Vol. 58, No. 7 (2009)
2009-04-05
GENERAL
2009, 58 (7): 4373-4378.
doi: 10.7498/aps.58.4373
Abstract +
By introducing and extending the （G′/G）-expansion method, the new exact general solutions are constructed for （2+1）-dimensional asymmetrical Nizhnik-Novikov-Veselov system. These solutions include general hyperbolic function solutions, general trigonometric function solutions and general rational function solutions. The solitary ware solutions are obtained when the constants involved in the general hyperbolic solutions are set at special values.
2009, 58 (7): 4379-4382.
doi: 10.7498/aps.58.4379
Abstract +
The generalized nonlinear Boussinesq equation is studied. Using the homotopic mapping perturbation method，firstly，the corresponding homotopic mapping transform is constructed；secondly，the suitable initial approximation is selected；and then by using the homotopic mapping，the approximate solution for the solitary wave is obtained. Finally，the small perturbation asymptotic expansions are obtained.
2009, 58 (7): 4383-4389.
doi: 10.7498/aps.58.4383
Abstract +
The Hopf bifurcation characteristic in a relative-rotation dynamical system with nonlinear damping force is analyzed and the necessary and sufficient condition of Hopf bifurcation is given under which the system has Hopf bifurcation emerging from the equilibrium. In order to make the Hopf bifurcation point transformed and control the stability and amplitude of limit cycle，a nonlinear feedback controller is proposed. The analytical results on controlling Hopf bifurcation points via the nonlinear controller agree well with the numerical results.
EDITOR'S SUGGESTION
2009, 58 (7): 4390-4395.
doi: 10.7498/aps.58.4390
Abstract +
The dynamical evolution of a periodically kicked Bose-Einstein condensate was investigated by simulation. Emphasis was placed on the influence of the nonlinear interaction between atoms on the quantum-resonance ratchet effect. It was found that weak nonlinear interaction reduces the ratchet effect while the strong nonlinear interaction causes the ratchet current to disappear or even reversed. The ratchet effect under high-order quantum resonance conditions was further studied. The ratchet effect is not obvious in this case and can be easily destroyed by the noise of the kicking potential period.
2009, 58 (7): 4396-4401.
doi: 10.7498/aps.58.4396
Abstract +
Taking Rosen-Zener tunneling as the underlying process，a scheme to achieve nonlinear Ramsey interferometry with a two-component trapped Bose-Einstein condensate is proposed. Abundant nonlinear Ramsey interference fringes are observed in time domain by the numerical simulations and these fringes are very different from the sinusoidal pattern for linear Ramsey interference due to the dominant nonlinear interaction between atoms. Furthermore，Fourier transformation results show that the fundamental frequency of Ramsey fringes can exactly reflect the information of nonlinearity or asymmetry. This finding provides an opportunity to measure atomic properties via measuring the frequency of fringes.
2009, 58 (7): 4402-4407.
doi: 10.7498/aps.58.4402
Abstract +
A stability theorem is proposed and proved for fractional system whose order is not greater than 1 according with Lyapunov stability theorem and Lyapunov converse theorem. Using this theorem，fractional unified chaotic systems with unknown parameter are synchronized adaptively. Numerical simulation certifies effectiveness of the theorem.
2009, 58 (7): 4408-4414.
doi: 10.7498/aps.58.4408
Abstract +
By rescaling the system，we obtain the Lorenz-Stenflo system with slow variation of the flow rotation. Based on Routh-Hurwitz criteria，the stabilities of the equilibria are analyzed and the bifurcation sets are sought to divide the parameter plane into different regions，corresponding to different types of the solutions. Different types of the solutions may bifurcate from the equilibria with the chang of the parameters. Furthermore，the symmetric bursting and the symmetric chaos are presented，and the mechanism of the symmetric bursting is explored by the method of slow-fast analysis.
2009, 58 (7): 4415-4420.
doi: 10.7498/aps.58.4415
Abstract +
In this paper，we propose a new method for calculating the main-axis Lyapunov exponents of a type of chaotic systems with delays，which we call Matrix iteration method. We discuss this new method theoretically by detailed analysis. Also，we introduce another method called Taylor expansion method. The method invented by Wolf is also introduced and we discuss the advantages and disadvantages of these three methods. Finally，we calculate the main-axis Lyapunov exponents of a type of chaotic systems with delay to test these method.
2009, 58 (7): 4421-4425.
doi: 10.7498/aps.58.4421
Abstract +
The spreading dynamics on scale-free networks is considered. Based on the SIS （susceptible-infected-susceptible） model，we investigate the critical point at which the epidemic breaks out in a very general setting. Some previous results，each of which is analyzed，can be seen as the special cases of our result，and the modeling reliability in each case is also discussed.
2009, 58 (7): 4426-4430.
doi: 10.7498/aps.58.4426
Abstract +
The synchronizability of the complex network has aroused much interest during the past years. But most studies pay little attention to the leaf nodes （whose degree equals 1），which do exist in the real networks. In this paper，we study the influence of leaf nodes on the synchronizability of the whole network systematically and find that as the proportion of the leaf nodes increases，the network becomes less synchronizable. Both numerical and theoretical results are given in the paper.
2009, 58 (7): 4431-4438.
doi: 10.7498/aps.58.4431
Abstract +
The dynamics of a compound oscillator with parametric and external excitation has been investigated. Local bifurcation analysis of the first order approximation shows that simple bifurcation as well as Hopf bifurcation may take place，as have been observed in the original system. The influence of several parameters on the dynamics has been explored，which reveals that different nonlinear behaviors can be obtained with the variation of the parameters. Furthermore，by employing global bifurcation theory，the necessary conditions for homoclinic and heteroclinic bifurcation has been presented，which agrees well with the numerical results.
2009, 58 (7): 4439-4448.
doi: 10.7498/aps.58.4439
Abstract +
The bifurcation and chaos should exist in Boost converter while the circuit stability conditions are not satisfied，and the stable state could not be gotten by its self control power. As a kind of OGY method without feedback，the parameter resonance can control the bifurcation and chaos in Boost converter. Based on current controlled Boost circuit，the regulating process and its control power are discussed with the reference value of inductor current as the resonant parameter，and the characteristic parameters in the stable state are also deduced in detail. The control power of parameter resonance with different resonant voltages and the stable characteristic parameters are analyzed for the bifurcation and chaos circuits，so as to realize the precise design of circuit parameters and ensure the Boost converter to work steadily and reliably.
2009, 58 (7): 4449-4456.
doi: 10.7498/aps.58.4449
Abstract +
Based on chaotic system and singular value decomposition （SVD）, a novel zero-bit watermarking algorithm is presented. The proposed approach changed the traditional ways that copyright is protected by modifying the contents of images. Taking advantage of the characteristics of extreme sensitivity to initial values, Logistic mapping is used to find the hiding position of information, which enhances the schemes security. The invariant characters of SVD are applied to make the watermarks of registration center, which make the watermarked image distortion-free. and can protect the copyright effectively. Binary image is selected as watermark image so that the watermarks of existing zero-bit watermarking algorithms are no longer invisible and meaningless. The relationship between watermark capacity and security is also analyzed. Simulation of the algorithm was performed on different types of images, including standard test image, cartoon image, medical image, poem image, remote sensing image and natural scenery image, and the results were compared with existing algorithms. Results show that the propsed algrithm is simple and applicable, and can resist many attacks such as filtering, noising, JPEG compression, and shearing.
2009, 58 (7): 4457-4462.
doi: 10.7498/aps.58.4457
Abstract +
A new four-dimensional continuous-time autonomous hyperchaotic system is proposed in this paper，which is built by adding a nonlinear controller to a three-dimensional continuous-time autonomous chaotic system. The hyperchaotic system is analyzed by studying the Lyapunov exponent spectrum and bifurcation diagram. The system will take on periodic and complex periodic，chaotic and hyperchaotic behaviors with change of the parameter. Also，an electronic circuit is designed and realized，the experimental results verify the conclusions.
2009, 58 (7): 4463-4468.
doi: 10.7498/aps.58.4463
Abstract +
Lag synchronization of spatiotemporal chaos in a weighted network with ring connection is studied in this paper. Spatiotemporal chaos systems are taken as the nodes of the network，and connections between the nodes are made through a weighted ring. According to stability theory，the condition to realize lag synchronization of the network is obtained by identifying the maximum Lyapunov exponent of the network. Lag synchronization exists in the whole network when the largest Lyapunov exponent is less than zero，and the coupling strength between the nodes can be given any weight value. Autocatalytic reaction-diffusion chaotic systems are taken as the nodes of the network，and simulation results show the effectiveness of the method.
2009, 58 (7): 4469-4476.
doi: 10.7498/aps.58.4469
Abstract +
The paper presents a new four-dimensional autonomous hyper-chaotic system by introducing a state variable into the three-dimensional autonomous chaotic system based on studying and analyzing some hyper-chaos. Compared with other hyper-chaotic systems，the bigger positive Lyapunov exponent of the new system is bigger, and the range of hyper-chaos is also wider when the parameter varies. Numerical simulations and bifurcation analysis are done in this paper, and an analog electronic circuit is designed to verify the new system. The experiment results show that various attractors of the hyper-chaotic circuit, including periodic attractor, quasi-periodic attractor，chaotic attractor，hyper-chaotic attractor，are well agree with the simulation results.
2009, 58 (7): 4477-4484.
doi: 10.7498/aps.58.4477
Abstract +
Biometric cryptosystems are widely studied recently, which bind a cryptographic key with the biometric characteristic of a user in such a way that the key cannot be recovered without a successful biometric authentication. By now, the key is generated from extra random number generators, which makes the system more complex. A novel method is proposed, which generates the key from biometric characteristics directly, and makes the biometric cryptosystems much cheaper, simpler and more convenient. To make the keys secure enough, the TD-ERCS is adopted and revised in the proposed algorithm. Experiments show that the algorithm is sensitive to biometric characteristics, free of collision, and the generated keys seem also random. It is believed that such a algorithm can be practically used for various biometric cryptosystems.
2009, 58 (7): 4485-4492.
doi: 10.7498/aps.58.4485
Abstract +
Multiscale entropy is a new method to analyze nonlinear time series on multiple temporal and spatial scales. Firstly，multiscale entropy characteristics of several typical nonlinear series were studied，and then based on this，the fluctuant conductance signals of 144 two-phase flow conditions were analyzed，which were collected by using array conductance sensors in upward vertical gas-liquid two-phase flow. The results indicated that the changing rate of sample entropy at small scales could be used to classify the three typical flow patterns （bubble flow，slug flow and churn flow），and the fluctuation of sample entropy of large scales reflected the dynamic characteristics of each flow pattern. The stochastic characteristic of bubble flow was shown as higher and oscillating sample entropy at large scales; the intermittence of gas slug and liquid slug of slug flow was represented as lower and stable sample entropy of large scales; the unstable and oscillating characteristics of churn flow behaved as the entropy between that of bubble flow and slug flow，and the entropy closed to that of bubble at larger scales. The multiscale entropy analysis of two-phase flow is helpful for understanding the dynamic characteristics of flow pattern transition，and the rate of multiscale entropy is a new indicator of flow pattern identification.
2009, 58 (7): 4493-4499.
doi: 10.7498/aps.58.4493
Abstract +
In this paper，we have studied the nonlinear waves using the Greenberg-Hasting Cellular Automaton model. The dependence of the propagation speed of plane wave on the neighbor radius and the excitation threshold is analyzed with the no-flux boundary condition by computer simulation, and then the excitation condition is obtained. The orbit of the tip of spiral wave is affected by the number of refractory states and the excitation threshold, and then the mechanism is analyzed.
2009, 58 (7): 4500-4505.
doi: 10.7498/aps.58.4500
Abstract +
In this paper，a cellular automaton model is proposed to describe the traffic flow at the grade roundabout crossing with the inner roundabout lane and the outer roundabout lane. Simulation results show that the traffic jam appears evidently at the intersection of the inner roundabout lane and the entering lane. The effect of the nonlinear reciprocity of the vehicle in the inner roundabout lane increases and the flux decreases with the increase of the deceleration probability. The flux of the system is improved by the increase of the turning probability.
2009, 58 (7): 4506-4513.
doi: 10.7498/aps.58.4506
Abstract +
Based on NaSch model, energy consumption in the mixed traffic flow was investigated. The formula of energy consumption was presented. There are three factors of mixed traffic flow to be studied, which are the maximum velocity, length of vehicle, and the ratio of the mixed traffic flow （Cn）, respectively. Simulation and analysis indicated that the energy consumption of the mixed traffic flow consisting of vehicles of the same length but different maximum velocity is associated with the lowest maximum velocity and its ratio. The slower the maximum velocity and its fraction, the more energy consumed. Moreover, for the mixed traffic flow consisting of vehicles with the same maximum velocity bot different length, the energy consumption is related to the ratio Cn of mixed vehicles. Longer vehicles lead to more energy consumption. For the mixed traffic flow involing vehicles of different maximum velocity and lengths, its energy consumption is determined by the ratio Cn of the mixed vehicles and delay probability p, the more the short and fast vehicles（Cn→0）, the less energy consumed, and vice versa.
2009, 58 (7): 4514-4522.
doi: 10.7498/aps.58.4514
Abstract +
Based on the WWH and SDNS cellular automata （CA） traffic model, an improved single lane traffic CA model with consideration of the change of driving rules is proposed. The model considers the drivers adopting different driving rules under mutative traffic condition, and introduces the rules of driving method change. Numerical simulations have been carried out. The results show the complicated evolution process of traffic flow. Different values of change probability have different effect on mixed traffic flow. With the increase of change probability, aggressive vehicles cause greater effect. In the evolution process, the change frequency can be controlled by the definition of the change probability and safety probability. The traffic flow is higher in the new model than in the NS and SDNS model. This proves that the stop status of the whole traffic flow has been improved.
2009, 58 (7): 4523-4530.
doi: 10.7498/aps.58.4523
Abstract +
Pedestrian facility planning and pedestrian flow management and control depend on the theory of pedestrian flow. Simulation of pedestrian evacuation flow is presented based on cellular automata （CA） in this paper. Two dynamic parameters are formulated to simplify tactically the decision-making process of pedestrians, which can reflect the pedestrian judgment on the surrounding conditions and decide the pedestrian’s choice of action. The relationships of evacuation time against pedestrian density and exit width for different sizes of systems are studied. Moreover, the effect of exits arrangement on evacuation time is analyzed. It is observed that there is a linear relationship between evacuation time and pedestrian density, however, there is a negative exponential one between evacuation time and exit width. And exits arrangement affects pedestrian evacuation time.
2009, 58 (7): 4531-4535.
doi: 10.7498/aps.58.4531
Abstract +
In this paper the temporal evolution processes of large numbers of metallic balls under high voltage were observed using a PC-camera. The fractal dimensions of the stable agglomerates of metallic balls were calculated by means of the sandbox method. Also, the relationship between the fractal dimension and the voltage were studied. The result showed that the stable agglomerates of metallic balls can have dendritic fractal structures through a self-organization evolution process and the fractal dimension decreases with the increasing of voltage. The results are of value for the growth mechanism and morphology study of dissipative structure.
2009, 58 (7): 4536-4541.
doi: 10.7498/aps.58.4536
Abstract +
A convenient 3ω method was developed to measure the thermal conductivity of multiwalled carbon nanotube/polypropylene （MWCNT/PP） composite，in which the metal strip heater was substituted for a platinum wire，and the commercial lock-in amplifier was replaced by a Labview-based virtual measurement system. The experimental result showed that，the thermal conductivity of multiwalled carbon nanotube/styrene-butadiene rubber/polypropylene （MWCNT/SBR/PP） ternary composite varied slightly with MWCNT/SBR powder loading; the thermal conductivity of MWCNT/PP composite increased with the increasing MWCNT loading; the composite thermal conductivity was far less than that expected from the law of mixtures，but in good agreement with the effective medium approach by incorporating the thermal interfacial resistance between the MWCNT and the surrounding polymer matrix.
NUCLEAR PHYSICS
2009, 58 (7): 4542-4547.
doi: 10.7498/aps.58.4542
Abstract +
A new viewpoint is proposed for understanding the quantum phase transition from a higher-ordered excitation model to other lower-ordered one, which suggests that a nucleus populated at high-momentum state decouples firstly to the yrast-band with the E2-trasitions mode, and then to the coexist region, wherein the structure-energy is released, the change in the nucleon coupling intensity ensues, finally the basic state of the lower-ordered excitation model is reconstructed, the evolution of basic states structure is accomplished. Therefore this is a smooth and calm transition. Using an example of the 76 Sr nucleus, the problem is discussed carefully.
Experimental investigation of L-band bifrequency magnetically insulated transmission line oscillator
2009, 58 (7): 4548-4555.
doi: 10.7498/aps.58.4548
Abstract +
Experimental investigation of an L-band bifrequency（BF） magnetically insulated transmission line oscillator（MILO） based on azimuthal partition of cavity-depth, is carried out for the first time. Experimental system and measurement system of the BFMILO are set up. Far-field patterns of the antenna of the L-band BFMILO are obtained, and the total microwave power is obtained by integral of the power density of radiation field. Employing an electron beam of 420kV, 34kA, an L-band BFMILO can generate high power microwave （HPM） with two stable and separate frequencies. They are 1.26GHz and 1.45GHz respectively with power of 398MW and 222MW. Preliminary experimental results of BFMILOs with different azimuthal partition proportion are presented. The investigation proves the feasibility that BFMILO can generate HPM with two stable frequencies, which indicates an L-band BFMILO is developed successfully for the first time. A new direction of HPM investigations is pointed out.
ATOMIC AND MOLECULAR PHYSICS
2009, 58 (7): 4556-4562.
doi: 10.7498/aps.58.4556
Abstract +
The interaction of diethyl ether clusters and laser was studied at laser intensity of 1011 W/cm2 using 5 ns, 1064 nm laser pulses. Multiply charged ions O6+ and C4+ were observed and the ratio of O6+/C4+ nearly kept constant in the range of laser intensity studied. The energy distribution of electrons ejected was measured via retarding potential method. At the laser intensity of 4.0×1011 W/cm2, the mean electron energy of 56eV and the maximum electron energy of 102eV were observed. It indicates that the multiply charged ions are mainly produced from energetic electron impact ionization. In addition, our calculated results show that the screening effect inside cluster can significantly decrease the ionization potentials of multiply charged ions, thus facilitating the ionization especially at low laser intensity. The above results further support our three stage model of “multiphoton ionization ignition-inverse bremsstrahlung heating-electron impact ionization”.
2009, 58 (7): 4563-4566.
doi: 10.7498/aps.58.4563
Abstract +
The Bi0.9Ba0.1Fe0.85Mn0.15O3 ceramic sample was prepared by sol-gel process. XRD analysis showed that it was the single-phase rhombohedral perovskite structure. A weak ferromagnetic and ferroelectric were obtain from the hysteresis loops. The dielectric constant which increased with an increase in the magnetic field showed that the Ba-Mn codoped BFO has greater magnetocapacitance effects.
2009, 58 (7): 4567-4572.
doi: 10.7498/aps.58.4567
Abstract +
The generalized gradient approximation （GGA） based on density functional theory（DFT）is used to analyze the structure, bonding and magnetic properties of the transition metal atom doped endohedral fullerene Ni@C20H20. The geometric optimization shows that the Ni atoms situated at the three possible positions off the cage center are all drawn back to the cage center. The binding energy and the energy gap calculations tell us that the cage center is the most thermodynamically and dynamically stable site for the Ni atom. It is discovered from the bonding analysis that the interaction between Ni and the cage could be neglected and the Ni atom keeps its atomic character. The electronic and magnetic properties reveal that the Ni atom with 2μB magnetic moments has no magnetism, while the cage with no magnetism still has no magnetic moment, mainly due to the zero electron transference between the Ni and the cage.
CLASSICAL AREA OF PHENOMENOLOGY
2009, 58 (7): 4573-4578.
doi: 10.7498/aps.58.4573
Abstract +
A novel finite-difference time-domain （FDTD） method combined with the semi-analytical recursive convolution （SARC） algorithm in DSP techniques for the analysis of electromagnetic characteristics in dispersive media is presented. In this method, the flexibility of FDTD in dealing with complicated objects is retained, while the advantages of absolute stability, high accuracy, low storage and high effectiveness of SARC in treating the linear system problem is introduced. The proposed SARC FDTD approach can therefore be applied to the analysis of general dispersive media provided the poles and corresponding coefficients in the dispersive medium model of interest are given. Three typical kinds of dispersive model, i.e. Debye, Drude and Lorentz medium are tested to demonstrate the feasibility of the present scheme.
2009, 58 (7): 4579-4589.
doi: 10.7498/aps.58.4579
Abstract +
The mixed-potential integral equation （MPIE） has been usually employed in numerical approach of electromagnetic scattering of the object, such as the method of moment （MoM）, due to its low-level singularity of vector and scalar potential Green functions. When an object is embedded partially in dielectric half-space medium, the Green’s function contains the Sommerfeld-type integrals, which embody the effect of the dielectric interface on scattering fields. Using the discrete complex image method （DCIM） and the Sommerfeld identity, the Sommerfeld integrals can be evaluated as the summation of finite complex image functions without directly numerical integration which always consumes large CPU time. As the points of the field and source are co-located in the same side of the interface, the spectrum function g（kzp） is not related with the field or source positions, and the complex image parameters fitted with the general pencil of functions （GPOF） method are approximate for all positions. However, if the points of the field and source are located, separately, in different sides of the interface, the spectral function is now related to z and z′. Generally, the GPOF is repetitiously used to find the complex image parameters for every z and z′, which consumes large CPU time and memory. This paper presents a novel method of Dual GPOF combining with DCIM for fast computation of the Sommerfeld integral. Firstly, the factors related with the variable z are separated, and the GPOF is used to find the complex image parameters for finite discrete source points z′l. Secondly, GPOF is used again to fit the relationship of each complex image parameter with variable z′. Then, the complex image parameter of any z′ can be evaluated as the direct function summation, and there is no need to perform GPOF for all z′ points. Comparing numerical values of Dual GPOF, point-by-point GPOF, and direct numerical integration, the Dual GPOF method is proved effective and efficient. Finally, Dual GPOF is applied to computation of electromagnetic scattering from a P.E.C. sphere object partially embedded in dielectric half-space, and the scattering patterns are presented and analyzed.
2009, 58 (7): 4590-4597.
doi: 10.7498/aps.58.4590
Abstract +
In this paper, an efficient new method named wave concept iterative process （WCIP） based on transformation between space domain and spectral domain is introduced. The derived WCIP theory formulation is presented in detail. A new kind of LTCC multilayer structure is analyzed using the iterative method. The approach involves the mixed magnetic and electric field equation and S parameter extraction technique based on a simple form of matched load simulation. The whole process is illustrated by MATLAB programming, so that frequency response of S parameter is determined. The numerical results are compared with the simulation results and good agreement shows the validity and usefulness of the iterative method given in this paper.
2009, 58 (7): 4598-4604.
doi: 10.7498/aps.58.4598
Abstract +
There are many advantages for frontally located beam smoothing elements in terminal optical system of the inertial confinement fusion （ICF） driver, but it will also exert an influence on the working state of the frequency conversion system at the same time. This paper studies the influence from the viewpoints of system integration and optimization. Based on the non-linear transmission theory of high power laser and adopting coupled-wave differential equations and perturbation theory, we calculate the third harmonic conversion efficiency, optical characteristics in near and far field as well as the shape of focal spot when CPP is laid before the frequency conversion system. Then we put forward the method for the optimization of detuning angle and thickness of the frequency conversion crystal, in order to reduce influence on ICF system when CPP is laid before the frequency conversion system. The third harmonic conversion efficiency, contrast ratio and encircled energy before and after optimization are compared and analyzed. The results show that the optimization of frequency conversion system is very effective in reducing the impact on the beam quality and the third harmonic conversion efficiency when CPP is laid before the frequency conversion system.
2009, 58 (7): 4605-4609.
doi: 10.7498/aps.58.4605
Abstract +
In the experiment for studying the nature of coupling between suface plasmon （SP） wave and terahertz radiation by subwavelenth-sized brass rods, we found that the polarization of the THz field, size of rods, and slit between rods critically affect the coupling efficiency. By the method of Fourier transform of the time domain data, we found the coupling frequency in THz range. With equal slits between the brass rods, we demonstrated thar the transmission resonance is attributed to the combined effects of surface plasmon and cavity mode resonance when the electric vector is perpendicular to the grating wires, while the cavity mode resonance dominates the resonance transmission when they are parallel.
2009, 58 (7): 4610-4617.
doi: 10.7498/aps.58.4610
Abstract +
Photoacoustic imaging combines advantages of both ultrasound imaging and optical imaging to provide high ultrasonic resolution and high optical contrast in noninvasive medical images. A photoacoustic computer tomography （PCT） system with a focused digital phased array transducer is proposed. In particular each element of the transducer array has a thin cylindrical ultrasonic lens to select 2D image plane and suppress the out-of-plane signals to realize photoacoustic computed tomography. A fast full-view photoacoustic imaging system for visualizing tissue structures using a linear transducer array with combined scan, whies; detects photoacoustic signals by combined electronic and mechanical scanning. An improved limited-field filtered back projection algorithm with directivity factors was applied to reconstruct the optical absorption distribution and further improve the lateral resolution. In the experiment, the spatial resolution of this imaging system is determined as 0.2mm, and the slice resolution of Z axis is 1.5mm. Mechanical step scanning was performed, leading to a measurement time of 6.4 seconds per position, and a total scan duration of 150 seconds to obtain the two-dimensional photoacoustic image. High spatial resolution of 2D photoacoustic vascular image and quasi-3D photoacoustic CT for breast cancer imaging are achicned. The experimental results demonstrate that this method and imaging system can realize the high resolution photoacoustic tomography wtich can be applied to noninvasive detection of early breast cancer.
2009, 58 (7): 4618-4623.
doi: 10.7498/aps.58.4618
Abstract +
Terahertz （THz） wireless communication system, which exhibits great potential in the future short-range wireless communications, is known to be a kind of ultra-broadband system with high data rates. THz sources, modulators and detectors are key elements in these systems, while in order to maintain high-quality connection, the THz reflectors are also very important. Here, a reflector which is a kind of one-dimensional photonic crystal made of polyvinylidene fluoride （PVDF）/polycarbonate （PC） is studied carefully. According to the refractive indices of materials in the THz range, the reflection coefficients for THz waves are obtained through transfer-matrix method and Fresnel formula calculation. We demonstrate that, in sub-THz waveband, this device shows high performance and wide reflection band. Furthermore, based on the calculation, the research and the design of THz reflectors are also discussed in this article.
2009, 58 (7): 4624-4629.
doi: 10.7498/aps.58.4624
Abstract +
The analytical expressions for the mean-squared beam width, the far-field divergence angle and the far-field radiant intensity of Gaussian Schell-model （GSM） array beams are derived, and the condition under which GSM array beams have the same far-field divergence angle as a single Gaussian beam. It is shown that GSM array beams and a Gaussian beam which have the same far-field divergence angle may have different far-field radiant intensity distribution, which is quite different from the behavior of GSM beams.
2009, 58 (7): 4630-4634.
doi: 10.7498/aps.58.4630
Abstract +
We investigate the influence of Kerr medium on the concurrence of the two-atom system under the condition of existence of dipole-dipole interaction. The entanglement of the system is calculated by making use of concurrence. We discuss the variation of the concurrence with time. The result shows that the appropriate choice for the nonlinear coupling constant χ of Kerr medium can improve the concurrence of the system and restrain effectively the intrinsic decoherence of the system. We also find that with respect to increasing concurrence, the multiple Λ may take different values as Ω varyies.
2009, 58 (7): 4635-4640.
doi: 10.7498/aps.58.4635
Abstract +
The squeezing properties of binomial state field interacting with Bose-Einstein condensate of two-level atoms are investigated within and without rotating-wave approximation by means of quantum theory. The influences of atomic eigenfrequency, the coupling constant between light field and atoms，the parameter of light field and the virtual photon field on squeezing properties have been discussed. The results show that two quadrature components can be squeezed periodically, and the duration of squeezing light has something to do with atomic eigenfrequency ω, and the degree of squeezing with the parameter of the binomial states field η and the virtual photon field. The frequency of light fluctuations collapse and revival is determined by the coupling constant between light field and atoms.
2009, 58 (7): 4641-4646.
doi: 10.7498/aps.58.4641
Abstract +
To obtain narrower linewidth and more intense THz radiation, long-pulse and high-peak power optical pumping source is attempted by performing a light amplification experiment in which a 6 W tunable continuous-wave CO2 seeding laser is amplified by a double-pass amplifier with TEA CO2 laser. The result of an amplification factor of 30 at 14 μs pulse duration （FWHM） is obtained experimentally. During the amplification, the output power of the amplified light increases with that of seeding light increasing, especially for the CO2 10P （20）line. Based on the optical pumping source of long pulse laser, a dynamic model for generating pulsed THz radiation is established, and the expression of saturation pumping intensity which yields THz radiation is given, thus the range of pumping intensity can be determined. In addition, by solving time-dependent equation of dynamic model, the expression of pumping energy absorbed by THz medium is deduced. Theoretical analysis shows further that the heavy molecules will be more appropriate for THz gain medium for long pulse pumping source, and no THz radiation can be generated due to the absence of population reversion below pumping intensity.
2009, 58 (7): 4647-4651.
doi: 10.7498/aps.58.4647
Abstract +
High-power and high efficiency continuous wave green laser has been realized by optimizing the plane-concave-plane resonator of three mirrors in an laser diode side-pumped configuration and KTP of Ⅱ-type phase-matching for frequency doubling. A maximum of 36.6 W continuous wave 532 nm green laser is obtained at pumping current of 36 A, corresponding to optical-to-optical conversion efficiency of 8.71％. The power stability of the laser is measured at output power of 33 W. During a half hour measurement time, the output power instability is 0.27%. The beam transfer factor is less than 8 measured by knife-edge method at output power of 33 W. Astigmatism compensation, misalignment sensitivity and mode profile inside resonator of three mirror folded resonator are also numerically simulated under high pump power.
2009, 58 (7): 4652-4658.
doi: 10.7498/aps.58.4652
Abstract +
Porous silicon can be fabricated by laser irradiation and annealing. The center wavelength of photoluminescence （PL） band is pinned in the region of 700—780 nm and its intensity increases obviously after oxidation of the sample. It was found that the PL intensity changes with time of laser irradiation and annealing. Calculation shows that some localized states appear in the band gap of the smaller nanocrystal when SiO bonds or Si—O—Si bonds are passivated on the surface. It was discovered that the number of SiO bonds or Si—O—Si bonds, which are related to irradiation and annealing time, obviously affects the generation of localized gap states.
2009, 58 (7): 4659-4663.
doi: 10.7498/aps.58.4659
Abstract +
We have developed a high power tunable single-frequency 1064 nm quasi-continuous-wave laser oscillator-amplifier system. By controlling precisely the temperature of etalon in the three-mirror ring traveling-wave oscillating cavity viz. adjusting refractive index and thickness of material to tune laser wavelength, 13.2 W 1064 nm single-frequency quasi-continuous-wave oscillating light with tunable range of 58 GHz is obtained, at repetition rate of 1 kHz and duty ratio of 10%. After the laser beam from the oscillator transfers through the amplifier with two laser heads, the output of the oscillator is amplified to be 44 W and the beam quality factor M2 is measured to be 1.58.
2009, 58 (7): 4664-4668.
doi: 10.7498/aps.58.4664
Abstract +
We use nonlinear polarization rotation technology to realize the self-start passive mode-locking in the erbium-doped fiber ring laser of positive dispersion, which can generate a new kind of gain-guided soliton pulse. The laser generates picosecond pulses with a rectangular-shaped spectrum, the FWHM width of which can be greater than 19 nm. The gain-guided soliton formation mechanism may include laser gain, spectral filtering effect, cavity dispersion and other fiber nonlinear effects. In this paper we also study the chirp property, peak power and other parameters of the pulse. With further increasing the pump power，the laser can operate on dual-pulse output mode.
2009, 58 (7): 4669-4674.
doi: 10.7498/aps.58.4669
Abstract +
Based on the framework of the spin-flip model （SFM）, the synchronization performance of two unidirectionally coupled vertical-cavity surface emitting lasers （VCSELs） subject to negative optoelectronic feedback has been theoretically investigated. The results show that, with the increase of the feedback intensity, the laser output power displays two different chaos regions, i. e., singles-polarization mode region and mixed mode region. The polarized mode in the single mode region can achieve good synchronization, but sharply decline in the quality of synchronization close to the mixed mode region. Within the mixed-mode region, the continuity and quality of synchronization of polarized modes are not ideal. The fact that the synchronization quality of the mixed mode is worse than that of the single mode, is on the contrary to reported results based on optical feedback. The reason may be due to that negative optoelectronic feedback has induced the competion between the polarized modes. In addition, we have found changes in internal parameters have a great impact on the selection of two chaotic regions.
2009, 58 (7): 4675-4679.
doi: 10.7498/aps.58.4675
Abstract +
A non-collinear laser beam combination based on Brillouin amplification was proposed. The influence of non-collinear Brillouin amplification on the combination efficiency was analyzed and discussed theoretically. It was shown that the efficiency of 80% can be achieved with the angle between the Stokes and the pump limited in the range of 90 mrad. According to the theoretical results, a four-beam non-collinear combination experiment was designed and carried out. The output energy of 189 mJ at 10 Hz was achieved with the energies of the four pumps being 70.4, 71.3 , 78.9 and 70.1 mJ， respectively. In order to simplify the scheme and reduce the optical loss, a construction of SBS cell for non-collinear combination of multiple beams was also designed.
2009, 58 (7): 4680-4684.
doi: 10.7498/aps.58.4680
Abstract +
The amplification of the signal of stimulated Brillouin scattering （SBS） is widely used in many area. A method for amplifying the signal of SBS using dual collinear beams in a single optical cell is introduced. The stimulated Brillouin scattering can be induced and amplified in one optical cell. Comparing with previous double-cell technique, the proposed method has the advantages of simple structure and low energy loss. It is necessary for some applications with weak signal of stimulated Brillouin scattering such as lidar for remote sensing, in which double-cell technique is not suitable.
EDITOR'S SUGGESTION
2009, 58 (7): 4685-4688.
doi: 10.7498/aps.58.4685
Abstract +
In the LiNbO3 nonlinear photonic crystals with a two-dimensional octagonal quasilattice, the collinear quasi-phase-matched second harmonic generation processes at multiple wavelengths were observed. The maximum conversion efficiency was 36%. Meanwhile, for the fist time we measured the change of the second harmonic conversion efficiency with the temperature and incident wavelength. The results show that the second harmonic has broader temperature and wavelength tuning bandwidth in long waveband. It is believed the study might be significant for the application of quasi-periodic nonlinear photonic crystal in practice.
2009, 58 (7): 4689-4696.
doi: 10.7498/aps.58.4689
Abstract +
In this paper, we use the amplification transfer function （ATF） to numerically calculate and analyze the spectral properties of parametric fluorescence （PF） via spontaneous parametric down-conversion. The results show that the output angle of single-wavelength PF which is nondegenerate is amplified and the whole spectrum of PF gradually becomes weak and has a continuous distribution. About the dense distribution of the broadband spectrum constituents near the degenerate state, we select two different pump pulses to numerically calculate it. With respect to the sensitive character of the phase-matching angle near the degenerate state, we use ATF and the spectrum probability distribution function （SPDC） to calculate and analysis it individually, the two results are consistent with each other.
2009, 58 (7): 4697-4701.
doi: 10.7498/aps.58.4697
Abstract +
In this paper a method is presented for the first time, witch can match completely group velocities of three pulses in femtosecond optical parametric amplification （OPA） of the KBe2BO3F2 （KBBF） crystal. We combine noncollinear configuration with tilting pulse front of pump, and collinear configuration with tilting pulse front of pump and signal, in type-Ⅰ and type-Ⅱ phase matching configurations respectively, to realize complete match of group velocities of three pulses in maximal parametric bandwidth of the femtosecond OPA, thus can obtain parametric pulses with higher gain and shorter duration.
2009, 58 (7): 4702-4707.
doi: 10.7498/aps.58.4702
Abstract +
We investigated by time-of-flight technique the slow light and superluminal effects in high-quality three-dimensional （3D） colloidal photonic crystals （PCs） with opal structure, fabricated by pressure controlled isothermal vertical deposition technique. They produce high transmission in the pass bands and sharp band edges due to low volume density of defects and dislocations. The variation of the group velocity across the short-wavelength band edge was measured. Group velocity as low as 0.43c was observed near the band edge while group velocity as high as 1.34c was found in the band gap. We have also simulated the transportation of ultrashort pulses through 3D PCs and compared the simulation results with the experimental ones.
2009, 58 (7): 4708-4716.
doi: 10.7498/aps.58.4708
Abstract +
The method of lines is used to analyze the influence of dielectric substrates on uniplanar EGB structure. The analysis process is introduced. According to periodic boundary conditions, one unit cell has been isolated as the calculation region, and the method of lines is used to get the propagation equation of potential function in different interfaces of medium layers. By combining the conversion relationship between field component and potential function on the continuity condition of the tangential field components in interfaces, eigen value equations are established, and the frequency band diagrams of eigen mode are obtained to describe the surface wave bandgap of EBG structure. The method is used to do a series of calculations on surface wave bandgap with dielectric substrates of different dielectric constants and different thicknesses. Through analysis of calculation results, six conclusions are obtained on the influence of dielectric substrates, which can provide some references for the design of uniplanar EGB structure.
2009, 58 (7): 4717-4723.
doi: 10.7498/aps.58.4717
Abstract +
Performances of a 20-element bimorph deformable mirror are measured and a closed-loop adaptive optics system, which consists of the bimorph mirror and a Shack-Hartmann wave-front sensor with 13×13 sub-aperture array, is established to test the mirror’s capability of correcting static aberrations. Experimental results show that, the mean displacement response to voltage of discrete actuators is 0.57 μm/100 V and that of defocus electrode is 1.09 μm/100 V. The hysteresis of actuator is limited to ten percents of the maximal displacement. With the ambient temperature increasing from 13℃ to 25℃, the P-V of defocus of the mirror surface is enlarged by 1.37 μm and the P-V of other distortions, such as astigmatism, coma and spherical aberration, is enlarged by 0.15 μm. The frequency response of the bimorph mirror is about 4 kHz. When the closed-loop control is on, intensity distributions at the image plane of distorted wave-fronts are effectively improved. The Strehl ratios increase from 0.02 to 0.95, and 0.03 to 0.54， respectively.
2009, 58 (7): 4724-4730.
doi: 10.7498/aps.58.4724
Abstract +
A simplified fluid-mechanics model is developed for the drawing of silica capillaries. Calculations are carried out to predict the influence of the technical parameters on the final geometry of the capillaries. Geometrical collapse is inevitable when drawing capillaries, and active control of the parameters only helps to some extent in tuning the final structures. Drawing experiments were performed to check the theoretical calcualtions. Good agreement was obtained, which certified the validity of the model. The predictions provide valuable practical information for the fabrication of photonic crystal fibers.
2009, 58 (7): 4731-4737.
doi: 10.7498/aps.58.4731
Abstract +
Many numerical approximations are introduced in solving the general nonlinear Schrdinger equation （GNLSE） using the split-step Fourier method （SSFM）. In addition, careful consideration of the spatial and the temporal step sizes and the temporal window is needed to ensure the numerical precision. Supercontinuum generation in photonic crystal fiber is analyzed to show the improvement and precision control. The nonlinear term is treated directly by an integral instead of any mathematical approximation. Then the integral term can be transformed to a convolution to be solved by the Fourier transform, which facilitates the accurate numerical computation of the nonlinear term. High precision is ensured since no factitious approximation is introduced. Computation precision with respect to the step size choice is analyzed. Criteria are proposed as follows. The temporal and spatial step sizes can be appropriately chosen from the spectrogram, and the temporal figure provides a criterion for the temporal step size. These conclusions present direct criteria for the difficuet job of step size choice in solving the GNLSE.
2009, 58 (7): 4738-4745.
doi: 10.7498/aps.58.4738
Abstract +
The structure of asymmetric LPFG with rotary refractive index modulation （R-LPFG） along the fiber axis is proposed and fabricated by using high frequency CO2 laser pulses. The transverse refractive index distribution of LPFG with asymmetric refractive index modulation in the cross-section is divided into multilayer circular waveguides, which are then discretized according to the magnitude of index. The mode coupling characteristics of this asymmetric LPFG is discussed by adopting coupled-mode equation sets and the revised Bragg condition. It’s observed that the transmission spectrum has different features depending on the rotation angle. For small angles, resonance peak is unique, however, when the angle becomes greater, the resonance peak will be split into two, which is different from the transmission spectrum of previous LPFGs. Moreover, the two split peaks have almost the same temperature sensitivity because they originated from the same two coupling modes. The high sensitivity torsion meter and strain meter without temperature compensation can be designed by adopting the wavelength spacing between the two split peaks.
2009, 58 (7): 4746-4751.
doi: 10.7498/aps.58.4746
Abstract +
Ultrasound radiation force has been shown promising in facilitating ultrasonic molecular imaging and target drug delivery. A chirp excitation technique is developed to improve the translational displacement of microbubbles in this study. Based on a modified RP equation and a particle trajectory equation, we investigate the dependence of the translation of microbubble clouds on the chirp parameters （center frequency and frequency shift） and bubble radius distribution. Results demonstrate that the chirp excitation performs better than the traditional sinusoidal excitation in displacing widely distributed microbubbles and bubble clouds with radius far from resonant size. For example, for Gaussian size-distributed microbubble clouds with mean radius 3.5 μm and variance of 1, a 1 MHz center frequency chirp with frequency range 0.75 MHz induces about 12% more microbubbles displaced over a distance of 30 μm during 200 μs insonification, compared with a 1 MHz sinusoidal excitation with equal acoustic pressure.
2009, 58 (7): 4752-4759.
doi: 10.7498/aps.58.4752
Abstract +
The band gap of two-dimensional metal tube structure with multi-width narrow slits is simulated and verified experimentally. The unique properties of the stop band exist in the band gap structure with combined parameters of slit tubes. It is found that the band gap of this structure is large and the starting frequency is low. In contrast to the band gap of the structure with narrow slits, the stop band of the combined structure of the parameters of the slits is continuous and large.
2009, 58 (7): 4760-4765.
doi: 10.7498/aps.58.4760
Abstract +
Photoacoustic imaging is a novel noninvasive imaging technique of biological tissue that overcomes the overwhelming scattering of light in biological tissue by utilizing ultrasonic wave to translate the image signal. In this study, near-infrared light and indocyanine green as an optical contrast agent were used to achieve the cerebrovascular distribution by photoacoustic imaging. Near-infrared light penetrates deep into the brain tissues through the skin and skull, and the indocyanine green enhances the photoacoustic signals of the blood vessels. A photoacoustic angiography of vascular distribution in the mouse brain in vivo was acquired that matches the anatomical photograph well. The experimental result demonstrates the photoacoustic imaging with near-infrared light and optical contrast agent has the potential for functional and molecular biomedical imaging.
2009, 58 (7): 4766-4771.
doi: 10.7498/aps.58.4766
Abstract +
Since the heat transfer irreversibility of a heating or cooling process should be measured by the entransy dissipation rate, an equivalent thermal resistance for a heat exchanger was defined based on the entransy dissipation rate of the heat exchanger. The equivalent thermal resistance includes both the overall heat transfer resistance and the extra thermal resistance caused by the non-counter-flow arrangement and the non-equilibrium heat capacity rate ratio of a heat exchanger. The reciprocal of the equivalent thermal resistance was defined as the equivalent thermal conductance. The relationship is established between the heat transfer irreversibility and the effectiveness of a heat exchanger in terms of the equivalent thermal resistance. Finally, a formula describing the relation among the effectiveness, the equivalent thermal resistance and the heat capacity rate ratio is obtained which does not depend on the different flow arrangements. Therefore the effectiveness-thermal resistance （conductance） method is more suitable for the performance comparison of different heat exchangers.
2009, 58 (7): 4772-4778.
doi: 10.7498/aps.58.4772
Abstract +
The influence of void configuration on void coalescence in single crystal copper under shock loading along ［100］ direction has been investigated with molecular dynamics （MD） simulation. The results reveal that the voids collapse and grow by means of emission of shear dislocation loops. In the tension stage, the voids first grow independently, then the plastic zone around the voids begin to interact and overlap, leading to the void coalescence. The pattern of void coalescence observed in our simulations coincides with the microscopic experimental results. We calculated for four different configurations, characterized by the angle （θ） formed by shock direction and the line connecting the two centers of voids, and found that among the four configurations，the coalescence of the voids prefers to occur where θ is 60°. Based on the resolved shear stress model around the void, we can clearly explain the simulation results.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2009, 58 (7): 4779-4786.
doi: 10.7498/aps.58.4779
Abstract +
Experimental study of aluminum wire array Z-pinch was conducted on Qiangguang-I facility （1.5 MA, 80 ns） to produce —1.7 keV characteristic X-ray radiation. With fixed wire diameter of 20 μm and annular diameter of 12 mm, K-shell yield up to 0.9 kJ/cm and 1.1 kJ/cm was obtained for wire number N=8 and N=12 arrays, respectively, which are much higher than that for N=16 and N=24 loads. X-ray power waveforms and time-resolved images showed that several implosions occurred due to the existence of trailing mass. During 60%—80% of the whole implosion time，the wire array almost stayed at its initial position rather than running inwards. Main implosion was completed in the next 25—30 ns, with parts of load mass left behind and forming so-called trailing mass. At later time of main implosion, load current was transferred to trailing mass left in outer radii, and caused secondary even tertiary implosion, which also contributed to total K-shell yield. Appearance of trailing mass might be caused by azimuthally correlated axial modulation, consisting of hot spots and gaps on wire plasma column, as well as the development of other inhomogeneities.
2009, 58 (7): 4787-4792.
doi: 10.7498/aps.58.4787
Abstract +
A weakly nonlinear model is proposed for the Kelvin-Helmholtz instability by expanding the perturbation velocity potential to third order. It is found that there is an important resonance in the process of mode coupling. This resonance makes the coupling processes very complex and interesting. Single-mode perturbation enters nonlinear stage quickly and produces lots of harmonics. The resonance reinforces the action of nonlinear process. The second and third harmonic generation efficiency of a single-mode disturbance is computed, as well as the nonlinear correction to the exponential growth of the fundamental modulation. Our simulations support the weakly nonlinear results from our analytic model. The nonlinear threshold phenomenon is also analyzed.
2009, 58 (7): 4793-4798.
doi: 10.7498/aps.58.4793
Abstract +
The effect of applied magnetic field on the plane wave attenuation by microwave plasma plume inside a closed and evacuated quartz vessel is studied though spatially propagating wave method. The objective is to analyze how the operating parameters affect the wave attenuation in plasma and to find the mechanism which controls the interaction between plasma and plane wave. The experimental results show that under the present experimenal condition and in the cases without and with applied field, collisions are the reason leading to plane wave attenuation in the plasma. Plasma resonance absorption and upper hybrid resonant absorption mechanisms can not be found. To increase the plane wave attenuation in magnetized plasma, magnetic flux density should be set to make the wave test frequency and upper hybrid frequency identical.
2009, 58 (7): 4799-4805.
doi: 10.7498/aps.58.4799
Abstract +
According to the interaction cavity structure of high-power microwave sources（HPM）, using magnetically insulated transmission line oscillator （MILO） as example, a one-surface multipactor discharge model is presented. Through the calculations of the susceptibility curves and the secondary electron evolution by using statistics and Monte Carlo simulation method, we analyzed the influence of the parameters of RF on multipactor discharge. Finally, measures for reducing and inhibiting multipactor discharge are proposed.
2009, 58 (7): 4806-4811.
doi: 10.7498/aps.58.4806
Abstract +
A new type of hexagonal superlattice pattern is observed under three different boundary conditions by using a dielectric barrier discharge device with water electrode. The power spectrum of hexagonal superlattice pattern and its angular distributions are shown. The phase diagram of the hexagonal superlattice pattern as a function of air concentration and applied voltage is obtained. The correlation measurements indicate that the hexagonal superlattice pattern is an interllacing of two different transient sublattices. The selection of pattern formation in quadrangular boundary with different area is studied. It is found that the hexagonal superlattice pattern is a typical type of nonlinear self-organized pattern, and the formation of square superlattice pattern is dependent upon the boundary conditions.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2009, 58 (7): 4812-4816.
doi: 10.7498/aps.58.4812
Abstract +
Based on the theoretical analysis, we performed finite element simulation of high pressure experiment in multi-anvil devices （XKY-6×2000 MN） to determine the temperature and the stress in WC anvil. The simulated results show that the temperature is not uniform, and the ununiform temperature can produce thermal-shear stress in the anvil. The peak value of thermal-shear stress lies on the beveling edge of the anvil, the peak value of thermal-shear stress is 0.62 GPa, which is 18 percent that of unheated anvil. The simulation results have been verified by high pressure synthesis experiment. We developed a new method to simulate the stress of WC anvil at high pressure and high temperature. This would greatly help the design of anvil with safe dimension.
2009, 58 (7): 4817-4825.
doi: 10.7498/aps.58.4817
Abstract +
The rapid solidification of Al50Mg50 alloy melt has been simulated by using molecular dynamics method, and the evolution mechanism of microstructures at different levels during the rapid solidification process have been deeply studied with the pair distribution function, the bond-type index method and the cluster-type index method. It is demonstrated that the simulated Faber-Ziman partial structure factors are in good agreement with the experimental results. The Al50Mg50 alloy melt possesses heredity. The Al50Mg50 amorphous structure is formed during the rapid solidification process, and the icosahedron short-rang-order structure plays a critical role in the formation of amorphous structure. The atomic cluster analyses based on the cluster-type index method shows that the microstructures of Al50Mg50 amorphous alloy is built from some different types of short-rang-order structures, and the icosahedron is the main short-rang-order structure. There are various cluster structures with different sizes in the short-rang-order regions. But it can not be described with the Bernal model.
2009, 58 (7): 4826-4834.
doi: 10.7498/aps.58.4826
Abstract +
The molecular dynamics method and quenching technique combined with the Gupta potential and the Sutton-Chen potential are used to study the melting properties of Irn （n=13, 14, 55, 56, 147 and 148） clusters. Our results show that the structures and melting behaviors of these clusters described by both potentials are generally in agreement with each other: the same ground state geometrical structure and the same trend of variation of the melting point and the pre-melting interval with cluster size are obtained for both potentials, and for the small Irn （n=13,14） clusters, the peak value of heat capacity is delayed from the saturation value of Lindemann index. While, for each of these iridium clusters, the melting point from the Gupta potential is higher than that from the Sutton-Chen potential, and we attribute the main reason to the higher energy gaps between the ground state and the first excited state of these iridium clusters from the Gupta potential than those from the Sutton-Chen potential.
2009, 58 (7): 4835-4839.
doi: 10.7498/aps.58.4835
Abstract +
Molecular statics simulation was used to study the yielding mechanism of 〈100〉 oriented single crystal copper nanowires under tension. The results show that, the yield of nanowires is initially realised via {111}〈112〉 twinning from random activation of partials, and subsequently via slip of {111}〈112〉 partial dislocation; in addition, the partial dislocations on different planes successively slip and meet to form stair-rod dislocation at the initial stage of plastic deformation, while some partial dislocations are piled up near the rigid boundary at the later stage of plastic deformation, both of which hinder dislocation slip, resulting in strengthening of the single crystal copper nanowires.
2009, 58 (7): 4840-4845.
doi: 10.7498/aps.58.4840
Abstract +
Under the condition of keeping the bipolar pulse voltage constant, the effect of cathodic pulse voltage duty cycle （dc） on characteristics of microarc oxidation coatings of titanium alloy have been studied by using a multifunction microarc oxidation power supply. Experimental results show that the variations of cathodic and anodic currents with treatment time show obviously several stages, and the beginning and ending time of various stages are closely related to dc. The coating consists mainly of rutile and anatase phases, and there is a sudden change in the rutile phase content at dc=50%. The thickness, morphology and microhardness of the coating strongly depend on dc change. The microhardness valucs of the coatings prepared at dc=50% and dc=70% are the minimum （4.96 GPa） and the highest （6.1 GPa）, respectively. In general, the MAO coatings prepared in the range of 65%≤dc≤80% have higher growth rate and quality.
2009, 58 (7): 4846-4852.
doi: 10.7498/aps.58.4846
Abstract +
The specimens of polycrystalline pure aluminum were irradiated with high-current pulsed electron beam （HCPEB）. The microstructure of vacancy defect clusters has been investigated in detail by using transmission electron microscopy （TEM）. The results reveal that large numbers of vacancy cells including dislocation loop and even stacking fault tetrahedra （SFT） can be formed in the specimens of polycrystalline pure aluminum irradiated with HCPEB. For the specimen irradiated with one pulse, vacancy dislocation loops were formed in the vacancy cells. SFTs became the dominating structures after five pulses. For the specimen irradiated with ten pulses, dislocation loops were frequently present and SFTs were only formed in some local zones of vacancy cells. In the vicinity of SFT formation, dislocation-free or very low dislocation densities were observed. It is suggested that high stress and strain rate induced by rapid heating and cooling due to HCPEB irradiation could cause the shifting of whole atomic planes synchronously. This is the more probable mechanism of the formation of SFTs.
2009, 58 (7): 4853-4861.
doi: 10.7498/aps.58.4853
Abstract +
Many new lightweight hydrogen storage materials developed in recent yesrs were reviewed, including carbon nanotubes, microporous metal-organic frameworks materials, complex aluminum/ boracium hydrides, metal-nitrogen-hydrogen systems, and metal-organic complexes, etc. Novel results and important progress studied through the state-of-the-art first-pricinples about the above materials were introduced, and the applications of the materials were analyzed and discussed in combination with experimental results. The relationship between these lightweight hydrogen storage materials were generalized, which suggests the methods of the design of material in the field of energy storage, and promotes new idea for research.
2009, 58 (7): 4862-4871.
doi: 10.7498/aps.58.4862
Abstract +
Transformation of various intrinsic Frank loops into stacking-fault-tetrahedron（SFT）-like structures in single crystal copper is investigated using the molecular dynamics（MD） method. We got stable SFT structures and explained their formation via a unified theory.Simulation results show that there is a critical size for the Frank loops at zero temperature. When being smaller than this critical size, Frank loops can not transform into SFT. Hexagonal and trapezoidal Frank loops exhibit oscillations in the relaxation processe, which is due to the elastic interactions between dislocations as well as the delay effects in the propagation of elastic waves.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2009, 58 (7): 4872-4877.
doi: 10.7498/aps.58.4872
Abstract +
The atomic cluster models of α-Al, Al3Sc,η phase in Al-Zn-Mg-Cu ultra high strength aluminum alloys have been constructed by cell translation and the atomic cluster models of the interfaces α-Al /liquid Al and α-Al /Al3Sc have been set up by self-programmed software. The electronic structure parameters such as the density of states, the bonding energy of atom, the Fermi energy of the microscopic structure in alloys, the interaction energy between Sc and vacancy, have been calculated by recursion method. The physical nature of grain refinement and corrosion behavior of Al-Zn-Mg-Cu alloys has been explained according to the calculated electronic structure parameter. The results show that the energy given off by Al3Sc precipitating from liquid Al is less than that by α-Al and Al3Sc can precipitate from liquid Al prior to α-Al. Because of the lower interface energy of α-Al /Al3Sc compared with α-Al /liquid Al, the Al3Sc phase may offer the condition for the nucleation and growth of α-Al, which increases the nucleation rate of α-Al and refines the grain. The calculated results also indicate that Sc can attract the vacancy, decreasing the diffusion ability of vacancy and solute atoms, which makes the precipitate free zone along grain boundaries become narrower, and decreases the potential difference between grain and grain boundary, leading to the improvement of the corrosion resistance.
2009, 58 (7): 4878-4882.
doi: 10.7498/aps.58.4878
Abstract +
Within the framework of relativistic mean field theory- the Dirac equation of electrons in solid states, the electron spin dynamics and the related problems are discussed.Firstly, we discuss the high-order effects of electron spin dynamics by virtue of non-relativistic expansion of the Hamiltonian for large component of Dirac equation, the electron spin dynamics Hamiltonian including the first-order and second-order Rashba effects have been derived. In the case of two-dimension and perpendicular constant electric field, the corresponding eigenenergies and eigenfunctions of the spin dynamics Hamiltonian are obtained. Finally, the physical meaning and the magnitude of the correction of the second-order Rashba effect are discussed.
2009, 58 (7): 4883-4887.
doi: 10.7498/aps.58.4883
Abstract +
The electronic structures of the intrinsic and nitrogen-doped silicon carbide nanotubes （SiCNTs） have been calculated by first-principles approach based on the density functional theory. The intrinsic （8, 0） SiCNT is a direct band-gap semiconductor with a gap value of 0.94 eV. The band-gap of the SiCNT with the doping concentration of nitrogen being 1.56% and 3.12％ is narrowed to 0.83 eV and 0.74 eV， respectively. The narrowing of the band-gap is the result of the weakening of the Si-N bonds compared with the corresponding Si-C bonds, which can be seen by comparing the charge density difference of the intrinsic SiCNT with that of the nitrogen-doped nanotube.
2009, 58 (7): 4888-4894.
doi: 10.7498/aps.58.4888
Abstract +
On atomic scale, using molecular dynamics （MD） simulation, the influence of different ion-beam incident angles on the structure of diamond-like carbon （DLC） films was discussed in the process of ion-beam-assisted deposition （IBAD）. The study focused on the model of film surface, average density and SP3 hybridization fraction under different ion beam incidence angles. The results showed that horizontal kinetic energy of the incident atoms was enhanced by the oblique incident assist ions, thereby the horizontal migration of the C-atoms were also enhanced. The ratio of migration of C-atoms with ion-beam oblique incidence became greater than with verticalineidace, however, the density of the film and SP3 hybridization fraction became smaller. The effect of different ion-beam incident angle on the films structure varies with the variation of assistance/deposition atomic ratio （Ar/C） and incident energy. Thus films with different structures could be obtained when the incidence of ion-beam is oblique.
2009, 58 (7): 4895-4900.
doi: 10.7498/aps.58.4895
Abstract +
The BMH empirical potentials of Pu-O bond in α-Pu2O3 are fitted by GULP program. The structure and optical properties of PuO2 and α-Pu2O3 crystal are calculated by molecular statics and molecular dynamics using BMH and shell potentials. The result shows that their simulated space groups, cell parameters, densities and entropies are approximately the samc as the data in published papers, with relative error less than 1.2%. Furthermore, their static （high frequency） dielectric constant tensor, IR dielectric function spectra and crystal lattice vibration peaks are all calculated. Part of the results are validated by experiment of spectroscopic ellipsometry, and the indirect energy bandgap of PuO2 is found to be 2.1 eV.
2009, 58 (7): 4901-4907.
doi: 10.7498/aps.58.4901
Abstract +
We have synthesized polycrystalline bars of Pr0.7（Sr1-xCax）0.3MnO3（x=0，0.1，…，1）with a sol-gel method using citrate as an agent, and grown epitaxial thin films of Pr0.7（Sr1-xCax）0.3MnO3 on （100）-oriented substrates of SrTiO3 （ STO ） by using pulsed laser deposition （ PLD ）. We systemically investigated their lattice structures and electronic transport property. Both polycrystalline and film samples have orthorhombic lattice structure, and their electronic transport behaviors nicely follow a adiabatic small polaron model in high temperature paramagnetic phase when T is larger than the Curie temperature TC. All resistivity data have been fitted by the small polaron model. The fitted results of polycrystalline samples qualitatively agree with that of corresponding film samples. With increasing contents of Ca, polaron hopping energy Ehop increases monotoniccaly, and ρ0 first decreases, then increases. The above results indicate that electronic transport property of high quality polycrystalline samples is less influenced by lattice defects and grain boundaries which are abundant in low quality samples, such that intrinsic small polaron transport behavior can appear in the former.
2009, 58 (7): 4908-4913.
doi: 10.7498/aps.58.4908
Abstract +
The effect of controlling laser field on broadband suppercontinuum generation is researched. By adjusting the wavelength of controlling field as much as ±0.2 μm at 1.6 μm, no effect has been found in suppercontinuum obviously, the cut off range and the bandwidth of suppercontinuum both vary less than 5%. With increasing of the intensity of the controlling field, the suppercontinuum is broadened. A 12 fs/800 nm and a 30 fs/1.6 μm pulses are combined to serve as the driving pulse with the relative phase of -0.1π and relative intensity of 20%, the short quantum path is selected and a 105 eV suppercontinuum is obtained.
2009, 58 (7): 4914-4919.
doi: 10.7498/aps.58.4914
Abstract +
A new method to generate isolated broadband attosecond pulses with stable pulse duration is proposed. By adopting a few-cycle driving pulse combined with its second harmonic field, of which the polarization angle is π/3 to the driving pulse, the short path is well selected and a smooth supercontinuum with the bandwidth of about 50 eV is observed. For the intensity of the second harmonic field higher than 1014 W/cm2, the intensity variation of the second harmonic field can hardly affect the location and the bandwidth of the generated supercontinuum. Pure isolated attosecond pulses with the duration of about 100 as can be generated by directly filtering out a supercontinuum from 85—125 eV, which facilitates the experimental implement for the isolated attosecond pulse generation.
2009, 58 (7): 4920-4924.
doi: 10.7498/aps.58.4920
Abstract +
Polycrystalline film of ZnTe/ZnTe:Cu is fabricated by co-vaporization. The influence of evaporation temperature on the structue of the film and Cu distribution in the film and performance of the cell fabricated with the films are studied by XRD,XPS, C-V and I-V analysis. The results indicate that, （1） the deposition temeperature has less effecton the structure of the film ZnTe/ZnTe:Cu, （2） the fact that the Cu concentration in the film deposited at 100℃ raises to a maximum and descend rapidly shows the function of ZnTe film in preventing Cu atoms diffusing in the film and the CdTe cell fabricated with the film shows a wider barrier （XD）,smaller capacitance, better diode characteristic and larger conversion efficiency.
2009, 58 (7): 4925-4930.
doi: 10.7498/aps.58.4925
Abstract +
GaN buffer with insertion of a 40-nm-thick high-temperature （HT） AlN interlayer is studied. The HT-AlN interlayer enhances the in-plane compressive strain of GaN film and thereby improves the electrical properties of AlGaN/GaN high electron mobility transistors （HEMTs）. Base on the precise measurement of Bragg angle, the strain states of GaN are calculated. It is found that the increased compressive stress enhances the piezoelectric polarization field in GaN, which consequently causes accumulation of more electrons at the AlGaN/GaN interface. On the other hand, the influence of AlGaN layer induced by the enhanced compressive stress on the two-dimensional electron gas （2DEG） sheet carrier density, both positive and negative, are proved to counteract each other. Meanwhile, the employment of the HT-AlN interlayer reduces the lattice mismatch between the GaN and AlGaN and smoothes the AlGaN/GaN interface, thus increases the 2DEG mobility by weakening the interface roughness scattering. The 1-μm gate-length HEMTs by using the GaN buffer layer with the HT-AlN interlayer are fabricated. The measurements show that the maximum drain current and transconductance are increased by 42% and 20% respectively compared with the conventional HEMTs without HT-AlN interlayer.
2009, 58 (7): 4931-4935.
doi: 10.7498/aps.58.4931
Abstract +
The effect of C60 doping in the hole transport NPB layer on the performance of the device was investigated by changing the C60 content from 0 to 20 wt%. The OLEDs had a structure of ITO/NPB:C60（0, 10, 15, and 20 wt %）/Alq3/LiF/Mg: Ag. The doping led to higher hole mobility in C60-doped NPB layer from 5 wt% to 15wt%. Moreover, the hole mobility decreased when the doping content is 20%. The higher hole mobility resulted in efficient hole injection and low driving voltage at high luminance. For example, the driving voltage is about 4V and the maximum luminance of the OLEDs is 11000 cd／m2 when the doping content is 15%.However, the driving voltage is about 6V and the maximum luminance of the OLEDs is 6500 cd／m2 when the doping content is 15%. The hole mobility change resulted in the different performance in the devices.
2009, 58 (7): 4936-4940.
doi: 10.7498/aps.58.4936
Abstract +
Photoluminescence of shallow Be acceptors in both bulk GaAs and a series of δ-doped GaAs/AlAs multiple quantum well samples with well width range from 30 to 200 ? were studied. A series of Be δ-doped GaAs/AlAs multiple quantum wells with the doping at the well center and a single epilayer of GaAs uniformly Be doped were grown by molecular beam epitaxy. The photoluminescence spectra were measured at 4.2 K. The two-hole transition of the acceptor-bound exciton from the ground state, 1s3/2（Γ6）, to the first-excited state, 2s3/2（Γ6）, have been clearly observed. A variational principle is used to obtain the 2s-1s transition energy of quantum confined Be acceptors as a function of the well width. It is found that the acceptor transition energy increases with decreasing quantum well width, and the experimental results are in good agreement with the theoretical calculation.
2009, 58 (7): 4941-4947.
doi: 10.7498/aps.58.4941
Abstract +
We have fabricated the top-contact organic thin film transistors （OTFTs） with, a 60 nm thick pentacene films as active layer and a 120 nm thermal growth SiO2 as gate insulator. Through using different self-assembled monolayers ,such as octadecyltrichlorosilane （OTS） and phenyltrimethoxysilane （PhTMS）, as a buffer between the organic semiconductor active layer and gate insulator, we studied the effect of different buffers on the performance of OTFTs device. At the same time, we also investigated the field-effect behavior and carrier transport mechanism of OTFTs device with PhTMS buffer under low gate modulation voltage. When |VGS|IDS of OTFTs device keeping balance. But the OTFT device still has good output characteristics, with the field-effect mobility （μEF） of 3.22×10-3 cm2/Vs, （on/off） current ratio of 1.43×102, and threshold voltage （VTH） of -0.66 V.
2009, 58 (7): 4948-4952.
doi: 10.7498/aps.58.4948
Abstract +
The model of nMOSFET threshold voltage was established based on study of voltage distribution in strained Si film,which was grown on relaxed SiGe virtual substrate.The model was analyzed with reasonable parameters, and the dependence of threshold voltage on Ge fraction and channel doping was revealed. The dependence of threshold voltage shift on Ge fraction was also obtained. The relationship between threshold voltage and strained Si layer thickness and doping was studied. The results indicates, the threshold voltage increases with increasing doping concentrations of relaxed SiGe layer, decreases with increasing Ge fraction of relaxed SiGe layer, and increases with increasing strained Si layer thickness. This threshold voltage model provides valuable reference to the strained-Si device design.
2009, 58 (7): 4953-4957.
doi: 10.7498/aps.58.4953
Abstract +
The compounds and films of La0.67Sr0.33CuxMn1-xO3（x= 0.05, 0.10, and 0.15） were prepared using the sol-gel technique and an RF magnetron sputtering method, respectively. The effect of Cu doping on the photoinduced properties of La0.67Sr0.33CuxMn1-xO3 films was investigated. Experimental results indicate that the increasing Cu doping content leads to a decrese of the insulator-metal transition temperature and an increase of the resistance. The laser irradiation induces an increase of the resistance in the metallic state. The maximum values of the photoinduced relative change in the resistance increase with increasing x, and it reaches 58.3％ at x =0.15. The intriguing mechanism of the photoinduced effect on the resistance was discussed, which may be the photoinduced electrons being trapped at a localized state outside the CuO2 planes in combination with and the lattice effect.
2009, 58 (7): 4958-4961.
doi: 10.7498/aps.58.4958
Abstract +
There has been much interest in the Si-based strained materials lately, which was widely adopted in the high-speed and high-performance devices and circuits. Based on the valence band E（k）-k relation of strained Si/（001）Si1-xGex, the hole effective mass along arbitrarily k wavevector direction were obtained. It was found that in comparison with relaxed Si, the obvious change occurs in the hole effective mass of first and second valence band in strained Si/（001）Si1-xGex along specific k wavevector directions. The hole effective mass plays a significant role in the hole mobility enhancement. The results can supply valuable references to the investigation on the Si-based strained PMOS device performance and the conduction channel design related to stress and orientation.
2009, 58 (7): 4962-4969.
doi: 10.7498/aps.58.4962
Abstract +
Using the principle of minimal energy and S-W model，the effect of the applied stress on exchange bias in ferromagnetic （FM） / antiferromagnetic （AFM） bilayers has been investigated.When the applied field is absent，according to the relation between the energy of the system and the orientation of ferromagnetic magnetization， the system will be in monostable state or bistable state，which is controlled by the competition between the uniaxial anisotropy and the exchange anisotropy.Monostable state or bistable state of the system determine the angular dependence of exchange bias immediately.When the external field is applied along the intrinsic easy axes and intrinsic hard axes， it is found by analyzing the magnetization processes that one of the switching field of the hysteresis loop shows a an abrupt change，while the other is kept constant，and consequently the exchange bias field and the coercivity will appear a jump phenomenon.The numerical calculations indicate that both the exchange bias field and the coercivity are larger in the magnitude at the points of the jump.Both the magnitude and the orientation of the applied stress will significantly affect the exchange bias by making a transition between monostable state and bistable state in the system.This transition induces a significant change in angular dependence of the exchange bias. Our results demonstrate that the applied stress is a viable way to control and tune the exchange bias of the FM/AFM bilayers.
2009, 58 (7): 4970-4975.
doi: 10.7498/aps.58.4970
Abstract +
Mn1-xZnxFe2O4 films were prepared by alternative rf-sputtering from targets of MnFe2O4 and ZnFe2O4. The films were deposited on substrates such as single-crystal Si（100）, oxidized single-crystal Si（100）, single-crystal Si（100） with ZnFe2O4 underlayer and single-crystal MgO（100）. The deposited films are amorphous. After annealing in vacuum furnace at 550℃, the polycrystalline Mn-Zn ferrite crystallizes. The film with composition of Mn0.5 Zn0.5 FeO4 shows better magnetic properties: high magnetization Ms and lower coercivity Hc. The coercivity of the film deposited on MgO（100） substrate is the lowest. In this paper, the influences of substrate on film structure and the magnetic properties are investigated. It is believed that the magneto-elastic anisotropy resulting from stress in films dominates the coercivity of Mn-Zn ferrite films. The grain size may be refined to nanometer scale instrumentally by using suitable substrate; the stronger exchange coupling between grains would lead to a lower magneto-elastic anisotropy by averaging to the grains. Therefore, the soft magnetic properties of Mn-Zn ferrite films are improved.
2009, 58 (7): 4976-4979.
doi: 10.7498/aps.58.4976
Abstract +
We have investigated the influence of oxygen vacancies on electronic transport and magnetic property of polycrystalline bulk La0.5Ca0.5MnO3 （LCMO）. With increasing content of oxygen vacancy, its resistivity at high temperature increases monotonically and follows the adiabatic small-polaron model. The resistivity at low temperature first deereases, then increases, and has the behavior of dR/dT>0 evidently. The resistivity at low temperature becomes insulating at last. Increasing the oxygen vacancies depresses the appearance of antiferromagnetic phase. As a result, antiferromagnetic transition does not occurs in oxygen-deficient LCMO samples. On more increasc of oxygen vacancies, ferromagnetic phase will eventually be suppressed.
2009, 58 (7): 4980-4986.
doi: 10.7498/aps.58.4980
Abstract +
SrTiO3 doped ceramics of （1-x）（K0.5Na0.5）NbO3-xSrTiO3 （0≤x≤0.15, （1-x）KNN-xST） were prepared using solid state reaction method and the phase components, crystal structure, and dielectric properties were investigated in this work. It is found that all samples （0≤x≤0.15） are single phases with a perovskite structure within the temperature range from 10 to 800 K. The （1-x）KNN-xST compound has the orthorhombic structure when SrTiO3 concentration is below 0.1, while it shows cubic structure when x ≥ 0.1. It changes from the normal ferroelectric phase to ferroelectric relaxors when increasing the SrTiO3 impurity below a certain temperature. The critical temperature of （1-x）KNN-xST decreases dramatically while the FWHM of D and the critical parameter γ increase with increasing SrTiO3. The temperature spectrum of permittivity loss ε″r indicates that the loss duc to relaxor polarization plays the key role at low temperatures, however, the loss duc to current leakage is significant in the region of high temperatures.
2009, 58 (7): 4987-4991.
doi: 10.7498/aps.58.4987
Abstract +
Infrared absorption spectra of congruent lithium niobate crystals are measured at low temperature, and a new peak is found at 3200 cm-1 in the spectra. Results show that this peak has no relationship with hydrogen in the crystals, and that with the increase of the temperature, both the intensity and shape of the peak change in a complex way. Based on these results, we suggest that the new peak results from the electron transfer from small polarons （Nb4+Li） to free polarons （Nb5+Nb）. In addition, we find that this peak can be decomposed into three Gauss peaks, and the three components are attributed to three transfer processes with different energies.
2009, 58 (7): 4992-4996.
doi: 10.7498/aps.58.4992
Abstract +
According to refractive indexes of nanomaterials （less than 1.4） and the AM0 solar spectrum, antireflection coatings of space Si solar cells are designed and analyzed. For double-layer and tri-layer antireflection coatings fabricated by normal and nanometer materials, the optimal parameters of coatings and reflection coefficient curves are obtained. The double-layer and tri-layer new nano antireflection coatings are optimized, and have better antireflection compared with those fabricated using the normal materials. It is interesting that, after optimizing, the minimal weighted reflectance of the new nano antireflection coatings is 15% and 24.5% less than that of the normal antireflection coatings for the double-layers and the tri-layers, respectively.
2009, 58 (7): 4997-5001.
doi: 10.7498/aps.58.4997
Abstract +
A free-standing, bidirectionally permeable and ultra-thin （500—1000 nm） porous anodic alumina membrane was fabricated using a two-step aluminium anodization process, which was then placed on top of a silicon film as an etching mask. The pattern was transferred to silicon using dry-etching technology, and the silicon nanopore array structure was formed. The factors which affict the pattern transfer process are discussed. Observation of the nanopatterned sample under a scanning electron microscope shows that the structure obtained by this method is made up of uniform and highly ordered holes, which attains to 125 nm depth. The photoluminescence spectrum from the nanopatterned sample，the surface of which has been thermal-oxidized, shows that the the luminesce is evidently enhanced, the mechanism of which is based on the normally weak TO phonon assisted bandgap light-emission process, and the physical reasons that underlic the enhancement have been analyzed. The PL results do show an attractive optical characteristic, which provides a promising pathway to achieve efficient light emission from silicon.
2009, 58 (7): 5002-5006.
doi: 10.7498/aps.58.5002
Abstract +
The theoretical model of quantum efficiency of the reflective varied doping GaAs photocathode is proposed, which adopts the weighted mean method based on the quantum efficiency formula of exponential doping and uniform doping photocathode. By this model the quantum efficiency curves of a reflective GaAs photocathode under varied doping structure have been fit and we obtained the precise best fit in different incident photon wavebands. The phenomena that the contributions of doping structures to the cathode quantum efficiency are different was discovered. This phenomena has been analysed and discussed detailedly in the paper.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2009, 58 (7): 5007-5012.
doi: 10.7498/aps.58.5007
Abstract +
MgO film was fabricated on （100） Si substrates with DC sputtering method, （100） highly preferred oriented MgO film and MgO/TiN bi-buffer layers were successfully obtained by optimizing the preparation parameters, and the preferred （100） orientation mechanism affected by temperature was also discussed in the paper. XRD, AFM, FESEM were used to investigate the crystalline orientation, surface morphology, as well as the cross-section morphology of MgO films deposited on substrates with and without buffer layers. Columnar structure of MgO film and a good epitaxial relationship between MgO and TiN films were shown in our samples. Refractive index of MgO film on Si was fitted with Sellmeier model in the visible wavelength range （1.692 @ 550 nm）, while on TiN/Si substrate, the refractive index was calculated to be 1.716 @ 550 nm （where part of the layer approaches to the air） with the help of an inhomogeneous model.
2009, 58 (7): 5013-5021.
doi: 10.7498/aps.58.5013
Abstract +
The （001） and （111）-oriented MgO buffer layers, on which ferroelectric Ba0.7Sr0.3TiO3（BST30） thin films were prepared subsequently, have been grown on Si （001） single-crystal substrates by sol-gel and magnetron sputtering methods. The microstructure of the thin films was investigated by XRD, AFM and SEM. The results showed that the （101）-prefer-oriented BST30 thin film was obtained on the thicker MgO（001） buffer layer, while only the competition between the （101） and （111） orientations of BST30 can be observed on the thinner MgO（111） buffer layer. With the increasing thickness of MgO（111） buffer layer, the BST30 thin film’s （101） orientation was restrained, whereas the （001） orientation was strengthened. The improved simplex method was utilized to obtain the optical constant （n） and thickness （d） of BST30 thin film and MgO buffer layer. It was also found that the MgO buffer layer can significantly decrease the leakage current and p-n junction effect of BST30 thin film.
2009, 58 (7): 5022-5028.
doi: 10.7498/aps.58.5022
Abstract +
GaP thick films were prepared on ZnS substrates by RF magnetron sputtering starting from a GaP target in an Ar atmosphere. Plasma emission diagnostics was employed to investigate the glow discharge during the deposition process. Only the emission lines of Ar atom appear and effects of the deposition parameters on the intensities of the emission lines were studied systematically. The deposition parameters were optimized through increasing the RF power and decreasing the working gas pressure of Ar simultaneously, with the intensity of the emission lines kept constant. And thus, stoichiometric GaP thick films with high IR transmission performance were deposited at an elevated deposition rate.
2009, 58 (7): 5029-5034.
doi: 10.7498/aps.58.5029
Abstract +
Si film anode was prepared by ion beam assisted deposition for Li ion batteries. The crystal structure, morphology and electrochemical performance of Si film were investigated in detail. The results show that the Si film is amorphous. The intercalation/deintercalation voltages of the Si film were at 0.03V and 0.16 V as well as 0.34 V and 0.49 V. The Si film exhibits high specific capacity and efficiency, which are 3134.4 mAh/g and 87.1％ in the first cycle, respectively. Furthermore, the Si film shows superior cycle performance. The capacity retention is 92.2% after 200 cycles at 0.5C rate.
2009, 58 (7): 5035-5040.
doi: 10.7498/aps.58.5035
Abstract +
High-refractive-index materials such as LaF3, NdF3 and GdF3 and low-refractive-index materials such as MgF2, AlF3 and Na3AlF6 single thin films are deposited by a resistive-heating boat at different deposited rates and specific substrate temperatures on single crystal MgF2 substrate. Transmittances of all fluoride thin films are measured through commercial spectrometer in the ambient atmosphere. Optical constants of these films are determined by the envelope method and the factors effecting optical constants are studied. Result of this work may play an important role in the design and manufacture of cost-effective, mechanically and optically stable optical coatings for the VUV region.
N-type window layer and its application in high deposition rate microcrystalline silicon solar cells
2009, 58 (7): 5041-5045.
doi: 10.7498/aps.58.5041
Abstract +
N-type phosphors doping layer as the window layer of microcrystalline silicon solar cells has been fabricated using conversional radio frequency plasma enhanced chemical vapor deposition. Because of hole and electron mobilities are of the same order for microcrystalline silicon thin film, microcrystalline silicon solar cells based on n-type doping layer as the window layer almost show the same efficiency as microcrystalline silicon solar cells based on p-type doping layer as the window layer. In addition, the results of quantum efficiency are also consistent with the I-V measurement results. Bilateral Raman measurement results using laser light of different wavelength indicated that p/i or n/i interface incubation layer has a disadvantageous influence on the current density of the solar cells. Through the optimization of n/i interface incubation layer, glass/ZnO/n/i/p/Al microcrystalline silicon solar cell with 7.7% conversion efficiency has been fabricated.
2009, 58 (7): 5046-5050.
doi: 10.7498/aps.58.5046
Abstract +
We find that C59N and C19N crystals can be synthesized by slow evaporation of C60 solvent in the atmosphere. It opens up a new and efficient route for the preparation and study of various kinds of Cn（n≤59）N crystals materials.
2009, 58 (7): 5051-5056.
doi: 10.7498/aps.58.5051
Abstract +
Radiation damages of 1 Mev and 1.8 Mev electrons to the triple-junction GaInP/GaAs/Ge solar cell are investigated. The results indicate that electric properties of the GaInP/GaAs/Ge triple-junction solar cell, such as open-circuit voltage, short-circuit current and maximum power, decrease obviously as the irradiation fluence increases. Under the irradiation fluence of 1×1015cm-2 of 1 MeV electrons, the maximum power output is degraded by 17.7%. Under higher energy electron irradiation, the dark I-V characteristic shows that the changes in both the series and parallel connection resistance of the triple-junction cell are reasons causing the degradational of electrical properties. By the analysis of spectral response, the degradation in electric properties is primarily duc to the severe damage of the GaAs sub-cell. The damage of the sub-cell is associated closely with the obvious decrease in the collecting efficiency of light-generated carriers in the bottom of the base-zone. The key to improve the radiation resistance of the triple-junction GaInP/GaAs/Ge solar cell is to reduce the damage of the base-zone of GaAs sub-cell as much as possible.
2009, 58 (7): 5057-5064.
doi: 10.7498/aps.58.5057
Abstract +
The chaotic time series prediction problem is considered. A novel type of cortex-like neural network model, i.e. multi-clusters echo state network model （MCESN）, regulated by a group of five growth-factors, is proposed. It is shown that characters of MCESN’ topology can be effectively determined by the growth-factors group; and that it is the MCESN possessing both small-world and scale-free properties of complex network that corresponds to the better prediction performance. In addition, Monte Carlo simulation experiments show that MCESN not only can be trained by easy algorithm, but also can achieve higher accuracy and less standard deviation prediction results than classical echo state networks.
2009, 58 (7): 5065-5069.
doi: 10.7498/aps.58.5065
Abstract +
The study of signal transmission in neural models is helpful to investigate and model the transmission mechanism of biological neural systems, and realize the signal processing of cognition mechanism. We investigate the transmission of weak signals in a noisy one-way coupled Hodgkin-Huxley neural system. The results show that there is stochastic resonance and it helps to detect and transmit the signal. In addition, the coupling enables the signal transmission in the system. The efficiency of the signal transmission is increased with the increasing coupling strength. At some strength the firing of the elements will be in synchronization. What’s more, the optimal noise intensity of the receptor declines with the increase of the coupling strength, in the end will be equal to that of the driver. When the coupling strength is too high, the receptor will fire by itself. But the spikes will be suppressed by the increasing noise. It may be used to interpret the self action of neurons and the self adjusting in neural system. In addition, these results show that the noise and coupling are important to the transmission of weak signal in neural system. This study maybe helpful to interpret some phenomena in biological experiments.
2009, 58 (7): 5070-5074.
doi: 10.7498/aps.58.5070
Abstract +
We study the metabolic energy cost of action potential in neuron for various membrane sizes and at different temperatures. Using the occupation number method to model stochastic Hodgkin-Huxley neuron, we found that there exists a minimal metabolic energy cost near the real size of the membrane at room temperature.
2009, 58 (7): 5075-5082.
doi: 10.7498/aps.58.5075
Abstract +
Investigation on human ears capability receives much attention in auditory science and target automatic recognition. In this paper, the relationship between auditory identification, nature of sound, training level and ambient noise are investigated by the test of subjective auditory capability approach. The test results are as follows: Human ears capability of identification is dependent on sound spectral structure and it will be better when the signal is of harmonic nature and in steady-state. The identification accuracy, being closely related with sound characteristic, can be improved by training of juries. The auditory identification capability under noise environment lies on sound spectral structure.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2009, 58 (7): 5083-5090.
doi: 10.7498/aps.58.5083
Abstract +
In this study the average monthly data anomalies of 1960-2007 for 740 stations in China were analyzed. This research uses EVT （extreme value theory） to explore the spatial EWME （extreme warm month events） properties in China. The effects of potentially explanatory factors - climate warming on the distribution of EWME are investigated using GPD model with parameters estimated via a spatial scheme. The results showed that: 1） The properties of EWME have strong spatial dependence. The EWME have smaller variability over Tibetan plateau and North China plain and the coastal area of South China, while the EWME have larger variability over North China. There are more EWME with higher quantile over Middle-lower Yangtze area than that over the other regions. 2） The stations have the return period of the 2002 EWME with 3-5 centigrade longer than 10 years, and some grids have the return period longer than 50 years for EWME with 5-7 centigrade. 3） It has a potential relationship between regional climate warming and the EWME over some of special region. The variability and high quantile of the EWME are affected obviously by climate warming over Tibetan plateau plain and North China plain and northeast China plain, namely over the regions between monsoons on and non-monsoons.
2009, 58 (7): 5091-5097.
doi: 10.7498/aps.58.5091
Abstract +
In a laser target positioning system, to correct the location error is essential for improving the measurement precision. In this paper, a novel method of correcting the tropospheric target location error in utilizing pure rotation lidar is presented. The vertical profiles of atmospheric refractive index are deduced by the pure rotation return signals of N2 and O2, which are detected by the pure Raman lidar. Then, the refraction angle and elevation orientation errors at different altitude are corrected based on error correcting theory. The results indicate that the target location error are well corrected by making use of the atmospheric refractive index which is detected by pure rotation lidar. At the same altitude, the target refraction angle and elevation orientation errors decrease as the visual angle increases. When the visual angle is 10°, the refraction angle of the target at 8 km reaches 3.15′ and the elevation orientation error is 14.55 m. When it is 30°, the refraction angle of the target at the same altitude is only 0.98′ and the elevation orientation error is 1.19 m.
2009, 58 (7): 5098-5103.
doi: 10.7498/aps.58.5098
Abstract +
Based on the coherent superposition principle and the generalized Huygens-Fresnel diffraction integral, the properties of a radial beam array with definite initial phase distributions propagating through a turbulent atmosphere are analytically investigated. It is shown that, when the propagation distance is short, the output intensity distribution will gradually take on a spiral shape, which means an optical vortex. However, under the effect of the atmospheric turbulence, the output vortex property gradually disappears, and changes to a bright-cantered beam without vortex property. Furthermore, the weaker the atmospheric turbulence is, the longer distance the output optical vortex （the center-dark beam） can propagate over.
2009, 58 (7): 5104-5110.
doi: 10.7498/aps.58.5104
Abstract +
Incoherent scattering spectrum of a collisional plasma with an arbitrary velocity distribution function is presented in this paper. Two integrals with complex singular points have been solved in order to obtain the results. The incoherent scatter spectrum during HF heating in low ionosphere region is computed. The effect of collision frequency, non-Maxwellian index, electron density, electron temperature and ion temperature on power spectrum is discussed. Under the action of collision frequency and non-Maxwellian factor, the original relationship between ionospheric parameters and the characteristics of spectral shape is invalid, which will bring about a large error in the inversion of ionosphere parameters, thus the original theoretical model must be revised.
2009, 58 (7): 5111-5117.
doi: 10.7498/aps.58.5111
Abstract +
The propagation of ULF waves （0.1—10 Hz） from magnetosphere to the conductive ground is studied in the presence of oblique magnetic field and the analytical solution is obtained. The effects on the parameters of the IAR, such as reflection coefficient, eigenfrequency and attenuation, are analyzed. It is shown by simulation that the change of oblique field results in the corresponding change of the scale length and the ionospheric conductivities. Along with the decrease of dip angle, the eigenfrequency declines and so does the attenuation of IAR. The conductivity corresponding to the maximum attenuation also decreases.
2009, 58 (7): 5118-5125.
doi: 10.7498/aps.58.5118
Abstract +
Cognitive radio decision engine based on particle swarm optimization is proposed. A population adaptive particle swarm optimization is also proposed to improve the convergence rate. Particle swarm optimization and population adaptive particle swarm optimization are used to adapt radio parameters respectively, and multi-carrier system is used for the performance analysis. Results show that cognitive decision engine based on binary particle swarm optimization has better convergence, precision and stability than the classic genetic algorithm, and population adaptive particle swarm optimization can further improve the performance at the initial stage of the search to meet real time requirement of cognitive radio.
2009, 58 (7): 5126-5132.
doi: 10.7498/aps.58.5126
Abstract +
Based on the theory of passive RFID technology and antenna scattering, some computation methods for radar cross-section of tag antennas with different loads are derived, and by integrating the radar cross-section with electromagnetic propagation environment, the identification range of passive radio frequency identification is theoretically analyzed. Furthermore, by measuring backscattered power in the case of different loads, and then combined with the calculating methods, radar cross-section of the tag is also found. Theoretical analysis results are found to be in good agreement with experimental data. The results obtained are useful for optimizing tag performance in passive RFID systems.
