Vol. 56, No. 8 (2007)
2007-04-20
GENERAL
2007, 56 (8): 4313-4318.
doi: 10.7498/aps.56.4313
Abstract +
Using genetic algorithm, we studied the evolution of strategies in the iterated prisoner's dilemma on complex networks. It is found that the agents located on complex networks can naturally develop some self-organization mechanics of cooperation by genome reproduction, recombination, mutation and selection, which can not only result in the emergence of cooperation, but also strengthen and sustain the persistent cooperation. At the same time, such mechanics punishes and takes revenge on defective agents, leading to a high cooperation rate on complex networks.
2007, 56 (8): 4319-4325.
doi: 10.7498/aps.56.4319
Abstract +
By an improved mapping approach, Bcklund transformation approach and a variable separation approach, a series of excitations of the (2+1)-dimensional generalized Breor-Kaup (GBK) system is derived. Based on the derived excitations, we find some complex waves of GBK equation. Then we discussed the evolution of dromion solitions in the background of the Jacobi sine waves.
2007, 56 (8): 4326-4330.
doi: 10.7498/aps.56.4326
Abstract +
A suitable transformation (trigonometric function method) is found to change nonlinear Boussinesq differential equations into nonlinear algebra equations, which are solved by Wu elimination method and therewith the general soliton solutions of Boussinesq differential equations are obtained.
2007, 56 (8): 4331-4336.
doi: 10.7498/aps.56.4331
Abstract +
Based on the proposal of Braunstein and Kimble，we studied the continuous variables quantum teleportation via minimum-correlation mixed state. The results show that the mixed entangled state as a generalized type of Einstein-Podolsky-Rosen entangled states is a good quantum channel. As an example，we analyzed in detail teleporting a coherent state by means of this channel. When the parameters of minimum-correlation mixed state are chosen properly，the fidelity will almost reach to unity. The minimum-correlation mixed state as a quantum channel is better than the two-mode squeezed vacuum state.
2007, 56 (8): 4337-4341.
doi: 10.7498/aps.56.4337
Abstract +
The entropy density，energy density and pressure of the spin fields near the event horizon in Taub-NUT spacetime are investigated by using the Wentzel-Kramers-Brillouin approximation. It is shown that any one of the thermodynamic quantities has three terms. The first term has the same form as that in flat spacetime, the second and the third terms are spin-dependent terms due to the spin fields.
2007, 56 (8): 4342-4347.
doi: 10.7498/aps.56.4342
Abstract +
With the development of intelligent transportation technologies, the variable message signs (VMS) have been widely used in guiding and managing the dynamic traffic. It is known that cell transmission model (CTM) can well reproduce such traffic dynamics as shock waves and jams. This paper employs the CTM to study the location problem of VMS. Simulation results show that there exists an optimal VMS location to minimize the total travel time of the traffic system. How changes of route choice probabilities before and after a traffic accident affect the optimal VMS locations is also investigated and it is concluded that the VMS should be placed far from the accident site if the anticipated accident is very serious.
2007, 56 (8): 4348-4352.
doi: 10.7498/aps.56.4348
Abstract +
Based on mean-field theory and single spatial mode approximation，we study the nonlinear dynamical properties of the susceptibility of total spin F=1 Bose-Einstein condensate with dipole-dipole interaction in a double-well potential. For certain initial states，we find that the susceptibility oscillation appears when λA+2λd=0. When λA+2λd≠0, the dynamical behavior of the system shows both oscillation and self-trapping of susceptibility.
2007, 56 (8): 4353-4360.
doi: 10.7498/aps.56.4353
Abstract +
We present in this paper some results on the temporal segmentation and retrieval of stored memories or patterns using neural networks composed of spiking neurons. Respecting the working environment，we present the network with stochastic or chaotic stimuli as their extremely working conditions and also with noise. We attempt to give an explanation to the function of memory retrieval of the brain system, where the stimuli usually may not be constant, sinusoidal or periodic, but rather chaotic or stochastic. For an input pattern which is a superposition of several stored patterns, it is shown that the proposed neuronal network model is capable of segmenting out each pattern one after another as synchronous firings of a subgroup of neurons，and if a corrupted input pattern is presented, the network is shown to be able to retrieve the perfect one，that is it has the function of associative memory. By thorougly adjusting the parameters, such as the coupling strength and the intensity of the noise, the temporal segmentation attains its optimal performance at intermediate noise intensity，which reminds of the stochastic resonance observed in the coupled spiking neuronal networks.
2007, 56 (8): 4361-4365.
doi: 10.7498/aps.56.4361
Abstract +
For a class of nonautonomous nonlinear vibration system，a corresponding derived system is constructed. A novel control method to realize the synchronization between the derived and the original system is presented. The method is proved by Lyapunov stability theory. The principle for choosing coupling parameters is given. For a Mathieu system excited by parameters and external force，the simulation is carried out. The result shows that the derived system can achieve fast synchronization with the original system under the action of chaos. The simulation for Duffing system excited by quasi-periodic force shows that the derived system can reach synchronization with the original system under the quasi-periodic motion.
2007, 56 (8): 4366-4371.
doi: 10.7498/aps.56.4366
Abstract +
Using improved state-dependent Riccati equation method，the state synchronization of unified chaotic systems is achieved. The state adjustors with given stability improve the dynamic performance of system synchronization. Theoretical deduction and simulation researches show that the error systems are asymptoticly stable. The comparative simulations with the traditional state-dependent Riccati equation method verify the improvement in dynamic performance of system synchronization.
2007, 56 (8): 4372-4377.
doi: 10.7498/aps.56.4372
Abstract +
A semiconductor laser that generates a chaotic signal with optical feedback can be treated as a chaotic transmitter. Based on single mode rate equations of semiconductor laser with optical feedback and external injection, the numerical simulation shows that the bandwidth of the transmitter is expanded by external optical injection. When the injection index kinj is 0.39, the bandwidth is increased from the value of 2.7 GHz without optical injection to 14.5GHz. The results also reveal that, under the same injection strength, the enhancement of bandwidth depends evidently on the frequency detuning between the external injection laser diode and the chaotic carrier transmitter. The maximum bandwidth of the chaotic transmitter can be obtained when the frequency detuning is in the range from 2 to 4GHz. The bandwidth of the chaotic transmitter can also be enhanced by increasing the bias current.
2007, 56 (8): 4378-4382.
doi: 10.7498/aps.56.4378
Abstract +
The optical phase-conjugate resonator consists of one normal mirror and one phase-conjugate mirror made of an instantaneous response lossless Kerr medium. Using the nonlinear feedback, we control the temporal chaos in the phase-conjugate resonator and spatiotemporal chaos in the coupled phase-conjugate map system. Stable control has been realized. The simulation shows that this controlling technique can be implemented easily in practical systems just by adjusting the feedback strength. The in-advance knowledge of the dynamic system is not required. The results are useful to theoretical analysis of chaos in phase-conjugate resonators.
2007, 56 (8): 4383-4392.
doi: 10.7498/aps.56.4383
Abstract +
A modified coupled map car-following model is proposed for a one-way with the application of intelligent transportation system to describe the dynamic characteristics of one-way traffic flow and the control of traffic congestion. Based on the present model and the theory of feedback control，the stability criteria are given for the change of speed of the lead vehicle. The theoretical analysis shows that the information on more leading vehicles of each vehicle could lead to a stabilization effect for the traffic flow，that is，the stability conditions could be considerably weakened. The corresponding numerical simulations confirm the correctness of the theoretical analysis. Through comparing the present results with those in previous works，it is concluded that the proposed control strategy is more effective in suppressing the formation of traffic congestion by applying and controlling the induction information provided by the intelligent transportation system.
2007, 56 (8): 4393-4400.
doi: 10.7498/aps.56.4393
Abstract +
The purpose of the present study is to investigate the presence of multifractal behaviors of the palmprint. By studying the probability distribution and the partition function，we show that the palmprints possess multifractal characteristics. The width spread，the maximum of multifractal spectrum，and a parameter which describes the asymmetry of the spectrum are proposed as the distinguishing palmprint features. This may provide some factors of importance for palmprint recognition，and shed some light on the problem of biometric recognition.
2007, 56 (8): 4401-4405.
doi: 10.7498/aps.56.4401
Abstract +
Using spectral sensitizing technology，silver halide microcrystals can be sensitized to the visible light region. The spectral sensitizing technology is important to the modern information recording and storage，solar energy transfer and storage, and the photoelectronic devices. The photoelectron decay characteristic of silver chloride microcrystals，which is adsorbed with green sensitizing cyanine dye，is studied by microwave absorption and dielectric spectrum detection technology. The model of photoelectron decay is set up. Electron trap effect of the dye in the silver halide surface is analyzed based on the photoelectron decay model and the photoelectron decay characteristic. The results show that when the mono-molecular dye is adsorbed on the surface，shallow electron traps are created; when the J-aggregated dye is adsorbed on the surface，deep electron traps are created. The critical dose of dye that seperates the shallow electron trap and deep electron trap effects is 0.2ml of dye (with concentration of 5.0mg/ml) per 40g AgCl emulsion.
NUCLEAR PHYSICS
2007, 56 (8): 4406-4411.
doi: 10.7498/aps.56.4406
Abstract +
A γ-ray integrated measurement prototype system named HAM-1 has been built up by integrating the technologies of γ-ray detection，spectral analysis and information barrier，which can be used for potential nuclear disarmament verification of plutonium components through the measurement of attributes such as the presence of plutonium，the weapon-grade plutonium and its age. A basic starting point for the development of HAM-1 was to prevent any possible leakage of sensitive weapon design information in the process of measurement. Two embedded ARM controllers were selected as the CPUs of HAM-1 according to the design requirements of information barriers. A code dedicated to HAM-1 has been developed for analyzing the high-resolution γ-ray spectra to accurately determine the attributes. The all-round process of HAM-1 from the spectra sampling to data processing and result display was automatic, during which the sensitive weapon design information was protected from leakage.
ATOMIC AND MOLECULAR PHYSICS
2007, 56 (8): 4412-4419.
doi: 10.7498/aps.56.4412
Abstract +
Quadratic configuration interaction method including single and duble substitutions has been used to optimize the possible ground-state structures of B2C and BC2 molecules with the 6-311++G(d, f) and 6-311G(df, pd) basis set. The results show that the ground state of B2C molecule is of C2v symmetry and of 1A1 state, the ground state of BC2 molecule is of Cs symmetry and of 2A′ state. And the equilibrium geometry, dissociation energy, harmonic frequency and force constant have been calculated. The potential energy functions of B2C and BC2 have been derived from the many-body expansion theory. The potential energy functions describe correctly the configurations and the dissociation energies of the two ground-state molecules. Molecular reaction kinetics of B+BC→B2C and B+CC→BC2 based on the potential energy functions is discussed briefly.
2007, 56 (8): 4420-4427.
doi: 10.7498/aps.56.4420
Abstract +
The potential energy curves for the ground state of SO and ClO molecules as well as their corresponding molecular ions have been computed using the multi-reference configuration interaction (MRCI) method with aug-cc-PVXZ (X=D,T,Q) basis sets. The MRCI energies are extrapolated to the complete basis sets limit with Feller's method. The equilibrium geometries and disassociation energies also have been determined. The analytical potential energy functions of these states have been fitted using Murrell-Sorbie function and least square fitting method. Based on the analytical potential energy functions, the vibrational levels have been determined by solving Schrdinger equation of nuclear motion, and corresponding spectroscopic constants accurately calculated respectively. The present values for the spectroscopic constants of each state are compared with the other theoretical and the experimental values currently available.
2007, 56 (8): 4428-4434.
doi: 10.7498/aps.56.4428
Abstract +
The total internal partition sums (TIPS) were calculated for nitrous oxide with the product approximation. For rotational partition sums Qrot, the centrifugal distortion corrections are under consideration. The calculation method for the vibrational partition sums Qvib used is the harmonic oscillator approximation. Using experimental transition moment squared and Herman-Waills factor coefficients and the calculated partition functions, we computed the line intensities of 3000—0200 and 1001—0110 transitions of nitrous oxide at several temperatures. The agreement between the calculated line intensities and experimental results and those extrapolated from HITRAN database is fairly good at temperatures up to 3000 K, which shows that the calculation of partition function and line intensity at high temperature is reliable. Furthermore, the line intensities and spectral simulations at the higher temperatures of 4000 and 5000 K are also presented.
2007, 56 (8): 4435-4440.
doi: 10.7498/aps.56.4435
Abstract +
Taking into consideration the overlapping effect of electron clouds between two bonded atoms in a molecule, a modificed potential method is presented which can be used to accurately calculate various cross sections for electron scattering by molecules at intermediate and high energies. The modified complex optical potential is directly employed to calculate the absolute differential, elastic integral and moment transfer cross sections for electron scattering by SO2 in the energy range 100—1000 eV by using the additivity rule at Hartree-Fork level. The quantitative results are compared with those obtained by measurement and other theories wherever available. It is shown that the additivity rule model together with the modified complex optical potential can give the encouraging results, which are much closer to the measurements than the unmodified ones.
2007, 56 (8): 4441-4447.
doi: 10.7498/aps.56.4441
Abstract +
The intermolecular potential energy surface for He-HI complex has been first calculated by employing supermolecule method and the single and double excitation coupled-cluster with a noniterative perturbation treatment of triple excitation CCSD(T) approach using a large basis set containing the bond function set 3s3p2d1f. The potential energy surface has two potential wells. The global minimum with a well depth of 4.473 meV has been found for the linear He-I-H configuration with R，the distance of the He atom and the center of mass of the HI molecule, of 0.363nm. In addition to the global minimum, there is a second minimum corresponding to the linear He-H-I configuration with a well depth of 2.996 meV and R of 0.442nm. Then the analytic expression of the interaction potential for the He-HI complex of the ground state has been obtained by utilizing the Barker potential-type analytic function to fit the calculated intermolecular energy data. On the basis of the above results, the differential scattering cross sections (DCS) at the energy of 100meV for collision between He atom and HI molecule have been calculated using the quantum close-coupling method. Finally, the validity of the potential energy surface of the He-HI system has been verified by comparing it with the CCSD(T) potential energy surface of the He-HX(X=F, Cl, Br) systems and by comparing their DCS at the same collision energy.
2007, 56 (8): 4448-4453.
doi: 10.7498/aps.56.4448
Abstract +
The direct Coulomb ionization process can be generally well described by the ECPSSR theory, which bases on the perturbed-stationary-state(PSS) and accounts for the energy-loss, Coulomb-deflection, and relativistic effects. But the ECPSSR calculation has significant deviations for heavy projectile at low impinging energies. In this paper we propose a new modified ECPSSR theory, i.e. MECUSAR, in which PSS is replaced by an united and separated atom model, and molecule-orbit effect is considered. The MECUSAR calculations give better agreement with the experimental data at lower impinging energies, and agree with the ECPSSR calculations at high energies. By using OBKN (Oppenheimer-Brinkman-Kramers formulas of Nikolaev) theory to describe the contribution of the electron capture, we further modified the proposed MECUSAR theory, and calculated the target ionization cross sections for different charge states of the projectile.
2007, 56 (8): 4454-4460.
doi: 10.7498/aps.56.4454
Abstract +
Using the symmetry-adapted-cluster-configuration-interaction (SAC-CI) method provided by the Gaussian 03 program package, the potential energy curve for 7Li2(23Πu) has been calculated with the basis set 6-311++G(d,p) over the internuclear separation range from 0.13 to 2.0 nm. And the ab initio calculated points have been subjected to a least squares fitting to Murrell-Sorbie function. Employing the Rydberg-Klein-Rees method, the harmonic frequency is derived from the analytic potential energy function and then other spectroscopic data are further computed. These spectroscopic data are Te=3.6701eV, De=1.0764eV, Re=0.3000nm, ωe=285.69cm-1, ωeχe=1.8351cm-1, αe=0.00942cm-1 and Be=0.5340cm-1, respectively. In particular, the present Te, De, Re and ωe values are in excellent agreement with other theoretical results. With the analytic potential energy function obtained on the SAC-CI/6-311++G(d,p) level, a total number of 67 vibrational states for the 23Πu state is found when J=0 by solving the radial Schrdinger equation of nuclear motion. The complete vibrational levels, classical turning points and inertial rotation constants for these vibrational states are also reported. The reasonable dissociation limit for the 23Πu state is deduced using the present results calculated.
2007, 56 (8): 4461-4466.
doi: 10.7498/aps.56.4461
Abstract +
This paper presents the measurement results of absolute cross-sections of ion-atom collision in detail. The pure cross-sections of Nei+(i=1—4) induced by C3+ were investigated using coincidence technique. Comparing our experimental results with those of n-body classical trajectory Monte Carlo method, it is found that the changes of pure cross sections with increasing energy have the same trend in the two cases. The causes of discrepancies between them are discussed. The mechanism of multiple-ionization is analyzed. The highly charged recoil ions come mainly from the Auger effect. The electron-electron interaction comes in to effect as the incident energy increases. The cross-sections of all kinds of reactions can be measured in this way using the same equipment and projectile ions can be various within a wide range of energy.
2007, 56 (8): 4467-4476.
doi: 10.7498/aps.56.4467
Abstract +
The geometric configuration, vibration frequency and thermodynamic properties of Al2O3X(X=H, D, T) molecular clusters with lower energy were optimized using B3LYP/6-311++G(d,p) method. The changes of entropy, enthalpy and Gibbs free energy of the reactions between Al2O3 and hydrogen (deuterium or tritium) gas have been calculated under the solid electron-vibration approximation method using formulae in thermodynamics under temperatures of 298—1098 K. Then the equilibrium pressures of hydrogen (deuterium or tritium) gas in these reactions are obtained. The results show that, the gaseous Al2O3X may have two possible ground states Al2O3X(X=H,D,T)(2A′)Cs and Al2O3X (2B2) C2v. Tritium can be displaced by deuterium; deuterium can be displaced by hydrogen in the reactions between Al2O3 and X2 with the production of solid Al2O3X which relates to ground Al2O3X with C2v symmetry. This displacement sequence is the same as that in the reactions between titanium and X2. These displacement effects are very weak. But hydrogen can be displaced by deuterium; deuterium can be displaced by tritium in the reactions between Al2O3 and X2 with the production of solid Al2O3X which relates to ground gaseous Al2O3X with Cs symmetry. This displacement sequence is opposite to that in the reactions between titanium and X2. In all, these displacement effects are very weak, and they grow still weaker as the temperature increases.
2007, 56 (8): 4477-4483.
doi: 10.7498/aps.56.4477
Abstract +
The geometrical structures and electronic properties of GenB (n=12—19) clusters have been studied using first principles with generalized gradient approximation. The results show that GenB (n=12—19) clusters have large energy gap. The lowest energy structures of GenB (n=12—19) clusters have two different element, such as Ge9 and Ge10, cluster configuration. There are two different models capable of forming GenB clusters, which are the cluster with B atom concaved in Gen, and the cluster in which B substitutes Gen+1. 17 is the magic number of GenB (n=12—19) clusters.
2007, 56 (8): 4484-4490.
doi: 10.7498/aps.56.4484
Abstract +
We report the results of calculations which were performed to investigate the equilibrium structures and electronic and magnetic properties of small BnNi(n≤5) clusters within the framework of density functional theory. We obtained the structures of BnNi(n≤5) clusters at different spin multiplicities and determined the lowest-energy structures of BnNi(n≤5) clusters. The results show that the spin multiplicities of the lowest-energy clusters are 2, 1, 2, 1 and 2, respectively. We also studied the magnetic properties of the lowest-energy clusters systematically and found that both the magnetic moment of Ni atom and the total magnetic moment of clusters oscillate with increasing size.
2007, 56 (8): 4491-4496.
doi: 10.7498/aps.56.4491
Abstract +
Diethyl ether clusters were studied using both laser multiphoton ionization mass spectrum and supersonic pulsed molecular beam technique. Only less intensity ions of (E)H+, (E)＋2 and (E)2H+(E stands for CH3CH2OCH2CH3), accompanying with some stronger fragmented ions of diethyl ether, were observed. Using ab initio calculation to optimize the geometric structure and calculate the energy of diethyl ether clusters at B3LYP/6-311++G(d,p) level, the results show that the most stable structure of (E)2 is a six-side-like ring, in which the oxygen atom and the ethyl group of one molecule are located close to the ethyl group and the oxygen atom of another molecule, respectively. The binding energy of (E)2 is 9.35meV when considering the basis set superposition error with counterpoise method, which is equivalent to the dipole-dipole interaction between the partners. Comparing with the binding energy 221.93meV of complex (CH3CH2OCH2CH3)-H2O, which is associated by hydrogen bonding, it is suggested that the diethyl ether clusters are formed by weak dipole-dipole interaction and the initially formed dimmer acts as a seed for further cluster growth, meanwhile, as the binding energy decreases with size increasing, so larger clusters are not easily observed in experiment.
CLASSICAL AREA OF PHENOMENOLOGY
2007, 56 (8): 4497-4503.
doi: 10.7498/aps.56.4497
Abstract +
The sheath-helix model is used to analyze the helical slow-wave structure in which the helix turns are considered effectively shorted by the resistive coating on dielectric helix-support rods. Meanwhile, a gap is introduced to simulate the helix tape thickness, and the helix support rods are emulated by an effective dielectric tube and the difference of the radical propagation constant in different regions is taken into consideration. The dispersion equation and interaction impedance of arbitrary field modes are derived. On this basis, the effects of attenuation coating on the attenuation constant, phase constant and interaction impedance are discussed. The results presented here are useful for designing attenuator and improving the high-frequency characteristics of helical slow-wave structure as well as preventing the reflection oscillation and backward-wave oscillation of traveling-wave tubes.
2007, 56 (8): 4504-4508.
doi: 10.7498/aps.56.4504
Abstract +
By introducing cavity resonators to microstrip structures, subwavelength cavity resonator microstrip antennas with left- and right-handed metamaterial bilayered substrates are proposed and investigated. Due to the phase compensation, the total height of the antennas is reduced rather than increased. With certain sets of parameters, as a breakthrough to the restricted bandwidth of the conventional microstrip antennas, greatly broadened bandwidth can be achieved. Under other conditions, antennas with narrow bandwidth and resonant sensitivity can be realized. Corresponding applications for sensor use are proposed, and distinct advantages over the nonresonant electromagnetic wave sensors are demonstrated.
2007, 56 (8): 4509-4514.
doi: 10.7498/aps.56.4509
Abstract +
The permittivity in the frequency domain is transformed to the time domain, and the complex electric susceptibility dyadic matrix and the complex electric displacement vector in time domain are introduced. The three_dimensional finite_difference time_domain method based on the recursive convolution principle(RC_FDTD) for the electric anisotropic dispersive medium is discussed in detail. To exemplify the availability of the RC-FDTD algorithm, the backscattering RCS of a non-magnetized plasma sphere is computed, and the numerical results are the same as that of shift operator_FDTD method, which shows that the RC-FDTD method is correct and efficient. In addition, the co-polarized and cross-polarized backscattering waves in time domain for a magnetized plasma sphere are obtained by the RC-FDTD algorithm. The results show that when the external magnetic field is implemented, the cross-polarized component appear evidently.
2007, 56 (8): 4515-4522.
doi: 10.7498/aps.56.4515
Abstract +
Gyrotron traveling wave amplifier is a very important high power millimeter wave source for radar applications. Second-harmonic operation is of special significance due to its low magnetic field operation. By using an interaction circuit with two-stage distributed-loss, the absolute instability and the gyrotron backward wave oscillation can be suppressed. Nonlinear analysis is used to study the stability and large signal performance of the amplifier. Particle in cell simulation of W-band TE02 gyrotron traveling wave amplifier are also performed. The results show that this amplifier can generate 300 kW at 94 GHz with 15% efficiency and 5% bandwidth in case of a 100 kV, 20 A electron beam input.
2007, 56 (8): 4523-4534.
doi: 10.7498/aps.56.4523
Abstract +
According to the symmetry, Taylor series expansion formulas were derived for the magnetic multipole fields and field parameters were defined. Theoretical calculation formulas of field parameters of magnetic multipole field were obtained for saddle coils with the Biot_Savart law and the magnitudes of field parameter were analyzed. The rules of calculating field parameters and the high order derivatives of field parameters were also given. Our results may provide a theoretical basis for some applications of the magnetic multipole fields.
2007, 56 (8): 4535-4541.
doi: 10.7498/aps.56.4535
Abstract +
In this paper, a simple technique to determine the geometrical properties of fractional Fourier transform of paraxial beams based on the phase space beam transformation matrix is presented. Taking the elliptic Gaussian beam as an example, we have compared our analysis technique with that of previous work and found that the present method is more reliable in predicting the geometrical properties of fractional Fourier transform of beams and has the advantage of clear intuitive physical insight into beam propagation and transformation process from a geometrical viewpoint. This technique provides a simple and convenient way to study propagation and transformation properties of light beams in a novel approach.
2007, 56 (8): 4542-4549.
doi: 10.7498/aps.56.4542
Abstract +
In the ultrashort pulse high-energy laser system based on chirped pulse amplification technique, multilayer dielectric grating with good output waveform and high laser induced damage threshold plays an important role. Based on Fourier spectral transform and rigorous modal method, optical characteristic of multilayer dielectric grating under irradiation of ultrashort optical pulse is analyzed. The results show that the reflected pulse at the -1 order exhibits asymmetrical Gaussian shape with oscillatory tail in the front edge and decreasing energy when the reflection bandwidth is narrower than the frequency bandwidth of the incident pulse with Guassian shape. The effect of structure parameters of multilayer dielectric grating on the reflection bandwidth are discussed in detail, which provides guidance to the design of multilayer dielectric grating with desired reflection bandwidth. Finally, near-field distribution of multilayer dielectric grating is analyzed to study its damages induced by ultrashort pulses.
2007, 56 (8): 4550-4555.
doi: 10.7498/aps.56.4550
Abstract +
In this paper, according to the analytical equation of the far-field intensity distribution in coherent combination of fiber laser arrays, we investigate the influence of fill factor of the laser arrays, elements and phase errors on the far-field patterns, the energy and angular width of the main lobe. At last, we describe the experimental study of the coherent combining of three-element fiber-laser arrays based on ytterbium polarization-maintaining single-mode power amplifiers with the fill factor of 12.5%.
2007, 56 (8): 4556-4564.
doi: 10.7498/aps.56.4556
Abstract +
The imaging quality of X-ray in-line outline imaging(XIOI)is mainly related to the size of samples, X-ray photon energy, X-ray scattering, distance from sample to detector Dsd and also distance from source to sample for point X-ray source and so on. To maximize the resolution of the image under the practical limitations of laboratorial equipment in distance from sample to detector and range of adjustable photon energy etc., the present paper studies the effect of the above factors on the imaging quality using numeral simulation. We obtained the relationship between imaging quality and photon energy, and not only found that there are certain X-ray photon energies which are equally well suitable for imaging but also found that we can get some high quality images for phase retrieval through which we can get the exact phase and absorption information. We have determined the precice Dsd values at which the imaging quality is the best for samples of different size, and found that Dsd is propertional to the sample size and determined the factor of proportionality. Though the diffuse scattering of X-ray reduces the contrast of images, we can still have good contrast when the variation in thickness is the order of the sample size when the scattering are only caused by the variation in thickness. So under these unfavorable conditions XIOI still performs effectively.
2007, 56 (8): 4565-4570.
doi: 10.7498/aps.56.4565
Abstract +
In order to reflect the statistics of high peak and heavy tail, speckle in synthetic aperture radar images is modeled as heavy-tailed Rayleigh distribution. First, based on Gamma prior distribution and heavy-tailed Rayleigh distribution of speckle, the maximum a posteriori filtering equation is proposed and its analytical form is provided in given characteristic parameter. Second, parameters of heavy-tailed Rayleigh distribution are estimated from the observed image using Mellin transformation. Last, maximum a posteriori de-speckling experiments and their quantitative measures are given. In order to eliminate the influence of window size and noise intensity on de-speckling results, dynamic relations of the de-speckling capability to noise variance and window size are suggested respectively. Results demonstrate that the heavy-tailed Rayleigh distribution accords with the real statistics of speckle, so the maximum a posteriori filter in heavy-tailed Rayleigh distribution of speckle has higher capability of noise reduction compared to the one in Rayleigh distribution of speckle and the Kuan filter.
2007, 56 (8): 4571-4577.
doi: 10.7498/aps.56.4571
Abstract +
The properties of the phase evolution of two coupled atoms Raman interacting with squeezed vacuum field are studied using the Pegg-Barnett phase theory. The phase probability distribution and variance are calculated. The evolution of the phase probability distribution functions in some cases are given by curves in the polar coordinate system. The influence of Raman interaction between the coupled atoms with squeezed vacuum field, the dipole-dipole interaction between the atoms and the initial states of atoms on the phase properties are discussed.
2007, 56 (8): 4578-4584.
doi: 10.7498/aps.56.4578
Abstract +
The exponential form of even and odd pair coherent state (EOPCS) is constructed. The EOPCS is generalized to a finite dimensional EOPCS in the Hilbert space, and then its orthonormalized property, completeness relations and some noncalssical properties are discussed. It is shown that the finite dimensional EOPCS has normalized and completeness relations, but not orthogonal relations. By virtue of the numerical computation method, we find that, for any value of q, in different ranges of the parameter |ξ| the 5-dimensional EOPCS for the modes 1 and 2 exhibit anti-bunching effect and the photons between the two modes are always correlated. The different peak structures of the phase probability distributions for the 7-dimensional EOPCS show remarkably different quantum interference properties.
2007, 56 (8): 4585-4589.
doi: 10.7498/aps.56.4585
Abstract +
The tuning characteristics of widely tunable wavelength vertical-cavity surface-emitting lasers (VCSELs) have been investigated based on transfer matrix model and finite-element structure-electric coupled-field analysis model and experiment, in which the electrostatic tuning of wavelength and microelectronic mechanical system (MEMS) cantilever of the tunable VCSELs with central wavelength 980nm were designed. The analysis shows that the characteristics of wavelength tuning of VCSELs will be affected by the maximal displacement of cantilever and the structure of wavelength resonator. The tuning range can be improved based on the optimization of sacrificial layer for a given geometry of cantilever and active region of the device. In this paper, the structure of tunable VCSEL is designed. Further numerical simulations show that a continuous tuning range up to 32nm is obtainable, tuning efficiency is 0.12, and the whole tuning range of wavelength lies within the high gain region of the InGaAs quantum wells with GaAs barrier.
2007, 56 (8): 4590-4595.
doi: 10.7498/aps.56.4590
Abstract +
A propagation model for spectral beam combining is set up in this paper. First, using ray tracing method, the phase change induced by the blazed grating is expressed with the optical path differences in the groove and between grooves and the calculation model for the phase change of the beam obliquely incident on the grating is constructed. Then, using the diffraction integral method and the principle of incoherent superposition, the analytical expression for the output field of each emitter after passing through the spectral beam combining system is given and the intensity distribution of combined beam is calculated. Finally, using to the intensity second-order moments method, the M2-factors of the array beam and the combined beam are calculated and the effect of the system parameters on the characteristics of the combined beam is analyzed quantitatively. The results show that the M2-factor of the combined beam after passing through the spectral beam combining system remains almost the same as that of an individual emitter. The spacing of center wavelength between each emitter decreases with the increasing focal length of the transforming lens, resulting in the narrower spectrum width of the combined beam. The encapsulation error of the array elements has relatively little effect on the intensity distribution of the combined beam, whereas the position error of the array source has relatively greater effect on the combined beam.
2007, 56 (8): 4596-4601.
doi: 10.7498/aps.56.4596
Abstract +
The process of doping of Zn into GaN induced by nanosecond pulse laser is analyzed in this paper. An analytical formula of temperature distribution in GaN material irradiated by pulsed laser is presented using one_dimensional model. The relation of surface temperature versus irradiation time and thermal deformation versus depth are obtained. The results show that the surface temperature is in proportion to the square root of irradiation time. The distributions of temperature and thermal deformation in the material are also obtained. The temperature gradiant and the thermal deformation are all highest near the surface. However, the temperature of surface increases in a saw_tooth manner when the material is irradiated continuously by pulseed laser.
2007, 56 (8): 4602-4607.
doi: 10.7498/aps.56.4602
Abstract +
Introducing a periodically poled LiNbO3 (PPLN) waveguide in fiber ring lasers, tunable wavelength conversion has been realized by the nonlinear interactions between the launched signals and the pump and control waves generated through the fiber ring lasers. The principle of all-optical wavelength conversion based on the cascaded sum-and difference_frequency generation in the quasi-phase-matched PPLN waveguide is introduced in the article. The evolution of the power of the pump, the signal, the control and the converted waves along the PPLN waveguide has been numerically simulated respcctively. The dependence of the conversion efficiency on the wavelength of the converted wave was calculated. Finally, the tunability of the wavelength conversion was experimentally verified.
2007, 56 (8): 4608-4614.
doi: 10.7498/aps.56.4608
Abstract +
Based on the Rayleigh diffraction integral, an analytical expression for the spectrum of polychromatic Gaussian beams diffracted at an aperture is derived， and the paraxial results are treated as a special case of our general expression. The main attention is focused on the effect of ratio of the waist width to the central wavelength w0/λ0 and truncation parameter δ on spectral shift and spectral switching. Only when w0/λ0 and δ satisfy certain conditions are the paraxial approximation results consistent with the nonparaxial ones. The nonparaxiality of field results in different spectral shifts and changes of the critical position where the spectral switching takes place.
2007, 56 (8): 4615-4621.
doi: 10.7498/aps.56.4615
Abstract +
Different from the traditional treatment, we use the third-order tensor κ(2)jkl to describe the optical activity and take the corresponding polarization as a perturbation, and directly derive a wave coupling equation for the phenomenon from Maxwell's equations. The analytic solution of the coupling equation, which involves the results from the previous macroscopic theory of optical activity, describing the propagation of monochromatic light in arbitrary polarization state traveling in any direction in an optical active crystal belonging to any point group, is given in this paper. Finally, as an application of this theory, the influence of the wave-vector mismatch on the optical rotation in quartz crystal is studied in detail by analyzing the polarization state of the output light.
2007, 56 (8): 4622-4626.
doi: 10.7498/aps.56.4622
Abstract +
A novel scheme using a main Brillouin amplifier in combination with a reshaping Brillouin amplifier is suggested to control the stimulated Brillouin scattering pulse shape. In this scheme, the main amplifier coarsely controls the pulse shape by changing the delay time between the pump and the Stokes seed, and the reshaping amplifier fine_tunes the pulse shape, especially the rise edge of the pulse by adjusting the distance between the two cells. Dependence of the pulse shape on the distance between the two amplifiers is experimentally researched. The results show that the encounter time and the distance between two amplifiers are the most important parameters for controlling the pulse shape. This scheme is a simple and efficient way to precisely control the pulse shape.
2007, 56 (8): 4627-4634.
doi: 10.7498/aps.56.4627
Abstract +
We use both the numerical method and the perturbation technique to investigate the self-deflection process of a dark photovoltaic spatial soliton in a photovoltaic photorefractive crystal with diffusion effect. It is found that the dark soliton does not propagate along a parabolic trajectory, and it becomes grayer during the propagation in the photorefractive crystal. In the normalized coordinate system(ξ,ζ), the dark soliton deflects to the direction of ξ>0, which is different from that of the bright soliton. Besides, the deflection of dark solitons can be controlled effectively by modulating the ratio of effective Glass coefficient r and the intensity of the signal beam. We also investigated numerically the self-deflection characteristic of gray solitons, which depend on the direction of its initial transverse velocity v. When v>0, gray soliton deflects to the direction of ξ>0 in the coordinate system(ξ,ζ); when vξξ>0 after propagating for a small distance.
2007, 56 (8): 4635-4641.
doi: 10.7498/aps.56.4635
Abstract +
The gain bandwidth of optical parametric chirped pulse amplification pumped by divergent beams has been theoretically studied. Several gain curves are simulated when the pump are non-diffraction-limited Gaussian beams or a beams having rectangular distribution in spatial spectrum respectively，using the method of Fourier transform and the fourth-order Runge-Kutta algorithm. The results show that the gain bandwidth of optical parametric chirped pulse amplification can be obviously improved by using divergent pump beams, either in the visible or in the near infrared region. Very high gain and broadband spectrum can be obtained if the divergence angle and intensity of the pump is selected properly.
2007, 56 (8): 4642-4647.
doi: 10.7498/aps.56.4642
Abstract +
We solved the equations of the Snyder-Mitchell model, which is a model describing the propagation of optical beams in nonlocal nonlinear media with strong nonlocality, in a rotating cylindrical coordinate system, and obtained a self-similar analytical solution of an optical vortex soliton. The result shows that the radial part of the solution is the product of the Whittaker and power function. The beam has a ring-shaped structure rotating around its core.
2007, 56 (8): 4648-4652.
doi: 10.7498/aps.56.4648
Abstract +
The electronic mechanism of Ca2+ doped sodium barium niobate crystals and the change in induced photorefractive properties are studied based on its transmission properties. Curves of the refractive index changing with time are plotted using Michelson interferometry. Experimental results show that proper Ca2+doping in sodium barium niobate crystals can improve the photorefractive properties effectively.
2007, 56 (8): 4653-4656.
doi: 10.7498/aps.56.4653
Abstract +
Characteristics of total reflection at the interface between left-handed and right-handed materials are derived from electromagnetic theory, and the phase is researched. The experimental idea of observing interference fringes of evanescent waves, when total reflection takes place at the interface is discussed, and the distribution of interference fringes is calculated from electromagnetic theory.
2007, 56 (8): 4657-4660.
doi: 10.7498/aps.56.4657
Abstract +
The transmission properties of pairing structure consisting of negative-permittivity and negative-permeability materials were studied using Maxwell equations. Most of energy of electromagnetic field is localized on the interface between the materials due to boundary conditions when electromagnetic wave is incident on the structure. Investigations show that the interface mode may evolve into resonance tunneling mode under specific incident angle, which leads to resonance transmission. The transmission properties can effect band-pass filtering.
2007, 56 (8): 4661-4667.
doi: 10.7498/aps.56.4661
Abstract +
It has been theoretically and experimentally shown that split ring resonators (SRRs) can be used as a negative permeability medium near its magnetic resonance frequency. In this paper, a new structure for magnetic resonance has been designed by introducing a short wire into the common SRRs. The effects of short wire on the transmission characteristics of SRRs and left-handed metamaterials (LHMs) are experimentally and numerically investigated. The results show that an additional capacitor, formed by short wire and the gap side of SRR, leads to higher capacitance around the gap and lower the resonance frequency of SRRs. The resonance frequency of SRRs decreases with the length of short wire, but increases with the distance between short wire and SRRs. The short wire has no significant influence on the negative permeability of SRR. It is also demonstrated that the left-handed transmission peak, negative permeability, antiresonance frequency of permittivity and negative refractive index of LHMs could be tuned by controlling the distance between the short wire and the SRR.
2007, 56 (8): 4668-4676.
doi: 10.7498/aps.56.4668
Abstract +
A novel high birefringence photonic crystal fiber (PCF) was proposed and the properties of this type of PCF were investigated with full vector finite difference frequency domain method. According to the results of numerical analysis, the two orthogonally polarized states of the fundamental mode become non-degenerate in the fiber and show strong linear polarization. The polarization properties are strongly dependent on the structure parameters of this PCF. Through choosing suitable relative structure parameters, it may exhibit birefringence as high as the order of 10-2, at least one order of magnitude higher than that of the conventional D-type and panda-type polarization-maintaining fiber. It is shown that it may also exhibit ideal dispersion effect in a properly designed geometrical structure of PCF. So this type of PCF can be effectively used to fabricate polarization-preserving fiber and correlative fiber elements with special dispersion and polarization characteristics.
2007, 56 (8): 4677-4685.
doi: 10.7498/aps.56.4677
Abstract +
Reflection characteristics of a novel fiber loop mirror (FLM), which is formed by inserting a fiber polarization controller (PC) into the loop, are investigated. Based on the equivalent optical-path analysis, a theoretical model for the FLM is presented. The reflection characteristics of the FLM are then simulated in detail with the model. Our simulation results show that, by varying the PC working conditions, i.e. the fast axis orientation or the intensity of the birefringence induced by the PC, the refrectivity of the FLM may be continuously tuned between 0 and 1. The spectral characteristic of reflectivity of the FLM is wide and flattened, limited only by the operating bandwidth of the optical fiber coupler used. Moreover, the reflection characteristics of the FLM are further investigated experimentally. The experimental results also show that by varying the working conditions of the PC, the reflectivity of the FLM may actually be continuously adjusted between its maximum and minimum values,which were measured to be 93% and 2%, respectively. The experimentally determined dependence of the fast axis orientation and intensity of the birefringence induced by the PC on the inclination angle of the three rigid discs with respect to the level of the PC verified the results of theoretical simulations.
2007, 56 (8): 4686-4693.
doi: 10.7498/aps.56.4686
Abstract +
The high frequency message filtering characteristics of the semiconductor laser which act as a receiver are numerically simulated in a scheme of closed-loop optical chaos communications system. Both transmitter and receiver are realized bysemiconductor lasers subject to strong optical injection. We examined the filtering properties of semiconductor laser with modulation message with different frequencies and amplitudes. The results demonstrate that the filtering effectiveness of the semiconductor laser receiver is higher for low frequency message and decreases as the message frequency approaches the relaxation oscillation frequency of the semiconductor laser. The quality of message recovery in such systems is shown to depend not only on the synchronization quality but also on the difference in amplitude of the message in the transmitter and receiver outputs.
2007, 56 (8): 4694-4699.
doi: 10.7498/aps.56.4694
Abstract +
The band structure of two kinds of phononic crystals (the square lattice of mercury (water) rods in a water (mercury) host) has been calculated using the plane-wave expansion method based on super cell. The translation group symmetry of phononic crystal is changed by changing the size of adjacent rods. We found that by changing the lengths of sides of adjacent rods one can effectively adjust the phononic band gaps. The present work shows that the translation group symmetry of phononic crystal has an important influence on the formation of its band gaps, and the method based on super cell is an effective method of studying the influence of translation group symmetry of phononic crystal on the formation of phononic band gaps.
2007, 56 (8): 4700-4707.
doi: 10.7498/aps.56.4700
Abstract +
Using an exact Mie scattering solution, the scattering and vibration mode of a single air sphere in unbounded rubber are analyzed with reference to the elements of the scattering matrix. Then, using the multiple_scattering method, the anechoic properties of the viscoelastic rubber coating (with a thickness of 8mm) containing a plane of periodically distributed air spheres are investigated under different backing conditions. The results show that the excellent anechoic performance of the coating originates from the dilatational resonance of each air sphere. The anechoic coating can attenuate most incident energy and minimize reflection in the resonance frequency region. Finally, a bi_layer coating (with a thickness of 20mm) is designed for broad_band anechoic material.
2007, 56 (8): 4708-4712.
doi: 10.7498/aps.56.4708
Abstract +
To describe different behaviors of a projectile into a granular media, a model with both viscous drag and hydrostatic drag has been suggested. Analyses show that the parameter Γ plays a key role in determining the forms of the solutions to this model. When ΓΓ≥2, the viscous drag is dominant. In these two cases, the projectile moves in the granular media with reverse deceleration variances. Experiments have been performed to verify this argument, and the obtained u(t) curves confirm this prediction.
2007, 56 (8): 4713-4721.
doi: 10.7498/aps.56.4713
Abstract +
In this article, the aluminium-plastic board and carbon paper technique are combined to investigate the contact force at bottom boundary of three-dimensional ordered granular arrays in response to a point load. Arching effects were observed with all arrays, and the force distribution markedly depends on the crystal structure and has a 120°-symmetry at every layer of the crystals， while the heterogeneity in force distribution in the pure hcp arrays is minimal of all the arrays. Non-main force chains were observed to have the “focalization" phenomena. These results can be explained by a force-transmission model.
2007, 56 (8): 4722-4727.
doi: 10.7498/aps.56.4722
Abstract +
The outstanding climbing skills on smooth solid surface of some insects like beetle, bee and cockroach or some small reptiles like gecko have been under investigation for a long time. When some kinds of insects crawl on a smooth glass surface, the micro_pads leave traces of self-secreted organic liquid, which suggests that, the thin layer of confined liquid may contribute to the bioadhesive force between the micro_pad and the smooth surface. To investigate the properties of the confined liquid thin film, a series of experiments were carried out on a home-made microtribometer with a trace amount of ionic liquid (［emim］ ［Tf2N］) or PAO(poly_α_olefin) oil confined between a nano-scale smooth steel sphere and a glass plate. For a critical confined volume of pitoliters to nanoliters, and a critical clearance of tens to hundreds of nanometers, confinement-induced spontaneous spreading and abrupt shrinking were observed, accompanied by the presence of a stable interfacial adhesive force of remarkable magnitude. This spreading/shrinking-induced adhesive force was proved to be fundamentally different from the common meniscus capillary force and was considered to stem from confinement-induced solidification, according to our further investigations. The confinement-induced adhesive force of organic liquid may shed significant light on the physical principle “employed” by insects to crawl quickly on smooth vertical surfaces. By subtly adjusting the volume of confined liquid or the clearance between the pads and the surface, insects can control (i.e., switch on or off) the interfacial force at will. The interfacial bonding force of confined liquid thin film discovered in this paper may provide a basis for the principle of biomimetic attachment systems.
2007, 56 (8): 4728-4732.
doi: 10.7498/aps.56.4728
Abstract +
The effect of surface roughness on the frictional characteristics of the liquid flow in the microchannels with distributed rectangular roughness elements is experimentally studied. The velocity fields are obtained by micro particle image velocimetry (micro-PIV). The friction coefficients in the microchannels with 3%—7% relative roughness are higher than the prediction based on the conventional theory due to the additional pressure drag caused by roughness elements. The existence of roughness elements gives rise to additional disturbance in the laminar stream, and the early transition to turbulent flow in the rough microchannels is validated.
2007, 56 (8): 4733-4741.
doi: 10.7498/aps.56.4733
Abstract +
In this paper, interfacial waves in three-layer stratified fluid with background current are investigated using a perturbation method, and the second-order asymptotic solutions of the velocity potentials and the second-order Stokes wave solutions of the associated elevations of the interfacial waves are presented based on the small amplitude wave theory, and the Kelvin-Helmholtz instability of interfacial waves is studied. As expected, for three-layer stratified fluid with background current, the first-order asymptotic solutions (linear wave solutions), dispersion relation and the second-order asymptotic solutions derived depend on not only the depths and densities of the three-layer fluid but also the background current of the fluids, and the second-order Stokes wave solutions of the associated elevations of the interfacial waves describe not only the second-order nonlinear wave-wave interactions between the interfacial waves but also the second-order nonlinear interactions between the interfacial waves and currents. It is also noted that the solutions obtained from the present work include the theoretical results derived by Chen et al (2005) as a special case. It also shows that with the given wave number k (real number) the interfacial waves may show Kelvin-Helmholtz instability.
2007, 56 (8): 4742-4748.
doi: 10.7498/aps.56.4742
Abstract +
For Rayleigh-Bénard convection in a binary fluid mixture, we obtained a traveling-wave state with periodic modulation of spatiotemporal dislocation defects in a rectangular cell by the numerical simulation of full hydrodynamic equations. The investigations show that the creation of pairs of convective rolls triggered by Eckhaus instability leading to this modulation. Because the generating frequencies of rolls at the left wall smaller than the annihilating frequencies of rolls at the right wall, the system soon move out of the stable region, in this way the system oscillates back and forth between the stable and unstable Echkaus wave number band. The properties of heat transfer and fluid mixing also studied too.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2007, 56 (8): 4749-4761.
doi: 10.7498/aps.56.4749
Abstract +
Stimulated trapped electron-acoustic wave scattering instability by a linearly-polarized laser interacting with a plasma layer at a subcritical density range is studied by particle simulation. Its early behavior is almost the same whether ion dynamics is taken into account or not. However, when ion dynamics is considered, a large ion acoustic wave is excited, which grows with time and eventually breaks up locally, followed by the generation of a large amplitude electromagnetic soliton. As a new phenomenon, an ion-vortex structure in ion phase-space is formed due to the ion acceleration and trapping by high local electromagnetic and electrostatic fields inside the soliton. As the electromagnetic soliton is accelerated backwards, several ion-vortices are formed in the wake behind. Ion-vortices are also found in inhomogeneous subcritical plasmas. These ion-vortices are recognized as the Kelvin-Helmholtz instability patterns, likely to be formed due to a topological defect, i.e., the plasma density cavity in the electromagnetic soliton region, which exhibit the well-known paradigmatic Ying-Yang pattern.
2007, 56 (8): 4762-4770.
doi: 10.7498/aps.56.4762
Abstract +
Plasma immersion ion implantation (PIII) has been developed as a low_cost and efficient surface modification technique of irregularly-shaped objects. The effect of six pulse waves with different rise_time patterns on the spatio-temporal evolution of plasma sheath, energy and dose of ion implantation has been simulated by particle-in-cell modeling. Statistical results may be obtained through assuming the Boltzmann distribution of electrons, and solving Poisson and Newton equations for tracing each ion in the plasma sheath. The results show that rise_time pattern has a critical influence on the evolution of plasma sheath. There exists maximum thickness difference of plasma sheath for different waveforms. The acceleration of ions is non_uniform due to the non-uniformity of electrical field strength. The maximum gradient of electrical field appears near the edge of plasma sheath. The results also show that optimization of dose and energy of incident ions may be achieved through modification of rise_time pattern. The numerical simulation of sheath expansion can be effectively used to provide a scientific basis for optimizing the PIII process.
2007, 56 (8): 4771-4777.
doi: 10.7498/aps.56.4771
Abstract +
The fuelling efficiency and injection depth are the fundamental problems of supersonic molecular beam fuelling. In the recent supersonic molecular beam injection (SMBI) experiments, the injection depths of SMBI have been found mainly dependent on the background electron density and temperature, the source gas pressure and temperature. The empirical scaling of SMBI depth was obtained at room temperature. Hydrogen clusters formed in the beam have been observed in the low temperature (source gas at liquid nitrogen temperature) SMBI experiments. It resulted in injection depth greater than 30 cm. Comparing the relationships between injection depths, plasma background parameters and molecular source temperature, the cluster formation phenomena during low temperature MBI are analyzed.
2007, 56 (8): 4778-4784.
doi: 10.7498/aps.56.4778
Abstract +
In this paper, the electric field of a symmetrical actuator system is simulated and the plasma flow control mechanism is analysed. The peristaltic acceleration process in electromagnetic field is simulated with finite-difference time_domain method and the result accurately consists with theoretical analysis, and the peristaltic acceleration mechanism is discussed. Our results may provide a theoretical basis for improving the peristaltic acceleration and optimizing the symmetrical plasma actuator configuration.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2007, 56 (8): 4785-4790.
doi: 10.7498/aps.56.4785
Abstract +
Based on the spin diffusion theory and the Ohm's law, we theoretically studied the spin polarized injection and transport through the ferromagnetic/organic semiconductor/ferromagnetic system and obtained the current spin polarization, which takes into account the special characteristics of organic semiconductors. Self-trapped states, such as spin polarons as well as spinless bipolarons are assumed to be the main carriers in organic semiconductors. From the calculation, it is found that polarons are the effective spin carriers in spin dependent injection and transport in organic semiconductors. The effect of the spin related interfacial resistances, conductivity match and organic semiconductor length on the current spin polarization are also discussed respectively.
2007, 56 (8): 4791-4797.
doi: 10.7498/aps.56.4791
Abstract +
The equation of state of gas detonation products is described by Ross's modification of hard-sphere variation theory and the improved one-fluid van der Waalsmixture model. The Gibbs free energy of dissociated carbon is calculated for the most probable state, which is determined by differentiating the four states of carbon, namely graphite, diamond, graphitelike and diamondlike. The equilibrium composition of detonation products are calculated by solving chemical equilibrium equations based on minimizing Gibbs free energy. The results are in good agreement with the results based on Becker-Kistiakowsky-Wilson (BKW) and Lennard-Jones-Devonshire equations of state. The detonation properties of explosives are calculated with this theory. The results of high density explosives based on BKW, Jones-Wilkins-Lee (JWL) and our equations of state are in good agreement with the experimental data. For low density explosives our results are better than that of BKW and JWL.
2007, 56 (8): 4798-4803.
doi: 10.7498/aps.56.4798
Abstract +
A series of blends with different compositions were prepared by melt-mixing on the basis of polystyrene and poly(ethylene oxide). Using the relative energy dissipation method, the dynamic behavior of the blends above the glass transition temperature was investigated. Two relaxational dissipation peaks (α and α′) were observed. It is suggested that peak α is associated with the glass transition of polystyrene and peak α′ with the liquid-liquid transition. The relaxation time τ of both relaxations does not satisfy the Arrhenius law. The effect of composition on the dynamic relaxation of the blends were also discussed qualitatively.
2007, 56 (8): 4804-4809.
doi: 10.7498/aps.56.4804
Abstract +
Rapid dendritic growth in undercooled liquid Ni80Cu10Co10 alloy has been investigated. This liquid ternary alloy was undercooled by a large degree up to 335 K(0.2TL) with electromagnetic levitation method. The rapidly solidified microstructure is revealed to be α-Ni solid solution phase by X-ray diffraction and differential scanning calorimetry. With the increase of undercooling, the microstructure is drastically refined. Its morphology transfers from coarse dendrite into equiaxial grains when undercooling exceeds 110 K. The solute microsegregation is suppressed with the enhancement of solute trapping effect under high undercooling condition. The dendritic growth velocity was measured as a function of undercooling. At extremely large undercoolings, the dendritic growth velocity of α-Ni phase is restrained by the solute diffusion of Cu and Co, and even decreases once undercooling exceeds 284 K.
2007, 56 (8): 4810-4816.
doi: 10.7498/aps.56.4810
Abstract +
The molecular dynamics method was used to investigate the effect of the impurities N,O,Si,P and S on the Young's moduli of armchair (5, 5) and zigzag (9, 0) single-walled carbon nanotubes. The results show that the Young's moduli of armchair (5, 5) and zigzag (9, 0) single-walled carbon nanotubes are 948 and 804 GPa, respectively. When the impurity concentration is less than 10%, the Young's moduli are approximately linearly decreasing with increasing of the impurity concentration, and the greatest decreasing ratio is induced by the impurity Si and the smallest by the impurity N. The decreasing rate of the Young's modulus increases with increase of the impurity atomic number when the impurity element is of the same period with the element C. The effect of the impurities on the Young's modulus of carbon nanotubes is the stronger when the period of the impurity element is different with the element C, and the decreasing rate of the Young's modulus decreases slightly with increasing of the impurity atomic number. The reasons are analyzed by the laws of the Young's potential energy variation of carbon nanotubes with impure and the electron cloud coupling between two atoms from the theory of the local density approximation based on the density functional theory.
2007, 56 (8): 4817-4822.
doi: 10.7498/aps.56.4817
Abstract +
Geometry optimization and total energy computation were carried out for TiS2, LiTiS2 and intercalation compound LixTiS2(x=1/4, 1/3, 1/2, 2/3, 3/4)ordered systems using first principles method based on local density approximation to density-functional theory. The computation results for these systems have been compared with other experimental and theoretical results available in the literature. The normalized increments of lattice parameters Δa0 and Δc0 increase monotonicly with the increasing concentration of Li ion. The curves show good agreement with lattice parameters obtained from experiment. The formation energies of ordered systems LixTiS2(x=1/4, 1/3, 1/2, 2/3, 3/4) are all negative, indicating their stability at low temperature. The formation energy of Li1/2TiS2 system with 3a0×a0 ordered structure is the lowest, implying the most stable structure. The results have shows that the local density approximation to density-functional theory can reasonably be applied to TiS2, LiTiS2 and intercalation compound LixTiS2(x=1/4, 1/3, 1/2, 2/3, 3/4)ordered systems.
2007, 56 (8): 4823-4828.
doi: 10.7498/aps.56.4823
Abstract +
We describe a novel technique for producing the p-type semiconductor. The homogeneous impurity doping of Ga and Al in the n-type silicon wafer is achieved by means of the combination of two technical processes namely aluminum emulsion coating and vaporizing Ga. This Ga-Al double-impurity doping technique, as well as its effect on the quality of the produced semiconductor devices, is investigated theoretically and tested in practice. The principle of the doping technique is also discussed in detail. By using SRP, four-probe needle and thyristor analyzer, we have examined the Ga-Al impurity concentration distribution within the silicon wafer, thin layer resistance RS, and other useful parameters of the fast switching thyristors made by this technique. The turn-off time of the fast switching thyristors, toff is 31.2—38.6 μs, turn-on time ton is 4.6—5.9 μs, and the forward voltage drop VTM measured is 1.9—2.1 V. Both experiment and some real applications indicated that this technique may obviously improve the semiconductor device in the overall performance, the electrical parameter uniformity and the end product rate. It provides a feasible technique for making fast switching thyristors.
2007, 56 (8): 4829-4833.
doi: 10.7498/aps.56.4829
Abstract +
We demonstrate experimentally that an irradiation-induced transformation of multiwalled carbon nanotubes to diamond nanocrystals (DNC) can be realized with double ions (40Ar+，C2H+6) bombardment. This approach is expected to form a new route for synthesis of DNC. The idea of multi-ion irradiation may also be valid for other materials and be used for fabricating nanostructures.
2007, 56 (8): 4834-4840.
doi: 10.7498/aps.56.4834
Abstract +
Hydrogenated silicon films were deposited on glass and single crystalline silicon substrate in a capacitively coupled radio-frequent plasma enhanced vapor deposition system aided by direct current bias excitation. Hydrogenated silicon films are used to realize a silicon material that consists of a two-phase mixture of amorphous and ordered silicon. Micro-Raman scattering is employed to investigate the thin film microstructure. The crystalline volume fraction (Xc) is obtained from the Raman spectra. Mesoscopic mechanical characterization of the thin film is done by nanoindentation based on the conventional depth-sensing indentation method. An analytical relation between Xc and elastic modulus is established. It is shown that the elastic modulus of the film on glass substrate is lower than that on silicon with the the same Xc. The grain size of phosphorus doped thin film is smaller than that of the intrinsic one and more ordered. The Xc is usually above 40%. The film with diborane doping is on the opposite, the Xc of which is usually below 40%. For P-doped, intrinsic and B-doped fims, when the Xc values are 45%, 30% and 15%, respectively, the values of elastic modulus are minimal.
2007, 56 (8): 4841-4846.
doi: 10.7498/aps.56.4841
Abstract +
The dependence of total strain energy of a pyramidal self-assembled Ge/Si semiconductor quantum dot on the aspect ratio, is investigated. The free energy consisting of the strain energy and surface energy is defined, and used to study the equilibrium shape of the systems. The results show that the strain energy of thesystem decreases with the increasing aspect ratio, and under the requirement ofminimum total free energy, the quantum dot with a given volume will take a particular height-to-width aspect ratio,i.e.the equilibrium aspect ratio. Meanwhile, the distributions of the stress, hydrostatic strain and biaxial strain are presented. These can serve as a basis for interpretation of experiments on strain self-assembled quantum dots.
2007, 56 (8): 4847-4855.
doi: 10.7498/aps.56.4847
Abstract +
By the first principles plane-wave pseudopotential calculations based on the density functional theory, we studied the equations of states and structural properties of transition metal compounds OsB2 and OsO2 and the possible high pressure phase transitions of OsO2. Three (rutile, pyrite and fluorite) phases of OsO2 are studied under high pressure. The calculated results support the opinion that OsB2 and the fluorite phase of OsO2 should be potential ultralow-compressible hard materials. The microscopic mechanism of large bulk modulus and high hardness can be understood from the calculated electronic structures. It is therefore suggested that incorporating light elements (boron, oxygen, carbon or nitrogen) into transition metals with high valence electron densities is possibly a new way to prepare superhard materials.
2007, 56 (8): 4856-4863.
doi: 10.7498/aps.56.4856
Abstract +
采用改进的Lee-Low-Pines中间耦合方法研究纤锌矿氮化物抛物量子阱材料中极化子能级,给出极化子基态能量、第一激发态到基态的跃迁能量以及不同支长波光学声子模对极化子能量的影响随阱宽L和外电场F变化的函数关系.在计算中考虑了纤锌矿氮化物GaN和AlxGa1-xN构成的抛物量子阱材料中准LO和准TO声子模的各向异性、电子有效带质量、介电常数、声子频率等随空间坐标z变化(SD)效应以及外电场作用对极化子能量的影响.结果表明，纤锌矿氮化物抛物量子阱材料中电子-声子相互作用、SD效应和外电场对极化子能量有明显影响，它们都使极化子能量降低.SD效应随阱宽L增大而减小，阱宽越大，SD效应越小，最后趋近于零.阱宽较大时，定域准LO声子对极化子能量的影响较大，而半空间准LO声子的影响较小；阱宽较小时，则正好相反.在阱宽L变化的整个范围，氮化物抛物量子阱材料中电子-声子相互作用对极化子基态能量的贡献(约为28—42 meV)比GaAs/AlxGa1-xAs抛物量子阱中的相应值(约为0.5—3.2 meV)大得多.而准TO声子对极化子基态能量的贡献比较小，要远远小于LO声子对极化子基态能量的总贡献.因此，讨论氮化物抛物量子阱(GaN/AlxGa1-xN)中电子态问题时要考虑电子与准LO声子相互作用，而可以忽略准TO声子的作用.
2007, 56 (8): 4864-4871.
doi: 10.7498/aps.56.4864
Abstract +
The nucleation mechanism of gas bubbles in melts was studied theoretically by three representative models, uamely the homogeneous nucleation in liquid metal, heterogeneous nucleation on the flat surface or in conic pits of refractory inclusions, under the assumption that the state of the system's Gibbs free energy change being maximal is the critical state of bubble nucleation. Introducing the contact angle and considering the effect of interfacial tension on bubble morphology, the following conclusions can be drawn: the critical nucleation radius of bubbles in the three models is the same, which will be reduced with increasing gas pressure above the liquid metal. The diameter of embryo bubbles, which is also the minimum pore size in lotus-type porous metal ingots, has a magnitude of micrometers under the common pressure of 0.1—1.0 MPa for the Gasar process. For heterogeneous bubble nucleation in conic pits of inclusions, there exists an optimal cone apex angle corresponding to the smallest volume of the embryo bubble, which is independent of gas pressure above liquid metals and increases with the increase of the contact angle. For alumina inclusion in Mg, Al, Fe, Ni and Cu melts, the optimal apex angles are 8.0°, 19.5°, 20.6°, 42.7° and 51.5°, respectively. Among these three bubble nucleation models, homogeneous nucleation is the most difficult, heterogeneous nucleation in conic pits of inclusions is the easiest, and heterogeneous nucleation on the flat surface of inclusions is intermediate.
2007, 56 (8): 4872-4876.
doi: 10.7498/aps.56.4872
Abstract +
The SnO2:Sb films have been prepared on sapphire (0001) substrates by radio frequency magnetron sputtering method. The structural and photoluminescence (PL) properties of the films were investigated. The prepared samples are polycrystalline films with rutile structure of pure SnO2. The photoluminescence of the samples was measured at room temperature. An ultraviolet luminescence peak near 334 nm was observed for the first time and the corresponding PL mechanism was investigated.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2007, 56 (8): 4877-4883.
doi: 10.7498/aps.56.4877
Abstract +
The total energies and electronic structures of differently configured Nb2H are calculated using pseudopotential plane wave method based on density functional theory and generalized gradient approximation. The results show that hydrogen atom favors tetrahedral (T) sites (T-wells) rather than octahedral (O) ones. There exist lower energy paths along adjacent T-T sites and saddle structures at the midpoint of them. When H is at T-site or along the T-T path, its 1s level is less broadened and the 4d, 5s bands of Nb extend downward from the metallic band area, forming an ionic band (sharp peak) separated by a gap of about 1 eV from the metallic valence bands, showing that the hydrogen atom captures one valence electron to form an anion. This low-lying ionic band loses its T-site hybridization features when H moves from T to O site, where the 1s band of H is widely broadened, extending into the metallic valence bands. The band structure changes only slightly when the hydrogen atom moves from one T site to a neighbor one along the lower energy path, keeping the T-configuration features. The calculated phonon spectra of H are dispersive only near special q-points for the system with H at a T site, showing that the hydrogen atom is probably vibraing locally and weakly interacts with other hydrogen atoms. The behavior of H at a trapping site plays an important role in bonding of the surrounding Nb atoms.
2007, 56 (8): 4884-4890.
doi: 10.7498/aps.56.4884
Abstract +
Based on ab initio methods and nonequilibrium Green's function theory, we have investigated the electronic transport properties of single and double 1，4-dithiolbenzene molecular devices which have different molecular relative orientations on molecule-metal contacts. Numerical results show that different molecular relative orientations have various influences on the electronic structure of molecules and current-voltage characteristics, which obviously affects the electronic transport properties of metal-molecule-metal systems. The equilibrium state of the extend molecule is not the best situation for electronic transportation. The characteristics of electronic transportation can thus be improved by adjusting the molecular relative orientation on molecule-metal contacts.
2007, 56 (8): 4891-4895.
doi: 10.7498/aps.56.4891
Abstract +
First principles full-potential linearized augmented plane wave method with local density approximation (LDA) was applied to calculate the ground state properties of ReSi1.75 crystal. Optimized results show that the equilibrium lattice constant of ReSi1.75 is smaller than experimental data by about 0.6%. On the basis of LDA results, LDA+U method was used to calculate the electronic structure. When Ueff=U-J=4.4eV, ReSi1.75 has a narrow gap semiconductor band structure with an indirect gap of 0.12 eV and a direct gap of 0.36eV. Effective mass calculation shows the highly anisotropic character of ReSi1.75 crystals. The density of sates changes sharply near Fermi level，thus the thermoelectric properties can be improved by doping.
2007, 56 (8): 4896-4900.
doi: 10.7498/aps.56.4896
Abstract +
V+ were implanted into anantase films by metal ion implantation. The electronic band structures of TiO2 films doped with V+ were calculated using a self-consistent full-potential linearized augmented plane-wave method within the first principles formalism. Influence of implantation on TiO2 films were examined by ultraviolet-visible spectrometry. The results of experiment and calculation show that the optical band gap of TiO2 films is narrowed by ion implantation. The calculation shows that the 3d state of V+ plays a significant role in red shift of ultraviolet-visible absorbance spectrum. It was also found that the optical band gap of TiO2 films decreases, with increasing amount of V+.
2007, 56 (8): 4901-4907.
doi: 10.7498/aps.56.4901
Abstract +
Equations of excitons in columned composite systems composed of quantum dots and quantum wires have been formulated. Then the energy of excitons is solved by the perturbation theory. From the example of CdS/HgS/CdS/HgS/CdS columned composite systems composed of quantum dots and quantum wires, the electric probability distribution in this system and the dependence of energy of excitons on the size of system are studied. The results show that：(1) The energies of electrons, holes and excitions in this system all decrease as the height, h0, of quantum dots increases and the impact of the interaction of electrons holes to the energies of the ground excitons is greater than that of the excitons. (2) The probability distribution of the electrons is undulating in the ridial direction, and the probability tends to zero on the axis and near the surface and is maximal near R/2. The probability distribution of electrons is vibratory near the quantum dots in the axial direction.
2007, 56 (8): 4908-4913.
doi: 10.7498/aps.56.4908
Abstract +
Spin-dependent electroresistance (ER) was observed in polycrystalline Nd0.67Sr0.33MnOy (y=3.00, 2.95, 2.90, 2.85, 2.80) samples, which were synthesized by solid-state reaction. For the sample with stoichiometry y=3.0, the I-V characteristic shows ohmic linearity over the whole range of measuring temperature. For the samples with oxygen-deficiency, i.e. yI-V curve is linear when temperature is above a certain point, similar to the case of y=3.0. However, the electric resistance strongly depends on load current, decreasing remarkably with increasing current load when temperature falls below the critical temperature. For the sample of y=2.85, the ER ratio reaches 80% when the load current increases from 1 to 30 μA. The spin-dependent ER effect should be strongly related with the oxygen-deficiency and grain boundary effect.
2007, 56 (8): 4914-4919.
doi: 10.7498/aps.56.4914
Abstract +
Low resistance p-type ZnO thin films were prepared by in situ oxidation of Zn3N2 films which were deposited by reactive radio-frequency magnetron sputtering of zinc in Ar-N2 mixture atmosphere. The effects of oxidation temperature and time on structural, electronic and optical properties of the samples were investigated by X-ray diffraction, scanning electron microscopy, Hall-effect measurements, transmittance spectra and photoluminescence spectra. It was found that the sample oxidized at 450 ℃ contains Zn2N3 besides ZnO. The sample oxidized at 500 ℃ for 2 h had good properties of low resistance (0.7 Ωcm), good transmittance rate (above 85%)，predominant excitonic ultraviolet emission with narrow full width at half maximum and weak deep level visible emission. The low resistance p-type ZnO films could be used in short wavelength industrial application.
2007, 56 (8): 4920-4923.
doi: 10.7498/aps.56.4920
Abstract +
La0.9Sr0.1MnO3 film has been fabricated on (100) LaAlO3 by direct-current magnetron sputtering. The quality of the film is good as monitored by atom force microscopy and X-ray diffraction. The resistivity-temperature curve shows that the film undergoes metal-insulator transition at 281 K. The peak resistivity decreases with increasing current and the current-induced electro-resistance is 30.5% at 4 mA. The results are simply explained by phase-separation theory.
2007, 56 (8): 4924-4929.
doi: 10.7498/aps.56.4924
Abstract +
The second-order cross correlation function of the photon emission and the polarization density matrix of the photon pairs emitted from the biexciton system of single semiconductor quantum dots under pulsed excitation were calculated using the dynamical population density matrix equations and quantum regression theorem. The entanglement of the photon pairs emitted from the single quantum dot with degenerate exciton states were investigated, and the relationship between the entanglement and the spin relaxation rate was also discussed. The simulations revealed that the spin relaxation destroys the entanglement of the photon pairs emitted from this system.
2007, 56 (8): 4930-4935.
doi: 10.7498/aps.56.4930
Abstract +
V+ were implanted into anantase films by metal ion implantation. The electronic band structures of TiO2 films doped with V+ were calculated using a self-consistent full-potential linearized augmented plane-wave method with
2007, 56 (8): 4936-4942.
doi: 10.7498/aps.56.4936
Abstract +
Synchrotron radiation photoemission was used to investigate the interface between the organic semiconductor (BZP) and polycrystalline silver film. Before complete coverage by one monolayer, the BZP molecules interacted weekly with silver and a gap state appeared at 0.9eV below Fermi level. After the coverage is complete, the growth of BZP is inferred to be in the 3D-island mode and the interaction with the substrate gradually diminishes. Because of the final state effect, the highest occupied molecular orbital (HOMO) level shifts to higher binding energy and reaches ultimately 2.3eV of the HOMO level in the bulk. The deposition of BZP on silver surface resulted in the decrease of sample work function, which demonstrats interfacial dipoles forming on the BZP/Ag contact (Δ=0.3eV) and indicats the electrons being deviated from the organic molecules to the metal substrate. In addition, the effect of annealing and exposure to oxygen on the BZP/Ag interface has been taken into account. It is found that the binding energies of the characteristic peaks of BZP and the gap state hardly change after interface annealing at 250 ℃, but after being exposed to oxygen, the gap state disappears and the valence bands shift to lower binding energies.
2007, 56 (8): 4943-4949.
doi: 10.7498/aps.56.4943
Abstract +
The electrical characteristics of NiSi metal gate and their thermal stability were studied. A physical model is proposed to explain the increase in NiSi work function when the forming temperature is higher than 500 ℃. By measuring the sheet resistance of NiSi film prepared at different temperatures, it is shown that NiSi has the lowest resistance when formed at 400 ℃, which is stable from 400 to 600 ℃. The X-ray diffraction measurement for the NiSi samples formed at various temperatures revealed that NiSi phase was the main component at temperatures from 400 to 600 ℃. The capacitors formed by furnace annealing has higher equivalent oxide charge Qox and lower breakdown electric field Ebd, which proves that furnace annealing is unsuitable for NiSi metal gate fabrication due to the long time of thermal processing (400 ℃ for 30 min). The electrical characteristics of NiSi gate metal oxide semiconductor capacitors formed at various rapid thermal annealing(RTA) temperatures were studied. By comparing theC-V curves, Ig-Vg curves and Qox of the capacitors, it was found that when the RTA temperature is higher than 500 ℃, reaction between NiSi and gate oxide will occurr, reducing the quality of the gate dielectric. In conclusion, the suitable forming temperature of NiSi metal gate should be from 400 to 500 ℃. Moreover, the NiSi work function and Qox formed at 400, 450 and 500 ℃ respectively were also determined.
2007, 56 (8): 4950-4954.
doi: 10.7498/aps.56.4950
Abstract +
High quality Pb1-xMnxSe(0≤x≤0.0681) thin films have been grown on BaF2(111) substrates by molecular beam epitaxy. Optical and structural properties of the Pb1-xMnxSe films have been studied using transmission spectrum and high resolution Ｘ-ray diffraction(HRXRD).HRXRD paterns indicate that Pb1-xMnxSe films have cubic-phase structure,and MnSe phase separation is not observed. The film orientation is parallel to (111) surface ofsubstrate. The lattice constant of Pb1-xMnxSe films decreases with increasing Mn content. The Mn content can be obtained by using Vegard's formula. Sharp absorption edges were observed in the transmission spectrum of Pb1-xMnxSe films.The fundamantal band gap of the Pb1-xMnxSe films was obtained by simulation, which increases almost exponentially from 0.28eV at x=0 to 0.49eV at x=0.0681. The refractive index in the wavelength ranged from 4 to 9.5 μm has also been obtained.
2007, 56 (8): 4955-4959.
doi: 10.7498/aps.56.4955
Abstract +
Magnetotransport properties of In0.53GaAs/In0.52AlAs high electron mobility transistor (HEMT) structures with different channel thickness of 10—35 nm have been investigated in magnetic fields up to 13 T at 1.4 K. Fast Fourier transform has been employed to obtain the subband density and mobility of the two-dimensional electron gas in these HEMT structures. We found that the thickness of channel does not significantly enhance the electron density of the two-dimensional electron gas, however, it has strong effect on the proportion of electrons inhabited in different subbands. When the size of channel is 20 nm, the number of electrons occupying the excited subband, which have higher mobility, reaches the maximum. The experimental values obtained in this work are useful for the design and optimization of InGaAs/InAlAs HEMT devices.
2007, 56 (8): 4960-4964.
doi: 10.7498/aps.56.4960
Abstract +
V+ were implanted into anantase films by metal ion implantation. The electronic band structures of TiO2 films doped with V+ were calculated using a self-consistent full-potential linearized augmented plane-wave method with
2007, 56 (8): 4965-4970.
doi: 10.7498/aps.56.4965
Abstract +
In this paper, a comparative experimental study on optical transparency of z-cut LiF, sapphire and LiTaO3 under 1011 Pa shock compression at our two-stage light-gas gun facility is presented. In our experiments, intensity contrast variation of light emitted from a modified experimental setup of Mallory-type was monitored by a high-speed electronic camera. The results demonstrated that the LiF under the shock pressure of 102 GPa retains its initial transparency in the whole observation time interval, which is in good agreement with the common view of good sustained transparency of LiF. The LiTaO3 became basically opaque at 139 GPa. Sapphire at 131 GPa shock pressure showed a continuous transparency degradation till became completely darkened when the shock wave was arriving at the free-surface.
2007, 56 (8): 4971-4976.
doi: 10.7498/aps.56.4971
Abstract +
The Er3+ and Er3+/Yb3+ co-doped bismuth glasses were prepared in this work. The absorption spectra and fluorescence spectra of the glasses were measured at room temperature. The strength parameters calculated according to the Judd-Ofelt theory from the absorption spectra are Ω2=(5.47—2.92)×10-20 cm2，Ω4=(2.16—1.22)×10-20 cm2, and Ω6=(1.29—0.80)×10-20 cm2. The absorption cross section at 980 nm and intensty of emission spectra at 1.5 μm were compared. FWHM of the broad fluorescence band of Er3+/Yb3+ co-doped bismuth glass at 1.5 μm is 91 nm, which is 15 nm larger than that of Er3+-doped bismuth glass. Emission cross section of σe=1.00×10-20 cm2 has been obtained using McCumber theory. The optical properties of Er3+ ion in bismuth-based glass are discussed by comparing with other glasses. The results show that Er3+ dopingof bismuth based glass is highly preferable for optical amplifier to realize broadband high gain amplification.
2007, 56 (8): 4977-4982.
doi: 10.7498/aps.56.4977
Abstract +
GaN-based green light emitting diodes was designed and fabricated. We calculated the internal electric field using the coupled method on the basis of analyzing the effect of the spontaneous polarization and the piezoelectric polarization. Taking into consideration of the effect of non-uniform carrier distribution in the active region, we obtained the fractions of the carriers and the rate of the recombination in different wells by calculating the steady state rate equation and Poisson equation. It was found that the calculation data are consistent with the experimental data for the changes of the peak wavelength, the light power and the halfwidth with the current in the range of 10—70 mA.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2007, 56 (8): 4983-4988.
doi: 10.7498/aps.56.4983
Abstract +
Ba8Ga16ZnxGe30-x(x=3, 4, 5, 6) clathrates with different Zn contente were synthesized by melt method reaction and spark plasma sintering. The effect of Zn composition on the thermoelectric properties were investigated. The results show that all samples behave p-type conductor. The electric conductivity and room-temperature carrier mobility decrease with increasing Zn content, and the room-temperature carrier density Np increases with increasing Zn content. Of all p-type Ba8Ga16ZnxGe30-x compounds, Ba8Ga16Zn3Ge27 has the highest Seebeck coefficient, the value of which is 234 μV·K-1 at 300 K, and reaches as high as 295 μV·K-1 around 700 K. The thermal conductivity of Ba8Ga16ZnxGe30-x decrease with increasing x. The maximum ZT value of 0.38 is obtained at 806 K for Ba8Ga16Zn3Ge27.
2007, 56 (8): 4989-4993.
doi: 10.7498/aps.56.4989
Abstract +
The technique of diffraction enhanced imaging computed tomography is studied in this paper and a new algorithm is given. The absorption, scattering and refractive index gradient can be obtained using the new algorithm with projection data of an object in the rotation range of 0°—360° and only needs to set the analyzer at one point of the rocking curve. The experimental process of the new algorithm is much simpler than the conventional.
2007, 56 (8): 4994-5002.
doi: 10.7498/aps.56.4994
Abstract +
Thermal stimulation on dissociation of methane hydrate was investigated with molecular dynamics simulation. The dissociation mechanism of methane hydrate with structure Ⅰ was investigated systematically by injecting heated, liquid water of 340 K. The results showed that when the water molecules on hydrate surface are made in contact with high temperature liquid water, they obtain heat energy, and with the obtained energy the water molecules move intensively, breaking the hydrogen bond between water molecules, and destroy the clathrate structure. In addition, methane molecules that have obtained heat energy, break away from the clathrate and diffuse into liquid. Due to heat energy being transferred into inside layer from outside layer through collision between molecules, the hydrate is dissociated layer by layer. Comparing the effects of liquid water with different temperatures of 340 and 277 K on hydrate dissociation, it is concluded that the thermal stimulation promotes dissociation of the hydrate.
2007, 56 (8): 5003-5008.
doi: 10.7498/aps.56.5003
Abstract +
The effects of different solvents on the morphology of poly(2-methoxy-5-(2-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV): N,N′-bis(1-ethylpropyl)-3,4,9,10-perylene bis(tetracarboxyl diimide) (EP-PTC) blend film and the performance of solar cells based on MEH-PPV: EP-PTC blend film were studied. The results show that the phase separation in the blend film using non-aromatic solvents is of micron (0.5—5 μm) size, which leads to low charge separation efficiency and consequently the low power conversion efficiency of the assembled solar cell; whereas the phase separation in the blend film using aromatic solvents is of nano-size, which increases the interface area of the two phases obviously and consequently increases the ratio of charge separation significantly. The power conversion efficiency of solar cells based on MEH-PPV: EP-PTC blend film using aromatic solvents is improved 30 times of magnitude compared with that of solar cell based on the blend film using non-aromatic solvents.
EDITOR'S SUGGESTION
2007, 56 (8): 5009-5012.
doi: 10.7498/aps.56.5009
Abstract +
Cu(In,Ga)Se2 layers with thickness of 2 μm were prepared by co-evaporation on different substrates of soda-lime glass, Mo foil, Mo film without preferred orientation and (100)-oriented Mo film. X-ray diffraction measurements were carried out to study the influence of substrates on the preferential growth of the films. The (112) texture of Cu(In,Ga)Se2 films deposited on the four substrates mentioned above decreases the given order, while the (220/204) texture begins to appear at first and its intensity increases gradually. On the soda-lime glass and Mo foil surfaces, the Cu(In,Ga)Se2 film shows almost a pure (112) texture. On the contrary, the (220/204) Cu(In,Ga)Se2 surface orientation dominates on the substrate of (110)-oriented Mo film. On Mo film without preferred orientation, no evidence of significant texture exists in Cu(In, Ga)Se2 layers. These results indicate that, of the substrates investigated, only the (110)-oriented Mo film has significant influence on the formation of (220) texture in Cu(In,Ga)Se2 film.
