Gas source molecular beam epitaxy and thermal stability of Si1-xGex/Si superlattice materials

Gas source molecular beam epitaxy has been used to grow Si1-xGex alloys and Si1-xGex/Si multi-quantum wells (MQWs) on (100) Si substrates with Si2H6 and GeH4 as sources. Heterostructures and MQWs with mirror-like surface morphology, good crystalline qualify, and abrupt interfaces have been studied by a variety of in situ and ex situ techniques. The structural stability and strain relaxation in Si1-xGex/Si heterostructures have been investigated, and compared to that in the As ion-implanted Si1-xGex epilayers. The results show that the strain relaxation mechanism of the non-implanted Si1-xGex epilayers is different from that of the As ion-implanted Si1-xGex epilayers.

Formation and magnetic properties of Fe16N2 films prepared by ion-beam-assisted deposition

Fe-N films containing the Fe16N2 phase were prepared in a high-vacuum system of ion-beam-assisted deposition (IBAD). The composition and structure of the films were analysed by Auger electron spectroscopy (AES) and X-ray diffraction (XRD), respectively. Magnetic properties of the films were measured by a vibrating sample magnetometer (VSM). The phase composition of Fe-N films depend sensitively on the N/Fe atomic arrival ratio and the deposition temperature. An Fe16N2 film was deposited successfully on a GaAs (1 0 0) substrate by IBAD at a N/Fe atomic arrival ratio of 0.12. The gram-saturation magnetic moment of the Fe16N2 film obtained is 237 emu/g at room temperature, the possible cause has been analysed and discussed. Hysteresis loops of Fe16N2 have been measured, the coercive force H-c is about 120 Oe, which is much larger than the value for Fe, this means the Fe16N2 sample exhibits a large uniaxial magnetocrystalline anisotropy. (C) 1998 Elsevier Science B.V. All rights reserved.

Characteristics of beam profile of Gaussian beam passing through an axicon

Bessel beam can overcome the limitation of the Rayleigh range of Gaussian beam with the same spot size propagation without any spreading due to diffraction, which is considered as an useful function in guiding particles in the next generation of optical tweezers. The mathematical description of the Bessel beam generated by an axicon is usually based on the Fresnel diffraction integral theory. In this paper, we deduce another type of analytic expression suitable for describing the beam profile generated from the axicon illuminated by the Gaussian beam based on the interferential theory. Compared with the Fresnel diffraction integral theory, this theory does not use much approximation in the process of mathematical analysis. According to the derived expression, the beam intensity profiles at any positions behind the axicon can be calculated not just restricted inside the cross region as the Fresnel diffraction integral theory gives. The experiments prove that the theoretical results fit the experimental results very well. (C) 2004 Elsevier B.V. All rights reserved.

Analytical method of predicating the instabilities of a micro arch-shaped beam under electrostatic loading

An arch-shaped beam with different configurations under electrostatic loading experiences either the direct pull-in instability or the snap-through first and then the pull-in instability. When the pull-in instability occurs, the system collides with the electrode and adheres to it, which usually causes the system failure. When the snap-through instability occurs, the system experiences a discontinuous displacement to flip over without colliding with the electrode. The snap-through instability is an ideal actuation mechanism because of the following reasons: (1) after snap-through the system regains the stability and capability of withstanding further loading; (2) the system flips back when the loading is reduced, i.e. the system can be used repetitively; and (3) when approaching snap-through instability the system effective stiffness reduces toward zero, which leads to a fast flipping-over response. To differentiate these two types of instability responses for an arch-shaped beam is vital for the actuator design. For an arch-shaped beam under electrostatic loading, the nonlinear terms of the mid-plane stretching and the electrostatic loading make the analytical solution extremely difficult if not impossible and the related numerical solution is rather complex. Using the one mode expansion approximation and the truncation of the higher-order terms of the Taylor series, we present an analytical solution here. However, the one mode approximation and the truncation error of the Taylor series can cause serious error in the solution. Therefore, an error-compensating mechanism is also proposed. The analytical results are compared with both the experimental data and the numerical multi-mode analysis. The analytical method presented here offers a simple yet efficient solution approach by retaining good accuracy to analyze the instability of an arch-shaped beam under electrostatic loading.

High-efficiency transfer and expression of AdCMV-p53 in human cervix adenocarcinoma cells induced by subclinical-dose carbon beam radiation

Purpose The aim of this study is to evaluate the eVect of carbon-beam irradiation on adenovirus-mediated p53 transfer in human cervix adenocarcinoma.Materials and methods The HeLa cells pre-exposed to carbon-beam or -ray, were infected with replication-deficient adenovirus recombinant vectors, containing human wild-type p53 (AdCMV-p53) and green Xuorescent protein (GFP) (AdCMV–GFP), respectively. The GFP transfer and p53 expression were detected by Xow cytometric analysis.Results The GFP transfer frequency in C-beam with AdCMV-GFP groups was 38–50% more than that inγ-ray with AdCMV–GFP groups. The percentage of p53 positive cells in the C-beam with AdCMV–p53 groups was 34–55.6% more than that in γ-ray with AdCMV-p53 groups (p < 0.05), suggesting that subclinical-dose C-beam irradiation could signiWcantly promote exogenous p53 transfer and p53 expression, and extend the duration of p53 expression in the HeLa cells. The expression of p21 increased with p53 expression in HeLa cells. The survival fractions for the 0.5–1.0 Gy C-beam with AdCMV-p53 groups were 38–43% less than those for the isodose γ-ray with AdCMV-p53 groups, and 31–40% less than those for the C-beam only groups (p <0.05).Conclusions The subclinical-dose C-beam irradiation could signiWcantly promote the transfer and expression of exogenous p53, extend the duration of p53 expression, and enhance the suppression of p53 on cervix adenocarcinoma cells.

Commissioning of electron cooling in CSRm

A new generation electron cooler has started operation in the heavy ion synchrotron CSRm which is used to increase the intensity of heavy ions. Transverse cooling of the ion beam after horizontal multi-turn injection allows beam accumulation at the injection energy. After optimization of the accumulation process an intensity increase in a synchrotron pulse by more than one order of magnitude has been achieved. In given accumulation time interval of 10 seconds, 108particles have been accumulated and accelerated to the final energy. The momentum spread after accumulation and acceleration in the 10−4 range has been demonstrated in six species of ion beams. Primary measurements of accumulation process varying with electron energy,electron beam current, electron beam profile, expansion factor and injection interval have been performed.The lifetimes of ion beams in the presence of electron beams were roughly measured with the help of DCCT signal.