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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 1,315 records · Page 73
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Shapiro spikes and negative mobility for skyrmion motion on quasi-one-dimensional periodic substrates

Using a simple numerical model of skyrmions in a two-dimensional system interacting with a quasi-one-dimensional periodic substrate under combined dc and ac drives where the dc drive is applied perpendicular to the substrate periodicity, we show that a rich variety of novel phase-locking dynamics can occur due to the influence of the Magnus term on the skyrmion dynamics. Instead of Shapiro steps, the velocity response in the direction of the dc drive exhibits a series of spikes, including extended dc drive intervals over which the skyrmions move in the direction opposite to the dc drive, producing negative mobility. Also, there are specific dc drive values at which the skyrmions move exactly perpendicular to the dc drive direction, giving a condition of absolute transverse mobility.

2-dimensional systems↗
DOE OSTI2017

Extremely large nonsaturating magnetoresistance and ultrahigh mobility due to topological surface states in the metallic Bi 2 Te 3 topological insulator

Here, weak antilocalization (WAL) effects in Bi 2 Te 3 single crystals have been investigated at high and low bulk charge carrier concentrations. At low charge carrier density the WAL curves scale with the normal component of the magnetic field, demonstrating the dominance of topological surface states in magnetoconductivity. At high charge carrier density the WAL curves scale with neither the applied field nor its normal component, implying a mixture of bulk and surface conduction. WAL due to topological surface states shows no dependence on the nature (electrons or holes) of the bulk charge carriers. The observations of an extremely large, non-saturating magnetoresistance, and ultrahigh mobility in the samples with lower carrier density further support the presence of surface states. The physical parameters characterizing the WAL effects are calculated using the Hikami-Larkin-Nagaoka formula. At high charge carrier concentrations, there is a greater number of conduction channels and a decrease in the phase coherence length compared to low charge carrier concentrations. The extremely large magnetoresistance and high mobility of topological insulators have great technological value and can be exploited in magneto-electric sensors and memory devices.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Shape of magnetic domain walls formed by coupling to mobile charges

Magnetic domain walls, which are crucially important in both fundamental physics and technical applications, often have a preference in their form due to many different origins, such as the crystalline shape, lattice symmetry, and magnetic anisotropy. Here we theoretically investigate yet another origin stemming from the coupling to mobile charges in itinerant magnets. Performing a large-scale numerical simulation in a minimal model for itinerant magnets, i.e., the Kondo lattice model with classical localized spins, we show that the shape of magnetic domain walls depends on the electronic band structure and electron filling. While Néel and 120° antiferromagnetic states do not show a strong preference in the shape of domain walls, noncoplanar spin states with scalar chiral ordering have distinct directional preferences of the domain walls depending on the electron filling. We find that the directional preference is rationalized by the wave-number dependence of the effective magnetic interactions induced by the mobile charges, which are set by the band structure and electron filling. Finally, we also observe that, in the noncoplanar chiral states, an electric current is induced along the domain walls owing to the spin Berry phase mechanism, with different spatial distributions depending on whether the bulk state is metallic or insulating.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Linear magnetoresistance caused by mobility fluctuations in n -doped Cd 3 As 2

Cd 3 As 2 is a candidate three-dimensional Dirac semimetal which has exceedingly high mobility and nonsaturating linear magnetoresistance that may be relevant for future practical applications. We report magnetotransport and tunnel diode oscillation measurements on Cd 3 As 2 , in magnetic fields up to 65 T and temperatures between 1.5 and 300 K. We find that the nonsaturating linear magnetoresistance persists up to 65 T and it is likely caused by disorder effects, as it scales with the high mobility rather than directly linked to Fermi surface changes even when approaching the quantum limit. As a result of the observed quantum oscillations, we determine the bulk three-dimensional Fermi surface having signatures of Dirac behavior with a nontrivial Berry phase shift, very light effective quasiparticle masses, and clear deviations from the band-structure predictions. In very high fields we also detect signatures of large Zeeman spin splitting (g~16).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Dislocation Structure and Mobility in hcp He 4

We assess the core structure and mobility of the screw and edge basal-plane dislocations in hcp 4 He using path-integral Monte Carlo simulations. Our findings provide key insights into recent interpretations of giant plasticity and mass flow junction experiments. First, both dislocations are dissociated into nonsuperfluid Shockley partial dislocations separated by ribbons of stacking fault, suggesting that they are unlikely to act as one-dimensional channels that may display Lüttinger-liquid-like behavior. Second, the centroid positions of the partial cores are found to fluctuate substantially, even in the absence of applied shear stresses. This implies that the lattice resistance to motion of the partial dislocations is negligible, consistent with the recent experimental observations of giant plasticity. Our results indicate that both the structure of the partial cores and the zero-point fluctuations play a role in this extreme mobility.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Effect of Chain Length Dispersity on the Mobility of Entangled Polymers

While nearly all theoretical and computational studies of entangled polymer melts have focused on uniform samples, polymer synthesis routes always result in some dispersity, albeit narrow, of distribution of molecular weights ( Ð M = M w / M n ~ 1.02 – 1.04 ). Here, the effects of dispersity on chain mobility are studied for entangled, disperse melts using a coarse-grained model for polyethylene. Polymer melts with chain lengths set to follow a Schulz-Zimm distribution for the same average M w = 36 kg / mol with Ð M = 1.0 to 1.16, were studied for times of 600 – 800 μ s using molecular dynamics simulations. This time frame is longer than the time required to reach the diffusive regime. We find that dispersity in this range does not affect the entanglement time or tube diameter. However, while there is negligible difference in the average mobility of chains for the uniform distribution Ð M = 1.0 and Ð M = 1.02 , the shortest chains move significantly faster than the longest ones offering a constraint release pathway for the melts for larger Ð M .

36 MATERIALS SCIENCE↗
DOE OSTI2018

Electron-Phonon Scattering in the Presence of Soft Modes and Electron Mobility in SrTiO 3 Perovskite from First Principles

Structural phase transitions and soft phonon modes pose a longstanding challenge to computing electron-phonon (e-ph) interactions in strongly anharmonic crystals. Here we develop a first-principles approach to compute e-ph scattering and charge transport in materials with anharmonic lattice dynamics. Our approach employs renormalized phonons to compute the temperature-dependent e-ph coupling for all phonon modes, including the soft modes associated with ferroelectricity and phase transitions. We show that the electron mobility in cubic SrTiO 3 is controlled by scattering with longitudinal optical phonons at room temperature and with ferroelectric soft phonons below 200 K. Our calculations can accurately predict the temperature dependence of the electron mobility in SrTiO 3 between 150-300 K, and reveal the microscopic origin of its roughly T -3 trend. Our approach enables first-principles calculations of e-ph interactions and charge transport in broad classes of crystals with phase transitions and strongly anharmonic phonons.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Mechanical Resonances of Mobile Impurities in a One-Dimensional Quantum Fluid

We study a one-dimensional interacting quantum liquid hosting a pair of mobile impurities causing backscattering. We determine the effective retarded interaction between the two impurities mediated by the liquid. Here, we show that for strong backscattering this interaction gives rise to resonances and antiresonances in the finite-frequency mobility of the impurity pair. At the antiresonances, the two impurities remain at rest even when driven by a (small) external force. At the resonances, their synchronous motion follows the external drive in phase and reaches maximum amplitude. Using a perturbative renormalization group analysis in quantum tunneling across the impurities, we study the range of validity of our model. We predict that these mechanical antiresonances are observable in experiments on ultracold atom gases confined to one dimension.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Working principles of doping-well structures for high-mobility two-dimensional electron systems

Suppressing electron scattering is essential to achieve high-mobility two-dimensional electron systems (2DESs) that are clean enough to probe exotic interaction-driven phenomena. In heterostructures it is common practice to utilize modulation doping, where the ionized dopants are physically separated from the 2DES channel. The doping-well structure augments modulation doping by providing additional screening for all types of charged impurities in the vicinity of the 2DES, which is necessary to achieve record-breaking samples. Despite its prevalence in the design of ultrahigh-mobility 2DESs, the working principles of the doping-well structure have not been reported. As such, here we elaborate on the mechanics of electron transfer from doping wells to the 2DES, focusing on GaAs/AlGaAs samples grown by molecular beam epitaxy. Based on this understanding we demonstrate how structural parameters in the doping well can be varied to tune the properties of the 2DES.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Monolayer 2D semiconducting tellurides for high-mobility electronics

Discovery and design of two-dimensional (2D) materials with suitable band gaps and high carrier mobility are of vital importance for the photonics, optoelectronics, and high-speed electronics. In this work, based on first principles calculations using density functional theory with Perdew-Burke-Ernzerhof and Heyd-Scuseria-Ernzerhoffunctionals, we introduce a family of monolayer isostructural semiconducting tellurides MNTe 4 , with M={Ti,Zr,Hf} and N={Si,Ge}. We find that these compounds have been identified to possess direct band gaps from 1.0 to 1.31 eV, which are well suited for photonics and optoelectronics applications. Additionally, anisotropic in-plane transport behavior is observed, and small electron and hole (0.11–0.15m e ) effective masses are identified along the dominant transport direction. Ultrahigh carrier mobility is predicted for this family of 2D compounds, which host great promise for potential applications in high-speed electronic devices. Furthermore, detailed analysis of electronic structures reveals the origins of the promising properties of this unique class of 2D telluride materials.

2-dimensional systems↗
DOE OSTI2024

Intrinsic Limits of Charge Carrier Mobilities in Layered Halide Perovskites

Layered halide perovskites have emerged as potential alternatives to three-dimensional (3D) halide perovskites due to their improved stability and larger material phase space, allowing fine tuning of structural, electronic, and optical properties. However, their charge carrier mobilities are significantly smaller than those of 3D halide perovskites, which has a considerable impact on their application in optoelectronic devices. Here, we employ state-of-the-art approaches to unveil the electron-phonon mechanisms responsible for the diminished transport properties of layered halide perovskites. Starting from a prototypical A M X 3 halide perovskite, we model the case of n = 1 and n = 2 layered structures and compare their electronic and transport properties to the 3D reference. The electronic and phononic properties are investigated within density functional theory (DFT) and density functional perturbation theory (DFPT), while transport properties are obtained via the Boltzmann transport equation. The vibrational modes contributing to charge carrier scattering are investigated and associated with polar-phonon scattering mechanisms arising from the long-range Fröhlich coupling and deformation-potential scattering processes. Our investigation reveals that the lower mobilities in layered systems primarily originate from the increased electronic density of states at the vicinity of the band edges, while the electron-phonon coupling strength remains similar. Such an increase is caused by the dimensionality reduction and the break in octahedra connectivity along the stacking direction. Our findings provide a fundamental understanding of the electron-phonon coupling mechanisms in layered perovskites and highlight the intrinsic limitations of the charge carrier transport in these materials. Published by the American Physical Society 2024

Cucco, Bruno (ORCID:0000000331564143)↗
DOE OSTI2019

Exploring the mobility and energy implications of shared versus private autonomous vehicles

The prevailing trend toward greater automation and connectivity requires modeling and analysis tools to explore connectivity, automation, decision science and other future mobility issues at multiple scales. This paper describes various modeling efforts in order to model the mobility and energy impact of autonomous and connected technologies; design of scenarios under different technological, behavioral, and socioeconomic assumptions; and finally, key findings from the scenario runs enabled by the advanced models developed. The integrated ABM-DTA software POLARIS has been extended to include transit, intra-household vehicle sharing, transportation network company (TNC) operations along with updates to the mesoscopic traffic models and value of time adjustments due to new technologies affecting the mode, destination, and route choice. The three scenarios are summarized as high sharing - low automation, high sharing - high automation, and low sharing - high automation, with VMT changes ranging from -13% to 42%.

Multi-autonomous Vehicle Studies, Models, Techniqu↗
DOE OSTI2022

Temperature Dependence of Charge Distributions and Carrier Mobility in an Undoped Si/SiGe Heterostructure

Capacitance–voltage (C–V ) characteristics and carrier transport properties of 2-D electron gases (2DEGs) in an undoped Si/SiGe heterostructure at T = 4 – 35 K are presented here. In this work, two capacitance plateaus due to density saturation of the 2DEG in the buried Si quantum well (QW) are observed and explained by a model of surface tunneling. The peak mobility at 4 K is 4.1 × 10 5 cm 2 /V·s and enhanced by a factor of 1.97 at an even lower carrier density compared to the saturated carrier density, which is attributed to the effect of remote carrier screening. At T = 35 K, the mobility enhancement with a factor of 1.35 is still observed, which suggests the surface tunneling is still dominant.

42 ENGINEERING↗
DOE OSTI2019

Closed-Loop Error Learning Control for Uncertain Nonlinear Systems With Experimental Validation on a Mobile Robot

This article develops a closed-loop error learning control (CLELC) algorithm for feedback linearizable systems with experimental validation on a mobile robot. Traditional feedback and feedforward controllers are designed based on the nominal model by using the feedback linearization control (FLC) method. Then, an intelligent controller is designed based on the sliding mode learning algorithm that utilizes closed-loop error dynamics to learn the system behavior. The controllers are working in parallel, and the intelligent controller can gradually replace the feedback controller from the control of the system. In addition to the stability of the sliding mode learning algorithm, the closed-loop stability of an nth-order feedback linearizable system is proven. The simulation results demonstrate that the CLELC algorithm can improve control performance (e.g., smaller rise time, settling time, and overshoot) in the absence of uncertainties, and also provides robust control performance in the presence of uncertainties as compared to traditional FLC method. To test the efficiency and efficacy of the CLELC algorithm, the trajectory tracking problem of a tracked mobile robot is studied in real time. The experimental results demonstrate that the CLELC algorithm ensures highly accurate trajectory tracking performance than traditional FLC method.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2013

Evaluation of the safety of mobile units for the conditioning of radioactive waste

In Germany mobile units are used to treat and condition radioactive waste. On behalf of the relevant authorities TUV SUD Industrie Service GmbH evaluates their safety. In this paper we outline the general procedure we apply and point out typical results. Generally, a generic safety case evaluates the effects of incidents and accidents and its consequences for the workers and the public. Special care is necessary to define the radioactive inventory, the nuclide composition and the mobility of the radioactive substances. A systems analysis is carried out. Typical aspects to be considered are the handling procedures, the measurement devices and automatic actions. From the various possible malfunctions the critical ones have to be identified. Generally one or only a few scenarios have to be considered in detail. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2013

Impact of proton irradiation on dc performance of AlGaN/GaN high electron mobility transistors

The effects of proton irradiation dose on dc characteristics and the reliability of AlGaN/GaN high electron mobility transistors (HEMTs) were investigated. The HEMTs were irradiated with protons at a fixed energy of 5 MeV and doses ranging from 109 to 2 1014 cm-2. For the dc characteristics, there was only minimal degradation of saturation drain current (IDSS), transconductance (gm), electron mobility and sheet carrier concentration at doses below 2 1013 cm-2, while the reduction of these parameters were 15%, 9%, 41% and 16.6%, respectively, at a dose of 2 1014 cm-2. At this same dose condition, increases of 37% in drain breakdown voltage (VBR) and of 45% in critical voltage (Vcri) were observed. The improvement of device reliability was attributed to the modification of the depletion region due to the introduction of a higher density of defects after irradiation at a higher dose.

Liu, L.↗
DOE OSTI2015

Effect of proton irradiation energy on AlGaN/GaN metal-oxide semiconductor high electron mobility transistors

The effects of proton irradiation energy on dc characteristics of AlGaN/GaN metal-oxide semiconductor high electron mobility transistors (MOSHEMTs) using Al 2 O 3 as the gate dielectric were studied. Al 2 O 3 /AlGaN/GaN MOSHEMTs were irradiated with a fixed proton dose of 5 × 10 15 cm -2 at different energies of 5, 10, or 15 MeV. More degradation of the device dc characteristics was observed for lower irradiation energy due to the larger amount of nonionizing energy loss in the active region of the MOSHEMTs under these conditions. The reductions in saturation current were 95.3%, 68.3%, and 59.8% and reductions in maximum transconductance were 88%, 54.4%, and 40.7% after 5, 10, and 15 MeV proton irradiation, respectively. Both forward and reverse gate leakage current were reduced more than one order of magnitude after irradiation. The carrier removal rates for the irradiation energies employed in this study were in the range of 127–289 cm -1 . These are similar to the values reported for conventional metal-gate high-electron mobility transistors under the same conditions and show that the gate dielectric does not affect the response to proton irradiation for these energies.

36 MATERIALS SCIENCE↗
DOE OSTI2014

TU-F-17A-05: Calculating Tumor Trajectory and Dose-Of-The-Day for Highly Mobile Tumors Using Cone-Beam CT Projections

Purpose: Cone-beam CT (CBCT) projection images provide anatomical data in real-time over several respiratory cycles, forming a comprehensive picture of tumor movement. We developed a method using these projections to determine the trajectory and dose of highly mobile tumors during each fraction of treatment. Methods: CBCT images of a respiration phantom were acquired, where the trajectory mimicked a lung tumor with high amplitude (2.4 cm) and hysteresis. A template-matching algorithm was used to identify the location of a steel BB in each projection. A Gaussian probability density function for tumor position was calculated which best fit the observed trajectory of the BB in the imager geometry. Two methods to improve the accuracy of tumor track reconstruction were investigated: first, using respiratory phase information to refine the trajectory estimation, and second, using the Monte Carlo method to sample the estimated Gaussian tumor position distribution. 15 clinically-drawn abdominal/lung CTV volumes were used to evaluate the accuracy of the proposed methods by comparing the known and calculated BB trajectories. Results: With all methods, the mean position of the BB was determined with accuracy better than 0.1 mm, and root-mean-square (RMS) trajectory errors were lower than 5% of marker amplitude. Use of respiratory phase information decreased RMS errors by 30%, and decreased the fraction of large errors (>3 mm) by half. Mean dose to the clinical volumes was calculated with an average error of 0.1% and average absolute error of 0.3%. Dosimetric parameters D90/D95 were determined within 0.5% of maximum dose. Monte-Carlo sampling increased RMS trajectory and dosimetric errors slightly, but prevented over-estimation of dose in trajectories with high noise. Conclusions: Tumor trajectory and dose-of-the-day were accurately calculated using CBCT projections. This technique provides a widely-available method to evaluate highly-mobile tumors, and could facilitate better strategies to mitigate or compensate for motion during SBRT.

60 APPLIED LIFE SCIENCES↗