NASA NTRS1999

A Hot-electron Direct Detector for Radioastronomy

A hot-electron transition-edge superconducting bolometer with adjustable thermal relaxation speed is proposed. The bolometer contacts are made from a superconductor with high critical temperature which blocks the thermal diffusion of hot carriers into the contacts. Thus electron-phonon interaction is the only mechanism for heat removal. The speed of thermal relaxation for hot electrons in a nanometer-size superconducting bolometer with T(sub c) = 100-300 mK is controlled by the elastic electron mean free path l. The relaxation rate behaves as T(sup 4)l at subkelvin temperatures and can be reduced by a factor of 10-100 by decreasing 1. Then an antenna- or wave guide-coupled bolometer with a time constant approx. = 10(exp -3) to 10(exp -4) s will exhibit photon-noise limited performance at millimeter and submillimeter wavelengths. The bolometer will have a figure-of-merit NEPtau = 10(exp -22) - 10(exp -21) W/Hz at 100 mK which is 10(exp 3) to 10(exp 4) times better (ie: smaller) than that of a state-of-the-art bolometer. A tremendous increase in speed and sensitivity will have a significant impact for observational mapping applications.

Karasik, Boris S.↗
NASA NTRS2000

A Hot-Electron Far-Infrared Direct Detector

A new approach is proposed to improve the sensitivity of direct-detection bolometers at millimeter, submillimeter and far-infrared wavelengths. The idea is to adjust a speed of the thermal relaxation of hot-electrons in a nanometer size normal metal or super-conductive transition edge bolometer by controlling the elastic electron mean free path. If the bolometer contacts are made of a superconductor with high critical temperature (Nb, Pb etc.) then the thermal diffusion into the contacts is absent because of the Andreev's reflection and the electron-phonon relaxation is the only mechanism for heat removal. The relaxation rate should behave as T(sup 4)l at subkelvin temperatures (l is the electron elastic mean free path) and can be reduced by factor of 10-100 by decreasing l. Then an antenna- or waveguide-coupled bolometer with a time constant about 10(exp -3) to 10(exp -5) s at T approximately equals 0.1-0.3 K will exhibit photon-noise limited performance in millimeter and submillimeter range. The choice of the bolometer material is a tradeoff between a low electron heat capacity and fabrication. A state-of-the-art bolometer currently offers NEP = 10(exp -17) W(Square root of (Hz)) at 100 mK along with a approximately equals 2 msec time constant. The bolometer we propose will have a figure-of-merit, NEP(square root (r)), which is 10(exp 3) times smaller. This will allow for a tremendous increase in speed which will have a significant impact for observational mapping applications. Alternatively, the bolometer could operate at higher temperature with still superior sensitivity. This device can significantly increase a science return and reduce the cost for future observational missions. This research was performed by the Center for Space Microelectronics Technology, Jet Propulsion Laboratory, California Institute of Technology, and was sponsored by NASA, Office of Space Science.

Karasik, B. S.↗
NASA NTRS2000

A Hot-electron Direct Detector for Radioastronomy

A new approach is proposed to improve the sensitivity of direct-detection bolometers. The idea is to adjust a speed of the thermal relaxation of hot-electrons in a nanometer size normal metal or superconductive transition edge bolometer by controlling the elastic electron mean free path. If the bolometer contacts are made of a superconductor with high critical temperature then the thermal diffusion into the contacts is absent because of the Andreev's reflection and the electron-phonon relaxation is the only mechanism for heat removal. The relaxation rate should behave as 7(exp 4)l at subkelvin temperatures (l is the electron elastic mean free path) and can be reduced by factor of 10 - 100 by decreasing l. Then an antenna- or waveguide-coupled bolometer with a time constant approx. 10(exp -3) to 10(exp -5) S at T approx. = 0.1 - 0.3 K will exhibit photon-noise limited performance in millimeter and subn-millimeter range. The bolometer will have a figure-of-merit NEk square root of tau approx. = 10(exp -22) 10(exp -21) W/Hz at 100 mK which is 10(exp 3) times smaller than that of a state-of-the-art bolometer. This will allow for a tremendous increase in speed which will have a significant impact for observational mapping applications. Alternatively, the bolometer could operate at higher temperature with still superior sensitivity This research was performed by the Center for Space Microelectronics Technology, JPL, California Institute of Technology, under the contract for NASA.

Karasik, B. S.↗