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Vol. 65, No. 23 (2016)

Topics
GENERAL
2016, 65 (23): 230201. doi: 10.7498/aps.65.230201
Abstract +
In the social and biological networks,each agent experiences a birth-and-death process.These evolving networks may exhibit some unique characteristics.Recently,the birth-and-death networks have gradually caught attention,and thus far,most of these studies on birth-and-death networks have focused on the calculations of the degree distributions and their properties.In this paper,a kind of random birth-and-death network (RBDN) with reducing network size is discussed,in which at each time step,with probability p(0pq=1-p.Unlike the existing literature,this study is to calculate the average degrees of the proposed networks under different network sizes.First,for the reducing RBDN,the steady state equations for each node's degree are given by using the Markov chain method based on stochastic process rule,and then the recursive equations of average degree for different network sizes are obtained according to these steady state equations.Second,by means of the recursive equations,we explore four basic properties of average degrees as follows:1) the average degrees are limited,2) the average degrees are strictly monotonically increasing,3) the average degrees are convergent to 2mq,and 4) the sum of each difference between the average degree and 2mq is a bounded number.Theoretical proofs for these four properties are also provided in this paper.Finally,on the basis of these properties,a generation function approach is employed to obtain the exact solutions of the average degrees for various network sizes.In addition to the theoretical derivations to the average degrees,computer simulation is also used to verify the correctness of exact solutions of the average degrees and their properties.Furthermore,we use numerical simulation to study the relationship between the average degree and node increasing probability p.Our simulation results show as follows:1) with the increasing of p,the convergent speed of the average degree to 2mq is increasing;2) with the increasing of m,the convergent speed of the average degree to 2mq is decreasing.In conclusion,for the proposed RBDN model,the main contributions of this study include 1) providing the recursive equations of the average degrees under different network sizes,2) investigating the basic properties for the average degrees,and 3) obtaining the exact solutions of the average degrees.
2016, 65 (23): 230301. doi: 10.7498/aps.65.230301
Abstract +
By studying the properties of the mixture of Bell state particles and single photons,in the paper we design a quantum code scheme with high coding capacity,and propose a novel quantum secure direct communication protocol with high transmission efficiency.Alice prepares Bell state particles and single photons,and divides Bell state particles into two sequences SA and SB.SB is sent to Bob for the first security check through using quantum correlation properties of particles.When the check result shows that the quantum channel is safe,by using the designed quantum code scheme, Alice encodes her classical message on the mixed quantum state sequence of Bell sequence SA and single photon sequence SS.Then,some single photons that are used for security check are re-inserted randomly into the encoded sequence,and the order of particles is rearranged to ensure checking Eve's attack.Alice sends the new sequence to Bob.Bob delays and receives it.And then,the quantum channel conducts the second-time security check.The transmission error rate is calculated,and if the error rate is lower than the tolerance threshold,the channel is safe.Bob decodes and reads Alice's message.The first security check is to determine whether quantum channel is safe.The second security check is to test whether there are eavesdroppers during information transmission.Safety analysis is done by applying the quantum information theory for the proposed protocol.The error rate introduced by Eve and the amount of information by Eve are calculated.It is shown that this pro-tocol can effectively resist measurement-resend attack,intercept-resend attack, auxiliary particle attack,denial of service attack and Trojan attack.Among them,auxiliary particle attack is analyzed in detail.The transmission efficiency and coding capacity are also analyzed.The transmission efficiency is 2,the quantum bit rate is 1,and the coding capacity is that a quantum state can encode three bits of classical messages.We also compare the proposed protocol with many existing popular protocols in the sense of efficiency,e.g.,Ping-Pong protocol, Deng F G et al.'s two-step and one-pad-time quantum secure direct communication protocol,Wang J et al.'s quantum secure direct communication protocol based on entanglement swapping and Quan D X et al.'s one-way quantum secure direct communication protocol based on single photon.It is proved that this proposed protocol has higher transmission efficiency.In addition,neither complex U operation nor entanglement swapping is used,and implementation process is simplified.However,this protocol is devoted to theoretical research of quantum secure direct communication.There are still some difficulties in the practical application.For example,the storage technology of quantum states is not mature at present.It is not easy to prepare and measure Bell state particles nor to combine them with single photons,and so on.The implementation of this protocol depends on the development of quantum technology in the future.
2016, 65 (23): 230501. doi: 10.7498/aps.65.230501
Abstract +
This paper is to deal with the blind extraction problem of chaotic signals by using a linear mixing model. In this model, a novel method to describe the distance function in a high dimensional space is proposed which relates the kernel function to objective function. When adopting the artificial bee colony algorithm (ABCA) as an alternative method to solve a multi-modal optimization problem, its analysis under a Markov chain model is also presented. The simulation results show that the objective function of this article has low complexity, and the artificial bee colony algorithm converges to a local minimum quickly. To be specific, the target function is constructed by combining the advantages of the proliferation exponent and the distance kernel function. The proliferation exponent can reflect the chaotic properties of a signal to a large extent, and the distance kernel can help to describe the statistical properties in a higher dimension. Due to the fact that only one frame of time-delay embedded signal is adopted, the computational complexity of our target function is low. The artificial bee colony algorithm is shown to be advantageous over other swarm algorithms. Although adopting ABCA for our evaluation function seems easy, we analyze why this algorithm can work, in contrast to the fact that most literature only runs some simulations to confirm its usefulness. Our analysis is only for a special case when the number of employed bees is set to be 2 and the process of onlooker bees and scouts are temporarily omitted. With smaller complexity than the methods based on proliferation exponents and kurtosises, simulations show that our method can have excellent performance when evaluated by correlation coefficients.
2016, 65 (23): 230502. doi: 10.7498/aps.65.230502
Abstract +
The Willis aneurysm system has some limitations in the description of the complex hemodynamic mechanism of blood with viscoelasticity. The fractional calculus has been used to depict some complex and disordered processes in organisms. Thus, we propose a fractional Willis aneurysm system (FWAS) byusing the Caputo fractional differential and its theory in the present article. Firstly, the existence and uniqueness of solution for FWAS are investigated theoretically. Then, we prove that the FWAS has a chaotic characteristic by analyzing the phase portraits and Poincar section, and it is a rational extension of its integer order form. We investigate the influences of pulse pressure and fractional order on the FWAS by means of bifurcation diagram and period doubling bifurcation. The results show that small changes of pulse pressure and fractional order canlead to a remarkable effect on the motion state of the FWAS. As the chaotic FWAS indicates that the brain blood flow is unstable, and the cerebral aneurysms are more likely to rupture in a very chaotic velocity field. Therefore we use two methods to control the chaotic FWAS. One is to design a suitable controller based on the stability theorem of fractional nonlinear non-autonomous system, and the other is to use a pulse control by taking the inspirit function of drug as impulse function. The numerical simulations show that the proposed two methods can control the blood flow velocity and speed up the periodic fluctuation within a small range, which shows that the cerebral aneurysm is not easy to rupture. The results obtained in this paper display that the fractional differential is a feasible method to characterize the Willis aneurysm system. The theoretical results in our article can provide some theoretical guidance for controlling and utilizing the actual FWAS system.

EDITOR'S SUGGESTION

2016, 65 (23): 231101. doi: 10.7498/aps.65.231101
Abstract +
Recently, much attention has been paid to an interesting subject, i.e., the interactions between surface plasmon polaritons (SPPs) and molecules. The interactions between SPPs and molecules often appear in two opposite cases, namely weak and strong coupling. When the interaction is weak, the absorption maximum simply coincides with the electronic transition energy of the molecule. In the weak coupling regime, the wave functions of the molecule and the SPP modes are considered to be unperturbed, only leading to enhancement of the absorption or fluorescence of the molecules. On the other hand, when the interaction is strong enough, the SPPs and molecules can form a coherent hybrid object, thus the excitation energy is shared by and oscillates between the SPPs and molecular systems (Rabi oscillations), leading to vacuum Rabi splitting of energy levels at the resonance frequency. Due to the fact that both the SPPs and the molecule components can be confined into the nanometer scale, the work on strong coupling with SPPs offers a very good opportunity to realize nanoplasmonic devices, such as thresholdless laser and room temperature B-E condensates.In this work, we investigate a hybrid system formed by strong coupled gold nanodisk arrays and J-aggregate molecules. Smooth gold nanodisk arrays are fabricated by a template-stripping process. In such an experimentally simple replicate process, mass-production of gold nanodisk arrays with the same morphology can be transferred from patterned indium tin oxides (ITO) glass. The structures on ITO glass are milled with a focused ion beam. Periodic gold nanodisk arrays have the capability of converting light into SPPs modes, resulting in a significant field confinement at the patterned metal surface. In particular, the desired SPP mode can be chosen by changing the nanodisk array period to match the absorbance peak of the J-aggregate molecule. On the other hand, J-aggregate molecule is chosen due to its large dipole moments and absorption coefficient, which makes it attractive for designing the strong exciton-plasmon interaction system. The strong coupled system is formed when the J-aggregate molecule is spin-coated on the gold nanodisk arrays. Through reflection measurements, Rabi splitting energy value 200 meV is observed when the period of the nanodisk array is 350 nm. Through tuning the coupling strength by changing the lattice period from 250 nm to 450 nm, the typical signature of strong coupling:anticrossing of energies is found in reflection spectra. This simple replicate process for strong coupling hybrid system fabrication should play an important role in designing novel ultrafast nanoplasmonic devices with coherent functionalities.
NUCLEAR PHYSICS
2016, 65 (23): 232501. doi: 10.7498/aps.65.232501
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The Monte Carlo intra-nuclear cascade program CBIM has been developed for describing spallation reactions involving protons, neutrons and pions on complex nuclei. In order to describe cascade process, several simplifications and assumptions are made in the following:firstly, neither reaction, nor reflection, nor refraction, nor ionization will be taken into account before the incident particle enters into the target nucleus; secondly, target nucleus is regarded as being spherical and the atom number should be greater than 2; thirdly, the knocked nucleon is determined by cross section sampling; finally, in the center-of-mass frame, the scattering angle is sampled based on differential cross section distribution. The basis physics model is based on the above assumptions and Bertini intra-nuclear cascade model; meanwhile, nucleon-nucleon angle differential distributions of INCL in the center-of-mass frame are introduced to overcome the shortage of Bertini model. The interactions between nucleon and nucleon or between nucleon and pion, such as elastic scattering, pion production and charge exchange, are included in the code. In the particles collision, the nucleon density changes with the target nucleus radius; and the interaction cross sections refer to 22 kinds of experimental cross sections in Bertini model. The intra-nuclear cascades induced by 45-3500 MeV neutron, proton or pion below 2500 MeV can be simulated by this code. Finally, comparisons between experimental reaction cross section over the energy range 60-378 MeV, and some simulation results by MCNPX, GEANT4 and PHITS over the energy range 65-3000 MeV show that they are in reasonable agreement with the CBIM results over the broad energy range considered.
ATOMIC AND MOLECULAR PHYSICS
2016, 65 (23): 233601. doi: 10.7498/aps.65.233601
Abstract +
An approach to breaking through the diffraction limitation in infrared microscopies is put forward in this paper. In this method, instead of Gaussian pump beam, an intensive vortex beam is first focused on the sample, leading to the saturation absorption of peripheral molecules in the point spread function (PSF). The vortex beam is followed by a Gaussian probe beam with the same wavelength. Because of the previous saturation absorption, the probe beam can only be absorbed by the molecules near the center, resulting in a shrunk PSF which means super-resolution. Furthermore, the PSF of a system based on this approach is numerically simulated. With a 100 nJ pulse energy vortex beam and a 0.1 nJ pulse energy probe beam, the theoretical resolution FWHM (full width at half maximum) is measured to be about 236 nm which is 14 times better than that of the traditional infrared microscopy.

EDITOR'S SUGGESTION

2016, 65 (23): 233701. doi: 10.7498/aps.65.233701
Abstract +
Single-atom-based single-photon source has several advantages, such as narrow bandwidth, wavelength matching with the absorption line of the same atomic ensemble, and insensitivity to the environment disturbing, and it is very important not only for basic researches in quantum optic field but also for applications in quantum information processing. In this paper, we report the generation of a 10-MHz-repetition-rate triggered single-photon source at 852 nm based on a trapped single cesium atom in a far-off-resonance microscopic optical dipole trap (FORT). To generate an optical dipole trap, a far-red-detuned 1064 nm laser beam is tightly focused by using a high numerical aperture lens, a typical trap depth is 2 mK and trap waist is 2.3 m. To obtain a maximum probability of pulsed excitation, the frequency of the pulsed laser should be resonant with the atomic energy levels and the trapped single atom must be excited with a -pulse. However, the interaction between the FORT laser and the atoms causes AC Stark shifts of the atomic energy levels. Thus, in order to demonstrate the resonant pulsed excitation, it is important to calculate and measure the shift of 6S1/2|Fg=4,mF=+4-6P3/2|Fe=5,mF=+5 cyclical transition in the FORT. For a two-level system, the probability of pulsed excitation can be described by Rabi oscillations with a characteristic Rabi frequency . With an optimized time sequence, we experimentally demonstrate the Rabi oscillation between the ground state and the excited state, and the peak power of -pulse laser is about 1.25 mW. We also measure the temporal envelope of single photons after a -pulse excitation. A gated pulsed excitation and cooling technique are used to reduce the possibility that atoms are heated by -pulse laser. The typical trapping lifetime of single cesium atom is extended from~108 ups to~2536 ms. The corresponding number of excitations is improved from 108 to 360000. The second-order intensity correlations of the emitted single-photon are characterized by implementing Hanbury Brown-Twiss setup. The statistics shows a strong anti-bunching with a value of 0.09 for the second-order correlation at zero delay. In the future, we will perform a Hong-Ou-Mandel two-photon interference experiment to analyze the indistinguishability of the single photons. We will also trap single atoms in a magic-wavelength optical dipole trap where the ground and the excited states have the same shift.
2016, 65 (23): 233301. doi: 10.7498/aps.65.233301
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The extended “ladder” transition controlled by two harmonic pulses is investigated by using a time-dependent quantum wave packet method. The molecular population transfers from the state|0, 0> angle to the states|5, 0> angle and|5, 2> angle induced by the fundamental and second-harmonic pulses, and to the states|5, 3> angle and |6, 2> angle induced by the fundamental and third-harmonic pulses. The calculated results show that the two harmonic pulses can induce a nearly 100% of population to transfer to the target state. The relative phase of two pulses can affect the population distribution. The variation of population has a period of π for the fundamental and second-harmonic pulses, and a period of 2π for the fundamental and third-harmonic pulses.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2016, 65 (23): 234101. doi: 10.7498/aps.65.234101
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In the past few years, the concept of an electromagnetic invisibility cloak has received much attention. Based on the pioneering theoretical work, invisibility cloaks have been greatly developed. Inspired by those theoretical researches, varieties of electromagnetic cloaks, acoustic cloaks, matter wave cloaks, mass diffusion cloaks, heat cloaks, magnetic cloaks, dc magnetic cloaks and electrostatic cloaks have been designed theoretically and demonstrated experimentally. The first experimentally demonstrated invisible cloak is made of metamaterial with simplified material parameters. The simplified cloak inherits some properties of the ideal cloak, but finite scattering exists. It is difficult to develop a perfectly invisible electromagnetic cloak having homogeneous and anisotropic components by using the natural materials. In this work, a bi-layer magnetic cloak made of neodymium iron boron (NdFeB) permanent magnetic material is designed. When the direction of the intrinsic magnetization intensity of the material is opposite to that of the applied magnetic field, the magnetic field lines will be repelled. When the direction of the intrinsic magnetization intensity is the same as the direction of applied magnetic field, the magnetic field lines will be attracted. With those properties, the two magnetic rings are designed, one is made of NdFeB, and the other is made of neodymium iron chromium boron (NdFeCrB). The direction of the intrinsic magnetization intensity is opposite or parallel to the applied magnetic field. The two magnetic rings nest a bi-layer magnetic ring. When a uniform magnetic field is applied, by using the formulas of the magnetic scalar potential in a cylindrical coordinate system and the constitute relations of magnetic rings, the distribution of magnetic field and scalar potential within the bi-layer concentric cylindrical permanent magnetic material are deduced. Based on theory as demonstrated, the bi-layer permanent magnetic material cylinder can cloak a magneto-static field. Under the conditions of the magnetic cloak with the specific relative permeability and the intrinsic magnetization intensity, the relation between the radius ratio and the applied magnetic field is obtained. The calculation results show that when the radius ratio and the applied magnetic field satisfy this relationship, the bi-layer permanent magnetic material cylinder can cloak the magneto-static field. The magnetic field distributions of both the magnetic non-cloak and magnetic cloak are simulated to show the effectiveness of the proposed theory.In summary, the results show that the cloak performance is influenced not only by the size parameters of the permanent magnetic material cylinder but also the relative permeability, the intrinsic magnetization intensity, and the applied magnetic field. The NdFeB permanent magnetic material used in the magnetic cloak is very common and can be easily obtained, which gives more convenience for the design and application of the magnetic cloak.
2016, 65 (23): 234102. doi: 10.7498/aps.65.234102
Abstract +
Ground penetrating radar (GPR) forward is one of the geophysical research directions.Through the forward of geological model,the database of radar model can be enriched and the characteristics of typical geological radar echo images can be understood,which in turn can guide the data interpretation of GPR measured profile,thereby improving the GPR data interpretation level.In this article,the interpolating wavelet scale function by using iterative interpolation method is presented,and the derivative of scale function is used in spatial differentiation of discrete Maxwell equations. The forward modeling formula of GPR based on the interpolation wavelet scale method is derived by using fourth-order Runge-Kutta method (RK4) for calculating the higher time derivative.Compared with the conventional finite difference time domain (FDTD) algorithm based on the central difference method,the interpolation wavelet scale algorithm improves the accuracy of GPR wave equation in both space and time discretization.Firstly,the FDTD algorithm and the interpolation wavelet scale method are applied to the forward modeling of a layered model with analytic solution. Single channel radar data and analytical solution fitting indicate that the interpolation wavelet scale method has higher accuracy than FDTD,with the same mesh generation used.Therefore,auxiliary differential equation perfectly matching layer (ADE-PML) boundary condition is used on an interpolation wavelet scale,and the comparisons between reflection errors obtained using CPML (FDTD),RK4ADE-PML (FDTD),and RK4ADE-PML (interpolating wavelet scales) in a homogeneous medium model show that the absorption effect of RK4ADE-PML (interpolating wavelet scales) is better than the other two absorbing boundaries.Finally,interpolation wavelet scale method,with both UPML,FDTD and RK4ADE-PML loaded,is used for two-dimensional GPR forward modeling,showing good absorption effect for evanescent wave.From all the experimental results,the following conclusions are obtained.1) Using the derivative of the interpolating wavelet scale function instead of central difference schemes for the spatial derivative discretization of Maxwell equations and time derivative calculated using the fourth-order Runge Kutta algorithm,the interpolating wavelet scale algorithm has higher accuracy than regular FDTD algorithm due to the improvement in the spatial and time accuracy of GPR wave equation.2) The best absorption layer parameters of interpolating wavelet scale RK4ADE-PML are selected, when the maximum value of the reflection error is the minimum.The maximum reflection error can reach-93 dB,which increases 20 dB compared with that of UMPL boundary in FDTD algorithm.And the higher simulation accuracy of interpolating wavelet scale algorithm than FDTD algorithm is confirmed after calculating single channel radar data.3) Comparing wave field snapshots of GPR forward modeling,radar pictures from wide-angle method and section method indicates that interpolating wavelet scale RK4ADE-PML reduces reflection error of absorption boundary,improves both spatial and time accuracy,is more effective than UPML boundary in eliminating false reflection of large angle incidence, and has better absorption effect for evanescent wave and low-frequency wave.
2016, 65 (23): 234204. doi: 10.7498/aps.65.234204
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Fiber laser can be used for fiber optic communications,laser cutting,industrial manufacture,defense security and many other fields because of its advantages of narrow output linewidth,good reproducibility,etc.However,due to nonlinear and thermal effects,only a limited output power of a single fiber can be obtained with a sharp attenuation of the output beam quality,which obstructs the applications of fiber lasers.Therefore,the research of expanding the power of a fiber laser source while maintaining its beam quality by combining coherent beam has become a hot subject at present.In this field,the performance of phase control of coherent laser beams is a key factor to influence the efficiency of combination.The phase-controlling methods mainly include stochastic parallel gradient descent control algorithm, dithering,and heterodyne detection.In this paper,based on the active phase lock technology,the traditional heterodyne detection method is improved by the use of a fiber electro-optic phase modulator (EOM) rather than an acousto-optic frequency shifter (AOFS) to avoid the complex designs of the RF driver and circuit,which makes the overall experimental setup simple and stable.Moreover,in order to achieve a stable and wide correction range of phase locking,two servo paths are designed by use of piezoelectric transducer (PZT) and EOM1 to correct the optical phase differences.Firstly, a single-frequency narrow-width fiber laser with its central wavelength of 1531 nm is split by a beam splitter to generate a signal and a reference beam,respectively.The reference beam is phase modulated by another EOM2 with a 15 MHz signal.The phase error signal is obtained by demodulating the detected heterodyne signal at the modulation frequency. After that the error signal is divided into two parts,and sent to two PID servos to control PZT and EOM1,respectively. The PZT,used in the slow feedback loop,eliminates the laser phase error induced by the ambient temperature drift, while the EOM1,in the quick feedback loop,can eliminate the influence of high frequency noise.Two PID servos are carefully designed according to the measurements of the dynamic response of the PZT and EOM1.A stable feedback loop with a bandwidth of 220 kHz (limited by the bandwidth of PID controller) is obtained according to the measurement of its phase error signal spectrum,thus a tight lock is expected.As a consequence,the error of phase locking is less than 0.88°,which indicates that the phase control accuracy is λ/400.The long-term stability of the system is assessed by a 2 hour monitoring of the lock error signal.According to the analysis of Allan deviation,the best phase lock value of 0.006° can be obtained for an integration time of 160 s.The overall phase lock experimental setup is simple and easy to operate;moreover the phase lock can be further improved by optimizing the parameters of the PID controller.
2016, 65 (23): 234501. doi: 10.7498/aps.65.234501
Abstract +
In this paper, in order to explore the movement characteristics of granular system under the horizontal and vertical excitation, the effective mass spectrum and dissipation power of granular material are studied by numerical simulation. We use LIGGGHTS software to simulate a granular system consisting of 13340 dispersed particles in a cubic container. For the two different vibration directions of granular system (horizontal and vertical), we carry out a pressure unloading experiment in a pressure range from 1012.10 kPa to 8.66 kPa. It is found that under the horizontal and vertical excitation, the resonance frequency fg and volume modulus k of granular system satisfy piecewise power-law with the change of pressure P applied to the top surface. It follows the laws, that is, fg∝P1/6 and k∝P1/3 at low pressure and fg∝P1/4 and k∝P1/2 at high pressure. At the same time, according to the effective mass of the imaginary part, we can obtain the dissipative characteristics of the granular system. Under the horizontal and vertical excitation, the reciprocal of quality factor of granular matter, 1/Q, decreases exponentially with the change of pressure P. In the relaxation dynamics of the granular system, both the acceleration and the stress play a role similar to the role of temperature in the thermal system. In order to further study the influence of acceleration on solid-fluid-like transition of granular system, we measure the relationships between the dissipation power and the vibration intensity (1g-30g) under different pressures (8.66-1012.10 kPa), in the horizontal vibration (500 Hz). At the fixed frequency and pressure, there is a characteristic vibration intensity Γ* in the curve of the average power dissipation of granular system with vibration intensity Γ. When ΓΓ*, the granular system exhibits a solid-like behavior, and the variation of the average power dissipation with the change of vibration intensity Γ shows a power-law scaling, p∝Γα (2αΓ > Γ*, the granular system exhibits a liquid-like behavior, and the variation of the average power dissipation of granular system with the vibration intensity Γ changes into a linear fashion. Then, the phase diagram of transition from the solid-like phase to fluid-like phase, i.e., Γ-P phase diagram, in granular system under the horizontal excitation, is obtained in this paper.
2016, 65 (23): 234202. doi: 10.7498/aps.65.234202
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It is significant to realize effective defocus image restoration for acquiring clear image in military and geological examination field. Most of existing algorithms have the problems of large computational cost, ringing and noise sensitivity, hence a novel approach by iterative joint bilateral filtering under Bayesian framework is proposed. Firstly, it utilizes defocus image depth estimation to compute the point spread function in the Bayesian framework. Then a minimum optimization problem is built to represent the blind restoration problem. After inferencing the solution procedure of the minimum optimization problem, we find that the joint bilateral filters can be used to search the optimal solution, which not only simplifies the searching procedure but also reduces the computational cost. Finally, an iterative joint bilateral filtering is designed to realize the image restoration. That means that the original restored image obtained from the bilateral filtering is used to design the guide image for the joint bilateral filters, and the guide image will serve as the input of the optimization problem for acquiring the better optimal result. This procedure is repeated until convergence. The experimental results indicate that this method can yield the ringing, reduce the computational cost, and remove the noise. Generally speaking, the average pixel error of 85% images is under 0.03, which has improved 19% comparing with the same error rang of existing algorithms, and 78% shorter than those of compared algorithms. It can be used in the engineering practice of blind restoration for single defocus image.
2016, 65 (23): 234201. doi: 10.7498/aps.65.234201
Abstract +
Much interest has been aroused in the polarization singularities. A new technique for metrology called singular Stokes polarimetry based on the detection of polarization singularities has been recently developed and used to detect deformations and displacements of samples on a submicron scale, to measure the topology of polarized speckle field and to study the biomedicine as well. The polarization singularities have been extensively studied theoretically, numerically and experimentally. However, most of the studiesare restricted within the frameworks of the fully coherent wave-fields. By using the spectral Stokes parameters introduced by Korotkova and Wolf[Korotkova O, Wolf E 2005 Opt. Lett. 30 198], Yan and L[Yan H, L B 2009 Opt. Lett. 34 1933] have extended the concept of the polarization singularities from fully coherent beams to partially coherent beams. On the other hand, Hajnal[Hajnal J V 1990 Proc. R. Soc. Lond. A 430 413] studied the electric and magnetic polarization singularities in free-space propagation experimentally with microwaves and confirmed that the electric and magnetic polarization singularities are not coincident in general. In this paper, taking the partially coherent edge dislocation beam for example, the explicit magnetic propagation expression for stochastic electromagnetic beam through an astigmatic lens is derived based on the representation of cross-spectral density matrix propagation. Using the spectral Stokes parameters the magnetic spectral singularities are studied in detail. It is shown that there exist magnetic spectral s12, s23 and s31 singularities of stochastic electromagnetic beams through an astigmatic lens. The magnetic spectral Stokes singularities correspond to the zero points of complex Stokes fields sij=0. s12 singularity corresponds to the circular polarization (C-point) of partially coherent beam, and s3 0(s30) means right-(left-) handedness, where the orientations of the major and minor axes of the polarization ellipse become undefined. s23 and s31 singularities must be located on L-lines, where the handedness of the polarization ellipse is undetermined (linear polarization). By suitably varying a control parameter, such as off-axis distance, slope of edge dislocation, spatial correlation length, and astigmatic coefficient or propagation distance, the motion, creation, and annihilation of magnetic spectral Stokes singularities may appear. It has been shown that a pair of C-points with equal but opposite topological charges and with similar handedness may be created or annihilated. The V point and handedness reversal of C point may take place. Compared with the electric spectral Stokes singularities of stochastic electromagnetic beams, the positions are not the same, and the left- and right-handedness spaces do not coincide. The results obtained in this paper would be useful for an in-depth understanding of polarization singularities of stochastic electromagnetic beams.
2016, 65 (23): 234301. doi: 10.7498/aps.65.234301
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Biot model has widely been used in geophysics, petroleum engineering, civil engineering, and ocean engineering since it was presented, and thus the research on the wave propagation in saturated porous medium has made much progress. However, fully saturated porous medium is rarely found in nature. Almost all the rocks or soils contain two kinds of fluids, such as gas and petroleum. Many researches have been done on the wave propagation in unsaturated porous medium. As is well known, a small volume of gas bubbles existing in a liquid can greatly change the velocity and attenuation of acoustic wave in the liquid. Evidences are beginning to be accumulated that the velocity and attenuation of acoustic wave in a saturated marine sediment can be affected by the gas bubbles existing in the saturated liquid. To investigate the sound propagation in a porous medium when the pore water contains a small number of air bubbles, in this paper we integrate the volume vibrations of bubbles in pore water into the continuity equation of pore-fluid filtration in porous medium based on Biot theory, so as to obtain the continuity equation of pore-fluid filtration with bubble volume vibration. On this basis, according to the relationship between the instantaneous radius of bubble and the background pressure of the medium under the linear vibration of bubble, as well as the equations of motion of the fluid medium and porous medium, a new displacement vector wave equation of porous medium under the influence of bubble is derived, which establishes the model for the sound velocity dispersion and attenuation prediction under the unsaturated porous medium. The presence of air bubbles increases the compressibility of pore fluid, which leads to the decrease in the sound velocity of the bubbly saturated porous medium. When the wave frequency equals the resonance frequency of the bubbles, the bubbles in pore water will produce resonance; the medium will present high dispersion and the velocity can greatly exceed the gas-free velocity. However, these have not been measured in field data. The absorption cross section of the air bubble can reach a maximum value, which leads to the maximum attenuation of the porous medium. It should be noted that the attenuation coefficient calculated with this model is related to the damping of the bubble motion due to radiation, thermal and internal friction, and the dissipation of the relative motion between the pore water and porous solid frame. The obtained numerical analysis is consistent with the above conclusion, which indicates that the volume concentration, the bubble size and the excitation frequency of the sound field are important parameters affecting the sound wave propagation in the saturated porous medium containing few bubbles.
2016, 65 (23): 234601. doi: 10.7498/aps.65.234601
Abstract +
Superlubricity may be the ideal and final solution for friction and wear.Superlubricity on a micrometer scale based on an excellent self-retraction phenomenon has been observed and realized under ambient conditions recently.But not all of the graphite interfaces can realize superlubricity even they are incommensurate.Therefore,in-depth studies of graphite interfaces are needed to find out the factors which prevent the superlubricity for being realized.For this reason, microscopic graphite mesas are fabricated on a highly oriented pyrolytic graphite in this paper to obtain superlubricity interfaces.After poor quality graphite layers are mechanically exfoliated from the highly oriented pyrolytic graphite,a silicon dioxide film is grown on a new graphite surface by plasma-enhanced chemical vapor deposition.Then the film is coated with photoresist.Microscopic photoresist square pattern is defined by electron beam lithography and used as a mask for reactive ion etching the SiO2 and highly oriented pyrolytic graphite to define graphite mesas.The graphite interfaces are obtained by shearing the graphite mesas by tungsten tips.Some of them are super lubricative,while others are not. To study the graphite interfaces,atomic force microscope is used to characterize the morphologies of graphite mesas.The edges of graphite contact surfaces are also tested by energy dispersive spectrometer (EDS) and X ray photoelectron spectroscopy (XPS).The morphologies of the four graphite surfaces show that the superlubricity surfaces are atomically flat while other surfaces have many defects such as steps and tears.These results are consistent with those from the stone wall model of graphite crystal structure.The results of EDS and XPS show that there are many oxygen-containing bonds at the edges of the graphite surfaces.It is found that the polycrystalline structure of the highly oriented pyrolytic graphite plays an important role in the forming process of graphite interface and can affect the quality of the graphite interface.The quality of the graphite surface will determine whether the superlubricity can be realized.Besides the inner of graphite interface,the edges of the interfaces can also hinder the superlubricity from being realized.There are a large number of induced chemical bonds and the adsorbed physical bonds adhered to the edge of the graphite contact surfaces.When these bonds are broken,the energy is required.These bonds are the origin of the resistance when the graphite mesa is sheared away from the contact surface and causes friction force when the contact surface is relatively sliding along the other contact surface even the interface is super lubricative. The results show that the polycrystalline structure of the highly oriented pyrolytic graphite can affect the quality of the graphite interface and determine whether the superlubricity can be realized.For the destruction of bonds sticking at the interface edge requires energy,the edge of the contact surface can cause the friction force of superlubricity.It is indicated that increasing the sizes of the graphite grains is beneficial to the realization of large area superlubricity.Using high temperature annealing or other methods to reduce the adsorbed bonds of the graphite edges will also reduce the frictional resistance in the process of superlubricity.
2016, 65 (23): 234701. doi: 10.7498/aps.65.234701
Abstract +
The spreading characteristics of a droplet on a heated substrate have direct influences on its spreading area and heat transfer, so the exploration in this aspect is of important significance for cooling electronic and aerospace equipments. In the present paper, the evolution model of a droplet on a heated solid substrate is established based on the lubrication theory, and spreading processes are simulated respectively when the wall temperature is uniform and decreases exponentially from the center to both sides. A method of assessing the heat flux and heat transfer capacity of a two-dimensional liquid droplet is proposed. Influences of spreading characteristics and heat convective condition at the liquid-gas interface on heat transfer feature of the droplet are examined, and the results are in good agreement with the published ones in the literature. The simulated results show that in the case of uniform wall temperature, the evolution of the droplet is dominated mainly by gravity and illustrates symmetrical spreading characteristics, and the thickness profile presents a single-peak feature of which the value diminishes with time. The heat flux across the droplet surface decreases from both sides to the center, and the surface area of the droplet increases with time slightly, so the performance of heat transfer is strengthened to a certain extent. When the wall temperature decreases exponentially from the center to both sides, the spreading process of the droplet manifests three obvious stages, in which a single-peak feature of thickness profile gradually evolutes into a double-peak feature after surviving for a short period of time, and the peak values of the double-peak first increase firstly and then decrease, resulting from the complex game of gravity and thermocapillary force and their alternative dominance in the evolution. The variations of the dynamic contact angle and travelling speed of the contact line with time can also reflect the above characteristics. The heat flux in the center of the droplet increases, while its values at the double-peak and contact lines decrease with time. In addition, the heat flux at the contact line has a distinct jump feature compared with that at the adjacent position. The droplet surface area expands significantly with time, so the heat transfer capability is improved apparently. Enhancing heat convective condition at the liquid-gas interface, namely greater Biot number, slows the droplet spreading process, which inhibits the expansion of the droplet surface area. However, it enables the droplet to stay in a higher temperature region, resulting in the enhancement of heat dissipation of the droplet. Therefore, the comprehensive interactions of the above aspects strengthen the heat transfer capability, and this phenomenon tends to be increasingly significant over time. Greater Biot number delays the variations of the dynamic contact angle and the travelling speed of the contact line, without changing their general characteristics.
REVIEW
2016, 65 (23): 234203. doi: 10.7498/aps.65.234203
Abstract +
Superoscillation is known as a counter-intuitive property of a band-limited function that oscillates faster than its highest Fourier component in a prescribed interval. Based on superoscillation, micro/nano optical devices, breaking through the diffraction limit in the far-field independent of evanescent waves, have potential applications, including super-resolution, nano-photolithography, high-density optical storage, etc. In this paper, superoscillation is introduced simply, and several optical superoscillatory designs with focusing and imaging abilities are summarized primarily, and some defects and future research emphases in these designs are pointed out.
2016, 65 (23): 237902. doi: 10.7498/aps.65.237902
Abstract +
Recently, all-solid state hybrid solar cells based on organic-inorganic metal halide perovskite (ABX3) materials have received much attention from the academic circle all over the world due to their unique physical and chemical properties. The perovskite materials exhibit advantages of high extinction coefficient, high charge mobility, long carrier lifetime, and long carrier diffusion distance. Furthermore, they are low cost and easily synthesized. The power conversion efficiency (PCE) has exceeded 20.8% since the PCE of 3.8% was first reported in 2009, making the perovskite solar cells the best potential candidate of the new generation solar cells to replace the high-cost and highly polluting silicon solar cells in the future. Meanwhile, because of the well-known special bipolar properties of the perovskite materials, various structures are designed such as the all-solid state mesoscopic heterojunctions, planar-heterojunctions, meso-superstructures, and HTM-free structures. In this review, we first introduce the development of the perovskite solar cells and then focus on the cell structure and its influence on the cell performance. Besides, the synthesis methods of the perovskite films and the performance characteristics and advantages of the perovskite solar cells with different cell structures are also discussed. It is found that although the perovskite crystals prepared by a one-step spin-coating method have bigger grain sizes, their morphologies are rougher and uncontrollable, which may suppress the charge carrier extraction efficiency and lead to a relatively low power conversion efficiency. Meanwhile, vapor-assisted method needs vaccum conditions, which significantly increases the manufacture cost of PSC. Compared with these methods mentioned above, solution-based sequential deposition method can not only enhance the reproducibility of PSC, but also obtain a higher PCE with a lower cost. Afterwards, the photogenerated carrier transport mechanism of the perovskite solar cells is discussed. The possible atomic interaction model and the electron structure between perovskite film and electron transport layer are proposed. There are two possible interface atomic structures at the interface of perovskite CH3NH3PbI3 and TiO2. It is supposed that the interaction between iodine atoms and titanium atoms dominates the atomic structure at the interface of CH3NH3PbI3 and TiO2, while the lead atoms are believed to bond to oxygen atoms. As is well known, charge extraction, transfer and recombination mainly occur at the interface of a cell. Therefore, the interface engineering including the design for energy level matching is important and necessary to enhance the charge transport efficiency, suppress the charge recombination and eventually improve the performance of perovskite solar cells. Moreover, the properties of the main electron transport layer (ZnO, TiO2, PCBM, Al2O3) and hole transport layer (spiro-OMeTAD, P3 HT, NiO, PTAA) and their influences on the PCE of the perovskite solar cells are discussed. The main challenges of the all-solid state hybrid perovskite solar cells such as environment pollution, the extremely small working areas and the instability are introduced. Finally, the development prospects of perovskite solar cells in the future are proposed in order to have a better understanding of the perovskite solar cells.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2016, 65 (23): 235201. doi: 10.7498/aps.65.235201
Abstract +
In order to explore and understand the spectroscopic characteristics of laser induced plasma and spectral intensity distribution under magnetic-spatially combined confinement,in this paper,the laser induced breakdown plasma spectral characteristics of Cu with magnetic-spatially combined confinement,obtained by the optical emission spectroscopy and the optical shadow graph are studied.The temporal evolutions of spectral intensity and the axial and transversal distributions of Cu I 521.8 nm plasma spectrum with magnetic-spatially combined confinement are analyzed.The experimental results show that the laser induced Cu plasma spectra are all enhanced under the conditions of magneticspatially combined confinement and spatial confinement.In addition,the maximum enhancement factors of Cu I 521.8 nm in these two kinds of confinement conditions are 2 and 1.2,respectively.The enhanced effect of plasma ion spectrum in the magnetic-spatial field is stronger than that of spatial confinement.Under the effect of magnetic-spatially combined confinement,spectral enhancement mechanisms are derived from the magnetic field and spatial mixed actions.At the early stage of plasma expansion,the magnetic field action is a dominant factor.The charged particles in plasma are affected by the Lorenz force in the magnetic field which induces the charged particles to do the Lamor cyclotron motion, then the plasma expansion is restrained and the plasma volume decreases.The frequency of collisions between the electron and ion in the plasma increases.Therefore,the spectral intensities of atoms and ions are strengthened.For the case of the larger delay time,the spectral enhancement is caused by the spatial confinement.The axial and transversal spatial intensity distributions of Cu I 521.8 nm are analyzed by the optical shadow graph method.The plasma is compressed by the shock wave because the shock wave generated by the Cu plasma is reflected by the space plate.The transversal expansion of plasma plume is constrained by the spatial confinement,which causes the spatial position of the plasma internal atoms with high densityto move forward,and also induces the maximum axial spatial location of Cu I 521.8 nm spectral intensity to be far from the Cu metal surface.The results indicate that the axial distribution of plasma plume,obtained from the optical shadow graph is corresponding to the axial distribution of plasma spectrum obtained by the optical emission spectroscopy.In summary,the spectrum enhancement of laser induced plasma with the magnetic-spatial combined confinement is influenced by two forces:one is the magnetic force and the other is the compressive force caused by the shock wave.The study of the laser induced breakdown plasma spectral characteristics of Cu with magnetic-spatially combined confinement provides a simple and powerful tool for improving the sensitivity of laser induced breakdown spectroscopy.
2016, 65 (23): 235202. doi: 10.7498/aps.65.235202
Abstract +
During cardiac development, the growth, remodeling and morphogenesis of embryonic hearts are closely linked to hemodynamic forces. An understanding of the interaction mechanism between hemodynamic forces and heart development is important for the early diagnosis and treatment of various congenital defects. The myocardial wall strain (MWS) in embryonic heart is a critical parameter for quantifying the mechanical properties of cardiac tissues. Here, we focus on the radial strain which is defined as the change of the myocardial wall thickness. An effective measurement of MWS is conductive to studies of embryonic heart development. Chick embryo is a popular animal model used for studing the cardiac development due to the similarity of cardiac development between the human heart and the chick heart at early developmental stages and its easy access. Although various imaging methods have been proposed, there still remain significant challenges to imaging of early stage chick embryo heart because it is small in size and beats fast. Optical coherence tomography (OCT) is a non-contact three-dimensional imaging modality with high spatial and temporal resolution which has been widely used for imaging the biological tissue. In this paper, we describe a method to measure in vivo MWS of chicken embryonic hearts with a high speed spectral domain OCT(SDOCT) system worked at 1310 nm. We perform four-dimensional (4D) (x, y, z, t) scanning on the outflow tract (OFT) of chick embryonic hearts in a non-gated way. The transient states of the OFT are extracted from the 4D data by using the beating synchronization algorithm. The OFT center line can be achieved by image processing. Assuming that the blood flow is parallel to the center line in the blood vessel, we calculate the Doppler angle of blood flow from the OFT center line. In a certain OFT cross-section, the OFT myocardial wall (inner and external borders) is segmented from the OCT images with a semi-automatic boundary-detection algorithm. Then, the myocardial wall thickness is calculated from the Doppler angle, area and sum of inner and external radii of the segmented myocardial wall. The radial strain is obtained by calculating the myocardial wall thickness variation. Previous methods calculated the myocardial wall thickness by directly subtracting inner and external radii. The measured result may be deteriorated by insufficient resolution of the system since the myocardial wall of OFT is very thin. The present method can solve this problem by calculating the thickness through using the sum of the radii instead of the subtraction. The experimental results on embryonic chick hearts demonstrate that the proposed method can measure the MWS of OFT along arbitrary orientation and it is a useful tool for studying the biomechanical characteristics of embryonic hearts.
CONDENSED MATTER:STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2016, 65 (23): 236201. doi: 10.7498/aps.65.236201
Abstract +
The nanocrystalline metals are widely investigated due to their unique mechanical properties. Currently, the available studies about deformation mechanisms of metals mainly focus on face-centered cubic metals such as Ni, Cu and Au. However, the body-centered cubic metals are still very limited, despite their industrial importance. Here, we investigate the effects of grain size and temperature on the mechanical behavior of nano-polycrystal -Fe under uniaxial tensile loading by using molecular dynamics (MD) simulation. The models of nanocrystalline -Fe with the grain sizes of 3.95, 6.80, 9.70, 12.50, 15.50, 17.50, 20.70 and 26.00 nm are geometrically created in three dimensions by using Voronoi construction, and these models are relaxed to reach an equilibrium state. Then, each of them has a strain of 0.001 along the Z-direction in each step, keeping zero pressure in the X- and Y-directions until the strain increases up to 0.2. A 1.0 fs time step is used in all of the MD simulations. Based on the data output, the stress-strain curves at different grain sizes are obtained. The results indicate that the peak stresses of nano-polycrystal -Fe decrease with the decrease of grain size, exhibiting a breakdown in the Hall-Petch relation when the grain size is smaller than a critical size. The major deformation mechanism is found to change from dislocation slips and twinning-mediated plasticity in a model with a larger grain size to grain boundary sliding in a model with a smaller grain size. It should be noted that twinning is formed by the emission of 1/6111 partial dislocations along the {112} slip plane. The results show that crack formation during tension is a cause of reducing the flow stress of nano-polycrystal -Fe with a large grain size and that the Young's modulus of nano-polycrystal -Fe decreases with the grain size decreasing. The main reason for the crack nucleation is here that grain boundaries perpendicular to the loading direction bear higher stress and the twin band interacts with grain boundaries at a larger grain size, causing the stress to concentrate at the intersections of grain boundaries. The results also show the detwinning behavior and migration of deformed twins in nano-polycrystal -Fe. The detwinning behavior occurs via the migration of the intersection of grain boundary and twin, and this intersection is incoherent boundary. The migration of deformed twins proceeds by repeating initiation and glide of 1/6111 partial dislocations on adjacent {112} planes. In addition, we find that the nucleation and propagation of dislocation become easier at higher temperature than at lower temperature.
2016, 65 (23): 236401. doi: 10.7498/aps.65.236401
Abstract +
In thermodynamics, the complete equation of state (EOS) for closed system is a functional relation defined by two independent state variables, and all other thermodynamic relations can be deduced by it. For example, Helmholtz free energy F as a function of specific volume v and temperature T of the system is a complete EOS. Unfortunately, the concrete expressions of these complete EOSs are unavailable. Here we establish a practical form of the complete EOS based on the pressure function pT(v) and constant-volume specific heat function Cv(v，T) This complete EOS is mathematically equivalent to the Helmholtz free energy F. Here pT(v) is determined by the measurement and Cv(v，T) can be expressed by two parts. One part is the lattice contribution based on the Debye model and the other part is electronic contribution obtained from the free electron model. Using this complete EOS we calculate the isothermal equation for six metals from the Hugoniot data. Good agreement between the isothermal equation and the experimental data verifies the reliability of the complete EOS. Through this complete EOS we can derive the concrete expression of physical parameters, and these physical parameters including the volume expansion coefficient, the volume speed of sound, the adiabatic modulus, and W-J coefficient are calculated by using the experimental data of Cu. Analyzing their variation trends we can timely adjust parameter in the calculation of the EOS. This kind of complete EOS is useful in the field of high temperature and high pressure physics.

EDITOR'S SUGGESTION

2016, 65 (23): 236801. doi: 10.7498/aps.65.236801
Abstract +
In our daily life, frictions are very common when two bodies in direct contact relatively move. However, when two bodies are separated by a finite distance, due to the quantum fluctuations inside the bodies, they may still experience a friction when they relatively move. Such a phenomenon is often called quantum friction, which has been studied for more than a decade. It has shown in previous studies that the surface modes, such as surface phonon polaritions (SPhPs) or surface plasmon polaritions (SPPs) have significant contribution to enhancing the quantum friction. However, to the best of our knowledge, the contribution of coupling from SPhPs and SPPs to quantum friction is still unknown. Here, we report a detailed study on the quantum frictions between two graphene sheets with the silicon carbide (SiC) substrates. For comparison, the quantum frictions between two other samples, i.e., SiC/SiC and graphene/graphene are also studied. As indicated in previous studies, SPhPs and SPPs, supported by SiC and graphene, respectively, can couple together in special frequency ranges. The coupling of SPhPs and SPPs can be tuned by varying the chemical potential of graphene. The coupling modes shift toward higher frequency as the chemical potential increases. Firstly, we analyze qualitatively the effects of coupled surface modes on quantum friction with the help of dispersion relation. Secondly, we calculate the quantum friction coefficients numerically for the three different samples. We find that due to the coupling of SPhPs and SPPs, the quantum friction between graphene sheets with SiC substrates is larger than that between the SiC or monolayer graphene sheets. We demonstrate that the coupling of SPhPs and SPPs can be modulated by chemical potential of graphene; therefore, the relationship between quantum friction coefficient and chemical potential is also studied. We observe that with the increase of chemical potential, quantum friction coefficient follows a non-monotonic trend, i.e., it first increases to its maximum value then decreases. We believe that our studies are not only helpful in understanding the micro mechanisms of friction, but also meaningful in the fabrications of micro- and nano-electromechanical systems.
2016, 65 (23): 236802. doi: 10.7498/aps.65.236802
Abstract +
Hydrogenated diamond film exhibits a high surface conductivity, which is very suitable for many in-plane microelectronic and microelectrochemical devices. However, the surface conductivity mechanism of hydrogenated diamond film remains unclear up to now. It inevitably retards its further applications. This work is to elucidate the effects of active adsorbate and water molecule on surface conductivity of hydrogenated diamond film. By the first principles method based on density functional theory, several models corresponding to hydrogenated and oxygenated diamond (100) surfaces physisorbed with various active adsorbates are built up. The adsorbed species include H3O+ ion mixed with H2O molecules with different concentrations. The adsorption energy, equilibrium geometry and density of states corresponding to the adsorption system are investigated. At the same time, the electron populations for different atoms of the physisorbed adsorbates are studied. The results show that the equilibrium geometry of H3O+ ion relaxes significantly after adsorption on hydrogenated diamond (100) surface. In addition, its adsorption energy increases dramatically compared with the system of individual H2O molecule adsorbed on hydrogenated diamond (100) surface. It follows that the strong interactions occur between H3O+ ion and hydrogenated diamond surface. With the concentration of the adsorbed H2O molecules increasing, the adsorption energy between the adsorbate and hydrogenated diamond (100) surface decreases gradually. It indicates that the interactions between H3O+ ion and the substrate weaken as the water molecule concentration increases. Concerning the electronic structure of H3O+ ion adsorbed on hydrogenated diamond (100) surface, shallow acceptors appear near Fermi level, which arises from charge transfer from hydrogenated diamond surface to adsorbed H3O+ ion. Therefore, hydrogenated diamond surface exhibits a p-type conductivity. With regard to the mixed adsorptions of H3O+ ion and H2O molecule, no significant effect on its conductivity is detected, though its surface energy band structure changes. At the same time, the electron transfers from hydrogenated diamond (100) surfaces to the adsorbates are also similar for all the systems with the adsorbates including one H3O+ ion and different H2O molecules. Thus, the adsorbed H2O molecule concentration in this work has no effect on the surface conductivity of hydrogenated diamond surface. However, the adsorbates containing H2O molecules and H3O+ ion physisorbed on oxygenated diamond (100) surfaces do not exist stably. The H3O+ ion will decompose into one H2O molecule and one H atom, which form HO bond with one O atom of oxygenated diamond surface. All the oxygenated diamond surfaces with various adsorbates exhibit an electric insulativity.
2016, 65 (23): 236803. doi: 10.7498/aps.65.236803
Abstract +
We report on the electron field emission (FE) from multi-layer AlGaN nanofilm grown by pulsed laser deposition, and the investigation of the multi-layer quantum structure effect on the field emission performance. The results show that the as-grown film has a good crystallinity, and the thickness values of GaN, AlN, and GaN film are 25 nm, 50 nm, and 25 nm, respectively. The FE measurement indicates that compared with single layer, the multilayer filmhas a low turn-on field and large threshold current. The turn-on filed is found to be 0.93 V/m, and the electric current density reaches to 30 mA/cm2 at 5.5 V/m. The improvement of the FE performance is attributed to resonant tunneling in the quantum well structure, and the accumulated electrons lower the effective surface barrier. The outstanding performance of multi-layer filed emission film should provide a feasible technical solution for large current and high power density thin film field emission device.
2016, 65 (23): 236101. doi: 10.7498/aps.65.236101
Abstract +
The long-period stacking ordered (LPSO) phases in magnesium alloys possess excellent mechanical performances, and have received considerable attention. The strengthening LPSO phases, such as 14H and 18R structures, are found experimentally in some Mg-Y-Cu alloys, which can significantly enhance the mechanical performances of the alloys.However, it is unknown which phase is more stable thermodynamically, and easier to form during the solidification. In this paper, thermodynamic stabilities and electronic characteristics of LPSO phases 14H and 18R (18R(m), 18R(t)) in Mg-Y-Cu alloys are investigated by the first-principles pseudopotential method based on the density functional theory. The present calculations are performed by using Vienna ab-initio simulation package (VASP) with projector-augmented plane wave pseudopotential, and the generalized gradient approximation is used to deal with and describe the exchange-correlation interaction. The plane wave cutoff energy is set to be 360 eV, the forces on all the atoms are less than 0.02 eV/. The k-point meshes of Brillouin zone sampling in a primitive cell are based on the Monkhorst-Pack scheme. The calculated enthalpies of formation indicate that the 14H and 18R phases coexist in Mg-Y-Cu alloys. The 18R phase has a larger absolute value of formation enthalpy, which means that it is easier to form than the 14H phase. The reaction energy is also computed for the transformation from the 18R phase to 14H phase, which shows that the 14H phase is more stable than the 18R phase. The results for density of states (DOS) reveal that the bondings of the 14H and 18R phases occur mainly among the valence electrons of Cu 3d, Y 4d, Mg 3s and Mg 2p orbits while those of Cu 4s, Y 4s and Y 4p orbits are very weak in the whole region. The bonding peaks of the 14H, 18R(m), and 18R(t) phases are localized, and the corresponding hybridization orbits, which are all or part of Mg 3s, Mg 2p, Cu 3d and Y 4d orbits, are determined. At the same time, there are sharp peaks on both sides of the Fermi level of the 14H, 18R(m) and 18R(t) phases, which shows that there exist pseudogaps in those phases. The presence of pseudogap indicates that the bonds in the 14H and 18R phases are noticeable covalent. In addition, the charge densities both on (0 0 0 1) plane of the 14H and 18R phases are analyzed in detail. The results show that the Cu-Y bond exhibits the covalent feature in the 14H and 18R phases, the covalent bonding of the 14H phase is stronger than that of the 18R phase, and it is the key reason that the 14H is more stable than the 18R. The calculated results for thermodynamic stabilities and electronic structures of LPSO phases will provide useful data for analyzing and designing Mg-Y-Cu alloys.
2016, 65 (23): 236402. doi: 10.7498/aps.65.236402
Abstract +
Coexistence of multiple wireless access technologies will be an indicator of next-generation wireless network, and the integration of heterogeneous wireless networks will meet the needs of high-performance services for mobile users. According to unique quality of service (QoS) requirements of different service type users in heterogeneous environment, the Markov decision model based handoff selection algorithm is proposed in this paper. A heterogeneous wireless network architecture based on the software defined network (SDN) is established to realize the transparency control of heterogeneous networks. Network state information of heterogeneous wireless networks is mastered by SDN controller. It is responsible for scheduling network resources dynamically according to the performance characteristics of each network. If the network state information in equal interval is sampled, the next moment state of network is only related to the current network state and action, but it is not related to the historical state. The problem of handoff selection for heterogeneous wireless networks is modeled as a Markov process with discrete time and continuous state. To predict the next moment state of network by Markov process to obtain a reward, when the reward is positive, it represents the income; when it is negative, it represents the cost. An immediate reward function is constructed for real-time service and non real-time service users respectively according to their different state attributes of the network. Considering five state attributes of wireless network as follows:delay, delay jitter, bandwidth, error rate and network load, the immediate reward function is constructed with weighted summation. Due to the difference in attribute weight distribution among different service type users, the attribute weights are determined by the analytic hierarchy process. In the long term, the objective function which consists of immediate reward function sequence is used to measure future long-term rewards. Then expected reward function based on the state action pair is constructed to obtain the handoff strategy of the maximum expected return by the iterative method of successive approximation. The proposed Markov decision model based handoff selection algorithm is used in simulation of the Matlab platform. The simulation results show that the proposed method can select the optimal handoff strategy for different service type users and reduce the blocking rate, thereby improving the QoS of users and resource utilization of wireless networks.
CONDENSED MATTER:ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2016, 65 (23): 237102. doi: 10.7498/aps.65.237102
Abstract +
With the development of down-scaling of CMOS technology for low power, mixed-signal, and high frequency applications, the optimal high frequency performance is shown to be shifted from lower moderate inversion toward weak inversion regimes. High-frequency noise model is a prerequisite for designing the radio frequency and millimeter-wave circuits, and is essential for the noise analysis of nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs). In this paper, based on the physical structure of 40 nm MOSFET and by considering the drift-diffusion equation and charge conservation law, accurate physics-based unified high-frequency noise model is developed for induced gatecurrent noise and its cross-correlation with drain-current noise under different bias conditions, which is used to describe the frequency and bias dependence of 40 nm MOSFET from weak inversion to strong inversion regime. Especially, the effective gate overdrive is explicitly included in unified noise model to offer excellent accuracy, continuity and smoothness, and this makes the proposed analytical models convenient to directly reflect the relationship between the noise model and bias condition. Besides, new analytical model is derived for the induced-gate current noise and its cross-correlation term of weakly inverted MOSFET. These simple expressions not only serve as the asymptotic limit for the validation of the proposed physics-based unified model, but also provide a clearer insight into and better understanding of the gate noise behavior and their cross-correlation in the weak-inversion region. Moreover, in terms of the proposed subthreshold noise model, the charge of weak inversion rather than the normal effective channel thickness approximation is involved. In this way, the model accuracy can be improved. Furthermore, a detailed derivation and discussion are presented by analyzing the physics-based noise generation mechanism of transistor including the channel thermal noise and the shot noise based on the small-signal equivalent circuit of the 40 nm MOSFET device. Using these expressions it is possible to extract the values of all the noise model parameters directly from measurement. The proposed model is demonstrated by using noise data from both measurement and the noise simulation. Excellent agreement between simulated and measured noise data shows that the proposed model can be used for predicting the noise behavior of 40 nm MOSFET under different dimensions and operating conditions. The applicability of reported model for drain-current noise is also verified. As far as small-signal (i.e., linear) bias-dependent operation is concerned, it is shown how most of the findings of this work can also be used to predict the data of long channel devices in the strong-inversion regimes.
2016, 65 (23): 237201. doi: 10.7498/aps.65.237201
Abstract +
Using Fresnel concentration to collect solar irradiation, the hot-end temperature of the semiconductor thermoelectric generator is enhanced, and the cold end is cooled through a radiator in air. For studying the performance of thermoelectric module under solar Fresnel concentration, a theoretical model of thermoelectric generator under steady condition is built from the perspective of energy flux. The model neglects the convection and radiation heat transfer between the cold and hot end and between the arms, and simplifies the heat conduction only along the arm. Utilizing this model, the temperature gradient on thermoelectric generator (dT/dx), the output current (I), the output voltage (V), and the output power (P) of thermoelectric generator are derived, and the influences of the resistance ratio a(=R/RH2) and the temperature difference ratio b(=T/TH2) on generator output performance under a certain structure parameters of thermoelectric generator are discussed. The results show that with the increase of resistance ratio (a), the output current (I) decreases, however the output power (P) and the conversion efficiency (he) first increase, then decreases. When the resistance ratio a=1, the output power (P) and the conversion efficiency (he) reach their maximum values. When the resistance ratio (a) is smaller, the output power (P) increases rapidly with the increase of the resistance ratio (a). When the resistance ratio (a) is larger, the output power (P) decreases slowly with the increase of the resistance ratio (a). With the increase of temperature difference ratio (b), the output power (P) and the conversion efficiency (he) increase, no matter what the value of the resistance ratio (a) is. It verifies the sensitivity of the output power (P) to the temperature difference. Therefore, with a certain figure of merit, the appropriate adjustment of temperature difference ratio (b) may improve the output power (P) and the conversion efficiency (he). Besides, the load residence should be larger than the internal residence for keeping the high output performance. A Fresnel concentration thermoelectric module, including 6 thermoelectric generators, is employed to experimentally explore its output performances. In experiment, the energy flux density on the surface of the thermoelectric generator is not uniform as desired. The uneven hot-end temperature will degrade the conversion efficiency, and even excessive local temperature may damage the semiconductor thermoelectric generator. A deviation of the thermoelectric generator from the focal plane of Fresnel lens will help to improve the energy flux uniformity and achieve an optimized output characteristics. The required output voltage and output power can be obtained through series/parallel connection of these thermoelectric generators. With the series connection of the thermoelectric generators, the output current is increased. With the parallel connection of the thermoelectric generators, the output voltage is increased.
2016, 65 (23): 237501. doi: 10.7498/aps.65.237501
Abstract +
Current-induced domain wall motion, which has potential application in the next-generation data storage and logic device, has attracted much interest in recent years. However, how the material defect and its joule heat influence current-driven domain wall motion in magnetic nanostripe is still unclear. This paper is to deal with these issues by using the Landau-Lifshitz-Gilbert spin dynamics. The results show that the material defect can pin domain wall motion and this pinning effect strongly depends on the defect concentration, location and shape. The pinning effect induced by the defect on domain wall motion results in the increase of threshold current, and the domain wall moves steadily and continuously. Specifically, the probability for domain wall motion induced by pinning effect is nonlinearly increasing with the increase of defect concentration. Namely, the increasing of the pinning ability with the increase of the defect concentration becomes fades away. Initially, when the defect is near to domain wall, the pinning ability is obvious. However, the pinning ability is not linearly increasing with the decrease of the initial distance between the defect and the domain wall. The results also show that the single defect is larger, the probability for domain wall motion induced by defect pining is bigger. Moreover, the material defect can suppress the domain wall trending toward breakdown and make domain wall move faster, but the suppressing ability is not obviously increasing with the increase of the defect concentration. On the other hand, the temperature field can remove the pinning phenomenon, which will result in the threshold current decrease. The decrease of the threshold current is of benefit to the working of the data storage and logic device. Also the temperature field can suppress the domain wall trending toward breakdown, but the suppressing ability is less than that of the defect. In addition, the Joule heat around defects can obviously eliminate the pinning effect of the defects, so the pinning effect for a few defects on current-induced domain wall motion can be ignored. Further analysis indicates that these effects are due to the change of the out-of-plane magnetization of the domain wall induced by the material defects and the temperature field, because the velocity of the domain wall motion induced by the applied current greatly depends on the out-of-plane magnetization of the domain wall.
2016, 65 (23): 237502. doi: 10.7498/aps.65.237502
Abstract +
Nowadays, the intense research effort is focused on exploring alternative emerging device to perform binary logical function. A promising device technology is multiferroic nanomagnet logic. The main reason for the interest in this nanomagnetic switch is its nonvolatility and comparatively low power consumption in combination with the ability to perform logic and storage in one single element. The basic element of multiferroic nanomagnet logic is a sub-100 nm size single domain magnet. Usually, the x-y direction defines the in-plane dimension, while the z axis direction depicts the thickness of nanomagnet. The in-plane magnetizations along easy axis are used to encode binary logic states 1 and 0, respectively; while along the hard axis they denote null logic. The logic operation and data transmission in magnetic logic are realized by the dipole-coupled magnetostatic interactions. In multiferroic nanomagnet logic, the interconnect wire is a very important component since it forms data transmission channel of any nanomagnetic logic circuit. There are two kinds of interconnected wires in this technology, namely antiferromagnetic coupling interconnected wire and ferromagnetic coupling interconnected wire. In this paper, the switching dynamics of a multiferroically and nanomagnetically interconnected wire employing ferromagnetic coupling is simulated by solving the Landau-Lifshitz-Gilbert equation with neglecting the thermal fluctuation effects. The wires are implemented with dipole-coupled two-phase (magnetostrictive/piezoelectric) multiferroic elements that are clocked with electrostatic potentials of 100 mV applied to the piezoelectric layer generating 20 MPa stress in the magnetostrictive layers for switching. Specifically, the ferromagnetic coupling effect model for multiferroic nanomagnet interconnected wire is established, and its magnetization dynamics is simulated by using different stress clocking. It is found that moderate strain (19.7-20.1 MPa) can ensure~180 magnetization reversal, and the logic state is successfully transferred in the ferromagnetic coupling interconnected wire. It is also found that the strong ferromagnetic coupling between multiferroic nanomagnets blocks effective magnetization reversal. This may arise from small spacing-induced out-of-plane magnetization, which does not favor the in-plane magnetization. These findings can provide some guidance for multiferroic logic circuit design.
2016, 65 (23): 237901. doi: 10.7498/aps.65.237901
Abstract +
In order to compute the multipactor thresholds of microwave devices with high efficiency and precision,a novel fast particle-in-cell (PIC) method is proposed,which takes advantage of the frequency-domain (FD) electromagnetic field solver of CST Microwave Studio (MWS).At the initial stage of multipactor (when there are not many electrons in the device),the self-consistent field generated by the electrons is much smaller than the applied electromagnetic field. Therefore it can be ignored in calculating the multipactor threshold and this will significantly reduce the computation burden.During simulations of multipactor process,the FD field pre-calculated by CST MWS is converted into timedomain (TD) scaling with the square root of the input power.Then the electron motion is investigated by Boris algorithm.When the electrons hit the boundaries of the simulation region,where triangular facets from CST are used for discretization,the secondary electrons will be emitted.After a series of simulations with variable input powers,the multipactor threshold is determined according to time evolution of the electron number.The multipactor thresholds in a parallel plate and a coaxial transmission line are investigated,and used as relevant verifications.Compared with the CST Particle Studio (PS),the fast method obtains almost the same thresholds,while the computational efficiency is improved by more than one order of magnitude.Since the self-consistent field generated by the electrons is ignored in the fast method and it is considered in CST PS,the results validate that the self-consistent field can be ignored in calculating the multipactor threshold.Finally,taking for example a parallel plate transmission line and a stepped impedance transformer,we study the effect of the number of initial macro-particles on the calculation precision.When the initial particles are so few that they can hardly reflect the randomness of the multipactor process,a higher calculated value will be resulted in.With the increase of the number of initial macro-particles,the calculated multipactor threshold is lower and more accurate.It is convergent when the number reaches about 2000 for the parallel plate transmission line and 4000 for the stepped impedance transformer,respectively.Taking into account other microwave devices with more complex electromagnetic field distribution,in order to ensure precision,it is recommended to select the number of initial macro-particles to be 8000.In addition,although CST MWS is used to obtain the electromagnetic field and boundary information in this paper,of course,other electromagnetic softwares (such as HFSS) can also be adopted as an alternative.
2016, 65 (23): 237101. doi: 10.7498/aps.65.237101
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Due to their spontaneous polarizations, ferroelectric materials have excellent dielectric, piezoelectric, pyroelectric properties, which enable them to be employed in many applications, such as capacitors, filters, sensors, detectors, and transducers, etc. In this paper, we use a first-principles-based effective Hamiltonian method to investigate perovskite SnTiO3, obtain essential coefficients for the effective Hamiltonian via ab initio computations, which are used in subsequent Monte-Carlo simulations to predict the phase transition temperature of SnTiO3, and different structural phases involved in such phase transition.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2016, 65 (23): 239901. doi: 10.7498/aps.65.239901
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