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Chin. Phys. B  
  Chin. Phys. B--2006, Vol.15, No.9
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Iterative solutions for low lying excited states of a class of Schr?dinger equation Hot!

Friedberg R., Lee T. D., Zhao Wei-Qin
Chin. Phys. B 2006, 15 (9): 01909;  doi: 10.1088/1009-1963/15/9/001
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The convergent iterative procedure for solving the groundstate Schr?dinger equation is extended to derive the excitation energy and the wavefunction of the low-lying excited states. The method is applied to the one-dimensional quartic potential problem. The results show that the iterative solution converges rapidly when the coupling g is not too small.

Relative entropy as a measure of entanglement for Gaussian states

Lu Huai-Xin, Zhao Bo
Chin. Phys. B 2006, 15 (9): 01914;  doi: 10.1088/1009-1963/15/9/002
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In this paper, we derive an explicit analytic expression of the relative entropy between two general Gaussian states. In the restriction of the set for Gaussian states and with the help of relative entropy formula and Peres--Simon separability criterion, one can conveniently obtain the relative entropy entanglement for Gaussian states. As an example, the relative entanglement for a two-mode squeezed thermal state has been obtained.

The quadratic-form identity for constructing Hamiltonian structures of the Guo hierarchy

Dong Huan-He, Zhang Ning
Chin. Phys. B 2006, 15 (9): 01919;  doi: 10.1088/1009-1963/15/9/003
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The trace identity is extended to the quadratic-form identity. The Hamiltonian structures of the multi-component Guo hierarchy, integrable coupling of Guo hierarchy and (2+1)-dimensional Guo hierarchy are obtained by the quadratic-form identity. The method can be used to produce the Hamiltonian structures of the other integrable couplings or multi-component hierarchies.

Homotopic method of solving a class of E1 Ni?o/La Ni?a-Southern Oscillation sea--air oscillator

Mo Jia-Qi, Wang Hui, Lin Wan-Tao
Chin. Phys. B 2006, 15 (9): 01927;  doi: 10.1088/1009-1963/15/9/004
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The EI Ni?/La Ni?a-Southern Oscillation (ENSO) is an interannual phenomenon involved in the tropical Pacific Ocean--atmosphere interactions. In this paper, an asymptotic method of solving the nonlinear equation for the ENSO model is created. And based on a class of oscillator of the ENSO model, the approximate solution of a corresponding problem is studied by employing the method of homotopic mapping. It is proved from the results that the homotopic method can be used for analysing the sea surface temperature anomaly in the equatorial eastern Pacific and the thermocline depth anomaly of the atmosphere--ocean oscillation for the ENSO model.

Stability with respect to partial variables for Birkhoff systems

Mei Feng-Xiang, Wu Hui-Bin, Shang Mei, Zhang Yong-Fa
Chin. Phys. B 2006, 15 (9): 01932;  doi: 10.1088/1009-1963/15/9/005
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In this paper, the stability with respect to partial variables for the Birkhoff system is studied. By transplanting the results of the partial stability for general systems to the Birkhoff system and constructing a suitable Liapunov function, the partial stability of the system can be achieved. Finally, two examples are given to illustrate the application of the results.

A new type of adiabatic invariants for nonconservative systems of generalized classical mechanics

Zhang Yi
Chin. Phys. B 2006, 15 (9): 01935;  doi: 10.1088/1009-1963/15/9/006
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The perturbations to symmetries and adiabatic invariants for nonconservative systems of generalized classical mechanics are studied. The exact invariant in the form of Hojman from a particular Lie symmetry for an undisturbed system of generalized mechanics is given. Based on the concept of high-order adiabatic invariant in generalized mechanics, the perturbation to Lie symmetry for the system under the action of small disturbance is investigated, and a new adiabatic invariant for the nonconservative system of generalized classical mechanics is obtained, which can be called the Hojman adiabatic invariant. An example is also given to illustrate the application of the results.

Linear optical implementation of optimal unambiguous discrimination among quantum states

Lu Jing, Zhou Lan, Kuang Le-Man
Chin. Phys. B 2006, 15 (9): 01941;  doi: 10.1088/1009-1963/15/9/007
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In this paper, we present a linear optical scheme for optimal unambiguous discrimination among nonorthogonal quantum states in terms of the multiple-rail and polarization representation of a single photon. In our scheme, discriminated quantum states are expressed by using the spatial degree of freedom of a single photon while the polarization degree of freedom of the single photon is used to act as an auxiliary qubit. The optical components used in our scheme are only passive linear optical elements such as polarizing beam splitters, wave plates, polarizers, single photon detectors, and single photon source.

Dissipative dynamics of dark solitons in elongated Bose--Einstein condensates

Tang Zheng-Hua, Yan Jia-Ren, Liu Ling-Hong
Chin. Phys. B 2006, 15 (9): 01947;  doi: 10.1088/1009-1963/15/9/008
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The dissipative dynamic stability is investigated of dark solitons in elongated Bose--Einstein condensates that can be described by the Gross--Pitaevskii equation including an additional term. Based on the direct perturbation theory for the nonlinear Schr?dinger equation, the dependence of the soliton velocity on time is explicitly given, and the shape of dark solitons remaining unchanged under the dissipative condition is confirmed theoretically for the first time. It is found that the dynamically stable dark solitons turn out to be thermodynamically unstable.

Influence of quantum degeneracy on the performance of a gas Stirling engine cycle

He Ji-Zhou, Mao Zhi-Yuan, Wang Jian-Hui
Chin. Phys. B 2006, 15 (9): 01953;  doi: 10.1088/1009-1963/15/9/009
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Based on the state equation of an ideal quantum gas, the regenerative loss of a Stirling engine cycle working with an ideal quantum gas is calculated. Thermal efficiency of the cycle is derived. Furthermore, under the condition of quantum degeneracy, several special thermal efficiencies are discussed. Ratios of thermal efficiencies versus the temperature ratio and volume ratio of the cycle are made. It is found that the thermal efficiency of the cycle not only depends on high and low temperatures but also on maximum and minimum volumes. In a classical gas state the thermal efficiency of the cycle is equal to that of the Carnot cycle. In an ideal quantum gas state the thermal efficiency of the cycle is smaller than that of the Carnot cycle. This will be significant for deeper understanding of the gas Stirling engine cycle.

Variational Monte Carlo analysis of Bose--Einstein condensation in a two-dimensional trap

Zheng Rong-Jie, Jin Jing, Tang Yi
Chin. Phys. B 2006, 15 (9): 01960;  doi: 10.1088/1009-1963/15/9/010
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The ground-state properties of a system with a small number of interacting bosons over a wide range of densities are investigated. The system is confined in a two-dimensional isotropic harmonic trap, where the interaction between bosons is treated as a hard-core potential. By using variational Monte Carlo method, we diagonalize the one-body density matrix of the system to obtain the ground-state energy, condensate wavefunction and the condensate fraction. We find that in the dilute limit the depletion of central condensate in the 2D system is larger than in a 3D system for the same interaction strength; however as the density increases, the depletion at the centre of 2D trap will be equal to or even lower than that at the centre of 3D trap, which is in agreement with the anticipated in Thomas--Fermi approximation. In addition, in the 2D system the total condensate depletion is still larger than in a 3D system for the same scattering length.

Controlling projective synchronization in coupled chaotic systems

Zou Yan-Li, Zhu Jie
Chin. Phys. B 2006, 15 (9): 01965;  doi: 10.1088/1009-1963/15/9/011
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In this paper, a new method for controlling projective synchronization in coupled chaotic systems is presented. The control method is based on a partially linear decomposition and negative feedback of state errors. Firstly, the synchronizability of the proposed projective synchronization control method is proved mathematically. Then, three different representative examples are discussed to verify the correctness and effectiveness of the proposed control method.

Synchronization of hyperchaotic Lorenz system based on passive control

Wang Fa-Qiang, Liu Chong-Xin
Chin. Phys. B 2006, 15 (9): 01971;  doi: 10.1088/1009-1963/15/9/012
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Synchronization of a hyperchaotic Lorenz system is discussed using passive control. Based on the properties of a passive system, a passive controller is designed and the synchronization between two hyperchaotic Lorenz systems under different initial conditions is realized. Simulation results show the proposed synchronization method to be effective.

The CP1 nonlinear sigma model with Chern--Simons term in the Faddeev--Jachiw quantization formalism

Wang Yong-Long, Li Zi-Ping
Chin. Phys. B 2006, 15 (9): 01976;  doi: 10.1088/1009-1963/15/9/013
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Using the Faddeev--Jackiw (FJ) quantization method,this paper treats the CP1 nonlinear sigma model with Chern--Simons term. The generalized FJ brackets are obtained in the framework of this quantization method, which agree with the results obtained by using the Dirac's method.

Potential energy curves and analytical potential energy functions of the metastable states of B22+

Zhang Xiao-Yan, Yang Chuan-Lu, Gao Feng, Ren Ting-Qi
Chin. Phys. B 2006, 15 (9): 01981;  doi: 10.1088/1009-1963/15/9/014
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The multi-reference configuration interaction method and aug-cc-pvqz (AVQZ) have been used to calculate potential energy curves (PECs) of the singlet and triplet states of the $\prod _{\rm u}$ and $\prod _{\rm g}$ symmetry of B$_{2}^{ + + }$. All of the four states (1!\prod _{\rm u}$, $^{ 1\!}\prod _{\rm g}$, $^{ 3\!}\prod _{\rm u }$ and $^{3\!}\prod _{\rm g})$ are found to be metastable states, though the potential well of $^{3\!}\prod _{\rm u }$ symmetry is very shallow. Based on the PECs, the analytical potential energy functions (APEFs) of these states have been fitted using the least square fitting method and two models of function. The spectroscopic parameters of each state are also calculated, and are compared with other investigations in the literature. The credibility and veracity of the two functions are evaluated. Some ideas to improve the fitting accuracy are presented. Also the vibrational levels for each state are predicted by solving the Schr\"{o}dinger equation of nuclear motion.

Forward--backward multiplicity correlations in 4.5 A GeV/c 16O--emulsion interactions

Zhang Dong-Hai, Zhao Hui-Hua, Liu Fang, He Chun-Le, Jia Hui-Ming, Li Xue-Qin, Li Zhen-Yu, Li Jun-Sheng
Chin. Phys. B 2006, 15 (9): 01987;  doi: 10.1088/1009-1963/15/9/015
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A detailed study of the mechanisms of the emissions of pions and protons in the forward and backward hemispheres in 4.5 A GeV/c oxygen-emulsion interactions has been carried out. The correlations between the multiplicities of secondary charged particles in the backward and forward hemispheres are investigated.

Photoionization of NaK molecule with a double-well potential in femtosecond pump--probe pulse laser fields

Yu Jie, Wang Sen-Ming, Yuan Kai-Jun, Cong Shu-Lin
Chin. Phys. B 2006, 15 (9): 01996;  doi: 10.1088/1009-1963/15/9/016
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The method of time-dependent quantum wave packet dynamics is used to calculate the femtosecond pump--probe photoelectron spectra and study the wave packet dynamic processes of the double-minimum potential state 61+ of NaK in intense laser fields. The evolutions of the wave packet and the photoelectron energy spectra with time and internuclear distance are described in detail. The wave packet dynamic information of the 61+ state can be extracted from the photoelectron energy spectra.

Resolution and noise in ghost imaging with classical thermal light

Cheng Jing, Han Shen-Sheng, Yan Yi-Jing
Chin. Phys. B 2006, 15 (9): 02002;  doi: 10.1088/1009-1963/15/9/017
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The resolution and classical noise in ghost imaging with a classical thermal light are investigated theoretically. For ghost imaging with a Gaussian Schell model source, the dependences of the resolution and noise on the spatial coherence of the source and the aperture in the imaging system are discussed and demonstrated by using numerical simulations. The results show that an incoherent source and a large aperture will lead to a good image quality and small noise.

The behaviours of optical novelty filter based on bacteriorhodopsin film

Chen Gui-Ying, Yuan Yi-Zhe, Liang Xin, Xu Tang, Zhang Chun-Ping, Song Qi-Wang
Chin. Phys. B 2006, 15 (9): 02007;  doi: 10.1088/1009-1963/15/9/018
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The quality of the novelty filter image is investigated at different intensities of the incident blue and yellow beams irradiating a bacteriorhodopsin (bR) film. The relationship between the transmitted blue beams and the incident yellow beams is established. The results show that the contrast of the novelty filter image depends on the lifetime of longest lived photochemical state (M state). These results enable one to identify the direction of a moving object and to improve the quality of the novel filter image by prolonging the lifetime of M state.

Entanglement in a system of two two-level atoms interacting with a single-mode field

Jin Li-Juan, Fang Mao-Fa
Chin. Phys. B 2006, 15 (9): 02012;  doi: 10.1088/1009-1963/15/9/019
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We investigate the entanglement in a system of two coupling atoms interacting with a single-mode field by means of quantum information entropy theory. The quantum entanglement between the two atoms and the coherent field is discussed by using the quantum reduced entropy, and the entanglement between the two coupling atoms is also investigated by using the quantum relative entropy. In addition, the influences of the atomic dipole--dipole interaction intensity and the average photon number of the coherent field on the degree of the entanglement is examined. The results show that the evolution of the degree of entanglement between the two atoms and the field is just opposite to that of the degree of entanglement between the two atoms. And the properties of the quantum entanglement in the system rely on the atomic dipole--dipole interaction and the average photon number of the coherent field.

Efficient and high-power laser-diode single-end-pumped Nd:YVO4 continuous wave laser at 1342nm

Zhang Yu-Ping, Zheng Yi, Zhang Hui-Yun, Wang Peng, Yao Jian-Quan
Chin. Phys. B 2006, 15 (9): 02018;  doi: 10.1088/1009-1963/15/9/020
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A compact, efficient and high-power laser diode (LD) single-end-pumped Nd:YVO4 laser with continuous-wave emission at 1342 nm is reported. With a single crystal single-end-pumped by fibre-coupled LD array, an output power of 7.36W is obtained from the laser cavity of concave-convex shape, corresponding to an optical-to-optical efficiency of 32.8%. The laser is operated in TEM00 mode with small rms amplitude noise of 0.3%. The influences of the Nd concentration, transmissivity of the output mirror and the cavity length on the output power have been studied experimentally.

Self-starting mode-locked picosecond Ti:sapphire laser by using of a fast SESAM

Zhu Jiang-Feng, Tian Jin-Rong, Wang Peng, Ling Wei-Jun, Li De-Hua, Wei Zhi-Yi
Chin. Phys. B 2006, 15 (9): 02022;  doi: 10.1088/1009-1963/15/9/021
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A stable continuous wave mode-locked picosecond Ti:sapphire laser by using a fast semiconductor saturable absorber mirror (SESAM) is demonstrated. The laser delivers pulse width of 20 ps at a central wavelength of 813 nm and a repetition rate of 100 MHz. The maximum output power is 1.34 W with pump power of 7 W which corresponds to an optical--optical conversion efficiency of 19.1%.

Generation of third harmonic emission in propagation of femtosecond laser pulses in air

Xi Ting-Ting, Zhang Jie, Lu Xin, Hao Zuo-Qiang, Yang Hui, Dong Quan-Li, Wu Hui-Chun
Chin. Phys. B 2006, 15 (9): 02025;  doi: 10.1088/1009-1963/15/9/022
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The main characteristics of the third harmonic emission generated by femtosecond laser pulses propagating in air are investigated by numerically solving the coupled nonlinear Schr?dinger equations. Strong third harmonic emission is observed with a maximum conversion efficiency as high as 0.43%. The on-axis phase difference between fundamental beam and third harmonic is investigated. The result is in good agreement with the phase-locking mechanism. Dependence of the conversion of third harmonic emission on focusing conditions is also studied. The results are also compared with those of experiments.

Ultrasonic study on organic liquid and binary organic liquid mixtures by using Schaaffs' collision factor theory

Lu Yi-Gang, Dong Yan-Wu
Chin. Phys. B 2006, 15 (9): 02030;  doi: 10.1088/1009-1963/15/9/023
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Based on Schaaff's collision factor theory (CFT) in liquids, the equations for nonlinear ultrasonic parameters in both organic liquid and binary organic liquid mixtures are deduced. The nonlinear ultrasonic parameters, including pressure coefficient, temperature coefficients of ultrasonic velocity, and nonlinear acoustic parameter B/A in both organic liquid and binary organic liquid mixtures, are evaluated for comparison with the measured results and data from other sources. The equations show that the coefficient of ultrasonic velocity and nonlinear acoustic parameter B/A are closely related to molecular interactions. These nonlinear ultrasonic parameters reflect some information of internal structure and outside status of the medium or mixtures. From the exponent of repulsive forces of the molecules, several thermodynamic parameters, pressure and temperature of the medium, the nonlinear ultrasonic parameters and ultrasonic nature of the medium can be evaluated. When evaluating and studying nonlinear acoustic parameter B/A of binary organic liquid mixtures, there is no need to know the nonlinear acoustic parameter B/A of the components. Obviously, the equation reveals the connection between the nonlinear ultrasonic nature and internal structure and outside status of the mixtures more directly and distinctly than traditional mixture law for B/A, e.g. Apfel's and Sehgal's laws for liquid binary mixtures.

Intensity tuning characteristics of double-mode He--Ne laser with optical feedback

Mao Wei, Zhang Shu-Lian, Fei Li-Gang
Chin. Phys. B 2006, 15 (9): 02036;  doi: 10.1088/1009-1963/15/9/024
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This paper investigates the intensity tuning characteristics of a double longitudinal modes He--Ne laser subjected to optical feedback. The intensity undulations of the total light and the two modes are observed for different external cavity length. Two modulations of the internal cavity length are performed. One is only for the internal cavity length being modulated and the other is for both the internal and the external cavity length being modulated. The undulation frequency of the total light is found to be determined by the ratio of external cavity length to internal cavity length in both modulations. When the external cavity length is integral times of the internal cavity length, the fringe frequency of the total light could be seven or even more times of that in conventional optical feedback. A simple theoretical analysis is presented, which is in good agreement with the experimental results. The potential use of the experimental results is also discussed.

Analysis of the axial electric field in a plasma-loaded-helix travelling wave tube

Xie Hong-Quan, Liu Pu-Kun
Chin. Phys. B 2006, 15 (9): 02042;  doi: 10.1088/1009-1963/15/9/025
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A helix type slow wave structure filled with plasma is immersed in a strong longitudinal magnetic field. Taking into account the effect of the plasma and the dielectric, the system is separated radially into three regions. By means of the sheath model and Maxwell equation, the distribution of the electromagnetic field is established. Using the boundary conditions of each region, the dispersion relation of the slow wave structure is derived. The trend of change for the radial profile of the axial electric field is analysed respectively in different plasma densities, plasma column radius and dielectric constant by numerical computation. Some useful results are obtained on the basis of the discussion.

Large amplitude electromagnetic solitons in intense laser plasma interaction

Li Bai-Wen, S Ishiguro, M M Skoric
Chin. Phys. B 2006, 15 (9): 02046;  doi: 10.1088/1009-1963/15/9/026
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This paper shows that the standing, backward- and forward-accelerated large amplitude relativistic electromagnetic solitons induced by intense laser pulse in long underdense collisionless homogeneous plasmas can be observed by particle simulations. In addition to the inhomogeneity of the plasma density, the acceleration of the solitons also depends upon not only the laser amplitude but also the plasma length. The electromagnetic frequency of the solitons is between about half and one of the unperturbed electron plasma frequency. The electrostatic field inside the soliton has a one-cycle structure in space, while the transverse electric and magnetic fields have half-cycle and one-cycle structure respectively. Analytical estimates for the existence of the solitons and their electromagnetic frequencies qualitatively coincide with our simulation results.

Disruption mitigation using laser ablation of high-Z impurities in HL-1M tokamak

Zheng Yong-Zhen, Feng Xing-Ya, Guo Gan-Cheng, Xu De-Ming, Zheng Yin-Jia
Chin. Phys. B 2006, 15 (9): 02053;  doi: 10.1088/1009-1963/15/9/027
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A preliminary experiment triggering a plasma current quench by laser ablation of high-Z impurities has been performed in the HL-1M tokamak. The injection of impurities with higher electric charges into tokamak plasmas can increase the radiation cooling of the plasma. Resistive, highly radiating plasma formed prior to the thermal quench can dissipate both the thermal and magnetic energies, which is possibly a simple and potential approach to reducing significantly the plasma thermal energy and magnetic energy before a disruption thereby a safe plasma termination is obtained.

X-ray observations of tungsten wire array Z-pinch implosions on QiangGuang-1 facility

Zhang Fa-Qiang, Li Zheng-Hong, Xu Ze-Ping, Xu Rong-Kun, Yang Jian-Lun, Guo Cun, Xia Guang-Xin, Chen Jin-Chuan, Song Feng-Jun, Ning Jia-Min, Wang Zhen, Xue Fei-Biao, Li Lin-Bo, Qin Yi, Ying Chun-Tong,
Chin. Phys. B 2006, 15 (9): 02058;  doi: 10.1088/1009-1963/15/9/028
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Z-pinch experiments with two arrays consisting, respectively, of 32 4-μm- and 6-μm-diameter tungsten wires have been carried out on QiangGuang-1 facility with a current rising up to 1.5MA in 80ns. At early time of implosion, x-ray framing images show that the initial emission comes from the central part of arrays, and double clear emission rings, drifting to the anode and the cathode at 5×106cm/s and 2.4×107cm/s respectively, are often produced near the electrodes. Later, in a 4-μm-diameter tungsten wire array, filamentation caused by ohmic heating is prominent, and more than ten filaments have been observed. A radial inward shift of arrays starts at about 30\,ns earlier than the occurrence of the x-ray peak power for both kinds of arrays, and the shrinkage rate of emission region is as high as 1.7×107cm/s in a 4-μm-diameter tungsten wire array, which is two times higher than that in a 6-μm one. Emission from precursor plasmas is observed in implosion of 6-μm-diameter tungsten wire arrays, but not in implosion of a 4-μm-diameter tungsten wire array. Whereas, in a 4-μm-diameter tungsten wire array, the soft x-ray emission shows the growth of m=1 instability in the plasma column, which is caused by current. The reasons for the discrepancy between implosions of 4-μm- and 6-μm-diameter tungsten wire arrays are explained.

Group-theoretical method for physical property tensors of quasicrystals

Gong Ping, Hu Cheng-Zheng, Zhou Xiang, Wang Ai-Jun, Miao Ling
Chin. Phys. B 2006, 15 (9): 02065;  doi: 10.1088/1009-1963/15/9/029
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In addition to the phonon variable there is the phason variable in hydrodynamics for quasicrystals. These two kinds of hydrodynamic variables have different transformation properties. The phonon variable transforms under the vector representation, whereas the phason variable transforms under another related representation. Thus, a basis (or a set of basis functions) in the representation space should include such two kinds of variables. This makes it more difficult to determine the physical property tensors of quasicrystals. In this paper the group-theoretical method is given to determine the physical property tensors of quasicrystals. As an illustration of this method we calculate the third-order elasticity tensors of quasicrystals with five-fold symmetry by means of basis functions. It follows that the linear phonon elasticity is isotropic, but the nonlinear phonon elasticity is anisotropic for pentagonal quasicrystals. Meanwhile, the basis functions are constructed for all noncrystallographic point groups of quasicrystals.

Seed-mediated growth of gold nanoparticles using self-assembled monolayer of polystyrene microspheres as nanotemplate arrays

Xiang Yan-Juan, Wu Xiao-Chun, Liu Dong-Fang, Zhang Zeng-Xing, Song Li, Zhao Xiao-Wei, Liu Li-Feng, Luo Shu-Dong, Ma Wen-Jun, Shen Jun, Zhou Wei-Ya, Zhou Jian-Jun, Wang Chao-Ying, Wang Gang
Chin. Phys. B 2006, 15 (9): 02080;  doi: 10.1088/1009-1963/15/9/030
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Arrays of noble metal nanoparticles show potential applications in (bio-)sensing, optical storage, surface-enhanced spectroscopy, and waveguides. For all such potential devices, controlling the size, morphology, and interparticle spacing of the nanoparticles is very important. Here, we combine seed-mediated growth with nanosphere lithography to study the controllable growth of gold nanoparticles (Au NPs), in which the self-assembly monolayer of polystyrene (PS) on a silicon surface is used to guide the modification of alkanesilanes and the subsequent adsorption of gold seeds; seed-mediated growth is applied to controlling the morphology and size of Au NPs. The size of adsorption region (determining the number of adsorbed gold seeds) is controlled by etching PS microspheres with oxygen plasma or annealing PS microspheres at the glass transition temperature. The size and morphology of the Au NPs are controlled by changing growth conditions. In such a way, we have achieved the dual control of the obtained Au NPs. Preliminary results show that this strategy holds a great promise. This approach can also be extended to a wide range of materials and substrates.

Molecular dynamics study of mosaic structure in the Ni-based single-crystal superalloy

Zhu Tao, Wang Chong-Yu
Chin. Phys. B 2006, 15 (9): 02087;  doi: 10.1088/1009-1963/15/9/031
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The mosaic structure in a Ni-based single-crystal superalloy is simulated by molecular dynamics using a potential employed in a modified analytic embedded atom method. From the calculated results we find that a closed three-dimensional misfit dislocation network, with index of $\langle 011\rangle${\{}100{\}} and the side length of the mesh 89.6\,{\AA}, is formed around a cuboidal $\gamma '$ precipitate. Comparing the simulation results of the different mosaic models, we find that the side length of the mesh only depends on the lattice parameters of the $\gamma $ and $\gamma '$ phases as well as the $\gamma $/$\gamma '$ interface direction, but is independent of the size and number of the cuboidal $\gamma '$ precipitate. The density of dislocations is inversely proportional to the size of the cuboidal $\gamma '$ precipitate, i.e.~the amount of the dislocation is proportional to the total area of the $\gamma $/$\gamma '$ interface, which may be used to explain the relation between the amount of the fine $\gamma '$ particles and the creep rupture life of the superalloy. In addition, the closed three-dimensional networks assembled with the misfit dislocations can play a significant role in improving the mechanical properties of superalloys.

Phonon spectrum and related thermodynamic properties of microcrack in bcc-Fe

Cao Li-Xia, Wang Chong-Yu
Chin. Phys. B 2006, 15 (9): 02092;  doi: 10.1088/1009-1963/15/9/032
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The phonon spectrum and the related thermodynamic properties of microcracks in bcc-Fe are studied with the recursion method by using the Finnis--Sinclair (F--S) N-body potential. The initial configuration of the microcracks is established from an anisotropic linear elastic solution and relaxed to an equilibrium by molecular dynamics method. It is shown that the local vibrational density of states of the atoms near a crack tip is considerably different from the bulk phonon spectrum, which is closely associated with the local stress field around the crack tip; meanwhile, the local vibrational energies of atoms near the crack tip are higher than those of atoms in a perfect crystal. These results imply that the crack tip zone is in a complex stress state and closely related to the structure evolution of cracks. It is also found that the phonon excitation is a kind of local effect induced by microcracks. In addition, the microcrack system has a higher vibrational entropy, which reflects the character of phonon spectrum related to the stress field induced by cracks.

Effect of electric field on the electronic spectrum and the persistent current of a quantum ring with two electrons

Wu Hong, Bao Cheng-Guang
Chin. Phys. B 2006, 15 (9): 02102;  doi: 10.1088/1009-1963/15/9/033
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The effect of an electric field E on a narrow quantum ring that contains two electrons and is threaded by a magnetic flux B has been investigated. Localization of the electronic distribution and suppression of the Aharonov--Bohm oscillation (ABO) are found in the two-electron ring, which are similar to those found in a one-electron ring. However, the period of ABO in a two-electron ring is reduced by half compared with that in a one-electron ring. Furthermore, during the variation of B, the persistent current of the ground state may undergo a sudden change in sign. This change is associated with a singlet--triplet transition and has no counterpart in one-electron rings. For a given E, there exists a threshold of energy. When the energy of the excited state exceeds the threshold, the localization would disappear and the ABO would recover. The value of the threshold is proportional to the magnitude of E. Once the threshold is exceeded, the persistent current is much stronger than the current of the ground state at E=0.

Calculation of phonon spectrum for noble metals by modified analytic embedded atom method (MAEAM)

Zhang Xiao-Jun, Zhang Jian-Min, Xu Ke-Wei
Chin. Phys. B 2006, 15 (9): 02108;  doi: 10.1088/1009-1963/15/9/034
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In the harmonic approximation, the atomic force constants are derived and the phonon dispersion curves along four major symmetry directions [00$\zeta $], [0$\zeta \zeta $], [$\zeta \zeta \zeta $] and [0$\zeta $1] (or $\De $, $\Si $, $\La $ and $Z$ in group-theory notation) are calculated for four noble metals Cu, Ag, Au and Pt by combining the modified analytic embedded atom method (MAEAM) with the theory of lattice dynamics. A good agreement between calculations and measurements, especially for lower frequencies, shows that the MAEAM provides a reasonable description of lattice dynamics in noble metals.

Accurate analysis of arbitrarily-shaped helical groove waveguide

Liu Hong-Tao, Wei Yan-Yu, Gong Yu-Bin, Yue Ling-Na, Wang Wen-Xiang
Chin. Phys. B 2006, 15 (9): 02114;  doi: 10.1088/1009-1963/15/9/035
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This paper presents a theory on accurately analysing the dispersion relation and the interaction impedance of electromagnetic waves propagating through a helical groove waveguide with arbitrary groove shape, in which the complex groove profile is synthesized by a series of rectangular steps. By introducing the influence of high-order evanescent modes on the connection of any two neighbouring steps by an equivalent susceptance under a modified admittance matching condition, the assumption of the neglecting discontinuity capacitance in previously published analysis is avoided, and the accurate dispersion equation is obtained by means of a combination of field-matching method and admittance-matching technique. The validity of this theory is proved by comparison between the measurements and the numerical calculations for two kinds of helical groove waveguides with different groove shapes.

Resonant tunnelling of electrons in multi-step single-barrier heterostructures

Wang Chang, Zhang Yong-Hua
Chin. Phys. B 2006, 15 (9): 02120;  doi: 10.1088/1009-1963/15/9/036
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We have studied the quantum transport of electrons in a three-step single-barrier AlGaAs heterostructure under electric field. Using the quantum transmitting boundary method and Tsu--Esaki approach, we have calculated the transmission coefficient and current--voltage characteristic. The difference of the effective mass among the three barriers is taken into account. Effects of the barrier width on transmission coefficient and peak-to-valley current ratios are examined. The largest peak-to-valley current ratio is obtained when the ratio of widths of the left, middle, and right barrier is fixed at 4:2:1. The calculated results may be helpful for designing devices based on three-step barrier heterostructures.

Influences of quantum noises on direct-modulated properties of 1.3-μm InGaAsP/InP laser diodes

Wang Jun, Ma Xiao-Yu, Bai Yi-Ming, Cao Li, Wu Da-Jin
Chin. Phys. B 2006, 15 (9): 02125;  doi: 10.1088/1009-1963/15/9/037
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Due to the zero dispersion point at 1.3μm in optical fibres, 1.3-μm InGaAsP/InP laser diodes have become main light sources in fibre communication systems recently. Influences of quantum noises on direct-modulated properties of single-mode 1.3-μm InGaAsP/InP laser diodes are investigated in this article. Considering the carrier and photon noises and the cross-correlation between the two noises, the power spectrum of the photon density and the signal-to-noise ratio (SNR) of the direct-modulated single-mode laser system are calculated using the linear approximation method. We find that the stochastic resonance (SR) always appears in the dependence of the SNR on the bias current density, and is strongly affected by the cross-correlation coefficient between the carrier and photon noises, the frequency of modulation signal, and the photon lifetime in the laser cavity. Hence, it is promising to use the SR mechanism to enhance the SNR of direct-modulated InGaAsP/InP laser diodes and improve the quality of optical fibre communication systems.

Quantum control of two interacting electrons in a coupled quantum dot

Song Hong-Zhou, Zhang Ping, Duan Su-Qing, Zhao Xian-Geng
Chin. Phys. B 2006, 15 (9): 02130;  doi: 10.1088/1009-1963/15/9/038
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Quantum-state engineering, i.e. active manipulation over the coherent dynamics of suitable quantum-mechanical systems, has become a fascinating prospect of modern physics. Here we discuss the dynamics of two interacting electrons in a coupled quantum dot driven by an external electric field. The results show that the two quantum dots can be used to prepare a maximally entangled Bell state by changing the strength and duration of an oscillatory electric field. Different from the suggestion made by Loss \textit et al (1998 Phys. Rev. A 57 120, the present entanglement involves the spatial degree of freedom for the two electrons. We also find that the coherent tunnelling suppression discussed by Grossmann \textit et al (1991 Phys. Rev. Lett. 67 516 persists in the two-particle case: i.e. two electrons initially localized in one dot can remain dynamically localized, although the strong Coulomb repulsion prevents them from behaving so. Surprisingly, the interaction enhances the degree of localization to a large extent compared with that in the non-interacting case. This phenomenon is referred to as the Coulomb-enhanced dynamical localization.

Ti--Al based ohmic contacts to n-type 6H-SiC with P+ ion implantation

Guo Hui, Zhang Yi-Men, Zhang Yu-Ming
Chin. Phys. B 2006, 15 (9): 02142;  doi: 10.1088/1009-1963/15/9/039
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The Ti--Al ohmic contact to n-type 6H-SiC has been fabricated. An array of TLM (transfer length method) test patterns with Au/Ti/Al/Ti/SiC structure is formed on N-wells created by P+ ion implantation into Si-faced p-type 6H-SiC epilayer. The specific contact resistance \rho c as low as 8.64×10-6\Omega\cdot cm2 is achieved after annealing in N2 at 900℃ for 5\,min. The sheet resistance Rsh of the implanted layers is 975\Omega/\sqcap\!\!\!\!\sqcup. X-ray diffraction (XRD) analysis shows the formation of Ti-3SiC2 at the metal/n-SiC interface after thermal annealing, which is responsible for the low resistance contact.

Crystal field analysis of the magnetization curves of R2Fe17 and R2Fe17H3 (R=Tb,Ho,Er)

Ma Ru-Gui, Yan Yu, Zhang Yan-Xiang, Du Xiao-Bo, Wang Xiang-Qun, Su Feng, Jin Han-Min
Chin. Phys. B 2006, 15 (9): 02146;  doi: 10.1088/1009-1963/15/9/040
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In this paper the values of the crystalline-electric-field parameters Anm for R2Fe17 and R2Fe17H3 (R=Tb,Ho,Er) are evaluated by fitting calculations to the magnetization curves measured on the single crystal at several temperatures. The fitted Anm for R2Fe17 are strikingly different from those for the corresponding R2Fe17H3. The energy gaps between the lowest four energy levels for Ho ions in Ho2Fe172 can be reproduced by using the fitted Anm and exchange field 2μBHex, which estimated from the fit of the temperature dependence of the spontaneous magnetization combined with inelastic neutron scattering experiment.

A full numerical calculation of the Franz--Keldysh effect on magnetoexcitons in a bulk semiconductor

Zhang Tong-Yi, Zhao Wei, Zhu Hai-Yan, Zhu Shao-Lan, Liu Xue-Ming
Chin. Phys. B 2006, 15 (9): 02151;  doi: 10.1088/1009-1963/15/9/041
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We have performed a full numerical calculation of the Franz--Keldysh (FK) effect on magnetoexcitons in a bulk GaAs semiconductor. By employing an initial value method in combination with the application of a perfect matched layer, the numerical effort and storage size are dramatically reduced due to a significant reduction in both computed domain and number of base functions. In the absence of an electric field, the higher magnetoexcitonic peaks show distinct Fano lineshape due to the degeneracy with continuum states of the lower Landau levels. The magnetoexcitons that belong to the zeroth Landau level remain in bound states and lead to Lorentzian lineshape, because they are not degenerated with continuum states. In the presence of an electric field, the FK effect on each magnetoexcitonic resonance can be identified for high magnetic fields. However, for low magnetic fields, the FK oscillations dominate the spectrum structure in the vicinity of the bandgap edge and the magnetoexcitonic resonances dominate the spectrum structure of higher energies. In the moderate electric fields, the interplay of FK effect and magnetoexcitonic resonance leads to a complex and rich structure in the absorption spectrum.

Effects of Nb2O5 on thermal stability and optical properties of Er3+-doped tellurite glasses

Zhao Chun, Zhang Qin-Yuan, Pan Yue-Xiao, Jiang Zhong-Hong
Chin. Phys. B 2006, 15 (9): 02158;  doi: 10.1088/1009-1963/15/9/042
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Er3+-doped tellurite glasses with molar compositions of xNb2O5-(14.7-x)Na2O--10ZnO--5K2O--10GeO2-- 60TeO2--0.3Er2O3 (x=0, 3, 5, 7 and 9) have been investigated for developing 1.5~μm fibre and planar amplifiers. The effects of Nb2O5 on the thermal stability and optical properties of Er3+-doped tellurite glasses have been discussed. It is noted that the incorporation of Nb2O5 (x=5) increases the thermal stability of tellurite glasses significantly. Er3+-doped niobium tellurite glasses exhibit a large stimulated emission cross-section (7.2\times 10-21- 10.7×10-21~cm2 and the gain bandwidth, FWHM×\sigmae^{\rm peak} (274\times 10-28 - 480×10-28~cm3), which are significantly higher than that of silicate and phosphate glasses. In addition, the intensity of upconversion luminescence of the Er3+-doped niobium tellurite glasses decreases rapidly with increasing Nb2O5 content. As a result, Er3+-doped niobium tellurite glasses might be a potential candidate for developing laser or optical amplifier devices.

Annealing behaviour of structure and morphology and its effects on the optical gain of Er3+/Yb3+ co-doped Al2O3 planar waveguide amplifier

Tan Na, Zhang Qing-Yu
Chin. Phys. B 2006, 15 (9): 02165;  doi: 10.1088/1009-1963/15/9/043
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Using transmission electron microscopy (TEM) and x-ray diffraction analysis, we have studied the structural and morphological evolution of highly Er/Yb co-doped Al2O3 films in the temperature range from $600\,^{\circ}\mkern-1mu$C--900$\,^{\circ}\mkern-1mu$C. By comparison with TEM observation, the annealing behaviours of photoluminescence (PL) emission and optical loss were found to have relation to the structure and morphology. The increase of PL intensity and optical loss above 800$\,^{\circ}\mkern-1mu$C might result from the crystallization of amorphous Al2O3 films. Based on the study on the structure and morphology, a rate equation propagation model of a multilevel system was used to calculate the optical gains of Er-doped Al2O3 planar waveguide amplifiers involving the variation of PL efficiency and optical loss with annealing temperature. It was found that the amplifiers had an optimized optical gain at the temperature corresponding to the minimum of optical loss, rather than at the temperature corresponding to the maximum of PL efficiency, suggesting that the optical loss is a key factor for determining the optical gain of an Er-doped Al2O3 planar waveguide amplifier.

Microwave backscattering from the sea surface with breaking waves

Liu Ye, Wei En-Bo, Hong Jie-Li, Ge Yong
Chin. Phys. B 2006, 15 (9): 02175;  doi: 10.1088/1009-1963/15/9/045
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Based on the effective medium approximation theory of composites, the whitecap-covered sea surface is treated as a medium layer of dense seawater droplets and air. Two electromagnetic scattering models of randomly rough surface are applied to the investigation of microwave backscattering of breaking waves driven by strong wind. The shapes of seawater droplets are considered by calculating the effective dielectric constant of the whitecap layer. The responses of seawater droplets shapes, such as sphere and ellipsoid, to the backscattering coefficient are discussed. Numerical results of the models are in good agreement with the experimental measurements of horizontally and vertically polarized backscattering at microwave frequency 13.9GHz and different incidence angles.

On the correlation of nonlinear variables containing secular trend variations: numerical experiments

Shi Neng, Yi Yan-Ming, Gu Jun-Qiang, Xia Dong-Dong
Chin. Phys. B 2006, 15 (9): 02180;  doi: 10.1088/1009-1963/15/9/046
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Due to global warming, the general circulation, underlying surfaces characteristics, and geophysical and meteorological elements all show evident secular trends. This paper points out that when calculating the correlation of two variables containing their own obvious secular trends, the interannual correlation characteristics between the two variables may be distorted (overestimated or underestimated). Numerical experiments in this paper show that if two variables have opposite secular trends, the correlation coefficient between the two variables is reduced (the positive correlation is underestimated, or the negative correlation is overestimated); and if the two variables have the same sign of secular trends, the correlation coefficient between the two variables is increased (the positive correlation is overestimated, or the negative correlation is underestimated). Numerical experiments also suggest that the effect of secular trends on the interannual correlation of the two variables is interchangeable, that is to say, as long as the values of the two trends are not changed, the two variables interchange their positions, and the effect of the secular trends on the interannual correlation coefficient of the two variables remains the same. If the two variables have the same-(opposite-) sign trends, the effect of secular trends on the interannual correlation coefficient is more (less) distinctive. A meteorological example is given.

Charge coupled devices (CCD) photometry and long-term optical variability of gamma-ray-loud BL Lacertae object Mkn 421

Zhang Xiong, Zheng Yong-Gang, Zhang Hao-Jing, Hu Shao-Ming, Xie Zhao-Hua
Chin. Phys. B 2006, 15 (9): 02185;  doi: 10.1088/1009-1963/15/9/047
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Evidence for the long-term optical curve variability and colour behaviour of the BL Lac object Mkn 421 is presented. Our results show that the amplitude of the optical variations of Mkn 421 is only about {\Delta} B=4.7 magnitude in the B-band for its photometric history from 1899 to 2002. The results of optical photometric monitoring of the Mkn 421 from April 2000 to Jan 2002 are provided. During our observation, Mkn 421 shows significant rapid variations and exhibits short time variability of 2.42 hours in the B band. A strong correlation between the B-V colour index and the magnitude in the B band is found. Our charge coupled device (CCD) photometry of Mkn 421 shows that the measured results are in good agreement with the predicted optical variability period of about 23 years.
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