Vol. 62, No. 3 (2013)
2013, 62 (3): 030303. doi: 10.7498/aps.62.030303
With the help of the newly proposed definition of entanglement, the entanglement dynamics of a system composed of two bosonic atoms trapped in a symmetric double-well potential is investigated. For any state of this system, the description of Majorana representation can be analytically derived. It is found that the entanglement dynamics depends on both the atomic interaction and the initial state of the system. Specifically, the atomic interaction determines the oscillation frequency of the entanglement degree, whereas the initial state controls the oscillation amplitude.
2013, 62 (3): 037202. doi: 10.7498/aps.62.037202
For an ultra-thin Bi2Se3 film sandwiched between two ferromagnetic insulators (FIs), we investigate how its topological properties change with the angle between the magnetizations of the two FIs. The Chern numbers are calculated from the low-energy effective Hamiltonian for electrons in the surface states, and the bulk energy band and the edge states are simulated from the tight-binding model of a long ribbon with armchair edges on a two-dimensional honeycomb lattice, from which the topological phase of the system can be determined. It is found that with the magnetizations of the FIs varying from parallel to antiparallel, there appears a topological phase transition from the anomalous quantum Hall phase to the trivial insulating one at a critical angle.
2013, 62 (3): 037501. doi: 10.7498/aps.62.037501
The crystal structures and magnetic properties of Mn2CoMxGa1-x and Mn2CoMxAl1-x (M=Cr, Fe, Co) alloys are investigated through experiment and calculation. Due to the covalent effect, the doped Fe and Co atoms preferentially occupy the A sites. It causes that some MnA (-2.1 B) atoms become MnD (3.2 B) and a local ferromagnetic structure of MnB-CoC-MnD is generated in the ferrimagnetic matrix, showing that an increment of molecular moment is as high as 6.18 B. The achievement of the ferromagnetic structure consumes the exchange interaction energy, consequently, reducing the TC in Fe doping alloys. It is found that the toleration for doping Co in Mn2CoAl reaches up to x=0.64, much more than that in Mn2CoGa (x=0.36), and the change from ordered B2 to A2 structures along with the decrease of Al content. These observations reveal the importance of the covalent effect in these intermetallic compounds. The Cr doping shows an abnormal increment of molecular moment of 3.65 B and increases the TC rapidly, which implies that Cr atoms may take an atomic configuration thereby disobeying to the occupation rule.
2013, 62 (3): 030201. doi: 10.7498/aps.62.030201
Using the T-matrix method, detailed numerical computation and comparative analysis on the distributions of light scattering matrix of fire smoke particles and non-fire smoke particles are carried out. The results show that when the wavelength of incident light changes, the degrees of variation on elements of fire smoke particles are different from those of non-fire smoke particles. By using dual-wavelength pulsed laser as incident ligh, through detecting appropriate element of light scattering matrix and combining XOR logical operation, the fire smoke particles can be distinguished from non-fire smoke particles, thereby reducing the misinformation caused by non-fire smoke particles.
2013, 62 (3): 030202. doi: 10.7498/aps.62.030202
A new method to detect carbon isotope ratio of carbon dioxide is presented, which is developed based on Fourier transform infrared spectroscopy (FTIR)technique. The method to obtain the absorption coefficient from HITRAN is introduced in detail. The nonlinear least square algorithm to determine δ13CO2 value is studied. The total system and experimental procedure are shown in the paper. The temperature and pressure effect on δ13CO2 value determination are analyzed from both theory and experiment analysis. For the same CO2 standard gas, the δ13CO2 value is detected by both FTIR and IRMS technology, and the results are also comparatively analyzed and discussed. The difference in mean between both methods is only 0.25%. The experimental results show that the FTIR technique to detect carbon isotope ratio of carbon dioxide is viable.
We present the spatially self-bending solutions (the partial Bessel functions) of the Maxwell equations, where the self-bending wave can be produced through launching a group of plane waves that interfere with each other by modulating their initial phases and the emission directions. The self-bending electromagnetic waves accelerating in a circular trajectory preserve the beam shape within a certain propagation distance. Unlike Ariy wave packet, the bending angle of the new self-accelerating wave can be much larger than that of Ariy beam. The Poynting vector of the half Bessel beam shows that the main lobe maintains a propagation-invariant structure and can complete a turn close to 180°. In addition, one pair of half Bessel electromagnetic waves is forbidden to propagate inside a certain area where the designed beam generates the self-shielding region.
Entanglement properties in the system of atoms interacting with three coupled cavities which are in weak coherent states
2013, 62 (3): 030302. doi: 10.7498/aps.62.030302
We study the entanglement dynamics of the system comprising three two-level atoms resonantly interacting with three coupled cavities which are in weak coherent states initially. The atom-atom entanglement and cavity-cavity entanglement are investigated. The influences of coupling constant between cavities and intensity of the cavity field on entanglement properties are discussed. The results obtained using the numerical method show that atom-atom entanglement and cavity-cavity entanglement are both strengthened with the increase of intensity of the cavity field; on the other hand, atom-atom entanglement is weakened and the entanglement between cavity A and cavity B is strengthened with the increase of the coupling constant between cavities, and the entanglement between cavity B and cavity C has a nonlinear relation with the increase of the cavity-cavity coupling coefficient. These results are different from those in the case that cavity field is in vacuum state initially.
Nonsingular terminal sliding-mode observer design for interior permanant magnet synchronous motor drive at very low-speed
2013, 62 (3): 030501. doi: 10.7498/aps.62.030501
A nonsingular high-order terminal sliding-mode observer of interior permanent magnet synchronous motor (IPMSM) is presented, of which the state variable is the current of rotor-oriented d-q reference frame, to obtain the accurate information about angular speed and rotor position for high-performance vector control system. A nonsingular terminal sliding-mode observer is used to improve dynamic response of the observer speed and robustness of the current observer, and the high-order sliding mode method is adopted to estimate the chattering phenomenon of the conventional sliding-mode. Meanwhile, to obtain the regulator parameters of speed and current loop, an integral-feedback method of estimating the rotor speed is adopted, and the inner current loop is realized using a decoupling and diagonal internal model control algorithm. Experimental results of 2 MW PMSM drive system show that the proposed method can accurately estimate the position and speed of the rotor, and the system has good dynamic and static performances.
Resonant behavior of a fractional linear oscillator with fluctuating frequency and periodically modulated noise
2013, 62 (3): 030502. doi: 10.7498/aps.62.030502
The resonant behavior of a fractional linear oscillator subjected to both parametric excitation of colored noise and external excitation of periodically modulated noise is considered. Using Laplace transformation technique and Shapiro-Loginov formula, exact expressions of the first moment for the system response and its long-time amplitude are presented. The influence of the system parameters on the long-time behavior of the system response is discussed, such as fractional order, friction coefficient, driving frequency, noise intensity and relevant rate. It is found that the long-time amplitude of the fractional oscillator behaves non-monotonical, that is, there exist stochastic resonances in a wide sense. Moreover, generalized stochastic resonance with two peaks can be found subject to some appropriate parameters.
2013, 62 (3): 030503. doi: 10.7498/aps.62.030503
In this paper, we studiy the Feynman's ratchet and pawl heat engine in a one-dimensional lattice. The dynamics of the particle is described by a master equation. The expressions of the current, efficiency and power output of the heat engine are derived analytically. The influences of the height of barrier, external load force and the temperature ratio of the heat reservoirs on the efficiency of heat engine are discussed. When the steady-state current is zero, there is a nonzero heat flux transterred from the hot bath to the cold bath, which is similar to the heat leak of the irreversible Carnot model. The curve of the power output versus the efficiency is a loop-shaped one. The heat engine is irreversible and the efficiency is less than the Carnot efficiency. The heat engine can work in optimal operation by optimizing the performance parameters of the heat engine.
Feedback compensation control on chaotic system with uncertainty based on radial basis function neural network
2013, 62 (3): 030504. doi: 10.7498/aps.62.030504
For the problem of controlling uncertain chaotic systems, a method of feedback compensation control based on the radial basis function neural network (RBFNN) is studied. In the proposed method, dynamic properties of chaotic system is first trained by RBFNN, and then feedback compensation control for chaotic system is implemented using trained good RBFNN model. The characteristics of this method is that this method can quickly track any given reference signal with on requirement for any mathematic model of controlled chaos system. The numerical simulation results show that the proposed control method not only has the fast response speed, high control accuracy, but also has a stronger ability to suppress parameter perturbation and to resist interference to chaos system.
2013, 62 (3): 030505. doi: 10.7498/aps.62.030505
Parameter estimation for chaotic system is, in fact, a multi-dimensional optimization problem. By combining biogeography-based optimization (BBO) with harmony search (HS) and opposition-based learning (OBL), a hybrid BBO scheme is proposed for solving the chaotic parameter estimation problem. The HS is used to enhance the local search ability of BBO, and OBL is employed to increase the diversity of the initial population, thereby improving the optimizing performance. The effectiveness and robustness of the proposed scheme are verified by numerical simulations on two typical chaotic systems.
2013, 62 (3): 030701. doi: 10.7498/aps.62.030701
The novel static polarization wind imaging interferometer adopts four-face pyramid prism and polarization array to obtain four different phase interferograms on four sections of CCD once. It can detect the target in real time and overcome the moving mirror scan detection mode defect that leads to be unable to accurately detect the rapily changing target. For the signal beam that is split into four equal parts, their intensities drop significantly, so whether the signal can be detected is a key problem. In this paper, the target spectral characteristic of the airglow is taken as the starting point of analysis, then the optical transmission properties and response of the NSPWII system and CCD signal-to-noise ratio are analyzed. Finally the conclusion is obtained that weak signal (such as night airglow) cannot be detected in the conventional detecting mode. Some improving measure is presented which extends light integration time, adopts pixel merger technology and select high sensitivity CCD (as electron multiplication CCD), they all can improve the signal-to-noise ratio effectively. After integrated using the these measures, the signal-to-noise ratio and responsiveness of NSPWII system are calculated. In view of the rapid changes of cost and objectives, only pixel binning is adopted, and the simulated curve of signal-to-noise ratio versus digital output is obtained. The result shows that the weak signal as night airglow can be detected.
2013, 62 (3): 030702. doi: 10.7498/aps.62.030702
The true surface temperature for an object can be determined by measuring emitted radiation. Primary spectrum pyrometry should be generally carried out by radiation measurement within an infinitesimal solid angle or within a finite solid angle in the case of diffuse emission, so that the radiation thermometry equations become a closed system for temperature and other undetermined parameters. The radiation thermometry equation with linear emissivity model is obtained, which can be used to measure the temperature of the object with non-diffuse emission within a finite solid angle. This equation is universal in radiation measurement, based on which equations for primary spectrum pyrometry are deduced. These equations are also universal in radiation measurement, in which the emissivity scales under different measurement conditions are limited to the same range, but have different physical meanings.
Dielectric property of binary phase composite and its interface investigated by electric force microscope
2013, 62 (3): 030703. doi: 10.7498/aps.62.030703
Dielectric property of two-phase stack-up sample is studied by electric force microscopy (EFM). Highly oriented pyrolytic graphite (HOPG)/polyethylene(PE) and mica/PE are fabricated. The phenomenon that phase shift (Δθ) of conducting probe varys with dielectric constant of material is discovered near the interface between the two materials by using phase detection EFM. The characteristic curves of tan(Δθ) versus tip voltage VEFM are of parabolic type. Quadratic coefficient increases with dielectric constant ε increasing. An approach to the qualitative analysis of the dielectric property near the interface between different material at the micro/nanometer scale, is provided in this paper.
2013, 62 (3): 030704. doi: 10.7498/aps.62.030704
Greenhouse effect is one of the main reasons for climate change. Carbon monoxide (CO) is a critical air pollutant because of its short-term harmful health effects. Furthermore, since CO emission has a global warming potential, it exacerbats the climate change as an indirect greenhouse gas. Due to the fact that the number of ground-based stations is less and the chemical model simulation is greatly dependent on assumption, satellite observation becomes the effective way to acquire the information about CO distribution. Sciamachy is the first satellite instrument to allow retrieval of CO by measuring absorption in the near infrared region from reflected and scattered sunlight, which is highly sensitive to the lower layers of the troposphere where the sources, such as biomass burning, are located, and where the bulk of the gases is usually found. In this study the Sciamachy CO vertical column density is successfully inversed; after applying the instrument and cloud correction, yearly and seasonal results are presented. Finally by comparing them with the fire counts of ATSR dataset and the distributions of population density and coal mines, the CO emission sources are preliminary deduced.
2013, 62 (3): 032801. doi: 10.7498/aps.62.032801
To determine the thorium capture ratio and thorium-uranium conversion ratio in the thorium cladding of the fusion-fission reactor model, a new approach, activated thorium sample decay γ -ray off-line measurement, is introduced in this paper. This method is based on the measurement of the 311.98 keV characteristic decay γ-ray emitted by 233Pa which is produced through decay of 233Th, the capture product of 232Th. Then some reverse mathematic approach is brought in and finally the 232Th (n, γ ) 233Th reaction ratio is determined, with an uncertainty of 6% in the test experiment (233Th/232Th under the order of magnitude 10-17). In the paper, we introduce the detail of the approach including the background, principle and the result of a simple test experiment which shows that this approach can work well in thorium capture ratio determination. Compared with prompt γ-ray method and mass spectrometer method, this decay γ-ray off-line method has many advantages in determining the thorium capture ratio and thorium-uranium conversion ratio in the thorium cladding. It is also promising in the measurement of other relevant parameters.
ATOMIC AND MOLECULAR PHYSICS
The differential cross sections and mechanisms of ionization and in the collisions of He2++H(1s) with strong magnetic fields
2013, 62 (3): 033401. doi: 10.7498/aps.62.033401
The ionization processes in collisions of H (1s) atoms with He2+ are studied by the classical trajectory Monte Carlo method. The first-order differential cross sections for ionization are calculated for different field strengths of both transverse and parallel magnetic field, and they increase obviously with the fields applied. Instead of monotonic decline, a peak appears for the differential cross section, with the fields applied. According to the analysis of trajectories, the ionization mechanisms become quite different. In this paper, the influences the strength and direction of the applied field including the incident energy on the ionization mechanisms are also explained.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
Implementation and analysis of the perfectly matched layer with auxiliary differential equation for the multiresolution time-domain method
2013, 62 (3): 034101. doi: 10.7498/aps.62.034101
A new implementation of perfectly matched layer (PML) with auxiliary differential equation (ADE-PML) is presented for the multiresolution time-domain method. The implementation is easier to obtain and can save more memories than the popularly used PML proposed by Berenger and the anisotropic perfectly matched layer (APML) when a more generalized medium is treated. Numerical results demonstrate that the ADE-PML is more superior to the PML and APML in absorbing propagation modes and low-frequency evanescent modes.
Finite difference time domain analysis on electromagnetic scattering characteristic of plasma thin layer based on surface impedance boundary condition method
2013, 62 (3): 034102. doi: 10.7498/aps.62.034102
Using the surface impedance boundary condition (SIBC)-finite difference time domain (FDTD) method, the electromagnetic scattering characteristic of non-magnetized plasma coating on metal material is obtained, under the one-dimensional (1D) oblique incident wave condition. The SIBC method can greatly reduce computational memory by ignoring the mesh division of the background material. Firstly, the expression of frequency domain surface impedance is derived, and substituted into boundary condition equation. Then the equation is transformed to time domain via Fourier inverse transformation method, and the formula is quantized to obtain the update equation by piecewise linear recursive convolution (PLRC) method. The algorithm is used to calculate the reflection coefficients of parallel and vertical polarization waves at oblique incident angels. The comparison of the SIBC-FDTD results with analytic solutions shows the validation and effectiveness of proposed method. Finally, the effect of incident angle on reflection coefficient is analyzed by this method.
2013, 62 (3): 034201. doi: 10.7498/aps.62.034201
For spatial-resolved diffuse reflectance, the sensitivity expression to the reduced scattering parameter μs' is derived and numerically analyzed in the two-point-source delta-P1 approximation of transport theory, and the influences of the scattering parameter μs' on the delta-P1 approximation reflectance, hybrid diffuse approximation reflectance, and diffusion approximation reflectance are also compared. The results show that the optical properties of biological tissue can be better described by the delta-P1 approxiamate model in relatively high scattering and absorbing medium for a short souce-detector separation; the optimum probe spacing ρopt is obtained and approximately equal to 2.7-4 transport mean free path for the reduced albedo a'>0.83, at which a variable reduced scattering coefficient has the smallest effect on the measurement of optical absorption in the turbid medium and ρopt reduces with the scattering parameter μs' increasing. The study is of great significance for optimizing the probe geometry and measuring the optical parameters of biological tissues.
Distributed-feedback laser based on low-scattering and high-gain holographic polymer dispersed liquid crystal grating
2013, 62 (3): 034202. doi: 10.7498/aps.62.034202
In this paper, we utilize the acrylate monomer of low functionality to fabricate a holographic polymer dispersed liquid crystal grating with polymer-scaffolding morphology. It is effective to reduce the scattering loss in the grating which acts as a resonator in distributed-feedback laser since there is no droplet of liquid crystal. Moreover, the use of the monomer with high refractive index (RI) ensures the formation of grating with high RI modulation and, correspondingly, high feedback gain. For these two factors mentioned above, an excellent laser property is obtained from the dye-doped laser pumped by Nd:YAG pulse laser at 532 nm. A spectral linewidth of 0.3 nm is observed at pump energy threshold 0.8 μJ/pulse and a conversion efficiency of 1.2% is achieved at operating wavelength 635 nm. The laser performance is improved in some aspects such as threshold energy, linewidth and conversion efficiency to some extent compared with those reports proeviously.
A nnovel wide field fluorescence sectioning microscope based on polarization filtering image enhancement and dynamic speckle illumination
2013, 62 (3): 034203. doi: 10.7498/aps.62.034203
We propose a novel wide-field fluorescence sectioning microscope based on polarization filtering image enhancement and dynamic speckle illumination in this article. A new type of polarization filtering image enhancement technology, based on the differences in polarization characteristic between excitation light and fluorescence, is used to filter out the excitation light and enhance image quality. The dynamic speckle illumination is employed to achieve the wide field fluorescence sectioning which has many advantages, such as simple configuration, low cost, rapid response and easy operation. The experimental results indicate that our filtering method can be utilized to improve the image quality significantly, and the dynamic using speckle illumination can be employed to achieve optical sectioning with a wide field and a high vertical resolution capability. Our study is not only to enrich the technological method to extract the weak fluorescence signal from the strong excitation light, but also to provide a significant reference for developing a light wavelength tunable multi-spectral fluorescence microscope with rapid response.
2013, 62 (3): 034204. doi: 10.7498/aps.62.034204
The formation conditions and the energy variation of surface waves at the interface between a metal and a photorefractive crystal are investigated by using the particle oscillator model and numerical techniques. Our analysis indicates that the positive and negative values of the propagation constant affect the type of surface waves and the energy distribution: formation surface waves are nonlocal surface waves for negative values of the propagation constant and are oscillating surface waves and localized surface waves for positive values of the propagation constant, and the energy of localized surface waves in photorefractive medium increases monotonically with the propagation constant value increasing. In a given physical system, one can control the different orders and the waveforms of localized surface waves by regulating the variable parameter determining the strength of nonlinear effects.
2013, 62 (3): 034205. doi: 10.7498/aps.62.034205
Breather for weak beam induced by strong beam in a strongly nonlocal nonlinear medium is investigated. Although the nonlinearity induced by the weak beam is weak enough to be negligible, the nonlinearity induced by the strong beam can balance the diffraction of the weak beam, thus the breather for weak beam comes into being. The analytical solution of the breather for weak beam is obtained. Based on the analytical solution, the evolution of the beam width is analysed, and the form as well as the direction of helix for the trajectory of the breather is systematically investigated.
2013, 62 (3): 034206. doi: 10.7498/aps.62.034206
We propose a broadband artificial magnetic conductor (AMC) composite structure for reducing radar cross section (RCS). By combining multiple periodic metal square patches together, we further broaden the working bandwidth of the AMC composite structure. The tested results indicate that the reflection is below -10 dB in a frequency range of 7.7—13.1 GHz, and the relative bandwidth is 51.9%. We study the scattering characteristics of the AMC composite structure at different frequencies by full wave simulation. Reflection peaks in the opposite quadrant bisector of plane shift close to the normal direction with the increase of frequency, but the backscattering RCS of the AMC composite structure is still about -8 dB smaller than that of the metal plate. The AMC composite structure has the advantages of broad working bandwidth, simple design, easy processing, etc, and has important application foreground.
Location-free optical microscopic imaging method with high-resolution based on microsphere superlenses
2013, 62 (3): 034207. doi: 10.7498/aps.62.034207
In this paper, we propose a novel location-free high-resolution optical microscopic imaging method based on micropipette-microsphere combined probe. According to optical principle, a physical model is established to determine the value of magnification. With SiO2 microspheres of different diameters such as 3.0, 4.4, 5.6, 7.5 and 10.0 μm, microscopic imaging experiments are carried out. The results show that the microstructure of un-etched DVD disk can be significantly magnified and the image contrast is obviously improved, which is consistent well with the theoretical calculation. Microscopic imaging of microstructures based on SiO2 microsphere array is realized by controlling the microspheres with a micropipette. Furthermore, a kind of micropipette-microsphere combined probe is prepared, by which location-free high-resolution optical microscopic imaging has implemented for the first time.
2013, 62 (3): 034208. doi: 10.7498/aps.62.034208
The Fourier mode coupling theory is applied to the analysis of the fiber Bragg grating based all-fiber acousto-optic modulator for the first time. Compared with the existing analysis methods, the algorithm of this model is simple and easy, and transmission characteristics of the modulator can be acquired effectively and efficiently. Based on the theory, the performances of the modulator, related to ultrasonic frequency and amplitude of acoustically induced strain, are investigated. Simulation results show that in reflection spectra of the modulator, the wavelength interval between the primary relection peak and the secondary relection peak is proportional to ultrasonic frequency, and the reflectivity of the reflection peak varies periodically with intensity change of the amplitude of acoustically induced strain. In addition, with the same amplitude of acoustically induced strain, more secondary reflections exist in the low-frequency ultrasonic modulated fiber Bragg grating, and the periodic variation of the energy reflected by fiber Bragg graing is more obvious. In the experiment, the fiber Bragg graing is modulated by an ultrasonic wave with a frequency of 885.5 KHz. The experimental results accord well with the simulation results.
2013, 62 (3): 034209. doi: 10.7498/aps.62.034209
Thus far, focusing a single beam and combining multi-optical Gaussian beams have become two major methods of locally enhancing output laser intensity. In this paper, we propose a method of generating free space auto-focusing beams by combining multiple one-dimensional Airy beams. By employing split-step beam propagation method, auto-focusing beams generated by four and eight one-dimensional Airy beams are numerically simulated, and their intensity distributions are displayed from both transverse plane and side-view propagation. By utilizing a spatial light modulator and computer generated phase masks, these auto-focusing beams are experimentally demonstrated. Our experimental results accord well with the numerical simulations.
2013, 62 (3): 034301. doi: 10.7498/aps.62.034301
The electro-mechanical equivalent circuit is the most common method of analyzing and designing composite transducers. However, for the thin disk, because of the complexity of flexural vibration, equivalent lumped parameters are difficult to obtain, and so this method is rarely used. From the point of equivalent view of the distributed parameter system and lumped parameter system, according to the kinetic energy equal principle and the potential energy equal principle in this paper, we give the lumped parameter equivalent mass and equivalent elasticity coefficient of the flexural vibration, and the resonance frequency equations as well. The results from the analytical method are in good agreement with those from the finite element method. Finally, the equivalent circuit of lumped parameter model of the thin disk in analyzing the composite vibration system is given. These results can serve as a reference for designing flexural vibration composite transducers.
Effects of Ge profile on thermal characteristics of SiGe heterojunction bipolar transistor with non-uniform doping profile in base region
2013, 62 (3): 034401. doi: 10.7498/aps.62.034401
As is well known, base region is the design focus of bipolar junction transistor (BJT). Generally, the doping distribution in base is non-uniform. In this paper, the effects of different Gaussian dopings in the base on bulk temperature distribution, temperature dependences of current gain and cut-off frequency are first studied. It is found that current gain and cut-off frequency of BJT have positive temperature coefficients, and the temperature in bulk is high. Then, the effect of Ge composition distribution on these device parameters is investigated. It is found that the SiGe heterojunction bipolar transistors (HBTs) with box Ge composition distribution and trapezoidal Ge composition distribution have negative temperature coefficients of current gain and cut-off frequency, and have good bulk temperature distributions. Furthermore, the SiGe HBT with trapezoidal Ge profile has higher current gain and cut-off frequency, and its temperature insensitivity is kept. The good trade-off is made among the magnitudes of current gain and cut-off frequency, temperature sensitivity and bulk temperature distribution.
2013, 62 (3): 034501. doi: 10.7498/aps.62.034501
In this paper, we show the numerical integration method of solving Lagrange-Maxwell equation by using the symplectic Runge-Kutta (R-K) method, and numerically study the motion of the plate in an RLC circuit spring coupled system and the current changes. Its result is consistent with that obtained by the traditional R-K method, which demonstrates symplectic integration algorithm is reasonable and effective in studying the electro-mechanical systems. And on this basis, the form invariance of Noether sense is studied by using the symplectic Runge-Kutta method.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
Neutral beam injection induced discrete Alfvn instabilities on experimental advanced superconducting tokamak
2013, 62 (3): 035201. doi: 10.7498/aps.62.035201
The neutral beam injection (NBI) system is applied to the experimental advanced superconducting tokamak (EAST). It will excite some Alfvn instabilities when the plasma characteristics are improved. The numerical research on the NBI-induced discrete Alfvn eigenmode (TAE) and toroidal effect-induced Alfvn eigenmode (TAE) in the pedestal region is presented in the paper. The research results show that plenty of TAEs appear in this region. The TAE is very different from the TAE. These modes are trapped by the -induced potential wells along the magnetic field line. Due to negligible continuum damping via wave energy tunneling, similar to TAE, the TAE can also be readily destabilized by energetic particles. Differently, TAE frequency spectrum is more broad than TAE, and they are existed not only inside the gap but also outside the gap. The growth rate increases with injected power increasing. This instability maybe affects the physical behavior of the tokamak and the confinement of the plasma.
Experimental research on X-ray radiation and ablation of an Ag foil targets irradiated by high intensity 2ω0 laser light beam
2013, 62 (3): 035202. doi: 10.7498/aps.62.035202
The characteristics of radiation and ablation are investigated for an Ag foil irradiated with 2 ns, ～5×1014 W/cm2, 526.5 nm laser at SGII laser facility. The flight trajectory and velocity of the Ag foil are measured by X-ray streak camera. The experimental results show that they are in good agreement with simulations of one-dimensional radiation hydrodynamics code multi-1d using a flux-limited f=0.01. A rocket model is used to fit the experimental data, and the mass ablation rate and ablation pressure are obtained. The L-shell X-ray conversion and spectra of the laser-produced Ag plasma are measured with a Bragg crystal spectrometer and an array of X-ray diodes. The design for X-ray backlighting radiograph experiments will benefit from these experimental results. The result and method presented in this article are significant for the ablation research on the capsule shell and cavity wall material in laser fusion ignition experiment.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2013, 62 (3): 036101. doi: 10.7498/aps.62.036101
We investigate the isothermal crystallizations of nanoparticles composed, respectively, of 4000 Cu atoms (Cu4000) and 13500 Cu atoms (Cu13500), and bulk Cu according to on embedded atom model, using molecular dynamics simulations. We note that different sizes of Cu nanoparticles display multistep crystallization at low temperature, and their crystallization time distribution is wider than at high temperature, shown by analyzing the structural and dynamic properties of isothermal crystallization. Moreover, the size of particle plays an important role in the crystallization process. The larger the size, the longer the crystallization time is. However, we find that there is a critical size rc. The crystallization time increases with particle size increasing when the size is less than rc. On the contrary, when the size is more than rc, the crystallization time decreases with particle size increasing.
Optimization of the longitudinal structure of intrinsic layer in microcrystalline silicon germanium solar cell
2013, 62 (3): 036102. doi: 10.7498/aps.62.036102
Using radio-frequency plasma enhanced chemical vapor deposition, based on the influences of discharge power on structural and photoelectric properties of μc-SiGe:H thin films, RF power profiling technique is developed during the deposition of μc-SiGe:H intrinsic layer. The optimized μc-SiGe:H intrinsic layer not only maintains homogeneity of the crystalline volume fraction along the depth profile, but also forms a band gap profiling configuration from wide to narrow in the direction of growth. By this method, the fill factor and the short-circuit current density of μc-SiGe:H solar cell are significantly improved, and an efficiency of 9.54% for the a-Si:H/μc-SiGe:H tandem solar cell is achieved.
2013, 62 (3): 036103. doi: 10.7498/aps.62.036103
By using the first-principles method based on the density-functional theory, twisting- deformation-dependent electrical characteristics of graphene nanoribbons (GNRs) are studied systematically. It is shown that the energy gap of the zigzag-edge graphene nanoribbon (ZGNR) is the most insensitive to twisting deformation, and it almost keeps metallicity unchanged, next is the armchair-edge graphene nanoribbons (AGNRs) by width W=3p-1 (p is a positive integer), and its gap has only a small change when twisting deformation occurs. However, the gap of AGNR with width W=3p+1 is extremely sensitive to twisting deformation, and it can display a variation from wide-gap semiconductor to moderate-gap semiconductor, quasi-metal, and metal, next is AGNR with W=3p. In other words, the larger the band gap for GNR in the absence of twisting deformation, the more significant the change (becoming small) of its band gap with twisting deformation. Additionally, for the whole electronic structure and transmission behavior, one can find that there is a much larger influence under twisting deformation in AGNR than in ZGNR. These studies suggest that it is necessary to take the effect of twisting deformation on the electrical characteristics into account in designing GNR-based nanodevices.
2013, 62 (3): 036104. doi: 10.7498/aps.62.036104
Surface photonic crystal (PC) structure can improve the external quantum efficiency of light-emitting diode (LED). However, it is very difficult to fabricate large area and uniform nanometer photonic crystal structure in this field. In this paper, two-dimensional PC with hole-like structure is successfully transferred to the surface of gallium nitride LED (GaN-LED) by the mental-polymer double-layer mask dry etching technology combined with the nanoimprint lithography. The large area nanometer PC patterns with pore diameter of 240 nm and period of 450 nm are obtained. The results show that the photoluminescence peak intensity of LED with the PC structure is 7.2 times higher than that of the conventional LED.
2013, 62 (3): 036105. doi: 10.7498/aps.62.036105
The variations in ID-VD characteristic of 0.8 μm SOINMOS transistors are studied, which are exposed to 60Co γ ray at a dose rate of 50 rad (Si)/s. The results show that the linear kink effects of these samples at each dose level ane not presente due to the optimizations of manufacture process and layout design. The drain voltage that corresponds to the impact ionization induced kink effect, increases, with dose level. An anomalous "Kink" effect in the back gate ID-VSUB characteristics of the partially depleted SOINMOS transistors is observed at a high dose level, which is attributed to interface trap states generated at the buried oxide/silicon film interface during irradiation.
2013, 62 (3): 036201. doi: 10.7498/aps.62.036201
The elasto-plastic deformation behavior, yield strength and strain rate of material under shock compression can be represented by shock front, and the shock front is also related to the variation of strength after shock compression. In this paper, we study the dynamic plastic deformation processe of nanocrystalline copper under shock compression through molecular dynamics simulations. We also explore the dependences of the shock front and the mechanism of elasto-plastic deformation on grain boundary, and make a comparison with the case of the shock response of nanocrystalline aluminum. This investigation shows that the contribution of grain boundary to the shock-front width of nanocrystalline copper are smaller than that of nanocrystalline aluminum. The plastic mechanism of nanocrystalline copper is dominated by the emission and propagation of partial dislocations, and the full dislocation and deformation twin are rarely found in the samples. From the simulations are also found that the shock-front width decreases with the increase of loaded shock stress. A quantitative inverse relationship between the shock wave front width and the shock intensity is obtained. This quantitative inverse relationship is close to other simulation result of nanocrystalline copper and quite different from results of coarse-grained copper compression experiments.
2013, 62 (3): 036301. doi: 10.7498/aps.62.036301
Based on density functional theory, the single O2 and CO adsorption on pristine and palladium (Pd) doped graphene are studied using first-principles calculations. By calculating the system adsorption energy, charge transfer, band structure and density of states (DOS), we find that compared with O2 and CO adsorbed on the pristine graphene, the Pd doped systems have high adsorption energies and large charge transfers. The reason is that the new energy levels which are brought into pristine graphene by the dopant Pd strengthened the interaction between graphene and the adsorbed gas molecule. Oxidizing gas O2 and reducing gas CO have obviously different effects on band structure and DOS of graphene. The DOS near the Fermi level of graphene has great change after adsorbing O2 and the change becomes smaller when O2 is adsorbed on Pd doped graphene, while there is almost no change in DOS when graphene adsorbs CO, which indicates that doping Pd on graphene adsorbing CO will not enhance the gas sensitivity. However, the adsorption energy increases, which can improve the gas sensing response speed when graphene adsorbs reducing gas.
Reverse-impact experiments are performed on bismuth (Bi) to probe into the release melting from shock pressures in a pressure range of 11-16 GPa. A displacement interferometer system for any reflector (DISAR) is employed to measure the particle velocity history at the impact interface of LiF window with Bi flyer. The obtained experimental data, together with the results from characteristic formulations and one-dimensional hydrodynamic simulations, indicate that bismuth is converted into the body-center-cubic phase under shock loading, and then melted with the releasing of state from the initial shock (Hugoniot). The inflexion on the release wave profiles is attributed to the release melting. The proposed method and extracted results are of importance for developing the phase-change diagnostics and understanding phase-transition behavior of Bi and its analogues.
2013, 62 (3): 036402. doi: 10.7498/aps.62.036402
Novel Ga30Sb70/Sb80Te20 nanocomposite multilayer films are prepared by alternate sputter deposition of two independent targets of Ga30Sb70 and Sb80Te20 in a magnetron sputtering system. The influence of layer thickness of Ga30Sb70 on the phase-change behavior of Ga30Sb70/Sb80Te20 multilayer film is investigated. The results show that the crystallization temperature can be controlled by adjusting the layer thickness of Ga30Sb70. The crystallization temperature increases with increasing the layer thickness of Ga30Sb70. The optical band gap is also found to increase with increasing in the layer thickness of Ga30Sb70. Transient crystallization dynamics of Ga30Sb70/Sb80Te20 multilayer film induced by single picosecond laser pulse pumping, is studied. The reversible phase transition between amorphous and crystalline state can be achieved by using picosecond laser pulses with different fluences.
2013, 62 (3): 036801. doi: 10.7498/aps.62.036801
A series of aluminum matrix nanocomposite films are synthesized by magnetron sputtering of Al and TiN targets. The composition, microstructure and mechanical property of the composite film are characterized by energy dispersive spectroscopic, X-ray diffraction, transmission electron microscope and nanoindenter. The influences of (Ti, N) content of supersaturated solute Ti,N atoms on the microstructure and mechanical property of the composite films are investigated. The results reveal that the composite film with adding Ti,N atoms together forms a dual-supersaturated solid solution exhibiting both features of substitutional and interstitial solid solution. Higher solute content induced gradual evolutions of nanocrystallization and amorphization of grains in film and solute enrichment occured at the grain boundaries. Correspondingly, the composite film containing 1.8 at.% (Ti, N) can rapidly reach a hardness of 3.9 GPa, and further increasing TiN content to 17.1 at% (Ti, N) the film hardness achieves 8.8 GPa demonstrating the significant strengthening effect of dual-supersaturation of Ti and N on aluminum film.
2013, 62 (3): 036802. doi: 10.7498/aps.62.036802
Roughening and pre-roughening processes on InGaAs surface are studied using scanning tunneling microscopy. There are different roughening and pre-roughening processes for InGaAs films at different substrate temperatures and As beam equivalent pressure. Under low temperature and low As beam equivalent pressure, pits is main mechanism in the beginning of InGaAs morphology evolution, with the increase of annealing time, a great number of pits and islands are observed which make the surface rough. Small islands should play a leading role during the InGaAs morphology evolution at high temperature and high As beam equivalent pressure, and the number of islands will increase gradually with the increase of annealing time till it reaches an equilibrium state.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
First-principles study on the effect of high Mn doped on the band gap and absorption spectrum of ZnO
2013, 62 (3): 037101. doi: 10.7498/aps.62.037101
According to the density functional theory, using first-principles plane-wave ultrasoft pseudopotential method, we set three different concentration Mn doped ZnO models, and perform the geomertry optimizations for the three modes. The total density of states, the band structures and the optical absorption are also calculated. The results show that in the case of non-spin state, the smaller the doping concentration of Mn is, the smaller the formation energy of ZnO is and the easier the Mn doping is, thus the stabler the crystal struetuer is; the Mn doping leads to the degenerations of the impurity energy band and the conduction band, and also to the optical absorption blue-shift. These calculation results accord with the experimental results. Moreover, the magnetism exists in the system under the situation of spin polarization, the absorption spectrum has a red-shift, which is consistent with the experimental result.
2013, 62 (3): 037102. doi: 10.7498/aps.62.037102
First-principles plan-wave pseudopotential method is used to calculate the energy bandgap properties, bowing parameters and structural properties of wurtzite BexZn1-xO alloys. The results show that the energy bandgaps and bowing parameters of BexZn1-xO alloys increase with Be content increasing. We find the energy bandgaps corrected of BexZn1-xO alloys to be in accordance with the experimental value. The reason about the energy bandgap broadening is also analyzed. The average bowing parameter of BexZn1-xO alloys is 6.02 eV, which is in good agreement with previous experimental result. The bowing parameter mostly arises from volume deformation of alloy and charge transfer effect. Futhermore, we analyze the changes of lattice parameter, average bond length and average second-neighboring distance with Be content in BexZn1-xO alloys.
2013, 62 (3): 037103. doi: 10.7498/aps.62.037103
We study the electronic properties of Cr- and W-doped single-layer MoS2 using an ab initio method according to the density functional theory. Our calculated results show the energy band structures of MoS2 are significantly affected by Cr doping, but not by W doping at a high doping concentration. The effects of Cr doping manifest as the transition of energy band structure from direct to indirect, and the decrease of band gap. Our further analysis reveals that strain is the direct reason for the change of electronic structure in the Cr-doped MoS2.
The DFT analyses of effect of chromium and nickel additions on the mechanical properties of Fe3Al based alloys
2013, 62 (3): 037104. doi: 10.7498/aps.62.037104
Site preference behaviors of transition metal (TM) Cr and Ni in Fe3Al alloy and alloying effect are studied by using first-principles. Computational results show that the addition of TM is beneficial to the improvement of the stability of Fe3Al alloy; Cr atoms prefer to stay at FeI sites, while Ni atoms occupy FeII sites. Our investigations reveal that Fe2NiAl has lower bulk modulus B, shear modulus G and B/G than other (Fe3-yXy)Al (X=Cr, Ni; y=0,1,2) alloys, which turns out to be a brittle alloy. The analyses of density of states and electronic charge density indicate that additional TM improves the interaction between TM and neighboring host atoms of Fe3Al but weakens the interaction between Al and Fe.
2013, 62 (3): 037105. doi: 10.7498/aps.62.037105
GaN:Tb nanoparticles are synthesized by simple ammonification of a mixture of Ga(NO3)3 and Tb(NO3)3. The XRD result shows that the sample predominantly presents hexagonal phase of GaN and its average grain size is 22.1 nm. TEM images show that the sizes of the particles are almost uniform. Besides conventional GaN Raman shifts, two extra peaks at 251 and 414 cm-1 observed in the Raman spectra can be attributed to the phonons activated by surface disorders or finite-size effects and vibration mode of N-rich octahedral Ga-N6 bonds, respectively. From photoluminescence spectra, four characteristic peaks of Tb3+ions are clearly observed: 5D4 →7F6(493.9 nm), 5D4 →7F5(551.2 nm), 5D4 →7F4(594.4 nm), 5D4 →7F3(630.1 nm).
2013, 62 (3): 037201. doi: 10.7498/aps.62.037201
Photo-conductivity transient processes of Fe:LiNbO3 congruent crystals are investigated by electro-chemical analyzer. The experiments are executed with Fe:LiNbO3 crystals of different Fe concentrations in the conditions of different laser intensities. The results show that the transient photo-conductivity of the Fe-doped lithium niobate crystal is formed through a complex process of electron transport; the decay of photo-conductivity can be fitted to an exponential function and a stretched-exponential function. The dependences of the fitting parameters on laser intensity and iron-doped concentration are measured. The values of amplitudes σ1max, σ2max, time constant τ2 and stretching factor β increase strongly at low intensities, and τ2 and β reach their saturation value for higher intensities; with the increase of the concentration of Fe ions, the values of σ1max, σ2max and τ2 incerease, but β decreases. With experimental results, we propose a charge transfer model which includes the migration of electrons in the conduction band and the jumping of electrons between small-polarons. The model better explains the main features of photo-conductivity decay for Fe-doped congruent lithium niobate crystals.
Investigation of anti-reflection properties of crystalline silicon solar cell surface silicon nanowire arrays
2013, 62 (3): 037301. doi: 10.7498/aps.62.037301
In order to trap more sunlight onto the crystalline silicon solar cell and improve the photo-electric conversion efficiency, it is very important to study the optical scattering properties of silicon nanowire arrays on silicon wafer. The rigorous coupled wave analysis method is used for optical simulation, and the Taguchi method is used for efficient optimization. The simulation results show that at the above-mentioned wavelengths the reflectance of the optimized structure is less than 2%, and also able to achieve the wide-angle antireflection. At room temperature and ambient pressure, the silicon nanowire arrays each with a period of 50 nm, duty ratio of 0.6 and height of 1000 nm are successfully prepared on mono-crystalline Si wafers using a novel metal-catalyzed chemical etching technique, the reflectance test results are consistent with simulation values. The average reflectance of the optimized structure over the above-mentioned wavelength range is 4%-5%, showing that the antireflection effect is obvious compared with the reflectivity of about 35% of the single crystal silicon. The minus reflection microstructures reduce the sun battery microstructure costs, at the same time, reduce the monocrystalline silicon surface light reflecting loss, improve the photoelectric conversion efficiency.
2013, 62 (3): 037302. doi: 10.7498/aps.62.037302
In this paper, we present a new kind of surface plasma resonance (SPR) sensor. It comprises three layers, an upper coupling prism, a middle metal layer, and a bottom semi-infinite two-level atom medium. Based on theoretical analysis on this SPR system, it is found that there are some new phenomena due to the coexistence of two resonant effects, i.e., the surface plasmon resonance and the resonant quantum transition between atomic energy levels. An interesting phenomenon is that the reflectivity of the incident light is very sensitive to the shift of atomic level induced by external field. This phenomenon is absent in ordinary SPR systems. Because the shift of atomic level is determined by external field, ultimately, the reflectivity of the incident light is sensitive to the external field. In this paper calculations are presented in the case where the external field is a magnetic field. It is found that the reflectivity of the incident light is very sensitive to the magnetic field. This property can be used for measuring a very weak magnetic field near surface of matter , thus it can be used for developing a new detection technology.
2013, 62 (3): 037303. doi: 10.7498/aps.62.037303
A reflection-mode GaAlAs photocathode and a reflection-mode GaAs photocathode using exponential-doping technique are prepared by metal organic chemical vapor deposition, and the Al content of GaAlAs emission layer is 0.63. The two photocathodes are activated in an ultra-high vacuum system, and the spectral response curves are measured after activation. The quantum efficiency formula for exponential-doping reflection-mode photocathode is used to fit the experimental curves of the two photocathodes respectively, and the effects of some performance parameters on photoemission are analyzed, such as electron diffusion and drift length, back-interface recombination velocity, surface electron escape probability, etc. The results show that the Al content of the GaAlAs photocathode plays a bad role in the photoemission compared with that the GaAs photocathode, but it solves the problem that the GaAs photocathode cannot be well used in the area of detecting the narrow wavelength light due to the broad spectral response. The reflection-mode GaAlAs photocathode prepared is responsive to the blue and green light.
A series of Ge films with varying thickness is prepared by plasma enhanced chemical vapor deposition technology. With the thickness of the sample becoming thinner, the sample shows ferromagnetism. When the 12-nm-thick sample is annealed at 300℃, the partile size becomes smaller, and thus the number of interface defects between the particles increases, so the sample gives a largest magnetic signal at room temperature (50 emu/cm3). FC-ZFC measurement shows that Curie temperature is 350 K. For a higher temperature (600 ℃, the coalescence of small Ge particles makes surface area decline, so magnetic signal becomes weak. With the thickness being 6 nm, the paramagnetism and the ferromagnetism coexist in the 6-nm-thick Ge film. When the 6-nm-thick sample is annealed under nitrogen atmosphere at 300 ℃, the sample only shows ferromagnetism. However, annealed at 600 ℃, the sample only presents paramagnetism. With the annealing temperature changing, the 12-nm-thick film and the 6-nm-thick film show different magnetic phenomena. Particle size and particle distribution cause different magnetic phenomena. It is supposed that the Ge nannostructure unpaired electrons are provided mainly by the interface defect between Si matyix and Ge film and the surface defect of Ge particles. The ferromagnetism coupling of the unpaired electrons is related to the distribution of sample particles and the junction among particles. The fusion between particles will reduce the ferromagnetism of the sample.
Quantitative analysis on the influences of the precursor and annealing temperature on Nd2O3 film composition
2013, 62 (3): 037701. doi: 10.7498/aps.62.037701
In this paper, ultra-thin Nd2O3 dielectric films are deposited on p-type silicon substrates by advanced atomic layer deposition method. Nd (thd)3 and O3 are used as the reaction precursors separately. The as-grown samples are annealed in N2 atmosphere in a temperature range of 700900 ℃. The samples are investigated at room temperature by X-ray photoelectron spectroscopy and the changes of the film composition at different annealing temperatures are discussed in detail. For a higher precursor temperature of 185 ℃ in the deposition process, the ratio of oxygen to neodymium atoms for the as-grown film is 1.82, which is close to the stoichiometry. Dielectric constant increases from 6.85 to 10.32.
Method of accurately measuring silicon sphere density difference based on hydrostatic suspension principls
2013, 62 (3): 037702. doi: 10.7498/aps.62.037702
The micro density difference between silicon single crystal spheres is not only important for the research on the redefinition of Avogadro constant based on quantum standard, but also a key solution for quality control for the production of silicon single crystal with ultra-high purity in semi-conductor industry. To overcome the complexity of non-contact laser interferometer method and improve the accuracy of hydro-weight method, a method based on the hydrostatic suspension principle is realized. The silicon single spheres to be measured are immersed into mixture liquid including 1,2,3-tribromopropane and 1,2-dibromoethane, and floated freely by adjusting the temperature and pressure of the liquid. The micro density difference between two silicon single crystal spheres is calculated based on a mathematical model by using liquid temperature, pressure, and central floatation height difference in the floatation condition. The stable constant temperature liquid with maximal error ± 100 μ K is realized by two-cycle water bath and PID control system. The floatation height of silicon single crystal sphere is determined by binary image and iterative algorithm. The stable suspension is achieved by the PID pressure control system, and the temperature fluctuation due to Joule-Thomson effect is reduced. By means of linearity between changes of temperature and pressure in hydrostatic suspension model, the compressibility of mixture liquid is measured. The experimental results show that the influence from liquid surface tension is avoided by using the hydrostatic suspension method, and accurate measurement of density difference between silicon single crystal spheres can be achieved with an uncertainty of 2.1× 10-7 (expand factor k=1).
Study on the p-type conductivities and Raman scattering properties of N+ ion-implanted O-rich ZnO thin films
2013, 62 (3): 037703. doi: 10.7498/aps.62.037703
The p-type N doped ZnO thin films are fabricated using radio-frequency magnetron sputtering technique in O-rich growth condition together with the direct N+ ion-implantation and annealing. The conductivities and Raman scattering properties of the samples are studied by Hall measurements and Raman spectra respectively. Hall measurements indicate that the optimal p-type ZnO film can be obtained when the sample is annealed at 600 ℃ for 120 min in N2 ambience, and its hole concentration is about 2.527×1017 cm-3. N+-implantation induces three additional vibrational modes in ZnO, which are located at 274.2, 506.7 and 640.4 cm-1 respectively. In the process of the annealing, by comparing the electrical properties and Raman speetra of the samples, we find that the competition between intrinsic donor defects and the activation of N acceptors plays a crucial role in the p-type formation of ZnO:N films during annealing.
2013, 62 (3): 037801. doi: 10.7498/aps.62.037801
The resonance Raman spectra of nonpolar molecule all-trans--carotene and polar molecule canthaxanthin in nonpolar CS2 and polar 1,2-dicholoroethane in a temperature range from 243K to 293K are measured. The results show that polarities of the solute and solvent have a great effect on Raman spectrum. Raman scattering cross-section of nonpolar all-trans--carotene in nonpolar solvent CS2 is biggest and its bandwidth is narrowest. Raman scattering cross-section of polar canthaxanthin in polar solvent 1,2-dicholoroethane is smallest and its bandwidth is widest. The experimental phenomena are explained by solvent effects, coherent weakly damped electron-lattice vibration and effective conjugated length.
2013, 62 (3): 037802. doi: 10.7498/aps.62.037802
In this paper, the lattice vibrational modes of the nonlinear optical crystal BaBPO5 are studied using polarized Raman spectrum and the first-principles calculation. The polarized Raman spectra are obtained in different configurations and recorded in a range of 100-1600 cm-1. Combining the experimental data of the FT-IR spectrum, the lattice vibrational modes of the internal and external vibrations are analyzed by the factor group analysis method. The results indicate that the primary Raman peaks of the BaBPO5 crystal are ascribed to the internal vibrations of the PO4 and BO4 tetrahedrons, and the vibrations of the PO4 tetrahedron show strong Raman- and IR-activity. On the other hand, the first-principles calculation indicates the correlation of the vibrational modes with the atomic activities. Especially, the Raman peak located at 672 cm-1 is ascribed to the vibration of the B-O-P bond, which is the connector between the PO4 and BO4 tetrahedron. These results obtained in this work are important for understanding the micro-structures of the BaBPO5 crystal, and the further study on the crystal growth mechanism.
2013, 62 (3): 037803. doi: 10.7498/aps.62.037803
Sr0.8-xBaxEu0.2WO4 red phosphors with different Ba2+ doping concentrations is prepared by the co-precipitation method at different sintering temperatures. We study the influences of the crystal orientation and lattice distortion of the samples on the luminescence property. The results show that the Sr0.8-xBaxEu0.2WO4 synthesized by the process has a phase with tetragonal structure and that the red emission 5D0-7F2 of Eu3+ in the sample can be excited effectively by the near-UV light and blue light. The part of Sr2+ replaced with Ba2+ leads to the increase in emission intensity of Sr0.8-xBaxEu0.2WO4. The doping concentration of Ba2+ has a great influence on the crystal parameter, the crystal symmetry and the luminescent property. The optimized doping concentration of Ba2+ is found to be 30%.
2013, 62 (3): 037804. doi: 10.7498/aps.62.037804
xBi2O3-50B2O3-(50-x)BaO glasses are prepared by melting method. Their infrared luminescence spectra, fluorescence decay curves and Raman scattering spectra are measured. No obvious near-infrared (NIR) luminescence is observed in 50Bi2O3-50B2O3 glass under 808 nm laser diode (LD) pumping. Adding BaO, broadband NIR luminescences are observed in 40Bi2O3-50B2O3-10BaO glass, 45Bi2O3-50B2O3-5BaO glass and 49Bi2O3-50B2O3-1BaO glass. With the increase of BaO concentration, no obvious NIR luminescences are observed in 30Bi2O3-50B2O3-20BaO glass, 20Bi2O3-50B2O3-30BaO glass and 10Bi2O3-50B2O3-40BaO glass. The NIR emissions with several emission peaks appear in 0.5Bi2O3-50B2O3-50BaO glass and 1Bi2O3-50B2O3-50BaO glass. The mechanism for the emission is investigated preliminarily.
2013, 62 (3): 037805. doi: 10.7498/aps.62.037805
The effects of shell thickness, inner core size, and dielectric constants of core and embedding medium on the localized surface plasmon resonance wavelength of gold nanoshell are investigated by Mie theory. The results show that the extinction peak of gold nanoshell is first blue-shifted and then red-shifted with the increase of shell thickness, that the shift shell thickness corresponding to quadrupolar wavelength is greater than that corresponding to dipolar wavelength, and that the ratio of the shift shell thickness to the inner core size decreases with the increase of inner core size, and increases with the increase of dielectric constant of inner core or embedding medium. The resonance peaks in the scattering spectra have similar phenomena to those in extinction spectra as the shell thickness increases. The shift of the spectral peak is ascribed to the plasmon hybridization and phase retardation effect.
Extraction of effective constitutive parameters of active terahertz metamaterial with negative differential resistance carbon nanotubes
2013, 62 (3): 037806. doi: 10.7498/aps.62.037806
A strategy is presented to acquire active terahertz (THz) metamaterial by incorporating negative differential resistance carbon nanotubes. Furthermore, we propose a method of extracting active metamaterial effective parameters. This new method can effectively solve the problems of signs and multi-branches, while the traditional parameter extraction method becomes powerless for active case. Our results indicate that the active THz metamaterial with metal wires array not only has negative value of the imaginary part of the permittivity but also presents magnetic-dispersion characteristics.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
One-step synthesis of a carbon nano sheet-scarbon nanotubes composite and its field emission properties
2013, 62 (3): 038101. doi: 10.7498/aps.62.038101
One-step synthesis of a carbon nano sheets-carbon nanotubes composite by plasma enhanced chemical vapor deposition and its field emission properties are investigated. We obtain the carbon nano sheets-carbon nanotube composite on the Co thin film with 20 nm Ti interlayer. We gain carbon nano sheets only on the Co thin films without Ti interlayer in the same growth conditions. The carbon nano sheets are distributed on the side wall and the top of carbon nanotubes. The Ti interlayer hinders the diffusion of Co into the silicon substrate and improves the catalytic capability of Co, thus it will help the growth of carbon nanotubes. When the thickness of Co film is 11 nm, the carbon nanotubes are vertically aligned on the flat Co film surface. Most of the carbon nano sheets are distributed on the top of the carbon nanotubes under this condition, which increases the number of emitters and enhances the field emission properties of the composites.
2013, 62 (3): 038402. doi: 10.7498/aps.62.038402
The past communication behaviors that guide the system communication in the future to satisfy the requirements of users and adapt to the changes of environment are the core part of cognitive radio system. In this paper, a cognitive engine based on Bayesian network is proposed to solve the parameters self-adaptive-adjusting problem of cognitive radio system under the complicated and highly varying radio environment and user requirement. Through structure learning and parameter learning of the sample data from the past communication behaviors, cognitive engine is established. The states of radio environment and requirements of users are made as inference evidences by data preprocessing, and the cognitive engine is used to make decision of the configuration parameters of communication system, and then the reconfiguration system is completed. A mobile wireless network is modeled to finish reconfiguration simulation using OPNET tool in this paper. Simulation results show that the proposed cognitive engine can make the wireless mobile network adapt to environment and effectively improve end-to-end communication performance. The feasibility of the method to model cognitive engine with Bayesian network is validated in this paper.
Nonlinear dynamic model and chaotic characteristics of mechanical thastic energy storage unit in energy storage process
2013, 62 (3): 038403. doi: 10.7498/aps.62.038403
A new way of energy storage based on mechanical elasticity is proposed. Nonlinear dynamic model of permanent magnet motor based mechanical elastic energy storage unit is derived, and the chaotic behaviors under certain parameter values and operation conditions are demonstrated. The linear stability of the nonlinear dynamic model is analyzed. Based on the operation parameters of the 0.16 kWh/0.8 kW unit that is being developed in our laboratory, three-dimensional nonlinear model is reduced to two-dimensional projection subsystem with varying parameters. Then the chaotic property of the unit is analyzed by coordinate plane projection method, the changing relationship between the locations of equilibrium point, characteristic equation of reduction subsystem and varying parameters is discussed, and the chaotic behavior is numerically verified. The issue caused by poor applicability of analytical methods and solved only by simulation is solved to some extent at a theoretical analysis level.
Effect of annealing atmosphere on characteristics of MONOS with LaTiON or HfLaON as charge storage layer
2013, 62 (3): 038501. doi: 10.7498/aps.62.038501
Charge-trapping memory capacitor with LaTiON or HfLaON serving as charge storage layer is fabricated by reactive sputtering method, and influences of post-deposition annealing (PDA) in NH3 or N2 ambient on its memory characteristics are investigated. It is found that before PDA, the LaTiON sample exhibits better retention characteristic than the HfLaON sample, but the later shows larger memory window (4.8 V at +/-12 V/1 s), and after PDA, the NH3-annealed sample has faster program/erase speed, better retention and endurance properties than the N2-annealed sample, owing to nitridation role of NH3. Furthermore, the HfLaON sample with PDA in NH3achieves a large memory window of 3.8 V at +/-12 V/1 s, and also shows better retention and endurance properties than the LaTiON sample with PDA in NH3.
2013, 62 (3): 038701. doi: 10.7498/aps.62.038701
In this letter, we show that the trajectory of electroencephalogram (EEG) possesses the character of time reversal asymmetry, which can provide information about the entropy production of EEG. We develop a kind of new method to estimate symbolic relative entropy and entropy production using forward and backward trajectories. Finally, we use this method to dispose and analyse the EEGs of younger and elder people, and is turned out that this method works and the average energy dissipation can be used as a parameter to detect nonequilibrium.
The grand canonical Monte Carlo simulation of hydrogen physisorption on single-walled carbon nanotubes at the low and normal temperatures
2013, 62 (3): 038801. doi: 10.7498/aps.62.038801
In this article, the grand canonical Monte Carlo (GCMC) method included in GULP software system is adopted to study the hydrogen storage properties of armchair-type single-walled carbon nanotubes (SWNT) with at the low and normal temperatures. The adsorption isotherms with five different radii of SWNT at T=77 K and T=280 K are obtained. The manifold configuration diagrams of hydrogen molecule in the carbon nanotubes with the same caliber at different temperatures and pressures are also given. A further study on hydrogen physisorption is carried out under different pressures and different diameters of carbon nanotubes, separately, at temperatures of 77 K and 280 K, and the results are compared with each other. Finally, we put forward some constructive suggestions about how to improve the adsorption capacity of SWNT according to the results of our GCMC simulation. This may be useful for further investigation.
2013, 62 (3): 038802. doi: 10.7498/aps.62.038802
Solar thermoelectric conversion is another way to convert solar radiation directly into electricity besides photovoltaic technology, and has become a new hot spot of solar energy utilization in recent years. In this paper a model of flat-panel thermal concentrated solar thermoelectric device is built based on the material of Bi2Te3. And finite element analysis is used to analyze the temperature distribution of the device under AM1.5 illumination. Furthermore, the influences of thermal concentration, cross section area and length of thermal legs on open voltage, maximum output power and conversion efficiency of the device are calculated based on temperature-dependent physical parameters. The results indicate that thermal concentration and length of thermal legs haved a significant influence on device performance, while the cross section area changes the conversion efficiency of device relatively weakly, and the conversion efficiency of the device reaches 1.56% in this model.
Influences of deposition rate in second stage on the Cu(In,Ga)Se2 thin film and device prepared by low-temperature process
2013, 62 (3): 038803. doi: 10.7498/aps.62.038803
Polycrystalline Cu(In,Ga)Se2 (CIGS) thin ?lms are deposited onto soda-lime glass substrates by the low-temperature three-stage process (below substrate temperature of 420℃). The influences of growth rate in the second stage on structural and electrical properties of CIGS thin film and device performance are investigated. With the increase of deposition rate during the second stage, the crystallinity and grain compactness of CIGS thin film are promoted, and the double-peak reflection pattern is reduced obviously,which can reduce the recombination in the grain boundary and help to improve the conversion efficiency of the CIGS solar cell significantly. However, according to the experimental results, higher growth rate during the second stage leads to rough surface and low carrier concentration. The larger surface roughness can be attributed to the larger grain size of secondary-phase Cu2-xSe, and the lower carrier concentration results from the reduction of passivation donor defect effect which is induced by the hindrance of Na diffusion from the glass substrate. High growth rate in the second stage is found to be able to increase the interface recombination and induce shunt paths in the solar cell and then the open circuit voltage and the cell parameters are deteriorated. Finally, a high conversion efficiency of 11.24% is achieved by optimizing the growth rate in the second stage.
2013, 62 (3): 038804. doi: 10.7498/aps.62.038804
In cognitive radio network (CRN), TCP end to end throughput is one of the key issues to measure its performance. However, most of existing research efforts devoted to TCP performance improvement have two weaknesses as follows. First, most of them only consider the underlying parameters to optimize the physical performance, but the TCP performance is neglected. Second, they are largely formulated as a Markov decision process (MDP), which requires a complete knowledge of network and cannot be directly applied to CRNs. To solve the above problems, a Q-BMDP algorithm is proposed in this paper. Each user in CRN combines modulation type and transmitting power at the physical layer, access channels at the media access control layer and TCP congestion control factor to maximize the TCP throughput. Due to the existence of perception error of environment, this issue is formulated as a partial observable Markov decision process (POMDP) which is then converted to belief state MDP, with Q-value iteration to find the approximately optimal strategy. Simulation and analysis results show that the proposed algorithm can be approximately converged to optimal strategy under a maximum error limit, and can effectively improve TCP throughput in a dynamic wireless network under the premise of the limited power consumption.
2013, 62 (3): 038901. doi: 10.7498/aps.62.038901
The identifying of the most influential nodes in the complex network is of great significance for information dissemination and control. We collect actual data from Sina Weibo and establish two user relationship networks based on bi-directional concern. By analyzing the statistical characteristics of the network topology, we find that each of them has a small world and scale free characteristics. Moreover, we describe four network centrality indicators, including node degree, closeness, betweenness and k-Core. Through empirical analysis of four-centrality metric distribution, we find that the node degrees follow a segmented power-law distribution; betweenness difference is most significant; both networks possess significant hierarchy, but not all of the nodes with higher degree have the greater k-Core values; strong correlation exists between the centrality indicators of all nodes, but this correlation is weakened in the node with higher degree value. The two networks are used to simulate the information spreading process with the SIR information dissemination model based on infectious disease dynamics. The simulation results show that there are different effects on the scope and speed of information dissemination under different initial selected individuals. We find that the closeness and k-Core can be more accurate representations of the core of the network location than other indicators, which helps us to identify influential nodes in the information dissemination network.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2013, 62 (3): 039201. doi: 10.7498/aps.62.039201
Sea spray droplets change the distribution of air-sea momentum fluxes in the sea surface, and then influence sea surface drag coefficient considerably. In order to estimate the influence of sea spray droplets on drag coefficient exactly, a new sea spray generation function (SSGF) which depends on wind and wave, is deduced. Then the SSGF is integrated into the spray momentum flux formula to calculate the spray momentum flux. The result shows that revised spray momentum flux formula are sensitive to the wave state remarkably. It indicates that the new sea spray flux formulae can be used in any wave state. The total momentum flux of sea surface contains sea spray momentum flux and air-sea interface momentum flux. Based on this theory, the formula of drag coefficient which is influenced by sea spray droplets at high wind speed is acquired. The theoretical result of the drag coefficient formula shows that the drag coefficient reduces at the high wind speed, which indicates that the sea spray droplets can restrict the increase of drag coefficient at high wind speed. Comparison between theoretical value and measured value of sea surface drag coefficient in field and laboratory shows that the theoretical value of drag coefficient cover the measured value almost. Meanwhile, with the new sea surface drag coefficient, ocean wave model can model the significant wave height very well in typhoon condition. All the results show that the new drag coefficient can be used properly at high wind speed.
Dynamic modification of the shallow water equation on the slowly changing underlying surface condition
2013, 62 (3): 039202. doi: 10.7498/aps.62.039202
In this paper, the shallow water equation is discussed, when the underlying surface is slowly changing. From the continuity equation and the equation of motion controlling the movement of the atmosphere, the slowly changing of the underlying surface is superimposed on the topography function as a small quantity, using the upper and lower boundary conditions of the atmosphere, the modified shallow water equation is obtained. In the modified shallow water equation, the slowly changing local horizontal divergence modifies the equilibrium between the local horizontal divergence and the local change of the thickness of the atmosphere. On the basis of this, the vorticity equation is obtained, which contains the slowly changing underlying surface.
The variation features of the surface mixed layer depth in Erhai Lake and Taihu Lake in spring and autumn and their mechanism analyses
2013, 62 (3): 039203. doi: 10.7498/aps.62.039203
The variation of the surface mixed layer depth may affect not only the evolution of aquatic ecosystem, but also the temporal-spatial distribution of precipitation and climate in the basin. Based on the meteorological data and water temperature profiles observed in Erhai Lake (located in the Tibetan Plateau) and Taihu Lake (located in the Taihu Plain), the variation features and the mechanisms of the surface mixed layer depths are investigated. The stratification in Erhai Lake can be established and sustained in summer; the diurnal stratification in Erhai Lake can also be established, However, in both summer and autumn, stratification may exist in Taihu Lake. The time length of stratification is longer in Erhai Lake than that in Lake Taihu in the autumn. And the surface mixed layer depths in Erhai Lake are shallower than those in Taihu Lake in summer and autumn. The transformation frequency between establishment and destruction of stratification in Taihu Lake is faster than that in Erhai Lake, which illustrated that the response of water body in Taihu Lake to atmospheric variation is quicker than that in Erhai Lake. The water depth is a key factor which prevents such shallow lakes as Taihu Lake from establishing and maintainaning stratifications and in a suitable radiation condition the stratification will exist. The net radiation is a key factor that determines the stratification and the length of the time when the stratification can be sustained in lakes whose depths are the same as that of Erhai Lake. The research result in this paper is helpful for exploring the coupling mechanism of the turbulence of water and air and the evolution law of aquatic ecosystem.
Wind partitioning and reconstruction with variational method in a limited domain I: theoretical frame and simulation experiments
2013, 62 (3): 039204. doi: 10.7498/aps.62.039204
As is well known, the efficient method to wind partitioning and reconstruction is to introduce the velocity potential and stream function which are calculated from divergence and vorticity by solving two Poisson's equations. Since velocity potential and stream function are coupled at the boundary of limited domain, the wind partitioning problem is nonunique. To vercome the nonuniqueness of the wind portioning, a new variational adjoint method combined with regularization is proposed in this paper, which is based on the control of velocity potential and stream function boundary values under Dirichlet conditions. The cost function is composed of two parts, one is the observation term to minimize the error of the reconstructed wind field, and the other is the regularization term to guarantee the uniqueness of the reconstruction problem by seeking a stable regularization solution within meteorological content. The results of numerical experiments demonstrate that after choosing an appropriate regularization parameter, the new variational adjoint method combined with regularization is efficient and suitable for wind portioning and reconstruction in a limited domain.
2013, 62 (3): 039205. doi: 10.7498/aps.62.039205
The global distribution of the second lapse-rate tropopause (LRT2) is investigated with the radio occultation measurements from the constellation observing system for meteorology, ionosphere and climate (COSMIC) covering December 2006-November 2008 Comparisons between COSMIC and radiosonde in terms of the tropopause are examined in three stations to check the difference. The research results are as follows. 1) In the winter, occurrence frequency for LRT2 in the northern hemisphere (NH) is of 50%-70% and in the southers hemisphere is of 20%-40%. 2) The second tropopause over the equatorial zone with 20%-26% occurrence seems to be related to the equatorial jet stream and subvisual cirrus above the first tropopause. 3) In the tropic, the difference in altitude and occurrence between the first and second tropopause decrease with latitude. In the extratropic zone, the difference increases with latitude and reaches a maximum of 7-8 km in the winter of the NH. 4) The second tropopause occurs frequently over the subtropical jet stream region, in which the first tropopause temperature is almost higher than the second one. 5) The more intense the single station daily variations of tropopause height, the bigger the difference between COSMIC and radiosonde is.
2013, 62 (3): 039206. doi: 10.7498/aps.62.039206
In recent years, critical slowing down phenomenon has shown great potentials in disclosing whether a complex dynamic tends to critical cataclysm. Based on the concepts of critical slowing down, the observed data of temperature and the pacific decadal oscillation (PDO) index and the intensity of Aleutian low pressure, which have different noises are processed in this article to study the precursory signals of abrupt climate change. Take the abrupt climate change in the late1970s-early 1980s as an example, then variances and autocorrelation coefficients which can characterize critical slowing down will be calculated separately; the occurring time of precursory signal of abrupt climate change under the noises influence is studied. The results show that the critical slowing down phenomenon appears in the data with different signal-to-noise ratios before the abrupt climate change takes place, which indicates that critical slowing down phenomenon is a possible early warning signal for abrupt climate change and the noise has less influence on the test results for precursory signals of abrupt climate change. Accordingly, it demonstrates the reliability of critical slowing down phenomenon to test the precursory signals of abrupt climate change, which provides an experimental basis for the wide applications of the present method in real observation data.