Vol. 56, No. 9 (2007)
2007-05-05
GENERAL
2007, 56 (9): 5013-5020.
doi: 10.7498/aps.56.5013
Abstract +
In this paper, the competitive mechanism is introduced to the production and propagation processes of the autowave of neural network. The autowave-competition neural network (ACNN) is proposed to successfully resolve the problem of single-source shortest paths (SSSP). The algorithm for shortest paths based on ACNN is presented. Compared with other neural network based approaches, the new algorithm has the following advantages: less number of neurons needed, simple structure of neurons and networks, readily available software and hardware. When ACNN is employed to resolve the shortest path problem, the computational complexity is only related to the hop number of the shortest path, but independent of the complexity of path graph, the number of the existing paths in the graph and the precision of the length of edges. Simulations show that the proposed algorithm is plausible and effictive.
2007, 56 (9): 5021-5025.
doi: 10.7498/aps.56.5021
Abstract +
A frequency matching method for stochastic resonance control is presented on the assumption that stochastic synchronization of stochastic resonance will be generated when the nonlinear bistable system is stimulated by noise and weak periodic signal. Theoretical analysis and numerical simulation show that stochastic resonance can be controlled. Not only the frequency range of stochastic resonance can be broadened, but also the intensity of stochastic resonance can be strengthened by controlling the frequency of the input signal and statistical characteristics of noise, resulting in more noise energy being converted into signal energy.
2007, 56 (9): 5026-5032.
doi: 10.7498/aps.56.5026
Abstract +
Based on the exact solution by multi-linear variable separation approach and introducing Jacobi elliptic functions in the variable separation functions, two types of doubly periodic propagating wave patterns for the Maccari system are derived. When the moduli of elliptic functions are set different, these periodic waves show different features with interesting properties. Especially, in the limiting case, the elliptic function waves may degenerate to dromion and peakon excitations. Graphical investigation of the interaction of elliptic function waves shows it to be inelastic.
2007, 56 (9): 5033-5036.
doi: 10.7498/aps.56.5033
Abstract +
The strict solutions of superconducting unbounded slab model are given for its field distribution, the validity of approximate results avceilable from literature is verified.
2007, 56 (9): 5037-5040.
doi: 10.7498/aps.56.5037
Abstract +
The unified symmetry of mechanico-electrical systems are studied in this paper. The definition and criterion of unified symmetry of mechanico-electrical systems are derived from the Lagrange-Maxwell equations. The Noether conserved quantity, Hojman conserved quantity and a new conserved quantity deduced from the unified symmetry are obtained. An example is given to illustrate the application of the results.
2007, 56 (9): 5041-5044.
doi: 10.7498/aps.56.5041
Abstract +
The methods of analytical mechanics for solving the Emden-Fowler equation are presented in this paper, including the Hamilton-Poisson method, the Lagrange-Noether method, the Lie-Hojman method and the Hamilton-Jacobi method.
2007, 56 (9): 5045-5048.
doi: 10.7498/aps.56.5045
Abstract +
This paper studies the form invariance of Maggi equation. Its definition and criterion are presented. A Hojman conserved quantity can be deduced using the form invariance. An example is given to illustrate the application of the result.
2007, 56 (9): 5049-5053.
doi: 10.7498/aps.56.5049
Abstract +
By the transforming with general KdV equation, the solutions of the variable coefficient generalized KdV equation are constructed firstly. As a result, we successfully obtain the new exact Jacobi elliptic function solutions, solitary-wave-like solutions, trigonometric function solutions and Weierstrass elliptic function solutions of variable coefficient generalized KdV equation.
2007, 56 (9): 5054-5059.
doi: 10.7498/aps.56.5054
Abstract +
We generalized the Jacobi elliptic function expansion method and obtained some new exact periodic solutions of Zakharov equations, thus replenished the known results of the equation using this method.
2007, 56 (9): 5060-5065.
doi: 10.7498/aps.56.5060
Abstract +
When the Schrdinger equation involves high-order power and inverse power potential functions or the superposed potential function of high-order anharmonic oscillatory potentials, introduced by the presence of electric dipole moment potential, molecular crystal potential, or the polarized equivalent potential, the solution of the Schrdinger equation becomes very complicated. In this paper, with the help of a combination of series solutions and asymptotic solutions utilized near the singular points, a series analytic solution of the wave functions of stationary state for radial Schrdinger equation with potential function V(r)=a1r6+a2r2+a3r-4+a4r-6 and the corresponding energy level structure are obtained under the tightly-coupled condition of the interacting power potential functions. Meanwhile, the paper gives a proper discussion and some important conclusions are drawn.
2007, 56 (9): 5066-5070.
doi: 10.7498/aps.56.5066
Abstract +
By exploiting the entanglement correlation in quantum mechanics, a quantum communication, protocol for data link layer is proposed. The protocol divides the link into busy time and leisure time, where the data is sent via classical channel and the distribution of quantum entanglement is supposed to be completed at leisure time and the acknowledge frame is sent via quantum entanglement channel. The transmission of quantum information can be instantaneous; therefore the minimum time span between two successfully delivered data frames can be significantly reduced. It is shown that the proposed protocol enhances the maximum throughput effectively and improves the performance of the stop-and-wait protocol of data link layer.
2007, 56 (9): 5071-5076.
doi: 10.7498/aps.56.5071
Abstract +
We investigate the nonlinear Rosen-Zener transition in two-mode systems. A non-trivial collective behavior, in which all the atoms tend to stay at only one of the modes, emerges with the appearance of nonlinear interaction under the adiabatic approximation. The application of an external field can transfer the atoms from one mode to the other completely. Such phenomena can be fully understood with the help of the phase diagram. We expect the predicted effect will be detected in current BEC experiments.
2007, 56 (9): 5077-5082.
doi: 10.7498/aps.56.5077
Abstract +
Using entropy density near event horizon of the accelerating non-stationary black hole, the instantaneous radiant emittance is calculated, and we arrive at the conclusion that the instantaneous radiant emittance of black hole in a direction at any time is always proportional to the quartic power of temperature of the event horizon of black hole in that direction. It is found that the generalized Stefan-Boltzmann coefficient is no longer a constant, but a coefficient dynamically related to the rate of change of event horizon, the structure of space-time near event horizon and the radiation absorption coefficient of the black hole. It shows that an intrinsic relation between the gravitational field around the black hole and its thermal radiation must exist.
2007, 56 (9): 5083-5087.
doi: 10.7498/aps.56.5083
Abstract +
We investigated the effects of asymmetry of the potential and noises on the mean first-passage time (MFPT) in two opposite directions in an asymmetric bistable system subjected to multiplicative colored and additive white noise with a correlated noise. The expression of the stationary probability distribution is obtained by means of an extended unified colored-noise approximation and the expressions of the MFPTs are derived through the steepest-descent approach. Numerical results show that the influence of the asymmetry coefficient r on T± is different and because of the correlated noise，even for r=0, T+(xs1→xs2,r) is not equal to T-(xs2→xs1,r). The curves of lnT± versus r in both cases have mono-valleys but the mono-valleys move to the opposite directions with increasing correlation intensity λ.
2007, 56 (9): 5088-5092.
doi: 10.7498/aps.56.5088
Abstract +
A model of dodging (random) walk is proposed for the excluded volume effect between the chains in linear polymers, and computer simulations were performed for four model systems. The calculated results indicate that: (1) the relationship between the end-to-end distance of the coils 〈R2〉 and the walk steps N is still linear, same as that of the random coil; (2) but the coil size is suppressed compared with that of the random coils; and (3) the suppression of the coil size produced by dodging in the case of headon encounter is stronger than that in the case of walking abreast.
2007, 56 (9): 5093-5097.
doi: 10.7498/aps.56.5093
Abstract +
By adding modulation wave to the equation of the laser intensity in the gain-noise model of the single-mode laser, we use the linear approximation method to calculate the power spectrum and signal-to-noise ratio(SNR) of the laser intensity, which is driven by two color noises correlated in the form of exponential function. The result shows that the correlated time and the correlated strength between the noises have great effect on the stochastic resonance(SR), and the variations of SNR with the frequency of the carrier signal ω and the modulator signal Ω are quite different from the curve of variation of white noise.
2007, 56 (9): 5098-5102.
doi: 10.7498/aps.56.5098
Abstract +
Based on the principle of chaos suppression with nonresonant parametric drive in autonomous chaotic systems, detection of weak square wave signals in strong noise is realized. Firstly a square wave signal whose frequency is far higher than the system characteristic frequency is introduced as the system internal drive signal. According to the parametric equivalent relation between the controlled system and the original system obtained by the averaging method, the critical value of the detection parameter is determined, which marks the sudden change from chaos state to periodic orbit of the system. Numerical simulations show that extremely low signal-to-noise ratio limit in detection can be attained by the proposed system. Compared with the known weak signal detection methods based on the parametric resonant perturbation principle, this method gives more accurate estimates of the detection parameter through strict theoretical analysis, which enables its generalization and application in related fields.
2007, 56 (9): 5103-5110.
doi: 10.7498/aps.56.5103
Abstract +
In the present paper, homoclinic chaos in averaged oscillator subjected to combined deterministic and narrow-band random excitations is investigated in detail. The method of multiple-scale is first used to reduce the oscillator subjected to combined deterministic and narrow-band random excitations to an averaged oscillator only under narrow-band random excitation. In order to determine the threshold of random excitation amplitude for the onset of chaos, the stochastic Melnikov technique is then applied to the averaged oscillator with mean-square criterion and it is found that the threshold of random excitation amplitude for the onset of chaos in the oscillator turns from increasing to decreasing as the intensity of the noise increases. On the other hand, another threshold of random excitation amplitude for the onset of chaos is obtained by calculating the largest Lyapunov exponents numerically. The Poincare maps are also used for verifying the conclusion. Qualitatively consistent results are obtained by the analytical and numerical methods.
EDITOR'S SUGGESTION
2007, 56 (9): 5111-5118.
doi: 10.7498/aps.56.5111
Abstract +
In this paper, we propose an approach using the Bernstein polynomial to model the dynamics of chaotic time series. Combining it with RLS algorithm, we can predict the chaotic time series adaptively. Theoretical analysis and computer simulation have demonstrated that this approach can provide high precision and satisfactory percentage of prediction for some typical chaotic time series. Because of the fast convergence of RLS algorithm, this approach can be applied to predicting short record chaotic time series in real time.
2007, 56 (9): 5119-5123.
doi: 10.7498/aps.56.5119
Abstract +
The chaotic synchronization in a one-way coupled oscillator array is studied. We find that, in certain coupling schemes (for example, the variable y driving x in the Lorenz system) and under proper coupling strengths, a nonlocal approximate synchronization happens between the first and the third oscillator, or all succeeding ones, which are not directly connected, accompanied with a desynchronization between the first and the closest, namely the second one. More detailed observation finds that under sufficiently strong coupling, there exists a single-driving-signal (not all three variables) synchronization between the first and the second oscillator that transfers the synchronization information to all remaining oscillators in the array. The nonlocal synchronization is a general phenomenon that has been observed in other systems.
2007, 56 (9): 5124-5130.
doi: 10.7498/aps.56.5124
Abstract +
This paper studies the chaotic behaviors of the fractional-order Liu system and the fractional-order unified system which were presented several years ago. It is found that chaos exists in the both systems with order less than three and the lowest order to have chaos is 0.3. The calculation results of the maximum Lyapunov exponents when the system is chaotic illustrate the existence of chaos. Chaos synchronization between fractional-order Liu system, fractional-order Lorenz system and fractional-order Lü sytem is realized by employing active control technique. Theoretieal analysis and numerical simulations demonstrate the effectiveness of the proposed method.
2007, 56 (9): 5131-5135.
doi: 10.7498/aps.56.5131
Abstract +
A class of four-dimensional switchable hyperchaotic systems is built by adding an additional state to the three-dimensional switchable chaotic system. When subsystems are hyperchaotic, an identical system parameter is determined according to the bifurcation diagrams of the subsystems. Some of its basic dynamical properties are studied detailedly, such as the feature of equilibrium, the phase portraits of hyperchaotic attractor,the Lyapunov exponent and the fractal dimension. A practical circuit is designed to realize these systems.
2007, 56 (9): 5136-5141.
doi: 10.7498/aps.56.5136
Abstract +
This paper presents a four-dimensional hyperchaotic Lorenz system, obtained by adding a nonlinear controller to Lorenz chaotic system. The hyperchaotic Lorenz system is studied by bifurcation diagram, Lyapunov exponent spectrum and phase diagram. Numerical simulations show that the new system's behavior can be convergent, divergent, periodic, chaotic and hyperchaotic as the parameter varies.
2007, 56 (9): 5142-5148.
doi: 10.7498/aps.56.5142
Abstract +
The phase synchronization problem of autonomous chaotic systems is discussod. Based on the nonlinear state observer algorithm and the pole placement technique, a phase synchronization scheme is designed. The phase synchronization of a new chaotic system is achieved by using this observer controller. Numerical simulations are carried out to verify its effectiveness.
2007, 56 (9): 5149-5157.
doi: 10.7498/aps.56.5149
Abstract +
Using the high sensitivity differential sensor,we collected 80 groups of fluctuating signals in vertical upward gas/liquid two phase flow. We established the general description method of chaotic attractor morphological characteristic using referenced sections, and then put forward a new flow pattern classification method of gas/liquid two phase flow by combining the chaotic attractor morphological feature parameters of different dimensions. The study shows that the proposed method can get a good clustering of gas/liquid two phase flow patterns, including the complex transitional flow pattern, and it implies that the method of chaotic attractor morphological characterization is an effective approach to study nonlinear time series in practice.
2007, 56 (9): 5158-5164.
doi: 10.7498/aps.56.5158
Abstract +
Based on the cellular automaton concept, this paper presents a train-following model for the moving block system with different types of trains running. We apply the model to the rail network and simulate the running of trains. We also simulate the traffic phenomenon of delay propagation in moving block system. We investigate how the main factors including train deliver time interval and initial delay affect the delay propagation of trains. The simulation results are compared with the theoretical results to verity the reliabity and feasibility of the model.
2007, 56 (9): 5165-5171.
doi: 10.7498/aps.56.5165
Abstract +
For speed-limited sections of railway traffic in the four-aspect fixed-block system, we propose a cellular automata model to simulate the traffic flow to analyze how the length of speed-limited section, train time interval and the speed-limit value affect the traffic flow. The simulation results demonstrate that the decrease of the length of speed-limited section, the moderate increase of train time interval and the increase of the speed-limit value can improve the green light runtime of the trains.
2007, 56 (9): 5172-5175.
doi: 10.7498/aps.56.5172
Abstract +
The C60@SWNT (peapod) samples were prepared by the vapor diffusion method. Polymerization of C60 molecules in single walled carbon nanotubes (SWNTs) under high pressure and simultaneous irradiation of UV laser (325 nm) has been carried out for the first time by using diamond anvil cell. Raman spectra of the peapod samples decompressed from high pressure indicated that C60 molecules form one-dimensional orthorhombic polymer in SWNTs under UV laser irradiation at a high pressure of 21.5GPa, which is lower than that for the polymerization of samples induced by high pressure only. The polymerization is an irreversible phase transition in the peapod.
2007, 56 (9): 5176-5179.
doi: 10.7498/aps.56.5176
Abstract +
Al-doped ZnO nanowire arrays were synthesized with Au catalysis on Si(100) substrate using the chemical vapor deposition technique. Only (002) diffraction peaks of ZnO can be found in the XRD patterns of the samples, which shows that the as-grown nanowires are highly crystalline in nature and grow along the ［001］ direction. The SEM images show that the ZnO nanowires are perpendicular to the substrate surface. Room-temperature photoluminescence (PL) measurement shows 3 near band-edge emission peaks at 373, 375 and 389nm. Analysis shows that the band gap of Al-doped ZnO nanowaires is 3.343eV and the exciton binding energy is 0.156eV. Room-temperature PL spectrum of pure ZnO nanowaires shows 3 near band-edge emission peaks at 377, 379 and 389nm. The band gap of pure ZnO nanowaires is 3.301eV and the exciton binding energy is 0.113eV, which shows that the band gap increases due to Al doping.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2007, 56 (9): 5180-5184.
doi: 10.7498/aps.56.5180
Abstract +
The research of dipole source localization has great significance in both clinical research and applications. For example, the EEG recording from the scalp is widely used for the localization of sources of electrical activity in the brain. This paper presents a closed formula that describes the electric field of dipoles at arbitrary position, which is a linear transformer called the transfer matrix. The expression of transfer matrix and its many useful characteristics are given, which can be used for the analysis of the electrical fields of dipoles.This paper also presents the closed formula for determining the location and magnitude of single dipole or multi-dipoles according to its electrical field distribution. A calculation result for a single dipole shows that the dipole will be located at the midpoint of a line segment if there are equivalent fields at its two ends.
NUCLEAR PHYSICS
2007, 56 (9): 5185-5190.
doi: 10.7498/aps.56.5185
Abstract +
The neutron 3PF2 pairing gap in pure neutron matter has been studied by using the Brueckner-Hartree-Fock(BHF) approach and the BCS theory. We have concentrated our attention on investigating the three-body force effect on the neutron superfluidity in the 3PF2 channel. The calculated results indicate that the three-body force enhances remarkably the 3PF2 superfluidity in neutron matter. When adopting the BHF single-particle spectrum, the three-body force turns out to increase the maximum value of the pairing gap from about 0.22MeV to about 0.5MeV.
2007, 56 (9): 5191-5195.
doi: 10.7498/aps.56.5191
Abstract +
By using the Glauber model of nucleus-nucleus collision, we present the distribution equations for the numbers of participants and nucleon-nucleon collisions versus the impact parameter in high-energy heavy-ion collisions, and analyze with them the centrality dependence of both numbers in \{sNN\}1/2=200GeV Au+Au collisions. The results fit well with those given by PHENIX Collaboration.
ATOMIC AND MOLECULAR PHYSICS
2007, 56 (9): 5196-5201.
doi: 10.7498/aps.56.5196
Abstract +
The present paper is devoted to the calculation of transition wavelengths, oscillator strength f, Einstein An0 and B0n coefficients of methyl vinyl siloxane from ground state Cs(X1A′) to the first ten different excited states by employing density functional theory and single substitute configuration interaction approach with basis sets 6-311++G**. At the same time, the excited states of methyl vinyl siloxane under different external electric fields are investigated. It is shown that the HOMO-LUMO gaps become smaller, and the excitation energies of the first ten excited states of methyl vinyl siloxane decrease sharply as the external electric field intensity become strong. The conclusion can be drawn that the transition between the ground state X1A′ and the excited state of methyl vinyl siloxane is easy to bring about.
2007, 56 (9): 5202-5208.
doi: 10.7498/aps.56.5202
Abstract +
The triple differential cross section for electron impact ionization of helium in the special geometry of coplanar large energy loss and close to minimum momentum transfer are calculated by use of BBK model and modified BBK model. The structure of the cross section is discussed.The results of the present work are compared with current experimental data. It is found that both BBK and modified BBK models are in good agreement with the experimental results of binary-to-recoil peak ratio and their angular distribution.
2007, 56 (9): 5209-5215.
doi: 10.7498/aps.56.5209
Abstract +
The geometries，total energies，and frequencies of Zrn(n=2—16) clusters have been systematically investigated by using density functional theory with the generalized gradient approximation.Ｔhe equilibrium geometries for different spin multiplicities as well as the ground-state structures have been determined.The calculated results on the averaged binding energy， fragmentation energy， second-order difference of cluster energies as well as the HOMO-LUMO gap of the Zrn(n=2—16) clusters indicate that the relative stabilities ofZr2，Zr5，Zr7，Zr13，Zr15 are stronger than clusters of othersizes. The true ground state for Zr13 cluster has icosahedral structure with Ih symmetry，and the stability of Zr13 is the strongest of all the investigated clusters.
2007, 56 (9): 5216-5220.
doi: 10.7498/aps.56.5216
Abstract +
The geometric structures and magnetism of the three probable isomers for Ni2Sn17，Mn2Sn17，［Ni2Sn17］4- and ［Mn2Sn17］2- have been studied using the generalized gradient approximation based on density functional theory. It was found that the D2d structures of Ni2Sn17，Mn2Sn17，［Ni2Sn17］4- and ［Mn2Sn17］2- are the most stable structures among the three possible isomers. The study shows that the D2d structure and the D4d structure of Ni2Sn17 have spin-splitting, leading to the magnetic moment 4μB and 2μB， respectively, and there is weak ferromagnetic interaction between the Ni and Sn atoms for the D2d structure and the D4d structure. However, there is no magnetic moment in the D4h structure. On the other hand, the D2d, D4d, and D4h Mn2Sn17 have 2μB，6μB and 2μB magnetic moments, respectively. It is interesting that there is weak antiferromagnetic interaction and weak ferromagnetic interaction between Mn and Sn atoms for the D2d and D4h structures, respectively, while there is not only weak ferromagnetic interaction but also weak antiferromagnetic interaction between Mn and Sn atoms in the D4d structure. Finally, It is worth noting that all their anion counterparts of ［Ni2Sn17］4- and ［Mn2Sn17］2- have closed electronic configurations, showing no magnetic moment.
CLASSICAL AREA OF PHENOMENOLOGY
2007, 56 (9): 5221-5226.
doi: 10.7498/aps.56.5221
Abstract +
VO2 based Mott transition field effect transistor is a new element to be incorporated between microbolometer sensitive pixels and on-chip read-out circuitry to fully realize the planar integrated infrared focal plane array. Based on their dielectric dispersions in Lorentz model, its infrared absorption was studied by the theory of reflection from Al surface plasma and absorption by optical phonons in VO2 and especially in α-SiN: H films. The present paper reports on the properties of the absorption in different wavelength regions and the dependence on the thickness of the nitride passivation layer, the absorbance spectra for the layer with thickness of λ/4n after the phase delay is corrected, and the changes in the spectra caused by the transition in the VO2 film.
2007, 56 (9): 5227-5232.
doi: 10.7498/aps.56.5227
Abstract +
In terms of Fourier optics, a single-pass tiled-grating compressor based on in-pair compensation is studied and the amplitude expression of the output pulse from the compressor in the far-field is obtained, which disclose that effect of the group delay, the pulse front tilting and the residual chirping on the pulse-width in the far field. Numerical simulation shows that the pulse in the far field is broadened with increasing spectral width. For a typical single-pass tiled-grating compressor, the tolerance of each error is obtained and the allowed spectral width is obtained when all errors are considered.
2007, 56 (9): 5233-5236.
doi: 10.7498/aps.56.5233
Abstract +
A detailed scheme by which the status of tiled gratings can be inspected in a real-time way is put forward. The He-Ne laser is shown to be inappropriate for the grating-tiling experiment where 1740/mm grating is used. A method is proposed by which the wavelength can be selected for the real-time measurement, so that the impact of all sorts of errors of the tiled gratings on the main beam(1053nm) and the beam for measurement is completely identical, which has been demonstrated by experiment.
2007, 56 (9): 5237-5242.
doi: 10.7498/aps.56.5237
Abstract +
The evolution of atomic population in the model of two different atoms interacting with two-mode field are investigated. The effect of the relative coupling constant (R=g1／g2) and initial two-mode cavity field on the inversion of atomic population are discussed.
2007, 56 (9): 5243-5247.
doi: 10.7498/aps.56.5243
Abstract +
In this paper, we promote a principle of polarization feedback which keeps polarization states stable in long-distance fiber. It is quite difficult to realize long-term stable quantum key distribution (QKD) with polarization encoding due to the effect of irregular polarization fluctuation in single mode fiber. We have designed a polarization feedback control system to compensate for the birefringence. And a polarization encoded QKD experiment has been realized in 100km fiber.
2007, 56 (9): 5248-5251.
doi: 10.7498/aps.56.5248
Abstract +
Strongly amplified X-ray laser observed in the recent transient collisional excitation (TCE) experiments pumped by a single short pulse suggests that with the help of low level prepulse producing lowly ionized preplasma it is also possible to obtain high gain in the TCE scheme. The TCE Ne-like Ge 19.6nm X-ray laser pumped by low level prepulse were studid with our code series. Time evolution of plasma status shows that the position of peak gain drifts outward and the gain region is widened as time goes on, which is similar to the QSS scheme.
2007, 56 (9): 5252-5257.
doi: 10.7498/aps.56.5252
Abstract +
We report here the experimental observations on the nature of the reverse saturable absorption over the visible region(450—470 nm) for two novel porphyrin-like complexes and tetraphenylporphyrin using open aperture z-scan. They all exist large reverse saturable absorption between the Soret and Q bands. A theoretical study has been carried out using 5-level models. The two novel samples have better optical limiting than tetraphenylporphyrin.
2007, 56 (9): 5258-5262.
doi: 10.7498/aps.56.5258
Abstract +
THz wave radiation in periodically poled lithium niobate (PPLN) during optical rectification induced by femtosecond pulse is studied theoretically in this paper. Attention is focused mainly on the distribution of fields in the frequency domain and time domain, and the spectrum width of THz wave radiation. The radiation field pulse duration, the amplitude and the spectrum width varying with the length of crystal and the angle of radiation are also discussed in detail.
2007, 56 (9): 5263-5268.
doi: 10.7498/aps.56.5263
Abstract +
Adopting noise initiation model of stimulated Brillion scattering (SBS), the characteristic of transmitted energy and power in CS2 liquid media under non-focusing pump is investigated numerically. The results show that, the waveform of transmitted pulse with increasing input energy shows the characteristic of power limiting. And the transmitted energy varies linearly with input energy withont the energy limiting effect. When 1053 nm 20 ns Nd:YLF laser was used as pump in a 3∶1 inverse expansion system are using attenuators to adjust input energy in the rang of 2 to 92 mJ, the rules of transmitted energy and waveform versus input energy are acquired, and compared with the results of focusing pump. The experimental results are in good agreement with the theoretical simulations. Because the optical breakdown takes place at high input energy under focusing pump, the power limiting with non-focusing pump is applicable under the circumstances of high power and energy.
2007, 56 (9): 5269-5275.
doi: 10.7498/aps.56.5269
Abstract +
The basic idea is to impose sinusoidal phase modulation to an optical beam and then make it propagate in a strongly nonlocal nonlinear medium with normalized symmetrical real spatial response.The spatial phase modulation splits the input beam into multiple sub-beams, while the nonlocal nonlinear medium shapes the sub-beams into spatial solitons, whose directions and power can be controlled by changing the modulation parameters/We present numerical results to show how the deflection and the detachment of beams are achieved.
2007, 56 (9): 5276-5280.
doi: 10.7498/aps.56.5276
Abstract +
The solid-state dye laser exhibits attractive features such as free from pollution, compact in structure and low-cost, the development of tunable solid-state dye lasers is an area of active research. PMMA is a popular host for solid-state dyes. When ethanol was added to PMMA, the host can have some improved properties. In the present paper,research results or the transmission spectra, laser spectra, fluorescence spectra and slope efficiency of the polymer dye lasers are presented. When ethanol concentration is 15%, the polymer has the highest transmission rate in 600—750 nm region. Ethanol has a slight impact on the fluorescence spectra. The bandwidth of laser spectra, of whicl the peak positions shift with the ethanol concentration, is about 7—9 nm FWHM. The relation of output energy versus input energy was also studied, and the experimental results confirmed the theoretical results. The highest slope efficiency attained in the experiment was 49.7% with the ethanol concentration of 15%.
2007, 56 (9): 5281-5286.
doi: 10.7498/aps.56.5281
Abstract +
The coherently coupled nonlinear Schrdinger (NLS) equation of the propagation of light pulse in fiber Bragg gratings has been utilized to study the modulation instability of laser pluses in fiber Bragg gratings with Gaussian apodization function, not only in the anomalous dispersion regime, but also in the normal dispersion regime. The results show that modulation instability can occur in both regimes. The regular gain spectra are observed obviously when input power reaches a definite value in the anomalous dispersion regime. The modulation instability can increase from a fixed value to infinity when the input power is in the region of modulation instability in normal dispersion regime. Furthermore, the gain spectra are restrained by Gaussian apodization function in both regimes.
2007, 56 (9): 5287-5292.
doi: 10.7498/aps.56.5287
Abstract +
A novel ultra-long-period fiber grating with periods up to several millimeters induced by using CO2 laser pulses is reported and discussed based on the mode coupling theory. The refractive index change occurs mainly within the cladding area of the fiber and has an unsymmetrical distribution on the cross-section of the ULPFG. The characteristics of temperature, strain, torsion and the effect of the refractive index of environment were investigated in theory and by experiment. The experimental results showed that different resonant peaks of the ULPFG have different sensitivities in the characteristics above mentioned, which offers possible applications in optical communications and sensing.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2007, 56 (9): 5293-5297.
doi: 10.7498/aps.56.5293
Abstract +
We triggered and guided the stationary electric field discharges in 3—23 cm long air gaps by using plasma channels produced by 40mJ ultrashort laser pulses. We have observed that the plasma channel reduced the discharge breakdown voltage to 40% of its natural value. By analyzing the delay between the laser pulse and the white light emission of discharge, we evaluated the speed of the stepped leader with laser guiding to be about 107 cm/s.
2007, 56 (9): 5298-5303.
doi: 10.7498/aps.56.5298
Abstract +
We investigated some important plasma antenna parameters, such as the transmission characteristics of a microwave propagating along the cylindrical symmetric plasma antenna, dispersion relations, radiation pattern and gain. By analyzing the dielectric constant of the tube material, the outer and inner radius, the density of the plasma and the collision frequency, we found that to minimize the attenuation of wave propagation along the plasma antenna, one should increase the density of the plasma, decrease the collision frequency, increase the radius of the plasma, increase the thickness of tube，and decrease the dielectric constant. We also calculated the radiation pattern of the plasma antenna and the change of gain under different conditions. These results are useful for the design of plasma antenna.
Numerical simulation of plasma nonlinear phenomena excited by radio-frequency wave using FDTD method
2007, 56 (9): 5304-5308.
doi: 10.7498/aps.56.5304
Abstract +
Radio-frequency excitation enables the plasma to contain two, three or more harmonic waves, and the modulating signal produced by signal source and excitation source under the plasma nonlinear influence should not be ignored. A new wave-plasma interaction model is set up considering the electron thermal motion, recombination and diffusion effect. It verifies the modulations of signal in different frequencies and explains the production of the second and high order harmonic waves in plasma excitation. The variation of the EM wave pedigree is analyzed taking into account the parameters such as input power, ionization field threshold and electron average energy. The result is of use in tuning the matching circuit or selecting the coupling element in plasma discharge, and it is helpful in electric magnetic interface design, plasma antenna transmission, etc.
2007, 56 (9): 5309-5313.
doi: 10.7498/aps.56.5309
Abstract +
Plasma impedance and power consumption were measured by modified voltage-current method at different applied voltages on the substrate electrode. The results indicated that discharge current increases with the increase of applied voltage on the substrate electrode when the applied voltage of RF electrode is fixed. As a result, plasma impedance decreases. In addition, only a small part of power was used by the plasma and a large part of power was consumed on the matching network and cables. Through the analysis of plasma electrical properties, it was found that the deposition rate of thin films will be increased with the increase of applied voltage on the substrate electrode so long as there is enough silane.
2007, 56 (9): 5314-5317.
doi: 10.7498/aps.56.5314
Abstract +
A dynamic model of plasma explosion is put forward. The inertia factor is considered daring plasma expansion, and the dynamic simulation is carried out. The dynamic mechanism of film damage induced by plasma blast is discussed.
2007, 56 (9): 5318-5322.
doi: 10.7498/aps.56.5318
Abstract +
Laser micro-spectral analysis coupled with CCD spectrometer was used in this experiment. With Fe Ⅰ356.54 nm and Fe Ⅰ358.12 nm as analysis spectral lines, the micro-plasma temperature and its spatial distribution were investigated in different matrices, namely Mg, Al, Si, and steel alloy 6-0. The electron temperature as a function of location in each matrix and the differences of that at the same location in different matrices were determined and an explanation was given. Finally, with Cu Ⅰ324.75 nm and Zn Ⅰ394.50 nm as analysis spectral lines, we have successfully used the calculated micro-plasma temperature to discuss the matrix effect.
2007, 56 (9): 5323-5329.
doi: 10.7498/aps.56.5323
Abstract +
Based on the Shafranov current moment theory, reconstruction of the plasma boundary in HL-2A is studied by using the movable current filament (VCF) method. Compared with the fixed current filament (FCF) method and the finite current element (FCE) method that we've used before, this method has the advantage of accurately rebuilding the fast evolving plasma boundary by using only 1—3 filaments, which offsets the drawbacks of the FCF and the FCE methods. Combining the movable current filament method with the FCF or the FCE method, the plasma boundary can be displayed in real time and the plasma shape can be controlled during the whole discharge process.
2007, 56 (9): 5330-5334.
doi: 10.7498/aps.56.5330
Abstract +
Area density is a an important parameter for measuring compression in inertial confinement fusion experiment. Using the MULTI code, physical parameters of fuel plasma are simulated at time of maximum suppression in indirectly driven implosion experiment at Shenguang Ⅱ. Secondary proton energy range in CR-39 is calculated. The average area density 〈ρR〉 of fusion fuel is obtained by measuring secondary protons from D2-filled fuel in the back of CR-39 for the first time. The result shows that CR-39 can be used to measure secondary proton yields of Shenguang Ⅱ. Secondary fusion reaction is affected largely by the suppression state in indirect drive.
2007, 56 (9): 5335-5340.
doi: 10.7498/aps.56.5335
Abstract +
Two different carbon nanotube (CNT) cathodes have been made by using screen-printing and also by coating. Pulse emission properties of the two cathodes were studied, and the cathode's surface morphology was characterized. Research results show that the CNT cathode is capable of generating pulsed beams of a few kilo-amperes. The pulsed field emission properties of the cathode were tested in a diode structure powered by a high-voltage generator(under conditions of 2×10-3 Pa, 100ns, 1.64MV). The results reveal that the emission current of the cathode made by coating can attain 5.11 kA, and the highest emission current density is 260 A/cm2. The stability of cathode made by screen-printing is better than that made by coating. The pulsed emission mechanism of carbon nanotubes is shown to be explosive electron emission. This carbon nanotube cathode appears to be suitable for high-power microwave device applications.
2007, 56 (9): 5341-5346.
doi: 10.7498/aps.56.5341
Abstract +
This paper systematically reviews the researches on the heat effect of a single steady-state microdischarge during plasma electrolytic oxidation (PEO) and explaines the causes for the appareut changes of microdischarge population which may be related with the evolution of the type and quantity of the disintegrated gas bubbles formed in the interface between the electrolyte and the substrate. The temperature field inside a microdischarge was estimated by virtue of a cylindrical channel model of microdischarge, and the temperature effect in the coatings adjacent to the microdischarge channel was calculated through the heat transfer equation of point heat source during the decay process of microdischarge, which provides a preliminary theoretical analysis for the types and crystalline states of different phase compositions (MgAl2O4,Mg2SiO4, etc.) contained in the coating formed on the magnesium alloy.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2007, 56 (9): 5347-5352.
doi: 10.7498/aps.56.5347
Abstract +
The thickness dependence of strain and magnetic properties of La0.8Ca0.2MnO3 (LCMO) thin films grown on SrTiO3 (STO) substrates were investigated by X-ray diffraction techniques and magnetic measurements. The results show that all the LCMO films are well oriented in the (00l) direction on the (001) STO substrate. The LCMO thin films have a pseudo-cubic structure. With the film thickness increasing, the in-plane lattice parameters decrease while the out-of-plane lattice parameter increases. The lattice parameter a is close to b, but a little smaller than c. The in-plane strain in the film is tensile due to the lattice mismatch between the LCMO film and STO substrate, whereas that out-of-plane is compressive. The LCMO films have a slightly perceptible mosaic structure along the qz direction, and there exists an angular deviation of about 0.1° between ［001］LCMO and ［001］STO. The physical properties of the films strongly depend on the film thickness. With the increase of the film thickness the TMI is enhanced, and the MR is weakened.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2007, 56 (9): 5353-5358.
doi: 10.7498/aps.56.5353
Abstract +
MgB2 thin films were deposited by e-beam on Al2O3(001)substrates. Resistive transition of the MgB2/Al2O3 was investigated in different magnetic fields applied parallel or perpendicular to the ab plane of the films by the standard four-probe method. An activation energy model is suggested to analyze quantitatively the activation energy of flux lines and resistive transition in whole transition temperature range. The anisotropy parameter γ=Hc2ab(0)/Hc2c(0)=2.26 was obtained by analyzing the upper critical field using the polynomial Hc2(t)=Hc2(0)+At+Bt2.
2007, 56 (9): 5359-5365.
doi: 10.7498/aps.56.5359
Abstract +
The band structure, density of states, electron density difference and optical properties of ZnO co-doped with transition metal (Mn, Fe, Co, or Cu) and N have been investigated by using density functional theory based on first-principles ultrasoft pseudopotential method. The optical properties of co-doped ZnO are similar to those of ZnO doped only with transition metal (Mn, Fe, Co and Cu), but rather different from those of pure ZnO. Moreover, our calculation indicates that co-doping of transition metal (Mn, Fe, Co, and Cu) and N is in more favor of the formation of p-type ZnO.
2007, 56 (9): 5366-5370.
doi: 10.7498/aps.56.5366
Abstract +
Perovskite-type BaHfO3 and SrHfO3 are studied using the first principle density functional method. The lattice parameters, elastic constants, bulk and shear moduli, density of states, band structures and charge densities of BaHfO3 and SrHfO3 are calculated after structural relaxation. The calculation results show that both BaHfO3 and SrHfO3 are indirect semiconductors with relatively higher bulk moduli. In the unit cell, the ionic bond is formed between Sr (Ba) atom and HfO3, while the covalent bond is formed between Hf atom and O atom.
2007, 56 (9): 5371-5375.
doi: 10.7498/aps.56.5371
Abstract +
We have investigated the electronic structure of p-type ZnO by adopting the ab-initio study of plane wave ultra-soft pseudo potential technique based on the density function theory (DFT). The cell parameters, total density of states (TDOS) and partial density of states (PDOS) of N atoms were obtained after optimizing the structure of ZnO. It was shown that the volume of the primitive cell decreases with increasing content of the dopant. The p-type conductivity was realized easily by codoping with N and Al in the ratio of 2∶1, compared with doping with N alone. As the doping proportion increases, the carrier concentration was increased and the mobility was enhanced, which leads to the improvement of the conductivity.
2007, 56 (9): 5376-5381.
doi: 10.7498/aps.56.5376
Abstract +
The change of atom configuration in hexagonal AlN, caused by native point defects (N and Al vacancies, N and Al antisites, N and Al interstitials), are calculated firstly by plan-wave pseudopotential method with the generalized gradient approximation in the frame of density functional theory, and the most stable structure are obtained. Then the formation energy of each kind of native point defect is calculated, by which the possibilities of the six kinds of native defects to be formed during crystal growth are analyzed. Finally, the defect energy levels responding to every kind of native point defect and their electron occupancy are analyzed from the aspect of density of states. The results show that all the native defects form very deep energy levels in the band gap except N vacancy, and foreign impurities are needed to realize n- or p-type AlN. The values of defect energy levels obtained will be helpful in ascertaining the luminescence mechanism of some AlN non-band-edge emissions.
2007, 56 (9): 5382-5388.
doi: 10.7498/aps.56.5382
Abstract +
The adsorption properties of Pd on CeO2(111) surface are studied using the first principle projector-augmented-wave (PAW) method based density functional theory (DFT) within generalized gradient approximation (GGA) and with the inclusion of on-site Coulomb interaction (DFT+U). It is found that there exist different adsorption features for different coverages of Pd: (1) For one monolayer (ML) Pd adsorption on CeO2(111) surface, Pd prefers to be adsorbed on the atop O site leaning toward the Ce-bridge site; while, for the 1/4 ML adsorption, Pd prefers to be adsorbed on the O-bridge site leaning toward the atop subsurface O site. (2) The interaction between the adsorbed Pd atoms is very strong when the coverage is one ML; on the other hand, there is almost no interaction between the Pd atoms for the 1/4 ML Pd adsorption, correspondingly, the interaction between Pd adatoms and CeO2(111) substrate is stronger for lower coverage adsorption (1/4 ML) than that for higher coverage adsorption (1 ML). (3) The adsorption of Pd disturbes the CeO2(111) surface structure in the vicinity of the adsorption site. (4) The Pd adsorption makes the Pd/CeO2 more active as compared with the clean CeO2(111) and bulk Pd metal, and there may exist some active sites at the Pd/CeO2 interface. These studies may lead to a better understanding for the Pd/CeO2 catalysts and give some hints to improve the efficiency of TWC.
2007, 56 (9): 5389-5393.
doi: 10.7498/aps.56.5389
Abstract +
Molecular dynamics simulations are used to investigate the shock-induced phase transformation of iron at a certain initial temperature, which shows that the temperature lowers the threshold pressure of shock-induced phase transformation. Also, the evolution of the loading and unloading waves is calculated and analyzed, and it is found that the velocity of the inverse phase transformation wave(during ε phase →α phase)equals the longitudinal sound speed of the wave front and is less than that of the wave rear. The pressure and density of unloading traces is also calculated, which affords a good explanation to the evolution of the unloading waves, and the phase transformation band of unloading is obtained, which shows the hysteresis in the reverse transformation.
2007, 56 (9): 5394-5399.
doi: 10.7498/aps.56.5394
Abstract +
Based on a tight-binding disordered model describing a single electron band, we establish a model of one-dimensional binary disordered systems with off-diagonal correlations, and derive the alternating current (ac) conductance formula. By calculating the ac conductivity, the function of disorder and off-diagonal correlations in electronic transport are analyzed, and the dependence of the ac conductivity on the field frequency and the temperature is studied. The results indicate that the ac conductivity of one-dimensional binary disordered system decreases with the increasing of the degree of lattices energy disorder. And the ac conductivity of the system is also dependent on the probabilities p, which represents the degree of compositional disorder of the system to some extent. The ac conductivity of system first decreases with the increasing of p, then increases with farther increasing of p. We also find that off-diagonal correlations lead to delocalization and enhance the ac conductivity of the system，namely, the ac conductivity of one-dimensional binary disordered system with off-diagonal correlations is larger than that of uncorrelated system in the Anderson limit. Moreover, the ac conductivity of one-dimensional binary disordered system decreases with the increasing of temperature, and increases drastically with the increasing of the frequency of the electric field.
2007, 56 (9): 5400-5407.
doi: 10.7498/aps.56.5400
Abstract +
The existence of an initial amorphous layer and a turbostratic boron nitride transition layer is one of the main hindrance to heteroepitaxial growth of cubic boron nitride on silicon susbtrate. In this paper, we systematically studied the effect of substrate pretreatment on the amorphous layer composition of cBN films prepared by plasma enhanced CVD, and found that the possible causes for the amorphous layer formation are the native silicon oxide, the oxygen remaining in the vacuum chamber, high energy ion bombardment, and the formation of silicon nitride at high substrate temperature. By 1200 K heating of silicon susbtrate in H2 atmosphere and by controlling the deposition temperature at below 900 K, the crystallinity of silicon susbtrate surface could be retained and the amorphous initial layer removed. Moreover, when the substrate bias is near the puttering effect dominated region, only pure turbostratic boron nitride film could be deposited. By reducing the time span of the substrate bias in puttering effect dominated region during the time dependent biasing technique, the turbostratic boron nitrde transition layer could be partitally eliminated. Nucleation of cubic boron nitride directly on silicon substrate was confirmed by high-resolution transmission electron microscopy. Our results may open up a possible route to cubic boron nitrde eptaxial growth on silicon substrate and lead to the potential high-temperature electronic applications of cubic boron nitride thin films.
2007, 56 (9): 5408-5412.
doi: 10.7498/aps.56.5408
Abstract +
A novel organic phosphorescent white-light-emitting device (WOLED) with configuration of ITO/NPB/NPB:Ir(piq)2(acac)/CBP:TBPe /BAlq:rubrene/BAlq/Alq3/Mg:Ag is fabricated successfully, in which the phosphorescent dye (Ir(piq)2(acac)) doped into NPB (hole transport material), fluorescent dye (TBPe) doped into CBP (ambipolar conductivity material) and another fluorescent dye (rubrene) doped into BALq (electron transport material) work as the red, blue and green emitting layer, respectively. The emitting layers are sandwiched between the undoped NPB and BALq layers. The charge distribution has been balanced for the introduction of heterojunctions between the different emitting layers, which results in a high fabrication reproducibility of the device. White emission is obtained by tuning the thickness and doping proportions of each layer. The device turns on at the driving voltage of 4.5 V. It shows a maximum external quantum efficiency and brightness of 1.5 % and 16260 cd/m2, respectively. The Commission Internationale de l'Eclairage (CIE) coordinates varies from (0.396, 0.432) at 5 V to (0.411, 0.414) at 20 V. The emission spectra of the device mainly consist of blue (464 nm), green (552 nm) and red (620 nm) emissions peaks, which are particular suitable to be combined with color filters to obtain the three primary colors and further achieve full color displays.
2007, 56 (9): 5413-5417.
doi: 10.7498/aps.56.5413
Abstract +
The adsorption of carbon on Ni (111) surface is studied with density functional theory in this paper. The adsorption energy and density of states is calculated. We analyzed the density of states of carbon on diamond (111) surface and carbon adsorbed on Ni(111) surface, the result shows that they have similar electronic structure, namely, they both have nonbonding and bonding sp3 hybridized electrons. Furthermore, it is found that the carbon atoms on diamond (111) surface and those adsorbed on Ni(111) surface are prone to bonding and form a diamond-like structure.
2007, 56 (9): 5418-5423.
doi: 10.7498/aps.56.5418
Abstract +
Two InAs/In0.15Ga0.85As quantum dots-in-a-well (DWELL) samples have been grown by solid source molecular beam epitaxy (MBE). The increased size of InAs dots and more homogeneous dot-size distribution have been found in one InAs DWELL sample with growth optimized by changing both the growth temperature and the thickness in In0.15Ga0.85As well layers. The improved optical properties of this sample have also been confirmed by photoluminescence (PL) and piezomodulated reflectance (PzR) spectra. The numerical calculations based on effective mass approximation indicate that the increase of dot size and the improved optical properties are dominantly due to the strain driven alloy decomposition of In0.15Ga0.85As well layers.
2007, 56 (9): 5424-5428.
doi: 10.7498/aps.56.5424
Abstract +
A study of the carrier transport performance of GaAs/AlGaAs very long wavelength quantum well infrared photodetectors (peak wavelength: 15 μm; period: >40) has been carried out based on quantum wave transport theory. It was shown that the thermoexcitation effect dominates in the devices. By the current continuity and self-consistent calculation based on Schrdinger equation and Poisson equation, we found that the main feature resulting from the model is the redistribution of the carriers and the electric field along the whole structure being made uneven to maintain current conservation. The high-field region extends over a few barriers near the emitter contact, which takes up a considerable part of the applied voltage. The conventional flat band model assumes that the applied voltage drops linearly across the structure, leading to the numerical value of dark current to be far from the experiments, especially at small bias. The numerical result of self-consistent calculation well explains the measurement data.
2007, 56 (9): 5429-5435.
doi: 10.7498/aps.56.5429
Abstract +
The subbands of the ground state EC1, the first excited state EC2 and heavy hole state EHH1 are calculated by solving the eigenvalues of effective-mass Hamiltonian H0 which is derived from eight-band k·p theory and the calculations are performed at kx=ky=kz=0 for the three-dimensional array of InGaAs/GaAs quantum dots (QDs). With indium content in InGaAs QDs gradually increasing from 30% to 100%, the intersubband transition wavelength of EC2 to EC1 blue-shifts from 18.50 to 11.87 μm，while the transition wavelength of EC1 to EHH1 red-shifts from 1.04 to 1.73 μm. With the sizes of In0.5Ga0.5As and InAs QDs increasing from 1.0 to 5.0 nm, the intersubband transition from EC1 to EC2 transforms from bound-state-to-continuum-state to bound-state-to-bound-state, and the corresponding intersubband transition wavelengths red-shift from 8.12 μm (5.90 μm) to 53.47 μm (31.87 μm), respectively, and the transition wavelengths of EC1 to EHH1 red-shift from 1.13 μm (1.60 μm) to 1.27 μm (2.01 μm), respectively.
2007, 56 (9): 5436-5440.
doi: 10.7498/aps.56.5436
Abstract +
The electronic structure of a two-electron quantum-dot molecular has studied using the effective mass approximation. The quantum-dot molecular consists of two vertically coupled self-assembled quantum disks in a magnetic field with varying orientation. Based on the calculation, a qubit modulated by the orientation of magnetic field is proposed with the total spin of the system. Electron correlations may lead to the switching of the total spin between S=0 and S=1 states. The switching is realized not by the changing strength but by the changing orientation of an external magnetic field. Our results provide a possibility to use the total spin of the system as a qubit modulated by varying the orientation of the magnetic field. Since high-quality vertically stacked quantum disks can be fabricated successfully, it is realistic to obtain qubits of this type.
2007, 56 (9): 5441-5445.
doi: 10.7498/aps.56.5441
Abstract +
The performances of organic semiconductor thin film devices are largely determined by the transport properties of the organic-organic (OO) interfaces，but many present theories concerning charge transport at OO interfaces are not suitable for practical device modelling. Based on the Miller-Abrahams formalism for hopping conduction, an analytical model for charge transport at OO interfaces has been developed by considering the difference between OO interfaces and metal-organic interfaces. The result shows that the transport of carriers at an OO interface depends not only on the energy barrier of the interface, but also on the carrier concentration and the electric field at the two sides of the interface. This model can be used to self-consistent calculations of the current density, field distribution and other characteristics in multiplayer organic semiconductor devices.
2007, 56 (9): 5446-5451.
doi: 10.7498/aps.56.5446
Abstract +
In order to obtain greater radiation hardness for SIMOX (separation by implanted oxygen) materials, nitrogen was implanted into SIMOX BOX (buried oxide). However, it has been found by the C-V technique employed in this work that there is an obvious increase of the fixed positive charge density in the nitrogen-implanted BOX with a 150 nm thickness and 4×1015cm-2 nitrogen implantation dose, compared with that unimplanted with nitrogen. On the other hand, for the BOX layers with a 375 nm thickness and implanted with 2×1015 and 3×1015cm-2 nitrogen doses respectively, the increase of the fixed positive charge density induced by implanted nitrogen has not been observed. The post-implantation annealing conditions are identical for all the nitrogen-implanted samples. The increase in fixed positive charge density in the nitrogen-implanted 150 nm BOX is ascribed to the accumulation of implanted nitrogen near the BOX/Si interface due to the post-implantation annealing process according to SIMS results. In addition, it has also been found that the fixed positive charge density in initial BOX is very small. This means SIMOX BOX has a much lower oxide charge density than thermal SiO2 which contains a lot of oxide charges in most cases.
2007, 56 (9): 5452-5457.
doi: 10.7498/aps.56.5452
Abstract +
Temperature dependence of the aggregation mechanism of silver atoms deposited on liquid substrates is reported. The experiment shows that the aggregation mechanism of the silver atoms conforms to the two-stage growth model approximately when the substrate temperature changes. The fringe effect of the substrates was visible in the experiment and the coverage ratio of the silver atom clusters in the central region of the sample was smaller than that on the edge of the sample. As the nominal film thickness increased, the coverage ratio went up rapidly first, then its growth speed slowed down and finally the coverage ratio approached a saturation value. The film thickness corresponding to the saturation coverage ratio decreased obviously with the increase of the substrate temperature. For a fixed nominal film thickness, as the oil temperature increased, the coverage ratio decreased. The evolvement behavior of the average length of the silver aggregate branches is similar to that of the coverage ratio in the central region of the sample. The diffusion mechanism of the silver atomic clusters towards the sample edge is also presented.
2007, 56 (9): 5458-5465.
doi: 10.7498/aps.56.5458
Abstract +
Regular mixed arrays of superconducting π rings and 0 rings have been investigated. Our calculation shows that the free energy of inverse spontaneous magnetization of two π rings indirectly coupled via a 0 ring is lower. The state of the full antiparallel pattern of π rings is favorable in the regular mixed arrays. This is a quantum phase coherence result of superconducting wave functions.
2007, 56 (9): 5466-5470.
doi: 10.7498/aps.56.5466
Abstract +
The total energy of the Heisenberg ferromagnetic system is calculated by the many-body Green's function method under random phase approximation when the correlation function 〈S+S-〉 is considered. A general expression of the total energy universally applicable to any spin quantum number S is obtained. Numerical results are computed. The energy is lower than the case when the correlation function is not included.
2007, 56 (9): 5471-5475.
doi: 10.7498/aps.56.5471
Abstract +
The structure of MnxGe1-x dilute magnetic semiconductor thin films prepared by magnetron co-sputtering has been studied by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) techniques. The XRD results show that in the MnxGe1-x thin film with low Mn doping concentration (x=0.070), only diffraction peaks attributed to crystalline Ge can be observed. In samples with high Mn doping concentrations (x=0.250, 0.360), the secondary phase Ge3Mn5 appears, and its content enhances with Mn doping concentration. The XAFS results indicate that for the Mn0.07Ge0.93 thin film, Mn atoms are mainly incorporated into the Ge lattice and located at the substitutional sites of Ge atoms with the ratio of 75%, while for the Mn0.25Ge0.75 and Mn0.36Ge0.64 samples, most of the Mn atoms are aggregated to form Ge3Mn5.
2007, 56 (9): 5476-5482.
doi: 10.7498/aps.56.5476
Abstract +
The phases of the antiferromagnetic magnetization and the corresponding exchange bias and coercivity in exchange-biased bilayers(FM/AF) with interface quadratic and biquadratic exchange coupling have been studied comprehensively. The results show that there are four possible cases for the antiferromagnetic magnetization, namely the reversible recovering case, irreversible half-rotating case, irreversible reversing and irreversible half-reversing cases. However, the realization of the cases strongly depends on interface quadratic coupling, interface biquadratic coupling and AF thickness. In the reversible recovering case the exchange coupling between FM and AF results in the exchange bias, and there is no exchange bias in the other cases. Specially, for exchange-biased bilayers without interface quadratic coupling, there exists a critical value of the biquadratic coupling constant J2crit=0.1σw(σw denotes AF domain wall energy), below which the positive exchange bias appears, otherwise there is no exchange bias, and the coercivity can be enhanced. For the FM/AF bilayers with interface quadratic and biquadratic coupling, the interface biquadratic coupling can weaken or eliminate the exchange bias, but always enhances the coercivity.
2007, 56 (9): 5483-5488.
doi: 10.7498/aps.56.5483
Abstract +
The ferromagnetic resonance (FMR) technique has been used to investigate the properties of ferromagnetic/nonmagnetic/ferromagnetic systems. We derived the FMR dispersion relation, taking into account the competition between the indirect interlayer coupling and stress anisotropy. In this case，one of the layers A has a strong in-plane anisotropy compared with the coupling strength and the in-plane anisotropy of the other layer B. As the applied field increases，magnetization of layer B may suddenly switch from the antiparallel configuration to the saturated state. The discontinuities are more important for the optical than for the acoustic modes. For the optical mode，the effect of interlayer coupling and stress anisotropy on the HR vs φH curves is more apparent.
2007, 56 (9): 5489-5493.
doi: 10.7498/aps.56.5489
Abstract +
The CaCu3Ti4-xNbxO12(x=0，0.01，0.04，0.08，0.2) ceramics have been prepared by solid-state reaction. The XRD of the sintered ceramics indicated that complete solutions are formed for all compositions x. Dielectric properties have been investigated in the frequency range from 40Hz to 110MHz. Compared with the un-doped CCTO sample，a Debye-type dielectric relaxation has been observed for the Nb-doping CCTO samples in the frequency range from 40Hz to 10kHz at room temperature，apart from the one already known in the frequency range higher than 10kHz. Moreover，the dielectric constant reaches 106 below the characteristic frequency of the lower frequency dielectric relaxation peak which shifts to higher-frequency with increasing Nb content. The experimental results are well explained in terms of the barrier layer capacitance model and an equivalent circuit containing three RC elements by showing three semicircles in the impedance spectroscopic plane.
2007, 56 (9): 5494-5501.
doi: 10.7498/aps.56.5494
Abstract +
Due to the limitations of specific insulation structures and the influence of technical process，the growth of electrical trees in high-voltage power equipments with polymer as main insulation materials is more complicated than those in uniform materials，especially in equipment employing semi-crystalline polymer，so it is difficult to characterize by mathematical means. The experiment reported in this paper is the first attempt to focus on the electrical treeing rules in both inner and outer insulation layers of high-voltage XLPE power cable. Through statistical experiments and the SEM observation，large difference in treeing character between the inner and outer layers of cable insulation due to the different crystalline status are found. Electrical trees initiating in the inner layer of insulation has shorter initiation time，higher growth rate and greater variety of shapes compared than in the outer layers. The initiation and propagation mechanisms are also discussed.
2007, 56 (9): 5502-5507.
doi: 10.7498/aps.56.5502
Abstract +
The properties of space charges formed in the hot compression molding process of low density polyethylene (LDPE) have been studied by measuring the open-circuit thermally stimulated discharge (TSD) current， in situ TSD charge and the isothermal charge decay. The results showed that the space charges with high stability at room temperature (RT) are trapped in two kinds of traps: shallow traps and deep traps with central depths of about 0.92eV and 1.31eV, respectively. Preliminary analysis indicates that the shallow traps and deep traps should be located in the surface layer and in the bulk of the sample, forming surface traps and bulk traps，respectively.
2007, 56 (9): 5508-5512.
doi: 10.7498/aps.56.5508
Abstract +
Based on the mutual interaction between the electromagnetic wave and pigment, the theoretical model is formulated. The influence of various factors, such as the size of the pigment, the volume ratio of pigment, and the optical constants, on the spectral reflective properties is systematically analyzed. Experiment has been performed to testify the theoretical model. The theoretical design for the spectral reflective properties of the stealth coating is further proposed.
2007, 56 (9): 5513-5517.
doi: 10.7498/aps.56.5513
Abstract +
A new GaN-based ultraviolet photodetector with Schottky barrior structure is proposed. Comparied with the conventional i-GaN/n+-GaN structure，there is an additional thin n-AlGaN cap layer on the i-GaN in the new structure. The simulation result demonstrates that the new structure leads to an increased quantum efficiency in GaN photodetection，since the negative effect of surface states on the photodetector is reduced in the new structure. In addition，it is suggested that the performance of device with the new structure could be further improved by employing an even thinner AlGaN cap layer with higher carrier concentration.
2007, 56 (9): 5518-5525.
doi: 10.7498/aps.56.5518
Abstract +
A series of Er3+/Yb3+-codoped，Ho3+/Yb3+-codoped，and Er3+/Yb3+/Ho3+-triply doped tellurite glasses were prepared by high-temperature melting. Under 975nm laser light excitation，intense green and red upconversion luminescence was observed in all the three kinds of codoped tellurite glasses. The influence of Yb3+ ions on the upconversion emission properties of Er3+ and Ho3+ ions was investigated as well as the energy transfer efficiencies of Yb3+→Er3+ and Yb3+→Ho3+. Upconversion luminescence mechanisms of Er3+ and Ho3+ sensitized by Yb3+ in tellurite glasses were discussed. When the concentrations of Er3+ and Ho3+ ions were fixed at low values，the intensity of green and red upcoversion emission in Er3+/Yb3+/Ho3+-triply doped tellurite glasses increases with increasing Yb2O3 content, which are due to the interaction overlap of Er3+：4S3/2→4I15/2 and Ho3+：5S2(5F4)→5I8 transitions，Er3+：4F9/2→4I15/2 and Ho3+：5F5→5I8 transitions，respectively. In Er3+/Yb3+/Ho3+-triply doped tellurite glasses，the upconversion luminescence mechanisms were affected by the interaction energy transfers of Er3+/Yb3+, Ho3+/Yb3+ and Er3+/Ho3+. The cross-relaxation between Er3+ and Ho3+ enhances the upconversion emission intensities of Ho3+ in the visible region.
2007, 56 (9): 5526-5530.
doi: 10.7498/aps.56.5526
Abstract +
The key to improve the efficiency of SSCL (solid state cathodoluminescence) is to improve the injection and acceleration of electrons in the accelerating layer. The electron acceleration ability of SiO is better than that of ZnS，while the latter is better than the former in the ability of injecting charges. So our attention was turned to the complex accelerating layer SiO2/ZnS，and prepared two kinds of devices to see whether the short peak occurrs. One of them is the low voltage mono-side recombination device, the othe the high voltage impacted two-side device. In result，under low voltage，the performance of the complex accelerating layer is superior to that of SiO2 and inferior to that of ZnS. Under high voltage，the complex accelerating layer is the best of all devices. Meanwhile，SiO2 was still found to be the main accelerating layer，yet ZnS is very useful for improving the performance in electron injection of SSCL device.
2007, 56 (9): 5531-5535.
doi: 10.7498/aps.56.5531
Abstract +
GaN-based dual-wavelength light emitting diodes (LEDs) with different In contents in the wells and barriers was designed and fabrieated in our experiment. Electroluminescence of the fabricated LED at the typical driving current of 20mA has two colors，blue and green respectively. The ratio of the intensities of the green light to the blue light increases with increasing driving current. At larger current，the blue shift of the peak wavelength of the green light was greater than that of the blue light. We are also concerned with the effect of polarization and non-uniform carrier distribution in the active region. Using coupled method of simulation，the calculated data were consistent with the experiment.
2007, 56 (9): 5536-5541.
doi: 10.7498/aps.56.5536
Abstract +
As-grown Fe-doped semi-insulating InP single crystal has been converted into n-type low-resistance material after high temperature annealing. Defects in the InP materials have been studied by conventional Hall effect measurement, thermally stimulated current spectroscopy, deep level transient spectroscopy and X-ray diffraction respectively. The results indicate that Fe atoms in the InP material change from the substitutional to the interstitial sites under thermal activation. Consequently, the InP material loses its deep compensation centers which results in the change in types of conduction. The mechanism and cause of the phenomena have been analyzed through comparison of the sites of Fe atom occupation and activation in doping, diffusion and ion implantation processes of InP.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2007, 56 (9): 5542-5547.
doi: 10.7498/aps.56.5542
Abstract +
By multiphase field model method，the unidirectional solidification of eutectic alloy CBr4-C2Cl6 is simulated，and the adjustment of lamellar spacing and morphology evolution have been investigated under three schemes of variable pulling speed，i.e., the stepped change，the linear change and the oscillatory change. The simulation results show that，for the growth under these three schemes，both the interface average velocity and the interface average undercooling show hysteresis effect. When the stepped increase or decrease in velocity is imposed，the lamellar spacing adjustment is realized through abrupt branching or progressive merging and annihilation accompanying the lamellar growing，respectively，and there is strong asymmetry in these two processes during lamellar adjustment. In the case of linear increase in velocity，the lamellar spacing changes by progressive branching，while in the case of linear decrease in velocity，the mechanism of adjustment of lamellar spacing is similar to that of growth under stepped decrease of velocity. The processes for linear increase and decrease in velocity show inverse symmetrical characteristic. A quasi-1λ oscillation mode is present when an oscillatory pulling speed is applied.
2007, 56 (9): 5548-5553.
doi: 10.7498/aps.56.5548
Abstract +
We have prepared polycrystalline CdTe thin films by close-spaced sublimation，then the film surface was been etched by nitric-phosphoric acid. After etching，the grain boundaries of CdTe thin films are broadened and it could be seen clearly that the surface became polished and more smooth, when observed by scanning electron microscope (SEM). After NP etching，highly conductive Te-rich layer is formed on the surface of CdTe thin film，as detected by X-ray diffraction (XRD). Four types of back-contact layers, including Cu, Cu/ZnTe：Cu, ZnTe：Cu and ZnTe/ZnTe：Cu were deposited respectively on the etched CdTe thin film，and the influences on the solar cells performance were compared. Our studies showed that the performance of CdTe solar cells with ZnTe/ZnTe：Cu complex back-contact layer was better than those with other back-contact layers，and the highest conversion efficiency of 13.38% has been obtained for CdTe polycrystalline thin film solar cells of 0.5cm2 size.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2007, 56 (9): 5554-5557.
doi: 10.7498/aps.56.5554
Abstract +
The extraterrestrial spectrum in the visible wavelength range of 340.5—818.6nm at the height of 9.6 km has been detected，furthermore，the upper sunlightand skylight spectra have been collected in different regions of Chengdu and Lijiang. The date and diagram indicated that 482.1nm is the hightest point in extraterrestrial direct solar spectra, and that of the extraterrestrial skylight spectra is shifted to 412nm. The skylight spectra registered on sunny and cloudy days are notably different. It was also found that the firn on top the Yulong Mountain has different albedos in different wavebands visible range.
2007, 56 (9): 5558-5563.
doi: 10.7498/aps.56.5558
Abstract +
Blazar is an extreme subset of AGNs，constituting less than a few percent of the known AGN population. While Blazar have some properties in common with other AGNs，they have some characteristics that set them apart. The observation properties are probably the cause of strong beaming effect，and these properties would be mutually correlated. The relationship between the polarization and the magnitude variation in brightness，as well as the core-dominance parameter are derived and used statistically to compare with the observational data of Blazar smaples. The result suggests that the polarization, variation and the core-dominance parameters are possible indications of the beaming effect.
