Size dependence of second-order hyperpolarizability of finite periodic chain under Su-Schrieffer-Heeger model

The second hyperpolarizability $\gamma_N(-3\omega\omega,\omega,\omega)$ of $N$ double-bond finite chain of trans-polyactylene is analyzed using the Su-Schrieffer-Heeger model to explain qualitative features of the size-dependence behavior of $\gamma_N$. Our study shows that $\gamma_N/N$ is {\it nonmonotonic} with $N$ and that the nonmonotonicity is caused by the dominant contribution of the intraband transition to $\gamma_N$ in polyenes. Several important physical effects are discussed to reduce quantitative discrepancies between experimental and our resultsComment: 3 figures, 1 tabl

Possible TeV Source Candidates In The Unidentified EGRET Sources

We study the $\gamma$-ray emission from the pulsar magnetosphere based on outer gap models, and the TeV radiation from pulsar wind nebulae (PWNe) through inverse Compton scattering using a one-zone model. We showed previously that GeV radiation from the magnetosphere of mature pulsars with ages of $\sim 10^5-10^6$ years old can contribute to the high latitude unidentified EGRET sources. We carry out Monte Carlo simulations of $\gamma$-ray pulsars in the Galaxy and the Gould Belt, assuming the pulsar birth rate, initial position, proper motion velocity, period, and magnetic field distribution and evolution based on observational statistics. We select from the simulation a sample of mature pulsars in the Galactic plane ($|b|\leq 5^\circ$) and in the high latitude ($|b|> 5^\circ$) which could be detected by EGRET. The TeV flux from the pulsar wind nebulae of our simulated sample through the inverse Compton scattering by relativistic electrons on the microwave cosmic background and synchrotron seed photons are calculated. The predicted fluxes are consistent with the present observational constraints. We suggest that strong EGRET sources can be potential TeV source candidates for present and future ground-based TeV telescopes.Comment: Minor changes, MNRAS in pres

Gas Sensing with h-BN Capped MoS2 Heterostructure Thin Film Transistors

We have demonstrated selective gas sensing with molybdenum disulfide (MoS2) thin films transistors capped with a thin layer of hexagonal boron nitride (h-BN). The resistance change was used as a sensing parameter to detect chemical vapors such as ethanol, acetonitrile, toluene, chloroform and methanol. It was found that h-BN dielectric passivation layer does not prevent gas detection via changes in the source-drain current in the active MoS2 thin film channel. The use of h-BN cap layers (thickness H=10 nm) in the design of MoS2 thin film gas sensors improves device stability and prevents device degradation due to environmental and chemical exposure. The obtained results are important for applications of van der Waals materials in chemical and biological sensing.Comment: 3 pages; 4 figure

Ebolavirus is evolving but not changing: No evidence for functional change in EBOV from 1976 to the 2014 outbreak

The 2014 epidemic of Ebola virus disease (EVD) has had a devastating impact in West Africa. Sequencing of ebolavirus (EBOV) from infected individuals has revealed extensive genetic variation, leading to speculation that the virus may be adapting to humans, accounting for the scale of the 2014 outbreak. We computationally analyze the variation associated with all EVD outbreaks, and find none of the amino acid replacements lead to identifiable functional changes. These changes have minimal effect on protein structure, being neither stabilizing nor destabilizing, are not found in regions of the proteins associated with known functions and tend to cluster in poorly constrained regions of proteins, specifically intrinsically disordered regions. We find no evidence that the difference between the current and previous outbreaks is due to evolutionary changes associated with transmission to humans. Instead, epidemiological factors are likely to be responsible for the unprecedented spread of EVD

Phase stability and the arsenic vacancy defect in In_xGa_1-xAs

The introduction of defects, such as vacancies, into InxGa1-xAs can have a dramatic impact on the physical and electronic properties of the material. Here we employ ab initio simulations of quasirandom supercells to investigate the structure of InxGa1-xAs and then examine the energy and volume changes associated with the introduction of an arsenic vacancy defect. We predict that both defect energies and volumes for intermediate compositions of InxGa1-xAs differ significantly from what would be expected by assuming a simple linear interpolation of the end member defect energies/volumes