Table of contents

12 February 2014, Volume 23 Issue 2 Previous issue    Next issue

GENERAL

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On the exact solutions to the long–short-wave interaction system Fan Hui-Ling (范慧玲), Fan Xue-Fei (范雪飞), Li Xin (李欣) Chin. Phys. B, 2014, 23 (2):  020201.  DOI: 10.1088/1674-1056/23/2/020201 Abstract ( 643 )   PDF (174KB) ( 642 )   The complete discrimination system for polynomial method is applied to the long–short-wave interaction system to obtain the classifications of single traveling wave solutions. Compared with the solutions given by the (G’/G)-expansion method, we gain some new solutions.

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Simulation of shock-induced instability using an essentially conservative adaptive CE/SE method Fu Zheng (付峥), Liu Kai-Xin (刘凯欣), Luo Ning (罗宁) Chin. Phys. B, 2014, 23 (2):  020202.  DOI: 10.1088/1674-1056/23/2/020202 Abstract ( 648 )   PDF (1346KB) ( 506 )   An essentially conservative adaptive space time conservation element and solution element (CE/SE) method is proposed for the effective simulation of shock-induced instability with low computational cost. Its implementation is based on redefined conservation elements (CEs) and solution elements (SEs), optimized interpolations and a Courant number insensitive CE/SE scheme. This approach is used in two applications, the Woodward double Mach reflection and a two-component Richtmyer–Meshkov instability experiment. This scheme reveals the essential features of the investigated cases, captures small unstable structures, and yields a solution that is consistent with the results from experiments or other high order methods.

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A novel (G’/G)-expansion method and its application to the Boussinesq equation Md. Nur Alam, Md. Ali Akbar, Syed Tauseef Mohyud-Din Chin. Phys. B, 2014, 23 (2):  020203.  DOI: 10.1088/1674-1056/23/2/020203 Abstract ( 699 )   PDF (1061KB) ( 1102 )   In this article, a novel (G’/G)-expansion method is proposed to search for the traveling wave solutions of nonlinear evolution equations. We construct abundant traveling wave solutions involving parameters to the Boussinesq equation by means of the suggested method. The performance of the method is reliable and useful, and gives more general exact solutions than the existing methods. The new (G’/G)-expansion method provides not only more general forms of solutions but also cuspon, peakon, soliton, and periodic waves.

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Chaos control in the nonlinear Schrödinger equation with Kerr law nonlinearity Yin Jiu-Li (殷久利), Zhao Liu-Wei (赵刘威), Tian Li-Xin (田立新) Chin. Phys. B, 2014, 23 (2):  020204.  DOI: 10.1088/1674-1056/23/2/020204 Abstract ( 691 )   PDF (885KB) ( 1072 )   The nonlinear Schrödinger equation with Kerr law nonlinearity in the two-frequency interference is studied by the numerical method. Chaos occurs easily due to the absence of damping. This phenomenon will cause the distortion in the process of information transmission. We find that fiber-optic transmit signals still present chaotic phenomena if the control intensity is smaller. With the increase of intensity, the fiber-optic signal can stay in a stable state in some regions. When the strength is suppressed to a certain value, an unstable phenomenon of the fiber-optic signal occurs. Moreover we discuss the sensitivities of the parameters to be controlled. The results show that the linear term coefficient and the environment of two quite different frequences have less effects on the fiber-optic transmission. Meanwhile the phenomena of vibration, attenuation and escape occur in some regions.

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Exponential synchronization of complex dynamical networks with Markovian jumping parameters using sampled-data and mode-dependent probabilistic time-varying delays R. Rakkiyappan, N. Sakthivel, S. Lakshmanan Chin. Phys. B, 2014, 23 (2):  020205.  DOI: 10.1088/1674-1056/23/2/020205 Abstract ( 711 )   PDF (357KB) ( 657 )   In this paper, the problem of exponential synchronization of complex dynamical networks with Markovian jumping parameters using sampled-data and Mode-dependent probabilistic time-varying coupling delays is investigated. The sampling period is assumed to be time-varying and bounded. The information of probability distribution of the time-varying delay is considered and transformed into parameter matrices of the transferred complex dynamical network model. Based on the condition, the design method of the desired sampled data controller is proposed. By constructing a new Lyapunov functional with triple integral terms, delay-distribution-dependent exponential synchronization criteria are derived in the form of linear matrix inequalities. Finally, two numerical examples are given to illustrate the effectiveness of the proposed methods.

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Analysis of Cr atom focusing deposition properties in the double half Gaussian standing wave field Chen Sheng (陈晟), Ma Yan (马艳), Zhang Ping-Ping (张萍萍), Wang Jian-Bo (王建波), Deng Xiao (邓晓), Xiao Sheng-Wei (肖盛炜), Ma Rui (马蕊), Li Tong-Bao (李同保) Chin. Phys. B, 2014, 23 (2):  020301.  DOI: 10.1088/1674-1056/23/2/020301 Abstract ( 647 )   PDF (1074KB) ( 413 )   The use of the dipole force on atoms is a new technology that is used to build nanostructures. In this way, a high quality standard nano-grating can be obtained. Based on the semi-classical model, the motion equation is investigated and the trajectories of atoms in double half Gaussian standing wave field are simulated. Compared with the Gaussian standing wave field, the double half Gaussian standing wave can well focus the Cr atoms. In order to obtain this kind of beam, a prism is designed and the experimental result shows that the beam is well generated.

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Photon number cumulant expansion and generating function for photon added- and subtracted-two-mode squeezed states Lu Dao-Ming (卢道明), Fan Hong-Yi (范洪义) Chin. Phys. B, 2014, 23 (2):  020302.  DOI: 10.1088/1674-1056/23/2/020302 Abstract ( 725 )   PDF (183KB) ( 506 )   For the first time, we derive the photon number cumulant for two-mode squeezed state and show that its cumulant expansion leads to normalization of two-mode photon subtracted-squeezed states and photon added- squeezed states. We show that the normalization is related to Jacobi polynomial, so the cumulant expansion in turn represents the new generating function of Jacobi polynomial.

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Teleportation of three-dimensional single particle state in noninertial frames Wu Qi-Cheng (吴奇成), Wen Jing-Ji (文晶姬), Ji Xin (计新), Yeon Kyu-Hwang Chin. Phys. B, 2014, 23 (2):  020303.  DOI: 10.1088/1674-1056/23/2/020303 Abstract ( 769 )   PDF (625KB) ( 452 )   Considering the spin degree of freedom of the Dirac field, we study the entanglement behavior of a different class of communication channel and teleportation of three-dimensional single particle state in noninertial frames. Numerical analysis shows that the communication channel in our scheme can offer enough distillable entanglement for the teleportation protocol. Moreover, the teleportation protocol could work well if Rob’s acceleration is not very big, but the fidelity of the teleportation is still reduced due to the Unruh effect.

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Preserving entanglement and the fidelity of three-qubit quantum states undergoing decoherence using weak measurement Liao Xiang-Ping (廖湘萍), Fang Mao-Fa (方卯发), Fang Jian-Shu (方见树), Zhu Qian-Quan (朱钱泉) Chin. Phys. B, 2014, 23 (2):  020304.  DOI: 10.1088/1674-1056/23/2/020304 Abstract ( 829 )   PDF (2183KB) ( 832 )   We demonstrate a method to preserve entanglement and improve fidelity of three-qubit quantum states undergoing amplitude-damping decoherence using weak measurement and quantum measurement reversal. It is shown that we are able to enhance entanglement to the greatest extent, and to circumvent entanglement sudden death by increasing the weak measurement strength both for the GHZ state and the W state. The weak measurement technique can also enhance the fidelity to the quantum region and even close to 1 for the whole range of the decoherence parameter in both of the two cases. In addition, the W state can maintain more fidelity than the GHZ state in the protection protocol. However, the GHZ state has a higher success probability than the W state.

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Quantum state measurement in double quantum dots with a radio-frequency quantum point contact Yan Lei (严蕾), Wang Hai-Xia (王海霞), Yin Wen (殷雯), Wang Fang-Wei (王芳卫) Chin. Phys. B, 2014, 23 (2):  020305.  DOI: 10.1088/1674-1056/23/2/020305 Abstract ( 665 )   PDF (485KB) ( 417 )   We study the dynamics of two electron spins in coupled quantum dots (CQDs) monitored by a quantum point contact (QPC) detector. Their quantum state can be measured by embedding the QPC in an LC circuit. We derive the Bloch-type rate equations of the reduced density matrix for CQDs. Special attention is paid to the numerical results for the weak measurement condintion under a strong Coulomb interaction. It is shown that the evolution of QPC current always follows that of electron occupation in the right dot. In addition, we find that the output voltage of the circuit can reflect the evolution of QPC current when the circuit and QPC are approximately equal in frequency. In particular, the wave shape of the output voltage can be improved by adjusting the circuit resonance frequency and bandwidth.

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Realization of quantum Fourier transform over ZN Fu Xiang-Qun (付向群), Bao Wan-Su (鲍皖苏), Li Fa-Da (李发达), Zhang Yu-Chao (张宇超) Chin. Phys. B, 2014, 23 (2):  020306.  DOI: 10.1088/1674-1056/23/2/020306 Abstract ( 876 )   PDF (237KB) ( 404 )   Since the difficulty in preparing the equal superposition state of amplitude is 1/√N, we construct a quantile transform of quantum Fourier transform (QFT) over ZN based on the elementary transforms, such as Hadamard transform and Pauli transform. The QFT over ZN can then be realized by the quantile transform, and used to further design its quantum circuit and analyze the requirements for the quantum register and quantum gates. However, the transform needs considerable quantum computational resources and it is difficult to construct a high-dimensional quantum register. Hence, we investigate the design of t-bit quantile transform, and introduce the definition of t-bit semiclassical QFT over ZN. According to probability amplitude, we prove that the transform can be used to realize QFT over ZN and further design its quantum circuit. For this transform, the requirements for the quantum register, the one-qubit gate, and two-qubit gate reduce obviously when compared with those for the QFT over ZN.

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Correlation dynamics of a qubit–qutrit system in a spin-chain environment with Dzyaloshinsky–Moriya interaction Hot! Yang Yang (杨阳), Wang An-Min (王安民) Chin. Phys. B, 2014, 23 (2):  020307.  DOI: 10.1088/1674-1056/23/2/020307 Abstract ( 646 )   PDF (1771KB) ( 582 )   We study the dynamics of correlations for a hybrid qubit–qutrit system in an XY spin-chain environment with Dzyaloshinsky–Moriya interaction. Our discussion involves a comparative analysis of negativity, quantum discord, and measurement-induced disturbance. It is found that the quantum discord is optimal of the three quantum correlations to detect the critical point of quantum phase transition. Only when the qubit interacts with the environment, is the phenomenon of sudden transition between the classical correlation and the quantum discord observed. Moreover, the Dzyaloshinsky–Moriya interaction enhances the decay of quantum correlations.

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Condensate fraction of asymmetric three-component Fermi gas Du Jia-Jia (杜佳佳), Liang Jun-Jun (梁军军), Liang Jiu-Qing (梁九卿) Chin. Phys. B, 2014, 23 (2):  020308.  DOI: 10.1088/1674-1056/23/2/020308 Abstract ( 679 )   PDF (275KB) ( 365 )   In this paper, we investigate the condensate fraction (CF) of fermionic pairs in the BCS–BEC crossover for three-component Fermi gas with both asymmetric interactions and unequal chemical potentials in two-dimensional free space. By using the functional-path-integral method, we have analytically derived the number densities and bound-state energy, from which the off-diagonal long-range order is analyzed in terms of the asymptotic behavior of the two-body density matrix. The explicit formula of CF is obtained as a function of the bound-state energy and population imbalance. It is demonstrated that the CF spectrum with respect to the bound-state energy can be used to characterize the quantum phase transition between the two kinds of Sarma phases as well as the transition from three-component to two-component superfluid. Moreover we obtain the same analytic formula of CF in the BCS superfluid phase as that of homogeneous Fermi gas with equal chemical potentials.

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Nonlinear resonance phenomenon of one-dimensional Bose–Einstein condensate under periodic modulation Hua Wei (花巍), Liu Shi-Xing (刘世兴) Chin. Phys. B, 2014, 23 (2):  020309.  DOI: 10.1088/1674-1056/23/2/020309 Abstract ( 670 )   PDF (1431KB) ( 476 )   We investigate the effect of an external periodic modulation on the one-dimensional (1D) Bose–Einstein condensate with harmonic trapping potential. By numerically solving the Gross–Pitaevskii equation with symplectic algorithm, the nonlinear resonance phenomenon is shown and the corresponding Fourier spectrum is given. The autoresonance phenomenon is also presented under almost periodic external modulation, and it shows that the condensate eventually evolves into quasi-periodic oscillation.

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Dynamics of entanglement under decoherence in noninertial frames Shi Jia-Dong (石甲栋), Wu Tao (吴韬), Song Xue-Ke (宋学科), Ye Liu (叶柳) Chin. Phys. B, 2014, 23 (2):  020310.  DOI: 10.1088/1674-1056/23/2/020310 Abstract ( 576 )   PDF (1727KB) ( 549 )   In this paper, we investigate the entanglement dynamics of a two-qubit entangled state coupled with its noisy environment, and plan to utilize weak measurement and quantum reversal measurement to study the entanglement dynamics under different decoherence channels in noninertial frames. Through the calculations and analyses, it is shown that the weak measurement can prevent entanglement from coupling to the amplitude damping channel, while the system is under the phase damping and flip channels. This protection protocol cannot prevent entanglement but will accelerate the death of entanglement. In addition, if the system is in the noninertial reference frame, then the effect of weak measurement will be weakened for the amplitude damping channel. Nevertheless, for other decoherence channels, the Unruh effect does not affect the quantum weak measurement, the only exception is that the maximum value of entanglement is reduced to √2/2 of the original value in the inertial frames.

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Resonant interaction scheme for GHZ state preparation and quantum phase gate with superconducting qubits in a cavity Liu Xin (刘欣), Liao Qing-Hong (廖庆洪), Fang Guang-Yu (方光宇), Wang Yue-Yuan (王月媛), Liu Shu-Tian (刘树田) Chin. Phys. B, 2014, 23 (2):  020311.  DOI: 10.1088/1674-1056/23/2/020311 Abstract ( 650 )   PDF (236KB) ( 456 )   A scheme is proposed to generate GHZ state and realize quantum phase gate for superconducting qubits placed in a microwave cavity. This scheme uses resonant interaction between the qubits and the cavity mode, so that the interaction time is short, which is important in view of decoherence. In particular, the phase gate can be realized simply with a single interaction between the qubits and the cavity mode. With cavity decay being considered, the fidelity and success probability are both very close to unity.

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Deterministic joint remote preparation of an arbitrary two-qubit state in the presence of noise Chen Zhong-Fang (陈忠芳), Liu Jin-Ming (刘金明), Ma Lei (马雷) Chin. Phys. B, 2014, 23 (2):  020312.  DOI: 10.1088/1674-1056/23/2/020312 Abstract ( 670 )   PDF (918KB) ( 451 )   Using two tripartite Greenberger–Horne–Zeilinger (GHZ) states as the shared channels, we investigate the noise effects on the deterministic joint remote preparation of an arbitrary two-qubit state. By unitary matrix decomposition procedure, we first construct the quantum logic circuit of the deterministic joint remote state preparation protocol. Then, we analytically derive the fidelity and the average fidelity for the deterministic joint remote preparation of an arbitrary two-qubit state and of four types of special two-qubit states under the influence of the Pauli noises. It is found that the fidelity depends on the noise types, the qubit-environment coupling strength, and the state to be remotely prepared. Moreover, even if the two GHZ channels are subject to the same environmental noises, the average fidelities for remotely preparing different two-qubit states display different time evolution behaviors. The remote preparation of the identical two-qubit states also shows that the average fidelities affected by different noisy environments exhibit different evolution actions.

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Electronic cluster state entanglement concentration based on charge detection Liu Jiong (刘炯), Zhao Sheng-Yang (赵圣阳), Zhou Lan (周澜), Sheng Yu-Bo (盛宇波) Chin. Phys. B, 2014, 23 (2):  020313.  DOI: 10.1088/1674-1056/23/2/020313 Abstract ( 692 )   PDF (346KB) ( 389 )   We propose an efficient entanglement concentration protocol (ECP) based on electron-spin cluster states assisted with single electrons. In the ECP, we adopt the electron polarization beam splitter (PBS) and the charge detector to construct the quantum nondemolition measurement. According to the result of the measurement of the charge detection, we can ultimately obtain the maximally entangled cluster states. Moreover, the discarded items can be reused in the next round to reach a high success probability. This ECP may be useful in current solid quantum computation.

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Nonlinear dissipative dynamics of a two-component atomic condensate coupling with a continuum Zhong Hong-Hua (钟宏华), Xie Qiong-Tao (谢琼涛), Xu Jun (徐军), Hai Wen-Hua (海文华), Li Chao-Hong (李朝红) Chin. Phys. B, 2014, 23 (2):  020314.  DOI: 10.1088/1674-1056/23/2/020314 Abstract ( 602 )   PDF (748KB) ( 393 )   We investigate the nonlinear dissipative coherence bifurcation and population dynamics of a two-component atomic Bose–Einstein condensate coupling with a continuum. The coupling between the two-component condensates and the continuum brings effective dissipations to the two-component condensates. The steady states and the coherence bifurcation depend on both dissipation and the nonlinear interaction between condensed atoms. The coherence among condensed atoms may be even enhanced by the effective dissipations. The combination of dissipation and nonlinearity allows one to control the switching between different self-trapped states or the switching between a self-trapped state and a non-self-trapped state.

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Dynamical Casimir effect in superradiant light scattering by Bose–Einstein condensate in an optomechanical cavity Sonam Mahajan, Neha Aggarwal, Aranya B Bhattacherjee, ManMohan Chin. Phys. B, 2014, 23 (2):  020315.  DOI: 10.1088/1674-1056/23/2/020315 Abstract ( 618 )   PDF (738KB) ( 306 )   We investigate the effects of dynamical Casimir effect in superradiant light scattering by Bose–Einstein condensate in an optomechanical cavity. The system is studied using both classical and quantized mirror motions. The cavity frequency is harmonically modulated in time for both the cases. The main quantity of interest is the number of intracavity scattered photons. The system has been investigated under the weak and strong modulations. It has been observed that the amplitude of the scattered photons is more for the classical mirror motion than the quantized mirror motion. Also, initially, the amplitude of scattered photons is high for lower modulation amplitude than higher modulation amplitude. We also found that the behavior of the plots are similar under strong and weak modulations for the quantized mirror motion.

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Studies of phase return map and symbolic dynamics in a periodically driven Hodgkin–Huxley neuron Ding Jiong (丁炯), Zhang Hong (张宏), Tong Qin-Ye (童勤业), Chen Zhuo (陈琢) Chin. Phys. B, 2014, 23 (2):  020501.  DOI: 10.1088/1674-1056/23/2/020501 Abstract ( 667 )   PDF (706KB) ( 520 )   How neuronal spike trains encode external information is a hot topic in neurodynamics studies. In this paper, we investigate the dynamical states of the Hodgkin–Huxley neuron under periodic forcing. Depending on the parameters of the stimulus, the neuron exhibits periodic, quasiperiodic and chaotic spike trains. In order to analyze these spike trains quantitatively, we use the phase return map to describe the dynamical behavior on a one-dimensional (1D) map. According to the monotonicity or discontinuous point of the 1D map, the spike trains are transformed into symbolic sequences by implementing a coarse-grained algorithm – symbolic dynamics. Based on the ordering rules of symbolic dynamics, the parameters of the external stimulus can be measured in high resolution with finite length symbolic sequences. A reasonable explanation for why the nervous system can discriminate or cognize the small change of the external signals in a short time is also presented.

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A modified equation of state for Xe at high pressures by molecular dynamics simulation Xiao Hong-Xing (肖红星), Long Chong-Sheng (龙冲生) Chin. Phys. B, 2014, 23 (2):  020502.  DOI: 10.1088/1674-1056/23/2/020502 Abstract ( 564 )   PDF (256KB) ( 638 )   The exact equation of state (EOS) for the fission gas Xe is necessary for the accurate prediction of the fission gas behavior in uranium dioxide nuclear fuel. However, the comparison with the experimental data indicates that the applicable pressure ranges of existing EOS for Xe published in the literature cannot cover the overpressure of the rim fission gas bubble at the typical UO2 fuel pellet rim structure. Based on the interatomic potential of Xe, the pressure–volume–temperature data are calculated by the molecular dynamics (MD) simulation. The results indicate that the data of MD simulation with Ross and McMahan’s potential [M. Ross and A. K. McMahan 1980 Phys. Rev. B 21 1658] are in good agreement with the experimental data. A preferable EOS for Xe is proposed based on the MD simulation. The comparison with the MD simulation data shows that the proposed EOS can be applied at pressures up to 550 MPa and 3 GPa and temperatures 900 K and 1373 K respectively. The applicable pressure range of this EOS is wider than those of the other existing EOS for Xe published in the literature.

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An improved car-following model with consideration of the lateral effect and its feedback control research Zheng Ya-Zhou (郑亚周), Zheng Peng-Jun (郑彭军), Ge Hong-Xia (葛红霞) Chin. Phys. B, 2014, 23 (2):  020503.  DOI: 10.1088/1674-1056/23/2/020503 Abstract ( 706 )   PDF (27695KB) ( 484 )   A car-following model is presented, in which the effects of non-motor vehicles on adjacent lanes are taken into account. A control signal including the velocity differences between the following vehicle and the target vehicle is introduced according to the feedback control theory. The stability condition for the new model is derived. Numerical simulation is used to demonstrate the advantage of the new model including the control signal; the results are consistent with the analytical ones.

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Employment of Jacobian elliptic functions for solving problems in nonlinear dynamics of microtubules Slobodan Zeković, Annamalai Muniyappan, Slobodan Zdravković, Louis Kavitha Chin. Phys. B, 2014, 23 (2):  020504.  DOI: 10.1088/1674-1056/23/2/020504 Abstract ( 631 )   PDF (437KB) ( 375 )   We show how Jacobian elliptic functions (JEFs) can be used to solve ordinary differential equations (ODEs) describing the nonlinear dynamics of microtubules (MTs). We demonstrate that only one of the JEFs can be used while the remaining two do not represent the solutions of the crucial differential equation. We show that a kink-type soliton moves along MTs. Besides this solution, we also discuss a few more solutions that may or may not have physical meanings. Finally, we show what kind of ODE can be solved by using JEFs.

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A unified drain current 1/f noise model for GaN-based high electron mobility transistors Liu Yu-An (刘宇安), Zhuang Yi-Qi (庄奕琪), Ma Xiao-Hua (马晓华), Du Ming (杜鸣), Bao Jun-Lin (包军林), Li Cong (李聪) Chin. Phys. B, 2014, 23 (2):  020701.  DOI: 10.1088/1674-1056/23/2/020701 Abstract ( 753 )   PDF (306KB) ( 669 )   In this work, we present a theoretical and experimental study on the drain current 1/f noise in the AlGaN/GaN high electron mobility transistor (HEMT). Based on both mobility fluctuation and carrier number fluctuation in a two-dimensional electron gas (2DEG) channel of AlGaN/GaN HEMT, a unified drain current 1/f noise model containing a piezoelectric polarization effect and hot carrier effect is built. The drain current 1/f noise induced by the piezoelectric polarization effect is distinguished from that induced by the hot carrier effect through experiments and simulations. The simulation results are in good agreement with the experimental results. Experiments show that after hot carrier injection, the drain current 1/f noise increases four orders of magnitude and the electrical parameter degradation Δgm/gm reaches 54.9%. The drain current 1/f noise degradation induced by the piezoelectric effect reaches one order of magnitude; the electrical parameter degradation Δgm/gm is 11.8%. This indicates that drain current 1/f noise of the GaN-based HEMT device is sensitive to the hot carrier effect and piezoelectric effect. This study provides a useful reliability characterization tool for the AlGaN/GaN HEMTs.

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Formation and dissociation of protonated cytosine–cytosine base pairs in i-motifs by ab initio quantum chemical calculations Zhang Xiao-Hu (张小虎), Li Ming (黎明), Wang Yan-Ting (王延颋), Ouyang Zhong-Can (欧阳钟灿) Chin. Phys. B, 2014, 23 (2):  020702.  DOI: 10.1088/1674-1056/23/2/020702 Abstract ( 728 )   PDF (646KB) ( 556 )   Formation and dissociation mechanisms of C–C+ base pairs in acidic and alkaline environments are investigated, employing ab initio quantum chemical calculations. Our calculations suggest that, in an acidic environment, a cytosine monomer is first protonated and then dimerized with an unprotonated cytosine monomer to form a C–C+ base pair; in an alkaline environment, a protonated cytosine dimer is first unprotonated and then dissociated into two cytosine monomers. In addition, the force for detaching a C–C+ base pair was found to be inversely proportional to the distance between the two cytosine monomers. These results provide a microscopic mechanism to qualitatively explain the experimentally observed reversible formation and dissociation of i-motifs.

ATOMIC AND MOLECULAR PHYSICS

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Theoretical study on K, L, and M X-ray transition energies and rates of neptunium and its ions Ismail Abdalla Saber, Dong Chen-Zhong (董晨钟), Wang Xiang-Li (王向丽), Zhou Wei-Dong (周卫东), Wu Zhong-Wen (武中文) Chin. Phys. B, 2014, 23 (2):  023101.  DOI: 10.1088/1674-1056/23/2/023101 Abstract ( 563 )   PDF (314KB) ( 370 )   The transition energies and electric dipole (E1) transition rates of the K, L, and M lines in neutral Np have been theoretically determined from the MultiConfiguration Dirac–Fock (MCDF) method. In the calculations, the contributions from Breit interaction and quantum electrodynamics (QED) effects (vacuum polarization and self-energy), as well as nuclear finite mass and volume effects, are taken into account. The calculated transition energies and rates are found to be in good agreement with other experimental and theoretical results. The accuracy of the results is estimated and discussed. Furthermore, we calculated the transition energies of the same lines radiating from the decaying transitions of the K-, L-, and M-shell hole states of Np ions with the charge states Np1+ to Np6+ for the first time. We found that for a specific line, the corresponding transition energies relating to all the Np ions are almost the same; it means the outermost electrons have a very small influence on the inner-shell transition processes.

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Na decorated B6 cluster and its hydrogen storage properties Ruan Wen (阮文), Wu Dong-Lan (伍冬兰), Luo Wen-Lang (罗文浪), Yu Xiao-Guang (余晓光), Xie An-Dong (谢安东) Chin. Phys. B, 2014, 23 (2):  023102.  DOI: 10.1088/1674-1056/23/2/023102 Abstract ( 664 )   PDF (845KB) ( 857 )   The structures and hydrogen storage properties of sodium atoms decorated B6 clusters are investigated by the B3LYP method with a 6-311+G (d, p) basis set. For NamB6 (m=1–3) clusters, Na atoms are always inclined to separate far enough from each other and not cluster together on a B6 cluster surface so that each Na atom has sufficient space to bind hydrogen molecules. The hydrogen storage gravimetric density of a two Na atoms decorated B6 cluster is 17.91 wt% with an adsorption energy per H2 molecule (AAE/H2) of 0.6851 kcal·mol-1. The appropriate AAE/H2 and preferable gravimetric density of the two Na atoms decorated B6 cluster complex indicate that it is feasible for hydrogen storage application in ambient conditions.

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The Coulomb effect on a low-energy structure in above-threshold ionization spectra induced by mid-infrared laser pulses Lin Zhi-Yang (林志阳), Wu Ming-Yan (吴明艳), Quan Wei (全威), Liu Xiao-Jun (柳晓军), Chen Jing (陈京), Cheng Ya (程亚) Chin. Phys. B, 2014, 23 (2):  023201.  DOI: 10.1088/1674-1056/23/2/023201 Abstract ( 693 )   PDF (544KB) ( 453 )   We investigate the low-energy structure (LES) in the above-threshold ionization spectrum at a mid-infrared laser wavelength with a semiclassical model. Using a softened Coulomb potential (CP) and changing the softening parameter, we show that though the very low-energy structure (VLES) and high low-energy structure (HLES) are both due to the interaction between the ionic CP and the electron, the two structures have different physical mechanisms: the VLES can be attributed to the electron–ion Coulomb interaction at a rather small distance and the HLES is more likely to be ascribed to the electron–ion Coulomb interaction at a large distance.

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Accurate equilibrium inversion barrier of ammonia by extrapolation to the one-electron basis set limit Li Yong-Qing (李永庆), Song Peng (宋朋), Ma Feng-Cai (马凤才) Chin. Phys. B, 2014, 23 (2):  023301.  DOI: 10.1088/1674-1056/23/2/023301 Abstract ( 612 )   PDF (253KB) ( 504 )   A scheme based on treating uniform singlet-pair and triplet-pair interactions is suggested to extrapolate electron correlation energy of ammonia, calculated at two basis-set levels of ab initio theory in the infinite one-electron basis-set limit. The dual-level method is tested on the extrapolation of the full correlation in coupled-cluster singles and doubles and in the case also a noniterative perturbative correction for connected triple energies for the C3v and D3h structures of ammonia, with correlation-consistent basis sets of the type cc-pVXZ (X=D,T,Q,5,6) and aug-cc-pVXZ (X=D,T,Q,5). For testing and comparison purposes, the energies reported by Klopper [J. Comput. Chem. 22 1306 (2001)] have been taken. From a corresponding extrapolation of CCSD(T)/AVXZ energies for X =4, 5, we obtain total inversion barriers of 1833.87 cm-1/1832.33 cm-1 for the two/three-parameter extrapolation rules, which are in good agreement with other theoretical extrapolation and empirical values in the literature.

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High-pressure-activated carbon tetrachloride decomposition Chen Yuan-Zheng (陈元正), Zhou Mi (周密), Sun Mei-Jiao (孙美娇), Li Zuo-Wei (里佐威), Sun Cheng-Lin (孙成林) Chin. Phys. B, 2014, 23 (2):  023302.  DOI: 10.1088/1674-1056/23/2/023302 Abstract ( 735 )   PDF (765KB) ( 513 )   The pressure-induced molecular dissociation as one of the fundamental problems in physical sciences has aroused many theoretical and experimental studies. Here, using a newly developed particle swarm optimization algorithm, we investigate the high-pressure-induced molecular dissociation. The results show that the carbon tetrachloride (CCl4) is unstable and dissociates into C2Cl6 and Cl2 under approximately 120 GPa and more. The dissociation is confirmed by the lattice dynamic calculations and electronic structure of the Pa3 structure with pressure evolution. The dissociation pressure is far larger than that in the case of high temperature, indicating that the temperature effectively reduces the activation barrier of the dissociation reaction of CCl4. This research improves the understanding of the dissociation reactions of CCl4 and other halogen compounds under high pressures.

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Stereodynamics study of the H’(2S)+NH(X3∑-→N(4S) +H2 reaction Wei Qiang (魏强) Chin. Phys. B, 2014, 23 (2):  023401.  DOI: 10.1088/1674-1056/23/2/023401 Abstract ( 634 )   PDF (1410KB) ( 456 )   The stereodynamics and reaction mechanism of the H’(2S)+NH(X3∑-→N(4S) +H2 reaction are thoroughly studied at collision energies in the 0.1 eV–1.0 eV range using the quasiclassical trajectory (QCT) on the ground 4A" potential energy surface (PES). The distributions of vector correlations between products and reagents P(θr), i>P(φr) and P(θr, φr) are presented and discussed. The results indicate that product rotational angular momentum j’ is not only aligned, but also oriented along the direction perpendicular to the scattering plane; further, the product H2 presents different rotational polarization behaviors for different collision energies. Furthermore, four polarization-dependent differential cross sections (PDDCSs) of the product H2 are also calculated at different collision energies. The reaction mechanism is analyzed based on the stereodynamics properties. It is found that the abstraction mechanism is appropriate for the title reaction.

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Control of the photodetachment of H- near a metallic sphere surface by an elastic interface Li Shao-Sheng (李绍晟), Wang De-Hua (王德华) Chin. Phys. B, 2014, 23 (2):  023402.  DOI: 10.1088/1674-1056/23/2/023402 Abstract ( 601 )   PDF (533KB) ( 449 )   According to the closed-orbit theory, we study the influence of elastic interface on the photodetachment of H- near a metallic sphere surface. First, we give a clear physical description of the detached electron movement between the elastic interface and the metallic sphere surface. Then we put forward an analytical formula for calculating the photodetachment cross section of this system. Our study suggests that the photodetachment cross section of H- is changed with the distance between the elastic interface and H-. Compared with the photodetachment cross section of H- near a metallic sphere surface without the elastic interface, the cross section of our system oscillates and its oscillation is strengthened with the decrease of the distance from the elastic interface to H-. In additon, our calcuation results suggest that the influence of the elastic interface becomes much more significant when it is located in the lower half space rather than in the upper half space. Therefore, we can control the photodetachment of H- near a metallic sphere surface by changing the position of the elastic interface. We hope that our work is conducive to the understanding of the photodetachment process of negative ions near interfaces, cavities and ion traps.

ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

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Role of shape of hole in transmission and negative refractive index of sandwiched metamaterials Zhong Min (钟敏), Ye Yong-Hong (叶永红) Chin. Phys. B, 2014, 23 (2):  024101.  DOI: 10.1088/1674-1056/23/2/024101 Abstract ( 538 )   PDF (543KB) ( 626 )   Transmission and negative refractive index (NRI) of metal–dielectric–metal (MDM) sandwiched metamaterial perforated with four kinds of shapes of holes are numerically studied. Results indicate that positions of all transmission peaks of these kinds of holes are sensitive to the shape of the hole. Under the same conditions, the circular hole can obtain the maximum NRI and the rectangular hole can obtain the maximum frequency bandwidth of NRI. Moreover, the figure of merit (FOM) of the circular hole is the maximum too. As a result, we can obtain a higher NRI and FOM metamaterial by drilling circular hole arrays on MDM metamaterial.

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Effects of self-fields on gain in two-stream free electron laser with helical wiggler and an axial guiding magnetic field Saviz S, Lashani E, Ashkarran A Chin. Phys. B, 2014, 23 (2):  024102.  DOI: 10.1088/1674-1056/23/2/024102 Abstract ( 500 )   PDF (771KB) ( 847 )   The theory for the two-stream free electron laser (FEL) consisting of a relativistic electron beam transporting along the axis of a helical wiggler in the presence of an axial guiding magnetic field is proposed and investigated. In the analysis, the effects of self-fields are taken into account. The electron trajectories and the small signal gain are derived. The characteristics of the linear-gain and the normalized maximum gain are studied numerically. The results show that there are seven stable groups of orbits in the presence of self-fields instead of two groups reported in the absence of the self-fields. It is also shown that the normalized gains of three groups decrease while the rest increase with the increasing of normalized cyclotron frequency Ω0. Furthermore, it is found that the two-stream instability and the self-field lead to a decrease in the maximum gain except for group 3. The results show that the normalized maximum gain is enhanced in comparison with that of the single stream.

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Effect of normalized plasma frequency on electron phase-space orbits in a free-electron laser Ji Yu-Pin (吉驭嫔), Wang Shi-Jian (王时建), Xu Jing-Yue (徐竟跃), Xu Yong-Gen (徐勇根), Liu Xiao-Xu (刘晓旭), Lu Hong (卢宏), Huang Xiao-Li (黄小莉), Zhang Shi-Chang (张世昌) Chin. Phys. B, 2014, 23 (2):  024103.  DOI: 10.1088/1674-1056/23/2/024103 Abstract ( 718 )   PDF (1903KB) ( 536 )   Irregular phase-space orbits of the electrons are harmful to the electron-beam transport quality and hence deteriorate the performance of a free-electron laser (FEL). In previous literature, it was demonstrated that the irregularity of the electron phase-space orbits could be caused in several ways, such as varying the wiggler amplitude and inducing sidebands. Based on a Hamiltonian model with a set of self-consistent differential equations, it is shown in this paper that the electron-beam normalized plasma frequency functions not only couple the electron motion with the FEL wave, which results in the evolution of the FEL wave field and a possible power saturation at a large beam current, but also cause the irregularity of the electron phase-space orbits when the normalized plasma frequency has a sufficiently large value, even if the initial energy of the electron is equal to the synchronous energy or the FEL wave does not reach power saturation.

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Amplifying device created with isotropic dielectric layer Wang Shen-Yun (王身云), Liu Shao-Bin (刘少斌) Chin. Phys. B, 2014, 23 (2):  024104.  DOI: 10.1088/1674-1056/23/2/024104 Abstract ( 740 )   PDF (936KB) ( 349 )   Using the concept of optical transformation, we report on an amplifying device, which can make an arbitrary object enlarged. Its potential application to small object identification and detection is foreseeable. The cylindrical anisotropic amplifying shell could be mimicked by radially symmetrical “sectors” alternating in composition between two profiles of isotropic dielectrics; the permittivity and permeability in each “sector” can be properly determined by the effective medium theory. Both the magnetic and nonmagnetic amplifying devices are validated by full-wave finite element simulations. Good amplifying performance is observed.

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Design of a tunable frequency selective surface absorber as a loaded receiving antenna array Lin Bao-Qin (林宝勤), Zhao Shang-Hong (赵尚弘), Wei Wei (魏伟), Da Xin-Yu (达新宇), Zheng Qiu-Rong (郑秋容), Zhang Heng-Yang (张衡阳), Zhu Meng (朱蒙) Chin. Phys. B, 2014, 23 (2):  024201.  DOI: 10.1088/1674-1056/23/2/024201 Abstract ( 724 )   PDF (650KB) ( 756 )   An effective approach to designing a tunable electromagnetic absorber is presented and experimentally verified; it is based on an idea that an existing frequency selective surface (FSS) absorber is regarded as a loaded receiving antenna array. The existing absorber is effectively simplified by withdrawing half of the loaded resistors; a more compact one is obtained when lumped capacitors are introduced. Building on this, a varactor-tunable absorber with a proper bias network is proposed. Numerical simulations of one tunable absorber with 1.6 mm in thickness show that a wide tuning range from 3.05 GHz to 1.96 GHz is achieved by changing the capacitance of the loaded varactor from 0.5 pF to 5.0 pF. An experiment is carried out using a rectangular waveguide measurement setup and excellent agreement between the simulated and measured results is demonstrated.

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Single photon transport properties in coupled cavity arrays nonlocally coupled to a two-level atom in the presence of dissipation Hai Lian (海莲), Tan Lei (谭磊), Feng Jin-Shan (冯金山), Xu Wen-Bin (徐文斌), Wang Bin (王彬) Chin. Phys. B, 2014, 23 (2):  024202.  DOI: 10.1088/1674-1056/23/2/024202 Abstract ( 633 )   PDF (529KB) ( 572 )   We discuss the effects of dissipation on the behavior of single photon transport in a system of coupled cavity arrays, with the two nearest cavities nonlocally coupled to a two-level atom. The single photon transmission amplitude is solved exactly by employing the quasi-boson picture. We investigate two different situations of local and nonlocal couplings, respectively. Comparing the dissipative case with the nondissipative one reveals that the dissipation of the system increases the middle dip and lowers the peak of the single photon transmission amplitudes, broadening the line width of the transport spectrum. It should be noted that the influence of the cavity dissipation to the single photon transport spectrum is asymmetric. By comparing the nonlocal coupling with the local one, one can find that the enhancement of the middle dip of single photon transmission amplitudes is mostly caused by the atom dissipation and that the reduced peak is mainly caused by the cavity dissipation, no matter whether it is a nonlocal or local coupling case. Whereas in the nonlocal coupling case, when the coupling strength gets stronger, the cavity dissipation has a greater effect on the single photon transport spectrum and the atom dissipation affection becomes weak, so it can be ignored.

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Investigation on the intensity noise characteristics of the semiconductor ring laser Kang Ze-Xin (康泽新), Cai Xin-Lun (蔡鑫伦), Wen Xiao-Dong (温晓东), Liu Chao (刘超), Jian Shui-Sheng (简水生), Yu Si-Yuan (余思远) Chin. Phys. B, 2014, 23 (2):  024203.  DOI: 10.1088/1674-1056/23/2/024203 Abstract ( 656 )   PDF (1764KB) ( 527 )   Based on the frequency-domain multimode theoretical model, detailed investigations on the noise characteristic of the semiconductor ring laser (SRL) are first performed in this paper. The comprehensive nonlinear terms related to the third order nonlinear susceptibility χ3 are included in this model; the Langevin noise sources for electric field and carrier density fluctuations are also taken into account. As the injection current increases, the SRL may present several operation regimes. Remarkable and unusual low frequency noise enhancement in the form of a broad low frequency tail extending all the way to the relaxation oscillation peak is observed in any of the operation regimes of SRLs. The influences of the backscattering coefficient on the relative intensity noise (RIN) spectrum in typical operation regimes are investigated in detail.

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Effects of water and ice clouds on cloud microphysical budget：An equilibrium modeling study Gao Shou-Ting (高守亭), Li Xiao-Fan (李小凡), Zhou Yu-Shu (周玉淑) Chin. Phys. B, 2014, 23 (2):  024204.  DOI: 10.1088/1674-1056/23/2/024204 Abstract ( 641 )   PDF (329KB) ( 333 )   The effects of water and ice clouds on the cloud microphysical budget associated with rainfall are investigated through the analysis of grid-scale data from a series of two-dimensional cloud-resolving model equilibrium sensitivity simulations. The model is imposed without large-scale vertical velocity. In the control experiment, the contribution from rainfall (cM) associated with net evaporation and hydrometeor loss/convergence is about 29% of that from the rainfall (Cm) associated with net condensation and hydrometeor gain/divergence and about 39% of that from the rainfall (CM) associated with net condensation and hydrometeor loss/convergence. The exclusion of ice clouds enhances rainfall contribution of CM, whereas it reduces rainfall contributions of Cm and cM. The removal of radiative effects of water clouds increases rainfall contribution of CM, barely changes rainfall contribution of Cm and reduces the rainfall contribution of cM in the presence of the radiative effects of ice clouds. Elimination of the radiative effects of water clouds reduces the rainfall contributions of CM and Cm, whereas it increases the rainfall contribution of cM in the absence of the radiative effects of ice clouds.

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Spectroscopic properties and mechanism of Tm3+/Er3+/Yb3+ co-doped oxyfluorogermanate glass ceramics containing BaF2 nanocrystals Hu Yue-Bo (胡曰博), Qiu Jian-Bei (邱建备), Zhou Da-Cheng (周大成), Song Zhi-Guo (宋志国), Yang Zheng-Wen (杨正文), Wang Rong-Fei (王荣飞), Jiao Qing (焦清), Zhou Da-Li (周大利) Chin. Phys. B, 2014, 23 (2):  024205.  DOI: 10.1088/1674-1056/23/2/024205 Abstract ( 767 )   PDF (504KB) ( 535 )   Transparent Tm3+/Er3+/Yb3+ co-doped oxyfluorogermanate glass ceramics containing BaF2 nanocrystals are prepared. Under excitation of a 980-nm laser diode (LD), compared with the glass before heat treatment, the Tm3+/Er3+/Yb3+ co-doped oxyfluorogermanate glass ceramics can emit intense blue, green and red up-conversion luminescence and Stark-split peaks; X-ray diffraction (XRD) and transmission electron microscope (TEM) results show that BaF2 nanocrystals with an average diameter of 20 nm are precipitated from the glass matrix. Stark splitting of the up-conversion luminescence peaks in the glass ceramics indicates that Tm3+, Er3+ and (or) Yb3+ ions are incorporated into the BaF2 nanocrystals. The up-conversion luminescence intensities of Tm3+, Er3+ and the splitting degree of luminescence peaks in the glass ceramics increase significantly with the increase of heat treat temperature and heat treat time extension. In addition, the possible energy transfer process between rare earth ions and the up-conversion luminescence mechanism are also proposed.

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Photon statistical properties of photon-added two-mode squeezed coherent states Xu Xue-Fen (许雪芬), Wang Shuai (王帅), Tang Bin (唐斌) Chin. Phys. B, 2014, 23 (2):  024206.  DOI: 10.1088/1674-1056/23/2/024206 Abstract ( 547 )   PDF (938KB) ( 477 )   We investigate photon statistical properties of the multiple-photon-added two-mode squeezed coherent states (PA-TMSCS). We find that the photon statistical properties are sensitive to the compound phase involved in the TMSCS. Our numerical analyses show that the photon addition can enhance the cross-correlation and anti-bunching effects of the PA-TMSCS. Compared with that of the TMSCS, the photon number distribution of the PA-TMSCS is modulated by a factor that is a monotonically increasing function of the numbers of adding photons to each mode; further, that the photon addition essentially shifts the photon number distribution.

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Applications of quantum Fourier transform in photon-added coherent state Ren Gang (任刚), Du Jian-Ming (杜建明), Yu Hai-Jun (余海军) Chin. Phys. B, 2014, 23 (2):  024207.  DOI: 10.1088/1674-1056/23/2/024207 Abstract ( 637 )   PDF (256KB) ( 341 )   Quantum Fourier transform is realized by the Hadamard gate in a quantum computer, which can also be considered as a Hadamard transform. We introduce the Hadamard transformed photon-added coherent state (HTPACS), which is obtained by letting the photon-added coherent state (PACS) across the quantum Hadamard gate, from this result. It is found that the HTPACS can be considered as a coordinate–momentum mutual exchanging followed by a squeezing transform of the PACS. In addition, the non-classical statistical properties of HTPACS, such as squeezing coefficient, Mandel parameter, etc., are also discussed.

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Maximal entanglement from photon-added nonlinear coherent states via unitary beam splitters K. Berrada Chin. Phys. B, 2014, 23 (2):  024208.  DOI: 10.1088/1674-1056/23/2/024208 Abstract ( 718 )   PDF (261KB) ( 825 )   In this paper, we construct photon-added f-deformed coherent states (PAf-DCSs) for nonlinear bosonic fields by discussing Klauder’s minimal set of conditions required to obtain coherent states. Using this set of nonlinear states, we propose a very useful scheme for generating the maximal amount of entanglement via unitary beam splitters for different strength regimes of the input field α, deformation q and excitation number m. Therefore, the possibility to create highly entangled states and to control the entanglement is proposed. Moreover, the condition for a maximal and separable output beam state is obtained. Finally, we examine the statistical properties of the PAf-DCSs through the Mandel parameter and exploit a connection between this quantity and the behavior variation of the output state entanglement. Our result may open new perspectives in different tasks of quantum information processing.

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A compact graphene Q-switched erbium-doped fiber laser using optical circulator and tunable fiber Bragg grating Li He-Ping (李和平), Xia Han-Ding (夏汉定), Wang Ze-Gao (王泽高), Zhang Xiao-Xia (张晓霞), Chen Yuan-Fu (陈远富), Zhang Shang-Jian (张尚剑), Tang Xiong-Gui (唐雄贵), Liu Yong (刘永) Chin. Phys. B, 2014, 23 (2):  024209.  DOI: 10.1088/1674-1056/23/2/024209 Abstract ( 748 )   PDF (381KB) ( 600 )   We demonstrate a passively Q-switched tunable erbium-doped fiber laser (EDFL) based on graphene as a saturable absorber (SA). A three-port optical circulator (OC) and a strain-induced tunable fiber Bragg grating (TFBG) are used as the two end mirrors in an all-fiber linear cavity. The Q-switched EDFL has a low pump threshold of 23.8 mW. The pulse repetition rate of the fiber laser can be widely changed from 9.3 kHz to 69.7 kHz by increasing the pump power from 23.8 mW to 219.9 mW. The minimum pulse duration is 1.7 μs and the highest pulse energy is 25.4 nJ. The emission wavelength of the laser can be tuned from 1560.43 nm to 1566.27 nm by changing the central wavelength of the strain-induced TFBG.

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Evolution of modes in double-clad Raman fiber amplifier Wang Wen-Liang (王文亮), Huang Liang-Jin (黄良金), Leng Jin-Yong (冷进勇), Guo Shao-Feng (郭少锋), Jiang Zong-Fu (姜宗福) Chin. Phys. B, 2014, 23 (2):  024210.  DOI: 10.1088/1674-1056/23/2/024210 Abstract ( 711 )   PDF (260KB) ( 424 )   Stimulated Raman scattering in a double cladding optical fiber is studied with a continuous wave laser used as a pump source. Under various launch conditions, pump modes are differently excited. Considering the mode coupling effect among the pump modes, the evolution of the power in the Stokes modes is studied. The results show that the scattered waves (the Stokes waves) in the fiber core with 9-μm diameter and 0.14 NA could propagate predominantly in the fundamental mode of the fiber by carefully adjusting the pump light launching conditions.

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Beam control in tri-core photonic lattices Ye Zhuo-Yi (叶卓艺), Xia Shi-Qiang (夏世强), Song Dao-Hong (宋道红), Tang Li-Qin (唐莉勤), Lou Ci-Bo (楼慈波) Chin. Phys. B, 2014, 23 (2):  024211.  DOI: 10.1088/1674-1056/23/2/024211 Abstract ( 589 )   PDF (743KB) ( 400 )   We report on theoretical investigations of beam control in one-dimensional tri-core photonic lattices (PLs). Linear splitting is illustrated in tri-core PLs; the effect of defect strength on the splitting is discussed in depth for single-wavelength light. We reveal that splitting disappears when the defect strength trends to zero, while reoccurring under nonlinearity. Multi-color splitting and active control are also proposed in such photonic structures.

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Nonlinear optical properties of an azobenzene polymer Zeng Yi (曾艺), Pan Zhi-Hua (潘志华), Zhao Fu-Li (赵付丽), Qin Mu (秦苜), Zhou Yan (周延), Wang Chang-Shun (王长顺) Chin. Phys. B, 2014, 23 (2):  024212.  DOI: 10.1088/1674-1056/23/2/024212 Abstract ( 737 )   PDF (367KB) ( 640 )   The nonlinear optical properties of an azobenzene polymer azo12–MO were investigated by a Z-scan technique. The polymer was synthesized by assembling the liquid-crystalline polymer azo12 with methyl orange. The nonlinear refractive index (1.39×10-15 cm2/W) and the nonlinear absorption coefficient (0.11 cm/GW) of azo12–MO were determined with 532-nm picosecond laser pulses at the irradiance of 92.40 GW/cm2. When compared to the nonlinear properties of azo12 and methyl orange, azo12–MO possesses the advantages of its two constituents and shows larger nonlinear optical properties.

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On the modeling of synchronized flow in cellular automaton models Jin Cheng-Jie (金诚杰), Wang Wei (王炜), Jiang Rui (姜锐) Chin. Phys. B, 2014, 23 (2):  024501.  DOI: 10.1088/1674-1056/23/2/024501 Abstract ( 647 )   PDF (1008KB) ( 478 )   In this paper, we further analyze our cellular automaton (CA) traffic flow model. By changing some parameters, the characteristics of our model can be significantly varied, ranging from the features of phase transitions to the number of traffic phases. We also review the other CA models based on Kerner’s three-phase traffic theory. By comparisons, we find that the core concepts for modeling the synchronized flow in these models are similar. Our model can be a good candidate for modeling the synchronized flow, since there is enough flexibility in our framework.

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Heat transfer for boundary layers with cross flow Krishnendu Bhattacharyya, Ioan Pop Chin. Phys. B, 2014, 23 (2):  024701.  DOI: 10.1088/1674-1056/23/2/024701 Abstract ( 534 )   PDF (953KB) ( 518 )   An analysis is presented to study the dual nature of solutions for the forced convective boundary layer flow and heat transfer in a cross flow with viscous dissipation terms in the energy equation. The governing equations are transformed into a set of three self-similar ordinary differential equations by similarity transformations. These equations are solved numerically using the very efficient shooting method. This study reveals that the dual solutions of the transformed similarity equations for velocity and temperature distributions exist for certain values of the moving parameter, Prandtl number, and Eckert numbers. The reverse heat flux is observed for larger Eckert numbers; that is, heat absorption at the wall occurs.

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

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A thin radar-infrared stealth-compatible structure：Design, fabrication, and characterization Tian Hao (田浩), Liu Hai-Tao (刘海韬), Cheng Hai-Feng (程海峰) Chin. Phys. B, 2014, 23 (2):  025201.  DOI: 10.1088/1674-1056/23/2/025201 Abstract ( 710 )   PDF (811KB) ( 1283 )   A thin radar-infrared stealth-compatible structure with reflectivity below –10 dB in the whole radar X wave band and infrared emissivity less than 0.3 in the infrared region of 8 μm–14 μm is reported. The designed stealth-compatible structure consists of metallic frequency selective surface (MFSS), resistive frequency selective surface (RFSS), and metal backing from the top down, and it is only 2.1-mm thick. The MFSS is made up of some divided low infrared emissivity metal copper films, and the RFSS consists of a capacitive array of square resistive patches. They are placed close together, working as an admittance sheet because of a mutual influence between them, and the equivalent admittance sheet greatly reduces the thickness of the whole structure. The proposed stealth-compatible structure is verified both by simulations and by experimental results. These results indicate that our proposed stealth-compatible structure has potential applications in stealth fields.

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Parametric instabilities in single-walled carbon nanotubes He Cai-Xia (何彩霞), Jian Yue (简粤), Qi Xiu-Ying (祁秀英), Xue Ju-Kui (薛具奎) Chin. Phys. B, 2014, 23 (2):  025202.  DOI: 10.1088/1674-1056/23/2/025202 Abstract ( 573 )   PDF (417KB) ( 453 )   Parametric instabilities induced by the coupling excitation between the high frequency quantum Langmuir waves and the low frequency quantum ion-acoustic waves in single-walled carbon nanotubes are studied with a quantum Zakharov model. By linearizing the quantum hydrodynamic equations, we get the dispersion relations for the high frequency quantum Langmuir wave and the low frequency quantum ion-acoustic wave. Using two-time scale method, we obtain the quantum Zaharov model in the cylindrical coordinates. Decay instability and four-wave instability are discussed in detail. It is shown that the carbon nanotube’s radius, the equilibrium discrete azimuthal quantum number, the perturbed discrete azimuthal quantum number, and the quantum parameter all play a crucial role in the instabilities.

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Out-of-plane shear flow effects on fast magnetic reconnection in a two-dimensional hybrid simulation model Wang Lin (王琳), Wang Xian-Qu (王先驱), Wang Xiao-Gang (王晓钢), Liu Yue (刘悦) Chin. Phys. B, 2014, 23 (2):  025203.  DOI: 10.1088/1674-1056/23/2/025203 Abstract ( 1046 )   PDF (1679KB) ( 385 )   The effects of out-of-plane shear flows on fast magnetic reconnection are numerically investigated by a two-dimensional (2D) hybrid model in an initial Harris sheet equilibrium with flows. The equilibrium and driven shear flows out of the 2D reconnection plane with symmetric and antisymmetric profiles respectively are used in the simulation. It is found that the out-of-plane flows with shears in-plane can change the quadrupolar structure of the out-of-plane magnetic field and, therefore, modify the growth rate of magnetic reconnection. Furthermore, the driven flow varying along the anti-parallel magnetic field can either enhance or reduce the reconnection rate as the direction of flow changes. Secondary islands are also generated in the process with converting the initial X-point into an O-point.

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Mechanical properties of Al/a-C nanocomposite thin films synthesized using a plasma focus device Z. A. Umar, R. S. Rawat, R. Ahmad, A. K. Kumar, Y. Wang, T. Hussain, Z. Chen, L. Shen, Z. Zhang Chin. Phys. B, 2014, 23 (2):  025204.  DOI: 10.1088/1674-1056/23/2/025204 Abstract ( 618 )   PDF (483KB) ( 584 )   The Al/a-C nanocomposite thin films are synthesized on Si substrates using a dense plasma focus device with aluminum fitted anode and operating with CH4/Ar admixture. X-ray diffractometer results confirm the formation of metallic crystalline Al phases using different numbers of focus shots. Raman analyses show the formation of D and G peaks for all thin film samples, confirming the presence of a-C in the nanocomposite thin films. The formation of Al/a-C nanocomposite thin films is further confirmed using X-ray photoelectron spectroscopy analysis. The scanning electron microscope results show that the deposited thin films consist of nanoparticles and their agglomerates. The sizes of th agglomerates increase with increasing numbers of focus deposition shots. The nanoindentation results show the variations in hardness and elastic modulus values of nanocomposite thin film with increasing the number of focus shots. Maximum values of hardness and elastic modulus of the composite thin film prepared using 20 focus shots are found to be about 10.7 GPa and 189.2 GPa, respectively.

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The combined effect of optical laser and microwave radiations on a metal surface Anatoli P Gavrilyuk, Nikolai Ya Shaparev Chin. Phys. B, 2014, 23 (2):  025205.  DOI: 10.1088/1674-1056/23/2/025205 Abstract ( 604 )   PDF (215KB) ( 350 )   In this paper, it is shown that the laser radiation intensity required for complete ionization of vapors produced on an irradiated metal surface can be reduced by more than an order of magnitude through using pulsed laser radiation in combination with microwave radiation.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

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Effects of annealing process on characteristics of fully transparent zinc tin oxide thin-film transistor Chen Yong-Yue (陈勇跃), Wang Xiong (王雄), Cai Xi-Kun (才玺坤), Yuan Zi-Jian (原子健), Zhu Xia-Ming (朱夏明), Qiu Dong-Jiang (邱东江), Wu Hui-Zhen (吴惠桢) Chin. Phys. B, 2014, 23 (2):  026101.  DOI: 10.1088/1674-1056/23/2/026101 Abstract ( 723 )   PDF (1013KB) ( 1162 )   Annealing effect on the performance of fully transparent thin-film transistor (TTFT), in which zinc tin oxide (ZnSnO) is used as the channel material and SiO2 as the gate insulator, is investigated. The ZnSnO active layer is deposited by radio frequency magnetron sputtering while a SiO2 gate insulator is formed by plasma-enhanced chemical vapor deposition. The saturation field-effect mobility and on/off ratio of the TTFT are improved by low temperature annealing in vacuum. Maximum saturation field-effect mobility and on/off ratio of 56.2 cm2/(V·s) and 3×105 are obtained, respectively. The transfer characteristics of the ZnSnO TFT are simulated using an analytical model and good agreement between measured and the calculated transfer characteristics is demonstrated.

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Dielectric and infrared properties of SrTiO3 single crystal doped by 3d (V, Mn, Fe, Ni) and 4f (Nd, Sm, Er) ions S. Maletic, D. Maletic, I. Petronijevic, J. Dojcilovic, D. M. Popovic Chin. Phys. B, 2014, 23 (2):  026102.  DOI: 10.1088/1674-1056/23/2/026102 Abstract ( 612 )   PDF (1092KB) ( 2086 )   In this study, the effects of doping by 3d (V, Mn, Fe, Ni) and 4f (Nd, Sm, Er) ions on dielectric and infrared properties of SrTiO3 (STO) single crystals are investigated. It is well known that doping of the SrTiO3 can change the dielectric properties of the STO from an insulator to an n-type semiconductor, and even to a metallic conductor. Dielectric and infrared (IR) properties of the undoped STO and doped STO single crystals are analyzed using dielectric spectroscopy (80 kHz–5 MHz), transmission (200 cm-1–4000 cm-1), and reflection spectroscopy (50 cm-1–2000 cm-1). It is found that doping by the 3d ions reduces the value of dielectric permittivity, but the trend of temperature dependence of the dielectric permittivity remains almost unchanged. On the other hand, dielectric spectroscopy measurements for samples doped by 4f ions show the anomalous behaviors of the dielectric permittivity at temperatures around the temperature of the structural phase transition. There are two fractures of temperature dependences of inverse dielectric permittivity εr-1(T). Transmittance spectroscopy measurements show that there are differences in the shape of the spectrum in the mid-IR region between the undoped STO and the one doped by 4f ions. The differences in the reflectance spectrum between the STO:Nd and STO are analyzed in detail.

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New observations on hydrogen bonding in ice by density functional theory simulations Zhang Peng (张鹏), Liu Yang (刘扬), Yu Hui (于惠), Han Sheng-Hao (韩圣浩), Lü Ying-Bo (吕英波), Lü Mao-Shui (吕茂水), Cong Wei-Yan (丛伟艳) Chin. Phys. B, 2014, 23 (2):  026103.  DOI: 10.1088/1674-1056/23/2/026103 Abstract ( 583 )   PDF (511KB) ( 588 )   In this paper, we report on a series of computational simulations on hydrogen bonding in two ice phases (Ih and Ic) using CASTEP with PW91 and RPBE exchange–correlation based on ab initio density functional theory. The strength of the H-bond is correlated with intramolecular O–H stretching, and the energy splitting exists for both the H-bond and covalent O–H stretching. By analyzing the dispersion relationship of ω(q), we observe the separation of the longitudinal optic (LO) mode from transverse optic (TO) mode at the gamma point, seemingly interpreting the controversial two H-bond peaks in the vibrational spectrum of ice recorded by inelastic incoherent neutron scattering experiments. The test of ambient environment on phonon density of sates (PDOS) shows that the relaxed tetrahedral structure is the most stable structural configuration for water clusters.

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Effects of cold rolling deformation on microstructure, hardness, and creep behavior of high nitrogen austenitic stainless steel Sun Shi-Cheng (孙世成), Sun Gui-Xun (孙贵训), Jiang Zhong-Hao (江忠浩), Ji Chang-Tao (季长涛), Liu Jia-An (刘家安), Lian Jian-She (连建设) Chin. Phys. B, 2014, 23 (2):  026104.  DOI: 10.1088/1674-1056/23/2/026104 Abstract ( 707 )   PDF (3067KB) ( 496 )   Effects of cold rolling deformation on the microstructure, hardness, and creep behavior of high nitrogen austenitic stainless steel (HNASS) are investigated. Microstructure characterization shows that 70% cold rolling deformation results in significant refinement of the microstructure of this steel, with its average twin thickness reducing from 6.4 μm to 14 nm. Nanoindentation tests at different strain rates demonstrate that the hardness of the steel with nano-scale twins (nt-HNASS) is about 2 times as high as that of steel with micro-scale twins (mt-HNASS). The hardness of nt-HNASS exhibits a pronounced strain rate dependence with a strain rate sensitivity (m value) of 0.0319, which is far higher than that of mt-HNASS (m=0.0029). nt-HNASS shows more significant load plateaus and a higher creep rate than mt-HNASS. Analysis reveals that higher hardness and larger m value of nt-HNASS arise from stronger strain hardening role, which is caused by the higher storage rate of dislocations and the interactions between dislocations and high density twins. The more significant load plateaus and higher creep rates of nt-HNASS are due to the rapid relaxation of the dislocation structures generated during loading.

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Elastic and thermodynamic properties of vanadium nitride under pressure and the effect of metallic bonding on its hardness Pu Chun-Ying (濮春英), Zhou Da-Wei (周大伟), Bao Dai-Xiao (包代小), Lu Cheng (卢成), Jin Xi-Lian (靳希联), Su Tai-Chao (宿太超), Zhang Fei-Wu (张飞武) Chin. Phys. B, 2014, 23 (2):  026201.  DOI: 10.1088/1674-1056/23/2/026201 Abstract ( 1003 )   PDF (338KB) ( 986 )   By the particle-swarm optimization method, it is predicted that tetragonal P42mc, I41md, and orthorhombic Amm2 phases of vanadium nitride (VN) are energetically more stable than NaCl-type structure at 0 K. The enthalpies of the predicted three new VN phases, along with WC, NaCl, AsNi, CsCl type structures, are calculated each as a function of pressure. It is found that VN exhibits the WC-to-CsCl type phase transition at 256 GPa. For the considered seven crystallographic VN phases, the structures, elastic constants, bulk moduli, shear moduli, and Debye temperatures are investigated. Our calculated equilibrium structural parameters are in very good agreement with the available experimental results and the previous theoretical results for the NaCl phase. The Debye temperatures of VN predicted three novel phases, which are all higher than those of the remaining structures. The elastic constants, thermodynamic properties, and elastic anisotropies of VN under pressure are obtained and the mechanical stabilities are analyzed in detail based on the mechanical stability criteria. Moreover, the effect of metallic bonding on the hardness of VN is also investigated, which shows that VNs in P42mc, I41md, and Amm2 phases are potential superhard phases. Further investigation on the experimental level is highly recommended to confirm our calculations presented in this paper.

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First-principles study of the structural, elastic, and optical properties for Sr0.5Ca0.5TiO3 Yang Chun-Yan (杨春燕), Zhang Rong (张蓉) Chin. Phys. B, 2014, 23 (2):  026301.  DOI: 10.1088/1674-1056/23/2/026301 Abstract ( 545 )   PDF (309KB) ( 583 )   A detailed theoretical study of the structural, elastic, and optical properties for Sr0.5Ca0.5TiO3 is carried out by first-principles calculations. The band structure exhibits a direct bandgap of 2.08 eV at the Γ point in the Brillouin zone. The bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio are derived based on the calculated elastic constants. The bulk modulus B=153 GPa and shear modulus G=81GPa are in good agreement with available experimental data. Poisson’s ratio ν=0.275 suggests that Sr0.5Ca0.5TiO3 should be classified as being a ductile material. Using the electronic band structure and density of states, we analyze the interband contribution to the optical properties. The real and imaginary parts of the dielectric function, as well as the optical properties such as the optical absorption coefficient, refractive index, extinction coefficient, and energy-loss spectrum are calculated. The static dielectric constant ε1(0) and the refractive index n(0) are also investigated.

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Phase transition of Bose–Einstein condensate under decoherence Zheng Qiang (郑强), Yi Shan-Feng (易善峰), Hu Chang-Gang (胡长刚) Chin. Phys. B, 2014, 23 (2):  026401.  DOI: 10.1088/1674-1056/23/2/026401 Abstract ( 666 )   PDF (285KB) ( 441 )   The effect of decoherence on the phase transition of a Bose–Einstein condensate in a symmetric double-well potential is determined by the mean atom number difference. It still has two phases, the tunneling phase and the self-trapping phase, even under decoherence. The density matrix and the operator fidelity also show very different behaviors in the two phases. This suggests that operator fidelity can be used to characterize the phase transition of this Bose–Einstein condensate model, even under decoherence.

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Subsolidus phase relation in the Bi2O3–Fe2O3–La2O3 system Hu Qi-Chang (胡启昌), Chen Ye-Qing (陈业青), Lü Pei-Wen (吕佩文), Huang Feng (黄丰), Wang Xian (王娴) Chin. Phys. B, 2014, 23 (2):  026402.  DOI: 10.1088/1674-1056/23/2/026402 Abstract ( 680 )   PDF (500KB) ( 706 )   Bismuth-containing semiconductor material is a hot topic in photocatalysts because of its effective absorption under the visible light. In this paper, we expect to explore a new bismuth-based photocatalyst by studying the subsolidus phase relations of the Bi2O3–Fe2O3–La2O3 system. The X-ray diffraction data shows that in this ternary system the ternary compound does not exist, while seven binary compounds (including one solid solution series Bi1-xLaxO1.5 with 0.167 ≤ x≤ 0.339) are obtained and eight compatibility triangles are determined.

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Subcooled pool boiling heat transfer in fractal nanofluids：A novel analytical model Xiao Bo-Qi (肖波齐), Yang Yi (杨毅), Xu Xiao-Fu (许晓赋) Chin. Phys. B, 2014, 23 (2):  026601.  DOI: 10.1088/1674-1056/23/2/026601 Abstract ( 757 )   PDF (287KB) ( 515 )   A novel analytical model to determine the heat flux of subcooled pool boiling in fractal nanofluids is developed. The model considers the fractal character of nanofluids in terms of the fractal dimension of nanoparticles and the fractal dimension of active cavities on the heated surfaces; it also takes into account the effect of the Brownian motion of nanoparticles, which has no empirical constant but has parameters with physical meanings. The proposed model is expressed as a function of the subcooling of fluids and the wall superheat. The fractal analytical model is verified by a reasonable agreement with the experimental data and the results obtained from existing models.

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Condensation of Fermions in the double-well potential Chen Xin-Wei (陈欣委), Liu Zhong-Qiang (刘中强), Kong Xiang-Mu (孔祥木) Chin. Phys. B, 2014, 23 (2):  026701.  DOI: 10.1088/1674-1056/23/2/026701 Abstract ( 614 )   PDF (315KB) ( 381 )   The density distribution of ultracold two-component fermionic gases 6Li, which are confined in a gradient magnetic field and a symmetrical double-well potential, is investigated by employing local-density approximation. It is found that three different regimes including quasi-molecular Bose–Einstein condensation (BECm), the dimers in the unitarity limit (ULd), and Bardeen–Cooper–Schrieffer superfluid (BCS) can coexist at the same time. Furthermore, the ranges of these regimes can be controlled to some extent by tuning the gradient of the magnetic field and the parameters characterizing the properties of the double-well potential. This study is of guidance and significance for experimentally realizing the coexistence of BECm, ULd, and BCS in the double-well potential.

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Effects of V/Ⅲ ratio on a-plane GaN epilayers with an InGaN interlayer Wang Jian-Xia (王建霞), Wang Lian-Shan (汪连山), Yang Shao-Yan (杨少延), Li Hui-Jie (李辉杰), Zhao Gui-Juan (赵桂娟), Zhang Heng (张恒), Wei Hong-Yuan (魏鸿源), Jiao Chun-Mei (焦春美), Zhu Qin-Sheng (朱勤生), Wang Zhan-Guo (王占国) Chin. Phys. B, 2014, 23 (2):  026801.  DOI: 10.1088/1674-1056/23/2/026801 Abstract ( 728 )   PDF (690KB) ( 598 )   The effects of V/Ⅲ growth flux ratio on a-plane GaN films grown on r-plane sapphire substrates with an InGaN interlayer are investigated. The surface morphology, crystalline quality, strain states, and density of basal stacking faults were found to depend heavily upon the V/Ⅲ ratio. With decreasing V/Ⅲ ratio, the surface morphology and crystal quality first improved and then deteriorated, and the density of the basal-plane stacking faults also first decreased and then increased. The optimal V/Ⅲ ratio growth condition for the best surface morphology and crystalline quality and the smallest basal-plane stacking fault density of a-GaN films are found. We also found that the formation of basal-plane stacking faults is an effective way to release strain.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

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AA bilayer graphene on Si-terminated SiO2 under electric field Liu Hai-Long (刘海龙), Liu Yan (刘艳), Wang Tao (汪涛), Ao Zhi-Min (敖志敏) Chin. Phys. B, 2014, 23 (2):  026802.  DOI: 10.1088/1674-1056/23/2/026802 Abstract ( 709 )   PDF (861KB) ( 476 )   The AA-stacked bilayer graphene/α-SiO2 (001) interfaces with Si terminated atoms are studied in the presence of an electric field F with different intensities by first principles. AA-stacked bilayer graphene is slightly mis-oriented on SiO2 substrate without electric field and the band gap is 0.557 eV. However, as F increases, the AA-stacked bilayer graphene has its layers gradually vertically shifted with each other and, finally, transfers into AB-stacked bilayer graphene and the band gap reduces to 0.252 eV under 0.015 Hartree.

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

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Comparison of electrical characteristic between AlN/GaN and AlGaN/GaN heterostructure Schottky diodes Lü Yuan-Jie (吕元杰), Feng Zhi-Hong (冯志红), Lin Zhao-Jun (林兆军), Gu Guo-Dong (顾国栋), Dun Shao-Bo (敦少博), Yin Jia-Yun (尹甲运), Han Ting-Ting (韩婷婷), Cai Shu-Jun (蔡树军) Chin. Phys. B, 2014, 23 (2):  027101.  DOI: 10.1088/1674-1056/23/2/027101 Abstract ( 755 )   PDF (268KB) ( 743 )   Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated. Based on the measured current–voltage and capacitance–voltage curves, the electrical characteristics of AlN/GaN Schottky diode, such as Schottky barrier height, turn-on voltage, reverse breakdown voltage, ideal factor, and the current-transport mechanism, are analyzed and then compared with those of an AlGaN/GaN diode by self-consistently solving Schrödinger’s and Poisson’s equations. It is found that the dislocation-governed tunneling is dominant for both AlN/GaN and AlGaN/GaN Schottky diodes. However, more dislocation defects and a thinner barrier layer for AlN/GaN heterostructure results in a larger tunneling probability, and causes a larger leakage current and lower reverse breakdown voltage, even though the Schottky barrier height of AlN/GaN Schottky diode is calculated to be higher that of an AlGaN/GaN diode.

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Different influences of Schottky metal on the strain and relative permittivity of barrier layer between AlN/GaN and AlGaN/GaN heterostructure Schottky diodes Lü Yuan-Jie (吕元杰), Feng Zhi-Hong (冯志红), Gu Guo-Dong (顾国栋), Dun Shao-Bo (敦少博), Yin Jia-Yun (尹甲运), Wang Yuan-Gang (王元刚), Xu Peng (徐鹏), Han Ting-Ting (韩婷婷), Song Xu-Bo (宋旭波), Cai Shu-Jun (蔡树军), Luan Chong-Biao (栾崇彪), Lin Zhao-Jun (林兆军) Chin. Phys. B, 2014, 23 (2):  027102.  DOI: 10.1088/1674-1056/23/2/027102 Abstract ( 621 )   PDF (281KB) ( 538 )   Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated. Based on the measured current–voltage and capacitance-voltage curves, the polarization sheet charge density and relative permittivity are analyzed and calculated by self-consistently solving Schrödinger’s and Poisson’s equations. It is found that the values of relative permittivity and polarization sheet charge density of AlN/GaN diode are both much smaller than the ones of AlGaN/GaN diode, and also much lower than the theoretical values. Moreover, by fitting the measured forward I–V curves, the extracted dislocations existing in the barrier layer of the AlN/GaN diode are found to be much more than those of the AlGaN/GaN diode. As a result, the conclusion can be made that compared with AlGaN/GaN diode the Schottky metal has an enhanced influence on the strain of the extremely thinner AlN barrier layer, which is attributed to the more dislocations.

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Damage mechanism of hydroxyl radicals toward adenine–thymine base pair Tan Rong-Ri (谈荣日), Wang Dong-Qi (王东琪), Zhang Feng-Shou (张丰收) Chin. Phys. B, 2014, 23 (2):  027103.  DOI: 10.1088/1674-1056/23/2/027103 Abstract ( 740 )   PDF (580KB) ( 532 )   The adenine–thymine base pair was studied in the presence of hydroxyl radicals in order to probe the hydrogen bond effect. The results show that the hydrogen bonds have little effect on the hydroxylation and dehydrogenation happened at the sites, which are not involved in a hydrogen bond, while at the sites involved in hydrogen bond formation in the base pair, the reaction becomes more difficult, both in view of the free energy barrier and the exothermicity. With a 6-311++G(d,p) level of description, both B3LYP and MP2 methods confirm that the C8 site of isolated adenine has the highest possibility to form covalent bond with the hydroxyl radicals, though with different energetics: B3LYP predicts a barrierless pathway, while MP2 finds a transition state with an energy of 106.1 kJ/mol. For the dehydrogenation reactions, B3LYP method predicts that the free energy barrier increases in the order of HN9 < HN61 < HN62 < H2 < H8.

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The effect of k-cubic Dresselhaus spin–orbit coupling on the decay time of persistent spin helix states in semiconductor two-dimensional electron gases Chai Zheng (柴政), Hu Mao-Jin (胡茂金), Wang Rui-Qiang (王瑞强), Hu Liang-Bin (胡梁宾) Chin. Phys. B, 2014, 23 (2):  027201.  DOI: 10.1088/1674-1056/23/2/027201 Abstract ( 662 )   PDF (435KB) ( 469 )   We study the theoretical effect of k-cubic (i.e. cubic-in-momentum) Dresselhaus spin–orbit coupling on the decay time of persistent spin helix states in semiconductor two-dimensional electron gases. We show that the decay time of persistent spin helix states may be suppressed substantially by k-cubic Dresselhaus spin–orbit coupling, and after taking the effect of k-cubic Dresselhaus spin–orbit interaction into account, the theoretical results obtained accord both qualitatively and quantitatively with other recent experimental results.

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Electronic band gap and transport in graphene superlattice with a Gaussian profile potential voltage Zhang Yu-Ping (张玉萍), Yin Yi-Heng (尹贻恒), Lü Huan-Huan (吕欢欢), Zhang Hui-Yun (张会云) Chin. Phys. B, 2014, 23 (2):  027202.  DOI: 10.1088/1674-1056/23/2/027202 Abstract ( 629 )   PDF (1877KB) ( 540 )   We study the electronic properties for the graphene-based one-dimensional superlattices, whose potential voltages vary according to the envelope of a Gaussian function. It is found that an unusual Dirac point exists and its location is exactly associated with a zero-averaged wave number (zero-k) gap. This zero-k gap is less sensitive to incident angle and lattice constants, properties opposing those of Bragg gap. The defect mode appearing inside the zero-k gap has an effect on transmission, conductance, and shot noise, which will be useful for further investigation.

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AlOx prepared by atomic layer deposition for high efficiency-type crystalline silicon solar cell Qiu Hong-Bo (仇洪波), Li Hui-Qi (李惠琪), Liu Bang-Wu (刘邦武), Zhang Xiang (张祥), Shen Ze-Nan (沈泽南) Chin. Phys. B, 2014, 23 (2):  027301.  DOI: 10.1088/1674-1056/23/2/027301 Abstract ( 588 )   PDF (321KB) ( 1045 )   The influence of atomic layer deposition parameters on the negative charge density in AlOx film is investigated by the corona-charge measurement. Results show that the charge density can reach up to -1.56×1012 cm-2 when the thickness of the film is 2.4 nm. The influence of charge density on cell conversion efficiency is further simulated using solar cell analyzing software (PC1D). With AlOx passivating the rear surface of the silicon, the cell efficiency of 20.66% can be obtained.

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Kink effect in current–voltage characteristics of a GaN-based high electron mobility transistor with an AlGaN back barrier Ma Xiao-Hua (马晓华), Lü Min (吕敏), Pang Lei (庞磊), Jiang Yuan-Qi (姜元祺), Yang Jing-Zhi (杨靖治), Chen Wei-Wei (陈伟伟), Liu Xin-Yu (刘新宇) Chin. Phys. B, 2014, 23 (2):  027302.  DOI: 10.1088/1674-1056/23/2/027302 Abstract ( 791 )   PDF (409KB) ( 33742 )   The kink effect in current–voltage (IV) characteristic s seriously deteriorates the performance of a GaN-based HEMT. Based on a series of direct current (DC) IV measurements in a GaN-based HEMT with an AlGaN back barrier, a possible mechanism with electron-trapping and detrapping processes is proposed. Kink-related deep levels are activated by a high drain source voltage (Vds) and located in a GaN channel layer. Both electron trapping and detrapping processes are accomplished with the help of hot electrons from the channel by impact ionization. Moreover, the mechanism is verified by two other DC IV measurements and a model with an expression of the kink current.

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High-order plasmon resonances in an Ag/Al2O3 core/shell nanorice Hot! Chen Li (陈立), Wei Hong (魏红), Chen Ke-Qiu (陈克求), Xu Hong-Xing (徐红星) Chin. Phys. B, 2014, 23 (2):  027303.  DOI: 10.1088/1674-1056/23/2/027303 Abstract ( 556 )   PDF (516KB) ( 648 )   Using numerical simulation, we investigate the high-order plasmon resonances in individual nanostructures of an Ag nanorice core surrounded by an Al2O3 shell. The peak positions of localized surface plasmon resonances (LSPRs) are red-shifted exponentially with the increase of the dielectric shell thickness. This is due to the exponential decay of electromagnetic field intensity in the direction perpendicular to the interface. This exponential red-shift depends on the wavelength of the resonance peak instead of the resonance order. In addition, we find that the LSPRs in an Ag nanorice of 60-nm width can be perfectly described by a single linear function. These features make nanorice an ideal platform for sensing applications.

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

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Investigation of the mode splitting induced by electro-optic birefringence in a vertical-cavity surface-emitting laser by polarized electroluminescence Zhang Jie (章杰), Yu Jin-Ling (俞金玲), Cheng Shu-Ying (程树英), Lai Yun-Feng (赖云锋), Chen Yong-Hai (陈涌海) Chin. Phys. B, 2014, 23 (2):  027304.  DOI: 10.1088/1674-1056/23/2/027304 Abstract ( 657 )   PDF (250KB) ( 434 )   The mode splitting induced by electro-optic birefringence in a P–I–N InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser (VCSEL) has been studied by polarized electroluminescence (EL) at room temperature. The polarized EL spectra with E‖[110] and E‖[110] directions, are extracted for different injected currents. The mode splitting of the two orthogonal polarized modes for a VCSEL device is determined, and its value increases linearly with the increasing injected current due to electro-optic birefringence. This article demonstrates that the polarized EL is a powerful tool to study the mode splitting and polarization anisotropy of a VCSEL device.

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A strategy of enhancing the photoactivity of TiO2 containing nonmetal and transition metal dopants Li Wei (李伟), Wei Shi-Hao (韦世豪), Duan Xiang-Mei (段香梅) Chin. Phys. B, 2014, 23 (2):  027305.  DOI: 10.1088/1674-1056/23/2/027305 Abstract ( 685 )   PDF (503KB) ( 547 )   An effective structural codoping approach is proposed to modify the photoelectrochemical (PEC) properties of anatase TiO2 by being doped with nonmetal (N or/and C) and transition metal (Re) elements. The electronic structures and formation energies of different doped systems are investigated using spin-polarized density functional theory. We find that (C, Re) doped TiO2, with a low formation energy and a large binding energy, reduces the band gap to a large extent, thus it could contribute to the significant enhancement of the photocatalytic activity in the visible-light region. It should be pointed out that, to be successful, the proper proportion of the dopants C and Re should be controlled, so that reasonable PEC properties can be achieved.

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Effect of magnetic properties of soft magnetic phase on the energy product of an exchange-spring magnet Jia Li-Ying (贾立颖), Yin Jin-Hua (阴津华), Ma Xing-Qiao (马星桥) Chin. Phys. B, 2014, 23 (2):  027501.  DOI: 10.1088/1674-1056/23/2/027501 Abstract ( 636 )   PDF (316KB) ( 369 )   Research on exchange-spring magnets has focused on the microstructures of the materials. However, research has seldom been concerned with the effect of magnetic properties of soft magnetic phase on the energy product of an exchange-spring magnet. In this paper, a simple one-dimensional numerical simulation is used to investigate this effect in a Nd2Fe14B-based exchange-spring magnet. The results reveal that the larger the anisotropy constant, the stronger the exchange coupling, and the higher the magnetization of the soft magnetic phase, the larger the energy product of an exchange-spring magnet. This provides evidence for choosing a soft magnetic phase in an exchange-spring magnet.

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Ferrimagnetism and metal–insulator transition in an organic polymer chain Ding Lin-Jie (丁林杰), Zhong Yuan (钟园), Fan Shuai-Wei (樊帅伟) Chin. Phys. B, 2014, 23 (2):  027502.  DOI: 10.1088/1674-1056/23/2/027502 Abstract ( 508 )   PDF (749KB) ( 505 )   The ferrimagnetism and quantum phase transition of a bipartite lozenge periodic Anderson-like organic polymer, in which the localized f electrons hybridize with the odd site conduction orbitals, are investigated by means of Green’s function theory. The ground state turns out to be gapless ferrimagnetism. At a finite temperature, the ferrimagnetic-to-paramagnetic phase transition takes place. The Kondo screenings and Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction can reduce and increase the transition temperature, respectively. Two Kondo screenings compete with each other, giving rise to the localized f electron spin screened antiferromagnetically. Accordingly, in a magnetic field, all spins are aligned along the chain easily, which is associated with metal–insulator transition. Furthermore, in a temperature-field plane, we reveal the gapless and spin polarized phases, which are characterized by susceptibility and specific heat, and whose behaviours are determined by the competition between the up-spin and down-spin hole excitations.

TOPICAL REVIEW—Magnetism, magnetic materials, and interdisciplinary research

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Exchange bias in ferromagnet/antiferromagnet bilayers Shi Zhong (时钟), Du Jun (杜军), Zhou Shi-Ming (周仕明) Chin. Phys. B, 2014, 23 (2):  027503.  DOI: 10.1088/1674-1056/23/2/027503 Abstract ( 704 )   PDF (600KB) ( 1038 )   Since the exchange bias (EB) effect was discovered in the Co/CoO core-shell nanoparticles, it has been extensively studied in various ferromagnet (FM)/antiferromagnet (AFM) bilayers due to its crucial role in spintronics devices. In this article, we review the investigation of the EB in our research group. First, we outline basic features of the EB, including the effects of the constituent layer thickness, the microstructure and magnetization of the FM layers, and we also discuss asymmetric magnetization reversal process in wedged-FM/AFM bilayers. Secondly, we discuss the mechanisms of the positive EB and the perpendicular EB. Thirdly, we demonstrate the hysteretic behavior of the angular dependence of the EB and analyze the EB training effect. Finally, we discuss the roles of the rotatable anisotropy in the two phenomena.

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

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High contrast atomic magnetometer based on coherent population trapping Yang Ai-Lin (杨爱林), Yang Guo-Qing (杨国卿), Xu Yun-Fei (徐云飞), Lin Qiang (林强) Chin. Phys. B, 2014, 23 (2):  027601.  DOI: 10.1088/1674-1056/23/2/027601 Abstract ( 760 )   PDF (248KB) ( 620 )   We present an experimental and theoretical investigation of the coherent population trapping (CPT) resonance excited on the D1 line of 87Rb atoms by bichromatic linearly polarized laser light. The experimental results show that a lin‖lin transition scheme is a promising alternative to the conventional circular–circular transition scheme for an atomic magnetometer. Compared with the circular light transition scheme, linear light accounts for high-contrast transmission resonances, which makes this excitation scheme promising for high-sensitivity magnetometers. We also use linear light and circular light to detect changes of a standard magnetic field, separately.

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Resonance-mode effect on piezoelectric microcantilever performance in air, with a focus on the torsional modes Qiu Hua-Cheng (邱华诚), Dara Feili, Wu Xue-Zhong (吴学忠), Helmut Seidel Chin. Phys. B, 2014, 23 (2):  027701.  DOI: 10.1088/1674-1056/23/2/027701 Abstract ( 679 )   PDF (673KB) ( 730 )   A high quality factor is preferred for a microresonator sensor to improve the sensitivity and resolution. In this paper we systematically investigate the performance of the microcantilever in different resonance modes, which are the first three flexural modes, the first lateral mode, and the first and the second torsional modes. An aluminum nitride-based piezoelectric cantilever is fabricated and tested under controlled pressure from an ultra-high vacuum to a normal atmosphere, using a custom-built vacuum chamber. From the experiment results, it can be seen that the torsional modes exhibit better quality factors than those of the flexural and lateral ones. Finally, an analytical model for the air damping characteristics of the torsional mode cantilever is derived and verified by comparing with experimental results.

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Asymmetric reversible diode-like resistive switching behaviors in ferroelectric BaTiO3 thin films Zhang Fei (张飞), Lin Yuan-Bin (林远彬), Wu Hao (吴昊), Miao Qing (苗青), Gong Ji-Jun (巩纪军), Chen Ji-Pei (陈继培), Wu Su-Juan (吴素娟), Zeng Min (曾敏), Gao Xing-Sen (高兴森), Liu Jun-Ming (刘俊明) Chin. Phys. B, 2014, 23 (2):  027702.  DOI: 10.1088/1674-1056/23/2/027702 Abstract ( 547 )   PDF (1223KB) ( 820 )   In this work, the resistive switching behaviors of ferroelectrictric BaTiO3/La0.67Sr0.33MnO3 heterostructures deposited by pulsed laser deposition are investigated. The BaTiO3 films show both well-established P–E hysteresis loops, and asymmetric reversible diode-like resistive switching behaviors, involving no forming process. It is found that both the ON/OFF ratio and the stability of resistive switching are substantially dependent on operation voltage (Vmax). At a Vmax of 15 V, a large ON/OFF resistance ratio above 1000 is obtained at a Vmax of 15 V, which is able to maintain stability up to 70-switching cycles. The above resistive switching behaviors can be understood by modulating interface Schottky barriers as demonstrated by I–V curve fitting.

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Microscopic degradation mechanism of polyimide film caused by surface discharge under bipolar continuous square impulse voltage Luo Yang (罗杨), Wu Guang-Ning (吴广宁), Liu Ji-Wu (刘继午), Peng Jia (彭佳), Gao Guo-Qiang (高国强), Zhu Guang-Ya (朱光亚), Wang Peng (王鹏), Cao Kai-Jiang (曹开江) Chin. Phys. B, 2014, 23 (2):  027703.  DOI: 10.1088/1674-1056/23/2/027703 Abstract ( 612 )   PDF (1005KB) ( 1008 )   Polyimide (PI) film is an important type of insulating material used in inverter-fed motors. Partial discharge (PD) under a sequence of high-frequency square impulses is one of the key factors that lead to premature failures in insulation systems of inverter-fed motors. In order to explore the damage mechanism of PI film caused by discharge, an aging system of surface discharge under bipolar continuous square impulse voltage (BCSIV) is designed based on the ASTM 2275 01 standard and the electrical aging tests of PI film samples are performed above the partial discharge inception voltage (PDIV). The chemical bonds of PI polymer chains are analyzed through Fourier transform infrared spectroscopy (FTIR) and the dielectric properties of unaged and aged PI samples are investigated by LCR testers HIOKI 3532-50. Finally, the micro-morphology and micro-structure changes of PI film samples are observed through scanning electron microscopy (SEM). The results show that the physical and chemical effects of discharge cut off the chemical bonds of PI polymer chains. The fractures of ether bond (C–O–C) and imide ring (C–N–C) on the backbone of a PI polymer chain leads to the decrease of molecular weight, which results in the degradation of PI polymers and the generation of new chemical groups and materials, like carboxylic acid, ketone, aldehydes, etc. The variation of microscopic structure of PI polymers can change the orientation ability of polarizable units when the samples are under an AC electric field, which would cause the dielectric constant ε to increase and dielectric loss tan δ to decrease. The SEM images show that the degradation path of PI film is initiated from the surface and then gradually extends to the interior with continuous aging. The injection charge could result in the PI macromolecular chain degradation and increase the trap density in the PI polymer bulk.

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Red-shift law of intense laser-induced electro-absorption in solids Deng Hong-Xiang (邓洪祥), Zu Hao-Yue (祖皓月), Wu Shao-Yi (邬邵轶), Sun Kai (孙凯), Zu Xiao-Tao (祖小涛) Chin. Phys. B, 2014, 23 (2):  027801.  DOI: 10.1088/1674-1056/23/2/027801 Abstract ( 643 )   PDF (357KB) ( 753 )   A theoretical study on the red-shift of laser-induced electro-absorption is presented. It is found that laser-induced red-shift scales with the cube root of the pump laser intensity in the optical tunneling regime and has an obvious deviation from this scale in the multi-photon regime. Our results show that in the optical tunneling regime, the laser-induced red shift has the same law as that in the direct current (DC) approximation. Though the scales are the same in the optical tunneling regime, the physical pictures in the two cases are quite different. The electro-absorption in the DC case is a tunneling-assisted transition process, while the laser-induced electro-absorption is a mixed multi-photon process.

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A generalized method of converting CT image to PET linear attenuation coefficient distribution in PET/CT imaging Wang Lu (王璐), Wu Li-Wei (武丽伟), Wei Le (魏乐), Gao Juan (高娟), Sun Cui-Li (孙翠丽), Chai Pei (柴培), Li Dao-Wu (李道武) Chin. Phys. B, 2014, 23 (2):  027802.  DOI: 10.1088/1674-1056/23/2/027802 Abstract ( 506 )   PDF (652KB) ( 547 )   The accuracy of attenuation correction in positron emission tomography scanners depends mainly on deriving the reliable 511-keV linear attenuation coefficient distribution in the scanned objects. In the PET/CT system, the linear attenuation distribution is usually obtained from the intensities of the CT image. However, the intensities of the CT image relate to the attenuation of photons in an energy range of 40 keV–140 keV. Before implementing PET attenuation correction, the intensities of CT images must be transformed into the PET 511-keV linear attenuation coefficients. However, the CT scan parameters can affect the effective energy of CT X-ray photons and thus affect the intensities of the CT image. Therefore, for PET/CT attenuation correction, it is crucial to determine the conversion curve with a given set of CT scan parameters and convert the CT image into a PET linear attenuation coefficient distribution. A generalized method is proposed for converting a CT image into a PET linear attenuation coefficient distribution. Instead of some parameter-dependent phantom calibration experiments, the conversion curve is calculated directly by employing the consistency conditions to yield the most consistent attenuation map with the measured PET data. The method is evaluated with phantom experiments and small animal experiments. In phantom studies, the estimated conversion curve fits the true attenuation coefficients accurately, and accurate PET attenuation maps are obtained by the estimated conversion curves and provide nearly the same correction results as the true attenuation map. In small animal studies, a more complicated attenuation distribution of the mouse is obtained successfully to remove the attenuation artifact and improve the PET image contrast efficiently.

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Optimization of InAs/GaAs quantum-dot structures and application to 1.3-μm mode-locked laser diodes Li Mi-Feng (李密锋), Ni Hai-Qiao (倪海桥), Ding Ying (丁颖), Bajek David, Kong Liang, Cataluna Maria Ana, Niu Zhi-Chuan (牛智川) Chin. Phys. B, 2014, 23 (2):  027803.  DOI: 10.1088/1674-1056/23/2/027803 Abstract ( 768 )   PDF (895KB) ( 507 )   The self-assembled growth of InAs/GaAs quantum dots by molecular beam epitaxy is conducted by optimizing several growth parameters, using a one-step interruption method after island formation. The dependence of photoluminescence on areal quantum-dot density is systematically investigated as a function of InAs deposition, growth temperature and arsenic pressure. The results of this investigation along with time-resolved photoluminescence measurements show that the combination of a growth temperature of 490 ℃, with a deposition rate of 0.02 ML/s, under an arsenic pressure of 1×10-6 Torr (1 Torr=1.33322×102 Pa), provides the best compromise between high density and the photoluminescence of quantum dot structure, with a radiative lifetime of 780 ps. The applicability of this 5-layer quantum dot structure to high-repetition-rate pulsed lasers is demonstrated with the fabrication and characterization of a monolithic InAs/GaAs quantum-dot passively mode-locked laser operating at nearly 1300 nm. Picosecond pulse generation is achieved from a two-section laser, with a ～ 19.7-GHz repetition rate.

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Numerical demonstration of three-dimensional terahertz metamaterials based on the causality principle Saeid Jamilan, Javad Nourinia, Mohammad Naghi Azarmanesh Chin. Phys. B, 2014, 23 (2):  027804.  DOI: 10.1088/1674-1056/23/2/027804 Abstract ( 568 )   PDF (521KB) ( 520 )   A parameter retrieval algorithm based on the causality principle and Kramers–Kronig (KK) relations is employed to calculate the effective parameters of three-dimensional (3D) metamaterials. Using KK relations, the branch selecting problem, which is the challenge of effective parameter retrieval method, can be removed. To reveal the validity of the proposed algorithm, the constitutive refractive index of a homogeneous polymide cube is extracted. The result is in excellent agreement with the intrinsic refractive index of the polymide. Finally, the two terahertz metamaterials with 3D structures are designed and their effective parameters are then retrieved using the proposed algorithm. Numerical simulations are performed using the full-wave electromagnetic solver, CST Microwave Studio.

INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

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Occurrence and elimination of in-plane misoriented crystals in AlN epilayers on sapphire via pre-treatment control Wang Hu (王虎), Xiong Hui (熊晖), Wu Zhi-Hao (吴志浩), Yu Chen-Hui (余晨辉), Tian Yu (田玉), Dai Jiang-Nan (戴江南), Fang Yan-Yan (方妍妍), Zhang Jian-Bao (张健宝), Chen Chang-Qing (陈长清) Chin. Phys. B, 2014, 23 (2):  028101.  DOI: 10.1088/1674-1056/23/2/028101 Abstract ( 567 )   PDF (650KB) ( 489 )   AlN epilayers are grown directly on sapphire (0001) substrates each of which has a low temperature AlN nucleation layer. The effects of pretreatments of sapphire substrates, including exposures to NH3/H2 and to H2 only ambients at different temperatures, before the growth of AlN epilayers is investigated. In-plane misoriented crystals occur in N-polar AlN epilayers each with pretreatment in a H2 only ambient, and are characterized by six 60°-apart peaks with splits in each peak in (1012) phi scan and two sets of hexagonal diffraction patterns taken along the [0001] zone axis in electron diffraction. These misoriented crystals can be eliminated in AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2 ambient. AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2 ambient are Al-polar. Our results show the pretreatments and the nucleation layers are responsible for the polarities of the AlN epilayers. We ascribe these results to the different strain relaxation mechanisms induced by the lattice mismatch of AlN and sapphire.

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Controlled construction of nanostructures in graphene Li Zhong-Jun (李忠军), Li Qiang (李强), Cheng Zeng-Guang (程增光), Li Hong-Bian (李红变), Fang Ying (方英) Chin. Phys. B, 2014, 23 (2):  028102.  DOI: 10.1088/1674-1056/23/2/028102 Abstract ( 412 )   PDF (570KB) ( 464 )   We report on the laser-assisted fabrications of nanostructures in graphene membranes supported on polymer films. By using a laser beam to deposit heat locally, irradiated polymer instantaneously melts and vaporizes. During laser drilling of the polymer, the single-layer graphene membrane adheres to the polymer surface and consequently forms tens of nanometer deep wells. Due to the short time scale of laser irradiation, heat diffusion in the polymer is negligible, and the excitation energy is highly confined in the polymer. As a result, graphene nanowells of hundreds of nanometers in diameter can be patterned with high fidelity. With the increasing of nanowell density, we observe the spontaneous formation of nanowrinkles connecting pairs of nanowells in the graphene membranes. Importantly, Raman spectra confirm that no defects are introduced in graphene membranes by laser irradiation under our experimental conditions. Our results highlight the possibility to construct nanostructures and to design novel devices based on graphene.

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Transparent conductive graphene films prepared by hydroiodic acid and thermal reduction Qin Meng-Meng (秦盟盟), Ji Wei (纪伟), Feng Yi-Yu (冯奕钰), Feng Wei (封伟) Chin. Phys. B, 2014, 23 (2):  028103.  DOI: 10.1088/1674-1056/23/2/028103 Abstract ( 747 )   PDF (1681KB) ( 1048 )   Transparent conductive graphene films are fabricated by the transfer printing of graphene aqueous dispersion followed by hydrohalic acids and thermal reduction. Results indicate that the graphene film reduced by hydroiodic acid (HI) reduction combined with thermal treatment shows a higher electrical conductivity than that reduced only by thermal treatment at the same transparency. A film with a sheet resistance of ～2400 Ω/sq at a transparency over 72% is obtained at a typical wavelength of 550 nm.

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Microstructure and its influence on CH4 adsorption behavior of deep coal Feng Yan-Yan (冯艳艳), Jiang Cheng-Fa (江成发), Liu Dai-Jun (刘代俊), Chu Wei (储伟) Chin. Phys. B, 2014, 23 (2):  028201.  DOI: 10.1088/1674-1056/23/2/028201 Abstract ( 580 )   PDF (682KB) ( 362 )   In this paper we investigate the influence of microstructure on the CH4 adsorption behavior of deep coal. The coal microstructure is characterized by N2 adsorption at 77 K, scanning electron microscopy (SEM), Raman spectroscopy, and Fourier transform infrared spectroscopy (FT-IR). The CH4 adsorptions are measured at 298 K at pressures up to 5.0 MPa by the the volumetric method and fitted by the Langmuir model. The results show that the Langmuir model fits well with the experimental data, and there is a positive correlation with surface area, pore volume, ID/IG, and CH4 adsorption capacity. The burial depth also affects the methane adsorption capacity of the samples.

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Study of a millimeter-wave squint indirect holographic algorithm suitable for imaging with large field-of-view Gao Xiang (高翔), Li Chao (李超), Fang Guang-You (方广有) Chin. Phys. B, 2014, 23 (2):  028401.  DOI: 10.1088/1674-1056/23/2/028401 Abstract ( 727 )   PDF (3303KB) ( 421 )   In this paper a millimeter-wave (MMW) squint indirect holographic method is presented, which is suitable for imaging with a large field-of-view. The proposed system employs the squint operation mode to remove the background and twin-image interferences, which achieves a similar effect to off-axis holography but leaves out the large-aperture quasi-optical component. The translational scanning manner enables a large field of view and ensures the image uniformity, which is difficult to realize in off-axis holography. In addition, a corresponding imaging algorithm for the presented scheme is developed to reconstruct the image from the recorded hologram. Some imaging results on typical objects, obtained with electromagnetic simulation, demonstrate good performance of the imaging scheme and validate the effectiveness of the image reconstruction algorithm.

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Impact of multiplexed reading scheme on nanocrossbar memristor memory’s scalability Zhu Xuan (朱玄), Tang Yu-Hua (唐玉华), Wu Chun-Qing (吴纯青), Wu Jun-Jie (吴俊杰), Yi Xun (易勋) Chin. Phys. B, 2014, 23 (2):  028501.  DOI: 10.1088/1674-1056/23/2/028501 Abstract ( 730 )   PDF (321KB) ( 489 )   Nanocrossbar is a potential memory architecture to integrate memristor to achieve large scale and high density memory. However, based on the currently widely-adopted parallel reading scheme, scalability of the nanocrossbar memory is limited, since the overhead of the reading circuits is in proportion with the size of the nanocrossbar component. In this paper, a multiplexed reading scheme is adopted as the foundation of the discussion. Through HSPICE simulation, we reanalyze scalability of the nanocrossbar memristor memory by investigating the impact of various circuit parameters on the output voltage swing as the memory scales to larger size. We find that multiplexed reading maintains sufficient noise margin in large size nanocrossbar memristor memory. In order to improve the scalability of the memory, memristors with nonlinear I–V characteristics and high LRS (low resistive state) resistance should be adopted.

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Performance improvement of GaN-based light-emitting diode with a p-InAlGaN hole injection layer Yu Xiao-Peng (喻晓鹏), Fan Guang-Han (范广涵), Ding Bin-Bin (丁彬彬), Xiong Jian-Yong (熊建勇), Xiao Yao (肖瑶), Zhang Tao (张涛), Zheng Shu-Wen (郑树文) Chin. Phys. B, 2014, 23 (2):  028502.  DOI: 10.1088/1674-1056/23/2/028502 Abstract ( 643 )   PDF (947KB) ( 1350 )   The characteristics of a blue light-emitting diode (LED) with a p-InAlGaN hole injection layer (HIL) is analyzed numerically. The simulation results indicate that the newly designed structure presents superior optical and electrical performance such as an increase in light output power, a reduction in current leakage and alleviation of efficiency droop. These improvements can be attributed to the p-InAlGaN serving as hole injection layers, which can alleviate the band bending induced by the polarization field, thereby improving both the hole injection efficiency and the electron blocking efficiency.

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Influences of polarization effect and p-region doping concentration on the photocurrent response of solar-blind p–i–n avalanche photodiodes Li Xiao-Jing (李晓静), Zhao De-Gang (赵德刚), Jiang De-Sheng (江德生), Liu Zong-Shun (刘宗顺), Chen Ping (陈平), Wu Liang-Liang (吴亮亮), Li Liang (李亮), Le Ling-Cong (乐伶聪), Yang Jing (杨静), He Xiao-Guang (何晓光), Wang Hui (王辉), Zhu Jian-Jun (朱建军), Zhang Shu-Ming (张书明), Zhang Bao-Shun (张宝顺), Yang Hui (杨辉) Chin. Phys. B, 2014, 23 (2):  028503.  DOI: 10.1088/1674-1056/23/2/028503 Abstract ( 713 )   PDF (658KB) ( 550 )   The influences of polarization and p-region doping concentration on the photocurrent response of Al0.4Ga0.6N/Al0.4Ga0.6N/Al0.65Ga0.35N p–i–n avalanche photodetector are studied in a wide range of reverse bias voltages. The simulation results indicate that the photocurrent under high inverse bias voltage decreases with the increase of polarization effect, but increases rapidly with the increase of effective doping concentration in p-type region. These phenomena are analyzed based on the calculations of the intensity and distribution of the electric field. A high p-region doping concentration in the p–i–n avalanche photodetector is shown to be important for the efficient compensation for the detrimental polarization-induced electrostatic field.

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Large-scale SiO2 photonic crystal for high efficiency GaN LEDs by nanospherical-lens lithography Hot! Wu Kui (吴奎), Wei Tong-Bo (魏同波), Lan Ding (蓝鼎), Zheng Hai-Yang (郑海洋), Wang Jun-Xi (王军喜), Luo Yi (罗毅), Li Jin-Min (李晋闽) Chin. Phys. B, 2014, 23 (2):  028504.  DOI: 10.1088/1674-1056/23/2/028504 Abstract ( 705 )   PDF (588KB) ( 847 )   Wafer-scale SiO2 photonic crystal (PhC) patterns (SiO2 air-hole PhC, SiO2-pillar PhC) on indium tin oxide (ITO) layer of GaN-based light-emitting diode (LED) are fabricated via novel nanospherical-lens lithography. Nanoscale polystyrene spheres are self-assembled into a hexagonal closed-packed monolayer array acting as convex lens for exposure using conventional lithography instrument. The light output power is enhanced by as great as 40.5% and 61% over those of as-grown LEDs, for SiO2-hole PhC and SiO2-pillar PhC LEDs, respectively. No degradation to LED electrical properties is found due to the fact that SiO2 PhC structures are fabricated on ITO current spreading electrode. For SiO2-pillar PhC LEDs, which have the largest light output power in all LEDs, no dry etching, which would introduce etching damage, was involved. Our method is demonstrated to be a simple, low cost, and high-yield technique for fabricating the PhC LEDs. Furthermore, the finite difference time domain simulation is also performed to further reveal the emission characteristics of LEDs with PhC structures.

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A novel strong green phosphor：K3Gd(PO4)2：Ce3+, Tb3+ for a UV-excited white light-emitting-diode Jiang Ting-Ming (姜庭明), Yu Xue (余雪), Xu Xu-Hui (徐旭辉), Zhou Da-Cheng (周大成), Yu Hong-Ling (余红玲), Yang Peng-Hui (杨鹏辉), Qiu Jian-Bei (邱建备) Chin. Phys. B, 2014, 23 (2):  028505.  DOI: 10.1088/1674-1056/23/2/028505 Abstract ( 773 )   PDF (561KB) ( 506 )   A series of K3Gd1-x-y(PO4)2:xCe3+, yTb3+ phosphors are synthesized by the solid-sate reaction method. X-ray diffraction and photoluminescence spectra are utilized to characterize the structures and luminescence properties of the as-synthesized phosphors. Co-doping of Ce3+ enhances the emission intensity of Tb3+ greatly through an efficient energy transfer process from Ce3+ to Tb3+. The energy transfer is confirmed by photoluminescence spectra and decay time curves analysis. The efficiency and mechanism of energy transfer are investigated carefully. Moreover, due to the non-concentration quenching property of K3Tb(PO4)2, the photoluminescence spectra of K3Tb1-x(PO4)2:xCe3+ are studied and the results show that when x=0.11 the strongest Tb3+ green emission can be realized.

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Bifurcation diagram globally underpinning neuronal firing behaviors modified by SK conductance Chen Meng-Jiao (陈梦娇), Ling Heng-Li (令恒莉), Liu Yi-Hui (刘一辉), Qu Shi-Xian (屈世显), Ren Wei (任维) Chin. Phys. B, 2014, 23 (2):  028701.  DOI: 10.1088/1674-1056/23/2/028701 Abstract ( 679 )   PDF (497KB) ( 498 )   Neurons in the brain utilize various firing trains to encode the input signals they have received. Firing behavior of one single neuron is thoroughly explained by using a bifurcation diagram from polarized resting to firing, and then to depolarized resting. This explanation provides an important theoretical principle for understanding neuronal biophysical behaviors. This paper reports the novel experimental and modeling results of the modification of such a bifurcation diagram by adjusting small conductance potassium (SK) channel. In experiments, changes in excitability and depolarization block in nucleus accumbens shell and medium-spiny projection neurons are explored by increasing the intensity of injected current and blocking the SK channels by apamin. A shift of bifurcation points is observed. Then, a Hodgkin–Huxley type model including the main electrophysiological processes of such neurons is developed to reproduce the experimental results. The reduction of SK channel conductance also shifts the bifurcations, which is in consistence with experiment. A global bifurcation paradigm of this shift is obtained by adjusting two parameters, intensity of injected current and SK channel conductance. This work reveals the dynamics underpinning modulation of neuronal firing behaviors by biologically important ionic conductance. The results indicate that small ionic conductance other than that responsible for spike generation can modify bifurcation points and shift the bifurcation diagram and, thus, change neuronal excitability and adaptation.

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Application of radial basis functions to evolution equations arising in image segmentation Li Shu-Ling (李淑玲), Li Xiao-Lin (李小林) Chin. Phys. B, 2014, 23 (2):  028702.  DOI: 10.1088/1674-1056/23/2/028702 Abstract ( 562 )   PDF (1125KB) ( 696 )   In this paper, radial basis functions are used to obtain the solution of evolution equations which appear in variational level set method based image segmentation. In this method, radial basis functions are used to interpolate the implicit level set function of the evolution equation with a high level of accuracy and smoothness. Then, the original initial value problem is discretized into an interpolation problem. Accordingly, the evolution equation is converted into a set of coupled ordinary differential equations, and a smooth evolution can be retained. Compared with finite difference scheme based level set approaches, the complex and costly re-initialization procedure is unnecessary. Numerical examples are also given to show the efficiency of the method.

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Multi-objective optimization of gradient coil for benchtop magnetic resonance imaging system with high-resolution Wang Long-Qing (王龙庆), Wang Wei-Min (王为民) Chin. Phys. B, 2014, 23 (2):  028703.  DOI: 10.1088/1674-1056/23/2/028703 Abstract ( 748 )   PDF (1921KB) ( 898 )   Significant high magnetic gradient field strength is essential to obtaining high-resolution images in a benchtop magnetic resonance imaging (BT-MRI) system with permanent magnet. Extending minimum wire spacing and maximum wire width of gradient coils is one of the key solutions to minimize the maximum current density so as to reduce the local heating and generate higher magnetic field gradient strength. However, maximum current density is hard to optimize together with field linearity, stored magnetic energy, and power dissipation by the traditional target field method. In this paper, a new multi-objective method is proposed to optimize the maximum current density, field linearity, stored magnetic energy, and power dissipation in MRI gradient coils. The simulation and experimental results show that the minimum wire spacings are improved by 159% and 62% for the transverse and longitudinal gradient coil respectively. The maximum wire width increases from 0.5 mm to 1.5 mm. Maximum gradient field strengths of 157 mT/m and 405 mT/m for transverse and longitudinal coil are achieved, respectively. The experimental results in BT-MRI instrument demonstrate that the MRI images with in-plane resolution of 50 μm can be obtained by using the designed coils.

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Analysis of each branch current of serial solar cells by using an equivalent circuit model Yi Shi-Guang (易施光), Zhang Wan-Hui (张万辉), Ai Bin (艾斌), Song Jing-Wei (宋经纬), Shen Hui (沈辉) Chin. Phys. B, 2014, 23 (2):  028801.  DOI: 10.1088/1674-1056/23/2/028801 Abstract ( 676 )   PDF (340KB) ( 596 )   In this paper, based on the equivalent single diode circuit model of the solar cell, an equivalent circuit diagram for two serial solar cells is drawn. Its equations of current and voltage are derived from Kirchhoff’s current and voltage law. First, parameters are obtained from the I–V (current–voltage) curves for typical monocrystalline silicon solar cells (125 mm×125 mm). Then, by regarding photo-generated current, shunt resistance, serial resistance of the first solar cell, and resistance load as the variables. The properties of shunt currents (Ish1 and Ish2), diode currents (ID1 and ID2), and load current (IL) for the whole two serial solar cells are numerically analyzed in these four cases for the first time, and the corresponding physical explanations are made. We find that these parameters have different influences on the internal currents of solar cells. Our results will provide a reference for developing higher efficiency solar cell module and contribute to the better understanding of the reason of efficiency loss of solar cell module.

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Hybrid solar cell based on polythiophene and GaN nanoparticles composite Feng Qian (冯倩), Shi Peng (时鹏), Li Yu-Kun (李宇坤), Du Kai (杜锴), Wang Qiang (王强), Feng Qing (冯庆), Hao Yue (郝跃) Chin. Phys. B, 2014, 23 (2):  028802.  DOI: 10.1088/1674-1056/23/2/028802 Abstract ( 631 )   PDF (1846KB) ( 805 )   Hybrid solar cells based on poly(3-hexylthiophene) (P3HT) and Galium nitride (GaN) nanoparticle bulk heterojunction are fabricated and analyzed. The GaN nanocrystal is synthesized by means of a combination of sol–gel process with high temperature ammoniation using Ga(OC2H5) as a precursor. Their characteristics are determined by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. With the addition of GaN nanoparticle to P3HT, the device performance is greatly enhanced.

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Modified-DBR-based semi-omnidirectional multilayer anti-reflection coating for tandem solar cells Ali Bahrami, Shahram Mohammadnejad, Nima Jouyandeh Abkenar Chin. Phys. B, 2014, 23 (2):  028803.  DOI: 10.1088/1674-1056/23/2/028803 Abstract ( 768 )   PDF (248KB) ( 559 )   In this paper, multilayer antireflection coatings are designed by modifying the thickness of two and three paired layer distributed Bragg reflector (DBR) structure. Our proposed DBR-based structures show antireflection behaviors, in spite of the reflection treatment in traditional DBR structures. Firstly, the proposed structures are designed to be equivalent to the theoretical ideal triple-layer (TL) antireflection coating (ARC). Therefore, the problem of finding a suitable material for the middle layer of triple structure is solved. Simulation results show the significant equivalency for the reflectance of proposed structures to the ideal TL ARC at the same wavelengths and incident angles. Also, the design of the structure is changed in order to present the constant reflectance coefficient over a wide range of wavelengths. This structure enhances the omni-directionality of the multilayer ARC.

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Generation of minimally persistent circle formation for a multi-agent system Luo Xiao-Yuan (罗小元), Shao Shi-Kai (邵士凯), Zhang Yu-Yan (张玉燕), Li Shao-Bao (李绍宝), Guan Xin-Ping (关新平), Liu Zhi-Xin (刘志新) Chin. Phys. B, 2014, 23 (2):  028901.  DOI: 10.1088/1674-1056/23/2/028901 Abstract ( 629 )   PDF (1283KB) ( 574 )   In this paper, two methods of generating minimally persistent circle formation are presented. The proposed methods adopt a leader-follower strategy and all followers are firstly motivated to move into the leader’s interaction range. Based on the information about relative angle and relative distance, two numbering schemes are proposed to generate minimally persistent circle formation. Distributed control laws are also designed to maintain the desired relative distance between agents. The distinctive features of the proposed methods are as follows. First, only 2n-3 unilateral communication links for n agents are needed during the circle formation process and thus the communication complexity can be reduced. In addition, the formation topology is kept fixed for the whole motion and achieves a self-stability property. Finally, each follower keeps a regualr interval with its neighbors and the formation converges to a uniform circle formation. Simulation results are also provided to demonstrate the effectiveness of the proposed methods.

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Optimal network structure to induce the maximal small-world effect Zhang Zheng-Zhen (张争珍), Xu Wen-Jun (许文俊), Zeng Shang-You (曾上游), Lin Jia-Ru (林家儒) Chin. Phys. B, 2014, 23 (2):  028902.  DOI: 10.1088/1674-1056/23/2/028902 Abstract ( 538 )   PDF (277KB) ( 449 )   In this paper, the general efficiency, which is the average of the global efficiency and the local efficiency, is defined to measure the communication efficiency of a network. The increasing ratio of the general efficiency of a small-world network relative to that of the corresponding regular network is used to measure the small-world effect quantitatively. The more considerable the small-world effect, the higher the general efficiency of a network with a certain cost is. It is shown that the small-world effect increases monotonically with the increase of the vertex number. The optimal rewiring probability to induce the best small-world effect is approximately 0.02 and the optimal average connection probability decreases monotonically with the increase of the vertex number. Therefore, the optimal network structure to induce the maximal small-world effect is the structure with the large vertex number (>500), the small rewiring probability (≈ 0.02) and the small average connection probability (<0.1). Many previous research results support our results.

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MDSLB：A new static load balancing method for parallel molecular dynamics simulations Wu Yun-Long (武云龙), Xu Xin-Hai (徐新海), Yang Xue-Jun (杨学军), Zou Shun (邹顺), Ren Xiao-Guang (任小广) Chin. Phys. B, 2014, 23 (2):  028903.  DOI: 10.1088/1674-1056/23/2/028903 Abstract ( 696 )   PDF (1289KB) ( 482 )   Large-scale parallelization of molecular dynamics simulations is facing challenges which seriously affect the simulation efficiency, among which the load imbalance problem is the most critical. In this paper, we propose, a new molecular dynamics static load balancing method (MDSLB). By analyzing the characteristics of the short-range force of molecular dynamics programs running in parallel, we divide the short-range force into three kinds of force models, and then package the computations of each force model into many tiny computational units called “cell loads”, which provide the basic data structures for our load balancing method. In MDSLB, the spatial region is separated into sub-regions called “local domains”, and the cell loads of each local domain are allocated to every processor in turn. Compared with the dynamic load balancing method, MDSLB can guarantee load balance by executing the algorithm only once at program startup without migrating the loads dynamically. We implement MDSLB in OpenFOAM software and test it on TianHe-1A supercomputer with 16 to 512 processors. Experimental results show that MDSLB can save 34%–64% time for the load imbalanced cases.

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Analyzing the causation of a railway accident based on a complex network Ma Xin (马欣), Li Ke-Ping (李克平), Luo Zi-Yan (罗自炎), Zhou Jin (周进) Chin. Phys. B, 2014, 23 (2):  028904.  DOI: 10.1088/1674-1056/23/2/028904 Abstract ( 583 )   PDF (449KB) ( 823 )   In this paper, a new model is constructed for the causation analysis of railway accident based on the complex network theory. In the model, the nodes are defined as various manifest or latent accident causal factors. By employing the complex network theory, especially its statistical indicators, the railway accident as well as its key causations can be analyzed from the overall perspective. As a case, the “7.23” China–Yongwen railway accident is illustrated based on this model. The results show that the inspection of signals and the checking of line conditions before trains run played an important role in this railway accident. In conclusion, the constructed model gives a theoretical clue for railway accident prediction and, hence, greatly reduces the occurrence of railway accidents.

GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

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Improvement of surface flux calculation：A study based on measurements over alpine meadow in the eastern Tibet Plateau in summer Li Sen (李森), Zhong Zhong (钟中) Chin. Phys. B, 2014, 23 (2):  029201.  DOI: 10.1088/1674-1056/23/2/029201 Abstract ( 517 )   PDF (757KB) ( 390 )   An improved flux-gradient relationship between momentum φm (ζ) and sensible heat φh (ζ) is obtained by the use of the observational data over an alpine meadow in the eastern Tibet Plateau, in Maqu of China during the period June to August, 2010. The empirical coefficients of Businger–Dyer type function for the cases of unstable and stable stratification are modified. Non-dimensional vertical gradients of wind and potential temperature are calculated by three fitting functions; that is, the log–linear, log–square, and log–cubic functions, respectively. It is found that the von Karman constant approaches 0.4025 and the Prandtl number is about 1.10 based on the measurements in near-neutral conditions, which are within reasonable range proposed in previous studies. The revised flux-gradient profile functions of -1/5 power law for momentum and -1/3 power law for sensible heat are best fitted in unstable stratification conditions. Meanwhile, 2/5 power law, instead of linear functions, is more appropriate in stable stratification cases for momentum and sensible heat. Compared with results from previous studies in which traditional functions are used, the momentum and sensible heat fluxes estimated by the revised profile functions in the current study are much closer to the observations for the unstable and stable stratification conditions.

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Vertical structure of predictability and information transport over the Northern Hemisphere Feng Ai-Xia (冯爱霞), Wang Qi-Gang (王启光), Gong Zhi-Qiang (龚志强), Feng Guo-Lin (封国林) Chin. Phys. B, 2014, 23 (2):  029202.  DOI: 10.1088/1674-1056/23/2/029202 Abstract ( 594 )   PDF (1406KB) ( 6418 )   Based on nonlinear prediction and information theory, vertical heterogeneity of predictability and information loss rate in geopotential height field are obtained over the Northern Hemisphere. On a seasonal-to-interannual time scale, the predictability is low in the lower troposphere and high in the mid-upper troposphere. However, within mid-upper troposphere over the subtropics ocean area, there is a relatively poor predictability. These conclusions also fit the seasonal time scale. Moving to the interannual time scale, the predictability becomes high in the lower troposphere and low in the mid-upper troposphere, contrary to the former case. On the whole the interannual trend is more predictable than the seasonal trend. The average information loss rate is low over the mid-east Pacific, west of North America, Atlantic and Eurasia, and the atmosphere over other places has a relatively high information loss rate on all-time scales. Two channels are found steadily over the Pacific Ocean and Atlantic Ocean in subtropics. There are also unstable channels. The four-season influence on predictability and information communication are studied. The predictability is low, no matter which season data are removed and each season plays an important role in the existence of the channels, except for the winter. The predictability and teleconnections are paramount issues in atmospheric science, and the teleconnections may be established by communication channels. So, this work is interesting since it reveals the vertical structure of predictability distribution, channel locations, and the contributions of different time scales to them and their variations under different seasons.