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

k-deformed Poincare algebras and quantum Clifford-Hopf algebras

The Minkowski spacetime quantum Clifford algebra structure associated with the conformal group and the Clifford-Hopf alternative k-deformed quantum Poincare algebra is investigated in the Atiyah-Bott-Shapiro mod 8 theorem context. The resulting algebra is equivalent to the deformed anti-de Sitter algebra U_q(so(3,2)), when the associated Clifford-Hopf algebra is taken into account, together with the associated quantum Clifford algebra and a (not braided) deformation of the periodicity Atiyah-Bott-Shapiro theorem.Comment: 10 pages, RevTeX, one Section and references added, improved content

Stability properties and asymptotics for N non-minimally coupled scalar fields cosmology

We consider here the dynamics of some homogeneous and isotropic cosmological models with $N$ interacting classical scalar fields non-minimally coupled to the spacetime curvature, as an attempt to generalize some recent results obtained for one and two scalar fields. We show that a Lyapunov function can be constructed under certain conditions for a large class of models, suggesting that chaotic behavior is ruled out for them. Typical solutions tend generically to the empty de Sitter (or Minkowski) fixed points, and the previous asymptotic results obtained for the one field model remain valid. In particular, we confirm that, for large times and a vanishing cosmological constant, even in the presence of the extra scalar fields, the universe tends to an infinite diluted matter dominated era.Comment: 10 page

Evaluation of corrosion resistance of multi-layered Ti/gass-ceramic interfaces by electrochemical impedance spectroscopy

Practical applications of metal/ceramic joints can be found in the biomedical field regarding the encapsulation of implantable telemetric devices, the fabrication of crowns and bridges for dental restoration, or in the production of drug delivery systems, biomedical sensors and electrodes. Most of metal/ceramic joints are produced by the active metal brazing technique, which originates a multi-layered interface which should be able of accommodating the abrupt electronic, crystallographic, chemical, mechanical and thermo-mechanical discontinuity that characterize these systems. Additionally, when considering biomedical applications, corrosion resistance becomes of prime importance. In this work, the corrosion resistance of Ti/glass-ceramic interfaces obtained by active metal brazing was evaluated by electrochemical impedance spectroscopy (EIS) tests. The electrochemical behaviour of the interface was monitored, as a function of time, in a simulated physiological solution at room temperature. In order to evaluate the contribution of each layer and galvanic interactions between them, to the degradation mechanism of the interface, individual samples, representative of reaction layers present at the interface, were fabricated and electrochemically ested. Results show that the corrosion behaviour, of the whole interface was strongly influenced by the chemical composition of its constitutive layers. Thus, layers containing high contents of both titanium and silver showed a polarisation resistance increase with the immersion time, as a result of the formation of a thermodynamically stable passive film. On the other hand, the copper rich layer, appears to be the main responsible for the interface degradation. In fact, for high immersion times, an instable passive film is formed and, as a consequence, large amounts of copper are released. Galvanic interactions between the copper and the silver rich layers where also dentified.Fundação para a Ciência e a Tecnologia (FCT), Portugal Portugal (projects POCTI/CTM/33384/2000 and SFRH/BPD/5518/2001)

Superinflation, quintessence, and the avoidance of the initial singularity

We consider the dynamics of a spatially flat universe dominated by a self-interacting nonminimally coupled scalar field. The structure of the phase space and complete phase portraits for the conformal coupling case are given. It is shown that the non-minimal coupling modifies drastically the dynamics of the universe. New cosmological behaviors are identified, including superinflation ($\dot{H}>0$), avoidance of big bang singularities through classical birth of the universe from empty Minkowski space, and spontaneous entry into and exit from inflation. The relevance of this model to the description of quintessence is discussed.Comment: RevTex, 10 pages, 4 figures, To appear in the proceedings of the 5th Peyresq meetin

Markov Chain Beam Randomization: a study of the impact of PLANCK beam measurement errors on cosmological parameter estimation

We introduce a new method to propagate uncertainties in the beam shapes used to measure the cosmic microwave background to cosmological parameters determined from those measurements. The method, which we call Markov Chain Beam Randomization, MCBR, randomly samples from a set of templates or functions that describe the beam uncertainties. The method is much faster than direct numerical integration over systematic `nuisance' parameters, and is not restricted to simple, idealized cases as is analytic marginalization. It does not assume the data are normally distributed, and does not require Gaussian priors on the specific systematic uncertainties. We show that MCBR properly accounts for and provides the marginalized errors of the parameters. The method can be generalized and used to propagate any systematic uncertainties for which a set of templates is available. We apply the method to the Planck satellite, and consider future experiments. Beam measurement errors should have a small effect on cosmological parameters as long as the beam fitting is performed after removal of 1/f noise.Comment: 17 pages, 23 figures, revised version with improved explanation of the MCBR and overall wording. Accepted for publication in Astronomy and Astrophysics (to appear in the Planck pre-launch special issue

1.5V fully programmable CMOS Membership Function Generator Circuit with proportional DC-voltage control

A Membership Function Generator Circuit (MFGC) with bias supply of 1.5 Volts and independent DC-voltage programmable functionalities is presented. The realization is based on a programmable differential current mirror and three compact voltage-to-current converters, allowing continuous and quasi-linear adjustment of the center position, height, width and slopes of the triangular/trapezoidal output waveforms. HSPICE simulation results of the proposed circuit using the parameters of a double-poly, three metal layers, 0.5 μm CMOS technology validate the functionality of the proposed architecture, which exhibits a maximum deviation of the linearity in the programmability of 7 %