583 research outputs found
Multidisciplinary, Multisite Evaluation of Alternative Sagebrush Steppe Restoration Treatments: The SageSTEP Project
This special issue presents short-term ecological effects of restoration treatments imposed as part of the Sagebrush Steppe Treatment Evaluation Project (SageSTEP), and summarizes public attitude survey results related to restoration efforts. Funded by the US Joint Fire Science Program (JFSP; 2005–2011), the Bureau of Land Management (BLM; 2011 to present), the National Interagency Fire Center (2011 to present), and the US Fish and Wildlife Service (2010), SageSTEP was designed and implemented to provide treatment-related information to managers concerned about the rapidly changing condition of sagebrush steppe ecosystems in the US Interior West (McIver et al. 2010). At lower elevations, cheatgrass has become more dominant at the expense of native perennial bunchgrasses, in some locations shifting fire return intervals from &spigt;50–100 yr to &spilt; 20 yr, and greatly increasing mean fire size (Whisenant 1990; Miller et al. 2011; Balch et al. 2012). At higher elevations, piñon pine and juniper woodlands have expanded and displaced sagebrush and other shrubs, in some places shifting fire return intervals from 10–50 yr to &spigt;&spigt; 50 yr, and significantly increasing mean fire severity (Miller and Heyerdahl 2008)
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Nonlinear optical and optoelectronic studies of topological insulator surfaces
Since their experimental discovery in 2008, topological insulators have been catapulted to the forefront of condensed matter physics research owing to their potential to realize both exciting new technologies as well as novel electronic phases that are inaccessible in any other material class. Their exotic properties arise from a rare quantum organization of its electrons called ``topological order,'' which evades the conventional broken symmetry based-classification scheme used to categorize nearly every other state of ordered matter. Instead, topologically ordered phases are classified by topological invariants, which characterize the phase of an electron's wavefunction as it moves through momentum space. When a topologically ordered phase is interfaced with an ordinary phase, such as the vacuum, a novel metallic state appears at their shared boundary. In topological insulators, this results in the formation of a two-dimensional metallic state that spans all of its surfaces. The surface state electronic spectrum is characterized by a single linearly dispersing and helically spin-polarized Dirac cone that is robust against disorder. The helical nature of the surface Dirac cone is highly novel because the Dirac electrons carry a net magnetic moment and are capable of transporting 100% spin-polarized electrical currents, which are the long-sought electronic properties needed for many spin-based electronic applications. However, owing to the small bulk band gap and intrinsic electronic doping inherent to these materials, isolating the surface electronic response from the bulk has proven to be a major experimental obstacle.
In this thesis, we demonstrate the means by which light can be used to isolate and study the surface electronic response of topological insulators using optoelectronic and nonlinear optical techniques. In chapter 1, we overview the physics of topological order and topological insulators. In chapter 2, we show how polarized light can be used to generate and control surface electrical currents that originate from the helical Dirac cone. In chapter 3, we demonstrate that the nonlinear second harmonic generation of light from a topological insulator is a sensitive surface probe and can be used to detect the breaking of space-time symmetries and monitor changes in the surface carrier density.Physic
Valley-selective optical Stark effect in monolayer WS2
Breaking space-time symmetries in two-dimensional crystals (2D) can
dramatically influence their macroscopic electronic properties. Monolayer
transition-metal dichalcogenides (TMDs) are prime examples where the
intrinsically broken crystal inversion symmetry permits the generation of
valley-selective electron populations, even though the two valleys are
energetically degenerate, locked by time-reversal symmetry. Lifting the valley
degeneracy in these materials is of great interest because it would allow for
valley-specific band engineering and offer additional control in valleytronic
applications. While applying a magnetic field should in principle accomplish
this task, experiments to date have observed no valley-selective energy level
shifts in fields accessible in the laboratory. Here we show the first direct
evidence of lifted valley degeneracy in the monolayer TMD WS2. By applying
intense circularly polarized light, which breaks time-reversal symmetry, we
demonstrate that the exciton level in each valley can be selectively tuned by
as much as 18 meV via the optical Stark effect. These results offer a novel way
to control valley degree of freedom, and may provide a means to realize new
valley-selective Floquet topological phases in 2D TMDs
Optical Stark effect in 2D semiconductors
Semiconductors that are atomically thin can exhibit novel optical properties beyond those encountered in the bulk compounds. Monolayer transition-metal dichalcogenides (TMDs) are leading examples of such semiconductors that possess remarkable optical properties. They obey unique selection rules where light with different circular polarization can be used for selective photoexcitation at two different valleys in the momentum space. These valleys constitute bandgaps that are normally locked in the same energy. Selectively varying their energies is of great interest for applications because it unlocks the potential to control valley degree of freedom, and offers a new promising way to carry information in next-generation valleytronics. In this proceeding paper, we show that the energy gaps at the two valleys can be shifted relative to each other by means of the optical Stark effect in a controllable valley-selective manner. We discuss the physics of the optical Stark effect, and we describe the mechanism that leads to its valleyselectivity in monolayer TMD tungsten disulfide (WS[subscript 2]).United States. Department of Energy (DE-FG02-08ER46521)United States. Department of Energy (DESC0006423)National Science Foundation (U.S.) (DMR-0845358)National Science Foundation (U.S.) (DMR-1231319
Light-induced topological magnons in two-dimensional van der Waals magnets
Driving a two-dimensional Mott insulator with circularly polarized light
breaks time-reversal and inversion symmetry, which induces an optically-tunable
synthetic scalar spin chirality interaction in the effective low-energy spin
Hamiltonian. Here, we show that this mechanism can stabilize topological magnon
excitations in honeycomb ferromagnets and in optical lattices. We find that the
irradiated quantum magnet is described by a Haldane model for magnons that
hosts topologically-protected edge modes. We study the evolution of the magnon
spectrum in the Floquet regime and via time propagation of the magnon
Hamiltonian for a slowly varying pulse envelope. Compared to similar but
conceptually distinct driving schemes based on the Aharanov-Casher effect, the
dimensionless light-matter coupling parameter at
fixed electric field strength is enhanced by a factor . This
increase of the coupling parameter allows to induce a topological gap of the
order of meV with realistic laser pulses, bringing an
experimental realization of light-induced topological magnon edge states within
reach.Comment: 21 pages, 4 figure
Bolus ingestion of whey protein immediately post-exercise does not influence rehydration compared to energy-matched carbohydrate ingestion
Whey protein is a commonly ingested nutritional supplement amongst athletes and regular exercisers; however, its role in post-exercise rehydration remains unclear. Eight healthy male and female participants completed two experimental trials involving the ingestion of 35 g of whey protein (WP) or maltodextrin (MD) at the onset of a rehydration period, followed by ingestion of water to a volume equivalent to 150% of the amount of body mass lost during exercise in the heat. The gastric emptying rates of the solutions were measured using 13C breath tests. Recovery was monitored for a further 3 h by the collection of blood and urine samples. The time taken to empty half of the initial solution (T1/2) was different between the trials (WP = 65.5 ± 11.4 min; MD = 56.7 ± 6.3 min; p = 0.05); however, there was no difference in cumulative urine volume throughout the recovery period (WP = 1306 ± 306 mL; MD = 1428 ± 443 mL; p = 0.314). Participants returned to net negative fluid balance 2 h after the recovery period with MD and 3 h with WP. The results of this study suggest that whey protein empties from the stomach at a slower rate than MD; however, this does not seem to exert any positive or negative effects on the maintenance of fluid balance in the post-exercise period
Nonequilibrium Quasiparticle Relaxation Dynamics in Single Crystals of Hole and Electron doped BaFeAs
We report on the nonequilibrium quasiparticle dynamics in BaFeAs on
both the hole doped (BaKFeAs) and electron doped
(BaFeCoAs) sides of the phase diagram using ultrafast
pump-probe spectroscopy. Below , measurements conducted at low
photoinjected quasiparticle densities in the optimally and overdoped
BaKFeAs samples reveal two distinct relaxation processes: a
fast component whose decay rate increases linearly with excitation density and
a slow component with an excitation density independent decay rate. We argue
that these two processes reflect the recombination of quasiparticles in the two
hole bands through intraband and interband processes. We also find that the
thermal recombination rate of quasiparticles increases quadratically with
temperature in these samples. The temperature and excitation density dependence
of the decays indicates fully gapped hole bands and nodal or very anisotropic
electron bands. At higher excitation densities and lower hole dopings, the
dependence of the dynamics on quasiparticle density disappears as the data are
more readily understood in terms of a model which accounts for the
quasiequilibrium temperature attained by the sample. In the
BaFeCoAs samples, dependence of the recombination rate on
quasiparticle density at low dopings (i.e., ) is suppressed upon
submergence of the inner hole band and quasiparticle relaxation occurs in a
slow, density independent manner.Comment: Accepted to Phys. Rev.
A Synopsis of Short-Term Response to Alternative Restoration Treatments in Sagebrush-Steppe: The SageSTEP Project
The Sagebrush Steppe Treatment Evaluation Project (SageSTEP) is an integrated long-term study that evaluates ecological effects of alternative treatments designed to reduce woody fuels and to stimulate the herbaceous understory of sagebrush steppe communities of the Intermountain West. This synopsis summarizes results through 3 yr posttreatment. Woody vegetation reduction by prescribed fire, mechanical treatments, or herbicides initiated a cascade of effects, beginning with increased availability of nitrogen and soil water, followed by increased growth of herbaceous vegetation. Response of butterflies and magnitudes of runoff and erosion closely followed herbaceous vegetation recovery. Effects on shrubs, biological soil crust, tree cover, surface woody fuel loads, and sagebrush-obligate bird communities will take longer to be fully expressed. In the short term, cool wet sites were more resilient than warm dry sites, and resistance was mostly dependent on pretreatment herbaceous cover. At least 10 yr of posttreatment time will likely be necessary to determine outcomes for most sites. Mechanical treatments did not serve as surrogates for prescribed fire in how each influenced the fuel bed, the soil, erosion, and sage-obligate bird communities. Woody vegetation reduction by any means resulted in increased availability of soil water, higher herbaceous cover, and greater butterfly numbers. We identified several trade-offs (desirable outcomes for some variables, undesirable for others), involving most components of the study system. Trade-offs are inevitable when managing complex natural systems, and they underline the importance of asking questions about the whole system when developing management objectives. Substantial spatial and temporal heterogeneity in sagebrush steppe ecosystems emphasizes the point that there will rarely be a “recipe” for choosing management actions on any specific area. Use of a consistent evaluation process linked to monitoring may be the best chance managers have for arresting woodland expansion and cheatgrass invasion that may accelerate in a future warming climate
Innovation in Rangeland Monitoring: Annual, 30 M, Plant Functional Type Percent Cover Maps for U.S. Rangelands, 1984-2017
Innovations in machine learning and cloud‐based computing were merged with historical remote sensing and field data to provide the first moderate resolution, annual, percent cover maps of plant functional types across rangeland ecosystems to effectively and efficiently respond to pressing challenges facing conservation of biodiversity and ecosystem services. We utilized the historical Landsat satellite record, gridded meteorology, abiotic land surface data, and over 30,000 field plots within a Random Forests model to predict per‐pixel percent cover of annual forbs and grasses, perennial forbs and grasses, shrubs, and bare ground over the western United States from 1984 to 2017. Results were validated using three independent collections of plot‐level measurements, and resulting maps display land cover variation in response to changes in climate, disturbance, and management. The maps, which will be updated annually at the end of each year, provide exciting opportunities to expand and improve rangeland conservation, monitoring, and management. The data open new doors for scientific investigation at an unprecedented blend of temporal fidelity, spatial resolution, and geographic scale
Bolus Ingestion of Whey Protein Immediately Post-Exercise Does Not Influence Rehydration Compared to Energy-Matched Carbohydrate Ingestion
Whey protein is a commonly ingested nutritional supplement amongst athletes and regular exercisers; however, its role in post-exercise rehydration remains unclear. Eight healthy male and female participants completed two experimental trials involving the ingestion of 35 g of whey protein (WP) or maltodextrin (MD) at the onset of a rehydration period, followed by ingestion of water to a volume equivalent to 150% of the amount of body mass lost during exercise in the heat. The gastric emptying rates of the solutions were measured using 13C breath tests. Recovery was monitored for a further 3 h by the collection of blood and urine samples. The time taken to empty half of the initial solution (T1/2) was different between the trials (WP = 65.5 ± 11.4 min; MD = 56.7 ± 6.3 min; p = 0.05); however, there was no difference in cumulative urine volume throughout the recovery period (WP = 1306 ± 306 mL; MD = 1428 ± 443 mL; p = 0.314). Participants returned to net negative fluid balance 2 h after the recovery period with MD and 3 h with WP. The results of this study suggest that whey protein empties from the stomach at a slower rate than MD; however, this does not seem to exert any positive or negative effects on the maintenance of fluid balance in the post-exercise period
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