12,689 research outputs found
Enhancing single-parameter quantum charge pumping in carbon-based devices
We present a theoretical study of quantum charge pumping with a single ac
gate applied to graphene nanoribbons and carbon nanotubes operating with low
resistance contacts. By combining Floquet theory with Green's function
formalism, we show that the pumped current can be tuned and enhanced by up to
two orders of magnitude by an appropriate choice of device length, gate voltage
intensity and driving frequency and amplitude. These results offer a promising
alternative for enhancing the pumped currents in these carbon-based devices.Comment: 3.5 pages, 2 figure
Mechanically-Induced Transport Switching Effect in Graphene-based Nanojunctions
We report a theoretical study suggesting a novel type of electronic switching
effect, driven by the geometrical reconstruction of nanoscale graphene-based
junctions. We considered junction struc- tures which have alternative
metastable configurations transformed by rotations of local carbon dimers. The
use of external mechanical strain allows a control of the energy barrier
heights of the potential profiles and also changes the reaction character from
endothermic to exothermic or vice-versa. The reshaping of the atomic details of
the junction encode binary electronic ON or OFF states, with ON/OFF
transmission ratio that can reach up to 10^4-10^5. Our results suggest the
possibility to design modern logical switching devices or mechanophore sensors,
monitored by mechanical strain and structural rearrangements.Comment: 10 pages, 4 figure
The silicon stable isotope distribution along the GEOVIDE section (GEOTRACES GA-01) of the North Atlantic Ocean
The stable isotope composition of dissolved silicon in seawater (δ30SiDSi) was examined at 10 stations along the GEOVIDE section (GEOTRACES GA-01), spanning the North Atlantic Ocean (40–60∘ N) and Labrador Sea. Variations in δ30SiDSi below 500 m were closely tied to the distribution of water masses. Higher δ30SiDSi values are associated with intermediate and deep water masses of northern Atlantic or Arctic Ocean origin, whilst lower δ30SiDSi values are associated with DSi-rich waters sourced ultimately from the Southern Ocean. Correspondingly, the lowest δ30SiDSi values were observed in the deep and abyssal eastern North Atlantic, where dense southern-sourced waters dominate. The extent to which the spreading of water masses influences the δ30SiDSi distribution is marked clearly by Labrador Sea Water (LSW), whose high δ30SiDSi signature is visible not only within its region of formation within the Labrador and Irminger seas, but also throughout the mid-depth western and eastern North Atlantic Ocean. Both δ30SiDSi and hydrographic parameters document the circulation of LSW into the eastern North Atlantic, where it overlies southern-sourced Lower Deep Water. The GEOVIDE δ30SiDSi distribution thus provides a clear view of the direct interaction between subpolar/polar water masses of northern and southern origin, and allow examination of the extent to which these far-field signals influence the local δ30SiDSi distribution
A computationally efficient method for calculating the maximum conductance of disordered networks: Application to 1-dimensional conductors
Random networks of carbon nanotubes and metallic nanowires have shown to be
very useful in the production of transparent, conducting films. The electronic
transport on the film depends considerably on the network properties, and on
the inter-wire coupling. Here we present a simple, computationally efficient
method for the calculation of conductance on random nanostructured networks.
The method is implemented on metallic nanowire networks, which are described
within a single-orbital tight binding Hamiltonian, and the conductance is
calculated with the Kubo formula. We show how the network conductance depends
on the average number of connections per wire, and on the number of wires
connected to the electrodes. We also show the effect of the inter-/intra-wire
hopping ratio on the conductance through the network. Furthermore, we argue
that this type of calculation is easily extendable to account for the upper
conductivity of realistic films spanned by tunneling networks. When compared to
experimental measurements, this quantity provides a clear indication of how
much room is available for improving the film conductivity.Comment: 7 pages, 5 figure
Avaliação do lençol freático em duas áreas exploradas com a cultura do coqueiro (Cocos nucifera L.).
bitstream/item/90507/1/CPATC-PESQ.-AND.-36-86.pd
The star-formation history of the Milky Way Galaxy
The star-formation histories of the main stellar components of the Milky Way
constrain critical aspects of galaxy formation and evolution. I discuss recent
determinations of such histories, together with their interpretation in terms
of theories of disk galaxy evolution.Comment: Invited review, IAU Symposium 258; 12 pages, 1 figur
Upper bound for the conductivity of nanotube networks
Films composed of nanotube networks have their conductivities regulated by
the junction resistances formed between tubes. Conductivity values are enhanced
by lower junction resistances but should reach a maximum that is limited by the
network morphology. By considering ideal ballistic-like contacts between
nanotubes we use the Kubo formalism to calculate the upper bound for the
conductivity of such films and show how it depends on the nanotube
concentration as well as on their aspect ratio. Highest measured conductivities
reported so far are approaching this limiting value, suggesting that further
progress lies with nanowires other than nanotubes.Comment: 3 pages, 1 figure. Minor changes. Accepted for publication in Applied
Physics Letter
AC transport in graphene-based Fabry-Perot devices
We report on a theoretical study of the effects of time-dependent fields on
electronic transport through graphene nanoribbon devices. The Fabry-P\'{e}rot
interference pattern is modified by an ac gating in a way that depends strongly
on the shape of the graphene edges. While for armchair edges the patterns are
found to be regular and can be controlled very efficiently by tuning the ac
field, samples with zigzag edges exhibit a much more complex interference
pattern due to their peculiar electronic structure. These studies highlight the
main role played by geometric details of graphene nanoribbons within the
coherent transport regime. We also extend our analysis to noise power response,
identifying under which conditions it is possible to minimize the current
fluctuations as well as exploring scaling properties of noise with length and
width of the systems
Black string corrections in variable tension braneworld scenarios
Braneworld models with variable tension are investigated, and the corrections
on the black string horizon along the extra dimension are provided. Such
corrections are encrypted in additional terms involving the covariant
derivatives of the variable tension on the brane, providing profound
consequences concerning the black string horizon variation along the extra
dimension, near the brane. The black string horizon behavior is shown to be
drastically modified by the terms corrected by the brane variable tension. In
particular, a model motivated by the phenomenological interesting case
regarding Eotvos branes is investigated. It forthwith provides further physical
features regarding variable tension braneworld scenarios, heretofore concealed
in all previous analysis in the literature. All precedent analysis considered
uniquely the expansion of the metric up to the second order along the extra
dimension, what is able to evince solely the brane variable tension absolute
value. Notwithstanding, the expansion terms aftermath, further accomplished in
this paper from the third order on, elicits the successive covariant
derivatives of the brane variable tension, and their respective coupling with
the extrinsic curvature, the Weyl tensor, and the Riemann and Ricci tensors, as
well as the scalar curvature. Such additional terms are shown to provide sudden
modifications in the black string horizon in a variable tension braneworld
scenarioComment: 12 pages, 5 figures, accepted in PR
Separation of the pyro- and piezoelectric response of electroactive polymers for sensor applications
One of the main problems for the use of ferroelectric materials for sensor applications,
especially for tactile sensors, is the separation of the pyro- and piezoelectric responseproduced by
temperature and pressure changes, respectively. We present two different approaches to achieve
this goal. The first approach is based on the study of the signal in the frequency domain and uses a
signal processing to separate the piezoelectric response from the pyroelectric one. The second
approach is based on a double layer configuration of electroactive polymers. In this case, the
piezoelectric response is equal in each layer, whereas, the pyroelectric response dominates in the
layer exposed to the temperature variations. The pyro- and piezoelectric responses are separated by
estimating the difference between the signals of two layers and the signal from one of the layers,
correspondingly.Fundação para a Ciência e a Tecnologia (FCT) - Grants POCTI/CTM/33501/99, POCI/CTM/59425/2004.Measurement Specialties Inc
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