3,324 research outputs found
Transport properties of graphene quantum dots
In this work we present a theoretical study of transport properties of a
double crossbar junction composed by segments of graphene ribbons with
different widths forming a graphene quantum dot structure. The systems are
described by a single-band tight binding Hamiltonian and the Green's function
formalism using real space renormalization techniques. We show calculations of
the local density of states, linear conductance and I-V characteristics. Our
results depict a resonant behavior of the conductance in the quantum dot
structures which can be controlled by changing geometrical parameters such as
the nanoribbon segments widths and relative distance between them. By applying
a gate voltage on determined regions of the structure, it is possible to
modulate the transport response of the systems. We show that negative
differential resistance can be obtained for low values of gate and bias
voltages applied.Comment: 9 pages, 9 figures, accepted to Phys. Rev.
Structure of Compact Stars in R-squared Palatini Gravity
We analyse configurations of compact stars in the so-called R-squared gravity
in the Palatini formalism. Using a realistic equation of state we show that the
mass-radius configurations are lighter than their counterparts in General
Relativity. We also obtain the internal profiles, which run in strong
correlation with the derivatives of the equation of state, leading to regions
where the mass parameter decreases with the radial coordinate in a
counter-intuitive way. In order to analyse such correlation, we introduce a
parametrisation of the equation of state given by multiple polytropes, which
allows us to explicitly control its derivatives. We show that, even in a
limiting case where hard phase transitions in matter are allowed, the internal
profile of the mass parameter still presents strange features and the
calculated M-R configurations also yield NSs lighter than those obtained in
General Relativity.Comment: 9 pages, 5 figures. Accepted for publication in General Relativity
and Gravitatio
Time Dependent Current Oscillations Through a Quantum Dot
Time dependent phenomena associated to charge transport along a quantum dot
in the charge quantization regime is studied. Superimposed to the Coulomb
blockade behaviour the current has novel non-linear properties. Together with
static multistabilities in the negative resistance region of the I-V
characteristic curve, strong correlations at the dot give rise to
self-sustained current and charge oscillations. Their properties depend upon
the parameters of the quantum dot and the external applied voltages.Comment: 4 pages, 3 figures; to appear in PR
A mesoscopic ring as a XNOR gate: An exact result
We describe XNOR gate response in a mesoscopic ring threaded by a magnetic
flux . The ring is attached symmetrically to two semi-infinite
one-dimensional metallic electrodes and two gate voltages, viz, and
, are applied in one arm of the ring which are treated as the inputs of
the XNOR gate. The calculations are based on the tight-binding model and the
Green's function method, which numerically compute the conductance-energy and
current-voltage characteristics as functions of the ring-to-electrode coupling
strength, magnetic flux and gate voltages. Our theoretical study shows that,
for a particular value of () (, the elementary
flux-quantum), a high output current (1) (in the logical sense) appears if both
the two inputs to the gate are the same, while if one but not both inputs are
high (1), a low output current (0) results. It clearly exhibits the XNOR gate
behavior and this aspect may be utilized in designing an electronic logic gate.Comment: 8 pages, 5 figure
Conductance and persistent current of a quantum ring coupled to a quantum wire under external fields
The electronic transport of a noninteracting quantum ring side-coupled to a
quantum wire is studied via a single-band tunneling tight-binding Hamiltonian.
We found that the system develops an oscillating band with antiresonances and
resonances arising from the hybridization of the quasibound levels of the ring
and the coupling to the quantum wire. The positions of the antiresonances
correspond exactly to the electronic spectrum of the isolated ring. Moreover,
for a uniform quantum ring the conductance and the persistent current density
were found to exhibit a particular odd-even parity related with the ring-order.
The effects of an in-plane electric field was also studied. This field shifts
the electronic spectrum and damps the amplitude of the persistent current
density. These features may be used to control externally the energy spectra
and the amplitude of the persistent current.Comment: Revised version, 7 pages and 9 figures. To appear in Phys. Rev.
Genome-wide diel growth state transitions in the diatom Thalassiosira pseudonana
Marine diatoms are important primary producers that thrive in diverse and dynamic environments. They do so, in theory, by sensing changing conditions and adapting their physiology accordingly. Using the model species Thalassiosira pseudonana, we conducted a detailed physiological and transcriptomic survey to measure the recurrent transcriptional changes that characterize typical diatom growth in batch culture. Roughly 40% of the transcriptome varied significantly and recurrently, reflecting large, reproducible cell-state transitions between four principal states: (i) "dawn," following 12 h of darkness; (ii ) "dusk," following 12 h of light; (iii ) exponential growth and nutrient repletion; and (iv) stationary phase and nutrient depletion. Increases in expression of thousands of genes at the end of the reoccurring dark periods (dawn), including those involved in photosynthesis (e.g., ribulose-1,5- bisphosphate carboxylase oxygenase genes rbcS and rbcL), imply large-scale anticipatory circadian mechanisms at the level of gene regulation. Repeated shifts in the transcript levels of hundreds of genes encoding sensory, signaling, and regulatory functions accompanied the four cell-state transitions, providing a preliminary map of the highly coordinated gene regulatory program under varying conditions. Several putative light sensing and signaling proteins were associated with recurrent diel transitions, suggesting that these genes may be involved in light-sensitive and circadian regulation of cell state. These results begin to explain, in comprehensive detail, how the diatom gene regulatory program operates under varying environmental conditions. Detailed knowledge of this dynamic molecular process will be invaluable for new hypothesis generation and the interpretation of genetic, environmental, and metatranscriptomic data from field studies
Accretion vs. colliding wind models for the gamma-ray binary LS I + 61 303: An assessment
Context. LS I +61 303 is a puzzling Be/X-ray binary with variable gamma-ray emission up to TeV energies. The nature of the compact object and the origin of the high-energy emission are unclear. One family of models invokes particle acceleration in shocks from the collision between the B-star wind and a relativistic pulsar wind, whereas another centers on a relativistic jet powered by accretion from the Be star decretion disc onto a black hole. Recent high-resolution radio observations showing a putative "cometary tail" pointing away from the Be star near periastron have been cited as support for the pulsar-wind model.
Aims. We wish to carry out a quantitative assessment of these competing models.
Methods. We apply a "Smoothed Particle Hydrodynamics" (SPH) code in 3D dynamical simulations for both the pulsar-windinteraction and accretion-jet models. The former yields a dynamical description of the shape of the wind-wind interaction surface. The latter provides a dynamical estimation of the accretion rate under a variety of conditions, and how this varies with orbital phase.
Results. The results allow critical evaluation of how the two distinct models confront the data in various wavebands. When one accounts for the 3D dynamical wind interaction under realistic constraints for the relative strength of the B-star and pulsar winds, the resulting form of the interaction front does not match the putative "cometary tail" claimed from radio observations. On the other hand, dynamical simulations of the accretion-jet model indicate that the orbital phase variation of accretion power includes a secondary broad peak well away from periastron, thus providing a plausible way to explain the observed TeV gamma ray emission toward apastron.
Conclusions. Contrary to previous claims, the colliding-wind model is not clearly established for LSI +61 303, whereas the accretionjet model can reproduce many key characteristics, such as required energy budget, lightcurve, and spectrum of the observed TeV gamma-ray emission.Facultad de Ciencias AstronĂłmicas y GeofĂsicasInstituto Argentino de RadioastronomĂ
- …