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Chin. Phys. B  
  Chin. Phys. B--2005, Vol.14, No.8
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Poisson structure and Casimir functions for a noncentral dynamical system in four-dimensional phase space

Lou Zhi-Mei, Chen Zi-Dong, Wang Wen-Long
Chin. Phys. B 2005, 14 (8): 01483;  doi: 10.1088/1009-1963/14/8/001
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In this paper, we express the differential equations of a noncentral dynamical system in Ermakov formalism to obtain the Ermakov invariant. In term of Hamiltonian theories and using the Ermakov invariant as the Hamiltonian, the Poisson structure of a noncentral dynamical system in four-dimensional phase space are constructed. The result indicates that the Poisson structure is degenerate and the noncentral dynamical system possesses four invariants: the Hamiltonian, the Ermakov invariant and two Casimir functions.

Painlevé property, symmetries and symmetry reductions of the coupled Burgers system

Lian Zeng-Ju, Chen Li-Li, Lou Sen-Yue
Chin. Phys. B 2005, 14 (8): 01486;  doi: 10.1088/1009-1963/14/8/002
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The Painlev\'e property, inverse recursion operator,infinite number of symmetries and Lie symmetry reductions of the coupled Burgers equation are given explicitly. Three sets of infinitely many symmetries of the considered model are obtained by acting the recursion operator and the inverse recursion operator on the trivial symmetries such as the identity transformation, the space translation and the scaling transformation respectively. These symmetries constitute an infinite dimensional Lie algebra while its finite dimensional Lie point symmetry subalgebra is used to find possible symmetry reductions and then the group invariant solutions.

Finite symmetry transformation groups and exact solutions of Lax integrable systems

Ma Hong-Cai, Lou Sen-Yue
Chin. Phys. B 2005, 14 (8): 01495;  doi: 10.1088/1009-1963/14/8/003
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In this paper, the finite symmetry transformation groups and then symmetries of Lax integrable nonlinear physical systems, the Davey--Stewartson equation and the (2+1)-dimensional Camassa--Holm equation are investigated by means of a simple direct method.

Doubly coupled vibration band gaps in periodic thin-walled open cross-section beams

Yu Dian-Long, Liu Yao-Zong, Qiu Jing, Wang Gang, Wen Ji-Hong
Chin. Phys. B 2005, 14 (8): 01501;  doi: 10.1088/1009-1963/14/8/004
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This paper studies flexural-torsional doubly coupled vibration through periodic thin-walled open cross-section beams composed of two kinds of materials.Based on the doubly coupled vibration equation, plane wave expansion method for the thin-walled beams is provided. If the filling fraction keeps constant, the lattice is one of the factors that affect the normalized gap width. If the lattice and filling fraction keeps constant, the Young modulus contrast plays a fundamental role for the band gap width, but not density contrast. Finally, the frequency response of a finite periodic binary beam is simulated by finite element method, which provides an attenuation of about 40dB in the frequency range of the band gaps. The findings will be significant in the application of phononic crystals.

Effect of noise on trace distance of remote state preparation

Chen Ai-Xi, Li Jia-Hua
Chin. Phys. B 2005, 14 (8): 01507;  doi: 10.1088/1009-1963/14/8/005
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The influence of noise on the remote preparation of a qubit by an entangled pair is investigated. In Bloch sphere representation, we use the trace distance to describe how close the final state is to the original state to be prepared. Our studies include two cases. Firstly, we consider the sender and the receiver sharing a mixture of two Bell states as quantum channel in remote state preparation (RSP), and calculate the trace distance as a function of mixture ratio and angle of a state to be prepared. Secondly, the system--environment interaction is taken into account by including stochastic fluctuating terms in the system Hamiltonian. Solving the Bloch equations, we obtain the evolution density matrix of the system. We then apply this stochastic model to study the effect of noise on the trace distance of RSP.

Quantum teleportation and Kerr--Newman spacetime

Ge Xian-Hui, Shen You-Gen
Chin. Phys. B 2005, 14 (8): 01512;  doi: 10.1088/1009-1963/14/8/006
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We consider the teleportation in the background of Kerr--Newman spacetime. Because of the Hawking effect,the fidelity of the teleportation is reduced. The results also show the fidelity is closely related to the mass, charge and rotating velocity of the black hole: high fidelity can be reached for massive, slowly rotating Kerr--Newman black holes.

Chaotic dynamics and synchronization of fractional-order Genesio--Tesi systems

Lu Jun-Guo
Chin. Phys. B 2005, 14 (8): 01517;  doi: 10.1088/1009-1963/14/8/007
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In this paper, we investigate numerically the chaotic behaviours in the fractional-order Genesio--Tesi system. We find that chaos exists in the fractional-order Genesio--Tesi system with order less than 3. The lowest order we find to have chaos is 2.4 in this fractional-order Genesio--Tesi system. We propose a drive-response synchronization method for synchronizing the fractional-order chaotic Genesio--Tesi systems only using a scalar drive signal. This synchronization approach, based on stability theory of fractional-order systems, is simple and theoretically rigorous. It does not require the computation of the conditional Lyapunov exponents.Simulation results are used to visualize and illustrate the effectiveness of the proposed synchronization method.

Master--slave synchronization of coupled fractional-order chaotic oscillators

Gao Xin, Yu Jue-Bang
Chin. Phys. B 2005, 14 (8): 01522;  doi: 10.1088/1009-1963/14/8/008
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The dynamics of fractional-order systems have attracted increasing attentions in recent years. In this paper, we study the synchronization technique based on the master--slave synchronization scheme and apply it to the synchronization of two coupled nonlinear fractional-order electronic chaotic oscillators. Simulations show that two coupled fractional-order chaotic oscillators can be brought to an exact synchronization with appropriate coupling strength. It is interesting that the synchronization rate of the fractional-order chaotic oscillators is slower than its integer-order counterpart; however, with the increase of system order, the curves of synchronization error can be smoothened, which indicates that the master--slave synchronization of two coupled fractional order oscillators can be smoothened and stabilized.

Phase synchronization and anti-phase synchronization of chaos for degenerate optical parametric oscillator

Feng Xiu-Qin, Shen Ke
Chin. Phys. B 2005, 14 (8): 01526;  doi: 10.1088/1009-1963/14/8/009
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We have investigated chaotic synchronization in the generalized sense for the degenerate optical parametric oscillator (DOPO). The numerical results show that two unidirectional coupling DOPOs in chaos can be completely phase synchronization or anti-phase synchronization with a suitable coupling coefficient under which the maximum condition Lyapunov exponent (MCLE) is negative. Phase synchronization and anti-phase synchronization of chaos can be realized through positive and negative coupling. On the other hand, the different synchronization states depend on the coupling types used in the DOPO systems.

Laser-modulated ac measurement of high temperature thermopower

Fan Zhen-Jun, Zhang Dian-Lin
Chin. Phys. B 2005, 14 (8): 01533;  doi: 10.1088/1009-1963/14/8/010
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An experimental set-up for ac-measurement of thermopower between room temperature and 1200K by using modulated laser-heating has been developed. The method shows many advantages compared with the conventional dc-measurements: convenience, applicability to small samples, possibility to measure the anisotropy of single crystals, and capability to continuously trace the temperature dependence of thermopower. The thermopower of a very short piece of pure Pt was measured with the present set-up to test the reliability and accuracy of our new method.

Four-probe scanning tunnelling microscope with atomic resolution for electrical and electro-optical property measurements of nanosystems

Lin Xiao, He Xiao-Bo, Lu Jun-Ling, Gao Li, Huan Qing, Shi Dong-Xia, Gao Hong-Jun
Chin. Phys. B 2005, 14 (8): 01536;  doi: 10.1088/1009-1963/14/8/011
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We demonstrate a special four-probe scanning tunnelling microscope (STM) system in ultrahigh vacuum (UHV), which can provide coarse positioning for every probe independently with the help of scanning electron microscope (SEM) and fine positioning down to nanometre using the STM technology. The system allows conductivity measurement by means of a four-point probe method, which can draw out more accurate electron transport characteristics in nanostructures, and provides easy manipulation of low dimension materials. All measurements can be performed in variable temperature (from 30K to 500K), magnetic field (from 0 to 0.1T), and different gas environments. Simultaneously, the cathodoluminescence (CL) spectrum can be achieved through an optical subsystem. Test measurements using some nanowire samples show that this system is a powerful tool in exploring electron transport characteristics and spectra in nanoscale physics.

Nuclear halo effect on nucleon capture reaction rates at stellar energies

Liu Zu-Hua, Zhou Hong-Yu
Chin. Phys. B 2005, 14 (8): 01544;  doi: 10.1088/1009-1963/14/8/012
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The capture cross sections at stellar energies are very difficult to measure directly. Hence, data are usually evaluated by using indirect methods or extrapolations from the experimental data obtained at the lowest possible energies. The asymptotic normalization coefficient (ANC) approach of the transfer reactions provides a reliable way for the determination of the capture cross sections at stellar energies. By virtue of its reliability, we have calculated the capture cross sections of the 10Be(n,γ)11Be reaction by using nuclear ANC method. 11Be is a well-known neutron halo nucleus with two weakly bound states.As a typical example, we have shown that the radiative cross sections for a nucleon captured into a halo state are obviously enhanced. The enormous enhancement of the capture cross section is just due to the large overlap of the incident neutron wave with the extended tail of the halo. The 10Be(n,γ)11Be capture reaction is involved in the inhomogeneous big-bang nucleosynthesis. We have evaluated its reaction rates at stellar energies with the nuclear ANC method.

Several observables sensitive to the symmetry energy in heavy-ion collisions induced by high energy radioactive beams

Yong Gao-Chan, Li Bao-An, Zuo Wei
Chin. Phys. B 2005, 14 (8): 01549;  doi: 10.1088/1009-1963/14/8/013
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Using an isospin- and momentum-dependent hadronic transport model,effects of the symmetry energy on several observables in heavy-ion collisions induced by radioactive beams at intermediate energies are studied. It is found that rapidity distribution of the isospin asymmetry of nucleon emissions, transverse momentum distribution of the ratio of free neutrons to protons at mid-rapidity, kinetic energy distribution of the ratio of \pi^{-}/\pi^{+} as well as evolution of the isospin fractionation are all sensitive to the symmetry energy.

Relativistic energy, fine structure and hyperfine structure of the low-lying excited states for Be-like system

Zhang Meng, Gou Bing-Cong
Chin. Phys. B 2005, 14 (8): 01554;  doi: 10.1088/1009-1963/14/8/014
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Variational calculations are carried out with a multiconfiguration-interaction wavefunction on the 1s22p2p 1De and 1s22p3p 3Pe states to obtain the energies including the mass polarization and relativistic corrections for the beryllium isoelectronic sequence (Z=4--10). The oscillator strengths,transition rates and wavelengths are also calculated. Our results are compared with other theoretical and experimental data in the literatures. The fine structure and hyperfine structure of 1s22p3p3Pe state are also explored.

Transition energy and oscillator strength of Sc18+ ion

Wang Zhi-Wen, Yang Di, Hu Mu-Hong, Han Qiu-Ju, Li Jin-Ying
Chin. Phys. B 2005, 14 (8): 01559;  doi: 10.1088/1009-1963/14/8/015
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The transition energies and the dipole oscillator strengths for the 1s22s--1s2np (2≤n≤9) and 1s22p--1s2nd (3≤n ≤9) of lithium-like Sc18+ ion are calculated by using the full core plus correlation method. The fine structure splittings of 1s2np and 1s2nd (n ≤9) states are determined from the expectation values of spin-orbit and spin-other-orbit interaction operators. The quantum defects of these series, as function of principal quantum number n, are obtained. The agreement between the f-values obtained from three alternative formulae is excellent. Comparisons of our results with experimental data available in the literature are carried out.Combining the single-channel quantum defect theory with the discrete oscillator strengths obtained in this work, this paper obtains the behaviour of discrete oscillator strengths and oscillator strength densities corresponding to the bound-free transitions adjacent to ionization threshold.

Ab initio calculation on the analytic potential energy functions for the state a3Σ+u and the state b3Πu of spin-aligned trimer 3Li2

Shi De-Heng, Sun Jin-Feng, Yang Xiang-Dong, Zhu Zun-Lue, Liu Yu-Fang
Chin. Phys. B 2005, 14 (8): 01566;  doi: 10.1088/1009-1963/14/8/016
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The energies, equilibrium geometries and harmonic frequencies of the triplet excited states (a3Σ+uand b3Πu of spin-aligned trimer 7Li2 are firstly calculated by using a symmetry adapted cluster-configuration interaction method. The potential curves for the two excited states have least squares fitted by the Murrell--Sorbie function.The spectroscopic data (Be, αe, ωe , and ωeχe) and the force constants (f2, f3 and f4 are calculated. It is found that the spin-aligned triplet excited state b3Πu is more stable than the ground state X1Σ+g, and that the Murrell--Sorbie function form is suitable not only for the ground state but also for the spin-aligned triplet excited states. Comparison between the theoretical determinations of dissociation energies, equilibrium interatomic distances and harmonic frequencies with the experimental data about a3Σ+u and b3Πu clearly shows that the present work represents a significant improvement in agreement between theories and experiments.

Intrinsic optical bistability between left-handed material and nonlinear optical materials

Shi Hong-Yan, Jiang Yong-Yuan, Sun Xiu-Dong, Guo Ru-Hai, Zhao Yi-Ping
Chin. Phys. B 2005, 14 (8): 01571;  doi: 10.1088/1009-1963/14/8/017
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The electromagnetic properties of the interface between a left-handed material and a conventional nonlinear material were investigated theoretically and numerically. We found a new phenomenon---optical bistability of the interface. It was shown that the incident intensity, incident angle and permeability ratio between the left-handed and the nonlinear materials could dramatically affect the optical bistable behaviour. We also compared the bistable behaviours of different electromagnetic modes. The results indicated that the TE mode was prior to the TM mode to obtain optical bistability for the same parameter.

Femtosecond laser-induced transient grating in CeO2-doped 75TeO2-20Nb2O5-5ZnO glass

Zhang Shi-An, Sun Zhen-Rong, Yang Xi-Hua, Wang Zu-Geng, Lin Jian, Huang Wen-Hai
Chin. Phys. B 2005, 14 (8): 01578;  doi: 10.1088/1009-1963/14/8/018
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Multiple order self-diffractions by femtosecond laser-induced transient grating in CeO2 doped 75TeO2-25Nb2O5-5ZnO glass are investigated by pump-probe technique. The glass structure and component are investigated by Raman and visible--NIR absorption spectra. It is showed that there exists Ce3+ ion in tellurite glass, and the transient grating is created by the optical Kerr effects and improved by the excited particle grating from the resonant absorption of the 1S0to 1F3 electronic energy transition of Ce3+ ion, and the conversion efficiency of the first order diffraction signal reaches about 11%. The results indicate that CeO2 doped 75TeO2-25Nb2O5-5ZnO glass has potential applications in ultra-fast all-optical switching.

Self-assembling three-dimensional colloidal photonic crystal multilayers from aqueous ethanol mixture solutions

Wang Jing, Yuan Chun-Wei, Tang Fang-Qiong
Chin. Phys. B 2005, 14 (8): 01581;  doi: 10.1088/1009-1963/14/8/019
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Vertical deposition technique to fabricate thin film solid artificial opals is becoming widely used. In the present work, we report our research on solvent modification and its effect on the quality of colloidal crystals. We used aqueous ethanol mixture solution to replace the ethanol solution, and used the vertical deposition technique to pack the spherical colloids into close-packed arrays. High quality samples can be prepared with thickness up to 20μm in one step. Furthermore, large spheres (diameters greater than 500nm) were successfully crystallized. Scanning electron microscopy (SEM) and optical methods were used to measure sample thickness and uniformity. The number of layers was calculated from the spectral separation of the Fabry--Perot fringes.

The validity of high-pass angular spectrum filter in solid immersion lens system

Zheng Chong-Wei, Dai Yu-Xing
Chin. Phys. B 2005, 14 (8): 01585;  doi: 10.1088/1009-1963/14/8/020
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The focal field of a solid immersion lens (SIL) system with a high-pass angular spectrum filter is calculated by using a vector method. Numerical results show that for a radially (azimuthally) polarized input field, a high-pass angular spectrum filter can reduce the light-spot (dark-spot) size of the SIL system. For a linearly polarized input field, however, the focal field cannot be optimized and the optical storage density of the SIL system cannot be improved by using a simple high-pass filter, either.

Analysis of pulse broadening induced by the second-order PMD

Fu Song-Nian, Wu Chong-Qing, Shum Ping
Chin. Phys. B 2005, 14 (8): 01591;  doi: 10.1088/1009-1963/14/8/021
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We propose a new conception of depolarization vector to describe the effect of depolarization induced by the second-order polarization mode dispersion (PMD). Deriving the formula of pulse broadening induced by the second-order PMD, we find that the polarization-dependent chromatic dispersion (PCD) always enhances the pulse broadening. However the depolarization vector decreases the pulse broadening. The pulse broadening is correlated with the bit-rate of a transmission system. By adjusting the directions of the Stokes vector of initial state of polarization, initial first-order polarization dispersion vector and depolarization vector to be parallel to each other,one can obtain an optimum dispersion compensation. But when the PCD is not equal to zero, the optimum dispersion cannot achieve a complete compensation,and the minimum pulse broadening is equal to σ= (21/2/4)( DCF/T0).

Nonlinear propagation of focused ultrasound in layered biological tissues based on angular spectrum approach

Zhu Xiao-Feng, Zhou Lin, Zhang Dong, Gong Xiu-Fen
Chin. Phys. B 2005, 14 (8): 01594;  doi: 10.1088/1009-1963/14/8/022
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Nonlinear propagation of focused ultrasound in layered biological tissues is theoretically studied by using the angular spectrum approach (ASA), in which an acoustic wave is decomposed into its angular spectrum, and the distribution of nonlinear acoustic fields is calculated in arbitrary planes normal to the acoustic axis. Several biological tissues are used as specimens inserted into the focusing region illuminated by a focused piston source. The second harmonic components within or beyond the biological specimens are numerically calculated. Validity of the theoretical model is examined by measurements. This approach employing the fast Fourier transformation gives a clear visualization of the focused ultrasound, which is helpful for nonlinear ultrasonic imaging.

Particle confinement and density profile behaviour on HL-1M

Jiao Yi-Ming, Zhou Yan, Deng Zhong-Chao, Ding Xuan-Tong, Liu Yong, Wang En-Yao
Chin. Phys. B 2005, 14 (8): 01600;  doi: 10.1088/1009-1963/14/8/023
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In this paper the density profile behaviour and the particle confinement operation regime on HL-1M have been studied under the pellet injection (PI),supersonic molecular beam injection (SMBI), gas puffing (GP) and lower hybrid current drive experimental situations. The relationships between density profile, particle confinement time and edge safe factor have been explored. The density profile, which is measured by six-channel far-infrared ray laser interferometer has been analysed by using the peaking coefficient calculation code. Changes of the outward and inward diffusion velocities before and after the peaking of the central density profile have been calculated using the global particle balance equations. The particle confinement operation regimes have been discussed. The peaking density profile can be easily obtained under the condition of efficient fuelling. In ohmic discharges, confinement time increases as the peaking density profile factor rises, and is saturated at a critical value related to the fuelling efficiency. The particle confinement time of SMBI lies between the values of GP and PI, and its value is about 3--5 times of the energy confinement time.

Stabilizing effect of ion pressure gradient on magnetic curvature-driven drift modes located at rational surface of tokamak plasma

Wang Ai-Ke
Chin. Phys. B 2005, 14 (8): 01604;  doi: 10.1088/1009-1963/14/8/024
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In the fluid model, we derive a dispersion relation for the toroidal drift modes of tokamak plasmas, including the ion pressure gradient and the magnetic field gradient and curvature. It is shown that the magnetic field gradient and curvature (MFGC) can cause instabilities at the rational surface, which are of toroidicity-induced (TI) modes. On the other hand, it is discovered that the ion pressure gradient can stabilize the present MFGC instabilities. The critical threshold of ion pressure gradient, which makes the growth rate reduced to zero, is obtained both analytically and numerically.

Study of effect of H2 addition on the production of fluorocarbon radicals in H2C4F8 inductively coupled plasma via optical emission spectroscopy actinometry

Huang Song, Xin Yu, Ning Zhao-Yuan
Chin. Phys. B 2005, 14 (8): 01608;  doi: 10.1088/1009-1963/14/8/025
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C4F8 plasma with the addition of H2 is generated by the inductively coupled plasma (ICP) method. The relative densities of CF,CF2, H and F radicals are determined by actinometric optical emission spectroscopy (AOES) as a function of the gas flow rate ratio R=H2/(H2+C4F8 at a pressure of 0.8 Pa and an input r.f.power of 400W, while that of HF is measured by quadrupole mass spectrometry (QMS). The results show that plasma activity increases firstly and then decreases with increasing R. As the gas flow rate ratio R changes from 0 to 0.625, relative densities of both CF and CF2 decrease, and the relative [CF] has a similar tendency as the calculated [CF], indicating that CF radicals are generated mainly by the electron impact dissociation of CF2 radicals. Production of HF is also discussed.

Study of tungsten wire array Z-pinch implosion on Qiang-Guang I facility

Xu Rong-Kun, Li Zheng-Hong, Yang Jian-Lun, Xu Ze-Ping, Ding Ning, Guo Cun, Jiang Shi-Lun, Ning Jia-Min, Xia Guang-Xin, Li Lin-Bo, Song Feng-Jun, Chen Jin-Chuan
Chin. Phys. B 2005, 14 (8): 01613;  doi: 10.1088/1009-1963/14/8/026
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The main results of investigation on the tungsten wire array Z-pinch implosion experiment performed on Qiang-Guang I facility in 2003 are reported in this paper. A set of diagnostic equipments including an x-ray power meter (XRPM), a 1D spatial-temporal x-ray meter and a pinhole camera was used to study implosion process and x-ray radiation characteristics of the tungsten wire array. In the experiment, the maximum x-ray yield of 36.6kJ was obtained for an optimizing load with a diameter of 8mm and a length of 20mm, which consists of 32 5-μm-diameter tungsten wires. The experimental results show that the region of x-ray emission decreased at a rate of 6.4×106cm/s by analysing the data of the 1D spatial-temporal x-ray meter. It also shows that the peak time of x-ray radiation was prior to the time when plasmas were compressed into a near-axis region.

Spatial uniformity of dielectric barrier discharge at moderate pd values

Dong Li-Fang, Mao Zhi-Guo, Ran Jun-Xia
Chin. Phys. B 2005, 14 (8): 01618;  doi: 10.1088/1009-1963/14/8/027
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The plasma with high spatial uniformity at moderate pd values is achieved by using a dielectric barrier discharge reactor with two liquid electrodes. The electrical and optical characteristics of the discharge are investigated by spatiotemporally resolved measurement. Results show that the plasma with high spatial uniformity can be maintained in a wide range of applied voltages and corresponding input powers as well. The influence of wall charges on the discharge power and the spatial uniformity is discussed.

Electronic states of C20 isomers

Wang Zhi-Gang, Zhang Cun-Hua, Fan Xian-Hong, Pan Shou-Fu, Yan Bing, Jin Ming-Xing
Chin. Phys. B 2005, 14 (8): 01622;  doi: 10.1088/1009-1963/14/8/028
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Using quantum chemistry ab initio method, geometry optimization of the singlet and triplet electronic states are performed for three isomers (ring,bowl and cage structures) of C20. From the optimized geometries, MP2 calculation gives a result which does not agree with Hund's rule, i.e. the singlet state has a lower energy for all the three isomers. Further more,total charge densities and electrostatic potentials of three isomers are given by the UHF wavefunctions, the results indicate negative charges located in the central area of cage and bowl structures, and the bond character of ring structure is analysed.

Surface morphology evolution of Si(110) by ion sputtering as a function of sample temperature

Qi Le-Jun, Ling Li, Li Wei-Qing, Yang Xin-Ju, Gu Chang-Xin, Lu Ming
Chin. Phys. B 2005, 14 (8): 01626;  doi: 10.1088/1009-1963/14/8/029
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Si(110) surface morphology evolution under normal-incident Ar+ ion sputtering has been studied as a function of Si temperature with the ion energy of 1.5keV and the ion flux 20μA/cm2. During temperature rising from room temperature to 800℃, Si(110) surface morphology changes from a dim dot/hole pattern to a distinct dot one, meanwhile the surface roughness increases steadily. The usually-accepted Bradley--Harper model fails to explain these data. By taking into account the Ehrlich--Schwoebel effect in the nanostructuring process, a simulation work was conducted based on a continuum dynamic model, which reproduces the experimental results.

Band structures, chemical bonding, and frequency-dependent optical properties of nonlinear optical crystals HgGa2S4 and Hg0.5Cd0.5Ga2S4

Huang Shu-Ping, Wu Dong-Sheng, Li Xiao-Dong, Lan You-Zhao, Zhang Hao, Gong Ya-Jing, Li Fei-Fei, Shen Juan, Cheng Wen-Dan
Chin. Phys. B 2005, 14 (8): 01631;  doi: 10.1088/1009-1963/14/8/030
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Band structures, density of states, chemical bonding properties, and frequency-dependent linear and nonlinear optical properties have been investigated in terms of the density functional theory and an anharmonic oscillator model for HgGa2S4 (HGS) and Hg0.5Cd0.5Ga2S4 (HCGS) crystals. The results obtained show that the top valence bands (VBs) are contributions from Ga--S bonding states and the bottom conduction bands (CBs) are mostly due to Ga--S antibonding states for the HGS crystal, and the top VBs mostly originates from the S-3p states and the bottom CBs are mainly composed of the Ga--S antibonding states for the crystal HCGS. The population analysis shows that both crystals have mixed ionic-covalent bonds. The interactions between Ga and S atoms mostly appear as covalent character, and the Hg--S and Cd--S bonds have substantially ionic characters in HGS and HCGS crystals, respectively. The calculated refractive indices of HGS are compared with the experimental ones, and are found to be in agreement with the experiment data in the low-energy region. It is also found that the band gap of HGS is smaller than that of HCGS, and that the second-order susceptibilities of HGS are larger than those of HCGS.

I-V-T studies on Ni-silicide/n-Si(100) contacts formed by Ti-Ni-Si solid state reaction

Zhu Shi-Yang, Ru Guo-Ping, Zhou Jia, Huang Yi-Ping
Chin. Phys. B 2005, 14 (8): 01639;  doi: 10.1088/1009-1963/14/8/031
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The current--voltage (I--V) characteristics of Ni silicide/n-Si(100) contacts,which were formed from solid-state reaction of Ni--Si with a thin Ti capping layer at different annealing temperatures, were measured at temperatures ranging from 80K to room temperature. The low temperature I--V curves exhibit an excess current at the low bias region which is significantly larger than that predicted by the traditional thermionic emission (TE) model. A double-Schottky barrier height (SBH) model simplified from Tung's pinch-off model is used to analyse the measured I--V curves, from which the extent of the SBH inhomogeneity can be extracted. Higher annealing temperature results in larger SBH inhomogeneity, implying the degradation of the silicide film uniformity. The thin Ti capping layer increases slightly both the NiSi phase transfer temperature and the thermal stability of the formed NiSi film.

Hot-carrier degradation characteristics and explanation in 0.25μm PMOSFETs

Liu Hong-Xia, Hao Yue, Hawkins I. D., Peaker A. R.
Chin. Phys. B 2005, 14 (8): 01644;  doi: 10.1088/1009-1963/14/8/032
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The hot-carrier effect (HCE) of deep-submicron PMOSFETs has been investigated. It is found that the HCE includes both generation of interface states and formation of positive fixed charges in the gate oxide. We present experimental evidences showing that two degradation mechanisms are important in the case of deep-submicron PMOSFETs. Firstly, the generation of positive fixed oxide charges is significant in the case of deep-submicron PMOSFETs, which degrades the threshold voltage and even limits the transistor lifetime. For advanced analogy and mixed signal applications, process and device reliability limits need to be set up based also on threshold voltage shift, in addition to traditional methods of the transconductance degradation or gate oxide lifetime. Secondly, the generation of interface states by holes influences the device characteristics. Some speculation on the HCE formation process is included.

Dependence of coercivity on phase distribution and grain size in nanocomposite Nd2Fe14B/ɑ-Fe magnets

Feng Wei-Cun, Gao Ru-Wei, Li Wei, Han Guang-Bing, Sun Yan
Chin. Phys. B 2005, 14 (8): 01649;  doi: 10.1088/1009-1963/14/8/033
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The dependence of coercivity on the grain size in nanocomposite Nd2Fe14B/ɑ-Fe magnets with different distributions of magnetically soft and hard phases is investigated by means of statistical mean. The calculations show that when there exists no soft phase, the coercivity of magnets decreases monotonically with hard grain size reducing. For a given volume fraction of hard phase, the coercivity of nanocomposite Nd2Fe14B/ɑ-Fe magnets with a random distribution of soft and hard grains shows a peak value as a function of hard grain size. When the hard grain size is larger than an optimum value of soft grain size (15nm), the nanocomposite Nd2Fe14B/ɑ-Fe magnets with the multilayer structure of soft and hard grains can possess a higher coercivity than that with the random distribution of soft and hard grains.

Enhancement of “intrinsic” magnetoresistance ratio and activation energy of La0.67Ca0.33MnO3 single crystals by Fe doping

Kou Zhi-Qi, Di Nai-Li, Ma Xiao, Li Qing-An, Cheng Zhao-Hua
Chin. Phys. B 2005, 14 (8): 01653;  doi: 10.1088/1009-1963/14/8/034
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In order to avoid the complicated influence of grain boundaries on the resistivity in the investigation of the ``intrinsic'' magnetoresistance (MR) effect, the magnetic and magnetotransport properties of La0.67Ca0.33MnO3 and La0.67Ca0.33Mn0.96Fe0.04O3 single crystals were investigated. Owing to the absence of grain boundaries in the single crystals, MR ratio under 40×105A/m is about two orders of magnitude larger than that in polycrystalline counterparts. A further enhancement of MR ratio from 3400% to 17600% was achieved by a few percent of Fe doping. On the basis of the small polaron model, the activation energy derived from fitting the resistivity above Tc was found to increase upon introducing Fe. A connection between the strong localization of electrons arising from magnetic polarons and the enhancement of MR was observed in the single crystals.

Effective dielectric response of graded composite materials containing anisotropic particles

Sang Zhi-Fang, Li Zhen-Ya
Chin. Phys. B 2005, 14 (8): 01657;  doi: 10.1088/1009-1963/14/8/035
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The effective dielectric response of granular composites, in which spheroidal particles with graded shells are randomly distributed in a host matrix, is investigated. General expressions for the effective dielectric constant of the composites and partial resonant condition are obtained in the dilute limit by use of a quasi-static approximation. In particular, spheroidal particles with a power-law gradation profile in the shells are studied in detail. We find that, by adjusting the dielectric gradient profile in the shells, the shape and structure of particles, it is possible to enhance the effective dielectric constant of the composite and to realize partial resonance. Under the partial resonant conditions, the coated spheroidal particles with graded shells within the host matrix can be regarded as equivalent homogeneous spheroids embedded in the same host. The equivalent spheroids have the same dielectric constant as the original cores and semiaxes equal to those of the original shells: i.e., the partial resonant system behaves as if the cores of the particles were enlarged and the shells were absent.

Influence of external field and particle size upon theoretical photoelectron emission spectral response of silver nano-particles embedded in BaO thin film

Yang Hai, Yang Li-Dong, Ma Yong, Yang Zhi
Chin. Phys. B 2005, 14 (8): 01665;  doi: 10.1088/1009-1963/14/8/036
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The dependence of the spectral distribution of photoelectron emission from silver nano-particles embedded in BaO semiconductor thin film on external voltage and the size of silver nano-particles is predicted and investigated theoretically. The photoelectron emission response curves to light of wavelength between 0.2 and 0.8μm are given. The dependence of the wavelength threshold on the size of silver nano-particles and external field strength is also shown. Photoemission in the visible spectrum is explained as due to the optical resonance absorption in the silver particles. This could be of importance in designing field-assisted photocathodes and finding their optimum operation conditions.

Interaction of femtosecond laser pulses with metal photocathode

Liu Yun-Quan, Zhang Jie, Liang Wen-Xi
Chin. Phys. B 2005, 14 (8): 01671;  doi: 10.1088/1009-1963/14/8/037
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The features of interaction of femtosecond laser pulses with photocathode are studied theoretically in this paper. The surface temperature of the metal cathode film while femtosecond laser pulses irradiation is studied with two-temperature model. With a simple photoelectric model we obtain the optimum metal film thickness for the back-illuminated photocathode. The generated ultrashort photocurrent pulses are strongly dependent on the temperature of the electron gas and the lattice during the femtosecond laser pulse irradiation on the photocathode.

Possible mechanism of quasi-periodic oscillation of x-ray and TeV emissions for Mkn 501

Yuan Yu-Hai, Yang Jiang-He
Chin. Phys. B 2005, 14 (8): 01683;  doi: 10.1088/1009-1963/14/8/039
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We propose that the emissions of an advection-dominated accretion flows (ADAF) disc are the seed photons up-scattered to x-ray and TeV γ-rays in Mkn 501 and the instability of an ADAF disc may explain the 23-day quasi-periodic oscillation period observed in the x-rays and TeV lightcurves of Mkn 501.In this model, the ADAF emissions of optical and x-rays go into the jet and are up-scattered to high energies by relativistic electrons. In this process, the instability occurring in ADAF results in the quasi-periodic variation in the seed photons and therefore causes the variation in high energy x- and γ-rays.
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