Table of contents

15 September 2001, Volume 10 Issue 9 Previous issue    Next issue

GENERAL

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COHERENT INFORMATION ON THERMAL RADIATION NOISE CHANNEL: AN APPROACH OF INTEGRAL WITHIN ORDERED PRODUCT OF OPERATORS Chen Xiao-yu (陈小余), Qiu Pei-liang (仇佩亮) Chinese Physics, 2001, 10 (9):  779-782.  DOI: 10.1088/1009-1963/10/9/301 Abstract ( 1087 )   PDF (207KB) ( 605 )   An analytical expression is given to the coherent information of the thermal radiation signal transmitted over the thermal radiation noise channel, one of the most essential quantum Gaussian channels. Focusing on the single normal mode of the thermal radiation signal and noise, we resolve the entangled state density operator, which characterizes quantum information transmission, into a direct product of two parts, with each part being a thermal radiation density operator. The calculation is aided by the technique known as "integral within ordered product of operators".

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OPTICAL SPACE OF THE KERR SOLUTIONS Lü Jun-li (吕君丽) Chinese Physics, 2001, 10 (9):  783-786.  DOI: 10.1088/1009-1963/10/9/302 Abstract ( 936 )   PDF (206KB) ( 375 )   In this paper, we discuss the optical geometry of the Kerr solutions, and show the embedding diagram at the equator $\theta=\pi/2$ for the optical space of the Kerr solution with $a/M=1$. It is noticed that at the equator $\theta=\pi/2$, the Gaussian curvature K for the ordinary space is negative, and the $\stackrel{\sim}{K}$ for the optical space is positive.

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STATIC SPHERICALLY SYMMETRIC SOLUTION OF EINSTEIN GRAVITY COUPLED TO ELECTRO-MAGNETIC AND SCALAR FIELDS Chen Guang (陈光) Chinese Physics, 2001, 10 (9):  787-789.  DOI: 10.1088/1009-1963/10/9/303 Abstract ( 1209 )   PDF (160KB) ( 612 )   The static spherically symmetric solution of Einstein gravity coupled to electromagnetic and scalar fields is obtained under the consideration of the self-gravitational interaction of the electromagnetic and scalar fields, which is singularity-free and stable.

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NUMERICAL ANALYSIS OF A CHAOTIC SYSTEM Ren Zhi-jian (任志坚) Chinese Physics, 2001, 10 (9):  790-795.  DOI: 10.1088/1009-1963/10/9/304 Abstract ( 1013 )   PDF (247KB) ( 327 )   This paper further proves that a single spiral strange attractor can be observed in an extremely simple autonomous electrical circuit by computer simulation. It is of third order and has only one nonlinear element: a three-segment piece-wise linear resistor. The digital analyses show that the strange attractor has peculiar features compared with other third-order differential systems.

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ON FEEDBACK CONTROL OF DELAYED CHAOTIC SYSTEM Li Li-xiang (李丽香), Peng Hai-peng (彭海朋), Lu Hui-bin (卢辉斌), Guan Xin-ping (关新平) Chinese Physics, 2001, 10 (9):  796-804.  DOI: 10.1088/1009-1963/10/9/305 Abstract ( 1260 )   PDF (256KB) ( 549 )   In this paper two different types of feedback control technique are discussed: the standard feedback control and the time-delay feedback control which have been successfully used in many control systems. In order to understand to what extent the two different types of control technique are useful in delayed chaotic systems, some analytic stabilization conditions for chaos control from the two types of control technique are derived ased on Lyapunov stabilization arguments. Similarly, we discuss the tracking problem by applying the time-delay feedback control. Finally, numerical examples are provided.

NUCLEAR PHYSICS

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MULTI-LAMBDA MATTER IN A CHIRAL HADRONIC MODEL Guo Hua (郭华), Yang Shu (杨树), Hu Xiang (胡翔), Liu Yu-xin (刘玉鑫) Chinese Physics, 2001, 10 (9):  805-808.  DOI: 10.1088/1009-1963/10/9/306 Abstract ( 1109 )   PDF (240KB) ( 556 )   Multi-lambda matter is investigated in the framework of a chiral hadronic model. It is shown that multi-lambda matter consisting of {N, $\varLambda$} is a metastable state as the strangeness per baryon and the density of hadronic matter are varied. The effective lambda mass decreases as the baryon density increases, and remains larger than that of the nucleon.

ATOMIC AND MOLECULAR PHYSICS

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THEORETICAL CALCULATION OF THE RELATIVISTIC SUBCONFIGURATION-AVERAGED TRANSITION ENERGIES Zhang Ji-yan (张继彦), Yang Xiang-dong (杨向东), Yang Guo-hong (杨国洪), Zhang Bao-han (张保汉), Lei An-le (雷安乐), Liu Hong-jie (刘宏杰), Li Jun (李军) Chinese Physics, 2001, 10 (9):  809-813.  DOI: 10.1088/1009-1963/10/9/307 Abstract ( 1075 )   PDF (221KB) ( 413 )   A method for calculating the average energies of relativistic subconfigurations in highly ionized heavy atoms has been developed in the framework of the multiconfigurational Dirac－Fock theory. The method is then used to calculate the average transition energies of the spin-orbit-split 3d－4p transition of Co-like tungsten, the 3d－5f transition of Cu-like tantalum, and the 3d－5f transitions of Cu-like and Zn-like gold samples. The calculated results are in good agreement with those calculated with the relativistic parametric potential method and also with the experimental results.

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FIELD IONIZATION OF MOLECULES IN AN INTENSE LASER FIELD Wu Cheng-yin (吴成印), Gong Qi-huang (龚旗煌) Chinese Physics, 2001, 10 (9):  814-820.  DOI: 10.1088/1009-1963/10/9/308 Abstract ( 1317 )   PDF (247KB) ( 852 )   In order to predict the field ionization probabilities, the accurate ab initio electrostatic potential of molecules has to be calculated. However, the calculation of the full ab initio electrostatic potential of molecules is complicated, even impossible for some larger molecules with low symmetry. Here, we present a semi-empirical model to treat the field ionization of molecules in an intense laser field. In this model, a modified Coulomb potential is used to take the place of the complicated ab initio electrostatic potential of molecules. The analytic equations of the Keldysh adiabatic parameter using the Coulomb potential and the modified Coulomb potential have first been given. Using our semi-empirical model, we have calculated the field ionization probabilities and the Keldysh adiabatic parameters of O2, N2, SO2, C2H4, CH3CN and C6H6 in an intense laser field. The results agree excellently with the calculations using the ab initio electrostatic potential of molecules. As the modified parameter for the Coulomb potential can be found from experimental measurements, the field ionization mechanism of molecules can be immediately predicted with our semi-empirical model.

CLASSICAL AREAS OF PHENOMENOLOGY

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UNCORRELATED TWO-MODE SU(1,1) COHERENT STATES Hou Li-zhen (侯丽珍), Zeng Gao-jian (曾高坚) Chinese Physics, 2001, 10 (9):  821-826.  DOI: 10.1088/1009-1963/10/9/309 Abstract ( 1244 )   PDF (243KB) ( 899 )   We have constructed a new kind of two-mode bosonic realization of SU(1,1) Lie algebra, on the basis of which the SU(1,1) generalized coherent states in the two-mode Fock space are derived. These two-mode SU(1,1) coherent states, which are called uncorrelated two-mode SU(1,1) coherent states, include three special cases. For these states, we study the mean photon number distribution and their non-classical properties, which are photon anti-bunching, violations of Cauchy－Schwarz inequality and two-mode squeezing.

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ANALYSIS OF LASING PERFORMANCES OF Yb3+-DOPED YAG, FAP, KYW CRYSTALS Huang Li-lei (黄莉蕾), Hong Zhi (洪治), Fang Da-wei (方达伟) Chinese Physics, 2001, 10 (9):  827-831.  DOI: 10.1088/1009-1963/10/9/310 Abstract ( 1284 )   PDF (238KB) ( 426 )   The Yb3+ laser emitting at a 1.03μm wavelength has been evoking strong interest recently due to its advantages of long fluorescence lifetime, broad absorption band and the fact that it never shows concentration quenching. On the other hand, as a laser of three-level system it has, in general, a relatively high threshold power, which makes it important to seek some suitable host crystals to reduce this. Here, we present a comparison of the lasing performances of Yb3+-doped YAG [Y3Al5O12], FAP [Ca5(PO4)3F] and KYW [KY(WO4)2] crystals, including threshold power and slope efficiency, with those of the Nd:YAG laser based on the threshold formula of three-and four-level systems deduced by the authors. The results show that the Yb3+ laser can output a power larger than the Nd:YAG laser does in the case of comparably higher pumping power, if the length of the lasing rod and the concentration of the active ions satisfy some conditions. The theoretical results are also close to the experimental results reported.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

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STUDY OF THE Al/GRAPHITE INTERFACE Lu Hua (陆华), Shen Dian-hong (沈电洪), Deng Xin-fa (邓新发), Xue Qi-kun (薛其坤), N. Froumin, M. Polak Chinese Physics, 2001, 10 (9):  832-835.  DOI: 10.1088/1009-1963/10/9/311 Abstract ( 1242 )   PDF (215KB) ( 958 )   Thin Al films with a thickness of 20－30nm were prepared by ultra-high vacuum deposition of Al onto a graphite surface parallel to a (0001) basal plane. The samples were annealed up to 1070K. X-ray photoelectron spectroscopy analysis has shown that for temperatures just higher than 770K, a little carbide occurs in the Al film and only an Al－C phase is present at the Al/graphite interface. After annealing at 970K, the Al4C3 phase can be observed, and the binding energy of the Al2p electrons increases continuously from 72.7 to 74.2eV with increasing temperature up to 1070K. Auger electron spectroscopy depth profiles are measured to investigate the phases existing in the Al film as well as at the Al/graphite interface. It is found that the Al4C3 phase at the interface is the final product of a series of Al carbides from the interfacial reaction between Al and graphite.

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

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INFLUENCES OF NONLINEAR INTERACTIONS ON POLARITONS Niu Jia-sheng (牛家胜), Luo Ying (罗莹), Ma Ben-kun (马本堃) Chinese Physics, 2001, 10 (9):  836-839.  DOI: 10.1088/1009-1963/10/9/312 Abstract ( 1073 )   PDF (197KB) ( 388 )   In this paper, we have analysed theoretically the polarization and dielectric constant generated by the nonlinear interactions between ions in ionic crystals. The spectrum of polaritons (coupling modes of photons and optical phonons) under nonlinear interactions has been developed. A new branch of dispersion relations has emerged in the original frequency gap between $\omega$TO and $\omega$LO.

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TERAHERTZ SPECTRA EMITTED FROM HOT CARRIERS IN TWO-DIMENSIONAL SEMICONDUCTORS Lei Xiao-lin (雷啸霖), Liu Shi-yong (刘世勇) Chinese Physics, 2001, 10 (9):  840-843.  DOI: 10.1088/1009-1963/10/9/313 Abstract ( 1099 )   PDF (225KB) ( 666 )   We propose a simple method to calculate the spectrum of spontaneous terahertz (THz) emission from hot carriers in two-dimensional semiconductors by means of transport quantities, which are easily obtained during the process of solving the transport problem using the recently developed balance-equation approach for THz-driven transport. The method has been applied to examine the surface emission from a GaAs/AlGaAs heterojunction and the edge emission from a GaAs-based multiple quantum-well system subjected to strong dc biases. The theoretical results obtained are in reasonably good agreement with measurements.

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COULOMB BLOCKADE OSCILLATIONS OF Si SINGLE-ELECTRON TRANSISTORS Wang Tai-hong (王太宏), Li Hong-wei (李宏伟), Zhou Jun-ming (周均铭) Chinese Physics, 2001, 10 (9):  844-846.  DOI: 10.1088/1009-1963/10/9/314 Abstract ( 1194 )   PDF (171KB) ( 670 )   Coulomb blockade oscillations of Si single-electron transistors, which are fabricated completely by the conventional photolithography technique, have been investigated. Most of the single-electron transistors clearly show Coulomb blockade oscillations and these oscillations can be periodic by applying negative voltages to the in-plane gates. A shift of the peak positions is observed at high temperatures. It is also found that the fluctuation of the peak spacing cannot be neglected.

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INVESTIGATION OF EMPIRICAL LAWS FOR SUPERCONDUCTIVITY OF ALLOY AND COMPOUND SUPERCONDUCTORS Liu Long-jian (刘隆鉴), Zhang Zhuang-jian (章壮健) Chinese Physics, 2001, 10 (9):  847-852.  DOI: 10.1088/1009-1963/10/9/315 Abstract ( 1245 )   PDF (237KB) ( 575 )   By studying the average effect of the electronegativity of alloy and compound superconductors, some empirical laws are found. The average value of the electronegativity of alloy superconductors $\overline{\chi}$ is from 1.3 to 1.9, most of which is concentrated in the region $1.5<\overline{\chi}<1.8$. The $\overline{\chi}$ of many higher Tc superconductors is nearly 1.60. All these characteristics are similar to element superconductors. The average value of the electronegativity $\overline{\eta}$ of positive valence elements in oxide superconductors is from 1.3 to 1.6, and the average effect of an electronegativity of zero on every element $\overline{\zeta}$ is from 1.75 to 2.00. The mean of $\overline{\eta}$ and  $\overline{\zeta}$ is from 1.5 to 1.8. When both $\overline{\eta}$ and $\overline{\zeta}$ are near the limit value (1.3 or 1.6 and 1.75 or 2.00), the material has a very low Tc, even is non-superconductive. The $\overline{\eta}$ of other compound superconductors is either too high or too low, and the $\overline{\zeta}$ is too low so that the Tc of these superconductors is quite low. Studying the average effect of the electronegativity of non-superconducting compounds further confirms the laws.

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CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF La1.2Sr1.8-xCaxMn2O7 He Lun-hua (何伦华), Zhang Pan-lin (张泮霖), Yan Qi-wei (严启伟) Chinese Physics, 2001, 10 (9):  853-856.  DOI: 10.1088/1009-1963/10/9/316 Abstract ( 1392 )   PDF (214KB) ( 463 )   The effect of divalent cation substitution on the structure and magnetic properties of La1.2Sr1.8-xCaxMn2O7 (x = 0－0.900) is investigated in this paper. Partly replacing divalent cation Sr2+ by Ca2+ ions results in the weakening and then disappearance of long-range ferromagnetic ordering, and the formation of spin canting and low-temperature spin-glass. Based on structural analysis by Rietveld profile fitting, we suggest that this variation of magnetic property be related to a Jahn－Teller-type attice distortion of MnO6 octahedra due to the introduction of the smaller sized Ca2+ ions.

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THE STRUCTURE AND MAGNETIC PROPERTIES OF La2-2xSr1Ca2xMn2O7 (x=0.25－1.00) He Lun-hua (何伦华), Zhang Pan-lin (张泮霖), Yan Qi-wei (严启伟) Chinese Physics, 2001, 10 (9):  857-861.  DOI: 10.1088/1009-1963/10/9/317 Abstract ( 1136 )   PDF (238KB) ( 504 )   In this paper, the effect of divalent cation substitution on the structure and magnetic properties in La2-2xSr1Ca2xMn2O7 have been investigated systematically using bulk samples with a wide doping concentration range 0.25≤x≤1.00. Replacing trivalent La ions by divalent Ca ions results in the weakening and then disappearance of the long-range ferromagnetic (FM) ordering, the formation of spin canting, antiferromagnetic (AFM) ordering and low-temperature spin-glass. These results show that increasing the hole-doping concentration significantly suppresses the FM state. We suggest that this variation of magnetic properties is related to the competition of the FM and AFM interactions resulting from the change of Mn3+/Mn4+ ratio and Jahn-Teller-type lattice distortion of MnO6 octahedra due to the introduction of Ca2+ ions.

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ANISOTROPIC CHARACTERISTICS OF DEMAGNETIZATION CURVE FOR NANOCRYSTALLINE Nd-Fe-B MAGNET CALCULATED BY MICROMAGNETICS Jin Han-min (金汉民), Wang Xue-feng (王学凤), Zhao Su-fen (赵素芬), Yan Yu (闫羽) Chinese Physics, 2001, 10 (9):  862-868.  DOI: 10.1088/1009-1963/10/9/318 Abstract ( 1133 )   PDF (292KB) ( 802 )   The demagnetization curves for nanocrystalline Nd-Fe-B magnets of a stoichiometric composition were calculated by making use of the finite element technique of micromagnetics. The curve, especially iHc, varies in a wide range with the direction of applied field if the grain number N is taken to be small. With the increase of N, the range becomes smaller and the average of iHc decreases and approaches a limit iHc(N = ∞). iHc for finite N is larger than, or at least equal to, iHc(N = ∞). Jr/Js is weakly affected by N and the field direction. Jr/Js(N = ∞) decreases with the increase of grain size L. These are larger than the experimental values for the Nd-rich Nd2.33Fe14B1.06Si0.21 magnets by ～0.05. iHc(N = ∞) increases with the increase of L, and is close to or somewhat smaller than the experimental values of the Nd-rich magnet, as would be expected. In contrast, the curve calculated for the non-interacting grain system (Stoner-Wohlfarth model) of N ≥ 30 depends neither on the field direction nor on N.

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REMARKABLE IMPROVEMENT OF THE COERCIVITY OF TbMn6Sn6 COMPOUND BY MELT-SPINNING PROCESS Zhao Peng (赵鹏), Zhang Shao-ying (张绍英), Zhang Hong-wei (张宏伟), Yan A-ru (阎阿儒), Shen Bao-gen (沈保根) Chinese Physics, 2001, 10 (9):  869-873.  DOI: 10.1088/1009-1963/10/9/319 Abstract ( 1126 )   PDF (212KB) ( 563 )   The melt-spinning process has been carried out to improve the hard-magnetic properties of the TbMn6Sn6 compound. For the TbMn6Sn6 ribbons quenched at a rate of 40m/s and annealed at 545K for 30min, the highest coercivity of about 0.6T is achieved at room temperature, which is much higher than that of the TbMn6Sn6 ingot. Both the ingot and the ribbon coercivities will increase with decreasing temperature. For ribbons, a greater improvement of coercivity has been made at lower temperatures. Microstructural studies show the uniform nanocrystalline distribution in the TbMn6Sn6 ribbons and a small amount of Tb-rich phase in grain boundaries. The observed remarkable improvement of magnetic hardening in ribbons is believed to arise from the uniform nanoscale microstructure and the domain-wall pinning at the grain boundaries.

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TIME-RESOLVED PHOTOLUMINESCENCE OF SINTERED ZnO CERAMICS Wen Xiao-ming (文小明), N. Ohno, Zhang Zhong-ming (张中明) Chinese Physics, 2001, 10 (9):  874-876.  DOI: 10.1088/1009-1963/10/9/320 Abstract ( 978 )   PDF (198KB) ( 682 )   The time-resolved photoluminescence (TRPL) of sintered ZnO ceramics was measured at low temperatures. A broad luminescence band was observed in the visible region. The TRPL experiment shows that photoluminescence decay behaviour can be depicted as t-n(r). The decay rate n(r) and lifetime are wavelength dependent, and the former varies exponentially with wavelength. The power-lowering behaviour of the luminescence intensity indicates that the luminescence band originates from the recombination of donor-acceptor pairs.