Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “VALLEYS”

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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

The Wintertime Covariation of CO 2 and Criteria Pollutants in an Urban Valley of the Western United States

Numerous mountain valleys experience wintertime particulate pollution events, when persistent cold air pools (PCAPs) develop and inhibit atmospheric mixing, leading to the accumulation of pollutants. Here we examine the relationships between trace gases and criteria pollutants during winter in Utah’s Salt Lake Valley, in an effort to better understand the roles of transport versus chemical processes during differing meteorological conditions as well as insights into how targeted reductions in greenhouse gases will impact local air quality in varying meteorological conditions. CO 2 is a chemically inert gas that is coemitted during fossil fuel combustion with pollutants. Many of these coemitted pollutants are precursors that react chemically to form secondary particulate matter. Thus, CO 2 can serve as a stable tracer and potentially help distinguish transport versus chemical influences on pollutants. During the winter of 2015–2016, we isolated enhancements in CO 2 over baseline levels due to urban emissions (“CO 2ex ”). CO 2ex was paired with similar excesses in other pollutant concentrations. These relationships were examined during different wintertime conditions and stages of pollution episodes: (a) Non-PCAP, (b) beginning, and (c) latter stages of an episode. We found that CO 2ex is a good indicator of the presence of gaseous criteria pollutants and a reasonable indicator of PM2.5. Additionally, the relationships between CO 2ex and criteria pollutants differ during different phases of PCAP events which provide insight into meteorological and transport processes. Lastly, we found a slight overestimation of CO:CO 2 emission ratios and a considerable overestimation of NO x :CO 2 by existing inventories for the Salt Lake Valley.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

The Importance of Avoided Crossings in Understanding High Valley Degeneracy in Half‐Heusler Thermoelectric Semiconductors

Abstract Half‐Heusler (hH) compounds are promising candidates for inexpensive, low‐toxicity thermoelectric materials. It is well known that engineering electronic bands with high valley degeneracy is an effective approach for enhancing the performance of thermoelectric materials, and there are several routes for achieving high valley degeneracy in hH systems. For instance, there are multiple locations in the first Brillouin zone where the valence band maximum can be found (at the Γ‐, L‐, or W‐point), and there are two competing low‐lying conduction bands at the X‐point, where the conduction band minimum is located. By converging the multiple valence band and conduction band extrema, the valley degeneracy, and hence, performance of these materials can be improved. Here, group theoretical and tight‐binding approaches, in addition to first‐principles density functional theory calculations, are used to study the chemical origins of various band extrema in both the n‐type and p‐type compounds, with particular focus on ZrNiSn and NbFeSb. Specifically, the importance of avoided crossings is explained. The results of this work can be used to better understand and develop design strategies for engineering better performing hH thermoelectrics.

Brod, Madison K.↗
DOE OSTI2022

Control of valley polarization in gapped graphene by linearly polarized ultrashort optical pulse

We present a versatile model of gapped graphene to manipulate valley polarization by using ultrafast linearly polarized pulses of zero areas. First, a circularly polarized pulse of a single oscillation produces a valley-selective population of the conduction band by means of topological resonance. Then we apply a linearly polarized femtosecond-long pulse that changes the valley polarization. The magnitude of such a change depends on the amplitude and the direction of polarization of the pulse. In conclusion, our protocol provides a favorable platform for applications in valleytronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Organic matter distribution in the icy environments of Taylor Valley, Antarctica

Glaciers can accumulate and release organic matter affecting the structure and function of associated terrestrial and aquatic ecosystems. Here we analyzed 18 ice cores collected from six locations in Taylor Valley (McMurdo Dry Valleys), Antarctica to determine the spatial abundance and quality of organic matter, and the spatial distribution of bacterial density and community structure from the terminus of the Taylor Glacier to the coast (McMurdo Sound). Our results showed that dissolved and particulate organic carbon (DOC and POC) concentrations in the ice core samples increased from the Taylor Glacier to McMurdo Sound, a pattern also shown by bacterial cell density. Fluorescence Excitation Emission Matrices Spectroscopy (EEMs) and multivariate parallel factor (PARAFAC) modeling identified one humic-like (C1) and one protein-like (C2) component in ice cores whose fluorescent intensities all increased from the Polar Plateau to the coast. The fluorescence index showed that the bioavailability of dissolved organic matter (DOM) also decreased from the Polar Plateau to the coast. Partial least squares path modeling analysis revealed that bacterial abundance was the main positive biotic factor influencing both the quantity and quality of organic matter. Marine aerosol influenced the spatial distribution of DOC more than katabatic winds in the ice cores. Certain bacterial taxa showed significant correlations with DOC and POC concentrations. Collectively, our results show the tight connectivity among organic matter spatial distribution, bacterial abundance and meteorology in the McMurdo Dry Valley ecosystem.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

Controlling Valley-Specific Light Emission from Monolayer MoS 2 with Achiral Dielectric Metasurfaces

Excitons in two-dimensional transition metal dichalcogenides have a valley degree of freedom that can be optically manipulated for quantum information processing. Here, we integrate MoS 2 monolayers with achiral silicon disk array metasurfaces to enhance and control valley-specific absorption and emission. Through the coupling to the metasurface electric and magnetic Mie modes, the intensity and lifetime of the emission of neutral excitons, trions, and defect bound excitons can be enhanced and shortened, respectively, while the spectral shape can be modified. Additionally, the degree of polarization (DOP) of exciton and trion emission from the valley can be symmetrically enhanced at 100 K. The DOP increase is attributed to both the metasurface-enhanced chiral absorption of light and the metasurface-enhanced exciton emission from the Purcell effect. Finally, combining Si-compatible photonic design with large-scale 2D materials integration, our work makes an important step toward on-chip valleytronic applications approaching room-temperature operation.

2D materials↗
DOE OSTI2017

Emptying Dirac valleys in bismuth using high magnetic fields

The Fermi surface of elemental bismuth consists of three small rotationally equivalent electron pockets, offering a valley degree of freedom to charge carriers. A relatively small magnetic field can confine electrons to their lowest Landau level. This is the quantum limit attained in other dilute metals upon application of sufficiently strong magnetic field. Here in this paper we report on the observation of another threshold magnetic field never encountered before in any other solid. Above this field, B empty , one or two valleys become totally empty. Drying up a Fermi sea by magnetic field in the Brillouin zone leads to a manyfold enhancement in electric conductance. We trace the origin of the large drop in magnetoresistance across B empty to transfer of carriers between valleys with highly anisotropic mobilities. The non-interacting picture of electrons with field-dependent mobility explains most results but the Coulomb interaction may play a role in shaping the fine details.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Field-tunable valley coupling in a dodecagonal semiconductor quasicrystal

Quasicrystals are characterized by atomic arrangements having long-range order without periodicity. Van der Waals bilayers provide an opportunity to controllably vary the atomic alignment between two layers from a periodic moiré crystal to an aperiodic quasicrystal. Here, in this work, we reveal that in a dodecagonal WSe 2 quasicrystal, two separate valleys in separate layers are brought arbitrarily close in momentum space through higher-order Umklapp scatterings. A modest perpendicular electric field is then sufficient to induce strong interlayer valley hybridization, manifested as another hybrid excitonic doublet. Concurrently, we observe the disappearance of the trion that exists at low field, which we attribute to a modified spatial distribution of the wavefunction due to the quasicrystal potential. This is possibly a precursor to localization. Our findings highlight the ability of incommensurate systems to bring any pair of momenta into close proximity, thereby introducing opportunities for valley engineering.

electronic properties and materials↗
DOE OSTI2018

Carrier dynamics and spin–valley–layer effects in bilayer transition metal dichalcogenides

Transition metal dichalcogenides are an interesting class of low dimensional materials in mono- and few-layer form with diverse applications in valleytronic, optoelectronic and quantum devices. Therefore, the general nature of the band-edges and the interplay with valley dynamics is important from a fundamental and technological standpoint. Bilayers introduce interlayer coupling effects which can have a significant impact on the valley polarization. The combined effect of spin–orbit and interlayer coupling can strongly modify the band structure, phonon interactions and overall carrier dynamics in the material. In this paper we use first-principles calculations of electron–electron and electron–phonon interactions to investigate bilayer MoS 2 and WSe 2 in both the AA' and AB stacking configurations. We find that in addition to spin–orbit coupling, interlayer interactions present in the two configurations significantly alter the near-band-edge dynamics. Scattering lifetimes and dynamic behavior are highly material-dependent, despite the similarities and typical trends in TMDCs. Additionally, we capture significant differences in dynamics for the AA' and AB stacking configurations, with lifetime values differing by up to an order of magnitude between them for MoS 2 . Further, we evaluate the valley polarization times and find that maximum lifetimes at room temperature are of the scale of 1 picosecond for WSe 2 in the AB orientation. These results present a pathway to understanding complex heterostructure configurations and ‘magic angle’ physics in TMDCs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Universal conductance fluctuations as a direct probe to valley coherence and universality class of disordered graphene

We demonstrate that the universal conductance fluctuations (UCF) can be used as a direct probe to study the valley quantum states in disordered graphene. The UCF magnitude in graphene is suppressed by a factor of four at high carrier densities where the short-range disorder essentially breaks the valley degeneracy of the K and K' valleys, leading to a density dependent crossover of symmetry class from symplectic near the Dirac point to orthogonal at high densities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Valley-dependent spin polarization and long-lived electron spins in germanium

Spin orientation and relaxation of conduction band electrons in bulk Ge are addressed by studying the steady-state circular polarization of the indirect gap photoluminescence (PL) at low temperatures. This provides a direct experimental proof of recently predicted spin-dependent selection rules for phonon-mediated optical transitions in Ge. In addition, we observe valley-dependent circularly polarized emission, and map the concomitant redistribution of electron spins within the multi-valley conduction band of Ge by gaining simultaneous access to the circular dichroism of light emitted across the direct and the indirect gap transitions. Finally, the lifetime of L-valley electrons is measured by means of decay curves of the indirect gap PL emission, yielding spin relaxation times in the order of hundreds of ns.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Electric field induced spin and valley polarization within a magnetically confined silicene channel

We study the electronic structure and transport properties of Dirac electrons along a channel created by an exchange field through the proximity of ferromagnets on a silicene sheet. The multiple total internal reflection induces localized states in the channel, which behaves like an electron waveguide. An effect of spin- and valley-filtering originating from the coupling between valley and spin degrees is predicted for such a structure. Interestingly, this feature can be tuned significantly by locally applying electric and exchange fields simultaneously. The parameter condition for observing fully spin- and valley-polarized current is obtained. These findings may be observable in todays' experimental technique and useful for spintronic and valleytronic applications based on silicene.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The unexpected beneficial effect of the L-valley population on the electron mobility of GaAs nanowires

The impact of the L-valley population on the transport properties of GaAs cylindrical nanowires (NWs) is analyzed by numerically calculating the electron mobility under the momentum relaxation time approximation. In spite of its low contribution to the electron mobility (even for high electron populations in small NWs), it is demonstrated to have a beneficial effect, since it significantly favours the Γ-valley mobility by screening the higher Γ-valley energy subbands.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Generation of valley-polarized electron beam in bilayer graphene

We propose a method to produce valley-polarized electron beams using a bilayer graphene npn junction. By analyzing the transmission properties of electrons through the junction with zigzag interface in the presence of trigonal warping, we observe that there exist a range of incident energies and barrier heights in which transmitted electrons are well polarized and collimated. From this observation and by performing numerical simulations, it is demonstrated that valley-dependent electronic currents with nearly perfect polarization can be generated. We also show that the peak-to-peak separation angle between the polarized currents is tunable either by incident energy or by barrier height each of which is controlled by using top and back gate voltages. The results can be used for constructing an electron beam splitter to produce valley-polarized currents.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Energies of the X- and L-valleys in In{sub 0.53}Ga{sub 0.47}As from electronic structure calculations

Several theoretical electronic structure methods are applied to study the relative energies of the minima of the X- and L-conduction-band satellite valleys of In{sub x}Ga{sub 1−x}As with x = 0.53. This III-V semiconductor is a contender as a replacement for silicon in high-performance n-type metal-oxide-semiconductor transistors. The energy of the low-lying valleys relative to the conduction-band edge governs the population of channel carriers as the transistor is brought into inversion, hence determining current drive and switching properties at gate voltages above threshold. The calculations indicate that the position of the L- and X-valley minima are ∼1 eV and ∼1.2 eV, respectively, higher in energy with respect to the conduction-band minimum at the Γ-point.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Valley splitting of single-electron Si MOS quantum dots

Here, silicon-based metal-oxide-semiconductor quantum dots are prominent candidates for high-fidelity, manufacturable qubits. Due to silicon's band structure, additional low-energy states persist in these devices, presenting both challenges and opportunities. Although the physics governing these valley states has been the subject of intense study, quantitative agreement between experiment and theory remains elusive. Here, we present data from an experiment probing the valley states of quantum dot devices and develop a theory that is in quantitative agreement with both this and a recently reported experiment. Through sampling millions of realistic cases of interface roughness, our method provides evidence that the valley physics between the two samples is essentially the same.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Ultrafast valley polarization in bilayer graphene

We study theoretically the interaction of a bilayer graphene with a circularly polarized ultrafast optical pulse of a single oscillation at an oblique incidence. The normal component of the pulse breaks the inversion symmetry of the system and opens up a dynamical bandgap due to which a valley-selective population of the conduction band becomes sensitive to the angle of incident of the pulse. We show that the magnitude of the valley polarization can be controlled by the angle of incidence, the amplitude, and the angle of in-plane polarization of the chiral optical pulse. Subsequently, a sequence of a circularly polarized pulse followed by a linearly polarized femtosecond-long pulse can be used to control and probe the valley polarization created by the preceding pulse. Our protocol provides a favorable platform to design ultrafast all-optical valleytronic information processing.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Full configuration interaction simulations of exchange-coupled donors in silicon using multi-valley effective mass theory

Abstract Donor spins in silicon have achieved record values of coherence times and single-qubit gate fidelities. The next stage of development involves demonstrating high-fidelity two-qubit logic gates, where the most natural coupling is the exchange interaction. To aid the efficient design of scalable donor-based quantum processors, we model the two-electron wave function using a full configuration interaction method within a multi-valley effective mass theory. We exploit the high computational efficiency of our code to investigate the exchange interaction, valley population, and electron densities for two phosphorus donors in a wide range of lattice positions, orientations, and as a function of applied electric fields. The outcomes are visualized with interactive images where donor positions can be swept while watching the valley and orbital components evolve accordingly. Our results provide a physically intuitive and quantitatively accurate understanding of the placement and tuning criteria necessary to achieve high-fidelity two-qubit gates with donors in silicon.

Joecker, Benjamin (ORCID:0000000302635440)↗
DOE OSTI2019

Spin-valley density wave in moiré materials

We introduce and study a minimum two-orbital Hubbard model on a triangular lattice, which captures the key features of both the trilayer ABC-stacked graphene-boron nitride heterostructure and twisted transition metal dichalcogenides in a broad parameter range. Our model comprises first- and second-nearest neighbor hoppings with valley-contrasting flux that accounts for trigonal warping in the band structure. For the strong-coupling regime with one electron per site, we derive a spin-orbital exchange Hamiltonian and find the semiclassical ground state to be a spin-valley density wave. Furthermore, we show that a relatively small second-neighbor exchange interaction is sufficient to stabilize the ordered state against quantum fluctuations. Effects of spin- and valley Zeeman fields as well as thermal fluctuations are also examined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗