Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Dependability”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,243 records · Page 69
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Spin-dependent transport behavior in C{sub 60} and Alq{sub 3} based spin valves with a magnetite electrode (invited)

The spin-dependent transport behavior in organic semiconductors (OSs) is generally observed at low temperatures, which likely results from poor spin injection efficiency at room temperature from the ferromagnetic metal electrodes to the OS layer. Possible reasons for this are the low Curie temperature and/or the small spin polarization efficiency for the ferromagnetic electrodes used in these devices. Magnetite has potential as an advanced candidate for use as the electrode in spintronic devices, because it can achieve 100% spin polarization efficiency in theory, and has a high Curie temperature (850 K). Here, we fabricated two types of organic spin valves using magnetite as a high efficiency electrode. C{sub 60} and 8-hydroxyquinoline aluminum (Alq{sub 3}) were employed as the OS layers. Magnetoresistance ratios of around 8% and over 6% were obtained in C{sub 60} and Alq{sub 3}-based spin valves at room temperature, respectively, which are two of the highest magnetoresistance ratios in organic spin valves reported thus far. The magnetoresistance effect was systemically investigated by varying the thickness of the Alq{sub 3} layer. Moreover, the temperature dependence of the magnetoresistance ratios for C{sub 60} and Alq{sub 3}-based spin valves were evaluated to gain insight into the spin-dependent transport behavior. This study provides a useful method in designing organic spin devices operated at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Sequence-dependent theory of oligonucleotide hybridization kinetics

A theoretical approach to the prediction of the sequence and temperature-dependent rate constants for oligonucleotide hybridization reactions has been developed based on the theory of relaxation kinetics. One-sided and two-sided melting reaction mechanisms for oligonucleotide hybridization reactions have been considered, analyzed, modified, and compared to select a physically consistent as well as robust model for prediction of the relaxation times of DNA hybridization reactions that agrees with the experimental evidence. The temperature- and sequence-dependent parameters of the proposed model have been estimated using available experimental data. The relaxation time model that we developed has been combined with the nearest neighbor model of hybridization thermodynamics to estimate the temperature- and sequence-dependent rate constants of an oligonucleotide hybridization reaction. The model-predicted rate constants are compared to experimentally determined rate constants for the same oligonucleotide hybridization reactions. Finally, we consider a few important applications of kinetically controlled DNA hybridization reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Influence of Gaussian white noise on the frequency-dependent first nonlinear polarizability of doped quantum dot

We investigate the profiles of diagonal components of frequency-dependent first nonlinear (β{sub xxx} and β{sub yyy}) optical response of repulsive impurity doped quantum dots. We have assumed a Gaussian function to represent the dopant impurity potential. This study primarily addresses the role of noise on the polarizability components. We have invoked Gaussian white noise consisting of additive and multiplicative characteristics (in Stratonovich sense). The doped system has been subjected to an oscillating electric field of given intensity, and the frequency-dependent first nonlinear polarizabilities are computed. The noise characteristics are manifested in an interesting way in the nonlinear polarizability components. In case of additive noise, the noise strength remains practically ineffective in influencing the optical responses. The situation completely changes with the replacement of additive noise by its multiplicative analog. The replacement enhances the nonlinear optical response dramatically and also causes their maximization at some typical value of noise strength that depends on oscillation frequency.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Characterization of multielectron dynamics in molecules: A multiconfiguration time-dependent Hartree-Fock picture

Using the framework of multiconfiguration theory, where the wavefunction Φ(t) of a many-electron system at time t is expanded as Φ(t)=∑{sub I}C{sub I}(t)Φ{sub I}(t) in terms of electron configurations (Φ{sub I}(t)), we divided the total electronic energy E(t) as E(t)=∑{sub I}|C{sub I}(t)|{sup 2}E{sub I}(t) . Here E{sub I}(t) is the instantaneous phase changes of C{sub I}(t) regarded as a configurational energy associated with Φ{sub I}(t). We then newly defined two types of time-dependent states: (i) a state at which the rates of population transfer among configurations are all zero; (ii) a state at which (E{sub I}(t)) associated with the quantum phases of C{sub I}(t) are all the same. We call the former time-dependent state a classical stationary state by analogy with the stationary (steady) states of classical reaction rate equations and the latter one a quantum stationary state. The conditions (i) and (ii) are satisfied simultaneously for the conventional stationary state in quantum mechanics. We numerically found for a LiH molecule interacting with a near-infrared (IR) field ε(t) that the condition (i) is satisfied whenever the average velocity of electrons is zero and the condition (ii) is satisfied whenever the average acceleration is zero. We also derived the chemical potentials μ{sub j}(t) for time-dependent natural orbitals ϕ{sub j}(t) of a many-electron system. The analysis of the electron dynamics of LiH indicated that the temporal change in Δμ{sub j}(t) ≡ μ{sub j}(t) + ε(t) · d{sub j}(t) − μ{sub j}(0) correlates with the motion of the dipole moment of ϕ{sub j}(t), d{sub j}(t). The values Δμ{sub j}(t) are much larger than the energy ζ{sub j}(t) directly supplied to ϕ{sub j}(t) by the field, suggesting that valence electrons exchange energy with inner shell electrons. For H{sub 2} in an intense near-IR field, the ionization efficiency of ϕ{sub j}(t) is correlated with Δμ{sub j}(t). Comparing Δμ{sub j}(t) to ζ{sub j}(t), we found that energy accepting orbitals of Δμ{sub j}(t) > ζ{sub j}(t) indicate high ionization efficiency. The difference between Δμ{sub j}(t) and ζ{sub j}(t) is significantly affected by electron-electron interactions in real time.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Growth-temperature dependence of optical spin-injection dynamics in self-assembled InGaAs quantum dots

The growth-temperature dependence of the optical spin-injection dynamics in self-assembled quantum dots (QDs) of In{sub 0.5}Ga{sub 0.5}As was studied by increasing the sheet density of the dots from 2 × 10{sup 10} to 7 × 10{sup 10} cm{sup −2} and reducing their size through a decrease in growth temperature from 500 to 470 °C. The circularly polarized transient photoluminescence (PL) of the resulting QD ensembles was analyzed after optical excitation of spin-polarized carriers in GaAs barriers by using rate equations that take into account spin-injection dynamics such as spin-injection time, spin relaxation during injection, spin-dependent state-filling, and subsequent spin relaxation. The excitation-power dependence of the transient circular polarization of PL in the QDs, which is sensitive to the state-filling effect, was also examined. It was found that a systematic increase occurs in the degree of circular polarization of PL with decreasing growth temperature, which reflects the transient polarization of exciton spin after spin injection. This is attributed to strong suppression of the filling effect for the majority-spin states as the dot-density of the QDs increases.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Composition dependent valence band order in c-oriented wurtzite AlGaN layers

The valence band order of polar wurtzite aluminum gallium nitride (AlGaN) layers is analyzed for a dense series of samples, grown heteroepitaxially on sapphire substrates, covering the complete composition range. The excitonic transition energies, found by temperature dependent photoluminescence (PL) spectroscopy, were corrected to the unstrained state using input from X-ray diffraction. k∙p theory yields a critical relative aluminum concentration x{sub c}=(0.09±0.05) for the crossing of the uppermost two valence bands for strain free material, shifting to higher values for compressively strained samples, as supported by polarization dependent PL. The analysis of the strain dependent valence band crossing reconciles the findings of other research groups, where sample strain was neglected. We found a bowing for the energy band gap to the valence band with Γ₉ symmetry of b{sub Γ₉}=0.85eV, and propose a possible bowing for the crystal field energy of b{sub cf}=-0.12eV. A comparison of the light extraction efficiency perpendicular and parallel to the c axis of Al{sub x}Ga{sub 1-x}N/Al{sub y}Ga{sub 1-y}N quantum well structures is discussed for different compositions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Temperature-dependent templated growth of porphine thin films on the (111) facets of copper and silver

The templated growth of the basic porphyrin unit, free-base porphine (2H-P), is characterized by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy measurements and density functional theory (DFT). The DFT simulations allow the deconvolution of the complex XPS and NEXAFS signatures into contributions originating from five inequivalent carbon atoms, which can be grouped into C–N and C–C bonded species. Polarization-dependent NEXAFS measurements reveal an intriguing organizational behavior: On both Cu(111) and Ag(111), for coverages up to one monolayer, the molecules adsorb undeformed and parallel to the respective metal surface. Upon increasing the coverage, however, the orientation of the molecules in the thin films depends on the growth conditions. Multilayers deposited at low temperatures exhibit a similar average tilting angle (30° relative to the surface plane) on both substrates. Conversely, for multilayers grown at room temperature a markedly different scenario exists. On Cu(111) the film thickness is self-limited to a coverage of approximately two layers, while on Ag(111) multilayers can be grown easily and, in contrast to the bulk 2H-P crystal, the molecules are oriented perpendicular to the surface. This difference in molecular orientation results in a modified line-shape of the C 1s XPS signatures, which depends on the incident photon energy and is explained by comparison with depth-resolved DFT calculations. Simulations of ionization energies for differently stacked molecules show no indication for a packing-induced modification of the multilayer XP spectra, thus indicating that the comparison of single molecule calculations to multilayer data is justified.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Effects of reagent rotational excitation on the H + CHD{sub 3} → H{sub 2} + CD{sub 3} reaction: A seven dimensional time-dependent wave packet study

Seven-dimensional time-dependent wave packet calculations have been carried out for the title reaction to obtain reaction probabilities and cross sections for CHD{sub 3} in J{sub 0} = 1, 2 rotationally excited initial states with k{sub 0} = 0 − J{sub 0} (the projection of CHD{sub 3} rotational angular momentum on its C{sub 3} axis). Under the centrifugal sudden (CS) approximation, the initial states with the projection of the total angular momentum on the body fixed axis (K{sub 0}) equal to k{sub 0} are found to be much more reactive, indicating strong dependence of reactivity on the orientation of the reagent CHD{sub 3} with respect to the relative velocity between the reagents H and CHD{sub 3}. However, at the coupled-channel (CC) level this dependence becomes much weak although in general the K{sub 0} specified cross sections for the K{sub 0} = k{sub 0} initial states remain primary to the overall cross sections, implying the Coriolis coupling is important to the dynamics of the reaction. The calculated CS and CC integral cross sections obtained after K{sub 0} averaging for the J{sub 0} = 1, 2 initial states with all different k{sub 0} are essentially identical to the corresponding CS and CC results for the J{sub 0} = 0 initial state, meaning that the initial rotational excitation of CHD{sub 3} up to J{sub 0} = 2, regardless of its initial k{sub 0}, does not have any effect on the total cross sections for the title reaction, and the errors introduced by the CS approximation on integral cross sections for the rotationally excited J{sub 0} = 1, 2 initial states are the same as those for the J{sub 0} = 0 initial state.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Temperature dependence of the electrical properties of hydrogen titanate nanotubes

The temperature dependence of the electrical properties of hydrogen-rich titanate nanotubes (H-TNTs) in the 90–270 °C range was investigated by impedance spectroscopy. Three types of dominant conduction were found which depend on the previous thermal treatment of the samples. For untreated samples, at low temperatures (T < 100 °C), electrical conductivity is relatively high (>10{sup −4} S/cm at T ≈ 90 °C) and is dominated by protonic transport within structural water molecules. For thermal annealing in inert atmosphere up to 150 °C, water molecules are released from the nanotube structure resulting in a dehydrated H{sub 2}Ti{sub 3}O{sub 7} phase. Such phase has a low, thermally-dependent, electrical conductivity (10{sup −8} S/cm at T ≈ 90 °C) with activation energy of 0.68 eV. For samples annealed up to 260 °C, loss of OH groups, and consequent generation of oxygen vacancies, occurs that result in the non-stoichiometric H{sub 2(1−z)}Ti{sub 3}O{sub 7−z} phase. This phase has much higher conductivity (10{sup −5} S/cm at T ≈ 90 °C) and lower associated activation energy (0.40 eV). The generation of oxygen vacancies is confirmed by electron paramagnetic resonance measurements at room temperature, which revealed the presence of single-electron-trapped oxygen vacancies. The activation energy value found is consistent with the thermal ionization energy of the oxygen vacancies. Such defect formation represents the initial stage of the phase transformation from titanate to TiO{sub 2} (B). X-ray diffraction and Raman spectroscopy measurements also support such interpretation.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Estimation of the temperature dependent interaction between uncharged point defects in Si

A method is described to estimate the temperature dependent interaction between two uncharged point defects in Si based on DFT calculations. As an illustration, the formation of the uncharged di-vacancy V{sub 2} is discussed, based on the temperature dependent attractive field between both vacancies. For that purpose, all irreducible configurations of two uncharged vacancies are determined, each with their weight given by the number of equivalent configurations. Using a standard 216-atoms supercell, nineteen irreducible configurations of two vacancies are obtained. The binding energies of all these configurations are calculated. Each vacancy is surrounded by several attractive sites for another vacancy. The obtained temperature dependent of total volume of these attractive sites has a radius that is closely related with the capture radius for the formation of a di-vacancy that is used in continuum theory. The presented methodology can in principle also be applied to estimate the capture radius for pair formation of any type of point defects.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Relativistic mean-field model with energy dependent self-energies

Conventional relativistic mean-field theory is extended with the introduction of higher-order derivative couplings of nucleons with the meson fields. The Euler-Lagrange equations follow from the principle of stationary action. From invariance principles of the Lagrangian density the most general expressions for the conserved current and energy-momentum tensor are derived. The nucleon self-energies show the explicit dependence on the meson fields. They contain additional regulator functions which describe the energy dependence. The density dependence of meson-nucleon couplings causes the apperance of additional rearrangement contributions in the self-energies. The equation of state of infinite nuclear matter is obtained and the thermodynamical consistency of the model is demonstrated. This model is applied to the description of spherical, non-rotating stars in β-equilibrium. Stellar structure is calculated by solving the Tolman-Oppenheimer-Volkov (TOV) equations. The results for neutron stars are shown in terms of mass-radius relations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Temperature dependence of the spin relaxation in highly degenerate ZnO thin films

Zinc oxide is considered a potential candidate for fabricating next-generation transparent spintronic devices. However, before this can be achieved, a thorough scientific understanding of the various spin transport and relaxation processes undergone in this material is essential. In the present paper, we are reporting our investigations into these processes via temperature dependent Hanle experiments. ZnO thin films were deposited on c-axis sapphire substrates using a pulsed laser deposition technique. Careful structural, optical, and electrical characterizations of the films were performed. Temperature dependent non-local Hanle measurements were carried out using an all-electrical scheme for spin injection and detection over the temperature range of 20–300 K. From the Hanle data, spin relaxation time in the films was determined at different temperatures. A detailed analysis of the data showed that the temperature dependence of spin relaxation time follows the linear-in-momentum Dyakonov-Perel mechanism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Towards time-dependent current-density-functional theory in the non-linear regime

Time-Dependent Density-Functional Theory (TDDFT) is a well-established theoretical approach to describe and understand irradiation processes in clusters and molecules. However, within the so-called adiabatic local density approximation (ALDA) to the exchange-correlation (xc) potential, TDDFT can show insufficiencies, particularly in violently dynamical processes. This is because within ALDA the xc potential is instantaneous and is a local functional of the density, which means that this approximation neglects memory effects and long-range effects. A way to go beyond ALDA is to use Time-Dependent Current-Density-Functional Theory (TDCDFT), in which the basic quantity is the current density rather than the density as in TDDFT. This has been shown to offer an adequate account of dissipation in the linear domain when the Vignale-Kohn (VK) functional is used. Here, we go beyond the linear regime and we explore this formulation in the time domain. In this case, the equations become very involved making the computation out of reach; we hence propose an approximation to the VK functional which allows us to calculate the dynamics in real time and at the same time to keep most of the physics described by the VK functional. We apply this formulation to the calculation of the time-dependent dipole moment of Ca, Mg and Na{sub 2}. Our results show trends similar to what was previously observed in model systems or within linear response. In the non-linear domain, our results show that relaxation times do not decrease with increasing deposited excitation energy, which sets some limitations to the practical use of TDCDFT in such a domain of excitations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Temperature dependent magnetic anisotropy of epitaxial Co{sub 2}FeAl films grown on GaAs

Co{sub 2}FeAl films with different thickness were prepared at different temperature by molecular beam epitaxy. Their magnetic anisotropy was studied by rotating magneto-optical Kerr rotation measurements under different temperature. It is found that the cubic anisotropy depends only on the temperature-dependent fourth order magneto-elastic coefficients. However, the results of growth and measurement temperature-dependent uniaxial anisotropy suggest that the uniaxial anisotropy of Co{sub 2}FeAl films may be attributed to contributions from both shear strain and anisotropic interfacial bonding. Our experimental findings proposed a new point of view to understand the origin of magnetic anisotropy in ferromagnet/GaAs(001) heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Time-dependent projected Hartree-Fock

Projected Hartree-Fock (PHF) has recently emerged as an alternative approach to describing degenerate systems where static correlation is abundant, when the spin-symmetry is projected. Here, we derive a set of linearized time-dependent equations for PHF in order to be able to access excited states. The close connection of such linear-response time-dependent PHF (TDPHF) to the stability condition of a PHF wave function is discussed. Expanding this analysis also makes it possible to give analytical expressions for the projected coupling terms of Hamiltonian and overlaps between excited Slater determinants. TDPHF with spin-projection (TDSUHF) and its Tamm-Dancoff approximation are benchmarked for several electronically degenerate molecules including the dissociating H{sub 2}, F{sub 2} and O{sub 3} at equilibrium, and the distorted ethylene. It is shown that they give consistently better descriptions of excited states than does time-dependent HF (TDHF). Furthermore, we demonstrate that they offer not only singly but also doubly excited states, which naturally arise upon spin-projection. We also address the thermodynamic limit of TDSUHF, using non-interacting He gas. While TDPHF singly excited states tend to converge to those of HF with the size of the system due to the lack of size-extensivity of PHF, doubly excited states remain reasonable even at the thermodynamic limit. We find that the overall performance of our method is systematically better than the regular TDHF in many cases at the same computational scaling.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

A nanoscale temperature-dependent heterogeneous nucleation theory

Classical nucleation theory relies on the hypothetical equilibrium of the whole nucleation system, and neglects the thermal fluctuations of the surface; this is because the high entropic gains of the (thermodynamically extensive) surface would lead to multiple stable states. In fact, at the nanometer scale, the entropic gains of the surface are high enough to destroy the stability of the thermal equilibrium during nucleation, comparing with the whole system. We developed a temperature-dependent nucleation theory to elucidate the heterogeneous nucleation process, by considering the thermal fluctuations based on classical nucleation theory. It was found that the temperature not only affected the phase transformation, but also influenced the surface energy of the nuclei. With changes in the Gibbs free energy barrier, nucleation behaviors, such as the nucleation rate and the critical radius of the nuclei, showed temperature-dependent characteristics that were different from those predicted by classical nucleation theory. The temperature-dependent surface energy density of a nucleus was deduced based on our theoretical model. The agreement between the theoretical and experimental results suggested that the developed nucleation theory has the potential to contribute to the understanding and design of heterogeneous nucleation at the nanoscale.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The multi-configurational time-dependent Hartree approach revisited

The multi-configurational time-dependent Hartree (MCTDH) approach facilitates accurate high-dimensional quantum dynamics simulations. In the approach, the wavefunction is expanded in a direct product of self-adapting time-dependent single-particle functions (SPFs). The equations of motion for the expansion coefficients and the SPFs are obtained via the Dirac-Frenkel variational principle. While this derivation yields well-defined differential equations for the motion of occupied SPFs, singularities in the working equations resulting from unoccupied SPFs have to be removed by a regularization procedure. Here, an alternative derivation of the MCTDH equations of motion is presented. It employs an analysis of the time-dependence of the single-particle density matrices up to second order. While the analysis of the first order terms yields the known equations of motion for the occupied SPFs, the analysis of the second order terms provides new equations which allow one to identify optimal choices for the unoccupied SPFs. The effect of the optimal choice of the unoccupied SPFs on the structure of the MCTDH equations of motion and their regularization is discussed. Generalized equations applicable in the multi-layer MCTDH framework are presented. Finally, the effects resulting from the initial choice of the unoccupied SPFs are illustrated by a simple numerical example.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Magnetic field dependence of electrical resistivity and thermopower in Ni{sub 50}Mn{sub 37}Sn{sub 13} ribbons

We report magnetization, magnetoresistance (MR) and magnetothermopower (MTEP) of melt spun Ni{sub 50}Mn{sub 37}Sn{sub 13} ribbons which exhibit an austentite to martensite phase transition at a temperature (T{sub M}) ≈ 294 K. Upon cooling from 400 K, dc-resistivity and thermopower show abrupt changes at T{sub M}, indicating a change in the electronic density of states. The thermopower is negative from 400 K down to 10 K. Application of a magnetic field of μ{sub 0}H = 5 T decreases T{sub M} by 5 K and induces large negative MR (-23%) but positive MTEP (9%) near T{sub M}. While the MR is appreciable from T{sub M} down to 10 K, MTEP is significant only below 60 K (MR = -2.5% and MTEP = +300% at 10 K). The magnetic field dependence of resistivity and thermopower show either reversible or irreversible behavior near T{sub M}, depending on whether the sample is zero-field cooled or field-cooled, which indicates that the electronic band structure near T{sub M} is magnetic history dependent.

36 MATERIALS SCIENCE↗