Magneto-transport characteristics of two-dimensional electron gas for Si delta-doped InAlAs/InGaAs single quantum well

Magneto-transport measurements have been carried out on a Si heavily delta-doped In0.52Al0.48As/In(0.53)G(0.47)As single quantum well in the temperature range between 1.5 and 60 K under magnetic field up to 10 T. We studied the Shubnikov-de Haas(SdH) effect and the Hall effect for the In0.52Al0.48As/In(0.53)G(0.47)As single quantum well occupied by two subbands, and have obtained the electron concentration, mobility, effective mass and energy levels respectively. The electron concentrations of the two subbands derived from mobility spectrum combined with multi-carrier fitting analysis are well consistent with the result from the SdH oscillation. From fast Fourier transform analysis for d(2)rho/dB(2)-1/B, it is observed that there is a frequency of f(1)-f(2) insensitive to the temperature, besides the frequencies f(1), f(2) for the two subbands and the frequency doubling 2f(1), both dependent on the temperature. This is because That the electrons occupying the two different subbands almost have the same effective mass in the quantum well and the magneto-intersubband scattering between the two subbands is strong.

Magnetotransport in two-dimensional electron gas: The interplay between spin-orbit interaction and Zeeman splitting

We investigate theoretically the interplay between Zeeman splitting, Rashba spin-orbit interaction (RSOI), and Dresselhaus spin-orbit interaction (DSOI) and its influence on the magnetotransport property of two-dimensional electron gas (2DEG) at low temperature. Our theoretical results show that the nodes of the beating patterns of the magnetoresistivity rho(xx) for 2DEG with RSOI or DSOI alone depend sensitively on the total spin splitting induced by these three spin splitting mechanisms. It is interesting to find that the eigenstates in the presence of RSOI alone are connected with those in the presence of DSOI alone but with opposite Zeeman splitting by a time-reversal transformation. Consequently, the magnetoresistivities exhibit exactly the same oscillation patterns for these two cases. For strong RSOI or DSOI alone, the magneto-oscillation of rho(xx) shows two distinct periods. For 2DEG with both RSOI and DSOI, the beating patterns vanish for equal RSOI and DSOI strengths and vanishing Zeeman splitting. They will appear again, however, when Zeeman splitting or the difference between RSOI and DSOI strengths increases.

Ras 超家族蛋白的起源演化研究

Ras 超家族蛋白是真核生物中普遍存在的一类小分子GTP 结合蛋白。它们 具有高度保守的GTP 结合结构域，根据序列结构和细胞功能被分为七个家族： Sar1、Arf、SRβ、Ran、Rab、Rho 和Ras。这些蛋白分别行使着真核生物特有的 细胞功能，诸如运输小泡的形成和转运（Sar1、Arf、Rab），胞质骨架的建成（Rho）， 细胞核-胞质运输及核膜重建（Ran）等，其起源演化和真核细胞的起源密切相关。 本文利用生物信息学手段和分子生物学实验调查研究了原核生物和原生生物中 Ras 超家族蛋白同源物的存在情况，并进行了分子系统分析，对Ras 超家族蛋白 的起源演化问题进行了较为深入、系统的探讨。获得了以下结果和结论： 1）通过原核生物基因组的搜索和序列结构分析，在一些真细菌中首次鉴定 出了高度相似于真核生物Ras 超家族蛋白的原核生物同源物，且实验证明它们的 基因具有表达活性；在原细菌中的产甲烷菌和热原体中也发现有序列分歧较大的 同源物。并在更多的真细菌种类中鉴定出了更多的前人已报道的另一种小分子 GTP 结合蛋白—MglA。序列比对分析表明MglA 蛋白具有自己独特的序列特征， 与真核生物的Ras 超家族蛋白序列差异较大。进一步的分子系统分析显示：真核 生物Ras 超家族蛋白的七个家族中，Ran、Rab、Rho 和Ras 等四个家族聚在一 起，上述我们所鉴定的真细菌的Ras 超家族蛋白同源物则紧聚在其外围；真核生 物的另三个家族（Sar1、Arf、SRβ）聚成另一枝，并接着与产甲烷原细菌的的同 源物及真细菌的MglA 蛋白聚在一起。这些结果表明：Ras 超家族蛋白不是前人 所认为的为真核生物所特有，实际上在一些原核生物中就已产生；真核生物Ras 超家族蛋白的祖先也不太可能是前人所认为的为真细菌的MglA；真核生物Ras 超家族蛋白的七个家族可能有两种不同的起源：Ran、Rab、Rho 和Ras 等可能 来源于蓝细菌或蛋白菌，或二者的共同祖先，而Sar1、Arf 和SRβ 可能来源于产 甲烷原细菌，这也可能反映了真核细胞“融合起源”的历史。 2）通过搜索一些较为低等的单细胞真核生物——原生生物基因组中Ras 超 家族蛋白，并结合一系列其他处在不同进化地位真核生物的Ras 超家族蛋白进行 分析，发现Sar1、Arf、Rab 和Ran 家族的蛋白在真核生物中普遍存在，而SRβ、 Rho 和Ras 家族蛋白在有些真核生物中未找到。根据各家族蛋白在真核生物中的分布情况推测在真核生物的最近共同祖先中存在的Ras 超家族蛋白可能有下列 两种情况：（1）最近的共同祖先已经具有了所有七个家族的蛋白，并且至少有 11 个成员：1 个Sar1、1 个SRβ、3 个Arf（Arf1、Arl1、Arl2）、3 个Rab（Rab1、 Rab6、Rab11）、1 个Ran、1 个Rho（Rac）和1 个Ras（RheB）。因而，部分真 核生物中缺少SRβ、Rho 和Ras 家族蛋白很可能是因基因丢失所致。植物中Ras 家族蛋白的缺少应该是由于在进化早期，其祖先绿藻丢失了单个Ras 家族蛋白基 因所致；（2）根据Cavalier-Smith 的真核生物划分为单鞭毛（变形虫类、真菌和 后生动物）和双鞭毛（藻类、植物和除变形虫外的原生动物）两大类的分类观点， 真核生物最近的共同祖先可能只具有除Ras 家族而外的六个家族的成员，而Ras 家族蛋白则是在此两大类群分化以后在单鞭毛类生物中才产生的，多数双鞭毛类 生物如原生动物、绿藻和植物中没有Ras 的情况应该是一种祖征，而个别双鞭毛 类生物如红藻具有的Ras 家族蛋白则很可能是从单鞭毛类生物那里水平基因转 移而来的。至于SRβ 和Rho 家族蛋白在部分物种中的缺少，则还是可能因为基 因丢失所致。此外，变形虫类生物中大量的Ras 超家族蛋白提示基因组的大小或 进化地位的高低并不是Ras 超家族蛋白成员多少的决定性因素，而细胞相应生理 活动的需求才是家族成员增多的关键。

A colorimetric assay for screening microcystin class compounds in aquatic systems

Secondary metabolites produced by water-blooming cyanobacteria in eutrophic waters include some potent hepatotoxins, These compounds also have tumour-promoting properties, attributable to their inhibition and activation of protein phosphatases and kinases respectively. The inhibitory effect of these toxins on protein phosphatases have been employed in a commonly used radiometric assay, involving the use of a P-32-labeled substrate, for the detection and quantitation of these compounds. This paper investigates and describes a colorimetric method in which the activity of protein phosphatase 2A is determined by measuring the rate of colour production from the release of yellow p-nitrophenol using p-nitrophenyl phosphate as the substrate. Results of this study suggest that the colorimetric protein phosphatase inhibition assay is a simple, inexpensive tool for screening substances that may have tumour-promoting characteristics in aquatic systems. The detection limit of the colorimetric method is comparable to the radiometric assay. (C) 1998 Elsevier Science Ltd. All rights reserved.

A new structure of In-based ohmic contacts to n-type GaAs

Electrical, structural and reaction characteristics of In-based ohmic contacts to n-GaAs were studied. Attempts were made to form a low-band-gap interfacial phase of InGaAs to reduce the barrier height at the metal/semiconductor junction, thus yielding low-resistance, highly reliable contacts. The contacts were fabricated by e-beam sputtering Ni, NiIn and Ge targets on VPE-grown n(+)-GaAs film (approximate to 1 mu m, 2 x 10(18) cm(-3)) in ultrahigh vacuum as the structure of Ni(200 Angstrom)/NiIn(100 Angstrom)/Ge(40 Angstrom)/n(+)-GaAs/SI-GaAs, followed by rapid thermal annealing at various temperatures (500-900 degrees C). In this structure, a very thin layer of Ge was employed to play the role of heavily doping donors and diffusion limiters between In and the GaAs substrate. Indium was deposited by sputtering NiIn alloy instead of pure In in order to ensure In atoms to be distributed uniformly in the substrate; nickel was chosen to consume the excess indium and form a high-temperature alloy of Ni3In. The lowest specific contact resistivity (rho(c)) of (1.5 +/- 0.5)x 10(-6) cm(2) measured by the Transmission Line Method (TLM) was obtained after annealing at 700 degrees C for 10 s. Auger sputtering depth profile and Transmission Electron Microscopy (TEM) were used to analyze the interfacial microstructure. By correlating the interfacial microstructure to the electronical properties, InxGa1-xAs phases with a large fractional area grown epitaxially on GaAs were found to be essential for reduction of the contact resistance.

Numerical simulation of a bubble rising in shear-thinning fluids

The motion of a single bubble rising freely in quiescent non-Newtonian viscous fluids was investigated experimentally and computationally. The non-Newtonian effects in the flow of viscous inelastic fluids are modeled by the Carreau theological model. An improved level set approach for computing the incompressible two-phase flow with deformable free interface is used. The control volume formulation with the SIMPLEC algorithm incorporated is used to solve the governing equations on a staggered Eulerian grid. The simulation results demonstrate that the algorithm is robust for shear-thinning liquids with large density (rho(1)/rho(g) up to 10(3)) and high viscosity (eta(1)/eta(g) up to 10(4)). The comparison of the experimental measurements of terminal bubble shape and velocity with the computational results is satisfactory. It is shown that the local change in viscosity around a bubble greatly depends on the bubble shape and the zero-shear viscosity of non-Newtonian shear-thinning liquids. The shear-rate distribution and velocity fields are used to elucidate the formation of a region of large viscosity at the rear of a bubble as a result of the rather stagnant flow behind the bubble. The numerical results provide the basis for further investigations, such as the numerical simulation of viscoelastic fluids. (C) 2010 Elsevier B.V. All rights reserved.

Circumstantial Evidence for a Soft Nuclear Symmetry Energy at Suprasaturation Densities

Within an isospin- and momentum-dependent hadronic transport model, it is shown that the recent FOPI data on the pi(-)/pi(+) ratio in central heavy-ion collisions at SIS/GSI energies [Willy Reisdorf , Nucl. Phys. A 781, 459 (2007)] provide circumstantial evidence suggesting a rather soft nuclear symmetry energy E-sym(rho) at rho >= 2 rho(0) compared to the Akmal-Pandharipande-Ravenhall prediction. Some astrophysical implications and the need for further experimental confirmations are discussed.

Nuclear matter at a HIRFL-CSR energy regime

We report some recent progress in constraining the symmetry energy E-sym(rho) at high densities using high-energy heavy-ion collisions. Circumstantial evidence of a soft E-sym(rho) at supra-saturation density is obtained by comparing the pion ratio pi(-)/pi(+) measured recently with FOPI at GSI and the IBUU04 model calculations. Detailed studies indicate that the power of determining the E-sym(rho)from pi(-)/pi(+) is enhanced with decreasing the beam energy to near the pion production threshold, showing a correlation to the increasing nuclear stopping. Among several heavy-ion reaction facilities in the world, the cooling storage ring (HIRFL-CSR), newly commissioned at Lanzhou, delivering heavy-ion beams up to 1 A GeV, to be coupled with advanced detectors will contribute significantly to further studies of the equation of state of asymmetric nuclear matter.

Doubly excited 2s2p P-1,3(1) resonances in photoionization of helium

The multi-configuration Dirac Fock (MCDF) method is implemented to study doubly excited 2s2p P-1,3(1) resonances of the helium atom and the interference between photoionization and photo excitation autoionization processes. In order to reproduce the total photoionization sprectra, the excited energies from the ground 1s(2) S-1(0) state to the doubly excited 2s2p P-1,3(1) states and the relevant Auger decay rates and widths are calculated in detail. Further more, the interference profile determined by the so-called Fano parameters q and rho(2) is also reproduced. Good agreement is found between the present results and other available theoretical and experimental results. This indeed shows a promising way to investigate the Fano resonances in photoionization of atoms within the MCDF scheme, although there are some discrepancies in the present calculations of the 2s2p P-3(1) state.

eta-production on the proton via electromagnetic and hadronic probes

The reactions pi(-)p -> eta n and gamma p -> eta p are investigated within a dynamical coupled-channels model of meson production reactions in the nucleon resonance region The meson-baryon channels included are pi N, pi Delta, sigma N, and rho N The direct eta-photoproduction process is studied within a formalism based on a chiral constituent quark model approach, complemented with a one-gluon-exchange mechanism, to take into account the breakdown of the SU(6)circle times O(3) symmetry In the models search, the following known nucleon resonances are embodied S-11(1535), S-11(1650), P-11(1440), P-11(1710), P-13(1720), D-13(1520), D-13(1700), D-15(1675), and F-15(1680). Data for the pi(-)p -> eta n reaction from threshold up to a total center-of-mass energy of W approximate to 2 GeV are satisfactorily reproduced For the photoproduction channel: two additional higher mass known resonances, P-13(1900) and F-15(2000), are also considered However, reproducing the data for gamma p -> eta p requires, within our approach, two new nucleon resonances, for which we extract, mass and width

Neutron-proton bremsstrahlung from intermediate energy heavy-ion reactions as a probe of the nuclear symmetry energy?

Hard photons from neutron-proton bremsstrahlung in intermediate energy heavy-ion reactions are examined as a potential probe of the nuclear symmetry energy within a transport model. Effects of the symmetry energy on the yields and spectra of hard photons are found to be generally smaller than those due to the currently existing uncertainties of both the in-medium nucleon-nucleon cross sections and the photon production probability in the elementary process pn -> pn gamma. Very interestingly, nevertheless, the ratio of hard photon spectra R-1/2(gamma) from two reactions using isotopes of the same element is not only approximately independent of these uncertainties but also quite sensitive to the symmetry energy. For the head-on reactions of Sn-132 + Sn-124 and Sn-112 + Sn-112 at E-beam/A = 50 MeV, for example, the R-1/2(gamma) displays a rise up to 15% when the symmetry energy is reduced by about 20% at rho = 1.3 rho(0) which is the maximum density reached in these reactions. (C) 2008 Elsevier B.V. All rights reserved.

Nuclear stopping and compression in heavy-ion collisions at intermediate energies

The nuclear stopping and the radial flow are investigated with an isospin-dependent quantum molecular dynamics (IQMD) model for Ni + Ni and Pb + Pb from 0.4 to and 1.2 GeV/u. The expansion velocity as well as the degree of nuclear stopping are higher in the heavier system at all energies. The ratio between the flow energy and the total available energy in center of mass of the colliding systems exhibits a positive correlation to the degree of nuclear stopping. The maximum density (rho(max)) achieved in the compression is comparable to the hydrodynamics prediction only if the non-zero collision time effect is taken into account in the later. Due to the partial transparency, the growing of the maximum density achieved in the central region of the fireball with the increase of beam energy becomes gradually flat in the 1 GeV/u energy regime. (C) 2008 Elsevier B.V. All rights reserved.

Delta-resonance effective mass in medium

Based on the relativistic chiral effective field theory, we study the effective mass of the Delta-resonance in medium by investigating the self-energy of the Delta-resonance related to the pi N decay channel in symmetric nuclear matter. We find that the effective mass of Delta-resonance decreases evidently with increasing nuclear density rho. In our calculation, we also consider the influence of the shifts of the nucleon mass, pion mass and its decay constant due to the restoration of chiral symmetry in medium. The results are roughly consistent with the data given by Lawrence Berkeley National Laboratory.

Effective masses of pentaquark Theta(+) in nuclear matter

The properties of baryons in nuclear matter are analysed in the relativistic mean-field theory(RMF). It is found that the scalar field sigma meson affects the properties of baryon at high density. A density dependent scalar coupling g(sigma)(N) is determined according to the idea of quark-meson coupling model and extended to RMF. It is shown that g(sigma)(N), affects the property of nuclear matter weakly at low density, but strongly at high density. The relation between the scalar density rho(S) and the nuclear density rho and the effective mass of the pentaquark circle minus(+) are studied with the density dependent coupling constant. The density dependent scalar coupling obviously affects the effective masses of baryons in nuclear matter, especially at high density.

System size and beam energy effects on probing the high-density behavior of nuclear symmetry energy with pion ratio

Based on the isospin-and momentum-dependent hadronic transport model IBUU04, we have investigated the pi(-)/pi(+) ratio in the following three reactions: Ca-48+Ca-48, Sn-124 +Sn-124 and Au-197+Au-197 with nearly the same isospin asymmetry but different masses, at the bombarding energies from 0.25 to 0.6 A GeV. It is shown that the sensitivity of probing the E-sym (rho) with pi(-)/pi(+) increases with increasing the system size or decreasing the beam energy, showing a correlation to the degree of isospin fractionation. Therefore, with a given isospin asymmetry, heavier system at energies near the pion threshold is preferential to study the behavior Of nuclear symmetry energy at supra-saturation densities.