3,030 research outputs found
Bound states in the continuum driven by AC fields
We report the formation of bound states in the continuum driven by AC fields.
This system consists of a quantum ring connected to two leads. An AC side-gate
voltage controls the interference pattern of the electrons passing through the
system. We model the system by two sites in parallel connected to two
semi-infinite lattices. The energy of these sites change harmonically with
time. We obtain the transmission probability and the local density of states at
the ring sites as a function of the parameters that define the system. The
transmission probability displays a Fano profile when the energy of the
incoming electron matches the driving frequency. Correspondingly, the local
density of states presents a narrow peak that approaches a Dirac delta function
in the weak coupling limit. We attribute these features to the presence of
bound states in the continuum.Comment: 5 pages, 3 figure
Enhancing thermoelectric properties of graphene quantum rings
We study the thermoelectric properties of rectangular graphene rings
connected symmetrically or asymmetrically to the leads. A side-gate voltage
applied across the ring allows for the precise control of the electric current
flowing through the system. The transmission coefficient of the rings manifests
Breit-Wigner line-shapes and/or Fano line-shapes, depending on the connection
configuration, the width of nanoribbons forming the ring and the side-gate
voltage. We find that the thermopower and the figure of merit are greatly
enhanced when the chemical potential is tuned close to resonances. Such
enhancement is even more pronounced in the vicinity of Fano like
anti-resonances which can be induced by a side-gate voltage independently of
the geometry. This opens a possibility to use the proposed device as a tunable
thermoelectric generator.Comment: 6 pages, 5 figures, accepted for publication in Physical Review
Dynamic instability in resonant tunneling
We show that an instability may be present in resonant tunneling through a
quantum well in one, two and three dimensions, when the resonance lies near the
emitter Fermi level. A simple semiclassical model which simulates the resonance
and the projected density of states by a nonlinear conductor, the Coulomb
barrier by a capacitance, and the time evolution by an iterated map, is used.
The model reproduces the observed hysteresis in such devices, and exhibits a
series of bifurcations leading to fast chaotic current fluctuations.Comment: 7 pages, 2 figure
On hadronic beam models for quasars and microquasars
Most of the hadronic jet models for quasars (QSOs) and microquasars (MQs)
found in literature represent beams of particles (e.g. protons). These
particles interact with the matter in the stellar wind of the companion star in
the system or with crossing clouds, generating gamma-rays via proton-proton
processes. Our aim is to derive the particle distribution in the jet as seen by
the observer, so that proper computation of the -ray and neutrino
yields can be done. We use relativistic invariants to obtain the transformed
expressions in the case of a power-law and power-law with a cutoff particle
distribution in the beam. We compare with previous expressions used earlier in
the literature. We show that formerly used expressions for the particle
distributions in the beam as seen by the observer are in error, differences
being strongly dependent on the viewing angle. For example, for
( is the Lorentz factor of the blob) and angles larger than , the earlier-used calculation entails an over-prediction (order of
magnitude or more) of the proton spectra for , whereas it always
over-predicts (two orders of magnitude) the proton spectrum at lower energies,
disregarding the viewing angle. All the results for photon and neutrino fluxes
in hadronic models in beams that have made use of the earlier calculation are
affected. Given that correct gamma-ray fluxes will be in almost any case
significantly diminished in comparison with published results, and that the
time of observations in Cherenkov facilities grows with the square of the
flux-reduction factor in a statistically limited result, the possibility of
observing hadronic beams is undermined.Comment: Accepted for publication in A&A Letter
Graphene nanoring as a tunable source of polarized electrons
We propose a novel spin filter based on a graphene nanoring fabricated above
a ferromagnetic strip. The exchange interaction between the magnetic moments of
the ions in the ferromagnet and the electron spin splits the electronic states,
and gives rise to spin polarization of the conductance and the total electric
current. We demonstrate that both the current and its polarization can be
controlled by a side-gate voltage. This opens the possibility to use the
proposed device as a tunable source of polarized electrons.Comment: 12 pages, 7 figures, accepted in Nanotechnolog
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
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