69 research outputs found
Dissipative dynamics of spins in quantum dots
We present a theory for the dissipation of electronic spins trapped in
quantum dots due to their coupling to the host lattice acoustic phonon modes.
Based on the theory of dissipative two level systems for the spin dynamics, we
derive a relation between the spin dissipative bath, the electron confinement,
and the electron-phonon interaction. We find that there is an energy scale,
typically smaller than the electronic lateral confinement energy, which sets
the boundary between different dissipative regimes .Comment: 4 pages, 2 eps figure
A EVOLUÇÃO DA GESTÃO DE PESSOAS NAS UNIVERSIDADES FEDERAIS: DO PATRIMONIALISMO AO ESTRATÉGICO.
O presente artigo objetiva mostrar, através de pesquisa bibliográfica e documental, a caminhada da Gestão de Pessoas dentro das Universidades Federais, desde a criação do Departamento Administrativo do Serviço Público (DASP) até a instituição da Política Nacional de Desenvolvimento de Pessoal (PNDP) e do Plano de Desenvolvimento Institucional (PDI), apresentando os principais momentos, que referenciam a mudança de pensamento da área no decorrer do tempo, transformando a Gestão de Pessoas de um ente limitado em estratégico dentro das instituições, com suas motivações legais e conceituais para o processo de desenvolvimento. O trabalho servirá de subsídio para reflexão sobre os avanços da Gestão de Pessoas nas Instituições Federais de Ensino no Brasil, que, na suma do trabalho, apresenta, mesmo perante um avanço inquestionável na área descrita, um longo caminho a ser percorrido, como causa de anos de estagnação das políticas e práticas de gestão dentro do serviço público
Polarization-selective excitation of N-V centers in diamond
The nitrogen-vacancy (N-V) center in diamond is promising as an electron spin
qubit due to its long-lived coherence and optical addressability. The ground
state is a spin triplet with two levels () degenerate at zero
magnetic field. Polarization-selective microwave excitation is an attractive
method to address the spin transitions independently, since this allows
operation down to zero magnetic field. Using a resonator designed to produce
circularly polarized microwaves, we have investigated the polarization
selection rules of the N-V center. We first apply this technique to N-V
ensembles in [100] and [111]-oriented samples. Next, we demonstrate an imaging
technique, based on optical polarization dependence, that allows rapid
identification of the orientations of many single N-V centers. Finally, we test
the microwave polarization selection rules of individual N-V centers of known
orientation
Size quantization effects in InAs self-assembled quantum dots
3 páginas, 3 figuras. PACS: 73.21.-b; 78.55.Cr; 78.66.Fd; 72.40.+w; 68.35.Ct; 73.40.KpWe study size quantization effects in InAs self-assembled quantum dots (QDs) that are embedded in GaAs. Using capacitance, photoluminescence and photovoltage spectroscopy, we correlate the measured quantized level energies with the quantum dot sizes and densities obtained from transmission electron microscopy. With increasing dot size, we observe a strong redshift of the QD features in all our data. In the capacitance spectra, a band gap renormalization of the two-dimensional wetting layer system appears when the first excited QD state crosses the wetting layer ground state. The relative size dependence and absolute energetic position of the QD transitions determined with photoluminescence provide some information about the influence of lateral confinement and height of the QD.This work was supported
by QUEST, a NSF science and technology center (Grant No.
DMR91-20007), the Alexander von Humboldt Foundation
(KHS), and the Brazilian agency CNPq (GMR).Peer reviewe
The minimum model for the iron-based superconductors
A single band --- model is proposed as the minimum model to
describe the superconductivity of the newly discovered iron-based
superconductors and (). With the mean-field approach, it is found that the pairing occurs in
the d-wave channel. In the likely parameter region of the real materials, by
lowering temperature, the system enters firstly the superconducting
phase with -symmetry and then enters the time-reversal-symmetry-broken
superconducting phase with -symmetry.Comment: 3 pages, 3 figure
Quantized conductance coincides with state instability and excess noise in tantalum oxide memristors
Tantalum oxide memristors can switch continuously from a low-conductance semiconducting to a high-conductance metallic state. At the boundary between these two regimes are quantized conductance states, which indicate the formation of a point contact within the oxide characterized by multistable conductance fluctuations and enlarged electronic noise. Here, we observe diverse conductance-dependent noise spectra, including a transition from 1/f 2 (activated transport) to 1/f (flicker noise) as a function of the frequency f, and a large peak in the noise amplitude at the conductance quantum GQ¼2e2/h, in contrast to
suppressed noise at the conductance quantum observed in other systems. We model the
stochastic behaviour near the point contact regime using Molecular Dynamics–Langevin
simulations and understand the observed frequency-dependent noise behaviour in terms of thermally activated atomic-scale fluctuations that make and break a quantum conductance channel. These results provide insights into switching mechanisms and guidance to device operating ranges for different applications
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