11,299 research outputs found
Decoherence processes during active manipulation of excitonic qubits in semiconductor quantum dots
Using photoluminescence spectroscopy, we have investigated the nature of Rabi
oscillation damping during active manipulation of excitonic qubits in
self-assembled quantum dots. Rabi oscillations were recorded by varying the
pulse amplitude for fixed pulse durations between 4 ps and 10 ps. Up to 5
periods are visible, making it possible to quantify the excitation dependent
damping. We find that this damping is more pronounced for shorter pulse widths
and show that its origin is the non-resonant excitation of carriers in the
wetting layer, most likely involving bound-to-continuum and continuum-to-bound
transitions.Comment: 18 pages, 4 figure
Mathematical models for vulnerable plaques
A plaque is an accumulation and swelling in the artery walls and typically consists of cells, cell debris, lipids, calcium deposits and fibrous connective tissue. A person is likely to have many plaques inside his/her body even if they are healthy. However plaques may become "vulnerable", "high-risk" or "thrombosis-prone" if the person engages in a high-fat diet and does not exercise regularly.
In this study group, we proposed two mathematical models to describe plaque growth and rupture.
The first model is a mechanical one that approximately treats the plaque as an inflating elastic balloon. In this model, the pressure inside the core increases and then decreases suggesting that plaque stabilization and prevention of rupture is possible.
The second model is a biochemical one that focuses on the role of MMPs in degrading the fibrous plaque cap. The cap stress, MMP concentration, plaque volume and cap thickness are coupled together in a system of phenomenological equations. The equations always predict an eventual rupture since the volume, stresses and MMP concentrations generally grow without bound. The main weakness of the model is that many of the important parameters that control the behavior of the plaque are unknown.
The two simple models suggested by this group could serve as a springboard for more realistic theoretical studies. But most importantly, we hope they will motivate more experimental work to quantify some of the important mechanical and biochemical properties of vulnerable plaques
Quantum wires from coupled InAs/GaAs strained quantum dots
The electronic structure of an infinite 1D array of vertically coupled
InAs/GaAs strained quantum dots is calculated using an eight-band
strain-dependent k-dot-p Hamiltonian. The coupled dots form a unique quantum
wire structure in which the miniband widths and effective masses are controlled
by the distance between the islands, d. The miniband structure is calculated as
a function of d, and it is shown that for d>4 nm the miniband is narrower than
the optical phonon energy, while the gap between the first and second minibands
is greater than the optical phonon energy. This leads to decreased optical
phonon scattering, providing improved quantum wire behavior at high
temperatures. These miniband properties are also ideal for Bloch oscillation.Comment: 5 pages revtex, epsf, 8 postscript figure
Quantum-Information Processing with Semiconductor Macroatoms
An all optical implementation of quantum information processing with
semiconductor macroatoms is proposed. Our quantum hardware consists of an array
of semiconductor quantum dots and the computational degrees of freedom are
energy-selected interband optical transitions. The proposed quantum-computing
strategy exploits exciton-exciton interactions driven by ultrafast sequences of
multi-color laser pulses. Contrary to existing proposals based on charge
excitations, the present all-optical implementation does not require the
application of time-dependent electric fields, thus allowing for a
sub-picosecond, i.e. decoherence-free, operation time-scale in realistic
state-of-the-art semiconductor nanostructures.Comment: 11 pages, 5 figures, to be published in Phys. Rev. Lett., significant
changes in the text and new simulations (figure 3
Surface and bulk electronic structures of LaOFeAs studied by angle resolved photoemission spectroscopy
The electronic structure of LaOFeAs, a parent compound of iron-arsenic
superconductors, is studied by angleresolved photoemission spectroscopy. By
examining its dependence on photon energy, polarization, sodium dosing and the
counting of Fermi surface volume, both the bulk and the surface contributions
are identified. We find that a bulk band moves toward high binding energies
below structural transition, and shifts smoothly across the spin density wave
transition by about 25 meV. Our data suggest the band reconstruction may play a
crucial role in the spin density wave transition, and the structural transition
is driven by the short range magnetic order. For the surface states, both the
LaO-terminated and FeAs-terminated components are revealed. Certain small band
shifts are verified for the FeAs-terminated surface states in the spin density
wave state, which is a reflection of the bulk electronic structure
reconstruction. Moreover, sharp quasiparticle peaks quickly rise at low
temperatures, indicating of drastic reduction of the scattering rate. A kink
structure in one of the surface band is shown to be possibly related to the
electron-phonon interactions.Comment: 9 pages, 8 figure
Statistical nature of non-Gaussianity from cubic order primordial perturbations: CMB map simulations and genus statistic
We simulate CMB maps including non-Gaussianity arising from cubic order
perturbations of the primordial gravitational potential, characterized by the
non-linearity parameter . The maps are used to study the characteristic
nature of the resulting non-Gaussian temperature fluctuations. We measure the
genus and investigate how it deviates from Gaussian shape as a function of
and smoothing scale. We find that the deviation of the non-Gaussian
genus curve from the Gaussian one has an antisymmetric, sine function like
shape, implying more hot and more cold spots for and less of both
for . The deviation increases linearly with and also
exhibits mild increase as the smoothing scale increases. We further study other
statistics derived from the genus, namely, the number of hot spots, the number
of cold spots, combined number of hot and cold spots and the slope of the genus
curve at mean temperature fluctuation. We find that these observables carry
signatures of that are clearly distinct from the quadratic order
perturbations, encoded in the parameter . Hence they can be very useful
tools for distinguishing not only between non-Gaussian temperature fluctuations
and Gaussian ones but also between and type
non-Gaussianities.Comment: 18+1 page
Chemical and physical fractions of soil organic matter under various management regimes in Roraima, Brazil.
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Eficiência agronômica da mistura de uréia com zeolita natural aplicada na cultura da roseira (Rosa spp.), Nova Friburgo, RJ.
Com objetivo de avaliar a eficiência agronômica da mistura de uréia com zeolita natural (arenito zeolítico), realizou-se experimento em área de produção comercial de flores de corte em Nova Friburgo, RJ, Brasil. Foram realizadas duas adubações de cobertura na cultura da roseira com duas doses de uréia (60 e 120 kg N. ha-1 ) misturadas ou não com arenito zeolítico, na proporção de 20% p/p. As colheitas foram avaliadas quantitativamente, através do total de hastes (HT), massa fresca (MFT) e massa seca (MST) produzidas, e qualitativamente, através da divisão em hastes curtas (HC), médias (HM) e longas (HL). Determinou-se também a extração total de N pelas hastes comerciais colhidas (EXTNH) e a eficiência agronômica dos fertilizantes (EAF). A eficiência agronômica dos fertilizantes (EAF) sobre as produções de HT, MFT, MST e EXTNH é elevada pelo efeito do arenito zeolítico, e a maior influência sobre HT provém do aumento nas produções de HL, elevando a qualidade das hastes produzidas. A mistura de arenito zeolítico com uréia mantém maior quantidade de N-uréia no sistema solo-planta, liberando o nutriente lentamente para a solução do solo, em sincronia com as demandas nutricionais das plantas, resultando no aumento da eficiência agronômica dos fertilizantes avaliados. O arenito zeolítico (CETEM) apresenta características favoráveis para uso em desenvolvimento de fertilizantes nitrogenados de liberação lenta, principalmente com objetivo de aumentar a eficiência agronômica dos fertilizantes
Closed Path Integrals and Renormalisation in Quantum Mechanics
We suggest a closed form expression for the path integral of quantum
transition amplitudes. We introduce a quantum action with renormalized
parameters. We present numerical results for the potential. The
renormalized action is relevant for quantum chaos and quantum instantons.Comment: Revised text, 1 figure added; Text (LaTeX file), 1 Figure (ps file
Decomposição e liberação de nutrientes da palhada de braquiária, sorgo e soja em áreas de plantio direto no cerrado goiano.
A avaliação da decomposição dos resíduos vegetais adicionados ao solo pelas plantas de cobertura permite uma melhor compreensão do fornecimento de nutrientes para as culturas de interesse comercial. O objetivo do trabalho foi avaliar as taxas de decomposição e a dinâmica da liberação de N, P e K de resíduos culturais na entressafra da soja cultivada em Latossolo Vermelho, sob plantio direto. Os resíduos utilizados foram braquiária (Brachiaria ruziziensis R. Germ. & Evrard) e o sorgo (Sorghum bicolor L. Moench). A produção média de biomassa seca de braquiária + soja foi de 6,1 Mg ha-1 e do sorgo + soja foi de 3,8 Mg ha-1 . A decomposição da matéria seca e a liberação de nutrientes foram monitoradas aos 15,30, 60, 90, 120 dias por meio de sacolas contendo resíduos culturais depositados na superfície do solo. Utilizou-se um modelo matemático exponencial negativo simples para descrever a decomposição dos resíduos e a liberação de N, P, K, sendo calculados a constante de decomposição (k) e o tempo de meia vida (T1/2). Os valores de T1/2 da matéria seca foram de 154 e 258 dias para braquiária + soja e sorgo + soja, respectivamente, no período seco e de 99 e 119 dias no período chuvoso. O nutriente que apresentou o menor T1/2 foi o P, seguido por K e N nos dois resíduos avaliados e nas diferentes épocas
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