Vol. 63, No. 13 (2014)
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2014, 63 (13): 134202. doi: 10.7498/aps.63.134202
In virtue of wave-front coding theory, a computer simulation analysis of the depth of field extension of negative refraction flat lens is carried out. The depth of field extension can be achieved by 6.7 times in the slab, when its index is -1, through computer simulation. When the refractive index of the tablet is slightly offset, taking -0.99 for example, 6.9 times of the depth of field extension can be achieved after the introduction of the calibration lens.
Study of electromagnetic scattering from two-dimensional rough sea surface based on improved Wen’s spectrum
2014, 63 (13): 134203. doi: 10.7498/aps.63.134203
By combining the improved Wen's spectrum with the classic Monte Carlo algorithm and Donelan's experimental conclusion, a two-dimensional sea surface model suitable for different depths and different stages of wave growth is proposed in this paper. Based on the classic two-scale method for the calculation of sea backscattering coefficient, and the introduction of the simulation results obtained by the SSA (small slope approximation) method, the electromagnetic scattering calculation method is modified. Compared with the fitting result based on the backscattering coefficient model of GIT, it proves that the improved Wen's spectrum is suitable for Chinese coastal waves, and the proposed electromagnetic scattering theory is accurate and efficient.
2014, 63 (13): 134204. doi: 10.7498/aps.63.134204
The multi-mode Dicke model, which describes many atoms interacting with the multi-mode photons, has attracted much attention; it not only exhibits rich quantum phases, but also has an important application in quantum information. In this paper, we explore the ground-state properties of the two-mode Dicke model by the Holstein-Primakoff transformation and Boson expansion method, and theoretically predict a new first-order quantum phase transition. In the experiment, this quantum phase transition could be detected by measuring the mean-photon number or the atom population.
High-power narrow linewidth fiber lasers are extensively applied in coherent detection and power-spectrum beam combination etc. The suppressing method of stimulated Brillouin scattering is analyzed. And the theory of sinusoidal phase modulation is analyzed also. A single frequency laser is broadened to become a 2.9 GHz linewidth seed by sinusoidal phase modulation technology. The power of the seed is then boosted from 50 mW to 780 W through a three-stage power amplifiber configuration. Central wavelength and linewidth of the laser are 1064.34 nm and 2.9 GHz respectively, with an optical-optical efficiency being 79%. And the beam quality is Mx2 =1.44 and My2 =1.43. The output powers before and after phase modulation are compared with each other. And the reason why output power increases is analyzed. The stimulated Brillouin scattering threshold is promoted by added longitudinal mode. Finally, the output power is promoted after phase modulation, so that the output power of this laser is only limited by the pump power. If the pump power is increased, the higher output power of narrow linewidth fiber laser will be achieved.
Experimental study on mode hopping triggered by transient characteristics of saturable absorber gratings in Er-doped fiber ring lasers
2014, 63 (13): 134206. doi: 10.7498/aps.63.134206
It is preferred to insert a saturable absorber grating in Er-doped fiber ring lasers for obtaining a stable single-longitudinal-mode laser operation; however, mode hopping is hardly avoided in various applications. A new mode hopping mechanism is found by utilizing the interferometric phase-demodulation method to transfer the optical frequency hops to the phase changes in real time. The regular mode hops triggered by fast cavity-length modulation are measured, and the characteristics and the origin of the mode hopping are obtained. Experimental results show that this kind of mode hopping, usually occurring between two neighboring longitudinal modes, may appear near the maximum slope of the modulation curve, and the laser frequency with shift about the space of the longitudinal modes before mode hopping. In addition, both the threshold frequency of the optical frequency modulation and the minimal frequency shift, which can triggered a mode hop, increase with the pump power at the same modulation amplitude. These experimental results can provide the stable operation condition if vibration or modulation exists, and they are helpful for optimal designing of the isolated assemblage or determining the operation range under the modulation condition.
2014, 63 (13): 134207. doi: 10.7498/aps.63.134207
The picosecond pulse from 1064 nm Yb-doped all-fiber mode-locked laser is amplified by Nd: YAG regenerative amplifier. Research is focused on the influence of Yb-doped all-fiber mode-locked laser on the efficiency of energy extraction of Nd: YAG regeneration amplifier. In order to increase the efficiency of energy extraction, spectral oscillatory fringe is decreased by means of restricting the self-phase modulation of Yb-doped all-fiber mode-locked laser. The Nd:YAG regenerative amplifier produces stable pulse energy of 1.3 mJ at a repetition rate of 1 kHz, which is seeded by a Yb-doped all-fiber mode-locked laser, with a low energy of 3.2 nJ, center wavelength of 1064.1 nm, 3 dB bandwidth of 0.35 nm 11 ps duration.
Semi-empirical analytical mode with highorder parameter of the biological tissue optical diffuse reflectance
2014, 63 (13): 134208. doi: 10.7498/aps.63.134208
Optical diffuse spectroscopy is crucial in non-invasive measurements and quick analysis of biological tissues. However, the lack of exact analytical solution limits its application. In this paper, a semi-empirical formula for diffuse reflectance is elaborated on the basis of the photon migration theory and the second-order similarity relation, and the influence of second-order parameter γ on the semi-empirical formula is studied by use of Monte Carlo method. Results show that the diffuse reflectance changes with γ nonlinearly when a small aperture detector is used to collected diffuse light. Finally, the proposed semi-empirical formula is mathematically simple compared with the other theoretical mode used at present, and offers a new theoretical and technical support in the measurement of the tissue optical parameters and the application of diffuse reflectance spectroscopy.
Experimental study on propagation properties of spherically aberrated beams through atmospheric turbulence
2014, 63 (13): 134209. doi: 10.7498/aps.63.134209
The propagation properties of spherically aberrated beams through atmospheric turbulence are studied experimentally, where the spherically aberrated beams are generated by a spatial light modulator (SLM), and the atmospheric turbulence is simulated by the rotary random phase plate. It is shown that both for the positive and negative-spherical aberrated beams, the intensity distribution is multi-annular in free space, but it becomes a Gaussian-like profile in turbulence. The positive spherical aberration results in a beam spreading, while the negative spherical aberration causes a beam focusing. The larger the positive spherical aberration, the worse the power in the bucket. However, the dependence of the negative spherical aberration on the power in the bucket is non-monotonic. In particular, the effect of spherical aberration on beam spreading decreases due to turbulence.
Photonic crystal fibers with high nonlinearity, large birefringence and multiple zero dispersion-wavelength
2014, 63 (13): 134210. doi: 10.7498/aps.63.134210
A novel kind of total internal reflection photonic crystal fiber (TIR-PCF) with highly nonlinear, large birefringence and multiple zero-dispersion wavelengths is designed. Characteristics such as birefringence, effective mode areas, nonlinearity and dispersion are investigated by finite element method (FEM). Numerical results demonstrate that the birefringence is 2.54×10-2 at the wavelength of 1.55 μm, and high nonlinear coefficients (50.22 W-1·km-1 and 54.61 W-1·km-1 in X, Y polarization directions respectively) are obtained by setting the appropriate structure parameters. In addition, two zero-dispersion wavelength points appear in the infrared band, one of which emerges near the wavelength of 1.55 μm. The design provides a new structure for large birefringence, highly nonlinear and photonic crystal fiber with multiple zero-dispersion wavelengths, and it could be widely used in polarization control, nonlinear optics, dispersion management and super-continuum generation.
2014, 63 (13): 134211. doi: 10.7498/aps.63.134211
Study on the bidirectional reflectance distribution function (BRDF) of seawater with spilt oil will provide a theoretical basis for detection of oil film on sea surface and oil emulsion in sea water, which will be of great significance for protection of marine environment and marine ecological balance. In this paper a model of BRDF for oil-polluted seawater has been developed by applying Monte Carlo method and Mie scattering theory. The BRDFs of clean seawater, oil-covered seawater, and oil-emulsion seawater are investigated at wavelengths of 532 nm and 355 nm. Simulation results of different film thicknesses and oil emulsion concentrations are presented. Results show that the BRDF is rather sensitive to the degree of pollution in seawater. The presence of oil film or oil emulsion will cause a decrease of the BRDF. At the same angles of observation, the BRDF value of seawater decreases with the increase of oil film thickness and oil emulsion concentration. Oil spill detecting and monitoring can potentially be achieved using BRDF data from optical sensors.
2014, 63 (13): 134301. doi: 10.7498/aps.63.134301
Based on the hypothesis that the wall of an elastic tube can be described as a membrane-type elastic structure, the coupled oscillation in a system of bubble clusters and local position of the elastic wall is explored, and the model of the nonlinear oscillation of bubbles is developed. According to the successive approximation method, the nonlinear resonance frequencies the and forced oscillation are analyzed theoretically. Results indicate that the resonance frequency of bubbles is mainly affected by the interaction of bubbles in clusters. Furthermore, there is a maximum frequency of ultrasound that will excite vibrations of the bubbles in clusters, and the response of multi-valued amplitudes exists in the region of high frequency.
2014, 63 (13): 134303. doi: 10.7498/aps.63.134303
The finite element method is introduced to investigate the absorption of a rubber slab (20 mm thick) with one layer of cylindrical cavities. It has a lower frequency absorption peak than that with spherical cavity in the same size, which is determined by the monopole resonance of cavities. Analysis of partial wave scattering verifies that the absorption peak is induced by the monopole resonance. Power dissipation density and displacement pattern of one unit cell is used to clarify intuitively the absorption mechanism of the cavity. Then the effect of damping of transverse modulus on the absorption is investigated under the condition of a steel backing. Both the physical and structural parameters of the rubber slab are optimized by a genetic algorithm for favorable absorption from 1.5 to 10 kHz.
Electromagnetic scattering characteristic of aluminous targets in the terahertz and far infrared region
2014, 63 (13): 134102. doi: 10.7498/aps.63.134102
In the terahertz and far infrared region, aluminum is in a state of transition from conductor to dielectric, and the research of the interaction between aluminous target and electromagnetic wave is meaningful for scattering prediction of targets. With the available error criterion model, dielectric function of aluminum is determined by fitting to experimental data in the terahertz and far infrared region. The transmitted parameters in aluminum are deduced by considering different loss mechanisms. Reflection and transmission characteristics on the interface of aluminum are investigated, and the reflection coefficients are given as a function of frequency. Results show that the transmitted parameters in aluminum keep their continuity and coherency from microwave to terahertz frequency. RCS (radar cross-section) results of aluminum plates computed by IBC method demonstrate that the increased wave impedance of aluminous targets has little impact on its backscattering, and the polished aluminous plate or sphere can still be treated as a perfect electrical conductor and used as a reference for RCS calibration.
Extraction of nonlinear characteristics from eddy current magnetic field of Al-alloy weld and their classification
2014, 63 (13): 134103. doi: 10.7498/aps.63.134103
GMR (giant magneto resistance) sensor can be used to measure the eddy current magnetic field of the Al-alloy weld. The nonlinear time series analysis method is introduced to study the signals from the eddy current magnetic field of the weld in three different states, which include good, lack of penetration, and clustered blowhole weld. Several nonlinear characteristic parameters, such as Lyapunov exponent and correlation dimension etc., are extracted. The results show that the eddy current magnetic field of the Al-alloy weld has distinguished chaotic features. Analysis of the Lempel-Ziv complexity and approximate entropy for different signals leads to the conclusion that these two complex measures are sensitive to the eddy current magnetic field of the weld zone. Through these methods, nonlinear characteristics of eddy current magnetic field of the weld can be obtained for identifying and classifying the type of the weld, which will serve as an efficient supplementary diagnostic tool to reveal different patterns of the welds.
2014, 63 (13): 134201. doi: 10.7498/aps.63.134201
Based on the optical transmission theory, the reason why front-surface particle contamination may induce the original damage of thin optical components is considered, and a damage mechanism is put forward: The localized thermal deformation of an optical element induced by the thermal effect of particle contamination together with the shading effect of it can disturb the laser beams. Simulated results show that for a high power laser, the localized thermal deformation of thin optical components, which disturbs the laser beam, is an important cause to produce strong light intensity modulations. The surface shape, phase delay, and thermal diffusion length of a localized thermal deformation are constantly changing with the increase of laser pulse shot number, so the highest light intensity modulation will be produced at different positions in the thickness direction or the xy direction on the rear-surface of an optical element. This not only can easily induce some damages on the rear-surface of the optical element, but also cause the interior damages scattered in the thickness direction.
Design and optimization of broadband polarization beam splitter made from a wave-structured multilayer film
2014, 63 (13): 134212. doi: 10.7498/aps.63.134212
Based on the 2D-PC wave multilayer film structure, a method to broaden the bandwidth of polarization beam splitter is proposed, which is composed of two different thickness periodic film stacks. Combined with the evaluation function of polarization splitting characteristic, the particle swarm optimization method is employed to design the optimal structural parameters. A broadband and compact polarization beam splitter is acquired, in which the center wavelength is 565 nm and its working range has achieved 220 nm with the average extinction ratio over 30 dB. In addition, by using the finite difference time domain method, the band structure and transmission spectrum of the wave-structure multilayer film are calculated, the angle sensitivity of the structure is investigated in detail. And we also study the electromagnetic field in the wavy-structure. Simulation results prove that the structure composed of the two different thickness periodic film stacks can avoid the discontinuity of bandgap, and PSO method can accelerate the convergence of the optimization algorithm and extend the bandwidth effectively.
2014, 63 (13): 134302. doi: 10.7498/aps.63.134302
Study on the precursor infrasound waves emitted before the occurrence of strong earthquakes has been performed, so as to discover the relationship among position, arriving time, intensity of the infrasound wave and earthquakes. With the special kind of infrasound microphone CASI-ICM-2011, a kind of infrasound waves with frequencies from 0.001 Hz to 0.01 Hz, peak sound pressure level from 50 Pa to 200 Pa, continuous time period from half hour to 4 hours, and propagation speed from 10 m/s to 30 m/s, arising no more than two weeks, was received before a series of earthquakes over Ms6.0 occur. Amplitude of the signal is higher when the earthquake is stronger. In the far field the sensor network was spread in north-east China with automatic data uploading to a central server computer in Beijing. Precursor infrasound waves emitted 4 days before Ms7.0 Lu-san earthquake have been positioned perfectly as a sound cloud map, also the infrasound wave emitted 12 days before Ms7.7 Pakistan earthquake has been positioned. A long-time continuous signal over 8 years has been analysed without a bit gap, showing several effective signals accompanying earthquakes. Law of the infrasound generation has been discussed with a suggestion for the mechanism that the infrasound could be radiated by a large scale surface vibration near one million square kilometers in earthquake developing. Two demonstrative signals received after Ms8.8 Chile earthquake and Yu-shu earthquake were provided to prove this suggestion. The detected signal shows that the infrasound wave arrives accompanying S wave at the same time. One model is provided as a piston sound source to simulate very low frequency infrasound radiated by large surface vibration. All the presented signals in this paper should be useful for precursor information obtained for close earthquake prediction.
2014, 63 (13): 134101. doi: 10.7498/aps.63.134101
Evaporation duct is a multipath environment on the sea, which tends to distort the signals, causes lower localization accuracy or even affects the normal work of radar when using the traditional localization algorithms. This paper presents a localization method in time reversal parabolic equation based on the radiation pattern loading. It could effectively handle the effects of evaporation duct and adaptively compensate the signal distortion, form time reversed waves matched with the propagation environment, and lead to a robust focusing and localization of the target. This method uses multipath effects flexibly to increase the effective aperture of antenna array for super resolution. In addition, it has a good tolerance in element spacing leading to a sparse array configuration which is more practical on the sea and widens its application fields. Simulation results show that the azimuth resolution in evaporation duct with the same aperture array has been improved 2 times more than in the free space by using this method; when 30λ is adopted as the element spacing, the sidelobe levels can be kept below -8.96 dB, so ghost images are effectively suppressed. The proposed method has strong robustness and high accuracy, thus may be useful in many practical applications, such as communication, search and rescue, pre-warning system on the sea, etc.
Underwater acoustic target classification and auditory feature identification based on dissimilarity evaluation
2014, 63 (13): 134304. doi: 10.7498/aps.63.134304
The purpose of this study is to explore perceptual classification of underwater acoustic targets and auditory features used by human being. First, we design a paired comparison experiment. Then we use the CLASCAL algorithm to model the dissimilarity ratings as a perceptual space, and analyze the properties in three common dimensions, specialties, 3 subjects' latent classes and their roles in target perceptual classification. Finally, based on the gammatone filterbank, we find some auditory features that can effectively underlie 3 common dimensions and beat properties, so as to use them to construct a binary decision tree to classify some new samples; thus we can provide some guidance about how to use these features in practical applications.
Constant-excitation caused response in a class of parametrically excited systems with two degrees of freedom
2014, 63 (13): 134501. doi: 10.7498/aps.63.134501
This paper focuses on the response in parametrically excited systems caused by constant excitation. Taking a maneuvering cracked rotor system as an example, we formulate the vibration equations with two degrees of freedom, in which the breathing of the crack constitutes parametric excitation, and the maneuver load of the maneuvering rotor is simplified as a constant excitation, and it is supposed that the rotor system is balanced without the consideration of eccentricity. By solving the equations with harmonic balance method, each order of harmonic components related with the rotating speed and the constant excitation is derived to analyze the corresponding resonance of the system. Results show that the constant excitation plays a decisive role in the parametrically excited primary and super-harmonic resonances of the system that agrees with the gravity dominance in common cracked rotor systems without maneuver load. And the stronger the constant excitation, the greater the resonances. Moreover, the orientation of the constant excitation makes a great impact on the parametrically excited primary resonance, but does not have a significant effect on the parametrically excited super-harmonic resonances. Results implies that constant excitation may increase the parametrically excited super-harmonic resonances of the cracked rotor systems, which is disadvantageous to the operating of the system. From another point of view, however, constant excitation can be used for early detection of crack faults in rotor systems.
Fine structures and the unsteadiness characteristics of supersonic flow over backward facing step via NPLS
2014, 63 (13): 134701. doi: 10.7498/aps.63.134701
In a low-noise supersonic wind tunnel at a Mach number 3.4, visualization of flow structure around backward facing step (BFS) with a 3 mm high step is carried out via schlieren and nano-tracer-based planar laser scattering (NPLS) respectively. The time-averaged flow characteristic of the reattachment region and the rich instantaneous structures of the redeveloping boundary layer are both revealed. By contrasting the NPLS images at different times, the unsteady evolvement characteristic of the coherent vortices in the redeveloping boundary layer is discussed. And the results are compared with the schlieren of Mach 4.2 and the prior data published. Results indicate that with either of the two flow visualization ways, the shock waves and the expansion waves can be captured; however, the NPLS technique has the obvious advantages to reveal the instantaneous structures on a small scale in a certain section plane with a time resolution of 6 ns and spatial resolution about micron magnitude; under the flow condition in this contribution, the growth rate of redeveloping boundary layer is 0.07519; the characteristic time is around 10 μs of the hairpin vortex shedding. At the same expansion rate, the reattachment occurs later with increasing Mach numbers, while if the expansion rate increases, the reattachment occurs earlier, and the flow turn angle is larger.
Velocity fluctuation and self diffusion character in a dense granular sheared flow studied by discrete element method
2014, 63 (13): 134502. doi: 10.7498/aps.63.134502
The distribution of average velocities, fluctuation of velocities, regional definition, and granular self-diffusion characters in dense granular flows between sheared parallel plates are discussed. In order to study the above problems, we use computer-established discrete element model with an average solid fraction of 0.80. Theoretical results show that the average velocities decrease with increasing height, and are larger for the case of lower plate with greater velocity; the average velocities in y direction are close to 0 because there is no bulk motion in y direction. Flows of the lower plate with a greater velocity induce relatively greater fluctuation of velocities in the x and y directions, the fluctuation of velocities increases with the height and is larger in the area close to the upper plate. The flows consist of a “solid-like” area in the lower test region, but a “fluid-like” region in the upper, and an “oscillating” region in the middle of the channel. By tracking the movements of granules continually, variations of the mean-square self-diffusion relative displacements with square time are plotted, and the mean self-diffusion relative coefficients are determined. As the fluctuation and self-diffusion analysis directly reflect the macroscopic properties of granules and provide bases and references for researching the flow mechanisms of “dense granular sheared flows”.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Study on the effect of X-ray diffractometer inherent angle scale error on the precision of lattice parameter calculation
2014, 63 (13): 136001. doi: 10.7498/aps.63.136001
This paper studies the effect of measuring angle error in X-ray powder diffractometer, which is caused by diffractometer inherent angle scale error resulting from mechanical manufacture, on the precision of calculated lattice parameter. It represents the theoretical limit of the consistency of the lattice parameters obtained by different diffractometers and laboratories. We use the calculated polysilicon diffraction patterns with random angle error to simulate the results measured by many sets of diffractometers of some manufacturing precision, then calculate and analyze the lattice parameters by three methods.
2014, 63 (13): 136101. doi: 10.7498/aps.63.136101
Based on the Nordholm's concept of Coulomb repulsive hole for plasma, a model of effective Coulomb potential is proposed to describe the charged fluids. Employing the classical density functional theory, the equilibrium structures of charged fluids confined in nano-cavities are calculated. Through the comparison with the numerical results, the effect of Coulomb correlation on the structure and excess adsorption is studied. In addition, the influence of Coulomb correlation on the structure is also calculated and studied under the condition of larger confinement. It is shown that the effective pair potential proposed here can be successfully used to predict the effects of Coulomb correlation on the structure and other physical chemical properties. Results obtained can provide some useful clues to the understanding of the correlation in other complex model potential system.
2014, 63 (13): 136501. doi: 10.7498/aps.63.136501
Many polyalcohols can change from one crystal structure to another in solid state at certain temperatures. A lot of papers reported their phase transition enthalpy, transition temperatures, and phase diagrams. This paper investigates the relation between transition enthalpy and hydrogen bond for the NPG (Neopentylglycol) and PE (Pentaerythritol) binary system on the basis of infrared spectrum experimental data and calorimetric results. It is shown that in the binary system some of the associated hydrogen bonds become weaker and are easier to break up. When temperature rises to a certain value, the hydrogen bonds, which retain in the course of phase transformation from the phase of NPG in PE to the plastic phase, will break up and form the second endothermal peak on the calorimetric curve. The number of retained hydrogen bonds is larger for the binary system NPG/PE with higher NPG concentration, and as a result, the corresponding enthalpy for the second endothermal peak is larger. While the influence of NPG on the retained hydrogen bonds is larger for binary system NPG/PE with higher NPG concentration, and the corresponding temperature of the second endothermal peak is lower.
2014, 63 (13): 136801. doi: 10.7498/aps.63.136801
In order to reveal the reason why mechanical properties of alloy films increase continuously after amorphizing, a series of Al-Zr alloy films with different Zr contents are prepared by magnetron co-sputtering of Al and Zr targets. The microstructure and mechanical properties of the films are characterized through a number of techniques, including X-ray energy dispersive spectroscopy, X-ray diffraction, transmission electron microscopy, and nanoindentation. Results show that the films with low Zr content form highly supersaturated solid solutions due to high dispersibility of vapor particles and non-equilibrium growth of the film in co-sputtering process. The film grains are refined to nanoscale particles due to dramatic lattice distortion and the film hardness increases rapidly. As Zr content increases, the film hardness increases continuously because of the increase of Al-Zr chemical bonds after amorphizing, and reaches a high value of 9.8 GPa at 33.3 at.% Zr. The research results reveal the effect of the Al-Zr chemical bonds on mechanical properties in amorphous films
Modification of surface morphology of a textured silicon substrate and its application in heterojunction solar cells
2014, 63 (13): 136802. doi: 10.7498/aps.63.136802
Texture on a mono-crystalline silicon substrate can effectively enhance the light trapping, and increase the short-circuit current of silicon hetero-junction solar cell. However, when the amorphous silicon layer deposited on the crystalline silicon substrate, the amorphous and crystalline epitaxial growth of mixed phases is created in the valleys with sharp edges, which leads to reducing the output characteristic parameters of the solar cell. In this paper, we use two methods to modify the microstructure of the textured substrate, and apply them to the silicon heterojunction solar cell. The textured silicon substrate is smoothed through the mixed acid, which makes the pyramid shape become round off and the open circuit voltage of the solar cell change from 564.6 mV to 609.4 mV. In addition, we use tetramethyl ammonium hydroxide (TMAH) instead of alkali solution. It is found that the textured substrate has an effective light trapping, and its micro-morphology is more rounded, the short-circuit current and open circuit voltage of the solar cell have been improved significantly.
2014, 63 (13): 136803. doi: 10.7498/aps.63.136803
To achieve nanoscale infrared photodetector electrodes with low resistivity, ion-implantation is used to implant high dose of As ion into high-resistivity silicon, and followed by rapid thermal annealing (RTA). A 200 nm thick Si:As layer with resistivity of 10-4 Ω · cm is obtained. Characterization by atomic force microscopy shows that the surfaces of the ion-implanted samples are smooth with a root-mean-square (RMS) coarseness of 0.5 nm. Although introduction of As ions destroys the lattice structure of crystalline silicon and causes a plenty of defects, proper annealing can restore the crystal lattice, as evidenced by the HRTEM observation of the annealed sample prepared by using focused ion beam (FIB) technology. Besides, the measurements of hall effect and spreading resistance indicate that the Si:As layer has good electrical properties including high carrier concentrations 2.5 × 1020 cm-3, high electron mobilities 40 cm2/V · s, and high electrical conductivities. The low resistivity Si:As material obtained is suitable to be used as the back electrodes of silicon-based optoelectronic devices.
2014, 63 (13): 136804. doi: 10.7498/aps.63.136804
Nanocrystalline silicon nc-Si:H/SiC:H multilayers were fabricated by thermal annealing of the hydrogenated amorphous Si α-Si:H/hydrogenated amorphous silicon carbide α-SiC:H stacked structures prepared by plasma enhanced chemical vapor deposition (PECVD) system at 900–1000℃. The microstructures of annealed samples were investigated by Raman scattering, cross-section transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy. Results demonstrate that the size of Si grains formed can be controlled by the α-Si:H layer thickness and annealing temperature. Optical absorption measurements show that the optical bandgap of the multilayered structures increases and the absorption coefficient decreases with diminishing Si grain size. However, the absorption coefficient and the optical bandgap of the multilayers are not influenced by the α-SiC:H layer thickness when the size of Si grains is kept constant.
2014, 63 (13): 136102. doi: 10.7498/aps.63.136102
Molecular dynamics simulation with a coarse grain model is performed to study the influence of single nanoparticle on the polymer crystallization behavior. By changing the mode of action of the polymer-nanoparticle (i.e. attraction or repulsion), the strength of the polymer-nanoparticle interactions, as well as the chain length of the polymer molecular, and by calculating the bond order parameter to characterize the influence in the cooling process, different effects of single nanoparticle on the polymer crystallization behavior are studied. This study has shown that the nanoparticle has no obvious effect on the whole polymer system composed of single nanoparticles. However, nanoparticles can promote the degree of order of polymer chains in crystallization process and enhance partially the polymer crystallization. Under the attraction and strong strength of the polymer-nanoparticle interaction, it is found that obviously the nanoparticle enhances the polymer crystallization partially. Furthermore, the chain length of the polymer molecular also shows some effect on the crystallization and the long-chain sample has a better enhancement for the polymer crystallization than the short-chain one under a strong attraction strength.
2014, 63 (13): 136103. doi: 10.7498/aps.63.136103
We have investigated the catalyst-free selective-area growth of GaAs and GaAs/InxGa1-xAs/GaAs (0x3. GaAs nanowire length would become longer by reducing the mask opening size. Thus we can form the GaAs nanowire uniform arrays with appropriate length and width by controling growth conditions and mask opening size. Then the photoluminescence measurement of GaAs/InxGa1-xAs/GaAs (0x<1) core-shell nanowires is carried out.
2014, 63 (13): 136502. doi: 10.7498/aps.63.136502
We have measured the temperature raised by laser irradiation on the basis of difference between Stokes and anti-Stokes Raman scattering cross-sections, and further estimated the thermal conductivity of the material. GeSbSe glasses with compositions of GexSb10Se90-x, GexSb15Se85-x, and GexSb20Se80-x are systematically studied with the aim of verifying the practicability of the new method and understanding the role of chemical composition in determining the structure and thermal conductivity of the glasses. All of the results are in agreement with those reported on thermal conductivity measured by different methods in the literature. It is indicated that Raman scattering method is convenient and efficient to measure thermal conductivity of the materials. For each group of glasses, it is found that the thermal conductivity increases with increasing Ge concentration up to a transition point corresponding to the glass with chemically stoichiometric composition. We ascribe the threshold behaviour of the thermal conductivity to the demixing of the structural units from glass network.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2014, 63 (13): 138801. doi: 10.7498/aps.63.138801
Integration of wind power into grids requires accurate forecasting, however, traditional wind power point forecast errors are unavoidable and they cannot be eliminated due to the highly volatile and uncertain in the chaotic time series of wind power. Unlike point prediction, which conveys no information about the prediction accuracy, probabilistic interval forecasts can provide a range, within which the target will lie with a certain probability, for estimating the potential impacts and risks facing the system operation. Most existing prediction interval (PI) construction methods are often placed after a deterministic forecasting model with or without prior assumptions, this paper propose a novel lower-upper bound estimation approach using extreme learning machine to directly construct PIs for wind power series. Based on the analysis of the interval forecasting error information in training dataset, a new problem formulation is developed in this method to get better PIs. In addition, in order to obtain the global optimal solution of the above model, a quantum bacterial foraging optimization algorithm is proposed by introducing the theory of quantum mechanics into bacteria foraging behavior. The testing results from two real wind farms with different confidence probability and optimization criterion demonstrate the excellent quality of PIs in terms of both reliability and sharpness, which provide a support for the steady operation of power system with wind power integration.
Study on electrical performance of metal copper films deposited by magnetron sputtering on polyimide flexible substrates
2014, 63 (13): 138101. doi: 10.7498/aps.63.138101
For preparing flexible copper clad laminate, copper films are deposited by magnetron sputtering on polyimide substrates. During the experiment, the prepared copper films show good conductivity while changing the technological parameter like preparation temperature, substrate bias, preparation time, and so on. The composition, structure, and surface morphology of the thin film are investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). The stylus profilometer and four-point probe resistance measuring instrument are used to examine the thickness and resistance of the thin films, and the resistivity of the film is calculated. Finally, the optimum processing conditions for the copper films are obtained according to the standard of industrial application: the preparation temperature is 100℃, the DC substrate bias is 50 V, with no pulsed substrate bias.
Study on the optimal design of frequency selective surfaces based on the discrete particle swarm optimization
2014, 63 (13): 138401. doi: 10.7498/aps.63.138401
The requirements of frequency-selective surface (FSS) between high transparency in pass band and high reflectance in stop band are contradictory, when they have loaded medium on one side and receive a large range of illumination. In order to solve the contradiction, this paper employs a discrete particle swarm optimization approach (hereafter referred to as a DPSO). In order to seek a balanced FSS with high transparency in pass band and high reflectance in stop band, the periodic intervals and geometrical dimensions of FSS-structures are optimized and designed by using the DPSO method. Simulation and test results indicate that the FSS of super dense Y loop elements in a half-loaded medium structure is presented in this paper: the transparency in pass band and stop band are 80% and 30% respectively. The DPSO method will offer an excellent FSS for the radome which receives a large range of illumination, and on the other hand, it provides a theoretical guidance for the requirements of FSS between high transparency in pass band and high reflectance in stop band.
Excessive spin-torque critical current has long been a problem received much attention. In this paper, we suggest that by introducing the out-of-plane stress or the stress anisotropy field, the out-of-plane demagnetizing field can be compensated effectively, and in this way the spin-torque critical current can be reduced. Specifically, the four-component distributed spin-circuit model is used to calculate the polarization current which is transferred from the polarizer to the detector (free layer).The properties of magnetization switching in the free layer of the lateral spin valve are studied under the influence of stress by using the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. Results show that, if the stress direction is appropriately selected, the out-of-plane demagnetizing field can be effectively compensated, thereby the spin torque critical current can be significantly reduced. Furthermore, as the stress is increased and the demagnetizing field is reduced, the magnetization reversal time is greatly reduced.
Random matrix theory is applied to study the correlation between different financial correlation coefficient matrices in the financial field. Correlation coefficient matrix is a key factor for constructing a network. In this paper we relate the random matrix theory to the network construction to study the financial networks model in terms of the random matrix. We select the stock data of Shanghai stock market, and divide them into four stages. We discuss the statistical properties of eigenvalues in financial correlation coefficient matrix and random matrix based on the random matrix theory, and improve the existing denoising method to construct the correlation coefficient matrix and to make it more suitable for building financial networks. After that we can build the financial network model. Then we analyze and compare the original financial network, the denoising financial network and the noise financial network in terms of the random matrix theory and the key node of networks. It is found that the primary important information is still in the original network, and the noise information corresponds to the information which the nodes of small degree in the original network include. Finally we analyze the topological structure of the financial networks, such as the minimum spanning tree, the motif and community structure. We also find that the topological properties of the improved financial networks are more remarkable and the topological structure is more compact.
Ordered subset image reconstruction studied by means of total variation minimization and fast first-order method in low dose computed tomograhpy
2014, 63 (13): 138701. doi: 10.7498/aps.63.138701
Low-dose computed tomography(CT) has an advantage to reduce X-rays that are harmful to the body. This paper considers the issue of reconstructing high-quality low-dose CT images from incomplete projection data. Generally, this can be done by statistical image reconstruction methods. However, the huge number of iterations of the statistical reconstruction algorithms leads to long computing time, making them difficult to be of practical value. To solve this problem, we propose a method to alleviate the issue by using total variation minimization and fast first-order method for the ordered subsets. We use Split Bregman alternating direction method to solve the optimization problem. Then, the projection onto convex sets method is used to speed up the convergence rate of the iterative method. Numerical experiments show that the relative reconstruction error of the proposed method can decrease faster than the first-order method of ordered subsets with the same iterative number.
2014, 63 (13): 130201. doi: 10.7498/aps.63.130201
A class of nonlinear evolution equation is considered by taking a simple and valid technique. Using the method of undetermined functions, firstly we introduce the solitary traveling wave solutions to the corresponding non-disturbed equation. And then the solitary wave solutions to the nonlinear disturbed dispersive equation are obtained using the generalized variational iteration method.
2014, 63 (13): 130301. doi: 10.7498/aps.63.130301
A concept of infrastructure quantum communication network is proposed, and an identification scheme for wireless communication networks is realized by combining classical certification and quantum teleportation.This identification scheme is discussed through the wireless LAN authentication and extended to the entire wireless communication network. In the wireless local area network, the information is transmitted between STA and AP who obtained the SK and EPR pair via quantum channel. Then AP will obtain the information through unitary transformation and calculate the fidelity with the original backup information, so as to determine whether the identification is successful or not.
2014, 63 (13): 130202. doi: 10.7498/aps.63.130202
To overcome the drawback of discrete particle model (DPM) and Euler-Euler two-fluid model (TFM) in solving gas-solid two phase flow, a new method called SPH-FVM coupled method is presented, and then it is used to simulate aerolian sand transport problems. Based on a pseudo fluid model the smoothed particle hydrodynamics (SPH) is used to solve the discrete particle phase by tracing the movement of each individual particle, and the finite volume method (FVM) is used to discretize the continuum flow field on the stationary mesh by capturing fluid characteristics. Two phases are coupled through contributions due to the effects of drag, pressure gradient and volume fraction, and then the coupled framework of SPH-FVM is established. The properties of SPH are redesigned to be suited for the discrete phase named SDPH. The relationship between SPH particles and discrete particles is illustrated, and the SPH discrete equations of pseudo fluid are derived. Saltation processes of sands in aerolian sand transport, sand movement under free-air wind, and creeping processes of dune, are simulated; while the particle trajectories, the distribution characteristics of mean downwind velocity, and the changes of gas velocity under the sand reaction are analyzed. Through comparison with experiments, it is shown that the accuracy of the new method is high, and it can also reduce the computational cost. This indicates that the new method can be applied to aerolian sand transport even to other gas-solid multiphase flows.
Effect of output capacitance time-constant on dynamic characteristics of V2-controlled buck converter
2014, 63 (13): 130501. doi: 10.7498/aps.63.130501
The second-order discrete iterative map model of V2-controlled Buck converter is established, based on which, the bifurcation diagrams with variation of output capacitance and its equivalent series resistance (ESR) are obtained, and the effect of output capacitance time-constant on the dynamic characteristics of V2-controlled Buck converter is investigated. It is found that with gradual reduction of output capacitance time-constant, the V2-controlled Buck converter shows the evolutive dynamic behavior from continuous conduction mode (CCM) period-1 to CCM period-2, CCM period-4, CCM period-8, CCM chaos, and discontinuous conduction mode (DCM) chaos. Jacobi matrix at a fixed point is also derived. According to this, the converter stability is analyzed by using characteristic values and maximum Lyapunov exponent, which validates the correctness of bifurcation analysis. Finally, the simulation and experimental circuits are set up, and the correctness of the theoretical analysis is verified by simulation and experimental results.
2014, 63 (13): 130503. doi: 10.7498/aps.63.130503
In order to be able to identify the hyper-chaotic l system with uncertain parameters effectively in real time, so that hyper-chaotic system control and synchronization tracking can be applied, this paper presents a system identification method for the hyper-chaotic system based on Wiener model. The linear part of the Wiener model consists of linear transversal filters, while the nonlinear part is represented approximately by piecewise linear filters. According to the minimum mean square error criterion, the filter parameter updated algorithm is derived, and the convergence condition is also obtained. Simulation results confirm the effectiveness of the adaptive filter for the identification of hyper-chaotic systems. The presented method not only overcomes the difficulty to identify a strongly nonlinear system only by adaptive linear filters, but also have a lower computational complexity compared with other non-linear adaptive filters.
2014, 63 (13): 130502. doi: 10.7498/aps.63.130502
A new method based on symbobic dynamics and relative entropy theory is proposed to examine the nonlinear behaviours of converters. Firstly, the discrete numerical sequence is obtained from iteration map, which is then transferred to a symbol-time series according to the topological conjugation, and the relative entropy is calculated by means of forward and backward probabilities. This paper takes a first one-order voltage feedback DCM Boost converter as an example, and the result shows that the relative entropy, which can measure quantitatively the distance apart from equilibrium when converter lies in a chaotic state, is a new and quantified nonlinear dynamic behaviours which has not been used in converters yet.
A local least square support vector machine prediction algorithm of small scale network traffic based on correlation analysis
2014, 63 (13): 130504. doi: 10.7498/aps.63.130504
Real-time monitoring and forecasting technology for network traffic has played an important role in network management. Effective network traffic prediction could analyze and solve problems before overload occurs, which significantly improves network availability. In this paper, after the vulnerability of traditional nonlinear prediction method in forecasting modeling is analyzed, the relevant local (RL) forecast which is based on correlation analysis and the parameter optimization method based on pattern search (PS) is introduced. Using the correlation analysis, the optimal training subset is chosen from time-and distance-correlated training samples. On this basis, the prediction model is established by LSSVM. Finally network traffic dataset collected from wired campus networks is studied for our experiments. And the results show that the relevant local LSSVM prediction method whose training set and parameters have been automatically optimized can effectively predict the small scale traffic measurement data, and RL-LSSVM traffic forecasting algorithm exhibits significantly good prediction accuracy for the data set compared with previous algorithm.
2014, 63 (13): 130701. doi: 10.7498/aps.63.130701
A novel maximum likelihood estimation model for time delay is constructed to estimate the passive time delay; the signal of emitter is unknown in this model. According to the model characteristics, the fast Fourier transformation (FFT) method is used to achieve time delay estimation (TDE). In order to improve the accuracy of estimator, the Markov Chain Monte Carlo (MCMC) sampling method is used to estimate the time delay value directly. Unlike traditional algorithms, MCMC method can obtain time delay without peak detection. Furthermore, the Cramer-Rao lower bound (CRLB) of this model is derived. Finally, simulations show that the proposed approach is suitable for both narrowband and broadband TDE, and the MCMC algorithm can achieve more precise time delay value with the same sample, and it has lower computational complexity than IS algorithm. The novel approach can estimate also the time delay of non-integer multiple of the sampling interval.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2014, 63 (13): 137303. doi: 10.7498/aps.63.137303
Electron transport properties of GaAs cluster, which is sandwiched between two semi-infinite Au(100)-3×3 electrodes in four different anchoring configurations (top-top, top-hollow, hollow-top, hollow-hollow), is investigated using the combination of density functional theory and non-equilibrium Green's function method. We optimize the geometry of junctions at different distances, simulate the breaking process of Au-GaAs-Au junctions, calculate the cohesion energy and conductance of the junctions as functions of distance dz, and obtain the most stable structure when the distances are set at 1.389 nm, 1.145 nm, 1.145 nm, 0.861 nm, respectively. For stable structures, the Ga-As bond lengths of the junctions is 0.222 nm, 0.235 nm, 0.227 nm, 0.235 nm, respectively. The equilibrium conductances are 2.33 G0, 1.20 G0, 1.90 G0, 1.69 G0,respectively. All junctions have large conductance. In the range of voltage from -1.2–1.2 V, the I-V curve of the junctions shows linear characteristics.
Finite-difference time domain method for the analysis of radar scattering characteristic of metal target coated with anisotropic ferrite
2014, 63 (13): 137501. doi: 10.7498/aps.63.137501
A metal cylinder coated with anisotropic ferrite material is analyzed using the finite-difference time-domain method (FDTD) in this paper, and the bistatic radar cross-section (RCS) is obtained. The new method of electromagnetic scattering by magnetized ferrite medium is analyzed in detail based on the FDTD. To exemplify the availability of the algorithm, bistatic RCS of a magnetized ferrite cylinder is computed, and the numerical results are the same as the shift operator FDTD (SO-FDTD), which shows that the proposed FDTD method is correct and efficient.
Study on the effect of In-2N co-doping at preferential locality on the photoelectric function of ZnO (GGA+U)
2014, 63 (13): 137201. doi: 10.7498/aps.63.137201
Nowadays although the study of In-N co-doping effect on the photoelectric function of ZnO is relatively common, all of the In-N co-doped ZnO are of random doping, and the preferential locality doping using the unpolarized structure of ZnO has not been considered so far. Therefore, in this paper, based on the density functional theory using first-principles plane-wave ultrasoft pseudopotential method, the un-doped and the In-N heavily co-doped Zn1-xInxO1-yNy (x= 0.0625, y=0.125) in different orientations have been set up, and band structures and density of states have been calculated respectively. The calculated results show that the In-N atoms along the c-axis orientation has the advantages of high stability over those in the vertical c-axis direction, the band gap is narrower, the effective mass is smaller, the mobility is greater, and the hole concentration is higher, so that the conductivity of ZnO is higher in the In-N heavily co-doped materials. We believe that these results may be helpful to the design and preparation of the conductivity of In-N heavily co-doped ZnO.
2014, 63 (13): 137301. doi: 10.7498/aps.63.137301
Equivalent circuit method is a principal one to analyze the active frequency selective surface (FSS). Extracting its lumped parameters is the key to the equivalent circuit method. We have constructed the transfer function based on the traditional equivalent circuit method and the transmission line theory. A matrix equation composed of lumped parameters is set up utilizing the relationship between the equivalent impedance and transmission peak. The equivalent lumped parameters are solved by the least square method, and the FSS frequency response curves are obtained from the transfer function. Compared with the full wave analysis method, the calculated results are in good agreement with that of simulation. Such results verify the accuracy and reliability of the method presented in this paper, and provide a theoretical reference to active FSS analysis using the equivalent circuit method.
2014, 63 (13): 137302. doi: 10.7498/aps.63.137302
Heterogeneous structure of a molecule semiconductor is the essential part of dye-sensitized solar cell, and the charge injection in it is the key factor of efficiency of solar energy conversion. A heterogeneous system is investigated where a metal nano-particle is used to decorate the structure of dye molecules and TiO2 semiconductor. Photoinduced charge injection dynamics from the molecule dye to TiO2 lattice is studied using density matrix theory. Simulations can account for the semiconductor lattice structure, the reflection of electron wave function in the lattice boundary, as well as the plasmon effect of the metal nano-particles. The compound treatment of density matrix theory and wave function approach is verified to be an efficient way for calculating the plasmon effect in the heterogeneous system. It is found that the plasmon enhancement due to the photoexcitation of metal nano-particles can reach as high as 3 orders of magnitude, which is shown to be an efficient way of improvement of charge conversion. The approach of density matrix theory and wave function treatment makes it possible to simulate the charge transfer in large-scale bulk semiconductor, the result of which is helpful for the theoretical analysis of plasmon enhancement in charge transfer dynamics.
Time domain theory of the electronic transport property of semiconductors measured by means of square-wave-modulated free carrier absorption technique
2014, 63 (13): 137801. doi: 10.7498/aps.63.137801
A time-domain model for the square-wave-modulated free carrier absorption (MFCA) is developed in measuring the electronic transport properties (the carrier lifetime, the carrier diffusivity, and the front surface recombination velocity) of semiconductor wafers. The dependences of time-domain MFCA signals on the electronic transport properties at different modulation frequencies are investigated by computer simulation. It is found that there are the high sensitivities of MFCA signal to the individual transport parameter. Furthermore, compared with the results by frequency-domain MFCA, in time-domain MFCA measuring ranges the transport properties can be improved.
2014, 63 (13): 137802. doi: 10.7498/aps.63.137802
Hot phosphor acid (H3 PO4) etching and/or SiOxNy surface passivation are used to change the surface states of high-resistance intrinsic GaN films. The films are investigated to reveal the influence of controlled surface states on photoluminescence (PL) emission. It is found that H3 PO4 etching cannot improve the ultraviolet (UV) PL emission obviously, but the PL spectrum in the range of visible light is considerably enhanced. After passivation with SiOxNy film, the quantum efficiency of UV PL is increased by a factor of 12-13. Meanwhile, the visible PL is significantly enhanced. By analyzing the PL spectra of the etched and passivated samples obtained at room temperature and low temperatures, we discuss the role of surface states in PL emission in the range of UV, blue and yellow bands, and the related physical mechanisms.
Wideband and large incident angle wave transparent material based on frequency selective surface with miniaturized elements
2014, 63 (13): 137803. doi: 10.7498/aps.63.137803
Different from the traditional frequency selective surface, the working frequency of a frequency selective surface with miniaturized elements (mFSS) is based on the intrinsic capacitance and inductance of its unit cell, but not the structure resonance. Focusing on the application of mFSS in wave transparent material (WTM), we have designed a band-pass WTM with unit cell of metal patch and wire, analyzed their intrinsic capacitance and inductance, and explored the parameters of the mFSS unit and its equivalent circuit on the performances of WTM. A bandpass WTM sample working at 10 GHz is designed and fabricated. Measurements demonstrate that the sample is wideband, insensitive to the incident angle, and polarization independent; the -1 dB bandwidth is over 40% at normal incidence and up to 20% even at large incident angles 60°. Experimental results are in good agreements with the calculations. The advantages of mFSS based WTM can expand its applications in microwave engineering, especially the radomes.
2014, 63 (13): 137701. doi: 10.7498/aps.63.137701
Zinc nitride (Zn3N2) thin films were deposited on glass substrates by reactive radio-frequency magnetron sputtering from a pure Zn target in nitrogen-argon ambient. X-ray diffraction analysis indicates that the films just after the deposition are polycrystalline with a preferred orientation of (400). With increasing substrate temperature, the grain size in zinc nitride film increases from 26.5 nm (100 ℃) to 33.6 nm (200 ℃), and then decreases to 17.8 nm (300℃). Atomic force microscopy reveals that the film surface morphology is dependent on the substrate temperature. With reflective spectroscopic ellipsometry, the ellipsometric parameters ψ and Δ of Zn3N2 films are measured. Then, a new model for Zn3N2 films is built. With the Tauc-Lorentz dispersion formula, the ellipsometric data are fitted, and both the thickness and optical constants (refractive index and extinction coefficient) of the films are obtained at a wavelength of 430–850 nm. The optical band gap is calculated from the extinction coefficient by using the Tauc formula, and a direct band gap of 1.73–1.79 eV is obtained.
ATOMIC AND MOLECULAR PHYSICS
2014, 63 (13): 133201. doi: 10.7498/aps.63.133201
The highly excited atoms have a strong Van der Waals interaction as compared with the ground-state atoms, which can block the excitation of neighboring atoms and forming the blockade effect. In this work, the interaction between highly excited atoms is calculated using the perturbation theory, the interaction of nS and nD pair states and the dispersion coefficients are obtained. This shows that the interactions of the SS for both Rb and Cs atoms are repulsive, whereas the interaction of DD states is repulsive for Cs and attractive for Rb.
2014, 63 (13): 133102. doi: 10.7498/aps.63.133102
Based on DFT-GGA calculations, we investigate systematically the structural, electronic and magnetic properties of ComAln (m+n ≤ 6) clusters. The calculated results show that the most stable structure of ComAln (m+n ≤ 6) clusters prefers to form the maximized number of Co–Al bonds, and is more similar to the most stable structure of pure cobalt clusters. With increasing Al atom numbers, the average magnetism of the clusters is reduced linearly. The magnetism of the ComAl (m=2–5) clusters is 4 μB smaller than that of Com+1 clusters, this agrees well with the recent Stern-Gerlach's experimental result of magnetism detection for a larger size of CoNAlM cluster. Reduction of the magnetism of ComAln clusters is mainly attributed to the non-magnetic Al element embeded and the weakening of spin polarization of the Co atoms.
2014, 63 (13): 133202. doi: 10.7498/aps.63.133202
A velocity-map-imaging (VMI) method is employed to investigate systematically the dynamical process of ejected electrons from autoionizing states of the Eu atom for the first time as far as we know. An atom is excited stepwise from the 4f76s6s8 S7/2 ground state to the 4f76s8s8 P7/2 Rydberg state via the 4f76s6p6 P5/2 intermediate state, then further excited to the 4f76p1/2(J=3)8s and 4f76p1/2(J=4)8s autoionizing states using the three-step isolated-core excitation method. According to the excitation pathways and selection rules, the value of total angular momentum of the autoionizing state can be calculated. The energy conservation and angular momentum parity conservation would enable us to determine the final states during the autoionizing process. The ejected electron, which decays from the autoionizing process, can be focused and imaged by the electron lens and the kinetic energy of it is resolved by the position sensitive detector. By combining velocity-map-imaging method with the mathematical transformation, the ejected electron energy distribution can be obtained, also the branching ratio is confirmed. Simultaneously, by tuning the wavelength of the third laser, the characteristic of the branching ratio following the variation of the photon energy, and the possibility of the population inversion have been discussed.
Microscopic characteristics mechanism of optical gas sensing material rutile titanium dioxide (110) surface adsorption of CO molecules
2014, 63 (13): 133101. doi: 10.7498/aps.63.133101
Using the optical gas sensing materials to adsorb gases can cause the changes of the optical properties of materials. This method can be used to measure the gas composition and is a hot topic of current research in the field of gas sensitive sensors. This paper studies the micro-characteristics of rutile TiO2 (110) surface adsorption of CO molecules. By using the first-principles plane-wave ultrasoft pseudopotential method based on the density functional theory (DFT), the adsorption energy, electron density of states, optical properties and charge density of the surface are calculated. Results show that the TiO2 (110) surface terminating in two coordinated O atoms is the most stable surface, and the structure with C-terminal of CO molecules adsorbed on the surface is the most stable. The higher the oxygen vacancy concentration, the more helpful it is to the adsorption of surface CO molecules. This process is exothermic. When the oxygen vacancy concentration is 33%, the adsorption energy can reach 1.319 eV. After adsorption, the structure of the surface tends to be more stable. Oxygen vacancy oxidizing the CO molecule is the essence of the adsorption process, and the charge of a CO molecule is transferred to the material surface. The CO molecules adsorbed on TiO2 (110) surface containing oxygen vacancies can improve its optical properties in visible light range; moreover, the higher the concentration of oxygen vacancy, the more obvious the improvement of absorption, reflection ability and optical gas sensing performance.
2014, 63 (13): 133203. doi: 10.7498/aps.63.133203
Low-frequency collective vibrational modes of biomolecules which often lie in terahertz (THz) band, make the terahertz time-domain spectroscopy (THz-TDS) an important technique for molecular identification and medicine quality inspection. Distinctive THz spectra between L-asparagine and its monohydrate were observed and the dehydration process of L-asparagine monohydrate was tracked by THz-TDS. Experiments indicate that THz wave is sensitive to phase transitions in crystals, dehydration process, and weak molecular interactions. Multi-techniques including differential scanning calorimetry and thermogravimetry, Fourier transform infrared spectroscopy, and powder X-ray diffraction are performed to investigate the thermodynamic properties, intermolecular and intramolecular vibrations, and molecular packing patterns of L-asparagine and its monohydrate. These measurements support the reliability of THz spectroscopy. To simulate and analyse the vibration modes of L-asparagine monohydrate, density functional theory calculations are performed using a Perdew Burke and Ernzerhof generalized gradient approach; the results agree well with the experimental observations.