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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Intersubband transitions in In{sub x}Ga{sub 1−x}N/In{sub y}Ga{sub 1−y}N/GaN staggered quantum wells

Intersubband transition energies and absorption lineshape in staggered InGaN/GaN quantum wells surrounded by GaN barriers are computed as functions of structural parameters such as well width, In concentrations, and the doping level in the well. Schrödinger and Poisson equations are solved self-consistently by taking the free and bound surface charge concentrations into account. Many-body effects, namely, depolarization and excitonic shifts are also included in the calculations. Results for transition energies, oscillator strength, and the absorption lineshape up to nonlinear regime are represented as functions of the parameters mentioned. The well width (total and constituent layers separately) and In concentration dependence of the built-in electric field are exploited to tune the intersubband transition energies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Landau damping of Langmuir twisted waves with kappa distributed electrons

The kinetic theory of Landau damping of Langmuir twisted modes is investigated in the presence of orbital angular momentum of the helical (twisted) electric field in plasmas with kappa distributed electrons. The perturbed distribution function and helical electric field are considered to be decomposed by Laguerre-Gaussian mode function defined in cylindrical geometry. The Vlasov-Poisson equation is obtained and solved analytically to obtain the weak damping rates of the Langmuir twisted waves in a nonthermal plasma. The strong damping effects of the Langmuir twisted waves at wavelengths approaching Debye length are also obtained by using an exact numerical method and are illustrated graphically. The damping rates of the planar Langmuir waves are found to be larger than the twisted Langmuir waves in plasmas which shows opposite behavior as depicted in Fig. 3 by J. T. Mendoça [Phys. Plasmas 19, 112113 (2012)].

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Disformal invariance of curvature perturbation

We show that under a general disformal transformation the linear comoving curvature perturbation is not identically invariant, but is invariant on superhorizon scales for any theory that is disformally related to Horndeski's theory. The difference between disformally related curvature perturbations is found to be given in terms of the comoving density perturbation associated with a single canonical scalar field. In General Relativity it is well-known that this quantity vanishes on superhorizon scales through the Poisson equation that is obtained on combining the Hamiltonian and momentum constraints, and we confirm that a similar result holds for any theory that is disformally related to Horndeski's scalar-tensor theory so long as the invertibility condition for the disformal transformation is satisfied. We also consider the curvature perturbation at full nonlinear order in the unitary gauge, and find that it is invariant under a general disformal transformation if we assume that an attractor regime has been reached. Finally, we also discuss the counting of degrees of freedom in theories disformally related to Horndeski's.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Moderately converging ion and electron flows in two-dimensional diodes

Flow of particles in diodes is solved selfconsistently assuming an approximated system of flow lines, that can be easily represented by an analytic transformation in a complex plane, with assumed uniformity in the third spatial direction. Beam current compression is tunable by an angle parameter {alpha}{sub 0}; transformed coordinate lines are circular arcs, exactly matching to the curved cathode usually considered by rectilinear converging flows. The curvature of flow lines allows to partly balance the transverse effect of space charge. A self-contained discussion of the whole theory is reported, ranging from analytical solution for selfconsistent potential to electrode drawing to precise numerical simulation, which serves as a verification and as an illustration of typical electrode shapes. Motion and Poisson equation are written in a curved flow line system and their approximate consistency is shown to imply an ordinary differential equation for the beam edge potential. Transformations of this equation and their series solutions are given and discussed, showing that beam edge potential has a maximum, so supporting both diode (with {alpha}{sub 0}{approx_equal}{pi}/3) and triode design. Numerical simulations confirm the consistency of these solution. Geometrical details of diode design are discussed: the condition of a zero divergence beam, with the necessary anode lens effect included, is written and solved, as a function of beam compression; accurate relations for diode parameters and perveance are given. Weakly relativistic effects including self-magnetic field are finally discussed as a refinement.

43 PARTICLE ACCELERATORS↗
DOE OSTI2018

Self-Consistent Simulation of GaAs/InGaAs/AlGaAs Heterostructures Photoluminescence Spectra and Its Application to pHEMT Structures Diagnostics

We performed numerical self-consistent solution of Schrödinger and Poisson equations for GaAs/InGaAs/AlGaAs pHEMT structures with quantum well. Based on the results we calculated optical transition matrix elements and photoluminescence spectra of such structures with the same design and different parameters (such as doping level and epitaxial layers width). In the photoluminescence spectra calculations three fitting parameters have been used. These parameters are GaAs/InGaAs valence band offset in strained quantum well, hole quasi Fermi level and inhomogeneous broadening. The PL peaks amplitudes and positions dependencies on the structure parameters were established. These dependencies can be used as the basis for pHEMT structure non-destructive diagnostics.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Theoretical and experimental studies of electric field distribution in N-polar GaN/AlGaN/GaN heterostructures

Electric field distribution in N-polar GaN(channel)/AlGaN/GaN(buffer) heterostructures was studied theoretically by solving Schrodinger and Poisson equations in a self-consistent manner for various boundary conditions and comparing results of these calculations with experimental data, i.e., measurements of electric field in GaN(channel) and AlGaN layers by electromodulation spectroscopy. A very good agreement between theoretical calculations and experimental data has been found for the Fermi-level located at ∼0.3 eV below the conduction band at N-polar GaN surface. With this surface boundary condition, the electric field distribution and two dimensional electron gas concentration are determined for GaN(channel)/AlGaN/GaN(buffer) heterostructures of various thicknesses of GaN(channel) and AlGaN layers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Enhanced numerical design of two-barrier infrared detectors with III–V compounds heterostructures considering the influence of lattice strain and misfit dislocations on the band gap

The goal of this work is the design of efficient infrared radiation detectors based on InAsSb compounds with two energy barriers around the absorber region. Two types of two-barrier detectors that work in the 3–5.5 μm wavelength range at 230 K is designed. Using our computer program to iteratively solve the Poisson equation, the spatial distribution of energy band edges in III–V heterostructures was calculated The influence of lattice stress, bending of the structure and doping on the energy shift of the edge of the bands is considered. Lattice strain is the cause of the formation of the misfit dislocations at the boundaries of the individual layers of the heterostructure. These dislocations partially relax the resulting stress and affect the lattice deformation in individual layers. From the minimum elastic energy condition, the density of these dislocations is determined. It has been shown that the band offset can be eliminated in the areas of both barriers in the designed two-barrier detectors.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

AN ADAPTIVE PARTICLE-MESH GRAVITY SOLVER FOR ENZO

We describe and implement an adaptive particle-mesh algorithm to solve the Poisson equation for grid-based hydrodynamics codes with nested grids. The algorithm is implemented and extensively tested within the astrophysical code Enzo against the multigrid solver available by default. We find that while both algorithms show similar accuracy for smooth mass distributions, the adaptive particle-mesh algorithm is more accurate for the case of point masses, and is generally less noisy. We also demonstrate that the two-body problem can be solved accurately in a configuration with nested grids. In addition, we discuss the effect of subcycling, and demonstrate that evolving all the levels with the same timestep yields even greater precision.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

The electron–hole overlap in the parabolic quantum well light emitting diode is much superior to the rectangular: even to that of a staggered quantum well

The strong piezoelectric field in the In{sub x}Ga{sub 1−x}N/GaN quantum well (QW) LEDs, separates the electrons and holes spatially, which decreases the luminescence. Various shapes and compositions of such QWs are studied to improve the performance. We have studied the transition energy (TE), overlap of electron and hole wave functions, band structures and field distributions of the parabolic QWs (PQW) through the self consistent solutions of Schrödinger and Poisson equations. The shape of the PQW is varied along with the compositions and dopings. The square of the overlap of electron and hole wave functions i.e. the transition probability (TP) is strikingly increased, compared to the rectangular QW and it is even higher than the symmetrically staggered QW. At a particular current density, for the same TE, the TP of the PQW increases more than two times that of the rectangular QWs. An important feature, desirable for the QW LEDs emerge. The change of the TE with increase in the current density is minimized. A brief theory, computational procedures and the results will be presented in details with suitable discussions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Surface waves on quantum plasma half-space with electron exchange-correlation effects

The propagation of surface waves on a quantum plasma half-space is investigated, taking into account the electron exchange-correlation effect. Using the modified quantum hydrodynamic model in conjunction with the Poisson equation, the dispersion relation of surface waves is obtained. It is found that due to the presence of electron exchange-correlation effect the wave frequency is shifted to lower frequencies. For different ranges of Brueckner parameter r{sub s}, the effect of electron exchange-correlation is investigated. It is indicated that for weak coupling region with r{sub s} < 0.1, the wave frequency remains unchanged and in this region the effect of electron exchange-correlation is negligible. For moderate coupling region, i.e., (0.1 < r{sub s} < 1), the influence of electron exchange-correlation is important and as r{sub s} increases, the electron exchange-correlation effect also increases.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Effect of ferroelectricity on charge conduction in a PCBM/perovskite device structure

The ferroelectricity in hybrid perovskite solar cells has attracted much attention and its effect on the device transport and photovoltaic properties is still under debate. Here in this paper, we investigate theoretically the effect of ferroelectricity on the redistribution of charge in a PCBM/perovskite device through the Poisson equation. The spontaneous polarization is set to be a hyperbolic tangent function of applied electrical field. The carriers in both the layers are assumed to be in quasi-equilibrium, and the (dark) current stems from the recombination at the PCBM/perovskite interface. The electrostatic potential profiles throughout the device and the current-bias (I-V) curves are obtained numerically. It is found that for the bias forward scan, larger potential drop will be on the PCBM layer, and for the bias reverse scan larger potential drop will be on the perovskite layer. The hysteresis of curves is also obtained. Lastly, these results shed new light on the effect of ferroelectricity in hybrid perovskite materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Influence of alloy disorder scattering on the hole mobility of SiGe nanowires

In this work, we analyze the influence of the alloy disorder (AD) scattering on the low-field hole mobility of Si{sub 1-x}Ge{sub x} nanowires (NWs). To do it, the electrostatic description is achieved through a self-consistent solution of the Poisson equation and the six-band k⋅p method in the cross section of the NW. The momentum relaxation time approximation is used to calculate the hole mobility, including alloy disorder and phonon scattering mechanisms, and the use of approximations to calculate the overlap integrals for the scattering matrix elements is discussed. We study the influence of the alloy disorder scattering on the total mobility compared to the phonon contribution, for different values of the AD scattering parameter proposed in the literature, and analyze the performance of SiGe NWs as a function of the Ge molar fraction for both low and high inversion charge densities.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

ON THE PROBLEM OF DEFORMED SPHERICAL SYSTEMS IN MODIFIED NEWTONIAN DYNAMICS

Based on Newtonian dynamics, observations show that the luminous masses of astrophysical objects that are the size of a galaxy or larger are not enough to generate the measured motions which they supposedly determine. This is typically attributed to the existence of dark matter, which possesses mass but does not radiate (or absorb radiation). Alternatively, the mismatch can be explained if the underlying dynamics is not Newtonian. Within this conceptual scheme, Modified Newtonian Dynamics (MOND) is a successful theoretical paradigm. MOND is usually expressed in terms of a nonlinear Poisson equation, which is difficult to analyze for arbitrary matter distributions. We study the MONDian gravitational field generated by slightly non-spherically symmetric mass distributions based on the fact that both Newtonian and MONDian fields are conservative (which we refer to as the compatibility condition). As the non-relativistic version of MOND has two different formulations (AQUAL and QuMOND) and the compatibility condition can be expressed in two ways, there are four approaches to the problem in total. The method involves solving a suitably defined linear deformation potential, which generally depends on the choice of MOND interpolation function. However, for some specific form of the deformation potential, the solution is independent of the interpolation function.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Resonant tunneling diode based on band gap engineered graphene antidot structures

The present work demonstrates the operation and performance of double barrier Graphene Antidot Resonant Tunnel Diode (DBGA-RTD). Non-Equilibrium Green’s Function (NEGF) frame work with tight-binding Hamiltonian and 2-D Poisson equations were solved self-consistently for device study. The interesting feature in this device is that it is an all graphene RTD with band gap engineered graphene antidot tunnel barriers. Another interesting new finding is that it shows negative differential resistance (NDR), which involves the resonant tunneling in the graphene quantum well through both the electron and hole bound states. The Graphene Antidot Lattice (GAL) barriers in this device efficiently improved the Peak to Valley Ratio to approximately 20 even at room temperature. A new fitting model is developed for the number of antidots and their corresponding effective barrier width, which will help in determining effective barrier width of any size of actual antidot geometry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Transition times between the extremum points of the current–voltage characteristic of a resonant tunneling diode with hysteresis

A numerical solution to the problem of transient processes in a resonant tunneling diode featuring a current–voltage characteristic with hysteresis is found for the first time in the context of a coherent model (based on the coupled Schrödinger and Poisson equations) taking into account the Fermi distribution of electrons. The transitions from the high-current to the low-current state and vice versa, which result from the existence of hysteresis and are of great practical importance for ultrafast switches based on resonant tunneling diodes, are studied in detail. It is shown that the transition times for such processes initiated by the application of a small voltage can significantly exceed the characteristic time ℏ/Γ (where G is the width of the resonance level). It is established for the first time that the transition time can be reduced and made as short as the characteristic time ℏ/Γ by applying a sufficiently high voltage. For the parameters of the resonant-tunnelingdiode structure considered in this study, the required voltage is about 0.01 V.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Gyrokinetic simulations of reverse shear Alfven eigenmodes in DIII-D plasmas

A gyrokinetic ion/mass-less fluid electron hybrid model as implemented in the GEM code [Y. Chen and S. E. Parker, J. Comput. Phys. 220, 837 (2007)] is used to study the reverse shear Alfven eigenmodes (RSAE) observed in DIII-D, discharge no. 142111. This is a well diagnosed case with measurement of the core-localized RSAE mode structures and the mode frequency, which can be used to compare with simulations. Simulations reproduce many features of the observation, including the mode frequency up-sweeping in time and the sweeping range. A new algorithmic feature is added to the GEM code for this study. Instead of the gyrokinetic Poisson equation itself, its time derivative, or the vorticity equation, is solved to obtain the electric potential. This permits a numerical scheme that ensures the E Multiplication-Sign B convection of the equilibrium density profiles of each species cancel each other in the absence of any finite-Larmor-radius effects. These nonlinear simulations generally result in an electron temperature fluctuation level that is comparable to measurements, and a mode frequency spectrum broader than the experimental spectrum. The spectral width from simulations can be reduced if less steep beam density profiles are used, but then the experimental fluctuation level can be reproduced only if a collision rate above the classical level is assumed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Optical anisotropy of InAs/GaSb broken-gap quantum wells

We investigate in detail the optical anisotropy of absorption of linearly polarized light in InAs/GaSb quantum wells grown on GaSb along the [001] direction, which can be used as an active region of different laser structures. The energy level positions, the wave functions, the optical matrix elements, and the absorption coefficients are calculated using the eight-band k {center_dot} p model and the Burt-Foreman envelope function theory. In these calculations, the Schroedinger and Poisson equations are solved self-consistently taking the lattice-mismatched strain into account. We find that a realistic Hamiltonian, which has the C{sub 2v} symmetry, results in considerable anisotropy of optical matrix elements for different directions of light polarization and different directions of the initial-state in-plane wave vector, including low-symmetry directions. We trace how the optical matrix elements and absorption are modified when spin-orbit interaction and important symmetry breaking mechanisms are taken into account (structural inversion asymmetry, bulk inversion asymmetry, and interface Hamiltonian). These mechanisms result in an almost 100% anisotropy of the absorption coefficients as the light polarization vector rotates in the plane of the structure and in a plane normal to the interfaces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

A comprehensive method of estimating electric fields from vector magnetic field and Doppler measurements

Photospheric electric fields, estimated from sequences of vector magnetic field and Doppler measurements, can be used to estimate the flux of magnetic energy (the Poynting flux) into the corona and as time-dependent boundary conditions for dynamic models of the coronal magnetic field. We have modified and extended an existing method to estimate photospheric electric fields that combines a poloidal-toroidal decomposition (PTD) of the evolving magnetic field vector with Doppler and horizontal plasma velocities. Our current, more comprehensive method, which we dub the 'PTD-Doppler-FLCT Ideal' (PDFI) technique, can now incorporate Doppler velocities from non-normal viewing angles. It uses the FISHPACK software package to solve several two-dimensional Poisson equations, a faster and more robust approach than our previous implementations. Here, we describe systematic, quantitative tests of the accuracy and robustness of the PDFI technique using synthetic data from anelastic MHD (ANMHD) simulations, which have been used in similar tests in the past. We find that the PDFI method has less than 1% error in the total Poynting flux and a 10% error in the helicity flux rate at a normal viewing angle (θ = 0) and less than 25% and 10% errors, respectively, at large viewing angles (θ < 60°). We compare our results with other inversion methods at zero viewing angle and find that our method's estimates of the fluxes of magnetic energy and helicity are comparable to or more accurate than other methods. We also discuss the limitations of the PDFI method and its uncertainties.

79 ASTRONOMY AND ASTROPHYSICS↗