Tunnel spectroscopy in ac-driven quantum dot nanoresonators

Electronic transport in a triple quantum dot shuttle device in the presence of an ac field is analyzed within a fully quantum mechanical framework. A generalized density matrix formalism is used to describe the time evolution for electronic state occupations in a dissipative phonon bath. In the presence of an ac gate voltage, the electronic states are dressed by photons and the interplay between photon and vibrational sidebands produces current characteristics that obey selection rules. Varying the ac parameters allows to tune the tunneling current features. In particular, we show that coherent destruction of tunneling can be achieved in our device

Quasienergy spectrum and tunneling current in ac-driven triple quantum dot shuttles

The dynamics of electrons in ac driven double quantum dots have been extensively analyzed by means of Floquet theory. In these systems, coherent destruction of tunneling has been shown to occur for certain ac field parameters. In the present work we analyze, by means of Floquet theory, the electron dynamics of a triple quantum dot in series attached to electric contacts, where the central dot position oscillates. In particular, we analyze the quasienergy spectrum of this ac driven nanoelectromechanical system, as a function of the intensity and frequency of the ac field and of external dc voltages. For strong driving fields, we derive, by means of perturbation theory, analytical expressions for the quasienergies of the driven oscillator system. From this analysis we discuss the conditions for coherent destruction of tunneling (CDT) to occur as a function of detuning and field parameters. For zero detuning, and from the invariance of the Floquet Hamiltonian under a generalized parity transformation, we find analytical expressions describing the symmetry properties of the Fourier components of the Floquet states under such transformation. By using these expressions, we show that in the vicinity of the CDT condition, the quasienergy spectrum exhibits exact crossings which can be characterized by the parity properties of the corresponding eigenvectors

Quantum dynamics, dissipation, and asymmetry effects in quantum dot arrays

We study the role of dissipation and structural defects on the time evolution of quantum dot arrays with mobile charges under external driving fields. These structures, proposed as quantum dot cellular automata, exhibit interesting quantum dynamics which we describe in terms of equations of motion for the density matrix. Using an open system approach, we study the role of asymmetries and the microscopic electron-phonon interaction on the general dynamical behavior of the charge distribution (polarization) of such systems. We find that the system response to the driving field is improved at low temperatures (and/or weak phonon coupling), before deteriorating as temperature and asymmetry increase. In addition to the study of the time evolution of polarization, we explore the linear entropy of the system in order to gain further insights into the competition between coherent evolution and dissipative processes.Comment: 11pages,9 figures(eps), submitted to PR

Quantificação de perdas na produção de grãos causadas pela antracnose foliar em diferentes genótipos de sorgo.

Trabalho publicado também no 7º Seminário de Iniciação Científica PIBIC/BIC Júnior, 2014, Sete Lagoas

A semi-automatic framework for the design of rational resilient collaborative systems

Collaborative systems (e.g., P2P instant messaging, file sharing, live streaming applications) constitute the largest traffic of today\u2019s Internet. Common to all these systems is the assumption that, in return to the service offered by the collaborative system, users are willing to participate by sharing their resources with others. However, in practice, these systems suffer from rational users, i.e. users that benefit from the system without contributing their fair share to it. A number of solutions have been devised in the literature to deal with the problem of rational users in collaborative systems. However, most of these solutions are tailored to specific systems and thus lack flexibility and re-usability. In this paper, we propose RACOON, the first framework for RAtional resilient COllabOrative system desigN. RACOON relies on an extensible model that allows a system designer to specify the protocol steps and the different types of rational users he wants to consider. Furthermore, RACOON relies on game theory to reason on the behaviour of rational users. Finally, RACOON includes a simulation module that allows performance-oriented tuning of the system. Throughout the paper, we show how we used RACOON for the design of a rational-resilient, collaborative live-streaming application. Performance evaluation realised on one hundred real machines shows that the configuration proposed by RACOON allows all users to visualise a quality stream even in presence of rational users

First report of Lasiodiplodia brasiliense causing maize stalk rot.

Maize stalk rot can lead to major corn yield losses in Brazil and worldwide. Maize stalks exhibiting tiny black specks or light brown lesions were collected in the city of Luís Eduardo Magalhães. Based on morphological characteristics and molecular assays, the fungus was identified as Lasiodiplodia brasiliense. This is the first report of maize stalk rot caused by L. brasiliense.Article 41