Vol. 63, No. 23 (2014)
2014-12-05
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
EDITOR'S SUGGESTION
2014, 63 (23): 238501.
doi: 10.7498/aps.63.238501
Abstract +
Within the framework of k p perturbation theory, models of the hole quantization and conductivity effective mass for the inversion layer in uniaxially tensile/compressive and Si-based baixially strained p-channel metal-oxid-semiconductor (PMOS) have been established. Results show that: 1) uniaxially compressive technique should be chosen for the carrier mobility enhancement in uniaxially strained PMOS; 2) the magnitude of uniaxial stress will be less than that of the biaxial case to improve PMOS performance using strained technique; 3) strained Si1-xGex is preferred to use instead of using strained Si, when we choose the biaxially strained materials for the PMOS channel. Our results can provide valuable references to Si-based and other strained device and materials design.
2014, 63 (23): 238201.
doi: 10.7498/aps.63.238201
Abstract +
Frequency-dependent changes in the extracellular potassium ion concentration have been added to the Luo-Rudy phase I heart model. Effects of the delayed restoration of extracellular potassium ion concentration on spiral waves are studied. Numerical simulation results show that due to the state of spiral wave, the delayed restoration of extracellular potassium ion concentration can lead to periodic oscillations of concentration, and the period and amplitude of the oscillation increase with the delayed recovery time, resulting in the emergence of various phenomena, such as the breathing spiral wave, the coexistence of multiple spiral waves, the meandering of spiral waves in the manner of Lévy flight, and the disappearance of spiral wave through different ways. These results are compatible with the experimental results.
2014, 63 (23): 238202.
doi: 10.7498/aps.63.238202
Abstract +
We have addressed numerically and analytically the diffusion mechanism of Turing pattern formation for the chlorine-iodine-malonic-acid system with a local concentration depended diffusivity (LCDD) in a two-dimensional space as well as its reciprocal counterpart. It is found that the Turing pattern develops in a normal way of Fickian diffusion when LCDD is not involved, namely with a zero LCDD adjusting parameter. The formation of a Turing pattern evolves in an abnormal way of either subdiffusion or superdiffusion when a non-zero LCDD adjusting parameter is taken into account, and a negative or a positive value of LCDD parameter corresponds to a subdiffusion or a superdiffusion. The sensitivity of the steady amplitude of the activator concentration of a Turing system to a random initial condition increases with increasing LCDD parameter.
2014, 63 (23): 238401.
doi: 10.7498/aps.63.238401
Abstract +
In order to identify the DC-DC converter system behavior with different feedback coefficient k, we propose a method, which adopts the ideas of limit and the information about entropy to estimate the DC-DC converter nonlinear behavior by considering the characteristics that the stability of the system in a state of cycle and when the system is in chaos will not be repeated. This method analyses the entropy of the system in periodic and chaotic states and can quantify the period-doubling and chaos behaviors in DC-DC converters. In this paper, we simulate the first-order voltage feedback DCM Boost converter and DCM Buck converter. Results indicate that, according to the proposed information entropy, the bifurcation point, cycle number, and the location of the chaos can be accurately reflected. The above method improves the theory and method of the converter nonlinear dynamics analysis.
2014, 63 (23): 238402.
doi: 10.7498/aps.63.238402
Abstract +
We have investigated the pulse shortening problem of relativistic klystron amplifier. It is shown that non-working mode self-excitation is an important cause of pulse shortening. Theoretical and numerical analyses show the mechanism of non-working mode self-excitation caused by the coupling between cavities. By the use of radio frequency absorbers, the problem of non-working mode self-excitation can be solved. In the experiment, the RF output power is 920 MW, the output power pulse width of the device can be increased from 77 ns to 137 ns.
2014, 63 (23): 238101.
doi: 10.7498/aps.63.238101
Abstract +
Based on the nanocomposite structure and doping modification, we have studied the preparation technology of high performance nanocomposite thin film and its characterization methods. The W-doped VO2/ZnO nanocomposite thin films are prepared successfully on SiO2 substrates by the three-step method. The structure and morphology of the W-doped VO2/ZnO/SiO2 films are analyzed by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscope. Results show that the films are mainly composed of VO2 and high valence cation W6+ replacing the V ion in the W-doped VO2/ZnO/SiO2 films. It is found that the flake nanocrystallines resemble a flower in shape, and its size and orientational growth are reduced. The thermochromic properties of W-doped VO2/ZnO films are measured and compared with the single-layer W-doped VO2 films on SiO2 substrates with the same thickness. The variation of infrared transmittance of the W-doped VO2/ZnO/SiO2 nanocomposite film is increased nearly two times, the phase transition temperature reduced approximately to 39 °C, and the width of the thermal hysteresis loop is about 6 °C. The W-doped VO2/ZnO/SiO2 nanocomposite film has a high infrared modulation ability, a lower phase transition temperature, and a narrower thermal hysteresis loop. Thus the potential application of this nanocomposite film is significantly improved.
2014, 63 (23): 238301.
doi: 10.7498/aps.63.238301
Abstract +
A super-elliptical cylinder surface can shorten the distance between coils and target, enhance the space utilization, and enlarge the homogeneous imaging volumes. This paper proposes a method to design magnetic resonance imaging (MRI) gradient coils using the stream function and the developable property of the super-elliptical cylindrical surface. Based on the Biot-Savart law, the relationship between the magnetic flux density and stream function is established firstly, and the objective is chosen in the least-square form with the additional Tikhonov regularization term. Numerical accuracy of the magnetic flux density in the region of interest is maintained through transforming the cylindrical surface to the corresponding flat surface, and the value of regularization coefficient of the dissipated powers is chosen automatically by using the L-curve method. Via imposing specified boundary conditions to the stream function on the developed surface, the optimization of gradient coils is gained by directly solving well-posed linear algebraic equations. Numerical examples illustrate the feasibility of the proposed design method. The designed coils on the super-elliptical cylindrical surface show that the electric current and the dissipated powers are adequately optimized under the condition that the linear gradient deviation is less than 5%.
2014, 63 (23): 238403.
doi: 10.7498/aps.63.238403
Abstract +
A general equivalent model for slow-wave structure is established in this paper, if the unit cell of the periodic slow-wave structures is represented by a three-port network and the parameters of the equivalent model are determined by a high frequency structural simulator. In this method, there is no need to find a complicated equivalent circuit, and the fields in the tunnels can be calculated by the high frequency software directly. Therefore, the three-port network model is simpler than the analytic method and no more complex than the common equivalent circuit model. A one-dimensional nonlinear beam-wave interaction theory suitable for the folded-waveguide traveling wave tubes (TWTs) is accomplished based on the three-port network model. And a one-dimensional nonlinear beam-wave interaction code, which complies with the MATLAB language, is used to simulate a folded-waveguide TWT. The difference between the results obtained from the code and the experimental data is less than 10%. This theory can be used in the design of novel folded-waveguide TWTs and research on nonlinear simulations.
2014, 63 (23): 238404.
doi: 10.7498/aps.63.238404
Abstract +
To improve the current and output power of the THz traveling wave tube (TWT), a fundamental mode multi-beam folded waveguide (FMMBFW) TWT scheme is proposed. Firstly, an equivalent circuit model FMMBFW for calculating the high-frequency characteristic is established and compared with numerical simulation. Secondly, the transmission characteristic of 60 periods FMMBFW is analyzed. Finally, the beam-wave interaction characteristic of 0.14 THz FMMBFW TWT is completed by numerical simulation and theoretical calculation. When the DC current is 12 mA and the applied voltage is 15.75 kV, the 3 dB bandwidth of 0.14 THz FMMBFW TWT is 25 GHz (128-153 GHz), the maximum gain is 33.61 dB and the maximum output power is 23 W. When the DC current is 30 mA and the voltage is 15.75 kV, the maximum gain is 38 dB and the maximum pulse output power is 63.1 W at 0.14 THz. Compared with the fundamental single-beam folded waveguide (FW) TWT under the same working condition, the 3 dB bandwidth is doubled, its output power is raised by a factor of 9.66 and the interaction efficiency is increased by 3.22 times. Based on the same gain, the length of FMMBFW TWT is just 52.6 mm while the length of single beam FW-TWT is 78.2 mm. The proposed method can increase effectively the current of FMMBFW TWT; and the interaction gain, efficiency, 3 dB bandwidth, output power can be improved. When the gain is the same, a shorted and compact FMMBFW TWT can be constucuted.
2014, 63 (23): 238405.
doi: 10.7498/aps.63.238405
Abstract +
In this paper, in order to overcome the self-excited oscillation in the high-gain relativistic klystron amplifier, an X-band high-gain relativistic klystron amplifier (RKA) is designed driven by the relativistic beam with its current at a kA-level. The corresponding diode impedance is 800 Ω. Its output power reaches 284 MW at the frequency of 9.47GHz, and the gain and efficiency are 50.6 dB and of 37.4% respectively when the beam voltage is of 800 kV. And for the obtained GW-level RF power in low-impedance pulsed power sources, the power microwave combination technology of off-axis eight-tube high-gain RKA is used. In the three-dimensional model, the microwave output of this kind of RKA is almost constant when the device is at off-axis 54 mm. Simulation is based on the 4.5 T superconducting magnet which is 4.5 m long in the laboratory, and the eight-tube high-gain RKA is simulated using a magnet with the microwave output of 284 MW. Then an eight to one power combiner is designed using HFSS software, and the combiner with eight-tube high-gain RKAs is simulated by a three-dimensional model. Its output power is 1.84 GW, with a gain of 50.7 dB and efficiency of 28.8% respectively.
2014, 63 (23): 238406.
doi: 10.7498/aps.63.238406
Abstract +
When the size of target or the rotation angle is big, migration through resolution cell may occur in the bistatic inverse synthetic aperture radar (ISAR), which will affect the imaging quality. Aiming at the problem of migration through resolution cell of bistatic ISAR in the presence of time-varying bistatic angle, a correction algorithm is proposed in this paper. Firstly, the echo model of bistatic ISAR is built, and the mechanism of migration through resolution cell is analyzed. Migration through range cell may be corrected through the generalized Keystone transformation, and the effect of non-uniform rotation is eliminated at the same time. Then the rotating center is estimated according to the maximum criterion of image contrast, and the range bin is scaled absolutely. A phase compensation term is constructed and the correction of migration through Doppler cell is finished. Finally, the simulations are carried out and the results show that the method proposed in this paper can solve the problem of migration through resolution cell and improve the image quality.
2014, 63 (23): 238407.
doi: 10.7498/aps.63.238407
Abstract +
Using supplementary controller of static var compensator (SVC) is an effective way for enhancing the transient stability of an interconnected power system. In conventional controller design, SVC is usually considered as a first-order inertial model with known parameters. In this paper, a nonlinear controller design method based on adaptive sliding mode control is proposed to design a SVC supplementary controller. The imprecise of the model and the external disturbances such as time-delay are taking into consideration in the SVC model, and the interconnected system with SVC is considered by the center of inertia model. Then the SVC supplementary controller is designed based on the adaptive sliding control theory to improve the transient stability of the power system. Finally, the effectiveness of the proposed controller is verified using two simple systems. Simulation results show that the designed SVC sliding mode controller is robust to the variation of operation conditions and time-delays, and it has a better performance in enhancing the system stability as compared with the conventional SVC controller.
2014, 63 (23): 238502.
doi: 10.7498/aps.63.238502
Abstract +
Indirect bandgap Ge can be turned to a direct bandgap semiconductor by the alloy-modified technique, which can be applied to advanced photonic devices and electronic devices. Based on 8 bands Kronig-Penny Hamilton, this paper focuses on the physical parameters of direct bandgap Ge1-xSnx, such as conduction band effective density of states, valence band effective density of states and the intrinsic carrier concentration, and aims to provide valuable references for understanding the direct bandgap modified Ge materials and device physics as well as their applications. Results show that: conduction band effective density of states in direct bandgap Ge1-xSnx alloy decreases obviously with increasing Sn fraction, while its valence band effective density of states almost does not change with increasing Sn fraction. Compared with bulk Ge, the conduction band effective density of states and valence band effective density of states in direct bandgap Ge1-xSnx alloy are lower by two and one orders of magnitude respectively; the intrinsic carrier concentration in direct bandgap Ge1-xSnx alloy increases with increasing Sn fraction, and its value is an order of magnitude higher than that of bulk Ge.
2014, 63 (23): 238503.
doi: 10.7498/aps.63.238503
Abstract +
A kind of illumination calculation of array antenna composed of light emitting diode (LED) source based on Lambert radiation model is proposed. This paper puts emphasis on the optimal design for the transmitting antenna of indoor visible light communication. And it also analyzes the influence of the forms of the spatial distribution of the light source, the distance between light sources, and the angle between the center beam of light source and optical axis of the system, as well as the layer spacing of space distribution on illumination uniformity. A circular array antenna is superior to a rectangular array antenna containing the same number of light sources in illumination uniformity and stability of signal transmission, which may increase by 10%. As the distance between light sources and the angle between the center beams of light sources and optical axis keep increasing, the illumination uniformity of array antenna composed of LED source increases first and then starts to drop. As a result, there exist the optimal values in the distance and the angle respectively. Illumination uniformity increases with the decrease of the distance of space distribution. The optimal value of design parameter of transmitting antenna array in a common room has been given, therefore, it optimizes launch performance, saves as much as 13% the number of light source and reduces the cost. These studies for the design of the transmitting antenna system provide a theoretical basis and practical values.
GENERAL
2014, 63 (23): 230203.
doi: 10.7498/aps.63.230203
Abstract +
Synchronization control methods for a class of tele-operation system with stochastically time-varying communication delays are surveyed, aiming at the existing communication delays, packet loss, and so on, during the ground-based tele-operation of unmanned aerial vehicles. A control model for this class of master-slave tele-operation system with communication time delays is established. Then, the time delays within the remote control communication network are depicted by a time-varying-delay function, of which the modal parameters are transferred based on an ergodic Markov process. After that, the synchronization control problem of the system is solved by stability analysis of the mean square within the framework of the stochastic systems. Finally, the semi-physical simulation platforms of an unmanned micro-quadrotor are carried out to verify the methods presented in this paper; and the elementary simulation results show that the loci of the master-slave sides are all bounded through designing the controllers by the feedback linearization separately in the methods.
2014, 63 (23): 230204.
doi: 10.7498/aps.63.230204
Abstract +
Based on the standardized precipitation index data of 89 meteorological stations in southwest China (Sichuan Province, Yunnan Province, Guizhou Province, Chongqing) during 1961-2010, the probability and interval time were analyzed by employing the theory of run and the Copula function. The droughts with duration less than 4 months and the severity less than 3 account for 80% of all the droughts in southwest China. Drought types are mainly light within a month, moderate within a month, severe within a season and severe within inter-seasons. The frequency of occurring of drought with a duration less than 3 months and the severity less than 1 is higher in the eastern and western areas, and the central region has higher frequencies of occurrence of drought with a duration greater than 3 months and with severity greater than 1.5. Return periods of drought in eastern and western areas are longer than that of central region. More obvious spatial difference appears with longer duration and greater severity. The probability differences of the same drought type in different climatic types are within 0.05, indicating that there is no wide-range fluctuation in the occurrence probabilities of all the drought types as the time goes on.
2014, 63 (23): 230502.
doi: 10.7498/aps.63.230502
Abstract +
Based on the solution to the strictly piecewise linear state equation of a photovoltaic (PV) supplied micro-grid inverter, the nonlinear dynamic behavior of PV supplied micro-grid inverter is analyzed in this paper, and each PV supplied micro-grid inverter is considered as a network node, then a synchronous method for PV supplied micro-grid is studied using the small-world network model. Studies have found that, compared with the nearest-neighbor coupling network model, the PV supplied micro-grid based on the small-world network model has a faster synchronization time, and also has a faster recovery time in case of plus disturbance.
2014, 63 (23): 230205.
doi: 10.7498/aps.63.230205
Abstract +
Spatial fading correlation (SFC) mainly depends on power azimuth spectrum (PAS) of arrival signals and the transceiver mode of multi-antenna arrays. This paper investigates in depth the approximate algorithm and its complexity in SFC of multi-antenna arrays in a mobile communication system. First, we derive the closed-form formulas for SFCs under three typical PAS: i. e. a uniform distribution, a Gaussian distribution and a Laplace distribution. Based on these theoretical formulas, we study the approximate algorithm when the angle spread in PAS for arrival signals is small. From this, we develop a multi-antenna reception channel model and analyze in detail the impact of the antenna array and electric wave propagation parameters we choose on the capacity of multiple-input multiple-output (MIMO) channel. By using theoretical calculations and simulation experiments, we find that in a particular situation the approximate algorithm provides a good approximation for SFC. Furthermore, a method is used to quantify the applicability and calculation efficiency while analyzing the MIMO multi-antenna array. Finally, it can be concluded that the approximate method has a good approximation in particular situations, and it will greatly reduce the theoretical computational complexity. The method we suggest will improve the efficiency of analyzing and simulating a complex MIMO multi-antenna system.
2014, 63 (23): 230202.
doi: 10.7498/aps.63.230202
Abstract +
The (G'/G)-expansion method is further studied, the solution to the second-order nonlinear auxiliary equation is changed into solving of one unknown quadratic equation and Riccati equation by two function transformations. An infinite sequence solution of auxiliary equation is obtained with the help of Bcklund transformation of Riccati equation and nonlinear superposition formula of the solution. In this way, the infinite sequence solution to the nonlinear evolution equation can be obtained by the (G'/G)-expansion method, this method is an extension of existing methods, which can get more infinite series solutions. Take the (2+1)-dimensional Zakharov-Kuznetsov modified equal width equation as an example to obtain the new infinite sequence solution. This method can be used to get the infinite sequence solution to other nonlinear evolution equations.
2014, 63 (23): 230206.
doi: 10.7498/aps.63.230206
Abstract +
Heat transfer between particles and that between gas phase and particle phase in gas-particle two-phase flow cannot be ignored. Smoothed discrete particle hydrodynamics, as a new method for solving the gas-particle two-phase flow, has been used in simulating the aerolian sand transport successfully. Based on the smoothed discrete particle hydrodynamics method, a heat conduction model is presented in this paper and is used to simulate the heat transfer processes and the particle evaporation in gas-particle two-phase flow. Firstly, the equations to be solved are presented in which the energy equations are introduced for each phase and the second derivative item in conduction is treated by combining a standard smoothed particle hydrodynamics first derivative with a finite difference approximation of a first derivative. The heat conduction between particle and gas is computed from temperature difference and heat transfer coefficient. The disc-type particle cluster problem and bubble fluidized bed are simulated and the results are in close agreement with the two fluid model simulation results. The vaporization law for discrete phase droplet is used to deal with the particle evaporation and then a jet evaporation is simulated. Numerical results all show a good agreement with the discrete particle model results. It is indicated that the new method is of good accuracy and practical applicability.
2014, 63 (23): 230501.
doi: 10.7498/aps.63.230501
Abstract +
In order to improve the security of secure communication combined with the generalized dislocated projective synchronization and lag projective synchronization, a new generalized dislocated lag projective synchronization (GDLPS) is investigated. This paper takes the fractional order Chen system and L system as examples. for the parameters of the two systems are uncertain, based on the fractional stability theory and adaptive control method, the nonlinear controller and parameter update laws are designed for the GDLPS between the two chaotic systems with uncertain parameters. Under the controller and parameter update laws, GDLPS of the two uncertain parameters chaotic systems is achieved and all uncertain parameters of the drive system and response system are identified. Theoretical analyses and numerical simulation show that this method is feasible and effective.
2014, 63 (23): 230503.
doi: 10.7498/aps.63.230503
Abstract +
Physical significance of fractional damping for order 0 p 2 is demonstrated from the perspective that it can be explained as the memory of acceleration. Based on Caputo's fractional derivatives, the transport phenomenon of fractional overdamped deterministic motors in spatial symmetric potentials driven by biharmonic forces is investigated numerically. Relationships between transport velocity and model parameters are analyzed. The effect of fractional order is discussed in detail. Research shows that the contribution of historical acceleration increases or decreases monotonously with the historical moment varying with different fractional orders. With certain parameters the transport velocity can show generalized resonance when the spatial potential depth or the external force frequency varies. Furthermore, for some large orders, the velocity varies in step with the variation of potential depth and is in a direct proportional to the frequency if there is transport. Effect of fractional damping is intimately linked with the shape of the force. The memory of damping force can promote or inhibit the particle transport under different conditions, thus triggering abundant transport behaviors.
2014, 63 (23): 230701.
doi: 10.7498/aps.63.230701
Abstract +
Spherical conformal microstrip antenna array which is used widely in the field of aeronautics is analyzed by the full wave analysis in this paper. Using the surface integral equation with spherical dyadic Green's function can decrease the number of unknowns remarkably, and also reduce the demand of memory as compared with the method using volume-surface integral equation. The curvilinear triangle is proposed to mesh the surface of the microstrip antenna array, which can simulate the characteristic of spherical surface accurately. Firstly, the problem about probe feed model is solved successfully by introducing the half Rao-Wilton-Glisson function and boundary charge, and the image method is used to treat the line integral singularity problem. Then, the characteristic basis function method is employed to further save memory and computation time further by reducing the rank of impedance matrix. Finally, the input impedance and far-field spherical conformal microstrip antenna array of different sizes are analyzed. Results are in good agreement with those in the literature and simulation software, and thus the validity and effectiveness of the analysis method are demonstrated.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2014, 63 (23): 235201.
doi: 10.7498/aps.63.235201
Abstract +
An empirical formula of collision frequency has been used for years to calculate the collision frequency of aircraft plasma sheath. But the formula ignores the influences of electron-electron impact, electron-ion impact, and electro-magnetic (EM) wave driving effect on the collision frequency. To remedy these deficiencies, this paper proposes a segmentation calculation method. Based on the plasma kinetic theory, combined with real flow conditions and synthesizing the influences of electron-electron impact, electron-ion impact and EM wave driving effect together, this method defines a new parameter, ionization-to-thermal motion ratio, to calculate the collision frequency segmentally. Theoretical analysis and simulation results demonstrate that this method is closer to the truth than the empirical formula when ITR is greater than 5.
2014, 63 (23): 235202.
doi: 10.7498/aps.63.235202
Abstract +
Characteristics of helicon plasma in a 45 cm long discharge tube were diagnosed by the Langmuir electrostatic probe, optical emission spectroscopy (OES), and integrated capacitively coupled detector (ICCD). The discharge in helical wave mode was confirmed by the sharply variation of electron density and electron temperature based on the Langmuir data. We have noticed that the variation of electron density measured by the Langmuir electrostatic-probe is consistent with the OES measurement. Intensities in the spectra of argon atoms and ions are strongly related to discharge modes. The photos taken by ICCD can distinguish the discharge modes in the radial region. Intensity changes in the radial region reflect the electron motivation and the energy transfer path in the helicon plasma.
2014, 63 (23): 235203.
doi: 10.7498/aps.63.235203
Abstract +
Ablative Rayleigh-Taylor growth was measured with single-mode modulated planar CH foil with different ratio of Br dopant at Shenguang Ⅱ laser facility. Results show that CH (6% Br) sample first enters the nonlinear regime and has the largest perturbation amplitude of second harmonic. The reason is that the density gradient effects can suppress the generation of the second harmonic, the more the Br is doped, the smaller the density gradient scale length can be achieved. The density gradient effects also suppress the feedback of third-order harmonic to the fundamental mode, which induces the nonlinear saturation amplitude to exceed 0.1λ, as the classical prediction shows.
2014, 63 (23): 235204.
doi: 10.7498/aps.63.235204
Abstract +
A novel electromagnetic (EM) radiation regime, the plasma-based multistage virtual cathode radiation model is proposed. Recent study indicates that, as an electron beam passes through a high-dense ion background, due to the cooperation of the ion background and the modulation of the virtual cathode formed at the focusing point, a multistage virtual cathode can be formed. Further studies show that the electrons reflected at different stages of the virtual cathode will come into oscillation, and the EM radiation will be excited, which is different from the betatron emission in an ion channel. As an example, a beam-ion channel system bound in a cylindrical cavity is studied by using particle in cell simulation. It is confirmed that a multistage virtual cathode is formed and EM radiation is induced. Finally, the characteristics of the radiation are discussed in the present paper.
2014, 63 (23): 235205.
doi: 10.7498/aps.63.235205
Abstract +
A laser shadowgraphy system was built based on a Nd:YAG laser backlight with a half width of 9 ns and a wavelength of 532 nm. It has the capacity of time resolution and integration simultaneously during the laser lighting time by utilizing a streak camera and a commercial digital camera as the image recording devices. Experimental study of the insulated-ordinary mixed planar wire array Z pinches was carried out on the Qiangguang-I facility. Experimental results indicate that the expansion of corona plasma of the insulated tungsten wires is slower than that of ordinary wires over the ablation stage. Average velocities of the insulated wire and the ordinary wire located at the array outmost edge are 1.1×104 m·s-1 and 1.7×104 m·s-1 between t=44 ns and t=56 ns respectively. In the fastest implosion stage of 10 ns just before the stagnant time, the average imploding velocities were 5.5×105 m·s-1 and 9.3×105 m·s-1 respectively for the plasma on the insulted wires side and the ordinary wires side. The duration of stagnant stage on the insulated wires side is 5.9 ns, while it is 9.5 ns on the ordinary wires side. The collision boundary is deflected to the insulated wires side. A similar magneto Rayleigh-Taylor Instability structure can be observed on the both sides at the stagnant time.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2014, 63 (23): 237102.
doi: 10.7498/aps.63.237102
Abstract +
The atomic and electronic structures of a Cu grain boundary with segregated Zn have been calculated by the first-principles method based on density functional theory and the effect of Zn segregation on Cu grain boundary is also analyzed. Results show that Zn is segregated to the Cu grain boundary in the way of substitution. Both Cu and Zn have the similar bonding characteristic with their neighbors, which are metallic bonds with a little covalentlike component. The Cu grain boundary with segregated Zn has strengthened the cohesion across the boundary slightly as compared with the clean Cu grain boundary because a small amount of charge accumulation is found between Zn and near neighboring Cu atoms due to the segregation of Zn. Grain boundary with segregated Zn would be fractured between Zn and Cu atoms because the d orbit of Zn is much more localized during the tensile test, resultsing in the weakness of Zn–Cu bond.
2014, 63 (23): 237101.
doi: 10.7498/aps.63.237101
Abstract +
The conductivities of InI with different concentrations of Ge-doping have been investigated by the ultra-soft pseudopotential approach of the plane-wave based on the density functional theory under the same condition. Models of the In1-xGexI (x=0, 0.125, 0.25) with In atoms substituted by different fraction of Ge are set up. The optimized structural parameters, total electron density of states, and energy band structures of Ge heavily doped In1-xGexI semiconductors at low temperature are calculated. Results show that the volumes are slightly reduced and the total energies are increased in the In1-xGexI systems and that the systems become instable. As the concentration of Ge increases, the electronic mobility decreases, but the relative number of electrons jumping to the conduction band increases, and the resistivity and the minimum optical band gap increase at the same time, which is beneficial to improving the performance of nuclear detection in the system.
2014, 63 (23): 237103.
doi: 10.7498/aps.63.237103
Abstract +
Group Ⅱ-VI and Ⅲ-V highly mismatched alloys are promising material systems in the application of high efficiency intermediate-band solar cell (IBSC), however, the key issues including band engineering of intermediate band still remain challenging. In this study, ZnTe:O alloys have been produced by isoelectric oxygen implantation into ZnTe single crystal, and the influences of implantation on the microstructural and optical properties of ZnTe:O have been investigated in detail. It is found that a proper dose of oxygen ions can lead to a compressive strain in the lattice and induce the formation of intermediate band located on the energy level of ～ 0.45 eV below the conduction band. While a high dose of oxygen ions causes ZnTe surface layer to become amorphous and enhances the deep level emission around 1.6 eV, which is related to Zn vacancies. Results of resonant Raman and time-resolved photoluminescence spectra indicate that implantation induced intermediate band is related to the localized exciton emission bound to oxygen isoelectric trap, and the associated photo excited carriers have a relatively long decay time. This suggests that the reduction of lattice distortion and alloy disorder may be needed for converting localized states of the intermediate band into extended states, which is crucial to realize high efficiency ZnTe:O based IBSCs.
2014, 63 (23): 237301.
doi: 10.7498/aps.63.237301
Abstract +
Investigation of the stability and electronic properties of a series of MC compounds and classic MAX phases, Mn+1ACn (M=Sc, Ti, V, Cr, and Mn; A=Al, Si, P, and S; n=1, 2, and 3), contributes to finding the intrinsic mechanism of the stability of Mn+1ACn and to the design of new Mn+1ACn phases. First-principles calculations show that the formation enthalpy of both MC and Mn+1ACn is directly correlated with the charge transfer from M-3d to C-2s and 2p orbitals. Correspondingly, the early transition metals with high electron donation ability are able to form stable MC phases. Among the various MC phases, MC is found to be electron-deficient, which is thus favorable to react with electron-abundant MA to form Mn+1ACn. Therefore, M2AlC and M2SiC can be more readily separated into two-dimensional M2C structures, compared to M2PC and M2SC.
2014, 63 (23): 237302.
doi: 10.7498/aps.63.237302
Abstract +
In this work, the strained SiGe p-channel metal-oxide-semiconductor field-effect transistor (PMOSFET) with poly-Si1-xGex gate has been studied. Based on the analysis of vertical electric field and potential distribution, the equipment oxide thickness of strained SiGe PMOSFET with poly Si1-xGex gate is established. The mechanism and the influence of hot carriers induced are studied. A model of the drift of threshold voltage is established; its relationships with the duration of the applied electrical stress, the voltage of gate, the Ge content of the poly Si1-xGex gate and the strained SiGe are also obtained. Based on the above results, the simulation results have been compared with the experimental data. The drift of threshold voltage is 0.032 V under 10000 s electrical stress. A good agreement is observed, which indicates the validation of our proposed model.
2014, 63 (23): 237303.
doi: 10.7498/aps.63.237303
Abstract +
Within the framework of the effective mass and adiabatic approximation, the electron transport through an InAs/InP cylindrical quantum wire is studied by using the transfer matrix method. The coherent and escape tunneling processes are analyzed in detail. Influence of external voltage and structure size on the dwell time and escape time are discussed theoretically. A resonant phenomenon of the dwell time for different electron longitudinal energies is observed. A peak value of dwell time appearing at some positions of the bound state increases as the energy level decreases. When a bias is applied on this system along the growth direction, all the peaks of the dwell time shift towards the lower energy and become higher with increasing bias. Furthermore, it can be seen that the asymmetry of structure affects the dwell time obviously. Different results are obtained with the increase of asymmetry of the structure, which can be attributed to a competition between the transmission probabilities through the whole structure and that through a single barrier. Besides, the coherent and escape tunneling processes are also investigated by using a finite-difference method between two asymmetrically coupled quantum disks. It is found that the coherent electron remains oscillating in the two coupled disks. When the right barrier thickness of the nanowire is decreased, a roughly exponential decay of the oscillation charge trapped in both quantum disks is observed. The oscillating period is not affected by the right barrier thickness. However, a great influence of the middle barrier on the oscillation period can be found easily.
2014, 63 (23): 237304.
doi: 10.7498/aps.63.237304
Abstract +
An analytical model for drain current of high-k gate dielectric fully-depleted nanoscale germanium-on-insulator (GeOI) p-channel metal-oxide-semiconductor field-effect transistor (PMOSFET) is established by solving two-dimensional Poisson's equation to derive the surface potential and inversion charge in the channel region. This drain current model includes velocity-saturation, channel-length modulation and mobility-modulation effects; and it simultaneously considers the impacts of the interface-trapped charges at both gate oxide/channel and buried oxide/channel interfaces and the fixed oxide charges on the drain current. A good agreement between the simulated drain current and experimental data is achieved in both the saturation and non-saturation regions, confirming the validity of the model. Using the model, the influences of the main structural and physical parameters on transconductance, output conductance, cut-off frequency, and voltage gain of the device are investigated. These can be served as a guide for the design of the GeOI PMOSFET.
2014, 63 (23): 237305.
doi: 10.7498/aps.63.237305
Abstract +
To improve the breakdown voltage and reduce the specific on-resistance of a small size silicon on insulator (SOI) device, a dual-trench SOI high voltage device with an L-shaped source field plate is proposed. The device has the features as follows: first, a trench gate is adopted. The trench gate widens the current conduction area and makes the current conduction path shorter, thus lowering the specific on-resistance. Second, a SiO2 dielectric layer is introduced into the drift region. This dielectric layer can hold a high electric field, which makes the breakdown voltage greatly increased. Third, an L-shaped source field plate is introduced. This field plate modulates the electric field in the drift region, so increases the optimized doping concentration of the drift region significantly and reduces the specific on-resistance. The results from the two-dimensional semiconductor simulator show that as compared with a conventional SOI device at the same cell pitch, the breakdown voltage is increased by 151%, and the specific on-resistance is reduced by 20%. The specific on-resistance is reduced by 80% at the same breakdown voltage. Compared with a dual-trench SOI device with the same cell pitch, the proposed device maintains the same high breakdown voltage as the dual- trench SOI device, and at the same time, the specific on-resistance is decreased by 26%.
2014, 63 (23): 237306.
doi: 10.7498/aps.63.237306
Abstract +
With the development of sensors, a study on carbon nanotube composites (CNT) used as force sensing elements is presented in this paper, which consists of carbon nanotubes with polydimethylsiloxane (PDMS) as a matrix. Nanocomposites of carbon nanotube and polydimethylsiloxane, CNT-PDMS with different filler concentrations have been successfully prepared via ultrasonic and mixed method. With different density, the electrical characteristics change as a function of the strain. The piezo-resistance and piezo-capacitance properties of these composites have been studied in detail. In our experiment, the gauge factor has reached 40 for piezo-resistance and 70 for piezo-capacitance. It is shown that there is an effective and reliable way, which is to change the density of CNT-PDMS nanocomposites, to set the features to sensing strain and stress for resistance and capacitance of the composites. This nanomaterial has a decent potential in mechanical quantity sensors field.
2014, 63 (23): 237401.
doi: 10.7498/aps.63.237401
Abstract +
This report is focused on the MgB2 nanosheets which have a hexagonal single crystal structure with variable thickness and different radial dimensions. The nanosheets are fabricated by hybird physical-chemical vapor deposition for the first time, as far as we know, and meanwhile they can be site-specific transferred so as to make physical properties measurement. Results of electrical and magnetic measurements indicate that the nanosheets is superconductive with a Tconset=38 K, Tc (0)=33 K. The images of scanning electrical microscope show that the nanosheets have a nanoscale thickness and have not only a large scale in wildth from several microns to hundreds of microns but also a flat cleaning surface. The selected area electrical diffraction data is consistent with the early report of MgB2 diffraction. According to those results, the single crystal nanosheets with high quality can be surely indentified as MgB2. It suggests a new technique for MgB2 single crystal fabrication, and a zero electrical behavior is observed in nanoscale single crystal MgB2. This could be a new opportunity to make a right material for the afterward research such as flux vortices, nanoscale mechanical properties.
2014, 63 (23): 237501.
doi: 10.7498/aps.63.237501
Abstract +
Samples of (La0.8Eu0.2)4/3Sr5/3Mn2O7 were prepared by solid state reaction method. X-ray diffraction patterns indicated that the sample shows no any asymmetry and no any trace of secondary phase. The magnetization curve as a function of temperature (M-T), the magnetization versus magnetic field (M-H) at different temperatures, and the electron spin resonance spectrum have been detected. The magnetization measurement reveals that with lowing temperature, all of the samples undergo a complex magnetic transition. They transform from the two-dimensional short-range ferromagnetic order at TC2D ≈ 282 K, and enter the three-dimensional long-range ferromagnetic state at TC3D ≈ 259 K. Then they step into the antiferromagnetic state at TN ≈ 208 K and enter electric charge temperature order at TCO ≈ 35 K. The antimagnetic phase is found in the sample (La0.8Eu0.2)4/3Sr5/3Mn2O7below TN. When TC3D=370 K, the paramagnetic phase and antimagnetic phase co-exist. When TC3D is above 370 K, only paramagnetic phase exists in the sample. Besides, through electrical resistivity versus temperature curve ρ-T, the sample shows the maximum magnetization electrical resistivity when metal-insulator transition temperature is reached TP ≈ 80 K, TP being far from TC3D. And the transition shows the phenomenon of intrinsic magnetization electrical resistance, MR ≈ 61%. The resistance begins to increase below TCO. Because of the lowing temperature, the itinerant electron eg becomes increasingly spontaneously localized. One can see from the fitted ρ-T curves that (La0.8Eu0.2)4/3Sr5/3Mn2O7 in high temperature range is in accordance with the small polaron mode range hopping conduction.
2014, 63 (23): 237502.
doi: 10.7498/aps.63.237502
Abstract +
We have investigated the Fe/Cr doping effects on the magnetic property of a charge-ordered antiferromagnetic manganite La0.4Ca0.6MnO3. Magnetic measurements reveal interesting doping effects. While all the Fe-doped samples still have antiferromagnetic ground state, strong ferromagnetic tendency can be seen in the Cr-doped samples. Meanwhile, Cr-doped samples show clear metallic transport behavior, indicating an inherent double exchange mechanism responsible for the ferromagnetic metallic state. We thus propose that the magnetic exchange interaction between Mn and dopants, and the d-orbital electronic structure of Cr/Fe are essential for the distinct doping effect of Fe and Cr.
2014, 63 (23): 237801.
doi: 10.7498/aps.63.237801
Abstract +
Cylindrical vector beams (CVB) can exhibit a unique optical field distribution and focusing characteristic, due to the cylindrical symmetry in polarization. They are widely used in optical micro-manipulation, super-resolution imaging etc. and can be extended to subwavelength scale applications rapidly. Usually, the focusing CVB in subwavelength dimensions is realized by using plasmonic lens. However, this method is restricted by the state of polarization of electromagnetic waves. Nevertheless, when the negative refraction effect of photonic crystals is utilized, subwavelength focusing or imaging can be achieved in orthogonal states of polarization simultaneously. In this paper, the one-dimensional metallic photonic crystal (1D-MPC) with stronger manipulation ability is discussed. The calculated band structure and equi-frequency surfaces show negative refraction for both orthogonal states of polarization in a specific wavelength band. A cylindrical 1D-MPC plano-concave lens is designed to simultaneously focus radially and azimuthally polarized beams to subwavelength dimensions in visible spectrum. This phenomenon is simulated using the finite element method. Furthermore, variation of the polarization components in CVB can directly modulate the spacial intensity and the polarization distribution in the focal field. In fact, subwavelength focusing of CVB with arbitrary polarization components can be achieved by using the 1D-MPC plano-concave lens. The scheme proposed in this paper can be taken as reference for other wavelength bands as well. This study is also valuable for optical micro-manipulation of small particle, super-resolution imaging, and other related areas.
REVIEW
2014, 63 (23): 230201.
doi: 10.7498/aps.63.230201
Abstract +
In this paper, the author presents an overview on his own research works. In recent ten years, we extended the present static statistical information theory to dynamic processes and established a dynamic statistical information theory whose core is the dynamic information evolution equation describing the evolution law of dynamic information. Starting from the idea that the state variable probability density evolution equations of the stochastic dynamic system, the classical and quantum nonequilibrium statistical physical systems obeying stochastic law and the electrodynamic system obeying decterministic law can be regarded as their information symbol evolution equations and the definitions of dynamic information and dynamic entropy, we derived the evolution equations of dynamic information and dynamic entropy that express the evolution laws of dynamic information. These show that for the dynamic systems obeying a stochastic law, the time rate of change of dynamic information densities originates from their drift, diffusion and dissipation in state variable space inside the systems and coordinate space in the transmission processes, and that the time rate of change of dynamic entropy densities is caused by their drift, diffusion and production in state variable space inside the systems and coordinate space in the transmission processes. For the dynamic systems obeying the deterministic law, the evolution equations of dynamic information and dynamic entropy are the same mathematical type as the former except that dynamic information (entropy) density only has drift in state variable space inside the systems. Information and entropy have been connected with the state and change law of the system. Information diffusion and information dissipation occur at the same time. When the space noise can be neglected, information wave will appear. If we only consider the information change inside the systems, the dynamic information evolution equations reduce to information equations corresponding to the dynamic equations which express evolution laws for the above dynamic systems. This reveals that the evolution laws of the respective dynamic systems can be expressed by information equations in a unified fashion. Furthermore, we have presented the formulas for drift and diffusion information flow, information dissipation rate, and entropy production rate and a unified information expression for degradation and self-organizing evolution. Obtained the dynamic mutual information and dynamic channel capacity reflecting the dynamic dissipative character in transmission process, in when in the limiting case the ratio of channel length to signal transmission rate approaches zero, reduces itself to the present static mutual information and static channel capacity. All these new theoritical formulas and results are derived from the dynamic information evolution equation.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2014, 63 (23): 234101.
doi: 10.7498/aps.63.234101
Abstract +
Since electric components and printed circuit board in the enclosure can be destroyed by electromagnetic pulse weapons through “front door and back door” coupling, which is a great threat to the operational security, the study of the shielding effectiveness is of important significance. A formulation for shielding effectiveness analysis of a rectangular enclosure with an electrically large aperture is proposed in this paper. Firstly, the plane wave with oblique incidence and polarization is decomposed. Secondly, based on the Cohn model, the equivalent electric and magnetic dipole of the electrically large aperture is computed. Thirdly, the total Hertz electric and magnetic vector potential is obtained through mirror procedure. Finally, the electric field inside an enclosure with electrically large aperture is formulated, which is used for shielding effectiveness calculation. Five verification experiments are designed. Simulation result shows that the mean square error and absolute error of this method compared to computer simulation technology (CST) microwave studio are 11.565 dB and 8.015 dB respectively, the correlation coefficient is 0.921, through which the accuracy of this method is verified. The simulation time of this method is 0.183 s, which is only 1/7530 times of CST, so its efficiency is obvious.
2014, 63 (23): 234201.
doi: 10.7498/aps.63.234201
Abstract +
At present the visual tracking model based on sparse representation is mainly divided into two types: one is to use the template set to reconstruct candidate samples, which is called forward model; the other is to project the template set into a candidate space, which is called reverse model. What the two models have in common is to compute the sparse correlation coefficient matrix of candidate sample and template set. Based on this, the paper establishes a bidirectional cooperative sparse representation tracking model. Using L2-norm constraint item, the forward and reverse sparse correlation matrix coefficients could be uniformly convergent. In comparison to conventional unidirectional sparse tracking model, bidirectional sparse tracking model could fully excavate the sparse mapping relation of the whole candidate sample and template set. And the candidate that scores highest in the sparse mapping table for the positive and negative templates is the tracking result. Based on the accelerated proximal gradient fast method, the paper derives the optimum solution (in matrix form) of bidirectional sparse tracking model. As a result, it allows the candidates and templates to be calculated in parallel, which can improve the calculation efficiency to some extent. Numerical examples show that the proposed tracking algorithm has certain priority over against the conventional unidirectional sparse tracking methods.
2014, 63 (23): 234202.
doi: 10.7498/aps.63.234202
Abstract +
For the sensing applications based on the self-mixing interference technology of fiber laser, the self-mixing interference in the linear cavity fiber laser is theoretically analyzed through a four-mirror cavity model. The output power and frequency equation are deduced, and the behaviors of the laser under different optical feedback strengths are analyzed and simulated as well. The intensity of the laser is modulated by the length of the external cavity, and one fringe of the signal corresponds to the displacement of half wavelength of the target. Experimental setup is developed to validate the theoretical analysis. The obtained results provide both the theoretical and experimental basis for further studying self-mixing interferemetric sensing applications of fiber lasers.
EDITOR'S SUGGESTION
2014, 63 (23): 234203.
doi: 10.7498/aps.63.234203
Abstract +
The soft X-ray spectrograph is an important instrument for plasma diagnostics. As the core optical element of spectrograph, holographic flat-field grating is fabricated by aspheric wave-front recording optics, so grooves on the surface are curve. The curve grooves of the grating would affect the spectral image properties, thus influencing spectral resolutions. In the design of recording optics, only the groove density distribution on the surface in meridian line should be guaranteed, so optimized recording optics is not unique. Thus gratings with different curvatures of grooves but with expected groove density distribution could be obtained. For holographic flat-field gratings used in a 0.8-6 nm region, we analyze the influences of different curve grooves on the spectral image by ray tracing, and find that the almost straight grooves which are obtained by means of cylinder mirror can obtain the better spectral images. The theoretical results show that the spectral resolutions of grating with almost straight grooves are obviously improved compared with curve grooves, the theoretical spectral resolutions increase from 626 to 953 at 3 nm and from 635 to 1222 at 5 nm, respectively.
EDITOR'S SUGGESTION
2014, 63 (23): 234601.
doi: 10.7498/aps.63.234601
Abstract +
The adhesive contact processes between a rigid spherical tip and substrates with nanogrooves of different sizes have been investigated with a large-scale molecular dynamics simulation method. Influences of the surface grooves on the load-displacement curves, the attractive forces in the loading/unloading processes, and material transfer have been discussed. Results show that compared with the contact between a tip and a smooth surface, the attractive force range becomes larger in the loading process, accompanied by several jumps of the load, and the maximum attractive forces both in the loading and unloading processes are smaller. When the groove depths are the same, the maximum attractive forces in the loading and unloading processes decrease gradually with the increase of the groove width. However, when the groove width becomes close to the contact diameter between the tip and the smooth surface, the maximum attractive force would increase slowly, tending to be close to the case of smooth surface. When the groove width is kept the same, the maximum attractive force in the loading process decreases with the increase of the groove depth, while the maximum attractive force in the unloading process is almost unchanged.
ATOMIC AND MOLECULAR PHYSICS
2014, 63 (23): 233301.
doi: 10.7498/aps.63.233301
Abstract +
In a coherent anti-Stokes Raman scattering (CARS) microscope, when samples with different shapes and dimensions are excitated by collinearly introduced and tightly focused Gaussian beams, the microscopic structure will be determined by the spatial distributions of generated CARS signals. Therefore, we build a theoretical model for CARS signals from spherical sample under the tightly focused condition. The intensity and phase distributions of tightly focused linear polarization Gaussian beams are analyzed with vector wave equations. The vector wave equation of CARS signals is derived from Green's function. The far-field CARS radiation patterns of spherical scatters with different diameters are simulatively calculated. Theoretical analysis and simulative calculation results show that the intensities of forward and backward CARS signals from the small spherical sampler are similar. The images with high contrast can be obtained by backward detection method from an objective with a high numerical aperture. For big spherical samplers, intensities of CARS signals are greatly increased. The emission direction is mainly concentrated in a spatial angle. The forward CARS signals can be effectively collected by an objective with low numerical aperture.
2014, 63 (23): 233104.
doi: 10.7498/aps.63.233104
Abstract +
The lower energy structures of Al2O3H3 molecular clusters are optimized through DFT/B3LYP connected with 6-311g++(d, p) all electrons basis set. It is found that the ground state configuration of Al2O3H3 has 1A' electronic state and Cs symmetry. Based on the research on energy, heat capacity at constant volume, entropy of Al2O3M3 and M2 (M=H, D, T), the hydrogen isotope effects of reactions between Al2O3 and hydrogen (deuterium or tritium) gas are studied by means of the solid electron-vibration approximation. In addition, the changes of enthalpy, entropy and Gibbs free energy, and the relation between equilibrium pressures and temperatures are presented. The investigation suggests that hydrogen can be replaced by deuterium, and deuterium can be replaced by tritium in the reactions between Al2O3 and M2 with the productions of solid Al2O3M3 (M=H, D, T). This replacement sequence is opposite to the metallic isotope effect e. g. for titanium, however these replacement effects are very weak, and they are weaker and weaker as the temperature increases.
2014, 63 (23): 233101.
doi: 10.7498/aps.63.233101
Abstract +
The ab initio configuration interaction method and coupled-cluster theory have been used to optimize the possible ground-state structures of SiF2. The method QCISD/6-311G(2df) is most suitable for the calculation of SiF2 by comparing the experimental value and the calculated value; and the calculated equilibrium structure, harmonic frequency, dissociation energy, force-constant are in good agreement with the experimental data. The potential energy functions of SiF2 have been derived from the many-body expansion theory. In the symmetry of stretching vibration and rotation potential energy diagram of SiF2, there is a saddle point in the reaction kinetics SiF+F→SiF2. A stable SiF2 molecule could be formed when F atom with an energy surpassing 4.38 eV. These are completely reported so far as we know for the first time. In addition, it is found that a stable SiF2 molecule could be formed through two equivalent channels of SiF+F→SiF2. And the reaction is endothermic with a threshold energy.
2014, 63 (23): 233102.
doi: 10.7498/aps.63.233102
Abstract +
Structures and stabilities of Ti2Bn (n=1-10) clusters have been systematically investigated by using the density-functional theory B3LYP method and ab initio CCSD(T) method. It is found that the ground state structures of the Bn clusters are substantially modified by doping two Ti atoms. Ti2Bn clusters have very clear growth patterns, namely to form bipyramid. All the most stable Ti2Bn can be visualized as bipyramids with the two Ti atoms located at the two apexes. Ti2B6, Ti2B7 and Ti2B8 are confirmed to be the magic number clusters based on the analysis of the second-order difference of energies. The dissociation energies, vertical ionization potentials and vertical electron affinities of Ti2Bn isomers are discussed. Ti2B6 cluster is found to be stable both kinetically and thermodynamically.
2014, 63 (23): 233103.
doi: 10.7498/aps.63.233103
Abstract +
In this paper phase stability and functional properties of Pd-doped NiTi with different Pd concentrations (Ni24-nPdnTi24, n=2, 3, 4, 5, 6, 9, 12; CPd=4.2 at.%, 6.3 at.%, 8.4 at.%, 10.4 at.%, 12.5 at.%, 18.8 at.% and 25 at.%) are calculated by first-principles method. Results show that B19' is the most stable when CPd is less than 10.4 at.%, whereas B19 is the most stable for CPd is equal to or larger than 10.4 at.%. The formation energy decreases with increasing Pd concentration. With increasing CPd, the energy difference between austenite and martensite decreases slightly and increases for CPdCPdCPd=10.4 at.%, which agrees well with the experimental results.
2014, 63 (23): 233201.
doi: 10.7498/aps.63.233201
Abstract +
Photodetachment of hydrogen negative ion near a dielectric sphere has been studied by using the image method combined with the semiclassical closed orbit theory. Firstly, we analyze the image charge distribution of the detached electron near the dielectric sphere; then we put forward the Hamiltonian for this system. By solving the Hamiltonian canonical equations, we can find the closed orbits of the detached electrons moving near the dielectric sphere. With the help of the semiclassical closed orbit theory, we derive the formula for calculating the photodetachment cross section of this system. Then we can calculate and analyze the photodetachment cross section. Calculated results suggest that the photodetachment cross section of the hydrogen negative ion near a dielectric sphere is not only related to the photon energy, but also the dielectric constant of the sphere. For a given dielectric sphere, with the increase of photon energy, the oscillating amplitude in the photodetachment cross section decreases while the oscillation frequency increases. When the photon energy is increased to a critical value, the oscillating structures in the cross section disappear. In addition, with the increase in the dielectric constant of the dielectric sphere, the oscillating structure in the photodetachment cross section becomes much more complicated. When the dielectric constant is increased to infinity, the photodetachment cross section of this system is consistent with the photodetachment cross section of the hydrogen negative ion near a metal sphere. Therefore, we can control the photodetachment cross section of the hydrogen negative ion near a dielectric sphere by changing the photon energy and the dielectric constant. Our study may provide some theoretical guidance and reference values for the experimental research of photodetachment of negative ion near the dielectric sphere.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2014, 63 (23): 236101.
doi: 10.7498/aps.63.236101
Abstract +
Zirconium (Zr) has a hexagonal close-packed crystal structure, which exhibits elastic and plastic anisotropy. Internal stresses can be easily generated in the rolling process and the subsequent plastic deformation process. It is critical to evaluate the internal stresses and the deformation mechanisms of Zr alloy materials. The deformation behaviors of Zr alloy influence directly its service life and safety. In this work, compression deformation behaviors of zircaloy-4 (Zr-4) alloy have been studied by the in situ neutron diffraction technique combined with the elastic-plastic self-consistent (EPSC) simulation. A compressive external load is applied along the thickness direction of the rolled plate, which is called through-thickness compression. Electron back-scattered diffraction is used to analyze the texture evolution during the plastic deformation. Transmission electron microscopy (TEM) is used to measure the distribution of the defects in the deformed sample. The EPSC simulation provides the deformation mechanism quantitatively by fitting the in situ neutron diffraction data, and the simulated results is confirmed by the TEM observations. Results show that when the true strain is small (less than 0.55%), prismatic {1010}20> (a> type) slip dominates; however, as the plastic strain is increased, the percentage of pyramidal {1011}23> (c+a> type) slip becomes larger than that of prismatic {1010}20> (a> type) slip, and the pyramidal {1011}20> (a> type) slip and pyramidal {1012}20> (a> type) slip may exist.
2014, 63 (23): 236102.
doi: 10.7498/aps.63.236102
Abstract +
In this work, the carrier microscopic transport process of biaxial strained Si p-channel metal-oxide-semiconductor field-effect transistor (PMOSFET) under γ -ray radiation has been studied. Effect of γ-ray on devices and the relationship between the variation of device electrical characteristics and the total dose are investigated. A model for considering the degradation of threshold voltage and transconductance due to the total dose radiation is established. Based on this model, numerical simulation has been carried out. Results show that the threshold voltage of PMOSFET decreases with increasing radiation dose. At a lower total dose, the threshold voltage decreases linearly. However, at a higher total dose, it becomes saturated. The degradation can be explained by the generation of trapped charges which increase the impact possibility of carriers in the channel and induce the reduction of mobility and transconductance accordingly. Finally, the simulation results are compared with the experimental data. A good agreement is observed, indicating the validation of our proposed model.
2014, 63 (23): 236601.
doi: 10.7498/aps.63.236601
Abstract +
Sierpinski carpet fractal structure is introduced into the construction of Si/Ge nanocomposites in this paper so as to regulate and control the thermal conductivity of the nanocomposites. Non-equilibrium molecular dynamics simulation is applied to investigate the thermal conduction performance of nanocomposites embedded with fractal structure. Effects of the silicon atom percent, axial length and cross-sectional dimensions on the thermal conductivity of nanocomposites embedded with fractal structure are analyzed and compared with the corresponding nanocomposites embedded with traditional rectangular structure. It is indicated that, owing to the enhanced scattering at the Si/Ge interfaces of nanocomposites embedded with fractal structure, their thermal conductivity are lower than that embedded with rectangular structure, thus providing an effective way to improve the thermoelectric efficiency. And it is also demonstrated that the thermal conductivity of nanocomposites embedded with fractal structure are affected by the silicon atoms percent, axial length and cross-sectional size. The thermal conductivity is first decreased and then increased with the increase of Si atom percent. In addition, the increase in axial length of nanocomposites may lead to the enhancement of thermal conduction.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2014, 63 (23): 239201.
doi: 10.7498/aps.63.239201
Abstract +
Using the precipitation data given by high-density regional automated weather stations during main flood season in Hunan Province, in 2013, Cressman algorithm is introduced to calculate the regional precipitation value in 0.01°×0.01° resolution and is compared with the methods of a reciprocal of distance square, distance exponent, and distance square exponent. It is shown that the precipitation construction of Cressman algorithm is finer and closer to the features of gage data than other methods. Precipitation accumulation amount given by Cressman algorithm is in agreement with those obtained using methods of reciprocal of distance square and range exponent as well. Cross-validation tests show that the number with the absolute error less than 0.5 mm accounts for 98.71% of total gage number. The standard deviation of error is 0.24 mm, which is the least of all methods. As to 0.01°×0.01° resolution, the regional precipitation calculated by Cressman algorithm is adaptable to validate the radar estimation rainfall.
2014, 63 (23): 239202.
doi: 10.7498/aps.63.239202
Abstract +
Seasonal evolution of atmospheric temperature with winter-summer-the following winter (winter-to-winter) characteristics in the northern area of East Asia (40–50°N, 100–130°E) from 1953 to 2012 has been investigated in this paper. Results indicate that: from 2008 on, the atmospheric temperature in this area had a typical characteristic of cold winter-warm summer-the following cold winter for three consecutive years from the lower layer to the upper layer (1000–400 hPa), and the similar situation also appeared in the period from 1950s to 1960s; however, the opposite variation characteristics of warm winter-cool summer-the following warm winter happened in 1990s. This typical seasonal evolution of atmospheric temperature was defined as a new variation mechanism: winter-to-winter recurrence (WWR). The recent 60 years may be divided into four different types according to the variation of the years from 1953 to 2012: negative (positive) winter-to-winter recurrence (negative/positive WWR) and negative (positive) non winter-to-winter recurrence (negative/positive non-WWR). In the 23 WWR years the probability of occurrence was close to 40%. This WWR characteristic still exists and is independent of the variation of the El Niño southern oscillation index. The synthetic anomalies analysis of geopotential height, vertical speed, and the winter vector at 850 hPa indicates that as for the WWR pattern, the dynamic process of inner atmosphere also has WWR characteristics corresponding to the atmospheric temperature from the lower layer (1000 hPa) to the upper layer (500 hPa) while the non-WWR pattern does not have the similar characters. This study will provide a new idea to further investigate the frequent occurrence of the winter low temperature events on the background of global warming.
2014, 63 (23): 239501.
doi: 10.7498/aps.63.239501
Abstract +
When using the implicit Monte Carlo method to simulate the thermal radiative photon transfer in materials, the radiative source particles emitted by the material should be treated carefully. In traditional sampling method for the location of radiative source particles can be regarded as uniform in a single cell, and this is reasonable for most of thermal radiative transfer problems. But it may result in unacceptable error in the problem which has a large absorption cross-section and remarkable temperature difference in a single cell. The reason for this error is discussed in this paper. A new method based on radiation energy density distribution is proposed and the sampling formula is derived. Numerical test indicates that the new method will give results much better than the old method and in agreement with the analytical results.
2014, 63 (23): 239801.
doi: 10.7498/aps.63.239801
Abstract +
Theoretical study shows that stochastic oscillations of an accretion disk should induce the variability of active objects, however, observations indicate that it contains both chaotic factor and random noise. We develop a “chaotic + stochastic” oscillating accretion disk model for the variability. In this model, the chaotic factor is introduced into the oscillating accretion disk, and the chaotic attractor is reproduced through analyzing the phase pattern of disturbance. Results show that: when the random factor dominates the variability, the light curve exhibits a strong randomness; when the random factor is equal to the chaotic factor, the light curve shows a “heartbeat” which is similar to an electrocardiogram; when the chaotic factor dominates the variability, the light curve shows a certain orderliness. The correlated dimension of the simulated light curve is consistent with the correlated dimension of observational data. This suggests that there is an intrinsic relationship between the simulated light curve and observational data.
