arXiv subjects

R. Ribeiro

Publications and source records attributed to R. Ribeiro.

8 recordsLinked to original sources

arXiv2025

Unoccupied bands in the molybdenum dichalcogenides MoS$_2$, MoSe$_2$, and MoTe$_2$

We present angle-resolved reflected electron spectroscopy (ARRES) data for the three molybdenum-based transition metal dichalcogenides (TMDs) \mos, \mose, and \mote. To follow the changes as the series moves from S to Se to Te in more detail, we determine accurate IV-spectra for monolayers and bulk TMDs. These experimental data sets are then compared with theoretical predictions for both the unoccupied band structure and the scattering density of states. We find good agreement, especially for lower energies where inelastic effects are relatively unimportant. Furthermore, we identify a series of interlayer resonances for which the dependence of the hybridization effects on the layer count is observed. Although these resonances bear similarity to interlayer resonances in hBN and graphene, they differ in their character, being dominated by unoccupied $d$-states of the chalcogen-atoms. The unoccupied states studied and analyzed here play a key role in all processes that require an electron to temporarily reside in a state above the vacuum level, such as in photoemission and secondary electron emission experiments.

cond-mat.mes-hall↗

arXiv2022

On cyclicity in discontinuous piecewise linear near-Hamiltonian differential systems with three zones having a saddle in the central one

In this paper, we study the number of limit cycles that can bifurcate from a periodic annulus of discontinuous planar piecewise linear Hamiltonian differential system with three zones separated by two parallel straight lines, such that the linear differential system, given by the piecewise one, in the region between the two straight lines (called of central subsystem) has a saddle at a point equidistant from these lines (obviously, the others subsystems have saddles and centers). We prove that the maximum number of limit cycles that bifurcate from the periodic annulus of this kind of piecewise Hamiltonian differential systems, by linear perturbations, is at least six. For this, we obtain normal forms for the systems and study the number of zeros of its Melnikov functions defined in two and three zones.

math.DS↗

arXiv2021

Limit Cycles from Planar Piecewise Linear Hamiltonian differential Systems with Two or Three Zones

In this paper, we study the existence of limit cycles in continuous and discontinuous planar piecewise linear Hamiltonian differential systems with two or three zones separated by straight lines and such that the linear systems that define the piecewise one have isolated singular points, i.e. centers or saddles. In this case, we show that if the planar piecewise linear Hamiltonian differential system is either continuous or discontinuous with two zones, then it has no limit cycles. Now, if the planar piecewise linear Hamiltonian differential system is discontinuous with three zones, then it has at most one limit cycle, and there are examples with one limit cycle. More precisely, without taking into account the position of the singular points in the zones, we present examples with the unique limit cycle for all possible combinations of saddles and centers.

math.DS↗

arXiv2018

Systems engineering applied to ELT instrumentation: The GMACS case

An important tool for the development of the next generation of extremely large telescopes (ELTs) is a robust Systems Engineering (SE) methodology. GMACS is a first-generation multi-object spectrograph that will work at visible wavelengths on the Giant Magellan Telescope (GMT). In this paper, we discuss the application of SE to the design of next-generation instruments for ground-based astronomy and present the ongoing development of SE products for the GMACS spectrograph, currently in its Conceptual Design phase. SE provides the means to assist in the management of complex projects, and in the case of GMACS, to ensure its operational success, maximizing the scientific potential of GMT.

astro-ph.IM↗

arXiv2013

On Various Types of Shadowing for Geometric Lorenz Flows

We show that Lorenz flows have neither limit shadowing property nor average shadowing property nor the asymptotic average shadowing property where the reparametrizations related to these concepts relies on the set of increasing homeomorphisms with bounded variation.

math.DS↗

arXiv2012

Growth of carbon nanotubes on quasicrystalline alloys

We report on the synthesis of carbon nanotubes on quasicrystalline alloys. Aligned multiwalled carbon nanotubes (MWNTs) on the conducting faces of decagonal quasicrystals were synthesized using floating catalyst chemical vapor deposition. The alignment of the nanotubes was found perpendicular to the decagonal faces of the quasicrystals. A comparison between the growth and tube quality has also been made between tubes grown on various quasicrystalline and SiO2 substrates. While a significant MWNT growth was observed on decagonal quasicrystalline substrate, there was no significant growth observed on icosahedral quasicrystalline substrate. Raman spectroscopy and high resolution transmission electron microscopy (HRTEM) results show high crystalline nature of the nanotubes. Presence of continuous iron filled core in the nanotubes grown on these substrates was also observed, which is typically not seen in MWNTs grown using similar process on silicon and/or silicon dioxide substrates. The study has important implications for understanding the growth mechanism of MWNTs on conducting substrates which have potential applications as heat sinks.

cond-mat.mtrl-sci↗

arXiv2012

Application of Neural Networks to the study of stellar model solutions

Artificial neural networks (ANN) have different applications in Astronomy, including data reduction and data mining. In this work we propose the use ANNs in the identification of stellar model solutions. We illustrate this method, by applying an ANN to the 0.8M$_\odot$ star CG Cyg B. Our ANN was trained using 60,000 different 0.8M$_\odot$ stellar models. With this approach we identify the models which reproduce CG Cyg B's position in the HR diagram. We observe a correlation between the model's initial metal and helium abundance which, in most cases, does not agree with a helium to metal enrichment ratio $Δ$Y/$Δ$Z=2. Moreover, we identify a correlation between the model's initial helium/metal abundance and both its age and mixing-length parameter. Additionally, every model found has a mixing-length parameter below 1.3. This means that CG Cyg B's mixing-length parameter is clearly smaller than the solar one. From this study we conclude that ANNs are well suited to deal with the degeneracy of model solutions of solar type stars.

astro-ph.SR↗

arXiv2007

Scalability and Optimisation of a Committee of Agents Using Genetic Algorithm

A population of committees of agents that learn by using neural networks is implemented to simulate the stock market. Each committee of agents, which is regarded as a player in a game, is optimised by continually adapting the architecture of the agents using genetic algorithms. The committees of agents buy and sell stocks by following this procedure: (1) obtain the current price of stocks; (2) predict the future price of stocks; (3) and for a given price trade until all the players are mutually satisfied. The trading of stocks is conducted by following these rules: (1) if a player expects an increase in price then it tries to buy the stock; (2) else if it expects a drop in the price, it sells the stock; (3)and the order in which a player participates in the game is random. The proposed procedure is implemented to simulate trading of three stocks, namely, the Dow Jones, the Nasdaq and the S&P 500. A linear relationship between the number of players and agents versus the computational time to run the complete simulation is observed. It is also found that no player has a monopolistic advantage.

cs.MA↗