2012年, 第58卷, 第06期　
刊出日期：2012-12-15

  

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  Stair and Step Soliton Solutions of the Integrable (2+1) and (3+1)-Dimensional Boiti-Leon-Manna-Pempinelli Equations

  M.T. Darvishi, M. Najafi, L. Kavitha, M. Venkatesh

  理论物理通讯. 2012, 58(06): 785-794.

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  The multiple exp-function method is a new approach to obtain multiple wave solutions of nonlinear partial differential equations (NLPDEs). By this method one can obtain multi-soliton solutions of NLPDEs. In this paper, using computer algebra systems, we apply the multiple exp-function method to construct the exact multiple wave solutions of a (2+1)-dimensional Boiti-Leon-Manna-Pempinelli equation. Also, we extend the equation to a (3+1)-dimensional case and obtain some exact solutions for the new equation by applying the multiple exp-function method. By these applications, we obtain single-wave, double-wave and multi-wave solutions for these equations.
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  Nonlocalization of Nonlocal Symmetry and Symmetry Reductions of the Burgers Equation

  金艳, 贾曼, 楼森岳

  理论物理通讯. 2012, 58(06): 795-799.

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  Symmetry reduction method is one of the best ways to find exact solutions. In this paper, we study the possibility of symmetry reductions of the well known Burgers equation including the nonlocal symmetry. The related new group invariant solutions are obtained. Especially, the interactions among solitons, Airy waves, and Kummer waves are explicitly given.
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  Spin Squeezing and Fixed-Point Bifurcation

  黄亦筱, 马健, 井晓幸, 王晓光

  理论物理通讯. 2012, 58(06): 800-806.

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  We study spin squeezing and classical bifurcation in a nonlinear bipartite system. We show that the spin squeezing can be associated with a fixed-point bifurcation in the classical dynamics, namely, it acts as an indicator of the classical bifurcation. For the ground state of a system with coupled giant spins, we find that the spin squeezing achieves its minimum value near the bifurcation point. We also study the dynamics of the spin squeezing, for an initial state corresponding to one of the fixed point, we find that in the stable regime, the spin squeezing exhibits periodic oscillation and always persists except at some fixed times, while in the unstable regime, the periodic oscillation phenomenon disappears and the spin squeezing survives for a short time. Finally, we show that the mean spin squeezing, which is defined to be averaged over time, attains its minimum value near the bifurcation point.
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  Spin and Pseudospin Symmetries of Dirac Equation and the Yukawa Potential as the Tensor Interaction

  H. Hassanabadi, E. Maghsoodi, S. Zarrinkamar

  理论物理通讯. 2012, 58(06): 807-814.

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  Considering of a tensor interaction in Dirac equation removes the degeneracy in spin and pseudospin doublets and consequently leads to results consistent with the experimental data. Here, instead of the commonly used Coulomb or linear terms, we investigate a tensor interaction of Yukawa form. We obtain arbitrary state solutions of Dirac equation under vector, scalar and tensor Yukawa potentials via a physical approximation and the Nikiforov-Uvarov methodology. The solutions are discussed in detail.
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  Morse Potential in the Momentum Representation

  孙国华, 董世海

  理论物理通讯. 2012, 58(06): 815-818.

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  The momentum representation of the Morse potential is presented analytically by hypergeometric function. The properties with respect to the momentum p and potential parameter β are studied. Note that |Ψ(p)| is a nodeless function and the mutual orthogonality of functions is ensured by the phase functions arg[Ψ(p)]. It is interesting to see that the |Ψ(p)| is symmetric with respect to the axis p=0 and the number of wave crest of |Ψ(p)| is equal to n+1. We also study the variation of |Ψ(p)| with respect to β. The amplitude of |Ψ(p)| first increases with the quantum number n and then deceases. Finally, we notice that the discontinuity in phase occurs at some points of the momentum p and the position and momentum probability densities are symmetric with respect to their arguments.
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  Generalized Conditional Entropies of Partitions on Quantum Logic

  Alexey E. Rastegin

  理论物理通讯. 2012, 58(06): 819-822.

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  For partitions on quantum logic, the R閚yi and Tsallis conditional entropies are introduced. Several relations between the conditional entropies of such partitions are derived.
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  Generation of a Genuine Six-Atom Entangled State in Cavity QED

  李渊华, 王贤平, 桑明煌, 聂义友

  理论物理通讯. 2012, 58(06): 823-826.

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  We propose a potential scheme to generate a genuine six-atom entangled state [J. Phys. A 42 (2009) 415301] by using atoms in cavity QED system, where the atoms interact simultaneously with the highly detuned single-mode cavity and the strong classical driving field. Thus our approach is insensitive to both the cavity decay and thermal field.
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  Mathieu Equation and Elliptic Curve

  贺伟, 缪炎刚

  理论物理通讯. 2012, 58(06): 827-834.

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  We present a relation between the Mathieu equation and a particular elliptic curve. We find that the Floquet exponent of the Mathieu equation, for both q<<1 and q>>1, can be obtained from the integral of a differential one form along the two homology cycles of the elliptic curve. Certain higher order differential operators are needed to generate the WKB expansion. We make a few conjectures about the general structure of these differential operators.
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  Quantum Communication Through a Two-Dimensional Spin Network

  王兆明, 顾永建

  理论物理通讯. 2012, 58(06): 835-839.

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  We investigate the state or entanglement transfer through a two-dimensional spin network. We show that for state transfer, better fidelity can be gained along the diagonal direction but for entanglement transfer, when the initial entanglement is created along the boundary, the concurrence is more inclined to propagate along the boundary. This behavior is produced by quantum mechanical interference and the communication quality depends on the precise size of the network. For some number of sites, the fidelity in a two-dimensional channel is higher than one-dimensional case. This is an important result for realizing quantum communication through high dimension spin chain networks.
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  Opto-Mechanical Effects in Superradiant Light Scattering by Bose-Einstein Condensate in an Optical Cavity

  Aranya B. Bhattacherjee

  理论物理通讯. 2012, 58(06): 840-846.

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  We investigate the effects of a movable mirror (cantilever) of an optical cavity on the superradiant light scattering from a Bose-Einstein condensate (BEC) in an optical lattice. We show that the mirror motion has a dynamic dispersive effect on the cavity-pump detuning. Varying the intensity of the pump beam, one can switch between the pure superradiant regime and the Bragg scattering regime. The mechanical frequency of the mirror strongly influences the time interval between two Bragg peaks. We find that when the system is in the resolved side band regime for mirror cooling, the superradiant scattering is enhanced due to coherent energy transfer from the mechanical mirror mode to the cavity field mode.
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  Modeling and Simulation for Urban Rail Traffic Problem Based on Cellular Automata

  许琰, 曹成铉, 李明华, 罗金龙

  理论物理通讯. 2012, 58(06): 847-855.

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  Based on the Nagel-Schreckenberg model, we propose a new cellular automata model to simulate the urban rail traffic flow under moving block system and present a new minimum instantaneous distance formula under pure moving block. We also analyze the characteristics of the urban rail traffic flow under the influence of train density, station dwell times, the length of train, and the train velocity. Train delays can be decreased effectively through flexible departure intervals according to the preceding train type before its departure. The results demonstrate that a suitable adjustment of the current train velocity based on the following train velocity can greatly shorten the minimum departure intervals and then increase the capacity of rail transit.
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  Controllability of Discrete-Time Multi-Agent Systems with Multiple Leaders on Fixed Networks

  刘波, 胡卫娜, 张杰, 苏厚胜

  理论物理通讯. 2012, 58(06): 856-862.

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  This paper investigates controllability of discrete-time multi-agent systems with multiple leaders on fixed networks. The leaders are particular agents playing a part in external inputs to steer other member agents. The followers can arrive at any predetermined configuration by regulating the behaviors of the leaders. Some sufficient and necessary conditions are proposed for the controllability of discrete-time multi-agent systems with multiple leaders. Moreover, the case with isolated agents is discussed. Numerical examples and simulations are proposed to illustrate the theoretical results we established.
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  Electron-Positron Pair Production in an Arbitrary Polarized Ultrastrong Laser Field

  何良勇, 谢柏松, 郭新恒, 王虹宇

  理论物理通讯. 2012, 58(06): 863-871.

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  Electron-positron pair production in an arbitrary polarized ultrastrong laser field is investigated in the first order perturbation approximation in which the Volkov states are used for convenient calculation of scattering amplitude and cross section. It is found surprisingly that the optimal pair production depends strongly on the polarization. For some cases of field parameters, the optimal field is elliptically polarized or evenly circularly polarized one, rather than the usual linear polarization as indicated by previous works. Some insights into pair generation are given and some interesting unexpected features are also discussed briefly.
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  Improved Light-Cone QCD Sum Rule Analysis of Rare Decays Λ_b → Λ_γ and Λ_b → Λl⁺l^-

  甘龙飞, 刘永录, 陈文博, 黄明球

  理论物理通讯. 2012, 58(06): 872-882.

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  We present a systematic light-cone QCD sum rule study of the exclusive rare radiative decay Λ_b → Λ_γ and rare semileptonic decay Λ_b → Λl⁺l^- within the framework of the standard model. Although some light-cone sum rule (LCSR) studies on these rare processes can be found in different literatures, it is necessary to reanalyze them systematically for the reason that either the baryonic distribution amplitudes are improved or different interpolating currents for the Λ_b baryon may lead to quite different results. In addition, the rare process Λ_b → Λ_γ has not yet been analyzed by LCSR with the Ioffe-type current. Taking all these reasons into account, we perform LCSR calculations of both the processes with two types of interpolating currents. Our calculations show that the choice of the interpolating current for the Λ_b baryon can affect the predictions significantly, especially for the rare radiative decay process.
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  Decentralized Cooperation Strategies in Two-Dimensional Traffic of Cellular Automata

  方峻, 覃征, 陈喜群, 冷彪, 徐朝晖, 江子能

  理论物理通讯. 2012, 58(06): 883-890.

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  We study the two-dimensional traffic of cellular automata using computer simulation. We propose two type of decentralized cooperation strategies, which are called stepping aside (CS-SA) and choosing alternative routes (CS-CAR) respectively. We introduce them into an existing two-dimensional cellular automata (CA) model. CS-SA is designed to prohibit a kind of ping-pong jump when two objects standing together try to move in opposite directions. CS-CAR is designed to change the solution of conflict in parallel update. CS-CAR encourages the objects involved in parallel conflicts choose their alternative routes instead of waiting. We also combine the two cooperation strategies (CS-SA-CAR) to test their combined effects. It is found that the system keeps on a partial jam phase with nonzero velocity and flow until the density reaches one. The ratios of the ping-pong jump and the waiting objects involved in conflict are decreased obviously, especially at the free phase. And the average flow is improved by the three cooperation strategies. Although the average travel time is lengthened a bit by CS-CAR, it is shorten by CS-SA and CS-SA-CAR. In addition, we discuss the advantage and applicability of decentralized cooperation modeling.
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  Diffusive Thermal Conductivity of Superfluid Fermi Gas in p-Wave State at Low Temperatures

  M. Khademi Dehkordi, S. Nasirimoghadam, F. Nabipoor, M.A. Shahzamanian

  理论物理通讯. 2012, 58(06): 891-894.

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  The diffusive thermal conductivity tensor of p-wave superfluid at low temperatures is calculated by using the Boltzmann equation approach. We use the Sykes and Brooker procedure and show that K_xx is equal to K_yy and these are related to T^-1, also K_zz is proporated to T^-3.
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  Diffusion-Limited Aggregation with Polygon Particles

  邓礼, 王延颋, 欧阳钟灿

  理论物理通讯. 2012, 58(06): 895-901.

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  Diffusion-limited aggregation (DLA) assumes that particles perform pure random walk at a finite temperature and aggregate when they come close enough and stick together. Although it is well known that DLA in two dimensions results in a ramified fractal structure, how the particle shape influences the formed morphology is still unclear. In this work, we perform the off-lattice two-dimensional DLA simulations with different particle shapes of triangle, quadrangle, pentagon, hexagon, and octagon, respectively, and compare with the results for circular particles. Our results indicate that different particle shapes only change the local structure, but have no effects on the global structure of the formed fractal cluster. The local compactness decreases as the number of polygon edges increases.
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  Near-Origin Structure of the Hooke's Atoms

  王雪梅

  理论物理通讯. 2012, 58(06): 902-908.

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  The Hooke's atoms with two or more than two electrons give rise to an interesting quantum mechanical model with valuable practical applications. In this work, we study the electronic properties near the origin of the harmonic potential. It is seen that the spherically averaged density, ρ, exhibits an interesting character — it has only even order terms in its small r expansion. The spherical average of the Hartree potential, v_H, and the spherical average of the Kohn-Sham exchange-correlation potential, v_xc, are also shown to have the same property — all odd order terms in their expansions vanish. Furthermore, the analysis and results extend also to the case of two-dimensional models. While only models interacting via. the Coulomb potential are primarily considered in the article, the results also extend to models interacting via. other potentials (viz. Van der Waals potential).
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  Spinor Boltzmann Equation with Two Momenta at the Fermi Level

  王正川

  理论物理通讯. 2012, 58(06): 909-914.

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  Based on the formalism of Keldysh's nonequilibrium Green function, we establish a two momenta spinor Boltzmann equation for longitudinal scalar distribution function and transverse vector distribution function. The longitudinal charge currents, transverse spin currents and the continuity equations satisfied by them are then studied, it indicates that both the charge currents and spin currents decay oscillately along with position, which is due to the momenta integral over the Fermi surface. We also compare our charge currents and spin currents with the corresponding results of one momentum spinor Boltzmann equation, the differences are obvious.
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  Influence of Mg Composition on Optical Properties of Exciton Confinement in Strained Wurtzite ZnO/Mg_xZn_1-xO Cylindrical Quantum Dots

  郑冬梅, 王宗篪

  理论物理通讯. 2012, 58(06): 915-922.

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  Based on the effective-mass approximation and variational approach, excitonic optical properties are investigated theoretically in strained wurtzite (WZ) ZnO/Mg_xZn_1-xO cylindrical quantum dots (QDs) for four different Mg compositions: x=0.08, 0.14, 0.25, and 0.33, with considering a three-dimensional carrier confinement in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The ground-state exciton binding energy, the interband emission wavelength, and the radiative lifetime as functions of the QD structural parameters (height and radius) are calculated in detail. The computations are performed in the case of finite band offset. Numerical results elucidate that Mg composition has a significant influence on the exciton states and optical properties of ZnO/Mg_xZn_1-xO QDs. The ground-state exciton binding energy increases with increasing Mg composition and the increment tendency is more prominent for small height QDs. As Mg composition increases, the interband emission wavelength has a blue-shift if the dot height L<3.5 nm, but the interband emission wavelength has a red-shift when L>3.5 nm. Furthermore, the radiative lifetime increases rapidly with increasing Mg composition if the dot height L>3 nm and the increment tendency is more prominent for large height QDs. The physical reason has been analyzed in depth.