## Vol. 65, No. 24 (2016)

##### 2016-12-20

###### REVIEW

2016, 65 (24): 240701.
doi: 10.7498/aps.65.240701

Abstract +

In recent years,the modeling and application of biological neurons have gained more and more attention.By now, the research on neuron models has become one of the most important branches of neuroscience.Neuron models can be used in various areas,such as biomimetic applications,memory design,logical computing,and signal processing. Furthermore,it is significant to study the dynamic characteristics of neural system by using neuron models.In this paper,the historical development of neuron models is reviewed.The neuron models have experienced three development stages.In the pioneering stage,a group of scientists laid the experimental and theoretical foundation for later research. Then,the whole study started to blossom after the publication of Hodgkin-Huxley model.In the 1970s and 1980s,various models were proposed.One of the research focuses was the simulation of neural repetitive spiking.Since the 1990s, researchers have paid more attention to setting up models that are both physiologically meaningful and computationally effective.The model put forward by Izhikevich E M has been proved to solve the problem successfully.Recently,IBM presented a versatile spiking neuron model based on 1272 ASIC gates.The model,both theoretically understandable and physically implementable,has been used as a basic building block in IBM's neuro-chip TrueNorth.In the paper, seventeen neuron models worth studying are listed.To give a more explicit explanation,these models are classified as two groups,namely conductance-dependent and conductance-independent models.The former group's goal is to model the electrophysiology of neuronal membrane,while the latter group is only to seek for capturing the input-output behavior of a neuron by using simple mathematical abstractions.The complexity and features of each model are illustrated in a chart,while the prominent repetitive spiking curves of each model are also exhibited.Five of the models are further detailed,which are the Hodgkin-Huxley model,the Integrate-and-fire model,the Fitzhugh-Nagumo model,the Izhikevich model,and the most recent model used by IBM in its neuro-chip TrueNorth.Finally,three questions are put forward at the end of the paper,which are the most important problems that today's researchers must consider when setting up new neuron models.In conclusion,the feasibility of physical implementation remains to be a challenge to all researchers. Through the aforementioned work,the authors aim to provide a reference for the study that follows,helping researchers to compare those models in order to choose the properest one.

###### GENERAL

2016, 65 (24): 240201.
doi: 10.7498/aps.65.240201

Abstract +

For wireless sensor networks, with energy constrained, topology optimization can reduce energy consumption and improve the structure of communication link. Based on the minimum rigid graph, a new topology optimization algorithm is presented in this paper, by considering the weights of communication links in graph and the generated algebraic properties of rigid graph. The proposed algorithm not only ensures the communication link is shorter which can prolong the network life cycle, but also keeps the graph structure more stable, which means that the network has good robustness. It is shown that communication link obtained by the proposed algorithm is shorter than that obtained by the related existing algorithms. As a result, the proposed algorithm has good network connectivity and structure stability. At the same time the trace of the generated rigid matrix is very big so that the proposed algorithm has excellent algebraic rigidity properties of a network.

2016, 65 (24): 240202.
doi: 10.7498/aps.65.240202

Abstract +

Finding explicit solutions of nonlinear partial differential equation is one of the most important problems in mathematical physics. And it is very difficult to find interaction solutions among different types of nonlinear excitations except for soliton-soliton interactions. It is known that Painlev analysis is an important method to investigate the integrable property of a given nonlinear evolution equation, and the truncated Painlev expansion method is a straight way to provide auto-Bcklund transformation and analytic solution, furthermore, it can also be used to obtain nonlocal symmetries. Symmetry group theory plays an important role in constructing explicit solutions, whether the equations are integrable or not. By applying the nolocal symmetry method, many new exact group invariant solution can be obtained. This method is greatly valid for constructing various interaction solutions between different types of excitations, for example, solitons, cnoidal waves, Painlev waves, Airy waves, Bessel waves, etc. It has been revealed that many more integrable systems are consistent tanh expansion (CTE) solvable and possess quite similar interaction solutions which can be described by the same determining equation with different constant constraints.
In this paper, the (2+1)-dimensional higher-order Broer-Kaup (HBK) system is studied by the nonlocal symmetry method and CTE method. By using the nonlocal symmetry method, the residual symmetries of (2+1)-dimensional higher order Broer-Kaup system can be localized to Lie point symmetries after introducing suitable prolonged systems, and symmetry groups can also be obtained from the Lie point symmetry approach via the localization of the residual symmetries. By developing the truncated Painlev analysis, we use the CTE method to solve the HBK system. It is found that the HBK system is not only integrable under some nonstandard meaning but also CTE solvable. Some interaction solutions among solitons and other types of nonlinear waves which may be explicitly expressed by the Jacobi elliptic functions and the corresponding elliptic integral are constructed. To leave it clear, we give out four types of soliton+cnoidal periodic wave solutions. In order to study their dynamic behaviors, corresponding images are explicitly given.

2016, 65 (24): 240501.
doi: 10.7498/aps.65.240501

Abstract +

The Helmholtz theorem confirms that any vector field can be decomposed into gradient and rotational field. The supply and transmission of energy occur during the propagation of electromagnetic wave accompanied by the variation of electromagnetic field, thus the dynamical oscillators and neurons can absorb and release energy in the presence of complex electromagnetic condition. Indeed, the energy in nonlinear circuit is often time-varying when the capacitor is charged or discharged, and the occurrence of electromagnetic induction is available. Those nonlinear oscillating circuits can be mapped into dynamical systems by using scale transformation. Based on mean field theory, the energy exchange and transmission between electronic field and magnetic field can be estimated by appropriate nonlinear dynamical equations for oscillating circuits. In this paper, we investigate the calculation of Hamilton energy for a class of dimensionless dynamical systems based on Helmholtz's theorem. Furthermore, the scale transformation can be used to develop dynamical equations for the realistic nonlinear oscillating circuit, so the Hamilton energy function could be obtained effectively. These results can be greatly useful for self-adaptively controlling dynamical systems.

2016, 65 (24): 240502.
doi: 10.7498/aps.65.240502

Abstract +

A mechanical system is often modeled as a multi-body system with non-smoothness. Typical examples are the noises and vibrations produced in railway brakes, impact print hammers, or chattering of machine tools. These effects are due to the non-smooth characteristics such as clearance, impact, intermittent contact, dry friction, or a combination of these effects. In a non-smooth system, neither of the time evolutions of the displacements and the velocities are requested to be smooth. Beam-cam device is an important kind of impacting system which has a wide range of applications. The rotation of the cam at some constant speed provides a force to operate the beam. The most common example is the valve trains of internal combustion engines, where the rotation of the cam imparts the proper motion to the engine valves through the follower while a spring provides a restoring force necessary to maintain contact between the components. The impact on beam-cam is a typical oblique-impact. It has been observed that under variations of the cam rotational speed and other parameters, the follower can exhibit a complex behavior including bifurcations and chaos.
We study a rigid flexible coupling system, which moves in the horizontal plane, and is composed of the hub, the flexible beam and a cam with constant rotating speed. Considering the second-order coupling of axial displacement which is caused by the transverse deformation of the beam, the kinetic energy and the potential energy of the whole system are calculated. The governing equations of the flexible beam-cam oblique-impact system are derived from Hamilton theory, when taking into account the second-order coupling quantity of axial displacement caused by the transverse displacement of the beam. Hertz contact theory and nonlinear damping theories are used to establish the contact model. By the equivalent conversion method in structural mechanics, the deflection curve of flexible beam is calculated. The acceleration at the contact point of beam and cam is used to judge whether they are separate, or contacted, or impacting. Due to the flexibility of beam, the impact point of beam-cam always changes with time and speed. We propose a method, which is a trial calculation method, to determine the impact point of flexible beam-cam.
Simulation results show that there is transverse vibration at the free end of flexible beam. There is inter-coupling among the flexible of beam, the large range of motion, and the impact. After the impact, the rotation angle of the flexible beam changes with time and the angle amplitude mainly decreases with the increase of the time, but the regularity is poor.

2016, 65 (24): 240702.
doi: 10.7498/aps.65.240702

Abstract +

During the last decade, fiber sensor has drawn extensive attention due to its flexible, insulating, and readily operating in most measurement environment. But generally, fiber sensor is sensitive to more than one environmental parameter at the same time, so the cross sensitivity limits the application of the sensor. In the present work, a novel design scheme of sensing simultaneously temperature and strain via guided acoustic-wave Brillouin scattering is proposed for resolving the cross sensitivity induced by temperature and strain in single mode fibers. In the guided acoustic-wave Brillouin scattering which occurs due to the interaction between two optical co-propagating waves and the transverse acoustic wave in optical fiber, multi spectrum peaks appear when the frequencies of pump and Stokes are appropriate. Brillouin frequency shift is dependent on elastic property of fiber material such as sound velocity, density, Young's modulus, etc. and these elastic properties are influenced by the surroundings. So Brillouin spectrum changes with temperature and strain. Because different acoustic modes of guided acoustic-wave Brillouin scattering have different sensitivities to temperature and strain, characteristic frequencies of different acoustic modes shift at different levels. Then the influences of temperature and strain on elastic property of fiber material, and the relationship between material properties and characteristic frequency of each acoustic mode can be worked out, therefore the temperature and strain can be calculated by the different influences of temperature and strain on each acoustic mode. The simulation results indicate that the temperature sensitivity of R02 mode is 0.86% lower than that of TR25 in the SMF-28 fiber, but the strain sensitivity of R02mode is 54.1% higher than that of TR25. Temperature sensitivity of R02 is approximately equal to that of TR25, but strain sensitivity of R02 is obviously diferent from that of TR25. So the influences of temperature and strain on Brillouin frequency shift can be effectively distinguished, thereby simultaneous measurements of temperature and strain can be realized by guided acoustic-wave Brillouin scattering.

###### ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

## EDITOR'S SUGGESTION

2016, 65 (24): 244201.
doi: 10.7498/aps.65.244201

Abstract +

High average power femtosecond lasers based on Ti:sapphire are widely used in strong-field physics and ultrafast dynamics.Continued advances include isolated attosecond pulse generation,few-cycle pulse generation,ultrafast spectroscopy,time-resolved photo-chemical reaction dynamics and laser micro-machining benefit greatly from use of such laser systems.The regenerative amplifiers are mostly utilized and have inherent advantages over multipass ones for applications in chirped pulse amplification.In this paper we describe a design,performance,and the characterizations of a novel linear cavity regenerative amplifier which has produced 4.8 W average power with 35 fs pulse durations at 1 kHz repetition rate.
The main difficulty in designing and constructing a high average power Ti:sapphire regenerative cavity is thermal lensing effect.In order to generate amplified pulses with an output power exceeding 5 W at 1 kHz,a green pump power higher than 20 W is required.Meanwhile,the focal pump beam diameter on the surface of Ti:sapphire crystal should have sub-millimeter mode size to demonstrate large pump fluence,inducing a focal length of a thermal lens about 100 mm,i.e.,which is much less than the scale of the cavity length.For our experiments,a cavity mode size adjustable geometry is employed to counteract thermal lensing effect and to optimize the conversion efficiency of the amplifier.We first characterize the cavity stability by applying the well-known ABCD matrix formalism.The cavity consisting of R=900 mm concave mirror,an 2=800 mm lens and a plane mirror has two stability ranges with increasing the focal length of the thermal lens.In order to obtain a highest thermal tolerance,the optimal cavity parameters are resolved when two stability zones merge into one.After characterizing the cavity in detail,we calculate the cavity mode and the pump beam size at the position of the Ti:sapphire rod as a function of the thermal focal length.The optimal mode radius occurs at 312 m,corresponding to the intersection point of two curves.Stability curve exhibits a weak thermal sensitivity which is defined as the change of radius of cavity mode size per unit focal power change of thermal lens, keeping well below 10 m/D in a range of 2 D-4 D.The calculated results show that the active compensation for thermal lens focal length from 100 mm to could be achieved by adjusting the lens position,without changing the cavity.
20 fs,3 nJ pulses at a repetition rate of 82 MHz produced by a home-made Kerr-lens mode-locked oscillator are first sent to a Martinez stretcher by using a 1200 lines/mm holographic reflectance grating,which temporally stretches the laser pulses to 200 ps.The seed pulses out of the stretcher is then injected into the regenerative cavity depicted above. The 20 mJ pumping energy at 1 kHz is focused through the R=900 mm concave mirror into a 10 mm Brewster-cut Ti:sapphire rod,which is cooled to 250 K by thermoelectric elements.Condensation was avoided by placing the crystal into a small evacuated chamber.Mode matchings of pump and laser beam are found to be of critical importance for high energy extraction efficiency and high beam quality.In our experiments it is accomplished by fine adjusting the F=800 mm cavity lens and the pump beam size.The amplified power of 6.5 W at 1 kHz is obtained with minimum beam distortion,giving a 33.6% slope efficiency.The trapped pulse is built-up quickly and saturated after 8-round trips. The beam size of the amplified laser is expanded to 15 mm in diameter before compressor.A transmission efficiency of 73.8% is achieved through the grating-pair Treacy-type compressor,leading to a 4.8 mJ pulse energy.The grating has a groove density of 1500 lines/mm,and the compressed output spectrum has a full width at half maximum of 29 nm. The pulse duration measurement is performed by using an interferometric autocorrelation.As a result,a typical autocorrelation trace corresponding to a 35 fs pulse width is displayed,and agrees well with the 32 fs transform limit.The far-field beam profile after the compressor is round and Gaussian in both s and p planes,respectively.This scheme is also sufficiently reliable and robust so that no components of the laser system were damaged over a year of operation. In summary,the theoretical analysis and experimental results show that the regenerative cavity developed in this work exhibits a high conversion efficiency and an extraordinary thermal stability,and it is very suitable for high power and high efficient amplification of femtosecond Ti:sapphire pulses.

2016, 65 (24): 244202.
doi: 10.7498/aps.65.244202

Abstract +

The propagation and interactions of Airy-Gaussian beams in a saturable nonlinear medium are investigated numerically based on the split-step Fourier transform method. We show that the propagation of a single Airy-Gaussian beam in the saturable nonlinear medium can generate breathing solitons under steady state conditions. The generation and propagation of these breathing solitons can be affected by the initial amplitude and the field distribution factor of the single Airy-Gaussian beam. In a certain power range, these breathing solitons propagate along the acceleration direction with a controllable tilted angle. In the domain existing in these breathing solitons and for a given value of the field distribution factor of the single Airy-Gaussian beam, when the initial amplitude of the single Airy-Gaussian beam increases gradually, the periodicity of these breathing solitons becomes from small to larger and the tilted angle of these breathing solitons increases monotonically. When the value of the initial amplitude of the single Airy-Gaussian beam is given, the bigger the value of the field distribution factor of the single Airy-Gaussian beam, the smaller the tilted angle of these breathing solitons. Furthermore, the stability of these breathing solitons has been investigated by using the beam propagation method, and it has been found that they are stable. We find that the propagations of two Airy-Gaussian beams in the saturable nonlinear medium can generate not only soliton pairs but also interactions between two Airy-Gaussian beams. When the two Airy-Gaussian beams interact with each other, it is found that the in-phase Airy-Gaussian beams attract each other and exhibit a single breathing soliton with strong intensity in the beam center and some symmetric soliton pairs with weak intensity near both sides of the beam center. The smaller the interval between the two incident Airy-Gaussian optical components, the stronger the attraction between two Airy-Gaussian beams, and the less the numbers of the soliton pairs. The energies of both the main lobes of two Airy-Gaussian beams and the single breathing soliton increase with the value of the field distribution factor of two Airy-Gaussian beams. On the other hand, the out-of-phase Airy-Gaussian beams repel each other and exhibit only symmetric soliton pairs on both sides of the beam center. Our analysis indicates that the repellant of two out-of-phase Airy-Gaussian beams becomes big when the interval between two incident Airy-Gaussian optical components decreases and the number of the soliton pairs becomes less when the field distributions of two beams are close to the Gaussian distribution.

2016, 65 (24): 244203.
doi: 10.7498/aps.65.244203

Abstract +

A novel method by demodulating the sideband of Brillouin gain spectrum (BGS) is proposed and demonstrated in order to enhance temperature measurement accuracy in a Brillouin optical time domain reflectometry (BOTDR) sensing system in this paper.Firstly,the characteristic of frequency shift of an acoustic optical modulator (AOM) is utilized to generate the sideband of BGS,and the influence of the peak power of the probe optical pulse on the temperature measurement accuracy is also investigated.Moreover,the theoretical analysis shows that benefiting from the reference continuous light from the source laser by the coherent detection,the intensity of the sideband is higher than that of the central peak,which indicates that the higher signal-to-noise ratio (SNR) can be expected by demodulating the sideband of BGS instead of the central peak.Thus the demodulating the sideband of BGS can further improve temperature measurement accuracy in the BOTDR sensing system theoretically.Secondly,the experimental setup of the distributed temperature sensing system based on BOTDR is built.The AOM is selected as the optical pulse modulator to produce high-extinction-ratio probe pulse light,following the frequency upshift of the injection light.The beat signal generated by coherently detecting the backscattering light from the fiber under test (FUT) and the reference light from the source laser is acquired.Furthermore,the central peak and the left sideband of BGS are respectively scanned by using microwave heterodyne frequency shift technique.The time domain waveforms at each frequency point are then obtained and Lorentzian curve fitting is performed at each sampling position,thus Brillouin frequency shift (BFS) along the FUT is plotted and the temperature is demodulated along the FUT based on the linear dependence of the BFS on the temperature in the optical fiber.Finally,the experimental results show that the peak power of the left sideband of Brillouin gain spectrum is about 3.27 dB stronger than that of the central peak.Meanwhile,the linewidth of left sideband of BGS is about 14.7 MHz narrower than that of the central peak at -1 dB point in the same conditions.When the left sideband of BGS is scanned,the SNR of the BOTDR system is improved by 4.35 dB due to the contribution of the reference light by coherently detecting and eliminating the effect of the coherent Rayleigh noise,and then the temperature measurement accuracy of 0.5℃ is achieved over a 10.2 km sensing fiber.

2016, 65 (24): 244204.
doi: 10.7498/aps.65.244204

Abstract +

The binary one-dimensional plasma photonic crystal (1DPPC) based on Fibonacci quasiperiodic structure is studied systematically in this paper. We consider the two simplest cases. In one case, the initial sequences F0 and F1 are both of single layer structure. In another case, one initial sequence (F0 or F1) is of a single layer structure, while the other one (F1 or F0) is of a double layer structure. Thus there are ten different kinds of initial sequences in total. The photonic bandgap characteristics of the 1DPPC with these different initial sequences and numbers of period are analyzed. On these bases, a novel structure of one-dimensional plasma photonic crystal (F3)3 with initial sequence of F0=AP, F1=P and F0=PA, F1=P is proposed in this paper to enlarge the omnidirectional photonic bandgap (OPBG). Compared with previously reported structures in the literature, this structure is simple in configuration with fewer layers and materials, and its OPBG width is wide. The influences of the parameters of the plasma material, such as the thickness, plasma frequency and collision frequency, on the OPBG characteristics of this structure are also discussed. The OPBG width increases with the increase of the thickness and plasma frequency of the plasma layer. Compared with the structures in the literature, the change of OPBG width is the fastest for the proposed structure when the parameters are relatively small. And with the same parameters, the OPBG width for the proposed structure is the widest when the parameters are greater than a certain value. The plasma collision frequency has no effect on the OPBG width for all the structures. But the OPBG width for the proposed structure is the widest when this parameter has the same value. The reason why the proposed structure has an optimal OPBG width is explained by analyzing the dispersion properties of the plasma. The real and imaginary part of the dielectric constant of plasma change with frequency significantly only in the low frequency region. Since the imaginary part of dielectric constant is nearly zero when the frequency is larger than 2 GHz, only the dispersion effect of the real part of dielectric constant needs to be considered in the frequency range we investigate. And its value is much greater than that of conventional medium in the same frequency range. This makes the high-reflectance bands of the 1DPPC broader than those in the case of pure photonic interference phenomena with conventional medium. On the other hand, the corresponding highest proportion of plasma layers in the whole quasiperiodic structure can also be used to explain the broadest band gap of (F3)3. These results can provide important theoretical guidance for designing the novel omnidirectional reflectors.

2016, 65 (24): 244401.
doi: 10.7498/aps.65.244401

Abstract +

In this paper, lattice Boltzmann method is used to simulate the phonon heat transport in NaCl@Al2O3 mesoporous composite material. Based on the Debye model, temperature coupling method is first proposed in the thermal simulation of mesoporous composite material, to calculate the effective thermal conductivity of mesoporous composite material with pores of various interface condition coefficient value, pore size, porositiy, shape and arrangement. Studies show that for the same porosity, the estimated thermal conductivity increases with increasing the value of diameter, showing the scale effect; for the same diameter, the estimated thermal conductivity decreases with increasing the value of porosity; for the same porosity and diameter, the estimated thermal conductivity decreases with increasing the value of interface condition coefficient p; porous shape and arrangement will affect thermal conductivity value, and the influences greatly increase with increasing the value of p.

2016, 65 (24): 244601.
doi: 10.7498/aps.65.244601

Abstract +

In the mold filling process, polymer melt will suffer the shear stress and stretch, which has important influences on the mechanical properties and surface quality of the final plastic products. In this paper a gas-liquid two-phase flow model for a viscoelastic fluid is proposed and used to simulate the mold filling process, in which the finitely extensible nonlinear elastic dumbbell with Peterlin closure (FENE-P) model and cross-WLF viscosity model combined with Tait state equation are used to describe the constitutive relationship and viscosity change of the viscoelastic melt, respectively. Meanwhile, the improved coupled level-set and volume-of-fluid method is used to trace the melt front, and the finite volume method on non-staggered grid is used to solve the mass, momentum, and energy conservation equations. Firstly, the R-function, an excellent implicit modeling tool of constructive solid geometry, is employed to establish the shape level-set function to describe the complex mold cavities based on the signed distance functions that represent basic geometries. And the immersed boundary method is applied to dealing with the complex mold cavities by using the shape level-set function. The benchmark problem of the flow past a cylinder is simulated to verify the validity of the FENE-P model, where the orientational ellipses are used to describe the molecular orientation and deformation. Moreover, the visualization of polymer molecular deformation is achieved. Then, the non-isothermal filling process of the viscoelastic fluid is simulated in an annular mold cavity with two circular insets, and the behaviors of the molecular orientation, temperature and stress in the filling process are shown and analyzed in detail. Finally, the problems are also discussed that how the injection velocity, melt and mold temperatures influences on the molecular conformation and solidified layer thickness. Numerical results show that the computational framework proposed in this paper can be successfully used to simulate the non-isothermal mold filling process in the complex mold cavity. Increasing properly the injection velocity can reduce the heat loss and improve the strength of the weld line. The higher the melt or mold temperature, the thinner the solidified layer is. Thus, increasing the injection velocity, as well as raising the melt and the mold temperatures will improve or remove the weld line in melt filling process.

2016, 65 (24): 244701.
doi: 10.7498/aps.65.244701

Abstract +

Smoothed particle hydrodynamics (SPH) method is a kind of meshless method, which is used to solve the problem of fluid simulation without complex operations of the grids. To reduce the computational complexity, SPH method based on the two-dimensional shallow water equations is employed to establish a fluid model. In large scale scenes, taking into account the high computational complexity and the serious distortion problems, in this paper we introduce an improved two-dimensional SPH algorithm according to the shallow water equations. The proposed method with two-dimensional complexity is obtained by discretizing the two-dimensional shallow water equations with SPH, and the depth of water is introduced as the particle's property. The problem of increased amount of calculation cannot be well solved by using traditional neighboring particle search method based on tree structure. To improve the efficiency of search and simplify the search operation of neighborhood particles, in this paper we introduce a point-in-box search algorithm and put forward a neighboring particles searching method on the basis of dynamic grid. Besides, for large scale scenes, by considering that the virtual particle method provides slow computation speed with complex boundary condition, the type-one virtual particles are utilized to ensure that the borders can be calculated precisely by combining the punish force to prevent the phenomenon of particle boundary penetrating. Therefore, a method is further obtained to handle boundary condition efficiently by combining the virtual particles with punish force in this paper. In the process of rendering, the fluid surface is first determined by mapping and interpolating particles into regular grids without the complex reconstruction of surface in three-dimensional. Then, we utilize the bilinear interpolation method to deal with the problem of missing values, and the surface grids are further densified. With OpenSceneGraph three-dimensional render engine, OpenGL Shading Language is adopted to speed up the rendering speed, and in this way, the real-time fluid simulation of large scale scenes can be further achieved. With the basic KD tree searching method employed in the simulations, the comparative experiments are provided to verify effectiveness of the proposed searching method based on dynamic grid. Given the data set obtained from random points, experimental results demonstrate that the method in this paper can be used to solve the problem of neighboring particles searching in large scale scenes. To show the effectiveness of the proposed method on the basis of the virtual particles and the punish force, another experiment based on the collapsing of a water column is further provided. Besides, in this paper we conduct an experiment on a certain actual reservoir terrain to prove that the proposed method can be applied to fluid simulation of large scale scenes.

2016, 65 (24): 244702.
doi: 10.7498/aps.65.244702

Abstract +

The electro-magnetic forces generated by electromagnetic field take control of the flow in the electrolyte solution. In this paper, the mechanism of two-degree-of-freedom vortex-induced vibration controlled by electro-magnetic forces is investigated numerically. With the coordinate at the moving cylinder, the stream function-vorticity equations, the initial and boundary conditions and distribution of hydrodynamic force are deduced in the exponential-polar coordinate. The equation of vorticity transport is solved by the alternative-direction implicit algorithm. The equation of stream function is integrated by means of a fast Fourier transform algorithm. The cylinder motion is calculated by the Runge-Kutta method. The flow field, pressure, lift/drag and cylinder displacement are interacted along the transverse and streamwise direction, where the instantaneous variations are discussed. The derivation shows that the vibration displacement in one direction, whose effects on the flow field influence the vortex-induced forces in both directions, affects the inertial force only in the corresponding direction and is independent of that in the other direction. The numerical calculations show that the vortex-induced vibration is affected by two factors, i.e., the vortex shedding and the cylinder shift. Both of the two factors have influences on the shear layers in the transverse direction and the secondary vortex in the streamwise direction, which further leads to the variations of lift/drag and the cylinder motion. Along the transverse direction, the strength of shear layer on one side is increased by the vortex shedding while the strength of shear layer on the other side is increased by the cylinder shift. Along the streamwise direction, the pressure of cylinder tail is varied with the effect of shedding vortex on the secondary vortex while the effect of cylinder shift on the secondary vortex is also opposite to that of shedding vortex. Notably, the effect of cylinder shift prevails over the effect of shedding vortex so that the former is dominated in the total effects. The flow separation and vortex shedding are suppressed as the fluid of boundary layer is accelerated under the action of electro-magnetic forces. Meanwhile, the vibration displacements decrease gradually along both the transverse and streamwise directions, which also suppresses the effects of pressure/suction sides. Therefore, the vibration is suppressed and the cylinder turns steady rapidly. In addition, the thrust generated by the wall electro-magnetic force counteracts the drag generated by the fluid electro-magnetic force, which means that the final position is at the upstream of the initial position. The experimental results show that the vortexes on the cylinder are suppressed fully and the flow field is steady under the action of electro-magnetic force, which agrees well with the numerical results.

2016, 65 (24): 244703.
doi: 10.7498/aps.65.244703

Abstract +

Spanwise-rotating turbulent plane Couette flow (RPCF) is one of the fundamental prototypes for wall-bounded turbulent flows in rotating reference frames. In this turbulent problem, there are large-scale roll cells which are widely studied. In this paper, a penta-decomposition method is proposed to separate the instantaneous velocity and the total kinetic energy into five parts, i.e., a mean part, a streamwise part and a cross-flow part of the secondary flow, and a streamwise part and a cross-flow part of the residual field. The transport equations for the last four shares, which contribute the total turbulent kinetic energy, are derived. According to these transport equations, the mechanisms of energy transfer among different fractions of turbulent kinetic energy can be revealed clearly. Our objective is to explore the energy balance and transfer among different fractions of the turbulent kinetic energy in RPCF based on a series of direct numerical simulation databases at a Reynolds number Rew=Uwh/=1300 (here, Uw is half of the wall velocity difference, and h is the channel half-width) and rotation number Ro=2zh/Uw (z is the constant angular velocity in the spanwise direction) in a range of 0Ro0.9. The results show that the energy is transferred between the streamwise part/cross-flow part of secondary flows and the residual field through the correlation between the vorticity of secondary flows and the shear stress of residual field. The rotation term acts as a bridge to transfer the energy between the streamwise part and the cross-flow part of either the secondary flows or the residual field. Moreover, pressure-strain redistribution term also plays an important role in the energy transfer between streamwise part and cross-flow part in residual field. For the streamwise part of residual field, in certain rotate rates, the energy obtained from the streamwise part of secondary flows by the correlation between the vorticity of secondary flows and the shear stress of residual field is larger than that obtained from mean flow through mean shear, implying that the streamwise motions of secondary flows have a significant influence on the streamwise motions of residual field.

###### PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

2016, 65 (24): 245201.
doi: 10.7498/aps.65.245201

Abstract +

The structure of an electronegative plasma sheath in an oblique magnetic field is investigated. Moreover, the collisions between positive ions and neutral particles are taken into account. It is assumed that the system consists of hot electrons, hot negative ions, and cold positive ions. Also the negative ions and the electrons are assumed to be described by the Boltzmann distributions of their own temperatures, and the accelerated positive ions are treated by the continuity and momentum balance equations through the sheath region. In addition, it is assumed that the collision cross section has a power law dependence on the positive velocity. After theoretical derivation, an exact expression of sheath criterion is obtained. The numerical simulation results include the density distributions of the positive ions for different invariable ion Mach numbers satisfying Bohm criterion, and the comparison of net space charge distribution between variable and invariable ion Mach numbers. Furthermore, three kinds of charged particle densities, the net space charges, and the spatial electric potentials in the sheath are studied numerically for different collision parameters under the condition of the fixed ion Mach number. The results show that the ion Mach number has not only the lower limit but also the upper limit. The ion Mach number affects the sheath structure by influencing the distribution of the positive ion density, and different conclusions can be obtained because ion Mach number is adopted as variable or invariable value when discussing the effects of the other variables which can result in a variety of the ion Mach numbers on the sheath formation. The reason is that the actual sheath structure modification brought on by the variation of a parameter can be divided into two parts. One is the sheath formation change caused directly by the variation of the parameter, and the other is the sheath formation change caused by the Bohm criterion modification which the variation of the parameter results in. Therefore, an identical ion Mach number should be adopted when studying the direct effects of a parameter variety on plasma sheath structure. In addition, it is concluded that the collisions between positive ions and neutral particles make positive ion density curve higher and electron density curve lower than the case without collisions. Negative ion density does not change significantly no matter whether there exists collision. Besides, there is a peak in the profile of the net space charge while in the presence of ion-neutral collision, and the net space charge peak moves toward the sheath edge. The spatial potential increases and the sheath thickness decreases on account of the presence of the collisions between ions and neutral particles.

## EDITOR'S SUGGESTION

2016, 65 (24): 245202.
doi: 10.7498/aps.65.245202

Abstract +

Based on the mass fraction model of multicomponent mixture, the interactions between weak shock wave and V shaped air/SF6 interface with different vertex angles are numerical simulated. The numerical scheme used in the simulation is the high-resolution finite volume method with minimized dispersion and controllable dissipation scheme, in which the dissipation can be adjusted without affecting the already optimized dispersion property of the scheme. The grid sensitivity study is performed to guarantee that the resolution is sufficient in the numerical simulation. After the shock wave interacts with the interface, the baroclinic vorticity is deposited near the interface due to the misalignment of the density and pressure gradient, which is the manifestation of the Richtmyer-Meshkov instability, leading to the vortical structures forming along the interface. The interface perturbations lead to the bubbles and spikes appearing. The predicted leftmost interface displacement and interface width growth rate in the early stage of interface evolution agree well with the experimental results. The process of transition to turbulence at the material interface is studied in detail. The numerical results indicate that with the evolution of the interfacial vortical structure due to Kelvin-Helmholtz instability, the array of vortices begins to merge. As a result, the vortices accumulate in several distinct regions. It is in these regions that the multi-scale structures are generated because of the interaction between vortices. It is shown clearly that in the regions where vortices are accumulated, the fluctuation energy spectrum has many large and smallscale elements, which indicates there may be turbulent structures in these regions. To further examine if there is mixing transition in these regions, the characteristic length scales of the flow fields are calculated. The separation between the Lipemann-Taylor scale and inner viscous scale is observed based on the circulation-based Reynolds number, leading to the appearance of an uncoupled inertial range. The classical Kolmogorov -5/3 power law is also shown in the fluctuation energy spectrum, which means that the inertial range is developed. The appearing of this inertial range confirms that the mixing transition does occur, and the flow field near the material interface will develop into turbulence.

###### CONDENSED MATTER:STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

2016, 65 (24): 246101.
doi: 10.7498/aps.65.246101

Abstract +

The equipment and devices which are long-time running in space are affected by space radiation effects and hot carrier injection effects at the same time which would reduce their optional times. Normally, the single mechanism test simulation method is used on the ground simulation test but the multi-mechanism effect affects the space equipments and devices, including total irradiation dose effect, hot carrier injection effect, etc. The total dose dependence of hot carrier injection (HCI) effect in the 0.35 m n-channel metal oxide.
semiconductor (NMOS) device is studied in this paper. Three samples are tested under different conditions (sample 1# with total irradiation dose (TID) and HCI test, sample 2# with TID, annealing and HCI test, sample 3# only with HCI test). The results show that threshold voltage of NMOS device with 5000 s HCI test after 100 krad (Si) total dose radiation has been negatively shifted then positively during total dose irradiation test and HCI test, and the threshold is higher than that of the device without radiation test. But the threshold voltage shift of NMOS device with 5000 s HCI test and 200 h annealing test after TID test is higher than that of the devices without radiation test and lower than that of the devices without annealing test. That is, the parameters of NMOS device vary faster with the combined effects (including the total dose irradiation effect and HCI effect) than with single mechanism effect. It is indicated that the hot electrons are trapped by the oxide trap charges induced by irradiation effect and then become a recombination centre. And then the oxide trap charges induced by irradiation effect reduce and become negative electronic. The interface trap charges induced by irradiation effect are reduced and then increased it is because the electrons of hole-electron pairs in the Si-SiO2 interface are recombined by oxide traps in the oxide during the forepart of HCI test but then the electrons are trapped by interface traps in the Si-SiO2 interface because the electrons from source area are injected to interface during the HCI test. So the threshold voltage is positively shifted due to the negative oxide trap charges and interface trap charges. The association effect is attributed to the reduction of oxide traps induced by recombination with hot electrons and the increase of the interface traps induced by irradiation trapped hot electrons.

2016, 65 (24): 246102.
doi: 10.7498/aps.65.246102

Abstract +

Laser-induced damage of fused silica optics at 351 nm is a key factor limiting the output energy of high-power laser facility, especially the damage growth process. A comprehensive understanding of its damage growth behavior is of critical importance for high-power laser facility. Thus we study the laser-induced damage growth on the exit surface of fused silica under the subsequent illumination of 5 ns square pulses at 351 nm on a large-aperture high-power laser facility. Experiment is conducted with a 36 cm thick UV grade fused silica focus lens in clean atmosphere and at room temperature. 56 laser shots of 3 fluence in a range from 0.1 J/cm2 to 8.1 J/cm2 are fired during the experiment. And the damage initiation process and growth process are monitored and recorded with an online optics damage inspection instrument which has an optical resolution of about 50 m.
Experimental results demonstrate that the sizes of exit-surface damage sites exponentially or linearly grow with laser shots and the damage growth rate increases with laser fluence. However, it is found that even under the same laser conditions the damage grow rate is not a fixed value, which means that besides the laser fluence other parameters also influence the damage grow process. In order to highlight some tendencies, we consider the single-shot damage growth rate and calculate the average of inside fluence bins. Statistical analysis shows that smaller sites tend to grow with larger growth rates than larger sites under the irradiation of the same laser fluence. This result indicates that damage growth rate is influenced by both laser fluence and damage site size. It suggests that the damage growth rule needs to be incorporated into a size-dependent growth effect.
The result that higher growth rates are obtained for small damage sites may be related to the damage growth mechanism of fused silica. Damage crater of fused silica consists of a central core and numerous surrounding cracks. The defects in the central core absorb laser energy and yield plasma, then the plasma pressure will open the cracks on the periphery of the crater and lead to lateral and axial expansion of cracks which can be identified as damage growth. The fact that smaller sites grow faster than larger sites implies that smaller sites more efficiently couple laser energy into fracture energy. Our results have important implications for both the prediction of fused silica optics lifetime and the fundamental understanding of laser damage mechanism.

###### CONDENSED MATTER:ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

2016, 65 (24): 247201.
doi: 10.7498/aps.65.247201

Abstract +

Optical pump-terahertz (THz) probe spectroscopy is employed to investigate the photo-excited carrier relaxation process and the evolution of terahertz conductivity in ZnSe.With the pump pulse at a wavelength of 400 nm,the carrier relaxation process can be well fitted to a biexponential function.We find that the recombination process in ZnSe occurs through two components,one is the fast carrier recombination process,and the other is the slow recombination process.The fast carrier relaxation time constant is in a range from a few tens of picoseconds to hundreds of picoseconds, and slow carrier relaxation time constant ranges from one to several nanoseconds.We find that both the fast and the slow carrier relaxation time constant increase with the power density of pump beam increasing,which is related to the density of defects in the sample.Upon increasing the excitation power density,the defects are filled by the increased photo-excited carriers,which leads to an increase in the fast carrier relaxation time.While,the slow carrier relaxation time increasing with pump flux can be attributed to the filling of surface state.We also present the THz complex conductivity spectra of ZnSe at different delay times with a pump flux of 240 J/cm2.It is shown that the real part of the conductivity decreases with increasing the pump-probe delay time.The real part of the conductivity is positive and increases with frequency in each of the selective three delay times (2,20,and 100 ps),while the imaginary part is negative and decreases with frequency.The transient conductivity spectra at terahertz frequency in different delay times are fitted with Drude-Smith model.According to the fitting results from Drude-Smith model,with the pump-probe delay time increasing,the average collision time and the value of c1 decrease.Generally,a higher carrier density leads to a more frequent carrier-carrier collision,which means that the collision time should decrease with carrier density increasing. The abnormal carrier density dependence of collision time implies a predominance of backscattering in our ZnSe.The predominance of backscattering is also observed for the negative value of c1.The negative value of c1 indicates that some photocarriers are backscattered in ZnSe.With a delay time of 2 ps,the value of c1 approaches to -1,which indicates that the direct current (DC) conductivity is suppressed,and the maximum conductivity shifts toward higher frequency. With increasing the delay time,the value of c1 decreases:in this case DC conductivity dominates the spectrum.The study of the dynamics of photoinduced carriers in ZnSe provides an important experimental basis for designing and manufacturing the high speed optoelectronic devices.

2016, 65 (24): 247501.
doi: 10.7498/aps.65.247501

Abstract +

Due to its magnetostructural phase transition (the structural phase transition and the magnetic phase transition are strongly coupled together and occur simultaneously),Mn-based Heusler alloys exhibit attractive physical effects,such as ferromagnetic shape memory effect,magnetostrain effect,magnetocaloric effect,magnetoresistance effect,and exchange bias effect.These effects are receiving increasing attentions from the applications in actuating,sensing,magnetic cooling,heat pump,and energy conversion.However,Mn-based Heusler alloys display these potentially useful magnetic effects only in the vicinity of the magnetostructural transformation temperature.Therefore,from the application point of view,being able to tune the magnetostructural transformation temperature and the magnetism simultaneously is highly desirable.Recently,our group has developed a new Mn-based Heusler alloy (Mn2NiSn) with magnetostructural phase transition.Considering that the magnetostructural transformation temperature of Mn50Ni41Sn9 alloy is relatively high (278 K) and its magnetism is relatively weak (19.5 emu/g at 5 K,1 emu/g=1 Am2kg-1),we expect to lower its magnetostructural transformation temperature and enhance its magnetism in order to expand its scope of application.In this paper,the role of Ni-Mn hybridization on the martensitic transformation temperature and the magnetism of the martensitic state of Mn50Ni41Sn9Cux alloys was studied.XRD measurement shows that the lattice constants increase with increasing Cu content in Mn50Ni41-xSn9Cux (x=0,1,3,5) alloys,and thus Ni-Mn hybridizatiidion between normal Ni 3d e g and excess Mn 3d decreases due to the lattice expansion and the decrease in the Ni content. The weakened Ni-Mn hybridization leads to the decrease of both the martensitic transformation temperature and the austenitic Curie temperature from 278 K and 290 K to 129 K and 237 K,respectively.It should be pointed out that the phenomenological and conventional valence electron concentration rule has not been able to explain the change of the martensitic transformation temperature in Mn50Ni41-xSn9Cux alloys,and only the microscopic Ni-Mn hybridization theory can explain that.Ni-Mn hybridization not only affects the martensitic transformation but also influences the magnetism of the martensitic state.It is found that the martensite is changed from a canonical spin glass to a cluster spin glass and its saturation magnetization increases from 19.5 emu/g to 24.1 emu/g.Furthermore,both the ac magnetic susceptibility and the magnetic relaxation measurements show that the system has changed gradually from a spin glass state with coexistence of ferromagnetic and antiferromagnetic interaction to a single ferromagnetic state.Therefore, increasing the Cu content in Mn50Ni41-xSn9Cux alloys has been proven to be an effective way of enhancing the ferromagnetic interaction of the martensitic state.Tuning the exchange interaction of the system is very crucial to tailoring the exchange bias effect of the system.With different Cu contents,a continuous tailoring of the spontaneous exchange bias field from 0 Oe (1 Oe=79.5775 A/m) to 1182 Oe is realized.The method of changing the Ni-Mn hybridization strength mentioned above provides a new way to control the martensitic transformation temperature and the magnetic properties of the martensitic state.

2016, 65 (24): 247502.
doi: 10.7498/aps.65.247502

Abstract +

The CoFeB/Ni multilayers with Pt underlayer are prepared by magnetron sputtering technique and the perpendicular magnetic anisotropy (PMA) of each of the samples is studied by anomalous Hall effect (AHE) method. The PMA of CoFeB/Ni multilayer is dependent on the thickness of Pt, Co, CoFeB and the number of CoFeB/Ni bilayers strongly. It is found that the sample structured as Pt(4)/[CoFeB(tCoFeB)/Ni(0.3)]2/Pt(1.0) has a good PMA when the CoFeB thickness is 0.4 nm for the interface anisotropy dominated in the multilayer. So the CoFeB thickness is fixed at 0.4 nm. The effect of Ni thickness on multilayer PMA is also studied. The PMA of the sample is kept relatively well and the Hall resistance (RHall) decreases as the Ni thickness increases. Meanwhile the coercivity (HC) fluctuates in a small range. When the Ni thickness is 0.3 nm, the remanence squareness of the sample is very good and the Hall effect is strongest. The influence of period number n on the sample PMA is significant for it changes the interface of the sample. When n is 3, the sample has a very good remanence squareness, for the interface effect is obvious and the magnetization reversal process is consistent. The Pt underlayer shows a great effect on the PMA performance of the sample, for it can change the (111) texture of the multilayer. The results show that when the Pt thickness is 4 nm, the remanence squareness is good and the sample has a suitable HC. So the optimum CoFeB/Ni multilayer with an excellent performance of PMA is structured as Pt(4)/[CoFeB(0.4)/Ni(0.3)]3/Pt(1.0). Its anisotropy constant Keff is 2.2106 erg/cm3 (1 erg/cm3=10-1 J/m3) which indicates that the sample has an excellent PMA and its interface anisotropy is the main reason for making the Keff have a larger value. The magnetic layer thickness of the optimum sample is 2.1 nm and the total thickness of it is less than 8 nm. The integration with device can be studied further. Furthermore, HC of the CoFeB/Ni multilayer is relatively small and can be increased by inserting the oxidation layer or other ways.

2016, 65 (24): 247801.
doi: 10.7498/aps.65.247801

Abstract +

In the past few years, many interesting optical phenomena, such as electromagnetically induced transparency, coherent optical control of a biexciton, slow light and optical solitons, have been investigated in single quantum dot (QD). However, in an actual semiconductor device there exist many quantum dots (QDs). Recently, QD molecule, which is comprised of double semiconductor QDs coupled by tunneling coupling, has been proposed. In this new semiconductor structure, many complex but interesting phenomena have been discovered. In fact, three QD molecules may also be composed of three QDs, which can be coupled by interdot tunneling coupling. For the three semiconductor QDs molecules, the influence of the interdot tunneling coupling strength must be considered. So, in this paper, with considering that a weak, -linear-polarized probe field can form left- and right-polarized components under the control of the parallel magnetic field, and when they are combined with the tunneling coupling among the QDs, an electromagnetically induced transparency medium of a five-level M configuration semiconductor three QDs is proposed. Subsequently, the nonlinear Faraday rotation in the semiconductor three QDs is analytically studied.
For the linear case, the linear dispersion relation is driven by a method of multiple scales. Then, by studying the linear optical properties, it is found that the system exhibits a single tunneling induced transparency window due to the quantum destructive interference effect driven by the interdot tunneling coupling under appropriate conditions, and the width of the tunneling induced transparency window can be effectively controlled by the strength of the interdot tunneling coupling. Meanwhile, the switch regulatory effect, which changes from the anomalous dispersion regime to the normal dispersion regime, is likely to be achieved by changing the strength of the interdot tunneling coupling.
For the nonlinear case, two coupled nonlinear Schrdinger equations, which govern the evolutions of left- and right-polarized components of the weak, -linear-polarized probe field under the applied longitudinal magnetic field, are derived. By studying the nonlinear properties, it is shown that a large nonlinear Faraday rotation angle can be obtained due to the quantum interference effect which is induced by the interdot tunneling coupling with a very low absorption of the weak, -linear-polarized probe field. In addition, it is also found that the nonlinear Faraday rotation direction is opposite to line Faraday rotation for the same magnetic field. What is more, the nonlinear Faraday rotation angle grows bigger than the linear Faraday rotation. These results mean that the Faraday rotation of the three semiconductor QDs with the electromagnetically induced transparency can be more effectively controlled by the nonlinear effect.

2016, 65 (24): 247802.
doi: 10.7498/aps.65.247802

Abstract +

Polymer nanocomposites have advantage over traditional materials in electrical properties from the standpoint of dielectrics and electrical insulation. The influences of nanoparticle dispersion in the matrix, which is mainly caused by different preparation methods, on the dielectric properties of composites have been given in the past work. In order to investigate the relationship between the dispersion of nanoparticles in the matrix and the dielectric properties of composites, nano-SiO2/epoxy composites are prepared by different methods. Nano-SiO2 is first modified by silane coupling agent to obtain nano-SiO2 powder and nano-SiO2 dispersing liquid, then unmodified and modified nano-SiO2 powder are mixed into epoxy by mechanical mixing method, and the modified nano-SiO2 dispersing liquid is mixed into epoxy by bubble mixing method to prepare nano-SiO2/epoxy composites. The amounts of nano-SiO2 content in the composites are 2 wt%, 3 wt%, 4 wt%, 5 wt% and 6 wt%, respectively. Breakdown strength and corona-resistance characteristics of the composites are tested. The results show that with the increase of the nano-SiO2 content, the breakdown strength and corona-resistance of nano-SiO2/epoxy composites increase. The maximal breakdown strength appears in the composites with 5 wt% nano-SiO2. This appearance accords with percolation theory. The composites prepared by bubble mixing method have better breakdown strengths and corona-resistances than the composites prepared by mechanical mixing method. The scanning electron microscope images of the nano-SiO2/epoxy composites are analyzed by Image J software to obtain the information about the nanoparticle number in the special grid. Morisita's index is used to characterize the dispersion of nano-SiO2 in the matrix quantitatively. It is concluded that the composites prepared by bubble mixing method have better dispersion than those prepared by mechanical mixing method. Compared with the unmodified nano-SiO2, modified one has good dispersion in the composite because of the improved compatibility between the nanoparticles and the matrix. Based on the role that nano-SiO2 particles block discharge from developing in the composite, the better dispersion means that there are more nanoparticles and more barriers on the discharge path. Meanwhile, the better dispersion also means that more interface areas form between nano-SiO2 and matrix. The shallower traps supplied by the interface area will contribute less energy when current carriers jump into and out off the traps. So the better the dispersion of nano-SiO2 in the matrix, the superior the breakdown strength and corona-resistance of the composites are.

###### INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

First-principles investigation on partitioning behavior of rare earth elements between α-Fe and Fe3C

2016, 65 (24): 248101.
doi: 10.7498/aps.65.248101

Abstract +

2016, 65 (24): 248102.
doi: 10.7498/aps.65.248102

Abstract +

Electric field induced semiconductor-metal transition characteristics of VO2 indicate extensive application prospects in smart window,storage device,intelligent radiator,signal generator,optical switch,etc.In order to explore the electric field induced semiconductor-metal transition characteristics of VO2,AZO/VO2/AZO sandwiched structure is prepared to study the problem of optical modulation under the action of applied electrical drive.Firstly,V thin film is fabricated by direct current magnetron sputtering on a ZnO-doped Al (AZO) conductive glass substrate.The operating pressure during sputtering is kept at 3.610-1 Pa,and the sputtering current and voltage are 2 A and 400 V,respectively.The VO2/AZO composite film is prepared by annealing under the air atmosphere for 3.5 h at 400℃.Secondly,another AZO conductive film is deposited by radio frequency magnetron sputtering on the top of the VO2 thin film.Thirdly, Pt electrodes are patterned on the bottom and top of AZO conductive glass by using photolithography and chemical etching processes,and finally AZO/VO2/AZO sandwiched structure is achieved.The crystal structure of the thin film is analyzed by X-ray diffraction (XRD) apparatus.The surface morphologies of the samples were studied by atomic force microscope (AFM).X-ray photoelectron spectroscopy (XPS) system is used to study the relative quantity of the surface elements.The current-voltage characteristics are measured by semiconductor parameter analyzer.The optical properties of the AZO/VO2/AZO sandwiched structure are determined by spectrophotometer.XRD results show that the VO2 thin film has a distinct (011) preferred orientation and well-crystallized structure.AFM results indicate that the VO2 thin film has compact nanostructure and smooth surface with a surface roughness of 5.975 nm.XPS results reveal that the VO2 thin film has high purity.Optical transmittance curves show that the maximum change of the optical transmittance measured from VO2/AZO composite film during the phase transformation is 24% at 800-2300 nm,while the maximum modulation of the transmittance of AZO/VO2/AZO sandwiched structure reaches 31% in the same wavelength range. When applying different voltages to AZO/VO2/AZO sandwiched structure at different ambient temperatures,the current abrupt change can be seen at the threshold voltage.The threshold voltage of the thin film phase transition is 8.1 V at 20℃,while the threshold voltage is 5.9 V at 40℃.However,the threshold voltage is zero at 60℃,which indicates that the semiconductor-metal transition of the VO2 thin film happens at that temperature.It can be found that the higher the ambient temperature,the lower the threshold voltage is.AZO/VO2/AZO sandwiched structure has stable properties with simple preparation technology,and its modulation property meets the performance requirements for electro-optic modulator under applying the electrical drive,which is expected to be applied to the integrated infrared modulator.

2016, 65 (24): 248103.
doi: 10.7498/aps.65.248103

Abstract +

Since its discovery in 2004, the graphene has attracted great attention because of its unique chemical bonding structure, which has excellent chemical, thermal, mechanical, electrical and optical properties. Due to the graphene being a zero band gap material, it has a limited development in the field of nano electronics. Therefore, in order to broaden its application scope, it is very important to carry out a study on opening the band gap of graphene. In this paper, we construct three models, i.e., the intrinsic graphene model, the N-doped graphene model, and the B-doped graphene model. We study the energy band structures and the electronic densities of states for the intrinsic graphene and the N/B doped graphenes with different doping concentrations. Furthermore, we study their optical and electronic properties including the absorption spectra, the reflection spectra, the refractive indexes, the conductivities, and the dielectric functions. The results are as follows. 1) The electronic states in the vicinity of the Fermi level for the intrinsic graphene are mainly generated by the C-2p orbits, while the electronic states in the vicinity of the Fermi level for the N/B doped graphenes are mainly generated through the hybridization between C-2p and N-2p/B-2p orbits. N doped graphene is of n-type doping, while B doped graphene is of p-type doping. 2) Compared with that of the intrinsic graphene, the Fermi level of N doped graphene moves up 5 eV. In the meantime, the band gap is opened, and the Dirac cone disappears. On the contrary, the Fermi level of B doped graphene moves down 3 eV compared with that of the intrinsic graphene. However, like the N doping, the band gap is also opened, and the Dirac cone disappears. Furthermore, the N doping is more effective than the B doping in opening the energy gap of the graphene for the same N/B doping concentration. 3) The N/B doping can cause the optical and electronic properties of the graphene to change, and exert great influences on the absorption spectrum, reflection spectrum, the refractive index, and the dielectric function, however it has little influence on the conductivity. When the energy of the incident wave is larger than a certain value, the optical and electrical properties of the intrinsic graphene remain unchanged. Besides, for the above case, the corresponding energies for the N/B doped graphenes are smaller than that for the intrinsic graphene. In addition, the energy for the B doped graphene is smallest. The conclusions of this paper can provide a theoretical basis for the application of graphene in optoelectronic devices.

2016, 65 (24): 248104.
doi: 10.7498/aps.65.248104

Abstract +

By using the nonequilibrium Green's function method, the ballistic thermal rectification in the three-terminal graphene nanojunction is studied. The dynamics of atoms is described by the interatomic fourth-nearest neighbor force-constant model. The nanojunction has a Y-shaped structure, created by a combination of a straight graphene nanoribbon and a leaning branch as the control terminal holding a fixed temperature. No heat flux flows through the control terminal. There exists a temperature bias between the two ends of the graphene nanoribbon serving as the left and right terminals, respectively. The primary goal of this paper is to demonstrate that the ballistic thermal rectification can be introduced by the asymmetric structure with different connection angles between terminals. The control terminal has a smaller connection angle with respect to the left terminal than to the right terminal. The forward direction is defined as being from the left terminal to the right terminal. The results demonstrate that, given the same control temperature and absolute temperature bias, the heat flux in the graphene nanoribbon tends to run preferentially along the forward direction. When the difference between the connection angles increases, the rectification ratio rises. Compared with that of the zigzag graphene nanoribbon, the rectification ratio of the armchair nanoribbon is much sensitive to the direction the control terminal. However, the greatest rectification ratio is found in the zigzag graphene nanoribbon which has a connection angle of 30 degrees with respect to the armchair branch. In addition, the direction of the control terminal can be adjusted to raise more than 50% of the rectification ratio of the graphene thermal rectifier based on the width discrepancy between the left and right terminals. The mechanism of the ballistic thermal rectification is also discussed. In the three-terminal graphene nanojunction, a smaller connection angle with respect to the control terminal leads to more phonon scatterings. The confirmation of this conclusion comes from a comparison of phonon transmission between different couples of terminals, which shows that in most of the frequency spectrum, the phonon transmission between the control terminal and the left terminal is smaller than between the control terminal and the right terminal. Given the same control terminal temperature and temperature bias, the asymmetric connection angles therefore will introduce a higher average temperature of the left and right terminals, and a larger heat flux in the forward process. Moreover, the average temperature difference between in the forward process and in the reverse process is found to be proportional to the temperature bias, and the proportionality coefficient will become bigger if the asymmetry is strengthened.

## EDITOR'S SUGGESTION

2016, 65 (24): 248401.
doi: 10.7498/aps.65.248401

Abstract +

Relativistic magnetron is a kind of compact cross-field high power microwave source. It has the virtues of wide frequency tunability and ability to operate with relative lower external magnetic field. To improve the compactness and reduce the size and weight of the relativistic magnetron further, a novel relativistic magnetron using all-cavity output and semi-transparent cathode is investigated theoretically and numerically. By using the all-cavity output structure, the radial dimension is reduced markedly (from 10.5 cm to 6.6 cm) and the axial dimension is also shortened considerably (from larger than 40 cm to less than 30 cm). Since the radiation fields in the interaction cavity are coupled through the coupling hole to the output fan waveguide, the cutoff frequencies of the fundamental mode and three higher order modes in the fan waveguide with different outer radii are calculated. The calculation results show that the mode separation is wide enough for the single mode operation on the fundamental mode. And by using the semi-transparent cathode, the high output efficiency can be obtained and the output characteristics are insensitive to the depth and width of each cathode slot. To verify the characteristic of the proposed magnetron, numerical simulations are carried out by using the three-dimensional particle-in-cell code. After careful optimization, simulations show that with a beam voltage of 395 kV and beam current of 5.6 kA, 1.15 GW output microwave with an efficiency of about 50% can be obtained at S-band with purer mode. The corresponding applied magnetic field is 4.75 kGs (1 Gs=1-4 T). In a relatively large range, both radiation power and the optimal magnetic field increase with the beam voltage. But the output efficiency keeps almost unchanged.
The effects of the depth, width and length of the coupling hole, width of the fan waveguide and the distance from the beginning position of the fan waveguide to the coupling hole center Lsc on the output characteristics are also analyzed. Simulation results show that when the dimension of the coupling hole is small, the output power is low. But there is no mode competition and the device works on the up mode. With the increase of the coupling hole, the output power increases accordingly. When the coupling hole is large enough, the mode competition between the up mode and /3 mode becomes so serious that the mode cannot win any more. At the same time, the output power decreases markedly. There also exist optimal values of both the fan width and the beginning position of the fan waveguide (Lsc) for maximal output power.

2016, 65 (24): 248801.
doi: 10.7498/aps.65.248801

Abstract +

Organic solar cells based on small molecules and conjugated polymers are attracting much attention due to their merits of low costs, simple fabrication processes, light weights, and mechanical flexibilities. Metals are usually considered as promising candidates for the semi-transparent electrodes. In such devices, a strong microcavity resonance can be supported between the two electrodes, resulting in a narrowed bandwidth of light absorption, which, unfortunately, will lower the performances of organic solar cells since broadband absorption is always highly desired. To overcome this obstacle, people have proposed many designs such as using ultra-thin electrodes or using dielectric-metal hybrid electrodes. Although the light absorption bandwidth can be improved considerably, the absorption efficiency would be lowered due to the weakened microcavity resonance. This is a tough problem that always bothers both researchers and engineers. To solve this problem, we propose a light trapping scheme based on broadband hybrid modes due to the hybridization between microcavity resonance and antireflection resonance. By introducing a capping layer outside the device structure, antireflection resonance can be excited inside the capping layer and can then couple with the intrinsic microcavity resonance, inducing dual microcavity-antireflection resonance hybrid modes. The hybrid modes are of broadband and their resonant wavelengths can be easily designed by tuning the capping layer thickness and cavity length, since the capping layer thickness would affect the antireflection resonance while the cavity length would affect the microcavity resonance. By matching the resonance with the high absorption region of the active layer, the overall absorptivity of the proposed device can be greatly enhanced by~37% compared to the conventional microcavity based device where only one mode, that is, the microcavity resonance can be supported. Moreover, we compare our light trapping scheme with the surface plasmon-polaritons based scheme where surface waves are excited to help improve the light absorption. We find that the overall absorptivity of the proposed device cannot be further improved when we introduce grating structure into the device in order to excite surface plasmon-polaritons. This is mainly because the light absorption based on our hybrid mode scheme is already thorough so that the introduction of grating structure can only improve the light loss dissipated in the metal electrodes due to scatterings and diffractions by the gratings. Therefore, the proposed hybrid mode based scheme can be considered as a simple and effective light trapping scheme for organic solar cells and may find applications in both polymer and small molecular based organic solar cells.

2016, 65 (24): 248901.
doi: 10.7498/aps.65.248901

Abstract +

The infrastructures such as the internet networks, and phone networks, and their traffic capacity are well discussed in the field of network science. However, there is another type of communication infrastructure, such as the wireless sensor networks, which are usually deployed in tough environments to perform specific tasks. This kind of network usually has limited power supply, and thus the main issue is how to make good use of the energy and prolong the network lifetime. In this paper, we investigate the transport process in power-limited communication networks. We use the complex network models to generate the scale-free networks. We assign each node E0 (a constant) unit of energy and an infinite queue with the first-in-first-out rule for buffering packets. In the traffic model, every node generates packets with a constant rate . The packets' destination nodes are randomly chosen from the network. At each time step, every node delivers at most C packets. If a packet's destination node is among the neighbors of the current node, the packet will be delivered to the destination node directly and then be discarded from the destination node. Otherwise, the packet will be forwarded to a neighbor of the current node with a given routing strategy. In the delivery of a packet, the node consumes a fixed amount of energy, and will die out when it uses up its energy. We propose a hybrid routing strategy for the power-limited scale-free networks based on both the node energy and the shortest path. Specifically, in the routing strategy, we consider the residual energy of neighbor nodes and the shortest path lengths between the neighbor nodes and the destination, and utilize a free parameter to adjust their relative importance. Simulation results demonstrate that there are optimal control parameters which correspond to the maximum network lifetime and the maximum number of delivered packets. According to the proposed routing strategy, we further study the relation between the network topological structure and network lifetime. We find that the more homogeneous the network, the larger the maximum network lifetime is. Moreover, we obtain that the maximum network lifetime gradually increases with the average node degree increasing, but almost decreases linearly with the network scale increasing. In this paper we discuss the network lifetime from the perspective of network science, and give more insights into the transport process on complex networks. In addition, our work provides some clues of how to design the efficient routing strategies for the power-limited communication networks.

###### GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

2016, 65 (24): 249101.
doi: 10.7498/aps.65.249101

Abstract +

The presence of cycle slips corrupts the carrier phase measurement which is critical for high precision global navigation satellite system static or kinematic positioning. The process of cycle slips is comprised of detecting the slips, estimating its exact integer and making a repair. In this paper, a novel approach to cycle slip detection and repair based on Bayesian compressive sensing is proposed, in order to reduce the noise effects on the performances of cycle slip detection and repair. Unlike traditional cycle slip detection and repair methods, we exploit the sparse property of the cycle slip signal, aiming to obtain the perception matrix and establish the sparse cycle slip detection model. Then in order to estimate and repair the value of cycle slips, the residuals of carrier phase double difference and the interference noise between multiple satellites, when more than one satellite has cycle slips, are taken into consideration, which is used as prior information to obtain the likelihood expression for cycle slip signal. Finally, we use the prior information about signals based on relevance vector machine principle derived from sparse Bayesian learning to predict cycle slip distribution and then estimate the value of cycle slips. The novel approach is tested with the actual collection of satellite data in the experiment. It is shown that the novel approach proposed in this paper can effectively estimate cycle slips and achieve better performance than orthogonal matching pursuit and l1 norm based algorithm when the redundancy of carrier phase is large enough. In the case of single frequency carrier phase observation, when redundancy is not less than 7, the novel approach can completely detect and repair cycle slips; in the case of dual-frequency carrier phase observation, when cycle slips happen in four of the eight satellites, 97.6% probability of accuracy is accomplished by the new approach.

2016, 65 (24): 249501.
doi: 10.7498/aps.65.249501

Abstract +

Laser three-dimensional (3D) image is a novel non-cooperative target 3D image acquisition technology, and the improvements in detection capability and imaging accuracy of the system are critically dependent on efficient echo-signal processing technique and 3D reconstruction method. The registration process is an essential step in array 3D imaging laser point cloud data processing. Registration of point clouds is an effective method that solves the problem caused by the target self-occlusion in the laser 3D imaging system. The accurate registration result will help provide better support for subsequent applications, such as object reconstruction and target recognition. In this study, a set of thresholds in the iterative closest point (ICP) algorithm is analysed on the basis of the characteristics of the laser array 3D imaging system and is combined with the range error and visual lateral resolution of the system, which are both important parameters in the imaging system. To improve the accuracy and speed of registration, the stop threshold of the iterative algorithm and the corresponding point-distance threshold in the algorithm are established in a novel way based on the range error and visual lateral resolution of the system. This forms the foundation, based on which an adaptive threshold ICP algorithm is proposed. The principal idea of the algorithm is to improve the threshold set that has a considerable effect on the accuracy and speed of registration. At first, the characteristics of the imaging point clouds of the laser array 3D imaging system are analysed in the algorithm. Based on this analysis, the distance between the two point clouds and corresponding points with ideal registrations are estimated theoretically, according to the range error and visual lateral resolution of the system. The simulation results show that the theoretically estimated results and actual results have the same variation tendency, thus providing a theoretical basis for subsequent improvements. Next, the estimated results are added according to the iterative closest point algorithm. This implies that the registration thresholds are capable of changing and adapting under different iterations and imaging systems, thus improving the speed and accuracy of registrations. This phenomenon is not seen in other algorithms. Experiments involving laser array imaging of a point cloud and laser scanning of depth imaging data show that the algorithm is practical and effective for both imaging types of point clouds and can improve the speed and accuracy of registration notably. The effectiveness and feasibility of the proposed algorithm are thus verified. In addition, for its full consideration of the imaging system, the basic idea of the proposed algorithm can be used for designing future applications as required.