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At least 343 records · Page 19

All sources NASA NTRS DOE OSTI

DOE OSTI2016

Distributed modeling of ablation (1996–2011) and climate sensitivity on the glaciers of Taylor Valley, Antarctica

Here, the McMurdo Dry Valleys of Antarctica host the coldest and driest ecosystem on Earth, which is acutely sensitive to the availability of water coming from glacial runoff. We modeled the spatial variability in ablation and assessed climate sensitivity of the glacier ablation zones using 16 years of meteorological and surface mass-balance observations collected in Taylor Valley. Sublimation was the primary form of mass loss over much of the ablation zones, except for near the termini where melt, primarily below the surface, dominated. Microclimates in ~10 m scale topographic basins generated melt rates up to ten times higher than over smooth glacier surfaces. In contrast, the vertical terminal cliffs on the glaciers can have higher or lower melt rates than the horizontal surfaces due to differences in incoming solar radiation. The model systematically underpredicted ablation for the final 5 years studied, possibly due to an increase of windblown sediment. Surface mass-balance sensitivity to temperature was ~–0.02 m w.e. K –1 , which is among the smallest magnitudes observed globally. We also identified a high sensitivity to ice albedo, with a decrease of 0.02 having similar effects as a 1 K increase in temperature, and a complex sensitivity to wind speed.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2019

Valley and Zeeman Splittings in Multilayer Epitaxial Graphene Revealed by Circular Polarization Resolved Magneto-infrared Spectroscopy

Circular-polarization-resolved magneto-infrared studies of multilayer epitaxial graphene (MEG) are performed using tunable quantum cascade lasers in high magnetic fields up to 17.5 T. Landau level (LL) transitions in the monolayer and bilayer graphene inclusions of MEG are resolved, and considerable electron–hole asymmetry is observed in the extracted electronic band structure. For monolayer graphene, a four-fold splitting of the $n$ = 0 to $n$ = 1 LL transition is evidenced and attributed to the lifting of the valley and spin degeneracy of the zeroth LL and the broken electron–hole symmetry. The magnetic field dependence of the splitting further reveals its possible mechanisms. The best fit to experimental data yields effective $g$-factors, $g^*_{vs}$ = 6.7 and $g^*_{ZS}$ = 4.8, for the valley and Zeeman splittings, respectively.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Physical Controls on the Hydrology of Perennially Ice-Covered Lakes, Taylor Valley, Antarctica (1996–2013)

The McMurdo Dry Valleys, Antarctica, are a polar desert populated with numerous closed-watershed, perennially ice-covered lakes primarily fed by glacial melt. Lake levels have varied by as much as 8 m since 1972 and are currently rising after a decade of decreasing. Precipitation falls as snow, so lake hydrology is dominated by energy available to melt glacier ice and to sublimate lake ice. To understand the energy and hydrologic controls on lake level changes and to explain the variability between neighboring lakes, only a few kilometers apart, we model the hydrology for the three largest lakes in Taylor Valley. Here, we apply a physically based hydrological model that includes a surface energy balance model to estimate glacial melt and lake sublimation to constrain mass fluxes to and from the lakes. Results show that lake levels are very sensitive to small changes in glacier albedo, air temperature, and wind speed. We were able to balance the hydrologic budget in two watersheds using meltwater inflow and sublimation loss from the ice-covered lake alone. A third watershed, closest to the coast, required additional inflow beyond model uncertainties. We hypothesize a shallow groundwater system within the active layer, fed by dispersed snow patches, contributes 23% of the inflow to this watershed. The lakes are out of equilibrium with the current climate. If the climate of our study period (1996–2013) persists into the future, the lakes will reach equilibrium starting in 2300, with levels 2–17 m higher, depending on the lake, relative to the 2020 level.

58 GEOSCIENCES↗

DOE OSTI2025

Exploring Water System Vulnerabilities in California's Central Valley Under the Late Renaissance Megadrought and Climate Change

Abstract California faces cycles of drought and flooding that are projected to intensify, but these extremes may impact water users across the state differently due to the region's natural hydroclimate variability and complex institutional framework governing water deliveries. To assess these risks, this study introduces a novel exploratory modeling framework informed by paleo and climate‐change based scenarios to better understand how impacts propagate through the Central Valley's complex water system. A stochastic weather generator, conditioned on tree‐ring data, produces a large ensemble of daily weather sequences conditioned on drought and flood conditions under the Late Renaissance Megadrought period (1550–1580 CE). Regional climate changes are applied to this weather data and drive hydrologic projections for the Sacramento, San Joaquin, and Tulare Basins. The resulting streamflow ensembles are used in an exploratory stress test using the California Food‐Energy‐Water System model, a highly resolved, daily model of water storage and conveyance throughout California's Central Valley. Results show that megadrought conditions lead to unprecedented reductions in inflows and storage at major California reservoirs. Both junior and senior water rights holders experience multi‐year periods of curtailed water deliveries and complete drawdowns of groundwater assets. When megadrought dynamics are combined with climate change, risks for unprecedented depletion of reservoir storage and sustained curtailment of water deliveries across multiple years increase. Asymmetries in risk emerge depending on water source, rights, and access to groundwater banks.

Gupta, Rohini S. [School of Civil and Environmenta↗

DOE OSTI2022

Inter-valley coherent order and isospin fluctuation mediated superconductivity in rhombohedral trilayer graphene

Superconductivity was recently discovered in rhombohedral trilayer graphene (RTG) in the absence of a moiré potential. Superconductivity is observed proximate to a metallic state with reduced isospin symmetry, but it remains unknown whether this is a coincidence or a key ingredient for superconductivity. Using a Hartree-Fock analysis and constraints from experiments, we argue that the symmetry breaking is inter-valley coherent (IVC) in nature. We evaluate IVC fluctuations as a possible pairing glue, and find that they lead to chiral unconventional superconductivity when the fluctuations are strong. We further elucidate how the inter-valley Hund’s coupling determines the spin-structure of the IVC ground state and breaks the degeneracy between spin-singlet and triplet superconductivity. Remarkably, if the normal state is spin-unpolarized, we find that a ferromagnetic Hund’s coupling favors spin-singlet superconductivity, in agreement with experiments. Instead, if the normal state is spin-polarized, then IVC fluctuations lead to spin-triplet pairing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2026

Hidden Spin-Valley Locking Stabilizes Nanosecond Spin Polarization in 2D Perovskites

Room-temperature spin control in semiconductors is fundamental to spin-optoelectronics. Although inversion symmetry breaking offers one path for spin control in semiconductors, strong spin dephasing at elevated temperatures remains a persistent limitation. Hidden spin polarization without global symmetry breaking provides another promising material design strategy, but robust spin stabilization from this effect has yet to be experimentally realized. Here we show spin stabilization at room temperature in two-dimensional hybrid organic-inorganic perovskites through hidden spin-valley locking. We use functional non-primary ammonium cations to induce symmetry-breaking distortions in the metal-halide layers, producing giant local spin splitting while preserving global inversion symmetry. Time-resolved circular dichroism measurements reveal optically generated spin-polarized carriers that persist for 686 ps in (AzOH)2PbI4 and 4.7 ns in the lead-free analogue (AzOH)2SnI4 at room temperature, without an external magnetic field. First-principles calculations suggest that these long lifetimes arise from hidden spin valleys, enhanced dielectric screening and reduced spin-orbit-induced scattering in the Sn-based compound. Our design paradigm unlocks inversion-symmetric semiconductors with ultralong spin lifetimes, broadening the materials options for light-driven spin control.

14 SOLAR ENERGY↗

DOE OSTI2019

Identification of spin, valley and moiré quasi-angular momentum of interlayer excitons

Moiré superlattices provide a powerful way to engineer properties of electrons and excitons in two-dimensional van der Waals heterostructures. The moir\'e effect can be especially strong for interlayer excitons, where electrons and holes reside in different layers and can be addressed separately. In particular, it was recently proposed that the moiré superlattice potential not only localizes interlayer exciton states at different superlattice positions, but also hosts an emerging moiré quasi-angular momentum (QAM) that periodically switches the optical selection rules for interlayer excitons at different moiré sites. Here we report the observation of multiple interlayer exciton states coexisting in a WSe 2 /WS 2 moiré superlattice and unambiguously determine their spin, valley, and moiré QAM through novel resonant optical pump-probe spectroscopy and photoluminescence excitation spectroscopy. We demonstrate that interlayer excitons localized at different moiré sites can exhibit opposite optical selection rules due to the spatially-varying moiré QAM. Finally, our observation reveals new opportunities to engineer interlayer exciton states and valley physics with moiré superlattices for optoelectronic and valleytronic applications.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2023

Ga 4 C-family crystals, a new generation of star thermoelectric materials, achieved by band degeneracies, valley anisotropy, and strong phonon scattering among others

Using density functional theory combined with the Boltzmann transport equation, we uncover a new class of high-performance thermoelectric materials, i.e., supertetrahedral Ga 4 C-family materials characterized by an ultrahigh thermoelectric figure of merit (ZT), which is produced both by a large power factor and ultralow lattice thermal conductivity. The former is contributed by multiple electronic band degeneracies, flat bands and valley anisotropy, while the latter is contributed both by strong phonon scattering and low phonon group velocity. Furthermore, we find that tensile strain engineering may further suppress lattice thermal conductivity, while well retaining the multi-band degeneracies, valley anisotropy and flat bands. In addition to the band degeneracy, the scattering mechanism of the system can also dominate the power factor in the process of strain implementation. Consequently, a novel thermoelectric phenomenon, i.e., a high thermoelectric figure of merit, is observed in Ca 4 C with a ZT value larger than 4.4 under a small strain ratio (0.75%). In conclusion, our theoretical studies not only uncover a new class of high-performance thermoelectric materials integrating multiple effective mechanisms, but also establish a realistic material plateau to investigate the competitive effect of multiple factors on enhancing the thermoelectric figure of merit.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

Resonant spin and valley polarization in ferromagnetic silicene quantum well

We propose a silicene-based lateral resonant tunneling device by placing silicene under the modulation of top nonmagnetic/ferromagnetic/nonmagnetic sandwich nanogates. Following the electric-tunable bandgap of silicene, lateral double-barrier structure is formed by imposing the flexible electrostatic modulation on top gates. By aligning the spin and valley-resolved confined states in magnetic well, remarkable spin/valley polarization can be accessed through spinor-relying resonant tunneling mechanism. Under the electrostatic, magnetic, and size manipulation, the confined well state can be efficiently engineered, and the observed spin and valley polarization can be further flexibly tuned, offering some helpful strategies to construct spinor-electronic logic atomically.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2014

Persistent currents in absence of magnetic field in graphene nanorings: The ambiguous role of inter valley scattering

Persistent currents can arise in normal-metal rings due to a magnetic flux threading the ring in equilibrium. However, can persistent currents arise in absence of magnetic flux in the same normal-metal rings? Yes they can but in a non-equilibrium set-up. This is known as current magnification. In this work, we show that current magnification can be seen in graphene nanorings. Further, graphene can have electrons polarized with a valley quantum number. Electron scattering between valleys can have a non-trivial effect on these persistent currents including inducing a sign change and generating them for parameters where none existed to begin with.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Valley filter from magneto-tunneling between single and bi-layer graphene

We consider tunneling transport between two parallel graphene sheets; where one is a single-layer sample and the other one a bi-layer. In the presence of an in-plane magnetic field, the interplay between combined energy and momentum conservation in a tunneling event and the distinctive chiral nature of charge carriers in the two systems turns out to favor tunneling of electrons from one of the two valleys in the graphene Brillouin zone. Adjusting the field strength enables manipulation of the valley polarization of the current, which reaches its maximum value of 100% concomitantly with a maximum of the tunneling conductance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Polarized spin and valley transport across ferromagnetic silicene junctions

We study ballistic transport of Dirac fermions through silicene barriers, of width d, with an exchange field M and metallic gates above them that provide tunable potentials of height U. Away from the Dirac point (DP), the spin- and valley-resolved conductances, as functions of U, exhibit resonances and close to it a pronounced dip that becomes a transport gap when an appropriate electric field E{sub z} is applied. The charge conductance g{sub c} of such a junction changes from oscillatory to a monotonically decreasing function of d beyond a critical E{sub z}. This change of g{sub c} can be used to realize electric-field-controlled switching. The field M splits each resonance of g{sub c} in two spin-resolved peaks. The spin p{sub s} and valley p{sub v} polarizations of the current near the DP increase with E{sub z} or M and become nearly perfect above certain of their values. We also show that p{sub s} and p{sub v} can be inverted either by reversing the polarity of U or the direction of M. For two barriers, there is no splitting in g{sub c} when the fields M are in opposite directions. Most of these phenomena have no analogs in graphene.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

The sensitivity of the electron transport within bulk zinc-blende gallium nitride to variations in the crystal temperature, the doping concentration, and the non-parabolicity coefficient associated with the lowest energy conduction band valley

Within the framework of a semi-classical three-valley Monte Carlo simulation approach, we analyze the steady-state and transient electron transport that occurs within bulk zinc-blende gallium nitride. In particular, we examine how the steady-state and transient electron transport that occurs within this material changes in response to variations in the crystal temperature, the doping concentration, and the non-parabolicity coefficient associated with the lowest energy conduction band valley. These results are then contrasted with those corresponding to a number of other compound semiconductors of interest.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2024

Ultrafast field-driven valley polarization of transition metal dichalcogenide quantum dots

Abstract We study theoretically the electron dynamics of transition metal dichalcogenide (TMDC) quantum dots (QDs) in the field of an ultrashort and ultrafast circularly polarized optical pulse. The QDs have the shape of a disk and their electron systems are described within an effective model with infinite mass boundary conditions. Similar to TMDC monolayers, a circularly polarized pulse generates ultrafast valley polarization of such QDs. The dependence of the valley polarization on the size of the dot is sensitive to the dot material and, for different materials, show both monotonic increase with the dot radius and nonmonotonic behavior with a local maximum at a finite dot radius.

Physics↗

DOE OSTI2010

NGC 404: A REJUVENATED LENTICULAR GALAXY ON A MERGER-INDUCED, BLUEWARD EXCURSION INTO THE GREEN VALLEY

We have discovered recent star formation in the outermost portion ((1-4) x R {sub 25}) of the nearby lenticular (S0) galaxy NGC 404 using Galaxy Evolution Explorer UV imaging. FUV-bright sources are strongly concentrated within the galaxy's H I ring (formed by a merger event according to del RIo et al.), even though the average gas density is dynamically subcritical. Archival Hubble Space Telescope imaging reveals resolved upper main-sequence stars and conclusively demonstrates that the UV light originates from recent star formation activity. We present FUV, NUV radial surface brightness profiles, and integrated magnitudes for NGC 404. Within the ring, the average star formation rate (SFR) surface density ({Sigma}{sub SFR}) is {approx}2.2 x 10{sup -5} M {sub sun} yr{sup -1} kpc{sup -2}. Of the total FUV flux, 70% comes from the H I ring which is forming stars at a rate of 2.5 x 10{sup -3} M {sub sun} yr{sup -1}. The gas consumption timescale, assuming a constant SFR and no gas recycling, is several times the age of the universe. In the context of the UV-optical galaxy color-magnitude diagram, the presence of the star-forming H I ring places NGC 404 in the green valley separating the red and blue sequences. The rejuvenated lenticular galaxy has experienced a merger-induced, disk-building excursion away from the red sequence toward bluer colors, where it may evolve quiescently or (if appropriately triggered) experience a burst capable of placing it on the blue/star-forming sequence for up to {approx}1 Gyr. The green valley galaxy population is heterogeneous, with most systems transitioning from blue to red but others evolving in the opposite sense due to acquisition of fresh gas through various channels.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2019

Topological resonance and single-optical-cycle valley polarization in gapped graphene

For gapped graphene, we predict that an intense ultrashort (single-oscillation) circularly polarized optical pulse can induce a large population of the conduction band and a large valley polarization. With an increase in the band gap, the magnitude of the valley polarization gradually increases from zero (for the native gapless graphene) to a value on the order of unity. The energy bandwidth of the electrons excited into the conduction band can be very large (≳10 eV for a reasonable pulse amplitude of ~0.5 V/Å). These phenomena are due to the effect of topological resonance: The matching of the topological (geometric) phase and the dynamic phase. Gapped graphene with a tunable band gap can be used as a convenient generic model of two-dimensional semiconductors with honeycomb generic lattice structures and broken inversion symmetry, such as transition-metal dichalcogenides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Valley-Polarized Quantum Anomalous Hall Effect in Ferrimagnetic Honeycomb Lattices

The valley-polarized quantum anomalous Hall effect (VP-QAHE), which combines valleytronics and topology in one material, is of significant fundamental and practical importance in condensed-matter physics and materials science. In previous model studies, VP-QAHE occurs under strong Rashba spin-orbit coupling (SOC), which is an extrinsic effect. In this report, using a low energy k • p model, we propose a different mechanism of VP-QAHE by introducing an intrinsic staggered magnetic exchange field and develop a general picture of valley dependent band inversion in honeycomb lattice. Using first-principles calculation, this new mechanism is further demonstrated in the Co decorated In-triangle adlayer on a Si(111) surface. This system is equivalent to a ferrimagnetic honeycomb lattice, and the supported adlayer is experimentally more feasible in synthesis, thus exhibiting advantages over the existing studies based on Rashba SOC and free-standing sheets. The underlying physical mechanism is generally applicable, opening a new avenue for exploration of substrate supported VP-QAHE.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2017

Gate-Controlled Spin-Valley Locking of Resident Carriers in WSe 2 Monolayers

Using time-resolved Kerr rotation, we measure the spin-valley dynamics of resident electrons and holes in single charge-tunable monolayers of the archetypal transition-metal dichalcogenide (TMD) semiconductor WSe 2 . In the n-type regime, we observe long (~ 130 ns) polarization relaxation of electrons that is sensitive to in-plane magnetic fields B y , indicating spin relaxation. In marked contrast, extraordinarily long (~ 2 μs) polarization relaxation of holes is revealed in the p-type regime, which is unaffected by B y , directly confirming long-standing expectations of strong spin-valley locking of holes in the valence band of monolayer TMDs. Finally, supported by continuous-wave Kerr spectroscopy and Hanle measurements, these studies provide a unified picture of carrier polarization dynamics in monolayer TMDs, which can guide design principles for future valleytronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗