55 research outputs found
Effect of a gap opening on the conductance of graphene superlattices
The electronic transmission and conductance of a gapped graphene superlattice
were calculated by means of the transfer-matrix method. The system that we
study consists of a sequence of electron-doped graphene as wells and hole-doped
graphene as barriers. We show that the transmission probability approaches
unity at some critical value of the gap. We also find that there is a domain
around the critical gap value for which the conductance of the system attains
its maximum value.Comment: 14 pages, 5 figures. To appear in Solid State Communication
Graphene-based modulation-doped superlattice structures
The electronic transport properties of graphene-based superlattice structures
are investigated. A graphene-based modulation-doped superlattice structure
geometry is proposed and consist of periodically arranged alternate layers:
InAs/graphene/GaAs/graphene/GaSb. Undoped graphene/GaAs/graphene structure
displays relatively high conductance and enhanced mobilities at elevated
temperatures unlike modulation-doped superlattice structure more steady and
less sensitive to temperature and robust electrical tunable control on the
screening length scale. Thermionic current density exhibits enhanced behaviour
due to presence of metallic (graphene) mono-layers in superlattice structure.
The proposed superlattice structure might become of great use for new types of
wide-band energy gap quantum devices.Comment: 5 figure
Dirac Spectrum in Piecewise Constant One-Dimensional Potentials
We study the electronic states of graphene in piecewise constant potentials
using the continuum Dirac equation appropriate at low energies, and a transfer
matrix method. For superlattice potentials, we identify patterns of induced
Dirac points which are present throughout the band structure, and verify for
the special case of a particle-hole symmetric potential their presence at zero
energy. We also consider the cases of a single trench and a p-n junction
embedded in neutral graphene, which are shown to support confined states. An
analysis of conductance across these structures demonstrates that these
confined states create quantum interference effects which evidence their
presence.Comment: 10 pages, 12 figures, additional references adde
Goos-H\"{a}nchen-like shifts for Dirac fermions in monolayer graphene barrier
We investigate the Goos-H\"{a}nchen-like shifts for Dirac fermions in
transmission through a monolayer graphene barrier. The lateral shifts, as the
functions of the barrier's width and the incidence angle, can be negative and
positive in Klein tunneling and classical motion, respectively. Due to their
relations to the transmission gap, the lateral shifts can be enhanced by the
transmission resonances when the incidence angle is less than the critical
angle for total reflection, while their magnitudes become only the order of
Fermi wavelength when the incidence angle is larger than the critical angle.
These tunable beam shifts can also be modulated by the height of potential
barrier and the induced gap, which gives rise to the applications in
graphene-based devices.Comment: 5 pages, 5 figure
Monitoring the initial pulmonary absorption of two different beclomethasone dipropionate aerosols employing a human lung reperfusion model
BACKGROUND: The pulmonary residence time of inhaled glucocorticoids as well as their rate and extend of absorption into systemic circulation are important facets of their efficacy-safety profile. We evaluated a novel approach to elucidate the pulmonary absorption of an inhaled glucocorticoid. Our objective was to monitor and compare the combined process of drug particle dissolution, pro-drug activation and time course of initial distribution from human lung tissue into plasma for two different glucocorticoid formulations. METHODS: We chose beclomethasone dipropionate (BDP) delivered by two different commercially available HFA-propelled metered dose inhalers (Sanasthmax(®)/Becloforte™ and Ventolair(®)/Qvar™). Initially we developed a simple dialysis model to assess the transfer of BDP and its active metabolite from human lung homogenate into human plasma. In a novel experimental setting we then administered the aerosols into the bronchus of an extracorporally ventilated and reperfused human lung lobe and monitored the concentrations of BDP and its metabolites in the reperfusion fluid. RESULTS: Unexpectedly, we observed differences between the two aerosol formulations Sanasthmax(®)/Becloforte™ and Ventolair(®)/Qvar™ in both the dialysis as well as in the human reperfusion model. The HFA-BDP formulated as Ventolair(®)/Qvar™ displayed a more rapid release from lung tissue compared to Sanasthmax(®)/Becloforte™. We succeeded to explain and illustrate the observed differences between the two aerosols with their unique particle topology and divergent dissolution behaviour in human bronchial fluid. CONCLUSION: We conclude that though the ultrafine particles of Ventolair(®)/Qvar™ are beneficial for high lung deposition, they also yield a less desired more rapid systemic drug delivery. While the differences between Sanasthmax(®)/Becloforte™ and Ventolair(®)/Qvar™ were obvious in both the dialysis and lung perfusion experiments, the latter allowed to record time courses of pro-drug activation and distribution that were more consistent with results of comparable clinical trials. Thus, the extracorporally reperfused and ventilated human lung is a highly valuable physiological model to explore the lung pharmacokinetics of inhaled drugs
Absence of N-terminal acetyltransferase diversification during evolution of eukaryotic organisms
Protein N-terminal acetylation is an ancient and ubiquitous co-translational modification catalyzed by a highly conserved family of N-terminal acetyltransferases (NATs). Prokaryotes have at least 3 NATs, whereas humans have six distinct but highly conserved NATs, suggesting an increase in regulatory complexity of this modification during eukaryotic evolution. Despite this, and against our initial expectations, we determined that NAT diversification did not occur in the eukaryotes, as all six major human NATs were most likely present in the Last Eukaryotic Common Ancestor (LECA). Furthermore, we also observed that some NATs were actually secondarily lost during evolution of major eukaryotic lineages; therefore, the increased complexity of the higher eukaryotic proteome occurred without a concomitant diversification of NAT complexes
Impact of livestock manure, nitrogen and biofertilizer (Azotobacter) on yield and yield components of wheat (Triticum aestivum L.)
Integrated nutrient
management strategies involving chemical
and biologic fertilizer is a real challenge to
stop using the high rates of agrochemicals
and to enhance sustainability of crop
production. In order to study the effects of
livestock manure, chemical nitrogen, and
biologic (Azotobacter) fertilizers on yield
and yield components of wheat, an
agricultural experiment in the form of split
factorial design with three replications was
conducted in Elam region, Iran. The aim of
this research was assessment of the effects
of these fertilizers separately and in
integrated forms; and setting out the best
fertilizer mixture. The results showed that
treatment with livestock manure,
Azotobacter and chemical nitrogen
increased plant height, biological and grain
yield. Using livestock manure and
Azotobacter increased biologic yield
through increase in plant height which
cause to increase in grain yield without any
significant changes in harvest index and
other yield components, but the use of
chemical nitrogen caused an increase in
plant height, No. of spikelete/spike, No. of
grain/spike, one thousand grain weight and
harvest index, biologic and grain yield. In
the light of the results achieved, we may
conclude that using livestock manure and
chemical nitrogen fertilizer together with
the Azotobacter had the maximum impact
on yield; and that we can decrease use of
chemical fertilizers through using livestock
manure and biologic fertilizers and to reach
to the same yield when we use only
chemical fertilizers
Magneto-Thermo-Mechanical Buckling Analysis of Mindlin Plate Reinforced with FG-Carbon Nanotubes
A buckling analysis of temperature-dependent embedded plates reinforced by single-walled carbon nanotubes (SWCNTs) subjected to a magnetic field is investigated. The SWCNTs are distributed as uniform (UD) and three types of functionally graded nanotubes (FG), in which the material properties of the nano-composite plate are estimated based on the mixture rule. The surrounding temperature-dependent elastic medium is simulated as Pasternak foundation. Based on the orthotropic Mindlin plate theory, the governing equations are derived using Hamilton's principle. The buckling load of the structure is calculated based on an exact solution by the Navier method. The influences of elastic medium, magnetic field, temperature and distribution type, and volume fractions of SWCNT are shown on the buckling of the plate. Results indicate that CNT distribution close to the top and bottom are more efficient than that distributed near the mid-plane for increasing the stiffness of the plates
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