Vol. 56, No. 6 (2007)
2007, 56 (6): 3306-3312. doi: 10.7498/aps.56.3306
Based on the mass-energy relation of Einstein's relativity theory, the mass of solid lattices is divided into two parts: the rest mass of solid atoms and the equivalent mass of thermal vibration energy of lattices. The latter is exactly the equivalent mass of the phonon gas in a solid. The vibration energy of the solid lattices includes the thermal energies consisting of the rest mass of the solid lattices and the equivalent mass of the phonon gas. The state equations for the lattice rest mass and the phonon gas are deduced based on the state equation of solids. The heat conduction is just the motion of the phonon gas in a solid. The conservation equations for the phonon gas motion are established. It is found that the conservation equation of phonon gas momentum degenerates to the Fourier's conduction law when the inertial force of the phonon gas can be ignored. The physical nature of the Fourier's law is the balance between the driving force and the resistance for the motion of the phonon gas. Under ultrahigh heat flux conditions where the inertial force is too high to be ignored, the Fourier's law is no longer valid even under the steady condition.
2007, 56 (6): 3630-3635. doi: 10.7498/aps.56.3630
By combining femtosecond laser with the scanning near-field optical microscope (SNOM), we have successfully realized the femtosecond-resolved near-field optical system. Through high frequency acousto-optic modulation and difference frequency lock-in detection, the signal-to-noise ratio was greatly improved and the background signals of the pump and probe light were removed. Therefore, we can measure a femtosecond-resolved weak transmission change in the collection mode SNOM. Spatial resolution of 80nm and temporal resolution better than 200fs were achieved simultaneously. Using this system, we studied the relaxation processes of hot electrons in gold nanostrucures and the difference in relaxation dynamics was observed at different sample positions.
2007, 56 (6): 3031-3038. doi: 10.7498/aps.56.3031
We apply the direct perturbation method to an integrable nonlinear Schrdinger equation with a correction term to obtain its asymptotic solutions. It is shown that there is a good qualitative agreement between the asymptotic and the exact solutions when ε is small enough. Then the direct perturbation method is applied to the perturbed N-component nonlinear Schrdinger equations and their asymptotical solutions are obtained.
2007, 56 (6): 3039-3042. doi: 10.7498/aps.56.3039
A new type of conserved quantity which is directly induced by Mei symmetry of Hamilton system is studied. Firstly, the definition and criterion of Mei symmetry for Hamilton system are given. Secondly, a coordination function is introduced; the conditions from which the new type of conserved quantity can be induced by Mei symmetry and the form of the new type of conserved quantity are obtained. Lastly, an illustration example is given. The result indicates that the coordination function should be selected properly according to the demand of the gauge function, thereby the gauge function can be find out more easily. Furthermore, since the choice of the coordination function is not unique more conserved quantities of Mei symmetry for Hamilton system can be obtained.
Perturbation of Lie symmetries and a type of generalized Hojman adiabatic invariants for variable mass systems with unilateral holonomic constraints
2007, 56 (6): 3043-3049. doi: 10.7498/aps.56.3043
In this paper, the perturbation problem of Lie symmetries and adiabatic invariants for variable mass systems with unilateral holonomic constraints are studied. Firstly, a type of generalized Hojman conserved quantity under general infinitesimal transformation is obtained. Then, based on the definition of high-order adiabatic invariants of a mechanical system, the perburbation of Lie symmetries for variable mass systems with unilateral holonomic constraints under small disturbance is discussed and a type of generalized Hojman adiabatic invariants are given. At last, an example to illustrate the application of the results is given.
2007, 56 (6): 3050-3053. doi: 10.7498/aps.56.3050
The differential equations are expressed partially by the equations of Hamilton system and then they can be written in the contravariant algebraic form. The Poisson integration theory of the equations is presented and an example is given to illustrate the application of the result.
2007, 56 (6): 3054-3059. doi: 10.7498/aps.56.3054
The perturbations of Lie symmetries and adiabatic invariants for holonomic mechanical systems in event space are studied. Based on the definition of high-order adiabatic invariants of a mechanical system, the perturbation of Lie symmetries for the system under the action of small disturbance is investigated, and a type of Hojman high-order adiabatic invariants of the system are obtained. The conditions for the existence of the adiabatic invariants and the form of the adiabatic invariants are given. An example is presented to illustrate the results.
This paper studies the Lie symmetries of discrete Lagrange systems. Based on discrete variational principle, the equations of motion for the discrete systems are established. Definition of Lie symmetry and determining equations for the discrete equations of motion are given. An example is presented to illustrate the application of the results.
2007, 56 (6): 3064-3069. doi: 10.7498/aps.56.3064
We solved the Burgers equation by using the homotopy analysis method (HAM) and obtained its approximate kink solitary wave solutions. The solutions agree very well with the exact solutions. The results indicate that the HAM is valid for finding the solitary wave solutions of a class of nonlinear evolution equations. We also made some efforts to extend the HAM. As an example, we got the bell-like solitary wave solutions of KP equation by this approach. Our extended method can be used to find the approximate solitary wave solutions for more nonlinear evolution equations in an easy way.
There have been great theoretical and practical interests in investigating time problem in quantum mechanics. Because time enters quantum mechanics as a parameter rather than a dynamical operator, people have to consider how to construct a time operator and calculate average time whenever they are faced with a time problem related to a physical process. In this paper we present a general investigation on time operator and average time.
2007, 56 (6): 3076-3082. doi: 10.7498/aps.56.3076
For the Bose-condensed gas in a combined potential consisting of an axially-symmetric harmonic magnetic trap and one-dimensional (1D) optical lattice, using the mean-field Gross-Pitaevskii (G-P) equation and the propagator method, we study the formation of matter-wave interference upon releasing the condensate from the combined potential. The analytical result of the wave function for matter wave interference at any time t is given, and the relation between the interference fringe period and the optical lattice period is obtained. The interference pattern of Bose-condensed gas in the optical lattices is analogous to the multiple slit interference pattern in optics, and coincides with the results of experiment performed by Andrews et al. and Peil et al. and the numerical solution of the G-P equation by Sadhan K. Adhikari.
2007, 56 (6): 3083-3087. doi: 10.7498/aps.56.3083
By taking into account the particle's self-gravitation interaction, Hawking radiation of the uncharged and charged particles as tunneling from a Reissner-Nordstrm-anti-de Sitter black hole has been studied firstly by using the Parikh-Wilczek's semi-classical tunneling approach. Next, we generalize the complex path method to reexamine the same question. Results obtained from both methods support the viewpoint of Parikh and Wilczek's, that is, the tunneling rate is related to the change of Bekenstein-Hawking entropy, and the actual emission spectrum is not exactly thermal.
2007, 56 (6): 3088-3091. doi: 10.7498/aps.56.3088
By using Darboux transformation method, we study analytically the nonlinear dynamics properties of Bose-Einstein condensates trapped in an stable external poten tial. It is shown that the strength of inter-atomic interaction condensate has an important effect on its nonlinear dynamics properties. When atom-atom repulsive interaction of condensate is strong enough, there only exhibits bright soliton. While the inter-atomic repulsive interaction is less than a critical value, the alternative evolvement phenomenon between bright and dark soliton appears.
2007, 56 (6): 3092-3100. doi: 10.7498/aps.56.3092
This paper presents a novel chaos-based self-embedding secure watermarking algorithm based on the chaotic pseudorandom ness and sensitivity to initial value of chaotic map. In the proposed approach, the real-valued chaotic sequence, which is generated by the given initial value as secret key, is used to produce the ordered indices as the watermark embedding positions. The proposed algorithm has a larger key space than the existing self-embedding watermarking algorithms due to its watermark embedding positions being randomly selected by the secret key. Furthermore, the problem, how to locate the tampered image block in the self-embedding watermarking algorithm has been solved successfully. Theoretical analysis and simulation results show that our scheme not only improves the abilities of tamper localization, but also has better security against many attacks, including the vector quantization and synchronous counterfeiting attacks.
2007, 56 (6): 3101-3106. doi: 10.7498/aps.56.3101
In this paper, the problem of lag synchronization for a class of chaotic systems with unknown parameters is proposed. Based on Lyapunov stability theory, lag controller and update law of parameters are obtained. This method is simple and systemic. A new chaotic system is taken as an example to illustrate the effectiveness of this proposed method. Numerical simulation illustrates the feasibility of this technique. To demonstrate the robustness against the effective of bounded noise of the proposed control strategy, it is applied to the new system and perfect simulation results are obtained.
A class of switchable three-dimensional chaotic systems is built. These systems have seven terms on the right-hand side, three of which are quadratic nonlinear. Some of its basic dynamical properties are studied detailedly, such as the feature of equilibrium, the chaos attractor, Lyapunov exponent and fractal dimension. A practical circuit is designed to realize these systems.
2007, 56 (6): 3113-3120. doi: 10.7498/aps.56.3113
In this paper, a new hyperchaotic system is constructed by introducing an additional state variable into the third-order Lorenz system. Some basic properties, including dissipativity, equlibria, stability and Hopf bifurcation, of this hyperchaotic system are analyzed in detail, and the bifurcation routes to hyperchaos from periodic, chaotic evolutions are observed. The existence of hyperchaos is verified with Lyapunov exponent spectrum. Moreover, an analog electronic circuit is designed, and various hyperchaotic attractors of this system are observed from the circuit experiments.
2007, 56 (6): 3121-3126. doi: 10.7498/aps.56.3121
This paper deals with the problem of generalized synchronization via T-S fuzzy models for two non-identical chaotic systems.A H∞ control method based on T-S fuzzy model is presented. A sufficient condition for synchronization of two non-identical systems is obtained based on Lyapunov functional method and matrix inequality techniques. Finally, an example is provided to demonstrate the effectiveness of the proposed method.
Approximate solving method for a generalized perturbed mechanism of western boundary undercurrents in equator pacific
2007, 56 (6): 3127-3131. doi: 10.7498/aps.56.3127
A class of perturbation mechanisms for the western boundary undercurrents in the equator Pacific is considered. The generalized governing equations are studied. Firstly, an optimal homotopic mapping is introduced. And the approximate solution is obtained for original model using the corresponding method of the homotopic mapping. Finally, an example shows that a good precision is altained for the approximate solution using the method of homotopic mapping.
2007, 56 (6): 3132-3136. doi: 10.7498/aps.56.3132
Given a chaotic drive system and the synchronization function, a response system was developed to implement linear generalized synchronization (GS) to the drive system. The method proposed here mainly utilizes the unidirectional coupling determined by white noise distribution. It is analytically proved that GS could be realized by using the white noise that satisfies certain conditions. In addition, the Chua's circuit was taken for illustration and verification.
A four-winged chaotic attractor was first observed numerically in a new 4-dimensional system. However, it was found to be a numerical artifact upon further analysis. It actually consists of two (upper and lower) coexisting double-wing chaotic attractors with domains of attraction independent of each other. The reason leading to the confusion is that both double-wing attractors are arbitrarily close to each other so as to cause a numerical error as well as a misunderstanding. By adding a simple linear state feedback term to the system, some similaritics of the system are destroyed, then the controlled system is able to generate diagonal double chaotic attractor which can cross the boundary between the upper and lower attractive domains. With the evolution of dynamical modes, the upper and the lower double-wing chaotic attractors as well as diagonal chaotic attractor are merged into a true four-winged chaotic attractor. At last, the frequency spectrum analysis shows that the four-winged chaotic attractor has extremely wide frequency bandwidth compared with that of the Lorenz system and Chua circuit, which is important in practical applications in communication encryption etc.
2007, 56 (6): 3145-3151. doi: 10.7498/aps.56.3145
The Hopf bifurcation characteristic in Langford system is analyzed and the control of Hopf bifurcation in this system is investigated by means of the method of linear state feedback control. The condition in which Hopf bifurcation occurs in the controlled system is arrived at in analysis and the analytical expressions of the limit cycles are obtained. Hopf bifurcation points are transformed and the stability of the limit cycles is controlled. The analytical results on controlling Hopf bifurcation points via the controllers agree well with the numerical results.
Research on the correlation between the mutual information and Lempel-Ziv complexity of nonlinear time series
2007, 56 (6): 3152-3157. doi: 10.7498/aps.56.3152
To explore the correlation between the mutual information and complexity of nonlinear dynamic system, the nonlinear time series of Logistic map， Lorenz model and cardiac RR intervals were used as the experimental data. The multi-segmented time-delayed mutual information， multi-segmented Lempel-Ziv complexity and their correlation coefficients were calculated. The results show that the mutual information of these series are strongly negatively correlated with the complexity. For the 201 series generated by Logistic equation, the absolute value of all correlation coefficients between the mutual information and the complexity of various segments are 0.9126 plus and the maximum reach to 0.9923， and for the 94 series of cardiac RR intervals， they are 0.8555 plus and the maximum reach to 0.9860. The investigations also indicate that the mutual information is more sensitive than the complexity in characterizing a nonlinear dynamic system.
2007, 56 (6): 3158-3165. doi: 10.7498/aps.56.3158
Based on the NS traffic model, we propose a cellular automaton traffic model for four-aspect fixed autoblock systems. We simulate the traffic phenomenon of train delay. We also investigate the effects of main factors including train interval, proportion of freight train, dwell time of the train in the station and unmber of platforms. Simulation results show that: (i) the freight train can restrain the running velocity of the passenger train following it; (ii) the delay may be relaxed by adjustment of the train's dwell time in the station; (iii) the increase in number of platforms in the station may improve the traffic density, however, the using frequency of the platform may be reduced when the train interval is large.
Research on the amplitude frequency characteristics compensation based on wavelet neural network for vibration velocity transducer
2007, 56 (6): 3166-3171. doi: 10.7498/aps.56.3166
A method of amplitude frequency characteristics compensation is presented to realize ultra-low frequency vibration measurement based on wavelet neural network(WNN) for vibration velocity transducer. In this method, a dynamic compensation network can be set up according to measurement data of dynamic response of vibration velocity transducer. The compensation principle is introduced and the geometrical structure of the network is analyzed and the algorithms for the training and initialization of network parameters are given. The weights of network, scale factor and displacement factor are trained by the steepest descent method and the network parameters initialization is integrated with the wavelet type, time-frequency parameters of wavelet and the training samples. The results show that the proposed wavelet neural network has good robustness, on-line correction ability, and higher precision and faster training speed than the BP neural network when used in the amplitude frequency characteristics compensation of vibration velocity transducer, and has practical value in measurement field.
2007, 56 (6): 3172-3177. doi: 10.7498/aps.56.3172
Using the infrared thermal image real-time monitoring system developed independently in our laboratory, the infrared characteristics of H2SO4-H2O2-H2O droplet in different movement states on the surface of GaAs substrate is studied, including, for instance, the static state, the slow motion state and the quick motion state. The obtained experimental results and the merit of our system are discussed. The main conclusions drawn from the experiments includes: In the static state, the created heat produces convection inside the droplet, the peak value of infrared radiation distribution is located at the top of droplet and the heat diffuses to the ambient along GaAs substrate, which causes the radiation to drop steeply at the edge of droplet. In the slow motion state, the residual heat phenomenon revealing as “comet tail" is observed when the droplet moves across the substrate surface, and as the temperature decreases, the radiation peak “co-moves” with the droplet in the same direction, in addition, the displacement curve of the radiation peak lags behind the actual displacement of the droplet and the maximum temperature may appear in the “comet tail”. In the quick motion state, the phenomenon shows as “liquid film trace" because the droplet leaves the substrate without reacting with GaAs, and the value of infrared radiation varies smoothly, showing in the gray_scale pattern as a half ellipse extending from the edge to the center of liquid film. The distribution and characteristic variety of grey-scale in the center of the trace is also analyzed. The proposed infrared monitoring system of thermal behavior of moving droplets will prompt the research of the droplet itself as well as the interdisciplinary studies, in which, for example, the infrared technology being introduced into material sciences and chemistry sciences and so on.
2007, 56 (6): 3178-3184. doi: 10.7498/aps.56.3178
The absorption, Raman and upconversion luminescence spectra of Er3+-doped barium gallogermanate glasses have been measured and the upconversion mechanisms of Er3+-doped barium gallogermanate glasses have been analyzed. The absorption and Raman spectra show that barium gallogermanate glasses have the ultraviolet absorption edge of 275nm and the maximum vibrational energy of 828cm-1, respectively. Intense green and red emissions, excited by 800nm and 980nm laser diode, were observed at room temperature. With increasing Er3+ ion concentration, the green emission increases initially and then decreases, while the red emissions increases monotonicly. Under the excitation of 800nm laser diode, the green emission is mainly attributed to the excited state absorption from the 4I13/2 level and the red emission to the energy transfer between the 4I13/2 level and the 4I11/2 level. Under the 980nm laser diode excitation, the green emission mainly originates from the energy transfer between the 4I11/2 levels and the red emission from the energy transfer between the 4I13/2 and 4I11/2 levels as well as the excited state absorption of the 4I13/2 level. The calculation results of quantum efficiency for different pumping wavelengths show that the upconversion efficiency of the green emission in barium gallogermanate glass with 1mol% Er3+ ions reaches a maximum value under the excitation of 800nm laser diode.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2007, 56 (6): 3185-3191. doi: 10.7498/aps.56.3185
By using one-dimensional photon state-vector function obtained previously, a general analytic expression is deduced for the optical two-hole interference experiment, which not only greatly enlarges the range of the observable, but also completely represents the probability distribution for a photon in the whole space. On the basis of the basic rules of quantum optics, we quantitatively analyze and discuss the domain of definition of the experiment, and the quantum behavior of the interference item on the average of space.
Calculating physical failure probability of HTR-10’s residual heat removal system by response surface method
2007, 56 (6): 3192-3197. doi: 10.7498/aps.56.3192
Probability safety assessment has been widely used in nuclear plant safety research, but the reliability research of passive system is at a beginning level. The failure of passive system consists of components failure and physical process failure. Calculating the failure probability of physical process is a very difficult task due to the uncertainties of the factors which affect the physical process, and the method for the calculation is quite different from that of active system. The paper introduces response surface method to calculate the failure probability, and applies the method to get the approximate failure probability of residual heat removal system in HTR-10, the 10MW high temperature gas reactor designed by the Institute of Nuclear and New Energy Technology at Tsinghua University.
ATOMIC AND MOLECULAR PHYSICS
Model potential calculations of oscillator strength spectra of lithium atoms in parallel electric and magnetic fields
2007, 56 (6): 3198-3204. doi: 10.7498/aps.56.3198
We present oscillator strength spectra of Rydberg lithium atoms in parallel electric and magnetic fields obtained via the B-spline basis set method combined with model potential. Both the radial and angular wavefunctions are expanded in terms of B-spline functions. The results are in good agreement with previous high-precision experiments. By analyzing the components of partial-wave, we give an explanation to the oscillator strength of part of the spectrum. The method of this paper is simple and effective. It is suggested that the present method can be applied to deal with the problem of Rydberg atom in crossed magnetic and electric field.
2007, 56 (6): 3205-3211. doi: 10.7498/aps.56.3205
UV (390nm) light was used to modulate the rubidium partial pressure by inducing desorption of Rb atoms from the inner walls of the vacuum cell. A few mW/cm2 of UV light was enough to increase the rubidium partial pressure by a factor of 30 in about 1 s. After the source was turned off, the pressure quickly decayed back to its previous value with a time constant as short as 120 ms, allowing for long tapping lifetimes after the MOT holding phase. The number-lifetime product of the magnetic trap was shown as a function of the duration of the MOT holding phase, the maximum value was achieved after a MOT holding time of 1.25 s.
Study of the point defect, Re site, and Re cluster in Re embedded Ni3Al using the embedded atom method
2007, 56 (6): 3212-3218. doi: 10.7498/aps.56.3212
An analytical embedded atom method model of Ni-Al-Re system is constructed on the basis of the physical properties of pure constituents Ni, Al, and Re. The characteristic properties of Ni3Al, including equilibrium lattice constant, elastic constants, cohesive energy, vacancy formation energy, and antisite defect formation energy, are calculated. The types of point defects in Ni3Al are discussed, and it is shown that the point defect is the antisite defect in the nonstoichiometric case. Furthermore, the occupation site of Re in Ni3Al is analyzed, the clustering of Re in Ni3Al and Ni are also studied. The calculation results show that the occupational site of Re is the Al site, and that the growth tendency of Re cluster becomes weak when the size of Re cluster reaches to 11?. The present calculations are in agreement with the experiment and the theoretical results obtained by other authors.
2007, 56 (6): 3219-3226. doi: 10.7498/aps.56.3219
Geometric structures of CoBen(n=1—12) clusters are optimized using the generalized gradient approximation (GGA) density functional theory. The energy, vibrational frequency and magnetism are calculated. The results indicate that the spin multiplicities of the ground-state clusters are 2 and 4. Furthermore, the investigated magnetic moments confirm that the Co atomic magnetic moments of CoBen(n=1—12) clusters display an odd-even oscillation feature. In addition, the Co atomic magnetic moments of CoBe6 is the smallest of all clusters due to the strong hybridization between the 4s, 3d state of Co and 2s, 2p state of Be and short Co-Be average bond distance and low symmetry. By analyzing the properties of electrons, it is concluded that doping of impurity increases the stability and the chemical activity of Be cluster. It is found that CoBe5 and CoBe10 clusters are more stable than the neighboring ones.
CLASSICAL AREA OF PHENOMENOLOGY
2007, 56 (6): 3227-3233. doi: 10.7498/aps.56.3227
Gray-scale spatial correlation reflects the definition of images, and the main purpose of fusing images is to improve the definition of images. Based on the spectrum characteristics of low light level (LLL) whole-wave image and LLL short-wave image and the analysis of the one-dimension at gray-scale histograms and the two-dimensional gray-scale spatial correlation charts of LLL whole-wave image and LLL short-wave image, the new method of fusing dual-spectrum LLL images based on gray-scale spatial correlation is proposed. The method is realized by gray-scale selection based standard deviation and gray-scale statistics balance, and the method can effectively improve the definition of images and is easy to execute on hardware compared with gray-scale modulation method and spectrum-field method. The theorefical expressions of the fusion method are expatiated particularly, and the experiment results with different scenes are analyzed.
2007, 56 (6): 3234-3239. doi: 10.7498/aps.56.3234
This paper proposes a 8-neighborhood based generalized Hough transform (SGHT) method developed from the existing generalized Hough transform (GHT) method. The difference chiefly shows in the process of generating the Reference-Table and recognizing the graphics. The new method resolves the difficulty in the process of recognizing fragmented graphics from images by conventional GHT, which can be employed not only in the un-fragmented graphics recognition but also in the fragmented graphics recognition with significant precision and rapidity. The effectiveness of ENHT has been proved in our experiments.
2007, 56 (6): 3240-3245. doi: 10.7498/aps.56.3240
Thermal focal length of amplifier is investigated by analyzing the variation of beam radius after thermal lens. According to the theory of laser beam propagation, the expressions of the thermal focal length are obtained. Also, the preconditions and the deviations of the expressions are analyzed. Based on the theoretical analysis, the thermal focal length of the laser diode (LD) end-pumped Tm,Ho∶GdVO4 amplifier is measured.
2007, 56 (6): 3246-3250. doi: 10.7498/aps.56.3246
A new type of Gaussian-like broadband superfluorescent fiber light source suitable for optical coherence tomography (OCT) is introduced in this paper. Erbium-doped optical fiber simultaneously pumped by 980nm laser diode (LD) and 1480 nm LD is used as the gain medium. Both optical controllers and temperature controllers are employed to adjust the output of LDs and to improve the stability of output power of light source. A fiber coupler with a specific splitting ratio plus multi-stage long period fiber gratings are used to modulate the output spectrum so that the spectral profile of the broadband light source is near Gaussian. The output power is about 27mW with center wavelength at 1.57μm and the bandwidth over 75 nm. The experiment results reveal that the sidelobes of the autocorrelation function of the output spectrum are effectively suppressed, which can meet the need of optical coherence tomography.
Experimental investigation of a novel tunable all-optical wavelength converter with a double-ring cavity
2007, 56 (6): 3251-3254. doi: 10.7498/aps.56.3251
Exploiting the cascaded sum- and difference-frequency generation in a periodically poled lithium niobate (PPLN) waveguide, a novel scheme of tunable all-optical wavelength conversion is proposed and experimentally demonstrated by use of a PPLN-based double-ring cavity configuration. Wavelength conversion of picosecond pulses from the signal wavelength to the idler wavelength is realized. A pulsed signal source with a repetition rate of 40 GHz and a pulse width of 1.57ps is adopted. The pump and control waves are generated from the double-ring fiber laser. Therefore, no external continuous-wave (CW) light is required.
2007, 56 (6): 3255-3260. doi: 10.7498/aps.56.3255
Based on the split-step-Fourier-transform and the diffraction theory of hot-image model we made the simulation of the evolvement of hot-image in case of thick nonlinear medium. The changes of some parameters influencing the intensity and location of hot-image, such as the modulation coefficient (including amplitude modulation and phase modulation), the distance between scatterer and the front surface of nonlinear medium, the size of the scatterer, the intensity of incident beams and so on, were detailedly analysed respectively. Numerical results were found to be in agreement with analytical theory for the changes in modulation coefficient and the intensity of incident beams, but not in agreement with analytical theory for the changes in the size of the scatterer. Moreover, when the distance between scatterer and the front surface of nonlinear medium is zero, hot-image will also be found on the back surface of nonlinear medium.
2007, 56 (6): 3261-3265. doi: 10.7498/aps.56.3261
The coupling of two mutually incoherent optical beams with the same polarization and wavelength in two-photon photorefractive crystals is studied. It is shown that incoherently coupled dark-dark, bright-bright, and bright-dark soliton pairs are possible due to two-photon photorefractive effect.
Study of temperature characteristic of rigid holographic bright solitons based on photorefractive two-wave mixing
2007, 56 (6): 3266-3271. doi: 10.7498/aps.56.3266
The temperature dependence of the dynamical evolution of rigid holographic bright solitons in photorefractive dissipative systems based on two-wave mixing is studies. Numerical results show that the stability of rigid holographic bright solitons in such systems depends strongly on the crystal temperature. The soliton formed at a given temperature can propagate stably in the crystal over a distance that is long enough. The input solitary beam can evolve into a stable soliton and propagate in the crystal when the temperature drift is quite small, whereasit will not evolve into a stable soliton and its intensity becomes increasing or decreasing with propagation if the temperature difference is large. A potential application of temperature properties of rigid holographic solitons in optical attenuators or repeaters is discussed.
2007, 56 (6): 3272-3278. doi: 10.7498/aps.56.3272
By numerical simulation we theoretically study one type of high order coupled photovoltaic spatial solitons that can be realized by using four parallel beams with two different frequency components in photovoltaic crystals whose light-induced refractive index perturbation is positive. The numerical results show that this kind of coupled soliton can be approached by hyperbolic secant beams undersome suitable conditions. The optical steering properties of such a high ordercoupled soliton are investigated. It is shown that such a coupled soliton cannot be realized when the incident beams remarkably deviated from the lateral symmetry.
Vectorial chaotic synchronization characteristics of unidrectionally coupled and injected vertical-cavity surface-emitting lasers based on optical feedback
2007, 56 (6): 3279-3291. doi: 10.7498/aps.56.3279
Based on the Spin Flip Model (SFM), vectorial chaotic synchronization characteristics of vertical-cavity surface-emitting semiconductor lasers (VCSELs) with isotropic optical feedback is numerically investigated. The results are as follows. First, when the external cavity round-trip time equals the propagation time of light from the transmitter to the receiver, both the x-linear polarization (LP) mode and the y-LP mode can realize very high complete synchronization. Second, when external cavity round-trip time is not equal to the propagation time and μ is near the lasing threshold, the better completely synchronized quality of the dominant y-LP mode can be temporarily achieved. In comparison, the governing x-LP mode can only realized poor synchronization quality. In addition, with the system governed by the mixed LP modes, each LP mode can obtain inferior synchronization quality. However, with injection current much greater than the threshold current, the completely governed y-LP mode can steadily achieve the best chaotic synchronization quality. At last, each LP mode of the injection can achieve good injection-locking synchronization with that of the receiver. While each dominant LP mode can obtaine poorer injection-locking synchronization quality than the corresponding suppressed LP mode, with equal power, two LP modes both can achieve almost identical high injection-locking synchronization quality. Namely, the LP state with less power can be obtain better injection-locking synchronization.
2007, 56 (6): 3292-3299. doi: 10.7498/aps.56.3292
A novel device for generating femtosecond double pulses is constructed, which mainly consists of three Dammann reflection gratings. Being free from transmission elements, pulse distortion is avoided. Measuring the generated double pulses with a home-made second-harmonic generation frequency-resolved optical gating, experimental results demonstrated that this device can generate double pulses exactly equal in pulse intensity and pulse width with arbitrary delay time. This method should be highly interesting for double pulses generation and its practical applications.
2007, 56 (6): 3300-3305. doi: 10.7498/aps.56.3300
Based on self-imaging effect and overlapping-imaging properties of multimode interference couplers, the phenomenon of fully destructive interference of overlapping-images in MMI couplers is revealed, for the first time. The condition for overlapping-image fully destructive interference is derived. From the point of view of overlapping-image fully destructive interference, we explain the paired interference and the symmetric interference. The expressions of the positions and number of output images for two especial overlapping-images are also given. We prove that this kind of phenomena exists not only in the paired interference and the symmetric interference by way of an example. The influence of the positional number on the fully destructive interference images is also analyzed. And the guided-mode propagation analysis method is used to confirm the analytical results.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
The self-consistent theory of the electron distribution and electro-magnetic field of a relativistic hollow electron beam in ion-channel
2007, 56 (6): 3313-3317. doi: 10.7498/aps.56.3313
The effect of space-charge and relativistic self magnetic-field on the self-consistent equilibrium of the system with a relativistic hollow electron beam and ion-channel is studied. The distribution function and the relationship between location and energy of electron beam are derived by use of kinetic theory. The self-consistent field in ion-channel has also been established. It is clearly shown that the self-consistent equilibrium of the system exists. The relation of location of beam and radius of ion-channel was presented using numerical calculation. The distribution of electro-magnetic field in ion-channel is also given. The theory which is used in apparatus design is afforded by the research.
Study of high frequency characteristics of the rectangular waveguide grating slow-wave structure with arbitrary grooves
2007, 56 (6): 3318-3323. doi: 10.7498/aps.56.3318
The rectangular waveguide grating slow-wave structure (SWS) is a new type of RF system of millimeter traveling wave tube (TWT). However, it has narrow pass band. For the purpose of broadening the bandwidth of this circuit, it is necessary to study the influence of groove shapes on the characteristics. In this paper, the dispersion equation of a rectangular waveguide grating SWS with arbitrary grooves is derived by means of an approximate field-theory analysis, in which the continuous profile of the groove is approximately replaced by a series of steps, and the field continuity at the interface of two neighboring steps and the matching conditions at the interface between the groove region and the interaction region are ensured. The cold test on dispersion characteristics of a rectangular groove SWS shows that the theoretical results are in good agreement with the experimental results. We have calculated the dispersion characteristics and the coupling impedance of the slow-wave structures with some special groove shapes. It shows that the dispersion characteristics of the triangle-groove structure is the weakest and the coupling impedance of it is the lowest, while the dispersion characteristics of the inverted-trapezoid-groove structure is the strongest and the coupling impedance of it is the highest.
2007, 56 (6): 3324-3331. doi: 10.7498/aps.56.3324
In high perveance klystrons, violent variation of electron beam radius and electron transverse velocity is met with, and the energy exchange between kinetic electrons and gap field is a nonlinear process. For accurately describing these physical phenomena, we have put forward the corresponding physical model and further developed the 2.5-dimensional computation program LSP. Moreover, by use of this tool, we analyzed a practical S-band high peak-power klystron in detail. By comparing with the calculation results using 1-dimensional program and experimental data, we noticed that this poly-dimensional program can approach the real situation with higher precision.
Two kinds of hexagon emission patterns with different spatio-temporal symmetry in dielectric barrier discharge
2007, 56 (6): 3332-3336. doi: 10.7498/aps.56.3332
Two kinds of emission hexagonal patterns with different spatio-temporal symmetry are observed in lower and higher applied voltage range respectively by using a dielectric barrier discharge device with water electrodes. The spatial wavelength and filament diameter of the hexagon in higher applied voltage range are larger than that in lower voltage range. The hexagon in higher applied voltage range has a brighter background，while no background exists in the hexagon in lower applied voltage range. The spatio-temporal dynamics of the two kinds of hexagon patterns is investigated. It is found that the hexagonal pattern in lower voltage range is an interlacing of two rectangular sublattices, which have a time sequence inversion behavior. The filaments in the hexagon in higher voltage range discharge simultaneously. The influence of the wall charges on the spatio-temporal dynamics of the emission patterns is discussed.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2007, 56 (6): 3337-3343. doi: 10.7498/aps.56.3337
Solute diffusion controlled solidification model was employed to simulate the columnar to equiaxial transition (CET) of TiAl alloy casting. The simulation results show that solute shock wave occurs when the diffusion fields of columnar and equiaxial dendrites meet. It could be one of the main factors blocking the columnar dendrite growth. Using this model, the CET in small castings was qualitatively predicted. It was found that the thermal gradient and solute undercooling ahead of the columnar tips of CET act jointly on CET, being in good agreement with experimental observations.
Local structures of Si/Gen/Si(001) hetero-structure films studied by grazing incidence fluorescence X-ray absorption fine structure
2007, 56 (6): 3344-3349. doi: 10.7498/aps.56.3344
The local structures around Ge atoms in the Si/Gen/Si(001) hetero-structure films (consisting of 1, 2, 4 and 8 monolayers) prepared by molecular beam epitaxy at 400℃ have been investigated by grazing incidence fluorescence X-ray absorption fine structure (XAFS). The results show that for the Si/Ge1/Si(001) or Si/Ge2/Si(001) hetero-structure film, the Ge atoms are dominantly surrounded by Si atoms as the nearest neighbor. For the Si/Ge4/Si(001) hetero-structure film, the coordination environment around Ge atoms is close to that of Si0.70Ge0.30 alloy. Even for the Si/Ge8/Si(001) hetero-structure film, the fraction of Ge-Ge coordination pair in the first shell is only 55%. This suggests that under the growth temperature of 400℃, the Ge atoms have a strong ability to migrate into the Si capping layer. With the thickness of Ge layer increasing from 1 to 2, 4 and 8ML, the amount of migrated Ge atoms increases from about 0.5 to 1.5, 2.0 and 3.0 nominal ML. We consider that the migration of Ge atoms during the growth of the Si cap is mainly attributed to the surface segregation of Ge atoms, which leads to the decrease in surface energy as well as strain energy in the Ge layer.
Strain in AlInGaN thin films caused by different contents of Al and In studied by Rutherford backscattering/channeling and high resolution X-ray diffraction
2007, 56 (6): 3350-3354. doi: 10.7498/aps.56.3350
A series of AlInGaN films with different contents of Al and In were grown by metal-organic chemical vapor deposition on sapphire with GaN (>2μm) intermediate layer. Rutherford backscattering/channeling was used to measure the compositions and the crystal quality of the AlInGaN films. Combining with the high resolution x-ray diffraction, the lattice constants and the strain in perpendicular andthe parallel directions of the AlInGaN epilayer can be determined accurately bytheθ-2θ scan of (0002) and (1015) reflections and the reciprocal-space x-ray mapping. We find that different contents of In and Al can effect the strain in the quaternary AlInGaN films obviously, and with the help of Vegard's law we give an explanation for this phenomenon.
Optimization of GaAs (110) quantum well material growth technology by reflection high energy electron diffraction
2007, 56 (6): 3355-3359. doi: 10.7498/aps.56.3355
Recently, there is an increasing interest in the molecular beam epitaxy growth of various high quality heterostructures on the nonpolar GaAs(110) surface for the unique properties which arise from this unconventional orientation. Considering that the Ga and As atoms are coplanar in GaAs(110) surface, the range of bestgrowth temperature is small. It is difficult to find the best growth condition by observing the change of reflection high energy electron diffraction (RHEED) pattern because this kind of (1×1) RHEED pattern is insensitive to growth temperature and V/III beam equivalent pressure ratio. In the process of the GaAs(110) quantum well growth, we observed the single and double period variation of oscillation of RHEED intensity. This implies that there are two growth modes (monolayer-by-monolayer and bilayer-by-bilayer) of GaAs quantum wells growing on the GaAs (110) substrate under different growth conditions. The measurements of transmission electron microscopy and photoluminescence at room temperature showed that the quantum wells have very bad optical property under the bilayer_by_bilayer growth mode, while the quantum wells grown under the monolayer-by-monolayer growthmode have much better optical property with rough interfaces. By means of RHEEDoscillations, high quality quantum wells have been grown on GaAs (110) substrate under optimized growth conditions.
2007, 56 (6): 3360-3365. doi: 10.7498/aps.56.3360
We report the structural characterization and lattice damage induced by the energetic boron (four atoms in all) and nitrogen atoms (eight atoms in all ) with the energy of 500eV co-doped into diamond film and subsequent annealing by computer simulation based on Tersoff empirical potential. With the increasing number of implanted atoms, the affected volume is enlarged, the radius of the disrupted region reaches to 0.68nm for twelve implanting atoms. As a result of annealing, the atoms of the disrupted core can move for rather long distances up to several lattice sites. The mean bond length after annealing approaches the diamond bond length, as obtained from the pair distribution function. Since it is rich in vacancies on the near surface layer, after annealing the interstitial-vacancy recombination could take place, leading to the redistribution of the stress. Annealing at 800℃ is adequate for the recovery of the implanted diamond lattice.
Equivalent parallel RLC model in the mesoscopic structure of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy
2007, 56 (6): 3366-3373. doi: 10.7498/aps.56.3366
Based on the AFM observation of the mesoscopic structure and the XRD experimental results of Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy, we proposed a model for exploring the influence of the mesoscopic structure on the soft magntic properties of this kind of Fe-based nanocrystalline alloys, and calculate the frequency function——D-function for the soft magntic properties of Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy. With the frequency function, we succeeded in exploring the influence of the frequency on the soft magntic properties of Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy. Analysis shows that the D-function is a complex function, the real part Re(D) shows the inductance and capacitance of nanocrystalline Fe73.5Cu1Nb3Si13.5B9, the imaginary part Im(D) shows its resistance. We have built an equivalent parallel RLC model for the mesoscopic structure of Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy. Based on the model we have calculated vextGMI=v|Re(D)=0, which is the condition of maximum GMI value for Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy, as well as influence of the factors μ,σ,ω,R,Hex and the micro-magntic-structure on the maximum GMI value.
2007, 56 (6): 3374-3378. doi: 10.7498/aps.56.3374
To determinate the nucleation region of Si nanoparticles formed in gas phase, the single crystalline Si target with high resistivity was ablated by a XeCl excimer laser in pure Ar gas under the ambient pressure of 10Pa, and the nanocrystalline Si films were systemically deposited on pieces of glass or single crystalline (111) Si substrates lined up at a distance of 2.0cm under the plasma. The Raman and X-ray diffraction spectra, scanning electron microscope and atomic force microscope images of the films show that Si nanoparticles were formed on the pieces placed at horizontal distances 0.5 to 2.8cm from the target, the average size of Si nanoparticles monotonically decreased with the distance increasing. The region that Si nanoparticles form in gas phase was estimated on the basis of the PLA dynamics.
2007, 56 (6): 3379-3387. doi: 10.7498/aps.56.3379
A series of samples of Ti1-xCrxO2±δ were prepared by Sol-Gel process. The particle size, morphology, valence, solid solution range, phase relations and magnetic properties of Ti1-xCrxO2±δ samples were studied by XRD, SEM, XPS and SQUID measurements. Meanwhile, the Rietveld refinement method was employed to study the effect of Cr doping on the crystal structure ofTiO2. The results show that the solid solution range of both Ti1-xCrxO2±δ systems annealed at 1000℃ are x＝0—0.03. Their lattice parameters decrease with increasing Cr concentration. The results of both XRD analysis and magnetic measurements show that the solid solution range of both Ti1-xCrxO2±δ systems annealed at 1000℃ are x＝0—0.02. Cr ions of samples sintered at 500℃ and 1000℃ have two chemical states, Cr+3 and Cr+6， respectively, while the samples sintered at 1000℃ have much greater Cr+6 content than the ones at 500℃. According to the results of M-H and M-T measurements, magnetic properties of Ti1-xCrxO2±δ samples sintered at 500 ℃ with x=0—0.02 are ferromagnetic. When x≥0.04, they consisted of both ferromagnetic phase and paramagnetic phase. They have stronger ferromagnetic properties at low temperature (50K). The paramagnetic phase is the main constituent and the ferromagnetic phase has less volume fraction at room temperature.
2007, 56 (6): 3388-3392. doi: 10.7498/aps.56.3388
Using the Cottrell-Bilby kinetics model for solute atoms, evolution of the solute concentration around dislocation line has been investigated with respect to the strain rate. Three types of interactions between dislocation and solutes were obtained: At low strain rates, the dislocation is effectively pinned by its condensed solute cloud and the solute concentration remains almost at its saturation value. At high strain rates, the npinning effect predominates and dislocation moves at high velocity with little solute disturbance. While at the intermediate strain rates, the aging and unpinning effects take place alternately and dynamic strain ageing operates. Morevover, the N-shaped relationship between the rate-dependent flow stress and strain rate has also been naturally deduced from the model equations without additional assumptions.
2007, 56 (6): 3393-3399. doi: 10.7498/aps.56.3393
A complete model for studying the effect of radiation induced strain relaxation on electric performance of AlGaN/GaN HEMT was built utilizing the degree of strain relaxation related to radiation dose and energy as a basic variable. The effects of radiation induced strain relaxation on carrier density, threshold voltage and I-V characteristic were systematically investigated. The calculation results indicated that decrease in polarization induced charges as a result of strain relaxation causes decrease in carrier density, increase in threshold volgate and degradation in saturation drain current according to a linear rule. All results showed that the effect of radiation induced strain relaxation on HEMTs with high Al content appears more prominent. The calculation results also imply that strain relaxation is one of the important radiation damage mechanisms, the sensitivity of Al content to radiation induced strain relaxation degree need to be considered when improving the figure of merit of HEMTs by virtue of enhancement in Al content.
2007, 56 (6): 3400-3406. doi: 10.7498/aps.56.3400
Based on the two-step interface trap buildup model and the statistical thermodynamics mechanism of point defects in solids, a relation between the radiation induced increase of oxide hole-traps and the buildup of interface traps in MOSFET is proposed. Then, based on the correlation between pre-irradiation 1/f noise power spectral density and post-irradiation oxide-trap charge, a sim-empirical expression of pre-irradiation 1/f noise and post-irradiation interface traps buildup is established which agrees well with the experimental results. This model shows that the process of interface trap buildup was influenced by the increasing of oxide hole-traps that could dissociated hydrogen into proton under irradiation. So, the magnitude of pre-irradiation 1/f noise is directly proportional to post-irradiation interface-trap charge and sub-threshold slope. Our results not only proves that the 1/f noise can play a important role in MOSFET irradiation effects, but also proves a new nondestructive method to estimate MOSFET radiation response.
2007, 56 (6): 3407-3410. doi: 10.7498/aps.56.3407
The interaction between the charged particle and the crystal is described by quantum mechanics. The schrdinger equation describing a paticle moving in the channel is reduced to Mathieu equation by using a sine-squared potential. The band structure of this system is discussed based on Bloch theorem. The instability zone and the stop-band width are found by the perturbation techniques. It is shown that the width of the 1-st order and 2-nd order instability zones are ΔE1=Kβ/2 and ΔE2=md2p(Kβ)2/16π22, respectively, and this system presents automatical the band structure, which is not present in Lindhard potential and Moliere potentials.
Effect of solute atoms and precipitated phase in Al-4.5wt% Cu alloys on the spatiotempopal characteristics of the serrated yielding
2007, 56 (6): 3411-3417. doi: 10.7498/aps.56.3411
It is well accepted that the interaction between solute atom and precipitated phase leads to dynamic strain aging (DSA). In this research solution treatments at different temperatures are adopted to control the solute concentration in solid solution and precipitated phase content in the Al-4.5wt%Cu alloy material, and their influences on dislocation movement are analyzed by subsequent tensile experiments on these treated material specimens, and the corresponding micro-mechanism of DSA is discussed. It is found that, when the solution treatment temperature is reduced stepwise from 500℃, the amplitude of the serrated yielding decreases until it reaches a minimum at 300℃, whereas the amplitude begins to increase as the solution treatment temperature is reduced continually to 100℃. Meanwhile, the heat-treatment also affects the propagation characteristics of shear bands. The experimental results also show that, when the solution treatment temperature is higher than 300℃, the solute concentration is the key factor influencing DSA. However, when the solution treatment temperature is lower than 300℃, the intensity of DSA is determined by precipitated phase density. In the case of room temperature, the micro-mechanism of DSA is found to be primarily governed by precipitated phases.
2007, 56 (6): 3418-3427. doi: 10.7498/aps.56.3418
Phase transitions between the polytypes of boron nitride and the influence of defect and impurity on preparation of c-BN thin film are studied from energy and structure aspects. A pathway from h-BN to c-BN is analysed, namely: h-BN→r-BN→c-BN. The transformation from h-BN to c-BN is more difficult than that from r-BN to c-BN. The energy barrier is very high in direct transformation from h-BN to c-BN, but it is very low from r-BN to c-BN. In fact, defects and impurities in c-BN thin films may favorably drive the transformation from h-BN to c-BN. Defects and impurities can reduce the energy barrier for the transformation sufficiently enough for it to proceed under conditions obtainable in common laboratories. Based on the theoreticd model, we developed a new method to prepare c-BN thin film (three-step method). The effect of time and substrate bias voltage on the first step of preparation of c-BN thin film is investigated. The study proves that 5 min and -180V is very favorable. The c-BN thin film that content with cubic phase exceeding 80% can be repeatedly prepared using the three-step method. Results of experiments accord with the theoreticat model very well.
Sulphur-doped and boron-sulphur co-doped diamond thin films were prepared using chemical vapour deposition (CVD) on Si substrates under different conditions. The influence of doping on stress in CVD diamond films were investigated with X-ray diffraction and Raman spectra. The results show that the sp2-carbon content, the concentration of defects and the residual compressive stress in CVD diamond films increase with increasing of sulphur content. Compared with sulphur doping of diamond films, the boron-sulphur co-doping with few boron atoms facilitates sulphur atom incorporation into diamond crystal. The boron-sulphur complexes in diamond can reduce crystal lattice distortion and crystal imperfection. As a result, sp2-carbon content and residual compressive stress in boron-sulphur co-doped diamond are reduced, and diamond crystal prefection is improved.
Effect of N2 partial pressure on the defect properties and mechanical behaviors of nanoscale ZrN/WN multilayers
2007, 56 (6): 3435-3439. doi: 10.7498/aps.56.3435
ZrN/WN nanoscale multilayer coatings were synthesized with different N2 partial pressure by radio frequency magnetron sputtering. The vacancy-defect properties were measured by slow positron annihilation, and mechanical behaviors were studied using nanoindentation. The results indicate that the multilayer coating with N2 partial pressure of 0.4 Pa has the lowest concentration of vacancy-defects, and the average S-parameters of the central and the combination layers are 0.4402 and 0.4641, respectively. Lower or higher N2 partial pressure results in an increase in vacancy-defect concentration. With vacancy-defect concentration decreasing, the hardness and critical load of multilayer coatings increase. The multilayer coating with lowest concentration of vacancy-defects also exhibits maximum hardness and critical load (34.8 GPa and 100 mN). The decrease of vacancy-defect concentration is helpful to improve the mechanical properfies of multilayer coatings.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2007, 56 (6): 3440-3445. doi: 10.7498/aps.56.3440
The electronic and optical properties of ZnO doped with 3d transition metals with wurtzite structure are studied with density functional theory based on ultra-soft psuedopotential. The cohesive energy, density of state, band structure, and optical absorption properties are calculated. Furthermore, we analyzed the change of electron structure, bonding and optical properties after doping in comparison with the experimental results.
2007, 56 (6): 3446-3452. doi: 10.7498/aps.56.3446
The surface of the glass channel of microfluidic chip is treated with dichlorodimethylsilan. After the treatment, the silicon hydroxyl on the surface of the glass channel is silylated, which results in the reduction or even the elimination of the electroosmotic flow. Furthermore, full-potential linear-muffin-tin-orbital molecular dynamics method is used to investigate the micro -mechanism of surface reaction theoretically. The calculation results indicate that the hydrogen atom in silicon hydroxyl binds with the chlorine atom in dichlorodimethylsilan to form a stable HCl molecule and goes off, thereby the surface of the channel is covered with silylane.
Curie-temperature modulation by polarization-induced built-in electric fields in Mn δ-doped GaN/AlGaN quantum wells
2007, 56 (6): 3453-3457. doi: 10.7498/aps.56.3453
Transfer matrix method has been used to analyze Curie-temperature (TC) modulation by polarization-induced built-in electric fields in Mn δ-doped GaN/AlGaN quantum wells. Schrdinger equation is employed to calculate the quantum-confined subband energies and the distribution of their corresponding envelope fuctions. Based on these，we investigate the dependence of TC on the built-in electric fields in different structures of quantum well. By changing the asymmetry of double quantum wells (DQW)，TC can be raised up to 3 times.
2007, 56 (6): 3458-3463. doi: 10.7498/aps.56.3458
In the framework of density functional theory, the static-lattice equation of state and structural phase stability of gold have been calculated to 2 TPa by using the augmented plane wave plus local orbital method with two distinct exchange-correlation energies; corresponding to the generalized gradient approximation (GGA) and the local density approximation (LDA), respectively. The fcc structure is predicted to be the most stable phase at zero pressure, transforming to the hcp structure at 380GPa (GGA) or 490GPa (LDA), and the hcp structure transforming to the bcc structure at 1250GPa (GGA) or 790GPa (LDA), which then remains stable to up 2 TPa. The possible reasons of the phase-transition are analyzed from the electronic density of state at the different phases and volumes. The equation of state at T=0K is also given based on the calculated structural result.
2007, 56 (6): 3464-3467. doi: 10.7498/aps.56.3464
In this paper, we report the difference in electric conduction of monocrystal and polycrystal graphite. Under a magnetic field of 8.15T, the positive magnetoresistance ratio of monocrystal and polycrystal graphite samples are found to be 69400%, 170% respectively at 4.5K. The polycrystal graphite presents accordingly the insulator-like character in a wide interval of temperature under 0 and 8T applied magnetic field, but the magnetic-field-correlated semimetal-insulator-like Transition has been reported for the monocrystal graphite (highly oriented pyrolitic graphite, HOPG). On analysis, it is revealed that large difference in electric conduction originates predominantly from the Coulomb interaction in HOPG graphite layers in contrast with the crystal interface scattering in the polycrystal graphite.
2007, 56 (6): 3468-3472. doi: 10.7498/aps.56.3468
The growth, interfacial reaction, and electronic structure of Fe on ZnO(0001) surface were studied using synchrotron radiation photoemission spectroscopy(SRPES). It was found that Fe layer grows in a way similar to the Stranski-Krastanov mode at room temperature. After～2? Fe deposited on the surface, the growth mode changes from the layer-by-layer to a mixed mode. At low coverage, about one monolayer of iron (ca. 1.5?) is oxidized by the outer oxygen atoms on ZnO(0001) surface. Analysis of the valence band and the Fe3p photoemission spectra indicated that below 2? coverage, Fe2+ species exists at the Fe/ZnO interface, followed by the formation of a metallic iron overlayer. A sharp Fermi edge appears at 5.1? coverage, indicating the formation of the bulk metallic Fe film. Furthermore, work-function of the sample decreases to a minimum of 4.5eV at 0.2? Fe, then goes to 4.9eV with the formation of a dipole layer at the interface.
2007, 56 (6): 3473-3478. doi: 10.7498/aps.56.3473
Ag1-xPb18SbTe20(x＝0，0.25，0.5，0.75) samples were prepared by melt-quench-spark plasma sintering(SPS). As the Ag content decreased, a second phase Sb2Te3 occurred in samples and the carrier concentration increased to 5×1018cm-3 and then ceased to increase as Ag content decreased further. The carrier mobility decreased first, and then increased, as the Ag content decreased. As temperature increased, the carrier scattering mechanism shifted gradually from ionized impurity scattering to acoustic phonon scattering. As the Ag content decreased, the electric conductivity increased while the Seebeck coefficient decreased, and the thermal conductivity show an increasing trend.
2007, 56 (6): 3479-3482. doi: 10.7498/aps.56.3479
The ultraviolet light emitting diode based on n-ZnO-nanowire/p-Si heterojunction was fabricated. Quasi-arrays of ZnO nanowires were grown on p-Si substrates using a simple low-temperature hydrothermal method that would be easily extended to mass production. As-grown ZnO nanowires showed good crystallinity, a preferable c axial orientation, and strong ultraviolet emission under optical excitation. Different kinds of cathodes were made to form the electrical contact. The I-V characteristics were diode-like. Under forward bias, the heterojunction diode emitted strong ultraviolet light at 387nm and weaker green light.
2007, 56 (6): 3483-3487. doi: 10.7498/aps.56.3483
Based on the temperature-dependent current-voltage (I-V-T) measurements and the temperature-dependent capacitance-voltage (C-V-T) measurements of Schottky diodes fabricated on n-type GaN，the mechanism of the electrical current transport was discussed using thin surface barrier (TSB) model. The experiment results indicated that there are different mechanisms at different temperatures and bias. Based on this assumption we give a modified I-V characteristic formula which gives excellent fit to the experiment data. The SBHs determined from high-temperature I-V curves，low-temperature C-V curves，and the metal work function agree well each other.
2007, 56 (6): 3488-3492. doi: 10.7498/aps.56.3488
GaP nanorods with widths of 20—40nm and lengths of 200—500nm, and nanoparticles with diameters of 20—40nm were synthesized via the mild benzene-thermal route, using Na, P and GaCl3 as the starting materials. X-ray powder diffraction, Transmission electron microscopy and X-ray photoelectron spectroscopy were used to characterize the morphology and crystal structure of the products. The results indicated that when the reaction temperature is lower than 250℃, the products consist almost is GaP nanorods. The nanorods change into nanoparticles gradually with increasing temperature. When the reaction temperature exceed 280℃, the products are regularly particles. The reaction proceeds through a metallic gallium intermediate and a solution-liquid-solid mechanism is proposed for the one-dimensional growth.
Research of tricolor microcavity top-emitting organic light-emitting devices with white emitting layer
2007, 56 (6): 3493-3498. doi: 10.7498/aps.56.3493
RGB pixel generation by microcavity top-emitting organic light-emitting diodes (TOLEDs) is beneficial to both the reduction of the light loss and the improving the color purity and efficiency. Based on the multi-emitting layers, white organic light-emitting diodes (OLEDs) and microcavity TOLEDs were prepared. TOLEDs were made using Ag/ITO as the reflector and adjusting layer, Al/Ag as the semi-transparent cathode, and Alq：DCJTB/TBADN：TBPe/Alq：C545 as white light emitting layer. By adjusting the thickness of ITO, optical length of cavity the colour of the device can be changed, so the RGB tricolor devices are obtained. The peak wavelengths are 476，539 and 601nm, the commission Internationale d'Eclairage (CIE) coordinates are (0.133，0.201)，(0.335，0.567) and (0.513，0.360), and the FWHM are 32，50 and 73nm for Blue, Green and Red，respectively.
2007, 56 (6): 3499-3503. doi: 10.7498/aps.56.3499
Different types of organic quantum well structures have been grown by organic molecular beam deposition. Energy level and optical property of the quantum well are determined by cyclic voltammetry, optical absorption and photoluminescence. Cyclic voltammetry and optical absorption results indicate that PBD/ Alq3 multilayer can be classified as type Ⅰ, NPB/Alq3 and BCP/ Alq3 as type Ⅱ. The PL experimental results indicate that the structures of PBD/ Alq3 and BCP/ Alq3 exhibit complete energy transfer from PBD or BCP to Alq3, and the structures of NPB/Alq3 exhibit partial energy transfer from NPB to Alq3. The reason for energy transfer in organic quantum well structures is discussed in light of the theoretical and experimental results published in the literature.
2007, 56 (6): 3504-3508. doi: 10.7498/aps.56.3504
A SOI PMOSFET with SiGe quantum well channel was formed by growing the SiGe film on the Si SOI structure. Not only does the device have the advantage of SOI structure, but also the performance of the device is improved because the carrier mobility in SiGe quantum well is much higher than that in Si. In this paper, the threshold voltage model of strained SiGe SOI quantum well channel PMOSFET is established on the basis of general Si SOI MOSFET, also the model of voltage-current characterisfic is founded. The characteristic of voltage-current, transconductance and leak conductance were simulated and analyzed using MATLAB. The results of the simulation confirm that the strained SiGe SOI quantum well channel PMOSFET performs much better than the conventional PMOSFET.
Dependence of anisotropically conductive adhesive high-frequency signal transmission on conductive particle property
2007, 56 (6): 3509-3514. doi: 10.7498/aps.56.3509
Using differential method, we have calculated the dependence of anisotropically conductive adhesive high-frequency signal transmitting property on conductive particle property (including the shape and size of particles). Under AC bias, the smaller the size of conductive filler particle in the conductive adhesive, the better conductance it has. Through the theoretical calculation, we found that the conductance is inversely proportional to the radius (side length) if the shape of the conductive metal particle is a sphere (cube). If the shape of the metal particle is a cuboid or cylinder, the conductance is inversely proportional to the height, independent of the changes in the base.
2007, 56 (6): 3515-3520. doi: 10.7498/aps.56.3515
In this letter we report the fabrication and structural properties of ZnO thin films grown on GaAs，Au/GaAs，Si and glass substrates by rf magnetron sputtering. The x-ray diffraction data indicated that all the ZnO films were pure c-axis oriented and the rocking curves showed that the full-width at half maximum(FWHM) of ZnO(002) peak of ZnO/Au/GaAs films is as small as 2.41°. The SEM images showed better surface for the ZnO films grown with Au buffer. The surface acoustic wave properties of IDT/ZnO/GaAs filter were measured by HP8753C network analyzer. The results indicated that the insertion loss is only 10.4dB and the electric efficiency can be up to 84.1% using the 5.6μm-wavelength IDT.
2007, 56 (6): 3521-3526. doi: 10.7498/aps.56.3521
Using a method of free energy minimization, the spin wave, namely the ferromagnetic resonance, of ferromagnetic (FM)/antiferromagnetic (AFM) bilayers under the stress field has been investigated. The thin FM film is taken to be a single crystal with cubic or uniaxial magnetocrystalline anisotropy, while the thickness of AFM layer is semi-infinite and has single uniaxial magnetocrystalline anisotropy. Numerical calculation shows that stress field and the interface coupling strength will affect the behavior of FM resonance only under low magnetic field, and there are two branches of FMR modes at the critical field, which distinguishes between the weak and strong external field. The critical field depends on the direction of stress field. On the other hand, the change of the direction of the stress field can weakly affect magnetocrystalline anisotropies axis of FM layer.
2007, 56 (6): 3527-3532. doi: 10.7498/aps.56.3527
The effect of quenching rate on texture formation and magnetic properties of nanocomposite Pr2Fe14B/α-Fe melt-spun ribbons has been investigated. The c-axis texture of Pr2Fe14B phase perpendicular to the ribbon plane was found on free surface of melt-spun ribbons prepared by optimized melt-spun technique. The formation of c-axis texture mainly result from the crystallites of Pr2Fe14B growing up along the flow of heat, which is induced by the contact of melt alloy with the surface of Cu wheel. The texture of Pr2Fe14B phase does not penetrate through the thickness of melt-spun ribbons, and the incompletely textured structure does not bring about remanence enhancement. Anisotropic nanocomposite Pr2Fe14B/α-Fe ribbons were obtained by eliminating the unaligned part of the ribbons melt-spun at 16m/s through acid-etching and polishing the side of ribbons in contact with the roller surface during spinning. Both Br and iHc of the anisotropic ribbon measured along the direction perpendicular to the plane were improved obviously due to the c-axis alignment of Pr2Fe14B phase.
2007, 56 (6): 3533-3538. doi: 10.7498/aps.56.3533
CoPtCu/Ag nanocomposite films have been prepared using CoPt/Cu/Ag multilayer precursor by magnetron sputtering and subsequent vacuum thermal annealing. Low-temperature ordered CoPtCu/Ag nanocomposite films with (001) texture have been prepared successfully with the optimized content of Ag and Cu in the films. The CoPtCu/Ag films start ordering at a low annealing temperature of 450℃, which is roughly lower by 150℃ than that needed for most CoPt-based films. Our present study shows that the Ag plays a dominant role in inducing the (001) texture of the film and the Cu additive is very effective to reduce the ordering temperature in CoPtAg system. The Co40Pt36Cu8Ag16 film after annealing at 500℃ exhibits a large perpendicular coercivity of 5.0×105 A/m with a small average grains size of 4—5 nm, implying the potential of the films as future ultrahigh density magnetic recording media.
The resonant magnetoelectric response of magnetostrictive/piezoelectric laminated composite under the consideration of losses
2007, 56 (6): 3539-3545. doi: 10.7498/aps.56.3539
The theoretical analysis of resonance magnetoelectric(ME) performances in longitude-transverse type magnetostrictive/piezoelectric laminated composite is presented in this paper based on the equivalent circuit method, and the formula of ME voltage coefficient is obtained, which is useful to the composite design and optimization. To evaluate the ME voltage coefficient near the resonance, the losses such as eddy-current loss, mechanical loss, and dielectric loss are considered and formulated, which indicates that the mechanical loss plays the key role in dissipation. The analysis, which takes losses into account, gives better explanations to current experimental values.
2007, 56 (6): 3546-3552. doi: 10.7498/aps.56.3546
The mode field and effective index of two structures (square air core and circular air core) of two-dimensional magnetophotonic crystals is analyzed using finite-difference frequency-domain (FDFD) method. According to the result of numerical analysis, the two degenerate fundamental modes become non-degenerate (L-mode and R-mode) in the structures, and the two corresponding effective indices are different. The Faraday rotation of these two structures is enhanced compared with that of the continuous magneto-optical materials. By choosing proper parameters of the air core, the Faraday rotation can be increased by one order for the square air core structure and 4 times for the circular core structure. The polarization state of optical waves is changed after passing through the structure. With the increasing of Faraday rotation, the ellipticity of outgoing optical waves is increased. Finally, we discuss the cause of the phenomena and present a method to resolve the problem of variation of ellipticity by introducing defect in the structure.
2007, 56 (6): 3553-3558. doi: 10.7498/aps.56.3553
Basing on measuring the luminescence spectra at room temperature pumped by 975nm light, the action mechanism of Na+ in the novel Yb3+,Na+:CaF2 crystal was systemically investigated. The relation between the concentration of doping Na+ and the laser operation threshold was analyzed. The optimum match concentration for Yb3+,Na+:CaF2 with low laser threshold was obtained. In the experiment, the lowest laser operation threshold at only 70mW of absorbing pumped power for 2%Yb3+, 3%Na+:CaF2 crystal is realized, which is in good agreement with the analysis of luminescence properties.
2007, 56 (6): 3559-3563. doi: 10.7498/aps.56.3559
Three types of double emitting layer white organic light-emitting devices (POLEDs) were fabricated. The efficiency of the device can be increased largely by the introduction of phosphorescent dye (Ir(ppy)3) and the chroma of the device can be tuned by inserting a BCP layer between the two light-emitting layers. The device with the configuration of: ITO/PVK:NPB (30nm)/BALq:3.0 wt%TBPe(30nm)/BCP(5.0nm)/CBP:5.0 wt%Ir(ppy)3:2.0 wt%rubrene(15nm)/BCP(10nm)/Mg:Ag shows white emission and high efficiency. A white light with Commission Internationale de l'Eclairage (CIE) coordinates of (0.31, 0.37) and brightness of 4670 cd/m2 are obtained at 17V. The device shows a maximum external efficiency of 1.4%. The CIE coordinates are well within the white-light region when the applied voltage varies between 5.0 to 17V.
2007, 56 (6): 3564-3569. doi: 10.7498/aps.56.3564
Ag/SiO2multilayers were prepared by RF magnetron sputtering. Via rapid thermal treatment, most of Ag nanoparticles move up to the surface. By changing the layer thickness of Ag and SiO2, Ag particles of different size, density and shape were obtained. The effect of factors such as size, density and shape of the Ag particle, on resonance absorption is also investigated. We change the annealing time according to the diffusion length. Ag particles with a facet parallel to substrate are obtained under adequate annealing time (17.5min). Since the atoms easily transport between particles within the facets, the radius of Ag particle decreases, and tends to a critical size. Correspondingly, the resonance absorption band becomes narrower and blueshift occurrs.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2007, 56 (6): 3570-3576. doi: 10.7498/aps.56.3570
Theoretical expressions of the epitaxial lateral overgrowth rate in the metalorganic chemical vapor deposition (MOCVD) have been formulated in this paper, with respect to two separate processes: vapor phase diffusion and mask surface diffusion. In the case of InP deposition on GaAs substrate, a parametric study was accomplished in order to determine the impact of the mask/window width to the growth rate. The model, which uses a new parameter “effective mask length” Lmask, reveals that the key factors determining the growth rate are mask/window width and mask width/effective mask length. This model can be used as a tool for predict the growth conditions leading to expected growth rate.
Monte Carlo simulation of effect of neutron scattering on image information in 14MeV fast neutron radiography
2007, 56 (6): 3577-3583. doi: 10.7498/aps.56.3577
Referring to the experimental parameters, the physical model of neutron radiography with 14MeV neutrons is built and the process of radiography is simulated using the Monte Carlo method. Effect of neutron scattering by a polyethylene sample on radiography is analyzed as a function of d, the distance between the sample and the detector, and parameters of the sample. The calculated results indicates that for a d values smaller than 5cm, intensity of scattered neutron component strongly depends on d, while when d is greater than 20cm, the influence of scattered neutrons to the image is reduced to a negligible level. The results also indicates that the effect of neutron scattering is relatively large when density of the sample ranges from 3g/cm3 to 5g/cm3, and intensity of scattered neutron component increases with width of the sample and is almost saturated at width greater than 3cm.
2007, 56 (6): 3584-3588. doi: 10.7498/aps.56.3584
Zinc oxide (ZnO) and magnesium doped ZnO (MgxZn1-xO) nanoparticles were prepared by flame spray synthesis. X-ray diffraction and field emission scanning electron microscopy were used to investigate the composition and structure of the samples. The results indicated that the samples contained nanoparticles of uniform size ranging between 10—20nm; and after introducing Mg into ZnO the lattice constant decreased. According to the UV-VIS transmission spectra, the band gap of MgxZn1-xO nanoparticles was wider than that of ZnO nanoparticles. The photoluminescence of the samples showed high intensity UV emission, and the mechanisms of emission were discussed.
Laser pulse control of ultrafast heterogeneous electron transfer——The multiple vibrational mode three electronic state system
2007, 56 (6): 3589-3595. doi: 10.7498/aps.56.3589
With the combination of the optimal control theory and MCTDH method, optimal controlling of multi-dimensional multi-state molecule such as pyrazine is realized. The quantum dynamics of this molecule under the excitation of optimal pulse is simulated. On the basis of the simulation of two-state multi-dimensional system, we consider the two excited states as target states, and the effect of target state on the quantum dynamics is discussed.
Studies on the laser-induced damage resistance of sol-gel derived ZrO2-TiO2 composite high refractive index films
2007, 56 (6): 3596-3601. doi: 10.7498/aps.56.3596
ZrO2-TiO2 composite films with mol fraction of Ti in the total amount of Zr and Ti ranging from 0 to 100, have been prepared by the sol-gel spin-coating method. The microstructure, optical properties, and laser-induced damage resistance (LIDR) of these films were investigated, and emphasis was given to the compositional dependence of the laser-induced damage threshold (LIDT). The experimental results indicate that at a small expense of refractive index, the sol-gel process can considerably improve the LIDR of the films. Moreover, with increasing mol fraction of Ti from 0 to 100, a general decrease of the LIDT in a wide range of 57.1—20.9J/cm2(at 1053nm，10ns pulse duration and in “R/1” testing mode) was observed. Combining the structural and optical analysis results, a possible correlation between the LIDR and optical band gap of the prepared films is presented on the basis of the multiphoton absorption model.
We have investigated experimentally the effect of series resistance to the dye-sensitized solar cell, and found a very effective way to improve the performance of the dye-sensitized solar cell. We have obtained a photo-electric conversion efficiency of 6.89% with the strip cell (active area:0.8×18cm2), and 5.7% with the module(15×20cm2). It is an encouraging step forward in the research on the dye-sensitized solar cell module, and it may provide the theoretical basis as well as experimental data for future applications.
2007, 56 (6): 3607-3612. doi: 10.7498/aps.56.3607
We used a total internal reflection fluorescence microscope based on the near field optics principle to study the Ca2+ signals in single heart cells of Sprague-Dawley rat. Due to the high signal to noise ratio and high speed of data acquisition of this microscope, the Ca2+ images show complex 2-dimensional wave patterns. The local elementary Ca2+ release units play important role in the formation and transformation of Ca2+ wave. Simulations based on Fire-Diffuse-Fire model show that the Ca2+ wave, composed of Ca2+ release units, may steadily exist in single heart cell. Our results will be helpful for understanding the micro-dynamics of living excitable medium.
2007, 56 (6): 3613-3618. doi: 10.7498/aps.56.3613
An adaptive wavelet analysis approach based on recursive least square (RLS) algorithm is proposed for analyzing the complicated spectra emitted by the interaction between femto-second laser and gases. The simulation results on twenty-seven sets of spectral data show that the proposed method has high efficiency in data compression. The data of the original spectra with 3968 points can be compressed to 124 points, and the correct recognition rate of support vector machine (SVM) can be as high as 100%, when thirteen or more compressed spectra are used for training and the others for testing.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2007, 56 (6): 3619-3629. doi: 10.7498/aps.56.3619
Reconstruction and analysis of proxy series is an important program of climate research. Using methods of wavelet transformation and power spectrum, the main periods of Dulan tree ring and other 7 temperature proxy series are analyzed in this paper. Through filtering, the 8 original series were divided into many different scale components, the similarities in dynamics and external features of components on each scale are studied based on the dynamical correlation factor exponent and correlation coefficient. Research results show that quasi 100a scale might be the common period of these proxy series, meanwhile, both dynamics and external features of these proxy series are similar on the quasi 100a and even higher scales. This means that the quasi 100a and even higher scale components meet with comparable conditions in these aspects. More attention must be paid to this scale when analyzing proxy series. We also found that the similarities in dynamics and external features of these proxy series are reduced as the scale of the components decreaces; On quasi 60—70a scale, the component similarities in dynamics features disappear. On quasi 30a and even lower scale the similarities of external features also disappear.