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 $\gamma$-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 $\Gamma =10$ ($\Gamma$ is the Lorentz factor of the blob) and angles larger than $\sim 20^o$, the earlier-used calculation entails an over-prediction (order of magnitude or more) of the proton spectra for $E>\Gamma mc^2$, 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