Chin. Phys. B

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Element-free Galerkin (EFG) method for a kind of two-dimensional linear hyperbolic equation

Cheng Rong-Jun, Cheng Yu-Min, Ge Hong-Xia

Chin. Phys. B, 2009, 18 (10): 4059-4064 doi: 10.1088/1674-1056/18/10/001
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The present paper deals with the numerical solution of a two-dimensional linear hyperbolic equation by using the element-free Galerkin (EFG) method which is based on the moving least-square approximation for the test and trial functions. A variational method is used to obtain the discrete equations, and the essential boundary conditions are enforced by the penalty method. Compared with numerical methods based on mesh, the EFG method for hyperbolic problems needs only the scattered nodes instead of meshing the domain of the problem. It neither requires any element connectivity nor suffers much degradation in accuracy when nodal arrangements are very irregular. The effectiveness of the EFG method for two-dimensional hyperbolic problems is investigated by two numerical examples in this paper.

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An improved boundary element-free method (IBEFM) for two-dimensional potential problems

Ren Hong-Ping, Zhang Wu

Chin. Phys. B, 2009, 18 (10): 4065-4073 doi: 10.1088/1674-1056/18/10/002
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The interpolating moving least-squares (IMLS) method is discussed first in this paper. And the formulae of the IMLS method obtained by Lancaster are revised. Then on the basis of the boundary element-free method (BEFM), combining the boundary integral equation (BIE) method with the IMLS method, the improved boundary element-free method (IBEFM) for two-dimensional potential problems is presented, and the corresponding formulae of the IBEFM are obtained. In the BEFM, boundary conditions are applied directly, but the shape function in the MLS does not satisfy the property of the Kronecker δ function. This is a problem of the BEFM, and must be solved theoretically. In the IMLS method, when the shape function satisfies the property of the Kronecker δ function, then the boundary conditions, in the meshless method based on the IMLS method, can be applied directly. Then the IBEFM, based on the IMLS method, is a direct meshless boundary integral equation method in which the basic unknown quantity is the real solution of the nodal variables, and the boundary conditions can be applied directly and easily, thus it gives a greater computational precision. Some numerical examples are presented to demonstrate the method.

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Application of higher-order KdV--mKdV model with higher-degree nonlinear terms to gravity waves in atmosphere

Li Zi-Liang

Chin. Phys. B, 2009, 18 (10): 4074-4082 doi: 10.1088/1674-1056/18/10/003
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Higher-order Korteweg-de Vries (KdV)-modified KdV (mKdV) equations with a higher-degree of nonlinear terms are derived from a simple incompressible non-hydrostatic Boussinesq equation set in atmosphere and are used to investigate gravity waves in atmosphere. By taking advantage of the auxiliary nonlinear ordinary differential equation, periodic wave and solitary wave solutions of the fifth-order KdV--mKdV models with higher-degree nonlinear terms are obtained under some constraint conditions. The analysis shows that the propagation and the periodic structures of gravity waves depend on the properties of the slope of line of constant phase and atmospheric stability. The Jacobi elliptic function wave and solitary wave solutions with slowly varying amplitude are transformed into triangular waves with the abruptly varying amplitude and breaking gravity waves under the effect of atmospheric instability.

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Simulation of natural convection under high magnetic field by means of the thermal lattice Boltzmann method

Zhong Cheng-Wen, Xie Jian-Fei, Zhuo Cong-Shan, Xiong Sheng-Wei, Yin Da-Chuan

Chin. Phys. B, 2009, 18 (10): 4083-4093 doi: 10.1088/1674-1056/18/10/004
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The thermal lattice Boltzmann method (TLBM), which was proposed by J. G. M. Eggels and J. A. Somers previously, has been improved in this paper. The improved method has introduced a new equilibrium solution for the temperature distribution function on the assumption that flow is incompressible, and it can correct the effect of compressibility on the macroscopic temperature computed. Compared to the previous method, where the half-way bounce back boundary condition was used for non-slip velocity and temperature, a non-equilibrium extrapolation scheme has been adopted for both velocity and temperature boundary conditions in this paper. Its second-order accuracy coincides with the ensemble accuracy of lattice Boltzmann method. In order to validate the improved thermal scheme, the natural convection of air in a square cavity is simulated by using this method. The results obtained in the simulation agree very well with the data of other numerical methods and benchmark data. It is indicated that the improved TLBM is also successful for the simulations of non-isothermal flows. Moreover, this thermal scheme can be applied to simulate the natural convection in a non-uniform high magnetic field. The simulation has been completed in a square cavity filled with the aqueous solutions of KCl (11wt%), which is considered as a diamagnetic fluid with electrically low-conducting, with Grashof number Gr=4.64× 10^4 and Prandtl number Pr=7.0. And three cases, with different cavity locations in the magnetic field, have been studied. In the presence of a high magnetic field, the natural convection is quenched by the body forces exerted on the electrically low-conducting fluids, such as the magnetization force and the Lorentz force. From the results obtained, it can be seen that the quenching efficiencies decrease with the variation of location from left, symmetrical line, to the right. These phenomena originate from the different distributions of the magnetic field strengths in the zones of the symmetrical central line of the magnetic fields. The results are also compared with those without a magnetic field. Finally, we can conclude that the improved TLBM will enable effective simulation of the natural convection under a high magnetic field.

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New explicit and exact solutions of the Benney--Kawahara--Lin equation

Xie Yuan-Xi

Chin. Phys. B, 2009, 18 (10): 4094-4099 doi: 10.1088/1674-1056/18/10/005
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In this paper, we present a combination method of constructing the explicit and exact solutions of nonlinear partial differential equations. And as an illustrative example, we apply the method to the Benney-Kawahara-Lin equation and derive its many explicit and exact solutions which are all new solutions.

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Entanglement dynamics between an isolated atom and a moving atom in the cavity

Deng Xiao-Juan, Fang Mao-Fa, Kang Guo-Dong

Chin. Phys. B, 2009, 18 (10): 4100-4104 doi: 10.1088/1674-1056/18/10/006
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The entanglement dynamics between an isolated atom and a moving atom interacting with a cavity field is investigated. The results show that there appears sudden death of entanglement between the isolated atom and the moving atom and that the time of entanglement sudden death (ESD) is independent of the initial state of the system. It is interesting that the isolated atom can also entangle with a cavity field, though they do not interact with each other originally, which stems from the fact that the entanglement between the isolated atom and the moving atom may turn into the entanglement between the isolated atom and the cavity.

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An efficient deterministic secure quantum communication scheme based on cluster states and identity authentication

Liu Wen-Jie, Chen Han-Wu, Ma Ting-Huai, Li Zhi-Qiang, Liu Zhi-Hao, Hu Wen-Bo

Chin. Phys. B, 2009, 18 (10): 4105-4109 doi: 10.1088/1674-1056/18/10/007
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A novel efficient deterministic secure quantum communication scheme based on four-qubit cluster states and single-photon identity authentication is proposed. In this scheme, the two authenticated users can transmit two bits of classical information per cluster state, and its efficiency of the quantum communication is 1/3, which is approximately 1.67 times that of the previous protocol presented by Wang et al [Chin. Phys. Lett. 23 (2006) 2658]. Security analysis shows the present scheme is secure against intercept-resend attack and the impersonator's attack. Furthermore, it is more economic with present-day techniques and easily processed by a one-way quantum computer.

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Effects of periodic modulation on the nonlinear Landau--Zener tunneling

Wu Li-Hua, Duan Wen-Shan

Chin. Phys. B, 2009, 18 (10): 4110-4116 doi: 10.1088/1674-1056/18/10/008
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We study the Landau-Zener tunneling of a nonlinear two-level system by applying a periodic modulation on its energy bias. We find that the two levels are splitting at the zero points of the zero order Bessel function for high-frequency modulation. Moreover, we obtain the effective coupling constant between two levels at the zero points of the zero order Bessel function by calculating the final tunneling probability at these points. It seems that the effective coupling constant can be regarded as the approximation of the higher order Bessel function at these points. For the low-frequency modulation, we find that the final tunneling probability is a function of the interaction strength. For the weak inter-level coupling case, we find that the final tunneling probability is more disordered as the interaction strength becomes larger.

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Entanglement evolution and transfer in a double Tavis--Cumming model in cavity QED

Xu Qing-Jun, Zhang Shi-Ying

Chin. Phys. B, 2009, 18 (10): 4117-4121 doi: 10.1088/1674-1056/18/10/009
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We have studied entanglement evolution and transfer in a double Tavis--Cumming model where two pairs of entangled two-level atoms AB and CD interact with two single-mode cavity fields a and b. We show that the Bell-like initial state of atoms AB can exhibit entanglement sudden death which should be independent of the initial entanglement of atoms CD. Also, we show that the initial entanglement of one atomic pair can be transferred into another pair, as well as the possible subsystems, that become entangled during evolution.

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Critical rotation of an anharmonically trapped Bose--Einstein condensate

Ma Juan, Li Zhi, Xue Ju-Kui

Chin. Phys. B, 2009, 18 (10): 4122-4129 doi: 10.1088/1674-1056/18/10/010
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We consider rotational motion of an interacting atomic Bose-Einstein condensate (BEC) with both two- and three-body interactions in a quadratic-plus-quartic and harmonic-plus-Gaussian trap. By using the variational method, the influence of the three-body interaction and the anharmonicity of the trap on the lowest energy surface mode excitation and the spontaneous shape deformation (responsible for the vortex formation) in a rotating BEC is discussed in detail. It is found that the repulsive three-body interaction helps the formation of the vortex and reduces the lowest energy surface mode frequency and the critical rotational frequency of the system. Moreover, the critical rotational frequency for the vortex formation in the harmonic-plus-Gaussian potential is lower than that in the quadratic-plus-quartic potential.

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The nonlinear Wannier function in square Kr?nig--Penney model

Xue Rui, Liang Zhao-Xin, Li Wei-Dong

Chin. Phys. B, 2009, 18 (10): 4130-4135 doi: 10.1088/1674-1056/18/10/011
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The nonlinear Wannier functions in square Kr?nig-Penney potential are investigated with the help of a set of exact nonlinear Bloch solutions. The nonlinear interaction makes the Wannier functions fall off as non-exponential law with distance and enhances the tunneling coupling between the neighbor sites.

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The influence of strain-reducing layer on strain distribution and ground state energy levels of GaN/AlN quantum dot

Liu Yu-Min, Yu Zhong-Yuan, Ren Xiao-Min, Xu Zi-Huan

Chin. Phys. B, 2009, 18 (10): 4136-4142 doi: 10.1088/1674-1056/18/10/012
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This article deals with the strain distributions around GaN/AlN quantum dots by using the finite element method. Special attention is paid to the influence of Al_0.2Ga_0.8N strain-reducing layer on strain distribution and electronic structure. The numerical results show that the horizontal and the vertical strain components are reinforced in the GaN quantum dot due to the presence of the strain-reducing layer, but the hydrostatic strain in the quantum dot is not influenced. According to the deformation potential theory, we study the band edge modifications and the piezoelectric effects. The result demonstrates that with the increase of the strain reducing layer, the transition energy between the ground state electron and the heavy hole increases. This result is consistent with the emission wavelength blue shift phenomenon observed in the experiment and confirms that the wavelength shifts toward the short wavelength range is realizable by adjusting the structure-dependent parameters of GaN/AlN quantum dot.

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Einstein--Podolsky--Rosen entanglement in time-dependent periodic pumping non-degenerate optical parametric amplifier

Zhao Chao-Ying, Tan Wei-Han

Chin. Phys. B, 2009, 18 (10): 4143-4153 doi: 10.1088/1674-1056/18/10/013
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The solution of the time-dependent periodic pumping non-degenerate optical parametric amplifier (NOPA) is derived when the pump depletion is considered both above and below thresholds. Based on this solution, the quantum fluctuation calculated shows that a high entanglement and a good squeezing degree of the parametric light beams are achieved near and above thresholds. We adopt two kinds of pump fields: (i) a continuously modulated pump with a sinusoidal envelope; (ii) a sequence of laser pulses with Gaussian envelopes. We analyse the time evolution of continuous variable entanglement by analytical and numerical calculations, and show that the periodic driven pumping also improves the degree of entanglement. The squeezing and Einstein-Podolsky-Rosen (EPR) entanglement by using the two pumping driven functions are investigated from below to above the threshold regions, the tendencies are nearly the same, and the Gaussian driven function is superior to that of the sine driven function, when the maximum squeezing and the minimum variance of quantum fluctuation are considered. In the meantime, the generalization of frequency degenerate OPA to frequency non-degenerated OPA problem is investigated.

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Quantum quasi-cyclic low-density parity-check error-correcting codes

Li Yuan, Zeng Gui-Hua, Moon Ho Lee

Chin. Phys. B, 2009, 18 (10): 4154-4160 doi: 10.1088/1674-1056/18/10/014
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Due to the fault of the author(s) of the article entitled “Quantum quasi-cyclic low-density parity-check error-correcting codes”, published in Chinese Physics B, 2009, Vol. 18, Issue 10, pp 4154--4160, has been found to partly copy from the article arXiv:quant-ph/0701020v2 on the arXiv preprint. So the above article in Chinese Physics B has been withdrawn from the publication. [2 February 2010]

In this paper, we propose the approach of employing circulant permutation matrices to construct quantum quasi-cyclic (QC) low-density parity-check (LDPC) codes. Using the proposed approach one may construct some new quantum codes with various lengths and rates of no cycles-length 4 in their Tanner graphs. In addition, these constructed codes have the advantages of simple implementation and low-complexity encoding. Finally, the decoding approach for the proposed quantum QC LDPC is investigated.

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Asymptotic quasinormal modes of scalar field in a gravity's rainbow

Liu Cheng-Zhou, Zhu Jian-Yang

Chin. Phys. B, 2009, 18 (10): 4161-4168 doi: 10.1088/1674-1056/18/10/015
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In the framework of the gravity's rainbow, the asymptotic quasinormal modes of the modified Schwarzschild black holes undergoing a scalar perturbation are investigated. By using the monodromy method, we analytically calculated the asymptotic quasinormal frequencies, which depend on not only the mass parameter of the black hole, but also the particle's energy of the perturbation field. Meanwhile, the real parts of the asymptotic quasinormal modes can be expressed as T_H\ln 3, which is consistent with Hod's conjecture. In addition, for the quantum corrected black hole, the area spacing is independent of the particle's energy, even though the area itself depends on the particle's energy. And that, by relating the area spectrum to loop quantum gravity, the Barbero-Immirzi parameter is given and it remains the same as from the usual black hole.

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Effect of road structure on the capacity of a signalized road intersection

Pan Jia-Xiu, Xue Yu, Liang Yu-Juan, Tang Tie-Qiao

Chin. Phys. B, 2009, 18 (10): 4169-4176 doi: 10.1088/1674-1056/18/10/016
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In this paper, we use the stochastic Nagel--Schreckenberg (NaSch) model to investigate the influence of a special right-turning lane connecting two main roads on the capacity of a signalized road intersection. It is found that the magnitude of right-turning traffic flow and the linking position of the special right-turning lane can greatly influence the capacity of the signalized road intersection. The relation between traffic flow and entering probability for different distances between the entrance (exit) of the special right-turning lane and the road intersection is simulated and analysed. The corresponding spatiotemporal pattern and phase diagram on different sections of the main road are given under the condition of close proximity to the signalized road intersection, stop-and-go traffic occur and obstruct the intersection. On the contrary, unchanged flux is maintained as the distance exceeds a critical values. All the studies indicate that setting a special right-turning lane by choosing a suitable location near a signalized road intersection can relieve the load of current traffic on the main road and maintain traffic flow.

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Modulational instability for a self-attractive two-component Bose--Einstein condensate

Li Sheng-Chang, Duan Wen-Shan

Chin. Phys. B, 2009, 18 (10): 4177-4181 doi: 10.1088/1674-1056/18/10/017
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By means of the multiple-scale expansion method, the coupled nonlinear Schr?dinger equations without an explicit external potential are obtained in two-dimensional geometry for a self-attractive Bose-Einstein condensate composed of different hyperfine states. The modulational instability of two-component condensate is investigated by using a simple technique. Based on the discussion about two typical cases, the explicit expression of the growth rate for a purely growing modulational instability and the optimum stable conditions are given and analysed analytically. The results show that the modulational instability of this two-dimensional system is quite different from that in a one-dimensional system.

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Effects of signal modulation and coloured cross-correlation of coloured noises on the diffusion of a harmonic oscillator

Liu Li, Zhang Liang-Ying, Cao Li

Chin. Phys. B, 2009, 18 (10): 4182-4186 doi: 10.1088/1674-1056/18/10/018
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The diffusion in a harmonic oscillator driven by coloured noises ξ(t) and η(t) with coloured cross-correlation in which one of the noises is modulated by a biased periodic signal is investigated. The exact expression of diffusion coefficient d as a function of noise parameter, signal parameter, and oscillator frequency is derived. The findings in this paper are as follows. 1) The curves of d versus noise intensity d and d versus noises cross-correlation time τ ₃ exist as two different phases. The transition between the two phases arises from the change of the cross-correlation coefficient λ of the two Ornstein--Uhlenbeck (O-U) noises. 2) Changing the value of τ ₃, the curves of d versus Q, the intensity of colored noise that is modulated by the signal, can transform from a phase having a minimum to a monotonic phase. 3) Changing the value of signal amplitude A, d versus Q curves can transform from a phase having a minimum to a monotonic phase. The above-mentioned results demonstrate that a like noise-induced transition appears in the model.

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Synchronization transition of limit-cycle system with homogeneous phase shifts

Zhang Ting-Xian, Zheng Zhi-Gang

Chin. Phys. B, 2009, 18 (10): 4187-4192 doi: 10.1088/1674-1056/18/10/019
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The behaviors of coupled oscillators, each of which has periodic motion with random natural frequency in the absence of coupling, are investigated when phase shifts are considered. In the system of coupled oscillators, phase shifts are the same between different oscillators. Synchronization and synchronization transition are revealed with different phase shifts. Phase shifts play an important role for this kind of system. When the phase shift α<0.5π, the synchronization state can be attained by increasing the coupling, and the system cannot reach the synchronization state while α≥q0.5π. A clear scaling between complete synchronization critical coupling strength K_pc and α-0.5π is found.

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Spin-3/2 Ising model AFM/AFM two-layer lattice with crystal field

Erhan Albayrak, Ali Yigit

Chin. Phys. B, 2009, 18 (10): 4193-4207 doi: 10.1088/1674-1056/18/10/020
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The spin-3/2 Ising model is investigated for the case of antiferromagnetic (AFM/AFM) interactions on the two-layer Bethe lattice by using the exact recursion relations in the pairwise approach for given coordination numbers q=3, 4 and 6 when the layers are under the influences of equal external magnetic and equal crystal fields. The ground state (GS) phase diagrams are obtained on the different planes in detail and then the temperature-dependent phase diagrams of the system are calculated accordingly. It is observed that the system presents both second- and first-order phase transitions for all q, therefore, tricritical points. It is also found that the system exhibits double-critical end points and isolated points. The model also presents two Néel temperatures, T_N, and the existence of which leads to the reentrant behaviour.

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A control method for congested traffic in the coupled map car-following model

Shen Fei-Ying, Ge Hong-Xia, Zhang Hui, Yu Han-Mei, Lei Li

Chin. Phys. B, 2009, 18 (10): 4208-4216 doi: 10.1088/1674-1056/18/10/021
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Based on the pioneer work of Konishi et al, a new control method is presented to suppress the traffic congestion in the coupled map (CM) car-following model under an open boundary. A control signal concluding the velocity differences of the two vehicles in front is put forward. The condition under which the traffic jam can be contained is analyzed. The results are compared with that presented by Konishi et al [Phys. Rev. 1999 E 60 4000--4007]. The simulation results show that the temporal behavior obtained by our method is better than that by the Konishi's et al. method, although both the methods could suppress the traffic jam. The simulation results are consistent with the theoretical analysis.

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Improving consensual performance of multi-agent systems in weighted scale-free networks

Qi Wei, Xu Xin-Jian, Wang Ying-Hai

Chin. Phys. B, 2009, 18 (10): 4217-4221 doi: 10.1088/1674-1056/18/10/022
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This paper studies consensus problems in weighted scale-free networks of asymmetrically coupled dynamical units, where the asymmetry in a given link is determined by the relative degree of the involved nodes. It shows that the asymmetry of interactions has a great effect on the consensus. Especially, when the interactions are dominant from higher- to lower-degree nodes, both the convergence speed and the robustness to communication delay are enhanced.

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Collision avoidance for a mobile robot based on radial basis function hybrid force control technique

Wen Shu-Huan

Chin. Phys. B, 2009, 18 (10): 4222-4228 doi: 10.1088/1674-1056/18/10/023
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Collision avoidance is always difficult in the planning path for a mobile robot. In this paper, the virtual force field between a mobile robot and an obstacle is formed and regulated to maintain a desired distance by hybrid force control algorithm. Since uncertainties from robot dynamics and obstacle degrade the performance of a collision avoidance task, intelligent control is used to compensate for the uncertainties. A radial basis function (RBF) neural network is used to regulate the force field of an accurate distance between a robot and an obstacle in this paper and then simulation studies are conducted to confirm that the proposed algorithm is effective.

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Images of triple gas electron multiplier with pixel-pads

Dong Jing, Hu Bi-Tao, Chen Yuan-Bo, Xie Yi-Gang

Chin. Phys. B, 2009, 18 (10): 4229-4233 doi: 10.1088/1674-1056/18/10/024
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Testing of a triple gas electron multiplier (GEM) with pixel-pads is described. Images by scanning and suspending radioactive sources were obtained by using 96 channels digital data acquisition (DAQ) system which was composed of 96 8× 8~mm² pads and associated electronics channels.

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Magnetically insulated transmission line oscillator oscillated in a modified HEM₁₁ mode

Wang Dong, Chen Dai-Bing, Qin Fen, Fan Zhi-Kai

Chin. Phys. B, 2009, 18 (10): 4281-4286 doi: 10.1088/1674-1056/18/10/031
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This paper puts forward a novel magnetically insulated transmission line oscillator (MILO) for the first time which takes a modified HEM₁₁ mode as its main interaction mode. The excitation of the oscillation mode is made possible by carefully adjusting the arrangements of each resonant cavity in a two-dimensional (2-D) slow wave structure. The high frequency characteristics are analyzed and a PIC simulation is carried out; the detailed results are discussed to get a better understanding of this new MILO. Employing an electron beam of about 441~kV and 39.7~kA, it finds that the modified HEM₁₁ mode MILO generates a high power microwave output of about 1.47~GW at 1.45~GHz. The power conversion efficiency is about 8.4% and the generated microwave is in a TE₁₁-like circularly polarized mode; its polarization direction is decided by the rotation direction of the SWS.

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Investigation of transmission resonances on the one-dimensional metallic cylindrical gratings in THz frequency range

Chen Hua, Sun Yi-Min, Wang Li

Chin. Phys. B, 2009, 18 (10): 4287-4291 doi: 10.1088/1674-1056/18/10/032
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In the experiments of THz wave transmitting through the metallic cylindrical gratings fabricated by sub-wavelength brass wires, this paper reports that the discrepancy in the sharp resonances occurred as the grating perpendicular or parallel to the electric vector are observed. A simulation based on the finite difference time domain (FDTD) indicated that the enhanced transmission through the grating is attributed to the combined effects of surface plasmons and cavity modes in the perpendicular condition, while the cavity modes dominate the resonant transmission under the other conditions. Additional experimental data and calculated results show that ～1 enhanced coupling efficiency can be realized in some THz frequency, which could be applied to the design and improvement of various optoelectronic devices, or detection of biological molecule and powder samples, etc.

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Experimental study on microdeposition of the copper thin film by femtosecond laser-induced forward transfer

Yang Li, Wang Ching-Yue

Chin. Phys. B, 2009, 18 (10): 4292-4297 doi: 10.1088/1674-1056/18/10/033
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The morphologies of the deposited dots on the 40~nm-thick copper film by the femtosecond laser-induced forward transfer that depend on the irradiated laser fluence have been studied, and the variations of orderliness of the diameter of deposited dots on the quartz substrate and forward ablated dot on the donor substrate with increasing pulse fluence have been obtained experimentally. The experimental results show that a thinner copper film would generate larger-sized ablated dot and deposited dot at the threshold fluence for transfer. By x-ray diffraction measurement, it is demonstrated that the crystal form of the transferred copper films is unaltered and the size of the crystallites is diminished.

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The (1+1)-dimensional spatial solitons in media with weak nonlinear nonlocality

Ding Na, Guo Qi

Chin. Phys. B, 2009, 18 (10): 4298-4302 doi: 10.1088/1674-1056/18/10/034
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We study the propagation of (1+1)-dimensional spatial soliton in a nonlocal Kerr-type medium with weak nonlocality. First, we show that an equation for describing the soliton propagation in weak nonlocality is a nonlinear Schr?dinger equation with perturbation terms. Then, an approximate analytical solution of the equation is found by the perturbation method. We also find some interesting properties of the intensity profiles of the soliton.

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Interaction of light filaments in air

Xi Ting-Ting, Lu Xin, Hao Zuo-Qiang, Ma Yuan-Yuan, Zhang Jie

Chin. Phys. B, 2009, 18 (10): 4303-4307 doi: 10.1088/1674-1056/18/10/035
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This paper analytically investigates the interaction of light filaments generated by a femtosecond laser beam in air. It obtains the Hamiltonian of a total laser field and interaction force between two filaments with different phase shifts and crossing angles. The property of the interaction force, which leads the attraction or repulsion of filaments, is basically dependent on the phase shift between filaments. The crossing angle between two filaments can only determine the magnitude of the interaction force, but does not change the property of the force.

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Efficient collinear frequency tripling of femtosecond laser with compensation of group velocity delay

Wang Yan-Ling, Zhou Xu-Gui, Wu Hong, Ding Liang-En

Chin. Phys. B, 2009, 18 (10): 4308-4313 doi: 10.1088/1674-1056/18/10/036
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This paper demonstrates an approach that negative uniaxial crystal has a relative anomalous dispersion effect which can compensate group velocity delay, and applies this approach to nonlinear frequency conversion of an ultrafast laser field. High efficiency of the third harmonic generation is experimentally fulfilled by adopting a collinear configuration of doubing-compensation-tripling system. Through finely adjusting the incident angle and optical axis direction of the compensation plate, it obtains ultraviolet (UV) output energy of 0.32~mJ centered at 270~nm with spectral bandwidth of 2~nm when input beam at 800~nm was 70~fs pulse duration and 6~mJ pulse energy which was extracted from Ti:sapphire laser system by a diaphragm, corresponding to an 800-to-270~nm conversion efficiency of 5.3% and a factor-of-1.6 improvement in the third harmonic generation of UV band in comparison with a general conventional configuration. Furthermore, when the full energy of 18~mJ from a Ti:sapphire laser system was used and optimized, the UV emission could reach 0.83~mJ.

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High efficiency continuous-wave tunable signal output of an intracavity singly resonant optical parametric oscillator based on periodically poled lithium niobate

Ding Xin, Sheng Quan, Chen Na, Yu Xuan-Yi, Wang Rui, Zhang Heng, Wen Wu-Qi, Wang Peng, Yao Jian-Quan

Chin. Phys. B, 2009, 18 (10): 4314-4318 doi: 10.1088/1674-1056/18/10/037
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[PDF 195 KB] (Downloads:1117)

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In this paper we report on a continuous-wave (CW) intracavity singly resonant optical parametric oscillator (ICSRO) based on periodically poled LiNbO₃ (PPLN) pumped by a diode-end-pumped CW Nd:YVO₄ laser. Considering the thermal lens effects and diffraction loss, an optical ballast lens and a near-concentric cavity are adopted for better operation. Through varying the grating period and the temperature, the tunable signal output from 1406~nm to 1513~nm is obtained. At a PPLN grating period of 29~μ m and a temperature of 413~K, a maximum signal output power of 820~mW at 1500~nm is achieved when the 808~nm pump power is 10.9~W, leading to an optical-to-optical conversion efficiency of 7.51%.

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Surface defect gap solitons in one-dimensional dual-frequency lattices

Zhu Wei-Ling, Luo Li, He Ying-Ji, Wang He-Zhou

Chin. Phys. B, 2009, 18 (10): 4319-4325 doi: 10.1088/1674-1056/18/10/038
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[PDF 1409 KB] (Downloads:649)

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We study the surface defect gap solitons in an interface between a defect of one-dimensional dual-frequency lattices and the uniform media. Some unique properties are revealed that such lattices can broaden the region of semi-finite gap, and the semi-finite gap exists not only in the positive and zero defects but also in the negative defect; unlike in the regular lattices, the semi-finite gap exists in the positive and zero defects but does not exist in the negative defect. In particular, stable solitons exist almost in the whole semi-finite gap for the positive and zero defects. These properties are different from other lattices with defects. In addition, it is found that the existence of surface dual-frequency lattice solitons does not need a threshold power.

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Photochemical kinetics for holographic grating formation in phenanthrenequinone doped poly (methyl methacrylate) photopolymer

Wang Jian, Sun Xiu-Dong, Luo Su-Hua, Jiang Yong-Yuan, Meng Qing-Xin

Chin. Phys. B, 2009, 18 (10): 4326-4332 doi: 10.1088/1674-1056/18/10/039
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[PDF 219 KB] (Downloads:512)

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The photochemical kinetics of phenanthrenequinone (PQ) doped poly (methyl methacrylate) photopolymer in holographic recording was studied theoretically and experimentally. The diffusion of PQ molecules during holographic recording was negligible because of its small diffusion coefficient at room temperature. A photochemical reaction kinetics model of PQ/PMMA was established. The analytical expressions for the temporal variations of transmittance and diffraction efficiency were derived. By fitting the experimental curves, some parameters related with the polymer components were obtained by the proposed model, which can be used to analyze the photochemical process and will be helpful to the optimization of material preparation.

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Modification of spontaneous emission rate of micrometer-sized light sources using hollow-core photonic crystal fibers

Lu Jiao-Hua, Meng Zi-Ming, Liu Hai-Ying, Feng Tian-Hua, Dai Qiao-Feng, Wu Li-Jun, Guo Qi, Hu Wei, Lan Sheng

Chin. Phys. B, 2009, 18 (10): 4333-4338 doi: 10.1088/1674-1056/18/10/040
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[PDF 2884 KB] (Downloads:720)

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We investigate numerically and experimentally the modification of the spontaneous emission rate for micrometer-sized light sources embedded in a hollow-core photonic crystal fiber (HCPCF). The diameter of the light source is deliberately chosen such that they could be easily introduced into the central hole of the hollow-core photonic crystal fiber by capillary force. The photoluminescence from the microparticles is measured by using an inverted microscope in combination with a spectrometer. The modification of the spontaneous emission rate is observed in a wavelength region where there is no band gap. The experimental observations are consistent with the simulation results obtained by the plane wave expansion and finite-difference time-domain techniques.

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First-principles calculations for the elastic properties of Ni-base model superalloys: Ni/Ni₃Al multilayers

Wang Yun-Jiang, Wang Chong-Yu

Chin. Phys. B, 2009, 18 (10): 4339-4348 doi: 10.1088/1674-1056/18/10/041
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[PDF 927 KB] (Downloads:2372)

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A model system consisting of Ni[001](100)/Ni₃Al[001](100) multi-layers are studied using the density functional theory in order to explore the elastic properties of single crystal Ni-based superalloys. Simulation results are consistent with the experimental observation that rafted Ni-base superalloys virtually possess a cubic symmetry. The convergence of the elastic properties with respect to the thickness of the multilayers are tested by a series of multilayers from 2γ'+2γ to 10γ'+10γ atomic layers. The elastic properties are found to vary little with the increase of the multilayer's thickness. A Ni/Ni₃Al multilayer with 10γ'+10γ atomic layers (3.54 nm) can be used to simulate the mechanical properties of Ni-base model superalloys. Our calculated elastic constants, bulk modulus, orientation-dependent shear modulus and Young's modulus, as well as the Zener anisotropy factor are all compatible with the measured results of Ni-base model superalloys R1 and the advanced commercial superalloys TMS-26, CMSX-4 at a low temperature. The mechanical properties as a function of the γ' phase volume fraction are calculated by varying the proportion of the γ and γ' phase in the multilayers. Besides, the mechanical properties of two-phase Ni/Ni₃Al multilayer can be well predicted by the Voigt-Reuss-Hill rule of mixtures.

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Force criterion of different electrolytes in microchannel

Ren Yu-Kun, Yan Hui, Jiang Hong-Yuan, Gu Jian-Zhong, Antonio Ramos

Chin. Phys. B, 2009, 18 (10): 4349-4352 doi: 10.1088/1674-1056/18/10/042
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[PDF 348 KB] (Downloads:541)

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The control and handling of fluids is central to many applications of the lab-on-chip. This paper analyzes the basic theory of manipulating different electrolytes and finds the two-dimensional model. Coulomb force and dielectric force belonging to the body force of different electrolytes in the microchannel were analyzed. The force criterion at the interface was concluded, and testified by the specific example. Three basic equations were analyzed and applied to simulate the phenomenon. The force criterion was proved to be correct based on the simulation results.

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Lattice Boltzmann simulation of fluid flows in two-dimensional channel with complex geometries

Wen Bing-Hai, Liu Hai-Yan, Zhang Chao-Ying, Wang Qiang

Chin. Phys. B, 2009, 18 (10): 4353-4359 doi: 10.1088/1674-1056/18/10/043
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[PDF 339 KB] (Downloads:878)

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Boundary conditions (BCs) play an essential role in lattice Boltzmann (LB) simulations. This paper investigates several most commonly applied BCs by evaluating the relative L₂-norm errors of the LB simulations for two-dimensional (2-D) Poiseuille flow. It is found that the relative L₂-norm error resulting from FHML's BC is smaller than that from other BCs as a whole. Then, based on the FHML's BC, it formulates an LB model for simulating fluid flows in 2-D channel with complex geometries. Afterwards, the flows between two inclined plates, in a pulmonary blood vessel and in a blood vessel with local expansion region, are simulated. The numerical results are in good agreement with the analytical predictions and clearly show that the model is effective. It is expected that the model can be extended to simulate some real biologic flows, such as blood flows in arteries, vessels with stenosises, aneurysms and bifurcations, etc.

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Single point velocity distributions in homogeneous isotropic turbulence

Yan Xiu-Lin, Ran Zheng

Chin. Phys. B, 2009, 18 (10): 4360-4365 doi: 10.1088/1674-1056/18/10/044
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[PDF 157 KB] (Downloads:544)

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The starting point for this paper lies in the results obtained by Tatsumi (2004) for isotropic turbulence with the self-preserving hypothesis. A careful consideration of the mathematical structure of the one-point velocity distribution function equation obtained by Tatsumi (2004) leads to an exact analysis of all possible cases and to all admissible solutions of the problem. This paper revisits this interesting problem from a new point of view, and obtains a new complete set of solutions. Based on these exact solutions, some physically significant consequences of recent advances in the theory of homogenous statistical solution of the Navier--Stokes equations are presented. The comparison with former theory was also made. The origin of non--Gaussian character could be deduced from the above exact solutions.

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Four-parameter analytical local model potential for atoms

Yu Fei, Sun Jiu-Xun, Tian Rong-Gang, Yang Wei

Chin. Phys. B, 2009, 18 (10): 4234-4241 doi: 10.1088/1674-1056/18/10/025
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[PDF 221 KB] (Downloads:699)

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Analytical local model potential for modeling the interaction in an atom reduces the computational effort in electronic structure calculations significantly. A new four-parameter analytical local model potential is proposed for atoms Li through Lr, and the values of four parameters are shell-independent and obtained by fitting the results of X_a method. At the same time, the energy eigenvalues, the radial wave functions and the total energies of electrons are obtained by solving the radial Schr?dinger equation with a new form of potential function by Numerov's numerical method. The results show that our new form of potential function is suitable for high, medium and low Z atoms. A comparison among the new potential function and other analytical potential functions shows the greater flexibility and greater accuracy of the present new potential function.

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Theoretical analysis of ionic autoionization spectra of lanthanum via an intermediate state [Xe]5d6d ¹P₁

Zhong Yin-Peng, Jia Feng-Dong, Zhong Zhi-Ping

Chin. Phys. B, 2009, 18 (10): 4242-4250 doi: 10.1088/1674-1056/18/10/026
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[PDF 1273 KB] (Downloads:590)

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In the framework of multi-channel quantum defect theory, eigenquantum defects μ_α and the transformation matrices U_iα of La⁺ are calculated from first principles by relativistic multi-channel theory, while the dipole matrix elements D_α are obtained by fitting with experimental data. Then the ionic autoionization spectra of lanthanum via the intermediate state [Xe]5d6d ¹P₁ in the energy region of 90213--91905~cm^-1 are obtained. Experimental peaks are classified and assigned by comparing with the corresponding calculated spectra. More specifically, four ionic autoionization Rydberg series converging to La²⁺ 5d_5/2²D_5/2 and several states converging to higher lying states of La²⁺ are found and assigned.

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Investigation of odd-parity Rydberg states of Eu I with autoionization detection

Xiao Ying, Dai Chang-Jian, Qin Wen-Jie

Chin. Phys. B, 2009, 18 (10): 4251-4258 doi: 10.1088/1674-1056/18/10/027
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[PDF 259 KB] (Downloads:611)

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Isolated-core-excitation (ICE) scheme and autoionization detection are employed to study the bound Rydberg states of europium atom. The high-lying states with odd parity have been measured using the autoionization detection method with three different excitation paths via 4f⁷6s6p[ ⁸P_5/2], 4f⁷6s6p[ ⁸P_7/2] and 4f⁷6s6p[ ⁸P_9/2] intermediate states, respectively. In this paper the spectra of bound Rydberg states of Eu atom are reported, which cover the energy regions from 36000~cm^-1 to 38250~cm^-1 and from 38900~cm^-1 to 39500 cm^-1. The study provides the information about level energy, the possible J values and relative line intensity as well as the effective principal quantum number n^* for these states. This work not only confirms the previous results of many states, but also discovers 11 new Rydberg states of Eu atom.

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Bose--Einstein condensation on an atom chip ^Hot!

Yan Bo, Cheng Feng, Ke Min, Li Xiao-Lin, Tang Jiu-Yao, Wang Yu-Zhu

Chin. Phys. B, 2009, 18 (10): 4259-4263 doi: 10.1088/1674-1056/18/10/028
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[PDF 1298 KB] (Downloads:1132)

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This paper reports an experiment of creating Bose-Einstein condensate (BEC) on an atom chip. The chip-based Z-wire current with a homogeneous bias magnetic field creates a tight magnetic trap, which allows for a fast production of BEC. After a 4.17-s forced radio frequency evaporative cooling, a condensate with about 3000 atoms appears. The transition temperature is about 300~nK. This compact system is quite robust, allowing for versatile extensions and further studying of BEC.

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Quantum stereodynamics study for the reaction F + HD

Liu Yu-Fang, Zhang Wei, Shi De-Heng, Sun Jin-Feng

Chin. Phys. B, 2009, 18 (10): 4264-4273 doi: 10.1088/1674-1056/18/10/029
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[PDF 1873 KB] (Downloads:661)

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This paper studies the quantum stereodynamics of the F + HD(υ = 0, j = 0) → HD + F/HF + D reaction at the collision energies of 0.52 and 0.87~kcal/mol. The quantum scattering calculations, based on Stark-Werner potential energy surfaces, show that the differential cross sections for the HF(υ' = 2) + D and DF(υ' = 3) + H channels are consistent with the recent theoretical results. Furthermore, the product rotational angular momentum orientation and alignment have been determined for some selected rovibrational states of the HF + D and DF + H channels.

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Surface-induced evaporative cooling ^Hot!

Ke Min, Yan Bo, Cheng Feng, Wang Yu-Zhu

Chin. Phys. B, 2009, 18 (10): 4274-4280 doi: 10.1088/1674-1056/18/10/030
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[PDF 865 KB] (Downloads:545)

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The effects of surface-induced evaporative cooling on an atom chip are investigated. The evolutions of temperature, number and phase-space density of the atom cloud are measured when the atom cloud is brought close to the surface. Rapid decrease of the temperature and number of the atoms is found when the atom-surface distance is < 100~μm. A gain of about a factor of five on the phase-space density is obtained. It is found that the efficiency of the surface-induced evaporative cooling depends on the atom-surface distance and the shape of the evaporative trap. When the atoms are moved very close to the surface, severe heating is observed, which dominates when the holding time is >8~ms. It is important that the surface-induced evaporative cooling offers novel possibilities for the realization of a continuous condensation, where a spatially varying evaporative cooling is required.

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High-resolution transmission electron microscopy and bulk magnetometry study of LaFe_11.5Si_1.5 compound

Zou Jun-Ding, Li Wei, Shen Bao-Gen

Chin. Phys. B, 2009, 18 (10): 4366-4369 doi: 10.1088/1674-1056/18/10/045
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[PDF 2734 KB] (Downloads:573)

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This paper studies the microstructural and magnetic properties of LaFe_11.5Si_1.5 compound by means of high-resolution transmission electron microscope and bulk magnetometry measurements. The crystalline structure is accompanied with the noncrystalline and nanocrystalline structures. This characteristic is the reflection of the crystalline process held by quenching. The inverse susceptibilities diverge and deviate from Curie--Weiss law under low applied magnetic fields. This paper proposes the possible mechanism between the anomalous susceptibilities and microstructure, and offers a perspective on the magnetic properties of metastable intermetallic compounds.

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Three-dimensional size and orientation of the precipitates in AZ91 magnesium alloys measured by TEM techniques

Zheng Ou, Ma Jia-Yan, Wang Jian-Bo, Zhou Jia-Ping, Jin Lei, Zhao Dong-Shan, Wang Ren-Hui

Chin. Phys. B, 2009, 18 (10): 4370-4379 doi: 10.1088/1674-1056/18/10/046
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[PDF 8871 KB] (Downloads:558)

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Knowledge of the microscopic structure, including three-dimensional (3-D) size and orientation of the precipitates, is essential to fully understand the mechanical properties of the magnesium alloys and designing the alloys with better performance. Analytical TEM with high spatial resolution offers the simultaneous measurements of 3-D size, structure, orientation, composition of the precipitates from one typical sample along an established crystallographic axis. Besides popular Burgers orientation relationship (OR), other ORs such as Pitsch--Schrader OR, Crawley OR, Potter OR and a new OR with the form of [0001]_α 1.0° from [311]_γ and (11\bar 20)_α 2.0° from (03\bar 3)_γ between the magnesium matrix and the precipitate γ -Mg₁₇Al₁₂ are identified by TEM imaging and diffraction techniques. As a case study, the thicknesses of the individual precipitates with Burgers OR are further measured to be 100--200~nm through both electron energy-loss spectroscopy and x-ray energy dispersive spectroscopy combining differential x-ray absorption and extrapolation, which are in agreement with the overall 3-D size statistic distribution results obtained through analysing various samples along various directions. Furthermore, the fabricated wedge-shape structure provides a platform on which to study the dependence of the interfacial strain on the variation of the thickness.

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Rotational viscosity of a liquid crystal mixture: a fully atomistic molecular dynamics study

Zhang Ran, Peng Zeng-Hui, Liu Yong-Gang, Zheng Zhi-Gang, Xuan Li

Chin. Phys. B, 2009, 18 (10): 4380-4384 doi: 10.1088/1674-1056/18/10/047
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[PDF 1109 KB] (Downloads:1512)

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Fully atomistic molecular dynamics (MD) simulations at 293, 303 and 313~K have been performed for the four-component liquid crystal mixture, E7, using the software package Material Studio. Order parameters and orientational time correlation functions (TCFs) were calculated from MD trajectories. The rotational viscosity coefficients (RVCs) of the mixture were calculated using the Nemtsov--Zakharov and Fialkowski methods based on statistical-mechanical approaches. Temperature dependences of RVC and density were discussed in detail. Reasonable agreement between the simulated and experimental values was found.

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Structural phase transition behaviour of Zn₄Sb₃ and its substitutional compounds (Zn_1.98M_0.02)₄Sb₃ (M= Al, Ga and In) at low temperatures

Liu Feng, Qin Xiao-Ying, Liu Mian

Chin. Phys. B, 2009, 18 (10): 4386-4392 doi: 10.1088/1674-1056/18/10/048
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[PDF 1077 KB] (Downloads:676)

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Structural phase transitions of Zn₄Sb₃ and its substitutional compounds (Zn_1.98M_0.02)₄Sb₃ (M= Al, Ga and In) are investigated by electrical transport measurement and differential scanning calorimetry below room temperature. The results indicate that both β → α and α → α' phase transitions of Zn₄Sb₃ are reversible and exothermic processes, which may be explained as that both the transitions originate from the ordering of the disordered interstitial Zn and vacancies in regular sizes. The derived activation energies of β → α and α → α' phase transition processes for Zn₄Sb₃ are E₁ =3.9~eV and E₂=4.1~eV, respectively. Although no remarkable influence on activation energy E₂ is observed after Al doping, Al substitution for Zn causes E₁ to increase to 4.6~eV, implying its suppression of β ≤ftrightarrow α transition to a great extent. Moreover, it is found that both β ≤ftrightarrow α and α ≤ftrightarrow α ' transitions are completely prohibited by substitution of either In or Ga for Zn in Zn₄Sb₃. The underlying mechanisms for these phenomena are discussed.

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Discrete gap breathers in a two-dimensional diatomic face-centered square lattice

Lü Bin-Bin, Tian Qiang

Chin. Phys. B, 2009, 18 (10): 4393-4406 doi: 10.1088/1674-1056/18/10/049
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[PDF 4014 KB] (Downloads:594)

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In this paper we study the existence and stability of two-dimensional discrete gap breathers in a two-dimensional diatomic face-centered square lattice consisting of alternating light and heavy atoms, with on-site potential and coupling potential. This study is focused on two-dimensional breathers with their frequency in the gap that separates the acoustic and optical bands of the phonon spectrum. We demonstrate the possibility of the existence of two-dimensional gap breathers by using a numerical method. Six types of two-dimensional gap breathers are obtained, i.e., symmetric, mirror-symmetric and asymmetric, whether the center of the breather is on a light or a heavy atom. The difference between one-dimensional discrete gap breathers and two-dimensional discrete gap breathers is also discussed. We use Aubry's theory to analyze the stability of discrete gap breathers in the two-dimensional diatomic face-centered square lattice.

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Phase-field simulation of the effect of interaction among ordered domains on interdiffusion in Ni--Al--Cr alloys

Zhang Yu-Xiang, Wang Jin-Cheng, Yang Yu-Juan, Yang Gen-Cang, Zhou Yao-He

Chin. Phys. B, 2009, 18 (10): 4407-4412 doi: 10.1088/1674-1056/18/10/050
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[PDF 2900 KB] (Downloads:589)

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The effect of interaction among γ ' ordered domains on the interdiffusion process in γ +γ ' /γ and γ +γ ' /γ +γ ' diffusion couples is investigated by using the phase-field method, in which bulk free energy and mobility are linked with thermodynamic and kinetic databases. Simulated results show that the interaction among γ ' ordered domains has great influence on the microstructure, the interdiffusion velocity and the volume fraction of γ ' phase on both sides of the diffusion couples.

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The effect of single AlGaN interlayer on the structural properties of GaN epilayers grown on Si (111) substrates

Wu Yu-Xin, Zhu Jian-Jun, Zhao De-Gang, Liu Zong-Shun, Jiang De-Sheng, Zhang Shu-Ming, Wang Yu-Tian, Wang Hui, Chen Gui-Feng, Yang Hui

Chin. Phys. B, 2009, 18 (10): 4413-4417 doi: 10.1088/1674-1056/18/10/051
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[PDF 2560 KB] (Downloads:1125)

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High-quality and nearly crack-free GaN epitaxial layer was obtained by inserting a single AlGaN interlayer between GaN epilayer and high-temperature AlN buffer layer on Si (111) substrate by metalorganic chemical vapor deposition. This paper investigates the effect of AlGaN interlayer on the structural properties of the resulting GaN epilayer. It confirms from the optical microscopy and Raman scattering spectroscopy that the AlGaN interlayer has a remarkable effect on introducing relative compressive strain to the top GaN layer and preventing the formation of cracks. X-ray diffraction and transmission electron microscopy analysis reveal that a significant reduction in both screw and edge threading dislocations is achieved in GaN epilayer by the insertion of AlGaN interlayer. The process of threading dislocation reduction in both AlGaN interlayer and GaN epilayer is demonstrated.

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Lattice strain and p-d repulsion affecting electronic structure of wurtzite Zn_1-xCd_xO alloys

Shi Li-Bin, Kang Li, Jin Jian-Wei, Chi Feng

Chin. Phys. B, 2009, 18 (10): 4418-4424 doi: 10.1088/1674-1056/18/10/052
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[PDF 1027 KB] (Downloads:784)

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In the paper, density of states, band structure and electron density difference of Zn_1-xCd_xO are calculated by first principles, here x varies from 0 to 0.75 at intervals of 0.125, and the band gap obtained from band structure changes from 0.968~eV to 0.043~eV. The lattice strain and p-d repulsion theory are used to investigate variation of the band gap, the results obtained show that the variation is mainly due to the lattice tensile strain. The p-d repulsion in Zn_1-xCd_xO cannot be neglected. In addition, electron density difference can be used to verify the results.

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Temperature-dependent far-infrared properties of Bi₁₂GeO₂₀ single crystal

Muhammad Kamran, Dai Yao-Min, Ma Ji-Yun, Li Bo-Hong, He Shi-Kun, Zhang Wei-Jun, Zheng Xiao-Rui, Qiu Xiang-Gang

Chin. Phys. B, 2009, 18 (10): 4425-4429 doi: 10.1088/1674-1056/18/10/053
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[PDF 193 KB] (Downloads:647)

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Far-infrared reflectivity spectra of bismuth germanium oxide (Bi₁₂GeO₂₀) single crystals are measured from room temperature down to 10~K. All the reflectivity spectra are fitted to a complex dielectric function ε(ω) in the factorised form. Phonon modes at low frequency are found to develop upon decreasing temperature and gain considerable spectrum weight below 150~K. The temperature-dependent static dielectric constants are obtained from the Lyddane--Sachs--Teller relation based on the obtained oscillator parameters. The dielectric constants are found to increase upon decreasing temperature, which is attributed to the charge transfer among the ions in the unit cell with the temperature varying.

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Magnetization of two-dimensional heavy holes with boundaries in a perpendicular magnetic field

Fang Cheng, Wang Zhi-Gang, Li Shu-Shen, Zhang Ping

Chin. Phys. B, 2009, 18 (10): 4430-4436 doi: 10.1088/1674-1056/18/10/054
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[PDF 975 KB] (Downloads:544)

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The magnetisation of heavy holes in III--V semiconductor quantum wells with Rashba spin-orbit coupling (SOC) in an external perpendicular magnetic field is studied theoretically. We concentrate on the effects on the magnetisation induced by the system boundary, the Rashba SOC and the temperature. It is found that the sawtooth-like de Haas--van Alphen (dHvA) oscillations of the magnetisation will change dramatically in the presence of such three factors. Especially, the effects of the edge states and Rashba SOC on the magnetisation are more evident when the magnetic field is smaller. The oscillation center will shift when the boundary effect is considered and the Rashba SOC will bring beating patterns to the dHvA oscillations. These effects on the dHvA oscillations are preferably observed at low temperatures. With increasing temperature, the dHvA oscillations turn to be blurred and eventually disappear.

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All-electron study of ultra-incompressible superhard material ReB₂: structural and electronic properties

Li Yan-Ling, Zhong Guo-Hua, Zeng Zhi

Chin. Phys. B, 2009, 18 (10): 4437-4442 doi: 10.1088/1674-1056/18/10/055
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[PDF 1939 KB] (Downloads:592)

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This paper investigates the structural and electronic properties of rhenium diboride by first-principles calculation based on density functional theory. The obtained results show that the calculated equilibrium structural parameters of ReB₂ are in excellent agreement with experimental values. The calculated bulk modulus is 361~GPa in comparison with that of the experiment. The compressibility of ReB₂ is lower than that of well-known OsB₂. The anisotropy of the bulk modulus is confirmed by c/a ratio as a function of pressure curve and the bulk modulus along different axes along with the electron density distribution. The high bulk modulus is attributed to the strong covalent bond between Re-d and B-p orbitals and the wider pseudogap near the Fermi level, which could be deduced from both electron charge density distribution and density of states. The band structure and density of states of ReB₂ exhibit that this material presents metallic behavior. The good metallicity and ultra-incompressibility of ReB₂ might suggest its potential application as pressure-proof conductors.

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First-principles calculations for electronic and optical properties of the zinc-blende structured BeS compound under pressure

Yang Huan, Chang Jing, Li Zhe, Chen Xiang-Rong

Chin. Phys. B, 2009, 18 (10): 4443-4448 doi: 10.1088/1674-1056/18/10/056
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[PDF 859 KB] (Downloads:750)

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The electronic and the optical properties of the cubic zinc-blende (ZB) BeS under high pressure have been investigated by using \it ab initio plane-wave pseudopotential density functional theory method in the generalised gradient approximation (GGA) for exchange-correlation interaction. The electronic band structure and the pressure dependence of the total and partial densities of state under pressure are successfully described. Our calculations show that the ZB BeS has large and indirect band gaps associated with (Γ → X) transitions in ambient conditions. The results obtained are consistent with the experimental data available and other calculations. The optical properties, including dielectric function, energy-loss function, complex refractive index, reflection and absorption spectra, are investigated and analysed at different external pressures. The results suggest that the optical absorption appears mostly in the ultra-violet region and the curve of refractive index shift toward high energies (blue shift) with pressure increasing.

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Pressure influence on the Stark effect of impurity states in a strained wurtzite GaN/Al_xGa_1-xN heterojunction

Zhang Min, Ban Shi-Liang

Chin. Phys. B, 2009, 18 (10): 4449-4455 doi: 10.1088/1674-1056/18/10/057
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[PDF 248 KB] (Downloads:887)

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A variational method is adopted to investigate the properties of shallow impurity states near the interface in a free strained wurtzite GaN/Al_xGa_1-xN heterojunction under hydrostatic pressure and external electric field by using a simplified coherent potential approximation. Considering the biaxial strain due to lattice mismatch or epitaxial growth and the uniaxial strains effects, we investigated the Stark energy shift led by an external electric field for impurity states as functions of pressure as well as the impurity position, Al component and areal electron density. The numerical result shows that the binding energy near linearly increases with pressure from 0 to 10~GPa. It is also found that the binding energy as a function of the electric field perpendicular to the interface shows an un-linear red shift or a blue shift for different impurity positions. The effect of increasing x on blue shift is more significant than that on the red shift for the impurity in the channel near the interface. The pressure influence on the Stark shift is more obvious with increase of electric field and the distance between an impurity and the interface. The increase of pressure decreases the blue shift but increases the red shift.

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Model and analysis of drain induced barrier lowering effect for 4H--SiC metal semiconductor field effect transistor

Cao Quan-Jun, Zhang Yi-Men, Jia Li-Xin

Chin. Phys. B, 2009, 18 (10): 4456-4459 doi: 10.1088/1674-1056/18/10/058
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[PDF 400 KB] (Downloads:743)

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Based on an analytical solution of the two-dimensional Poisson equation in the subthreshold region, this paper investigates the behavior of DIBL (drain induced barrier lowering) effect for short channel 4H--SiC metal semiconductor field effect transistors (MESFETs). An accurate analytical model of threshold voltage shift for the asymmetric short channel 4H--SiC MESFET is presented and thus verified. According to the presented model, it analyses the threshold voltage for short channel device on the L/a (channel length/channel depth) ratio, drain applied voltage V_DS and channel doping concentration N_D, thus providing a good basis for the design and modelling of short channel 4H--SiC MESFETs device.

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Metal/semiconductor hybrids consisting of self-assembled CdS nanoparticles on Cd nanowires

Fu Xiu-Li, Peng Zhi-Jian, Tang Wei-Hua, Guo Xi

Chin. Phys. B, 2009, 18 (10): 4460-4464 doi: 10.1088/1674-1056/18/10/059
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[PDF 4677 KB] (Downloads:649)

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We report on the synthesis and the characterisation of metal/semiconductor hybrids consisting of self-assembled CdS nanoparticles on Cd nanowires, which are grown by thermal evaporation of the mixture of CdS and Cr. The growth of the hybrids is attributed to the decomposition of CdS at high temperature and the strain relieving that arises mainly from the lattice mismatch between Cd and CdS. Temperature dependence of zero-field resistance of single nanohybrid indicates that the as-produced Cd/CdS nanohybrid undergoes a metal--semiconductor transition as a natural consequence of hybrid from metallic Cd and semiconducting CdS. The metal/semiconductor hybrid property provides a promising basis for the development of novel nanoelectronic devices.

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The modulation of Schottky barrier height of NiSi/n-Si Schottky diodes by silicide as diffusion source technique

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Chin. Phys. B, 2009, 18 (10): 4465-4469 doi: 10.1088/1674-1056/18/10/060
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This paper reports that the Schottky barrier height modulation of NiSi/n-Si is experimentally investigated by adopting a novel silicide-as-diffusion-source technique, which avoids the damage to the NiSi/Si interface induced from the conventional dopant segregation method. In addition, the impact of post-BF₂ implantation after silicidation on the surface morphology of Ni silicides is also illustrated. The thermal stability of Ni silicides can be improved by silicide-as-diffusion-source technique. Besides, the electron Schottky barrier height is successfully modulated by 0.11~eV at a boron dose of 10¹⁵~cm^-2 in comparison with the non-implanted samples. The change of barrier height is not attributed to the phase change of silicide films but due to the boron pile-up at the interface of NiSi and Si substrate which causes the upward bending of conducting band. The results demonstrate the feasibility of novel silicide-as-diffusion-source technique for the fabrication of Schottky source/drain Si MOS devices.

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Formation of the intermediate semiconductor larger for the Ohmic contact to silicon carbide using Germanium implantation

Wang Yang-Yuan, Guo Hui, Wang Yue-Hu, Zhang Yu-Ming, Qiao Da-Yong, Zhang Yi-Men

Chin. Phys. B, 2009, 18 (10): 4470-4473 doi: 10.1088/1674-1056/18/10/061
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By formation of an intermediate semiconductor layer (ISL) with a narrow band gap at the metallic contact/SiC interface, this paper realises a new method to fabricate the low-resistance Ohmic contacts for SiC. An array of transfer length method (TLM) test patterns is formed on N-wells created by P⁺ ion implantation into Si-faced p-type 4H-SiC epilayer. The ISL of nickel-metal Ohmic contacts to n-type 4H-SiC could be formed by using Germanium ion implantation into SiC. The specific contact resistance ρ_c as low as 4.23× 10^-5~Ωega \cdotcm² is achieved after annealing in N₂ at 800~°C for 3~min, which is much lower than that (>900~°C) in the typical SiC metallisation process. The sheet resistance R_sh of the implanted layers is 1.5~kΩega /\Box. The technique for converting photoresist into nanocrystalline graphite is used to protect the SiC surface in the annealing after Ge⁺ ion implantations.

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Physical simulations and experimental results of 4H—SiC MESFETs on high purity semi-insulating substrates

Chen Gang, Bai Song, Li Zhe-Yang, Wu Peng, Chen Zheng, Han Ping

Chin. Phys. B, 2009, 18 (10): 4474-4478 doi: 10.1088/1674-1056/18/10/062
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In this paper we report on DC and RF simulations and experimental results of 4H--SiC metal semiconductor field effect transistors (MESFETs) on high purity semi-insulating substrates. DC and small-signal measurements are compared with simulations. We design our device process to fabricate n-channel 4H--SiC MESFETs with 100~μm gate periphery. At 30~V drain voltage, the maximum current density is 440~mA/mm and the maximum transconductance is 33~mS/mm. For the continuous wave (CW) at a frequency of 2~GHz, the maximum output power density is measured to be 6.6~W/mm, with a gain of 12~dB and power-added efficiency of 33.7％. The cut-off frequency (f_T) and the maximum frequency (f_max) are 9~GHz and 24.9~GHz respectively. The simulation results of f_T and f_max are 11.4~GHz and 38.6~GHz respectively.

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Tunneling conductance in quantum wire/insulator/d_{x² -y²} + id_xy mixed wave superconductor junctions

Wei Jian-Wen

Chin. Phys. B, 2009, 18 (10): 4479-4485 doi: 10.1088/1674-1056/18/10/063
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Using the extended Blonder--Tinkham--Klapwijk (BTK) theory, this paper calculates the tunnelling conductance in quantum wire/insulator/d_{x² -y²} + id_xy mixed wave superconductor ( q / I /d_{x² -y²} + id_xy) junctions. That is different from the case in d- and p-wave superconductor junctions. When the angle α between a-axis of the d_{x² -y²} wave superconductor and the interface normal is π /4, there follows a rather distinctive tunnelling conductance. The zero-bias conductance peak (ZBCP) may or may not appear in the tunnelling conductance. Both the interface potential z and the quasi-particle lifetime factor Γ are smaller, there is no ZBCP. Otherwise, the ZBCP will appear. The position of bias conductance peak (BCP) depends strongly on the amplitude ratio of two components for d_{x² -y²} + id_xy mixed wave. The low and narrow ZBCP may coexist with the BCP in the tunnelling conductance. Using those features in the tunnelling conductance of \rm q / I /d_x² - y² + \rm id_xy junctions, it can distinguish d_x² - y² + \rm id_xy mixed wave superconductor from d- and p-wave one.

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Matching effect in superconducting NbN thin film with a square lattice of holes

Muhammad Kamran, He Shi-Kun, Zhang Wei-Jun, Cao Wen-Hui, Li Bo-Hong, Kang Lin, Chen Jian, Wu Pei-Heng, Qiu Xiang-Gang

Chin. Phys. B, 2009, 18 (10): 4486-4489 doi: 10.1088/1674-1056/18/10/064
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A square hole array is fabricated over a micro-bridge of NbN thin film by electron beam lithography and reactive ion milling. Magneto-resistance is measured across the micro-bridge filled with a hole array near the superconducting transition temperature. It is found that magneto-resistance minima occur when the number of vortices is an integer multiple or a fractional multiple of the number of holes. The temperature and the current dependences of the matching effect are studied.

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Electronic structures and magnetic properties in SmCo_7-xM_x

Wang Pei-Ji, Kao Hong, Zhang Chang-Wen, Yu Feng, Zhou Zhong-Xiang

Chin. Phys. B, 2009, 18 (10): 4490-4496 doi: 10.1088/1674-1056/18/10/065
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The electronic structures and magnetic properties of SmCo_7-xM_x (M=Ti, Si, Zr, Hf, Cu, B, Ag, Ga, Mn) compounds are investigated by using a spin-polarized MS-X.α method. The results show that the long-range ferromagnetic order is determined by a stronger 3d--5d interaction, rather than the traditional RKKY interaction, and the effects of doping element M on 3d--5d coupling are negligible in Sm--Co-based compounds. The nonmagnetic dopant Si atoms have a larger effect on the moments of 2e site although they preferably occupy the Co 3g sites, which results in the stronger uniaxial anisotropy of this compound. Analysis of the formation energies indicates that 5d-element doped compounds are more stable than other dopants, and furthermore, they have a higher Curie temperature above room temperature, which will be in favor of their potential application as high-temperature permanent magnets.

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Effects of anharmonic lattice distortion on orbital and magnetic orderings in KCuF₃

Liu Da-Yong, Chen Dong-Meng, Zou Liang-Jian

Chin. Phys. B, 2009, 18 (10): 4497-4505 doi: 10.1088/1674-1056/18/10/066
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Lattice, magnetic and orbital structures in KCuF₃ are self-consistently determined by our cluster self-consistent field approach based on a spin-orbital-lattice Hamiltonian. Two stable structures are obtained and found to be degenerate, which confirms the presence of the coexistent phases observed experimentally. We clearly show that due to the inherent frustration, the ground state of the system only with the superexchange interaction is degenerate; while the Jahn--Teller distortion, especially the anharmonic effect, stabilizes the orbital ordered phase at about 23\% in the x²--y² orbit and at 77\% in the 3z²--r² orbit. Meanwhile the magnetic moment of Cu is considerably reduced to 0.56μ_B, and magnetic coupling strengths are highly anisotropic, J_z/J_xy \approx 18. These results are in good agreement with the experiments, implying that the anharmonic Jahn--Teller effect plays an essential role in stabilising the orbital ordered ground state of KCuF₃.

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Hydrogen absorption of LaFe_11.5Si_1.5 compound under low hydrogen gas pressure

Fu Bin, Long Yi, Shi Pu-Ji, Ma Tao, Bao Bo, Yan A-Ru, Chen Ren-Jie

Chin. Phys. B, 2009, 18 (10): 4506-4510 doi: 10.1088/1674-1056/18/10/067
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Hydrogen absorptions of LaFe_11.5Si_1.5 compound in 1-atm hydrogen gas at different temperatures are investigated. The hydrogen content in the hydrogenated sample does not increase with the increase of temperature of hydrogen absorption but changes complicatedly. The characteristic of first-order transition in LaFe_11.5Si_1.5 compound is weakened after hydrogen absorption. It leads the peaks of magnetic entropy to become wider and the hysteresis loss to reduce significantly, but relative cooling power (RCP) is not changed considerably.

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Great enhancement in remnant polarization of SrBi₄Ti₄O₁₅ thin films by A-site doping of Fe³⁺ ions

Sun Jia-Bao, Sun Hui, Wang Wei, Cai Hua, Chen Xiao-Bing

Chin. Phys. B, 2009, 18 (10): 4511-4514 doi: 10.1088/1674-1056/18/10/068
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SrBi_4-xFe_xTi₄O₁₅ (SBFT-x) thin films (x =0.00, 0.05, 0.08, 0.15) have been synthesized on Pt/Ti/SiO₂/Si (100) substrates by sol-gel method. This paper finds that Fe-doping does not change the crystal structure of SrBi₄Ti₄O₁₅ (SBTi). The coercive filed (E_c) and remnant polarisation (P_r) increase at first, then decrease with the increase of Fe doping content. At a maximum applied field of 229~kV/cm, the 2P_r reaches a maximum value of 91.1~μ C/cm² at x = 0.05 and the corresponding E_c is 72~kV/cm. The 2P_r increases by about 260% and the E_c decreases by about 6%, respectively. Obviously, the ferroelectric property of SrBi₄Ti₄O₁₅ is greatly enhanced by Fe doping. The fatigue-endurance characteristic of the SBFT-0.05 is not improved. After 1.1×10^9 read/write cycles at a frequency of 50~kHz, the nonvolatile polarisations (P_nv= P*- P^Λ) decreased about 48% of its initial value.

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Transient surface photoconductivity of GaAs emitter studied by terahertz pump-emission spectroscopy

Shi Yu-Lei, Zhou Qing-Li, Zhang Cun-Lin

Chin. Phys. B, 2009, 18 (10): 4515-4520 doi: 10.1088/1674-1056/18/10/069
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This paper investigates the ultrafast carrier dynamics and surface photoconductivity of unbiased semi-insulating GaAs in detail by using a terahertz pump-emission technique. Based on theoretical modelling, it finds that transient photoconductivity plays a very important role in the temporal waveform of terahertz radiation pulse. Anomalous enhancement in both terahertz radiation and transient photoconductivity is observed after the excitation of pump pulse and we attribute these phenomena to carrier capture in the EL2 centers. Moreover, the pump power- and temperature-dependent measurements are also performed to verify this trapping model.

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Ultraviolet laser-induced voltages in LaSrAlO₄ single crystal

Lü Zhi-Qing, Zhao Kun, Zhao Hui, Zhao Song-Qing, Zhou Qing-Li

Chin. Phys. B, 2009, 18 (10): 4521-4523 doi: 10.1088/1674-1056/18/10/070
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[PDF 170 KB] (Downloads:547)

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Laser-induced ultrafast photovoltaic effect is observed in LaSrAlO₄ single crystal at ambient temperature without any applied bias. An open-circuit photovoltage is obtained when the wafer is irradiated by a 248-nm-KrF laser pulse of 20~ns duration. The response time and full width at half maximum of the photovoltage pulse are 6~ns and 19~ns, respectively, indicating that LaSrAlO₄ single crystal has potential application in ultraviolet detector.

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Synthesis and characterization of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors prepared by sol--gel--combustion processing

Huang Ping, Cui Cai-E, Wang Sen

Chin. Phys. B, 2009, 18 (10): 4524-4531 doi: 10.1088/1674-1056/18/10/071
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[PDF 1484 KB] (Downloads:1951)

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A type of red luminescent Sr₃Al₂O₊:Eu²⁺, Dy³⁺ phosphor powder is synthesised by sol--gel--combustion processing, with metal nitrates used as the source of metal ions and citric acid as a chelating agent of metal ions. By tracing the formation process of the sol--gel, it is found that it is necessary to reduce the amount of NO₃^- by dropping ethanol into the solution for forming a stable and homogeneous sol--gel. Thermogravimetric and Differential Scanning Calorimeter Analysis, x-ray diffractionmeter, scanning electron microscopy and photoluminescence spectroscopy are used to investigate the luminescent properties of the as-synthesised Sr₃Al₂O₆:Eu²⁺, Dy³⁺. The results reveal that the Sr₃Al₂O₆ crystallises completely when the combustion ash is sintered at 1250~℃. The excitation and the emission spectra indicate that the excitation broadband lies mainly in a visible range and the phosphors emit a strong light at 618~nm under the excitation of 472~nm. The afterglow of (Sr_0.097Eu_0.03Dy_0.03)₃Al₂O₆ phosphors sintered at 1250~℃ lasts for over 1000 s when the excited source is cut off.

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A novel Dy³⁺-doped GdPO₄ white-light phosphors under vacuum ultraviolet excitation for Hg-free lamps application

Han Guo-Cai, Wang Yu-Hua, Wu Chun-Fang, Zhang Jia-Chi

Chin. Phys. B, 2009, 18 (10): 4532-4535 doi: 10.1088/1674-1056/18/10/072
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Novel Dy³⁺-doped GdPO₄ white light phosphors with monoclinic system were successfully synthesised by hydrothermal method at 240~\du. This paper investigates the luminescence properties of white-light Gd_1-xPO₄,:xDy³⁺ under vacuum ultraviolet (VUV) excitation. The strong absorption at around 147~nm in excitation spectrum energy can be transferred to the energy levels of Dy³⁺ ion from the host absorption. Additionally, this white light phosphors are activated by a single Dy³⁺ion and with a lower preparation temperature, which tend to decrease the consumption of rare earth resource and energy. Therefore, the luminescence of Gd_1-xPO₄:xDy³⁺ under VUV excitation is effective, and proves to be promising in application to mercury-free lamp.

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Effects of doping concentration on properties of Mn-doped ZnO thin films

Gao Li, Zhang Jian-Min

Chin. Phys. B, 2009, 18 (10): 4536-4540 doi: 10.1088/1674-1056/18/10/073
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[PDF 406 KB] (Downloads:1091)

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This paper reports that the radio frequency magnetron sputtering is used to fabricate ZnO and Mn-doped ZnO thin films on glass substrates at 500~°C. The Mn-doped ZnO thin films present wurtzite structure of ZnO and have a smoother surface, better conductivity but no ferromagnetism. The x-ray photoelectron spectroscopy results show that the binding energy of Mn_2p3 / 2 increases with increasing Mn content slightly, and the state of Mn in the Mn-doped ZnO thin films is divalent. The chemisorbed oxygen in the Mn-doped ZnO thin films increases with increasing Mn doping concentration. The photoluminescence spectra of ZnO and Mn-doped ZnO thin films have a similar ultraviolet emission. The yellow green emissions of 4~wt.% and 10~wt.% Mn-doped thin films are quenched, whereas the yellow green emission occurs because of abundant oxygen vacancies in the Mn-doped ZnO thin films after 20~wt.% Mn doping. Compared with pure ZnO thin film, the bandgap of the Mn-doped ZnO thin films increases with increasing Mn content.

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Distribution of carriers in gradient-doping transmission-mode GaAs photocathodes grown by molecular beam epitaxy

Zhang Yi-Jun, Chang Ben-Kang, Yang Zhi, Niu Jun, Zou Ji-Jun

Chin. Phys. B, 2009, 18 (10): 4541-4546 doi: 10.1088/1674-1056/18/10/074
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[PDF 193 KB] (Downloads:815)

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The gradient-doping structure is first applied to prepare the transmission-mode GaAs photocathode and the integral sensitivity of the sealed image tube achieves 1420~μ A/lm. This paper studies the inner carrier concentration distribution of the gradient-doping transmission-mode GaAs photocathode after molecular beam epitaxy (MBE) growth using the electrochemical capacitance-voltage profiling. The results show that an ideal gradient-doping structure can be obtained by using MBE growth. The total band-bending energy in the gradient-doping GaAs active-layer with doping concentration ranging from 1× 10^19~cm^-3 to 1×10^18~cm^-3 is calculated to be 46.3 meV, which helps to improve the photoexcited electrons movement toward surface for the thin epilayer. In addition, by analysis of the band offsets, it is found that the worse carrier concentration discrepancy between GaAs and GaAlAs causes a lower back interface electron potential barrier which decreases the amount of high-energy photoelectrons and affects the short-wave response.

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Enhanced field emission characteristics of thin-Au-coated nano-sheet carbon films

Gu Guang-Rui, Ito Toshimichi

Chin. Phys. B, 2009, 18 (10): 4547-4551 doi: 10.1088/1674-1056/18/10/075
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This paper reports that the nano-sheet carbon films (NSCFs) were fabricated on Si wafer chips with hydrogen--methane gas mixture by means of quartz-tube-type microwave plasma chemical vapour deposition (MWPCVD). In order to further improve the field emission (FE) characteristics, a 5-nm Au film was prepared on the samples by using electron beam evaporation. The FE properties were obviously improved due to depositing Au thin film on NSCFs. The FE current density at a macroscopic electric field, E, of 9~V/μ m was increased from 12.4~mA/cm² to 27.2~mA/cm² and the threshold field was decreased from 2.6~V/μ m to 2.0~V/μ m for Au-coated carbon films. A modified F-N model considering statistic effects of FE tip structures in the low E region and a space-charge-limited-current effect in the high E region were applied successfully to explain the FE data of the Au-coated NSCF.