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Generalized Positive-Definite Operator in Quantum Phase Space Obtained by Virtue of the Weyl Quantization Rule
HU Li-Yun, FAN Hong-Yi
Chin. Phys. Lett. 2009, 26 (6):
060307
.
DOI: 10.1088/0256-307X/26/6/060307
We introduce a generalized positive-definite operator Δg(q,p) by smoothing out the Wigner operator Δw(q,p) and by averaging over the "coarse graining'' function. The function is then regarded as the classical Weyl correspondence of the operator Δg(q,p); in this way we can easily identify a quantum state |Φ> such that Δg(q,p)=|Φ><Φ|, and |Φ> turns out to be a new kind of squeezed coherent state. Correspondingly, the generalized distribution function for any state |φ> is <φ| Δg(q,p) |φ> =|<Φ|φ>|2 , which is obviously positive-definite and is a generalization of the Husimi function.
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Dynamical System Approach to a Coupled Dispersionless System: Localized and Periodic Traveling Waves
Gambo Betchewe, Kuetche Kamgang Victor, Bouetou Bouetou Thomas, , Timoleon Crepin Kofane,
Chin. Phys. Lett. 2009, 26 (6):
060503
.
DOI: 10.1088/0256-307X/26/6/060503
We investigate the dynamical behavior of a coupled dispersionless system describing a current-conducting string with infinite length within a magnetic field. Thus, following a dynamical system approach, we unwrap typical miscellaneous traveling waves including localized and periodic ones. Studying the relative stabilities of such structures through their energy densities, we find that under some boundary conditions, localized waves moving in positive directions are more stable than periodic waves which in contrast stand for the most stable traveling waves in another boundary condition situation.
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Behavior of a Logistic Map Driven by White Noise
YANG Zheng-Ling, GAO Yang, GAO Yong-Tao, ZHANG Jun
Chin. Phys. Lett. 2009, 26 (6):
060506
.
DOI: 10.1088/0256-307X/26/6/060506
In the real world, every nonlinear system is inevitably affected by noise. As an example, a logistic map driven by white noise is studied. Unlike previous studies which focused on the behavior under local parameters to find analytical results, we investigate the whole driven logistic map. For a white noise driven logistic map, its non-divergent interval decreases with increasing white noise. The white noise does not change the equilibrium point and two-cycle intervals in statistics, if the driven logistic map is kept non-divergent. In particular, chaos can be excited by white noise only after the four-cycle bifurcation begins. The latest result is a necessary condition which has not been given in the literature [Int. J. Bifur. Chaos 18(2008)509], and it can be deduced from Sharkovsky's theorem. Numerical simulations prove these analytical results.
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Stabilization and Shift of Frequency in an External Cavity Diode Laser with Solenoid-Assisted Saturated Absorption
HAN Shun-Li, CHENG Bing, ZHANG Jing-Fang, XU Yun-Fei, WANG Zhao-Ying, LIN Qiang
Chin. Phys. Lett. 2009, 26 (6):
063201
.
DOI: 10.1088/0256-307X/26/6/063201
A simple method to realize both stabilization and shift of the frequency in an external cavity diode laser (ECDL) is reported. Due to the Zeeman effect, the saturated absorption spectrum of Rb atoms in a magnetic field is shifted. This shift can be used to detune the frequency of the ECDL, which is locked to the saturated absorption spectrum. The frequency shift amount can be controlled by changing the magnetic field for a specific polarization state of the laser beam. The advantages of this tunable frequency lock include low laser power requirement, without additional power loss, cheapness, and so on.
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Theoretical Study on Inner Shell Electron Impact Excitation of Lithium
YANG Ning-Xuan, DONG Chen-Zhong, JIANG Jun
Chin. Phys. Lett. 2009, 26 (6):
063401
.
DOI: 10.1088/0256-307X/26/6/063401
Cross sections for electron impact excitation of lithium from the ground state 1s22s to the excited states 1s2s2, 1s2p2, 1s2snp (n=2-5), 1s2sns (n=3-5), 1s2pns (n=3-5), and 1s2pnp (n=3-5) are calculated by using a full relativistic distorted wave method. The latest experimental electron energy loss spectra for inner-shell electron excitations of lithium at a given incident electron energy of 2500eV [Chin. Phys. Lett. 25(2008)3649] have been reproduced by the present theoretical investigation excellently. At the same time, the structures of electron energy loss spectra of lithium at low incident electron energy are also predicted theoretically, it is found that the electron energy loss spectra in the energy region of 55-57eV show two-peak structures.
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Guiding of Highly Charged Ions through PC Nanocapillaries
LI De-Hui, WANG Yu-Yu, ZHAO Yong-Tao, XIAO Guo-Qing, ZHAO Di, XU Zhong-Feng, LI Fu-Li
Chin. Phys. Lett. 2009, 26 (6):
063402
.
DOI: 10.1088/0256-307X/26/6/063402
Angular distribution and current dependence of the transmitted ion fraction are investigated for 40keV Xe7+ bombarding on polycarbonate (PC) nanocapillaries. By measuring the angular distribution of the transmitted ion fraction, a strong guiding effect is found in PC nanocapillaries. Furthermore, with increase of the incident current, a turning point of the transmitted ion fraction is found, which is explained qualitatively by the discharge capacity of the nanocapillaries.
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Pulse Compression Based on Laser-Induced Optical Breakdown in Suspension
HASI Wu-Li-Ji, FU Mei-Ling, LU Huan-Huan, GONG Sheng, LU Zhi-Wei, LIN Dian-Yang, HE Wei-Ming
Chin. Phys. Lett. 2009, 26 (6):
064202
.
DOI: 10.1088/0256-307X/26/6/064202
Pulse compression based on laser-induced optical breakdown in suspension is investigated. The physical mechanism behind it is analyzed theoretically and validated in the Q-switched Nd:YAG laser system. A 12-ns pump pulse is suppressed to 5ns with good fidelity in the front edge and sharp steepness in the trailing edge. The HT-270, which has a small gain coefficient and absorption coefficient, is used as a solvent, and therefore the disturbance induced by stimulated Brillouin scattering and absorption are minimized and the transmittivity is enhanced.
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Coexisting Raman- and Rayleigh-Enhanced Four-Wave Mixing in Femtosecond Polarization Beats
NIE Zhi-Qiang, ZHAO Yan, ZHANG Yan-Peng, GAN Chen-Li, ZHENG Huai-Bin, LI Chang-Biao, LU Ke-Qing
Chin. Phys. Lett. 2009, 26 (6):
064205
.
DOI: 10.1088/0256-307X/26/6/064205
Based on the polarization interference of Raman- and Rayleigh-enhanced four-wave mixing processes, heterodyne detection of the Raman, Rayleigh and coexisting Raman and Rayleigh femtosecond difference-frequency polarization beats is investigated in the cw and the three Markovian stochastic models, respectively. These two processes exhibit asymmetric and symmetric spectra, respectively, and the thermal effect in them can be suppressed by a field-correlation method. Such studies of coexisting Raman- and Rayleigh-enhanced four-wave mixing processes can have important applications in coherence quantum control, and quantum information processing.
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Electrically Pumped Room-Temperature Pulsed InGaAsP-Si Hybrid Lasers Based on Metal Bonding
CHEN Ting, HONG Tao, PAN Jiao-Qing, CHEN Wei-Xi, CHENG Yuan-Bing, WANG Yang, MA Xiao-Bo, LIU Wei-Li, ZHAO Ling-Juan, RAN Guang-Zhao, WANG Wei, QIN Guo-Gang
Chin. Phys. Lett. 2009, 26 (6):
064211
.
DOI: 10.1088/0256-307X/26/6/064211
A pulsed InGaAsP-Si hybrid laser is fabricated using metal bonding. A novel structure in which the optical coupling and metal bonding areas are transversely separated is employed to integrate the silicon waveguide with an InGaAsP multi-quantum well distributed feedback structure. When electrically pumped at room temperature, the laser operates with a threshold current density of 2.9kA/cm2 and a slope efficiency of 0.02W/A. The 1542nm laser output exits mainly from the Si waveguide.
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High Power Er/Yb Codoped Double Clad Fiber Pulsed Amplifier Based on an All-Fiber Configuration
ZHOU Lei, NING Ji-Ping, CHEN Cheng, HAN Qun, ZHANG Wei-Yi, WANG Jun-Tao
Chin. Phys. Lett. 2009, 26 (6):
064215
.
DOI: 10.1088/0256-307X/26/6/064215
We report an all-fiber two-stage high power pulsed amplifier, seeded with a 1550nm, 1kHz repetition rate rectangular pulse, and based on Er/Yb co-doped double clad fiber. All the characteristics are measured in the experiment. The maximal slope efficiency is 22.56%, which is the highest we know of at such a low repetition rate, and the maximal output signal power is 1W. The various factors that affect the pulsed amplifier performance are analyzed. A high output power while keeping high power conversion efficiency can be obtained with careful selection of the input power, pump power and repetition rate. The experimental results show that the crucial parameters should be optimized when designing all-fiber pulsed amplifiers.
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Scalar Statistics along Inertial Particle Trajectory in Isotropic Turbulence
LIU Ya-Ming, LIU Zhao-Hui, HAN Hai-Feng, LI Jing, WANG Han-Feng, ZHENGChu-Guang
Chin. Phys. Lett. 2009, 26 (6):
064402
.
DOI: 10.1088/0256-307X/26/6/064402
The statistics of a passive scalar along inertial particle trajectory in homogeneous isotropic turbulence with a mean scalar gradient is investigated by using direct numerical simulation. We are interested in the influence of particle inertia on such statistics, which is crucial for further understanding and development of models in non-isothermal gas-particle flows. The results show that the scalar variance along particle trajectory decreases with the increasing particle inertia firstly; when the particle's Stokes number St is less than 1.0, it reaches the minimal value when St is around 1.0, then it increases if St increases further. However, the scalar dissipation rate along the particle trajectory shows completely contrasting behavior in comparison with the scalar variance. The mechanical-to-thermal time scale ratios averaged along particle, <r>p, are approximately two times smaller than that computed in the Eulerian frame r, and stay at nearly 1.77 with a weak dependence on particle inertia. In addition, the correlations between scalar dissipation and flow structure characteristics along particle trajectories, such as strain and vorticity, are also computed, and they reach their maximum and minimum, 0.31 and 0.25, respectively, when St is around 1.0.
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Antisite Defects of the L12 Structure Determined by the Phase Field Microelasticity Model
ZHANG Jing, CHEN Zheng, LU Yan-Li, WANG Yong-Xin, ZHAO Yan
Chin. Phys. Lett. 2009, 26 (6):
066101
.
DOI: 10.1088/0256-307X/26/6/066101
A phase field microelasticity simulation is performed to examine the antisite defect of L12-Ni3Al in Ni75Al5.3V19.7 ternary alloy. Combinimg strain energy with the phase field model leads to an atom configuration change as time proceeds. For the Ni sublattice, the antisite defect AlNi, the equilibrium occupancy probability (OP) of which declines, precedes NiNi and VNi in reaching equilibrium; subsequently, NiNi and VNi present a phenomenon of symmetrical rise and decline individually. Similarly, for the Al sublattice, the antisite defect NiAl, the OP of which eventually rises, takes fewer time steps than AlAl and VAl to attain equilibrium. Thereafter, AlAl rises while VAl declines symmetrically at the axes of the NiAl curve. Furthermore, the OP for the Al sublattice is much more sensitive to strain energy than that for the Ni sublattice.
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Numerical Simulation of Wave Propagation and Phase Transition of Tin under Shock-Wave Loading
SONG Hai-Feng, LIU Hai-Feng, ZHANG Guang-Cai, ZHAO Yan-Hong
Chin. Phys. Lett. 2009, 26 (6):
066401
.
DOI: 10.1088/0256-307X/26/6/066401
We undertake a numerical simulation of shock experiments on tin reported in the literature, by using a multiphase equation of state (MEOS) and a multiphase Steinberg Guinan (MSG) constitutive model for tin in the β, γ and liquid phases. In the MSG model, the Bauschinger effect is considered to better describe the unloading behavior. The phase diagram and Hugoniot of tin are calculated by MEOS, and they agree well with the experimental data. Combined with the MEOS and MSG models, hydrodynamic computer simulations are successful in reproducing the measured velocity profile of the shock wave experiment. Moreover, by analyzing the mass fraction contour as well as stress and temperature profiles of each phase for tin, we further discuss the complex behavior of tin under shock-wave loading.
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Modification of Band Gap of -SiC by N-Doping
LIU Hong-Sheng, FANG Xiao-Yong, SONG Wei-Li, HOU Zhi-Ling, LU Ran, YUAN Jie, CAO Mao-Sheng
Chin. Phys. Lett. 2009, 26 (6):
067101
.
DOI: 10.1088/0256-307X/26/6/067101
The geometrical and electronic structures of nitrogen-doped β-SiC are investigated by employing the first principles of plane wave ultra-soft pseudo-potential technology based on density functional theory. The structures of SiC1-xNx (x=0, 1/32, 1/16, 1/8, 1/4) with different doping concentrations are optimized. The results reveal that the band gap of β-SiC transforms from an indirect band gap to a direct band gap with band gap shrinkage after carbon atoms are replaced by nitrogen atoms. The Fermi level shifts from valence band top to conduction band by doping nitrogen in pure β-SiC, and the doped β-SiC becomes metallic. The degree of Fermi levels entering into the conduction band increases with the increment of doping concentration; however, the band gap becomes narrower. This is attributed to defects with negative electricity occurring in surrounding silicon atoms. With the increase of doping concentration, more residual electrons, more easily captured by the 3p orbit in the silicon atom, will be provided by nitrogen atoms to form more defects with negative electricity.
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Electrical Characteristics of Co/n-Si Schottky Barrier Diodes Using I-V and C-V Measurements
G. Güler, Ö, . Güllü, S. Karatas, Ö, . F. Bakkaloglu
Chin. Phys. Lett. 2009, 26 (6):
067301
.
DOI: 10.1088/0256-307X/26/6/067301
Electrical characteristics of Co/n-Si Schottky barrier diodes are analysed by current-voltage (I-V) and capacitance-voltage (C-V) techniques at room temperature. The electronic parameters such as ideality factor, barrier height and average series resistance are determined. The barrier height 0.76eV obtained from the C-V measurements is higher than that of the value 0.70eV obtained from the I-V measurements. The series resistance RS and the ideality factor n are determined from the d\ln(I)/dV plot and are found to be 193.62Ω and 1.34, respectively. The barrier height and the RS value are calculated from the H(I)-I plot and are found to be 0.71eV and 205.95Ω. Furthermore, the energy distribution of the interface state density is determined from the forward bias I-V characteristics by taking into account the bias dependence of the effective barrier height. The interface state density Nss ranges from 6.484×1011cm-2eV-1 in (EC-0.446)eV to 2.801×1010cm-2eV-1 in (EC-0.631,eV, of the Co/n-Si Schottky barrier diode. The results show the presence of a thin interfacial layer between the metal and the semiconductor.
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GaAs-Based Metamorphic Long-Wavelength InAs Quantum Dots Grown by Molecular Beam Epitaxy
WANG Peng-Fei, XIONG Yong-Hua, WANG Hai-Li, HUANG She-Song, NI Hai-Qiao, XU Ying-Qiang, HE Zhen-Hong, NIU Zhi-Chuan
Chin. Phys. Lett. 2009, 26 (6):
067801
.
DOI: 10.1088/0256-307X/26/6/067801
A bilayer stacked InAs/GaAs quantum dot structure grown by molecular beam epitaxy on an In0.05Ga0.95As metamorphic buffer is investigated. By introducing a InGaAs:Sb cover layer on the upper InAs quantum dots (QDs) layers, the emission wavelength of the QDs is extended successfully to 1.533μm at room temperature, and the density of the QDs is in the range of 4×109-8×109cm-2. Strong photoluminescence (PL) intensity with a full width at half maximum of 28.6meV of the PL spectrum shows good optical quality of the bilayer QDs. The growth of bilayer QDs on metamorphic buffers offers a useful way to extend the wavelengths of GaAs-based materials for potential applications in optoelectronic and quantum functional devices.
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Hydrothermal Synthesis and Vacuum Ultraviolet-Excited Luminescence Properties of Novel Dy3+-doped GdPO4 White Light Phosphors
HAN Guo-Cai, WANG Yu-Hua, WU Chun-Fang, ZHANG Jia-Chi, LU Yang-Hua
Chin. Phys. Lett. 2009, 26 (6):
067803
.
DOI: 10.1088/0256-307X/26/6/067803
Novel Dy3+-doped GdPO4 white light phosphors with a monoclinic system are successfully synthesized by the hydrothermal method at 240°C. The strong absorption at around 147nm in the excitation spectrum is assigned to the host absorption. It is suggested that the vacuum ultraviolet excited energy is transferred from the host to the Dy3+ ions. The f-d transition of the Dy3+ ion is observed to be located at 182nm, which is consistent with the calculated value using Dorenbos's expression. Under 147nm excitation, Gd0.92PO4:0.08Dy3+ phosphor exhibits two emission bands located at 572nm (yellow) and 478nm (blue), which correspond to the hypersensitive transitions 4F9/2-6H13/2 and 4F9/2-6H15/2. The two emission bands lead to the white light. Because of the strong absorption at about 147nm, Gd0.92PO4:0.08Dy3+ under vacuum ultraviolet excitation is an effective white light phosphor, and has promising applications to mercury-free lamps.
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Growth and Characteristics of Epitaxial AlxGa1-xN by MOCVD
ZHANG Jie, GUO Li-Wei, CHEN Yao, XU Pei-Qiang, DING Guo-Jian, PENGMing-Zeng, JIA Hai-Qiang, ZHOU Jun-Ming, CHEN Hong
Chin. Phys. Lett. 2009, 26 (6):
068101
.
DOI: 10.1088/0256-307X/26/6/068101
AlxGa1-xN epilayers with a wide Al composition range (0.2≤x≤ 0.68) were grown on AlN/sapphire templates by low-pressure metalorganic chemical vapour deposition (LP-MOCVD). X-ray diffraction results reveal that both the (0002) and (10-15) full widths at half-maximum (FWHM) of the AlxGa1-xN epilayer decrease with increasing Al composition due to the smaller lattice mismatch to the AlN template. However, the surface morphology becomes rougher with increasing Al composition due to the weak migration ability of Al atoms. Low temperature photoluminescence (PL) spectra show pronounced near band edge (NBE) emission and the NBE FWHM becomes broader with increasing Al composition mainly caused by alloy disorder. Meanwhile, possible causes of the low energy peaks in the PL spectra are discussed.
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Manipulation of Nanoparticles Using Dark-Field-Illumination Optical Tweezers with Compensating Spherical Aberration
ZHOU Jin-Hua, TAO Run-Zhe, HU Zhi-Bin, ZHONG Min-Cheng, WANG Zi-Qiang, CAI Jun, LI Yin-Mei,
Chin. Phys. Lett. 2009, 26 (6):
068701
.
DOI: 10.1088/0256-307X/26/6/068701
Based on our previous investigation of optical tweezers with dark field illumination [Chin. Phys. Lett. 25(2008)329], nanoparticles at large trap depth are better viewed in wide field and real time for a long time, but with poor forces. Here we present the mismatched tube length to compensate for spherical aberration of an oil-immersion objective in a glass-water interface in an optical tweezers system for manipulating nanoparticles. In this way, the critical power of stable trapping particles is measured at different trap depths. It is found that trap depth is enlarged for trapping nanoparticles and trapping forces are enhanced at large trap depth. According to the measurement, 70-nm particles are manipulated in three dimensions and observed clearly at large appropriate depth. This will expand applications of optical tweezers in a nanometre-scale colloidal system.
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An Efficient Control Strategy of Epidemic Spreading on Scale-Free Networks
HANG Hai-Feng, LI Ke-Zan, FU Xin-Chu, WANG Bing-Hong,
Chin. Phys. Lett. 2009, 26 (6):
068901
.
DOI: 10.1088/0256-307X/26/6/068901
We present a novel and effective method for controlling epidemic spreading on complex networks, especially on scale-free networks. The proposed strategy is performed by deleting edges according to their significances (the significance of an edge is defined as the product of the degrees of two nodes of this edge). In contrast to other methods, e.g., random immunization, proportional immunization, targeted immunization, acquaintance immunization and so on, which mainly focus on how to delete nodes to realize the control of epidemic spreading on complex networks, our method is more effective in realizing the control of epidemic spreading on complex networks, moreover, such a method can better retain the integrity of complex networks.
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Dynamical Stability and Attractor of the Variable Generalized Chaplygin Gas Model
FU Huan-Huan, WU Ya-Bo, CHENG Fang-Yuan
Chin. Phys. Lett. 2009, 26 (6):
069801
.
DOI: 10.1088/0256-307X/26/6/069801
For the variable generalized Chaplygin gas (VGCG) as a dynamical system, its stability is analyzed and the related dynamical attractors are investigated. By analysis it is shown that there are two critical points corresponding to the matter-dominated phase and the VGCG dark energy-dominated phase, respectively. Moreover, when the parameters n, α and γ take some fixed values, the phase with ωVGCG= -0.92 is a dynamical attractor and the equation of state of VGCG reaches it from either ωVGCG > -1 or ωVGCG< -1, independent of the initial values of the dynamical system. This shows a satisfactory cosmological model: the early matter-dominated era, followed by the dark energy-dominated era. Meanwhile, the evolutions of density parameters Ωγ and ΩVGCG are quite different from each other. For different initial values of x and y,Ωγ decreases and ΩVGCG increases as the time grows, they will eventually approach Ωγ=0 and ΩVGCG= 1. Furthermore, since different values of n or α may lead to different equation-of-state parameters ΩVGCG, we also discuss the constraints on the parameters n and α by the observation data.
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86 articles
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