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Chin. Phys. B  
  Chin. Phys. B--2009, Vol.18, No.12
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Successive defects asymmetric simple exclusion processes with particles of arbitrary size

Cai Jiu-Ju, Xiao Song, Wang Ruo-Hui, Liu Fei
Chin. Phys. B 2009, 18 (12): 5097 ;  doi: 10.1088/1674-1056/18/12/001
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This paper uses various mean-field approaches and the Monte Carlo simulation to calculate asymmetric simple exclusion processes with particles of arbitrary size in the successive defects system. In this system, the hopping probability p (p<1) and the size d of particles are not constant. Through theoretical calculation and computer simulation, it obtains the exact theoretical results and finds that the theoretical results are in agreement with the computer simulation. These results are helpful in analysing the effect of traffic with different hopping probabilities p and sizes d of particle.

Theoretical investigation of synchronous totally asymmetric simple exclusion process on lattices with two consecutive junctions in multiple-input-multiple-output traffic system

Xiao Song, Cai Jiu-Ju, Wang Rui-Li, Liu Ming-Zhe, Liu Fei
Chin. Phys. B 2009, 18 (12): 5103 ;  doi: 10.1088/1674-1056/18/12/002
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In this paper, we study the dynamics of the synchronous totally asymmetric simple exclusion process (TASEP) on lattices with two consecutive junctions in a multiple-input-multiple-output (MIMO) traffic system, which consists of m sub-chains for the input and the output, respectively. In the middle of the system, there are N (nN synchronously increasing, the vertical phase boundary moves toward the right and the horizontal phase boundary moves toward the upside in the phase diagram. The boundary conditions of the system as well as the numbers of input and output determine the no-equilibrium stationary states, stationary-states phases, and phase boundaries. We use the results to compare with computer simulations and find that they are in very good agreement with each other.

A stochastic epidemic model on homogeneous networks

Liu Mao-Xing, Ruan Jiong
Chin. Phys. B 2009, 18 (12): 5111 ;  doi: 10.1088/1674-1056/18/12/003
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In this paper, a stochastic SIS epidemic model on homogeneous networks is considered. The largest Lyapunov exponent is calculated by Oseledec multiplicative ergodic theory, and the stability condition is determined by the largest Lyapunov exponent. The probability density function for the proportion of infected individuals is found explicitly, and the stochastic bifurcation is analysed by a probability density function. In particular, the new basic reproductive number R*, that governs whether an epidemic with few initial infections can become an endemic or not, is determined by noise intensity. In the homogeneous networks, despite of the basic productive number R0>1, the epidemic will die out as long as noise intensity satisfies a certain condition.

Influence of velocity spatiotemporal correlations on the anomalous scaling exponents of passive scalars

Zhang Xiao-Qiang, Wang Guang-Rui, Chen Shi-Gang
Chin. Phys. B 2009, 18 (12): 5117 ;  doi: 10.1088/1674-1056/18/12/004
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In this paper, we consider spatial-temporal correlation functions of the turbulent velocities. With numerical simulations on the Gledzer--Ohkitani--Yamada (GOY) shell model, we show that the correlation function decays exponentially. The advecting velocity field is regarded as a colored noise field, which is spatially and temporally correlative. For comparison, we are also given the scaling exponents of passive scalars obtained by the Gaussian random velocity field, the multi-dimensional normal velocity field and the She--Leveque velocity field, introduced by She, et al. We observe that extended self-similarity scaling exponents H(p)/ H(2) of passive scalar obtained by the colored noise field are more anomalous than those obtained by the other three velocity fields.

Solving mKdV-sinh-Gordon equation by a modified variable separated ordinary differential equation method

Xie Yuan-Xi
Chin. Phys. B 2009, 18 (12): 5123 ;  doi: 10.1088/1674-1056/18/12/005
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By introducing a more general auxiliary ordinary differential equation (ODE), a modified variable separated ODE method is developed for solving the mKdV--sinh-Gordon equation. As a result, many explicit and exact solutions including some new formal solutions are successfully picked up for the mKdV--sinh-Gordon equation by this approach.

Probabilistic and robust preparation of a GHZ-type state via atomic ensembles and linear optics

Lu Xiao-Song, Shi Bao-Sen, Guo Guang-Can
Chin. Phys. B 2009, 18 (12): 5133 ;  doi: 10.1088/1674-1056/18/12/006
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This paper proposes two simple and robust schemes to generate an atomic-ensemble Greenberger--Horne--Zeilinger-type (GHZ-type) entangled state via linear optics and single photon detection. These schemes are based on two-photon Hong--Ou--Mandel-type interference, therefore they are insensitive to the phase fluctuation. This advantage will make the realizations of these two schemes easier. One scheme can scale efficiently with the number of ensembles because of the used quantum memory. Both schemes are also robust to the noise and within the reach of current technology.

The N-mode squeezed state with enhanced squeezing

Xu Xue-Xiang, Hu Li-Yun, Fan Hong-Yi
Chin. Phys. B 2009, 18 (12): 5139 ;  doi: 10.1088/1674-1056/18/12/007
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It is known that exp≤[iλ≤(Q1P1-i/2)] is a unitary single-mode squeezing operator, where Q1, P1 are the coordinate and momentum operators, respectively. In this paper we employ Dirac's coordinate representation to prove that the exponential operator Sn≡ exp≤[iλ\sum\limitsi=1n(QiPi+1 +Qi+1Pi))], (Qn+1=Q1, Pn+1=P1), is an N-mode squeezing operator which enhances the standard squeezing. By virtue of the technique of integration within an ordered product of operators we derive Sn's normally ordered expansion and obtain new N-mode squeezed vacuum states, its Wigner function is calculated by using the Weyl ordering invariance under similar transformations.

Finite-time disentanglement induced by single-mode thermal field

Han Feng, Xia Yun-Jie
Chin. Phys. B 2009, 18 (12): 5144 ;  doi: 10.1088/1674-1056/18/12/008
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In this paper, we study the entanglement dynamics of atoms locally coupled to a cavity field. By studying two different models within the framework of cavity QED, we show that the so-called atomic entanglement sudden death always occurs if initially the cavity field is in the thermal state, in clear contrast with that in the vacuum state where the same entanglement decay is in infinite time.

Exact solution for the thermo Jaynes--Cummings model

Yuan Hong-Chun, Fan Hong-Yi
Chin. Phys. B 2009, 18 (12): 5149 ;  doi: 10.1088/1674-1056/18/12/009
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Based on the construction of supersymmetric generators, we use the Lewis--Riesenfeld invariant method to deduce the exact and explicit eigen-energy spectrum with the time-dependent thermo Jaynes--Cummings model. One of the advantages of this approach is that it can transform the hidden form, related to the chronological product, of the time evolution operator into an explicit expression. Moreover, the dynamical and statistics properties of physical quantities are obtained for the given initial states in the thermo Jaynes--Cummings system.

Quantum information procession with fermions based on charge detection

Tang Li
Chin. Phys. B 2009, 18 (12): 5155 ;  doi: 10.1088/1674-1056/18/12/010
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This paper proposes a fermionic linear optical scheme for the teleportation and entanglement concentration via entanglement swapping based on charge detection. It also proves that this method is useful in generating entangled states such as GHZ states, W states, and cluster states by using fermionic polarizing beam splitters and single spin rotations assisted by a parity check on the fermionic qubits. This scheme is nearly deterministic (i.e., with 100\% successful probability) and does not need the joint Bell state measurement required in the previous schemes.

One-step implementing three-qubit phase gate via manipulating rf SQUID qubits in the decoherence-free subspace with respect to cavity decay

Shao Xiao-Qiang, Chen Li, Zhang Shou, Zhao Yong-Fang
Chin. Phys. B 2009, 18 (12): 5161 ;  doi: 10.1088/1674-1056/18/12/011
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We present a scheme for implementing a three-qubit phase gate via manipulating rf superconducting quantum interference device (SQUID) qubits in the decoherence-free subspace with respect to cavity decay. Through appropriate changes of the coupling constants between rf SQUIDs and cavity, the scheme can be realized only in one step. A high fidelity is obtained even in the presence of decoherence.

Generation of arbitrary four-atom entangled decoherence-free states

Ji Xin, Lü Tian-Quan, Zhang Shou
Chin. Phys. B 2009, 18 (12): 5168 ;  doi: 10.1088/1674-1056/18/12/012
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This paper proposes a scheme to generate arbitrary four-atom entangled decoherence-free states by using simple linear optical elements, four one-sided cavities in which four atoms are confined respectively. By conveniently tuning the titled angle of one half-wave plate, it can obtain arbitrary four-atom entangled decoherence-free states with a successful probability of 1 as long as there is no photon loss.

Quantum demonstration of a bio-molecular solution of the satisfiability problem on spin-based ensemble

Ren Ting-Ting, Feng Mang, Chang Weng-Long, Luo Jun
Chin. Phys. B 2009, 18 (12): 5173 ;  doi: 10.1088/1674-1056/18/12/013
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DNA computation (DNAC) has been proposed to solve the satisfiability (SAT) problem due to operations in parallel on extremely large numbers of strands. This paper attempts to treat the DNA-based bio-molecular solution for the SAT problem from the quantum mechanical perspective with a purpose to explore the relationship between DNAC and quantum computation (QC). To achieve this goal, it first builds up the correspondence of operations between QC and DNAC. Then it gives an example for the case of two variables and three clauses for details of this theory. It also demonstrates a three-qubit experiment for solving the simplest SAT problem with a single variable on a liquid-state nuclear magnetic resonance ensemble to verify this theory. Some discussions are made for the potential application and for further exploration of the present work.

Entanglement of two atoms in two-mode Raman coupled model with intrinsic decoherence

Zhang Jian, Shao Bin, Zou Jian
Chin. Phys. B 2009, 18 (12): 5179 ;  doi: 10.1088/1674-1056/18/12/014
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Considering intrinsic decoherence, the two-atom two-mode Raman coupled model is investigated in this paper. Utilizing the constants of motion in this model, we obtain the analytic expressions of the density operator of the system for investigating the entanglement of two atoms. The speed of entanglement decay increases with the increasing of the coupling coefficient of one atom. The difference between the oscillation periods when the initial state parameter of atomic subsystem belongs to two intervals becomes smaller with the increasing of the coupling coefficient of one atom. The increasing of the initial photon number of the second field can hasten the vanishing of entanglement of atomic subsystem. The robustness of atomic entanglement against decoherence depends on the interval of the initial state parameter of atomic subsystem.

Finite-size effects in a D-dimensional ideal Fermi gas

Su Guo-Zhen, Ou Cong-Jie, Wang A Qiu-Ping, Chen Jin-Can
Chin. Phys. B 2009, 18 (12): 5189 ;  doi: 10.1088/1674-1056/18/12/015
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By using the Euler--MacLaurin formula, this paper studies the thermodynamic properties of an ideal Fermi gas confined in a d-dimensional rectangular container. The general expressions of the thermodynamic quantities with the finite-size corrections are given explicitly and the effects of the size and shape of the container on the properties of the system are discussed. It is shown that the corrections of the thermodynamic quantities due to the finite-size effects are significant to be considered for the case of strong degeneracy but negligible for the case of weak degeneracy or non-degeneracy. It is important to find that some familiar conclusions under the thermodynamic limit are no longer valid for the finite-size systems and there are some novel characteristics resulting from the finite-size effects, such as the nonextensivity of the system, the anisotropy of the pressure, and so on.

M-ary information processing via an optimal nonlinear detector

Li Jian-Long
Chin. Phys. B 2009, 18 (12): 5196 ;  doi: 10.1088/1674-1056/18/12/016
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This paper presents a novel approach of M-ary baseband pulse amplitude modulated signal processing via a parameter-optimized nonlinear dynamic system. This nonlinear system usually shows the phenomenon of stochastic resonance by adding noise. To thoroughly discuss the signal processing performance of the nonlinear system, we tune the system parameters to obtain a nonlinear detector with optimal performance. For characterizing the output of the nonlinear system, the derivation of the probability of detection error is given by the system response speed and the probability density function of the nonlinear system output. By varying the noise intensity with fixed system parameters, the phenomenon of stochastic resonance is shown and by tuning the system parameters with fixed noise, the probability of detection error is minimized and the nonlinear system is optimized. The detection performance of the two cases is compared with the theoretical probability of detection error, which is validated by numerical simulation.

A delay-decomposition approach for stability of neural network with time-varying delay

Qiu Fang, Cui Bao-Tong, Ji Yan
Chin. Phys. B 2009, 18 (12): 5203 ;  doi: 10.1088/1674-1056/18/12/017
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This paper studies delay-dependent asymptotical stability problems for the neural system with time-varying delay. By dividing the whole interval into multiple segments such that each segment has a different Lyapunov matrix, some improved delay-dependent stability conditions are derived by employing an integral equality technique. A numerical example is given to demonstrate the effectiveness and less conservativeness of the proposed methods.

Anti-control of chaos in p--Ge photoconductor

Feng Yu-Ling, Zhang Xi-He
Chin. Phys. B 2009, 18 (12): 5212 ;  doi: 10.1088/1674-1056/18/12/018
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We present a scheme for the anti-control of chaos in the p--Ge photoconductor system by using a chaotic laser to irradiate and disturb this system. The numerical simulations show that this scheme can be effectively used to control periodic states in this p--Ge system into chaotic states. Moreover, the different chaos states with different chaotic orbits can be obtained by appropriately adjusting the disturbance intensity and disturbance frequency, and by increasing this intensity or reducing this frequency, this p--Ge system gradually evolves to fully developed chaotic states.

Digital chaotic sequence generator based on coupled chaotic systems

Liu Shu-Bo, Sun Jing, Xu Zheng-Quan, Liu Jin-Shuo
Chin. Phys. B 2009, 18 (12): 5219 ;  doi: 10.1088/1674-1056/18/12/019
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Chaotic systems perform well as a new rich source of cryptography and pseudo-random coding. Unfortunately their digital dynamical properties would degrade due to the finite computing precision. Proposed in this paper is a modified digital chaotic sequence generator based on chaotic logistic systems with a coupling structure where one chaotic subsystem generates perturbation signals to disturb the control parameter of the other one. The numerical simulations show that the length of chaotic orbits, the output distribution of chaotic system, and the security of chaotic sequences have been greatly improved. Moreover the chaotic sequence period can be extended at least by one order of magnitude longer than that of the uncoupled logistic system and the difficulty in decrypting increases 2128*2128 times indicating that the dynamical degradation of digital chaos is effectively improved. A field programmable gate array (FPGA) implementation of an algorithm is given and the corresponding experiment shows that the output speed of the generated chaotic sequences can reach 571.4~Mbps indicating that the designed generator can be applied to the real-time video image encryption.

Disturbance rejection and H pinning control of linear complex dynamical networks

Li Zhong-Kui, Duan Zhi-Sheng, Chen Guan-Rong
Chin. Phys. B 2009, 18 (12): 5228 ;  doi: 10.1088/1674-1056/18/12/020
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This paper concerns the disturbance rejection problem of a linear complex dynamical network subject to external disturbances. A dynamical network is said to be robust to disturbance, if the H norm of its transfer function matrix from the disturbance to the performance variable is satisfactorily small. It is shown that the disturbance rejection problem of a dynamical network can be solved by analysing the H control problem of a set of independent systems whose dimensions are equal to that of a single node. A counter-intuitive result is that the disturbance rejection level of the whole network with a diffusive coupling will never be better than that of an isolated node. To improve this, local feedback injections are applied to a small fraction of the nodes in the network. Some criteria for possible performance improvement are derived in terms of linear matrix inequalities. It is further demonstrated via a simulation example that one can indeed improve the disturbance rejection level of the network by pinning the nodes with higher degrees than pinning those with lower degrees.

Bifurcation control and chaos in a linear impulsive system

Jiang Gui-Rong, Xu Bu-Gong, Yang Qi-Gui
Chin. Phys. B 2009, 18 (12): 5235 ;  doi: 10.1088/1674-1056/18/12/021
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Bifurcation control and the existence of chaos in a class of linear impulsive systems are discussed by means of both theoretical and numerical ways. Chaotic behaviour in the sense of Marotto's definition is rigorously proven. A linear impulsive controller, which does not result in any change in one period-1 solution of the original system, is proposed to control and anti-control chaos. The numerical results for chaotic attractor, route leading to chaos, chaos control, and chaos anti-control, which are illustrated with two examples, are in good agreement with the theoretical analysis.

Combined bottleneck effect of on-ramp and bus stop in a cellular automaton model

Song Yu-Kun, Zhao Xiao-Mei
Chin. Phys. B 2009, 18 (12): 5242 ;  doi: 10.1088/1674-1056/18/12/022
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The combined bottleneck effect is investigated by modeling traffic systems with an on-ramp and a nearby bus stop in a two-lane cellular automaton model. Two cases, i.e. the bus stop locates in the downstream section of the on-ramp and the bus stop locates in the upstream section of the on-ramp, are considered separately. The upstream flux and downstream flux of the main road, as well as the on-ramp flux are analysed in detail, with respect to the entering probabilities and the distance between the on-ramp and the bus stop. It is found that the combination of the two bottlenecks causes the capacity to drop off, because the vehicles entering the main road from the on-ramp would interweave with the stopping (pulling-out) buses in the downstream (upstream) case. The traffic conflict in the former case is much heavier than that in the latter, causing the downstream main road to be utilized inefficiently. This suggests that the bus stop should be set in the upstream section of the on-ramp to enhance the capacity. The fluxes both on the main road and on the on-ramp vary with the distance between the two bottlenecks in both cases. However, the effects of distance disappear gradually at large distances. These findings might give some guidance to traffic optimization and management.

Complex network analysis in inclined oil--water two-phase flow

Gao Zhong-Ke, Jin Ning-De
Chin. Phys. B 2009, 18 (12): 5249 ;  doi: 10.1088/1674-1056/18/12/023
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Complex networks have established themselves in recent years as being particularly suitable and flexible for representing and modelling many complex natural and artificial systems. Oil--water two-phase flow is one of the most complex systems. In this paper, we use complex networks to study the inclined oil--water two-phase flow. Two different complex network construction methods are proposed to build two types of networks, i.e. the flow pattern complex network (FPCN) and fluid dynamic complex network (FDCN). Through detecting the community structure of FPCN by the community-detection algorithm based on K-means clustering, useful and interesting results are found which can be used for identifying three inclined oil--water flow patterns. To investigate the dynamic characteristics of the inclined oil--water two-phase flow, we construct 48 FDCNs under different flow conditions, and find that the power-law exponent and the network information entropy, which are sensitive to the flow pattern transition, can both characterize the nonlinear dynamics of the inclined oil--water two-phase flow. In this paper, from a new perspective, we not only introduce a complex network theory into the study of the oil--water two-phase flow but also indicate that the complex network may be a powerful tool for exploring nonlinear time series in practice.

Inverse Monte Carlo study on effective interaction potential of Ag--Rh alloy from pair correlation functions

Zhang Jing-Xiang, Li Hui, Song Xi-Gui, Zhang Jie
Chin. Phys. B 2009, 18 (12): 5259 ;  doi: 10.1088/1674-1056/18/12/024
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This paper presents an inverse Monte Carlo method to reconstruct pair interaction potential from pair correlation function. This approach adopts an iterative algorithm on interaction potential to fit known pair correlation function by compelling deviations of canonical average to meet with Hamiltonian parameters on a basis of statistical mechanism. The effective interaction potential between particles in liquid Ag--Rh alloys has been calculated with the inverse Monte Carlo method. It demonstrates an effective and simple way to obtain the effective potential of complex melt systems.

Coulomb effects on the formation of proton halo nuclei

Liang Yu-Jie, Li Yan-Song, Zhu Min, Liu Zu-Hua, Zhou Hong-Yu
Chin. Phys. B 2009, 18 (12): 5267 ;  doi: 10.1088/1674-1056/18/12/025
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This paper makes some qualitative and quantitative analyses about halo formation rules of some mirror nuclei with the relativistic mean-field (RMF) theory and the Woods--Saxon mean-field model. By analysing two opposite effects of Coulomb interaction on the proton halo formation, it finds that the energy level shift has a larger contribution than that of the Coulomb barrier when the mass number A is small, the hindrance of the Coulomb barrier becomes more obvious with the increase of the mass number A, and the overall effect of the Coulomb interaction almost disappears when A≈39 as its two effects counteract with each other.

Lithium atom population transfer by population trapping in a chirped microwave pulse

Jia Guang-Rui, Zhang Xian-Zhou, Ren Zhen-Zhong, Wu Su-Ling
Chin. Phys. B 2009, 18 (12): 5272 ;  doi: 10.1088/1674-1056/18/12/026
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Using a time-dependent multilevel approach, we demonstrate that lithium atoms can be transferred to states of lower principle quantum number by exposing them to a frequency chirped microwave pulse. The population transfer from n = 79 to n = 70 states of lithium atoms with more than 80% efficiency is achieved by means of the sequential two-photon Δ n = - 1 transitions. It is shown that the coherent control of the population transfer can be accomplished by the optimization of the chirping parameters and microwave field strength. The calculation results agree well with the experimental ones and novel explanations have been given to understand the experimental results.

Tunneling between double wells of atom in crossed electromagnetic fields

Shen Li, Wang Lei, Yang Hai-Feng, Liu Xiao-Jun, Liu Hong-Ping
Chin. Phys. B 2009, 18 (12): 5277 ;  doi: 10.1088/1674-1056/18/12/027
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The tunneling between double wells of atom in crossed electromagnetic fields is investigated by a one-dimensional Hamiltonian model. The crossed fields induced outer well is apart from the nuclear origin and it is very difficult to access by means of spectroscopy but it will be possible if there exists the tunneling of the electron between the outer well and the Coulomb potential predominated well at the nuclear origin. A one-dimensional quantum calculation with B-spline basis has been performed for hydrogen atom in crossed fields accessible in our laboratory, at B=0.8~T and F=-220~V.cm-1. The calculation shows that the wavefunctions of some excited states close to the Stark saddle point in the outer well extend over to the Coulomb potential well, making it possible to penetrate the quantum information of the outer well. However, the tunneling rate is very small and the spectral measurement of the transitions from the ground state should be of a high resolution and high sensitivity.

Interference effects on the photoionization cross sections between two neighbouring atoms: nitrogen as an example

Wu Jian-Hua, Yuan Jian-Min
Chin. Phys. B 2009, 18 (12): 5283 ;  doi: 10.1088/1674-1056/18/12/028
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Interference effects on the photoionization cross sections between two neighbouring atoms are considered based on the coherent scattering of the ionized electrons by the two nuclei when their separation is less than or comparable to the de Broglie wave length of the ionized electrons. As an example, the single atomic nitrogen ionization cross section and the total cross sections of two nitrogen atoms with coherently added photoionization amplitudes are calculated from the threshold to about 60~\AA (1~\AA=0.1~nm) of the photon energy. The photoionization cross sections of atomic nitrogen are obtained by using the close-coupling R-matrix method. In the calculation 19 states are included. The ionization energy of the atomic nitrogen and the photoionization cross sections agree well with the experimental results. Based on the R-matrix results of atomic nitrogen, the interference effects between two neighbouring nitrogen atoms are obtained. It is shown that the interference effects are considerable when electrons are ionized just above the threshold, even for the separations between the two atoms are larger than two times of the bond length of N2 molecules. Therefore, in hot and dense samples, effects caused by the coherent interference between the neighbours are expected to be observable for the total photoionization cross sections.

Multiphoton ionization of the hydrogen atom exposed to circularly or linearly polarized laser pulses

Wang Pei-Jie, He Feng
Chin. Phys. B 2009, 18 (12): 5291 ;  doi: 10.1088/1674-1056/18/12/029
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This paper studies the multiphoton ionization of the hydrogen atom exposed to the linearly or circularly polarized laser pulses by solving the time-dependent Schr?dinger equation. It finds that the ratio of the ionization probabilities by linearly and circularly polarized laser pulses varies with the numbers of absorbing photons. With the same laser intensity, the circularly polarized laser pulse favors to ionize the atom with more ease than the linearly polarized laser pulse if only two or three photons are necessary to be absorbed. For the higher order multiphoton ionization, the linearly polarized laser pulse has the advantage over circularly polarized laser pulse to ionize the atom.

An alternative view of condensed-phase photoionization

Ma Xiao-Guang, Yang Chuan-Lu, Gong Yu-Bing, Wang Mei-Shan
Chin. Phys. B 2009, 18 (12): 5296 ;  doi: 10.1088/1674-1056/18/12/030
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This paper proposes an accurate valuable interpretation scheme to study the evolvement of the photoionization processes from the isolated to the condensed atoms by a unique ab initio method. The variations of the photoionization cross sections of the atomic sodium with the photoelectron energy and the boundary radius of the atomic configuration space are studied in this new scheme by the R-matrix method. The discrepancy in the photoionization spectra of the isolated and the condensed sodium has been explained quantitatively and understood successfully by this alternative view in detail for the first time.

Energy spectrum and superfluidity of spin-2 ultracold bosons in optical lattices

Wang Yong-Jun, Liu Xian-Feng, Han Jiu-Rong
Chin. Phys. B 2009, 18 (12): 5301 ;  doi: 10.1088/1674-1056/18/12/031
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This paper studies the superfluidity of ultracold spin-2 Bose atoms with weak interactions in optical lattices by calculating the excitation energy spectrum using the Bogoliubov approach. The energy spectra exhibit the characteristics of the superfluid-phase explicitly and it finds the nonvanishing critical speeds of superfluid. The obtained results display that the critical speeds of superfluid are different for five spin components and can be controlled by adjusting the lattice parameters in experiments. Finally it discusses the feasibilities of implementing and measuring superfluid.

Effect of the reagent vibration on stereodynamics of the reaction O(1D)+HF →F+OH

Xu Yan, Zhao Juan, Yue Da-Guang, Liu Hao, Zheng Xiao-Yun, Meng Qing-Tian
Chin. Phys. B 2009, 18 (12): 5308 ;  doi: 10.1088/1674-1056/18/12/032
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This paper studies the influence of the reagent vibration on the reaction O(1D)+HF → HO+F by using a quasi-classical trajectory method on the new \textitab initio 1A' ground singlet potential energy surface (Gómez-Carrasco et al 2007 Chem. Phys. Lett. 435 188--193). The product angular distributions which reflect the vector correlation are calculated. Four polarization-dependent differential cross sections (PDDCSs) which are sensitive to many photoinitiated bimolecular reaction experiments are presented in the center of the mass frame, respectively. The differential cross section indicates that the OH product mainly tends to the forward scattering, and other PDDCSs are also influenced by the vibration levels of HF.

Band rules for the frequency spectra of three kinds of aperiodic photonic crystals with negative refractive index materials

Quan Xiao-Lin, Yang Xiang-Bo
Chin. Phys. B 2009, 18 (12): 5313 ;  doi: 10.1088/1674-1056/18/12/033
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By means of the theory of electromagnetic wave propagation and transfer matrix method, this paper investigates the band rules for the frequency spectra of three kinds of one-dimensional (1D) aperiodic photonic crystals (PCs), generalized Fibonacci GF(p,1), GF(1,2), and Thue--Morse (TM) PCs, with negative refractive index (NRI) materials. It is found that all of these PCs can open a broad zero-? gap, TM PC possesses the largest zero-? gap, and with the increase of p, the width of the zero-? gap for GF(p,1) PC becomes smaller. This characteristic is caused by the symmetry of the system and the open position of the zero-? gap. It is found that for GF(p,1) PCs, the possible limit zero-? gaps open at lower frequencies with the increase of p, but for GF(1,2) and TM PCs, their limit zero-? gaps open at the same frequency. Additionally, for the three bottom-bands, we find the interesting perfect self-similarities of the evolution structures with the increase of generation, and obtain the corresponding subband-number formulae. Based on 11 types of evolving manners Qi(i=1,2,....,11) one can plot out the detailed evolution structures of the three kinds of aperiodic PCs for any generation.

Optimal design of sub-wavelength metal rectangular gratings for polarizing beam splitter based on effective medium theory

Zhao Hua-Jun, Peng Yong-Jun, Tan Ju, Liao Chang-Rong, Li Peng, Ren Xiao-Xia
Chin. Phys. B 2009, 18 (12): 5326 ;  doi: 10.1088/1674-1056/18/12/034
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A novel optimal design of sub-wavelength metal rectangular gratings for the polarizing beam splitter (PBS) is proposed. The method is based on effective medium theory and the method of designing single layer antireflection coating. The polarization performance of PBS is discussed by rigorous couple-wave analysis (RCWA) method at a wavelength of 1550~nm. The result shows that sub-wavelength metal rectangular grating is characterized by a high reflectivity, like metal films for TE polarization, and high transmissivity, like dielectric films for TM polarization. The optimal design accords well with the results simulated by RCWA method.

Controllable grating fabrication by three interfering replicas of single femtosecond laser pulse

Han Yan-Hua, Qu Shi-Liang, Wang Qiang, Guo Zhong-Yi, Chen Xiang-Jun
Chin. Phys. B 2009, 18 (12): 5331 ;  doi: 10.1088/1674-1056/18/12/035
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The controllable periodic M-shape gratings are fabricated on the surface of silica glass by three coplanar interfering beams from a single femtosecond pulse. The configuration of the M-shape periodic structure is characterized by optical microscopy and atomic force microscopy. The experimental results and the theoretical simulation show that the period and the modulation depth ratio between the neighboring grooves of the fabricated gratings can be controlled by adjusting the collision angles and pulse energy of the three beams, respectively.

The entanglement between two isolated atoms in double mode--mode competition model

Wu Qin, Fang Mao-Fa, Cai Jian-Wu, Hu Yao-Hua
Chin. Phys. B 2009, 18 (12): 5336 ;  doi: 10.1088/1674-1056/18/12/036
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Extending the double Jaynes--Cummings model to a more complicated case where the mode--mode competition is considered, we investigate the entanglement character of two isolated atoms by means of concurrence, and discuss the dependence of atom--atom entanglement on the different initial state and the relative coupling strength between the atom and the corresponding cavity field. The results show that the amplitude and the period of the atom--atom entanglement evolution can be controlled by the choice of initial state and relative coupling strength, respectively. We find that the phenomenon of entanglement sudden death (ESD) is sensitive to the initial conditions. The length of the time interval for zero entanglement depends not only on the initial degree of entanglement between two atoms but also on the relative coupling strength of atom--field interaction. The ESD effect can be weakened by enhancing the mode--mode competition between the three- and single-photon processes.

Comparison between effects of Doppler broadening on pure and non-pure inversionless gains with frequency up-conversion

Fan Xi-Jun, Ma Hui, Liu Zhong-Bo, Tong Dian-Min
Chin. Phys. B 2009, 18 (12): 5342 ;  doi: 10.1088/1674-1056/18/12/037
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A study is made of the effects of Doppler broadening on pure gain without inversion, which means that neither one-photon nor two-photon inversions are allowed, and non-pure gain without inversion, which means that one-photon inversion does not occur but two-photon inversion is present, in a closed Λ-type three-level system with incoherent pumping. It is shown that when the driving field is resonant but the probe field is not, in a certain range of Doppler width, for the case of the lower degree of frequency up-conversion, generally, pure gain without inversion increases monotonically and non-pure gain without inversion does not monotonically increase or decrease with increasing Doppler width; for the case of the higher degree of frequency up-conversion, pure gain without inversion decreases monotonically but non-pure gain without inversion cannot be produced. In the case of two-photon resonance, in some range of Doppler width, pure gain without inversion does not monotonically increase or decrease while non-pure gain without inversion decreases monotonically with Doppler width increasing. Finally, an experimental scheme for examining our theoretical result is given.

GaN-based violet laser diodes grown on free-standing GaN substrate

Zhang Li-Qun, Zhang Shu-Ming, Jiang De-Sheng, Wang Hui, Zhu Jian-Jun, Zhao De-Gang, Liu Zong-Shun, Yang Hui
Chin. Phys. B 2009, 18 (12): 5350 ;  doi: 10.1088/1674-1056/18/12/038
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A violet laser diode (LD) structure is grown on a free-standing c-plane GaN substrate and 4~μ m× 800~μ m ridge waveguide LDs are fabricated. The electrical and the optical characteristics of LDs under different facet-coating and chip-mounting conditions are investigated under pulse mode operation. The active region temperatures of p-side up and p-side down mounted LDs are calculated with different injection currents. The calculated thermal resistances of p-side up and p-side down mounted LDs are 4.6~K/W and 3~K/W, respectively. The threshold current of the p-side down mounted LD is much lower than that of the p-side up mounted LD. The blue shift of the emission wavelength with increasing injection current is observed only for the LD with p-side down mounting configuration, due to the more efficient heat dissipation.

Four-wave mixing and six-wave mixing in a four-level confined atomic system

Li Chang-Biao, Zhang Yan-Peng, Nie Zhi-Qiang, Zheng Huai-Bin, Shi Mei-Zhen, Liu Dong-Ning, Song Jian-Ping, Lu Ke-Qing
Chin. Phys. B 2009, 18 (12): 5354 ;  doi: 10.1088/1674-1056/18/12/039
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We have investigated coexisting four-wave mixing and six-wave mixing (SWM) in ultra-thin, micrometre and long vapour cells. There exists competition between Dicke-narrowing features and polarization interference in the micrometre cell. The oscillation behaviour of SWM signal intensities and linewidths results from destructive interference. With a larger destructive interference, the SWM signal in ultra-thin cells shows a narrow spectrum, in contrast to the long cell case. Due to the Dicke-narrowing features, a narrow spectrum can be obtained, and such spectra can be used for high precision measurements and metrological standards.

Measurement of stimulated Brillouin scattering (SBS) threshold based on waveform variation of SBS optical limiting

Hasi Wu-Li-Ji, Lu Huan-Huan, Fu Mei-Ling, Gong Sheng, Lü Zhi-Wei, Lin Dian-Yang, He Wei-Ming, Gao Wei
Chin. Phys. B 2009, 18 (12): 5362 ;  doi: 10.1088/1674-1056/18/12/040
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This paper proposes a method for measuring the stimulated Brillouin scattering (SBS) threshold based on waveform variation of SBS optical limiting. The output waveforms for different pump power densities are numerically simulated, and validated in the Nd:YAG seed-injected laser system. The results indicate that SBS does not take place in the case of a low pump power density and thus the output power scales up linearly with pump power. Once the pump power density exceeds the SBS threshold, SBS takes place and thereby the energies are transferred from pump to Stokes. As a result, a small shoulder appears in the trailing edge of the output waveform, which provides another method to determine the SBS threshold.

Optimization of a solar-blind and middle infrared two-colour photodetector using GaN-based bulk material and quantum wells Hot!

Cen Long-Bin, Shen Bo, Qin Zhi-Xin, Zhang Guo-Yi
Chin. Phys. B 2009, 18 (12): 5366 ;  doi: 10.1088/1674-1056/18/12/041
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This paper calculates the wavelengths of the interband transitions as a function of the Al mole fraction of AlxGa1-xN bulk material. It is finds that when the Al mole fraction is between 0.456 and 0.639, the wavelengths correspond to the solar-blind (250~nm to 280~nm). The influence of the structure parameters of AlyGa1-yN/GaN quantum wells on the wavelength and absorption coefficient of intersubband transitions has been investigated by solving the Schr?dinger and Poisson equations self-consistently. The Al mole fraction of the AlyGa1-yN barrier changes from 0.30 to 0.46, meanwhile the width of the well changes from 2.9~nm to 2.2~nm, for maximal intersubband absorption in the window of the air (3~μm <λ <5~μm). The absorption coefficient of the intersubband transition between the ground state and the first excited state decreases with the increase of the wavelength. The results are finally used to discuss the prospects of GaN-based bulk material and quantum wells for a solar-blind and middle infrared two-colour photodetector.

Below-diffraction-limited hybrid recording using silicon thin film super-resolution structure

Jiao Xin-Bing, Wei Jing-Song, Gan Fu-Xi
Chin. Phys. B 2009, 18 (12): 5370 ;  doi: 10.1088/1674-1056/18/12/042
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We report on new experimental results for below-diffraction-limited hybrid recording. In our experiments, by means of focused laser assisted magnetic recording, the magnetic domains within TbFeCo thin films are obtained under an external perpendicular direct magnetic field. For a single magnetic medium, the domain size is mainly determined by the focused spot, which is about 620~nm for the laser wavelength λ =406~nm, and a numerical aperture of the lens of 0.80. However, when a silicon thin film structure is inserted between the substrate and the magnetic medium, the recording domains can be reduced obviously. By optimizing the experimental condition, even the size can be reduced to about 100~nm, which is below the diffraction limit, i.e. about 1/6 of the spot size. This is very useful for improving the hybrid recording density in practical applications.

Optimization control of modulation-instability gain in photonic crystal fibres with two-zero dispersion wavelengths

Wang He-Lin, Leng Yu-Xin, Xu Zhi-Zhan
Chin. Phys. B 2009, 18 (12): 5375 ;  doi: 10.1088/1674-1056/18/12/043
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We design three kinds of photonic crystal fibres (PCF) with two zero-dispersion wavelengths (ZDWs) using the improved full vector index method (FVIM) and finite-difference frequency domain (FDFD) techniques. Based on these designed fibres, the effect of fibre structure, pump power and wavelength on the modulation instability (MI) gain in the anomalous dispersion region close to the second ZDW of the PCFs is comprehensively analysed in this paper. The analytical results show that an optimal MI gain can be obtained when the optimal pump wavelength (1530~nm) is slightly shorter than the second ZDW (1538~nm) and the optimal pump power is 250~W. Importantly, the total MI gain bandwidth has been increased to 260~nm for the first time, so far as we know, for an optimally-designed fibre with Λ = 1.4~nm and d/Λ = 0.676, and the gain profile became much smoother. The optimal pump wavelength relies on the second ZDW of the PCF whereas the optimal pump power depends on the corporate operation of the optimal fibre structure and optimal pump wavelength, which is important in designing the most appropriate PCF to attain higher broadband and gain amplification.

Study of runaway electron behaviour during electron cyclotron resonance heating in the HL-2A Tokamak

Zhang Yi-Po, Yang Jin-Wei, Liu Yi, Song Xian-Ying, Yuan Guo-Liang, Li Xu, Zhou Yan, Zhou Jun, Yang Qing-Wei, Chen Liao-Yuan, Rao Jun, Duan Xu-Ru, Pan Chuan-Hong, HL-2A Team
Chin. Phys. B 2009, 18 (12): 5385 ;  doi: 10.1088/1674-1056/18/12/044
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During the current flat-top phase of electron cyclotron resonance heating discharges in the HL-2A Tokamak, the behaviour of runaway electrons has been studied by means of hard x-ray detectors and neutron diagnostics. During electron cyclotron resonance heating, it can be found that both hard x-ray radiation intensity and neutron emission flux fall rapidly to a very low level, which suggests that runaway electrons have been suppressed by electron cyclotron resonance heating. From the set of discharges studied in the present experiments, it has also been observed that the efficiency of runaway suppression by electron cyclotron resonance heating was apparently affected by two factors: electron cyclotron resonance heating power and duration. These results have been analysed by using a test particle model. The decrease of the toroidal electric field due to electron cyclotron resonance heating results in a rapid fall in the runaway electron energy that may lead to a suppression of runaway electrons. During electron cyclotron resonance heating with different powers and durations, the runaway electrons will experience different slowing down processes. These different decay processes are the major cause for influencing the efficiency of runaway suppression. This result is related to the safe operation of the Tokamak and may bring an effective control of runaway electrons.

Energetic-ion generation by the combination of laser pressure and Coulomb explosion

Jin Zhang-Ying, Shen Bai-Fei, Zhang Xiao-Mei, Wang Feng-Chao, Ji Liang-Liang
Chin. Phys. B 2009, 18 (12): 5395 ;  doi: 10.1088/1674-1056/18/12/045
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A scheme of generating energetic ions by the interaction of an ultrahigh-intensity laser pulse and a thin solid foil is studied. The combination of the effects of radiation pressure and Coulomb explosion makes the ion acceleration more effective. The maximum ion velocity variation with time is predicted theoretically while the temporal evolution of the electrostatic field due to the Coulomb explosion is taken into consideration. Two-dimensional particle-in-cell simulations are done to verify the theory.

The influence of the structures and compounds of DLC coatings on the barrier properties of PET bottles

Yang Li, Wang Zhen-Duo, Zhang Shou-Ye, Yang Li-Zhen, Chen Qiang
Chin. Phys. B 2009, 18 (12): 5401 ;  doi: 10.1088/1674-1056/18/12/046
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To reduce the oxygen transmission rate through a polyethylene terephthalate (PET) bottle (an organic plastic) diamond-like carbon (DLC) coatings on the inner surface of the PET bottle were deposited by radio frequency plasma-enhanced chemical vapour deposition (RF-PECVD) technology with C2H2 as the source of carbon and Ar as the diluted gas. As the barrier layer to humidity and gas permeation, this paper analyses the DLC film structure, composition, morphology and barrier properties by Fourier transform infrared, atomic force microscopy, scanning electron microscopy and oxygen transmission rate in detail. From the spectrum, it is found that the DLC film mainly consists of sp3 bonds. The barrier property of the films is significantly relevant to the sp3 bond concentration in the coating, the film thickness and morphology. Additionally, it is found that DLC film deposited in an inductively coupled plasma enhanced capacitively coupled plasma source shows a compact, homogeneous and crack-free surface, which is beneficial for a good gas barrier property in PET bottles.

Mitigation and prediction of disruption on the HL-2A Tokamak

Zheng Yong-Zhen, Qiu Ying, Zhang Peng, Huang Yuan, Cui Zheng-Ying, Sun Ping, Yang Qing-Wei
Chin. Phys. B 2009, 18 (12): 5406 ;  doi: 10.1088/1674-1056/18/12/047
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Injection of high-Z impurities into plasma has been proved to be able to reduce the localized thermal load and mechanical forces on the in-vessel components and the vacuum vessel, caused by disruptions in Tokamaks. An advanced prediction and mitigation system of disruption is implemented in HL-2A to safely shut down plasmas by using the laser ablation of high-Z impurities with a perturbation real-time measuring and processing system. The injection is usually triggered by the amplitude and frequency of the MHD perturbation field which is detected with a Mirnov coil and leads to the onset of a mitigated disruption within a few milliseconds. It could be a simple and potential approach to significantly reducing the plasma thermal energy and magnetic energy before a disruption, thereby achieving safe plasma termination. The plasma response to impurity injection, a mechanism for improving plasma thermal and current quench in major disruptions, the design of the disruption prediction warner, and an evaluation of the mitigation success rate are discussed in the present paper.

Polarized spin transport in mesoscopic quantum rings with electron--phonon and Rashba spin--orbit coupling

Liu Ping, Xiong Shi-Jie
Chin. Phys. B 2009, 18 (12): 5414 ;  doi: 10.1088/1674-1056/18/12/048
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The influence of electron--phonon (EP) scattering on spin polarization of current output from a mesoscopic ring with Rashba spin--orbit (SO) interaction is numerically investigated. There are three leads connecting to the ring at different positions; unpolarized current is injected to one of them, and the other two are output channels with different bias voltages. The spin polarization of current in the outgoing leads shows oscillations as a function of EP coupling strength owing to the quantum interference of EP states in the ring region. As temperature increases, the oscillations are evidently suppressed, implying decoherence of the EP states. The simulation shows that the magnitude of polarized current is sensitive to the location of the lead. The polarized current depends on the connecting position of the lead in a complicated way due to the spin-sensitive quantum interference effects caused by different phases accumulated by transmitting electrons with opposite spin states along different paths.

Multiple car-following model of traffic flow and numerical simulation

Peng Guang-Han, Sun Di-Hua
Chin. Phys. B 2009, 18 (12): 5420 ;  doi: 10.1088/1674-1056/18/12/049
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On the basis of the full velocity difference (FVD) model, an improved multiple car-following (MCF) model is proposed by taking into account multiple information inputs from preceding vehicles. The linear stability condition of the model is obtained by using the linear stability theory. Through nonlinear analysis, a modified Korteweg-de Vries equation is constructed and solved. The traffic jam can thus be described by the kink--antikink soliton solution for the mKdV equation. The improvement of this new model over the previous ones lies in the fact that it not only theoretically retains many strong points of the previous ones, but also performs more realistically than others in the dynamical evolution of congestion. Furthermore, numerical simulation of traffic dynamics shows that the proposed model can avoid the disadvantage of negative velocity that occurs at small sensitivity coefficients λ in the FVD model by adjusting the information on the multiple leading vehicles. No collision occurs and no unrealistic deceleration appears in the improved model.

Orbital magnetization in semiconductors

Fang Cheng, Wang Zhi-Gang, Li Shu-Shen, Zhang Ping
Chin. Phys. B 2009, 18 (12): 5431 ;  doi: 10.1088/1674-1056/18/12/050
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This paper theoretically investigates the orbital magnetization of electron-doped (n-type) semiconductor heterostructures and of hole-doped (p-type) bulk semiconductors, which are respectively described by a two-dimensional electron/hole Hamiltonian with both the included Rashba spin--orbit coupling and Zeeman splitting terms. It is the Zeeman splitting, rather than the Rashba spin--orbit coupling, that destroys the time-reversal symmetry of the semiconductor systems and results in nontrivial orbital magnetization. The results show that the magnitude of the orbital magnetization per hole and the Hall conductance in the p-type bulk semiconductors are about 10-2--10-1 effective Bohr magneton and 10-1--1 e2/h, respectively. However, the orbital magnetization per electron and the Hall conductance in the n-type semiconductor heterostructures are too small to be easily observed in experiment.

Screening influence on the Stark effect of impurity states in strained wurtzite GaN/AlxGa1-xN heterojunctions under pressure

Zhang Min, Ban Shi-Liang
Chin. Phys. B 2009, 18 (12): 5437 ;  doi: 10.1088/1674-1056/18/12/051
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The screening effect of the random-phase-approximation on the states of shallow donor impurities in free strained wurtzite GaN/AlxGa1-xN heterojunctions under hydrostatic pressure and an external electric field is investigated by using a variational method and a simplified coherent potential approximation. The variations of Stark energy shift with electric field, impurity position, Al component and areal electron density are discussed. Our results show that the screening dramatically reduces both the blue and red shifts as well as the binding energies of impurity states. For a given impurity position, the change in binding energy is more sensitive to the increase in hydrostatic pressure in the presence of the screening effect than that in the absence of the screening effect. The weakening of the blue and red shifts, induced by the screening effect, strengthens gradually with the increase of electric field. Furthermore, the screening effect weakens the mixture crystal effect, thereby influencing the Stark effect. The screening effect strengthens the influence of energy band bending on binding energy due to the areal electron density.

Kondo effect in a deformed molecule coupled asymmetrically to ferromagnetic electrodes

Wang Rui-Qiang, Jiang Kai-Ming
Chin. Phys. B 2009, 18 (12): 5443 ;  doi: 10.1088/1674-1056/18/12/052
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The nonequilibrium Kondo effect is studied in a molecule quantum dot coupled asymmetrically to two ferromagnetic electrodes by employing the nonequilibrium Green function technique. The current-induced deformation of the molecule is taken into account, modeled as interactions with a phonon system, and phonon-assisted Kondo satellites arise on both sides of the usual main Kondo peak. In the antiparallel electrode configuration, the Kondo satellites can be split only for the asymmetric dot-lead couplings, distinguished from the parallel configuration where splitting also exists, even though it is for symmetric case. We also analyze how to compensate the splitting and restore the suppressed zero-bias Kondo resonance. It is shown that one can change the TMR ratio significantly from a negative dip to a positive peak only by slightly modulating a local external magnetic field, whose value is greatly dependent on the electron--phonon coupling strength.

A theoretical study of harmonic generation in a short period AlGaN/GaN superlattice induced by a terahertz field

Chen Jun-Feng, Hao Yue
Chin. Phys. B 2009, 18 (12): 5451 ;  doi: 10.1088/1674-1056/18/12/053
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Based on an improved energy dispersion relation, the terahertz field induced nonlinear transport of miniband electrons in a short period AlGaN/GaN superlattice is theoretically studied in this paper with a semiclassical theory. To a short period superlattice, it is not precise enough to calculate the energy dispersion relation by just using the nearest wells in tight binding method: the next to nearest wells should be considered. The results show that the electron drift velocity is 30% lower under a dc field but 10% higher under an ac field than the traditional simple cosine model obtained from the tight binding method. The influence of the terahertz field strength and frequency on the harmonic amplitude, phase and power efficiency is calculated. The relative power efficiency of the third harmonic reaches the peak value when the dc field strength equals about three times the critical field strength and the ac field strength equals about four times the critical field strength. These results show that the AlGaN/GaN superlattice is a promising candidate to convert radiation of frequency ω to radiation of frequency 3ω or even higher.

The effects of vicinal sapphire substrates on the properties of AlGaN/GaN heterostructures

Xu Zhi-Hao, Zhang Jin-Cheng, Zhang Zhong-Fen, Zhu Qing-Wei, Duan Huan-Tao, Hao Yue
Chin. Phys. B 2009, 18 (12): 5457 ;  doi: 10.1088/1674-1056/18/12/054
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AlGaN/GaN heterostructures on vicinal sapphire substrates and just-oriented sapphire substrates (0001) are grown by the metalorganic chemical vapor deposition method. Samples are studied by high-resolution x-ray diffraction, atomic force microscopy, capacitance--voltage measurement and the Van der Pauw Hall-effect technique. The investigation reveals that better crystal quality and surface morphology of the sample are obtained on the vicinal substrate. Furthermore, the electrical properties are also improved when the sample is grown on the vicinal substrate. This is due to the fact that the use of vicinal substrate can promote the step-flow mode of crystal growth, so many macro-steps are formed during crystal growth, which causes a reduction of threading dislocations in the crystal and an improvement in the electrical properties of the AlGaN/GaN heterostructure.

Spin-dependent electron transport of a waveguide with Rashba spin--orbit coupling in an electromagnetic field

Xiao Xian-Bo, Li Xiao-Mao, Chen Yu-Guang
Chin. Phys. B 2009, 18 (12): 5462 ;  doi: 10.1088/1674-1056/18/12/055
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We investigate theoretically the spin-dependent electron transport in a straight waveguide with Rashba spin--orbit coupling (SOC) under the irradiation of a transversely polarized electromagnetic (EM) field. Spin-dependent electron conductance and spin polarization are calculated as functions of the emitting energy of electrons or the strength of the EM field by adopting the mode matching approach. It is shown that the spin polarization can be manipulated by external parameters when the strength of Rashba SOC is strong. Furthermore, a sharp step structure is found to exist in the total electron conductance. These results can be understood by the nontrivial Rashba subbands intermixing and the electron intersubband transition when a finite-range transversely polarized EM field irradiates a straight waveguide.

Energetics and electronic structure of a single copper atomic chain wrapped in a carbon nanotube: a first-principles study

Zhang Jian-Min, Du Xiu-Juan, Wang Su-Fang, Xu Ke-Wei
Chin. Phys. B 2009, 18 (12): 5468 ;  doi: 10.1088/1674-1056/18/12/056
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In the generalized gradient approximation, the energy and electronic structure are investigated for a single copper atomic chain wrapped in (4, 4), (5, 5) and (6, 6) armchair carbon nanotubes by using the first-principles projector-augmented wave potential within the framework of density functional theory. The results show that the (4, 4) and (5, 5) tubes are too narrow to wrap a Cu chain, but the (6, 6) tube is nearly ideal to wrap a Cu chain on its centre axis. Wider tubes are anticipated to wrap more than one Cu chain spontaneously with forces amounting to a fraction of a nanonewton. Although the tube--chain interaction decreases with the increase of the tube diameter of (4, 4), (5, 5) and (6, 6) successively, the charge density of the Cu@(6, 6) combined system still does not show complete superposition of that of the pristine (6, 6) tube and Cu chain. Successively reducing the restrictions of (4, 4), (5, 5) and (6, 6) tubes on the Cu chain leads to a reduction in shift of the highest peak of the Cu chain towards lower energies, that is from -0.5177~eV of the isolated Cu chain to -1.36785~eV, -0.668~eV and -0.588~eV for the Cu@(4, 4), Cu@(5, 5) and Cu@(6, 6) systems, respectively. In reverse, the strong metallic character of the Cu chain also enhances the metallic character of the combined systems so that the broader pseudogaps of the pristine carbon nanotubes around the Fermi level change into the narrow pseudogaps of the combined systems.

Analytical model for reverse characteristics of 4H--SiC merged PN--Schottky (MPS) diodes

Song Qing-Wen, Zhang Yu-Ming, Zhang Yi-Men, Lü Hong-Liang, Chen Feng-Ping, Zheng Qing-Li
Chin. Phys. B 2009, 18 (12): 5474 ;  doi: 10.1088/1674-1056/18/12/057
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A new analytical model for reverse characteristics of 4H--SiC merged PN--Schottky diodes (MPS or JBS) is developed. To accurately calculate the reverse characteristics of the 4H--SiC MPS diode, the relationship between the electric field at the Schottky contact and the reverse bias is analytically established by solving the cylindrical Poisson equation after the channel has pinched off. The reverse current density calculated from the Wentzel--Kramers--Brillouin (WKB) theory is verified by comparing it with the experimental result, showing that they are in good agreement with each other. Moreover, the effects of P-region spacing (S) and P-junction depth (Xj) on the characteristics of 4H--SiC MPS are analysed, and are particularly useful for optimizing the design of the high voltage MPS diodes.

Study on the degradation of NMOSFETs with ultra-thin gate oxide under channel hot electron stress at high temperature

Hu Shi-Gang, Hao Yue, Ma Xiao-Hua, Cao Yan-Rong, Chen Chi, Wu Xiao-Feng
Chin. Phys. B 2009, 18 (12): 5479 ;  doi: 10.1088/1674-1056/18/12/058
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This paper studies the degradation of device parameters and that of stress induced leakage current (SILC) of thin tunnel gate oxide under channel hot electron (CHE) stress at high temperature by using n-channel metal oxide semiconductor field effect transistors (NMOSFETs) with 1.4-nm gate oxides. The degradation of device parameters under CHE stress exhibits saturating time dependence at high temperature. The emphasis of this paper is on SILC of an ultra-thin-gate-oxide under CHE stress at high temperature. Based on the experimental results, it is found that there is a linear correlation between SILC degradation and Vh degradation in NMOSFETs during CHE stress. A model of the combined effect of oxide trapped negative charges and interface traps is developed to explain the origin of SILC during CHE stress.

Nonlocal Andreev reflection and spin current in a three-terminal Aharonov--Bohm interferometer

Peng Ju, Yu Hua-Ling, Wang Zhi-Guo
Chin. Phys. B 2009, 18 (12): 5485 ;  doi: 10.1088/1674-1056/18/12/059
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This paper theoretically reports the nonlocal Andreev reflection and spin current in a normal metal-ferromagnetic metal-superconducting Aharonov--Bohm interferometer. It is found that the electronic current and spin current are sensitive to systematic parameters, such as the gate voltage of quantum dots and the external magnetic flux. The electronic current in the normal metal lead results from two competing processes: quasiparticle transmission and nonlocal Andreev reflection. The appearance of zero spin-up electronic current (or spin-down electronic current) signals the existence of nonlocal Andreev reflection, and the presence of zero electronic current results in the appearance of pure spin current.

Differential conductance in normal-metal/insulator/metal/d-wave superconductor junction carrying a supercurrent

Li Xiao-Wei
Chin. Phys. B 2009, 18 (12): 5491 ;  doi: 10.1088/1674-1056/18/12/060
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This paper applies the Bogoliubov--de Gennes equation and the Blonder--Tinkham--Klapwijk approach to study the oscillatory behaviour of differential conductance in a normal metal/insulator/metal/d-wave superconductor junction carrying a supercurrent Is. We find that (i) a three-humped structure appears at a nearly critical supercurrent Is and z ≈ 0.5 for the normal metal/insulator/metal/d_x2 + y2-wave superconductor junction; (ii) the zero-bias conductance peak splits into two peaks with sufficiently large applied current for the normal metal/insulator/metal/dxy-wave superconductor junction; (iii) the conductance spectrum exhibits oscillating behaviour with the bias voltage and the peaks of the resonances are suppressed by increasing supercurrent Is.

The phenomenon of even bulk modes variance in a ferromagnetic A--A bilayer system

Zhou Wen-Ping, Yun Guo-Hong, Liang Xi-Xia
Chin. Phys. B 2009, 18 (12): 5496 ;  doi: 10.1088/1674-1056/18/12/061
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The eigenproblems of spin waves in a symmetrical ferromagnetic bilayered system with periodic boundary conditions are solved using the interface-rescaling approach (IRA). The results show that interface coupling between two sublayers would not change the excitation energy of odd bulk modes, but change excitation energy of even bulk modes. We call this peculiar phenomenon the phenomenon of even bulk mode variance (PEBMV). There are two kinds of mechanisms which cause PEBMV: phase reversal and phase translation of the magnon at the interface, corresponding, respectively, to the antiferromagnetic and ferromagnetic interface coupling cases. PEBMV embodies the selective effect of the interface on different bulk magnons.

Comparative studies on Zn0.95Co0.05O thin films on C- and R-sapphire substrates

Peng Ying-Zi, Thomas Liew, Song Wen-Dong, Chong Tow Chong
Chin. Phys. B 2009, 18 (12): 5501 ;  doi: 10.1088/1674-1056/18/12/062
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Zn_0.95Co_0.05O precipitate-free single crystal thin films were synthesized by a dual beam pulsed laser deposition method. The films form a wurtzite structure whose hexagonal axis is perpendicular or parallel to the plane of the surface depending on the C-plane (0001) or R-plane (11\bar 20) sapphire substrate. Based on the results of high-resolution transmission electron microscopy and x-ray diffraction, C-plane films show larger lattice mismatch. The films exhibit magnetic and semiconductor properties at room temperature. The coercivity of the film is about 8000 A/m at room temperature. They are soft magnetic materials with small remanent squareness S for both crystal orientations. There is no evidence to show that the anisotropy is fixed to the hexagonal axis (C-axis) for the wurtzite structure.

Self-consistent nonlinear analysis of a frequency-quadrupling terahertz gyroklystron

Liu Di-Wei, Yuan Xue-Song, Yan Yang, Liu Sheng-Gang
Chin. Phys. B 2009, 18 (12): 5507 ;  doi: 10.1088/1674-1056/18/12/063
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This paper analyses a three-cavity frequency-quadrupling terahertz gyroklystron with successive frequency-doubling in each cavity with self-consistent nonlinear theory. The beam--wave interaction efficiency and the electron bunching process are studied. The variation of output efficiency with the length of drift tubes and output power and the variation of Ohmic loss with the length of output cavity are considered. Numerical simulations predict an optimal output efficiency of 1.8%, a power output of more than 2~kW and a gain of 33~dB after taking into account Ohmic losses when the frequency-quadrupling gyroklystron, driven by a 40-kV, 3-A electron beam and 1 Watt input power, operates at 225~GHz.

Diffraction of terahertz waves after passing through a Fresnel lens

Shi Yu-Lei, Zhou Qing-Li, Zhang Cun-Lin
Chin. Phys. B 2009, 18 (12): 5511 ;  doi: 10.1088/1674-1056/18/12/064
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The spatiotemporal and spectral characteristics of ultrawide-band terahertz pulses after passing through a Fresnel lens are studied by using the scalar diffraction theory. The simulation shows that the transmitted terahertz waveforms compress with increasing propagation distance, and the multi-frequency focusing phenomenon at different focal points is observed. Additionally, the distribution of terahertz fields in a plane perpendicular to the axis is also discussed, and it is found that the diffraction not only induces focusing on-axis but also inhibits focusing at off-axis positions. Therefore, the Fresnel lens may be a useful alternative approach to being a terahertz filter. Moreover, the terahertz pulses travelling as a basic mode of a Gaussian beam are discussed in detail.

Anomalous behaviours of terahertz reflected waves transmitted from GaAs induced by optical pumping

Shi Yu-Lei, Zhou Qing-Li, Zhao Dong-Mei, Zhang Cun-Lin
Chin. Phys. B 2009, 18 (12): 5518 ;  doi: 10.1088/1674-1056/18/12/065
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Femtosecond pump-terahertz probe studies of carrier dynamics in semi-insulating GaAs have been investigated in detail for various pump powers. It is observed that, at high pump powers, the reflection peaks flip to the opposite polarity and dramatically enhance as the pump arrival time approaches the reflected wave of the terahertz pulse. The abnormal polarity-flip and enhancement can be interpreted by the pump-induced enhancement in the photoconductivity of GaAs and half-wave loss. Moreover, the carrier relaxation processes and surface states filling in GaAs are also studied in these measurements.

Improved anti-Stokes energy transfer between rare earth ions in Er(0.5)Yb(9.5):FOV oxyfluoride vitroceramics explains the strong color reversal

Chen Xiao-Bo, Wang Ce, Gregory J. Salamo, Naruhito Sawanobori, Kang Dong-Guo, Masaaki Ohtsuka, Yang Guo-Jian, Peng Fang-Lin
Chin. Phys. B 2009, 18 (12): 5523 ;  doi: 10.1088/1674-1056/18/12/066
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The widely used energy transfer theory is a foundation of luminescence, in which the rates of Stokes and anti-Stokes processes have the same calculation formula. An improvement on the anti-Stokes energy transfer to explain the fluorescence intensity reversal between the red and green fluorescence of Er(0.5)Yb(9.5):FOV is reported in the present article. The range of the intensity reversal \varSigma was measured to be 877. Dynamic processes for 16 levels were simulated. A coefficient, the improvement factor of the intensity ratio of Stokes to anti-Stokes processes in quantum Raman theory compared to classical Raman theory, is introduced to successfully describe the anti-Stokes energy transfer. A new method to calculate the distance between the rare earth ions, which is critical for the energy transfer calculation, is proposed. The validity of these important improvements is also proved by experiment.

Intraband dynamics and terahertz emission in biased semiconductor superlattices coupled to double far-infrared pulses

Li Min, Mi Xian-Wu
Chin. Phys. B 2009, 18 (12): 5534 ;  doi: 10.1088/1674-1056/18/12/067
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This paper studies both the intraband polarization and terahertz emission of a semiconductor superlattice in combined dc and ac electric fields by using the superposition of two identical time delayed and phase shifted optical pulses. By adjusting the delay between these two optical pulses, our results show that the intraband polarization is sensitive to the time delay. The peak values appear again for the terahertz emission intensity due to the superposition of two optical pulses. The emission lines of terahertz blueshift and redshift in different ac electric fields and dynamic localization appears. The emission lines of THz only appear to blueshift when the biased superlattice is driven by a single optical pulse. Due to excitonic dynamic localization, the terahertz emission intensity decays with time in different dc and ac electric fields. These are features of this superlattice which distinguish it from a superlattice generated by a single optical pulse to drive it.

Enhanced nonlinear optical absorption of Au/SiO2 nano-composite thin films

Zhao Cui-Hua, Zhang Bo-Ping, Shang Peng-Peng
Chin. Phys. B 2009, 18 (12): 5539 ;  doi: 10.1088/1674-1056/18/12/068
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Nano metal-particle dispersed glasses are the attractive candidates for nonlinear optical material applications. Au/SiO2 nano-composite thin films with 3~vol\% to 65~vol% Au are prepared by inductively coupled plasma sputtering. Au particles as perfect spheres with diameters between 10~nm and 30~nm are uniformly dispersed in the SiO2 matrix. Optical absorption peaks due to the surface plasmon resonance of Au particles are observed. The absorption property is enhanced with the increase of Au content, showing a maximum value in the films with 37~vol% Au. The absorption curves of the Au/SiO2 thin films with 3~vol\% to 37~vol% Au accord well with the theoretical optical absorption spectra obtained from Mie resonance theory. Increasing Au content over 37~vol% results in the partial connection of Au particles, whereby the intensity of the absorption peak is weakened and ultimately replaced by the optical absorption of the bulk. The band gap decreases with Au content increasing from 3~vol\% to 37~vol % but increases as Au content further increases.

Neutrino oscillation interference phase in Kerr space--time

Ren Jun, Jia Meng-Wen, Yuan Chang-Qing
Chin. Phys. B 2009, 18 (12): 5575 ;  doi: 10.1088/1674-1056/18/12/073
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The mass neutrino interference phases along the null trajectory and the geodesic line in Kerr space--time are studied on the plane θ=π/2. Because of the rotation object in Kerr space--time, a particle travelling along the radial geodesic must have a dragging effect produced by the angular momentum of the central object. We give the correction of the phase due to the rotation of the space--time. We find that the type-I interference phase along the geodesic remains the double of that along the null on the condition that the rotating quantity parameter a2 is preserved and the higher order terms are negligible (e.g. a4). In addition, we calculate the proper oscillation length in Kerr space--time. All of our results can return to those in Schwarzschild space--time as the rotating parameter a approaches zero.
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