Raman evidence for atomic correlation between the two constituent tubes in double-walled carbon nanotubes

Four well-resolved peaks with very narrow linewidths were found in the D-band and G'-band features of double-walled carbon nanotubes (DWNTs). This fact implies the occurrence of additional van Hove singularities (vHSs) in the joint density of states (JDOS) of DWNTs, which is consistent with theoretical calculations. According to their peak frequencies and theoretical analysis, the two outer peaks can be deduced to originate from a strong coupling between the two constituent tubes of commensurate DWNTs and the two inner peaks were curvature-related and assigned to originate from the two tubes with a weak coupling. This observation and elucidation constitute the first Raman evidence for atomic correlation and the resulting electronic structure change of the two constituent tubes in DWNTs. This result opens the possibility of predicting and modifying the electronic properties of DWNTs for their electronic applications.

Atomic hydrogen induced step bunching and fabrication of quantum wire arrays on GaAs (311)A substrate by molecular beam epitaxy

Atomic hydrogen assisted molecular beam epitaxy (MBE) is a novel type of epitaxial growth of nanostructures. The GaAs (311)A surface naturally forms one-dimensional step arrays by step bunching along the direction of (-233) and the space period is around 40nm. The step arrays extend over several mum without displacement. The InGaAs quantum wire arrays are grown on the step arrays as the basis. Our results may prompt further development of more uniform quantum wire and quantum dot arrays.

Collective and single-particle excitations in coupled quantum wires in magnetic fields

The full spectra of magnetoplasmons and single-particle excitations are obtained of coupled one-dimensional electron gases in parallel semiconductor quantum wires with tunneling. We show the effects of the interwire Coulomb interaction and the tunneling, as well as the magnetic-field-induced localization on the elementary excitations in symmetric and asymmetric coulped quantum wire structures. The interacton and resonance between the plasmon and the intersubband single-particle excitations are found in magnetic fields.

Atomic-force-microscopy investigation of the formation and evolution of Ge islands on GexSi1-x strained layers

A constant amount of Ge was deposited on strained GexSi1-x layers of approximately the same thickness but with different alloy compositions, ranging from x = 0.06 to x = 0.19. From their atomic-force-microscopy images, we found that both the size and density of Ge islands increased with the Ge composition of the strained layer. By conservation of mass, this implies that these islands must incorporate material from the underlying strained layer. (C) 2000 American Institute of Physics. [S0003-6951(00)03529-4].

The improvement characteristics of homoepitaxial GaAs grown by atomic hydrogen-assisted molecular beam epitaxy

The electrical activity of defects in GaAs grown on GaAs substrates doped with Si and Be by both conventional molecular beam epitaxy (MBE) and atomic hydrogen-assisted MBE (H-MBE) were characterized by deep level transient spectroscopy. The trap densities are significantly reduced in the homoepitaxial GaAs grown by H-MBE compared to that grown by MBE. The reduction of trap densities is attributed to in situ passivation of these defects by atomic H during the growth. The improvement characteristics of GaAs materials will be significance for fabrication of semiconductor devices.

Substrate surface atomic structure influence on the growth of InAlAs quantum dots

We have examined the influence of substrate surface orientation on self-assembled InAlAs/AlGaAs quantum dots grown on (0 0 1) and (n 1 1) A/B (n = 3, 5) GaAs substrates by molecular beam epitaxy (MBE). Preliminary characterizations have been performed using photoluminescence (PL) and transmission electron microscopy (TEM). The PL emission energies of quantum dots on high Miller index surface are found to be strongly dependent on the atomic-terminated surface (A or B surface) of the substrate. We observed that there were planar ordering larger islands on (3 1 1)B surface compared to (0 0 1) surface, in contrast, a rough interface and smaller "grains" on (3 1 1)A surface, this result is identical with PL emission energy from these islands. We propose that the rapid strain-induced surface "roughening" impedes the formation of 3D islands on A surface, and indicating that this is a promising approach of the realization of ordering distribution on (3 1 1)B plane for devices such as red-emitting semiconductor quantum dots lasers. (C) 1999 Elsevier Science B.V. All rights reserved.

Analysis of atomic force microscopic results of InAs islands formed by molecular beam epitaxy

Atomic force microscopy (AFM) measurements of nanometer-sized islands formed by 2 monolayers of InAs by molecular beam epitaxy have been carried out and the scan line of individual islands was extracted from raw AFM data for investigation. It is found that the base widths of nanometer-sized islands obtained by AFM are not reliable due to the finite size and shape of the contacting probe. A simple model is proposed to analyze the deviation of the measured value From the real value of the base width of InAs islands. (C) 1998 Elsevier Science B.V. All rights reserved.

Characterization of self-organized InAs/GaAs quantum dots under strain-induced and temperature-controlled nucleation mechanisms by atomic force microscopy and photoluminescence spectroscopy

Atomic force microscopy and photoluminescence spectroscopy (PL) has been used to study asymmetric bilayer InAs quantum dot (QD) structures grow by molecular-beam epitaxy on GaAs (001) substrates. The two InAs layers were separated by a 7-nm-thick GaAs spacer layer and were grown at different substrate temperature. We took advantage of the intrinsic nonuniformity of the molecular beams to grow the seed layer with an average InAs coverage of 2.0 ML. Then the seed layer thickness could be divided into three areas: below, around and above the critical thickness of the 2D-3D transition along the 11101 direction of the substrate. Correspondingly, the nucleation mechanisms of the upper InAs layer (UIL) could be also divided into three areas: temperature-controlled, competition between temperature-controlled and strain-induced, and strain-induced (template-controlled) nucleation. Small quantum dots (QDs) with a large density around 5 x 10(10) cm(-2) are found in the temperature-controlled nucleation area. The QD size distributions undergo a bimodal to a unimodal transition with decreasing QD densities in the strain-induced nucleation area, where the QD densities vary following that of the seed layer (templating effect). The optimum QD density with the UIL thickness fixed at 2.4 ML is shown to be around 1.5 x 10(10) cm(-2), for which the QD size distribution is unimodal and PL emission peaks at the longest wavelength. The QDs in the in-between area exhibit a broad size distribution with small QDs and strain-induced large QDs coexisting.

An Enhanced Particle Swarm Optimization Algorithm

Particle Swarm Optimization (PSO) algorithm is often used for finding optimal solution, but it easily entraps into the local extremum in later evolution period. Based on improved chaos searching strategy, an enhanced particle swarm optimization algorithm is proposed in this study. When particles get into the local extremum, they are activated by chaos search strategy, where the chaos search area is controlled in the neighborhood of current optimal solution by reducing search area of variables. The new algorithm not only gets rid of the local extremum effectively but also enhances the precision of convergence significantly. Experiment results show that the proposed algorithm is better than standard PSO algorithm in both precision and stability.

Interactions of three dual-dressing effects of four-wave mixing in a five-level atomic system

We study the four-wave mixing (FWM) in an opening five-level system with two dressing fields. There are three kinds of doubly dressing mechanisms (parallel cascade, sequential cascade, and nested cascade) in the system for doubly dressed four-wave mixing. These mechanisms reflect different correlations between two dressing fields and different effects of two dressing fields to the FWM. Investigation of these mechanisms is helpful to understand the generated high-order nonlinear optical signal dressed by multi-fields.

Dressed multi-wave mixing in a V-type four-level atomic system

The dressed four- and six-wave mixings in a V-type four-level system are considered. Under two different dressed conditions, two- and three-photon resonant Autler-Townes splittings, accompanied by enhancement and suppression of wave mixing signal, are obtained analytically. Meanwhile, an electromagnetic induced transparency of multi-wave mixing is presented, which shows multiple peaks and asymmetric effects caused by one-photon, two-photon and three-photon resonances, separately. The slow light propagation multiple region of multi-wave mixing signal is also obtained.