Vol. 61, No. 21 (2012)
Periodic solution of relative rotation nonlinear dynamic model with commonly damped force and forcing periodic force
2012, 61 (21): 210201. doi: 10.7498/aps.61.210201
The periodic problem of the corresponding autonomous system is discussed. Some results on the existence and uniqueness of periodic solutions of the system are obtained by using the continuation theorem of coincidence degree theory. The results available from the literature are generalized. Furthermore, an example is given to illustrate our results are new.
2012, 61 (21): 210202. doi: 10.7498/aps.61.210202
There exist fast scale and slow scale nonlinear dynamic behaviors in the voltage source inverter. Period doubling bifurcation and low-frequency oscillation instability phenomena are investigated. A unified bifurcation control strategy based on frequency-domain transfer function is proposed to suppress the fast-scale and slow-scale instabilities under the guarantee of the inverter's good dynamic and static performance. The stability boundary of the inverter is greatly expanded with the help of the controller. Harmonic balance method and small-signal averaged model are used to verify the effectiveness of the proposed control method when it is used to suppress the period-doubling bifurcation and the low-frequency oscillation, the stability domain border is also derived with the help of the analysis method. Theoretical analysis and simulation results show that the proposed control strategy is correct and feasible.
2012, 61 (21): 210203. doi: 10.7498/aps.61.210203
The lattice strain of large-scale KDP crystal is characterized by using high resolution X-ray diffraction technique, and the lattice stress is analysed quantitatively in detail. The results indicate that KDP crystal may cleave easily along the  direction, which corresponds to the cracking phenomenon in the practical work. The major factors of introducing the internal stress and causing crack in the crystal growth process are summarized. Those conclusions provide important theoretical basis for proposing crack control measures of the large-scale KDP crystals.
Restraining parasitic interference fringe phenomenon in detection imaging through the medium with inclined angle
2012, 61 (21): 210204. doi: 10.7498/aps.61.210204
Some periodic interference fringes due to medium with inclined angle are found in the echo image when the medium plate is inclined in the penetrating experiment. These interference fringes have serious influence on the imaging result of the interesting target. The study indicates that the interference fringes are caused by the reflected waves, which mainly come from the surface and below layer of the medium plate. To analyze the interference fringe phenomenon, a point source model is constructed in the paper, and the expression of the spacing between the adjacent interference fringes is derived. When the factor about antenna is taken into account, the pyramid horn near-field model is constructed. Based on it, the accurate electromagnetic simulations are performed. The results of the commercial software CST and MATLAB calculation both confirm the analysis about the interference fringes and the relation with the inclined angle of the medium plate. From the expression of the thin medium, the spacing between the adjacent interference fringes is related to incline degree of the medium. Controlling the influence factors of the spacing between fringes can restrain the interference fringe phenomenon in order to improve the target resolving power and imaging quality.
Integral-form solution of the Caldeira-Leggett density operator equation obtained by virtue of thermo entangled state representation
2012, 61 (21): 210301. doi: 10.7498/aps.61.210301
Open quantum system, namely system-reservoir model, is described by a master equation of density operator. For example, the Caldeira-Leggett eqaution describes dissipative phenomenon of solid physics. Although some efforts have been made to derive the exact expression of this master equation, so far as we know, it has not been reported in the literature. The purpose of this paper is to provide a new approach to solving the Caldeira-Leggett equation, via this approach the explicit integral-form expression of ρ(t) can be obtained. The main point of this approach is to convert equation of density operator into an equation of density state vector, and then project density state vector into thermo entangled state representation and convert it into wave function by using the technique of integration within an ordered product of operators. Thus the master equation for Caldeira-Leggett model is converted into an differential equation of wave function. Wave function is also a function. The wave function can be obtained via the approach to solving the differential equation in mathematics. It can be converted into a density state vector and density operator. Using the technique of integration within an ordered product of operators again, the integra-form solution of the Caldeira-Leggett equation is obtained.
2012, 61 (21): 210302. doi: 10.7498/aps.61.210302
The three-parameter two-mode squeezed number state is proposed by the technique of integration in an ordered product of operators. Its squeezing, antibunching effect and Cauchy-Schwartz inequality are analysed. The conditions under which squeezing or antibunching effect is displayed, are given. The effects of the complex parameter and photon number on squeezing , antibunching effect and Cauchy-Schwartz inequality of the field are discussed. The results indicate that its squeezing, antibunching effect and the degree of violation of Cauchy-Schwartz inequality of two-mode field are all weakened with the increase of photon number; on the other hand, its antibunching effect and the degree of violation of Cauchy-Schwartz inequality of two-mode field are weakened with the increase of the complex parameter modulus, while its squeezing is strengthened with the increase of the complex parameter modulus.
Atomic entropy squeezing of the Jaynes-Cummings model driven by classical fields in non-Markovian environment
2012, 61 (21): 210303. doi: 10.7498/aps.61.210303
The atomic entropy squeezing of the Jaynes-Cummings model driven by classical fields in non-Markovian environment is investigated according to the non-Markovian quantum theory and the entropy squeezing theory. Our attention is focused on the influences of the non-Markovian effects, the Rabi frequencys of classical fields and the detuning on the atomic entropy squeezing. And we explain the atomic entropy squeezing dynamic by the memory effects of the non-Markovian processes. It is found that the atomic entropy squeezing can be maintained for a long time when both the Rabi frequencys of classical fields and the non-Markovian effect are present simultaneously. And we can obtain the optimal squeezing states by choosing appropriate parameters of the Rabi frequency and the detuning of classical field. Our results provide a potential method of generating high-degree squeezed and longtime atomic squeezing states by controlling the atom-field interaction.
2012, 61 (21): 210304. doi: 10.7498/aps.61.210304
The validity of factorization law for the entanglement evolution of two qubits coupled to two independent non-Markovian channels is investigated. It is found that for some initial states, such as the Bell state and the maximally entangled mixed state, the entanglement can be characterized by the factorized entanglement when the effects of the channels are weak. For the general initial states, the factorized entanglement provides us with a good approximation to characterize the entanglement dynamics when the entanglement is large.
Soliton excitation with high-oder-nonlinearity of spinor Bose-Einstein condensate in an optical lattice
2012, 61 (21): 210305. doi: 10.7498/aps.61.210305
Soliton excitation with high-oder-nonlinearity of spinor Bose-Einstein condensate in an optical lattice is studied in detail. The exact solution for bright soliton which is expressed as an elliptic integral is found, and the analytic solution for dark soliton with particular parameters is presented. The energy is also found.
2012, 61 (21): 210501. doi: 10.7498/aps.61.210501
Adopting power function as a damping kernel function of generalized Langevin equation, flash ratchet potential as a potential field, the model of fractional Brownian motor is derived in the case of overdamped condition. With the memory effect of fractional derivatives, the motion characteristics of the particle in overdamped fractional Brownian motor are discussed. Inverse transport which is not seen in conventional Brownian motor, is found in an overdamped fractional Brownian motor. The influences of fractional order and noise density on transport speed are discussed separately. For a fixed fractional order, stochastic resonance appears in transport speed as noise density varies. For a fixed noise density, transport speed will oscillate as the fractional order varies, that is, multipeak generalized stochastic resonance takes place.
2012, 61 (21): 210502. doi: 10.7498/aps.61.210502
We use the multi-scale entanglement reorganization ansatz algorithm to simulate the spin 1/2 quantum XYX model and determine its ground-state phase diagram and entanglement phase diagram by using local order parameter and characterizing quantum entanglement: one-tangle, concurrence, and entanglement ratio R. We find that the information in entanglement phase diagram is more than in ground-state phase diagram. In addition, we extract the critical exponents β and ν from the numerical data near critical point.
2012, 61 (21): 210503. doi: 10.7498/aps.61.210503
In order to detect the weak characteristic signal submerged in heavy noise with extremely low signal-to-noise ratio, a method based on singular value decomposition (SVD) and stochastic resonance is proposed. The sampling signal is first preprocessed and reconstructed by means of SVD, and then we search for a component signal. In the component signal, the components of the characteristic signal match noise strength. Then the component signal is processed with the non-linear bistable system to obtain stochastic resonance response, thus the goal of detecting the weak characteristic signal submerged in a heavy background noise is realized.
Breaking chaotic direct sequence spreading spectrum communication systems using DM-UKF chaotic fitting method
2012, 61 (21): 210504. doi: 10.7498/aps.61.210504
In this paper, a dual model unscented Kalman filter chaotic fitting breaking method is proposed to break chaotic direct sequence spread spectrum communication systems in the cases of low spreading factor, low signal-to-noise ratio or severe multipath fading. Based on the characteristic that the range of information symbol is a finite set, the proposed algorithm fits the original chaotic signal through different filters which work in parallel. The fitting errors are used to choose the optimum matching filter, thus to estimate the information symbols. Furthermore, an error-controlling-factor is introduced to increase the distance of model based tracking errors, which can not only facilitate the information extracting process, but also reduce the influence of noise and multipath fading. Theoretical analysis and simulation results prove that the proposed algorithm is superior to the existing breaking method.
2012, 61 (21): 210505. doi: 10.7498/aps.61.210505
Dynamic modeling and analysis of a bistable piezoelectric cantilever power generation system are presented. Firstly, a magnetic force model is established which could induce bistability, and the force mathematical expression between the two magnets is given. Secondly, a lumped parameter model of piezoelectric cantilever power generation system is built, and the range of distance between the two magnets is analyzed while the bistable phenomenon occurs. Thirdly, the response characteristics of the system are studied numerically. The results show that the beam undergoes large-amplitude motion at low frequency and low amplitude, and that the voltage response has a broadband. The system has high energy escaped from potential well when excitation amplitude imereases. Finally, the result is verified experimentally. This study provides a theoretical basis for the design and application of bistable piezoelectric cantilever power generation system.
2012, 61 (21): 210506. doi: 10.7498/aps.61.210506
The control of spiral waves in three-layer coupled excitable media is studied by using the Bär model. The bidirectional coupling between adjacent layers is adopted. We use planar waves generated by a local periodic signal added to the second layer to eliminate spiral waves in the media. The numerical results show that when the couplings among three layer media meet some conditions the spiral waves in the media can be controlled. The control of spiral waves may be achieved by using the complementary coupling strategy. The interaction between planar wave and low-frequency spiral wave can produce high-frequency spiral wave, leading to failure to eliminate spiral waves. There exists an optimal drive width. Both larger and smaller drive width may need larger coupling strength between the first and third layers. The control results depending on the control opportunity are observed. We hope that this study will contribute to the design of the implantable cardioverter defibrillator.
2012, 61 (21): 210507. doi: 10.7498/aps.61.210507
In this paper, we propose method of generating grid multi-scroll chaotic attractors with the positive-type second generation current conveyors (CCII+). Firstly, a three-dimensional grid multi-scroll chaotic system is presented. Some basic dynamical properties are studied. Furthermore, we construct the saturation functions with current output and design the chaotic system with series structure by using current conveyers. In the whole design the port parasitic parameters of current conveyers are taken into considerations. By analyzing we could learn that the circuit can work in a higher frequency range. Finally, the effectiveness of this method is verified by circuit implementation.
Perturbation solution of single-phase AC-DC converter based on nonlinear oscillation and chaotic analysis
2012, 61 (21): 210508. doi: 10.7498/aps.61.210508
This paper deals with the issue of modeling method by perturbation solution to the single-phase AC-DC converter. A continuous circuit that includes a single-phase AC-DC converter is inducted first. And then a differential equation is presented with two parameters, which are controlling parameter and small perturbation parameter. A strict proof indicates that this equation can be solved by the regular perturbation. The compareison between the result and the simulation figures of PSCAD/EMTDC, shows that the result is correct and simple, thus avoiding the requirements for large computation capacity. Finally, the physical significance of the two parameters is analyzed, showing that they may incur the period-doubling chaos. This method not only fits the single-phase AC-DC converter circuit, but also meets the need of two-phase or N-phase circuits.
2012, 61 (21): 210509. doi: 10.7498/aps.61.210509
Synchronization of two spiral waves in two-dimensional excitable systems interacting through a passive medium is studied by using the Bär model. The passive medium is composed of excitable elements. There are no couplings among these elements. The numerical results show that synchronization of spiral waves is significantly affected by the passive medium. When two subsystems have the same initial spiral waves, the passive medium can induce meander of stable spiral waves and cause spiral waves to transform into multi-spiral waves or anti-target waves. When initial spiral waves are in an asynchronization state, the synchronization and phase-synchronization between two spiral waves are established if the relevant parameters are properly chosen. In addition, the following phenomena are observed: the tips of two spiral waves repel each other, multi-spiral waves coexist, synchronized spatiotemporal pattern repeats periodically, and the two systems evolves into the resting state. Wave patterns can generally be observed in passive medium. However, passive medium can exhibit synchronous oscillation in certain circumstances. These results can help one understand the formation of spatiotemporal patterns in the cardiac system.
2012, 61 (21): 210510. doi: 10.7498/aps.61.210510
In barotropic fluids, based on the quasi-geostrophic potential vorticity equation, an inhomogeneous mKdV-Burgers equation including slowly changing topography and an external source is derived by employing the perturbation method and stretching transforms of time and space. With the inspection of the evolution of the amplitude of Rossby waves, it is found that beta effect, topography effect, slowly changing topography and an external source are all the important factors, and that the solitary Rossby wave is induced thought the basic stream function has a shear flow . On the assumption that the nonlinear and topographic effects are in balance, an inhomogeneous mKdV-Burgers equation is derived, the results show that the topography and Rossby wave interact in the barotropic flows. The inhomogeneous mKdV-Burgers equation describing the evolution of the amplitude of solitary Rossby wave as the change of Rossby parameter β(y) with latitude y, topographic forcing, slowly changing topography and the external source is obtained.
2012, 61 (21): 210511. doi: 10.7498/aps.61.210511
The clustering of nodes is an important feature of complex network. Previous researches mainly focus on community discovery in unweighted network, with little attention paid to the weighted network because of the complexity of weighted network. The community discovery of the weighted network is believed to be a much more difficult task. In this paper, we perform a study on the effectivenesses of community evaluation criterion and the performances of the existing discovery algorithms. First, we summarize three classical community evaluation criterions of weighted network, and analyze their effectivenesses according to a simulated noisy dataset, which has different community sizes, densities and local characteristics. Second, we adopt five datasets to compare the performances of three typical community discovery algorithms. The study shows that the existing criterions encounter difficulties in evaluating the basic community structure and in evaluating the weighted community with complex structure, and the generalization ability of the typical community discovery algorithm of weighted network is unsatisfactory.
Influences of axial position manipulation and misalignments of optical elements on radial trap position manipulation
2012, 61 (21): 210701. doi: 10.7498/aps.61.210701
Optical tweezers in which a tightly focused laser beam is used to trap micron-sized or nanometer-sized particles have become indispensable tools for measuring the forces and displacements associated with molecular biomechanical events in a noninvasive manner. A complete beam manipulation system is composed of a beam expander input lens, beam expander output lens, focusing lens, piezoelectric translator mirror to control the trap position, with the overfilling degree of the objective entrance aperture retained. The accurate manipulations of trap position in three dimensions are the bases for the realization of the position clamp and force clamp. The optical path of optical tweezers based on infinity corrected conjugate microscope is calculated using matrix optics. The influences on radial trap position manipulation caused by axial position adjustment of focusing lens and objective, and by the installation location error of focusing lens and piezoelectric translator mirror are analyzed. The result shows that axial position adjustment of objective introduces a nominal error in radial trap position manipulation. The misalignments of focusing lens and piezoelectric translator mirror have a greater influence on optical tweezers performances. The theory points out the accurate dynamic axial position adjustment range, which is useful to optical tweezers design and experiments.
Relation of containment capacity with phase composition, density and Vickers hardness of simulated waste forms Gd2-xNdxZr2O7(Nd=An(Ⅲ), 0≤ x ≤ 2.0)
2012, 61 (21): 212801. doi: 10.7498/aps.61.212801
In order to investigate the capability of Gd2Zr2O7 pyrochlore as a waste form for immobilizing trivalent actinides nuclides, Nd(Ⅲ) is used as an alternative substance for An(Ⅲ). The compounds in the system Gd2-xNdxZr2O7 (0≤ x ≤ 2.0) are synthesized at 1500 ℃ for 72 h by high temperature solid state reaction method, using Gd2O3 and ZrO2 powders as the raw materials. The phase, intensity, Vickers hardness and microcosmic shape are characterized by X-ray diffraction, Vickers hardness tester, scanning electron microscopy and so on. The results indicate that the phase of synthesized waste form with plate-shape keeps the phase of pyrochlore. The intensity of compound slightly decreases with the increase of the containment capacity value x, but it is above 5.76 g·cm-3. The value of Vickers hardness also decreases with the increase of x. The values of x and Vickers hardness are linearly related by HV=695.18636-162.64091 x (HV≥ 400 kg·mm-2).
The effect of energetic electron impact ionization on radiation ionization process of high-altitude nuclear explosion
2012, 61 (21): 212802. doi: 10.7498/aps.61.212802
The X-ray ionizations and atmospheric temporal evolutions of different altitude nuclear explosions at different distances are numerically simulated. The effects of energetic electron impact ionization on radiation ionization process are analyzed in this paper. It is concluded that the energetic electron impact ionization process is important for radiation ionization, and in the case of 1 kt equivalent explosion at 80 km, the electron density at 1.5 km distance from explosion center increases two orders because of the energetic electron impact ionization. In 5 μs the spectral energy distribution of energetic electrons varies with time, and the number density of energetic electrons decaying with electron energy will present an approximately negative exponential distribution. The peak time of electron density and the influence area of ionization increase with explosion altitude increasing. The ionization effect for 1 kt equivalent explosion at 80 km has an important influence on micro-wave communication in a 100 km range.
ATOMIC AND MOLECULAR PHYSICS
Theoretical calculation of the vib-rotational interaction potential and the differential coefficient cross sections for He-HD (HT, DT) system
2012, 61 (21): 213101. doi: 10.7498/aps.61.213101
The interaction potential surfaces of He-HD (HT, DT) are calculated by employing ab initio method at the CCSD(T)/AUG-CC-PV5Z +bf(3s3p2d1f) calculational level when the key-lengths of target-molecule are different. The vib-rotational interaction potentials of He-HD (HT, DT) system are fitted using the method of center of mass transformation-fitting, the Tang-Toennies potential function and nonlinear least square method at the incident angles of 0°, 20°, 40°, 60°, 80°, 90°, 100°, 120°, 140°, 160° and 180°. The differential coefficient cross sections at the energies of 60, 90 and 120 meV are calculated by using the quantum close-coupling method. On the basis of the above result, the change rules of the differential coefficient cross section with incident energy, reduced mass of system and scattering angle are discussed.
Tetragonal distortion of Mn2NiGa Heusler alloy is calculated by first-principles based on density functional theory with projector augmented wave pseudopotential, and the magnetism, electronic structure, elastic constants and phonon frequencies are also calculated and analyzed. The contribution of the spin magnetic moments of Mn atom to the total moment is largest for Mn2NiGa, and the Mn2NiGa alloy shows ferrimagnetism in these two cases, owning to the antiparallel but unbalanced magnetic moments of Mn (A) atom and Mn (B) atom. Analysis of tetragonal distortion shows that there is a local minimum total energy at c/a=0.94 and c/a=1.27, which corresponds to a stable martensitic phase. Elastic constants of Mn2NiGa reveal that cubic structure does not satisfy stability conditions, but tetragonal structure (c/a=0.94 and c/a=1.27) does. The imaginary values of phonon frequencies in cubic structures validate that tetragonal structure (c/a=0.94 and c/a=1.27) of Mn2NiGa is more stable than cubic structure. The phase transition temperature of c/a=1.27 tetragonal structure converting to cubic structure is about 315 K.
2012, 61 (21): 213301. doi: 10.7498/aps.61.213301
Ultracold polar RbCs molecules are produced via photoassociation in a dual dark-SPOT magneto-optical trap, in which ultracold rubidium and cesium atoms with high densities are formed by laser cooling and trapping. The a3∑+ triplet metastable state ultracold RbCs molecules with 104/s production rate are detected using resonance-enhanced two-photon ionization. The ultra-high resolution rovibrational spectra of (2)0+, (3)0- and (2)0- rovibrational states are observed by scanning the photoassociation laser frequency. The rotational constants of (2)0+, (3)0- and (2)0- vibrational states are 0.00349 cm-1, 0.00649 cm-1 and 0.00372 cm-1 by fitting respectively.
2012, 61 (21): 213302. doi: 10.7498/aps.61.213302
A systematic research on ultra-low field magnetic resonance imaging (MRI) is conducted based on high-Tc dc-SQUID senor. The coil system is first updated to reach the requirements of MRI experiment. After that one-dimensional and two-dimensional imaging are performed and images consistent well with original phantoms are obtained successfully. Two different methods are used to rebuild the image: direct back projection and Fourier transform reconstruction. Both of them can obtain the profile of water phantom. A comparative discussion between these methods is proposed: the Fourier transform method has a better profile, while the direct back projection has a better signal-to-noise ratio. Imaging of biological sample such as green pepper and celery is also performed, and it is consistent well with the physical object.
Theoretical study on electron-impact excitation processes and the relevant polarization of radiation of 2s1/2—2p3/2 in W65+ through W71+
2012, 61 (21): 213401. doi: 10.7498/aps.61.213401
In this paper, the excitation energy and radiative transition probabilities are calculated for 2p3/2-2s1/2 transition in W65+ through W71+ ions, by using GRASP92 package based on the multi-configuration Dirac-Fock method. The present calculations are compared with other theoretical and experimental results [Podpaly et al. 2009 Phys. Rev. A 80 052504], and they are in good agreement each other. Furthermore, the total cross section and the magnetic sublevels cross section for the 2s1/2-2p3/2 excitation in W65+ through W71+ ions as well as the polarization of resulting transitional lines are calculated, with a fully relativistic distorted-wave method. Based on the calculation, the variations of the excitation cross sections and polarization with the increase of incident electron energy are discussed systematically.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
A novel optical element, vaulted axicon, is proposed for the first time in this paper. We analyze the distribution of light field with diffraction theory, and simulate the intensity distribution behind vaulted axicon. The result shows that multi-bottle beam can be obtained after a plane wave has passed through an vaulted axicon, moreover the intensity of the bottle beam is very high in the focal region because of the energy of spherical wave is significant concentrated in this region. The simulation and comparison show that the intensity around the bottle beam generated by vaulted axicon is far higher than that generated by superposition of two Bessel beams, therefore the particle trapping efficiency can be significantly increased. By comparing the scattering forces of bottle beam generated by the two methods, we demonstrate that the bottle beam generated by vaulted axicon is superior in particle trapping.
2012, 61 (21): 214202. doi: 10.7498/aps.61.214202
The concept and physical model of "beam moving" are proposed. For the scheme of angular spectral dispersion, taking the sinusoidally phase-modulated pulse and the linearly chirped pulse for example, the properties of the transverse movement of chirped broadband beam are analyzed theoretically. The effects of the transverse moving speed, the moving cycle and the moving mode on the smoothing of focal spot are discussed in detail. The results show that the linearly chirped pulse could achieve better smoothing effect than the sinusoidally frequency-modulated pulse with the same bandwidth.
2012, 61 (21): 214203. doi: 10.7498/aps.61.214203
By exploiting the entanglement properties of continuous variable quantum GHZ state, we propose a tripartite quantum deterministic key distribution, in which the key is generated from its amplitude and the phase can be used to test and verify the channel security. The existing quantum deterministic key distribution can only hand over deterministic key to one receiver in one communication. However, we always have to transmit deterministic key to more than one receiver in real life. The analysis results of information theory show that when the channel transmission efficiency is greater than 0.5, the proposed protocol can securely hand over the pre-deterministic key to two receivers simultaneously, and it can also be extended to multiparty quantum deterministic key distribution when preparing multiple entangled state, this will greatly improve the overall efficiency of the key transmission, furthermore, the continuous variable quantum GHZ state could have a higher channel capacity, so the protocol has the important practical significance.
2012, 61 (21): 214204. doi: 10.7498/aps.61.214204
Properties of transmission of a weak pulsed light in an asymmetric semiconductor quantum well are investigated. Under the condition of same physical parameters, we research absorption and chromatic dispersion of pulsed light via changing the cross coupling term resulting from the spontaneously generated coherence effect in an asymmetric semiconductor quantum well. It turns out that the absorption of quantum well medium is continually decreasing with the enhancement of spontaneously generated coherence effect. Simultaneously, strong chromatic dispersion turns into transparent window.
2012, 61 (21): 214205. doi: 10.7498/aps.61.214205
The emission of silicon quantum dots is weaker when their surface is passivated well. Oxygen or nitrogen on surface of silicon quantum dot can break the passivation to form the localized electronic state in band gap to generate active center where the stronger emission occurs. In this way we can build up the radiative matter for emission. Controlling the surface bonds on silicon quantum dots, various wavelengths of emission can be obtained. The annealing is important for the treatment of the activation. Experiments demonstrate that the stimulated emissions at about 600 nm and 700 nm appear on active silicon quantum dots, and the photoluminescence peaks are found in a range from 1500 nm to 1600 nm.
2012, 61 (21): 214206. doi: 10.7498/aps.61.214206
An amplified femtosecond pulse sequence is generated in a multi-TW Ti: sapphire chirped pulse amplification system (JG-II facility). The pulse interval of the pulse sequence is 14.8 ns, corresponding to a frequency of 67.5 MHz. The compressed pulse sequence can reach 122 mJ in 19 independent pulses with a pulse width of about 60 fs. The energy of the pulses declines from 20 mJ to 0.5 mJ, corresponding to a peak power from 1011 W to 1010 W. The pulse sequence has important applications in lengthening the lifetime of the laser produced plasma channel in air and in microfabrication.
The effects of proton implant energy on threshold and output power of vertical cavity surface emitting laser
2012, 61 (21): 214207. doi: 10.7498/aps.61.214207
A method of balancing the output power and threshold current property of vertical cavity surface emitting laser is studied. The relationship between proton implantation energy and device performance is analyzed by simulation and experiment. It is found that a higher injection energy can destroy the active region, thus reducing the output power property. The threshold current will be increased since a lower injection energy may weaken the restriction on the injection current. The results indicate that 315 keV injection energy is the right choice for our device structure. The output power and threshold current obtained under 10 μm aperture are 1.7 mW and 4.3 mW, respectively.
Wavelength-tunable linear-cavity passively mode-locked Yb-doped fiber laser based on volume Bragg grating
2012, 61 (21): 214208. doi: 10.7498/aps.61.214208
The linear-cavity all-normal-dispersion Yb-doped fiber laser based on semiconductor saturable absorber mirror and reflection type volume Bragg grating (VBG) is demonstrated. A passively mode-locked pulse stable output with a 0.32 nm spectral bandwidth, a 10.2 mW maximum average output power, a single pulse energy of 0.63 nJ and a 16.42 MHz repetition rate is achieved, when the central wavelength of pulse is 1030 nm. The central wavelength of the pulse can be tuned approximately from 1011.9 nm to 1050.6 nm by the spectral separation and mode selection effect of VBG with a tuning range of 38.7 nm at room temperature. Q-switched, second harmonic mode-locked, simultaneous dual wavelength and triple wavelength output are observed. This mode-locked fiber laser can be used as the optical source in wave division multiplexing optical time division multiplexing communication system and optical source of optical coherence tomography due to its tunable wavelength.
2012, 61 (21): 214209. doi: 10.7498/aps.61.214209
The graphene samples with different numbers of layers are prepared by chemical vapor deposition, the relation between the number of graphene layers and the transmission at 550 nm is analyzed by graphene transmission spectrum. Besides, the damage characteristics of graphene under high intensity laser irradiation are analyzed. The results show that under the laser irradiation, for the single layer graphene, G band and 2D band shift toward high frequeney but for the multilayer graphene, only G band shifts a litile; the ratio between intensities of G band and 2D band in Raman spectrum characterizes the number of graphene layers, and it increases with the irradiated time, so the high intensity laser can peel the graphene.
Photonic crystal fibers with an air hole in the core are analyzed by finite-element method. The relations of mode field distribution, confinement loss and dispersion characteristics with fiber structure parameter and wavelength are achieved. The principle of guiding light in air hole is explained by diffraction and the characteristics of photonic crystal fiber. For the fiber with low loss, single-mode, tightly confined light in the air core, the ranges of structure parameters and wavelengths are obtained. An optimal fiber structure is designed, of which the mode is tightly confined in the air core, and the confinement loss α=5.9× 10-5 dB/km. These provide theoretical instruction for the design and fabrication of photonic crystal fiber with an air hole in the core. The high intensity in an air hole, coupled with long interaction length, promises a new class of experiments in light-matter interaction and nonlinear fiber optics.
2012, 61 (21): 214211. doi: 10.7498/aps.61.214211
Quantum well lasers are irradiated by electrons with a total fluence of 1 10-16 cm-2. The output power is reduced and the threshold current is increased under electron irradiation. The displacement damage effect on quantum well laser is analyzed theoretically and we deduce the relationship between the radiation induced output power and threshold current change and the electron fluence. The formula fits the experiment data very well, and can describe the change trend of the laser performance under electron irradiation, it can also predict the behavior of quantum well laser under radiation environment and is valuable for practical application.
2012, 61 (21): 214301. doi: 10.7498/aps.61.214301
Acoustic scattering from a submerged finite cylindrical shell with periodically bulkheads, insonified by an incident plane wave, is studied. The motion of shell is described by Donnell equations, while the motion of bulkheads is described by thin plate motion equations and plane stress equations with considering axial force circumferential force radial force and bending moment simultaneously. Through angle-frequency spectrum besides the surface elastic waves, Bragg waves and Bloch-Floquet waves for the periodically bulkheads, the interactions between bulkheads resonance can be seen. Finally the theory is verified by experiment. Bragg waves, partial Bloch-Floquet waves and resonance of bulkheads highlight are all accordant with theory.
2012, 61 (21): 214302. doi: 10.7498/aps.61.214302
The low-frequency underwater sound absorption phenomenon induced by the localized resonance in phononic crystal shows a promising application for the design of underwater acoustic absorption material in recently study. To further reveal the sound absorption mechanism and optimize the low-frequency underwater sound absorption characteristic, the viscoelastic coating embedded with various shapes of scatterer is investigated. In this paper, to shorten the computing time of the original finite element program and save the core memory, the conventional finite element method is simplified due to the symmetry of the lattice and the scatterers, then the simplified finite element method is validated by comparing the results of the simplified finite element method with those of the conventional finite element method. The relationship between the resonance mode described with the displacement contours of one unit cell at specified frequencies and the corresponding absorption spectrum is discussed in detail, which reveals briefly the sound absorption mechanism of the viscoelastic coating embedded with cylindrical scatterer. Finally, the shape effect of the scatterer on the sound absorption characteristics is investigated, spherical scatterers and the three cylindrical scatterers with different shapes but the same volume are considered. Further, the influence of the density of the core on acoustic absorption characteristic under an air backing is discussed. The results show that lower sound absorption properties can be deduced by adopting the cylindrical scatterers and reducing the radius of the base of circular cylindrical scatterer, and the absorption bandwidth can be improved by choosing the optimal scatterer material.
2012, 61 (21): 214501. doi: 10.7498/aps.61.214501
A new symmetry of a relativistic mechanical system is put forward, and the corresponding conserved quantity is given. The new symmetry is defined in such a way that if each solution to the differential equations of motion of a relativistic mechanical system corresponding to a set of Birkhoff's dynamical functions satisfies the differential equations of motion obtained by other set of Birkhoff's dynamical functions and vice versa, then the corresponding invariance is called a symmetry of Birkhoffians. We prove that the coefficient matrix which relates to the relativistic Birkhoff's equations obtained from two sets of Birkhoff's dynamical functions, is such that the trace of all its integer powers is a conserved quantity of the system, and therefore a theorem known for nonsingular equivalent Lagrangians presented by Currie and Saletan is extended to a relativistic Birkhoffian system. Two examples are given to illustrate the application of the results.
2012, 61 (21): 214502. doi: 10.7498/aps.61.214502
According to the point of view that static dense granular matter is a nonlinear elastic body, we derive an analytic sound speed expression given by its elastic potential, for the case of samples with cylindrical symmetry and sound waves propagating in the directions along and perpendicular to the symmetry axis. By a detailed comparison of the expressions with experiments reported in the literature, we conclude that the acoustic method can determine all material parameters appearing in the elastic potential accurately, which provides us with an important way of studying detailed nonlinear elasticity behaviors of granular materials, if stresses, density, and their uniformity of samples are thoroughly controlled and measured.
2012, 61 (21): 214503. doi: 10.7498/aps.61.214503
In this paper, a multi-lane cellular automata model is adopted to investigate the evolution of traffic flow in a two-off-ramp system. The variable message sign (VMS) is used to realize traffic flow guidance. More attention is paid to the decision-making behaviors of the driver, i.e., choosing the target off-ramp and its influence on road capacity. The original model without VMS and the improved model with VMS are proposed respectively. Through numerical simulation, the corresponding phase diagram, the capacity, the flux of each section and typical patterns are obtained, then the effects of the injection probabilities, the fraction of leaving vehicles and the length of the exit lane on capacity are analyzed in detail. When the length of the exit lane is rather short but the entering probability and the fraction of leaving vehicles are rather large, the road capacity is increased noticeably with the help of VMS and therefore the state of traffic flow is improved. It is shown that the utilization efficiency of downstream off-ramp is enhanced and traffic jams are alleviated due to the guidance of VMS.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2012, 61 (21): 215201. doi: 10.7498/aps.61.215201
A new method to obtain the q-profile and current density distribution from Faraday rotation angle measured by polarimeter is introduced. The large aspect-ratio hypothesis is taken into consideration in plasma equilibria discharge analysis. The q-profiles and current density profiles are obtained in Ohmic and ECRH discharges. The calculation results accord well with the soft X-ray measurements.
2012, 61 (21): 215202. doi: 10.7498/aps.61.215202
Under intense electric field, the electrons emitted from the cathode by field emission have strong space charge effects, so the space charge limited current of a diode is an important parameter in the design of many intense electron beam apparatus, such as the high power microwave source devices. The field emission current density depends on cathode material and electric field at the cathode surface, while the space charge limited current density is a function of applied voltage and diode gap distance. To investigate the influence of the diode gap distance on space charge effect in field emission, in the paper we build a model of a planar vacuum diode operating with a field emission cathode. The time evolutions of the electric field at the cathode surface with various diode gap distance and applied voltage are studied using the particle-in-cell method, and the steady value of the electric field at the cathode surface is obtained. The electric field at the cathode surface first oscillates and finally reaches a steady state. At a given applied electric field, the longer the diode gap distance, the higher the absolute value of the electric field at the cathode surface is, and it takes more time to reach the steady state for longer diode gap distance; the distribution of the electric field in the diode gap region is steeper for shorter diode gap distance after the electric field at the cathode surface has reached a steady state.
2012, 61 (21): 215203. doi: 10.7498/aps.61.215203
Taking advantage of the glowing discharge technology and plasma mass spectrometry diagnosis technology, the variation rules of positive ion species and energy with power in H2/C4H8 mixture gas are investigated. The characteristic ions are measured and their forming process is discussed. The results show that the intensity of the C-H segment ions in the H2/C4H8 plasma increases with radio-frequency (RF) power increasing, the intensity reaches a maximum at an RF power of 20 W, and then reduces after the RF power has reached 25 W. The relative concentration of m/e 57 (C4H9+) is highest when the power is less than 10 W, and when the power is more than 10 W, the relative concentration of m/e 39 (C3H3+) reaches a maximum. The energy of the C-H segment ions increases gradually with the increase of RF power. The qualitative analyses of H2/C4H8 plasma composition and energy will be beneficial to improving the fabrication technology of glowing discharge polymer coating.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Study of pulsed field emission characteristics and simulation models of carbon nanotube cold cathodes
2012, 61 (21): 216101. doi: 10.7498/aps.61.216101
For the application requirement of a carbon nanotube clod cathode (CNCC) used in the microwave and millimeter wave electric vacuum radiation source devices, pulsed field emission characteristics of CNCC are experimentally investigated by a 2 μs/20 kV high-voltage modulator in this paper. The pulsed field emission currents for different distances between anode and cathode and those of CNCC after the pulsed high-tension arc has occurred many times are studied. A maximal emission current of 16 mA is tested for CNCC with a 4 mm diameter emission disk, and the emission current density is achieved to be 127 mA/cm2. Based on the experimental results, the field emission simulation model of CNCC is established by PIC simulation software. Simulation results show that this simulation model is suited for describing the experimental results and laying the foundation for the development of CNCC electron optical system and radiation source devices.
2012, 61 (21): 216102. doi: 10.7498/aps.61.216102
Ternary Ti61.2Cu32.5Fe6.3 quasiperitectic alloy is rapidly solidified in drop tube. The diameter of the obtained droplets varies from 80 to 1120 m. The theoretical analysis indicates that the range of undercooling is from 34 to 293 K (0.23TL). Due to the influences of containerless, microgravity, ultrahigh vacuum, etc, the microstructure of solidified alloy is composed of Cu0.8Fe0.2Ti phase, CuTi2 phase and CuTi3 phase. This result deviates appreciably from the equilibrium state. CuTi3 phase exhibits a conspicuous solute trapping effect during rapid solidification. The microstructure of alloy consists chiefly of eutectic (Cu0.8Fe0.2Ti and CuTi2 phases) and dendrites (Cu0.8Fe0.2Ti, CuTi3) structure. With the increase of undercooling, the microstructure of eutectic experiences a transition from strip eutectic cell to ellipsoidal eutectic cell to spherical eutectic cell; the morphology of Cu0.8Fe0.2Ti dendrite experiences a transition from coarse dendrites to broken dendrites to anomalous grain; while the morphology of CuTi3 dendrite changes from small block to coarse dendrite.
2012, 61 (21): 216801. doi: 10.7498/aps.61.216801
A compact hydrophobic nanoparticle (HNP) adsorption layer, which has miro- and nano-dual structural properties like lotus leaf, can be built by adsorbing HNP on core surfaces. A slip velocity on the surface can be produced with the purpose of reducing the water resistance and increasing water injection rate. The results are of significance for the study of HNP drag reduction technology. In this paper we give a briefing of the super hydrophobic properties of the lotus leaf, mosquito legs, and striders leg. The strong hydrophobic surface preparation method with HNP adsorption layer is presented, and physical mechanism of strong hydrophobic surfaces with regular arrangement of HNPs is given. According to the real HNP adsorption core samples, the contact angle range is given, the calculation results accord well with experimental data. Core displacement experimental results show that the average drag reduction rate can be up to 94%.
2012, 61 (21): 216802. doi: 10.7498/aps.61.216802
The long-wavelength laser on InP (001) is fabricated, the temperature dependence of threshold current density is investigated. The nomalous decrescence of threshold current density is observed with the increase of temperature, which leads to a negative characteristic temperature. The origin of this phenomenon is discussed. We attribute the anomalous temperature dependence of threshold current density to the carrier redistribution effect.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2012, 61 (21): 217101. doi: 10.7498/aps.61.217101
Based on the first-principles density functional theory, the electronic structures, densities of states and optical properties of the structures α-Nb5Si3, β-Nb5Si33 and γ-Nb5Si are calculated by using the local density approximation and plane wave pseudopotential method. The calculation results show that the valence band of Nb5Si3 near the Fermi energy is composed of Nb 4d, Si 3s, 3p orbits and the conduction band is comprised mainly of Nb 4d orbit; the optical properties of Nb5Si3 possess anisotropic characteristics, the static dielectric function ε1(0) of Nb5Si3 is about 207, and the refractive index n0 is 13. Their absorptions in a range above 15 eV approach to zero, showing the optical transparent behaviors.
2012, 61 (21): 217102. doi: 10.7498/aps.61.217102
The electronic structure and photoelectric properties of semiconductor material OsSi2 epitaxially grown on a Si(111) substrate are invesligated using the pseudo potential plane wave method based on first principles method. The calculated results show that OsSi2 is an indirect semiconductor material with a band gap of 0.625 eV. The valence band of OsSi2 epitaxially grown on a Si(111) substrate is composed mainly of Si 3s, 3p and Os 5d, and the conduction band is comprised mainly of Os 5d as well as Si 3s, 3p. The static dielectric function is 15.065, the reflectivity is 3.85, and the biggest peak of the absorption coefficient is 3.9665× 105 cm-1. Furthermore, the static dielectric function, refractivity index, reflectivity, absorption, conductivity and loss function of OsSi2 epitaxially grown on a Si(111) substrate are analyzed in terms of the calculated band structure and density of states. The results offer theoretical data for the design and application of OsSi2.
2012, 61 (21): 217103. doi: 10.7498/aps.61.217103
In In0.6Ga0.4As/GaAs quantum dot, using a one-dimensional effective potential model and the finite difference method, we theoretically study the properties of an exciton under the influence of an applied magnetic field, such as the transition energy, the binding energy, the spatial distributions of the electron and the hole. The effects due to the applied magnetic filed and the quantum confinement on the binding energy are analyzed, and the following results are obtained: the ground state transition energy of the heavy-hole exciton can split into four energy levels due to the Zeeman effect, of which the results are in good agreement with experimental results; the binding energy increases monotonically with the increase of lateral confinement or magnetic field; the size of the quantum dot has a significant influence on the binding energy of the exciton, which can be seen both from the average distance between the electron and the hole and from the wave function distributions of the exciton.
2012, 61 (21): 217104. doi: 10.7498/aps.61.217104
A series of Ag-doped p-type Agx(Pb0.5Sn0.5)1-xTe compounds is prepared by melting followed by slow-cooling process, and the phase compositions, microstructures and thermoelectric properties are also systematically investigated. The introduction of Ag in Pb/Sn site effectively increases the hole density which is much lower than the theoretically predicated value in the approximation of complete substitution and single acceptor of Ag, in spite of the fact that all samples show finely single phase for the 5% Ag-doped sample. This implies that part of Ag atoms enter into the interstitial sites acting as electron donor to reduce the hole density. With the increase of Ag content, the electrical conductivity increases gradually and the Seebeck coefficient shows an opposite variation tendency, mainly owing to the variation of hole density. Interestingly, the anomalous crossover of Seebeck coefficient at about 450 K indicates the transition of dominating valence valley from light-band to heavy-band while temperature is higher than 450 K. Consequently, due to the optimization of hole density and the domination of heavy band with large effective mass, 1% Ag-doped sample obtains a highest power factor of 2.1 mWm-1K-2 at 750 K, which results in a highest ZT of 1.05 combined with the suppressed lattice thermal conductivity via intensifying point defect phonon scattering. This high ZT is ～ 50% higher than that of Ag-free sample and also higher than commercial p-type PbTe material. Further, the 50% substitution of toxic and heavy Pb by Sn is beneficial for the practical application and environmental sustainability of PbTe-based materials.
2012, 61 (21): 217301. doi: 10.7498/aps.61.217301
In this article, we investigate the optical and electrical properties of surface plasmon polariton of Ag nanoparticles influenced by surrounding medium with different conductivities. Ultraviolet-visible infrared spectra and surface enhanced Raman scattering show that when the Al2O3 films are used as a surrounding medium, the optical loss caused by surface plasmon polariton of Ag nanoparticles will be reduced and the surface electromagnetic (EM) field will be enhanced, while conductive substrate may lead to the surface plasmons transmitting along the substrate layer then reduce the EM field and enhance the optical absorption. This property provides an effective approach to the use of optical and electrical properties of surface plasmon polariton in thin film solar cells when a thin film of Al2O3 is added as a cover layer.
2012, 61 (21): 217302. doi: 10.7498/aps.61.217302
In this paper, the characteristics of the two-dimensional hole gas (2DHG) in p-GaN/p-AlxGa1-xN heterojunction is investigated in detail, based on self-consistent solutions of one-dimensional Poisson and Schrdinger equations. The valence band structures and the 2DHG distributions are calculated in the cases of different Al components and piezoelectric polarization effects. Then, the influences of Al components and piezoelectric polarization effects on 2DHG are analysed specifically. The results show that with the increase of Al component, the quantum well at the heterojunction interface turns deeper and narrower, which leads to an accelerated growth of the 2DHG peak density and a line increase of the 2DHG sheet concentration. Furthermore, piezoelectric polarization effects also make the quantum well at the heterojunction interface deeper and narrower, at the same time, the Fermi level moves close to the top of the barrier and the location of peak density moves close to the heterojunction interface. In addition, the influences of valence band offset and acceptor doping concentration on 2DHG are relatively small. Ohmic contact of p-AlxGa1-xN is fabricatea with the 2DHG, and its I-V characteristic is much better than that without the 2DHG, which indicates that the 2DHG can significantly improve the performance of p-AlxGa1-xN ohmic contact.
2012, 61 (21): 217303. doi: 10.7498/aps.61.217303
An analysis of band structure, wave function distribution and absorption of linearly polarized light along the  direction in InAs/GaSb quantum well grown along the  direction is performed by the eight-band K-P model and finite difference method. Our study shows that the band structure and wave function distribution could be regulated effectively by changing the thickness of InAs or GaSb layer. When the bottom of conduction subband and the top of the valence subband are in resonance, the hybridization of ground electron and light-hole state at the zone-center is very weak, and the overlap between the wave function of the ground and the first-excited electron state is considerable, according to the theory of wave function engineering, so the transition rate between the ground and the first-excited electron state at the zone-center is larger than that when the bottom of conduction subband and the top of the valence subband are not in resonance. This is very important for designing advanced optoelectronic devices such as far-infrared or mid-infrared cascade lasers and detecters based on InAs/GaSb quantum wells.
2012, 61 (21): 217304. doi: 10.7498/aps.61.217304
High-mobility In0.6Ga0.4As channel metal oxide semiconductor high electron mobility transistor (MOSHEMT) and metal oxide semiconductor field effect transistor (MOSFET) are investigated based on simulation and experiment in this paper. It is found that InAlAs barrier layer has a great influence on the characteristics of In0.6Ga0.4As MOSHEMT. In0.6Ga0.4As MOSHEMT exhibits excellent electrical characteristics compared with In0.6Ga0.4As MOSFET. The experimental results show that the effective channel mobility of MOSHEMT is 2812 cm2/V·s-1, which is 3.2 times that of MOSFET. A 0.02 mm gate length MOSHEMT shows higher drive current, peak transconductance, Ion/Ioff ratio and gate breakdown voltage and lower sub-threshold swing than the MOSFET with the same gate length.
Dependence of the DC stress negative bias temperature instability effect on basic device parameters in pMOSFET
2012, 61 (21): 217305. doi: 10.7498/aps.61.217305
To analyze the dependence of the DC stress negative bias temperature instability (NBTI) effect on basic device paraments, such as the channel length, the gate oxide thickness, the doping concentration, we solve the hydrogen molecule drift-diffusion model of NBTI together with the semiconductor device equations. The results are compared with the existing experimental data and the basic laws and physics of devices, which is necessary for reliability studies of NBTI. The analysis results show that NBTI effect is not affected by the channel length change, but maily by the thickness of the gate oxide layer. Gate oxide thickness thinning and gate oxide layer electric field enhancement effect are consistent, which determines the device degradation in the manner of exponential law. With channel doping concentration increasing, NBTI effect will be reduced, which is because the device channel surface hole concentration is reduced, however with the doping concentration increases to such a value that the device source drain leakage current is very low (low leakage device), the MBTI effect is obviously enhanced. These are helpful for understanding NBTI and designing the high performance device.
2012, 61 (21): 217306. doi: 10.7498/aps.61.217306
Because of its excellent non-volatile storage characteristics, simple structure, fast storage, low energy consumption and high integration, memristor has aroused a widespread interest in the field of new electronic devices. In this paper, metal-insulator-metal stack of memristor is introduced and relative memristive material, its mechanism as well as the application in the field of electronic circuits and artificial intelligence are summarized. The significant role of interfacial effects on memristive behavior and improvement of its performance is emphasized on. Especially, the effects of interface nanodots on the optimization of memristor properties are proposed. The research prospects of memristor are also analyzed and discussed.
2012, 61 (21): 217307. doi: 10.7498/aps.61.217307
In order to analyze the influence of dielectric truss structure on frequency selective surfaces (FSS) transmission characteristics, as an example, the FSS of Y ring unit and the truss made of polyimide are investigated by the finite difference time domain method. The relevant physical model is developed and pore blocking rate is defined. Through the analysis it is proved that the variations of truss period and rib width will affect the FSS, so they become the main parameters to measure dielectric truss structure, and that the improvement of the aperture stop ratio will increase transmission loss. When truss period changes from infinity to 80 mm the transmittance of passband is reduced by more than 0.9 dB on an average; when the ribs width increases from 0 to 10 mm, the transmittance of passband is reduced by more than 0.6 dB on an average. When aperture stop ratio is lower than 12.11%, the passband produces no shipt. When aperture stop ratio is less than 4.12%, the influence of the truss on FSS is negligible. The FSS specimens is fabricated by coating and lithography process, the dielectric truss is machined by numerically controlled machine, and the microwave measurement is carried out in a dark room. The experimental results and the calculation results are verified to be in good agreement with each other. Therefore the present study presents an experimental and theoretical reference for designing the FSS stealth radar with the dielectric truss structure.
2012, 61 (21): 217701. doi: 10.7498/aps.61.217701
Based on the high dielectric tunability, high dielectric constant and small dielectric loss at paraelectric phase, lead-free relaxor ferroelectrics have been used in microwave devises widely. According to the present theory of dielectric tunability, the expression of dielectric tunability is derived by adjusting parameters properly. The derived expression can be used to deal with experimental results. It is found that there exists a great difference between experimental result and the theoretical result obtained under the assumption of unchanged paraelectric phase under an electric field, while the experimental result and the theoretical result of Johnson in 1962 is consistent. A modified relation of dielectric constant with higher electric field is derived by considering the Gibbs free energy modified with an external electric field and a spontaneous polarization. The result from the modified equation is in agreement with the experimental result. A concept of degree of tunability is proposed to express quantitatively the relationship between dielectric tunability and concentration of dopants.
2012, 61 (21): 217801. doi: 10.7498/aps.61.217801
Photoacoustic imaging is an advanced medical imaging technology based on the effect of ultrasound generation by electromagnetic irradiation. However in photoacoustic imaging, because the optical property of tissue is always very complex, the aberration of light in tissue is ineluctable and the image spot is dispersed, which greatly influences the photoacoustic image resolution. For improving the lateral resolution of photoacoustic imaging, the adaptive optics multispectral photoacoustic imaging system is proposed in this paper. In this system an adaptive optics sub-system is designed to correct the wavefront errors of the illuminating light for obtaining high-resolution images of biological tissues. Moreover, the multispectral imaging is combined to obtain different wavelength photoacoustic images, which is useful for distinguishing organization accurate structure, identifying organizational components, etc. The simulation results demonstrate that when the wavefront errors are corrected by the adaptive optics system, the image resolution and quality are improved significantly. This research will be helpful for improving the ability and application of photoacoustic imaging.
Preparation and luminescence properties of a red phosphor Mg5SnB2O10:Eu3+, Bi3+ for light emitting diode
2012, 61 (21): 217802. doi: 10.7498/aps.61.217802
A novel Mg5SnB2O10: Eu3+, Bi3+ red phosphor is synthesized by a solid-state reaction, whose structure and photoluminescence properties are investigated through X-ray diffraction, diffuse reflection spectrum and photoluminescence spectrum. The excitation spectrum centred at 393 nm reveals the 7F0-5L6 characteristic transition of Eu3+, which matches well with the near ultraviolet chip for light emitting diode (LED). In the emission spectrum under 393 nm excitation, peaks at 591, 612 and 701 nm could be attributed to the transitions of 5D0-7F1, 5D0-7F2 and 5D0-7F4 of Eu3+, respectively. With the introduction of Bi, the emission intensity is improved due to the energy transfer from Bi3+ to Eu3+. In this paper, the optimal doping concentration of Eu3+ and Bi3+ is determined to study the photoluminescence properties and the optimal sample Mg4.89Eu0.1Bi0.01SnB2O10 is obtained. The integral intensity of the emission spectrum for Mg4.89Eu0.1Bi0.01SnB2O10 phosphor excited at 393 nm is about 1.1 times stronger than that of Y2O2S: Eu3+ excited at 394 nm.
2012, 61 (21): 217803. doi: 10.7498/aps.61.217803
Amorphous SiOx:C particles are prepared by pyrolyzing method, and then they are calcined in an air ambient at different temperatures. The structures mophologies and optical properties of samples are analyzed with FTIR spectrum, scanning electron microscopy and fluorescent microscope, respectively. The results show that the luminescence band is blue-shifted with the increase of the annealing temperature. The particles exhibit the highest photoluminescence intensity with the 417 nm peak when annealed at 500 ℃. And the particles possess red, green or blue light emissions at room temperature when irradiated with appropriate wavelengths. Upon heating at a higher temperature (600 ℃ or 800 ℃), the fluorescence intensity of the SiOx:C sample decreases. We think that the phenomenon is attributed to the reduction of the number of oxygen defects in the sample heated at high temperature.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Growth evolution of Ge quantum dot modulated by the atom bombardment during ion beam sputtering deposition
2012, 61 (21): 218101. doi: 10.7498/aps.61.218101
The Ge quantum dots on Si substrate are prepared by ion beam sputtering deposition (IBSD). The growth evolution is observed to experience two stages with Ge coverage (θ) increasing. When θ increases from 6 monolayers (ML) to 10.5 ML, the average base width and height of quantum dots both increase, and the dome shape dots with small aspect ratio values are obtained. As the dots grow up, Ge atoms are also accumulated in the wetting layer, which contributes to the observed quantum dot density increasing mildly during this stage. When θ is in a range from 11.5 ML to 17 ML, vertical growth dominates the dot evolution. Another dome shape quantum dots are prepared with large aspect ratio values. Ge coverage gain results in the dot density increasing rapidly. A wetting layer decomposition process is demonstrated to give significant effect on that. The growth transition occurs as θ increases from 10.5 ML to 11.5 ML, and the dot density is enhanced 6.4 times in this course. So it is concluded that the evolution of Ge quantum dot prepared by IBSD is very different from that deposited on the thermal equilibrium condition. The observed characters of the dot shape and size distribution result from the kinetic behaviors of the surface atoms which are restricted by the thermodynamic limitation. Ge coverage is the one of the most important factors which can change the free energy. On the other hand, the energic sputtered atom bombardment enhances surface diffusion and defers nucleation of three-dimensional islands until the superstrain wetting layer is formed, which can also change the system free energy and the surface atom kinetic behaviors. So the growth evolution of Ge quantum dots prepared by IBSD is related so much with the effect of atom bombardment on the quantum dot growth.
For the numerical simulation of hot working process of spray-formed FGH4095, the high temperature deformation behavior of spray-formed FGH4095 is studied at different deformation temperatures (1010-1140 ℃) and different strain rates (0.001-10.0 s-1) using Gleeble 3800 thermal simulator. The constitutive equation is established with the sinh model. The calculative results are consistent with the experimental data. It indicates that the accurate material model is provided for the numerical simulation for hot working process of spray-formed FGH4095.
Two-dimensional numerical analysis of the collection mechanism of single event transient current in NMOSFET
2012, 61 (21): 218501. doi: 10.7498/aps.61.218501
The single event effects in NMOSFET at different values of drain bias, gate length and striking location are thoroughly analyzed by two-dimensional numerical simulator in this paper. The results show that single event transient current increases with the increase of drain bias and with the decrease of gate length. Furthermore, single event transient current varies with the electric field at some places in the device. The present study provides important guidance on the devices design of mitigating the single event effects.
The influence of modified electrodes by V2O5 film on the performance of ambipolar organic field-effect transistors based on C60/Pentacene
2012, 61 (21): 218502. doi: 10.7498/aps.61.218502
C60/Pentacene-based ambipolar organic heterostructure field-effect transistors (AOFETs) with Al source-drain (S/D) electrodes modified by inserting a transition metal oxide (V2O5) layer are fabricated. Compared with the device without V2O5 modified layer, the modified device shows good ambipolar characteristics with a hole mobility of 8.6× 10-2 cm2/V·s-1 and an electron mobility of 6.4× 10-2 cm2/V·s-1, and threshold voltages of 25 and -25 V, respectively. These performance improvements are ascribed to the presence of V2O5 layer at the Pentacene/Al interface which significantly reduces the source/drain contact resistance, increases the holes injection and makes electronic and hole injection close to balance. This result indicates that modified electrodes by V2O5 film is an effective approach to fabricating low cost and high performance AOFETs for realizing commercial applications.
A study on the mechanism of RecA in homologous recognition by using single molecule fluorescence tracking
2012, 61 (21): 218701. doi: 10.7498/aps.61.218701
RecA plays an important role in homologous recognition in prokaryotes, and it has become a hot point in homologous recognition related research since its discovery. We establish an assay by combining total internal reflection fluorescence and flow stretching to visualize in real time the motion of single RecA-ssDNA filaments which are tagged with fluorescent beads. This enables us to study the interaction of RecA-ssDNA filaments with their templates in the homologous recognition process. It is found that the searching and binding is a short-time (τ=0.2 s) and short-distance (l=1.05 μm) process. Two distinguished motion modes for the RecA-ssDNA filament are observed, a Brownian motion and a directed motion. The observations suggest a model that a RecA-ssDNA filament just interacts weakly with the template DNA before it binds firmly to the template DNA. If no homologous site is found in a searching process, the filament drops off the template and repeats the searching process again until it finally finds its target.
2012, 61 (21): 218702. doi: 10.7498/aps.61.218702
In site quantitative, high-contrast and high-resolution imaging of micro/nanoscale material is an important goal of the X-ray microscopy and imaging. A novel method which is called lensless imaging or coherent X-ray diffraction imaging, is a promising approach to solving these problems. In this review, a brief introduction to imaging theory and development of coherent X-ray diffraction imaging, and some typical applications in material science and biology are presented. For instance, two-dimensional (2D) imaging of Bi dopant distribution in a Si crystal, quantitative three-dimensional (3D) imaging of a GaN quantum dot with core shell structure, 2D imaging of stained Escherichia coli bacteria, nanoscale imaging and mechanisms of biomineralization of fish bones, 2D high-contrast imaging of an unstained herpes virus, 3D high-resolution imaging of an unstained yeast cell and in situ quantitative analysis are illuminated. Finally, the future prospect of coherent X-ray diffraction imaging is given. With the development of X-ray free electron lasers and combining cryogenic techniques with coherent X-ray diffraction microscopy, coherent diffraction imaging will be a powerful tool and widely used in materials science and biology.
2012, 61 (21): 218703. doi: 10.7498/aps.61.218703
The technique of laser range-Doppler image has been intimately noticed by aerospace and national defense circles. A analytical model of laser range-Doppler image of cone-cylinder combination is proposed in this paper. The analytical model includes the contribution of disk base. The analytical model can provide the effects of geometry parameters, the roughness of the surface, attitude, and pulse duration on laser range-Doppler image. This analytical model can reduce into the analytical model of range-Doppler image of cone and cylinder. This analytical model can reduce into the analytical model of Doppler spectra for plane waves. The influences of geometry parameters, and attitude are analyzed numerically by using the analytical model. The results indicate that the laser range-Doppler image of cone-cylinder combination can show the information about geometrical shape and attitude of target. Combining the theory and measurements, the analytical model can be used for indentifying physical parameters and geometrical parameters of cylinders, cones and cone-cylinder combination. This analytical solution is of significance for the laser Doppler velocimetry and ladar applications.
2012, 61 (21): 218901. doi: 10.7498/aps.61.218901
Community structure has an important influence on the structural and dynamic characteristics of the complex system. In the present study, a group similarity model is proposed for the measurement of similarity between two communities. So it can help us understand the mechanism of inter action between these communities. Moreover, based on this model, a hierarchical clustering based algorithm for network community structure detection is put forward. By this algorithm, one pair of communities with the largest similarity is merged in each iteration. And then an evaluation function is adopted for choosing the optimal partition. The algorithm gives a higher performance than many state-of-the-art community detection algorithms when tested on a series of real-world and synthetic networks. Especially, it performs better when the edge density of the network is high.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
The characteristics of turbulent momentum and heat similarity function and bulk transfer coefficient over grassland surface
2012, 61 (21): 219201. doi: 10.7498/aps.61.219201
The turbulent momentum and sensible heat transfer over land surface have a notable influence on the change of global climate and atmospheric circulation, and Monin-Obukhov similarity function is a most important method to calculate the turbulent momentum and sensible heat flux near the surface, and ascertaining the right bulk transfer coefficient is a most effective way of improving the atmospheric model simulation capabilities. The characteristic of Monin-Obukhov similarity function is analyzed and the empirical formula is fitted, and the changes of bulk transfer coefficients of momentum and sensible heat over grassland with mean wind speed at 10 m high are discussed by using the data of the flux observations over Xilin Gol grassland in Spring 2008. Comparison with the observation values by eddy correlation method shows that the revised Monin-Obukhov similarity function underestimates the momentum flux by 10.8% and over estimates the sensible heat flux by 6.5%, but the typical Businger-Dyer similarity function underestimates the momentum flux by 37.0% and over estimates the sensible heat flux by 16.1%. Under unstable stratification, the bulk transfer coefficients of momentum (CD) and sensible heat (CH) vary with mean wind speed at 10 m high (U) according to the power law, which take the forms CD=0.009U-0.322 and CH=0.184U-1.978 respectively. Under stable stratification, the bulk transfer coefficients are found to increase in the manner of the logarithm law over grassland surface and tend to neutral or nearly neutral values with wind speed increasing. The revised Monin-Obukhov similarity function can significantly improve the accuracy of turbulent momentum and sensible heat flux computed by average gradient data, and the relations between bulk transfer coefficients and wind speed at 10 m high provide the useful parameterization schemes for accurately expressing the transportation characteristics of near surface turbulence.
2012, 61 (21): 219401. doi: 10.7498/aps.61.219401
In the solar-terrestrial space environment, the ionosphere couples tightly with the upper magnetic layer as well as the lower middle atmosphere in various forms. Meanwhile, the ionosphere can affect radio-communication and satellite navigations, so the research on ionosphere prediction model is very important. Now, the accuracy of statistic prediction mode is about 60%, but cannot meet the practical requirements. In order to solve the problem, the prediction model of total electron content (TEC) data is achieved in three major phases: decomposition of the spatiotemporal variability of the TEC data, noise reduction of the encoded space, and time variability and the prediction, by a nonlinear forecasting technique of the time variability. Experiments show that the new prediction model is better than traditional prediction model. The prediction data shows realistic features and a reliable physical distribution, and the relative accuracies of prediction for 1, 2, 4, and 7 d obtained by our method is 0.32, 0.48, 0.68 and 0.94 TECU.
2012, 61 (21): 219402. doi: 10.7498/aps.61.219402
The determination of the varying position and shape of magnetopause is one of the important Gordian knots in geophysics and space physics. According to the solar wind-magnetosphere-ionosphere coupling global magnetohydrodynamic (MHD) simulation, and with the maximum electric current criterion, we study the position and shape of the magnetopause under several solar wind dynamic pressure (Dp) and interplanetary magnetic field conditions. The simulation results show that the subsolar position (r0) of the magnetopause is controlled mainly by Dp with the significant decrease of r0 as Dp increases. At a certain Dp, when southword Bz (Bz0) decreases to zero, then shifts to northward (Bz0) and increases, the subsolar position r0 keeps increasing. For all cases studie here, the flare angle () of the magnetopause experiences small changes. This provide an evidence for the structural self-similarity of magnetopause in equatorial plane. Compared with the empirical low-latitude magnetopause model of Shue98, MHD simulation can reproduce the dependence of the subsolar point r0 on Dp, while the saturation effect of r0 varying with Bz in empirical model is represented only with slow solar wind. As to the flare angle , although the difference between MHD simulation and empirical model is less than 2.5, the variation of with Bz in MHD simulations is nonlinear and different from the linear trend in empirical model.
2012, 61 (21): 219501. doi: 10.7498/aps.61.219501
The variability sky survey is a very effective method of searching for quasars. In the paper, we present a novel method to discriminate quasars from variable stars based on their intrinsic variabilities. The power-law model is used to fit the light-curve structure function in five wavebands, but the value of the structure function is normalized. The developed method is applied to 1411 spectroscopically confirmed quasars and 174 stars in SDSS stripe 82. We set a suitable criterion, then obtain a reasonable classified result. The classification accuracies about quasars and stars reach 92.2% and 83.6% respectively. Compared with the structure function only based on one physical parameter, the normalized structure function with amplitude and power-law index has a good efficiency to deal with these data. The method for selecting quasar candidates ensures big sky survey telescope a high survey efficiency and saving valuable astronomical observation time. The results support that the optical variabilities of most of quasars originate from the instabilities of accretion disks.