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Vol. 64, No. 1 (2015)

2015-01-05

INVITED REVIEW

Researches on the electrical properties of vanadium oxide thin films on Si substrates
Xiong Ying, Wen Qi-Ye, Tian Wei, Mao Qi, Chen Zhi, Yang Qing-Hui, Jing Yu-Lan
2015, 64 (1): 017102. doi: 10.7498/aps.64.017102
Abstract +
Quality enhanced VO2 thin films have been sputtering deposited on silicon substrates by introducing an ultrathin Al2 O3 buffer between the substrate and the film. With a preferred orientation (011), the VO2 films have an excellent thermal-induced metal-insulator transition (MIT). The electrically-driven MIT (E-MIT) characteristics have also been investigated by applying voltage to VO2 thin film based two-terminal device at particular temperatures. Sharp jumps in electric current are observed in the I-V curve with a variation of amplitude by two orders. The threshold voltage decreases with increasing temperature. At room tempature, the threshold voltage is 8.6V and the phase transition ccurs in a voltage width of only 0.1V. With the sharp and fast phase change, the VO2 thin films can be used in ultrafast switching electronic devices.

INVITED REVIEW

Persistent currents in mesoscopic graphene rings with armchair edges
Dai Nan, Deng Wen-Ji
2015, 64 (1): 017302. doi: 10.7498/aps.64.017302
Abstract +
Based on the tight-binding model, the energy spectrum and persistent currents of mesoscopic graphene rings with armchair edges are studied analytically and numerically. Characters of the persistent currents changing with Aharonov-Bohm (A-B) magnetic flux in rings in different geometry are investigated in datail. The periodicity and special symmetry of energy spectrum and persistent currents changing with the magnetic flux are revealed. It is demonstrated that the persistent currents are determined by the geometric structures of the rings; the quantum states with small eigen-energies may carry much larger currents than those quantum states with eigen-energies far away from zero.

INVITED REVIEW

Analysis of magnetic disturbance torque and drift error in a superconducting suspension system
Cui Chun-Yan, Hu Xin-Ning, Cheng Jun-Sheng, Wang Hui, Wang Qiu-Liang
2015, 64 (1): 018403. doi: 10.7498/aps.64.018403
Abstract +
In a superconducting suspension system, the disturbance torque acting on the superconducting rotor may not be generated if the rotor is an ideal sphere and in the complete Meissner state. However, in fact there exist always spherical tolerance during manufacturing process of the sphere and the centrifugal distortion due to the high speed rotation. Therefore, the disturbance torque will be generated due to the magnetic levitation force not getting through the mass center of the rotor when the rotor is levitated in the magnetic field. Based on the physical mechanism of the superconducting-magnetic bearing, the disturbance torque and the drift error are analyzed. The disturbance torques include the main torque due to asphericity of the sphere, the second torque generated by the combination of asphericity, uncentering and assembly errors. The model of drift rate is also deduced and the drift rate is calculated by substituting the rotor parameters into the formula. This analysis provides a reference for the rotor drift testing and error compensation, and is instructive for the optimization design of the rotor structure.
GENERAL
Fidelity recovery scheme for quantum teleportation in amplitude damping channel
Yang Guang, Lian Bao-Wang, Nie Min
2015, 64 (1): 010303. doi: 10.7498/aps.64.010303
Abstract +
During the course of quantum teleportation in amplitude damping channel, the quantum Bell entanglement state will suffer a de-coherence, which will lead to the quality decrease of quantum teleportation, or even communication failure. To overcome this influence, we propose a method to compensate for the de-coherence of Bell entanglement state. Based on the parameter estimation of the amplitude damping channel, the compensation is divided into two steps. The first step (called pre-compensation) is carried out before the occurrence of de-coherence; the second step (called match-compensation) is carried out after the quantum entanglement state has experienced the de-coherence in the amplitude damping channel. The former is done at the EPR source device, while the latter is done at the quantum user device. The parameters of pre-compensation and match-compensation are determined by the amplitude damping coefficient. The quantum teleportation is carried out after the entanglement compensation. We will give the theoretical derivation and performance analysis of this method. Compared with the method that has no compensation and the method that the compensation is only done after de-coherence, the method given in this paper has a higher quantum teleportation fidelity which is close to 1, when the compensation parameter is adjusted accurately. Our method shows an effective influence on the teleportation quality decrease due to the entanglement de-coherence.
Entanglement evolution of three interacting twolevel atoms within a common environment
Feng Ling-Juan, Xia Yun-Jie
2015, 64 (1): 010302. doi: 10.7498/aps.64.010302
Abstract +
Entanglement dynamical evolution of three two-level atoms coupled to a common environment is investigated. We utilize the tripartite negativity to quantify entanglement and analyze the effect of the initial state, pairwise dipole-dipole interactions on the entanglement dynamical properties of the system by means of numerical calculations. Results show that a novel quantum interference can be controlled by the relative phase of initially entangled states of the atoms. And the excited-state population can be trapped in the long time limit. Besides, the tripartite entanglement can be enhanced by choosing the appropriate conditions.
A BLT equation-based approach for calculating the shielding effectiveness of enclosures with apertures
Luo Jing-Wen, Du Ping-An, Ren Dan, Nie Bao-Lin
2015, 64 (1): 010701. doi: 10.7498/aps.64.010701
Abstract +
A calculation method based on BLT equation is proposed for analyzing the shielding effectiveness of enclosures containing apertures. This method can accurately and quickly obtain the shielding effectiveness of oblique incidence and polarization, off-centered apertures, and multi-apertures in two walls of enclosures. Aperture scattering matrix can be obtained according to equivalent circuit theory. Then, propagation relationships and reflection relationships are established, and finally the general BLT equation is derived, including the coupling to apertures. In order to verify the validity, the results are compared with these from equivalent circuit theory and CST. With the same aperture impedance, the aperture scattering matrix contains the interaction of the electromagnetic field so that the method proposed in this paper owns higher precision and can predict more resonant modes compared with the equivalent circuit theory. This method takes less time and computer resource compared with CST which cannot easily get the influence of enclosure parameters.
The variational iteration method for characteristic problem of strong damping generalized sine-Gordon equation
Xu Yong-Hong, Shi Lan-Fang, Mo Jia-Qi
2015, 64 (1): 010201. doi: 10.7498/aps.64.010201
Abstract +
A class of nonlinear strong damping sine-Gordon disturbed evolution differential equation is studied which appears widely in mathematics and mechanics. Firstly, we introduce a traveling wave transformation, and obtain the exact solution of degenerate equation. Then a functional calculating method for variational iteration is constructed, thus an iterative expansion is found. Finally, the approximate traveling wave analytic solutions for the original strong damping generalized sine-Gordon disturbed evolution equation are found. The arbitrary order approximate solutions, and the simple variational iteration method are obtained with higher accuracy. The approximate analytic solution can make up for the imperfection of the simple numerical simulation solution.
Scattering quantum walk search algorithm on star graph
Liu Yan-Mei, Chen Han-Wu, Liu Zhi-Hao, Xue Xi-Ling, Zhu Wan-Ning
2015, 64 (1): 010301. doi: 10.7498/aps.64.010301
Abstract +
Quantum walk is a typical quantum computing model which is receiving significant attention in recent years from theory researchers. In this paper, we prove that the two major formulations for discrete quantum walks, coined and scattering, are unitarily equivalent on star graph. We then propose a new quantum search algorithm on star graph based on the scattering quantum walk. It is shown that the temporal complexity of the algorithm is the same as that in Grover algorithm, but success probability is greater than that in Grover algorithm when the objects are more than one third of total items.
Control and synchronization in chaotic systems based on fast linear predictive control
Zhang Yuan, Xu Qi, Sun Ming-Wei, Chen Zeng-Qiang
2015, 64 (1): 010502. doi: 10.7498/aps.64.010502
Abstract +
A kind of fast linear generalized predictive control (GPC) algorithm is proposed based on the extended state observer for chaotic (hyperchaotic) systems. The linear extended state observer is employed to estimate and compensate the nonlinear dynamics and the existing uncertainties of the chaotic (hyperchaotic) systems so that an integrator can be obtained to serve as the model for GPC design. Using this scheme, the computational complexity can be substantially reduced. A step coefficient matrix can be derived analytically and a future output prediction can be explicitly calculated by only using the last two samples of the output. Therefore, the self-tuning algorithms and the Diophantine equation can be completely avoided. The proposed method can be used to control nonlinear targets in a straightforward manner. Simulation results show the effectiveness of this linear algorithm.
Dynamical modeling and multi-periodic behavior analysis on pulse train controlled DCM-DCM BIFRED converter
Shi Guo-Dong, Zhang Hai-Ming, Bao Bo-Cheng, Feng Fei, Dong Wei
2015, 64 (1): 010501. doi: 10.7498/aps.64.010501
Abstract +
According to four different operating modes of a BIFRED converter, a piecewise smooth linear time-varying model for pulse train (PT)-controlled BIFRED converter operating in dual discontinuous conduction modes (DCM-DCM) is established, from which time-domain waveforms and phase portraits under different loads are obtained by numerical simulations. By analyzing the variation of the charge of an output capacitor in a switching period, a one-dimensional approximate discrete mapping model for PT-controlled DCM-DCM BIFRED converter is derived, in which the multi-periodic behaviors with the variations of the load resistance and the input voltage are studied, and the stability analysis and the estimations of the load resistance ranges are performed. Research findings indicate that the dynamical behaviors depicted by the one-dimensional approximate discrete mapping model are in good agreement with those described by the piecewise smooth linear time-varying model, well revealing the complicated multi-periodic behaviors existing in the PT-controlled DCM-DCM BIFRED converter. By PSIM circuit simulations and physical circuit experimental measurements, the waveforms obtained are consistent with the results of theoretical analysis, effectively validating the feasibility of the two dynamical models.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
Influence of uneven wall changing over time on the characteristics of liquid surface wave evolution
Wang Song-Ling, Liu Mei, Wang Si-Si, Wu Zheng-Ren
2015, 64 (1): 014701. doi: 10.7498/aps.64.014701
Abstract +
This paper mainly studied the evolution of liquid surface waves along an uneven wall. Considering the characteristic of the uneven wall changing over time, the perturbation equation for the surface waves is derived by using the small parameter perturbation method to solve the control equations under the given boundary conditions. The method of derivative expansion is used to find the solution to the equation and numerical research is then carried out for the wall shape of a simple harmonic. By studying the influence of different parameters on the wavy wall, it can be found that when the frequency of the wall is small, the wavelengths of static waves and traveling waves are close to each other, promoting the merger between the surface waves, and the surface wave amplitude is obviously increased when the wall frequency and wall depth or Re increase. By contrast of the two cases of wavy wall and uneven wall, the surface wave on uneven wall has a periodical change at the same location with the increase of time, while the surface wave on wavy wall has an almost periodic change with a longer wavelength. Although the decrease of the wall amplitude and the wall frequency both can cause surface wave amplitudes reduced, the surface waveforms are different, for the surface wave on the wavy wall can be regarded as the superposition of wavy wall waveform and surface waveform caused by uneven wall.
Research on semi-supervising learning algorithm for target model updating in target tracking
Gao Wen, Tang Yang, Zhu Ming
2015, 64 (1): 014205. doi: 10.7498/aps.64.014205
Abstract +
Target detection and tracking technique is one of the hot subjects in image processing and computer vision fields, which has significant research value not only in military areas such as imaging guidance and military target tracking, but also for civil use such as security and monitoring and the intelligent man-machine interaction. In this paper, for target deformation, scale changing, rotation, and other issues in the long-term stable target tracking, a bootstrapping feedback learning algorithm is proposed, which may improve the target model and the classifier discriminating capacity as well as the fault tolerance ability; and it also makes fewer errors during the updating, and then the proof of convergence of the algorithm is given. Experimental results show that among the same tracking algorithms, utilization of the learning method to update the target model and classifier is more stable and more adaptable than unusing it in the processes of target scale changing, deformation, rotation, perspective changing and fuzzy. And compared with the existing conventional method, this method has a better robustness, and a high value in practical application and research.
Theoretical and experimental studies on the self-reconstruction property of non-diffracting Mathieu beam
Li Dong, Wu Feng-Tie, Xie Xiao-Xia, Sun Chuan
2015, 64 (1): 014201. doi: 10.7498/aps.64.014201
Abstract +
Self-reconstruction properties of the Mathieu beam are studied theoretically and experimentally. By means of the Mathieu-Hankel wave's theory, the self-reconstruction mechanism of Mathieu beam is analyzed. Based on the Fresnel diffraction theory, an analytical expression for the optical field of a Mathieu beam partially blocked by a circular opaque obstacle is derived, and the self-reconstruction process of Mathieu beam is simulated based on the analytical expression. Using a cylindrical lens and an axicon system to generate Mathieu beam, the self-reconstruction properties of the Mathieu beam partially blocked by a circular opaque obstacle on axis and off axis are verified. Experimental results agree well with the theoretical analysis.
Multiple speckle patterns differential compressive ghost imaging
Zhong Ya-Jun, Liu Jiao, Liang Wen-Qiang, Zhao Sheng-Mei
2015, 64 (1): 014202. doi: 10.7498/aps.64.014202
Abstract +
Equipment requirement, quality of reconstructed image, and reconstruction time are important factors in the realization of thermal-light ghost imaging system. In this paper, we propose a new ghost imaging scheme with multiple speckle patterns, named multiple speckle patterns differential compressive ghost imaging scheme. In this scheme, the high temporal resolution requirements for detectors is reduced by continuously detecting multiple independent speckle patterns. We eliminate the background and other noises in ghost imaging system by using differential ghost imaging method. And the reconstruction time is effectively reduced simultaneously to improve the reconstructed image quality by introducing the compressive sensing techniques. Numerical results show that for the two-level grayscale “N” image, the mean square error, in using the proposed scheme with 8000 measurements, is reduced by 96.9%, and the peak signal to noise ratio has improved by 15.1 dB, in comparison with those using the original multiple speckle patterns ghost imaging scheme with 35000 measurements. For the eight-level grayscale “Pepper” image, the peak signal to noise ratio is enhanced by 11.4 dB. The proposed scheme also can decrease the requirements of detection equipment to improve the image quality, and reduce the reconstruction time. Therefore, it may have a broad application prospect.
Laser diode double-end-direct-pumped slab laser with hybrid resonator
Mao Ye-Fei, Zhang Heng-Li, Xu Liu, Deng Bo, Sang Si-Han, He Jing-Liang, Xing Ji-Chuan, Xin Jian-Guo, Jiang Yi
2015, 64 (1): 014203. doi: 10.7498/aps.64.014203
Abstract +
To reduce the influence of thermal lensing and attain the high power laser with high beam quality, a laser diode stack dual-end-direct-pumped Nd:YVO4 slab laser with negative branch confocal folded hybrid cavity has been developed. Its maximum output power is 416 W, optical conversion efficiency is 54.3%, and slope efficiency is 61.6%. When the output laser power is 370 W, M2 values in the unstable resonator direction and stable resonator direction are 3.9 and 4.7, respectively.
Study on nonlinear surface waves along the boundary of LiNbO3 crystals
Chen Wei-Jun, Lu Ke-Qing, Hui Juan-Li, Wang Chun-Xiang, Yu Hui-Min, Hu Kai
2015, 64 (1): 014204. doi: 10.7498/aps.64.014204
Abstract +
The propagation of nonlinear surface waves along the boundary of LiNbO3 crystals with diffusion and photovoltaic nonlinearities is investigated theoretically and experimentally. Surface waves with different oscillating period are obtained by changing the values of the propagation constant, and the energy of surface waves would increase monotonically with the propagation constant. Our experimental results are in good agreement with the theoretical analysis. Experimental results show also that the generation of surface waves can be shortened by increasing the power of the input beam; and the excitation efficiency of the surface waves can be enhanced by increasing the angle (less than 90 degree) between the input beam and the crystal c-axis.
Quantitative analysis of flow structures in compression ramp based on flow visualization
Wu Yu, Yi Shi-He, He Lin, Quan Peng-Cheng, Zhu Yang-Zhu
2015, 64 (1): 014703. doi: 10.7498/aps.64.014703
Abstract +
Flow visualization studies on the laminar boundary layer flows over a 25° compression ramp are carried out via NPLS technique in a Mach 3.0 wind tunnel; fine flow structures such as boundary layer, shear layer, and shock waves may be visualized clearly. Based on the visualized data, quantitative analysis is conducted using intermittency, spatial correlations, and fractal theory. The intermittency function γ of the boundary layer and the interaction region is calculated, and the size of coherent structures and the structure angle θ for the boundary layer are obtained, so that the fractal dimension of the boundary layer in streamwise can be provided. Experimental data in the present paper have been compared with the results of Ringuette and Bookey, and the quantitative characteristics of flow structures are discussed in detail.
Pulsed microwave energy spatial distribution imaging by means of thermoacoustic tomography
Bi Xin, Huang Lin, Du Jing-Song, Qi Wei-Zhi, Gao Yang, Rong Jian, Jiang Hua-Bei
2015, 64 (1): 014301. doi: 10.7498/aps.64.014301
Abstract +
Microwave-induced thermoacoustic imaging is a noninvasive, high contrast, high resolution, and cost effective method for cancer detection. It has the potential to serve as a routine breast tumor screening. In the present study, simulation and experiment have been used for pulsed microwave energy spatial distribution investigation. The target to be imaged is a 99 square array composed of tubes 3 mm in diameter and 8 mm in separation. The simulation and experimental results both indicat that far away from the antenna, more tubes could be thermoacoustically recovered, which means a larger radiation area obtained. The thermoacooustically recovered tubes are 3.1 mm in diameter and 7.7 mm in separation. Obtained results suggest that it is feasible to detect microwave energy spatial distribution with the thermoacoustic imaging, which has paved the way to solve the inhomogeneous microwave energy problem in traditional quantitative thermoacoustic tomography.
Stability of surfactant-laden droplet spreading over an inclined heterogeneous substrate
Li Chun-Xi, Chen Peng-Qiang, Ye Xue-Min
2015, 64 (1): 014702. doi: 10.7498/aps.64.014702
Abstract +
For the spreading of a droplet covered with insoluble surfactant over an inclined random heterogeneous substrate, the base state and disturbance evolution equations for the film thickness and surfactant concentration are derived using the lubrication theory. Stability of the droplet spreading on the heterogeneous substrates, and effects of dimensionless parameters as well as the internal mechanism are investigated based on the non-modal stability theory. Results show that the disturbances of film thickness and surfactant concentration exihibit a double-hump shape and the crests lie in thinning regions at the bottom of the droplet. With increasing disturbance wave number, the stability of the droplet spreading is enhanced, but the maximal disturbance transfers from the right-side to the left-side of the droplet bottom. Increasing inclined angle leads to severe instability distinctly. Improving topographical height or increasing wave number of the substrate leads the spreading stability to firstly enhance and then to weaken. The droplet evolution displays an unstable spreading under smaller capillary number, with the maximum disturbance to be increasing before decreasing.
Experimental study on flow visualization of hypersonic flat plate boundary layer
Fu Jia, Yi Shi-He, Wang Xiao-Hu, Zhang Qing-Hu, He Lin
2015, 64 (1): 014704. doi: 10.7498/aps.64.014704
Abstract +
The classical problem of flat plate boundary layer which involves turbulence and transition is still hot, and a mass of work should be done to reach a high accuracy measurement of this flow, especially under the condition of high velocity. In the present paper, the application of the nano-based planar laser scattering (NPLS) method in a hypersonic short-duration facility is explored, and then the high accuracy measurement of a flat plate boundary layer is studied. The Mach number of the main flow is 7.3, the total pressure is 4.8 MPa, and the total temperature is 680 K. Through analysis and tests, the synchronization control of the NPLS system with the test facility is realized, and with the quantitative control, the tracer particle is uniformly seeded. Based on this, the transient boundary layer flow in the short-duration tunnel is visualized with high resolution, and the transition from laminar to turbulent flows is captured. The development characteristic of the flow is studied finally.
Spontaneous emission field excited by an atom near the μ-negative metamaterials slab
Qin Li, Li Ze-Ya, Xu Jing-Ping, Zhang Li-Wei, Yang Ya-Ping
2015, 64 (1): 014206. doi: 10.7498/aps.64.014206
Abstract +
This paper discusses the spontaneous emission field of a two-level atom near a μ-negative metamaterial(MNG) slab, in which the surface modes are excited. the μ-negative metamaterial is a kind of artificial-microstructured materials possessing effective negative permeability and positive permittivity. According to Maxwell's equations and boundary conditions, the MNG slab supports only TE-polarized surface modes.We analyze the properties of the surface mode, i.e.the number of the surface mode and its symmetry or antisymmetry profiles, supported by the MNG slab with different permeability and thickness, and then study the influence of these characteristics on the spatial distribution of the spontaneous emission field in detail. Results show that the distance between the atom and the slab can affect the ratio of surface mode to the total atomic emission field. When the surface mode plays the dominate role, the spontaneous emission field of the atom on the nearest surface of MNG slab are directionally propagating along y-axis if the atomic dipole is along x-axis due to the TE-polarized surface mode. The atomic emission field on the other surface depends on the symmetry of the surface modes and their percentage. If the symmetric and antisymmetric surface modes coexist, the field intensity on the right surface is weakened or even disappears completely, but if there exists only symmetric or antisymmetric surface mode, the field intensity on the right surface is nearly identical with that on the left surface. These phenomena are significantly different from the case of atoms near a metal slab or a dielectric slab. Our results are useful for the controllable atomic emission and have potential application to the single-photon source.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Study on surface relief related to reverse martensitic transformation in Mn-based high-temperature antiferromagnetic shape memory alloy
Yuan Feng, Liu Chuan, Geng Zheng, Cui Yan-Guang, Wang Lin, Wan Jian-Feng, Zhang Ji-Hua, Rong Yong-Hua
2015, 64 (1): 016801. doi: 10.7498/aps.64.016801
Abstract +
Evolution of surface relief and its intrinsic mechanism associated with martensitic transformation (MT) during heating and cooling in Mn79.5Fe15.6Cu4.9 high-temperature antiferromagnetic shape memory alloy (SMA) have been investigated in nano-scale by means of in-situ atomic force microscopy (AFM), X-ray diffraction (XRD), and dynamic mechanical analyzer (DMA). Experimental results show that the N-type surface relief originates from the reverse MT and is completely made of matrix which is different from the conventional ones. The reverse MT exhibits untwinning shear and the reverse shearing of twinned martensites mainly contribute to the surface relief. The measured surface relief angles are less than 1°, which are determined by the small difference of lattice constants between fcc and fct structures. Surface relief has a good recovery property because of the crystallographic reversibility rule in SMAs, implying that this kind of alloy has a good surface morphology memory effect.
Study of laser action in dye-doped polymer dispersed cholesteric liquid crystal film
Dai Qin, Wu Jie, Wu Xiao-Jiao, Wu Ri-Na, Peng Zeng-Hui, Li Da-Yu
2015, 64 (1): 016101. doi: 10.7498/aps.64.016101
Abstract +
In this work, laser dye DCM, nematic liquid crystal TEB30A, chiral dopant S-811 and PVA are used to prepare polymer dispersed cholesteric liquid crystal (LC) films by the method of microcapsule. Lasing mechanism and temperature-tunable characteristics of the films are investigated. The diameters of large droplets in the films are about 80 μm and LC molecules are planar-textured in the droplets when viewed between crossed polarisers through a polarization microscope. Pumped by the 532 nm Nd:YAG laser, narrow linewidths in laser emissions are obtained at 634.5 nm and 680.2 nm, and the linewidths are of 0.25 nm and 0.29 nm, respectively. The device is then compared with a dye-doped cholesteric liquid crystal laser. The wavelength of output laser will blueshift when the temperature of the device is increased, and a 22.8 nm tunable lasing, from 666.7 nm to 643.9 nm, is obtained. Analysis concludes that the lasing mechanism of the film is band-edge lasing, and the emission wavelengths are corresponding to the two edges of the photonic band gap.
Geometric stability and nitrogen adsorption properties of small BaxOy cluster-modified Ru(0001) surface
Yan Jing, Xu Wei-Yun, Guo Hui, Gong Yu, Mi Yi-Ming, Zhao Xin-Xin
2015, 64 (1): 016802. doi: 10.7498/aps.64.016802
Abstract +
Barium promoter is widely used in the secondary ammonia synthesis catalysis, which could greatly improve the performance of a catalyst. Although barium oxide is confirmed as the main component of barium promoter, the existence of metallic barium has been argued. In order to theoretically clarify this issue, the first principles calculations have been performed to study the geometric stability and the nitrogen adsorption properties of small BaxOy cluster-modified Ru(0001) surface. It is found that Ba2O cluster is more stable than other small clusters or atoms (BaO2, BaO, Ba and O) on the Ru(0001) surface under the condition that the pressure rate of H2O/H2 is below 1‰. This implies that BaO promoter could be partially reduced by hydrogen gas in the experiment. According to the results of the projected density of states and charge difference induced by modification of cluster, the O atom in Ba2O cluster gains electrons from dz2 orbit of the underlying Ru atom, and forms O–Ru bonds; while Ba atom in Ba2O clusters transfers electrons to the nearest Ru atoms and forms Ba-Ru metallic bonds. As the adsorption of nitrogen is an initial reactant in ammonia synthesis, we also study the nitrogen adsorption properties near the Ba2O cluster. Compared with the different chemical properties of O and Ba atoms, the adsorption properties of nitrogen molecules on the sites close to O and Ba atoms are similar. The nitrogen adsorption energies at the corresponding sites are calculated to be 0.88 and 0.78 eV, respectively. The bond lengths of nitrogen molecules are about 0.187 nm near O atom, and 0.190 nm near Ba atom, both of which are shorter than those on a clean surface (~ 0.197 nm). And the stretching vibrational frequency of a nitrogen molecule is calculated to be 1888 cm-1 near the O atom, 1985 cm-1 near the Ba atom, both of which are also less than those on a clean surface (~ 2193 cm-1). This suggests that Ba2O cluster may weaken the bond strength of nitrogen molecules. According to the charge difference induced by nitrogen adsorption, the electrostatic interactions of Ba2O clusters increase the occupation of π antibonding orbital and the electric polarization of the nitrogen molecule, and thus weaken the N–N bond.
Molecular dynamics study of ripples in graphene monolayer on silicon surface
Qin Ye-Hong, Tang Chao, Zhang Chun-Xiao, Meng Li-Jun, Zhong Jian-Xin
2015, 64 (1): 016804. doi: 10.7498/aps.64.016804
Abstract +
By using the classical molecular dynamics and the simulated annealing techniques, the evolutions of the rippled morphology in single atomic graphenes placed on the Si (100), Si (111) and Si (211) surfaces respectively are performed at an atomic level. Our results show that the monolayer graphene sheets on the different Si surfaces form atomic scale rippled structures. A graphene monolayer prepared on Si surface forms rippled structure due to the relative lattice mismatch between graphene and Si substrate. The rippled morphology of graphene sheet on Si surface is strongly dependent on the annealing temperature. Such ripples will directly affect the adhesion strength between graphene and Si substrate. These findings are useful for understanding the structural morphology and stability of graphene on the semiconductor Si substrate, which will provide an analysis reference for further applications of graphene.
Molecular dynamics study on the phase transition of high density helium
Zhang Bao-Ling, Song Xiao-Yong, Hou Qing, Wang Jun
2015, 64 (1): 016202. doi: 10.7498/aps.64.016202
Abstract +
The melting curve of helium has been calculated over the temperature range of 0–1000 K using molecular dynamics method. Comparisons of the calculated melting curve and the experimental data show that the consistence between the melting curve and the experimental data may reach 500 K. When the temperature is above 600 K, no reliable experimental data have been found for comparison. In addition, the pressures inside the helium bubbles of various sizes existing in metal titanium are calculated and have a comparison with the melting curve. Results show that, at low temperatures, the helium bubbles may be in solid state as the temperature decreases. When the temperature is above 300 K, no solid helium bubble may exist.
Simulation of precipitate splitting in an alloy (Ⅱ): Elastic energy collapse
Liu Lei, Chen Zheng, Wang Yong-Xin
2015, 64 (1): 016401. doi: 10.7498/aps.64.016401
Abstract +
The phenomenon of elastic energy collapse (EEC) accompanying the variation of the structure and temperature during the growth of a nucleus is discovered while we study the precipitate splitting in an alloy. There are two ways to introduce the EEC, one is the instantaneous elastic energy collapse (IEEC) and the other is the gradual elastic energy collapse (GEEC). Simulation shows that whether the precipitate is splitted is related to the time when EEC emerges, and that the kind of splitting of the precipitate depends on the rate of EEC. For IEEC, if it is introduced when the diameter of a nucleus reaches about 80l–90l (l=12.18 nm) the precipitate is splitted to 4 blocks. For GEEC, if it is intoduced when the time is larger than τ = 7.5× 103 s, the precipitate is splitted to 2 blocks.
Critical properties of the S4 model for Sierpinski carpet
Yin Xun-Chang, Liu Wan-Fang, Zhu Zu-Song, Kong Xiang-Mu
2015, 64 (1): 016402. doi: 10.7498/aps.64.016402
Abstract +
According to the bond-moving renormalization group technique, the critical behaviour of S4 model for Sierpinski carpet is investigated, then the critical points are obtained. From the results we find that there are a Wilson-Fisher fixed point and a Gaussian fixed point. In contrast to the Gauss model for Sierpinski carpet, the critical points have altered obviously. Results indicate that the two systems belong to two different universal classes.
Simulation studies on the influence of nanofilm thickness on the elastic properties of B2-NiAl
Yang Xi-Yuan, Zhang Jin-Ping, Wu Yu-Rong, Liu Fu-Sheng
2015, 64 (1): 016803. doi: 10.7498/aps.64.016803
Abstract +
In this paper the molecular dynamics (MD) technique and the modified analytic embedded atom method (MAEAM) are applied to study the influence of size and surface condition of the nanofilm on the elastic properties of B2-NiAl, The elastic properties of the bulk NiAl alloy and the size dependence of the surface energy of nanofilms are first calculated. It is found that the calculated results of the elastic properties are consistent with those from experiments and theories; and the surface energy, which is barely influenced by the thickness, is controlled by the surface atomic composition. On this basis, our investigations are mainly focused on the relationship between the elastic properties of nanofilms and their thickness. The obtained results indicate that the elastic properties of the nanofilm may change exponentially with the increasing thickness, which can also be regulated by the surface atomic composition. Furthermore, the inherent mechanism of the thicknes and surface that affects this relationship is analyzed in detail, showing that the surface atomic composition and the deviation of interplanar spacing in nanofims are two major factors for determining the thickness dependence of the elastic properties. These are in good agreement with the previous theoretical and experimental studies.
Effects of layer thickness and strain rate on mechanical properties of copper-gold multilayer nanowires
Fan Qian, Xu Jian-Gang, Song Hai-Yang, Zhang Yun-Guang
2015, 64 (1): 016201. doi: 10.7498/aps.64.016201
Abstract +
Effects of individual layer thickness and strain rate on the mechanical behavior of copper-gold multilayer nanowires as well as the dislocation nucleation mechanism under a uniform tensile loading are investigated using molecular dynamics method. Simulations indicate that the highest yield strength increases with the increase of the individual layer thickness. Furthermore, the result also shows that the mechanical properties in the tensile process at different strain rates are dramatically different from each other, where the dislocation motion and twinning deformation are at a lower strain rate, while the individual atoms are at a higher strain rate for leading to amorphization. The general conclusions derived from this work can provide a guideline for the design of high performance multilayer composite materials.
Synthesis and characterization of flower-like MoS2 microspheres by hydrothermal method
Fu Chong-Yuan, Xing Song, Shen Tao, Tai Bo, Dong Qian-Min, Shu Hai-Bo, Liang Pei
2015, 64 (1): 016102. doi: 10.7498/aps.64.016102
Abstract +
High-purity flower-like MoS2 microspheres have been successfully synthesized by hydrothermal method using Na2MoO4 and CH3CSNH2 as precursors, and H4O40SiW12 as an additive. Samples are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS). XRD and EDS patterns show that the as-prepared samples are MoS2, which have good crystallinity with a well-stacked layered structure. SEM images show that the as-prepared MoS2 are composed of flower-like microspheres with a mean diameter about 300 nm, the structures of which are constructed from dozens of hundreds of MoS2 nano-sheet with a thickness of several nanometers. It is also found that the silicotungstic acid plays an important role in the formation of the flower-like MoS2 microspheres, which could affect the size and morphology of the MoS2. Flower-like MoS2 is not found in the as-prepared product without adding silicotungstic acid. A formation mechanism of MoS2 microspheres is tentatively given.
Structure and phase change in Cu-Ge3Sb2Te5 films for use in phase change random access memory
Sun Jing-Yang, Wang Dong-Ming, Lü Ye-Gang, Wang Miao, Wang Yi-Man, Shen Xiang, Wang Guo-Xiang, Dai Shi-Xun
2015, 64 (1): 016103. doi: 10.7498/aps.64.016103
Abstract +
The Cu-Ge3Sb2Te5 thin films with different Cu contents were prepared by magnetron sputtering method. The dependence of film resistance on temperature is measured in situ by using the four-point probe heating platform. The crystal structure, microstructure, optical gap, and bond states of the Cu-Ge3Sb2Te5 films are investigated by X-ray diffraction, transmission electron microscopy, transmission and Raman spectra, respectively. It is shown that the crystallization temperature and activation energy of crystallization increase with increasing Cu content, suggesting the improvement in thermal stability and data retention ability, while optical gap decreases with increasing Cu concentration. It is observed that the Raman peak shifts from 129 cm-1 to 127 cm-1, which may be ascribed to the vibration of polar Cu–Te bonds. The Cu-Ge3Sb2Te5 crystallizes into the embedded Cu2Te and Ge2Sb2Te5 phases with evenly grown grains.
ATOMIC AND MOLECULAR PHYSICS
Evolution of average cluster size in supsonic cluster jet under high gas backing pressure
Xu Yi, A. S. Boldarev, Dong Eon Kim, Chen Guang-Long
2015, 64 (1): 013601. doi: 10.7498/aps.64.013601
Abstract +
Evolution of the average cluster size at the center of a cluster jet from the nozzle throat along the gas flow is investigated using simulations. The simulation is performed for the cluster jet from the expansion of Ar gas into vacuum through a long conical nozzle (with the length L of 30 mm) under a high backing pressure (~ 5×106 Pa). Results indicate that the cluster size increases gradually until it is close to the maximum with the increase of the distance from the nozzle throat, and the part of the jet with large-size clusters is located at the distance greater than 20 mm from the nozzle throat. Based on the simulation results about the evolution of the cluster size and the atom density in a cluster jet, the optimization of a nozzle length has been discussed under a given condition. This work shows that a proper nozzle length is about 20 mm for a usual conical nozzle with an opening angle of about 8.5 degree and a throat diameter of about 0.5 mm.
Investigation on stable structures of Au-Pd alloy nanoparticles with high-index facets
Shao Gui-Fang, Zheng Wen-Xin, Tu Na-Na, Liu Tun-Dong, Wen Yu-Hua
2015, 64 (1): 013602. doi: 10.7498/aps.64.013602
Abstract +
Based on the Monte Carlo simulation method, this paper employs the tight-binding potentials and the quantum-corrected Sutton-Chen type many-body potentials to investigate the stable structure, the distribution of surface atoms, the core-shell distribution, and the chemical short-range order parameter of tetrahexahedral Au-Pd nanoparticles. Different sizes and different Au contents are considered. Our results show that the surface atom distribution exhibits the same trend for the two types of potentials, that is, Au atoms tend to segregate on the surface while Pd atoms prefer to occupy the inner sites, this is beneficial to lowering the total energy of the structure. Nanoparticles are always present in a core-shell structure for small Au content. With increasing Au content, the Au-Pd nanoparticles will tend to form an onion-like multi-shell structure for the tight-binding potentials. The degree of the segregation of Au-Pd nanoparticles at the quantum-corrected Sutton-Chen type potentials is higher than that for the tight-binding potentials.
First-principles study on saturated adsorption of alkali metal atoms on silicene
Huang Yan-Ping, Yuan Jian-Mei, Guo Gang, Mao Yu-Liang
2015, 64 (1): 013101. doi: 10.7498/aps.64.013101
Abstract +
Based on density functional first-principles calculations, we study the stability, micro-geometry, and electronic properties of alkali metal atoms adsorbed on silicene, and perform the comparison between pure and hydrogen-saturated silicenes. We found that all the formation energies of SiX(X=Li, Na, K and Rb) are negative, indicating that the relative structural stability of these new compounds is higher than silicene. Bader charge analysis shows that electric charge is transferred from Si atoms to H atoms in SiH compound, but in SiX the direction of charge transfer is opposite, i.e., the charge is transferred from alkali metal atoms to Si atoms. From the viewpoint of chemical bonding, it can be regarded that valence bond is formed between Si atoms and H atoms, and the bonds between Si and alkali metal atoms are mainly ionic, but there exists covalent contribution. From the band structure calculations, it is also found that the new type compound SiLi is a semiconductor with a direct band gap of 0.34 eV; however, all the other compounds of SiX(X=Na, K and Rb) exhibit metallic property.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
Phenomenological modeling of nanosecond pulsed surface dielectric barrier discharge plasma actuation for flow control
Zhao Guang-Yin, Li Ying-Hong, Liang Hua, Hua Wei-Zhuo, Han Meng-Hu
2015, 64 (1): 015101. doi: 10.7498/aps.64.015101
Abstract +
One-zone inhomogeneous phenomenological nanosecond dielectric barrier discharge (NS-DBD) actuation model used for flow control simulation is established to investigate the flow control mechanisms, based on experiments and theoretical analysis. When the inhomogeneous phenomenological model is applied to a plate, the formation of spanwise vorticity is analyzed through the vorticity transport equation, and the spanwise vorticity is mainly engendered due to the baroclinicity of pressure gradient and density gradient, also due to the vorticity transfer by the flow convection in the vicinity of the actuation region. Agreement of the simulation with experiments on a column shows that the inhomogeneous phenomenological NS-DBD actuation model is reasonable. Separation control over NACA 0015 airfoil at high angle of attack indicates that the spanwise vortices induced by plasma actuation make the separated shear layer instable, promote interaction between shear layers, and downstream the separation point. Different excitation frequency has different effect on the lift, with the optimum reduced frequency F+ ≈ 6 in current simulation.
Crystallization behaviors of deuterium in glass microsphere
Yin Jian, Chen Shao-Hua, Wen Cheng-Wei, Xia Li-Dong, Li Hai-Rong, Huang Xin, Yu Ming-Ming, Liang Jian-Hua, Peng Shu-Ming
2015, 64 (1): 015202. doi: 10.7498/aps.64.015202
Abstract +
To develop deuterium-tritium (DT) cryogenic targets that meet the inertial confinement fusion (ICF) experiment requirements, the DT crystal seeding growth process needs to be controlled to obtain single crystalline DT-ice, thus reducing the crystal defects formed during crystal growth and improving ice-layering. In this paper, the close-packed hexagonal (hcp) single crystal growth mode has been established through kinetic theory of crystal growth morphology. Experimentally, the target chamber temperature is controlled to within ± 3 mK and the deuterium (D2) crystal growth process can be observed by backlit shadowgraphy. Results show that slow cooling can reduce the crystal defects significantly at the 20–100 Pa conducting helium pressure. When the cooling rate reaches 2 mK/min, two single crystal growth modes are observed with good reproducibility. Experimental results conform with the proposed hcp single crystal growth model. Compared with the results from Lawrence Livermore National Laboratory (LLNL), the methods of D2/DT single crystal growth in the cryogenic target are proposed.
Numerical investigations on the interaction of shock waves with spherical SF6 bubbles
SHA Sha, Chen Zhi-Hua, Zhang Qing-Bing
2015, 64 (1): 015201. doi: 10.7498/aps.64.015201
Abstract +
Based on the simulation of large eddy current, combined with the high-precision computational scheme, the study of planar shock wave and reflected shock waves at different reflected distances, which interact with spherical SF6 bubbles, is carried out numerically in 3D. Our numerical results show clearly the deformation of spherical interface that is induced by the Richtmyer-Meshkov instability due to the collision of shock wave. Jets induced by incident shock wave and reflected shock waves are revealed, and the flow field of the interactions between the reflected shock waves at different reflected distances and the SF6 bubbles is also discussed in detail.
Axial backlighting of two-wire Z-pinch using an X-pinch as an X-ray source
Zhao Shen, Zhu Xin-Lei, Shi Huan-Tong, Zou Xiao-Bing, Wang Xin-Xin
2015, 64 (1): 015203. doi: 10.7498/aps.64.015203
Abstract +
The axial backlighting of two-wire Z-pinch using an X-pinch as an X-ray source was conducted on PPG-1, a pulsed power generator with a current of 400 kA in amplitude and 100 ns in pulse width. Its time-resolved axial backlighting images are obtained. In the backlighting images all the physical processes similar to those occurring in the early stage of the wire-array Z-pinch are observed, including the expansion of the high-density wire core surrounded by low-density coronal plasmas, the motion of the coronal plasma toward the axis of the wire-array, and the formation of precursor plasma. The areal mass density of the plasma shown in the backlighting images is calibrated with step-wedge filters. And the time-resolved radial distributions of the areal mass density of the plasma from the two-wire Z-pinch are obtained.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Effects of Nb doping concentration on TiO2 electricel conductivity and optical performance
Hou Qing-Yu, Lv Zhi-Yuan, Zhao Chun-Wang
2015, 64 (1): 017201. doi: 10.7498/aps.64.017201
Abstract +
Nowadays, in the reports of Nb heavy-doped TiO2, when the doping mole of Nb is in the range of 0.050 to 0.0625, there is a current controversy between the two experimental results about the minimum resistance of the doped systems. To solve this contradiction, the models of un-doped and the three different concentrations of Nb doped Ti1-xNbxO2 (x=0.03125, 0.050, 0.0625) have been set up based on the first-principles plane wave ultra-soft pseudo potential method of density functional theory; then the geometry optimization of all models is carried out; and the band structures, the density of states, and optical properties are calculated. Results reveal that under the condition of limited doping amount as in this paper, when the doping moles of Nb is increased, the volume, the total energy, and the formation energy of the doped system are increased; the doped system has a lower stability and is hard to be redoped; the relative electronic concentration, and the electron effective mass are increased; the migration rate is reduced, and the conductivity is thus reduced. The wider the optical band-gap, the more obvious the shift of absorption edge to the short wavelength side, the lower the absorptivity and reflectivity; and the transmittance is increased. these are in agreement with the experimental results.
Theoretical and experimental studies on N, Fe, La co-doped anatase TiO2 band adjustment
Wang Qing-Bao, Zhang Zhong, Xu Xi-Jin, Lü Ying-Bao, Zhang Qin
2015, 64 (1): 017101. doi: 10.7498/aps.64.017101
Abstract +
Based on the plane wave method (PWPP) of densiy functional theory (DFT) we model the N, Fe, La three elements co-doped anatase TiO2 crystal structure and calculate its band structure and density of states with Material Studio. By the sol-gel method, the intrinsic anatase TiO2 and the anatase TiO2 with N, Fe, La three elements co-doping are prepared and investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). Results indicate that the changes of the N, Fe, La co-doped TiO2 lattice volume and its bond length will result in a decline of the crystal symmetry and the effective separation of the electron-hole pair. Impurity level appearing at the bottom of the conduction band and at the top of valence band leads to the decrease of the TiO2 forbidden band width(1.78 eV to 1.35 eV, reduced by 25%), the red shift of light absorption edge, the increase of density of states as well as, the improve ment of electron transition probability and the photocatalytic efficiency of TiO2. Ion doping makes the particles in doped TiO2 become smaller, i.e. the size of particles in TiO2>N/Fe_TiO2>N/Fe/La_TiO2, the emission peak of the N/Fe/La: TiO2 is 425 nm and its energy gap is smaller than that of the intrinsic TiO2. The measured N/Fe/La: TiO2 photocatalytic ability is stronger than the N/Fe: TiO2, the origin can be due to the increasing number of the electronic states and the impurity energy levels.
Research on the relationship between ideality factor and number of units of GaN-based high voltage light-emitting diode
Bai Jun-Xue, Guo Wei-Ling, Sun Jie, Fan Xing, Han Yu, Sun Xiao, Xu Ru, Lei Jun
2015, 64 (1): 017303. doi: 10.7498/aps.64.017303
Abstract +
Ideality factor can reflect the current, the carrier leakage, and the phenomenon of non-radiative recombination in light-emitting diode (LED). For the problem of ideality factor from current report about GaN-based LED, the value of ideality factor n is calculated by using the I-V curve fitting of high voltage LED. And the relationship between the ideality factor and the number of units is series in 12, 19, 51 and 80 V GaN-based high-voltage LED are discussed. In addition, the relationship between ideality factor and spectral half width (FWHM) is analyzed, and the impact of current crowding effect on the ideality factor is also studied. Results show that the ideality factor n increases nearly linearly with the number of units in series, indicating that the ideality factor n of high voltage LED is composed of its series units. It is a valuable result for understanding the ideality factor of GaN-based high voltage LED.
Photoluminescence properties and energy transfer from Ce3+ to Mn2+ in Na2SrMg(PO4)2
Xiong Xiao-Bo, Yuan Xi-Ming, Liu Jin-Cun, Song Jiang-Qi
2015, 64 (1): 017801. doi: 10.7498/aps.64.017801
Abstract +
Na2SrMg(PO4)2: Ce3+, Mn2+ phosphor was prepared by high temperature solid state reaction. Its luminescence properties and energy transfer from Ce3+ to Mn2+ were investigated. The emission bands of Ce3+ and Mn2+peaking at 334 nm and 617 nm were attributed to 5d→4f transition of Ce3+ and 4T1(4G)→6A1(6S) transition of Mn2+, respectively. Dependence of Ce3+ and Mn2+ luminescence properties on their concentrations was investigated as well. With the increase of Mn2+ content, the emission intensity of Ce3+ could be observed to decrease, however, the emission intensity of Mn2+ was found to increase. It is indicated that the luminescence of Mn2+ could be sensitized by energy transfer from Ce3+ to Mn2+ in Na2SrMg(PO4)2 host. According to the Dexter's energy transfer formula of multipolar interaction, it is demonstrated that the energy transfer between Ce3+ and Mn2+ is due to the electric dipole-quadripole interaction of the resonance transfer.
Study on cation distribution and magnetic moment of Ti ions in spinel ferrites Ni1-xTixFe2O4
Xu Jing, Qi Wei-Hua, Ji Deng-Hui, Li Zhuang-Zhi, Tang Gui-De, Zhang Xiao-Yun, Shang Zhi-Feng, Lang Li-Li
2015, 64 (1): 017501. doi: 10.7498/aps.64.017501
Abstract +
Spinel ferrite samples TixNi1-xFe2O4 (x=0, 0.1, 0.2, 0.3, 0.4) were prepared using conventional solid reaction method. The sample exhibit a single-phase cubic spinel structure with a space group obtained Fd3m. The lattice parameter a increases with the increase of Ti doping level x. But the specific saturation magnetizations, σs, gradually decrease with increasing Ti doping level x at 10 K and 300 K. It is interesting that when the doping level x≥0.2, two transition temperatures, TL and TN, are found: when the temperature is lower than TN, the magnetization is obviously decreased, while at the temperature TL, dσ/d T reaches a maximum value. This phenomenon indicates that an additional antiferromagnetic structure arises in the traditional spinel phase of ferrites which results from Ti doping, that Ti ions will show the form of Ti3+ and Ti2+ cations which have magnetic moments, and that the magnetic moments of the Ti cations are opposite to those of the Fe and Ni cations. The dependence of the magnetic moments of the samples on the Ti doping level x at 10 K was fitted successfully using the quantum-mechanical potential barrier model proposed earlier by our group. In the fitting process, the distributions of Ti, Ni and Fe cations in the samples are obtained. It is found that 80% of the Ti cations will occupy the [B] sites in Ti2+ form.
Long persistent and photo-stimulated luminescence properties of β-Sr2SiO4:Eu2+, La3+ phosphors
Cheng Shuai, Xu Xu-Hui, Wang Peng-Jiu, Qiu Jian-Bei
2015, 64 (1): 017802. doi: 10.7498/aps.64.017802
Abstract +
A series of β-Sr2SiO4: Eu2+, La3+ phosphors have been synthesized via high temperature solid state reaction. Photoluminescence (PL), long persistent luminescence (LPL), and the photo-stimulated luminescence (PSL) suggest that Eu2+ ions occupy Sr(1) and Sr(2) sites. Significant enhancement of PL, LPL and PSL has been observed by co-doping La3+ in β-Sr2SiO4: Eu2+, La3+. Meanwhile, the introduction of La3+ ions increases significantly the intensities of the TL bands (T1 and T3 regions), and promotes the formation of a large number of traps in LPL and PSL. In addition, the observation of the PSPL(photo-stimulated long persistent luminescence) phenomenon demonstrates the occurrence of electrons which are retrapped by the shallow traps in the LPL process.
Characteristics of charge transport in nano-sized TiO2 particles/submicron spheres multilayer thin-film electrode
Jiang Ling, Zhang Chang-Neng, Ding Yong, Mo Li-E, Huang Yang, Hu Lin-Hua, Dai Song-Yuan
2015, 64 (1): 017301. doi: 10.7498/aps.64.017301
Abstract +
In this work, we design the nano-sized TiO2 particles/submicron spheres multilayer structured photoanode, based on the fact of stronger light scattering properties of TiO2 submicron spheres. Effect of TiO2 submicron-spheres on the charge transport and interfacial properties in multilayer thin-film electrodes are investigated in detail using intensity-modulated photocurrent spectroscopy (IMPS), electrochemical impedance spectroscopy (EIS) and incident photon-to-current conversion efficiency (IPCE). Results obtained from IMPS for dye-sensitized solar cells (DSCs) indicate that submicron-spheres have fewer defects, but the poor contact at the interfaces between submicron spheres hinders the electron transport and makes the transit time longer. EIS results show that there are no obvious differences in interface recombination between the designed electrodes. It is interesting to find that the bottom section of the photoanode composed of nano-sized TiO2 thin film has a higher light utilization efficiency than that composed of submicron-spheres; meanwhile, the Fermi level of TiO2 and the photovoltaic properties of DSCs have been extended. Our results may provide an experiment basis for structure design of high-efficiency DSC photoanode.
Cellular automaton simulation of the molten pool of laser solid forming process
Wei Lei, Lin Xin, Wang Meng, Huang Wei-Dong
2015, 64 (1): 018103. doi: 10.7498/aps.64.018103
Abstract +
A two-dimensional multiscale model is presented for simulating laser melting process, which is the same as laser solid forming (LSF) without the addition of metallic powders into the molten pool. The metallurgical process in molten pool is simulated, including the temperature distribution, the pool shape, and the solidification microstructure. The shape of the molten pool and the microstructure in Fe-C single crystal substrate at a laser scanning speed of 15 mm/s is simulated. Results reveal the instability from planar interface to cell\ dendrite morphologies in the molten pool. At the bottom of the molten pool, the growth morphology is planar interface, which is also called bonding zone. Epitaxial dendrite arrays can be seen to grow above the bonding zone.
Molecular dynamics simulations of shock initiation of hexanitrohexaazaisowurtzitane/trinitrotoluene cocrystal
Liu Hai, Li Qi-Kai, He Yuan-Hang
2015, 64 (1): 018201. doi: 10.7498/aps.64.018201
Abstract +
Multiscale shock technique (MSST) has been shown to accurately reproduce the thermodynamic and chemical reaction paths throughout the shock wave fronts and reaction zone of shock initiation of energetic materials. A 1:1 cocrystal of hexanitrohexaazaisowurtzitane/trinitrotoluene (CL20/TNT) is shocked along the 110 orientations under the conditions of shock velocities lying in the range 610 kms-1 in ReaxFF molecular dynamics simulations. Products recognition analysis leads to reactions occurring with shock velocities of 7 kms-1 or stronger, and the shock initiation pressure is 24.56 GPa obtained from the conservation of Rankine-Hugoniot relation. Comparisons of the relationships are carried out between shock velocity and particle velocity, shock velocities and elastic-plastic transition. During shock initiation with the shock velocities lying in the range 78 kms-1, the shocked systems correspond to an elastic-plastic deformation, primary chemical reactions, and secondary chemical reactions. And the elastic-plastic transition coincides with the chemical reaction at higher shock velocity (9 kms-1), the cocrystal material response is over-driven, and all the thermodynamic properties show steep gradients, the compressed material by the shock wave steps into the plastic region, and a large number of carbon atoms appear in the early stage of over-driven shock initiation.
Molecular dynamics simulations on the growth of thin amorphous hydrogenated carbon films on diamond surface
Zhang Chuan-Guo, Yang Yong, Hao Ting, Zhang Ming
2015, 64 (1): 018102. doi: 10.7498/aps.64.018102
Abstract +
The growth of thin amorphous hydrogenated carbon films (a-C:H) on diamond (111) surface from the bombardment of CH2 radicals is studied using molecular dynamics simulations. Structural analysis shows that the local structure (e.g., the first coordination number of C atoms) of a-C:H depends critically on the content of hydrogen. The increase in kinetic energy of incident radicals leads to the decrease of hydrogen content, which subsequently changes the proportion of sp3 bonded C atoms in a-C:H.
Experimental study on a long pulse X-band coaxial multi-beam
Liu Zhen-Bang, Huang Hua, Jin Xiao, Yuan Huan, Ge Yi, He Hu, Lei Lu-Rong
2015, 64 (1): 018401. doi: 10.7498/aps.64.018401
Abstract +
An X-band coaxial multi-beam relativistic klystron amplifier is designed in order to increase the output microwave power and the operating frequency of the amplifier tube. Based on the results from PIC simulation, the experiment is performed on the long pulse accelerator. In the experiment, the transmittal and bunching of electron beam are analyzed, and then the amplifier is driven by a beam of 4.2 kA at 700 kV, and an external source injects microwave power of 60 kW into an input cavity, then 670 MW microwave power of 5 Hz repeating frequency is extracted, the amplifier gain is about 40 dB, the pulse width is 89 ns, and the conversion efficiency is 23%. The experiment proves that several tens kW-level input power can drive the X-band coaxial multi-beam relativistic klystron amplifier to generate several hundred MW-level long pulse output power.
Study on the electromagnetic properties of thin-film solar cell grown with graphene using FDFD method
Zhou Li, Wei Yuan, Huang Zhi-Xiang, Wu Xian-Liang
2015, 64 (1): 018101. doi: 10.7498/aps.64.018101
Abstract +
In recent years amorphous silicon solar cells have been receiving a great deal of interest due to their high energy conversion efficiency and low cost. The positions of absorption peak reflect the good absorption performance at specific frequency point or nearby spectra. However, the absorption peaks of amorphous silicon solar cell which are mainly determined by the properties of amorphous silicon and metal electrode, cannot be tuned. And the absorption efficiency can not be further enhanced also. Therefore, monolayer graphene film will be employed in the solar cells with periodic structure due to its remarkable electro-optic properties. With a suitable chemical potential applied, the dielectric constant of graphene can be tuned. This design mainly aims to tune the position of the absorption peak based on the graphene by using finite-difference frequency-domain method. Also, an approximate fitted function is developed in order to overcome the singularity in the exact expression. Numerical results show that the approximate closed form expression generates results within a maximum absolute error of 0.8%. Theoretical results provide the realistic organic thin-film solar cells with theoretical basis and technical support.
Increase in light extraction efficiency of vertical light emitting diodes by a photo-electro-chemical etching method
Gong Zhi-Na, Yun Feng, Ding Wen, Zhang Ye, Guo Mao-Feng, Liu Shuo, Huang Ya-Ping, Liu Hao, Wang Shuai, Feng Lun-Gang, Wang Jiang-Teng
2015, 64 (1): 018501. doi: 10.7498/aps.64.018501
Abstract +
The rate of photo-electro-chemical (PEC) etching on N-polar n-GaN using vertical light emitting diodes (V-LEDs) has been investigated in detail, by varying the etching parameters (etchant concentration, etching duration and light intensity). V-LED with optimal hexagonal pyramid structure (the side-wall angle is 31°) has been fabricated, and then the influence of the PEC etching on the electrical and optical properties of V-LED has been analyzed. After PEC etching, the sample has good ohmic contact with the electrode and has lower contact resistance than a reference sample. The electrical characteristics have a better improvement. And the light output power has improved obviously after PEC etching, which shows 86.1% enhancement at 20 mA. Effect of side-wall angle of the pyramids on light extraction efficiency (LEE) in V-LEDs is theoretically calculated by finite difference time domain (FDTD) method. Simulation results show that the LEE is significantly increased for the sidewall angle between 20° and 40°, and the maximum enhancement is realized at a side-wall angle of 23.6° (the total reflection angle at the GaN/air interface).
A method of adaptive pulse width modulation for multiple-valued data transmission
Sun Hua-Juan, Yan Xiao-Hong, Hao Xue-Yuan
2015, 64 (1): 018402. doi: 10.7498/aps.64.018402
Abstract +
The non-ideal characteristics such as dielectric loss and skin effect of coaxial cables etc can cause attenuation of the high-frequency component of the signals, making it difficult for the receiver to interpret the signal, thereby greatly reducing the data rate. Pulse width modulation (PWM) pre-emphasis technique, which has been very good to use in the binary signal, utilizes time-domain information processing to increase the data rate. We introduce the new adaptive PWM pre-emphasis method to further eliminate the data-dependent jitter, which can dynamically compensate for the transmission loss of multiple-valued data with strong or weak pre-emphasis. The results of eye diagrams show that this method can improve the quality of signal transmission.
Honeycomb architecture based mobile fault-tolerant recovery algorithm in WSANs
Du Jing-Lin, Zheng Ruo-Qin, Xie Li
2015, 64 (1): 018901. doi: 10.7498/aps.64.018901
Abstract +
This paper aims at finding an algorithm in wireless sensor and actor networks (WSANs) to recover the failure actor. First, this paper describes the real-time coverage model in WSANs, proves WSAN's coverage recovery is NP-hard. A cell-based mobile fault-tolerant algorithm HMFR is presented to recover the failure actor, which has a good performance under a limited condition of initial deployment of network. Through simulation experiments, the results show that the algorithm is more effective than the present algorithms in terms of actor coverage and move distance.
Evolutionary gambling dynamics for two growing complex networks
Xiang Hai-Tao, Liang Shi-Dong
2015, 64 (1): 018902. doi: 10.7498/aps.64.018902
Abstract +
The dynamic complex network is an important model of social structure and stability. Based on the single dynamic complex network, we propose a growing double-network evolutionary gambling model. When the two networks are separated, we find that the average of cooperation strategy has a jump as the payoff increases, which can be regarded as a phase transition. This result is a generalized result of static gambling network. When the two networks are connected, their averages of cooperation strategy are synchronized. When the intra-linkages are increased, the natural selection does not favor cooperation, while the fair selection does. When the inter-linkages are increased, the average of cooperation strategy decreases for both networks. As the ratio of inter- and intra- linkage is constant, the more the average degree, the less the cooperation. We find the existence of defection leader, and uncover its influence on the average of cooperation strategy and how it interacts with cooperation leader. These results provide some hints to understand the social structure, stability and evolution.
Randomness analysis of lane formation in pedestrian counter flow based on improved lattice gas model
Li Ming-Hua, Yuan Zhen-Zhou, Xu Yan, Tian Jun-Fang
2015, 64 (1): 018903. doi: 10.7498/aps.64.018903
Abstract +
In this paper, we extend a lattice gas model recently proposed by Li et al, which considers the view field of pedestrian. An improved lattice gas model takes into account the effect of pedestrians' walking preference feature of empty area in the view field to simulate traffic dynamics of pedestrian counter flow. Three dynamic evolution processes under different pedestrian density are reproduced. The randomness of lane formation for different pedestrian density is found, and the probability of lane formation is given. Numerical simulations of relationship diagrams between the probability of lane formation and parameters of the system geometry size, the probability and the proportion of right walker flow, the probability and the strength of the drift, also the probability and the view field size are investigated. Results show that the extended model cannot form for the lane formation under a low pedestrian density, which is associated with the real pedestrian traffic. It is found that the density of pedestrian counter flow could be divided into 5 intervals, and there are differences in the dynamic evolution processes between these 5 intervals. This model and its result is useful for the study of the dynamic evolution process, and is helpful for raising efficiency of pedestrian counter flow in the channel.
An adaptive global optimization algorithm of cooperative spectrum sensing with relay
Zhang Qian, Liu Guang-Bin, Yu Zhi-Yong, Guo Jin-Ku
2015, 64 (1): 018404. doi: 10.7498/aps.64.018404
Abstract +
In this paper is carried out an investigation that the performance of the cooperative spectrum sensing with relay may be interfered by redundant relays, the number of busy cognitive users (CUs) and the secondary relay(SR), detection threshold and transmission error of the channel, so that a global adaptive optimization algorithm is proposed. In this algorithm, based on the maximum interference-free power relay selection (MIFPRS), the set of secondary relay is defined adaptively; and in order to reach a maximum of the probability of detection, the single cognitive user minimizes the transmission error of the channel and selects the relay from the set of secondary relays. To achieve the maximum of the total channel throughput under the given probability of detection, ale ues the entire adaptive optimization algorithm. Numerical simulations demonstrate that the proposed algorithm has a high degree of channel transmission precision, and a large amount of channel throughput, so as to spend less frequency band.