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Chin. Phys. B  
  Chin. Phys. B--2004, Vol.13, No.7
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Characteristic functional structure of infinitesimal symmetry transformations of Birkhoffian systems

Gu Shu-Long, Zhang Hong-Bin
Chin. Phys., 2004, 13 (7): 979-983 doi: 10.1088/1009-1963/13/7/001
Full Text: [PDF 159 KB] (Downloads:378)
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In this paper, it is shown that infinitesimal symmetry transformations of Birkhoffian systems have a characteristic functional structure, which is formulated by means of an auxiliary symmetry transformation function Z_μ(t,a) (introduced by the relation ξ_μ(t,a)=Z_μ(t,a)+\dot{a}^μξ_0 (t, a)) and is manifestly dependent upon the constants of motion of the system. At the end of the paper, an example is given to illustrate the application of the results.

The soliton-like solutions to the (2+1)-dimensional modified dispersive water-wave system

Li De-Sheng, Zhang Hong-Qing
Chin. Phys., 2004, 13 (7): 984-987 doi: 10.1088/1009-1963/13/7/002
Full Text: [PDF 148 KB] (Downloads:415)
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By a simple transformation, we reduce the (2+1)-dimensional modified dispersive water-wave system to a simple nonlinear partial differential equation. In order to solve this equation by generalized tanh-function method, we only need to solve a simple system of first-order ordinary differential equations, and by doing so we can obtain many new soliton-like solutions which include the solutions obtained by using the conventional tanh-function method.

Generation of Bell states in two-component Bose-Einstein condensates

Zhang Guo-Feng, Yin Wen, Liang Jiu-Qing, Yan Qi-Wei
Chin. Phys., 2004, 13 (7): 988-989 doi: 10.1088/1009-1963/13/7/003
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We examine analytically the generation of Bell state in Bose condensates of two interacting species trapped in a double well configuration. The density of probability for finding the entangled Bell state is given. The effect of the tunnelling rate and the interspecies interaction strength on the generation of Bell state is discussed. We find that the oscillation amplitude of the density of probability for finding the entangled Bell state becomes greater as the tunnelling rate Ω increases, and the self-interaction strength of the component A(B) has no effect on it.

Lattice-gas simulation of escaping pedestrian flow in corridor

Qiu Bing, Tan Hui-Li, Kong Ling-Jiang, Liu Mu-Ren
Chin. Phys., 2004, 13 (7): 990-995 doi: 10.1088/1009-1963/13/7/004
Full Text: [PDF 252 KB] (Downloads:449)
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A lattice-gas model of biased-random walkers is used to simulate the escaping pedestrian flow under the open boundary condition in corridor. Given that the total number of people is unchanging, we have studied the evolution of pedestrian flow by varying parameters of system size. Relationships between parameters of system size and the transition time are discussed in this paper. Scaling behaviour is found as follows: the transition time t_c scales as t_c∝W^{-0.85±0.04}, and t_c∝D, where W is the width of corridor, and D is the strength of drift. However, the other parameters are found to have little influence on the transition time.

Synchronization of coupled dynamical chaotic networks via scalar controllers

Li Zhi, Shi Song-Jiao
Chin. Phys., 2004, 13 (7): 996-999 doi: 10.1088/1009-1963/13/7/005
Full Text: [PDF 157 KB] (Downloads:590)
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Some scalar linear controllers, which can ensure that the states of coupled chaotic dynamical networks asymptotically synchronize each other, are derived on the basis of high gain state feedback control. Numerical simulation is given to validate the proposed theoretical result.

Investigation of the mean free path of projectile fragments produced in 16O-Em collision at 60 A GeV

Zhang Dong-Hai, Gong Jin-Sheng
Chin. Phys., 2004, 13 (7): 1000-1004 doi: 10.1088/1009-1963/13/7/006
Full Text: [PDF 188 KB] (Downloads:284)
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Results are presented for an investigation of the mean free path of projectile fragments with charge 3≤Z≤8, produced by 60 A GeV {}^{16}O in nuclear emulsion. No dependence of mean free path on the distance from the point of the fragment emission is observed and our result is consistent with the nonexistence of anomalons.

Fractional Fourier transform for partially coherent beam in spatial-frequency domain

Cai Yang-Jian, Lin Qiang
Chin. Phys., 2004, 13 (7): 1025-1032 doi: 10.1088/1009-1963/13/7/010
Full Text: [PDF 689 KB] (Downloads:386)
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By using Fourier transform and the tensor analysis method, the fractional Fourier transform (FRT) in the spatial-frequency domain for partially coherent beams is derived. Based on the FRT in the spatial-frequency domain, an analytical transform formula is derived for a partially coherent twisted anisotropic Gaussian-Schell model (GSM) beam passing through the FRT system. The connections between the FRT formula and the generalized diffraction integral formulae for partially coherent beams through an aligned optical system and a misaligned optical system in the spatial-frequency domain are discussed, separately. By using the derived formula, the intensity distribution of partially coherent twisted anisotropic GSM beams in the FRT plane are studied in detail. The formula derived provide a convenient tool for analysing and calculating the FRTs of the partially coherent beams in spatial-frequency domain.

Correlation between randomly varying birefringence and random dispersion map in a dispersion-managed soliton system

Li Hong, Wang Tie-Jun, Huang De-Xiu
Chin. Phys., 2004, 13 (7): 1033-1038 doi: 10.1088/1009-1963/13/7/011
Full Text: [PDF 336 KB] (Downloads:395)
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The correlation between perturbations caused by randomly varying birefringence and a random dispersion map is considered in a dispersion-managed soliton system, and their effects on soliton propagation and interaction are investigated numerically. These perturbations lead to the disintegration of a soliton, and enhance the interaction between solitons. The correlation plays an important role, and reinforces these effects. Furthermore, there is a stochastic resonance between two perturbations in the system; here the effect is the largest, and the corresponding distance until disintegration is the shortest. Finally, nonlinear gain and a filter are introduced to effectively suppress these effects.

Generation of motional Fock states of a trapped ion in the mediate-excitation regime

Zheng Shi-Biao
Chin. Phys., 2004, 13 (7): 1039-1041 doi: 10.1088/1009-1963/13/7/012
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We propose a scheme to prepare Fock states for the vibrational motion of a trapped ion. Unlike previous schemes, the present scheme works in the mediate-excitation regime, in which the corresponding Rabi frequency is equal to the trap frequency. Thus, the required time is greatly shortened, which is important in view of decoherence.

Mid-infrared generation based on a periodically poled LiNbO3 optical parametric oscillator

Lin Xue-Chun, Kong Yu-Peng, Zhang Ying, Zhang Jie, Yao Ai-Yun, Bi Yong, Sun Zhi-Pei, Cui Da-Fu, Li Rui-Ning, Wu Ling-An, Xu Zu-Yan
Chin. Phys., 2004, 13 (7): 1042-1045 doi: 10.1088/1009-1963/13/7/013
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We report a nanosecond Nd:YVO_4-pumped optical parametric oscillator (OPO) based on periodically poled LiNbO_3 (PPLN). Tuning is achieved in this experiment by varying the temperature and period of the PPLN. The design of double-pass singly resonant oscillator (DSRO) and confocal cavity enables the OPO threshold to be lowered considerably, resulting in a simple, compact, all-solid-state configuration with the mid-infrared idler powers of up to 466mW at 3.41μm.

An all-optical switch of Mach-Zehnder interferometer type using an active fibre ring resonator

Li Jun-Qing, Alireza Bananej, Li Qiang-Hua, Chen Qiang, Li Chun-Fei
Chin. Phys., 2004, 13 (7): 1046-1051 doi: 10.1088/1009-1963/13/7/014
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We propose an all-optical switch of the Mach-Zehnder interferometer type using an active nonlinear ring resonator and analyse the significance of the parameter A, a product of gain and total loss, for performing an ideal 1 by 2 switch. We found that in the range of 1-κ≤A≤\sqrt{1-κ}, the increment of A can compensate the losses inside the ring, therefore increase the finesse of the ring and enhance the nonlinearity contribution to reduce the switching power threshold effectively. We also emphasize the importance of the initial switching point and discuss the feasibility of utilizing a high-nonlinear fibre in the ring.

Polarization-maintaining photonic crystal fibre

Lou Shu-Qin, Wang Zhi, Ren Guo-Bin, Jian Shui-Sheng
Chin. Phys., 2004, 13 (7): 1052-1058 doi: 10.1088/1009-1963/13/7/015
Full Text: [PDF 401 KB] (Downloads:509)
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Photonic crystal fibres (PCFs) offer new possibilities of realizing highly birefringent fibres due to a higher intrinsic index contrast compared to conventional fibres. In this paper, we analyse theoretically the modal properties of a kind of polarization-maintaining PCF, namely the elliptical hole photonic crystal fibre (EHPCF) with a central small elliptical hole introduced in the core region. We demonstrate the possibility of achieving large birefringence with zero walk-off in the single-mode regime. This PCF also exhibits unusual dispersion properties. Several hundred nanometres of ultra broadband flattened dispersion near the wavelength 1.55μm is achieved in this EHPCF with a central small hole of d_c=0.4Λ.

Finite-amplitude vibration of a bubble and sonoluminescence

Qian Zu-Wen, Xiao Ling, Guo Liang-Hao
Chin. Phys., 2004, 13 (7): 1059-1064 doi: 10.1088/1009-1963/13/7/016
Full Text: [PDF 219 KB] (Downloads:343)
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Numerical solutions of the differential equation for a bubble performing finite-amplitude vibration are given in detail for a variety of situations. The results demonstrate that in lower acoustic pressure (maximum Mach number very low) its vibration has bounce. When acoustic pressure is in excess of 1.18atm and the instantaneous radius of the bubble approaches its equivalent Van der Waals radius, the maximum velocity and acceleration on the surface of a bubble have a huge increase in a very short period, which seems to favour the sonoluminescence. In vacuum environment (0.1atm), an intensive sonoluminescence could be generated.

Many-body dipole-induced dipole model for electrorheological fluids

Huang Ji-Ping, Yu Kin-Wah
Chin. Phys., 2004, 13 (7): 1065-1069 doi: 10.1088/1009-1963/13/7/017
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Theoretical investigations on electrorheological (ER) fluids usually rely on computer simulations. An initial approach for these studies would be the point-dipole (PD) approximation, which is known to err considerably when the particles approach and finally touch each other due to many-body and multipolar interactions. Thus various works have attempted to go beyond the PD model. Being beyond the PD model, previous attempts have been restricted to either local-field effects only or multipolar effects only, but not both. For instance, we recently proposed a dipole-induced-dipole (DID) model which is shown to be both more accurate than the PD model and easy to use. This work is necessary because the many-body (local-field) effect is included to put forth the many-body DID model. The results show that the multipolar interactions can indeed be dominant over the dipole interaction, while the local-field effect may yield a correction.

The application of computational fluid dynamics to pedestrian level wind safety problem induced by high-rise buildings

Li Lei, Hu Fei, Cheng Xue-Ling, Han Hao-Yu
Chin. Phys., 2004, 13 (7): 1070-1075 doi: 10.1088/1009-1963/13/7/018
Full Text: [PDF 10880 KB] (Downloads:427)
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In this paper, the pedestrian level wind safety problem induced by high-rise buildings has been studied using the computational fluid dynamics (CFD) codes Fluent. The verification by use of wind tunnel data shows that Fluent can fairly reproduce the flow field in the areas adjacent to the structure when a realizable k-ε turbulence model is adopted in calculations. The results of the numerical simulations including seven cases show that the existence of high-rise buildings does increase the wind hazard probability at the pedestrian level; furthermore, the wind direction, the geometric size of structures and the layout of structures can obviously affect the pedestrian level wind environment. However, trees on the pavement do not contribute much in reducing the wind hazard probability.

Radiation pressure and laser cooling of a three-level atom in a ladder configuration

Tan Lei, Wang Zhi-Cheng, Gu Huai-Qiang
Chin. Phys., 2004, 13 (7): 1005-1012 doi: 10.1088/1009-1963/13/7/007
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Radiation pressure and laser cooling of a moving three-level ladder-type atom in bichromatic travelling fields are considered. The dependence of the force on parameters such as detunings, Rabi frequencies and spontaneous decay rates is calculated numerically and shown graphically, and analytical expressions for the force are obtained for some special parameter values. It is shown that the radiation pressure shows Doppler-shifted resonance peaks resulting respectively from one-photon and two-photon transitions. Using the present scheme, Doppler cooling of sodium exploiting the 3{}^2S_{1/2}-3{}^2P_{3/2}-3{}^2D_{5/2} cascade transitions is investigated. It is found that temperatures lower than the Doppler limit can be achieved.

Time-dependent quantum dynamics study for reaction of D+CH4→CH3+HD

Liu Xin-Guo, Zhang Qing-Gang, Zhang Yi-Ci, Wang Ming-Liang, John Zhang Zeng-Hui
Chin. Phys., 2004, 13 (7): 1013-1017 doi: 10.1088/1009-1963/13/7/008
Full Text: [PDF 232 KB] (Downloads:318)
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The semirigid vibrating rotor target (SVRT) model has been applied to the study of the reaction of D+CH_4→CH_3+HD using a time-dependent wave packet method. The energy dependence of the calculated reaction probability shows oscillatory structures similar to those observed in the abstraction reaction of H+H_2, H+CH_4 etc. We have also studied the influence of rotational and vibrational excitation of the reacting molecule (CH_4) on reaction probability. The excitation of the H-CH_3 stretching vibration gives significant enhancement of reaction probability, which rises significantly with the enhancement of rotational quantum number j. Finally, we have compared the cross section and the rate constant of the D+CH_4 system with that of the H+CH_4 system.

Total cross sections for scattering of electrons from O2, H2O, H2, O3, CO and CO2 at 100-2000eV

Shi De-Heng, Sun Jin-Feng, Yang Xiang-Dong, Zhu Zun-Lue, Liu Yu-Fang
Chin. Phys., 2004, 13 (7): 1018-1024 doi: 10.1088/1009-1963/13/7/009
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A complex optical model potential rewritten by the concept of bonded atom, which considers the overlap of electron clouds, is employed to calculate the total cross sections for electron scattering from several simple molecules (O_2, H_2O, H_2, O_3, CO and CO_2) consisting of C, H and O atoms in an incident energy range of 100-2000eV by the use of the additivity rule at Hartree-Fock level. In the study, the complex optical potential composed of static, exchange, correlation polarization plus absorption contributions firstly uses the bonded-atom concept. The quantitative molecular total cross section results are compared with experimental data and with the other calculations wherever available and good agreement is obtained. It is shown that the additivity rule along with the complex optical model potential rewritten by the concept of bonded atom can be used successfully to calculate the total cross section of electron-molecule scattering above 100eV, whereas the rule together with the complex optical model potential not rewritten by the concept of bonded atom is only successfully used above 300-500eV. So, the introduction of the bonded-atom concept in the complex optical potential can improve the accuracy of the total cross section calculations.

Novel material for nonvolatile ovonic unified memory (OUM)-Ag11In12Te26Sb51 phase change semiconductor

Liu Bo, Song Zhi-Tang, Zhang Ting, Feng Song-Lin, Gan Fu-Xi
Chin. Phys., 2004, 13 (7): 1167-1170 doi: 10.1088/1009-1963/13/7/036
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In this paper, Ag_{11}In_{12}Te_{26}Sb_{51} phase change semiconductor films have been prepared by dc sputtering. The crystallization behaviour of amorphous Ag_{11}In_{12}Te_{26}Sb_{51} thin films was investigated by using differential scanning calorimetry and x-ray diffraction. It was found that the crystallization temperature is about 483K and the melting temperature is 754.8K and the activation energy for crystallization, E_a, is 2.07eV. The crystalline Ag_{11}In_{12}Te_{26}Sb_{51} films were obtained using initializer. The initialization conditions have a great effect on the sheet resistance of Ag_{11}In_{12}Te_{26}Sb_{51} films. We found that the effect of the initialization condition on the sheet resistance can be ascribed to the crystallinity of Ag_{11}In_{12}Te_{26}Sb_{51} films. The sheet resistance of the amorphous (R_{amo}) film is found to be larger than 1×10^6Ω and that of the crystalline (R_{cry}) film lies in the range from about 10^3 to 10^4Ω. So we have the ratio R_{amo}/R_{cry}=10^2~10^3, which is sufficiently large for application in memory devices.

A nonequilibrium phase transition in immune response

Zhang Wei, Qi An-Shen
Chin. Phys., 2004, 13 (7): 1171-1176 doi: 10.1088/1009-1963/13/7/037
Full Text: [PDF 203 KB] (Downloads:397)
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The dynamics of immune response correlated to signal transduction in immune thymic cells (T cells) is studied. In particular, the problem of the phosphorylation of the immune-receptor tyrosine-based activation motifs (ITAM) is explored. A nonlinear model is established on the basis of experimental observations. The behaviours of the model can be well analysed using the concepts of nonequilibrium phase transitions. In addition, the Riemann-Hugoniot cusp catastrophe is demonstrated by the model. Due to the application of the theory of nonequilibrium phase transitions, the biological phenomena can be clarified more precisely. The results can also be used to further explain the signal transduction and signal discrimination of an important type of immune T cell.

Morphological study of the localized growth of materials in dielectric barrier discharge

Zhao Qing-Xun, Li Xue-Chen, Jiang Nan, Wang Long
Chin. Phys., 2004, 13 (7): 1076-1081 doi: 10.1088/1009-1963/13/7/019
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The localized growth of materials has been realized in a dielectric barrier discharge reactor in the mixture of acetylene and argon in previous work. In this paper, the morphology of the materials synthesized in the process is studied. The results indicate that the polymer's structure consists of three layers. The layer near the substrate is homogeneous with thickness of several micrometres; the middle layer is composed of dense bulges with height of about more than 10μm in average. The distance between two neighbouring bulges is about 230μm; the top layer is made up of a few large columns with the height up to 2mm, and with the average distance of about 3.5mm. The growth of the three layers corresponds to three types of discharge. The discharge mechanism is analysed through studying the morphology of the polymer. It can be deduced from the morphology that the first and second discharge phases should belong to the Townsend breakdown, and the last discharge phase should be explained on the basis of the streamer mechanism.

Calculation of the surface energy of fcc metals with modified embedded-atom method

Zhang Jian-Min, Ma Fei, Xu Ke-Wei
Chin. Phys., 2004, 13 (7): 1082-1090 doi: 10.1088/1009-1963/13/7/020
Full Text: [PDF 143 KB] (Downloads:1060)
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The surface energies for 38 surfaces of fcc metals Cu, Ag, Au, Ni, Pd, Pt, Al, Pb, Rh and Ir have been calculated by using the modified embedded-atom method. The results show that, for Cu, Ag, Ni, Al, Pb and Ir, the average values of the surface energies are very close to the polycrystalline experimental data. For all fcc metals, as predicted, the close-packed (111) surface has the lowest surface energy. The surface energies for the other surfaces increase linearly with increasing angle between the surfaces (hkl) and (111). This can be used to estimate the relative values of the surface energy.

Melting and Grüneisen parameters of NaCl at high pressure

Chen Qi-Feng, Cai Ling-Cang, Duan Su-Qing, Chen Dong-Quan
Chin. Phys., 2004, 13 (7): 1091-1095 doi: 10.1088/1009-1963/13/7/021
Full Text: [PDF 217 KB] (Downloads:455)
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The Buckingham potential has been employed to simulate the melting and thermodynamic parameters of sodium chloride (NaCl) using the molecular dynamics (MD) method. The constant-volume heat capacity and Grüneisen parameters have been obtained in a wide range of temperatures. The calculated thermodynamic parameters are found to be in good agreement with the available experimental data. The NaCl melting simulations appear to validate the interpretation of superheating of the solid in the one-phase MD simulations. The melting curve of NaCl is compared with the experiments and other calculations at pressure 0-30GPa range.

Application of shell model in molecular dynamics simulation to MgO

Liu Zi-Jiang, Cheng Xin-Lu, Chen Xiang-Rong, Zhang Hong, Lu Lai-Yu
Chin. Phys., 2004, 13 (7): 1096-1099 doi: 10.1088/1009-1963/13/7/022
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The P-V-T equation of state of MgO has been simulated under high pressure and elevated temperature using the molecular dynamics (MD) method with the breathing shell model (BSM). It is found that the MD simulation with BSM is very successful in reproducing accurately the measured molar volumes of MgO over a wide range of temperature and pressure. In addition, the MD simulation reproduces accurately the measured volume compression data of MgO up to 100GPa at 300K. It is demonstrated that the MD simulated P-V-T equation of state of MgO could be applied as a useful internal pressure calibration standard at elevated temperatures and high pressures.

Analytical model of electron transport characteristics for 4H-SiC material and devices

Lü Hong-Liang, Zhang Yi-Men, Zhang Yu-Ming
Chin. Phys., 2004, 13 (7): 1100-1103 doi: 10.1088/1009-1963/13/7/023
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Based on 4H-SiC material parameters, three different analytical expressions are used to characterize the electron mobility as the function of electric field. The first model is based on simple saturation of the steady-state drift velocity with electric field (conventional three-parameter model for silicon). The second GaAs-based mobility model partially reflects the peak velocity in high electric fields. The third multi-parameter model proposed in this paper is more realistic since it well reproduces the drift velocity-field characteristics obtained by Monte Carlo calculations, revealing the peak drift velocity with subsequent saturation at higher electric fields. Thus, the drift velocity model presented in this paper is much better for device simulation. In this paper, the influence of mobility model on DC characteristics of 4H-SiC MESFET is calculated and the better accordance with the experimental results is presented with multi-parameter model.

A compact I-V model for lightly-doped-drain MOSFETs

Yu Chun-Li, Yang Lin-An, Hao Yue
Chin. Phys., 2004, 13 (7): 1104-1109 doi: 10.1088/1009-1963/13/7/024
Full Text: [PDF 264 KB] (Downloads:363)
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A novel model for lightly-doped-drain (LDD) MOSFETs is proposed, which utilizes the empirical hyperbolic tangent function to describe the I-V characteristics. The model includes the strong inversion and subthreshold mechanism, and shows a good prediction for submicron LDD MOSFET. Moreover, the model requires low computation time consumption and is suitable for design of MOSFETs devices and circuits.

Analysis of the effect of sidewall on the performance of 6H-SiC Schottky barrier source/drain NMOSFETs

Tang Xiao-Yan, Zhang Yi-Men, Zhang Yu-Ming, Gao Jin-Xia
Chin. Phys., 2004, 13 (7): 1110-1113 doi: 10.1088/1009-1963/13/7/025
Full Text: [PDF 190 KB] (Downloads:347)
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Between source/drain and gate of SiC Schottky barrier source/drain MOSFET (SiC SBSD-MOSFET), there must be a sidewall as isolation. The width of sidewall strongly affects on the device performance. In this paper the effect of sidewall on the performance of 6H-SiC SBSD-NMOSFET is simulated with the 2D simulator MEDICI. The simulated results show that a sidewall with width less than 0.1μm slightly affects the device performance. However, when the width of sidewall exceeds 0.1μm, the conduction does not occur until the drain voltage is high enough and saturation current sharply decreases. The effect of the sidewall on device performance can be reduced by decreasing the doping concentration in the epitaxial layer.

A novel type of ultra fast and ultra soft recovery SiGe/Si heterojunction power diode with an ideal ohmic contact

Ma Li, Gao Yong, Wang Cai-Lin
Chin. Phys., 2004, 13 (7): 1114-1119 doi: 10.1088/1009-1963/13/7/026
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A novel type of p^+(SiGe)-n^--n^+ heterojunction switching power diode with high-speed capability is presented to overcome the drawbacks of existing power diodes. The improvement is achieved by using a p^+-n^+ mosaic layer as a substitute for the n^+ region in the conventional p^+(SiGe)-n^--n^+ diode to realize an `ideal ohmic' contact for electrons and holes simultaneously. Compared with conventional p^+(SiGe)-n^--n^+ diodes, the ideal ohmic contact p^+(SiGe)-n^--n^+ diodes have about one third of the reverse recovery time and a half of peak reverse recovery current. Furthermore, the softness factor increases nearly two times and the leakage current decreases 1-2 orders of magnitude. These improvements are achieved without resorting special process step to lower the carrier lifetime and thus the devices could be easily integrated into power ICs. The Ge percentage content of p^+(SiGe) layer is an important parameter for the optimal device design.

Anisotropy of the upper critical field in c-axis oriented MgB2 thin films

Wang Shu-Fang, Liu Zhen, Zhu Ya-Bin, Zhou Yue-Liang, Chen Zheng-Hao, Lü Hui-Bin, Yang Guo-Zhen
Chin. Phys., 2004, 13 (7): 1120-1123 doi: 10.1088/1009-1963/13/7/027
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C-axis oriented MgB_2 thin films were synthesized on single-crystal MgO (111) substrates using a chemical vapour deposition technique. The as-formed films revealed a sharp superconducting transition temperature of 38K with the transition width 0.2K. The temperature dependence of the upper critical magnetic field H_{c2}(T) in the films was determined via resistivity for magnetic field H parallel and perpendicular to the c axis of the films. Using the Werthamer-Helfand-Hohenberg formula, we obtained the anisotropy ratio of the upper critical field γ=1.2.

Comparison of calculation methods for the tunnel splitting at excited states of biaxial spin models

Cui Xiao-Bo, Chen Zhi-De
Chin. Phys., 2004, 13 (7): 1124-1128 doi: 10.1088/1009-1963/13/7/028
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We present the calculation and comparison of tunnel splitting at excited levels of biaxial spin models by various methods, including the generalized instanton method, the generalized path integral method for coherent spin states, the perturbation method, and the exact method by numerical diagonalization of the Hamiltonian. It is found that, for integer spin with spin number around 10, tunnel splitting predicted by the generalized path integral for coherent spin states is about 10^{-n} times of the exact numerical result for the nth excited level, while the ratio of the results of the perturbation method and the exact numerical method diverges in the large spin limit. We thus conclude that the generalized instanton method is the best approximate way for calculating tunnel splitting in spin models.

Magnetism and magnetoresistance of Er1-xSmxMn6Ge6(x=0.2-1.0) compounds

Wang Ru-Wu, Zhang Shao-Ying, Zhang Li-Gang, Yao Jin-Lei, Li Yun-Bao, Shen Bao-Gen
Chin. Phys., 2004, 13 (7): 1129-1133 doi: 10.1088/1009-1963/13/7/029
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Magnetic and transport properties of Er_{1-x}Sm_xMn_6Ge_6(x=0.2-1.0) have been investigated by x-ray diffraction (XRD) and magnetization measurement. Analysis of the XRD patterns indicates that the samples with x≤0.4 mainly consist of HfFe_6Ge_6-type phase and the samples with 0.6≤x≤1.0 mainly consist of YCo_6Ge_6-type phase (P6/mmm). The lattice constants and the unit cell volume increase with increasing Sm content. The antiferro-ferri-ferromagnetic transitions can be observed with increasing Sm content. The samples with x=0.2 and 0.4 order antiferromagnetically at 420 and 425K, respectively. The samples with x=0.6, 0.8 and 1.0 order from ferri- to ferromagnetically over the whole magnetic ordering temperature range. The corresponding Curie temperatures are 435, 441 and 446K, respectively. The magnetoresistance (MR) isotherms of the sample with x=0.8, measured at various temperatures, are analysed. The magnitude of MR is found to be positive below 55K and gradually increases to a relatively large value of about 5.02% at 5K in a field of 5T as the temperature is lowered. A possible explanation for the positive MR is given.

Effect of Co substitution on magnetic properties and magnetic entropy changes in LaFe11.83Si0.94Al0.23 compounds

Shen Jun, Li Yang-Xian, Wang Fang, Wang Guang-Jun, Zhang Shao-Ying
Chin. Phys., 2004, 13 (7): 1134-1138 doi: 10.1088/1009-1963/13/7/030
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Effect of Co substitution on magnetic properties and magnetic entropy changes in LaFe_{11.83}Si_{0.94}Al_{0.23} compounds has been investigated by means of magnetization measurements. X-ray diffraction shows the prepared compounds to be single phase with the cubic NaZn_{13}-type structure. Substitution of Co for Fe leads to an increase of Curie temperature of the material. The magnetic entropy changes in LaFe_{11.83}Si_{0.94}Al_{0.23} and LaFe_{11.03}Co_{0.80}Si_{0.94}Al_{0.23} compounds are 21.8J/(kg·K) to 16.9J/(kg·K) under a magnetic field change of 0-5T at Curie temperature, respectively. Giant magnetic entropy changes are attributed to the higher magnetization and the rapid change in magnetization at Curie temperature.

Magnetotransport properties and magnetocaloric effects of Mn1.95Cr0.05Sb0.95Ga0.05 compound

Liu Xi-Bin, Zhang Shao-Ying, Shen Bao-Gen, Yao Jin-Lei, Wang Fang
Chin. Phys., 2004, 13 (7): 1139-1143 doi: 10.1088/1009-1963/13/7/031
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The magnetotransport properties and magnetocaloric effects of the compound Mn_{1.95}Cr_{0.05}Sb_{0.95}Ga_{0.05} have been studied. With decreasing temperature, a spontaneous first-order magnetic phase transition from ferrimagnetic (FI) to antiferromagnetic (AF) state takes place at T_s=200K. A metamagnetic transition from the AF to FI state can be induced by an external field, accompanied by a giant magnetoresistance effect of 57%. The magnetic entropy changes are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation. Mn_{1.95}Cr_{0.05}Sb_{0.95}Ga_{0.05} exhibits a negative magnetocaloric effect, and the absolute values of ΔS_M^{max}(T,ΔH) are 4.4, 4.1, 3.6, 2.8 and 1.5 J/(kg·K) for magnetic field changes of 0-5T, 0-4T, 0-3T, 0-2T and 0-1T, respectively.

Magnetic properties and coercivity mechanism of precipitation-hardened Gd-Co based ribbons

Rong Chuan-Bing, Zhang Jian, Du Xiao-Bo, Zhang Hong-Wei, Zhang Shao-Ying, Shen Bao-Gen
Chin. Phys., 2004, 13 (7): 1144-1148 doi: 10.1088/1009-1963/13/7/032
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Gd(Co_{0.88-x}Cu_xFe_{0.09}Zr_{0.03})_z ribbons with x=0.075-0.200 and z=6.4-7.7 have been prepared by a melt-spinning technique. A cellular microstructure consisting of 2:17 cells surrounded by the 1:5 cell boundary phase is obtained after precipitation hardening. The dependence of room temperature coercivity on the heat treatment process suggests that the long-time isothermal aging is not helpful for the development of magnetic properties. Positive temperature coefficient of remanence from room temperature to about 673K is typical for all samples, while positive temperature coefficient of coercivity is obtained only in ribbons with low Cu content. The coercivity mechanism of the precipitation-hardened ribbons at different temperatures is also discussed.

Study of BaxSr1-xTiO3 thin films using transverse-field Ising model

Tao Yong-Mei, Jiang Qing
Chin. Phys., 2004, 13 (7): 1149-1155 doi: 10.1088/1009-1963/13/7/033
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In this paper, the effects of doping on the thermodynamic properties of Ba_xSr_{1-x}TiO_3 (BST) thin film are investigated, based on the transverse-field Ising model (TIM) within the framework of mean field theory. We apply the double-peak distribution model of related parameters to mimic doping. The lattice expansion arising from doping with large Ba^{2+} was also taken into account. We concentrate on the doping concentration dependence of peak temperature (T_m), spontaneous polarization and dielectric susceptibility. It is found that the doping concentration has great influence on the dielectric properties and phase transition properties of BST thin films. We also discuss the quantum effect arising from doping.

Infrared-to-green upconversion luminescence and mechanism of Ho3+, Nd3+ and Yb3+ ions in oxyfluoride glass ceramics

Zhang Jun-Jie, Dai Shi-Xun
Chin. Phys., 2004, 13 (7): 1156-1162 doi: 10.1088/1009-1963/13/7/034
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New oxyfluoride glasses and glass ceramics co-doped with Nd^{3+}, Yb^{3+} and Ho^{3+} were prepared. The upconversion of infrared radiation into green fluorescence has been studied for Nd^{3+}, Yb^{3+} and Ho^{3+} in the transparent oxyfluoride glass ceramics. At room temperature very strong green upconversion luminescence due to the Ho^{3+}: ({}^5F_4, {}^5S_2)→{}^5I_8 transition under 800 nm excitation was observed in the glass ceramics. The intensity of the green upconversion luminescence in a 1mol% YbF_3-containing glass ceramic was found to be about 120 times stronger than that in the precursor oxyfluoride glass. The reason for the highly efficient Ho^{3+} upconversion luminescence in the oxyfluoride glass ceramics is discussed. The upconversion mechanism is also investigated.

Quantum confinement analysis of nanostructures in oxidation of SiGe alloys

Huang Wei-Qi, Liu Shi-Rong
Chin. Phys., 2004, 13 (7): 1163-1166 doi: 10.1088/1009-1963/13/7/035
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We report the investigation on the oxidation behaviour of Si_{1-x}Ge_x alloys (x=0.05, 0.15, and 0.25). It was found for the first time that a nanocap (thickness: 1.6-2.0nm) was formed on the oxide film after fast oxidation. Some new peaks in photoluminescence spectra were discovered, which could be related to the Ge nanocap, the Ge nanolayer (thickness: 0.8-1.2nm) and the Ge nanoparticles (with various diameters from 2.6nm to 7.4nm), respectively. A suitable model and several new calculating formulae combined with the Unrestricted Hartree-Fock-Roothaan (UHFR) method and quantum confinement analysis have been proposed to interpret the PL spectra and the nanostructure mechanism in the oxide and Ge segregation.

Statistical properties of extragalactic radio sources

Zhang Jiang-Shui
Chin. Phys., 2004, 13 (7): 1177-1182 doi: 10.1088/1009-1963/13/7/038
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In this paper, a large sample of extragalactic radio sources is analysed to show their statistical properties. The core and total radio powers are used to determine the core-dominance parameter for galaxies, BL Lacertae objects and quasars; mutual correlations between core radio power, total radio power, redshift and core dominance parameter are examined for different subclasses. A statistically significant correlation between the total and core radio power is confirmed. There are no obvious correlations between core-dominance parameter and the total power for our whole sample and quasars, but there is a statistically significant anti-correlation for our galaxy sample. Some discussions and comparison of the correlations with those obtained by other authors are also given.
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