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High precision and fast method for absolute distance measurement based on resampling technique used in FM continuous wave laser ranging
Meng Xiang-Song, Zhang Fu-Min, Qu Xing-Hua
Acta Physica Sinica, 2015, 64 (23): 230601
Coupled oscillation of bubbles in a spherical bubble cluster
Wang Cheng-Hui, Mo Run-Yang, Hu Jing, Chen Shi
Acta Physica Sinica, 2015, 64 (23): 234301
Hydrogen storage properties of Li-decorated C24 clusters
Qi Peng-Tang, Chen Hong-Shan
Acta Physica Sinica, 2015, 64 (23): 238102
Acta Physica Sinica  
  Acta Physica Sinica--2015, 64 (23)   Published: 05 December 2015
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ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

A design of high-gain end-fire antenna based on split-ring resonator structures

Liu Hong-Xi, Gao Jun, Cao Xiang-Yu, Liu Yan-Fang, Zhang Di, Li Si-Jia
Acta Physica Sinica. 2015, 64 (23): 234101 doi: 10.7498/aps.64.234101
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Vivaldi antennas have wide applications in practice due to the ultra-wide band properties; however, their gain and directivity are relatively low. In this paper, a new method is presented to improve the gain and directivity of Vivaldi antennas in a broad band using split-ring resonator (SRR). Based on the peculiar electromagnetic properties of SRR, a novel high-gain SRR-Vivaldi end-fire antenna in C and X bands is designed and fabricated. The size of the antenna is only 0.33λ ×0.33λ ×0.013λ, a significant miniaturization. Equivalent analysis method has been adopted to study the resonance characteristic of an SRR structure. By adding the SRR structures the singular metamaterials in the front of the Vivaldi antenna have an exponential taper slot, and the SRR structures can play a role as a director which has the ability to enhance the antenna's directivity so that the surface currents will focus on the end-fire direction. The SRR structures have been analyzed, designed, and fabricated, which can be embedded into the original Vivaldi antenna smoothly and compactly. As a result, the gain of the SRR-Vivaldi antenna are enhanced effectively, while the size and bandwidth of the original antenna can be kept, with the reflection coeffcient less than -10 dB from 4 to 13.6 GHz after using SRR. The novel Vivaldi antenna based on the SRR has good features of high gain, high directivity, low return loss and low cross-polarization. Compared to the original Vivaldi antenna, the simulation and measured results demonstrate that the gain of the novel SRR-Vivaldi antenna in C band has been increased by an average value of 75.44% and the half-power beam width has been decreased by 20 degrees in xoy and xoz planes. Meanwhile, the gain has been increased by an average value of 24.46% in X band and the half-power beam width has been decreased by 25 degrees in xoz plane. Testing result of the fabricated antenna demonstrates the reliability of the design. A good agreement between simulations and measurements is obtained. The design owns the merits of low cost, simple design and ease in fabrication and conformation, thus provides a new idea for end-fire antenna gain and directivity improvement. The new antenna has great potentials in applications.

Watermarking algorithm based on ptychographical imaging

Liu Xiang-Lei, Pan Ze, Wang Ya-Li, Shi Yi-Shi
Acta Physica Sinica. 2015, 64 (23): 234201 doi: 10.7498/aps.64.234201
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We propose a method for watermarking algorithm using ptychographical imaging. The proposed method uses four probes and one fake probe for illumination, orderly. And five weighted double phase-encoded hidden images are added to a host image that is referred to as the transmitted image. Specifically, the amplitude of the transmitted image contains encoded and attenuated watermark image. And the phase of the transmitted image is the host image due to the application of the fake probe. We develop an analytical presentation for the experimental principle using mathematical derivation of the fake probe and ptychographical imaging. We test analytically the distortion of the transmitted image that is due to noise jamming and the effect of the occlusion of the pixels of the transmitted image. Moreover, we discuss the impact of the probe number and probe overlap rate on system robustness, respectively. Results of computer simulations are presented. First, the amplitude and phase of the watermark image are extracted well. Second, the contradiction between watermark imperceptible and watermark extraction quality can be resolved. Third, the simulations illustrate the system ability to extract the watermark image under distortions and the robustness of the transmitted image against removal trials. Furthermore, the robustness of the system is improved, as the number or overlap rate of the probe increases. In the end, the proposed method is applied to GIF images.

An evaluation criterion of infrared image complexity based on background optimal filter scale

Hou Wang, Mei Feng-Hua, Cheng Guo-Jun, Deng Xi-Wen
Acta Physica Sinica. 2015, 64 (23): 234202 doi: 10.7498/aps.64.234202
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An evaluation of infrared image complexity is proposed based on the background optimal filtering to solve the problem that the traditional methods have given poor results in the background evaluation. Meanwhile, the optimal filtering scale for infrared image filtering can be given by this method, it will provide a guidance for optimal infrared image filtering. First, we generate the Gaussian simulated target and fuse it to the infrared image to obtain the real infrared image with the simulated target. Then, this image is filtered in different scales and the signal-to-noise ratio of the target after filtering is calculated. Finally, the maximal value of signal-to-noise ratio of the target is used as the background optimal filter scale, to evaluate the infrared image complexity. Besides, the infrared filtering scale is deduced by establishing the mathematic model, and then the mathematical expression of optimal filtering scale is obtained. A lot of experiments indicate that: 1) The mathematical expression of optimal filtering scale agrees with the experimental results. 2) The result of our method is better than that of the traditional methods based on information entropy. Because the optimal filtering scale is obtained by using our method, we can use this scale to filter the infrared image to effectively detect a small target. 3) When the scale of simulated target increases, the optimal filtering scale increases accordingly. So, when we calculate the infrared image complexity, the scale of simulated target must be the same. We can compare the infrared image complexity between different images. Moreover, the optimal filtering scale is independent of the intensity of simulated target. 4) The effect of Gaussian and Butterworth high-pass filter is better than that of the ideal high-pass filter in the proposed method. 5) The infrared image complexity can be analyzed by the proposed method effectively. Moreover, changes of different image contents can be analyzed by using the optimal filtering scale.

Surface roughness modeling based on laser speckle imaging

Chen Su-Ting, Hu Hai-Feng, Zhang Chuang
Acta Physica Sinica. 2015, 64 (23): 234203 doi: 10.7498/aps.64.234203
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Surface roughness is an important parameter in measuring the roughness of surface formed by laser irradiation on the workpiece. Speckle images of rough surfaces in different classes and different surface roughness values are obtained by constructing a set of laser speckle image acquisition systems. First, the texture features of speckle images including coarseness, contrast and direction are extracted using Tamura texture theory. Then, the interactions these three features with the surface roughness are analyzed. Based on the analyses of their monotonic relations, the surface roughness functions, including flat grinding, external grinding and mill grinding craftworks, are established respectively between the texture coarseness feature of the speckle image Fcrs and surface roughness Ra. Through the establishment of surface roughness function for the above three classes of workpieces, the value of surface roughness can be computed directly. However, before obtaining the value of surface roughness, the classes of processing technic should be determined because of the inconsistency of function expressions for different classes. And based on the specific connection and related dependencies between Tamura texture features and workpiece class, Bayes network is proposed to describe this uncertainty relation among different classes. Through network structure learning and parameter learning, a model for reasoning is found which can be used to determine the class of workpiece after obtaining texture coarseness feature Fcrs. Thus, not only can the value of surface roughness be measured, also the class of work-piece can be recognized. Experiments are conducted to confirm the feasibility of the proposed model for measurement. The detection results indicate that high precision and accuracy are achieved for both workpiece class recognition and roughness measurement.

Dynamic evolution of an excited atom near the left-handed slab acted by the Casimir-Polder force

Xu Jing-Ping, Chang Sheng-Long, Qin Li, Yang Ya-Ping
Acta Physica Sinica. 2015, 64 (23): 234204 doi: 10.7498/aps.64.234204
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Influence of the Casimir-Polder force on a slowly moving atom near a left-handed slab is discussed. We focus on an initially excited atom and its dynamic evolution during the spontaneous decay process. The left-haned slab is adopted based on two factors: (1) It provides a relatively stronger Casimir-Polder force on the excited atom far away from the interface, and (2) it can lead to an inhibited spontaneous decay rate within such a region. Therefore, we can discuss the dynamic evolution of atoms acted only by the Casimir-Polder force. The dynamic evolution discussed here includes both the evolution of atomic population and the atomic displacement. As the Casimir-Polder force depends on the atomic population, while the decay rate is related to the atomic positions, the atomic dynamic evolution is determined by its initial conditions, i.e. its position and volecity. We choose two initial positions for discussion, i.e. (1) the position with the maximum resonant Casimir-Polder force, and (2) the edge of the resonant Casimir-Polder force of the atom with dipole parallel to the interface. Furthermore, we also consider two kinds of orientations of atomic dipole, i. e. parallel and normal to the interface. It is found that the atom can be repulsed away from a surface by the Casimir-Polder force with a proper initial velocity in certain dipole orientaion during the sponatneous decay process. As the atomic dynamics depends on the orientation of the atom dipole momentum, our result can be used as a reference to distinguish atoms with different dipole momenta. Though the force discussed here exists during the spontaneous decay process, it is much different from the recoil force of the atom when it emits a photon during the spontaneous decay. The statistical average of the recoil force is null, but that of the resonant Casimir-Polder force is not. After reasonable estimation, such a Casimir-Polder force can counteract the thermal fluctuation of temperature of 15 μupK during sponatneous decay. If combined with other constraint methods, it is helpful to control the dynamics of an atom more efficiently.

Generation of a family of Pearcey beams and their optical structure

Ren Zhi-Jun, Li Xiao-Dong, Jin Hong-Zhen
Acta Physica Sinica. 2015, 64 (23): 234205 doi: 10.7498/aps.64.234205
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Based on differential geometry and optical diffraction theory, the determinants of optical structure of Pearcey beams are examined. We theoretically propose and experimentally observe a bundle of Pearcey beams with different optical topology structures. Besides, we have studied their properties and the results show that the structure of Pearcey beams can be flexibly controlled, hence they will be expected to play a new role in some research fields.

Analysis of a novel four-mode micro-structured fiber with low-level crosstalk and high mode differential group delay

Xu Min-Nan, Zhou Gui-Yao, Chen Cheng, Hou Zhi-Yun, Xia Chang-Ming, Zhou Gai, Liu Hong-Zhan, Liu Jian-Tao, Zhang Wei
Acta Physica Sinica. 2015, 64 (23): 234206 doi: 10.7498/aps.64.234206
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In this paper, a novel four-mode micro-structured fiber with low-level crosstalk and high mode differential group delay is proposed to solve the large transmission capacity and low crosstalk problems in the mode division multiplexing system. Electromagnetic field distribution, crosstalk, mode differential group delay and dispersion of the fiber are studied by using the full-vector finite element method. To determine the particular parameters of the micro-structured fiber, the performances of the inter-core crosstalk and mode differential group delay (MDGD) are considered comprehensively under different conditions. Simulation results show that this fiber can support four-mode transmission with 19 cores over the whole C+L wavelength band when the cladding diameter is 125 μm. The inter-core crosstalks of LP01 mode, LP11 mode, LP21 mode and LP02 mode are -131.01, -96.36, -63.32, -49.96 dB respectively and the mode differential group delays are high as all of them are more than 160 ps/m. Therefore, compared with the previous work, this fiber has the lower inter-core crosstalk and larger MDGD. Owing to the large index difference between core and cladding, the n_eff differences between the linearly polarized modes are all larger than 10-3across the whole operating wavelength band, which is beneficial to low inter-mode corsstalk. Furthermore, the fabrication of this fiber is simple due to its preforming only need stacking technique to adjust the hexagonal structure geometry size without complex modified chemical vapor deposition process involved. The designed fiber can be used in short-distance and large-capacity transmission system, and it has potential applications in making the corresponding high power devices.

A demodulation method of high-speed fiber Bragg grating based on dispersion-compensating fiber

Li Zheng-Ying, Sun Wen-Feng, Li Zi-Mo, Wang Hong-Hai
Acta Physica Sinica. 2015, 64 (23): 234207 doi: 10.7498/aps.64.234207
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Fiber Bragg grating sensing is one of the most attractive researches in the field of optical fiber sensing. It has made considerable progress due to its advantages in high multiplexing, high precision, small size, light weight, good corrosion resistance and immunity to electromagnetic interference. However, the traditional fiber Bragg grating demodulation technology can hardly achieve high-speed demodulation of multiplexing gratings, which seriously limits its extensive application. A novel high-speed fiber Bragg grating demodulation method is proposed and demonstrated in this paper. Large dispersion will be generated when light going through the long-distance dispersion compensation fiber. Based on the dispersion effect of dispersion compensation fiber, a light beam of different wavelength will generate different time delay, and the wavelength shift of the fiber Bragg grating sensor is then transformed into time domain, and ultimately the fiber Bragg grating wavelength demodulation can be realized by measuring the delay of grating reflective light pulse. The reflective light pulse train of all the cascade fiber Bragg grating sensors can be obtained only through one pulse of light source. This method can be applied in all-fiber structure without wavelength scanning so that it can promote the demodulation speed greatly and can be applied to the demodulation of quasi-distributed fiber Bragg grating sensor network. Disturbing influence of dispersion compensating fibers can be eliminated by introducing the reference grating, and the demodulation process is immune to light intensity disturbance. A test system is set up to demodulate a quasi-distributed sensor network which is made up of three fiber Bragg grating sensors. Results show that the linearity of the demodulated wavelength is good and the demodulation speed can be up to 1 MHz. The demodulation linearity is about 0.9969, and the error is about 27.8 pm after 10 times average. The novel demodulation method proposed in this paper has been tested through theoretical analysis and experimental demonstration, its feasibility to realize high-speed demodulation of fiber grating has been proved, but significant improvements still can be made in the demodulation system. The next step of research work will focus on how to realize decoupling between the location information and the wavelength, to avoid the influence of temperature disturbance on wavelength demodulation, so as to further improve the wavelength resolution and demodulation accuracy.

Analysis and experimental investigation of the temperature property of sensors based on symmetrical metal-cladding optical waveguide

Luo Xue-Xue, Chen Jia-Bi, Hu Jin-Bing, Liang Bin-Ming, Jiang Qiang
Acta Physica Sinica. 2015, 64 (23): 234208 doi: 10.7498/aps.64.234208
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Symmetrical metal-cladding waveguide (SMCW) is a kind of new waveguide construction, and it consists of a planar glass slab sandwiched in two metal films with different thicknesses. The metal in this structure is usually a noble metal, such as Au, Ag and Cu etc. One of the characteristics of the glass is the sub-millimeter thickness, which is useful for exciting the ultrahigh order mode. Since the SMCW structure was proposed, it has received much attention from the researchers for its excellent characteristics of free-space coupling technique and ultrahigh order mode excitation. This free-space coupling technology has a higher sensitivity compared with the end-face coupling, prism coupling and grating coupling techniques. The ultrahigh order mode is very sensitive to the incident light wavelength, the thickness of guiding layer and the refractive index, but not sensitive to polarization. Based on the thermal-optical effect and thermal expansion effect of metal film and guiding layer materials, we research the temperature property of the SMCW structure. Researching methods include simulation analysis and experimental demonstration. First, we calculate the relation of the thickness and dielectric property of metal films, and the thickness and refractive index of the guiding layer with the temperature. Results show that these four factors are nearly proportional to the temperature difference. Then, we simulate the relationship of the reflectivity of the SMCW structure with those four factors by means of single-factor investigation under spectral and angular interrogation mode of operation, and find that the temperature-dependence of thickness of the guiding layer makes the chief contribution to the waveguide function of SMCW. Meanwhile, we analyze the sensitivity of the sensors based on SMCW structure, and the result shows that the sensitivity of this kind of sensor can be up to 21.89 pm/K (spectral mode) and 1.449×10-3 rad/K (angular mode). Finally, we demonstrate the simulation results by experiment. In our experiment, a series of reflectivity is measured at temperatures varying from 320 to 380 K, and the value is expressed in the form of voltage output of PSD (position sensitive diode). The sensor shows a good linearity and a high average resolution of 0.517×10-3 rad/K; furthermore, we fit the experimental data and get the linear function between angle shifts and temperature difference of Δθ = 0.02965×ΔT. So, once the temperature has any minute variation, it will easily give a change in the resonance incident angle and show the effect of sensor. Owing to the advantages of high sensitivity, low cast and easy fabrication, the temperature sensor based on SMCW will be a promising sensor in many fields.

Generation of no-diffraction hollow vertex beams with adjustable angular momentum by wave plate phase plates

Shi Jian-Zhen, Xu Tian, Zhou Qiao-Qiao, Ji Xian-Ming, Yin Jian-Ping
Acta Physica Sinica. 2015, 64 (23): 234209 doi: 10.7498/aps.64.234209
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In this article, a new scheme is proposed to generate approximately no-diffraction hollow vertex beams by wave plates. By selecting the appropriate thickness values of wave plates based on the properties of the double refraction, four-step-phase plates for o-light or e-light are formed. With linearly polarized light irradiated at the phase plate, the diffractions of o-light and e-light would overlap according to their intensities. By focusing effect of quasi-Galileo telescope system, a no-diffraction hollow vertex beam can be generated. In this scheme, the optical path is simple and convenient to adjust. Under the adaxial condition, the distributions of diffraction intensity and angular momentum of two wave plates at the numbers of cycles, s=1 and s=4, are numerically simulated according to Fresnel diffraction theory and classical electromagnetic field angular momentum theory. Simulation results indicate that the approximately no-diffraction hollow vertex beams can be generated by each of two phase plates within a long distance. The distributions of intensity and the angular momentum are essentially the same as those generated by spiral phase plates at the same number of cycles. The distributions of intensity and the angular momentum are different at different numbers of cycles s. If s increases, the diffraction bright ring radius increases, the intensity decreases and the average orbital angular momentum increases. At s=4, the length of no-diffraction region is significantly greater than at s=1 and the average orbital angular momentum is four times that at s=1. Within the no-diffraction region, the distribution of orbital angular momentum intensity varies with distance but the total angular momentum is constant. A phase compensator is inserted in the diffraction path to adjust the phase difference between o-light and e-light. Whereas the spin angular momentum of the diffraction light can be adjusted by them, and thus the total angular momentum intensity and average photon angular momentum can be adjusted. This scheme can be utilized to guide the cold atoms or molecules to obtain the adjustable torque throughout the interacting process of atoms and photons.

Coupled oscillation of bubbles in a spherical bubble cluster Hot!

Wang Cheng-Hui, Mo Run-Yang, Hu Jing, Chen Shi
Acta Physica Sinica. 2015, 64 (23): 234301 doi: 10.7498/aps.64.234301
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The pressure wave emitted by a pulsating bubble affects the motions of other bubbles, so in an acoustic field bubbles are in a state of coupled oscillation. In this paper, a cluster with cavitation bubbles inside is considered, and a mathematical model is developed to describe the dynamics of the bubbles of the same radius inside a spherical cluster when the effects of coupled oscillation are included. Based on this new model, the nonlinear acoustic response of cavitation bubbles is analyzed numerically. Comparison of our model with those in the literature, shows that bubbles are suppressed heavily. Because of the coupled oscillations of bubbles, the motions of a bubble are affected by more constraints in the system, which cause the decrease of natural frequency of the bubbles. The nonlinear acoustical response of bubbles is improved by the coupled oscillation in a bubble cluster. With the rise in number density of the cluster, the suppression of bubble oscillation is enhanced. For a cluster of 1 mm radius, when the bubble number is below 500, the change of bubble number may cause a sharp decrease of maximum radial displacement of the bubbles. In cavitation region, there are bubble clusters and large-sized bubble, and the moving large bubble can absorb small bubbles from the surface of bubble cluster, so the bubble numbers inside a cluster varies with time, which may change the acoustic response of coupled oscillating bubbles. The increase of the liquid static pressure can suppress the oscillation of bubbles too, and there is a sensitive region (1-2 atm) that affects remarkably the acoustical response of bubbles. Driving ultrasound can affect the motion of bubble greatly. The range of cavitation bubble size is narrowed when the wave frequency increases. The bubbles whose initial radii are close to 5 μm are easy to be activated by ultrasound under given acoustic conditions, i.e. sizes of bubble cluster, surrounding liquid and inner gas. The cluster oscillation of bubbles may suppress the motion of individual bubbles, and weaken the cavition effects caused by individual bubbles. However, the collapse time of the bubbles may be delayed, and the cavitation region may become larger than that for a single bubble. As a result, cavitation effects are amplified in the cluster region.

A gradient representation of holonomic system in the event space

Wu Hui-Bin, Mei Feng-Xiang
Acta Physica Sinica. 2015, 64 (23): 234501 doi: 10.7498/aps.64.234501
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The dynamics research in the event space has important geometric and mechanical meanings, and great progress has been made in this field. A gradient system is a kind of important systems in differential equations and dynamical systems, and is receiving more and more attention. In this paper, a gradient representation and a fractional gradient representation of a holonomic system in the event space are studied. First, the differential equations of motion for the system are established and expressed in the first order form. Second, we have obtained the condition under which the system can be considered as a gradient system and also the condition under which the system can be considered as a fractional gradient system. When a constrained mechanical system is transformed into a gradient system or a fractional gradient system, one can use the properties of the gradient system or the fractional gradient system to study the integration and the stability of a constrained mechanical system. Finally, two examples are given to illustrate the application of the results. The event space is known as more extensive than the configuration space, therefore, the result in the configuration space is a special case of this paper.

Granular axial flow characteristics in a grinding area studied by discrete element method

Han Yan-Long, Jia Fu-Guo, Zeng Yong, Wang Ai-Fang
Acta Physica Sinica. 2015, 64 (23): 234502 doi: 10.7498/aps.64.234502
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Granular grinding is one of the most important unit operations used in a wide variety of industries. Examples can be found in the food industry, for instance, rice processing, etc.. The performance of grinding can be characterized by the particle flow process. Thus in order to study the stable flow process of particles during grinding, we must establish a discrete element model (DEM) of granular axial flow in the grinding area between the grinding roller and the screen drum. DEM is a numerical method used for modelling the mechanical behaviour of granular materials. When DEM is used in grinding, the particle motion is controlled by contact models that are governed by physical laws. Using EDEM software, the process of grinding can be simulated and analyzed. The simulation system chooses continuous feeding; after a period of time, it reaches a steady flow. Research results show that the uneven distribution of particle flow density (PFD) is caused by the axial movement difference of particles in the grinding area. The form, flow rate and distribution of granular axial flow are influenced by static friction coefficient difference between particles and screen drum. Axial mean square deviation of single particles in the grinding area is positively correlated with the square of time, which follows a “super” diffusive behavior defined by some studies. By an overall consideration of the grinding area, we find that the axial average velocities increase, however, the average velocities that are synthesized by three-axis velocities gradually decrease along the axial direction. This is because in a different axial position with different PFI, the PFI plays the key role in energy transfer. More energy will be transferred between high PFI particles that may cause high particle velocity. We also find that the fluctuation velocity square of particles presents the trend of first increasing then decreasing and finally increasing along the axial direction. The difference between PFIs is also elucidated by the total energy dissipation in each collisional energy level for a single particle. Results show that the single particle can endure intenser collision, more energy loss in anterior half segment than those in the second half of the grinding area. As mentioned above, the particle flow was analyzed in terms of particle flow intensity, particle velocity, collision energy, collision number, and so on. Some experimental results confirm the validity of the simulation. The simulation reflects the stable flow characteristics of particles in the grinding area and provides bases and references for further studying the product quality control and grinding equipment parameters optimization.

Simulation study of the colliding bulge and surface micro-jet of metal flyers driven by detonation

Liu Jun, Fu Zheng, Feng Qi-Jing, Wang Pei
Acta Physica Sinica. 2015, 64 (23): 234701 doi: 10.7498/aps.64.234701
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In the cylindrical implosion problem, the phenomenon of colliding bulge and surface micro-jet formation of two-layer metal flyers, which are driven by two slip detonations in opposite direction of the pole, is studied by simulation using Euler's program. Simulation results of the inner surface travel times of the lead flyer coincide well with the experimental results. In the polar position, there is a fracture cavity in the lead flyer, and a blunt bulge is formed on the inner surface. At the equator, large-scale fracture particles are generated as the inner surface of the lead flyer is growing. It is considered that the colliding bulge at the equator which seem to be continuous in the X-ray images is actually discontinuous, and it is composed of large-scale fracture particles and small-scale micro-jet particles. By analysis of the inner surface position on the optical images at different times, the maximum velocity of the lead micro-jet particles is obtained. It is found that the maximum velocity of the micro-jet particles is declined in the pole region, but at the equator its maximum velocity is increased with time. It is considered that the subsequent loading waves on the colliding bulge area may cause higher speed of micro-jet particles than the first loading wave. And then, the groove micro-jet model of the lead, which is loaded by impact, is used to be equivalent to the uniform disfigurement surface micro-jet. It is proved that both the micro-jet maximum velocity in the pole region and the velocity at the equator can be formed by the same uniform disfigurement surface, and the correctness of the experimental optical image is also verified. Finally, the restrained method of the colliding bulge and surface micro-jet in this problem is studied by simulation. The micro-jet maximum velocity of the lead flyer can be declined by changing the two opposite initiation points to the points close to the metal flyers in the pole region, and the main cause of collision bulge at the equator is that the Mach reflection is formed in the collision area because of the low sound velocity of lead.

Horizontal jet characteristics of an underwater solid rocket motor at the beginning of working

Tang Yun-Long, Li Shi-Peng, Liu Zhu, Sui Xin, Wang Ning-Fei
Acta Physica Sinica. 2015, 64 (23): 234702 doi: 10.7498/aps.64.234702
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Compared with the conventional solid rocket motor which works in atmosphere, the underwater solid rocket motor has different performance characteristics, because the buoyancy of water cannot be neglected, while in the atmosphere only gravity is present. In order to get a deep understanding of the mechanism of thrust oscillation characteristics and of the jet structure of underwater solid rocket motor, a 3-D numerical simulation using the volume of fluid (VOF) model is given in this paper to investigate the horizontal jet structure and the oscillating flow field induced by the gravity and buoyance. The principle of momentum is carried out to analyze the oscillating flow field in the initial working period. Result of analysis indicates that the behavior of rising gas bubbles is remarkably influenced by gravity and buoyancy. Compared to the conventional solid rocket motor in the atmosphere environment, the phenomenon shows that the rising gas bubbles are more obvious in the water. It is concluded that thrust-gravity and buoyancy coupling can generate bigger roll moment in the vertical direction than the conventional solid rocket motor. Numerical calculations in conjunction with experiment in the references proves that the model contains gravity, buoyancy and the VOF model can be used to describe the jet structure and thrust oscillation of underwater solid rocket motor; gravity and buoyancy cannot be ignored in the research of underwater solid rocket motor performance characteristics.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

Relaxation of granular elasticity

Sun Qi-Cheng, Liu Chuan-Qi, Gordon G D Zhou
Acta Physica Sinica. 2015, 64 (23): 236101 doi: 10.7498/aps.64.236101
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In granular materials, particles constitute a complex force chains network through contact with each other, and elastic energies are stored due to deformation of particles. This elastic behavior is macroscopic manifestation of inter-particle contacts. Elastic constants or elastic moduli are of fundamental importance for granular material. Due to the hyper-static property of inter-particle forces, the bulk elastic energy stored in the contacts is metastable in the viewpoint of energy landscape, i.e. a high energy state may approaches a more stable state (i.e. relatively lower state) under the action of external perturbations or internal stress, resulting in the elastic modulus reduction. This process is the so-called elasticity relaxation. It may be more obvious in granular materials.
The time-dependent behavior of granular materials, especially the creep, has been studied in experiments and numerical simulations, while the stress relaxation has few reported investigations. Stress relaxation is defined as the process in vohich the initial strain is maintained and the stress decays with the time. From energetic viewpoint, elastic energy is stored in the deformation of particles. The granular system is in a metastable state when confined in a state easy to break the balance. Generally speaking, the shape and grading of particles, volume fraction, surface friction properties, initial structure features, ageing time, loading strain rate will all play important roles in stress relaxation.
In this work, it is believed that the elastic relaxation is the only mechanism to describe the stress relaxation, and the mechanism of it is analyzed from the viewpoint of the potential energy surface. Stress relaxation is calculated by means of the so-called two-granular temperature theory (TGT) we developed previously (Sun Q et al. 2015 Sci. Rep. 5 9652). The stress decays fast at the beginning, then decreases gradually slowly to a stable value. The logarithmic fit is first proposed to describe the stress decay in the compressed system. Calculated results of stress relaxation match well with the measured results in a recently published paper (Miksic A, Alava M J 2013 Phys. Rev. E 88 032207). Both elastic energy and granular temperature may be reduced with increasing time. It is found that the initial value of the granular temperature has a great influence on the stress relaxation, and at present its effect is input by trial and error. It would be a major problem how to determine the initial value of the granular temperature. Moreover, the relaxation coefficient of elastic stress is basically chosen as a function of granular temperature which is described by the Arrhenius equation that need be further investigated.

High-pressure structure prediction of Hf-C system and first-principle simulation of their electronic properties

Peng Jun-Hui, Zeng Qing-Feng, Xie Cong-Wei, Zhu Kai-Jin, Tan Jun-Hua
Acta Physica Sinica. 2015, 64 (23): 236102 doi: 10.7498/aps.64.236102
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Hafnium carbides (Hf-C system), known as ultra-high temperature ceramics, have attracted growing attention because of their unique features. In this paper, we carry out researches on the stable crystal structures in the Hf-C system at high pressures, using a variable-composition ab initio evolutionary algorithm implemented in the USPEX code. In addition to the ambient-pressure structures HfC (Fm3m), there are two new compounds Hf3C2 and Hf6C5 and two high-pressure structures of HfC. When pressures are lower than 100 GPa, no new structures are found other than those at ambient pressure, and Hf3C2 and Hf6C5 become metastable at 20 GPa and 100 GPa, respectively. At 200 GPa, a new compound Hf2C is found, and the stable structure HfC has changed from Fm3m to C2/m. At 300 GPa, another new compound HfC2 is found. At 400 GPa, the stable structure of HfC has changed again to the space group Pnma. And at 500 GPa, the stable structures are Hf2C, HfC2 and HfC (Pnma), no new structures are found except those at 400 GPa. The composition-pressure phase diagram that shows the pressure range of stable structures in Hf-C system is simulated by calculation of their enthalpies. When the pressures are lower than 15.5 GPa and 37.7 GPa, Hf3C2 and Hf6C5 are stable, respectively, and their space groups are both of C2/m. And Hf2C and HfC2, with space group I4/m and Immm, respectively become stable structures when the pressure is higher than 102.5 GPa and 215.5 GPa, respectively. The phase-transition route of HfC is Fm3mC2/mPnma, and the two phase-transition pressures are 185.5 GPa and 322 GPa, respectively, which are different from the conclusion of Zhao. Then we will show and discuss the newly predicted high-pressure structures and their crystallographic data, such as volume, lattice constants and atom positions. The crystal structures of HfC are described in the literature. The structure of Hf2C contains 12 atoms in the conventional cell, and carbon atoms lie at the center of decahedron consisting of 8 hafnium atoms. In the structure of HfC2, carbon atoms form the quasi-graphite sheets and hafnium atoms lie betweent the two sheets. The dynamical and mechanical stabilities of the high-pressure structures have been verified by calculations of their phonon dispersion curves and elastic constants. And the bulk modulus and shear modulus of HfC2 are larger than those of the other three high-pressure structures. Finally we will study their electronic properties, band structures, density of states (DOS), electron localization functions (ELFs), and the Bader charge analyses of these structures are simulated based on the first-principle. The band structure and density of states show that these four high-pressure structures have weak metallic and strong Hf-C covalent bond. The Bader charge analysis further proves the strong Hf-C covalent bond and weak ionic bond. And ELF shows the existence of C—C covalent bond. In summary, the Hf—C bond shows strong covalence, weak metallicity and ionicity, and the C—C bond is covalent.

The multi-scale theoretical models of Young's modulus of doped monocrystalline silicon nano-film

Lv Huan-Ling, Wang Jing
Acta Physica Sinica. 2015, 64 (23): 236103 doi: 10.7498/aps.64.236103
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The study of physical properties of silicon nano-materials is very important for its application in semiconductor technology. Doping is beneficial to improving the physical properties of silicon nano-materials, it can improve the application value as well. Young's modulus of the crystal in the direction of [100] of the doped silicon nano-film is studied by an analytical model, which is based on the semi-continuum approach. In the model, the strain energy is obtained from the Keating strain energy model. The relationship between the Young's modulus and film thickness are also discussed. Results show that the Young's modulus decreases with the increase of the thickness of the silicon film, especially with the small size; the variation tendency of the Young's modulus of doped silicon films is similar to the pure silicon film. And the Young's modulus decreases as the doping concentration decreases for different doping position. Neither the doping concentration nor the doping position, it is the thickness that shows the most important effect on the Young's modulus of the doped silicon nano-film. Findings in this paper may serve as a reference for similar study, and can offer a totally new idea of the doped monocrystalline silicon materials as well.

Preparation and characterization of the superlattice (Sm-doped ceria/yttria-stabilized zirconia)N electrolyte film

Liu Hua-Yan, Fan Yue, Kang Zhen-Feng, Xu Yan-Bin, Bo Qing-Rui, Ding Tie-Zhu
Acta Physica Sinica. 2015, 64 (23): 236801 doi: 10.7498/aps.64.236801
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As is well known, a solid oxide fuel cell(SOFC) is a device that can convert chemical energy directly into electrical energy. The possibility to grow thin films of oxide materials is of great importance in the development of SOFC. Recently, it is expected that oxide heterostructures, with almost ideal interfaces, should lead to the interesting artificial materials with some novel properties. So we have prepared superlattice electrolyte thin films.
On the MgO single-crystal substrates, multilayer epitaxial thin-film heterostructures of 20 mol% samarium-oxide-doped ceria (Ce0.8Sm0.2O2-δ, SDC) and 8 mol% yttria-stabilized zirconia (Y2O3: ZrO2YSZ) have been deposited in turn, using the pulsed laser deposition (PLD) technique. Five different superlattices (SDC/YSZ)N (N=3, 5, 10, 20, 30) films are fabricated, keeping the total thickness constant (300 nm), but with a different number of hetero-interfaces. The choice of coupling SDC and YSZ aims to have both layers in the superlattices made of oxygen-ion conductors with compatible crystallographic features. On one hand, we have to remove any potential contribution of the deposition substrate to the total conductivity, and the superlattices may be grown on <110>-oriented MgO single-crystalline wafers. On the other hand, the YSZ electrolyte film requires that the temperature should be higher, and SDC as the electrolyte leakage; both should be combined with each other, so as to have complementary advantages. In this way, not only the temperature of SOFCs is reduced, but also the leakage avoided. Because YSZ and SDC both belong to cubic fluorite structure and have similar lattice parameters, the lattice constant of YSZ is 5.14 Å, and the lattice constant of SDC is 5.44 Å. Compatible crystal properties, oxygen ion conductivity, and great matching defects can lead to a semi-coherent interface between the ZrO2 and CeO2 layers. The interface effect is obvious. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and AC impedance mearurements are used to characterize the surface morphology, phase structure and electrical properties of the electrolyte film. Results show that the superlattice (SDC/YSZ)N electrolyte films will form obviously interface and better superlattice structure. This kind of thin films do not show crack, with no element diffusion at the interfaces, growing uniformly, densely and smoothly. Electrochemical measurements show a sizable increase in conductivity with increasing number of SDC/YSZ interfaces. So it is an ideal low-temperature fuel cell electrolyte materials.

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

Propagation properties of the graphene surface plasmon in comb-like waveguide

Qiao Wen-Tao, Gong Jian, Zhang Li-Wei, Wang Qin, Wang Guo-Dong, Lian Shu-Peng, Chen Peng-Hui, Meng Wei-Wei
Acta Physica Sinica. 2015, 64 (23): 237301 doi: 10.7498/aps.64.237301
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We investigate theoretically the electromagnetic propagation properties of graphene plasmons in a comb-like dielectric-graphene-dielectric (DGD) waveguide. The effective index of surface plasmon mode supported by the waveguide is analysed numerically, and it is found that the effective refractive index increases with the refractive index of the dielectric and decreases with Fermi energy of the graphene sheet. For a comb-like DGD waveguide with a finite branch length, a subwavelength plasmon filter can be formed by Fabry-Perot resonance caused by the reflection of the guided mode at the branch. The central frequencies of the gaps can be changed by varying the length of the branch, Fermi energy, the refractive index of the dielectric and the layer number of graphene sheets. The analytic and simulated result reveals that a novel nanometric plasmonic filter in such a comb-shaped waveguide can be realized with ultracompact size in a length of a few hundred nanometers in the mid-infrared range. We find that the frequencies of the stopband increase with Fermi energy and the layer number of graphene sheets, while will they decrease nonlinearly with the length of the branch and the refractive index of the dielectric. In addition, the width of the gap can be increased with the number of comb branches. Such electromagnetic properties could be utilized to develop ultracompact photonic filters for high integration.

Experimental research on the new Al0.25Ga0.75N/GaN HEMTs with a step AlGaN layer

Yuan Song, Duan Bao-Xing, Yuan Xiao-Ning, Ma Jian-Chong, Li Chun-Lai, Cao Zhen, Guo Hai-Jun, Yang Yin-Tang
Acta Physica Sinica. 2015, 64 (23): 237302 doi: 10.7498/aps.64.237302
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In this paper, experimental results are reported about the new Al0.25Ga0.75N/GaN high electron mobility transistor (HEMT) with a step AlGaN layer. The rule of 2DEG concentration variation with the thickness of AlGaN epitaxial layer has been applied to the new AlGaN/GaN HEMTs: The step AlGaN layer is formed at the gate edge by inductively coupled plasma etching, the 2DEG concentration in the etched region is much lower than the other parts of the device. A new electric field peak appears at the corner of the step AlGaN layer. The high electric field at the gate edge is decreased effectively due to the emergence of the new electric field peak, and this optimizes the surface electric field of the new AlGaN/GaN HEMTs. The new devices have the same threshold voltage and transconductance as the conventional structure, -1.5 V and 150 mS/mm. That means, the step AlGaN layer does not affect the forward characteristics of the AlGaN/GaN HEMTs. As the more uniform surface electric field distribution usually leads to a higher breakdown voltage (BV), with the same gate to drain length LGD=4 μm, the BV can be improved by 58% for the proposed Al0.25Ga0.75N/GaN HEMTs as compared with the conventional structure. At VGS=1 V, the saturation currents (Isat) is 230 mA/mm for the conventional Al0.25Ga0.75N/GaN HEMT and 220 mA/mm for the partially etched Al0.25Ga0.75N/GaN HEMT (LEtch=4 μm, LGD=4 μm). The decrease of Isat is at most 10 mA/mm. However, as the BV has a significant enhancement of almost 40 V, these drawbacks are small enough to be acceptable. During the pulse I-V test, the current collapse quantity of the conventional structure is almost 40% of the maximum IDS(DC), but this quantity in the new devices is only about 10%, thus the current collapse effect in Al0.25Ga0.75N/GaN HEMTs has a significant remission for a step AlGaN layer. And as the high electric field peak at the gate edge is decreased, the effect of the gate electrode on electron injection caused by this electric field peak is also included. The injected electrons may increase the leakage current during the off-state, and these injected electrons would form the surface trapped charge as to decrease the 2DEG density at the gate. As a result, the output current and the transconductance would decrease due to the decreased electron density during the on-state. That means, with the region partially etched, the electron injection effect of the gate electrode would be remissed and the stability of Schottky gate electrode would be improved. In addition, due to the decrease of the high electric field at the gate edge, the degradation of the device, which is caused by the high electric field converse piezoelectric effect, will be restrained. The stability of the partially etched AlGaN/GaN HEMT will become better.

Research of coupling behavior based on series-parallel flux-controlled memristor

Wang Yan, Yang Jiu, Wang Li-Dan, Duan Shu-Kai
Acta Physica Sinica. 2015, 64 (23): 237303 doi: 10.7498/aps.64.237303
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Memristor is a nanoscale element with low power consumption and high integration, having great potential in applications. A single memristor has rich electrical properties, and its series-parallel circuit exhibits more abundant dynamic behaviors. However, memristors' coupled effects cannot be ignored in high-density integrated environment. Therefore, this paper first deduces the mathematical model of coupled memristor in detail based on the coupled flux controlled memristors. Second, considering the different polarity connection and coupling strength, we discuss the coupled condition of two flux-controlled memristors in series and parallel connections. Then the detailed theoretical analysis is illustrated, and the variation of memristance in terms of voltage, time and flux as well as the relations between voltage and currents are examined via numerical simulations to further explore the influence of coupled effects on the memristive system. At the same time, a graphical user interface of series-parallel coupled circuit based on Matlab is designed. Through this interface, we can adjust the initial value of memristor and coupling coefficient, select different connection modes, obtain corresponding connection diagram and output waveform which intuitively show the dynamic behavior of different parameters directly and provide experimental reference for further study of the circuit design. Furthermore, this paper shows the influence of initial value on the normal working range of memristors in the presence of coupling. From the table 1 it can be easily obtained that when the memristors are connected in the same direction, the range of memristance without coupling is greater than that with coupling. And the situation is opposite when the memristors are connected in different directions. Finally, the hysteresis curve with different coupling coefficients and the change of memristance with time are shown via building the Pspice simulator of coupled memristors, so the coupling effects of memristor is confirmed by circuit simulations. Experimental results reflect that the coupling with the same polarity enhances the change of resistance, and the coupling with different polarity with slow down it. Such dynamical properties can be well utilized in memristive networks and provide a strong theoretical basis for the comprehensive consideration of the design of memristive system.

Micromagnetics simulation on the microwave permeability of magnetic porous nano-flakes

Tu Kuan, Han Man-Gui
Acta Physica Sinica. 2015, 64 (23): 237501 doi: 10.7498/aps.64.237501
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Many modern electronic devices are operated on a frequency above 1 GHz. Frequencies of electromagnetic noises coming from these devices are usually larger than 10 GHz. High-frequency magnetic losses in the natural resonance mechanism can be used to dissipate the energy of electromagnetic noises. Ferromagnetic nanostructural materials (nano flakes or nanowires) in strong shape anisotropy fields are one of the promising anti electromagnetic interference (EMI) materials due to their large high-frequency magnetic losses. Application of EMI requires that the electromagnetic wave absorbing materials should be lightweight and have a wide absorbing bandwidth. However, most electromagnetic wave absorbing materials reported do not have these features. To meet these demands, the microwave magnetic properties of porous α-Fe nano flakes (length × width × thickness: 300 nm × 100 nm × 10 nm) have been simulated based on micromagnetics theory. Compared to the nano flakes without nano pores, simulation results reveal that the demagnetization fields will be altered if a nano flake contains several pores. Effect of nano pores (diameter =15 nm) in different arrangements (rows × columns: 2×10; 2×5; 2×2; 4×5) on the high-frequency magnetic properties is investigated in this paper. It is found that nano flakes can alter the configurations of magnetic domains. More domains in small sizes in an inhomogeneous localized magnetic anisotropic field have been achieved. Consequently, more high-frequency magnetic loss peaks can be found. Overlapping of magnetic loss peaks implies that it potentially enables to widen the bandwidth of electromagnetic absorption within 10–30 GHz. Furthermore, simulations reveal that the quantity, magnitude and resonance frequencies of the loss peaks are strongly dependent on the quantity and the arrangement of nano pores. Besides, the existence of multi magnetic loss peaks has been studied for ellipsoid objects from the perspective of inhomogeneously localized effective magnetic fields. Results reveal that the frequently observed wide magnetic loss peaks in experimental data may be due to the inhomogeneously localized effective magnetic fields of an absorber containing a plentiful of randomly oriented particles. Clearly, compared to the nano flakes without pores, the nano flakes with pores can significantly reduce the volume density. Therefore, our simulation results show that porous nano flakes can be a good lightweight electromagnetic wave absorber candidate with wide absorbing bandwidths.

Quantitative research into the influence of slider-disk contact force on the information intensity of the magnetic recording layer

Liu Yu-Liang, Chen Zhi-Gang, Sun Da-Xing, Zhang Guang-Yu
Acta Physica Sinica. 2015, 64 (23): 237502 doi: 10.7498/aps.64.237502
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In order to achieve the requirement of rapid growth of the magnetic storage density, the slider-disk spacing needs to be reduced to less than 2 nm. However, the slider-disk contact can easily occur within such a narrow spacing, and eventually result in the loss of the stored data in the magnetic recording film, i.e., demagnetization of the magnetic disk. Therefore, research into the magnetomechanical relationship related to the slider-disk contact demagnetization is significantly important to identify the demagnetization mechanism and further improve the anti-demagnetization performance of the magnetic disk. In this study, the nanoscratch experiment and the magnetic force microscope technology are used to investigate the magnetomechanical behavior induced by the slider-disk contact. And according to the phase imaging principle of the magnetic force microscope, the relationship between the information intensity of the magnetic recording layer and the magnetic contrast measured by the magnetic force microscope is found. Thus, a quantitative analysis method is proposed, which is different from the previous qualitative observation of the magnetic domain change. Experimental results show that the critical demagnetization load during the slider-disk contact is 120 μup N. When the slider-disk contact force exceeds the critical demagnetization load, the increase of slider-disk contact force can lead to the decrease of the information intensity of the magnetic recording layer. And the decay rate of the information intensity will be rapidly enhanced after the slider-disk contact force reaches 380 μup N. Moreover, the variation trend of the information intensity with the depth of the residual scratch is the same as that of the information intensity with the slider-disk contact force. Specially, before the slider penetrates the hard carbon layer of the magnetic disk, the slider-disk contact demagnetization still may occur, corresponding to the load cases from 120 μup N to 200 μup N. In addition, for any slider-disk contact force, the area of the surface damage of the hard carbon layer is always greater than that of the demagnetization of the magnetic recording layer. This phenomenon is related to the elasto-plastic force fields in the hard carbon layer and the magnetic recording layer. Moreover, when the slider repeatedly scratches the same location on the surface of the magnetic disk, the information intensity of the magnetic recording layer will decrease with the increase of scratching number. After the scratching number is beyond 20, the elastic shakedown status may occur in the magnetic recording layer, and correspondingly, the information intensity of the magnetic recording layer can be close to a constant value. This result is derived from the work hardening process during the slider-disk repeatedly scratching.

A calculation method for initial magnetization curve under constant magnetization based on time-space transformation

Deng Dong-Ge, Wu Xin-Jun
Acta Physica Sinica. 2015, 64 (23): 237503 doi: 10.7498/aps.64.237503
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It is of great significance to research on methods for obtaining the initial magnetization curve, the important magnetic property of ferromagnetic materials. In the existing methods, a time-varying magnetic field is adopted as the excitation field. To obtain the initial magnetization curve, magnetic field and induced magnetic flux density in the specimen have to be measured step-by-step as the excitation field changes, and this is inefficient. Thus, a calculation method for initial magnetization curve based on time-space transformation is proposed in this paper. In this method, an elongated rod or a circular ring is used as the specimen. A spatially varying magnetic field generated by constant magnetization is utilized as the excitation field. The strength of the excitation field changes with the spatial positions of the specimen. Under the action of the excitation field, the magnetic field strength within the specimen is calculated by means of the responding magnetic field strength on the surface of the specimen according to the continuity of the tangential magnetic field strength. While, based on the Gauss' law for magnetism, the law of approach to saturation and the basic equation of magnetization curve in Rayleigh region, the induced magnetic flux density within the specimen can be calculated from the responding magnetic flux density on the surface of the specimen. After obtaining the magnetic field strength and magnetic flux density in the specimen, the initial magnetization curve can be obtained. To verify theoretically the correctness of the method, simulations are carried out with an elongated rod and a circular ring. In experiments, a spatially varying magnetic field generated by DC coils is applied on the specimen as the excitation field. The initial magnetization curve calculated from the magnetic field strength and magnetic flux density on the surface of the specimen is similar to the known initial magnetization curve. Experimental results also show that when adopting an elongated rod rather than a circular ring as the specimen, this calculation method for initial magnetization curve is simpler and the error in the results is smaller, which are different from those obtained by existing measurement methods for initial magnetization curve. In addition, in order to study the influence of the limiting factors in practical applications of the calculated results, further research is conducted based on the simulation data. Results show that when choosing a proper elongated rod as the specimen, the initial magnetization curve can be calculated from the magnetic field strength and magnetic flux density on the surface of the specimen under the constant magnetization, also the induced magnetic field flux in the specimen does not have to be measured by the encircling detecting coil which makes this method easy to operate. Namely, this method is feasible in practice. This paper may be a theoretical guidance for exploring new measurement methods for initial magnetization curve.

Preparation and charge storage property of FEP thin film electret with grid electric field distribution in millimeter scale

Zhang Lin-Cheng, Chen Gang-Jin, Xiao Hui-Ming, Cai Ben-Xiao, Huang Hua, Wu Ling
Acta Physica Sinica. 2015, 64 (23): 237701 doi: 10.7498/aps.64.237701
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Electret has caught wide attention because it can produce a lasting and stable electrostatic field in the application of MEMS devices such as miniwatt electret generator, electret motor, electret sensor, electret transducer, and so on. Of all the above applications, a remarkable feature is that the electrostatic field distribution on electret surface is patterned in millimeter size or even smaller. However, the charge storage performance of electret in miniature size will dramatically get worse in contrast with the macro-electret. Therefore, it is very important to develop an applicable preparing method to maintain the stability of electrostatic field distribution in micro-patterned electret. In this paper, it is reported that a fluorinated ethylene propylene copolymer (FEP) with evaporated aluminum grid electrode a 25 μm thickness topped with at a step of 2 or 3 millimeter are successfully prepared to form the electret with well grid distribution of electric field(abbreviated as grid electret) by means of corona charging and thermal charging technology. Effect of grid width and charging temperature on the charge storage performance is studied. After stored for 150 days, the grid distribution of electric field on the FEP surface becomes clear and organized. The potentials of the area covered by aluminum electrode are close to zero, while that of uncovered area still remain high. The potential differences between the covered and uncovered by aluminum electrode area are identical in different charging methods, it is 110 V (electric field 44 kV/cm) for the sample with an electrode width of 2 mm, and 130 V (electric field 52 kV/cm) for the sample with an electrode width of 3 mm. Results also show that the initial surface potentials of the grid electrets prepared by corona charging is higher than that by thermal charging, but the former decays more rapidly. For the same charging method, the narrower the aluminum electrode area can lead to the lower initial surface potential, and the higher charging temperature causes the larger initial surface potential. According to the principle of corona charging and thermal charging technology it is concluded that the difference of charge storage capability between FEP and aluminum can account for the grid distribution of electric field on the FEP surface.

Study on charge storage characteristics of PP film electret charged by interface polarization method

Chen Gang-Jin, Rao Cheng-Ping, Xiao Hui-Ming, Huang Hua, Zhao Yan-Hai
Acta Physica Sinica. 2015, 64 (23): 237702 doi: 10.7498/aps.64.237702
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Electret attracts increasing attention nowadays because of its lasting and stable electrostatic field. To achieve the widespread use of electret material, higher density and better stability of the electret charge storage as well as well-distributed electrostatic field must be ensured at the same time. Based on the mechanism of interface polarization on double-layer media, a novel charging technology for electret is reported in this paper; and PP film is successfully charged through an auxiliary layer to form electret by this proposed method. Effect of charging temperature and charging voltage on the charge storage performance of the as-prepared PP film electret is investigated by means of surface potential measurement. Also its charge storage performance at high temperatures is explored by thermally stimulated discharge technique. Furthermore, its electrostatic field distribution in the directions of X and Y is measured. Results show that the interface polarization charging is more excellent than the corona charging. At a certain temperature, the surface potential of PP film electret increases with increasing charging voltage and both are in a good linear relationship. This is in good agreement with the theoretical analysis in terms of the equation of electret charge accumulation during the charging process. It is shown that in the case of constant charging voltage within the range of 0.5–3.0 kV, the effect of charging temperature is not obvious when the temperature is below 75 ℃; however, when the temperature is higher than 75 ℃, the surface potential of PP film electret increases with increasing temperature. In addition, its surface potential may change a little with time so it has an excellent charge storage stability. The distribution of its surface potential shows that it exhibits an homogeneous electrostatic field due to interface polarization charging.

Ultrafast dynamic optical properties of graphene

Jin Qin, Dong Hai-Ming, Han Kui, Wang Xue-Feng
Acta Physica Sinica. 2015, 64 (23): 237801 doi: 10.7498/aps.64.237801
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Graphene exhibits excellent ultrafast optical properties due to its unique electronic structure. In this paper we investigate theoretically the ultrafast dynamic optical properties of graphene based on the Bloch-equations, and introduce the theoretical model of graphene. First, we give the energy which has a linear relationship with the wave vector k. The behavior of electrons in the vicinity of the two Dirac points can be described by the massless Dirac-equation, thus we have the Dirac equation of graphene. Second, we discuss the interaction between graphene and light field. The Bloch-equations of graphene are obtained through the Heisenberg equation and then we discuss the photon carriers,electric polarization and optical current change over time by analyzing the Bloch-equations. It is found that the nonequilibrium carriers in graphene induced by a terahertz field can be built in 20-200 fs due to the Pauli blocking and the conservation of energy principle. The photon carrier density will increase with the frequency of enhanced light field. Thus an optical current can be created rapidly within 1 ps. A graphene system responds linearly to the external optical field for √2evFE0t<<ħω, while the graphene systems respond nonlinearly to the external optical field, where E0 and ω are respectively the intensity and the frequency of the light, t is the time and vF the Dirac velocity in graphene. The electric polarization and optical current increase with increasing photon energies. These theoretical results are in agreement with recent experimental findings and indicate that graphene exhibits important features and has practical applications in the ultrafast optic filed, especially in terahertz field.

Polarization-insensitive and broad-angle gradient metasurface with high-efficiency anomalous reflection

Liu Tong-Jun, Xi Xiang, Ling Yong-Hong, Sun Ya-Li, Li Zhi-Wei, Huang Li-Rong
Acta Physica Sinica. 2015, 64 (23): 237802 doi: 10.7498/aps.64.237802
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Polarization-insensitive metasurfaces are of great value in practical applications. In this paper, we present a polarization-insensitive reflective phase-gradient metasurface operating in optical communication band which has almost the same electromagnetic (EM) responses for both x-and y-polarized incident waves with high-efficiency anomalous reflection.
The reflective metasurface employs a typical metal (Au)-insulator (SiO2)-metal (Au) structure, in which the top metal layer consists of periodic arrays of isotropic cross-shaped gold antennas, i.e. unit cells. The supercell of the metasurface is composed of five unit cells with their dimensions different from each other. The normally incident waves are reflected by the metal-grounded plane, but the reflection phases of both x-and y-polarized waves are controlled by changing the dimensions of their unit cells. Based on the finite-difference time-domain simulations, we investigate the polarization-dependent EM responses of this metasurface under the illumination of linearly polarized incident plane waves. Selecting carefully five cross-shaped gold antennas in different dimensions, we obtain polarization-insensitive metasurface with high-performance anomalous reflection in optical communication band.
First, in order to investigate the polarization sensitivity of the proposed metasurface, we study the EM responses for x-and y-polarized incident waves, since arbitrary linearly-polarized EM waves can be separated into two orthogonally-polarized components. We find that the two orthogonally-polarized incident EM waves have almost the same phase and amplitude response with the phase nearly linearly changing from 0 to 2πup within a supercell, hence a constant gradient of phase discontinuity is introduced and anomalous reflection will occur. We further analyze the reflected electric-field patterns and the far-field intensity distributions, from which we find that the reflected beams exhibit low-distortion wavefronts and the scattered light is predominantly reflected into the anomalous mode. As a consequence, high-efficiency anomalous reflection is realized, with a 70% reflectivity at the operating wavelength of 1480 nm. Furthermore, we look into the incident-angle dependence of the proposed metasurface, and find that the designed metasurface can exhibit polarization insensitivity within a broad incident angle ranging from -30° to 0°.
In conclusion, we propose a broad-angle polarization-insensitive reflective gradient metasurface with high-efficiency anomalous reflection, which has potential applications in optical communications, signal processing, displaying, imaging and other fields.

Effect of synthesis temperature and N2/O2 flow on morphology and field emission property of SnO2 nanowires

Ma Li-An, Zheng Yong-An, Wei Zhao-Hui, Hu Li-Qin, Guo Tai-Liang
Acta Physica Sinica. 2015, 64 (23): 237901 doi: 10.7498/aps.64.237901
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A large amount of tin oxide (SnO2) nanowire arrays were synthesized on the flexible conductive carbon fiber substrate by thermal evaporation of tin powders in a tube furnace. The temperature, as well as the flow rate of the carrier N2 gas and the reaction O2 gas, plays an important role in defining the morphology of the SnO2 nanowires. Morphology and structure of the as-grown SnO2 samples are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Results show that all the samples possess a typical rutile structure, and no other impurity phases are observed. The morphology changes from rod to wire with the increase of reaction temperature. Ratio of length to diameter of the nanowires increases first and then decreases with the flow ratio of N2/O2 gas. The optimum synthesis conditions of SnO2 nanowire are: reaction temperature 780 ℃, N2 and O2 flow rates being 300 sccm and 3 sccm respectively. In our growth process, the nanowire grows mainly due to the vapor-liquid-solid (VLS) growth process, but both the VLS process and surface diffusion combined with a preferential growth mechanism play the important role in morphology evolution of the SnO2.
Field emission measurements for Samples 1-6 are carried out in a vacuum chamber and a diode plate configuration is used. Relationship between the growth orientation, aspect ratio, density and uniformity of the arrays and field emission performances will be investigated first. Results reveal that the field emission performance of SnO2 nanostructures depends on their morphologies and array density. The turn-on electric field (at the current density of 10 μupA/cm2) decreases and the emission site density increases with tin oxide array density, and the turn-on electric field of Sample 5 (synthesized at 780 ℃, nitrogen and oxygen flow rates being 300 sccm and 3 sccm respectively) is about 1.03 V/μm at a working distance of 500 μm. By comparison, for the turn-on electric fields of the not well-aligned SnO2 nanowire arrays we have 1.58, 2.13, 2.42, 1.82, and 1.97 V/μm at 500 μm. These behaviors indicate that such an ultralow turn-on field emission and marked enhancement in β (~ 4670) can be attributed to the better orientation, the good electric contact with the conducting fiber substrate where they grow, and the weaker field-screening effect. Our results demonstrate that well-aligned nanowire arrays, with excellent field-emission performance, grown on fiber substrate can provide the possibility of application in flexible vacuum electron sources.

INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Theoretical investigation of the effects of impurity on the properties of Nb2GeC

Chen Jun-Jun, Duan Ji-Zheng, Zhang Xue-Zhi, Jiang Xin, Duan Wen-Shan
Acta Physica Sinica. 2015, 64 (23): 238101 doi: 10.7498/aps.64.238101
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MAX phases are potential future materials used in the nuclear industry. Recently, a new MAX phase Nb2GeC is predicted as the most stable compound, and confirmed by thin film synthesis.In the operation of fusion reactor, the accumulation and aggregation of helium and hydrogen produced from transmutation reactions would induce bubble formation and void swelling and further result in embrittlement and irradiation-induced hardening of the materials. High solubility and permeability of tritium and solubility of interstitial impurities like O, C, and N can also lead to embrittlement. In order to further investigate the characters of Nb2Ge in irradiation environment, ab initio calculations are performed on the energetics of O, H and He impurities in Nb2Ge. The study of all the impurities is carried out in two ways, substitutionally and interstitially. Formation energies due to substitution and interstitial are calculated, lattice parameters and unit cell volume of Nb2GeC with substitutional or interstitial impurities are obtained, and its electronic property is analysed by Mulliken population and electron charge density.The formation energies of H substitution are lower than those of O substitution and He substitution, hence H atoms are trapped more easily by some irradiation-induced vacancies. The formation energies of O subtitution indicate the sequence Ef(Osub-Nb)>Ef(Osub-Ge) ≈ Ef(Osub-C), which is related to the strength of bonds. Analysis on electron charge density and Mulliken population shows that C-O bond is stronger than Nb-O and Ge-O bond, and the bond lengths of C-O, Nb-O and Ge-O are 3.256, 2.118 and 1.985 Å respectively. Due to the interaction of O atom with Nb, Ge and C atoms in Nb2Ge, the O atom would deviate from the vacancy, and goes to the deformed sites in the crystal structure. As for H substitution, the formation energies of substitution show the sequence Ef(Hsub-Nb)>Ef(Hsub-Ge) > Ef(Hsub-C). C-H and Nb-H are ionic bond and covalent bond respectively, and their bond lengths are 3.131 and 2.706 Å respectively. The formation energies of He substitution present the sequence: Ef(Hesub-C) > Ef(Hesub-Nb) > Ef(Hesub-Ge), and suggest that the He atom is the easiest to be trapped by C vacancy. All O, H and He interstitials make lattice parameter a increase, c decrease and unit cell V shrink. Besides, the results of substitution and interstitial formation energies show that O, H and He impurities prefer to stay on octahedral sites. These results could provide initial physical picture for further understanding the accumulation and bubble formation of impurities in Nb2GeC.

Hydrogen storage properties of Li-decorated C24 clusters Hot!

Qi Peng-Tang, Chen Hong-Shan
Acta Physica Sinica. 2015, 64 (23): 238102 doi: 10.7498/aps.64.238102
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Hydrogen is considered as a potentially ideal substitution for fossil fuels in the future sustainable energy system because it is an abundant, clean and renewable energy carrier. A safe, efficient and economic storage method is the crucial prerequistite and the biggest challenge for the wide scale use of hydrogen. The nanomaterial is one of the most promising hydrogen storage materials because of its high surface to volume ratio, unique electronic structure and novel chemical and physical properties. It has been demonstrated that pristine nanostructures are not suitable for hydrogen storage, since they interact weakly with hydrogen molecule and their hydrogen storage density is very low. However, the hydrogen storage capacity of the nanostructures can be significantly enhanced through substitutional doping or decoration by metal atoms. Using density functional theory, we investigate the properties of hydrogen adsorption on Li-decorated C24clusters. Results show that the preferred binding site for Li atom is the pentagonal rings. The interaction of Li atoms with the clusters is stronger than that among Li atoms, thus hindering effectively aggregation of Li atoms on the surface of the cluster. The decorated Li atoms are positively charged due to electron transfer from Li to C atoms. When H2 molecules approach Li atoms, they are moderately polarized under the electric field, and adsorbed around the Li atoms in molecular form. Each Li atom in the Li-decorated C24 complexes can adsorb two to three H2 molecules. The H-H bond lengths of the adsorbed H2 molecules are slightly stretched. The average adsorption energies are in the range of 0.08 to 0.13 eV/H2, which are intermediate between physisorption and chemisorption. C24Li6 can hold up to 12 H2 molecules, corresponding to a hydrogen uptake density of 6.8 wt%. This value exceeds the 2020 hydrogen storage target of 5.5 wt% proposed by the U. S. Department of Energy.

Study on the absorbing properties of critically coupled resonator with graphene

Xu Jie, Zhou Li, Huang Zhi-Xiang, Wu Xian-Liang
Acta Physica Sinica. 2015, 64 (23): 238103 doi: 10.7498/aps.64.238103
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A critically coupled resonator (CCR) is a thin-film structure that can absorb nearly all of the incident electromagnetic radiation, leading to null scattering. In order to effectively achieve and control the critical coupling (CC) phenomena, we replace the polymer absorbing layer by a graphene-based multi-film structure. FDFD (finite difference frequency domain) method is used to solve the Maxwell equation, and the graphene's surface conductivity is calculated by using the Kubo formula. Our results demonstrate that the CC phenomenon is realized at the near-infrared frequency and the frequency of absorption peak can be engineered by the Fermi energy of the graphene sheets. With increasing Fermi energy the absorption peak moves to the longer wavelength side. The effective permittivity of a multi-film structure has a strong dependence on the thickness of the dielectric and the layer number of the grapheme sheets in the multi-film structure. It is found that the central frequency of the absorption peak shifts towards longer wavelength side with increasing layer number of the graphene sheets M and the thickness of dielectric d1. Moreover, we also demonstrate that the absorption efficiency is affected by the electron-phonon relaxation time and the incident angle. It is clear that the central frequency of the absorption peak has a slight shift and the absorption is changed with the relaxing time and incident angle. The results offer the theoretical basis to the design of graphene-based critical coupling devices and optical detectors.

A fractional-order memristor model and the fingerprint of the simple series circuits including a fractional-order memristor

Yu Ya-Juan, Wang Zai-Hua
Acta Physica Sinica. 2015, 64 (23): 238401 doi: 10.7498/aps.64.238401
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A memristor is a nonlinear resistor with time memory. The resistance of a classical memristor at a given time is represented by the integration of all the full states before the time instant, a case of ideal memory without any loss. Recent studies show that there is a memory loss of the HP TiO2 linear model, in which the width of the doped layer of HP TiO2 model cannot be equal to zero or the whole width of the model. Based on this observation, a fractional-order HP TiO2 memristor model with the order between 0 and 1 is proposed, and the fingerprint analysis of the new fractional-order model under periodic external excitation is made, thus the formula for calculating the area of hysteresis loop is obtained. It is found that the shape and the area enclosed by the hysteresis loop depend on the order of the fractional-order derivative. Especially, for exciting frequency being bigger than 1, the memory strength of the memristor takes its maximal value when the order is a fractional number, not an integer. Then, the current-voltage characteristics of the simple series one-port circuit composed of the fractional-order memristor and the capacitor, or composed of the fractional-order memristor and the inductor are studied separately. Results demonstrate that at the periodic excitation, the memristor in the series circuits will have capacitive properties or inductive properties as the fractional order changes.

The novel feed forward and decision feedback equalizer structures and improved variable step algorithm

Hao Xue-Yuan, Yan Xiao-Hong, Qian Li-Xia
Acta Physica Sinica. 2015, 64 (23): 238402 doi: 10.7498/aps.64.238402
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Skin effect and dielectric loss in super-long cable will cause nonlinear attenuation at different signal frequency, and in addition, coupling noise and thermal noise also cause signal distortion at the receiver end. These factors seriously affect the signal transmission speed in the super-long cable. Especially, in the field of exploration of shale gas and bed methane, the transmission cable is also used to transport high-precision synchronization pulse signal, and the synchronization pulse must reach the microsecond accuracy, which is used for data phase calibration. A synchronization signal is a high frequency signal, which suffers more severe attenuation and noise interference. At the receiving end, the sync pulse signal will be drowned in the noise environment, and so it is difficult to restore the original signal.#br#Although fiber can achieve a high transfer rate, but the fiber cable cannot transmit power energy; in addition, the tensile strength and heat resistance of the fiber are much worse than copper cable, these weaknesses limit its application in such industry. Therefore, an effective balancing algorithm is necessary to overcome the propagation effects and interference in a super-long copper cable. However, conventional equalization techniques have well-balanced effect for the short-range communications, but for the long-distance communication, they often have poorly balanced results. In order to solve the above problem and improve the long cable signal transmission speed, this paper presents a new balanced portfolio structure; the new structure uses feed-forward equalizer (FFE) as the pre-stage, and decision-feedback equalizer (DFE) as the post stage to form a new structure. The combination structures can effectively utilize the flexibility of FFE and overcome the problem of error diffusion in DFE. By mathematical modeling and simulation, this paper gives the best combination factors. Furthermore, based on the improved structure, a new convergence algorithm is proposed, which uses the arc tangent function combined with three error converge factors to form a converging function, and it has the advantages of fast convergence and steady-state error. Simulation results show that the FFE-DFE combination equalizer has low computational complexity, fast convergence, and strong channel tracking capability; in addition, it can speed up the data processing speed, and better respond to the real variation of the channel. Simulation results show also that the performance is improved by 50% by eliminating inter-symbol interference and noise.#br#The real circuit board based on the new algorithm have been tested in the East China Petroleum Bureau, the test results show that the algorithm can rectify 160 dB signal distortion, and the transmission speed can reach 5 Mbps in 6 dB signal to noise ratio.

Analytic models for solid state plasma of Si/Ge/Si heterogeneous and lateral SPiN diode

Kang Hai-Yan, Hu Hui-Yong, Wang Bin, Xuan Rong-Xi, Song Jian-Jun, Zhao Chen-Dong, Xu Xiao-Cang
Acta Physica Sinica. 2015, 64 (23): 238501 doi: 10.7498/aps.64.238501
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Silicon-based reconfigurable antenna, which is fabricated by heterogeneous and lateral surface PiN (SPiN), is an effective technology to achieve antenna miniaturization and enhance radar and wireless communication system performance. In this paper, the heterogeneous and lateral Si/Ge/Si SPiN diode is presented, that can weaken the band gap narrowing of the P + and N + region to improve the injection ratio of PN junction. And the electrical properties of the solid state plasma within the intrinsic region are also studied. Based on the bipolar diffusion model and Fletcher boundary condition, the analytic models of the junction voltage, the current density and the solid state plasma concentration distribution are established for a large injection current, and the numerical simulation has been carried out. Results show that the junction voltage increases linearly with the increase of applied voltage and the solid state plasma concentration at the boundary; and the current density increases exponentially with increasing junction voltage and the applied voltage, respectively. And the applied voltage and the current density of heterogeneous SPiN diode is lower than that of homogeneous SPiN diode under the same condition due to the difference of barrier heights between the heterojunction and homojunction. In addition, results also show that the value of the concentration decreases with increasing length of the intrinsic region; otherwise, it increases with increasing applied voltage and the doping concentration in the P+ and N+ regions. Under the same conditions, the solid state plasma concentration of the heterogeneous SPiN diode is nearly seven times that of homogeneous diode, which is in excellent agreement with the data published, giving the evidence for the validity of our method. The proposed models provide an effective reference for the design and application of silicon-based reconfigurable antennas.

Physical mechanism of uniaxial strain in nano-scale metal oxide semiconductor transistor caused by sin film

Yang Min-Yu, Song Jian-Jun, Zhang Jing, Tang Zhao-Huan, Zhang He-Ming, Hu Hui-Yong
Acta Physica Sinica. 2015, 64 (23): 238502 doi: 10.7498/aps.64.238502
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Performance of a nano-scale MOS (metal-oxide-semiconductor) can be significantly improved by uniaxial stress, caused by the SiN film deposited on the surface of MOS. Although this technique has been widely used in the performance improvement of CMOS and integrated circuit, the physical mechanism for instance, how is the strain in MOS channel caused by the SiN film? how about the relation between the kinds of the structure of SiN film needed to be discussed in depth. On the basis of the ISE TCAD, three typical models for stress analysis——such as the segmentation structure model, the closed-loop structure model and the integrity structure model——are proposed. And then, this paper reveals the physical mechanism about how the stress in MOS channel is caused by the SiN film and how much the magnitude of the stress in MOS channel is induced. Results shows that: 1) The “step” structure is the necessary condition for the strain in the MOS channel to be caused by the SiN film. 2) With the tendency for SiN film to shrink or expand, the film may lead to the deformation along the MOS source/drain region of the Si material, which causes the deformation of Si in the channel. 3) The whole of the channel stress in SiN film is equal to the sum of the stress in the source/drain imposed by the SiN film above the source/drain, the stress which the “closed loop structure” applies to the channel, and the stress generated in the channel by the whole SiN film. Our conclusions may provide the valuable references to the manufacture of nano-scaled MOS and the research of the novel inducing stress technique.

Ultrasound imaging algorithm based on generalized sidelobe canceller

Wang Ping, Cheng Na, Gong Zhi-Hui, Wang Lin-Hong
Acta Physica Sinica. 2015, 64 (23): 238701 doi: 10.7498/aps.64.238701
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The traditional delay and sum (DAS) algorithm is the most widely adopted method in medical ultrasound imaging; although it can produce images quickly, it sacrifices the resolution and the contrast ratio. The adaptive method such as the minimum variance (MV) continuously updates the apodization weighting vectors according to the received signals, so that the variance of the weighted signals is minimized, and thus the quality of the ultrasound imaging can be improved, especially its resolution. Although the image quality may be improved in the contrast ratio as well as the resolution after combining the minimum variance with the coherence factor (MV-CF), it complicates the algorithm, and the robustness against noise is enhanced but a little. An improved ultrasound imaging algorithm based on the generalized side lobe canceller (GSC) is proposed, which is constructed according to the minimum variance principle. The canceller is designed to classify the signal into desired and noise signals, combined with wiping off the big interferential eigenvectors, so that the robustness against noise can be enhanced. Firstly, the canceller divides the weighting vector into non-adaptive and adaptive weights, then the eigenstructure subspace is established according to the covariance matrix of the received signals, and the renewed weighting vector is achieved finally by projecting the weighting vector into the left singular space of the eigenstructure subspace. Simulations of the point targets and the cyst phantom through the simulation tool Field II demonstrate that the ultrasound image acquired through the proposed method is better than the traditional DAS and MV-CF algorithms in terms of the contrast ratio and resolution. In practice, the contrast ratio increases by roughly 7 dB compared to DAS and 5 dB to MV-CF. Furthermore, the proposed method gives a more satisfactory lateral resolution as well as the lowest side lobe peak level. From the sound-absorbing speckle simulation, the contrast ratio increases by 3 dB more than that of DAS and over 4 dB than that of MV-CF when noise exists. In addition, MV-CF performs the worst in the robustness aspect while the proposed GSC method makes improvement on the basis of it. Besides, the image quality can be further improved by combining the proposed method with sign coherence factor (GSC-SCF). After such a combination, the noise added to the data sets is almost invisible in point targets simulation. It also possesses the maximum mean power in cyst region in sound-absorbing speckle simulation. Finally, an experiment is conducted on the basis of the complete data sets which are offered by the University of Michigan. Results indicate that the proposed methods can perform better than the conventional DAS and MV-CF in resolution, contrast ratio and the robustness against noise.

First-principles studies on the properties of Cu2ZnSnS4 grain-boundaries due to photovoltaic effect

Fan Wei, Zeng Zhi
Acta Physica Sinica. 2015, 64 (23): 238801 doi: 10.7498/aps.64.238801
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Microstructures and electronic structures of Cu2ZnSnS4 (CZTS) grain-boundaries (GB) are studied by the first-principles electronic structure method. Some special twist grain-boundaries have low grain-boundary energies and exhibit similar electronic structure as that in a perfect crystal. The twist grain-boundaries such as 3[221] and 6[221] have grain-boundary planes parallel to (112) plane, the easiest cleavage plane, so that they have small damages to the crystal structure and small influence on the properties of the materials. Grain-boundary plays two roles in CZTS thin-films: (1) capturing and trapping holes from p-n junctions, and (2) providing fast channels for transportation of majority carriers. As the majority of carriers, the positively charged holes need override a barrier before being trapped by a potential-well in the grain-boundary region. For the minority of carriers, the grain boundary is a high barrier to prevent electrons from transporting across it. The intrinsic nature of the potential barrier is not very clear. By calculating the distributions of static potentials across different grain boundaries of CZTS and also by comparing them with those across different surfaces, we find that the potential barriers at grain boundaries are the remnants of the potential barriers of surfaces, which trap the electrons in the bulk and prevent the electrons from escaping from the bulk to vacuum. When two surfaces get contact to form a grain boundary the corresponding surface barriers will be merged together as one potential barrier of the grain boundary. It is obvious that if a grain boundary intersects with the surface, the escaping work function near the grain boundary is lower than that near the prefect crystal surface. Experiment shows the coexistence of Sn4+ and Sn2+ions. The Sn4+ ions are located in the bulk by bonding 4 S atoms as neighbors. Our results show that Sn2+ ions can appear in the grain-boundary regions, on the surfaces or in the bulk with lattice defects so that Sn2+ ions have the lower coordination number by bonding 3 S atoms. The Sn atom is favored to be at the center of S octahedron with six neighboring S (or O) atoms in most sulfides (oxides) of tin. In CZTS, Sn atom is at the center of tetrahedron with 4 neighboring S atoms so that Sn atom is very active to move by structural relaxations. Most importantly the conduction-bands in CZTS are formed by the hybridizations between the s electrons of Sn and p electrons of S so that the conduction-bands of CZTS are sensitively dependent on the distributions and properties of Sn atoms. The appearing of Sn2+ ions and the strong structural relaxations of Sn atoms in grain-boundary regions and on surfaces induce extra in-gap states as a new source for the recombination of electron-hole pairs that are un-favored to the photo-voltage effects. Generally, the grain boundary plays a negative role in brittle photo-voltage materials such as Si and GaAs, and the positive role in ductile photo-voltage materials such as CdTe and CIGS (Cu(InGa)Se2). It means that the growth of the hard and brittle films is very difficult, the micro-cracks and micro-pores are easily created. Our calculations show that CdTe, CIGS and CZTS are all ductile with Poisson-ratio greater than 0.33. This means that CZTS can be used as the absorber of flexible solar cell. By comparing the optical absorption-coefficients of crystals, grain-boundaries, surfaces and nano-particles, we find that the internal surfaces in thin-films with high pore-ratio can create new energy-levels in band-gap, which enhances the recombination between electrons and holes and decreases the optical absorption-coefficients (>1.3 eV). As a result, the high dense CZTS thin-film is required for high-efficient CZTS solar-cell. The positive role of grain boundary is more important if the CZTS film has the large, unique oriented grains and the uniform distribution of grain sizes. The simple and regular grain-boundary network is more beneficial to the coherent transport of majority carriers.

REVIEW

Quantum precision measurement based on squeezed light

Sun Heng-Xin, Liu Kui, Zhang Jun-Xiang, Gao Jiang-Rui
Acta Physica Sinica. 2015, 64 (23): 234210 doi: 10.7498/aps.64.234210
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According to the Heisenberg uncertainty principle, the precision of any physical quantity measurement is limited by quantum fluctuation in general, which leads to the so-called standard quantum limit (SQL). The SQL can be beaten by using squeezed light, hence enhancing the measurement accuracy. Squeezed light is a typical nonclassical light, it exhibits reduced noise in one quadrature component. Since Caves proposed the scheme of phase measurement enhancement with squeezing, squeezed light has been used to enhance measurement precision in many areas. This review focuses on the following four kinds of precision measurements based on squeezed light: the measurements of relative phase, small lateral displacement and tilt, magnetic field, and clock synchronization. For all of these measurements, vacuum squeezing has been used to enhance measurement precision, while the types of squeezing (squeezing angle, transverse mode, polarization etc.) are different. For phase measurement, quadrature squeezing is injected into the conventionally unused input port of Mach-Zehnder interferometer (MZI) or Michelson interferometer (MI). For displacement or tilt measurement, a vacuum squeezing beam of a special transverse mode is coupled into an intense coherent beam, yielding a spatial-squeezed light whose transverse position or tilt angle noise is lower than that of a classical light beam. Based on the Faraday effect of polarization rotation, the magnetic field can be detected precisely. The precision can be increased further by using the polarization squeezing. The polarization squeezing can be generated by coupling two orthogonal polarized beams together, a coherent beam and a vacuum squeezed beam. Various polarization squeezing can be illustrated on the Poincaré sphere. Finally, in the clock synchronization based on the optical frequency comb, squeezed light can be used to enhance the time measurement precision. A theoretical scheme with multimode squeezing of supermode (a kind of mode describing the frequency mode of a pulse laser beam) is introduced. The squeezing has extensively been applied into the quantum precision measurements such as gravitational wave detection as well as biological measurement and will play a more important role in the near future.
GENERAL

Water permeability in carbon nanotube arrays

Han Dian-Rong, Zhu Xing-Feng, Dai Ya-Fei, Cheng Cheng-Ping, Luo Cheng-Lin
Acta Physica Sinica. 2015, 64 (23): 230201 doi: 10.7498/aps.64.230201
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The membrane composed of carbon nanotube arrays may be widely used in biological molecular devices, image display area and optoelectronic devices. In this paper, the water permeability of the (11, 11) carbon nanotube arrays is simulated by using the SPC/E water model and the molecular dynamics program LAMMPS at 300 K. It is found that the distance between carbon nanotubes has a significant impact on water density distribution and the electric dipole moment orientation. Regardless of the distance between the neighboring tubes, water molecules will get into the nanotubes and form a double-layer cylindrical ring structure inside the nanotubes. However, water molecules can fill into the interstitial space of the nanotube array only when the nearest distance between the neighbor the tubes is greater than 3.4 Å, or the interstitial cross area becomes greater than 57.91 Å2. As the interstitial space increases, the structure of water molecules in the interstitial space will evolve from disconnected single-file chains to boundary-shared close-packing-like columnar circles. Meanwhile, the radius of the water ring inside the nanotube will increase and its boundary becomes more sharp due to the attractions from those water molecules filled in the interstitial space. Relative to the tube axis, the distributions of the water molecular electric dipole moments in the interstitial space depend upon water structures. Under the condition of single-file chain, the distribution exhibits a bimodal characteristic, which is very similar to the distribution of water dipole moments inside the nanotube. Whereas, for the boundary-shared close-packing-like water columnar circle, the distribution of dipole moments shows a unimodal characteristic and the peak corresponds to the angle 90°. This indicates that the preferred orientation of the water dipoles points to the direction perpendicular to the tube axis. These conclusions are helpful in the understanding of the water transport properties in carbon nanotube arrays.

Fractal dimensions and escape rates in the two-dimensional Hénon-Heiles potential and its deformation form

Zhang Yan-Hui, Shen Zhi-Peng, Cai Xiang-Ji, Xu Xiu-Lan, Gao Song
Acta Physica Sinica. 2015, 64 (23): 230501 doi: 10.7498/aps.64.230501
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From the beginning of the twentieth century the development of nonlinear theory has guided us into a new field of exploration; chaos and fractals are important tools widely used in studying the nonlinear system. Fractal focuses on the description of the physical geometry while Chaos emphasizes the research on the developing process of dynamics coupled with geometry. We have studied the nonlinear behavior of the particles in non-integrable system by means of chaos and fractals.#br#There are fractal structures in a Hénon-Heiles system, by which we can investigate the general escape law of particles in chaotic systems. The motion of particles is traced by dynamical algorithm within the framework of Chin and Chen (2005 Celestial Mechanics and Dynamics Astronomy 91 301). For the first time, we study the escape property of particles in Hénon-Heiles system and make a comparison with it in its hexagon-shaped form. Particles with energy higher than the threshold (1/6) can escape from the Hénon-Heiles system. The self-similar structures are found by calculating the escape time of particles with an energy higher than the threshold for different exit angles. We calculate the escape rate of particles with different energies and make a statistical analysis on the fractal dimensions by ‘box-counting' method. It is that the escape rate and fractal dimensions change with the energy of particles in the Hénon-Heiles system. Moreover, we find that the fractal dimensions are strongly linear with the escape rate.#br#To testify the universality of the conclusion, we calculate the escape rate and fractal dimensions of particles in the hexagon-shaped Hénon-Heiles system. Unlike the motion of particles in the Hénon-Heiles system, the escape of particles is divided into two ranges of energy. In low energy range, the escape rate and fractal dimensions of particles change with energy, which is similar to that in the Hénon-Heiles system. However, the escape rate and fractal dimensions tend to become stable in high energy range. But the general law is still valid–the fractal dimensions are strongly linear with the escape rate.#br#Therefore, the fractal dimensions can be served as a tool to study the escape features of particles in a chaotic system. We can characterize the transport behaviors in a chaotic electronic equipment by investigating the fractal dimensions in the design of mesoscopic devices.

Study on real-time optical sampling of chaotic laser for all-optical physical random number generator

Li Pu, Jiang Lei, Sun Yuan-Yuan, Zhang Jian-Guo, Wang Yun-Cai
Acta Physica Sinica. 2015, 64 (23): 230502 doi: 10.7498/aps.64.230502
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Absolutely secure communication should be implemented only through the ‘one-time pad' proposed by Shannon, requires that physical random numbers with rates matched with the associated communication systems be used as secret keys. With the wide application of the WDM technology in optical communication, the single channel rate of the current digital communication system has exceeded 10 Gb/s and developed towards 100 Gb/s. To ensure the absolute security of such a large capacity communication, a large number of real-time, and secure random numbers are needed.#br#Secure random numbers are commonly produced through utilizing physical random phenomena, called physical random number generators. However, conventional physical random number generators are limited by the low bandwidth of the applied entropy sources such as thermal noise, photon-counting and chaotic electrical circuits, and thus have typical low bit rates of the order of Mb/s.#br#In recent years, chaotic lasers attracted wide attention due to their generation of secure, reliable and high-speed random number sequences, and so due to their coherent merits such as high bandwidth, large amplitude fluctuation and ease of integration. There have been lots of schemes based on laser chaos for high-speed random number generation, but most of them execute the random number extractions from the associated laser chaos in the electrical domain and thus their generation rates are faced with the well-known ‘electrical bottleneck'. On the other hand, all-optical random number generation (AO-RNG) methods are all signal processes in the optical domain, so they can efficiently overcome this rate limitation and have a great potential in generating ultrafast random numbers of several dozens or hundreds of Gb/s. However, there is no experimental report on its realization of AO-RNG. One of the obstacles in the way for the AO-RNG achievement is to implement the fast and real-time all-optical sampling of the entropy signals (i.e., laser chaos).#br#In this paper, we present a principal experimental demonstration of the feasibility in the all-optical sampling of the chaotic light signal through constructing a TOAD-based all-optical sampler with a polarization-independent semiconductor optical amplifier (SOA). Specifically, we experimentally generate chaotic laser signals using an optical feedback semiconductor laser and finally complete a 5 GSa/s real-time and high-fidelity all-optical sampling of the chaotic laser with a bandwidth of 6.4 GHz. Further experimental results show that whether the optical sampling period is proportional to the external cavity feedback time or not has a great effect on the weak periodic suppression of the chaotic signal: only when both of them are out of proportion, can the weak periodicity of the original chaotic signal be effectively eliminated; and this is favorable for the generation of high-quality physical random numbers. To the best of our knowledge, it is the first time to realize all-optical sampling of chaotic signal in experiments.

High precision and fast method for absolute distance measurement based on resampling technique used in FM continuous wave laser ranging Hot!

Meng Xiang-Song, Zhang Fu-Min, Qu Xing-Hua
Acta Physica Sinica. 2015, 64 (23): 230601 doi: 10.7498/aps.64.230601
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Frequency modulated continuous wave (FMCW) laser ranging is one of the most interesting techniques for precision distance metrology. It is a promising candidate for absolute distance measurement at large standoff distances (10 to 100 m) with high precision and accuracy, and no cooperation target is needed during the measuring process. How to improve the measurement resolution in practice has been the research focus of the FMCW laser ranging in recent years.
FMCW laser ranging system uses the method which may convert the measurement of flight time to the frequency measurement, while the ranging resolution can be determined by the tuning range of the optical frequency sweep in theory. The main impact-factor that reduces the resolution is the tuning nonlinearity of the laser source, which may cause an amount of error points within the sampling signal. So a dual-interferometric FMCW laser ranging system is adopted in this paper. Compared to the traditional Michelson scheme, an assistant interferometer is added. The assistant interferometer has an all-fiber optical Mach-Zehnder configuration, and the delay distance is at least 2 times longer than OPD (optical path difference) of the main interferometer. Because it provides the reference length, the length of the fiber must remain unchanged. The interference signal is obtained on the photodetector. At the time points of every peak and bottom of the auxiliary interferometer signal, the beating signal from the main interferometer is re-sampled. The original signal is not the equal time intervals, while the re-sampled signal is the equal optical frequency intervals. Based on the property of the re-sampled signal, a method by splicing the re-sampled signal to optimize the signal processing is proposed, by which the tuning range of the laser source limitation can be broken and high precision can be easily obtained. Also, a simple high-speed measuring method is proposed.
Based on all the above principles, the two-fiber optical frequency-modulated continuous wave laser ranging system is designed. The delay fiber in the FMCW laser ranging system is 40.8 m long, and the tuning speed and tuning range of the laser source are set to 10 nm/s and 40 nm respectively. Experiments show that the optimization method can effectively improve the measurement resolution and measuring efficiency; in the 26 measuring ranges, 50 μm resolution can be easily obtained and the error is less than 100 μm.

Study on in-situ real-time measurement for thin film growth of organic semiconductors

Xu Jia-Jia, Hu Chun-Guang, Chen Xue-Jiao, Zhang Lei, Fu Xing, Hu Xiao-Tang
Acta Physica Sinica. 2015, 64 (23): 230701 doi: 10.7498/aps.64.230701
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We propose an approach for in-situ real-time measuring the optical and electric properties of a thin film in parallel during the process of growth. The method is developed based on two techniques: differential reflectance spectroscopy (DRS) and field effect transistor (FET) structure based electrical characteristics testing method. In order to demonstrate the performance of the method, FETs with a bottom-gate structure are manufactured and the pentacene organic thin film is deposited by vacuum thermal evaporation as a transport layer on the top of the transistor, i.e. the insulator substrate of SiO2. The optical and electrical properties of the organic thin film are in-situ investigated during its growth. As obtained from the optical spectra, the DRS signal moves up and down along the wavelength. Its fluctuation amplitude increases quickly and is very sensitive to the variation of the thickness of the top most film since the shutter of the molecular evaporation source is open. A good agreement between the experimental data and the computational results with a four-layer structure model of Si/SiO2/pentacene/air suggests that the DRS signal here is mainly due to the interference that exists in the multilayer interfaces. In addition, there are two characteristic peaks at 629 nm (1.97 eV) and 673 nm (1.84 eV) appearing occurs clearly in the DRS spectra at the initial stage of the growth. It means that the pentacene layer forms a thin film phase structure. Furthermore, the growth rate is evaluated to be 0.23 nm/min. When the effective thickness of the pentacene layer reaches 28 nm, calculated from the growth rate and the measured time, the conductivity of the organic FET becomes noticeable. It implies that an electrical conducting layer is already formed. After that, the thickness of the conducting layer continuously increases, while the current between the drain and the source increases slowly and turns to be saturated. After a 15-hour film growth, the sample has a threshold voltage of -20 V and the charge carrier mobility is 3.1×10-3 cm2/(V· s). These data confirm that the sample is an FET although its electronic properties are not good enough. These results show that the proposed approach is a useful measurement tool to build the relationships among the data of the optical spectrum, the electrical property, and the structure of the thin films. Hence, it is valuable for both the explanation of the growth mechanism of the thin film in research and the optimization of its preparation process in industry.
ATOMIC AND MOLECULAR PHYSICS

Study on the photodetachment wave packet dynamics of H- ion in a gradient electric field

Chen Zhao-Hang, Wang De-Hua, Cheng Shao-Hao
Acta Physica Sinica. 2015, 64 (23): 233201 doi: 10.7498/aps.64.233201
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Using the combination of the time-dependent perturbation theory and the closed-orbit theory, we put forward a calculation formula for the autocorrelation function of H ion in a gradient electric field, and then calculate and analyze the autocorrelation function of the system. Especially, we discuss the effect of laser pulse width, electric field strength and the electric field gradient on the autocorrelation function of H ion in a gradient electric field. It is demonstrated that when the laser pulse width is very narrow, far less than the period of the detached electron, the quantum wave packet revival phenomenon is significant. A series of sharp reviving peaks appear in the autocorrelation function, which are caused by the interference between the returning electron wave packets travelling along the closed orbit and the outgoing electron wave packets. However, with the increase of laser pulse width, the quantum wave packet revival phenomenon becomes weakened. When the difference between the pulse width and the period of the closed orbit is not very large, the reviving peaks in the autocorrelation function become widely spread gradually and the oscillatory structures get flattened. This correspondence will vanish finally due to the interference between the adjacent peaks. In addition, our study also suggests that the background electric field strength and the electric field gradient in the gradient electric field can also have significant effects on the autocorrelation function. With the increase of background electric field strength and electric field gradient, the period of the detached electron's closed orbit gets shorter, the number of the revival peaks in the autocorrelation function is increased gradually, and the quantum wave packet revival phenomenon will be enhanced. Therefore, we can control the autocorrelation function of the hydrogen negative ion by changing the laser pulse width and the external electric field strength. Our results will provide some reference values for the experimental research on the wave packet dynamic property of atoms or ions in external fields.

Two-photon absorption properties of novel charge transfer molecules with divinyl sulfide/sulfone center

Wu Xiang-Lian, Zhao Ke, Jia Hai-Hong, Wang Fu-Qing
Acta Physica Sinica. 2015, 64 (23): 233301 doi: 10.7498/aps.64.233301
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Organic materials with strong two-photon absorption response have attracted a great deal of interest in recent years for their many potential applications such as two-photon fluorescence microscopy, optical limiting, photodynamic therapy, and so on. Theoretical study on the relationships between molecular structure and two-photon absorption property has great importance in guiding the experimental design and synthesis of functional materials. Nowadays, quantum chemical calculations become very useful and popular tools in investigating the structure-property relations. At the same computational level, the two-photon absorption properties of different compounds can be compared accurately, and thus provide reasonable structure-property relations. Recently, a series of novel divinyl sulfides/sulfonesbased molecules have been synthesized and it is found that their photophysical properties behave like quadrupolar charge-transfer chromophores. In order to explore their potential two-photon absorption applications, in this paper, the two-photon absorption properties of these new molecules are calculated by using quantum chemical methods. Their molecular geometries are optimized at the hybrid B3LYP level with 6-31+g(d, p) basis set in the Gaussian 09 program. The two-photon absorption cross sections are calculated by response theory using the B3LYP functional with 6-31g(d) and 6-31+g(d) basis sets respectively in the Dalton program. In response theory, the single residue of the quadratic response function is used to identify the two-photon transition matrix element. Using the same methods, the two-photon absorption properties of distyrylbenzene compounds are computed for comparison. The basis set effects on excitation energies and two-photon absorption cross sections have been checked. It is found that the use of large basis sets could probably provide better numerical results, but the overall property trends would not change. Calculations show that the molecule with a triphenylamine group has the largest cross-section due to its strong donor groups. The divinyl sulfones-based dyes have larger cross-sections than the corresponding sulfides-based ones, because divinyl sulfones have stronger capability to accept electrons and at the same time the torsional angles between benzene rings in sulfones-based molecules are smaller than in the sulfides-based molecules. In the applicable wavelength range, these new dyes exhibit large two-photon absorption cross-sections which have the same order of magnitude as the strong two-photon absorption molecules with similar conjugation length. The largest cross section comes to 1613.3 GM calculated by using 6-31g(d) basis set. Molecular orbitals involved in the strongest two-photon absorption excitations are plotted and the charge transfer process is analyzed at length. The divinyl sulfide and sulfone centers behave as electron withdrawing groups and can form effective charge transfer molecules. On the basis of these new molecules, the structure inducing two-photon absorption enhancement is designed by employing isomerism effect. When the benzene rings of carbazole groups are connected with the molecular center, the planarity and charge transfer intensity are increased, and then the two-photon absorption cross-section can be improved dramatically. This study provides theoretical guidelines for the synthesis of new type of active two-photon absorption materials.

Theoretical study on electron-impact excitation cross section and polarization for 5s2S1/2 → 5p2P3/2 of Cd

Jiao Fei, Jiang Jun, Xie Lu-You, Zhang Deng-Hong, Dong Chen-Zhong, Chen Zhan-Bin
Acta Physica Sinica. 2015, 64 (23): 233401 doi: 10.7498/aps.64.233401
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The electron-impact excitation (EIE) cross-sections of 5s2S1/2 → 5p2P3/2 of Cd+ have been calculated by using the relativistic distorted-wave (RDW) method which we have developed recently. In order to discuss the electronic correlation effects, four different models are used to describe the target wave functions, namely Model A, 8S, 5SD and 6SD. Model A is a single configuration model, it includes 5s1/2, 5p1/2 and 5p3/2 only. The 8S is a valence-valence correlation model, it considers the impacts of 6s, 7s, 8s and 6p, 7p, 8p orbitals on wave function of 5s1/2, 5p3/2, respectively. The 5SD is a core-valence correlation model, it includes all the virtual single and double excitations from the 4s, 4p, 4d, 5s and 5p shells into the unoccupied 4f, 5p, 5d, and 5f shells. The 6SD is also a core-valence correlation model, it includes all the virtual single and double excitations from the 4s, 4p, 4d, 5s and 5p shells into the unoccupied 4f, 5p, 5d, 5f, 6s, 6p and 6d shells. The oscillator strength calculated by Model A is 0.72, which is in disagreement with the experiment 0.55 measured by Xu et al., while the results of 6SD 0.57 is in agreement with these of the experiment very well. For the EIE cross-sections, the core-valence correlation is very important. The results of Model A and 8S are larger than the experimental results obtained by Gomonai et al., while the results of 5SD and 6SD is obviously smaller than the results of Model A. In low energy range (<10 eV), the EIE cross-sections of 5SD and 6SD is at least 70% smaller than that of Model A and it is in good agreement with the experimental results. However, the contributions of correlation effects to the linear polarization of 5p2P3/2 → 5s2S1/2 emission lines after excitation are very small, while the linear polarization of Model A, 5SD and 6SD are in consistent with each other and also in good agreement with other theories. But for high energies, the theoretical results have big difference from the experimental results obtained by Goto et al.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

Extraction efficiency enhancement of single InAs quantum dot emission through light scattering on the Au nanoparticles

Su Dan, Dou Xiu-Ming, Ding Kun, Wang Hai-Yan, Ni Hai-Qiao, Niu Zhi-Chuan, Sun Bao-Quan
Acta Physica Sinica. 2015, 64 (23): 235201 doi: 10.7498/aps.64.235201
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Single semiconductor quantum dots (QDs) have been considered as the promising solid-state single photon sources. To obtain bright quantum sources, the key issue is to enhance extraction efficiency of the QD emission, which is challenging since QDs normally emit isotropically in a high refractive index material. In this article, we investigate the influence of Au nanoparticles on the QD photoluminescence (PL) extraction efficiency based on the techniques of optically positioned QDs and single QD emission detection. The InAs QD samples studied are grown using the molecular beam epitaxy on a (001) GaAs substrate. The sample consists of, in sequence, a 200 nm GaAs buffer layer, a 100 nm AlAs sacrificed layer, a 30 nm GaAs, a QD layer, and a 100 nm GaAs cap layer. The QD sample is mounted in a cryostat cooled down to 5 K, and excited by illumination of a 640 nm diode laser (CW or pulsed with a repetition frequency of 80 MHz). Excitation laser beam is focused to an approximately 2 μm spot on the sample using a microscope objective (NA : 0.5) which is mounted on a nanocube XYZ piezo nanopositioning stage with a scanning range of 100×100×100 μm3. The QD PL is collected using the same objective and measured using a 0.5 m focal length monochromator equipped with a silicon charge-coupled device (CCD). The PL decay measurements are performed using a silicon avalanche photodiode (APD) and a time-correlated single-photon counting (TCSPC) board.#br#In order to study the influence of different environments surrounding the QDs on the spontaneous emission rate and the extraction efficiency, the same QD emissions are measured under the conditions that: (1) A typical QD is at first chosen and optically positioned and then its emission is measured. (2) A GaAs layer containing the QDs is lifted off from the as-grown sample by an AlAs sacrificed layer and placed on the Au film with or without Au nanoparticles. (3) Optical measurements are carried out to obtain the QD emission intensity. This technique enables us to compare the same QD emission intensity for the as-grown QD sample, which is placed on the Au film or on the Au nanoparticles.#br#In summary, it is found that the measured QD emission intensity increases up to 6 times that of the original for the QD placed on the Au nanoparticles, otherwise it is only doubled for the QD placed on the Au film. The time-resolved PL measurements show that the QDs have nearly the same decay time for the QDs in different environments, implying that the QD spontaneous emission rate has not been changed. Therefore, the enhanced PL is due to the increase of extraction efficiency. The physical mechanism underlying the Au nanoparticles-induced PL enhancement is attributed to the trapped QD emission light within the sample and scattered again by Au nanoparticles and collected by the microscopy objective.

Experimental research on laser-produced Gd target plasma source for 6.7 nm lithography

Dou Yin-Ping, Xie Zhuo, Song Xiao-Lin, Tian Yong, Lin Jing-Quan
Acta Physica Sinica. 2015, 64 (23): 235202 doi: 10.7498/aps.64.235202
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Extreme ultraviolet (EUV) lithography at λ =6.7 nm is a challenging subject for next generation semiconductor lithography beyond 13.5 nm. The availability of strong radiation at the operating wavelength and low-debris of the plasma source are the two most important aspects for the development of laser-produced Gd plasma source at 6.7 nm. In this paper, experimental research on the extreme ultraviolet source based on the laser-produced Gd plasma is performed. Strong radiation near 6.7 nm from the source has been obtained, which is attributed to the n=4-n=4 transitions in Gd ions that overlap to yield an intense unresolved transition array (UTA). Dependence of spectral variation near the strong emission region of Gd plasma on the incident laser power density and detection angles is given. It is found that the intensity of EUV radiation around 6.7 nm is increased with increasing laser power density, and the emission peak around 7.1 nm increases faster than that of emission peak around 6.7 nm after the laser intensity reaching 6.4×1011 W/cm2, which is ascribed to the unique spectroscopic behavior of Gd ions. In addition, the energy of the ion debris from laser-produced Gd plasma source as well as the angular distribution of the ion yield off the target normal are measured with Faraday cup. Results show that the ion energy corresponding to the peak position of Gd ion energy distribution is about 2.6 keV at 10° off the target normal, and the yield of Gd ions decreases with the increase of the angle from the target normal. Furthermore, the stopping ability of an ambient magnetic field for ion debris from laser Gd plasma source is evaluated, and the result shows that the energetic Gd ion can be effectively mitigated by applying a 0.9 T magnetic field.

Numerical studies on the formation process of Z-pinch dynamic hohlruams and key issues of optimizing dynamic hohlraum radiation

Xiao De-Long, Sun Shun-Kai, Xue Chuang, Zhang Yang, Ding Ning
Acta Physica Sinica. 2015, 64 (23): 235203 doi: 10.7498/aps.64.235203
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Dynamic hohlraum is a possible selection to drive inertial confinement fusion. Currently, the ~8 MA PTS facility in China has been completed, which provides a powerful experimental platform of relatively large drive current for researches of dynamic hohlraums and dynamic hohlraum driven inertial fusion. To understand the formation processes and the main characteristics of the dynamic hohlraum, and explore the most important issues affecting the optimization of hohlraum radiation, is not only fundamental in the research of dynamic hohlraums, especially for the experimental design, but also can provide a physical insight for the experimental diagnosis. In this paper the implosion dynamics of a tungsten wire-array Z-pinch embedded with a CH foam converter, especially the impaction interaction of the wire-array plasma with the converter plasma, is numerically investigated using a one-dimensional non-equilibrium radiation magnetohydrodynamic code. In simulations the tungsten plasma is assumed as a plasma shell with a width of 1 mm, and the CH converter plasma is assumed to be uniform with an initial temperature of 0.1 eV. The overall implosion is driven by an assumed current with a peak value of 8 MA and a rise time of 66.4 ns. It is shown that a local high pressure region, which is generated by the impaction of the tungsten plasma with the converter plasma, is crucial to launch the strongly radiating shock wave and to form the dynamic hohlraum. Due to the supersonic radiation transfer in the low opacity CH converter plasma, which is also produced in the high pressure region, there exists a hohlraum region inside the front of the shock wave, in which the radiation is high. At the same time, the plasma pressure is uniform in this hohlraum region, so the plasma will not be disturbed before the shock arrives. As the shock propagates to the axis, the hohlraum becomes small and the radiation temperature is also increased. Basically, the hohlraum radiation is determined by the detailed profiles of plasma conditions when the wire-array plasma impacts onto the CH converter plasma. And these profiles are determined by many factors, such as the drive current, initial masses and radii of the wire-array and the converter, as well as the material of the converter. When the drive current is fixed, the optimal wire-array can be determined. Firstly, the mass ratio of the wire-array to the CH converter is varied. Numerical calculations show that as this ratio is decreased, the shock velocity is increased and the radiation temperature is increased as well. Additionally, the time duration of the radiation pulse before the shock arrives at the axis is remarkably increased. It is also found that when this mass ratio is slightly lower than unity, for example 0.75, a relative optimal dynamic hohlraum can be produced. Secondly, if the mass ratio is fixed and the initial radius of the converter is decreased, it is found that the shock velocity is just slightly changed. However, the peak hohlraum radiation temperature is increased and the radiation pulse becomes remarkably narrow. A suitable radius ratio of the wire-array to the converter, neither too large to induce strong Magneto-Rayleigh-Taylor (MRT) instability nor too small to gain a small kinetic energy of the wire-array before impacting onto the converter surface, should be selected. In the future we will develop two-dimensional code to investigate the effect of MRT instability on the formation of dynamic hohlraums.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

One-dimension nonlinear and dispersive seismic wave modeling in solid media

Zhou Cong, Wang Qing-Liang
Acta Physica Sinica. 2015, 64 (23): 239101 doi: 10.7498/aps.64.239101
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The nonlinear theory in Earth Science is very important for solving the problems of the earth. When considering some of the nonlinear properties of the medium, solitary wave (a special wave with a finite amplitude and a single peak or trough) may appear. Previous studies showed that it may be related to the rupture in the earthquake process. Therefore, it would be very helpful to explain some special phenomena in actual observation data if we fully understand the characteristics of nonlinear waves.#br#In this paper, based on the nonlinear acoustic wave equation, we first perform 1-D nonlinear acoustic wave modeling in solid media using a staggered grid finite difference method. To get the stable and accurate results, a flux-corrected transport method is used. Then we analyze several different types of nonlinear acoustic waves by setting different parameters to investigate their nonlinear characteristics in the solid media. Compared with the linear wave propagation, our results show that the nonlinear coefficients have important influences on the propagation of the acoustic waves. When the equations contain only a third-order nonlinear term (consider the case β 1 ≠ 0, β 2=0, α =0), the main lobe of the wave is tilted backward and its amplitude gradually attenuates with the wave spreading, and the amplitude of its front side-lobe attenuates slowly while the back side-lobe attenuates quickly. The whole shape and amplitude of the wave remain unchanged after propagating a certain distance. When the equations contain only a fourth-order nonlinear term (consider the case β 2 ≠ 0, β 1=0, α =0), the main lobe and the two side-lobes of the wave are all slowly damped, but the shape of the whole wave is unchanged with the wave spreading.#br#In addition, for some combinations of nonlinear and dispersive parameters (consider the case β 1 ≠ 0, α ≠ 0, β 2=0), the wave acts like the linear wave, and the nonlinear acoustic wave is equal to solitary wave which is usually obtained by Kortewegde de Vries (KdV) equation. We validate our modeling method by comparing our results with the analytic solitary solutions. Solitary wave propagates with a fixed velocity slightly less than that of the linear compressional wave, which is probably due to the balance between nonlinear and dispersion effects, making the stress-strain constitutive relations show the nature of linear wave.

Effects of the surface albedo on short-wave infrared detection of atmospheric CO2

Chen Jie, Zhang Chun-Min, Wang Ding-Yi, Zhang Xing-Ying, Wang Shu-Peng, Li Yan-Fen, Liu Dong-Dong, Rong Piao
Acta Physica Sinica. 2015, 64 (23): 239201 doi: 10.7498/aps.64.239201
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The greenhouse gas carbon dioxide, for which short-wave infrared remote sensing detection is carried out by using satellite sensors to measure the Earth's atmosphere scattering solar radiation, and makes use of the inversion algorithm to achieve measurements. Most of the solar radiation enter the satellite sensors after surface reflection, so the surface albedo which reflects the surface features is one of the important parameters which affect the accuracy of the detection. Aiming at the great demands of high precision carbon dioxide for greenhouse gas, this study first investigate the effects of the Earth's surface albedo on the observed spectra. Simulation results show that the increase in the surface albedo will enhance the observed spectral intensity, especially larger in the O2-A band than in the 1.6 μm band. In other words, the surface albedo has a greater impact on O2-A ban. In the actual satellite inversio, the surface types of actual observation pointare uncertain, which will result in the error of surface albedo. Effect of surface albedo on the inverted XCO2 is analyzed when the surface albedo is changed by changing the type of surfac. Two observation cases are analyzed in detail. One is on April 23, 2009 for the desert surface, and another on May 21, 2013 for the grass surfac. Results show that when the O2-A band surface albedo approximates to the real surface albedo valu, the relative error of the inverted XCO2 is the smaller. If the relative changes of the O2-A band surface albedo exceed 0.25 in the grass surfac or 0.35 in the desert surface, the relative error of the inverted XCO2 will be greater than 1%, not satisfying the design requirement of the inversion system. In contrast, the changesin 1.6 μm band surface albedo have negligible effect on the inverted XCO2. This study shows the importance of surface albedo in the process of satellite remote sensin, and provides an important theoretical basis and guidance for improving the accuracy of remote sensing detectio. All these are significantly contributed to the hyperspectral satellite observation of the greenhouse gas, the investigation of global CO2 distributions, and the prediction and monitoring of the climate change.

Three dimensional Born geometrical factor of multi-component induction logging in anisotropic media

Wang Lei, Fan Yi-Ren, Huang Rui, Han Yu-Jiao, Wu Zhen-Guan, Xing Dong-Hui, Li Wei
Acta Physica Sinica. 2015, 64 (23): 239301 doi: 10.7498/aps.64.239301
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Geometrical factor has been widely used in the design of induction tool and analysis of complex logging responses, as well as the high resolution processing of logging data. Works in this area are usually limited to two-dimensional isotropic medium and are only available to cases of coaxial transmitter and coaxial receiver coupling. For a more thorough understanding of multicomponent induction logging in the presence of transverse anisotropy, a knowledge of anisotropic geometrical factor is often necessary. In this paper, the two-dimensional and isotropic geometrical factors are extended to the anisotropic media using Born approximation, and the expressions of 3D Born geometrical factors for multicomponent induction logging are derived. Then the sensitivity and detectability of coaxial, coplanar and cross-coupling measurements are investigated. Numerical results show that with increasing coefficients of conductivity anisotropy, the spatial distribution of geometrical factors becomes increasingly complicated, and more sensitivity information can be detected by multicomponent induction tools. The multicomponent induction tool is sensitive to conductivity anisotropy at arbitrary dipping angles. In vertical wells, coplanar measurements are significantly affected by the conductivity anisotropy. Compared with coaxial and coplanar measurements, cross-coupling component offers superior sensitivity information to the conductivity anisotropy with the dipping angle being 40~ 60°. In horizontal wells, coaxial measurements are the most sensitive to the conductivity anisotropy. The extended 3D Born geometrical factor directly exhibits the anisotropy sensitivity in terms of spatial contribution, and has made up for the shortage of previous geometrical factors. The new geometrical factor will create favorable conditions for the development of new multicomponent induction tool and the interpretation of anisotropic formations.
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