997 resultados para QUANTUM PROCESSES
Resumo:
Transient photocurrents induced by short light pulses at lattice-matched GaAs/AlxGa1-xAs multiple quantum well (MQW) electrodes were studied as a function of electrode potential. Dual exponential photocurrent decay transients were observed at various potentials. By analysis of the dual exponential decay transients, information on steady state photocurrents (I-s), surface collection of photoexcited minority carriers (G(0)) and lifetimes of surface states (T-s) was obtained. The kinetic behaviors of photoprocesses at illuminated MQW/electrolyte interface were discussed.
Resumo:
GaAs/AlGaAs quantum dot arrays with different dot sizes made by different fabrication processes were studied in this work. In comparison with the reference quantum well, photoluminescence (PL) spectra from the samples at low temperature have demonstrated that PL peak positions shift to higher energy side due to quantization confinement effects and the blue-shift increases with decreasing dot size, PL linewidths are broadened and intensities are much reduced. It is also found that wet chemical etching after reactive ion etching can improve optical properties of the quantum dot arrays.
Resumo:
GaAs/AlAs/GaAlAs double barrier quantum well (DBQW) structures are employed for making the 3 similar to 5 mu m photovoltaic infrared (IR) detectors with a peak detectivity of 5x10(11) cmHz(1/2)/W at 80K. The double crystal x-ray diffraction is combined with synchrotron radiation x-ray analysis to determine the exact thickness of GaAs, AlAs and GaAlAs sublayers. The interband photovoltaic (PV) spect ra of the DBQW sample and the spectral response of the IR photocurrent of the devices are measured directly by edge excitation method, providing the information about spatial separation processes of photogenerated carriers in the multiquantum wells and the distribution of built-in field in the active region.
Resumo:
The dark current characteristics and temperature dependence for quantum dot infrared photodetectors have been investigated by comparing the dark current activation energies between two samples with identical structure of the dots-in-well in nanoscale but different microscale n-i-n environments. A sequential coupling transport mechanism for the dark current between the nanoscale and the microscale processes is proposed. The dark current is determined by the additive mode of two activation energies: E-a,E-micro from the built-in potential in the microscale and E-a,E-nano related to the thermally assisted tunneling in nanoscale. The activation energies E-a,E-micro and E-a,E-nano decrease exponentially and linearly with increasing applied electric field, respectively.
Resumo:
In this report, we have investigated the temperature and injection power dependent photoluminescence in self-assembled InAs/GaAs quantum dots (QDs) systems with low and high areal density, respectively. It was found that, for the high-density samples, state filling effect and abnormal temperature dependence were interacting. In particular, the injection power-induced variations were most obvious at the temperature interval where carriers transfer from small quantum dots (SQDs) to large quantum dots (LQDs). Such interplay effects could be explained by carrier population of SQDs relative to LQDs, which could be fitted well using a thermal carrier rate equation model. On the other hand, for the low density sample, an abnormal broadening of full width at half maximum (FWHM) was observed at the 15-100 K interval. In addition, the FWHM also broadened with increasing injection power at the whole measured temperature interval. Such peculiarities of low density QDs could be attributed to the exciton dephasing processes, which is similar to the characteristic of a single quantum dot. The compared interplay effects of high-and low-density QDs reflect the difference between an interacting and isolated QDs system.
Resumo:
Charge transfer due to collisions of ground state O3+ (2s(2)2p P-2) ions with molecular hydrogen is investigated using the quantum-mechanical molecular-orbital close-coupling (MOCC) method, and electronic and vibrational state-selective cross sections along with the corresponding differential cross sections are calculated for projectile energies of 100, 500, 1000 and 5000 eV/u at the orientation angles of 25 degrees,45 degrees and 89 degrees. The adiabatic potentials and radial coupling matrix elements utilized in the QMOCC calculations were obtained with the spin-coupled valence-bond approach. The infinite order sudden approximation (IOSA) and the vibrational sudden approximation (VSA) are utilized to deal with the rotation of H-2 and the coupling between the electron and the vibration of H-2. It is found that the distribution of vibrationally resolved cross sections with the vibrational quantum number upsilon' of H-2(+) (upsilon') varies with the increment of the projectile energy; and the electronic and vibrational stateselective differential cross sections show similar behaviors: there is a highest platform within a very small scattering angle, beyond which the differential cross sections decrease as the scattering angle increases and lots of oscillating structures appear, where the scattering angle of the first structure decreases as E-P(-1/2) with the increment of the projectile energy E-P; and the structure and amplitude of the differential cross sections are sensitive to the orientation of molecule H-2, which provides a possibility to identify the orientations of molecule H-2 by the vibrational state-selective differential scattering processes.
Resumo:
Shot noise through a closed Aharonov-Bohm interferometer carrying a quantum dot in one of its two current paths is investigated. It is found that the shot noise can be modulated by the magnetic flux Phi, the dot level, and the direct tunneling. Due to the interference between the two transmission channels, the Kondo correlation manifests itself in the flux dependence of the shot noise, which exhibits oscillation behavior with a period of Phi(0)/2 (Phi(0) is the flux quantum) for small voltages below the Kondo temperature T-K. At voltages well above T-K or outside the Kondo regime, the shot noise is determined by high-energy Coulomb and hybridization processes, and its Aharonov-Bohm oscillations restore the fundamental period of Phi(0). As a result of its two-particle nature, the shot noise contains higher-order harmonics absent in the current, demonstrating the fact that the noise is more sensitive to the effects of quantum interference than the current.
Resumo:
The inhibition effect of metal-free phthalocyanine (H2Pc), copper phthalocyanine (CuPc) and copper phthalocyanine tetrasulfuric tetrasodium salt (CuPc center dot S(4)center dot Na-4) on mild steel in I mol/l HCl in the concentration range of 1.0 X 10(-5) to 1.0 X 10(-3) mol/l was investigated by electrochemical test, scanning electron microscope with energy dispersive spectrometer (SEM/EDS) and quantum chemical method. The potentiodynamic polarization curves of mild steel in hydrochloric acid containing these compounds showed both cathodic and anodic processes of steel corrosion were suppressed, and the Nyquist plots of impedance expressed mainly as a capacitive loop with different compounds and concentrations. For all these phthalocyanines, the inhibition efficiency increased with the increase in inhibitor concentration, while the inhibition efficiencies for these three phthalocyanines with the same concentration decreased in the order Of CuPc center dot S(4)center dot Na-4 > CuPc > H2Pc according to the electrochemical measurement results. The SEM/EDS analysis indicated that there are more lightly corroded and oxidative steel surface for the specimens after immersion in acid solution containing 1.0 x 10(-3) mol/l phthalocyanines than that in blank. The quantum chemical calculation results showed that the inhibition efficiency of these phthalocyanines increased with decrease in molecule's LUMO energy, which was different from the micro-cyclic compounds. (c) 2005 Elsevier B.V. All rights reserved.
Resumo:
The inhibiting effect and mechanism of 1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-7-(1-piperazinyl)-3-quinoline carboxylicacid(ciprofloxacin), 1-ethyl-6-fluoro-1,4-dihydro-4-oxo-7-(1-piperazinyl)-3-quinoline carboxylic acid (norfloxacin) and (-)-(S)-9-fluoro-2,3-dihydro-3-methyl-10-(4-methyl-1-piperazinyl)-7-oxo-7 H-pyrido(1,2,3-de)-1,4-benzoxazine-6 carboxylic acid (ofloxacin) on the corrosion of mild steel in 1 mol/L HCl have been studied using electrochemical method, quantum chemical method and SEM at 303 K. The potentiodynamic results showed that these compounds suppressed both cathodic and anodic processes of mild steel corrosion in 1 mol/L HCl. The impedance spectroscopy showed that R-p values increased, and C-dl values decreased with the rising of the working concentration. Quantum chemical calculation showed that there was a positive correlation between some inhibitors structure properties and the inhibitory efficiency. The inhibitors function through adsorption followed Langmuir isotherm, and chemisorption made more contribution to the adsorption of the inhibitors on the steel surface compared with physical adsorption. SEM analysis suggested that the metal had been protected from aggressive corrosion because of the addition of the inhibitors.
Resumo:
The purines and its derivatives, such as, guanine, adenine, 2,6-diaminopurine, 6-thioguanine and 2,6-dithiopurine, were investigated as corrosion inhibitors for mild steel in 1 M HCl solution by weight loss measurements, electrochemical tests and quantum chemical calculations. The polarization curves of mild steel in the hydrochloric acid solutions of the purines showed that both cathodic and anodic processes of steel corrosion were suppressed. The Nyquist plots of impedance expressed mainly as a depressed capacitive loop with different compounds and concentrations. For all these purines, the inhibition efficiency increased by increasing the inhibitor concentration, and the inhibition efficiency orders are 2,6-dithiopurine > 6-thioguanine > 2,6-diaminopurine > adenine > guanine with the highest inhibiting efficiency of 88.0% for 10(-3) M 2,6-dithiopurine. The optimized structures of purines, the Mulliken charges, molecular orbital densities and relevant parameters were calculated by quantum chemical calculations. The quantum chemical calculation results inferred that the adsorption belong to physical adsorption, which might arise from the pi stacking between the pi electron of the purines and the metal surface. (C) 2008 Elsevier Ltd. All rights reserved.
Resumo:
Gough, John, 'Quantum Stratonovich Stochastic Calculus and the Quantum Wong-Zakai Theorem', Journal of Mathematical Physics. 47, 113509, (2006)
Resumo:
Gough, John, (2004) 'Quantum Flows as Markovian Limit of Emission, Absorption and Scattering Interactions', Communications in Mathematical Physics 254 pp.498-512 RAE2008
Resumo:
Quantum dashes are elongated quantum dots. Polarized edge-photovoltage and spontaneous emission spectroscopy are used to study the anisotropy of optical properties in 1.5μm InGaAsP and AlGaInAs-based quantum dash lasers. Strain, which causes TM-polarized transitions to be suppressed at the band edge, coupled with carrier confinement and dash shape leads to an enhancement of the optical properties for light polarized along the dash long axis, in excellent agreement with theoretical results. An analysis of the integrated facet and spontaneous emission rate with total current and temperature reveals that, in both undoped and p-doped InGaAsP-based quantum dash lasers at room temperature, the threshold current and its temperature dependence remain dominated by Auger recombination. We also identify two processes which can limit the output power and propose that the effects of the dopant in p-doped InGaAsP-based lasers dominate at low temperature but decrease with increasing temperature. A high threshold current density in undoped AlGaInAs-based quantum dash laser samples studied, which degrade rapidly at low temperature, is not due to intrinsic carrier recombination processes. 1.3μm GaAs-based quantum dots lasers have been widely studied, but there remains issues as to the nature of the electronic structure. Polarized edge-photovoltage spectroscopy is used to investigate the energy distribution and nature of the energy states in InAs/GaAs quantum dot material. A non-negligible TM-polarized transition, which is often neglected in calculations and analyses, is measured close to the main TE-polarized ground state transition. Theory is in very good agreement with the experimental results and indicates that the measured low-energy TM-polarized transition is due to the strong spatial overlap between the ground state electron and the light-hole component of a low-lying excited hole state. Further calculations suggest that the TM-polarized transition reduces at the band edge as the quantum dot aspect ratio decreases.
Resumo:
The realization of nonclassical states is an important task for many applications of quantum information processing. Usually, properly tailored interactions, different from goal to goal, are considered in order to accomplish specific tasks within the general framework of quantum state engineering. In this paper, we remark on the flexibility of a cross-Kerr nonlinear coupling in hybrid systems as an important ingredient in the engineering of nonclassical states. The general scenario we consider is the implementation of high cross-Kerr nonlinearity in cavity-quantum electrodynamics. In this context, we discuss the possibility of performing entanglement transfer and swapping between matter qubits and light fields initially prepared in separable coherent states. The recently introduced concept of entanglement reciprocation is also considered and shown to be possible with our scheme. We reinterpret some of our results in terms of applications of a generalized Ising interaction to systems of different nature.
Resumo:
Modelling Joule heating is a difficult problem because of the need to introduce correct correlations between the motions of the ions and the electrons. In this paper we analyse three different models of current induced heating (a purely classical model, a fully quantum model and a hybrid model in which the electrons are treated quantum mechanically and the atoms are treated classically). We find that all three models allow for both heating and cooling processes in the presence of a current, and furthermore the purely classical and purely quantum models show remarkable agreement in the limit of high biases. However, the hybrid model in the Ehrenfest approximation tends to suppress heating. Analysis of the equations of motion reveals that this is a consequence of two things: the electrons are being treated as a continuous fluid and the atoms cannot undergo quantum fluctuations. A means for correcting this is suggested.