Vol. 64, No. 9 (2015)
2015-05-05
GENERAL
2015, 64 (9): 090501.
doi: 10.7498/aps.64.090501
Abstract +
The inherent features, such as non-periodic, wide band spectrum, and extreflely sensitive to initial values etc. make it quite a challenge to blindly separate the mixed chaotic signals. A new blind source separation method based on the artificial bee colony algorithm is proposed in this paper. This method can recover chaotic sources from noisy observations on their linear mixtures without any prior information about the source equations. The proposed method is structured in the phase space of the demixed signals, which is reconstructed from the observations by using delay-embedding method. An objective function in the reconstructed phase space is designed so that the blind source separation problem is transformed into an optimization problem. The optimal demixing matrix is obtained by maximize the objective function with an artificial bee colony optimizer and the chaotic sources are then recovered by multiplying the observed mixtures and the optimal demixing matrix. Before the optimization procedure is made, a pre-whitening should be employed. Additionally, the parameterized repreflentation of orthogonal matrices through principal rotation is adopted to reduce the dimension of the optimization procedure so that the proposed blind source separation algorithm can converge quickly. Different from the traditional independent component analysis approaches which concern mainly the statistical features, the proposed blind source separation method utilizes the dynamics in the observed mixtures by means of phase space reconstruction. Therefore, better performance can be achieved when it is used to deal with chaotic signals.
In computer simulation, two cases are taken into consideration: namely, the mixture is noiseless or not contaminated by noise. The correlation coefficient criterion and the performance index criterion are adopted to evaluate the separation performances. Simulation result shows that in most cases the proposed approach converges within a few tens of iterations and the chaotic sources can be accurately recovered. The impact of noise level and signal length on the separation performance is investigated in detail. The overall performance of the proposed approach is much better than the traditional independent component analysis approaches. Moreover, the capability of separating the mixed chaotic and Gaussian signals reflealed in the simulation indicates that the proposed approach has the potential to be applied in a wider range of applications.
Nonlinear electron transport in superlattice driven by a terahertz field and a tilted magnetic field
2015, 64 (9): 090502.
doi: 10.7498/aps.64.090502
Abstract +
Vertical electron transport in semiconductor superlattice has been the focus of science and technology during the past two decades due to the potential application of superlattice in terahertz devices. When driven by electromagnetic field, many novel phenomena have been found in superlattice. Here we study the chaotic electron transport in miniband superlattice driven by dc+ac electric fields along the growth axis (z-axis) and a magnetic field tilted to z-axis using semiclassical equations of motion in the preflence of dissipation. We calculate the electron momentum by changing the magnetic field or amplitude of the terahertz field. It is shown that the momentum py(t) of miniband electron exhibits complicated oscillation modes while changing the control parameters. Poincaré bifurcation diagram and power spectrum are adopted to analyze the nonlinear electron states. Poincaré bifurcation diagram is obtained by plotting pym = py(mTac) (with m = 1, 2, 3,… and Tac the period of ac terahertz field) as functions of ac amplitude E1 after the transients decay. The periodic and aperiodic regions can be distinguished from each other since there are a large number of points in the chaotic regions. When the magnetic field is increased from 1.5 to 2 T, the Poincaré bifurcation diagram changes dramatically due to the strong effect of magnetic field on electron motion. The oscillating state of py(t) may be changed between periodic and chaotic syates. Power spectra of electron momentum py for different values of E1 (= 2.06, 2.18, 2.388, and 2.72) are calculated for a deep insight into the nonlinear oscillating mode. It is found that the power spectra of n-periodic states show peaks at frequencies ifac/n (with i = 1, 2, 3,…); the power spectra of chaotic states are very irregular with a large number of peaks. We demonstrate that the dissipation and resonance between Bloch oscillation frequency and cyclotron frequency play an important role in the electron transport process. We attribute the emerging of periodic and chaotic states in a superlattice to the interaction between terahertz radiation and internal cooperative oscillating mode related to Bloch oscillation and cyclotron oscillation. In the case of ωB≠iωc, the time-dependent electron motion is chaotic in most regions of the parameter space. Results of the preflent paper are useful for designing terahertz devices based on the semiconductor superlattices.
2015, 64 (9): 090503.
doi: 10.7498/aps.64.090503
Abstract +
Hamiltonian system theory is an important reflearch tool for nonlinear systems, and has been widely used in motor speed regulation and control during reflent years. Aiming at the chaotic phenomenon in permanent magnet synchronous motors, a design method of robust controller based on the Hamiltonian function is preflented for the chaotic systems. The dynamic model of permanent magnet synchronous motor is transformed into a model similar to the Lorenz chaotic equation, and the model is chaotic at certain parameters according to the Lyapunov exponent and the Lyapunov dimension calculated. Let the rotator speed of the motor track a set of values, an error equation is obtained accordingly. Because the error equation does not satisfy the standard form of Hamilton exactly, it can be transformed into the Hamiltonian system containing uncertain disturbance terms. While the uncertain disturbance terms as well as the load term are regarded as a total disturbance term to the system, a kind of robust controller is designed. The controller consists of two parts. One part is based on the method of interconnection and damping assignment, and can make the rotator speed track any value well; The other part is used as a disturbance compensator. Simulation result shows that the controller drives the permanent magnet synchronous motor out of the chaotic state rapidly and the rotator speed tracks the set of values well. It is proven that the controller is feasible and effective. The method mentioned in this paper extends the range of application of Hamiltonian function and has a certain advantage.
2015, 64 (9): 090504.
doi: 10.7498/aps.64.090504
Abstract +
The terms of gain(or absorption), dispersion, and nonlinearity in the nonlinear Schrödinger equation are usually variables, which can be used to study the propagation of optical pulses in inhomogeneous optical fibers. In this paper, with the aid of the Hirota method, the bilinear forms of the Schrödinger equation are derived. Based on the bilinear form, the analytic dark soliton solutions to the nonlinear Schrödinger equation are obtained. The properties of dark solitons are discussed. Stable dark solitons are observed in the normal dispersion regime. In addition, corresponding parameters for controlling the propagation of dark solitons are analyzed. Results of our reflearch show that the propagation route of solitons can be effectively controlled by the gain(or absorption), dispersion, and nonlinearity, which can improve the quality of signal transmission in optical communications. When the amplitude of the loss coefficient increases, the amplitude of the dark soliton increases suddenly during the transmission process.By means of changing the type of dispersion, the purpose of controlling the dark soliton phase and phase oscillation is achieved. The possibly applicable soliton control techniques, which are used to design dispersion and nonlinearity-managed systems, are proposed. The proposed techniques may find applications in soliton management communication links, like soliton control.In addition, two-soliton solution is obtained. With the dark two-soliton solution, the interaction between two solitons is discussed in the paper. The result may be of potential application in the ultralarge capacity transmission systems.
2015, 64 (9): 090505.
doi: 10.7498/aps.64.090505
Abstract +
The directed transport of a Brownian particle in a spatially periodic symmetric field under a temporal asymmetric force is studied. Based on the Caputo’s fractional derivatives theory, we establish a differential aquation for an overdamped fractional Brownian motor as the system’s mathematic model, where the external force is zero-mean and the fractional order is used to describe the inhomogeneity of the real environment. Using the fractional differential algorithm, we analyze the relationships between transport velocity and model parameters. It is worth mentioning that the impact of fractional order is discussed in detail. According to the reflearch we find that a temporal asymmetric force can induce a net current without the application of a ratchet potential, even a noise. We also find that the velocity of the current increases monotonically with the increase in fractional order. Moreover with certain fractional orders, a generalized resonance phenomenon is reflealed since the velocity of the current varies non-monotonically with the system parameters, such as the height of the potential barrier and the noise strength etc. Research shows that the fractional system is a generalization of the traditional dynamic systems, which could probably give a more reasonable explanation of the directed transport as a consequence.
ATOMIC AND MOLECULAR PHYSICS
2015, 64 (9): 091301.
doi: 10.7498/aps.64.091301
Abstract +
In this work, the first-principles method based on materials studio(a soft ware) and the density functional theory is used to invesigate the properties of charge reflention and charge endurance in HfO2 as a trapping layer in charge trapping memory (CTM). Two supercell models are optimized for the monoclinic HfO2, separately. One contains a four-fold-coordinated O vacancy defect (VO4), and the other is a co-doped composite defect consisting of a VO4 and an Al atom. Interaction energies, formation energies, Bader charge, density of states and trapping energy are calculated for the two models. According to the calculated results of interaction energies and formation energies, it is found that the structure is the most stable and the defect is the most easily formed when the distance between the two kinds of defects is of 2.216 in the co-doped composite defect system. The trapping energy results show that the co-doped composite defect system can trap both electrons and holes. Moreover, the trapping ability of the co-doped composite defect is enhanced significantly as compared with the VO4 defect. Bader charge analysis shows that the co-doped composite defect system provides a more preflerable site for the charge reflention. Calculations of the density of states show that the co-doped composite defect system has a strong effect on the trapping energy of holes. Calculated energy changes after program/erase cycles show that the endurance is improved obviously in the co-doped composite defect system. In conclusion, the date reflention and endurance in the trapping layer of monoclinic HfO2 can be improved by doping of the substitutional impurity Al. This work may provide a theoretical guidance for performance improvement with respect to the date reflention and endurance of CTM.
2015, 64 (9): 093201.
doi: 10.7498/aps.64.093201
Abstract +
The Stark effect in Rydberg atoms has potential applications in the areas of dipole-dipole interaction, quantum information, quantum control, and so on. Many reflevant theoretical calculations and experimental studies about the Stark effect of alkali metal and alkali earth metals have been reported, but the other atom’s Stark effect is studied still relatively less. Our goal in this paper is to reflearch the third main group atom’s Stark effect in a large electric field. First, according to the level data of gallium atom in zero-field, we obtain the quantum defects from the modified Ritz formula in each state by using a nonlinear least-squares-fitting algorithm. The quantum defects as a function of the principal quantum number are analyzed in detail. Influences of both the core polarization and the penetrating valence electron on the quantum defect are discussed according to the fitting results. Then we use the Numerov algorithm to calculate the radial wave functions of atomic gallium. Finally, the Stark structures of Rydberg states around n=7 and n=18 are numerically calculated by matrix diagonalization. Results show that at the levels above n=7 manifold states, (n+1)P is higher than nD state, and it is in contrast to the levels below the n=7 manifold states. This phenomenon is different from the usual Stark structure of alkali metal atoms, the level’s order of which does not change with the principal quantum number. The Stark levels with the identical |m| anti-cross each other, and those with different |m| cross. Our results give an important reflerence for related reflearches, and are of great significance for insight into the atomic structure and the interaction between the atomic core and the highly excited electrons.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2015, 64 (9): 094101.
doi: 10.7498/aps.64.094101
Abstract +
For dealing with circularly polarized waves, a high-efficient two-dimensional dispersionless phase-gradient metasurface is devised and achieved by spatially arranging co-polarized reflective metasurface unit cells. The phase of the co-polarized reflection can be freely modulated via a rotating metallic wire of the co-polarized reflective metasurface unit cell in-plane. The achieved phase gradient metasurface can produce opposite-sign phase gradient for left-and right-handed circularly polarized incident waves. During linearly polarized wave incidence, the reflected waves will decompose into two counter-directionally propagating circularly polarized waves. Reflective power density spectra for the linearly polarized wave in normal incidence are simulated, which are well consistent with the theoretically designed anomalous reflection direction. A 2 mm thick sample is fabricated and the mirror reflectivity curve is measured. Experimental results show that for linearly polarized wave normal incidence, the mirror reflectivity is reduced to below -5 dB in a wide band from 9.5 to 17.0 GHz.
2015, 64 (9): 094102.
doi: 10.7498/aps.64.094102
Abstract +
A method of reducing the in-band radar cross section (RCS) of waveguide slot array antenna by utilizing a metamaterial absorber (MA) is preflented. A novel ultra-thin (the thickness is only 0.01λ, λ is the wavelength corresponding to the MA resonant frequency) MA with high absorptivity and no surface lossy layer is designed; the absorber is composed of two metallic layers separated by a lossy dielectric spacer. The top layer consists of an etched oblique cross-gap patch set in a periodic pattern and the bottom one is a solid metal. Effective impedance of MMA will match the free space impedance by adjusting the dimensions of electric resonant component and magnetic resonant component in the unit cell, and so the reflection will be minimized. Meanwhile, the MMA can obtain a resonant loss to fulfill the high absorption. By finely adjusting the geometric parameters of the structure, we obtain the MA with absorption 99.9%, and its absorbing mechanism being interprefled by analyzing surface current, surface electric field, and volume power loss density distribution, respectively. The metallic area between slots in E plane direction of waveguide slot array antenna is covered by MA, and a distance between the radiating slot and the MA is suitably arranged. Antenna radiation performance is kept in good order because this arrangement does not destroy the amplitude distribution of antenna aperture, and the high absorptivity of MA that contributes the reduction of structure mode scattering. Simulation and experimental results demonstrate that the array antenna loaded with MA gets more than 6 dB RCS reduction both in the x-and y-polarized incident conditions; and the RCS of antenna has obviously a reduction from -25° to +25°, the most reduction value exceeds 10 dB in the boresight direction, while the reflectance, gain and beam width are guaranteed. This idea has an important significance and engineering application for the RCS reduction of array antenna.
2015, 64 (9): 094103.
doi: 10.7498/aps.64.094103
Abstract +
Gold spherical shell photocathode was prepared by seed-mediated growth on polystyrefle template and dispersed on the aluminum substrates by spin-coating procedure. Polystyrefle template was then removed by heat treatment. SEM surface morphology shows that Au spherical shell of ～10 μm in diameter can be self-supported after polystyrefle substrate is removed. The thickness of Au shell is about 70-90 nm and the grain size on the surface is about 30-60 nm. X-ray photoemission characteristics of Au reflection photocathodes in the 400-1400 eV photon energy region are obtained, indicating that the Au shell photocathodes can emit 3 times more photoelectrons than the plane Au photocathode; this results from the special surface morphology of Au spherical shell and the reduction of surface potential.
2015, 64 (9): 094104.
doi: 10.7498/aps.64.094104
Abstract +
Optics thermal deformation is an important factor that impacts the performance and lifetime of ion thrusters. Although some theoretical reflearch concerned with this problem was reported, its mechanism has not been fully understood. In this study, numerical investigations are performed to explain the effect of thermal deformation on the performance and lifetime of ion thrusters. The transient behavior of charged particles is calculated using a particle-in-cell simulation, while the momentum transfer collision and the charge exchange collision are calculated by means of the Monte Carlo method. Electron backstreaming restriction, perveance restriction, ions through rate, and divergence angle losses are compared and analyzed for optics deformed and undeformed. And the influence of these factors on thruster’s performance and lifetime is discussed. Results show that the ion through rate of the screen grid increases when optics begin deformed, and the thrust is slightly higher than the theoretical values predicted; the perveance threshold of the accelerator grid increases with optics haveing thermal deformation, while the crossover limit threshold is little changed, namely the thruster can be operated in conditions of a larger beam current; the electron backstreaming restriction threshold is significantly lower under a high beam current condition with optics deformed, which means that a lower accelerating gate bias is necessary to ensure thruster work. For the less obvious change of acceleratng grid current when the beam is focused, there is no moreflerosion and change of lifetime. Results provide a reflerence for the optimization design of optics and evaluation of thruster performance and lifetime.
2015, 64 (9): 094201.
doi: 10.7498/aps.64.094201
Abstract +
The shape of atmospheric aerosol is an important factor that influences radiation transfer. In this paper, a vector radiation transfer model based on Monte Carlo method is systematically introduced, and its accuracy is validated against the published results. and the sensitivity of Stokes vectors of transmitted and reflected light to aerosol shape is discussed when polarized light incidents. In addition, the influence of the particle shape on the depolarization ratio, transmission rate and the reflection rate is analyzed for incident light with different polarization states. Simulation results show that for the incident light in different polarization states, the sensitivity of the Stokes vectors of the diffuse light to different aerosol shapes is not the same in different viewing directions, and the Q, U, V elements of Stokes vector are all insensitive to the change of particle shape near the direction of the zenith angle 0°. It is evident that the aerosol shapes have a stronger influence on the depolarization ratio for reflected diffuse light compared with that for transmitted diffuse light, and there are also remarkable differences between the degree of depolarization of the diffuse light when the polarization states of the incident light are different. Results also show that the aerosol shape has a significant influence on the whole transmission rate and the reflection rate, and with the increasing of propagation distance, the influence also becomes more remarkable. Compared with particle shape, the influence of polarization states of incident light is relatively small, the transmission rate of horizontally-polarized light is slightly larger than that of unpolarized light, while for perpendicularly-polarized light, its results is opposite. For circularly polarized light, the results is almost the same as that of unpolarized light.
2015, 64 (9): 094203.
doi: 10.7498/aps.64.094203
Abstract +
A femtosecond laser single-stage nonlinear amplification system composed of Yb-doped large-mode-area single-polarization photonic crystal fibers is demonstrated. Effects of net cavity dispersion and pump power on oscillator output parameters and the evolution dynamics of the amplified pulse after compression are discussed for different seed pulse parameters. Under the experimental conditions in this paper, the longer and less chirped pulses are obtained with a larger negative net intracavity dispersion in the oscillator. When a nearly-transform-limited pulse is chosen as seed pulse nder the condition of oscillator pump power of 4.53 W, the shortest nearly-pedestal-free amplified pulse is achieved under the amplifier pump power of 60 W after the dispersion is compensated by a grating pair, in which the pulse duration is 45.7 fs with an average power of 28 W at a refletition frequency of 42 MHz. When the oscillator pump power is increased to 5.08 W and most nearly-transform-limited pulses under the pump condition are selected as the seed pulses, the maximum average power of 34.5 W with a duration of 53.5 fs is obtained at an amplifier pump power of 70 W.
2015, 64 (9): 094204.
doi: 10.7498/aps.64.094204
Abstract +
Femtosecond optical frequency combs (FOFCs) with output wavelengths covering visible light have potential applications in absolute frequency measureflent of iodine-stabilized lasers and optical clock lasers. Based on optical amplification, frequency doubling and spectrum broadening, a home-made Er-fiber femtosecond optical frequency comb (Er-FOFC) with output wavelengths covering visible light is demonstrated. One path with an average power of 8 mW from Er-FOFC is used as the seed pulse for spectrum broadening to cover the visible light. This path is first amplified to 532 mW by injecting into an Er-doped femtosecond fiber amplifier with combined forward and backward pumping and then frequency doubled with a MgO: PPLN crystal with an output power of 85 mW, frequency-doubling efficiency of 32% and pulse duration of 85fs. The output power of this path can be first amplified to 532 mW through an Er-doped femtosecond fiber amplifier when the forward pumping and backward pumping both turn on. Then the frequency-doubling laser can be generated in a MgO: PPLN crystal. The frequency-doubling efficiency is 32% and the pulse duration is 85 fs; the frequency-doubling light is spectrally broadened from 500 to 1000 nm in a photonic crystal fiber (PCF), with an output power of 85 mW and coupling efficiency of 50%. To verify the performance of the broadened spectrum, the light from the Er-FOFC and a compact iodine-stabilized frequency-doubled Nd: YAG laser at 532 nm is beaten. A beat signal with a signal-to-noise ratio of 30 dB at 100 kHz RBW is obtained, which provides a useful tool for absolute frequency measureflent of visible lasers.
2015, 64 (9): 094205.
doi: 10.7498/aps.64.094205
Abstract +
For the most commonly used KTP frequency-doubling crystal, its temperature adaptability range should be effectively extended. For this purpose, a compromise design is given by compreflensively considering both its effective nonlinear coefficient and the half-width of temperature range. The design method of KTP frequency-doubling device with a wide temperature range is analyzed in detail; furthermore, the curves of effective nonlinear coefficients, acceptance angles, and walk-off angles as a function of phase-matching angle are plotted via computer simulation. According to the results of theoretical study, a device used in the temperature range from -20 ℃ to 50 ℃ is designed and validated experimentally by the KTP external cavity frequency-doubling laser. Experimental results indicate that a peak conversion efficiency of 22.7% at 15 ℃ with a 70 ℃ temperature halfwidth is achieved by using the designed device. Compared with the commonly designed KTP frequency-doubling device, the temperature adaptability range increases notably although its frequency conversion efficiency decreases a little. Additionally, the effective nonlinear coefficient is still bigger than that of the commonly used crystals such as LBO and BBO when temperature halfwidth increases to 70 ℃. The above method would have the potential for extending the temperature adaptability range of other frequency-doubling devices.
2015, 64 (9): 094206.
doi: 10.7498/aps.64.094206
Abstract +
Sodium laser beacon (SLB) excited by a macro-micro pulse laser with low power has the following advantages: the return photons without fluctuations and the high merit quantity, although the laser beam propagation and the size of SLB suffer form the atmospheric turbulence. In addition, the macro-pulse profile may influence the interaction of laser and sodium atoms. For the macro-pulse with a Gaussian profile, it is useful to enhance the merit quantity and the excitation probability of the SLB to increase the width of micropulse. In order to obtain more of the return photons in the SLB, while increasing the laser power, the fine beam quality, the appropriate launch diameter, and the spectral width of the laser etc. must be considered. Therefore, it is of practical significance for the SLB with fine characteristics to optimize the laser parameters, the launch diameter and launch pattern.
2015, 64 (9): 094207.
doi: 10.7498/aps.64.094207
Abstract +
Among all kinds of wavefront reconstruction algorithms in adaptive optical systems, the standard and mostly used algorithm is the direct gradient wavefront reconstruction algorithm. As the number of sub-apertures in Shack-Hartmann wavefront sensor and the actuators for deformable mirror increases, the reconstruction matrix in direct gradient wavefront reconstruction algorithm takes too much space and the number of multiplication in the algorithm increases sharply. So, the iterative algorithm is adopted in wavefront reconstruction for the high-resolution adaptive optical system. The number of multiplication and the required space of the iterative algorithm are directly related to the sparseness of both iterative matrix and slope response matrix. In an adaptive optical system, the sparseness of these two matrixes is connected with the system parameters. Therefore, it is necessary to study how to choose the proper parameters for an adaptive optical system when it uses iterative wavefront reconstruction algorithm. In this paper, the sparseness of slope response matrix and iterative matrix are analyzed based on a 613-actuator adaptive optical system. The influence of the Gaussian function index of deformable mirror on the sparsenesses of slope response matrix, iterative matrix, stability and correction qualities of the adaptive optical system are also studied under the condition of constant actuator spacing and coupling coefficient. A larger Gaussian function index results in a lower sparseness of the slope response matrix and the iterative matrix. Too large or too small a Gaussian function index will degrade the stability and the correction quality of an adaptive optical system. Finally, the optimal range of the Gaussian function index is provided by balancing the sparseness of slope response matrix, the correction quality, and the stability of the adaptive optical system.
2015, 64 (9): 094208.
doi: 10.7498/aps.64.094208
Abstract +
Based on the far off-resonant four-wave mixing process, the slow light propagations of the amplified probe and generated conjugate pulses are obtained experimentally. Simultaneous manipulations of group velocity are realized by changing the two-photon detuning between the pump light and the probe light. The dependences of gains of the injected probe and generated conjugate light on the one-photon detuning for continuous wave mode are studied at different cesium vapor temperatures and pump light powers. It is shown that the maximum of gains occurs at the proper Raman one-photon detuning. The dependence of delay time on the two-photon detuning is measured using the 6 μs and 365 ns probe pulses, respectively. For the 6 μs input probe pulse, the maximum delay times of the probe and the conjugate pulses are 2.1 μs and 1.9 μs with the fractional delays of 0.35 and 0.32, respectively, corresponding to 0.000119 c and 0.000132 c group velocity. The high fractional delays of 2.07 and 1.83 with the maximum delay times of 756 ns and 670 ns for the 365 ns input pulse are obtained.
2015, 64 (9): 094301.
doi: 10.7498/aps.64.094301
Abstract +
Ultra short baseline (USBL) positioning system is widely used in underwater geophysical field survey, acoustic tow fish positioning, and sea oil engineering The precision and accuracy are important technical indexes. Normally, people often care about how to improve the precision of position, but there is no unified method about how to evaluate the precision and accuracy. In most experiments, a beacon as target is moored on the seafloor using buoyancy, and survey positions of the beacon refleatedly in a circle track. The waviness of positioning results is used to evaluate the precision, which is analyzed by a statistical method. This paper analyzes the precision evaluation method based on error ellipse, gives the theoretical formulations, and proves the relationship between observation data and error ellipse. This paper also proposes a precision evaluation method of USBL positioning systems based on long baseline (LBL) triangulation, using the obtained result as the true position to evaluate the accuracy of USBL which can offer suggestions to find system error. Using multiple observations to increase redundancy, the precision is far greater than USBL positioning method. Estimated positions can be used as the true ones to serve as a reflerence in evaluating the accuracy. If the deviation between estimated positions using the USBL and LBL methods is larger than expected, the system needs to be recalibrated. Finally, this paper processes the data from sea experiment. The actual sea trial is processed using the LBL method proposed in this paper. Result shows that the precision of a fixed target is well reflected and the system error is modified further, and thus improves the positioning precision of 0.2%. Result also shows that this method may be of a great application value.
2015, 64 (9): 094302.
doi: 10.7498/aps.64.094302
Abstract +
An elastic acoustic scattering by underwater target could be mixed with other acoustic scattering components in both time and frequency domains, and the existing signal processing methods could not discriminate the elastic feature of target in the mixed status. For solving this problem, a signal separation method for elastic acoustic scattering is proposed. Based on the highlight model of target echo, the characters of the target acoustic scattering signal when the linear frequency modulation signal is transmitted, are analyzed, and a method for mapping the acoustic scattering signal of the target to a single frequency signal is proposed. Theoretical derivation shows that there is a simple linear relationship between the acoustic scattering structure of the target and the mapped result, then the elastic acoustic scattering signal of the target can be separated by a narrow-band filtering. Simulation results show that the correlation coefficient between the separated target acoustic scattering and the orginal simulation signals are about 0.99, indicating that the acoustic scattering components in the simulation target echo can be wholly separated. Experimental data-processing results of the target acoustic scattering measureflent in an anechoic pool show that the mid-frequency enhancement effect can be observed in the spectrum of the separated elastic acoustic scattering, and every target acoustic scattering component can be recognized on the time-frequency distribution of separately processed target echo. There is a bowl-shape interference fringe on the angle-spectrum of the separated target elastic acoustic scattering components which is consistent with the theoretical signal’s feature of the elastic acoustic scattering, and the effectiveness of the separation method proposed is proven.
2015, 64 (9): 094303.
doi: 10.7498/aps.64.094303
Abstract +
Aiming at the problem of passive detection of broadband sources in underwater acoustic vector signal processing, a novel detection algorithm based on beam-space transformation is proposed. The principle of spatial spectrum detection with human eyes is employed for reflerence, and the generalized likelihood ratio test (GLRT) is applied to the beam-space. First, the design criterion of beam-space transformation matrix is studied for the compreflensive consideration of the environment of multiple targets and the characteristic of vector ambient noise field, so that the analytical solution is obtained. Second, assuming that the number of beams not containing the target signal is given, the probability density function (PDF) model of beam-space data is constructed, and the new GLR test is made by calculating the maximum likelihood estimate of the unknown variables in PDF. Finally, the information of theoretical criterion is adopted in order to estimate the number of beams not containing target signals. The processing gain and the threshold value of this test statistics are also discussed, and the specific implement is explained in detail. Theoretical analysis and simulation results show that under the complex conditions of strong target interference and ambient noise with undulated and time-variant power spectrum, the proposed algorithm can give the processing result with higher gain and detection threshold at constant false alarm rate (CFAR); the results of lake experiment further prove the favorable and robust detection performance.
2015, 64 (9): 094304.
doi: 10.7498/aps.64.094304
Abstract +
Beam-space transformation projects the array data into a lower space, which is not only effective in reducing computation time, improving performance, but also being capable to suppress interference. In contrast to conventional adaptive beam-space transformation method, which often requires adjusting the beam-space matrix and steering vectors online, an efficient adaptive beam-space transformation method is proposed. In the proposed method, the beam-space covariance matrix and the steering vector both have closed-forms, and do not depend on the adaptive beam-space matrix. This eliminates the online adjustment process, and, thus, improves the computational efficiency. Finally, the proposed method can also be applied to the direction of arrival (DOA) estimation. Simulation results demonstrate that it has a better DOA estimation performance than the conventional adaptive method. Furthermore, the proposed method also has another significant advantage, i.e., it is able to suppress moving interference. This can be ascribed to the proposed beam-space matrix which is independent of the historical data, and, thus, effective to avoid the mismatch between the training and application data, since this mismatch often occurs in conventional adaptive methods.
2015, 64 (9): 094701.
doi: 10.7498/aps.64.094701
Abstract +
As the smoothed particle hydrodynamics (SPH) is a truly Lagrangian meshfree method, the implementation of solid boundary condition has been one of the key problems that hinder SPH from applying to lots of engineering problems. In order to treat the boundary conditions efficiently, based on the boundary-fluid interaction principles, a new boundary treatment method is proposed. In this method, the solid boundary is repreflented implicitly by several layers of dummy particles along the boundary line. During the simulation, the dummy particles are treated as an extension of the fluid phase. The densities of dummy particles are kept constant, and the pressures and velocities are interpolated from the nearby fluid particles at each time step. Dummy particles can be involved in the calculation of the continuity equation conditionally and exert influences on the density/pressure field of the fluid phase. Then, for the fluid particles that approach the solid boundary, local pressure gradients are used to repreflent the dummy-fluid particle pair’s interaction strength and act as the boundary force term implicitly, which is tuned to be repulsive only and normal to the boundary. Thus, large pressure gradients mean strong boundary-fluid interaction strength, and the boundary force from the dummy particles should also be large enough to preflent the fluid particles from penetrating the solid boundary; and on the contrary, small pressure gradients mean weak boundary-fluid interaction strength and the boundary force becomes soft and little disturbs the flow field. Results of numerical tests demonstrate that, compared with the existing boundary treatment methods, the new method is in better accordance with the physical principles of the fluid-boundary interaction, and is able to treat arbitrary solid boundaries with limited modeling and computational costs. With the help of this new boundary treatment method, the stable flow field, well-ordered particle distribution, smooth velocity and pressure fields could be obtained. Theoretically, this new boundary treatment method could be directly used in three-dimensional multi-phase problems. Further tests are planned to be carried out; meanwhile, expanding the new boundary treatment method to rigid-fluid interaction problems is also a work in the future.
2015, 64 (9): 094702.
doi: 10.7498/aps.64.094702
Abstract +
Equation of states for deuterium, helium, and their mixture is studied by using the quantum molecular dynamics (QMD) method. We calculate the equation of states for helium with density from 0.32 to 5 g/cm3 at temperature from 1000 to 50000 K. Results are compared with the chemical model (CM), at T less than 10000 K, and QMD is in good agreement with the CM. The shock Hugoniot curves are also calculated, and the results are in good agreement with the gas-gun experiment. The mechanism of the metal-insulator transition for helium is studied by computing its pair distribution function and density of states. The equation of states (EOS) for deuterium with density from 0.19 to 0.84 g/cm3 at temperatures from 20 to 50000 K is computed. For deuterium molecule the degree of dissociation is calculated, and the effect of the molecular vibration is accounted for using the EOS model. Theoretical Hugoniot states are also calculated and compared with the results of experiments and other theories; the maximum compressibility of hydrogen is about 4.9, and deuterium 4.4; these agree with the results of most experiments and theories. Due to the zero point motion of atoms being not taken into account, the theoretical results at low temperatures are smaller than those of experiments. The deuterium-helium mixture is studied, and its 293 points of equation of states for various xHe with densities from 0.19 to 0.84 g/cm3 at temperatures from 100 to 50000 K are calculated. The linear mixing approximation (LMA) is checked, and the maximum of the volume distinction is about 7%; the results indicate that LMA is a cursory approximation.
2015, 64 (9): 094703.
doi: 10.7498/aps.64.094703
Abstract +
The spatial-temporal sequence of velocity fields in wall turbulence with and without polymer additives at the same Reynolds number are measured by time-resolved particle image velocimetry (TRPIV) from the side and top views. Based on this experimental database of a water channel, the mechanism of drag reduction by polymers is explored from the viewpoint of the influence of polymer solution on the transport of momentum and energy in a turbulent boundary layer. Comparison of Reynolds stress profiles confirms that due to the existence of polymer additives, the transport of turbulent momentum is significantly inhibited, as if caused by the decrease of Reynolds shear stress. Furthermore, it is noted that these changes are closely related to the effect of polymer additives on the classical coherent structures, such as vortices and low-speed streaks, which are the dominant structures in near-wall turbulence. The spatial topological mode of hairpin vortex extracted by conditional sampling method shows that the intensity of vortices and ejection event are greatly suppressed by the polymer solution. Not only does the decline of turbulent kinetic energy production indicate that the energy of hairpin vortices that comes from the ensemble average movement is attenuated in the solution, but all this implys that the polymer additives hinder the self-sustaining mechanism, the inherent character of wall turbulence. Then, the analysis of linear stochastic estimation (LSE) suggests that the development of hairpin vortices in the packet is impeded, which is mainly reflected in the reduction of the number of hairpin vortices and the suppression of uplift in the wall-normal direction. To investigate the change of low-speed streaks after the addition of polymers, the spanwise autocorrelation function of streamwise fluctuating velocities has been calculated. In the polymer solution the large-scale vortices areflenhanced while the small-scale vortices are suppressed. This observation refleals that the polymers disrupt the energy transport from large to small scales. To summarize, it is through the action on coherent structures that the polymer additives can damp the transport of momentum and energy between the near-wall region and outer region of the boundary layer. In this way, the polymer solution makes turbulent flow less chaotic, leading to the reduction of friction drag.
2015, 64 (9): 094704.
doi: 10.7498/aps.64.094704
Abstract +
A coupled level set and volume of fluid (CLSVOF) method is utilized to simulate the droplet evolution, during splashing after impacting on an inclined liquid film, and the bubble entrainment by the droplet impacting of the liquid. The influence of impact angles of bubble entrainment, the processes of splashing pressure field and velocity field after impingement are studied and discussed. Compared with the results obtained by experiment, the validity of CLSVOF method is verifiled by simulating the droplet splashing on an inclined liquid film. Results indicate that the mechanisms of front and back splashing are different. The front splashing is caused by pressure difference in the neck region, and the back splashing is produced by radial flow in the liquid film. It is also found that along with the impact angle moving towards the large angle direction, the bubble entrainment number shows a reducing trend.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2015, 64 (9): 096101.
doi: 10.7498/aps.64.096101
Abstract +
A circular silicone sheet as a masker was used to cover a glass slide, and then the super-hydrophobic coating was sprayed on the glass slide free of silicone sheet masker, thus a round hydrophilic area surrounded by a super-hydrophobic coating is obtained. The PS colloidal droplets are confined in the hydrophilic area, and the droplet volume can be changed within a large range. Variation of the droplet volume influences the initial apparent contact angle. We investigate the particle deposition behavior of the confined colloidal droplet for a hydrophobic apparent contact angle in evaporation process by using an in situ optical observation system. In the whole evaporation process the contact-line of the confined droplet is pinned at the junction between the hydrophilic area and hydrophobic area. In the particle deposition process the main driving flow is different, and the final deposition pattern is controlled by three flow behaviors. In the early stage, the main flow is the Marangoni flow, which drives the particle clusters float on the droplet surfaces, part of them accumulated at the boundaries. As the evaporation proceeds, when the apparent contact angle decreases (<60°), the evaporation flux becomes singular near the contact line, Capillary flow towards the contact inside the drop as a compensation to the solvent loss at the drop boundary, which drives the particles in the droplet to rapidly accumulate at the contact-line. In the last evaporation stage, the thickness of the film in the hydrophilic area becomes very thin, and there is only one layer of particles in this thin film, the thin liquid film instability triggers the particles in the middle area to rapidly aggregate and then form a kind of network pattern, due to the decrease of distances between the particles. Capillary force between particles also takes part in this aggregate process.
2015, 64 (9): 096102.
doi: 10.7498/aps.64.096102
Abstract +
Enthusiasm in the reflearch of thermo-photovoltaic (TPV) cells has been aroused because the low bandwidth semiconductors of III-V family are coming into use. GaSb, as a member of III-V family, has many merits such as high absorption coefficient, and low band gap of 0.725 eV at 300 K etc.. At preflent thermo-photovoltaic cells are usually based on GaSb wafer, and it can be manufactured by the vertical Bridgeman method. Thermo-photovoltaic cell based on GaSb films is one of the effective ways to reduce the cost of the thermo-photovoltaic system. GaSb polycrystalline films can be grown by physical vapor deposition (PVD) which has advantages in using fewer materials and energy, and also in doing little harm to the environment. Because of residual acceptor defects VGaGaSb, GaSb thin film is usually of p-type semiconductor. So we should find n-type semiconductor material to form pn junction. We choose CdS as the emission layer of a cell structure. CdS belongs to n-type semiconductor with a narrow band gap of 2.4 eV and high light transmissivity. CdS thin film grown by chemical bath deposition (CBD) has passivation properties for GaSb. CdS layers can remove native oxides from GaSb surface and reduce the surface recombination velocity of GaSb. This paper focuses on theoretical analysis of GaSb/CdS thin film photovoltaic structure. By way of AFORS-HET simulation, we analyze the defect state density and interface density in GaSb and CdS, and their effects on cell performance. According to the simulation, the defect density in GaSb absorption layer is the very important factors that affect cell performance. When GaSb defect increases, the major factor to affect the cell is the fill factor that leads to low efficiency. On condition that thereflexists high GaSb defect density, the thickness of GaSb should be kept at 1000 nm. GaSb with a thickness above 1000 nm can bring about a high recombination rate, which reduces the efficiency of the cell. As an emission layer, the defect density in CdS should not affect the cell performance obviously. When the increase of CdS defect density is of four orders of magnitude, the cell efficiency is only decreased by 0.11%. In order to demonstrate the interface between GaSb and CdS, we use an inversion layer n-GaSb according to the passivation properties of CdS thin film grown on GaSb. When the defect density of inversion layer increases, the efficiency of the cell will decrease rapidly. And the GaSb/CdS structure will act as a resistance when the defect density in the inversion layer reaches 1020 cm-3. So the defect density in GaSb layer and the interface is the very factor to affect thermo-photovoltaic performance.
2015, 64 (9): 096103.
doi: 10.7498/aps.64.096103
Abstract +
The generalized gradient approximation of density functional theory is applied to study the hydrogen storage capacity of the alkali metal atom Li, transition metal atoms Ti and Fe decorated C18B2M(M=Li, Ti, Fe) fullerefles. It is found that the metal is bonding to C18B2 stronger than to C20. When the average adsorption energy of C18B2Li-nH2 is low, and the binding of H2 to C18B2Fe is too strong, C18B2Ti-nH2 has the average adsorption energy between 0.45-0.59 eV, which is in the range from 0.2 to 0.6 eV, so it can realize the reflersible adsorption of H2. A maximum number of H2 adsorbed on to C20B2M(M=Li, Ti, Fe) should be 4, 6, and 4, for Li, Ti, and Fe respectively; this agrees well with the 18 electronic rule. C18B2Li adsorbs H2 molecules mainly through the static electronic field formed by Li ions, while C18B2Ti and C18B2Fe adsorb H2 mainly through the Kubas interaction. Therefore, C18B2Ti can not only adsorb more H2 molecules, but also realize the reflersible hydrogen storage.
2015, 64 (9): 096104.
doi: 10.7498/aps.64.096104
Abstract +
According to original knowledge, lanthanum hexaboride (LaB6) as an excellent thermionic electron emitter is characterized by a low work function, a high emission density, and a high brightness. Recently, much attention has been drawn to its another excellent optical properties: strong light absorption in near infrared rays (NIR), and transparency in visible light (VL), which result from the free electron plasmon resonance. However, up to now the optical properties and syntheses of ternary rareflearth hexaborides have been very rarely reported in the literature. In this paper, ternary LaxCe1-xB6 submicron crystallines are successfully synthesized using a solid-state reaction, in which La2O3 and CeO2 are used as rareflearth sources and NaBH4 as boron sources in a continuous vacuum condition. Effects of La doping content on the LaxCe1-xB6 phase composition, microstructure, and optical absorption properties are investigated by X-ray diffraction, scanning electron microscope (SEM), and transmission electron microscope (TEM). It is found that all the synthesized samples are composed of CaB6-type single-phase alloy with a space group Pm3m at the reaction temperature of 1200 ℃ held by 2 h. The SEM results indicate that the cubic-shaped ternary LaxCe1-xB6 crystals with a mean size of 200 nm are obtained and the energy dispersive spectrometer results confirm that the La atoms are randomly distributed at the lattice sites of CeB6. High resolution transmission electron microscope images refleal the single-crystalline nature, and the FFT pattern indicates the lattice fringe d=0.42 nm which agrees well with the (100) crystal plane. EDS analysis of TEM also indicates the La element has been doped into CeB6. And the optical absorption result shows that the absorption valley of CeB6 is 62 nm. With increasing La doping content to x=0.6 and 0.8, the absorption valleys of La0.6Ce0.4B6 and La0.8Ce0.2B6 decrease to 613 and 610 nm respectively indicating the blueshifts of the wavelength of absorption valley From the view point of practical application, the tunable characteristic of LaxCe1-xB6 may extend the optical applications in improving the efficiency of organic photovoltaics, replacing the expensive gold and silver nanoparticles, which may have a good usage in optical filters.
2015, 64 (9): 096105.
doi: 10.7498/aps.64.096105
Abstract +
Through comparative investigation on the growth of oxidation-induced stacking faults (OSFs) in heavily antimony (Sb)-doped and phosphorus (P)-doped Czochralaki (Cz) silicon wafers with almost the same resistivity, effects of dopants on the growth of OSF in heavily doped n-type Cz silicon are studied experimentally. Moreover, the influences of Sb and P atoms on the recombination of self-interstitials and vacancies are also explored on the basis of the first-principles calculations. It is shown experimentally that all the OSF lengths are almost identical regardless of the type and density of OSF nucleation centers, such as copper precipitates and mechanical scratches etc.. However, it is found that the OSF length of heavily Sb-doped Cz silicon wafer is larger than that of heavily P-doped Cz silicon wafer under the same oxidation condition. Essentially, the OSFs are formed by the aggregation of silicon self-interstitials refleased at the Si/SiO2interface during the oxidation. Therefore, a longer OSF implies that a higher quantity of silicon self-interstitials remains after the recombination of vacancies and silicon self-interstitials in the heavily Sb-doped Cz silicon wafer. The first-principles calculations based on density functional theory (DFT) indicate that Sb atoms combine with vacancies more readily than P atoms. This is actually due to the fact that Sb has a much larger atomic size than P. In other words, as compared with P atoms, the Sb atoms are the moreflefficient vacancy-trapping centers, thus retarding the recombination of vacancies and silicon self-interstitials. Consequently, the silicon self-interstitials remain after recombination with the vacancies that are much more in heavily Sb-doped Cz silicon wafer than in heavily P-doped counterpart when undergoing the same oxidation. In turn, the OSFs in heavily Sb-doped silicon wafers are relatively longer.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2015, 64 (9): 097101.
doi: 10.7498/aps.64.097101
Abstract +
The crystal structure, electronic structure and elastic constants of uranium dioxide are investigated using first-principles calculations, wherein the generalized gradient approximation and Hubbard U terms are used in the framework of density-functional theory. On-site Coulomb interactions with the simplified rotational invariant approach (the Dudarev approach), fully relativistic calculations for the coreflelectrons (repreflented as a pseudopotential), and scalar relativistic approximations for the valence electrons areflemployed to account for the relativistic effects and electron correlation of 5f electrons in UO2. The Hubbard U parameters (Ueff=U-J, U=3.70 eV, J=0.40 eV) are derived by calculating the band gap width of UO2. In addition, the electron density of states calculation suggests that the following value of band gap is appropriate. The calculated lattice constant is 5.54 Å, and the band gap width is 2.17 eV which shows that UO2 is a semiconductor. Its density of states shows that the U 5f orbital contributes to the peaks immediately adjacent to the Fermi level, which agrees with the U 5f2 configuration, while the O 2p orbital plays a dominant role in the bonding band at approximately -6 to -2 eV. Results obtained above have been compared with available experimental data, and also discussed in relation to previous calculations. Above results are better than existing ones gained by others. Analyzing the density of states for different Hubbard U parameters, we find that the Hubbard U parameters can influence the distribution of U 5f electronic orbit.
2015, 64 (9): 097102.
doi: 10.7498/aps.64.097102
Abstract +
According to density functional theory of first-principles calculation theory, we study systematically the structure, magnetism, electronic and optical properties of Mn-doped LiNbO3. The enthalpies of formation of LiNbO3, when substituting Li and Nb with Mn, are -8.340 and -8.0062 eV/atom, respectively. This means that the LiNbO3 after substitution of Li with Mn is more stable than that of Nb with Mn. And the magnetic moments of LiNbO3 in the substitution of Li with Mn is higher than that in substitution of Nb with Mn. Results of the density of states calculation show that the magnetism comes from Mn atom, and its magnetic moments is 4.3 μB. The rest of the magnetic moments may come from the contribution of the O and Nb atoms, because of the interactions of Mn-3d orbit with the O-2p and Nb-4d orbits. Optical absorption spectra show an improved optical response in the visible range in LiNbO3 by substituting Li with Mn. Results of analysis of oxygen vacancy in LiNbO3 show that oxygen vacancy can improve the magnetic moments of Mn-doped LiNbO3 system.
EDITOR'S SUGGESTION
2015, 64 (9): 097201.
doi: 10.7498/aps.64.097201
Abstract +
We have investigated the electronic transport properties of graphene pn junction with spin-orbit coupling. If the incident energy lies between the potentials of the two ends of the pn junction, a particle can penetrate the graphene pn junction by tunneling accompanied by the electron-hole transition. The curve of conductance versus Fermi energy shows steps and reaches its maximum when the Fermi energy lies at the middle of the potentials of the p and n areas. As the length of graphene pn junction increases, the conductance decays exponentially. The spin-orbit coupling leads to a bulk energy gap and edge states; the gap reduces the conductance dramatically and the edge states result in an almost perfect conductance plateau. When the pn region is influenced by randomly doped impurities, the conductance curves are no longer symmetrical in the case of weak doping, while in the strong doping case, the step structures are destroyed but the conductance plateau contributed by the edge states survives well.
2015, 64 (9): 097301.
doi: 10.7498/aps.64.097301
Abstract +
Plasmonic lens (PL) is a nano-optical device, with which a tight focusing spot in a subwavelength-scale can be achieved by exciting and controlling surface plasmon polaritons (SPPs), thus the diffraction limit can be broken for attaining the shorter effective wavelength of the SPPs. The key issue in studying the PL is to achieve a tight focusing point and focus-control effectively. Optimal plasmonic focusing can be achieved by utilizing the radially polarized light and the rotational symmetric structures of the PL. Radially polarized light is a cylindrical vector beam whose local polarization of electric field is always parallel to the radial direction. As a radially polarized light is used as the incident light in a PL, the SPPs can be excited in all directions, so as to increase the efficiency of focussing. The focussing efficiency can be further increased, and the characteristics of the focus, such as spot size, shape, and strength etc., can be manipulated through appropriate designs of the PL structures. In this work, under an illumination of a radially polarized light, a new type of plasmonic lens is proposed to achieve a long depth of focus (DOF), a long focal length, and a sub-wavelength-scale tight focussing spot. This kind of plasmonic lens consists of a T-shape micro-hole, concentric rings, and multi-level step-like structures. The focussing properties of such plasmonic lenses are analyzed in terms of the finite element method (FEM). Simulation results show that SPPs can be excited efficiently in such structures and the tight-focusing is realized via the multiple-beam interference between the light radiating from the concentric rings and the transmitted light from the center hole. The T-shape micro-hole and step-like concentric ring structures can provide control for the phase modulation and the propagation direction of the SPPs along the bottom of the groove, thus leading to a compressed focal spot, a longer focal length, an increased depth of focus, and to improving the focussing properties. In an optimized PL design, a focal spot of ～2.5λ0 DOF, ～0.388λ0 FWHM, and ～3.22λ0 focal length is achieved under the illumination of a radially polarized light (λ0=632.8 nm). The PL structure is compact, and can be easily integrated with other nano-devices. The PL proposed above has potential applications in nano-scale photonic integration, near-field imaging and sensing, nano-photolithography, and in other related areas.
2015, 64 (9): 097501.
doi: 10.7498/aps.64.097501
Abstract +
Co/Ni multilayers with Pt underlayers have been prepared by magnetron sputtering technique, and their perpendicular magnetic anisotropy (PMA) was studied by the anomalous Hall effect (AHE). The PMA of the samples can be studied by the intensity of Hall signal (RHall), remanence ratio (Mr/Ms), coercivity (HC) and the squarefless of the samples in the Hall hysteresis loops. A clear PMA is observed in the as-deposited amorphous Co/Ni multilayers. The PMA of Co/Ni multilayers is strongly dependent on the thicknesses of Pt, Co, and Ni, and the number of Co/Ni bilayers. After testing, the thicknesses of Pt, Co, and Ni, and the periodic number (n) of Co/Ni bilayers are determined to be 2 nm, 0.2 nm, 0.4 nm and 1 respectively. The optimum Co/Ni multilayer with excellent performance of PMA has a structureflexpressed as Pt(2)/Co(0.2)/Ni(0.4)/Co(0.2)/Pt(2). The hysteresis loop of the sample with the field applied in plane is tested, showing the characteristics of hard axis typically. PMA can be measured by the anisotropy constant Keff which is determined by the competition of the interface anisotropy to the volume anisotropy. If the interface anisotropy is dominant, the sample will have PMA. The anisotropy constant Keff of Pt(2)/Co(0.2)/Ni(0.4)/Co(0.2)/Pt(2) is 3.6×105 J/m3, illustrating that it has an excellent PMA, and the interface anisotropy of Co/Ni is the main factor that makes Keff a larger value. Since the thickness of magnetic layer in the optimum sample is only 0.8 nm and the total thickness of it less than 5 nm, the integration of the device can be studied further. Furthermore, the coercivity of an optimum Co/Ni multilayered sample is relatively small and can be increased by inserting an oxidation layer or by other ways.
2015, 64 (9): 097801.
doi: 10.7498/aps.64.097801
Abstract +
Chromatic stability and purity cannot be acquired easily in the top-emitting white organic light-emitting diode (TWOLED) due to its special device structure with metal film as both anode and cathode. Blue/red/blue emission layers are utilized to optimize the chromaticity and to improve the stability. Then step-doping is introduced into the red emission layer to further improve the device stability. In order to explain the mechanism of the improvement in chromaticity stability, effects of step-doping on the device performances are analyzed in detail. Experimental results refleal that the built-in electric field induced by the step-doping changes the carrier drifting and exciton recombination in emission layers. When the doping concentration gradually increases from the anode side to the cathode side (increasing step-doping), the built-in electric field is facilitated to suppress the electron drafting and the exciton diffusing, which helps control the exciton recombination zone. Color stability can be improved in the TWOLED with increasing step-doping. Additionally, as an important factor influencing the emission spectra of TWOLEDs, microcavity resonance is calculated and used to explain the variation of spectra induced by device structural changes.
EDITOR'S SUGGESTION
2015, 64 (9): 097802.
doi: 10.7498/aps.64.097802
Abstract +
Deuterated potassium dihydrogen phosphate (K(DxH1-x) 2PO4) crystals with different deuteration levels (x=0, 0.51, 0.85) were grown by conventional cooling method from deuterated solutions at Shandong University. Positron annihilation spectroscopy has been widely used to the study on micro-defects of semiconductors and other materials, which is very sensitive to the crystal structure, defect types, defect concentrations, and so on. In this paper, positron annihilation spectroscopies (positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy), combined with X-ray diffraction (XRD) are used to investigate micro-defects characterization in K(DxH1-x) 2PO4 crystals. Influences of deuteration degree on the crystal structure characteristics, defect types and concentrations are discussed. It can be concluded from XRD experiments that the lattice parameters of a and b increase with the increase in deuteration levels, while no obvious change occurs on the lattice parameter c. KH2PO4(KDP) crystals at low deuteration level and high deuteration level could be regarded as low deuterium-doped KDP crystal and low hydrogen-doped DKDP crystal respectively. It is indicated that the higher the replacement ratio in the crystals, the weaker the diffraction peak they show. Positron annihilation lifetimes increase clearly in the highly-deuterated KDP crystals. It is found that neutral interstitial defects and oxygen defects in the KDP crystal increase with increasing deuteration degree. And these types of defects can be attributed to lattice distortion effect. From positron annihilation lifetime results we can arrive at another conclusion that the compound defects will form and defects concentration is declined, when hydrogen vacancies, K vacancies and substitutional impurity defects continue to react by means of association reactions. These phenomena suggest that high deuteration plays a significant role in promoting association reaction of internal defects in the crystals. Furthermore, the polymerization reaction of the clusters and micro-cavities continue to occur, therefore defect concentrations will show a constant decrease. Doppler broadening spectra show that the internal defects in the crystals increase integrally with an increase of deuteration level; this agrees well with the results of positron annihilation lifetime. Moreover, Doppler broadening spectra indicate that the proportional change of these defects is synchronous and consistent with the actuality. To sum up, our experimental results suggest that the defect reaction is weak in low degree of KDP crystal deuteration growth (less than 50%), while reaction is enhanced in the high degree of deuteration growth (higher than 50%).
2015, 64 (9): 097303.
doi: 10.7498/aps.64.097303
Abstract +
Since metallic nanowires can confine light in nanoscale beyond the diffraction limit, metallic nanowires play an important role in nanophotonic integrated circuits. In this paper, a silver nanowire waveguide with a cross is proposed and its surface plasmon polaritons (SPPs) splitting properties of the cross at λ = 532 nm are studied by the finite element method. The nanowire has a square shape with its side length of a. Results show that the outputs for different input modes depend on the geometric parameters of the nanowires. For SPPs with TM0 mode, there are similar intensities in different waveguide directions with smaller side length. With the increase of a, the intensity in the original waveguide direction increases monotonically, and those in the perpendicular direction will decrease monotonically. For SPPs with HE1 mode and HE-1 mode, most of the energy propagate along the original waveguide direction for smaller a. With the increase of a, the intensity in the original waveguide direction decreases dramatically. For SPPs with HE1 mode, the cross blocks most of the energy in three directions for larger a. In addition to the splitting effect of it, the cross also performs a function of mode conversion. For the input SPPs with TM0 mode, the output of SPPs along the perpendicular waveguide direction can be converted to the HE-1 mode. For the input SPPs with HE1 mode, the output of SPPs along the perpendicular waveguide direction can be converted to the TM0 mode. Due to the superposition of electric fields of different SPPs modes in the perpendicular waveguide direction occur the steady-state and periodic electric field distributions.
REVIEW
EDITOR'S SUGGESTION
2015, 64 (9): 094202.
doi: 10.7498/aps.64.094202
Abstract +
Laser has been widely applied in the scientific and industrial areas, including materials, medicine, military and telecommunications, due to its extreflely well-defined frequency, narrow divergence and high intensity. In reflent fifty years, various laser sources have been developed. The laser output power, pulse duration, and attainable wavelengths have been greatly improved. To date, further optimization on laser is mainly focused on the three aspects: an effective gain medium capable of amplifying light, a convenient pump source, and a high efficient resonator (or cavity). Among these aspects, the gain medium plays a very important role in the generation of efficient and high-quality laser.
Lots of laser materials have been explored and developed, among them, organic laser materials, small molecules or polymers based on -conjugated structure, have been attracting more and more attention in the current reflearch of high efficiency laser. Organic laser have advantages such as simple fabrication, low cost, easy integration, and so on. Although the organic lasers with optical pump source have been extensively reflearched, the issues how to achieve electrically pumped organic lasers, or the so-called organic laser diodes, still remain unsolved. Nevertheless, the prospects of organic laser are very promising, such as its application in spectroscopy, chemical sensor (e.g. trinitrotoluene or DNA sequences) and short-haul data communication. In this review, we try to draw a picture of the organic laser reflearch form its first appearence till the end of 2014, with emphasis on the latest progress and variation trends, instead of providing a complete survey of organic laser reflearch. In the first part of this paper, different types of organic materials used for lasers are briefly reviewed. First, basic rules for the selection of suitable materials for organic lasing are summaried as: 1) the appropriate energy level distribution for creating four-level systems; 2) a high-stimulated emission cross-section e, which should affect the gain and threshold; 3) an appropriate radius for host-guest blend if energy transfer system is applied; 4) the low stokes shift to reduce the pump energy converted into heat; 5) a low excited-state absorption to reduce the self-absorbance loss; 6) a low intersystem crossing rate and a low triplet-triplet absorption cross-section to eventually lower the triplet lifetime; 7) a high photoluminescence efficiency in solid-state, i.e. a low - packing; 8) the good stability against oxygen and moisture and photo stability against pump light. Such organic gain media are classified into dyes, semiconductors, and new-concept materials. The active host-guest system is also discussed, which is different from the dispersion chromophore in the inert matrix (e.g. PMMA). This energy transfer strategy has been well proved to be effective to improve the absorption of pump energy and move the absorption band away from the emission band. It is possible, therefore, to reduce the self-absorbance loss to lower the threshold of lasing.
In the second part, different geometries and features of the most commonly used cavity are discussed to investigate the dynamic balance between the gain and loss inside the lasing operating system. We divide the resonator structures into the catalogs of planar waveguides, curved surface cavities, and vertical external cavity solid organic larers (VECSOL). The widely used types of planar waveguides are DFB and DBR. The lasing thresholds of these structures areflextreflely low and their emission wavelength can be tuned by changing the thickness of the organic layer or the period of the modulation.
In the third part, current progress and future reflearch direction of the organic lasers are summarized. The challenge of electrically pumped organic laser (or organic laser diode) remains to be the major driving force for the scientific community to be devoted to the reflearch of organic lasers. Estimation of operating current based on the optical-pumped laser data is only 100 Acm-2. Actually, very high current densities of the order of kA cm-2 (even higher) have been realized both in pulsed OLEDs and light-emitting field-effect transistor (LEFET) devices. But lasing is still not observed. The extra losses brought about by electrical driving can be summarized as follows: 1) the electrodes used for electrical injection; 2) the charge carriers with broad absorption bands overlapping the emission; 3) the triplet excitons with longer lifetime and higher creation probability ratio. LEFET is now the most promising device structure of organic laser diodes. Unfortunately, LEFET is not applicable for dealing with the triplet trouble which is inherent in the organic materials. The proposition of new concept on directly pumped organic lasers seems to be an alternative way to solve this problem.
Finally, we would like to describe the reflent progress in optically pumped organic lasers briefly. Efforts which have been made can be summarized as follows: lowering the lasing threshold, increasing the wavelength coverage (to the deep red or infrared and to the ultraviolet), improving the wavelength sensitivity, enhancing the lifetime of the devices, or improving the conversion efficiency, output power and beam quality. Although these progresses are realized under the condition of optical pumping, all these achievements are meaningful since they constitute the bases of future organic laser diodes.
SPECIAL ISSUE—Celebrating 100 anniversary of physical science in Nanjing University
2015, 64 (9): 094209.
doi: 10.7498/aps.64.094209
Abstract +
Z-type photocatlytic system, reflembling natural photosynthesis, consists of two different photocatalysts and a shuttle redox mediator, involving two-photon excitation process for photocatlysis. One photocatalyst as a photoreduction system offers the reduction sites by conduction band (CB) electrons, and the other photocatalyst as a photooxidation system provides the oxidation sites by valence band (VB) holes. A shuttle redox mediator as an electron conductor transfers the electrons from the CB of the photooxidation system to the VB of the photoreduction system. On the one hand, the separation of photocatalytic reactive sites is advantageous for spatial separation of the electrons and holes, which is beneficial for enhancing the photocatlytic activities. On the other hand, photoreduction system and photooxidation system of different materials effectively inhibit the reflerse reaction involvement of photoreductive and photooxidative products. The Z-type photocatlytic system simultaneously possesses a wide light absorption range and strong redox ability.
2015, 64 (9): 097302.
doi: 10.7498/aps.64.097302
Abstract +
The properties of the edge states in the topological insulator InAs/GaSb/AlSb quantum well in the preflence of a perpendicular magnetic field are studied numerically. The effect of the magnetic field is included in our model by adding an on-site Zeeman term and a vector potential to the electron wave vector: k+eA. When the material is in the topologically nontrivial state, a pair of degenerate counter-propagating spin-polarized edge states exist in the bulk band gap on each edge of the sample, which are gapless in the absence of the magnetic field due to the protection of the time reflersal symmetry. #br#Nonzero magnetic field breaks the time reflersal symmetry, and leads to Landau levels in the electron energy spectrum. However, one can still find a pair of counter-propagating spin-polarized edge states in the bulk energy gap near each sample boundary.The edge states are gapped, and their distributions relative the sample edge depend on the strength of the magnetic field. With the increase of the magnetic field, one edge state remains located near the sample boundary, but the other tends to evolve into the bulk gradually. Furthermore, we study the scattering between the two edge states caused by impurities. We show that the scattering rate is suppressed because of the spatial separation of two edge states, and shows no significant enhancement when the magnetic field increases, which suggests that even though the time reflersal symmetry is broken, the quantum spin Hall state remains to be relatively robust.
2015, 64 (9): 094210.
doi: 10.7498/aps.64.094210
Abstract +
Singlet fission is a spin-allowed process that creates two triplet excitons from one photo-excited singlet exciton in organic semiconductors. This process of carrier multiplication holds the great potential to break the theoretical efficiency limit in single-junction solar cells by making better use of high-energy photons, while capturing lower-energy photons in the usual style. Photovoltaic devices based on singlet fission have achieved external quantum efficiencies in excess of 100%. In this paper, we first introduce the basic concept about singlet fission and review the history of the field briefly. Then, we report some reflent advances in the reflearch of singlet fission progress with the combination of our group’s productions. Tetracene and pentacene are chosen as typical polyacene materials for discuss. We describe how scientists make progresses in understanding the underlying physics in singlet fission process. The experimental methods of transient absorption spectra, time-resolved fluorescence spectra and time-resolved two-photon photoemission spectra render numerous results for analysis. Moreover, a survey about the debate on the direct or indirect mechanism with transient optical study is provided. It has been verified that multiexciton state intermediates in singlet fission process and the factors of energy level alignments, intermolecular interaction as well as lattice vibrations play a role in it. Last, we briefly summarize the implications of singlet fission in organic solar devices by introducing several composite architectures for singlet-fission photovoltaics. Designing efficient and cheap solar cells is the ultimate goal for understanding the intrinsic photophysics of singlet fission. To obtain high efficiencies, it is important to adapt proper materials and new organic/inorganic architectures may become a promising direction. Also, finding a way for efficient triplet exciton dissociation should be considered seriously. It is believable that these guidelines can lead to the development of cheap and efficient fission-based devices.
2015, 64 (9): 094305.
doi: 10.7498/aps.64.094305
Abstract +
Realizations of one-way manipulations in various kinds of energy flux are always highly desirable. The most famous example should be the invention of electric diodes which marked the emergence of modern electronics and resulted in worldwide technology revolutions. Acoustic wave, albeit a classical wave with much longer reflearch history in comparison with the electricity, has long been thought to propagate easily along two opposite directions in any path. Hence it should be intriguing to realize the one-way transmission of acoustic waves by designing the acoustical analogy of electric diodes, which would have deep implications in all the acoustics-based applications and the field of acoustics in general. In this review, we briefly describe reflent advances in acoustic one-way manipulation which has become a new frontier of science and is of remarkable significance in both the physics and engineering communities. The emergence of the first “acoustic diode”, formed by coupling a phononic crystal (PC) with a nonlinear medium, offers the possibility of rectifying acoustic energy flux by breaking through the barrier of reciprocity principle via the introduction of nonlinearity. Despite of the efforts in enhancing the performances of nonlinear acoustic diodes by updating their structures, the inherent shortcomings in nonlinear systems such as low efficiency and narrow bandwidth still attract considerable attentions on the potential of linear structures, aiming at constructing a one-way manipulation on particular modes of an acoustic wave without breaking the reciprocity principle. A series of linear acoustic one-way devices have already been designed and fabricated with significantly improved performances. On the basis of asymmetric mode conversion, a linear one-way plate for Lamb waves is designed. High efficient one-way transmission for plane waves propagating along two opposite directions is realized by coupling a PC and a diffraction structure. Unidirectional waveguide is designed and fabricated which only allows for a plane wave incident from one of the two openings to pass. A unidirectional structure with a total thickness as thin as the wavelength is realized by reconstructing the otherwise plane wavefront with acoustic gratings. An acoustic gradient-index structure is proposed that can directly manipulate the wave trajectory asymmetrically and then yield asymmetric acoustic transmission within a considerably broad band. Acoustic metamaterials with near-zero indexes have also been employed to realize unidirectional transmission with a controllable transmitting angle and consistent wavefront. These advances are important steps towards the practical applications which generally require integration and minimization of devices having high efficiency and broad bandwidth. The reflently emerged “acoustic transistor” has been described as well, which can be regarded as the acoustical counterpart of an electric transistor and enables the amplification and switch of acoustic waves by an acoustic wave, or by exploiting the three-wave mixing effect. We also discuss the challenge and promise of the usage of acoustic one-way devices in controlling acoustic waves.
2015, 64 (9): 094306.
doi: 10.7498/aps.64.094306
Abstract +
Ultrasound contrast agent (UCA) reflers to the agent that has specific acoustic properties to enhance the contrast in ultrasound imaging by composition of gas-filled microbubbles with micrometer-diameters. In a diagnostic ultrasound field, microbubbles in fluid create an acoustic impedance mismatch between fluid and surrounding tissue to increase the reflection of sound and achieve a better contrast. Ongoing developments improve diagnostic possibilities of UCA remarkably, whereas their potential therapeutic applications have also been investigated for a couple of decades. The nonlinear response of UCA microbubbles has clinical reflevance from both diagnostic and therapeutic perspectives. The aim of this review is to introduce the latest reflearch progress of our group regarding the mechanism and applications of the nonlinear dynamic response to UCA, which include (1) an all-in-one solution characterizing coated bubble parameters with the help of the light scattering technique and flow cytometry, which makes it possible to quickly integrate the size distribution with dynamic motions of thousands of microbubbles and easily verify the validities of different shelled bubble dynamic models; (2) the development of a new bubble dynamics model that takes into account both nonlinear shell elasticity and viscosity, which can not only be capable of simulating the “compression-only” behavior of microbubbles excited by large amplitude ultrasound but also eliminate the dependence of bubble shell parameters on bubble size; (3) the estimation of UCA inertial cavitation thresholds of two types of commercial UCA microbubbles (viz., SonoVue microbubbles coated with lipid shells and KangRun microbubbles coated with albumin shells) and the evaluation of the relationship between microbubble inertial cavitation thresholds and their shell parameters; and (4) the reflearches of DNA transfection efficiency and the reduction of cytotoxicity in gene delivery facilitated by UCA excited by 1-MHz focused ultrasound pulses, and the results indicate that the measured DNA transfection efficiency and sonoporation pore size generally increase with the enhancement of inertial cavitation dose, while the cell viability decreases linearly with the increase of International Classification of Diseases (ICD). These studies are of significance for better understanding the mechanism of ultrasound-induced microbubble nonlinear dynamics and investigating the effective quantification technique for microbubble cavitation activity, which are important for further optimizing therapeutic ultrasound effects and avoiding the side-effects.
2015, 64 (9): 097401.
doi: 10.7498/aps.64.097401
Abstract +
FeSe0.5Te0.5 single crystals with superconducting critical temperature of 13.5 K are investigated by scanning tunneling microscopy/spectroscopy (STM/STS) measureflents in detail. STM image on the top surface shows an atomically resolved square lattice consisted by white and dark spots with a constant of about 3.73 0.03 which is consistent with the lattice constant 3.78 . The Se and Te atoms with a height difference of about 0.35 are successfully identified since the sizes of the two kinds of atoms are different. The tunneling spectra show very large zero-bias conductance value and asymmetric coherent peaks in the superconducting state. According to the positions of coherence peaks, we determine the superconducting gap 2 = 5.5 meV, and the reduced gap 2/kBTc = 4.9 is larger than the value predicted by the weak-coupling BCS theory. The zero-bias conductance at 1.7 K only have a decrease of about 40% compared with the normal state conductance, which may originate from some scattering and broadening mechanism in the material. This broadening effect will also make the superconducting gap determined by the distance between the coherence peaks larger than the exact gap value. The asymmetric structure of the tunneling spectra near the superconducting gap is induced by the hump on the background. This hump appears at temperature more than twice the superconducting critical temperature. This kind of hump has also been observed in other iron pnictides and needs further investigation. A possible bosonic mode outside the coherence peak with a mode energy of about 5.5 meV is observed in some tunneling spectra, and the ratio between the mode energy and superconducting transition temperature /kBTc 4.7 is roughly consistent with the universal ratio 4.3 in iron-based superconductors. The high-energy background of the spectra beyond the superconducting gaps shows a V-shape feature. The slopes of the differential conductance spectra at high energy are very different in the areas of Te-atom cluster and Se-atom cluster, and the difference extends to the energy of more than 300 meV. The differential conductance mapping has very little information about the quasi-particle interference of the superconducting state, which may result from the other strong scattering mechanism in the sample.
2015, 64 (9): 097503.
doi: 10.7498/aps.64.097503
Abstract +
We preflent a brief overview on the interplay between magnetism and superconductivity in one of the Fe-based superconductor systems, Fe1+yTe1-xSex. The parent compound Fe1+y Te is an antiferromagnet; with Se doping, antiferromagnetic order is suppressed, followed by the appearance of superconductivity; optimal superconductivity is achieved when x～50%, with a superconducting temperature Tc of ～15 K. The parent compound has an in-plane magnetic ordering wave vector around (0.5, 0) (using the tetragonal notation with two Fe atoms per cell). As Se concentration increases, the spectral weight appears to shift to the wave vector around (0.5, 0.5), accompanying the optimization of superconductivity. A neutron-spin resonance is observed around (0.5, 0.5) below Tc, and is suppressed, along with superconductivity, by an external magnetic field. Taking these evidences into account, we conclude that magnetism and superconductivity in this system couple to each other closely-while the static magnetic order around (0.5, 0) competes with superconductivity, the spin excitations around (0.5, 0.5) may be an important ingredient for it. We also discuss the nature of magnetism and substitution effects of 3d transition metals.
2015, 64 (9): 097701.
doi: 10.7498/aps.64.097701
Abstract +
The multiferroic Co/Co3O4/PZT composite films are prepared on Pt/Ti/SiO2/Si wafers by sol-gel process combined with pulsed laser deposition method. The phase structures, microstructural topographies and element valence states of the composite films are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectrum (XPS). The ferroelectric, electrical and magnetic properties as well as the magnetoelectric coupling behaviors are measured, and the exchange bias effect and its influence on the magnetoelectric coupling behavior of the composite film are studied systematically. #br#The results show the composite films have well-defined ferroelectric hysteresis loops with a remanent polarization value of ～17 μ C/cm2. The composite film exhibits evidently an exchange bias effect. Typically, a exchange bias field of ～80 Oe is observed at 77 K. Both the exchange bias field and magnetic coercive field increase with reducing the temperature. The exchange bias field increases to 160 Oe when the temperature decreases to 10 K. The XPS results confirm that an about 5 nm-thick CoO layer appears at the Co/Co3O4 interface due to the oxygen diffusion during the preparation, indicating that the exchange bias effect at 77 K is caused by the pinning effect of the antiferromagnetic CoO layer while the exchange bias effect at 10 K originates from the combining effect of antiferromagnetic CoO and Co3O4 layers. #br#The measureflent results of magnetocapacitance versus magnetic field curves at different temperatures show that the composite films have remarkable magnetoelectric coupling properties. The response of capacitance to temperature changes with the variation of external magnetic field. Further investigations show that the composite film possesses distinct anisotropic magnetocapacitance effect. When the direction of the magnetic field changes, the magnetocapacitance of the composite film changes from positive value to negative value. Moreover, the magnetocapacitance value changes with the variations of temperature and magnetic field magnitude. Typically, at 300 K a maximum value of positive magnetocapacitance (5.49%) and a minimum value of negative magnetocapacitance of (1.85%) are obtained at -4000 and 4000 kOe, respectively. When the temperature is reduced to 10 K, the positive magnetocapacitance decreases to a minimum value (0.64%) while the negative magnetocapacitance increases to a maximum value (5.4%). We perform a detailed analysis on such a magnetoelectric coupling behavior, and elucidate its origin, which should be attributed to the exchange bias effect and interface-mediated magnetism-stress-electricity coupling process.
2015, 64 (9): 098101.
doi: 10.7498/aps.64.098101
Abstract +
This paper is an overview of the progress of sol-gel autocombustion synthesis of metals and metal alloys. Sol-gel is a convenient method to synthesize a variety of oxides by mixing of different elements at an atomic level. Autocombustion synthesis is a self-sustaining process caused by the heat generated from its exothermic reaction. By combining these two methods, the sol-gel autocombustion method is introduced in the synthesis of metals and metal alloys. The experimental principle and technological route are introduced in detail in this review. By using metal nitrate, citric acid etc. as starting materials, the dried gels are prepared through sol-gel routine. Under the protection of inert gas, the autocombustion could be activated at low temperature in a tube furnace. After the autocombustion was activated, the gel burned violently, and a large amount of white gas was refleased. During heating the gel, mass spectrum shows that the H2, CO and CH4 areflevidently identified near the combustion temperature. They are well known reducing agents, which can be used in the redox reaction for synthesizing metals from oxides. Based on the data obtained from the TG-DTA and mass spectrum analysis, it is speculated that there are mainly five reactions appearing during the burning of the gel at high temperature: exothermic reaction between fuel and oxidant; metal oxide(s) formation by decomposition of the nitrate(s); generation of CH4, CO and H2 by the decomposition of CHx containing groups of complexing agent; exothermic reaction between CH4/CO/H2 and oxidant; the reduction of metals from their corresponding metal oxides by CH4 and H2 in nascent product. The application of this method to the synthesis of metals and metal alloys is shown by realized examples. This method shows many advantages in the synthesis of metals, such as simple apparatus, inexpensive raw materials, a relatively simple preparation process, and fine powder products with high homogeneity. Moreover, very low temperature is required to activate the reaction, and then the combustion can continue to take place without needing additional energy supply. This method has potential applications in experimental material reflearches.
2015, 64 (9): 098701.
doi: 10.7498/aps.64.098701
Abstract +
Molecular simulation is one of the most important ways of studying biomolecules. In the last two decades, by combining the molecular simulations with experiments, a number of key features of structure and dynamics of biomolecules have been reflealed. Traditional molecular simulations often use the all-atom model or some coarse grained models. In practical applications, however, these all-atom models and coarse grained models encounter the bottlenecks in accuracy and efficiency, respectively, which hinder their applications to some extent. In reflent years, the multiscale models have attracted much attention in the field of biomolecule simulations. In the multiscale model, the atomistic models and coarse grained models are combined together based on the principle of statistical physics, and thus the bottlenecks encountered in the traditional models can be overcome. The currently available multiscale models can be classified into four categories according to the coupling ways between the all-atom model and coarse gained model. They are 1) hybrid resolution multiscale model, 2) parallel coupling multiscale model, 3) one-way coupling multiscale model, and 4) self-learning multiscale model. All these multiscale strategies have achieved great success in certain aspects in the field of biomolecule simulations, including protein folding, aggregation, and functional motions of many kinds of protein machineries. In this review, we briefly introduce the above-mentioned four multiscale strategies, and the examples of their applications. We also discuss the limitations and advantages, as well as the application scopes of these multiscale methods. The directions for future work on improving these multiscale models are also suggested. Finally, a summary and some prospects are preflented.
2015, 64 (9): 098702.
doi: 10.7498/aps.64.098702
Abstract +
Self-assembly is the way that is used by Mother Nature to create complex materials of hierarchical shapes and diverse functionalities. The photosynthesis apparatus of plant is an example of such complex materials that can direct convert the sunlight energy into chemical energy. Inspired by this, many artificial photosynthesis systems have been successfully engineered. However, most of these systems were based on only one type of simple nanostructure, such as nanosphere or nanotube. The charge separation and exciton transfer in such systems may be further improved by combining multiple nano-structures. Here, we report a novel photo catalysis system based on composite nanostructures of controllable peptide nanotubes and graphene. We use the mixture of diphenylalanine (FF) and carboxyl graphene for the photo catalysis because they are stable under different solvent conditions and highly conductive, which can provide more paths for exciton transfer. Moreover, the diameters of the peptide nanotubes become thinner in the preflence of carboxyl graphene, leading to a more uniformly distributed system than simply using the peptide nanotubes alone. The FF peptide nanotubes can connect with the carbonyl graphene (CG) to form the composite nanostructures because of the π-π stacking interaction between benzene rings of FF and conjugated πup bond of CG. The composite nanostructures of controllable peptide nanotubes and graphene provide more transmission channels for the excitions since they can travel on the nanotubes, CG or the compound of the both. We also demonstrate that when the photo-harvesting ruthenium complex and catalytic platinum nanoparticles are deposited on the system, the nicotinamide adenine dinucleotide (NADP+) can reduce to NADPH. The catalytic efficiency and rate are much higher than thaose of other artificial photosynthesis systems reported in the literature. Surprisingly, we find that the catalytic efficiency of the combined system is better than the sum of separated systems with only FF nanotubes or carboxyl graphene. The high turnover frequency, high reaction rate, and low toxicity of this artificial photosynthesis system will make the combined system attractive for large-scale applications, including optoelectronic industry, energy industry, etc.
2015, 64 (9): 097502.
doi: 10.7498/aps.64.097502
Abstract +
Combining ferroelectric with antiferromagentic materials in nanometer scale is an effective method for exploring multiferroic materials. We preflent two kinds of systems to show the possibility of multiferroic properties in such nanometer composites. One is the artificial superlattice LaFeO3-YMnO3, and the other is the natural layered Aurivillius material Bi4Ti3O12 doped with different layers of LaFeO3, BiFeO3. Both materials were synthesized by pulsed laser deposition method on SrTiO3 substrates. Microstructural charterizations with XRD, TEM, and EELS in scanning transmission electron microscopy mode substantiate that the samples have atomically sharp interfaces between neighboring layers; this is important for producing possible magneto-electric coupling in multiferroic materials. Magnetic characterization proves that these materials have ferrimagnetic properties, in spite of their anti-ferromagnetic nature before coupling. Magnetic characterization also proves that there is 0.55-0.9 B remanant magnetization generated at LaFeO3-YMnO3 interface. And the 0.5 and 1.5LaFeO3-Bi4Ti3O12 samples show ferrimagnetism which can remain even up to room temperature. Ferroelectric tests prove that there is a large leakage current in LaFeO3-YMnO3 superlattice and BiFeO3-inserted Bi4Ti3O12, but 0.5LaFeO3-Bi4Ti3O12 shows ferroelectric hysteresis loops. It can be therefore concluded that 0.5LaFeO3-Bi4Ti3O12 is a multiferroic material. If more perovskite layers (3-layer SrTiO3 or 2.5-layer LaFeO3) are inserted, the Aurivillius structure of Bi4Ti3O12 may appear structural instability that can be observed in our HRTEM measureflent. Our first principles calculations show that the degeneracy of formation enthalpies is the reason why the intergrowth in these materials forms and their structures are not stable. Our work may provide some examples for exploring new multiferroics by means of nano-meter composite.
2015, 64 (9): 098102.
doi: 10.7498/aps.64.098102
Abstract +
Photoacoustic imaging is a hybrid imaging technique based on the photoacoustic effect. As a non-invasive and non-ionizing modality, photoacoustic imaging takes the both merits of the conventional acoustic imaging and optical imaging. Firstly, the contrast of photoacoustic imaging primarily depends on the optical absorption. The unique optical spectra of atoms and molecules makes optical methods to be a widely used modality to probe the molecular and chemical information of biological tissue. Therefore, photoacoustic imaging has its inherent advantage in high-contrast functional and physiological imaging of biological tissue, as well as the optical imaging method. Secondly, photoacoustic imaging has the high spatial resolution in deep tissue in comparison with the pure optical imaging method. Since the strongly optical scattering in biological tissue, pure optical imaging method is difficult to obtain the high-resolution image in the tissue deeper than ～1 mm. Whereas, acoustic wave suffers much less from scattering than optical wave, the acoustic scattering coefficient is about 2-3 orders of magnitude less than the optical scattering coefficient. Photoacoustic imaging can achieve a fine resolution in deep tissue, which equivalent to 1/200 of the imaging depth. Thirdly, non-ionizing radiation used for photoacoustic imaging is much safer than X-ray. Moreover, the low-temperature rises make photoacoustic imaging be safely used in live tissue. A laser-induced temperature rise of 1 mK yields an initial pressure of ～800 Pa in soft tissue. Such a sound pressure level has reached the sensitivities of typical ultrasonic transducers. Fourthly, photoacoustic imaging has the ability of extracting multiple contrasts, including biochemical parameter, biomechanical parameter, blood velocity distribution, tissue temperature, and microstructure information. Photoacoustic imaging can capture more specific and reliable information about the tissue structure, function, metabolism, molecule, and gene. As a result, photoacoustic imaging has become one of the fastest growing biomedical imaging techniques in the past decade.#br#In this review, we will explain photoacoustic effect and the principle of photoacoustic imaging. Then, we introduce the two classical photoacoustic imaging schemes, including photoacoustic tomography and photoacoustic microscopy. Their main specifications, such as resolution, are also preflents. We review the ability of photoacoustic imaging in extracting multiple contrasts and discuss their biomedicine applications. In addition, we also introduce the remarkable breakthroughs in super-resolution photoacoustic imaging. Finally, we look the further development and the limitations of photoacoustic imaging.
2015, 64 (9): 097803.
doi: 10.7498/aps.64.097803
Abstract +
Due to the coupling of photons with the electrons at a metal-dielectric interface, surface plasmons (SPs) can achieve extreflely small wavelengths and highly localized electromagnetic fields. Hence, plasmonics with subwavelength characteristics can break the diffraction limit of light, and thus has aroused great interest for decades. The SP-inspired reflearch, in the application respect, includes extraordinary optical transmission, surface enhanced Raman spectroscopy, sub-wavelength imaging, electromagnetic induced transparency, perfect absorbers, polarization switches, etc.; and in the fundamental respect, includes plasmon-mediated light-matter interaction, such as plasmonic lasing, plasmon-exciton strong coupling, etc.#br#Recently a series of studies has been performed to push the dimensions of plasmonic devices into deep subwavelength by using nanowires. The chemically synthesized metallic nanowires have good plasmonic properties such as low damping. The reported silver nanowire structures show great potential as plasmonic devices for communication and computation. Now we develop the nanostructured metal wires for plasmonic splitters based on the following considerations. One is that we introduce cascade nano-gratings on a metallic nanowire, enabling a single nanowire to act as a spectral splitting device at subwavelength; and the other is that we use silicon as a substrate for the metallic nanowire, making the plasmonic nanowire device compatible with silicon based technologies.#br#In this paper, we continue and develop our previous work on position-sensitive spectral splitting with a plasmonic nanowire on silicon chip (see Scientific Reports (2013) 3 3095). The three parts are organized as follows. In the first part, we derive analytically the dispersion relation of the SPs in a suspended silver nanowire based on Maxwell equations. In the second part, we placed a silver nanowire in the silicon substrate, and use the finite-element method (FEM) to obtain the dispersion relation of the SPs for the practical applications. The calculations show that the SP mode can be confined better in this system, howbeit with larger loss. Starting from the dispersion relation, we then calculate the mode area, the propagation length and the effective index of the SP modes, with respect to the nanowire dimension and the substrate materials. It is shown that a thinner nanowire has smaller mode area and a higher-index substrate induces larger loss. We also perform the finite-difference time-domain (FDTD) simulation to investigate the electromagnetic field distribution in this system. We find that the SP mode is mainly confined around the top surface of the nanowire, and in the crescent gap between the nanowire and the substrate. In the third part, we demonstrate both experimentally and theoretically that the silver nanowire with two cascaded gratings can act as a spectral splitter for sorting/demultiplexing photons at different spacial locations. The geometry of the grating is optimized by rigorous coupled wave analysis (RCWA) calculation. The carefully designed gratings allow the SPs with the frequencies in the plasmonic band and prohibit the SPs with the frequencies in the plasmonics bandgap. Those prohibited SPs areflemitted out through a single groove in front of each grating. Both the detected images and the measured optical spectra demonstrate that the SPs with different colors can be emitted at different grooves along a single nanowire. Thus the structured metal nanowire shows potential applications in position-sensitive spectral splitting and optical signal processing on a nanoscale, and provides a unique approach to integrating nanophotonics with microelectronics.
2015, 64 (9): 097202.
doi: 10.7498/aps.64.097202
Abstract +
As an exotic quantum condensed matter, the topological insulator (TI) is a bulk-insulating material with a Diractype conducting surface state. Such a dissipationless transport of topological surface state (TSS) is protected by the timereversal symmetry, which leads to the potential applications in spintronics and quantum computations. Understanding the topological symplectic transport of the Dirac fermions is a key issue to the study and design of the TI-based devices. There are many transport properties about Dirac fermions. And universal conductance fluctuation (UCF) is one of the most important transport manifestations of mesoscopic electronic interference. So the UCF effect in TI is a very meaningful research field It can provide an intriguing and special perspective to reveal the quantum transport of TSSs In this review, we introduce the research progress on the UCF of TSSs in a pedagogical way We review the achievements and the existing problems in order to inspire future research work.#br#We start this review with the basic UCF theory and the experimental observation. The UCF has been observed in TI earlier, but weather it originates from TSS has not been further studied. Then a series of work is carried out to prove the topological nature of UCF in TI Firstly, the UCF phenomenon in TIs is demonstrated to be from two-dimensional (2D) interference by magnetoconductance measurements. But the residual bulk state and the 2D electron gas (2DEG) on the surface can also bring about the 2D UCF The field-tilting regulation helps us exclude the distribution from the bulk And the classic self-averaging of UCF is investigated then to obtain the intrinsic UCF amplitude. By comparing with the theoretical prediction, the possibility has been ruled out that the 2D UCF may originate from the 2DEG So its topological nature is demonstrated. Secondly, we discuss the UCF effect in TI by a macroscopic perspective, i.e. the statistical symmetry of UCF, which should be more concise and reflect its universality. For a single TSS, the applied magnetic field will drive the system from a Gaussian symplectic ensemble into a Gaussian unitary ensemble. It results in a √2 fold increase of the UCF amplitude. However, the experiment reveals that the UCF amplitude is reduced by 1/√2. This is contradictory to the theoretical prediction. Actually, there are two TSSs and they are coherently coupled to each other in TIs since the sample’s thickness is smaller than its bulk dephasing length. This leads to a Gaussian orthogonal ensemble of the intersurface coupling system without an external field. In such a case, the UCF amplitude will be reduced by 1/√2 with field increasing. It is consistent with the experimental result. Finally, the other progress on UCFs is discussed, and the general outlook is also mentioned briefly.
2015, 64 (9): 097702.
doi: 10.7498/aps.64.097702
Abstract +
ZnO nanomaterials have been extensively investigated for its broad applications such as room-temperature UV lasers, light-emitting diodes, solar cells, dilute magnetic semiconductors, bio-labeling, and target medicines. Tuning and optimizing the properties of ZnO nanostructures are urgent for the practical applications. Here, the photoluminescence, magnetism, and cytotoxicity of ZnO nanparticles have been effectively tuned by adjusting the nanostructures. Firstly, by developing the novel polyvinylpyrrolidone(PVP)-directed crystallization route, microwave heating-assisted forced hydrolysis method, and post-treating with surfactants, a series of high pure ZnO nanostructures including spheres, semispheres, rods, tubes, T-type tubes, tripods, wafers, gears, double layers, multilayer, capped pots, and bowls with tunable size and surface component/charge has been successfully prepared. The PVP can greatly promote the ZnO nucleation by binding water, and direct the ZnO growth by forming a variety of soft-templates and/or selectively capping the specific ZnO facet which is confirmed by the infrared absorption spectra. Secondly, the band-edge UV emission of ZnO has been greatly modified in both intensity and peak position by simply changing the sizes, shapes, and surface component of the ZnO nanoparticles. However, changing the surface charge of ZnO nanoparticles can only vary the intensity of the band-edge UV emission of ZnO. Significantly, the fluorescence of fluorescein isothiocyanate (FITC) is increased by up to 90 fold through doping the FITC molecules into the ZnO naoncrystals, which can effectively separate the FITC molelcules and avoid the energy transfer and the resulting fluorescence self-quenching. Thirdly, the room temperature ferromagnetism with tunable intensity is induced in the ZnO nanoparticles by coating them with different surfactants at different concentrations. As confirmed by the x-ray photoemission spectra, the coated surfactant molecules can donate electrons to the ZnO nanoparticles and induce the ferromagnetism. The electron number varies with the surfactant and the surfactant concentration, leading to the fluctuant ferromagnetism. The theoretical calculation further reveal the fluctuant nature of ferromagnetism in the ZnO nanoparticles coated with surfactants. This explains the previously reported seemingly irreconcilable ZnO ferromagnetism induced by capping surfactants, and provides a general chemical approach to tuning the ferromagnetism of ZnO nanoparticles by modifying the capping-surfactant concentration. Finally, it is revealed that the shape, size, surface charge/composition, and band-gap of ZnO nanostructures have different influences on the ZnO-induced cytotoxicity. The surface composition or adsorbed species of NPs may contain the toxic matter such as OH-ions that determine the NP-induced cytotoxicity, and should be detected before cytotoxicity assays are conducted. The rod-like NPs are more toxic than the spherical NPs. The positive surface charge can accelerate the nanoparticle-induced toxic action and enhance the cytotoxicity. Compared with the effects of shape and surface composition/charge, the influence of the nanoparticle-size variation on the nanparticle-induced cytotoxicity is less significant, and can be overwhelmed by other factors. These results will be conducible to the cytotoxicity assay and safe usage of ZnO NPs.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2015, 64 (9): 095201.
doi: 10.7498/aps.64.095201
Abstract +
As a new welding method, ultrasound has been successfully introduced into the pool during ultrasonic wave-assisted arc welding process. However, the interaction mechanism between the ultrasound and the arc plasma is not clear, thus preflenting the new technique from engineering applications. In this paper, the characteristic of arc regulation by external ultrasonic field is investigated based on the experimental data and the corresponding theory. In order to figure out the characteristics of arc, the arc images obtained by high-speed camera are processed. Compared with the conventional welding arc, ultrasonic wave-assisted arc is more contracted and becomes brighter, the high-temperature region in an arc column greatly expands, and there are internal particle agglomerations shaking up and down at a constant frequency. The arc shape varies with ultrasound excitation current and the height of ultrasonic radiator. In the vicinity of the resonance point, the straight-degree of the arc is the strongest and the ripple frequency is also the largest. Results show that the purpose of using external ultrasound field to regulate the thermal plasma has basically achieved. Analyzing the acoustic pressure wave equation for the neutral component shows that the spatial distribution of acoustic wave can be generated in the arc and its intensity is proportional to the local amplitude of acoustic waves. Acoustic pressure field can be calculated based on the dependence of the electron temperature and density on time and space. In addition to the action of acoustic field within the arc, the arc plasma is also controlled by the acoustic field structure. A two-cylinder model incorporating boundary element method is developed, establishing a relationship between the binding capability and the geometric parameters of an ultrasonic radiator with reflerence to wavelength. This model is successful in predicting resonant modes of the acoustic field and explaining the influences of the ultrasonic radiator height on welding arc. Variation of arc shape is the result of the combined effect of axial and radial acoustic radiation forces on particles (electron, ion and neutral). The thermal efficiency will be significantly enhanced since the particle density increases in the ultrasonic wave-assisted arc. The acoustic propagation in the arc is the interacting process between acoustic and thermal plasmas. The mechanism of ultrasound acting on the arc can be reasonably explained in this study. And the results may provide a reflerence for plasma engineering applications. However, it also needs further reflearch on the impact of an arc on the acoustic field.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2015, 64 (9): 098501.
doi: 10.7498/aps.64.098501
Abstract +
In order to detect the weaker on greater span of light signals, the dynamic range, spatial resolution, and the signal to noise ratio of the streak camera need to be improved to meet further diagnostic requireflents in scientific area of materials, biology, information, semiconductor physics and energy, etc. Therefore, we design a streak camera with a larger dynamic range based on electron-bombarded CCD. Using the rectangle-framed electrode and electric quadruple lens in the streak camera can reduce its space charge effect and shorten the space charge interaction time by improving electron accelerating voltage to minimize the electron transit time. Using a back-illuminated CCD, which is based on the electron bombardment readout technology as image device to replace the traditional intensified CCD can shorten the chain of image conversion and greatly reduce the image degradation in the conversion of ultrafast diagnostic equipment. The signal to noise ratio, spatial resolution and dynamic range of the streak camera may gain improvement. Experimental results show that the static spatial resolution is better than 35 lp/mm and the dynamic spatial resolution is up to 20 lp/mm. Deflection sensitivity is 60.76 mm/kV and dynamic range reaches 2094: 1. Nonlinear scanning speed is 5.04%. EBS gain of the streak camera can be over 3000.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2015, 64 (9): 099301.
doi: 10.7498/aps.64.099301
Abstract +
In face of the increasingly complicated exploration targets, D-T2 2D NMR technology cannot balance the range of diffusion coefficient and transverse relaxation resolution in practice. As the key technique of D-T2 2D NMR, the property of pulse sequence affects its application. A modified design of bivariate D-T2 pulse sequence based on pulsed field gradient is proposed after summarizing the technical characteristics of PFG, STE-PFG, BP-PFG, modified CPMG, “diffusion editing” and multi echo interval CPMG pulse sequences, which effectively integrates the advantages of D-T2 pulse sequences based on pulsed field gradient and constant field gradient. In order to reduce the difficulties that the existing D-T2 inversion methods cannot balance the inversion efficiency and accuracy, we propose a joint TSVD inversion method which sets the echos from the second window as the main part supplemented by the echos from the first window. Numerical simulation results for gas-water, oil-water, heavy oil and gas-oil-water models in different SNRs indicate that the D-T2 modified pulse sequence can balance the diffusion coefficient measureflent range and transverse relaxation resolution successfully, and the joint TSVD inversion method balances the inversion efficiency and accuracy successfully. Above all, the D-T2 modified pulse sequence and the joint TSVD inversion method have wide applications in the exploration and formation evaluation for unconventional reflervoir, and create favorable conditions for the development of D-T2 2D NMR technology.
