Simón Bolívar University

About: Simón Bolívar University is a education organization based out in Caracas, Venezuela. It is known for research contribution in the topics: Population & Crystallization. The organization has 5912 authors who have published 8294 publications receiving 126152 citations.

Exact analytical solution of channel surface potential as an explicit function of gate voltage in undoped-body MOSFETs using the Lambert W function and a threshold voltage definition therefrom

Abstract: Two useful applications of the Lambert W function to undoped-body MOSFET modeling are presented. Firstly, it is applied to the problem of inverting the gate voltage versus channel surface potential equation. The result is an exact analytical solution of the channel surface potential as an explicit function of the gate voltage for either n or p channel operation. Additionally an approximate but highly accurate analytical solution is presented which is continuously valid for all regions of operation. Secondly, we propose a new unambiguous analytical definition for the threshold voltage of these undoped-body devices. This definition overcomes the impossibility of using the traditional definition based on the bulk Fermi potential, and the ambiguities introduced by other definitions. The threshold voltage is mathematically described also using the Lambert W function at the transition point from subthreshold to superthreshold behavior. An approximation for the )1 branch of the Lambert W function is proposed to express the threshold voltage approximately using elementary logarithmic functions. These new descriptions are then verified against two-dimensional numerical device simulations. 2003 Elsevier Ltd. All rights reserved.

An aggregation pheromone modulates lekking behavior in the vector mosquito aedes aegypti (diptera: culicidae)

Abstract: Males of Aedes aegypti mosquitoes formed swarms in the laboratory, triggered by the onset of the photophase or by the presence of odors from a rat (which is a potential host for females). The swarm attracted both males and females and increased mating activity. The number of copulas per mosquito was positively correlated with the number of mosquitoes in the swarm and with the duration of the swarm. Swarming and mating activity increased with the presence of a host for females. Young sexually immature males, less than 24 h old, flew but did not swarm nor copulate. Observations using an olfactometer showed that swarming males produced a volatile pheromone that stimulates the flying activity of females at a distance. Females also produce a volatile attractant. The results suggest that males, and possibly also females, produce an aggregation pheromone that attracts males and females towards the swarm. The characteristics of the pheromone-mediated swarm may be described as a 3-dimensional lek. Our results suggest that the development of pheromone-based control systems and/or pheromone traps for the monitoring of vector populations is feasible, adding a new tool to combat this vector of several human pathogens.

Left-handed W bosons at the LHC

Abstract: The production of $W$ bosons in association with jets is an important background to new physics at the LHC. Events in which the $W$ carries large transverse momentum and decays leptonically lead to large missing energy and are of particular importance. We show that the left-handed nature of the $W$ coupling, combined with valence quark domination at a $pp$ machine, leads to a large left-handed polarization for both ${W}^{+}$ and ${W}^{\ensuremath{-}}$ bosons at large transverse momenta. The polarization fractions are very stable with respect to QCD corrections. The leptonic decay of the ${W}^{\ifmmode\pm\else\textpm\fi{}}$ bosons translates the common left-handed polarization into a strong asymmetry in transverse momentum distributions between positrons and electrons and between neutrinos and antineutrinos (missing transverse energy). Such asymmetries may provide an effective experimental handle on separating $W+\mathrm{\text{jets}}$ from top-quark production, which exhibits very little asymmetry due to C invariance, and from various types of new physics.

Nucleation and crystallization of isotactic poly(propylene) droplets in an immiscible polystyrene matrix

Abstract: In this work, isotactic poly(propylene) (iPP) was finely dispersed in immiscible atactic polystyrene (PS) matrices. When the dispersion obtained is fine enough (droplet size of approximately 1-2 μm), the iPP crystallizes in a fractionated fashion as temperatures between 104 and 42°C. By applying a self-nucleation procedure, we were able to corroborate that what causes the fractionated crystallization in most droplets is the lack of highly active heterogeneous nuclei (i.e. those normally active at low supercoolings in the bulk polymer) in every droplet. When a sulficient amount of a compatibilizer is used to obtain very small particle sizes and more homogenerous dispersions, the iPP crystallizes exclusively at a low temperature exotherm that exbibits an onset at 51°C and peaks at 46°C. Wide-angle X-ray diffraction measurements indicated that both the iPP in the bulk and in dispersed droplets crystallized in the monoclinic α-phase, this evidence may rule out the possibility that the crystallization observed at 46°C is due to the formation of another crystal modification or a mesomorphic phase as previously suggested in literature. The results presented in this work indicate that this low temperature exotherm may represent the dynamic crystallization during cooling of heterogeneity-free droplets that nucleate homogeneously at temperatures close to 51°C.

mGPT: a probabilistic planner based on heuristic search

Abstract: We describe the version of the GPT planner used in the probabilistic track of the 4th International Planning Competition (IPC-4). This version, called mGPT, solves Markov Decision Processes specified in the PPDDL language by extracting and using different classes of lower bounds along with various heuristic-search algorithms. The lower bounds are extracted from deterministic relaxations where the alternative probabilistic effects of an action are mapped into different, independent, deterministic actions. The heuristic-search algorithms use these lower bounds for focusing the updates and delivering a consistent value function over all states reachable from the initial state and the greedy policy.