Vol. 65, No. 3 (2016)
An improved centroid localization algorithm based on iterative computation for wireless sensor network
2016, 65 (3): 030101. doi: 10.7498/aps.65.030101
Wireless sensor network (WSN) is a basic component of internet and it plays an important role in many application areas, such as military surveillance, environmental monitoring and medical treatment. Node localization is one of the interesting issues in the field of WSN. Now, most of the existing node localization algorithms can be divided into two categories. One is range-based measurement and the other is range-free measurement. The localization algorithm of range-based measurement can achieve better location accuracy than the localization algorithm of range-free measurement. However, they are generally very energy consuming. Therefore, the range-free measurements are most widely used in practical applications. According to the application of localization algorithm in WSN by range-free measurements, an improved centroid localization algorithm based on iterative computation for wireless sensor network is proposed. In this algorithm, the position relationship of the closed area surrounded by the anchor nodes inside the unknown node's communication range and the unknown node is obtained by approximate point-in-triangulation test at first. Different position relationships determine different stopping criteria for iteration. Then, the centroid coordinates of the closed area surrounded by the anchor nodes inside the unknown node's communication range and the received signal strength (RSSI) between the centroid node and the unknown node are calculated. The anchor node with the weakest RSSI would be replaced by the centroid node. By this method, the closed area surrounded by the anchor nodes inside the unknown node's communication range is reduced. The location accuracy is increased by the cyclic iterative method. With the change of the anchor node ratio, the communication radius of the unknown node and the effect of RSSI error, the algorithm performance is investigated by using simulated data. The simulation results validate that although the improved centroid localization algorithm performance will be lost when the number of the anchor nodes inside the unknown node communication range decreases, the new approach can achieve good performance under the condition of few anchor nodes inside the unknown node communication range and this method is of strong robusticity against RSSI error disturbance.
2016, 65 (3): 030301. doi: 10.7498/aps.65.030301
We propose a new Ket-Bra entangled state (KBES) method to solve the master equation of finite-level system. The KBES method can convert the master equation into Schrdinger-like equation which is easier to solve than the master equation, and Schrdinger equation in a certain form can also be used to solve the Schrdinger-like equation. Thus the KBES method has a wider application range. In the paper, we mainly study the master equation of the two-level atom. The corresponding master equation is solved by the KBES method, and for the first time we obtain the opera-sum solution of the atom. Furthermore, we compare this result with the well known solution that describes the laser channel. There is much analogousness between both opera-sum solutions, which show that there is some supersymmetry between Bose creation-annihilation operator and upper-down transition operators of atom. Finally, we further analyze the supersymmetry between the bose and atom system, and find that the spin-up and spin-down operator can be represented by the creation and annihilation operator repectively, which can be achieved in infinite ways. It is easy to understand that the bose operator is infinite-level while the spin operator is two-level, thus the creation-annihilation operator is super-complete for the spin operator. Thus the representation is not unique, and all of this directly shows and proves the supersymmetry.
2016, 65 (3): 030302. doi: 10.7498/aps.65.030302
Most of quantum cryptography protocols are designed under the ideal conditions without considering the impact of noise in actual communication; thus they may result in that the confidential information cannot be transmitted to the receiver accurately or eavesdroppers can steal the confidential information by mixing in noise. Therefore, analyzing the security of quantum cryptography protocols under noise conditions is of great significance. For the purpose of analyzing the security of quantum BB84 protocol in collective-rotation noise, firstly this paper introduces the quantum BB84 protocol, and considers the influence of environmental noise on it. An explanation should be stated that in a noise environment, the effects of noise and eavesdropping cannot be distinguished between each other. So the mechanism for which the error bit is simply used as the criterion to judge whether there exists eavesdropping in the BB84 protocol, cannot be used in the noise environment. The mechanism to judge whether there exists eavesdropping in quantum noise channel needs to be modified and improved for protecting the information. An initial qubit error rate can be set according to the noisy quantum channel. If the qubit error rate of the quantum communication channel is larger than that, it can be determined that the quantum channel is not secure and exists eavesdropping, no matter what the reason is. And on this basis, the collective-rotation noise model will be established in quantum channel by using the particle deflection model and distinguish the noise from the eavesdropping in quantum channel quantitatively, and the relationship of the amount of information that eavesdroppers can steal, the quantum bits error rate and the noise level will be analyzed by using the von Neumann entropy. Finally, the noise critical point will be calculated by using the collective noise model and the relationship between the amount of information that eavesdroppers can steal, at the quantum bits error rate, and the noise level. Through the analysis, we can know that in the existing noise level, the most of the eavesdropping can steal 25% of the key from the communication. However, the Eve's eavesdropping behavior will be detected, so that Alice and Bob will give up the current consultation key, and restart the key negotiation. This result shows that the quantum BB84 protocol is safe and secure in the collective-rotation noise channel. The research results of this paper will enrich the theory of quantum cryptography, and the innovation of security detection methods in quantum cryptographic protocols will help promote the process of practical quantum cryptography.
2016, 65 (3): 030303. doi: 10.7498/aps.65.030303
Quantum information technology is mainly based on quantum entanglement. As an important coherent superposition state, the coherence of quantum entanglement source is easily affected by environment and becomes fragile, which will lead to the failure of the quantum information processing. Thus, it is critical to reveal the evolutions of quantum entanglement source under different noisy environments and different noisy channels. Firstly, we experimentally prepare a high-fidelity two-bit entangled state by several technical methods. The fidelity observed for the state prepared in our experiment is 0.993 and the signal-to-noise ratio can reach up to 299. Then, we simulate the bit-flip noise and phase-shift noise (collective and non-collective) using the all-optical experimental setup. Finally, based on the entanglement qubit state, we experimentally study the evolutions of entanglement characteristic under different noisy environments and the single, double and mixed noisy channels. The experimental results show that for the same type of noise, the entanglement properties disappear fast when entangled qubit passes through dual channel noisy environment. The upper bounds of noise intensity to destroy the entanglement property are 0.25 and 0.26 for the single bit-flip noise and phase-shift noisy channels, respectively. The comparison between the two different kinds of noisy environments shows that the entanglement properties disappear fast when entangled bit passes through non-collective environment. The upper bounds of noise intensity are 0.08 and 0.14 for non-collective bit-flip and phase-shift noise to destroy the entanglement property, while the noise intensities are 0.14 and 0.23 for collective bit-flip and phase-shift noise, respectively. For different kinds of noises, the results show that bit-flip noise is more likely to destroy the entanglement properties than the phase-shift. Our results have great significance for the theoretical and experimental studies of entanglement decoherence and have important application value for quantum information processing technology based on the nonlinear optical system.
2016, 65 (3): 030501. doi: 10.7498/aps.65.030501
Wind power is one of the most attractive renewable clean energies under development at present. On a global scale, wind power generation development was very rapid in recent years. As the wind power generation tends to develop toward large-scale and offshore, the traditional cooling methods gradually expose their own shortcomings. As the large wind turbine installation tower is high, and the installation site is dispersed, the installation and maintenance of wind turbine generator are difficult. So, the generator is required to have a small weight and less maintenance. Self-circulation inner evaporative cooling system (SCIECS) has the following advantages: self-circulation without pump, high cooling efficiency, safe and reliable operation, and basically maintain-free, etc. The self-circulation of cooling system can be realized by the 35 between wind turbine generator and the horizontal direction, and it is very suitable for being used in a large-scale wind power generator. Owing to the intrinsic nonlinearity of two-phase self-circulation system, changes of operation condition and circuit topology in a large range may lead to an instability of the cooling system, causing the system parameters to severly change. The instability of the cooling system can cause the local overheating and even burning of the generator, which provides a huge security risk for the cooling system, thus threatening the safe and stable operation of the generator. The stability of SCIECS is very important for the safe operation of wind turbine. In this paper, static stability of the SCIECS in wind power generator is studied based on the nonlinear bifurcation analysis theory and its numerical continuation method. System static bifurcation diagram is obtained to analyze the evolution characteristics of the SCIECS. Parameter effect of the system static bifurcation is analyzed at the same time. In order to verify the theoretical prediction of the static bifurcation of the small-angle two-phase natural circulation, an experimental platform is built. Static bifurcation of the SCIECS is observed experimentally. The experimental results show that the static bifurcation phenomenon exists in the natural circulation two-phase flow of small angle, and the theoretically predicted m-Q bifurcation curves are in good agreement with the experimental curves, which verifies the correctness of the theoretical analysis.
2016, 65 (3): 030502. doi: 10.7498/aps.65.030502
Studying the process of network public opinion reversal is of great significance for guiding public opinion toward a positive direction. Currently, the research on opinion reversal mainly focuses on the construction of dynamic models and analysis of simulations and the results of which have a certain theory value. However, whether these models are applicable to the real social network environment has not been tested. For studying the process of public opinion reversal, we build a model according with the realities, and make an in-depth analysis of the typical case of opinion reversal. Some rules are found from the observation and statistics: the fundamental reason of public opinion reversal is the conflicting news. Spreading of news affects the opinions of the group. The news properties, including transmission rate, credibility, opinion polarity, publication date and the degree of message source determine the extent to reverse. Based on these rules, parameters of news properties are set, and a model of opinion reversal is proposed by combing the information dissemination with opinion evolution. Simulation results show that the transmission rate of news, the credibility of news, and the degree of message source have a positive influence on the margin of reversal. The influence of credibility is more dramatic than that of transmission rate. Moreover, the public opinion would be reversed more quickly and completely if the conflicting news is released more easily. The proposed model can fit the actual data, which is helpful for understanding and explaining the process of network public opinion reversal, and provides theoretical basis for guiding the network public opinion.
2016, 65 (3): 030503. doi: 10.7498/aps.65.030503
Ever since the special characteristics hidden in the chaos was discovered, the chaotic behavior has been extensively studied as a ubiquitous and complex nonlinear dynamic phenomenon, which is gradually extending to various disciplines of natural and social science, and the significant values in the theoretical and the practical application have attracted much attention from scholars of different fields in the recent decades. Conventional methods of analyzing chaotic dynamic systems, including the Lyapunov exponent, correlation dimension, Poincar map, unavoidably encounter some common problems, such as reconstruction of the phase space, determination of the linear area, etc. Besides, the current approaches each also possess a poor capability of balancing the direct observation and the quantitative calculation. Based on the fact that the neighbor data relate to each other to some degree, taking those shortages into consideration, aiming at depicting the chaotic features efficiently, a new method of analyzing the complicated chaotic motion is proposed. During the processing of that novel approach, the Euclidean distance is continuously computed to represent the dependence of the adjacent unit, after that, the original complicated array is converted into a simpler series composed of the distance of neighbor sub-sequences with more distinct characteristics. The mean value and the standard deviation of the newborn series are exacted to assist in describing the chaotic changing law. The method is adopted for studying the typical chaotic models, like Logistic model, Chebychev model, Duffing oscillator, Lorenz system, etc., which proves the good performances in explaining the chaotic variation rules in different systems. Based on the model verification, it could be seen that the method could detect the chaotic motion both qualitatively and quantitatively, and the ability for that method to resist the noise is improved up to some degree, what is more, the information about the real model is not required, thereby simplifying the analysis of the complicated chaotic behavior whose authentic model is unavailable. In addition, the method is applied to decomposing the vibration signal to monitor the working condition of the rotating rotor, and the results show that the conditional variation could be detected obviously. The analyses above show that the proposed method, on the basis of the dependence between nearby data, could perform well in observing the chaotic feature in an efficient way which simplifies the operation and clarifies the chaotic variation, moreover, the application potential of this method is worthy of great attention.
In this paper, firstly we construct a quadratic chaotic system and prove that it is a topological conjugate system of Tent map. Secondly, having analyzed the probability density function of the system, we propose an anti-trigonometric function map. Additionally, the performances of the quadratic chaotic system such as information entropy, Kolmogorov entropy and discrete entropy are tested for both the original systems and the homogenized systems with different parameters. Numerical simulations show that the information entropy of the uniformly distributed sequence is close to the theoretical limit and the discrete entropy remains unchanged. This result shows that the homogenization method is effective. In conclusion, the chaotic sequence after homogenization not only inherits the diverse properties of the original sequence, but also exhibits better uniformity.
2016, 65 (3): 030701. doi: 10.7498/aps.65.030701
The growth mechanism of hydrogenated carbon films in plasma-enhanced chemical vapor deposition (PECVD) is complicated and much attention has to be paid to it for the unique properties of carbon films. In this paper molecular dynamics simulations are carried out to illustrate the collision behaviors of CH radical on the clear and hydrogenated diamond (111) surface with varying incident energy (from 1.625 to 65 eV), aiming at the growth mechanism of hydrogenated carbon film by PECVD. Our simulations show that the behaviors of incident CH radical can be divided into adsorbing, rebounding, reaction releasing one H atom and reaction releasing two H atoms, while the reaction releasing one H2molecule rarely occurs. At low incident energy, selective adsorption of CH at unsaturated surface C site is the dominated growth mechanism since no reactions can conduct. Such growth model results in films with rough surface, high hydrogen fraction, and loose structure. As the incident energy increases, two chemical reactions that one releases one H atom and the other releases two H atoms are important. Caused by these reactions, the saturated C site in the surface will be transferred into unsaturated one, so that it can further adsorb subsequently incident CH radicals. The occurrence of these reactions makes films grow more uniformly, leading to the smoothness and dense structure of the films. The hydrogen fraction in the films will be reduced by these reactions. To confirm the above growth mechanism, the carbon film growth from CH radicals are then simulated. The film obtained with low energy (3.25 eV) CH radicals is found to be loose, rough, and have many carbon chains with adsorbed hydrogen atoms on the surfaces, while the film produced with high energy (39 eV) radicals are dense, smooth and the chains on the surfaces are short and have less hydrogens. On the other hand, most of the C atoms in the films deposited with low energy have one H atom as coordination, while for high energy most of C atoms in the films have no H atom as coordination. These observations agree well with the proposed growth mechanism. The destruction effects caused by the incident CH radicals are also analyzed based on the variation of the sp2-C and sp3-C in the films.
Electromagnetic topology based fast algorithm for shielding effectiveness estimation of multiple enclosures with apertures
2016, 65 (3): 030702. doi: 10.7498/aps.65.030702
Shielding effectiveness (SE) estimation for an enclosure with apertures has been an attractive issue in electromagnetic compatibility (EMC) research area. Though many fast algorithms are developed for SE calculation, they mainly focus on the case of single cavity. Moreover, most of these methods neglect the wave coupling through apertures from enclosure to outside. A fast algorithm based on electromagnetic topology is proposed for calculating the SE of cascading multiple enclosures with apertures. In this algorithm, the wave coupling through apertures in both directions is taken into consideration. Firstly, the equivalent circuital model of cascading double enclosures and its signal flow graph of electromagnetic topology are given, followed by the derivation of scattering matrix of apertures node. Then propagation relationships at tube level and reflection relationships at node level are derived. As a result, the general BLT (Baum-Liu-Tesche) equation for voltage calculation at each node is established. Two major categories of cascading three enclosures with apertures are investigated. For serially cascading three enclosures, the general BLT equations are extended on the basis of BLT equations for cascading double enclosures, since the structures are a simple extension of them. For hybrid serially-parallelly cascading three enclosures, the common walls between the main enclosure and two sub-enclosures are considered as a topological node represented by a three-port network, whose scattering matrix is derived according to the definition of scattering parameters. Consequently, the general BLT equations for hybrid serially-parallelly cascading three enclosures are developed. Compared to the algorithms presented in the relevant literature, the topology-based algorithm proposed in this paper can not only calculate the shielding effectiveness for cascading multiple enclosures, but also lead to more accurate results in that the impedance of apertures is obtained through using diaphragms model. In order to validate the proposed method, a cascading double enclosures from a literature is chosen as an example. Calculated SE results are in good agreement with those in the literature. Then, three enclosures with different configurations and dimensions are also designed to validate the proposed method. Results from the proposed method are compared with those from the finite difference time domain (FDTD) method, and they are found to be in good agreement with each other. Experimental results also demonstrate the validation of the proposed method. Especially, the proposed method takes far less time to calculate SE than for FDTD method.
ATOMIC AND MOLECULAR PHYSICS
2016, 65 (3): 033101. doi: 10.7498/aps.65.033101
Low-lying electronic states (X2, A2+, a4, B2, b4, C2, F2-, E2+ and D2) of the 6Li32S molecule are computed at the aug-cc-pV5Z/MRCI level. The potential energy curves are presented for these states; the corresponding spectroscopic constants are reported. Electronic transition moment, Einstein coefficients, Frank-Condon factors and radiative lifetimes for the A2+-X2, B2 -X2, C2 -X2 systems are calculated. The balanced distance between two nuclei, harmonic frequencies and inertia moment of ground state X2 are predicted in this paper, and they are in accordance with their corresponding experimental data. The balance distances between the two nuclei in the electronic states of b4, C2, D2 are all longer than 4 , so they are very unstable. The D2 electronic state will dissociate to Li+ ion and S- ion: they are far from each other. The electronic transition dipole moment, Einstein coefficient, Franck-Condon factor and radiative lifetime in transition from lowest excited A2+ state to ground state X2 are predicted in this paper. The electronic transition dipole moments from three low lying electronic state A2+, B2 and C2 to the ground state X2 are calculated at the aug-cc-pV5 Z/MRCI level. The results show that the electronic transition dipole moment of A2+X2 has a small positive value while the nucleus distance is short, and rapidly decreases down to a small negative value with the nucleus distance increasing to around balance distance. Then it is stable about zero value while the nucleus distance continually increases. The electronic transition dipole moment of B2 X2 has a small negative value (which is larger than that of A2+ X2) at a short nucleus distance, and rapid increases up to a small positive value with the nucleus distance increasing to about balance distance. Then it slows down to zero while the nucleus distance increases to about 4. Finally it turns stable about zero value while the nuclei distance continually increases. The electronic transition dipole moment of C2 X2 is more sophisticated, but it has a large value than other two transitions. So the low-lying electronic state A2+ is stabler than B2, and B2 is stabler than C2 . The results also show that the ground state X2 and the lowest excited state A2+ have similar IR frequencies, their difference is within 8 cm-1, so they cannot be distinguished by IR spectrum. The A2+ has a balanced distance about 0.076 shorter than ground X2, which implies that A2+ has stronger chemical bond than ground X2 .
icMRCI+Q study on spectroscopic properties of twelve -S states and twenty-three states of the CF+ cation
2016, 65 (3): 033102. doi: 10.7498/aps.65.033102
The potential energy curves of twenty-three states generated from the twelve -S states (X1+, a3, 13+, 13, 11, 11-, 13-, 21+, 11, 23, 21 and 23+) correlating with the first dissociation channel C+(2Pu)+ F(2Pu) of the CF+ cation are obtained by using the internally contracted multireference configuration interaction approach with the Davidson modification (icMRCI+Q) on the basis of the correlation-consistent aug-cc-pV5Z and aug-cc-pV6Z basis sets for the first time. The spin-orbit coupling, core-valence correlation and relativistic corrections are taken into account, and all the potential energy curves are extrapolated to the complete basis set limit by separately extrapolating the Hartree-Fock and correlation energies scheme. Based on the calculated potential energy curves, the spectroscopic parameters of the bound and quasibound nine -S and sixteen states of the CF+ cation are obtained. And the spectroscopic parameters of X1+and a31st well-S states which are in very good agreement with experimental results are achieved. Furthermore, the vertical and adiabatic ionization potentials of ionization from the X2 state of CF radical to the bound and quasibound nine -S states of the CF+ cation are calculated, and the vertical and adiabatic ionization potentials of the CF+(X1+) CF(X2 ) and CF+(a31st well) CF(X2 ) ionizations are also in good agreement with the corresponding experimental values. Various curve crossings of -S states are revealed, and with the help of our computed spin-orbit coupling matrix elements, the predissociation mechanisms of the a31st well, 111st well and 21+ states are analyzed for the first time. The spin-orbit-induced predissociations for the a31st well, 111st well and 21+-S states could happen, and the predissociations of the a31st well, 111st well and 21 +-S states start around the vibrational levels ' = 15, ' = 1 and ' = 1, respectively. Relative energies of the twenty-three states in the dissociation limits are given, and our calculations match the experimental results very well. Finally, the Franck-Condon factors and radiative lifetimes of transitions from (2) 0+1st well (;'=05), (1) 11st well ('=05) and (2) 11st well ('=0) to X0+ states are predicted for the future laboratory research.
2016, 65 (3): 033103. doi: 10.7498/aps.65.033103
The vibration feature in a molecule solid is an important character of its structure. The different vibration frequencies of isolated nitrogen molecule (N2) and nitrogen molecule in the solid state are explored. Five solid-cluster models with the different numbers of nitrogen molecules (N46, N60, N76, N100, and N126) are constructed on the basis of -N2 crystal structures. The density functional theory is used to calculate the vibration frequencies of nitrogen molecules. The calculated infrared spectra and average vibration frequencies (AVFs) of the optimized structures for the five models are compared with each other. The results indicate that the AVF of nitrogen molecule in solid model is higher than that of isolated nitrogen molecule due to the collective effect. It is found that the AVF increases with increasing the number of molecules. The AVF of the inner molecules is always higher than that of surface molecules in the solid. On a whole, the vibration frequencies are ordered as vinner vsurface visolated for each case. The local coordination environment is believed to be mainly responsible for the differences in frequency among the isolated, surface and inner molecules. The bond length of molecule in solid is shorter than that in an isolated molecule, thus resulting in a stronger bond force and a higher vibration frequency. Similarly, due to a smaller number of molecules interacting with surface molecules, the bond force between molecules in the solid surface is weaker, thus resulting in a lower vibration frequency than in the inner region of solid.
Influence of atomic potential on the generation of high harmonic generation from the atoms irradiated by mid-infrared laser pulses
2016, 65 (3): 033201. doi: 10.7498/aps.65.033201
By numerically solving the time dependent Schrodinger equation, the harmonic spectra generated from the atoms are obtained. The atomic potentials are modeled by a short-range potential and a long-range soft Coulomb potential, respectively. It is found that using the same laser parameters, the intensity of harmonic spectrum from the short-range atom is lower than the one from the long-range atom. However, in a high energy (near the cutoff) region of harmonic spectra, their conversion efficiencies are almost the same. The differences in emission intensity among harmonic spectra decrease as the harmonic energy increases. We calculate the time dependent probabilities of the ground state and ionization. It is found that the ionization probability of the long-range potential is larger than that of the short-range potential. There is no large difference in ground probability between the potentials of two models. The high harmonic generation is a stimulated process, and its intensity is proportional to the product between the amplitude for ground state and the amplitude of the continuum state. Thus the product of the long-range atom is larger than that of the short-range atom, and the emission spectrum presents a similar character. In order to analyze the mechanism of the intensity difference between two models, we perform a time-frequency analysis of the harmonic emission spectrum. The analysis is selected of the wavelet of the time dependent dipole moment. From the emission profile of the harmonic analysis, we find that the harmonic generated from long orbit plays a dominant role for the short-range atom. The amplitudes of electric field are large for the long orbit harmonic emission, thus the ionization mechanism of the atom is the tunnel ionization. For the short orbit, the instant field for the ionization is weak. Thus the short orbit plays a small role in the harmonic emission from the short-range atom. Using this feature of the short-range atom, we generate an isolated attosecond pulse. The short model atom is widely used to study the ionization of the plasma. Thus this work will contribute to the research on the high-order harmonic generation from the plasma.
2016, 65 (3): 033401. doi: 10.7498/aps.65.033401
Dielectronic recombination (DR) rate coefficients of complex structure ions are very important for spectral simulation in some application researches, such as nuclear fusion and extreme ultraviolet lithography. Theoretical calculations are made for dielectronic recombination rate coefficients of Au34+ ions by using a flexible relativistic atomic code. Influences of excitation and radiation channels, configuration interactions, and decays to autoionizing levels possibly followed by radiative cascades (DAC) on DR rate coefficient are analyzed. The contribution of DAC is evident. The total DR rate coefficient is greater than either the radiation recombination coefficient or three-body recombination coefficient for electron temperature greater than 1 eV. In order to facilitate simple applications, the total DR rate coefficients for the ground state and the first excited state are fitted to an empirical formula. These results should be useful for further analyzing the DR process of complex structures ions.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2016, 65 (3): 034201. doi: 10.7498/aps.65.034201
Based on the phase information in time domain of a Mach-Zehnder interferometer (MZI), a method of calibrating the wavenumber of the source is proposed. Cross-correlation of wrapped phase in time-domain of the MZI is adopted to determine drifts among axial-lines. Owing to non-strictly periodic characteristics of wrapped phase in time-domain of the MZI, the determinable range of shift is unlimited. Synchronization of signals in time domain is then performed to correct their corresponding shifts. The obtained results demonstrate that it is feasible to realize phase measurement with high precision even under unstable swept source for the optical coherence tomography system.
2016, 65 (3): 034301. doi: 10.7498/aps.65.034301
Complex zone of the ocean is often characterized by horizontal variations of environmental parameters(bathymetry, sound speed profile, bottom properties etc.), initiating redistribution of the sound field in horizontal plane, which is the so-called three-dimensional (3D) effect. Based on the adiabatic mode parabolic equation method, modeling of 3D effects is carried out, in which the eigenvalues and eigenfunctions are calculated by the standard normal mode model KRAKEN, and the amplitude corresponding to each mode is computed by the wide-angle parabolic equation model RAM. The present 3D model is very efficient and can give clear physical meaning, but it can be only applied to a waveguide whose properties vary gradually with horizontal range due to the adiabatic assumption between different modes. This model is then used to analyze the horizontal refraction caused by internal waves and by a coastal wedge. The numerical results show that there are several areas in the horizontal plane, corresponding to different structures of intensity distributions. Moreover, the redistribution of the sound field in horizontal plane depends on source frequency and mode number. Frequency and modal dependences lead to variations of spectrum, distortion of signal with some spectrum, and spatiotemporal fluctuations of the sound field.
2016, 65 (3): 034701. doi: 10.7498/aps.65.034701
In order to better understand the variation of flow structure with delay time, we propose the element area (EA) of attractor morphology parameter in this paper. First, the conductance fluctuating signals and adaptive optimal kernel time-frequency representations of different gas-liquid flows are shown, we can find that flow pattern evolution is always accompanied by the numerical and frequency changes of large amplitude fluctuation (LAF). Then three kinds of signals, i. e., rossler signal, white noise and sinusoidal signal with multi-components, are used for analyzing the simulations, and the results indicate that the greater the frequency of LAF, the smaller the delay time of first crest of EA( peak ) is, and that the more the LAF, the bigger the peak value of first crest of EA(hpeak) is. Additionally, we use the above rule to analyze the conductance fluctuating signals measured from upward gas-liquid two-phase flow experiments and the signal length is selected to be 10 s for analysis. When the water superficial velocity is fixed to be 0.1138 m/s and the gas superficial velocity is gradually increased, we find that the peak is constant and hpeak changes up and down at bubble flow. When the flow pattern evolves into bubble-slug transition flow, the peak begins to turn bigger, and when the flow pattern evolves into slug flow, the peak becomes constant again while the hpeak increases monotonically with the gas flow rate increasing. The peak begins to become smaller as the flow pattern evolves from slug flow into churn flow, and we can find that the peak and hpeak of transition flow are alike. The peak and hpeak of bubble flow and churn flow are also alike because their dynamical mechanisms are similar but the downward trend of bubble flow is more gently than that of churn flow. When the water superficial velocity is fixed to be 0.2719 m/s, we can find similar variations of peak and hpeak to the above. Finally we determine the fall ratio (Rf) which is the ratio of the difference between the first crest and the first trough of EA and the hpeak, and then quantitatively distinguish three typical flow patterns, i.e., bubble flow, slug flow and churn flow by the Rf - peak distribution.
2016, 65 (3): 034702. doi: 10.7498/aps.65.034702
The unsteady electroosmotic flow characters of power-law fluids in a finite micro-diffuser are studied in this paper. Based on the Ostwald-de Wael model which is used to describe power-law fluids (the shear thinning, thickening and Newtonian fluids), high accuracy compact difference schemes are used to solve the two-dimensional Poisson-Nernst-Planck equations and the modified Cauchy momentum equations. Electroosmotic flow distributions of power-law fluids at initial instant and steady state are numerically simulated in this paper. It is presented that while the radius of the diffuser is increasing, the dimensionless apparent viscosity influenced by shear strain conduces to the different velocity profiles of power-law fluids. In the micro-diffuser, the shear strains of pseudo plastic and dilatant fluids are decreasing with the radius increasing and the apparent viscosity of pseudo plastic fluid is increasing with the shear strain decreasing, but the apparent viscosity of dilatant fluid is decreasing with the shear strain decreasing. The apparent viscosity of power-law fluids can estimate the flow performance, and the fluid with high viscosity flows more slowly than the one with low viscosity. The numerical results show that a fast speed response of power-law fluid is found near the wall at the beginning and the average dimensionless velocity of power-law fluids is decreasing with the radius increasing when fixing the diffuser angle and dimensionless electrokinetic diameter at the same dimensionless zeta potentials. At the initial instant, the different velocity distributions of power-law fluids from upstream to downstream near the wall in diffuser are essentially due to the change of dimensionless shear strain. Because the dimensionless shear strains of pseudo plastic and dilatant fluids are in a larger value zone in upstream, the dimensionless apparent viscosity of dilatant fluid is larger than that of the pseudo plastic fluid, and the velocity peak of pseudo plastic fluid is larger than that of the dilatant fluid. In downstream, the apparent viscosity of pseudo plastic fluid is larger than that of the dilatant fluid so that their velocity peaks are similar. At the steady state, the velocity profiles of power-law fluids are plug-like and the velocity is decreasing with increasing radius when the continuity conditions are satisfied, and the flow regularity of Newtonian is just like that on a macroscopic scale. The velocity profile of pseudo plastic fluid is larger than that of dilatant fluid in upstream and their velocity profiles in downstream are not much different. The power-law fluid flow distribution at initial instant is similar to that at the steady state. From the flow regularities respectively at initial instant and the steady state it follows that the flow rate of pseudo plastic fluid is larger than that of Newtonian fluid and the dilatant fluid flow rate is smaller than Newtonian fluid rate. At the initial instant, under the same electrokinetic diameter and different zeta potentials, the difference in shear strain among power-law fluids in the micro-diffuser near the wall leads to the difference in the apparent viscosity, and eventually leads to the velocity distribution difference between pseudo plastic and dilatant fluids.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
Study on transmission characteristics of electromagnetic waves in inhomogeneously magnetized plasma sheath
2016, 65 (3): 035201. doi: 10.7498/aps.65.035201
We use simulation software to simulate the plasma sheath flow field of RAM-II at four different altitudes, and get the distribution information of electron density, temperature, and Mach number of plasma sheath at four typical flight altitudes. On this basis, the paper analyzes the cause of the distribution at each altitude. Secondly, Z transform FDTD formula is used to solve the electromagnetic problem in the dispersed medium. According to RAM-II aircraft plasma sheath data, we calculate the power transmission coefficient and power reflectance coefficient of L, S-band electromagnetic waves by Z transform FDTD method at each altitude. Because of the strength of magnetic field used in actual aircraft cannot be uniform, in this calculation the plasma sheath and the magnetic induction are both non-uniform. The transmission characteristics are also different between left-hand circularly polarized waves and right-hand circularly polarized wave in the magnetized plasma. Finally, the paper gives some theoretical design advise about communication antenna for supersonic speed aircraft.
Effect of wall secondary electron distribution function on the characteristics of stable sheath near a dielectric wall
2016, 65 (3): 035202. doi: 10.7498/aps.65.035202
It is widely known that the energy distribution of secondary electrons induced by a single-energy electron beam presents typical bimodal configuration. However, the total velocity distribution of secondary electrons induced by a Maxwellian plasma electron group has not been revealed clearly, due to the lack of detailed theoretical calculation and calculation and experiment result. Therefore, researchers usually function satisfies single-energy distribution ( 0), half-Maxwellian distribution and so on, in order to study the characteristics of stable fluid sheath near a dielectric wall. For this reason, using the Monte Carlo method to simulate the wall secondary electron emission events based on a detailed probabilistic model of secondary electron emission induced by single-energy incident electron beam, we found that, when the incident electron follows an isotropic Maxwellian distribution, the total perpendicular-to-wall velocity distribution of the secondary electrons emitted from dielectric wall follows a three-temperature Maxwellian distribution. In the simulation, the incident angle of the plasma electrons and the emergence angle of the secondary electrons are considered, so the Monte Carlo method can discriminate whether the secondary electron velocity is perpendicular to or parallel to the wall surface. Then, a one-dimensional stable fluid sheath model is established under the wall boundary condition that the secondary electrons obey the three-temperature Maxwellian distribution; and some contrastive studies are made in order to reveal the effect of wall total secondary electron distribution functions such as single-energy distribution, half-Maxwellian distribution, and three-temperature Maxwellian distribution with the sheath characteristics. It is found that the total secondary electron distribution function can significantly influence the ion energy at the sheath interface, the wall surface potential, the potential and electron/ion-density distributions, and so on. Both the ion energy at sheath interface and the wall surface potential increase monotonously with the increase of wall total secondary electron emission coefficient. But the values of three-temperature Maxwellian distribution differ much from that of half-Maxwellian distribution and single-energy distribution. When the total secondary electron follows a three-temperature Maxwellian distribution, the critical space charge saturated sheath has no solution, indicating that with the increase of the wall total secondary electron emission coefficient, the sheath will directly transit from the classic sheath structure to the anti-sheath one. In the future work, a kinetic, static sheath model will be developed in order to study the characteristics of anti-sheath and space charge saturated sheath near a dielectric wall
Characterization and properties of polyimide films prepared in different monomer ratios by vapor deposited polymerization
2016, 65 (3): 035203. doi: 10.7498/aps.65.035203
The improvement of mechanical and thermal properties of the polyimide (PI) capsule, which is one of the most important ignition capsules in inertial confinement fusion experiment, has great significance to realize the ignition. PI capsules can be prepared by solution method and vapor deposited polymerization (VDP) method. In comparison with the traditional solution method, the polyimide film prepared by the method of vapor deposition has the characteristics of controllable thickness, uniformity, and better surface roughness. At the same time, it can be deposited on the surface of complex structures. Because of its excellent properties, VDP has great advantages in the preparation of PI films and capsules. The difference between the capsule and the film prepared by VDP is mainly caused from the geometric size and the substrate. By adjusting the monomer content, the performance of PI film can be improved, and it is important for enhancing the performance of PI capsules. In this paper, pyromellitic dianhydride (PMDA) and oxydianiline (ODA) are used to prepare poly amic acid (PAA) films in different monomer ratios by vapor deposited polymerization. The evaporation temperature of ODA is 95 ℃, and that of the PMDA may be 120 ℃, 123 ℃, 124 ℃ or 126 ℃ respectively. FT-IR spectra measurement shows that the absorbance of PMDA becomes stronger with increasing evaporation temperature. After heat treatment, the excessive PMDA is evaporated again. There exists only PI in the final product. But the structure, elastic modulus, and hardness of the PI thin films are influenced by the existence of excessive monomer in the progenitor. With the increase of PMDA evaporation temperature, the stretching vibration peaks of C-N bond intensity will be 72.1%, 91.6%, 69.2% and 63.5%, compared with the vibration peaks of benzene ring; this indicates that the molecular weight of PI is reduced by the imbalance of the composition. The XRD curve shows that the film with composition close to it has a higher crystallization degree. FT-IR and XRD curvs indicate that the presence of excess monomer could inhibit further growth of the molecular chain, resulting in the decrease of molecular weight. The elastic modulus and hardness of polyimide film are measured by the nano-indentation tester, and the thermal stability is analyzed by a thermogravimetric analyzer. Samples of low molecular weight show low elastic modulus and hardness, while the thermal stability becomes worse. The temperatures of different samples with 5% molecular weight are 487 ℃, 524 ℃, 542 ℃ and 533 ℃ respectively, showing that increase of molecular weight is beneficial to the improvement of thermal stability of the films. Scanning electron microscopy images show that the polyimide film has a layered structure, the samples having composition close to each other show better surface roughness, being in good agreement with the molecular growth theory of polyimide.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Cluster-plus-glue-atom model of FCC solid solutions and composition explanation of typical industrial alloys
2016, 65 (3): 036101. doi: 10.7498/aps.65.036101
It was found previously by us that the compositions of industrial alloy specializations are related to the chemical short-range ordering in solid solution alloys, which is in accordance with the cluster-plus-glue-atom model. This model identifies short-range-ordered chemical building units in solid solutions, which the specific alloy compositions rely on. For instance, substitutional-type FCC solid solution alloys are described by cluster-based units formulated as [cluster](glue atom)16, where the bracketed cluster is the nearest-neighbor coordination polyhedral cluster, cuboctahedron in this case, and one-to-six glue atoms occupy the inter-cluster sites at the outer-shell of the cluster. In the present paper, we investigate the atomic configurations of these local units in substitutional-type FCC solid solutions by exhausting all possible cluster packing geometries and relevant cluster formulas. The structural model of stable FCC solid solutions is first reviewed. Then, solute distribution configurations in FCC lattice are analyzed by idealizing the measured chemical short-range orders within the first and second neighborhoods. Two key assumptions are made with regards to the cluster distribution in FCC lattice. First, the clusters are isolated to avoid the short-range orders from extending to longer range ones. Second, the clusters are at most separated by one glue atom to confine the inter-cluster distances. Accordingly, only a few structural unit packing modes are identified. Among them, the configurations with glue atoms 0, 1, 3, and 6 show good homogeneities which indicate special structural stabilities. Finally, compositions of FCC Cu-Zn (representative of negative enthalpy systems) and Cu-Ni (positive enthalpy ones) industrial alloys are explained by using the structure units of cluster packing and the cluster formulas, expressed as [Zn-Cu12]Zn1-6 and [Zn-Cu12](Cu, Zn)6, where the cluster is Zn-centered, shelled with Cu atoms, and glued with one to six Zn or with a mixture of six Cu and Zn. In particular, the formula [Zn-Cu12]Zn6, with the highest Zn content, corresponds to the solubility limit in Cu-Zn alpha phase zone, which is also the composition of the specification C27400. The Cu-rich Cu-Ni alloys are explained by cluster formulas [Cu-Cu12](Cu, Ni) 6, where the cluster is Cu centered and glued with a mixture of six Cu and Ni. The Ni-rich Monel alloy is explained by cluster formulas [Ni-Ni12](Cu5Ni)-[Ni-Ni12]Ni6. The present work provides a new approach to alloy composition explanation and eventually to alloy composition design from the perspective of short-range ordering in solid solutions.
2016, 65 (3): 036301. doi: 10.7498/aps.65.036301
In this paper, the two-dimensional granular assemblies composed of 2048 mono-dispersed frictional disks are simulated by the discrete element method. A set of eigenvalues and corresponding eigenvectors is obtained by diagonalizing the Hessian matrix for each stable configuration. The effects of the friction coefficient of disk on mechanical and geometrical properties of these systems under isotropic confining are studied. Results show that at a fixed pressure, with increasing from 0.001 to 1.0, the crossover frequency *, which separates the Debye scale region from the platform of vibrational density of states, and the boson peak BP gradually shift towards lower frequency, and the intensity of the boson peak D(BP) / BP increases. These results are mainly attributed to the fact that the system becomes more and more disordered with the increase of (i.e., the decrease of the average coordination number), resulting in more excess modes at *. For a better understanding of the different vibration modes of the two-dimensional frictional granular systems, we plot the polarization vector diagrams for different frequencies ( 1 = 0.15, 2 = 1.5 and 3 = 6.0) for configurations with = 0.001 and = 1.0, respectively. Mode analysis results show that the mode at low ( 1.0) has a mixed translational-rotational but translational-dominated character; the mode at intermediate frequency (1.0 4.0) is localized and has a mixed translational-rotational but translational-dominated character; and the mode at high frequency ( 4.0) have a strongly rotational in character. It is worth noting that the low-frequency modes become more localized and the rotational participation fraction also increases as increases, implying that the rotational modes play more important role in the system with higher friction coefficient.
2016, 65 (3): 036801. doi: 10.7498/aps.65.036801
The formation of Nickel based germanosilicides (NiSiGe) has attracted growing interest in the state-of-the-art metal oxide semiconductor field effect transistor (MOSFET) technology, because silicon-germanium alloy (Si1-xGex) is used as embedded source/drain stressor or channel material to enhance the hole mobility in the channel region. However, a major problem of NiSiGe film is that it has a poor thermal stability after annealing at high temperature (550 ℃), which leads to its agglomeration. In this work, we study the reaction between Ni and Si0.7Ge0.3 in the presence of an Al interlayer. Pure Ni (10 nm) film and Ni (10 nm)/Al (3 nm) bi-layers are deposited respectively on Si0.7Ge0.3 substrates by electron beam evaporation. Solid-phase reactions between Ni or Ni/Al and Si0.7Ge0.3 during rapid thermal processing in N2 ambient for 30 s are studied at 700 ℃. The un-reacted metal is subsequently etched in H2SO4 solution. The NiSi0.7Ge0.3 films are characterized by Rutherford backscattering spectrometry (RBS), crosssection transmission electron microscopy (XTEM), energy dispersive X-ray spectrometer (EDX), and secondary ion mass spectroscopy (SIMS) techniques. For the Ni/Si0.7Ge0.3 sample, the segregation of Ge at grain boundaries of nickel germanosilicides during the interfacial reactions of Ni with Si0.7Ge0.3 films and the subsequent formation of Ge-rich Si1-wGew (w0.3) are confirmed by the RBS and XTEM measurements. However, in the case of Al incorporation, a very uniform and smooth NiSi0.7Ge0.3 film is obtained with atomic NiSi0.7Ge0.3/Si0.7Ge0.3 interface. The orthorhombic NiSi0.7Ge0.3 is finally epitaxial grown on cubic Si0.7Ge0.3substrate tilted at a small as demonstrated by the High resolution XTEM. Furthermore, based on the EDX and SIMS measurements, it is found that most of the Al atoms from the original interlayer diffuse towards the NiSi0.7Ge0.3 surface, and finally form an oxide mixture layer. It is proposed that the addition of Al reduce Ni diffusion, balance the Ni/Si0.7Ge0.3 reaction and mediate the NiSi0.7Ge0.3 lattice constant. In addition, the main mechanism of epitaxial growth of NiSi0.7Ge0.3 film is analyzed in detail. In summary, Al mediation is experimentally proved to induce the epitaxial growth of uniform and smooth NiSi0.7Ge0.3 layer on relaxed Si0.7Ge0.3 substrate, providing a potential method of achieving source/drain contact material for SiGe complementary metal oxide semiconductor devices.
2016, 65 (3): 036802. doi: 10.7498/aps.65.036802
Pure and Yb-doped In2O3 nanotubes have been successfully fabricated by using the single-capillary electrospinning method, followed by calcination. The morphological and structural characteristics of the as-synthesized nanotubes are investigated by scanning electron microscope (SEM) and X-ray powder diffraction (XRD). The SEM images reveal that all the pure and Yb-doped In2O3 nanotubes are distributed evenly, and the average diameter of the as-synthesized nanotubes is about 200 nm. The XRD analysis results show that the as-prepared samples are well-crystallized, and the diffraction peaks can be indexed according to cubic In2O3. Gas sensors based on pure and Yb-doped In2O3 nanotubes have been fabricated and investigated for formaldehyde detection in detail. As shown in the experimental results, Yb-doped In2O3 nanotubes exhibit enhanced formaldehyde sensing properties compared with pure In2O3 nanotubes. At the optimum operating temperature of 230 ℃, the response of the gas sensors based on pure In2O3 nanotubes to 100 ppm formaldehyde is 18.4, while the response of gas sensors based on Yb-doped In2O3 nanotubes is 69.8 in the same working condition, which is 3.8 times larger than that of pure In2O3 nanotubes. The improvement of Yb-doped In2O3 nanotubes gas-sensing property may be due to the formation of the heterojunction structure at the interface between the two different semiconducting oxides. The response and recovery times of Yb-doped In2O3 nanotubes to 100 ppm formaldehyde are about 4 s and 84 s respectively, indicating the fast response speed of Yb-doped In2O3 nanotubes. Moreover, even at 100 ppb of formaldehyde a detectable response can be observed and the value is 2.5. The low limit of formaldehyde detection shows that the as-synthesized Yb-doped In2O3 nanotube gas sensors can be used for the detection of dilute formaldehyde. Furthermore, the Yb-doped In2O3 nanotube gas sensors have excellent selectivity towards formaldehyde. In this experiment, acetone has the highest sensitivity in a variety of common interfering gases and the response value is 22 to 100 ppm at 230 ℃, which is less than one-third of the sensitivity of formaldehyde. Carbon monoxide has the lowest response value of 1.7, which is much lower than that of formaldehyde. In addition, the responses of gas sensors to different concentrations of formaldehyde almost unchanged during the test (50 days), indicating that the Yb-doped In2O3 nanotubes possess good repeatability and long-term stability. The excellent formaldehyde gas-sensing properties of Yb-doped In2O3 nanotubes indicate that the as-synthesized nanomaterials can be used as a promising candidate to detect formaldehyde in practical applications.
2016, 65 (3): 036803. doi: 10.7498/aps.65.036803
Recently, terahertz radiation has been a branch of cutting-edge science and technology involving many fields such as public security, military defense and national economy. In the past, far-field measurements were widely carried out based on terahertz time-domain spectroscopy. But the spatial resolution is limited by far-field diffraction effect. In order to break diffraction limit and gain sub-wavelength spatial resolution in terahertz frequency region, a series of near-field detection methods came into being, such as confocal microscopy, using an aperture, guided mode, scattering, direct detection in the near-field, etc. Each method has its own advantages and disadvantages. Using the photoconductive-antenna tip is one of the direct detection methods and it delivers the possibility of near-field measurements of terahertz waves. In this method, the photoconductive-antenna tip is a tapered photoconductive tip probe. So it can be close enough to the sample surface and receive the near-field signal on the basis of principle of photoconductivity. In this way, high spatial resolution can be gained. In this article, we introduce our recent progress of near-field and far- field scanning terahertz spectroscopy system with photoconductive-antenna in detail. Firstly, we analyze and summarize the near-field detection methods that have been developed in these years. And then, using the femtosecond laser whose center wavelength is 800 nm and the photoconductive-antenna tip detector coupled with fiber, we construct fiber near-field/ far-field scanning terahertz spectroscopy (N/F-STS). The frequency bandwidth is in a range from 0.2 THz to 1.5 THz and the terahertz spot is circular and uniform indicated by performance test. Also the amplitude and phase of the terahertz field are recorded simultaneously. It has the ability to perform three-dimension scan in various experiment conditions conveniently. Finally, we introduce the real applications in our laboratory. N/F-STS can be used to scan spatial electric distribution in three dimensions and test the spectral properties in terahertz range like other traditional far-field methods. Nevertheless, the most importantly, N/F-STS is used to scan the terahertz near-field of samples, such as terahertz surface plasmon polaritons, etc. The presented method thus is useful in some application areas, such as metamaterials, graphene, surface plasmons, waveguide transmission, near-field imaging, biological test, and chip inspection.
2016, 65 (3): 036804. doi: 10.7498/aps.65.036804
The dynamical evolution process of nanoscaled film on a solid substrate depends on many factors, such as the properties of thin film, the characteristics of the substrate, and the external environment. It is essential to elucidate the influences of these factors for our understanding self-organized growth of nanoparticles and the dewetting/detachment mechanism of nanofilm on a solid substrate. In the present paper, we investigate the dynamical dewetting/detachment of metal Au and Pt nanofilm on a graphene/graphite substrate at high temperature by using the molecular dynamics simulation technique. We discuss the influences of metal-substrate interaction, temperature and thickness of film on the dewetting dynamics. Our results reveal that the Au and Pt nanofilms with the same initial thickness on graphene substrates manifest different dewetting dynamical processes at high temperatures. Some nanoscale holes are formed randomly during the dewetting of Pt nanofilm with a thickness of less than 0.6 nm because of the strong interaction between the Pt films and substrate. In contrast, no hole is observed and a nanodroplet is formed directly by high temperature dewetting for Au nanofilm with the same initial thickness as that of Pt nanofilm. The resulting Au and Pt nanodroplets move in the vertical direction due to the surface tension and the constraint of the solid substrate. A high-temperature nanodroplet will be detached from the graphene substrate surface at a constant speed. Interestingly, the values of detachment velocity (vd) of nanodroplets show different dependences on initial thickness for Au and Pt nanofilm, respectively. In a thickness range of 0.2-2.3 nm, the vd of Pt nanodroplet increases and then decreases as the thickness of nanofilm increases. However, the vd of Au nanodroplet decreases gradually and then increases steeply as the Au nanofilm turns thicker. The different thickness dependences of vd for Au and Pt nanofilms are analyzed qualitatively by considering different metal-substrate viscous dissipations. In addition, the detachment time (td) of a dewetting metal film is also related to the temperature and the thickness of substrate. Our results demonstrate that the td decreases monotonically with the decrease of film thickness and the raise of temperature. These results provide a theoretical guideline for industrial production processes, such as metal coating, flotation, and the surface cleaning.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2016, 65 (3): 037101. doi: 10.7498/aps.65.037101
Based on density functional theory and using the first-principle method, the electronic structures of -Fe(C) with Cr, Mo, and Ni are calculated within the generalized gradient approximation (GGA)-PW91. Meanwhile, the effects of alloys on the bonding characters and austenitic stability are studied by the overlap population, mulliken charge population, density of states, charge density difference, and cohesive energy. The results show that there coexist the metallic bond, covalent bond and weak ionic bond in each of -Fe(C)-Cr and -Fe(C)-Mo unit cell, while only metallic bond and covalent bond coexist in -Fe(C)-Ni. The bonding orbit is mainly determined by the interactions between the d orbits of alloy atoms and the orbits of Fe 3d and C 2p. Moreover, the effects of Cr, Mo, Ni solution on austenitic stability are investigated by studying the influence of alloy element on -Fe(C) electronic structure.
First-principle study of the magnetism and photocatalyticactivity of RE(La/Ce/Pr/Nd) doping anatase TiO2
2016, 65 (3): 037102. doi: 10.7498/aps.65.037102
The models of pure TiO2 and La/Ce/Pr/Nd singly doped TiO2 are established by using the plane wave potential based on density functional theory. After geometry optimization, the stability of the mixed structure is analyzed by calculating the formation energy. Then the magnetic state of each doped model is analyzed by calculating the spin electronic state density. The analyses are verified by comparing magnetic ground state energies. Finally the influences of each rare earth element on band structure and absorption spectrum of TiO2 are discussed. The results show that La/Pr doped TiO2 presents ferromagnetism, Nd doped TiO2 exhibits anti-ferromagnetism, and Ce doped TiO2 is paramagnetic body. The band structure of TiO2 is affected less because Ce is doped and the red shift of absorption spectrum is not obvious. While visible light absorption coefficient of TiO2 is effectively improved because La/Nd is doped. Pr doped TiO2 manifests an absorption peak in the infrared region. If the electronic structure is considered in the further calculation research, one should make sure what magnetic state the system is in and whether there is spin-electron band splitting effect firstly. In order to obtain the correct results, the influence of the band structure should not be ignored.
2016, 65 (3): 037103. doi: 10.7498/aps.65.037103
The studies on absorption spectra of Y-doped ZnO have presented two distinctly different experimental results, which are the red shift and blue shift on the optical bandgap and absorption spectra when the mole fraction of impurity increases from 0.0313 to 0.0625. To solve this contradiction, the calculations in this paper are carried out by the CASTEP tool in the materials studio software based on the first-principal calculations of norm conserving pseudopotential of the density functional theory, and the geometric structures of ZnO, Zn0.9687Y0.0313O, Zn0.9583Y0.0417O and Zn0.9375Y0.0625O systems are constructed. By using the method of GGA+U, we calculate the band structure, density of state, electron density difference, population, orbital charges and absorption spectrum. The results show that with the doping amount increasing from 0.0313 to 0.0625, both the lattice parameters and the volume of doping system increase: the higher the total energy of the doping system the higher the formation energy of the doping system is, thereby making doping difficult and the stability of the doping system lower Increasing Y-doping concentration weakens the covalent bond, strengthens the ionic bond; as Y doping concentration increases, the Mulliken bond populations and bond lengths of Y-O parallel and vertical to c-axis decrease for the doping system. Meanwhile, the more the Y doping content, the wider the optical bandgap of the doping system becomes and thus more significant the blue shift of absorption spectra of Y-doped ZnO systems will be. The calculation results of absorption spectra of Y-doped ZnO system are consistent with the experimental data. And the contradiction between blue shift and red shift of absorption spectra of Y-doped ZnO system is explained reasonably. These results may contribute to the improvement of the design and the preparation of short wavelength optical devices from Y-doped ZnO.
2016, 65 (3): 037301. doi: 10.7498/aps.65.037301
Ti is intentionally added into La2O3 to prepare LaTiO gate dielectric Ge metal-oxide-semiconductor (MOS) capacitor with both high k value and good interface quality. In order to examine the effects of Ti content on the electrical properties of the device, LaTiO films with different Ti/La2O3 ratios (10.6%, 18.4%, 25.7% and 31.5%) are deposited by reactively co-sputtering Ti and La2O3 targets. Capacitance-voltage curves, gate-leakage current properties and high-field stress characteristics of the devices are measured and analyzed. It is found that some electrical properties, such as interface-sate density, gate-leakage current, device reliability and k value, strongly depend on Ti content incorporated into La2O3. Ti incorporation can significantly increase the k value: the higher the Ti content, the larger the k value is. The relevant mechanism lies in the fact that higher Ti content leads to an increase of Ti-based oxide in the LaTi-based oxide, because Ti-based oxide has larger k value than La-based oxide. On the contrary, interface quality, gate-leakage current and device reliability deteriorate as Ti content increases because Ti-induced defects at and near the interface increase with Ti content increasing. Of the Ti/La2O3 ratios in the examined range, the largest Ti/La2O3 ratio is 31.5%, which results in the highest k value of 29.4, the largest gate-leakage current of 9.710-2 Acm-2 at Vg=1 V, the highest interface-sate density of 4.51012 eV-1cm-2 and the worst device reliability, while the La2O3 film without Ti incorporation exhibits the lowest k value of 11.7, the smallest gate-leakage current of 2.510-3 Acm-2 at Vg=1 V, the lowest interface-sate density of 3.31011 eV-1cm-2 and the best device reliability. As far as the trade-off among the electrical properties is concerned, 18.4% is the most suitable Ti/La2O3 ratio, which leads to a higher k value of 22.7, lower interface-sate density of 5.51011 eV-1cm-2, an acceptable gate-leakage current of 7.110-3 Acm-2 at Vg=1 V, and a better device reliability. In view of the fact mentioned above, excellent electrical properties could be obtained by setting Ti content to be an optimal value. Therefore, the optimization of Ti content is critical for LaTi-based oxide Ge MOS device preparation.
2016, 65 (3): 037401. doi: 10.7498/aps.65.037401
LaNiC2 is one of ternary RNiC2 compounds, where R is a rare earth or Y. Its space group is Amm2. the symmetry along the c-axis of the crystal structure lacks inversion symmetry along the c-axis. In 2009, Hillier et al. performed the muon spin relaxation experiment (upSR) which implied that time-reversal symmetry is broken in LaNiC2. As a weak correlation noncentrosymmetric superconductor, LaNiC2 has attracted wide research interest in recent years. Though a lot of theoretical and experimental studies have been carried out, the order parameter of this compound remains highly controversial. The measurements of specific heat and nuclear quadrupole relaxation suggest that LaNiC2 is normally BCS-like, which is further supported by theoretical calculations. But recently another study showed that the London penetration depth depends on T2 below 0.4 Tc indicative of nodes in the energy gap. Evidence of possible nodal superconductivity can also be inferred from the early measurements of specific heat given by Lee et al. However, the experimental results obtained by Chen et al. supported the existence of two-gap superconductivity in LaNiC2.Based on the above case, the two-band Ginzburg-Landau theory is used to study the temperature dependence of the upper critical field for the superconductor LaNiC2 in this paper. Choosing the Ginzburg-Landau theory for calculating the upper critical field is just because Ginzburg-Landau theoretical model is simple, easy to understand, low-calculation, and the clear physical meanings of the parameters. The theoretical results in this paper accord with the experimental data very well in the whole temperature range. The curve of Hc2 (T) has an obvious positive curvature near the critical temperature, which is typical feature of multi-gap superconductor. Therefore, our results show strong evidence that two-gap scenario is better to account for the superconductivity of LaNiC2, consistent with the results of Chen Jian et al. The influences of two different energy bands on the upper critical field are also studied. It is found that the relatively small coherent length has a grester influence on the upper critical magnetic field of LaNiC2. So if we want to improve the upper critical field of LaNiC2, reducing the relatively small coherence length can be achieved in theory.
2016, 65 (3): 037701. doi: 10.7498/aps.65.037701
Traditionally, a diamond anvil cell (DAC) operated at low temperature can be pressurized by using a helium-driven piston or remote control tightening mechanism. This approach of pressurizing DAC is not convenient for operating at low temperature. Here we develop a low-temperature pressurizing technique for in situ tuning pressure in DAC at 20 K by an electrically driven method. The improved DAC pressure apparatus is composed of traditional DAC device and a piezoelectric actuator (PZT). Here the PZT used in the experiment is the PSt 150/1010/40 supplied by the Piezomechanik. Both parts are assembled together in a red copper or stainless steel cylinder. The DAC part is thermally contacted with a low temperature holder for cooling the chamber of the DAC in the experiment. The wires of the PZT connect with the voltage source through the wiring terminals of the cryostat. As the DAC apparatus cools down, two electrodes of the PZT are connected together when a voltage difference between the electrodes is generated. When the temperature of the DAC chamber arrives at the presetting value, two electrodes of the PZT are connected with the voltage source for applying voltage to the PZT. In this paper, we find that the PZT stroke shows a linear increase with increasing voltage at 300 K, whereas it is approximately linear at 80 and 6 K. The maximum strokes are 40, 26 and 15 upm at 300, 80 and 6 K respectively when the applied voltage is 120 V. The experimental results show that the PZT-driven DAC apparatus can continuously generate pressure from 0.49 to 4.41 GPa at low temperature and applied voltage of 0-290 V, where at zero voltage an initial pressure of 0.49 GPa is generated by using driven screws of the DAC device at room temperature. The pressure in the DAC chamber is determined by the red shift of ruby florescence line. The calibrated chamber temperature in DAC is determined as a function of pressure (PZT voltage) by using the intensity ration (R2/R1) of ruby R2 and R1 fluorescence lines. We find that the chamber temperature only slightly increases with increasing pressure in a range of (19 1) K. The main difference between the present device and the other tuning DAC apparatus is that the force on the DAC can be conveniently applied by using PZT voltage. This guarantees a high pressure-tuned resolution in the experiment, e. g., we tune a single InAs quantum dot (QD) emission wavelength to match the cavity mode. Such a tuning technique is found to have applications in realizing a compact tunable single photon source or completing two-photon interference of Hong-Ou-Mandel experiments between the QD and nitrogen vacancy center in diamond or atom, respectively.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2016, 65 (3): 037801. doi: 10.7498/aps.65.037801
There often appears unwanted elasto-optical birefringence in optical devices such as electro-optical, magneto-optical modulators and sensors when they are manufactured and used. This kind of elasto-optical birefringence causes unwanted effect for optical device and needs to be removed. Based on the method of index ellipsoid analysis, we theoretically analyze elasto-optical effects in various point groups of optical crystals and glasses, and accordingly propose some methods to eliminate unwanted elasto-optical birefringences in optical crystals and glasses. Main conclusions show that for orthorhombic biaxial crystal and the light wave propagating along any one crystalline axis of the crystal, if there is no shearing stress and the two external normal stresses applied to the other two crystalline axes can keep a constant ratio related to crystal parameters including refractive index and photoelastic constants, then unwanted elasto-optical birefringence can be eliminated from relevant optical devices, typical crystals include potassium titanium oxide phosphate (KTiOPO4, KTP) crystal and rubidium titanium oxide phosphate (RbTiOPO4, RTP) crystal, which are usually used as electrooptic Q-switchers in laser systems. For all the uniaxial crystyals such as potassium dihydron phosphate (KH2PO4, KDP), beta-barium borate ( -BaB2O4, BBO) and lithiun niobate (LiNbO3, LN) crystals, cubic crystals of 43 m, 432, m3 m point groups such as bismuth germanate (Bi4Ge3O12, BGO) crystal, and optical glasses, if the two normal stresses applied to the x1- and x2- crystalline axes of the crystal are equal to each other and there is no shearing stress, or there exists only one normal stress applied to the x3- crystalline axis of the crystal, then for the light wave propagating along the x3- crystalline axis, unwanted elasto-optical birefringence in relevant optical devices can also be eliminated. The above-proposed method to remove unwanted elasto-optical birefringence is benificial to design, manufacture, and usage of related optical devices.
Thermoluminescence and optically stimulated luminescence characteristics of Al2O3:C films annealed at different tempeartures
2016, 65 (3): 037802. doi: 10.7498/aps.65.037802
-Al2O3:C crystal is a high sensitive luminescence dosemeter, and it possesses a high thermoluminescence (TL) sensitivity, approximately 40-60 times greater than LiF: Mg, Ti. However, the crystal growth requires sophisticated laboratories, high temperatures and highly reducing atmosphere. The fluorescence and TL characteristics of -Al2O3:C ceramic are similar to those of -Al2O3:C crystal, however, it shows three TL peaks. In this work, porous alumina membranes are prepared by two-step anodization in 0.5 M/L oxalic acid at 5 ℃. We investigate the influence of annealing temperature ( 600 ℃) on thermoluminescence (TL) and optically stimulated luminescence (OSL) characteristics of Al2O3:C films and discuss the influence mechanism. The scanning electron microscopy measurement reveals that Al2O3:C film possesses highly ordered nanopores with homogeneous dimensions arranged in a closed-packed hexagonal pattern. The energy dispersive X ray spectroscopy and the Fourier transform infrared spectroscopy results indicate that oxalic acid impurity is incorporated into the porous alumina membrane in the synthesis process, after the annealing treatment, the oxalic acid impurity decomposes and C2+ replaces Al3+, which leads to the formation of F+and the C content of samples increasing with elevated annealing temperature. The X-ray diffraction measurement reveals that Al2O3:C films annealed at different temperatures are amorphous. TL measurements show that the dominated peak of Al2O3:C film is centered at around 310 ℃, owing to the number of F+increasing with the annealed temperature increasing, under the same irradiation dose, the sample annealed at 600 ℃ has the greatest TL intensity. With the increase of the irradiation dose, the TL intensity increases and the dominated peak gradually shifts to high temperature, which is consistent with the general order kinetic model. The sample annealed at 600 ℃ has the greatest TL sensitivity and its TL response shows excellent linear characteristic in as dose range of 1-10 Gy, but shows super-linear behavior in a dose range of 10-120 Gy. The OSL measurements show that with the increases of the annealed temperature and the irradiation dose, the OSL initial intensity increases and each of all samples shows a typical exponential decay. Compared with the case of -Al2O3:C crystal, the fast attenuation rate of film is dramatically accelerated. In a dose range of 1-200 Gy, the OSL responses of all samples each show an excellent linear characteristic, the sample annealed at 600 ℃ has the greatest OSL sensitivity. Compared with TL response, OSL response of Al2O3:C film shows a wider range of linear dose response. In this paper we have made a beneficial exploration for Al2O3:C films as OSL dosimerer.
Photoacoustic and surface photovoltaic characteristics of L-Cysteine-capped ZnSe quantum dots with a core-shell structure
2016, 65 (3): 038101. doi: 10.7498/aps.65.038101
The study on photoelectronic characteristics of ZnSe quantum dots (QDs) is of significance for investigating its microelectronic structure and expanding its potential applications because ZnSe QD has low biologic toxicity. In the present paper, the surface photovoltaic and photoacoustic technologies, and laser Raman, X-ray diffraction, transmission electron microscopy and Foureier transform infrared spectroscopy spectrum are jointly used to probe the microstructures, the photoacoustic and surface photovoltaic characteristics of L-Cysteine-capped ZnSe QDs prepared by water-phase synthesis at different reflux temperatures. The results indicate that the ZnSe QDs with a mean grain size of about 3 nm has a core-shell ZnSe/ZnS/L-Cys structure, in which the sulfhydryl groups in ligand prefer reacting with Zn atom at the (220) face to form the ZnS shell layer between the core-ZnSe and ligand L-Cys. The results show that the QDs with n-type photovoltaic property display a wide range of surface photovoltaic response and weak photoacoustic signal upon the illumination of near ultraviolet to visible light as compared with others QDs with similar core-shell structures in II-VI group. Especially, the strong SPV response and the weak PA signal in a wavelength region of 350-550 nm imply that the photon energies in the range are almost all used to produce the surface photovoltaic (SPV) phenomenon instead of the thermal lattice vibration caused by non-radiative de-excitation process. This reveals the energy complementary relationship between the photoacoustic and the surface photovoltaic phenomena of the QDs. The PA signals appearing in a short wavelength range of 300-350 nm and the Raman peaks located in a high frequency ranges of 1120 cm-1, 1340 cm-1 and 1455 cm-1 are identified as relating closely to the multi-phonon vibration modes of ligand L-Cys. At low reflux temperature, the photoelectric threshold of the SPV response that relates to the core-ZnSe displays a red shift to a certain extent as compared with the bulk ZnSe. The narrowed bandgap may be attributed to quantum confinement effect of the QDs. In addition, the intensity of the SPV response that relates to the core-ZnSe gradually increases with the decrease of the reflux temperature. The results show that the above improved surface photovoltaic characteristics of the QDs may benefit from the reduced average grain size of the ZnSe QDs, thus causing its surface and small-size effects.
2016, 65 (3): 038102. doi: 10.7498/aps.65.038102
Se and MoSe2 nanoflakes are prepared in N2 environment by hot filament chemical vapor deposition through using Se and MoO3 powders as the source materials. The structures and compositions of Se and MoSe2 nanoflakes are systemically studied by using field emission scanning electron microscope, transmission electron microscope, energy dispersive X-ray spectroscope, micro-Raman spectroscope, and X-ray photoelectron spectroscope. The results indicate that the mixing of the Se and MoO3 powders directly affects the formations and structures of Se and MoSe2 nanoflakes. When the Se and MoO3 powders are fully mixed, the Se nanoflakes are formed, however the MoSe2 nanoflakes are formed under no mixture of Se and MoO3 powders. This is due to the fact that different reactions of Se and MoO3 powders in gas environment with or without mixing the Se and MoO3 powders are generated. The study of photoluminescence properties indicates that the photoluminescence peaks are generated at about 774, 783 nm and 783, 784 nm for the Se and MoSe2 nanoflakes, respectively, which are different from the photoluminescence properties of monolayer MoSe2 nanosheet. These outcomes can enrich our knowledge of the synthesis and optical properties of two-dimensional Se-based nanomaterials and will contribute to the development of optoelectronic devices of two-dimensional Se-based nanomaterials.
Simulation of the influences of surface topography of deposited layer on arc shape and state in arc based additive forming
2016, 65 (3): 038103. doi: 10.7498/aps.65.038103
The stacking deposition and the overlapping deposition are usually employed in arc based additive forming process, which will result in different surface topographies of deposited layer. Consequently, the shape and state, heat and mass transfer of electric arc will be affected by the surface topography of deposited layer. A three-dimensional numerical model of electric arc based on magnetic fluid dynamics, local thermodynamic equilibrium and optical thin assumption for arc based additive forming process with pure argon shielding gas is presented. Simultaneously, four kinds of deposited layer model with different surface topographies are established, which are the deposited layer models of planar substrate, namely the substrate without weld bead, deposited layer model of single-pass single-layer, deposited layer model of single-pass two-layers, and deposited layer model of overlapping. The numerical calculation is performed on condition that deposition current and the distance between the electrodes are constant. And the simulation results include the profile of electric arc, corresponding temperature field, flow field, current density, electromagnetic force, and the arc pressure distribution. The temperature field of planar substrate accords well with other researcher's experimental result, and the profiles of electric arc are in good agreement with images captured by high-speed camera. Surface topography of deposited layer plays a decisive role in determining the profile of electric arc under the same process conditions. The comparison of evolvement among the distributions on specified paths shows that the electric arc of planar substrate has higher temperature, velocity, current density and pressure in the arc center, arising from completely symmetrical deposition layer model and smaller contact area between the arc and the substrate; the number of layers of single-pass multi-layer deposited layer has little influence on various parameters of electric arc, but because the deposited layer height changes, the temperature and pressure on the outside of deposited layer have small deviation; asymmetric arc profile will form when the overlapping deposition is performed. There is a relatively low temperature in the arc center, resulting from larger contact area between the arc and the surface of deposited layer. In addition, the distributions of current density, electromagnetic force and pressure deflect to the deposited layer. The above conclusions can provide a theoretical basis for basic research and process decision of arc based additive forming, and it can also provide the parameters for the subsequent weld pool dynamics and metal transfer simulation.
Influence of magnetic flux density and cooling rate on orientation behavior of Tb0.27Dy0.73Fe1.95 alloy during solidification process
2016, 65 (3): 038104. doi: 10.7498/aps.65.038104
The rare-earth giant magnetostrictive material Tb0.27Dy0.73Fe1.95 is one of the most important functional magnetic materials. Their superior properties include high saturation magnetostrictive coefficient at room temperature, high electromechanical coupling coefficients, high output power, fast response, high energy density, and non-contact drive. Thus, they can be used to build sensors, precision machinery, magnetomechanical transducers, and adaptive vibration-control systems. In this material, the magnetic phase (Tb, Dy)Fe2 has a typical MgCu2-type cubic Laves phase structure and exhibits different magnetostrictive properties along different crystal orientations. The 111 direction of this phase is the easy magnetization axis, along which the linear magnetostriction is higher than other directions. Thus, researchers have focused on preparing (Tb, Dy)Fe2 with a crystallographic orientation along or close to the 111 direction. Generally, the directional solidification method is used to prepare the Tb0.27Dy0.73Fe1.95 alloy. However, a crystal orientated along the 110 or 112 direction is always obtained and both of these directions require a high external magnetic field for improved magnetostrictive performance. The 111 preferred growth orientation can be acquired using seed crystal technology. However, the relatively low growth velocity can cause the appearance of the linear (Tb, Dy)Fe3 phase which induces a high brittleness of the material. Therefore, new methods to prepare Tb0.27Dy0.73Fe1.95 products with high 111 orientation at higher growth velocity are required. In this paper, we solidify the Tb0.27Dy0.73Fe1.95 alloys under various high magnetic field and cooling rate conditions. We study the effects of the magnetic flux density and cooling rate on the crystal orientation of the (Tb, Dy)Fe2 phase and the magnetization behavior of the alloys. It is found that after field-treated solidification, a high 111 orientation of (Tb, Dy)Fe2 along the magnetic field direction can be produced. As a consequence, the magnetostriction without applying stress remarkably increases. By increasing the magnetic flux density applied during the solidification of the Tb0.27Dy0.73Fe1.95 alloys, the 111 orientation of (Tb, Dy)Fe2 could be obtained at higher cooling rates. Ranging from 4 T to 10 T, with increasing cooling rate the magnetic flux density, at which the 111 or 110 orientation of (Tb, Dy)Fe2 occurs, increases or decreases, respectively. The saturated magnetization of the alloys increases with increasing cooling rate. The application of the magnetic fields does not affect the saturated magnetization.
2016, 65 (3): 038105. doi: 10.7498/aps.65.038105
Railway plays a major role in our daily life and national economy. In recent years, people payed much more attention to the safety operation of the high-speed train. In fact, the rail cracks originate from surface micro cracks will directly affect the safety of high-speed train. Therefore, it is vital to detect the rail surface micro cracks. Numerous nondestructive testing methods have been developed and applied in the detection of high speed rail cracks, such as magnetic particle testing, eddy current testing, and ultrasonic testing, etc. However, all the above conventional methods could only achieve crack information from the point of one-dimensional signal but not effective for the detection of surface micro cracks. A surface defect detection method based on photoacoustic (PA) signal from high speed rail is proposed soas to detect the surface crack more exactly and visually. Simulation and experiments are designed to validate the proposed method. Firstly, three models of high-speed rail with transverse crack, oblique crack, and scale stripping are established respectively. Meanwhile, the PA effect is simulated by finite element analysis and K-wave. Then, PA image of the rail surface is reconstructed by time inversion reconstruction algorithm, and some parameters, such as the center frequency of ultrasonic sensor and the laser power are also confirmed in further simulation. Subsequently, an experimental platform is established to collect the actual PA signal from a rail surface and to reconstruct PA images of the rail surface and shallow layer. The crack appearing in PA images are clear enough to show the receive crack information, such as sizes, propagating directions, and locations, which can be used to evaluate the rail states and decide processing scheme. It is proved that clear images of rail surface and shallow layer can be received by the detecting method of high-speed rail surface defects based on photoacoustic signal, and the surface cracks can be detected effectively.
2016, 65 (3): 038201. doi: 10.7498/aps.65.038201
Surface segregation is a significant phenomenon due to its influence on many surface processes, such as corrosion, oxidation and catalysis. Defects and vacancies produced by ion irradiation in alloys used in reactors or other radiation environments may also induce surface segregation. In this work, we deposit AuCu3 film on a Si(111) substrate by magnetic sputtering. He+ and Au+ produced by pelletron are used to simulate radiation fields in reactors, and surface segregation induced by ion irradiation is investigated. SRIM software is used to simulate ion range and displacements produced in sample. Rutherford backscattering spectrometry is used to determine concentration changes near the surface of sample before and after irradiation. The results show that two kinds of ion irradiations lead to different surface segregation trends. When irradiated by 2 MeV He+, Au elements are segregated at the surface of sample. Oppositely, when irradiated by 1 MeV Au+, Cu elements are observed at the surface of sample. After analysis and discussion, we consider that this phenomenon is induced by different vacancy distributions by He+ and Au+ irradiation. 2 MeV He+ produced Au and Cu vacancies are distributed in whole film from surface to substrate smoothly, except very near the surface the concentration of vacancies has an obvious reduction. As a result, a gradient of the vacancy concentration is formed between the surface and the interior of the film. As the concentration of vacancies on the surface is lower than in interior, it would lead to vacancy diffusion from interior to surface, equivalent to diffusions of Cu and Au atoms along the opposite directions. Because of lighter atomic mass, Cu atom has a faster diffusion rate than Au atom. As a result, the concentration of Au atoms near the surface increases. Unlike He+, Au+ produces a mass of vacancies near the surface of the film, consistent with the Bragg peak by energy deposition of Au+, but decreases rapidly inside the film. It leads to a gradient of the vacancy concentration from surface to interior of the film. When vacancies diffuse from surface to interior, Cu and Au atoms diffuse from interior to surface, the lighter Cu atom concentration increases faster than Au atom concentration. Our research results explain the different segregation trends by light ion with higher energy and heavy ion with lower energy. It may help to understand the surface segregation of alloys used in complex irradiation field.
A fast two dimensional joint linearized bregman iteration algorithm for super-resolution inverse synthetic aperture radar imaging at low signal-to-noise ratios
2016, 65 (3): 038401. doi: 10.7498/aps.65.038401
In practical inverse synthetic aperture radar (ISAR), the traditional imaging algorithms have low range and low cross-range resolutions while the echoes have limited bandwidth and sparse azimuth aperture in small coherent processing interval. To obtain super-resolution ISAR imaging at low signal-to-noise (SNR) ratios, this paper puts forward a novel fast two-dimensional joint linearized Bregman iteration (2D-JLBI) algorithm based on compressive sensing theory. Firstly, the radar echoes are established as a two-dimensional joint sparse representation model in the range frequency-azimuth Doppler domain. Consequently, the original two-dimensional super resolution imaging problem is converted into a two-dimensional jointly compressive reconstruction problem. Secondly, to avoid the reconstruction complexity from the vectorization of the echoes, the two-dimensional joint linearized Bregman iterative algorithm is proposed. Meanwhile, three strategies, namely the weighted residual iteration, estimation of the stagnation step, and optimizing the condition numbers of sensing matrices, are combined to improve the convergence speed. Both the ISAR imaging ability at low SNR and its speed are improved obviously. Finally, simulation experiments show that the proposed algorithm can shorten the imaging time and have better noise robustness under the condition of sub-Nyquist sampling rate and low SNR.
Damage effects and mechanism of the GaN high electron mobility transistor caused by high electromagnetic pulse
2016, 65 (3): 038402. doi: 10.7498/aps.65.038402
As electromagnetic environment of semiconductor device and integrated circuit deteriorates increasingly, electromagnetic pulse (EMP) of device and damage phenomenon have received more and more attention. In this paper, the damage effect and mechanism of the GaN high electron mobility field effect transistor(HEMT) under EMP are investigated. A two-dimensional electro-thermal theoretical model of GaN HEMT under EMP is proposed, which includes GaN polarization effect, mobility degradation in large electric field, avalanche generation effect, and self-heating effect. The internal transient response of AlGaN/ GaN HEMT is analyzed under the EMP injected into the gate electrode, and the damage mechanism is studied. The results show that the temperature of device keeps increasing, and the rate is divided into three stages, which present a tendency of rapid-slow-sharp till burn-out. The first rapid increasing of temperature is caused by the avalanche breakdown, and then rate becomes smaller due to the decrease of electric field. As the temperature is more than 2000 K, a positive feedback is formed between the hot electron emission and temperature of device, which causes temperature to sharply increase till burn-out. The maximum values of electric field and current density are located at the cylinder surface beneath the gate around the source, which is damage prone because of heat accumulation. Finally, the dependences of the EMP damage power, P, and the absorbed energy, E, on pulse width are obtained in a nanosecond range by adopting the data analysis software. It is demonstrated that the damage power threshold decreases but the energy threshold increases slightly with the increasing of pulse-width. The proposed formulas P = 38-0.052 and E = 1.1 0.062 can estimate the HPM pulse-width dependent damage power threshold and energy threshold of AlGaN/GaN HEMT, which can provide a good prediction of device damage and a guiding significance for electromagnetic pulse resistance destruction.
Enhancement mode AlGaN/GaN double heterostructure high electron mobility transistor with F plasma treatment
2016, 65 (3): 038501. doi: 10.7498/aps.65.038501
Effects of double heterostructure materials (AlGaN/GaN/AlGaN/GaN) with different GaN channel thickness values (14 nm, 28 nm, 60 nm) on the high electron mobility transistor (HEMT) are simulated by using silvaco, and furthermore, the differences in characteristic among the enhancement mode devices made from such double heterostructure materials with different F injection doses (150 W, 135 W) are also simulated. The simulation results show that the threshold voltage shifts towards positive direction and the saturation current decreases as the GaN channel thickness decreases. The two-dimensional electron gas (2 DEG) density could be reduced as GaN channel thickness decreases due to piezoelectric polarization weakened by backing AlGaN barrier. Combining F plasma treatment and double heterostructure material, the enhancement mode device with high positive threshold voltage is successfully developed. The DC characteristics of the enhancement mode devices with different GaN channel thickness values are analyzed comparatively, and the simulation results are validated by using the experimental results. The threshold voltages of these enhancement mode devices with GaN channel thickness values of 14 nm, 28 nm, and 60 nm reach 1.1 V, 0.8 V, and 0.3 V, respectively. The maximum transconductance values of these enhancement mode devices with GaN channel thickness values of 14 nm, 28 nm, and 60 nm reach 115 mS/mm, 137 mS/mm, and 198 mS/mm, respectively. The thinner GaN channel thickness in the double heterostructure could reduce the depth of quantum well and 2 DEG density, so that the device with a GaN channel thickness of 14 cm has a lower saturation current. The breakdown voltages and gate reverse leakage currents of the three kinds of devices are investigated, and the device with a thinner GaN channel has a lower leakage current and higher breakdown voltage due to weakened vertical electrical field in thinner channel double heterostructure. The damage of channel mobility in F plasma treatment is weakened by using a lower plasma power (135 W), and the enhancement mode device without annealing process demonstrates a better saturation current and transconductance characteristic. The results of the device with annealing confirm that the plasma damage is depressed at an F injection power of 135 W. The threshold voltage temperature stability of 14 nm GaN channel thickness device is studied, and Vth is only 0.4 V after 350 ℃ 2 min annealing process. Drain induced barrier lowering (DIBL) effects of the HEMTs with double heterostructures are investigated, and the DIBL value of the14 nm GaN channel device is 16 mV/V. The DIBL value indicates a good limiting property of the 2 DEG in double heterostructure device.
2016, 65 (3): 038701. doi: 10.7498/aps.65.038701
Dissolution has attracted considerable attention since the dissolution is a common phenomenon in nature, and is of fundamental interest to reveal the morphology evolutions and microstructures of materials in materials science and pharmaceutical industry. A lot of research has been made in the field of crystal dissolution. And the solid-liquid interfacial energy is recognized as playing a key role in a wide range of material phenomena.The goal of the present study is to present analytical results for the dissolution of spherical crystal with the consideration of surface tension. In this review, we introduce the recent progress of spherical particle dissolution through similar studies. In our paper, a mathematical model is proposed to describe the dissolution process of a spherical crystal with moving boundary. The effect of surface tension through the Gibbs-Thomson condition is included in the mathematical model. And the dissolution of the spherical crystal is considered from the perspective of the concentration change of the solution. An asymptotic solution of the concentration and morphology for a spherical crystal in the dissolution is obtained by using the matched asymptotic expansion method. The results show that the surface tension has great effects on the concentration and interface shape of spherical crystal dissolution. As the surface tension parameter increases, the radius of the crystal decreases, the velocity of the spherical dissolution and the concentration of the solution increase. We have the conclusion that surface tension accelerates the dissolution process of the spherical crystal. And the larger the surface tension parameter, the faster the dissolution rate is and the shorter the dissolution time. The particle radius decreases with time going by, and the dissolution velocity increases with time increasing until the dissolution is completed. The concentration of the dissolution and interface shape of the spherical crystal can be calculated with the results obtained in this paper. It is shown that our analytical results accord well with the results obtained from the numerical results of Vermolen et al. [Vermolen F J, Vuik C, Zwaag S V D 2003 Mater. Sci. Eng. A 347 265].
Time-varying dynamic Bayesian network model and its application to brain connectivity using electrocorticograph
2016, 65 (3): 038702. doi: 10.7498/aps.65.038702
Cortical networks for speech production are believed to be widely distributed and highly organized over temporal, parietal, and frontal lobes areas in the human brain cortex. Effective connectivity demonstrates an inherent element of directional information propagation, and is therefore an information dense measure for the relevant activity over different cortical regions. Connectivity analysis of electrocorticographic (ECoG) recordings has been widely studied for its excellent signal-to-noise ratio as well as high temporal and spatial resolutions, providing an important approach to human electrophysiological researches. In this paper, we evaluate two patients undergoing invasive monitoring for seizure localization, in which both micro-electrode and standard clinical electrodes are used for ECoG recordings from speech-related cortical areas during syllable reading test. In order to explore the dynamics of speech processing, we extract the high gamma frequency band (70-110 Hz) power from ECoG signals by the multi-taper method. The trial-averaged results show that there is a consistent task-related increase in high gamma response for micro-ECoG electrodes for patient 1 and standard-ECoG electrodes for both patients 1 and 2. We demonstrate that high gamma response provides reliable speech localization compared with electrocortical stimulation. In addition, a directed connectivity network is built in single trial involving both standard ECoG electrodes and micro-ECoG arrays using time-varying dynamic Bayesian networks (TV-DBN). The TV-DBN is used to model the time-varying effective connectivity between pairs of ECoG electrodes selected by high gamma power, with less parameter optimization required and higher computational simplicity than short-time direct directed transfer function. We observe task-related connectivity modulations of connectivity between large-scale cortical networks (standard ECoG) and local cortical networks (micro-ECoG), as well as between large-scale and local cortical networks. In addition, cortical connectivity is modulated differently before and after response articulation onset. In other words, electrodes located over sensorimotor cortex show higher connectivity before articulation onset, while connectivity appears gradually between sensorimotor and auditory cortex after articulation onset. Also, the connectivity patterns observed during articulation are significantly different for three different places of articulation for the consonants. This study offers insights into preoperative evaluation during epilepsy surgery, dynamic real-time brain connectivity visualization, and assistance to understand the dynamic processing of language pronunciation in the language cortex.
Automatic seizure detection of electroencephalogram signals based on frequency slice wavelet transform and SVM
2016, 65 (3): 038703. doi: 10.7498/aps.65.038703
Over 50 million people all over the world are suffering from epilepsy It is of great significance to achieve automatic seizure detection in electroencephalogram (EEG) signal for clinical diagnosis and treatment. In order to achieve automatic diagnosis of epilepsy, a multitude of automated computer aided diagnostic techniques have been proposed. However, only a few of studies lay emphasis on the effects of different rhythm signals. To explore the influence of rhythm signals on classification accuracy, a newly-developed time-frequency analysis method called frequency slice wavelet transform (FSWT), which is able to locate arbitrary time-frequency range with the use of frequency slice function and whose inverse transformation only relies on fast Fourier transform, is employed to extract five different rhythm signals, namely (0.5-4 Hz), (4-8 Hz), (8-13 Hz), (13-30 Hz) and (30-50 Hz) from original EEG signal. Subsequently, for extracting the nonlinear and linear features, the approximate entropy of each rhythm signal and fluctuation index of adjacent rhythm signals are calculated to reflect the variation characteristics of rhythm signals and they are flocked together to form the nine-dimensional feature vectors. Finally, the extracted vectors are fed into a support vector machine (SVM) which is optimized by genetic algorithms (GA) for classification. Specifically, since the parameters of SVM are associated with the final classification accuracy and appropriate parameters could lead to a remarkable result, GA is applied to parameter optimization, half of the obtained vectors are randomly selected as a training set for training, and the remaining vectors constitute a testing set to test the established model. Experimental results of the proposed approach, which is employed in a public epileptic EEG dataset obtained from department of epitology at Bonn University for validation indicate that the proposed method in this study can carry out the task of classifying normal, inter-ictal and epileptic seizure EEG signals with a high classification accuracy (98.33%), a sensitivity of 99%, a specificity of 99%, and a positive predictive value of 99.5%. The presented approach provides an outstanding scheme for the automatic diagnosis of epilepsy, and the directions of our further research may include the application of the proposed method to the diagnosis of other disorders.