Table of contents

05 July 2004, Volume 13 Issue 7 Previous issue    Next issue

GENERAL

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Characteristic functional structure of infinitesimal symmetry transformations of Birkhoffian systems Gu Shu-Long (顾书龙), Zhang Hong-Bin (张宏彬) Chinese Physics, 2004, 13 (7):  979-983.  DOI: 10.1088/1009-1963/13/7/001 Abstract ( 1037 )   PDF (159KB) ( 425 )   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 $Z_{\mu}(t,a)$ (introduced by the relation $\xi_{\mu}(t,a)=Z_{\mu}(t,a)+\dot{a}^{\mu}\xi_0 (t, a))$ 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.

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The soliton-like solutions to the (2+1)-dimensional modified dispersive water-wave system Li De-Sheng (李德生), Zhang Hong-Qing (张鸿庆) Chinese Physics, 2004, 13 (7):  984-987.  DOI: 10.1088/1009-1963/13/7/002 Abstract ( 1346 )   PDF (148KB) ( 529 )   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.

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Generation of Bell states in two-component Bose－Einstein condensates Zhang Guo-Feng (张国锋), Yin Wen (殷雯), Liang Jiu-Qing (梁九卿), Yan Qi-Wei (严启伟) Chinese Physics, 2004, 13 (7):  988-989.  DOI: 10.1088/1009-1963/13/7/003 Abstract ( 1215 )   PDF (138KB) ( 521 )   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 $\varOmega$ increases, and the self-interaction strength of the component A(B) has no effect on it.

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Lattice-gas simulation of escaping pedestrian flow in corridor Qiu Bing (邱冰), Tan Hui-Li (谭惠丽), Kong Ling-Jiang (孔令江), Liu Mu-Ren (刘慕仁) Chinese Physics, 2004, 13 (7):  990-995.  DOI: 10.1088/1009-1963/13/7/004 Abstract ( 1349 )   PDF (252KB) ( 544 )   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 $t_{\rm c}$ scales as $t_{\rm c}\varpropto W^{-0.85\pm0.04}$, and $t_{\rm c}\varpropto D$, 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.

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Synchronization of coupled dynamical chaotic networks via scalar controllers Li Zhi (李智), Shi Song-Jiao (施颂椒) Chinese Physics, 2004, 13 (7):  996-999.  DOI: 10.1088/1009-1963/13/7/005 Abstract ( 1079 )   PDF (157KB) ( 664 )   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.

NUCLEAR PHYSICS

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Investigation of the mean free path of projectile fragments produced in 16O－Em collision at 60 A GeV Zhang Dong-Hai (张东海), Gong Jin-Sheng (巩进生) Chinese Physics, 2004, 13 (7):  1000-1004.  DOI: 10.1088/1009-1963/13/7/006 Abstract ( 1164 )   PDF (188KB) ( 377 )   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.

ATOMIC AND MOLECULAR PHYSICS

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Radiation pressure and laser cooling of a three-level atom in a ladder configuration Tan Lei (谭磊), Wang Zhi-Cheng (汪志诚), Gu Huai-Qiang (顾怀强) Chinese Physics, 2004, 13 (7):  1005-1012.  DOI: 10.1088/1009-1963/13/7/007 Abstract ( 1323 )   PDF (257KB) ( 918 )   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$^2$S$_{1/2}$－3$^2$P$_{3/2}$－3$^2$D$_{5/2}$ cascade transitions is investigated. It is found that temperatures lower than the Doppler limit can be achieved.

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Time-dependent quantum dynamics study for reaction of D+CH4→CH3+HD Liu Xin-Guo (刘新国), Zhang Qing-Gang (张庆刚), Zhang Yi-Ci (张怿慈), Wang Ming-Liang (王明良), John Zhang Zeng-Hui (张增辉) Chinese Physics, 2004, 13 (7):  1013-1017.  DOI: 10.1088/1009-1963/13/7/008 Abstract ( 1721 )   PDF (232KB) ( 403 )   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.

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Total cross sections for scattering of electrons from O2, H2O, H2, O3, CO and CO2 at 100－2000eV Shi De-Heng (施德恒), Sun Jin-Feng (孔金锋), Yang Xiang-Dong (杨向东), Zhu Zun-Lue (朱遵略), Liu Yu-Fang (刘玉芳) Chinese Physics, 2004, 13 (7):  1018-1024.  DOI: 10.1088/1009-1963/13/7/009 Abstract ( 1265 )   PDF (206KB) ( 406 )   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$_2$O, 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.

CLASSICAL AREAS OF PHENOMENOLOGY

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Fractional Fourier transform for partially coherent beam in spatial-frequency domain Cai Yang-Jian (蔡阳健), Lin Qiang (林强) Chinese Physics, 2004, 13 (7):  1025-1032.  DOI: 10.1088/1009-1963/13/7/010 Abstract ( 1157 )   PDF (689KB) ( 514 )   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.

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Correlation between randomly varying birefringence and random dispersion map in a dispersion-managed soliton system Li Hong (李宏), Wang Tie-Jun (王铁军), Huang De-Xiu (黄德修) Chinese Physics, 2004, 13 (7):  1033-1038.  DOI: 10.1088/1009-1963/13/7/011 Abstract ( 1249 )   PDF (336KB) ( 511 )   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.

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Generation of motional Fock states of a trapped ion in the mediate-excitation regime Zheng Shi-Biao (郑仕标) Chinese Physics, 2004, 13 (7):  1039-1041.  DOI: 10.1088/1009-1963/13/7/012 Abstract ( 1079 )   PDF (133KB) ( 493 )   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.

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Mid-infrared generation based on a periodically poled LiNbO3 optical parametric oscillator Lin Xue-Chun (林学春), Kong Yu-Peng (孔宇鹏), Zhang Ying (张瑛), Zhang Jie (张杰), Yao Ai-Yun (姚爱云), Bi Yong (毕勇), Sun Zhi-Pei (孙志培), Cui Da-Fu (崔大复), Li Rui-Ning (李瑞宁), Wu Ling-An (吴令安), Xu Zu-Yan (许祖彦) Chinese Physics, 2004, 13 (7):  1042-1045.  DOI: 10.1088/1009-1963/13/7/013 Abstract ( 1407 )   PDF (169KB) ( 504 )   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.

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An all-optical switch of Mach－Zehnder interferometer type using an active fibre ring resonator Li Jun-Qing (李俊庆), Alireza Bananej, Li Qiang-Hua (励强华), Chen Qiang (陈强), Li Chun-Fei (李淳飞) Chinese Physics, 2004, 13 (7):  1046-1051.  DOI: 10.1088/1009-1963/13/7/014 Abstract ( 1239 )   PDF (221KB) ( 519 )   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 $1-\kappa\leq A\leq \sqrt{1-\kappa}$, 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.

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Polarization-maintaining photonic crystal fibre Lou Shu-Qin (娄淑琴), Wang Zhi (王智), Ren Guo-Bin (任国斌), Jian Shui-Sheng (简水生) Chinese Physics, 2004, 13 (7):  1052-1058.  DOI: 10.1088/1009-1963/13/7/015 Abstract ( 1257 )   PDF (401KB) ( 594 )   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 dc=0.4$\varLambda$.

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Finite-amplitude vibration of a bubble and sonoluminescence Qian Zu-Wen (钱祖文), Xiao Ling (肖灵), Guo Liang-Hao (郭良浩) Chinese Physics, 2004, 13 (7):  1059-1064.  DOI: 10.1088/1009-1963/13/7/016 Abstract ( 1287 )   PDF (219KB) ( 405 )   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.

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Many-body dipole-induced dipole model for electrorheological fluids Huang Ji-Ping (黄吉平), Yu Kin-Wah (余建华) Chinese Physics, 2004, 13 (7):  1065-1069.  DOI: 10.1088/1009-1963/13/7/017 Abstract ( 1119 )   PDF (180KB) ( 661 )   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.

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The application of computational fluid dynamics to pedestrian level wind safety problem induced by high-rise buildings Li Lei (李磊), Hu Fei (胡非), Cheng Xue-Ling (程雪玲), Han Hao-Yu (韩浩玉) Chinese Physics, 2004, 13 (7):  1070-1075.  DOI: 10.1088/1009-1963/13/7/018 Abstract ( 1401 )   PDF (10880KB) ( 535 )   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-$\varepsilon$ 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.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

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Morphological study of the localized growth of materials in dielectric barrier discharge Zhao Qing-Xun (赵庆勋), Li Xue-Chen (李雪辰), Jiang Nan (江南), Wang Long (王龙) Chinese Physics, 2004, 13 (7):  1076-1081.  DOI: 10.1088/1009-1963/13/7/019 Abstract ( 1195 )   PDF (1130KB) ( 427 )   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.

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Calculation of the surface energy of fcc metals with modified embedded-atom method Zhang Jian-Min (张建民), Ma Fei (马飞), Xu Ke-Wei (徐可为) Chinese Physics, 2004, 13 (7):  1082-1090.  DOI: 10.1088/1009-1963/13/7/020 Abstract ( 1641 )   PDF (143KB) ( 1221 )   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.

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Melting and Grüneisen parameters of NaCl at high pressure Chen Qi-Feng (陈其峰), Cai Ling-Cang (蔡灵仓), Duan Su-Qing (段素青), Chen Dong-Quan (陈栋泉) Chinese Physics, 2004, 13 (7):  1091-1095.  DOI: 10.1088/1009-1963/13/7/021 Abstract ( 1468 )   PDF (217KB) ( 560 )   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.

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Application of shell model in molecular dynamics simulation to MgO Liu Zi-Jiang (刘子江), Cheng Xin-Lu (程新路), Chen Xiang-Rong (陈向荣), Zhang Hong (张红), Lu Lai-Yu (逯来玉) Chinese Physics, 2004, 13 (7):  1096-1099.  DOI: 10.1088/1009-1963/13/7/022 Abstract ( 1409 )   PDF (174KB) ( 585 )   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.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

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Analytical model of electron transport characteristics for 4H-SiC material and devices Lü Hong-Liang (吕红亮), Zhang Yi-Men (张义门), Zhang Yu-Ming (张玉明) Chinese Physics, 2004, 13 (7):  1100-1103.  DOI: 10.1088/1009-1963/13/7/023 Abstract ( 1125 )   PDF (206KB) ( 445 )   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.

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A compact I－V model for lightly-doped-drain MOSFETs Yu Chun-Li (于春利), Yang Lin-An (杨林安), Hao Yue (郝跃) Chinese Physics, 2004, 13 (7):  1104-1109.  DOI: 10.1088/1009-1963/13/7/024 Abstract ( 1128 )   PDF (264KB) ( 417 )   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.

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Analysis of the effect of sidewall on the performance of 6H-SiC Schottky barrier source/drain NMOSFETs Tang Xiao-Yan (汤晓燕), Zhang Yi-Men (张义门), Zhang Yu-Ming (张玉明), Gao Jin-Xia (郜锦侠) Chinese Physics, 2004, 13 (7):  1110-1113.  DOI: 10.1088/1009-1963/13/7/025 Abstract ( 1187 )   PDF (190KB) ( 432 )   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.

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A novel type of ultra fast and ultra soft recovery SiGe/Si heterojunction power diode with an ideal ohmic contact Ma Li (马丽), Gao Yong (高勇), Wang Cai-Lin (王彩琳) Chinese Physics, 2004, 13 (7):  1114-1119.  DOI: 10.1088/1009-1963/13/7/026 Abstract ( 1162 )   PDF (296KB) ( 502 )   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.

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Anisotropy of the upper critical field in c-axis oriented MgB2 thin films Wang Shu-Fang (王淑芳), Liu Zhen (刘震), Zhu Ya-Bin (朱亚彬), Zhou Yue-Liang (周岳亮), Chen Zheng-Hao (陈正豪), Lü Hui-Bin (吕惠宾), Yang Guo-Zhen (杨国桢) Chinese Physics, 2004, 13 (7):  1120-1123.  DOI: 10.1088/1009-1963/13/7/027 Abstract ( 1314 )   PDF (180KB) ( 481 )   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 $\gamma$=1.2.

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Comparison of calculation methods for the tunnel splitting at excited states of biaxial spin models Cui Xiao-Bo (崔晓波), Chen Zhi-De (陈芝得) Chinese Physics, 2004, 13 (7):  1124-1128.  DOI: 10.1088/1009-1963/13/7/028 Abstract ( 970 )   PDF (222KB) ( 353 )   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.

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Magnetism and magnetoresistance of Er1-xSmxMn6Ge6(x=0.2－1.0) compounds Wang Ru-Wu (汪汝武), Zhang Shao-Ying (张绍英), Zhang Li-Gang (张立刚), Yao Jin-Lei (姚金雷), Li Yun-Bao (李云宝), Shen Bao-Gen (沈保根) Chinese Physics, 2004, 13 (7):  1129-1133.  DOI: 10.1088/1009-1963/13/7/029 Abstract ( 1184 )   PDF (226KB) ( 373 )   Magnetic and transport properties of Er$_{1-x}$Sm$_x$Mn$_6$Ge$_6$($x$=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$_6$Ge$_6$-type phase and the samples with 0.6≤x≤1.0 mainly consist of YCo$_6$Ge$_6$-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.

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Effect of Co substitution on magnetic properties and magnetic entropy changes in LaFe11.83Si0.94Al0.23 compounds Shen Jun (沈俊), Li Yang-Xian (李养贤), Wang Fang (王芳), Wang Guang-Jun (王光军), Zhang Shao-Ying (张绍英) Chinese Physics, 2004, 13 (7):  1134-1138.  DOI: 10.1088/1009-1963/13/7/030 Abstract ( 1519 )   PDF (284KB) ( 503 )   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$\cdot$K) to 16.9J/(kg$\cdot$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.

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Magnetotransport properties and magnetocaloric effects of Mn1.95Cr0.05Sb0.95Ga0.05 compound Liu Xi-Bin (刘喜斌), Zhang Shao-Ying (张绍英), Shen Bao-Gen (沈保根), Yao Jin-Lei (姚金雷), Wang Fang (王芳) Chinese Physics, 2004, 13 (7):  1139-1143.  DOI: 10.1088/1009-1963/13/7/031 Abstract ( 1446 )   PDF (220KB) ( 445 )   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 $T_{\rm s}=200$K. 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 $\Delta S_{\rm M}^{\rm max}(T, \Delta H)$ are 4.4, 4.1, 3.6, 2.8 and 1.5 J/(kg$\cdot$K) for magnetic field changes of 0－5T, 0－4T, 0－3T, 0－2T and 0－1T, respectively.

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Magnetic properties and coercivity mechanism of precipitation-hardened Gd－Co based ribbons Rong Chuan-Bing (荣传兵), Zhang Jian (张健), Du Xiao-Bo (杜晓波), Zhang Hong-Wei (张宏伟), Zhang Shao-Ying (张绍英), Shen Bao-Gen (沈保根) Chinese Physics, 2004, 13 (7):  1144-1148.  DOI: 10.1088/1009-1963/13/7/032 Abstract ( 1261 )   PDF (232KB) ( 487 )   Gd(Co$_{0.88-x}$Cu$_x$Fe$_{0.09}$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.

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Study of BaxSr1-xTiO3 thin films using transverse-field Ising model Tao Yong-Mei (陶永梅), Jiang Qing (蒋青) Chinese Physics, 2004, 13 (7):  1149-1155.  DOI: 10.1088/1009-1963/13/7/033 Abstract ( 1109 )   PDF (227KB) ( 492 )   In this paper, the effects of doping on the thermodynamic properties of Ba$_x$Sr$_{1-x}$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 ($T_{\rm m}$), 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.

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Infrared-to-green upconversion luminescence and mechanism of Ho3+, Nd3+ and Yb3+ ions in oxyfluoride glass ceramics Zhang Jun-Jie (张军杰), Kawamoto Yoji, Dai Shi-Xun(戴世勋) Chinese Physics, 2004, 13 (7):  1156-1162.  DOI: 10.1088/1009-1963/13/7/034 Abstract ( 1168 )   PDF (246KB) ( 458 )   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$^{3+}$: ($^5$F$_4$, $^5$S$_2$)→$^5$I$_8$ 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.

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Quantum confinement analysis of nanostructures in oxidation of SiGe alloys Huang Wei-Qi (黄伟其), Liu Shi-Rong (刘世荣) Chinese Physics, 2004, 13 (7):  1163-1166.  DOI: 10.1088/1009-1963/13/7/035 Abstract ( 1304 )   PDF (174KB) ( 464 )   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.

CROSS DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

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Novel material for nonvolatile ovonic unified memory (OUM)－Ag11In12Te26Sb51 phase change semiconductor Liu Bo (刘波), Song Zhi-Tang (宋志棠), Zhang Ting (张挺), Feng Song-Lin (封松林), Gan Fu-Xi (干福熹) Chinese Physics, 2004, 13 (7):  1167-1170.  DOI: 10.1088/1009-1963/13/7/036 Abstract ( 925 )   PDF (180KB) ( 342 )   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, $E_{\rm a}$, 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 ($R_{\rm amo}$) film is found to be larger than $1\times10^6\Omega$ and that of the crystalline ($R_{\rm cry}$) film lies in the range from about $10^3$ to $10^4\Omega$. So we have the ratio $R_{\rm amo}/R_{\rm cry}=10^2\sim 10^3$, which is sufficiently large for application in memory devices.

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A nonequilibrium phase transition in immune response Zhang Wei (张伟), Qi An-Shen (漆安慎) Chinese Physics, 2004, 13 (7):  1171-1176.  DOI: 10.1088/1009-1963/13/7/037 Abstract ( 1072 )   PDF (203KB) ( 481 )   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.

GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

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Statistical properties of extragalactic radio sources Zhang Jiang-Shui (张江水) Chinese Physics, 2004, 13 (7):  1177-1182.  DOI: 10.1088/1009-1963/13/7/038 Abstract ( 920 )   PDF (273KB) ( 322 )   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.