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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Linear Theory of Electron-Plasma Waves at Arbitrary Collisionality

The dynamics of electron-plasma waves are described at arbitrary collisionality by considering the full Coulomb collision operator. The description is based on a Hermite- Laguerre decomposition of the velocity dependence of the electron distribution function. The damping rate, frequency, and eigenmode spectrum of electron-plasma waves are found as functions of the collision frequency and wavelength. A comparison is made between the collisionless Landau damping limit, the Lenard-Bernstein and Dougherty collision operators, and the electron-ion collision operator, finding large deviations in the damping rates and eigenmode spectra. A purely damped entropy mode, characteristic of a plasma where pitch-angle scattering effects are dominant with respect to collisionless effects, is shown to emerge numerically, and its dispersion relation is analytically derived. It is shown that such a mode is absent when simplified collision operators are used, and that like-particle collisions strongly influence the damping rate of the entropy mode.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1974

Calculation of the total electron excitation cross section in the Born approximation using Slater wave functions for the Li (2s yields 2p), Li (2s yields 3p), Na (3s yields 4p), Mg (3p yields 4s), Ca (4s yields 4p) and K (4s yields 4p) excitations

Excitation of neutral atoms by inelastic scattering of incident electrons in gaseous nebulae were investigated using Slater Wave functions to describe the initial and final states of the atom. Total cross sections using the Born Approximation are calculated for: Li(2s yields 2p), Na(3s yields 4p), k(4s yields 4p). The intensity of emitted radiation from gaseous nebulae is also calculated, and Maxwell distribution is employed to average the kinetic energy of electrons.

Simsic, P. L.↗
DOE OSTI2020

Exact two-body expansion of the many-particle wave function

Progress toward the solution of the strongly correlated electron problem has been stymied by the exponential complexity of the wave function. Previous work established an exact two-body exponential product expansion for the ground-state wave function. By developing a reduced density-matrix analog of Dalgarno-Lewis perturbation theory, we prove here that (i) the two-body exponential product expansion is rapidly and globally convergent with each operator representing an order of a renormalized perturbation theory, (ii) the energy of the expansion converges quadratically near the solution, and (iii) the expansion is exact for both ground and excited states. The two-body expansion offers a reduced parametrization of the many-particle wave function as well as the two-particle reduced density matrix with potential applications on both conventional and quantum computers for the study of strongly correlated quantum systems. In this work, we demonstrate the result with the exact solution of the contracted Schrödinger equation for the molecular chains H 4 and H 5 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Observing pre-edge K -shell resonances in Kr, Xe, and XeF 2

The 1s→np Rydberg transitions below the 1s ionization thresholds of Kr and Xe are obscured in x-ray absorption spectra due to core-hole lifetime broadening. However, the np spectator electrons associated with those resonances can affect the core-hole decay spectra. We report on ion charge-state distributions of Kr and Xe measured in coincidence with KL 2,3 (K α1,2 ), KM 2,3 (Kβ 1,3 ), and KN 2,3 (Kβ 2 ) x-ray fluorescence as the incident x-ray energy is scanned through pre-edge resonances and ionization thresholds. The coincidence measurements select vacancy cascades that begin with a radiative transition that transfers 1s holes to the 2p, 3p, and 4p shells followed by emission of Auger electrons. We observe shifts of ion yields from higher to lower charge states that we attribute to np spectator electrons. For the special case of Kr 1+ that is produced in coincidence with Kβ 2 x rays, the ion yield decreases in the pre-edge region. This is attributed to production of neutral, metastable Kr 4p –1 np states that reduce the Kr 1+ yield. Model fits to the x-ray absorption spectra are presented to show the lifetime broadened pre-edge resonances and ionization thresholds. High-level relativistic coupled-cluster calculations that treat relativistic, electron correlation, and wave function relaxation effects on the same footing obtain agreement with the experimental 1s ionization energies of Kr and Xe to <2 eV. The Xe K-edge x-ray absorption spectrum and ion charge-state distributions of XeF2 were also recorded. Excitation of the 7σ u lowest unoccupied molecular orbital (LUMO) in XeF 2 is observed in the pre-edge region. Our ab initio calculations find that the 6p z Rydberg state is strongly perturbed by the presence of the 7σu LUMO. A fit to the measured LUMO, 6p x , 6p y , 6p z Rydberg states, and ionization threshold is guided by the relativistic coupled-cluster calculations. The F ligands modify the valence electron charge distribution and result in a ~2.3 eV chemical shift of the Xe 1s ionization energy relative to atomic Xe. Xe 1s –1 core-hole decay in XeF 2 results in ionization of the F ligands and energetic fragmentation into atomic ions. Ion charge-state spectra of XeF 2 were recorded in coincidence with x-ray fluorescence for excitation on the LUMO resonance and above the Xe 1s ionization threshold. For the ion time of flight spectra recorded on the LUMO resonance, the F q+ (q = 1–4) peaks are split into two peaks along the linear polarization direction of the incident x-ray beam. This effect is attributed to spatial alignment of XeF 2 molecules by resonant x-ray absorption, and the peak splittings are used to measure the F ion fragmentation energies following Kα 1,2 , Kβ 1,3 , and Kβ 2 x-ray fluorescence. Here, we observe variations of the F ion charge state yields and fragmentation energies for the three fluorescence pathways that leave the molecule in different outer-shell hole states.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

System of ideal quantum dots with Coulomb interaction

The wave functions of single-electron states localised on a system of single-level ideal quantum dots in a semiconductor matrix are constructed. The standard Kane theory, which describes the renormalisation of the effective mass of electrons in bulk III − V semiconductors, is transformed, for the first time as far as we know, for small-size quantum dots. The renormalised electron mass in a quantum dot depends on its ground state energy, which, in turn, depends on this mass. Thus, a self-consistent problem is obtained for calculating the electron binding energy. The radius of the Debye screening of the Coulomb interaction in a system of quantum dots is calculated at room temperature. The Coulomb repulsion of electrons localised on dots limits from above the possible number of filled dots. The condition is formulated for the optimal concentration of quantum dots. A classical distribution function for the probability of quantum dot filling is constructed. It is found the state, where electrons fill exactly half the maximum possible number of filled dots, exhibits the minimal energy (and therefore the most stable), the condition being fairly universal. In particular, it can be used to estimate the limiting efficiency of quantum-dot diode lasers. (quantum dots)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Convergence of variational Monte Carlo simulation and scale-invariant pre-training

We provide theoretical convergence bounds for the variational Monte Carlo (VMC) method as applied to optimize neural network wave functions for the electronic structure problem. Here, we study both the energy minimization phase and the supervised pre-training phase that is commonly used prior to energy minimization. For the energy minimization phase, the standard algorithm is scale-invariant by design, and we provide a proof of convergence for this algorithm without modifications. The pre-training stage typically does not feature such scale-invariance. We propose using a scale-invariant loss for the pretraining phase and demonstrate empirically that it leads to faster pre-training.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2025

Charge Separation at Organic Interfaces with Near-Zero Energy Offset: A Step toward Designing Inorganic-like Organic Semiconductors

For organic semiconductors, it is often presumed that an energy level offset at the donor–acceptor (D-A) interface is required to provide the driving force for charge separation (CS). This energy level offset unavoidably leads to a voltage loss in organic photovoltaics. In this work, by using zinc phthalocyanine (ZnPc) and fluorinated zinc phthalocyanine (F 4 ZnPc) as a model D/A interface, we found that spontaneous CS, with an enthalpy increase of ∼0.3–0.4 eV, can occur even with an interfacial energy offset as small as ∼0.1 eV. This enthalpy-increase CS process is driven by entropy. The entropic driving force can be enhanced by two factors: (1) a point-like spatial contact between the delocalized electron and hole wave function in the charge transfer exciton; (2) a small band bending near the interface originating from long-range electrostatic interaction. Our work demonstrates that effective CS can occur at interfaces with a near-zero energy level offset, which means that the energy loss at the D/A interface can be avoided.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Excitonic Dynamics in Janus MoSSe and WSSe Monolayers

We report here details of steady-state and time-resolved spectroscopy of excitonic dynamics for Janus transition metal dichalcogenide monolayers, including MoSSe and WSSe, which were synthesized by low-energy implantation of Se into transition metal disulfides. Absorbance and photoluminescence spectroscopic measurements determined the room-temperature exciton resonances for MoSSe and WSSe monolayers. Transient absorption measurements revealed that the excitons in Janus structures form faster than those in pristine transition metal dichalcogenides by about 30% due to their enhanced electron–phonon interaction by the built-in dipole moment. Here, by combining steady-state photoluminescence quantum yield and time-resolved transient absorption measurements, we find that the exciton radiative recombination lifetime in Janus structures is significantly longer than in their pristine samples, supporting the predicted spatial separation of the electron and hole wave functions due to the built-in dipole moment. These results provide fundamental insight in the optical properties of Janus transition metal dichalcogenides.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Variations of light emission and carrier dynamics around V-defects in InGaN quantum wells

Time- and spectrally-resolved scanning near-field optical microscopy was applied to study spatial variations of photoluminescence (PL) spectra and carrier dynamics in polar InGaN/GaN single quantum wells (QWs) emitting from 410 nm to 570 nm. The main attention was devoted to variations of PL properties and carrier dynamics around V-defects. The PL intensity, peak wavelength, and linewidth, as well as the radiative and nonradiative recombination times, were found to be different in V-defect-rich and defect-free regions. The radiative lifetime close to the defects was longer up to several times, which is attributed to an increased electron and hole wave function separation in the QW plane. PL decay times, measured using excitation and collection through the near-field probe, were one to two orders of magnitude shorter than PL decay times measured in the far field. This shows that the near-field PL decay and the integrated PL intensity are primarily determined by the carrier out-diffusion from under the probe. Only in the immediate vicinity of the V-defects, the near-field PL decays due to the nonradiative recombination at dislocations. The area of such enhanced nonradiative recombination is limited to just a few percent of the total QW area. This shows that recombination via dislocations and V-defects does not play a decisive role in the overall nonradiative recombination and internal quantum efficiency of polar InGaN/GaN QWs.

36 MATERIALS SCIENCE↗
NASA NTRS1980

Nonrelativistic hyperfine splitting in muonic helium by adiabatic perturbation theory

Huang and Hughes have discussed the hyperfine splitting delta mu of muonic helium using a variational approach. In this paper, the Born-Oppenheimer approximation is used to simplify the evaluation of delta mu in the nonrelativistic limit. The first-order perturbed wave function of the electron is obtained in closed form by modifying the method used by Dalgarno and Lynn. The result is close to those of Huang and Hughes which required a large Hylleraas expansion and considerable extrapolation.

Drachman, R. J.↗
DOE OSTI2019

On the Two-Phonon Relaxation of Excited States of Boron Acceptors in Diamond

The relaxation of holes from excited states of boron acceptors in diamond with the emission of two optical phonons is studied theoretically. To describe the wave function of acceptor states, an electron-like Hamiltonian with an isotropic effective mass is used. The wave function of the ground state is determined by the quantum-defect method. The probability of the transition is calculated in the adiabatic approximation. It is assumed that the phonon dispersion law is isotropic and the phonon frequency is quadratically dependent on the wave-vector modulus, with the maximum and minimum frequencies ω{sub max} and ω{sub min} at the center and boundary of the Brillouin zone, respectively. A high sensitivity of the probability of the transition to the characteristic of phonon dispersion ω{sub max} – ω{sub min} is revealed, especially for the transition with the energy E{sub T} in the range 2ℏω{sub min} ≤ E{sub T} < ℏω{sub min} + ℏω{sub max}. Depending on the energy of the transition and on the phonon dispersion, the two-phonon relaxation rate at the low-temperature limit varies from extremely small values (<10{sup 8} s{sup –1}) near the threshold E{sub T} = 2ℏω{sub min} to extremely large values (>10{sup 12} s{sup –1}) in the “resonance” range ℏω{sub min} + ℏω{sub max} ≤ E{sub T} ≤ 2ℏω{sub max}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2023

Simulating Electron Distribution Function in the Pulsating Aurora Using Particle and Wave Data of ARASE Satellite

Decades lasting research on the pulsating aurora suggested that this phenomenon forms as result of interactions between the magnetospheric keVs electrons and whistler-mode chorus waves. Arase satellite observation reported the direct evidence for this process confirming in situ measurement of highly correlated precipitated electrons and chorus wave activity. This paper presents the theoretical analysis of this observational event based on the SuperThermal Electron Transport (STET) code that simulates the highly dynamic environment of measured waves and particle data. Specifically, the STET code simulated results confirms the delicate loss-cone observation results of this mission and further reveals the broader energy range of precipitated electron fluxes that was not measurable by Arase satellite.

George V Khazanov↗
DOE OSTI2020

Automation of Active Space Selection for Multireference Methods via Machine Learning on Chemical Bond Dissociation

Predicting and understanding the chemical bond is one of the major challenges of computational quantum chemistry. Kohn–Sham density functional theory (KS-DFT) is the most common method, but approximate density functionals may not be able to describe systems where multiple electronic configurations are equally important. Multiconfigurational wave functions, on the other hand, can provide a detailed understanding of the electronic structures and chemical bonds of such systems. In the complete active space self-consistent field (CASSCF) method, one performs a full configuration interaction calculation in an active space consisting of active electrons and active orbitals. However, CASSCF and its variants require the selection of these active spaces. This choice is not black box; it requires significant experience and testing by the user, and thus active space methods are not considered particularly user-friendly and are employed only by a minority of quantum chemists. Our goal is to popularize these methods by making it easier to make good active space choices. Here, we present a machine learning protocol that performs an automated selection of active spaces for chemical bond dissociation calculations of main group diatomic molecules. The protocol shows high prediction performance for a given target system as long as a properly correlated system is chosen for training. Good active spaces are correctly predicted with a considerably better success rate than random guess (larger than 80% precision for most systems studied). Our automated machine learning protocol shows that a “black-box” mode is possible for facilitating and accelerating the large-scale calculations on multireference systems where single-reference methods such as KS-DFT cannot be applied.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Noninteracting electrons in a prototypical one-dimensional sinusoidal potential

A prototypical model of a one-dimensional metallic monatomic solid containing noninteracting electrons is studied, where the argument of the cosine potential energy, periodic with the lattice, contains the first reciprocal lattice vector G 1 =2π/a, where a is the lattice constant. The time-independent Schrödinger equation can be written in reduced variables as a Mathieu equation for which numerically exact solutions for the band structure and wave functions are obtained. The band structure has band gaps that increase with increasing amplitude q of the cosine potential. In the extended-zone scheme, the energy gaps decrease with increasing index n of the Brillouin-zone boundary ka=nπ, where k is the crystal momentum of the electron. The wave functions of the band electron are derived for various combinations of k and q as complex combinations of the real Mathieu functions with even and odd parity, and the normalization factor is discussed. The wave functions at the bottoms and tops of the bands are found to be real or imaginary, respectively, corresponding to standing waves at these energies. Irrespective of the wave vector k within the first Brillouin zone, the electron probability density is found to be periodic with the lattice. The Fourier components of the wave functions are derived versus q, which reveal multiple reciprocal-lattice-vector components with variable amplitudes in the wave functions unless q = 0. The magnitudes of the Fourier components are found to decrease exponentially as a power of n for n~3 to 45 for ka = π/2 and q = 2, and a precise fit is obtained to the data. The probability densities and probability currents obtained from the wave functions are also discussed. The probability currents are found to be zero for crystal momenta at the tops and bottoms of the energy bands, because the wave functions for these crystal momenta are standing waves. To conlcude, the band structure is calculated from the central equation and compared to the numerically exact band structure.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗