The gate-controllable complex conductivity of graphene offers unprecedented opportunities for reconfigurable plasmonics at terahertz and mid-infrared frequencies. However, the requirement of a gating ...electrode close to graphene and the single 'control knob' that this approach offers limits the practical implementation and performance of these devices. Here we report on graphene stacks composed of two or more graphene monolayers separated by electrically thin dielectrics and present a simple and rigorous theoretical framework for their characterization. In a first implementation, two graphene layers gate each other, thereby behaving as a controllable single equivalent layer but without any additional gating structure. Second, we show that adding an additional gate allows independent control of the complex conductivity of each layer within the stack and provides enhanced control on the stack equivalent complex conductivity. These results are very promising for the development of THz and mid-infrared plasmonic devices with enhanced performance and reconfiguration capabilities.
We prove an extended lifespan result for the full gravity-capillary water waves system with a 2 dimensional periodic interface: for initial data of sufficiently small size
ε
, smooth solutions exist ...up to times of the order of
ε
-
5
/
3
+
, for almost all values of the gravity and surface tension parameters. Besides the quasilinear nature of the equations, the main difficulty is to handle the weak small divisors bounds for quadratic and cubic interactions, growing with the size of the largest frequency. To overcome this difficulty we use (1) the (Hamiltonian) structure of the equations which gives additional smoothing close to the resonant hypersurfaces, (2) another structural property, connected to time-reversibility, that allows us to handle “trivial” cubic resonances, (3) sharp small divisors lower bounds on three and four-way modulation functions based on counting arguments, and (4) partial normal form transformations and symmetrization arguments in the Fourier space. Our theorem appears to be the first extended lifespan result for quasilinear equations with non-trivial resonances on a multi-dimensional torus.
We review recent progress on the long-time regularity of solutions of the Cauchy problem for the water waves equations, in two and three dimensions. We begin by introducing the free boundary Euler ...equations and discussing the local existence of solutions using the paradifferential approach. We then describe in a unified framework, using the Eulerian formulation, global existence results for three- and two-dimensional gravity waves, and our joint result (with Deng and Pausader) on global regularity for the gravity-capillary model in three dimensions. We conclude this review with a short discussion about the formation of singularities and give a few additional references to other interesting topics in the theory.
This article is part of the theme issue ‘Nonlinear water waves’.
Directional control of a processive molecular hopper Qing, Yujia; Ionescu, Sandra A; Pulcu, Gökçe Su ...
Science (American Association for the Advancement of Science),
08/2018, Letnik:
361, Številka:
6405
Journal Article
Recenzirano
Odprti dostop
Intrigued by the potential of nanoscale machines, scientists have long attempted to control molecular motion. We monitored the individual 0.7-nanometer steps of a single molecular hopper as it moved ...in an electric field along a track in a nanopore controlled by a chemical ratchet. The hopper demonstrated characteristics desired in a moving molecule: defined start and end points, processivity, no chemical fuel requirement, directional motion, and external control. The hopper was readily functionalized to carry cargos. For example, a DNA molecule could be ratcheted along the track in either direction, a prerequisite for nanopore sequencing.
Metal‐catalysed CO2 hydrogenation is considered a source of methane in serpentinized (hydrated) igneous rocks and a fundamental abiotic process germane to the origin of life. Iron, nickel, chromium ...and cobalt are the catalysts typically employed in hydrothermal simulation experiments to obtain methane at temperatures >200°C. However, land‐based present‐day serpentinization and abiotic gas apparently develop below 100°C, down to approximately 40–50°C. Here, we document considerable methane production in thirteen CO2 hydrogenation experiments performed in a closed dry system, from 20 to 90°C and atmospheric pressure, over 0.9–122 days, using concentrations of non‐pretreated ruthenium equivalent to those occurring in chromitites in ophiolites or igneous complexes (from 0.4 to 76 mg of Ru, equivalent to the amount occurring approximately in 0.4–760 kg of chromitite). Methane production increased with time and temperature, reaching approximately 87 mg CH4 per gram of Ru after 30 days (2.9 mgCH4/gru/day) at 90°C. At room temperature, CH4 production rate was approximately three orders of magnitude lower (0.003 mgCH4/gru/day). We report the first stable carbon and hydrogen isotope ratios of abiotic CH4 generated below 100°C. Using initial δ13CCO2 of ‐40‰, we obtained room temperature δ13CCH4 values as 13C depleted as −142‰. With time and temperature, the C‐isotope separation between CO2 and CH4 decreased significantly and the final δ13CCH4 values approached that of initial δ13CCO2. The presence of minor amounts of C2‐C6 hydrocarbons is consistent with observations in natural settings. Comparative experiments at the same temperatures with iron and nichel catalysts did not generate CH4. Ru‐enriched chromitites could potentially generate methane at low temperatures on Earth and on other planets.
Methane was abiotically produced in CO2 hydrogenation experiments at temperatures from 20 to 90° using concentrations of non‐pretreated ruthenium equivalent to those occurring in chromitites in ophiolites or igneous complexes. Stable C and H isotope ratios of abiotic CH4 generated below 100°C is reported for the first time. Comparative experiments at the same temperatures with iron and nichel catalysts did not generate CH4. Ru‐enriched chromitites can potentially generate methane at low temperatures on Earth and on other planets.
Chemists have long sought the ability to modify molecules precisely when presented with several sites of similar reactivity. We reasoned that the confinement of substrates within nanostructures might ...permit site-selective reactions unachievable in bulk solution, even with sophisticated reagents. In particular, the stretching and alignment of polymers within nanotubes might allow site-specific cleavage or modification. To explore this proposition, macromolecular disulfide substrates were elongated within members of a collection of tubular protein nanoreactors, which contained cysteine residues positioned at different locations along the length of each tube. For each nanoreactor, we defined the reactive location by using a set of polymer substrates (site-selectivity) and which of the two sulfur atoms was attacked (regioselectivity), and found that disulfide interchange occurs with atomic precision. Our strategy has potential for the selective processing of a wide variety of biomacromolecules, and the chemistry and substrates might be generalized yet further by using alternative nanotubes.
The important problem of the bubble formation and evolution on a bismuth vanadate anode, and their consequence on the efficiency of a photoelectrochemical cell is analyzed. Although bubbles represent ...the way in which the gas products are collected to be stored, they first tend to stick on the electrode surface, decreasing the effective area, increasing the interfacial electric resistance, and, thus, increasing the losses. Starting with general thermodynamic considerations on the process of bubble generation, two ideal surfaces have been imagined: the inner surfaceinside the poreswhere the main electrochemical processes take place, and the outer surface, on which the bubbles stick until they are large enough to leave the surface and be collected. A percolation approach has been used in order to explain the time variation (decrease) of the photoelectric process during the bubble generation. On the basis of the percolation approach, a fitting function has been proposed in order to analyze the experimental data recorded for a bismuth vanadate porous photoanode tested into a home-designed photoelectrochemical cell at several bias voltages, under dark and light conditions.
We report on the results of an extensive geochemical survey of fluids released in the Vardar zone (central‐western Serbia), a mega‐suture zone at the boundary between Eurasia and Africa plates. ...Thirty‐one bubbling gas samples are investigated for their chemical and isotopic compositions (He, C, Ar) and cluster into three distinct groups (CO2‐dominated, N2‐dominated, and CH4‐dominated) based on the dominant gas species. The measured He isotope ratios range from 0.08 to 1.19 Ra (where Ra is the atmospheric ratio), and reveal for the first time the presence of a minor (<20%) but detectable regional mantle‐derived component in Serbia. δ13C values range from −20.2‰ to −0.1‰ (versus PDB), with the more negative compositions observed in N2‐dominated samples. The carbon‐helium relationship indicates that these negative δ13C compositions could be due to isotopic fractionation processes during CO2 dissolution into groundwater. In contrast, CO2‐rich samples reflect mixing between crustal and mantle‐derived CO2. Our estimated mantle‐derived He flux (9.0 × 109 atoms m−2 s−1) is up to 2 orders of magnitude higher than the typical fluxes in stable continental areas, suggesting a structural/tectonic setting favoring the migration of deep‐mantle fluids through the crust.
Key Points
Chemical and isotopic composition of natural gas manifestations along the Serbian Vardar zone are controlled by mixing processes and fraction during water‐gas‐rock interactions in shallow crustal layers
Mantle‐derived He flux of 2 orders of magnitude higher than normally found in stable continental areas are estimated
Mantle volatiles and heat are sourced directly from the mantle supporting the asthenosphere up‐rise and delamination processes at the mantle‐crust boundary recognized in the studied area
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