Variable Step Random Walks and Self-Similar Distributions

We study a scenario under which variable step random walks give anomalous statistics. We begin by analyzing the Martingale Central Limit Theorem to find a sufficient condition for the limit distribution to be non-Gaussian. We note that the theorem implies that the scaling index $\zeta$ is 1/2. For corresponding continuous time processes, it is shown that the probability density function $W(x;t)$ satisfies the Fokker-Planck equation. Possible forms for the diffusion coefficient are given, and related to $W(x,t)$. Finally, we show how a time-series can be used to distinguish between these variable diffusion processes and L\'evy dynamics.Comment: 13pages, 2 figure

On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection

A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)

Effect of the embedded atom on the electronic, optical properties and kinetic stability of 3,6silaprismane

Using ab initio calculations it is found that endohedral complexes C@Si18H12, Si@Si18H12 and Ge@Si18H12 are less kinetically stable than pure [3,6]silaprismane Si18H12. Moreover, kinetic stability of Si18H12 is higher than the stability of its carbon molecular analogue C18H12. The height of the minimum energy barrier preventing the Si18H12 isomerization is found to be 1.09 eV and its lifetime at room temperature will reach several hours that is acceptable not only for its immediate experimental observation but for the laboratory synthesis without using the extreme temperature conditions as well. It is also found that embedded atoms can significantly change the electronic properties of Si18H12 cage that is directly affect the optical characteristics of the systems considered. Thus, the doped Si18H12s can be experimentally divided from each other due to the changes in their optical spectra. © 2017 Elsevier B.V

Kinetic Stability and Reactivity of Silicon and Fluorine-Containing CL-20 Derivatives

A CL-20 based cages in which carbon/oxygen atoms are replaced by silicon/fluorine ones are studied using the ab initio molecular dynamics, density functional theory, and time-dependent density functional theory. In contrast to the pristine CL-20, the first step of pyrolysis of these cages is the migration of oxygen/fluorine atoms to silicon. Molecules containing fluorine are unstable at room temperature. The high-energy silicon-containing molecule (CSi5H6N12O12) is approximately as stable as pristine CL-20. Energy barrier preventing its decomposition is about 200 kJ/mol. Energies of the frontier orbitals and reactivity descriptors of CSi5H6N12O12 are very close to the corresponding values of pure CL-20. All studied cages can form covalent dimers via the methylene molecular bridges. It is found that the reactions of dimerization are exothermic. Dimers� isomers in which silicon atoms are located closer to the methylene bridges possess lower internal energies. It is found that the mechanisms of dimers� thermal decomposition are similar to the analog mechanisms of corresponding monomers. Dimerization of the cages results in the redshifts of their ultraviolet spectra. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei

High robustness of epitaxial 4H-SiC graphene to oxidation processes

International audienceWe present an experimental prove of high robustness of epitaxial 4H-SiC graphene to oxidation processes. During a post-fabrication cleaning procedure we noticed that epitaxial graphene is extremely stable to ozone treatment. We analyse graphene properties using both electron transport measurements and numerical calculations. We ascribe this effect to the substrate topography, which significantly affects the graphene stability under UV/ozone treatment

Efficient cleaning of graphene from residual lithographic polymers by ozone treatment

cited By 3International audienceWe present an experimental study of time dependent ozone treatment on post-process epitaxial graphene using both electron transport measurements and resonant micro-Raman spectroscopy. We focus on a systematic analysis of residual polymer decomposition on the epitaxial graphene on SiC substrate. It was found that graphene could be effectively cleaned by ultraviolet (UV)/ozone treatment after nanofabrication from residual lithographic polymers. This procedure improves the charge carrier mobility, almost by a factor of two for strongly contaminated samples, decreases the doping level and does not introduce defect inside the graphene lattice. It was found that epitaxial SiC graphene is extremely stable when exposed to radical oxygen atoms. We ascribe this effect to the substrate topography, which significantly affects the graphene stability under UV/ozone treatment. Our calculations reveal that surface roughness of the SiC substrate can change the energy gain from epoxy group adsorption by a few tenths of electron volts

Quantitative assessment of the differential impacts of arbuscular and ectomycorrhiza on soil carbon cycling

A significant fraction of carbon stored in the Earth’s soil moves through arbuscular mycorrhiza(AM) and ectomycorrhiza (EM). The impacts of AM and EM on the soil carbon budgetare poorly understood.We propose a method to quantify the mycorrhizal contribution to carbon cycling, explicitlyaccounting for the abundance of plant-associated and extraradical mycorrhizal mycelium. Wediscuss the need to acquire additional data to use our method, and present our new globaldatabase holding information on plant species-by-site intensity of root colonization by mycorrhizas.We demonstrate that the degree of mycorrhizal fungal colonization has globally consistentpatterns across plant species. This suggests that the level of plant species-specific rootcolonization can be used as a plant trait.To exemplify our method, we assessed the differential impacts of AM : EM ratio and EMshrub encroachment on carbon stocks in sub-arctic tundra. AM and EM affect tundra carbonstocks at different magnitudes, and via partly distinct dominant pathways: via extraradicalmycelium (both EM and AM) and via mycorrhizal impacts on above- and belowground biomasscarbon (mostly AM).Our method provides a powerful tool for the quantitative assessment of mycorrhizal impacton local and global carbon cycling processes, paving the way towards an improved understandingof the role of mycorrhizas in the Earth’s carbon cycle.Conservation Biolog