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  • Dynamics of the entanglement witness for three qubits in common environment

    Lu Huai-Xin
    Chin. Phys. B 2007, 16 (7): 1878-1882
    In the measurement-based model of quantum computing, a one-way quantum computer consisting of many qubits can be immersed in a common environment as a simple decoherence mechanism. This paper studies the dynamics of entanglement witness for 3-qubit cluster states in the common environment. The resul...

Chin. Phys. B  
  Chin. Phys. B--2007, Vol.16, No.7
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Pseudospectral method with symplectic algorithm for the solution of time-dependent Schr?dinger equations

Bian Xue-Bin, Qiao Hao-Xue, Shi Ting-Yun
Chin. Phys., 2007, 16 (7): 1822-1826 doi: 10.1088/1009-1963/16/7/002
Full Text: [PDF 266 KB] (Downloads:754)
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A pseudospectral method with symplectic algorithm for the solution of time-dependent Schr\"{o}dinger equations (TDSE) is introduced. The spatial part of the wavefunction is discretized into sparse grid by pseudospectral method and the time evolution is given in symplectic scheme. This method allows us to obtain a highly accurate and stable solution of TDSE. The effectiveness and efficiency of this method is demonstrated by the high-order harmonic spectra of one-dimensional atom in strong laser field as compared with previously published work. The influence of the additional static electric field is also investigated.

Mei symmetry and generalized Hojman conserved quantity for variable mass systems with unilateral holonomic constraints

Jing Hong-Xing, Li Yuan-Cheng, Wang Jing, Xia Li-Li, Hou Qi-Bao
Chin. Phys., 2007, 16 (7): 1827-1831 doi: 10.1088/1009-1963/16/7/003
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This paper studies Mei symmetry which leads to a generalized Hojman conserved quantity for variable mass systems with unilateral holonomic constraints. The differential equations of motion for the systems are established, the definition and criterion of the Mei symmetry for the systems are given. The necessary and sufficient condition under which the Mei symmetry is a Lie symmetry for the systems is obtained and a generalized Hojman conserved quantity deduced from the Mei symmetry is got. An example is given to illustrate the application of the results.

The classification of travelling wave solutions and superposition of multi-solutions to Camassa-- Holm equation with dispersion

Liu Cheng-Shi
Chin. Phys., 2007, 16 (7): 1832-1837 doi: 10.1088/1009-1963/16/7/004
Full Text: [PDF 105 KB] (Downloads:790)
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Under the travelling wave transformation, the Camassa--Holm equation with dispersion is reduced to an integrable ordinary differential equation (ODE), whose general solution can be obtained using the trick of one-parameter group. Furthermore, by using a complete discrimination system for polynomial, the classification of all single travelling wave solutions to the Camassa--Holm equation with dispersion is obtained. In particular, an affine subspace structure in the set of the solutions of the reduced ODE is obtained. More generally, an implicit linear structure in the Camassa--Holm equation with dispersion is found. According to the linear structure, we obtain the superposition of multi-solutions to Camassa--Holm equation with dispersion.

An efficient quantum secure direct communication scheme with authentication

Yang Yu-Guang, Wen Qiao-Yan, Zhu Fu-Chen
Chin. Phys., 2007, 16 (7): 1838-1842 doi: 10.1088/1009-1963/16/7/005
Full Text: [PDF 105 KB] (Downloads:664)
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In this paper an efficient quantum secure direct communication (QSDC) scheme with authentication is presented, which is based on quantum entanglement and polarized single photons. The present protocol uses Einstein--Podolsky--Rosen (EPR) pairs and polarized single photons in batches. A particle of the EPR pairs is retained in the sender's station, and the other is transmitted forth and back between the sender and the receiver, similar to the `ping--pong' QSDC protocol. According to the shared information beforehand, these two kinds of quantum states are mixed and then transmitted via a quantum channel. The EPR pairs are used to transmit secret messages and the polarized single photons used for authentication and eavesdropping check. Consequently, because of the dual contributions of the polarized single photons, no classical information is needed. The intrinsic efficiency and total efficiency are both 1 in this scheme as almost all of the instances are useful and each EPR pair can be used to carry two bits of information.

The Holevo capacity of a generalized amplitude-damping channel

Hou Li-Zhen, Fang Mao-Fa
Chin. Phys., 2007, 16 (7): 1843-1847 doi: 10.1088/1009-1963/16/7/006
Full Text: [PDF 200 KB] (Downloads:483)
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The Holevo capacity of a generalized amplitude-damping channel is investigated by using a numerical method. It is shown that the Holevo capacity depends on the channel parameters representing the ambient temperature and fidelity. In particular, under a special condition, the Holevo capacity of the generalized amplitude-damping channel can be written as an analytical expression.

The projective Riccati equation expansion method and variable separation solutions for the nonlinear physical differential equation in physics

Ma Zheng-Yi
Chin. Phys., 2007, 16 (7): 1848-1854 doi: 10.1088/1009-1963/16/7/007
Full Text: [PDF 2143 KB] (Downloads:629)
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Using the projective Riccati equation expansion (PREE) method, new families of variable separation solutions (including solitary wave solutions, periodic wave solutions and rational function solutions) with arbitrary functions for two nonlinear physical models are obtained. Based on one of the variable separation solutions and by choosing appropriate functions, new types of interactions between the multi-valued and single-valued solitons, such as a peakon-like semi-foldon and a peakon, a compacton-like semi-foldon and a compacton, are investigated.

Sudden death of entanglement of a quantum model

Zhang Guo-Feng
Chin. Phys., 2007, 16 (7): 1855-1857 doi: 10.1088/1009-1963/16/7/008
Full Text: [PDF 266 KB] (Downloads:603)
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In this paper the so-called sudden death effect of entanglement is investigated in a quantum model. The results show that one can expect the resurrection of the original entanglement to occur in a periodic way following each sudden death event. The length of the time interval for the zero entanglement depends not only on the degree of entanglement of the initial state but also on the initial state.

An important property of entanglement: pairwise entanglement that can only be transferred by an entangled pair

Xi Xiao-Qiang, Liu Wu-Ming
Chin. Phys., 2007, 16 (7): 1858-1862 doi: 10.1088/1009-1963/16/7/009
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Based on the calculation of all the pairwise entanglements in the n ( n \leq 6 )-qubit Heisenberg XX open chain with system impurity, we find an important result: pairwise entanglement can only be transferred by an entangled pair. The non-nearest pairwise entanglements will have the possibility to exist as long as there has been even number of qubits in their middle. This point indicates that we can obtain longer distance entanglement in a solid system.

Exact solutions of the Klein--Gordon equation with Makarov potential and a recurrence relation

Zhang Min-Cang, Wang Zhen-Bang
Chin. Phys., 2007, 16 (7): 1863-1867 doi: 10.1088/1009-1963/16/7/010
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In this paper, the Klein--Gordon equation with equal scalar and vector Makarov potentials is studied by the factorization method. The energy equation and the normalized bound state solutions are obtained, a recurrence relation between the different principal quantum number n corresponding to a certain angular quantum number \ell is established and some special cases of Makarov potential are discussed.

Quantum broadcast communication

Wang Jian, Zhang Quan, Tang Chao-Jing
Chin. Phys., 2007, 16 (7): 1868-1877 doi: 10.1088/1009-1963/16/7/011
Full Text: [PDF 154 KB] (Downloads:498)
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Broadcast encryption allows the sender to securely distribute his/her secret to a dynamically changing group of users over a broadcast channel. In this paper, we just take account of a simple broadcast communication task in quantum scenario, in which the central party broadcasts his secret to multi-receiver via quantum channel. We present three quantum broadcast communication schemes. The first scheme utilizes entanglement swapping and Greenberger--Horne--Zeilinger state to fulfil a task that the central party broadcasts the secret to a group of receivers who share a group key with him. In the second scheme, based on dense coding, the central party broadcasts the secret to multi-receiver, each of which shares an authentication key with him. The third scheme is a quantum broadcast communication scheme with quantum encryption, in which the central party can broadcast the secret to any subset of the legal receivers.

Dynamics of the entanglement witness for three qubits in common environment Hot!

Lu Huai-Xin
Chin. Phys., 2007, 16 (7): 1878-1882 doi: 10.1088/1009-1963/16/7/012
Full Text: [PDF 164 KB] (Downloads:568)
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In the measurement-based model of quantum computing, a one-way quantum computer consisting of many qubits can be immersed in a common environment as a simple decoherence mechanism. This paper studies the dynamics of entanglement witness for 3-qubit cluster states in the common environment. The result shows that environment can induce an interesting feature in the time evolution of the entanglement witness: i.e., the periodical collapse and revival of the entanglement dynamics.

An optical scheme for conditional generation of W state and photon-added coherent state

Li Yan, Jing Hui, Zhan Ming-Sheng
Chin. Phys., 2007, 16 (7): 1883-1888 doi: 10.1088/1009-1963/16/7/013
Full Text: [PDF 327 KB] (Downloads:541)
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We propose a simple scheme to generate an arbitrary photon-added coherent state of a travelling optical field by combining an array of degenerate parametric amplifiers and corresponding single-photon detectors. Particularly, when the single-photon-added coherent state is observed by developing the novel technique of Zavatta et al (2004 Science 306 660), we can simultaneously obtain the generalized N -qubit W state.

Linear matrix inequality approach to exponential synchronization of a class of chaotic neural networks with time-varying delays

Wu Wei, Cui Bao-Tong
Chin. Phys., 2007, 16 (7): 1889-1896 doi: 10.1088/1009-1963/16/7/014
Full Text: [PDF 380 KB] (Downloads:719)
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In this paper, a synchronization scheme for a class of chaotic neural networks with time-varying delays is presented. This class of chaotic neural networks covers several well-known neural networks, such as Hopfield neural networks, cellular neural networks, and bidirectional associative memory networks. The obtained criteria are expressed in terms of linear matrix inequalities, thus they can be efficiently verified. A comparison between our results and the previous results shows that our results are less restrictive.

Experimental confirmation of a new reversed butterfly-shaped attractor

Liu Ling, Su Yan-Chen, Liu Chong-Xin
Chin. Phys., 2007, 16 (7): 1897-1900 doi: 10.1088/1009-1963/16/7/015
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This paper reports a new reverse butterfly-shaped chaotic attractor and its experimental confirmation. Some basic dynamical properties, and chaotic behaviours of this new reverse butterfly attractor are studied. Simulation results support brief theoretical derivations. Furthermore, the system is experimentally confirmed by a simple electronic circuit.

Synchronization in a unified fractional-order chaotic system

Wu Zheng-Mao, Xie Jian-Ying
Chin. Phys., 2007, 16 (7): 1901-1907 doi: 10.1088/1009-1963/16/7/016
Full Text: [PDF 1796 KB] (Downloads:991)
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In this paper, the synchronization in a unified fractional-order chaotic system is investigated by two methods. One is the frequency-domain method that is analysed by using the Laplace transform theory. The other is the time-domain method that is analysed by using the Lyapunov stability theory. Finally, the numerical simulations are used to illustrate the effectiveness of the proposed synchronization methods.

The sea--air oscillator model of decadal variations in subtropical cells and equatorial Pacific SST

Mo Jia-Qi, Lin Wan-Tao, Lin Yi-Hua
Chin. Phys., 2007, 16 (7): 1908-1911 doi: 10.1088/1009-1963/16/7/017
Full Text: [PDF 99 KB] (Downloads:502)
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In this paper a time delay equation for sea--air oscillator model is studied. The aim is to create an approximate solving method of nonlinear equation for sea--air oscillator model. Employing the method of variational iteration, it obtains the approximate solution of corresponding equation. This method is an approximate analytic method, which can be often used for analysing other behaviour of the sea surface temperature anomaly of the atmosphere--ocean oscillator model.

Impulsive generalized synchronization of chaotic system

Zhang Rong, Xu Zhen-Yuan, He Xue-Ming
Chin. Phys., 2007, 16 (7): 1912-1917 doi: 10.1088/1009-1963/16/7/018
Full Text: [PDF 948 KB] (Downloads:688)
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In this paper, with a given manifold y= H(x) , we have constructed a response system for a continuous-time chaotic system as a drive system, and used impulsive control theory to demonstrate theoretically that this response system can achieve impulsive generalized synchronization (GS) with the drive system. Our theoretical result is supported by numerical examples.

The open-plus-closed loop (OPCL) method for chaotic systems with multiple strange attractors

Song Yun-Zhong
Chin. Phys., 2007, 16 (7): 1918-1922 doi: 10.1088/1009-1963/16/7/019
Full Text: [PDF 584 KB] (Downloads:782)
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Based on the open-plus-closed-loop (OPCL) control method a systematic and comprehensive controller is presented in this paper for a chaotic system, that is, the Newton--Leipnik equation with two strange attractors: the upper attractor (UA) and the lower attractor (LA). Results show that the final structure of the suggested controller for stabilization has a simple linear feedback form. To keep the integrity of the suggested approach, the globality proof of the basins of entrainment is also provided. In virtue of the OPCL technique, three different kinds of chaotic controls of the system are investigated, separately: the original control forcing the chaotic motion to settle down to the origin from an arbitrary position of the phase space; the chaotic intra-attractor control for stabilizing the equilibrium points only belonging to the upper chaotic attractor or the lower chaotic one; and the inter-attractor control for compelling the chaotic oscillation from one basin to another one. Both theoretical analysis and simulation results verify the validity of the proposed means.

Stochastic period-doubling bifurcation analysis of stochastic Bonhoeffer--van der Pol system

Zhang Ying, Xu Wei, Fang Tong, Xu Xu-Lin
Chin. Phys., 2007, 16 (7): 1923-1933 doi: 10.1088/1009-1963/16/7/020
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In this paper, the Chebyshev polynomial approximation is applied to the problem of stochastic period-doubling bifurcation of a stochastic Bonhoeffer--van der Pol (BVP for short) system with a bounded random parameter. In the analysis, the stochastic BVP system is transformed by the Chebyshev polynomial approximation into an equivalent deterministic system, whose response can be readily obtained by conventional numerical methods. In this way we have explored plenty of stochastic period-doubling bifurcation phenomena of the stochastic BVP system. The numerical simulations show that the behaviour of the stochastic period-doubling bifurcation in the stochastic BVP system is by and large similar to that in the deterministic mean-parameter BVP system, but there are still some featured differences between them. For example, in the stochastic dynamic system the period-doubling bifurcation point diffuses into a critical interval and the location of the critical interval shifts with the variation of intensity of the random parameter. The obtained results show that Chebyshev polynomial approximation is an effective approach to dynamical problems in some typical nonlinear systems with a bounded random parameter of an arch-like probability density function.

Magnetocaloric effect in ErCo2 compound

Zou Jun-Ding, Shen Bao-Gen, Sun Ji-Rong
Chin. Phys., 2007, 16 (7): 1817-1821 doi: 10.1088/1009-1963/16/7/001
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The ErCo 2 compound is prepared by arc-melting and its entropy changes are calculated using Maxwell relation. Its entropy change reaches 38~J/(kg \cdot K) and its refrigerant capacity achieves 291 J/kg at 0--5 T. The mean field approximation is used to calculate the magnetic entropy of ErCo 2 compound. Results estimated by using the Maxwell relation deviate from mean field approximation calculations in ferrimagnetic state; however, the data obtained by the two ways are consistent in the vicinity of phase transition or at higher temperatures. This indicates that entropy changes are mainly derived from magnetic degree of freedom, and the lattice has almost no contribution to the entropy change in the vicinity of phase transition but its influence is obvious in the ferrimagnetic state below TC.

Critical behaviour in nuclear structure from spherical to axially symmetric deformed shape in IBM

Zhang Jin-Fu, Lü Li-Jun, Bai Hong-Bo
Chin. Phys., 2007, 16 (7): 1941-1946 doi: 10.1088/1009-1963/16/7/022
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To compare with the predictions of the transitional dynamical symmetry X(5) proposed by Iachello (2001 Phys. Rev. Lett. 87 052502), the critical behaviours of U(5)--SU(3) are studied in the space of two control parameters in the interacting boson model (IBM). A simple-shaped phased diagram has been presented. It is found that X(5) predictions cannot be exactly reproduced by our calculations and that the best agreement is close to the calculations with boson numbers N = 11 and 12. By comparing with experimental data on X(5)-like nuclei, we find that X(5) predictions and IBM calculations can reproduce the energy ratios and E2 transition ones.

Characteristics of surface waves in anisotropic left-handed materials

Jiang Yong-Yuan, Shi Hong-Yan, Zhang Yong-Qiang, Hou Chun-Feng, Sun Xiu-Dong
Chin. Phys., 2007, 16 (7): 1959-1962 doi: 10.1088/1009-1963/16/7/026
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We report the coexistence of TE and TM surface modes in certain same frequency domain at the interface between one isotropic regular medium and another biaxially anistotropic left-handed medium. The conditions for the existence of TE and TM polarized surface waves in biaxially anisotropic left-handed materials are identified, respectively. The Poynting vector and the energy density associated with surface modes are calculated. Depending on the system parameters, either TE or TM surface modes can have the time averaged Poynting vector directed to or opposite to the mode phase velocity. It is seen that the characteristics of surface waves in biaxially anisotropic left-handed media are significantly different from that in isotropic left-handed media.

Generation of four-particle GHZ states in bimodal cavity QED

Yang Zhen-Biao
Chin. Phys., 2007, 16 (7): 1963-1970 doi: 10.1088/1009-1963/16/7/027
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We propose a scheme for preparing four-particle Greenberger--Horne--Zeilinger states using two identical bimodal cavities, each supports two modes with different frequencies. This scheme is an alternative to another published work [Christopher C Gerry 1996 {\it Phys. Rev.} A {\bf 53} 4591]. Comparisons between them are discussed. The fidelity and the probability of success influenced by cavity decay for the generated states are also considered.

Atom--photon entanglement in the system with competing k-photon and l-photon transitions

Wu Qin, Fang Mao-Fa, Hu Yao-Hua
Chin. Phys., 2007, 16 (7): 1971-1975 doi: 10.1088/1009-1963/16/7/028
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We have investigated the evolution of the atomic quantum entropy and the entanglement of atom--photon in the system with competing k-photon and l-photon transitions by means of fully quantum theory, and examined the effects of competing photon numbers (k and l), the relative coupling strength between the atom and the two-mode field (\lambda/g), and the initial photon number of the field on the atomic quantum entropy and the entanglement of atom--photon. The results show that the multiphoton competing transitions or the large relative coupling strength can lead to the strong entanglement between atoms and photons. The maximal atom--photon entanglement can be prepared via the appropriate selection of system parameters and interaction time.

Simultaneous negative permittivity and permeability in a coherent atomic vapour

Shen Jian-Qi
Chin. Phys., 2007, 16 (7): 1976-1985 doi: 10.1088/1009-1963/16/7/029
Full Text: [PDF 409 KB] (Downloads:495)
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A new quantum optical mechanism to realize simultaneously negative electric permittivity and magnetic permeability is suggested. In order to obtain a negative permeability, we choose a proper atomic configuration that can dramatically enhance the contribution of the magnetic-dipole allowed transition via the atomic phase coherence. It is shown that the atomic system chosen with proper optical parameters can give rise to striking electromagnetic responses (leading to a negative refractive index) and that the atomic vapour becomes a left-handed medium in an optical frequency band. Differing from the previous schemes of artificial composite metamaterials (based on classical electromagnetic theory) to achieve the left-handed materials, which consist of anisotropic millimetre-scale composite structure units, the left-handed atomic vapour presented here is isotropic and homogeneous at the atomic-scale level. Such an advantage may be valuable in realizing the superlens (and hence perfect image) with left-handed atomic vapour.

Different operation states of soliton pulses in an erbium-doped fibre ring laser

Zhang Shu-Min, Lü Fu-Yun, Gong Yan-Dong, Yang Xiu-Feng
Chin. Phys., 2007, 16 (7): 1986-1990 doi: 10.1088/1009-1963/16/7/030
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We report on the experimental observation of soliton pulses in an erbium doped fibre ring laser. The passive mode-locking is achieved using the nonlinear polarization rotation technique. By adjusting the pump power and the intracavity polarization controllers, a normal soliton, a stable 8th harmonic mode-locked pulse and a noise-like pulse have been observed in our laser. The experimental results revealed that the noise-like pulse is not suitable for the optical telecommunication, and in order to obtain the stable harmonic mode-locked soliton, a strong unstable CW laser field is necessary to mediate global soliton interaction. The formation mechanism of the harmonic mode-locked pulse has also been analysed.

An all-solid-state high power quasi-continuous-wave tunable dual-wavelength Ti:sapphire laser system using birefringence filter

Ding Xin, Ma Hong-Mei, Zou Lei, Zou Yue, Wen Wu-Qi, Wang Peng, Yao Jian-Quan
Chin. Phys., 2007, 16 (7): 1991-1995 doi: 10.1088/1009-1963/16/7/031
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This paper describes a tunable dual-wavelength Ti:sapphire laser system with quasi-continuous-wave and high-power outputs. In the design of the laser, it adopts a frequency-doubled Nd:YAG laser as the pumping source, and the birefringence filter as the tuning element. Tunable dual-wavelength outputs with one wavelength range from 700~nm to 756.5~nm, another from 830~nm to 900mn have been demonstrated. With a pump power of 23~W at 532~nm, a repetition rate of 7~kHz and a pulse width of 47.6~ns, an output power of 5.1~W at 744.8~nm and 860.9~nm with a pulse width of 13.2~ns and a line width of 3~nm has been obtained, it indicates an optical-to-optical conversion efficiency of 22.2{\%}.

Polarization switching and synchronization of mutually coupled vertical-cavity surface-emitting semiconductor lasers

Zhang Wei-Li, Pan Wei, Luo Bin, Li Xiao-Feng, Zou Xi-Hua, Wang Meng-Yao
Chin. Phys., 2007, 16 (7): 1996-2002 doi: 10.1088/1009-1963/16/7/032
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Polarization switching (PS) dynamics and synchronization performances of two mutually coupled vertical-cavity surface-emitting lasers (VCSELs) are studied theoretically in this paper. A group of dimensionless rate equations is derived to describe our model. While analysing the PS characteristics, we focus on the effects of coupling rate and frequency detuning regarding different mutual injection types. The results indicate that the x-mode injection defers the occurrence of PS, while the y-mode injection leads the PS to occur at a lower current. Strong enough polarization-selective injection can suppress the PS. Moreover, if frequency detuning is considered, the effects of polarization-selective mutual injection will be weakened. To evaluate the synchronization performance, the correlation coefficients and output dynamics of VCSELs with both pure mode and mixed mode polarizations are given. It is found that performance of complete synchronization is sensitive to the frequency mismatch but it is little affected by mixed mode polarizations, which is opposite to the case of injection-locking synchronization.

A study of ultrafast electron diffusion kinetics in ultrashort-pulse laser ablation of metals

Yang Jian-Jun, Liu Wei-Wei, Zhu Xiao-Nong
Chin. Phys., 2007, 16 (7): 2003-2010 doi: 10.1088/1009-1963/16/7/033
Full Text: [PDF 428 KB] (Downloads:508)
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Temperature dependence of the electron diffusion in metallic targets, where the electron--electron collision is the dominant process, is investigated with the help of an extended two-temperature model. In sharp contrast to the low electron temperature case, where only the electron---phonon collisions are commonly considered, the electron diffusion process underlying the high electron temperatures evolves dramatically different in both temporal and spatial domains. Calculated results of the ablation yield at different pulse durations are presented for a copper plate impinged by ultrashort laser pulses with energy fluences ranging from 0.1 J/cm2 to 10 J/cm2. The excellent agreement between the simulation results and the experimental data indicates the significant role of electron--electron collisions in material ablations using intense ultrashort laser pulses.

A photonic crystal waveguide with silicon on insulator in the near-infrared band

Tang Hai-Xia, Zuo Yu-Hua, Yu Jin-Zhong, Wang Qi-Ming
Chin. Phys., 2007, 16 (7): 2011-2014 doi: 10.1088/1009-1963/16/7/034
Full Text: [PDF 621 KB] (Downloads:567)
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A two-dimensional (2D) photonic crystal waveguide in the \Gamma--K direction with triangular lattice on a silicon-on-insulator (SOI) substrate in the near-infrared band is fabricated by the combination of electron beam lithography and inductively coupled plasma etching. Its transmission characteristics are analysed from the stimulated band diagram by the effective index and the 2D plane wave expansion (PWE) methods. In the experiment, the transmission band edge in a longer wavelength of the photonic crystal waveguide is about 1590\,nm, which is in good qualitative agreement with the simulated value. However, there is a disagreement between the experimental and the simulated results when the wavelength ranges from 1607 to 1630\,nm, which can be considered as due to the unpolarized source used in the transmission measurement.

Semi-vectorial analysis of a compact wavelength demultiplexer based on the tapered multimode interference coupler

Xiao Jin-Biao, Liu Xu, Cai Chun, Sun Xiao-Han
Chin. Phys., 2007, 16 (7): 2015-2022 doi: 10.1088/1009-1963/16/7/035
Full Text: [PDF 1326 KB] (Downloads:544)
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Based on a parabolically tapered multimode interference (MMI) coupler with a deep-etched SiO2/SiON rib waveguide, a compact wavelength demultiplexer operating at 1.30 and 1.55\mum wavelengths is proposed and analysed by using three-dimensional semi-vectorial finite-difference beam propagation method (3D-SV-FD-BPM). The results show that a MMI section of 330.0\mum in length, which is only 76% length of a straight MMI coupler, is achieved with the contrasts of 42.3 and 39.2dB in quasi-TE mode, and 38.4 and 37.8dB in quasi-TM mode at wavelengths 1.30 and 1.55\mum, respectively, and the insertion losses below 0.2dB at both wavelengths and in both polarization states. The alternating direction implicit algorithm with the Crank--Nicholson scheme is applied to the discretization of the 3D-SV-FD-BPM formulation along the longitudinal direction. Moreover, a modified FD scheme is constructed to approximate the resulting equations along the transverse directions, in which the discontinuities of the derivatives of magnetic field components Hy and Hx along the vertical and horizontal interfaces, respectively, are involved.

Effect of initial linear chirp on collision characteristics of two solitons in the birefringent fibre

Zheng Hong-Jun, Liu Shan-Liang, Xu Jing-Ping
Chin. Phys., 2007, 16 (7): 2023-2027 doi: 10.1088/1009-1963/16/7/036
Full Text: [PDF 328 KB] (Downloads:464)
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The collision characteristics of the orthogonally polarized solitons with initial linear frequency chirp in the linear birefringent fibre for \beta 2<0 are numerically studied. It is found that initial chirp changes the threshold value of solitons to form the bound-state in the birefringent fibre. The effect of initial positive chirp on the threshold value is more obvious than that of negative chirp. In the case of \delta = 0.7 and initial interval 2\tau0 = 1.25, the two solitons are mutually bound for 0.2 \le C \le 1, and they do not form the bound-state for -1 \le C<0.2. Frequency shifts increase with the increase of chirp parameter C for -1 \le C<0.2, and have the oscillatory structure for C \ge 0.2. The effect of positive chirp on temporal FWHM is greater than that of negative chirp. The peak of temporal waveform oscillates with the propagation distance. The period and amplitude of the oscillation for the chirped case are greater than those for the unchirped case, and they vary with the increase of | C| . The peak of output temporal waveform can be controlled by changing the initial chirp.

Effect of multipolar interaction on the effective thermal conductivity of nanofluids

Zhou Xiao-Feng, Gao Lei
Chin. Phys., 2007, 16 (7): 2028-2032 doi: 10.1088/1009-1963/16/7/037
Full Text: [PDF 514 KB] (Downloads:538)
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Nanofluids or liquids with suspended nanoparticles are likely to be the future heat transfer media, as they exhibit higher thermal conductivity than those of liquids. It has been proposed that nanoparticles are apt to congregate and form clusters, and hence the interaction between nanoparticles becomes important. In this paper, by taking into account the interaction between nearest-neighbour inclusions, we adopt the multiple image method to investigate the effective thermal conductivity of nanofluids. Numerical results show that then the thermal conductivity ratio between the nanoparticles and fluids is large, and the two nanoparticles are close up and even touch, and the point-dipole theory such as Maxwell--Garnett theory becomes rough as many-body interactions are neglected. Our theoretical results on the effective thermal conductivity of CuO/water and Al$_{2}$O$_{3}$/water nanofluids are in good agreement with experimental data.

New expressions for the surface roughness length and displacement height in the atmospheric boundary layer

Lin Jian-Zhong, Li Hui-Jun, Zhang Kai
Chin. Phys., 2007, 16 (7): 2033-2039 doi: 10.1088/1009-1963/16/7/038
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An alternative model for the prediction of surface roughness length is developed. In the model a new factor is introduced to compensate for the effects of wake diffusion and interactions between the wake and roughness obstacles. The experiments are carried out by the use of the hot wire anemometry in the simulated atmospheric boundary layer in a wind tunnel. Based on the experimental data, a new expression for the zero-plane displacement height is proposed for the square arrays of roughness elements, which highlights the influence of free-stream speed on the roughness length. It appears that the displacement height increases with the wind speed while the surface roughness length decreases with Reynolds number increasing. It is shown that the calculation results based on the new expressions are in reasonable agreement with the experimental data.

Numerical exploration of coherent excitation in three-level ladder systems

Zhang Xian-Zhou, Li Xiao-Hong, Yang Xiang-Dong
Chin. Phys., 2007, 16 (7): 1947-1951 doi: 10.1088/1009-1963/16/7/023
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The technique of stimulated Raman adiabatic passage (STIRAP) is used to transfer potassium atoms from the 22p state to the 21p Rydberg state through the intermediate state 22s. The results show that complete population transfer is related to pulse duration and overlap, and occurs when the pulse duration and overlap have adequate values. At the same time, population trapping is also formed. Complete population transfer can also occurs when the two-photon resonance condition ({\it\Delta}s= {\it\Delta}p) is met.

Structure and stability of various states of the EuC and EuC2 molecules

Fan Xian-Hong, Wang Zhi-Gang, Yan Bing, Pan Shou-Fu, Chen Bo
Chin. Phys., 2007, 16 (7): 1952-1955 doi: 10.1088/1009-1963/16/7/024
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B3LYP level density functional theory (DFT) and multiconfiguration self-consistent-field (MCSCF) level \textit{ab initio} method calculations have been performed on the basis of relativistic effective core potentials to investigate the nature of EuC and EuC2 molecules. The computed results indicate that the ground states of EuC and EuC2 are 12\sum+ and 8A2 respectively. Dissociation potential energy curves of the low-lying electronic states of EuC have been calculated using the MCSCF method, and the same level calculation on EuC2 indicates that the dissociation energy of EuC2 of ground state compares well with the available experimental data. The bond characteristic is also discussed using Mulliken populations.

Electron impact excitation of tetrasulfur S2 molecule

Yan Bing, Pan Shou-Fu, Yu Jun-Hua
Chin. Phys., 2007, 16 (7): 1956-1958 doi: 10.1088/1009-1963/16/7/025
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Electron impact excitation of tetrasulfur S4 molecule is investigated in this work using R-matrix method. The twelve low-lying electronic states are used in close-coupling expansion to represent the target states, and the integral cross sections for elastic scattering and excitation into the six lowest electronic states are calculated.

Research on CeO2 cap layer for YBCO-coated conductor

Shi Dong-Qi, Ma Ping, Ko Rock-Kil, Kim Ho-Sup, Chung Jun-Ki, Song Kyu-Jeongc, Park Chan
Chin. Phys., 2007, 16 (7): 2142-2147 doi: 10.1088/1009-1963/16/7/058
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Two groups of coated conductor samples with different thicknesses of CeO2 cap layers deposited by pulsed laser deposition (PLD) under the same conditions have been studied. Of them, one group is of CeO2 films, which are deposited on stainless steel (SS) tapes coated by IBAD-YSZ (IBAD-YSZ/SS), and the other group is of CeO2/YSZ/Y2O3 multi-layers, which are deposited on NiW substrates by PLD for the fabrication of YBCO-coated conductor through the RABiTS approach. YBCO film is then deposited on the tops of both types of buffer layers by PLD. The effects of the thickness of the CeO$_{2}$ film on the texture of the CeO2 film and the critical current density (Jc) of the YBCO film are analysed. For the case of CeO2 film on IBAD-YSZ/SS, there appears a self-epitaxy effect with increasing thickness of the CeO2 film. For CeO2/YSZ/Y2O3/NiW, in which the buffer layers are deposited by PLD, there occurs no self-epitaxy effect, and the optimal thickness of CeO2 is about 50nm. The surface morphologies of the two groups of samples are examined by SEM.

Hyperon--hyperon interaction in relativistic mean field model

Li Ang, Zuo Wei, Mi Ai-Jun, Burgio G
Chin. Phys., 2007, 16 (7): 1934-1940 doi: 10.1088/1009-1963/16/7/021
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By using the new experimental data of \Lambda\Lambda potential, this paper has performed a full calculation for strange hadronic matter with different strangeness contents as well as its consequences on the global properties of neutron star matter in relativistic mean field model. It finds that the new weak hyperon--hyperon interaction makes the equations of state much stiffer than the result of the previous strong hyperon--hyperon interaction, and even stiffer than the result without consideration of hyperon--hyperon interaction. This new hyperon--hyperon interaction results in a maximum mass of 1.75M_{\odot} (where M_{\odot} stands for the mass of the Sun), about 0.2--0.5M_{\odot} larger than the previous prediction with the presence of hyperons. After examining carefully the onset densities of kaon condensation it finds that this new weak version of hyperon--hyperon interaction favours the occurrence of kaons in comparison with the strong one.

Investigation of transfer ionization in collisions of partially stripped carbon ions with helium at low to intermediate velocities

Chen Xi-Meng, Gao Zhi-Min, Liu Zhao-Yuan, Ding Bao-Wei, Lu Yan-Xia, Fu Hong-Bin, Shao Jian-Xiong, Cui Ying, Zhang Hong-Qiang, Liu Yu-Wen, Du Juan, Sun Guang-Zhi
Chin. Phys., 2007, 16 (7): 2040-2043 doi: 10.1088/1009-1963/16/7/039
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The ratios of transfer ionization (TI) to single-electron capture (SC) cross sections have been measured for the collisions of partially stripped Cq+ ions (q=1--4) with He. The collision velocity ranges from 0.7 to 4.4v0 (v0 is the Bohr velocity). The projectile-ion and recoil-ion coincidence technique is used to separate the processes of TI and SC. The ratios reach the maximum when the velocity is about 3.7v0 This can be explained qualitatively based on the two-step mechanism. The experimental results are also compared with the results calculated using the classical trajectory Monte Carlo (CTMC) method. The CTMC results are in agreement with the experimental data basically. The discrepancies in higher velocity region are interpreted by the effective charge effect.

RF electric field penetration and power deposition into nonequilibrium planar-type inductively coupled plasmas

Mao Ming, Wang Shuai, Dai Zhong-Ling, Wang You-Nian
Chin. Phys., 2007, 16 (7): 2044-2050 doi: 10.1088/1009-1963/16/7/040
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The RF electric field penetration and the power deposition into planar-type inductively coupled plasmas in low-pressure discharges have been studied by means of a self-consistent model which consists of Maxwell equations combined with the kinetic equation of electrons. The Maxwell equations are solved based on the expansion of the Fourier--Bessel series for determining the RF electric field. Numerical results show the influence of a non-Maxwellian electron energy distribution on the RF electric field penetration and the power deposition for different coil currents. Moreover, the two-dimensional spatial profiles of RF electric field and power density are also shown for different numbers of RF coil turns.

The increasing of localized free volume in bulk metallic glass under uniaxial compression

Deng Yu-Fu, Yang Fei, Yang Jian-Lin, Zhang Wei
Chin. Phys., 2007, 16 (7): 2051-2055 doi: 10.1088/1009-1963/16/7/041
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Using transmission electron microscopy and electron diffraction, we have investigated the microstructure of a Zr55Al10Ni5Cu30 bulk metallic glass under a uniaxial compression until a failure occurs at room temperature. It is shown that the amorphous structure has changed locally in the vicinity of the failure plane after the plastic deformation. An increase in free volume is observed within the localized areas due to the concentration of plastic flow, suggesting that the increasing of local free volume dominates the deformation mechanism of the metallic glass. The effect of free volume on the `serrated flow' behaviour in the stress--strain curves during the uniaxial compression is discussed.

Density functional theory study on LaNi4.5 Al0.5 hydride phase: electronic properties and sites occupation

Chen Dong, Yu Ben-Hai, Wang Chun-Lei, Gao Tao
Chin. Phys., 2007, 16 (7): 2056-2061 doi: 10.1088/1009-1963/16/7/042
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In this paper the crystal structure, electronic structure and hydrogen site occupation of LaNi4.5Al0.5Hy hydride phase (y=5.0, 6.0) have been investigated by using full-potential linearized augmented plane wave method. The hydrogen atoms were found to prefer the 6m, 12o and 12n sites, while no 4h sites were occupied. A narrowed Ni-d band is found due to the lattice expansion, the total density of states at EF increases with y, which indicates that the compounds become less stable. The interaction between Al and Ni, H plays a dominant role in the stability of LaNi4.5Al0.5Hy hydride phase. The smaller the shift of EF towards the higher energy region, the more stable the compounds will be. The obtained results are compared with experimental data and discussed in the light of previous works.

The charge transfer via a double helix DNA

Yan Xun-Ling, Dong Rui-Xin
Chin. Phys., 2007, 16 (7): 2062-2068 doi: 10.1088/1009-1963/16/7/043
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This paper reports a double helix model of charge transfer in DNA molecule. The calculated results show that the transmission characteristics of DNA are not only related to the longitudinal transfer but also to the helicity of molecule. It finds that there are four transmission bands centred at 0.92, 3.07, 7.75 and 8.87eV, also the width and intensity of bands corresponding to the helix direction are less than that of the longitudinal direction. With the increase of hopping energy, transmission bands become wide so that the transmission ability is enhanced.

Quantum transport through two series Aharonov--Bohm interferometers with zero total magnetic flux

Wang Jian-Ming, Wang Rui, Zhang Yong-Ping, Liang Jiu-Qing
Chin. Phys., 2007, 16 (7): 2069-2074 doi: 10.1088/1009-1963/16/7/044
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With the help of nonequilibrium Green's function technique, the electronic transport through series Aharonov--Bohm (AB) interferometers is investigated. We obtain the AB interference pattern of the transition probability characterized by the algebraic sum \phi and the difference \theta of two magnetic fluxes, and particularly a general rule of AB oscillation period depending on the ratio of integer quantum numbers of the fluxes. A parity effect is observed, showing the asymmetric AB oscillations with respect to the even and odd quantum numbers of the total flux in antiparallel AB interferometers. It is also shown that the AB flux can shift the Fano resonance peaks of the transmission spectrum.

Spin-polarized quantum transport through an Aharonov--Bohm quantum-dot-ring

Wang Jian-Ming, Wang Rui, Liang Jiu-Qing
Chin. Phys., 2007, 16 (7): 2075-2081 doi: 10.1088/1009-1963/16/7/045
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In this paper the quantum transport through an Aharonov--Bohm (AB) quantum-dot-ring with two dot-array arms described by a single-band tight-binding Hamiltonian is investigated in the presence of additional magnetic fields applied to the dot-array arms to produce spin flip of electrons. A far richer interference pattern than that in the charge transport alone is found. Besides the usual AB oscillation the tunable spin polarization of the current by the magnetic flux is a new observation and is seen to be particularly useful in technical applications. The spectrum of transmission probability is modulated by the quantum dot numbers on the up-arc and down-arc of the ring, which, however, does not affect the period of the AB oscillation.

Properties of GaN on different polarity buffer layers by hydride vapour phase epitaxy

Qiu Kai, Zhong Fei, Li Xin-Hua, Yin Zhi-Jun, Ji Chang-Jian, Han Qi-Feng, Chen Jia-Rong, Cao Xian-Cun, Wang Yu-Qi
Chin. Phys., 2007, 16 (7): 2082-2086 doi: 10.1088/1009-1963/16/7/046
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This paper reports on N-, mixed-, and Ga-polarity buffer layers are grown by molecular beam epitaxy (MBE) on sapphire (0001) substrates, with the GaN thicker films grown on the buffer layer with different polarity by hydride vapour epitaxy technique (HVPE). The surface morphology, structural and optical properties of these HVPE-GaN epilayers are characterized by wet chemical etching, scanning electron microscope, x-ray diffraction, and photoluminescence spectrum respectively. It finds that the N-polarity film is unstable against the higher growth temperature and wet chemical etching, while that of GaN polarity one is stable. The results indicate that the crystalline quality of HVPE-GaN epilayers depends on the polarity of buffer layers.

A study on the electrical property of HgSe under high pressure

Hao Ai-Min, Gao Chun-Xiao, Li Ming, He Chun-Yuan, Huang Xiao-Wei, Zhang Dong-Mei, Yu Cui-Ling, Guan Rui, Zou Guang-Tian
Chin. Phys., 2007, 16 (7): 2087-2090 doi: 10.1088/1009-1963/16/7/047
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Using a microcircuit fabricated on a diamond anvil cell, we have measured in-situ conductivity of HgSe under high pressures, and investigated the temperature dependence of conductivity under several different pressures. The result shows that HgSe has a pressure-induced transition sequence from a semimetal to a semiconductor to a metal, similar to that in HgTe. Several discontinuous changes in conductivity are observed at around 1.5, 17, 29 and 49GPa, corresponding to the phase transitions from zinc-blende to cinnabar to rocksalt to orthorhombic to an unknown structure, respectively. In comparison with HgTe, it is speculated that the unknown structure may be a distorted CsCl structure. For the cinnabar-HgSe, the energy gap as a function of pressure is obtained according to the temperature dependence of conductivity. The plot of the temperature dependence of conductivity indicates that the unknown structure of HgSe has an electrical property of a conductor.

Two-step dissociation of a polaron in conjugated polymers

Liu Xiao-Jing, Gao Kun, Li Yuan, Wei Jian-Hua, Xie Shi-Jie
Chin. Phys., 2007, 16 (7): 2091-2095 doi: 10.1088/1009-1963/16/7/048
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We have studied the electric-field-driven motion of a polaron by solving the time-dependent Schr\"{o}dinger equation nonadiabatically and the lattice equation of motion simultaneously. It is found that the polaron may experience two sequent transitions under high fields; one is the transition from the subsonic to the supersonic state, and the other from the supersonic to dissociated state. The acoustic mode is decoupled from the charge when the polaron moves at a speed faster than the sound speed, and then the optical mode is decoupled at the second transition to make the polaron dissociate completely.

Electronic transport properties of the single-impurity Anderson model

Chen Ming-Lun, Wang Shun-Jin
Chin. Phys., 2007, 16 (7): 2096-2100 doi: 10.1088/1009-1963/16/7/049
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We investigate the electronic transport properties of the single-impurity Anderson model. By employing the cluster expansions, the equations of motion of Green's functions are transformed into the corresponding equation of motion of connected Green's functions, which contains the correlation of two conduction electrons beyond the Lacroix approximation. With the method we show that the asymmetric line shape of zero bias conductance manifests itself as the Fano effect, and the Kondo effect is observed in the narrow peak of differential conductance curve of the system. The Fano and the Kondo effects can coexist in the single-impurity Anderson model when the impurity level is adjusted to an appropriate position.

The transport properties in an Aharonov--Bohm interferometer with a quantum dot

Chen Ming-Lun, Wang Shun-Jin
Chin. Phys., 2007, 16 (7): 2101-2105 doi: 10.1088/1009-1963/16/7/050
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This paper investigates the electronic transport properties in an Aharonov--Bohm interferometer with a quantum dot coupling to left and right electrodes. By employing cluster expansions, it transforms the equations of motion of Green's functions into the corresponding equation of motion of connected Green's functions, which provides a natural and uniform truncation scheme. With this method under the Lacroix's truncation approximation, it shows that the asymmetric line shape of zero bias conductance manifests itself as the Fano effect, and the Kondo effect has been observed in the narrow peak of differential conductance curve of the system. Our numerical results also show that the building of Fano state suppresses the amplitude of Kondo resonance.

Photon-assisted electronic structure and transport for a quantum dot

Liao Wen-Hu, Gao Qin-Xiang, Zhou Guang-Hui
Chin. Phys., 2007, 16 (7): 2106-2110 doi: 10.1088/1009-1963/16/7/051
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This paper investigates theoretically the electronic structure and transport of a two-level quantum dot irradiated under a strong laser field at low temperatures. Using the method of Keldysh equation of motion for nonequilibrium Green functions, it examines the time-averaged density of states and conductance for the system with photon polarization parallel with and perpendicular to the tunnelling current direction respectively. It is demonstrated that, by analysing some numerical examples, more photon sidebands resonance states and multi- and single-photon transitions are found when diagonal matrix elements dominate the interaction, while the electronic transitions due to multiphoton absorption are more or less suppressed when off-diagonal interaction dominates.

Actions of negative bias temperature instability (NBTI) and hot carriers in ultra-deep submicron p-channel metal--oxide--semiconductor field-effect transistors (PMOSFETs)

Liu Hong-Xia, Hao Yue
Chin. Phys., 2007, 16 (7): 2111-2115 doi: 10.1088/1009-1963/16/7/052
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Hot carrier injection (HCI) at high temperatures and different values of gate bias Vg has been performed in order to study the actions of negative bias temperature instability (NBTI) and hot carriers. Hot-carrier-stress-induced damage at Vg=Vd, where Vd is the voltage of the transistor drain, increases as temperature rises, contrary to conventional hot carrier behaviour, which is identified as being related to the NBTI. A comparison between the actions of NBTI and hot carriers at low and high gate voltages shows that the damage behaviours are quite different: the low gate voltage stress results in an increase in transconductance, while the NBTI-dominated high gate voltage and high temperature stress causes a decrease in transconductance. It is concluded that this can be a major source of hot carrier damage at elevated temperatures and high gate voltage stressing of p-channel metal--oxide--semiconductor field-effect transistors (PMOSFETs). We demonstrate a novel mode of NBTI-enhanced hot carrier degradation in PMOSFETs. A novel method to decouple the actions of NBTI from that of hot carriers is also presented.

A novel implantable multichannel silicon-based microelectrode

Sui Xiao-Hong, Zhang Ruo-Xin, Pei Wei-Hua, Chen Hong-Da
Chin. Phys., 2007, 16 (7): 2116-2119 doi: 10.1088/1009-1963/16/7/053
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Silicon-based microelectrodes have been confirmed to be helpful in neural prostheses. The fabricated 7-channel silicon-based microelectrode was feasible to be implanted into the brain cortex. The manufacturing process by micro-electromechanical system (MEMS) technology was detailed with four photolithographic masks. The microscopic photographs and SEM images indicated that the probe shank was 3mm long, 100\mum wide and 20\mu m thick with the recording sites spaced 120\mu m apart for good signal isolation. To facilitate the insertion and minimize the trauma, the microelectrode is narrowed down gradually near the tip with the tip taper angle of 6 degrees. Curve of the single recording site impedance versus frequency was shown by test in vitro and the impedance declined from 150.5k\Omega to 6.0k\Omega with frequency changing from 10\,k to 10MHz.

Photoresponse of the In0.3 Ga0.7 N metal--insulator--semiconductor photodetectors

Zhou Jian-Jun, Wen Bo, Jiang Ruo-Lian, Liu Cheng-Xiang, Ji Xiao-Li, Xie Zi-Li, Chen Dun-Jun, Han Ping, Zhang Rong, Zheng You-Dou
Chin. Phys., 2007, 16 (7): 2120-2122 doi: 10.1088/1009-1963/16/7/054
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In0.3 Ga0.7N metal--insulator--semiconductor (MIS) and metal--semiconductor (MS) surface barrier photodetectors have been fabricated. The In0.3 Ga0.7N epilayers were grown on sapphire by metalorganic chemical vapour deposition (MOCVD). The photoresponse and reverse current--voltage characteristics of the In0.3 Ga0.7N MIS and MS photodetectors were measured. A best zero bias responsivity of 0.18 A/W at 450~nm is obtained for the In0.3 Ga0.7N MIS photodetector with 10~nm Si3N4 insulator layer, which is more than ten times higher than the In0.3 Ga0.7N MS photodetector. The reason is attributed to the decrease of the interface states and increase of surface barrier height by the inserted insulator. The influence of the thickness of the Si3N4 insulator layer on the photoresponsivity of the MIS photodetector is also discussed.

Theoretical investigation of exchange bias

Xiong Zhi-Jie, Wang Huai-Yu, Ding Ze-Jun
Chin. Phys., 2007, 16 (7): 2123-2130 doi: 10.1088/1009-1963/16/7/055
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The exchange bias of bilayer magnetic films consisting of ferromagnetic (FM) and antiferromagnetic (AFM) layers in an uncompensated case is studied by use of the many-body Green's function method of quantum statistical theory. The effects of the layer thickness and temperature and the interfacial coupling strength on the exchange bias HE are investigated. The dependence of the exchange bias HE on the FM layer thickness and temperature is qualitatively in agreement with experimental results. When temperature varies, both the coercivity HC and HE decrease with the temperature increasing. For each FM thickness, there exists a least AFM thickness in which the exchange bias occurs, which is called pinning thickness.

Oxygen-vacancy-related dielectric relaxation and conduction mechanisms in Bi5 TiNbWO15 ceramics

Wang Xiao-Juan, Gong Zhi-Qiang, Qian Ya-Feng, Zhu Jun, Chen Xiao-Bing
Chin. Phys., 2007, 16 (7): 2131-2135 doi: 10.1088/1009-1963/16/7/056
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This paper reports that the intergrowth ceramics Bi5TiNbWO15 (BW-BTN) have been prepared with the conventional solid-state reaction method. The dielectric and conductivity properties of samples were studied by using the dielectric relaxation and AC impedance spectroscopy in detail. Two distinct relaxation mechanisms were detected both in the plots of dielectric loss (tan\delta) and the imaginary part ({Z}'') versus frequency in the frequency range of 10 Hz--13MHz. We attribute the higher frequency relaxation process to the hopping process of the oxygen vacancies inside the grains, while the other seems to be associated with the space charges bound at the grain boundary layers. The AC impedance spectroscopy indicates that the conductivities at 625K for bulk and grain boundary are about 1.12\times10-3S/m and 1.43\times10-3S/m respectively. The accumulation of the space charges in the grain boundary layers induces a space charge potential of 0.52eV.

Solvent and electric field dependence of the photocurrent generation in donor:acceptor blend system

Zhou Yin-Hua, Yang Zhong-Feng, Wu Wei-Cai, Xia Hai-Jian, Wen Shan-Peng, Tian Wen-Jing
Chin. Phys., 2007, 16 (7): 2136-2141 doi: 10.1088/1009-1963/16/7/057
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This paper reports that the blend films of poly (2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) and N,N'-bis(1-ethylpropyl)-3,4: 9,10-perylene bis (tetracarboxyl diimide) (EP-PDI) with the weight ratio of 1:2.5 have been prepared by spin-coating from chloroform (CF) and chlorobenzene (CB) solutions respectively. The absorption spectra and the morphology of the blend films show that large crystal-like EP-PDI aggregates are formed in film prepared from CB solution, which corresponds to a new absorption shoulder near 590nm, while there is no shoulder around 590\,nm in the UV--Vis absorption spectra of the blend film from CF solution. The electric-field dependence spectra of the photocurrent generation quantum yield of the film from CB solution shows that at weak electric field the EP-PDI aggregates act as more efficient sensitizers, but at strong electric field the quantum yields become almost invariable over the entire spectral range no matter what the state of EP-PDI, monomer or aggregate. At strong electric field, the photocurrent generation yields of both films from CF and CB solution saturate and their yield spectra become spectrally similar, mentioning that at strong electric field the photoexcitons dissociate efficiently and the free charges are collected by the electrodes almost completely.
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