Vol. 57, No. 6 (2008)
2008-03-20
GENERAL
2008, 57 (6): 3299-3304.
doi: 10.7498/aps.57.3299
Abstract +
The geometric structure of Friedmann universe in the discrete space time is researched. It is proved that the dust matter which forms the Friedmann universe is not distributed on the Friedmann spacetime points. The geodesic motion equations of test particles are derived, and the effects of clock and frequency shift are revealed.
2008, 57 (6): 3305-3311.
doi: 10.7498/aps.57.3305
Abstract +
An algebraic method to construct the exact solutions of nonlinear differential-difference equations is presented by introducing the coupled Riccati equations. As an example, we apply this method to the general lattice equation, the relativistic Toda lattice equations and the (2+1) dimensional Toda lattice equation. Some kink solitary wave solutions and complexiton solutions are obtained with the help of symbolic system Mathematica. Our method can also be applied to other nonlinear differential-difference equations.
2008, 57 (6): 3312-3318.
doi: 10.7498/aps.57.3312
Abstract +
By using the modified mapping method, we find some new exact solutions of the nonlinear Klein-Gordon equation, mKdV equation and the generalized Boussinesq equation. The solutions obtained in this paper include Jacobian elliptic function solutions, combined Jacobian elliptic function solutions, soliton-like solutions and triangular function solutions.
2008, 57 (6): 3319-3323.
doi: 10.7498/aps.57.3319
Abstract +
We propose a method for calculating the unitary evolution matrix of the arbitrary spin-s operators rigorously. This is an indirect method, which differs from the method of group theory or the method of direct calculation. The kernel of our method is to use the identity of two systems in their expressions, namely the Hamiltonian Hs=Sx of spin-s particle and the Hamiltonian of the Heisenberg XX open chain with interaction J=n(N-n). Because of this identity, the calculation of the unitary evolution matrix of the spin-s operators is substituted by the calculation of the state evolution matrix in Heisenberg XX open chain. As examples, the unitary evolution matrix of s=3/2, s=2 and s=5/2 are calculated by using our method. Since the evolution of the state |sm〉 under the operator e-itSx corresponds to the rotation of the initial state |sm〉 around the x-axis by an angle β＝t, and the evolution matrix element dsm′m(t)=〈sm′|e-itSx|sm〉 is just the projection of the finial state e-itSx|sm〉 on the initial state |sm′〉, the evolution matrix at t=π corresponds the perfect transmission of quantum state in the Heisenberg XX open chain.
2008, 57 (6): 3324-3327.
doi: 10.7498/aps.57.3324
Abstract +
In a parabolic quantum dot (QD), the exciton vacuum state and the exciton ground state may be employed as a two-level quantum system——a qubit. The pure dephaseing due to deformation potential exciton-bulk longitudinal acoustic phonon (LAP) interaction for the exciton qubit is investigated by using the reduce density matrix. The dependence of dephasing factor on time and the confinement length of the QD and temperature is calculated. It is shown that the pure dephasing factor increases quickly with increasing time when time is smaller than 2.5ps, and the dephasing time is of the order of ps. Even at absolute zero temperature, exciton-LAP interaction still causes excitonic dephasing. The pure dephasing factor increases with increasing temperature quickly when temperature is higher than 3K. At the same time, it is found that the confinement length of the QD has important effects on pure dephasing of qubit, the smaller the quantum size, the faster the pure dephasing is. The results show that using appropriate QD size at low temperature and using ultra-fast laser technology can effectively suppress pure dephasing due to exciton-LAP interaction.
2008, 57 (6): 3328-3332.
doi: 10.7498/aps.57.3328
Abstract +
Using the quantum statistical method, the difficulty of solving wave equation on the background of the black hole is avoided. We directly solve the partition functions of bosonic field and fermionic field on the background of the axisymmetric Kerr black hole through using the new equation of state density motivated by the generalized uncertainty relation in the quantum gravity theory. Then the entropy of the bosonic field and fermionic field near the horizon of the black hole are calculated. In our results the divergence appearing in the brick wall model is removed, as well as without using the small mass approximation. The series expression of the statistical entropy of the black hole is convergent. Therefore, it gives a better understanding of the black hole statistical entropy in non-spherical symmetry spacetimes.
2008, 57 (6): 3333-3336.
doi: 10.7498/aps.57.3333
Abstract +
By means of the linear approximation, we investigate the power spectrum and signal-to-noise ratio of the laser intensity for the gain-noise model of the single-mode laser with the noise correlation intensity λ modulated by the time period. Effects of the noise intensity D and Q, the noise correlation intensity λ, the period modulation frequency Ωλ and the input signal frequency Ω on the signal-to-noise ratio R are discussed. Some novel phenomena are found.
2008, 57 (6): 3337-3341.
doi: 10.7498/aps.57.3337
Abstract +
A method for constructing multi-scroll chaotic systems based on shift transformation is presented. Firstly a proper unstable linear system is selected, and the phase space is divided into several parts by a series of parallel interfaces. Then shift transformations of the linear system are implemented so that each space region is dominated by the corresponding new system. Finally the whole system is obtained using a proper nonlinear function based on signum functions. According to this method new multi-scroll chaotic systems with arbitrary odd number scrolls can be constructed, furthermore, some existing chaotic systems can also be modified into multi-scroll systems. Numerical simulations verify the feasibility of the proposed method.
2008, 57 (6): 3342-3352.
doi: 10.7498/aps.57.3342
Abstract +
In the present paper, the control of chaos in the Josephson junction with two different kinds of resonant harmonic excitations, namely the additive and parametric excitations, are investigated in detail. With the Melnikov method, we have obtained the regions of excitation amplitude in which heteroclinic chaos may be generated or suppressed. Meanwhile, for suppressing the heteroclinic chaos, we have determined the prereguisite relationships between parameters of the system excitation and the control excitation. The analytical results show that phase difference between the two excitations has important effect. Moreover, numerical methods show that the phase control method is feasible not only for controlling heteroclinic chaos, but also for controlling other types of chaos in nonautonomous systems. Comparing the effect of an additive harmonious excitation with that of a parametric one, we find the former one is more effective at the small resonant frequencies, while the latter one is more effective at the large ones.
2008, 57 (6): 3353-3358.
doi: 10.7498/aps.57.3353
Abstract +
We proposed a quantitative detection method for weak periodic signal in this paper. A result is obtained when analyzing the stable output of chaotic oscillator system in the large-scale periodic state, which shows that the average area of the periodic solution of the chaotic oscillator system is a steady geometric characteristic quantity of periodic solutions of the chaotic oscillator system. There is a steadily and monotonically increasing relation between the average area and the amplitude to be detected. When some parameters are defined, the precision of the geometric characteristic quantity can reach 10-6V2. The further quantitative detection of weak signals is built based on the two characteristics of the chaos system, namely the immunity to noise and sensitivity to weak signal. The simulation experiments shown that the immunity to noise will be enhanced along with the increase of amplitude detected while the precision can be guaranteed.
2008, 57 (6): 3359-3366.
doi: 10.7498/aps.57.3359
Abstract +
By observing the phase diagram and using the behavior complexity algorithm, the complexity of chaotic pseudo-random sequences generated by the new TD-ERCS discrete chaotic system is analyzed in detail, and the rules of complexity variety are investigated. Based on the Kolmogorov complexity, from one-dimensional time series to multidimensional phase space restructure, the complexity values of TD-ERCS discrete chaotic pseudo-sequences are calculated by using the Limpel-Ziv algorithm, ApEn algorithm and PE algorithm, respectively. The results show that the behavior complexity of TD-ERCS system is high, and the complexity value changes a little with the change of the parameters of TD-ERCS system. TD-ERCS system is a discrete chaotic system with the steady complexity, and the pseudo-random sequences generated by TD-ERCS are suitable for use in information encryption and spread spectrum communications.
2008, 57 (6): 3367-3373.
doi: 10.7498/aps.57.3367
Abstract +
Some algebraic criteria for synchronization with error bound of forced oscillators with different external forces are derived by Lyapunov direct method and Sylvester's criterion. Using algebraic inequalities, the synchronization error bound can be estimated. By controlling the estimated synchronization error bound, the real synchronization error bound can be controlled to be smaller than a prescribed precision of the synchronization. The Duffing-Van der Pol forced oscillator used as a numerical example verifies the effectiveness of the method.
2008, 57 (6): 3374-3379.
doi: 10.7498/aps.57.3374
Abstract +
A novel fourth-order Colpitts chaotic oscillator is presented. Theoretic design and circuit implementation shows that the fourth-order Colpitts chaotic oscillator can be obtained by parallelizing a capacitor C3 with inductor L in a canonical third-order Colpitts chaotic oscillator. The resonance frequency is changed by adjusting the value of C3, and the oscillator becomes chaotic through doubling period bifurcation. The dynamical behaviors of the fourth-order Colpitts chaotic oscillator are further investigated, including equilibrium points, bifurcations and Lyapunov exponents. Finally, this approach is verified in both numerical simulations and circuit experiments.
2008, 57 (6): 3380-3384.
doi: 10.7498/aps.57.3380
Abstract +
As the biological neural networks have small-world property and updating connection weights with time, we accordingly propose a new model of small-world biological neural networks based on biophysical Hodgkin-Huxley(H-H)neurons with updated weights. Then we study the statistical properties of excitement of this model and the updating of weights. The results show that, for networks with the same structure and parameters and external stimulation, there exists an optimal learning rate ralue b* which makes the excitement strength of biological neural networks strongest.
2008, 57 (6): 3385-3390.
doi: 10.7498/aps.57.3385
Abstract +
A kind of new dynamic modeling method is presented based on improved genetic algorithm (IGA) and wavelet neural networks (WNN) and the algorithm is applied to a new type of robot wrist force sensor. The dynamic model of the wrist force sensor is set up according to data of the dynamic calibration, where the structure and parameters of wavelet neural networks of the dynamic model are optimized by genetic algorithm. The results show that the proposed method can overcome the shortcoming of easy convergence to the local minimum points of BP algorithm, and the network complexity, the convergence and the generalization ability are well compromised and the speed and precision of modeling are increased.
2008, 57 (6): 3391-3396.
doi: 10.7498/aps.57.3391
Abstract +
We report measurements on the length-dependent thermal conductivity of individual single-walled carbon nanotubes (SWNTs) on a substrate using a four-pad 3ω method with the consideration of heat loss between the sample and substrate. An increase in thermal conductivity with length (0.5—7μm) was observed at room temperature. The length-dependence of thermal conductivity was theoretically predicted by the modified WV model with consideration of second order-3-phonon process. The predicted phonon mean free path is about 175nm. Both the measurement and the prediction show the scale effect of the length on the thermal conductivity of SWNTs.
2008, 57 (6): 3397-3401.
doi: 10.7498/aps.57.3397
Abstract +
Samples in a gasketed diamond anvil cell (DAC) are usually subjected to a uniaxial stress at high pressure due to the geometric structure of the DAC, namely a maximum stress along the cell loading axis and a minimum stress in the radial direction.Using conventional DAC in situ high pressure X-ray diffraction method， the measured compression curves often lie above the hydrostatic compression curves because of the effect of uniaxial stress field.Using the technique of DAC radial X-ray diffraction and the lattice strain theory, through the recent experiments with W, diamond and B6O samples，we discussed how the unixalxial stress filed, sample strength, and the sample difference between the sample material and pressure standard material affect the determination of equation of state.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2008, 57 (6): 3402-3409.
doi: 10.7498/aps.57.3402
Abstract +
By solving thermal conduction and thermo-elastic equations with an integral-transform method, a theory to describe the three-dimensional temperature and deformation distributions in an optical coating sample irradiated by a pulsed or square-wave modulated laser is developed for both transient and steady state cases. The theoretical simulations are compared with corresponding results of the finite-element method in time and space domains and good agreements are obtained. In addition, the influences of heating-beam radius and modulation frequency on the temperature and deformation distributions are discussed. Applications of the theoretical model to absorptance and deformation measurements of optical coatings are discussed.
NUCLEAR PHYSICS
2008, 57 (6): 3410-3418.
doi: 10.7498/aps.57.3410
Abstract +
X-ray refraction contrast CT is the computer tomography based on phase contrast. When the biological samples are taken from organs or tissues，X-ray refraction contrast CT provides more internal subtle details compared with conventional absorption CT images and it is considered a novel and very promising imaging technique. The iterative and analytical algorithms are the two main strategies in computer tomography. In the field of X-ray refraction contrast CT several analytical algorithms have been reported in the past. However the algorithm using iterative method have not been proposed. In this study，we tested the application of iterative algorithm in X-ray refraction contrast CT image reconstruction，investigated the influence of the order in which data are accessed in algebraic reconstruction techniques，and satisfactory CT images were obtained. Our data demonstrated that when biological samples are imaged or the complete projective data are not available，our iterative algorithm showed significant advantage over the analytical algorithms. It effectively reduces the number of the projection data，exposure time，as well as the radiation damage to the biology samples.
ATOMIC AND MOLECULAR PHYSICS
2008, 57 (6): 3419-3424.
doi: 10.7498/aps.57.3419
Abstract +
The structure and potential energy function of the ground states and low-lying excited states for AuZn and AuAl molecule are studied using B3LYP method. Their analytic potential energy functions are in good agreement with the Murrell-Sorbie function，and the ground states is X2Σ and X1Σ for AuZn and AuAl molecule，respectively. The adiabatic excitation energies of a4Σ for AuZn，and a3Σ for AuAl molecule is 43529 and 19991kJ/mol，respectively.The electronic and vibration energy or electronic and vibration entropy of the gas molecules are assumed to be the corresponding values of their solid states. The relationships of thermodynamic properties of formation reaction and temperature have been calculated based on this approximation.
2008, 57 (6): 3425-3428.
doi: 10.7498/aps.57.3425
Abstract +
With a hollow cathode lamp and!a highly-resolution grating monochromator，the emission spectrum of 7 energy levels from the even-parity configurations 4d96s and 4d95d of palladium (Pd) have been measured. Branching ratios of 15 lines produced by the transitions from these levels to 4d95p levels were derived. The majority of our results can provide corrections for the related atomic parameters of Pd atom in the literature.
2008, 57 (6): 3429-3434.
doi: 10.7498/aps.57.3429
Abstract +
Far-infrared absorption spectra and refractive indices of hypoxanthine and inosine have been measured by terahertz time-domain spectroscopy (THz-TDS) in the frequency range of 03—16THz at room temperature. The experimental results show that there is a distinct difference between the THz absorption spectra of hypoxanthine and inosine，which suggests that THz-TDS is highly sensitive to molecular structures and the components. Hartree-Fork calculation was used to optimize and simulate the THz spectra obtained by measurement. The low frequency vibrational mode at 14THz of inosine is attributed to the swing and torsion between purine and pentose rings according to the theoretical calculation. There is a linear relationship between the absorption intensity and the concentration of sample，which is consistent with Lambert-Beer's Law. Quantitative analysis of the components of mixtures of hypoxanthine and inosine was carried out by linear regression with non-negative constraint and the relative error was less than 7%. The sources of error were also discussed.
2008, 57 (6): 3435-3439.
doi: 10.7498/aps.57.3435
Abstract +
From the linear stability analysis，the modulational instability (MI) of two-component dipolar Bose-Einstein condensates (BECs) in an optical lattice is studied. The relation of the region of MI with the on-site interaction and the site-to-site interaction induced by the dipole-dipole interaction is obtained. The results show that there is a great influence of site-to-site interaction on the MI of the two-component dipolar BECs in the optical lattices. This gives us some useful information for manipulating two-component dipolar BECs in practice.
2008, 57 (6): 3440-3445.
doi: 10.7498/aps.57.3440
Abstract +
The line intensities of 100000-000000 transition of asymptotic asymmetric-top molecule H122C16O at several temperatures were calculated by directly calculating the partition functions and regarding the rotationless transition dipole moment squared as a constant. Results showed that the calculated line intensity data at 500 and 3000K are in excellent agreement with the data in HITRAN database，which provide a strong support for the calculations of partition function and line intensity at high temperature. Thereby，the line intensities and spectral simulations of 100000-000000 transition band at the higher temperature 4000 and 5000K were presented. The results are of significance for studying the high-temperature molecular spectrum by experimental measurement and theoretical calculation.
2008, 57 (6): 3446-3451.
doi: 10.7498/aps.57.3446
Abstract +
An analytic expression of the intermolecular interaction potential of the ground state of the Ne-HF system is obtained by nonlinear least square method to fit the intermolecular interaction energies，which have been calculated for the coupled cluster singles and doubles including connected triple excitations level andwith the augmented correlation consistent polarized quadruple zeta basis set CCSD (T)/aug-cc-pVQZ. Differential and partial cross sections for the Ne-HF scattering are obtained by means of converging close coupling calculations from the fitted ab initio CCSD (T) potential. The calculation is performed separately at the incident energies 60，75，100 and 150meV. The tendency of the scattering cross sections varying with incident energy and the influence of the state-to-state excitation cross sections on the total inelastic scattering cross section are discussed in detail.
2008, 57 (6): 3452-3457.
doi: 10.7498/aps.57.3452
Abstract +
Close-coupling equation is applied to the 3He(4He)-H2 system，and the rotational excitation cross sections for incidence energy of 05eV are calculated.By analyzing the differences between these partial wave cross sections and differential cross sections，the change of the partial wave cross sections and differential cross sections with increase of quantum number，as well as with the change of the mass of isotope atom are found out. Based on the calculation，the effect of isotopic substitution on atom-diatom collision is discussed.
2008, 57 (6): 3458-3463.
doi: 10.7498/aps.57.3458
Abstract +
DS3C model is applied to study single ionization of helium by 102eV electron impact. Fully differential cross sections are calculated at different scattering angles of θ1 (8°，10°，15°，20°) for the perpendicular plane geometry. The results of 3C and CCC model were compared with experimental data, which sho ws that DS3C model can give a successful description of the (e，2e) process for noncoplanar geometry qualitatively，indicating the existence of strong dynamic correlations effect between the three particles in the final channel for noncoplanar geometry.
CLASSICAL AREA OF PHENOMENOLOGY
2008, 57 (6): 3464-3472.
doi: 10.7498/aps.57.3464
Abstract +
Based on the 2-D ocean-like surface of linear model and the first-order small slope approximation，the characteristic of Doppler shift for different order Doppler spectra of scattered echoes is investigated and the formula of Doppler shift is derived. Comparing with the conventional formula of Doppler shift，the influence of the wind speed and the orbit motion of the sea wave in large scale is considered in our result. By employing the two-scale model of rough surface and physical mechanism of Doppler shift，a formula of the spectrum widening for the scattered field from the oceanic surface is also presented. The numerical results evaluated by the formulae in this study are compared with the simulated and the measured data，the dependence of the Doppler shift and the spectrum widening on the parameters，such as the wind speed，the frequency of the incident wave and the incident angle, etc. are discussed. It is found that the formulae obtained in this research can be used to predict the Doppler shift and the spectrum widening.
2008, 57 (6): 3473-3480.
doi: 10.7498/aps.57.3473
Abstract +
A current based hybrid method combining the MOM with the Kirchhoff approximation for analysis of scattering interaction between a two-dimensional infinitely long conducting target with arbitrary cross section and a 1-D rough sea surface with PM spectrum is proposed. The electromagnetic scattering region in the hybrid method is split into KA region and MOM region，respectively. The computational time of the hybrid method mainly depends on the number of unknowns of the target due to the induced current on the rough sea surface does not need to be calculated. The composite bistatic scattering cross section for the infinitely long cylinder above the rough sea surface with PM spectrum is calculated，the numerical results are compared and verified with those obtained by the conventional MOM，which shows the high efficiency of the hybrid method. Finally，the influence of the windspeed，the size and location of the target on the bistatic scattering cross section with different polarizations is discussed in detail.
2008, 57 (6): 3481-3485.
doi: 10.7498/aps.57.3481
Abstract +
The rigorous electro-magnetic diffraction theory of nonparaxial vectorial beam is proposed and illustrated by the diffraction of nonparaxial vectorial Gaussian beam diffracted at a slit. A numerical comparison with the analytical expressions based on the vectorial Rayleigh-Sommerfeld diffractiion integrals is given to show the consistency between them. A futher extension of the theory is briefly discussed.
2008, 57 (6): 3486-3493.
doi: 10.7498/aps.57.3486
Abstract +
On the base of perfect conductivity integral theory of diffraction gratings，after transforming the grating equation，the image equation of diffraction wave vector in Littrow mounting is given. Using the equation and in view of the power function property，the infinite series，which represents the kernel function，is transformed into the sum of a symmetrical series and a geometrical series. Compared with the primary integral method，from the view of numerical calculation，the new kernel function can reduce the computing time and improve the convergence. The extension results show that the power function character can improve convergence in the deviated Littrow mounting.
2008, 57 (6): 3494-3498.
doi: 10.7498/aps.57.3494
Abstract +
Based on the second-order moment theory of beam propagation，the properties of Lorentz beam have been investigated. The expressions of the beam waists，the transverse divergence angle and the beam propagation factor have been presented. The transverse beam waist only depends on the corresponding beam parameter. However，the transverse divergence angle and the beam propagation factor are both determined by the two transverse beam parameters. The curves of the beam propagation factor versus the two transverse beam parameters are also given. The numerical results show that variational laws of the beam propagation factor in the x-and y-directions versus the two transverse beam parameters are apparently different，while the variational rule of the integrated beam propagation factor versus the two transverse beam parameters is the composite manifestation of the above cases. In the paraxial case，the beam propagation factor trends to a constant value of 141. With the beam waist being the same，therefore，the divergence of Lorentz beam is 1.41 times that of the Gaussian beam in the paraxial case. Accordingly，Lorentz beam is an appropriate model to describe certain laser sources with high divergence.
2008, 57 (6): 3499-3505.
doi: 10.7498/aps.57.3499
Abstract +
Glan-Taylor prism is one of the most important polarization components of the new polarization interference imaging spectrometer developed in our laboratory. The principle of beam splitting and imaging of Glan-Taylor prism are presented. This paper deduces the propagation track of light in Glan-Taylor prism by ray-tracing method at random incidence angle and the theoretical expressions of image displacement and chromatic aberration are derived. The results will provide a theoretical basis for the study of new polarization interference imaging spectrum technology and the development of apparatuses.
2008, 57 (6): 3506-3513.
doi: 10.7498/aps.57.3506
Abstract +
In this paper，we show that the enhanced transmission and the surface plasmon distribution through a thin gold film with a periodic array of subwavelength holes with the finite-difference time-domain (FDTD) method. The transmission spectra exhibit two different surface plasmon resonance features: the localized waveguide resonance and the well-recognized photonic crystal resonance. For the localized waveguide resonance，the tangential electric field component Ey is seen to be concentrated entirely in the rectangular air hole region. And for the photonic crystal resonance，the tangential electric field components Ey is nonzero only on the two sides of the rectangular hole region. With the aid of the SP，light can form a strong focal spot at the bottom of the metal.
2008, 57 (6): 3514-3518.
doi: 10.7498/aps.57.3514
Abstract +
The dependence of the quantum correlation of twin beams generated from a non-degenerate optical parametric oscillator on the analysis frequency was theoretically studied with semi-classical method. It was pointed out that the quantum entanglement is higher at lower analysis frequency. By means of a pair of unbalanced fiber Mach-Zehnder interferometers，the correlation noise spectrum of the intensity-difference and the phase-sum of twin beams with non-degenerate frequencies and total power of 20mW were experimentally measured，which were 3.1 and 1.3dB below the corresponding shot noise limit at 2MHz，respectively. The experimental results are in agreement with the theoretical predictions.
2008, 57 (6): 3519-3524.
doi: 10.7498/aps.57.3519
Abstract +
The output performance of a laser diode end-pumped continuous wave Tm，Ho∶YLF laser is studied and the laser parameters are optimized. Based the quasi-three-level rate equations，the relation formulus between the output power and the input power is deduced with consideration of the energy transfer up-conversion. The laser parameters such as the concentration of doped ions，the crystal length，the pump-to-mode size ratio and the transmittance of output coupler are optimized by numerical analysis. The output power as a function of pump power is obtained in experiment. Comparison of experimental and theoretical results shows that the theoretical results are valid.
2008, 57 (6): 3525-3530.
doi: 10.7498/aps.57.3525
Abstract +
The characteristics of acoustic-optic double-clad fiber laser end-pumped by diode laser are reported for different pumping constructions. The comparison and discussions of average power, pulse width and stability of Q-switched pulses under the forward pump and back ward pump conditions are presented. In experiment, the output of Q-switched pulses with 60% slope efficiency of average power, 52ns pulse width and 03mJ single pulse were obtained at the repetition rate of 10kHz. Finally, the rate equations with different pump configurations are deduced to analyze the Q-switched pulse, and the theoretical results agree with well experimental results.
2008, 57 (6): 3531-3537.
doi: 10.7498/aps.57.3531
Abstract +
Three-dimensional analytical model for analyzing injection current vectors in gain-guided vertical-cavity surface-emitting lasers was developed based on the theory of steady-state electric field. Our calculation covered the spatial distribution of injected current density and its correlation with parameters of the top electric contact ring. The results showed that there exists a region between the two contacts where the radial carrier distribution has a maximum at the center. As the optical power is proportional to the injected current, to obtain fundamental mode or uniform light intensity output with low threshold, the active region should be set to overlap with the boundary layer where the uniform carrier distribution just occurs. The carrier distributions for top-and bottom-emitting configurations with various ring parameters as well as the influences on the near-field modes were discussed.
2008, 57 (6): 3538-3542.
doi: 10.7498/aps.57.3538
Abstract +
A theoretical investigation is carried into the effect of spontaneously generated coherence on absorption and dispersion in probe laser of Y-type atom system. It is found that the electromagnetically induce transparency window in the system narrows gradually as the spontaneously generated coherence intensifies. Moreover, the anomalous dispersion appears and the absorption changes from negative value to positive valve at resonance. At the same time, the absorption is zero or negative near resonance when the relative strength of the coupling field and pump field are different, and the atomic medium shows positive absorption when the two fields are exattly or nearly equal and close.
2008, 57 (6): 3543-3546.
doi: 10.7498/aps.57.3543
Abstract +
A reshaping Brillouin amplifier is combined with the main Brillouin amplifier in the conventional stimulated Brillouin scattering (SBS) phase conjugation mirror (PCM), which can control the SBS pulse shape, the main amplifier controls the pulse shape roughly, while the reshaping amplifier can control it finely. Effect of pump on pulse shape is studied, and the dependence of the pulse shape on the distance between the two amplifiers is achieved at various pump intensities. Results show that the pump intensity determines the amplifying efficiency of the seed, and that it is an important parameter to control the pulse shape.
2008, 57 (6): 3547-3552.
doi: 10.7498/aps.57.3547
Abstract +
The effect of background beam with different Glass constant on the relative spatial refractive perturbation in photovoltaic crystal was discussed, in which was introduced the concrete conditions for the formation of bright or dark photovoltaic spatial solitons in crystals with negative refractivity change. Based on the results, for what is to our knowledge the first time, the bright photovoltaic solitons was experimentally observed in LiNbO3:Fe by use of e-ray signal of 532nm and o-ray background illumination of 488nm.
2008, 57 (6): 3553-3561.
doi: 10.7498/aps.57.3553
Abstract +
The propagation of (2+1)D paraxial symmetrical hyperbolic secant beams in weakly nonlocal nonlinear media is studied by variational approach. A series of differential equations which describe the evolutions of the width, the phase, the phase front curvature, and the amplitude of the beam are obtained. The critical power for a beam propagating as a spatial optical soliton in weakly nonlocal nonlinear media is also obtained. A quantitative depiction of the steadying effect of nonlocality on the propagation of a beam is obtained through the steadiness analyse, which provides a self-consistent explanation of the transition from unsteady local kerr solitons to steady weakly nonlocal solitons. The results of numerical method agree with that of variational calculations, implying that hyperbolic secant function is a good approximation to a (2+1)D weakly nonlocal spatial optical soliton.
2008, 57 (6): 3562-3570.
doi: 10.7498/aps.57.3562
Abstract +
In this article, dielectric photonic crystals (PhC) with finite dimensions and exposed in air is analyzed for its potential value in traveling wave tubes (TWTs). The modes existing in finite dielectric PhC are EH and HE, and also E if the finite PhC is two-dimensiond. The E1 mode is the E polarization in infinite dielectric PhC. Calculation shows that only the existence of gaps of pass-bands is necessary for PhC in TWTs, while the complete photonic crystal band gaps are superfluous.
2008, 57 (6): 3571-3577.
doi: 10.7498/aps.57.3571
Abstract +
We experimentally studied the effects of certain factors, such as the ways of loading of electrodes, the concentration of ER fluids, the type of ER fluids and intensity of applied electric fields, on the resonant frequency of split-resonant ring array infiltrated in ER fluids. The experimental results show that the way of loading of electrodes has important effects on the magnetic resonance of SRR array. The resonant frequency of SRR array shows a red-shift when the ER fluid is infiltrated, the maximum shift of about 130MHz was obtained through the adjustment of the applied electric-field. Numerical simulation demonstrates that the permeability of SRR array is negative near the resonant frequency, and the frequency range of negative permeability can be dynamically tuned by the applied electric-field.
EDITOR'S SUGGESTION
2008, 57 (6): 3578-3582.
doi: 10.7498/aps.57.3578
Abstract +
The propagating characteristics of a symmetrical three-layer slab waveguide with mu-negative (MNG) media are studied in detail. It is found the MNG waveguide can propagate not only quick waves but also slow waves of TE0 and TE1 modes. The properties of this guided mode are different from those of the left-handed waveguide and the conventional waveguide. In MNG slab waveguide, the region of effective refractive index of the guided mode is wider than those of the other two. In addition, the TE0 fundamental guided mode does not exist at all, while the higher-order TE guided modes and all TM guided modes do exist. However the surface wave of TE0 and TE1 modes can emerge under a certain condition. The propagation constant of higher-order TE mode is greater than that of the TM mode of the same order. Two TE modes of the same order can coexist in the MNG, which is closely related to the structural parameters. The energy fluxes of two TE modes are oppositely directed inside and outside the inner layer, therefore the total energy flux can be negative.
2008, 57 (6): 3583-3587.
doi: 10.7498/aps.57.3583
Abstract +
The full-vector plane-wave method was used to calculate the bandgaps of the photonic crystal fiber with triangular structure. The transmission spectrum of the hollow-core photonic crystal fiber fabricated by Yanshan University was measured, and we obtained the relation between the transmission intensity and the wavelength in visible region. In the subsequent experiment, the mode field patterns were observed. The theoretical simulation is well consistent with the experimental result.
The influence of spectral lines and coupling coefficient on mutual injection locking of fiber lasers
2008, 57 (6): 3588-3592.
doi: 10.7498/aps.57.3588
Abstract +
Mutual injection locking is recognized as one of the most important and promising research directions for the fibre laser combining technology. Basing on the mutual injection locking of the two fiber lasers, the far field and the spectra of the two fiber lasers were inspected by infrared CCD and optical spectrum analyzer when the spectrum of one fiber laser and the coupling coefficient were changed. From the experiment, we conclude that the stronger was the mutual injection locking when the closer the spectra of the two fiber laser are; and the mutual injection locking would fail when the difference in the frequencies exceeds the locking range, which can be extended by increasing the coupling coefficient of the fiber lasers.
2008, 57 (6): 3593-3599.
doi: 10.7498/aps.57.3593
Abstract +
The optical stopping effect in undopeel and low concentration Sn- and P-doped As2S8 amorphous thin films is studied experimentally. The optical stopping response curve, experimental data of room temperature deexcitation and spectral testing data of the samples are provided. The results show that, the Sn-doped sample can significantly improve the ms range response in the fast-recovery process, and has the effect in decreasing the residual propagation loss. But in P-doped sample, the effects are contrary to that in the Sn-doped sample. Based on the experimental results, some analysis and discussions are presented.
2008, 57 (6): 3600-3606.
doi: 10.7498/aps.57.3600
Abstract +
Electric field distribution of the light modulator affected by the dioxide dielectric charging is deduced with Gauss law. Electrostatic force formula of the movable grating under such condition is obtained. Charging and discharging mechanisms of the dielectric have been analyzed. The relationship between movable grating's displacement and the dielectric charge is obtained. The results show that the grating will bounce back and then be pulled down when it is actuated by low frequency voltage. When the actuating voltage period is far shorter than the charging time constant, variation of the modulator's output light intensity will be increasingly small until the modulator is disabled thoroughly when the field produced by the storage charges is half that of the actuating electric field. Experiment was carried out to test the theoretical analysis and the results agree well. Finally, a method is proposed to eliminate the storage charge, and its feasibility is proved by the experiment.
2008, 57 (6): 3607-3612.
doi: 10.7498/aps.57.3607
Abstract +
The intrinsic factors of magneto-optical waveguide isolator based on 45° nonreciprocal mode conversion and 45° reciprocal mode conversion are introduced. A novel phase self-compensation isolator is designed and simulated by the full-vectorial FD-BPM. The insertion loss of this isolator is -12dB and the isolation is-34dB.
2008, 57 (6): 3613-3619.
doi: 10.7498/aps.57.3613
Abstract +
The reflection and refraction on the fluid-solid interface of the acoustic wave excited by a concave phased array are studied in this paper. The analytical presentations of the refraction and reflection fields of the focused acoustical wave on the fluid-solid interface are obtained for the first times by the ray approximation method. The refraction fields in the solid are analyzed and discussed. The ultrasonic phased fields of the refraction compressional and shear waves in the solid are numerical simulated and investigated by the analytical presentations. It is found that the compressional and shear waves are better focused in the solid with a cylindrical fluid-solid interface than in that with a planar fluid-solid interface.
2008, 57 (6): 3620-3624.
doi: 10.7498/aps.57.3620
Abstract +
In this paper, the model of a one-dimensional generalized Fibonacci quasi-periodical phononic crystal is firstly proposed, the transmission coefficients of the elastic waves through this 1D quasi-periodical phoninic crystal are numerically calculated, and the obtained transmission coefficients are compared with those of the phononic crystals with periodical structure and with normal Fibonacci structure. The results show that the band gap can also be found in the phononic crystals with the generalized Fibonacci quasi-periodical structure, and the frequency range of the gap is larger than that of both the periodical structure and the normal Fibonacci structure. However, there are more strongly localized resonant modes in the gap of the generalized Fibonacci quasi-periodical phononic crystals. This study to the localized modes is useful to the fabrication of the acoustic or elastic wave filters.
2008, 57 (6): 3629-3635.
doi: 10.7498/aps.57.3629
Abstract +
In single bubble sonoluminescence (SBSL), the forward Mie scattering signal of the bubble, which is waved due to the interference between the scattered light components, provides a scale for measuring the change of the bubble radius. The unit of the scale is decided by the scattered angle θ, the optical wavelength λ, and the relative refraction index n of the fluid; and the scale marks are the peaks and valleys of forward scattered light, which correspond to specific radius values. The forward Mie scattering method has been applied in measurement of the R(t) curve, especially the maximum radius value Rmax, and further in the fit of the equilibrium radius R0 of bubble in SBSL, and is proved to be a feasible and convenient technique in the study of the bubble dynamics in SBSL.
2008, 57 (6): 3636-3640.
doi: 10.7498/aps.57.3636
Abstract +
In this article, the discrete element method is used to research the force distribution of two-dimensional granular system under the condition of isotropic compression and pure shear. The simulation results are compared with the experimental results under the same conditions. Then we simulate the force distribution of two-dimensional granular system with point defects under the condition of isotropic compression and pure shear and compare the results with the results without point defects. The affects of point defects to granular system are found.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2008, 57 (6): 3641-3645.
doi: 10.7498/aps.57.3641
Abstract +
Wave-absorbing coating samples were fabricated with nano tetraleg ZnO as the absorbent and epoxy resin as the binder. The influence of the content of nano tetrapleg ZnO in the absorbing coatings and the thickness of the coatings on wave-absorbing property was investigated. The results showed that excellent absorption was observed in the wave band from 15GHz to 18GHz when the coating thickness was 15mm. When the coating thickness increased to 35mm, the wave-absorbing property of the samples was improved significantly, especially in the wave band from 6GHz to 11GHz. The frequency width reached 136GHz (44—18GHz) when the reflection value of the samples was below -5dB, its wave-absorbing property has obviously improved compared with that of the micrometer sized ZnO. Finally, the wave-absorbing mechanism of nano tetraleg ZnO was discussed. Such coatings are of great values in applications involving electromagnetic wave shielding.
2008, 57 (6): 3646-3652.
doi: 10.7498/aps.57.3646
Abstract +
We study the self-compression and splitting of a circularly polarized laser pulse propagating in plasmas with density window from 1/4 critical to slightly below critical density by solving the nonlinear Schrdinger equation numerically. It is demonstrated by the numerical calculation that the effective self-compression of laser pulse can be achieved in even shorter distance by increasing both the background plasma density and intensity of the laser pulse, or decreasing the width of pulse. The full-width at half maximum of the compressed pulse can reach 1/35 of the initial one's or even smaller. It has been found that this kind of self-compression occurs in the process of formation of a high-order soliton when a laser pulse propagates in a plasma, so that we can obtain greats compression ratio than in thin plasmas. We also obtained the splitting of a high-order soliton formed after self-compression of a laser pulse propagating in plasmas from the result of the numerical calculation in this situation. The phenomenon of self-compression and splitting is also observed by using one-dimensional particle-in-cell simulations and the result was consistent with the numerical calculation.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2008, 57 (6): 3653-3660.
doi: 10.7498/aps.57.3653
Abstract +
A tracing simulation has been performed for the solidification of liquid Pb at six different cooling rates by molecular dynamics method. The pair distribution function g(r) curves, the bond-type index method of Honeycutt-Andersen(HA), the cluster-type index method and average coordination numbers have been used to analyze the evolution of micro-cluster structures during solidification processes. The results show that there is a critical cooling rate (in the range of 1×1013 and 5×1012 K·s-1) for forming amorphous or crystal structure. When the cooling rate is higher than the critical cooling rate, amorphous structures are formed mainly with the 1551,1541 and 1431 bond-types. When the cooling rate is lower than the critical cooling rate, the crystal structures mainly with the 1441 and 1661 bond-types or the bcc basic cluster (14 6 0 8 0 0) are firstly formed, and keep stable for a period of time, and then rapidly transform to the partial crystalline structure mainly with the 1421 and 1422 bond-types, or the fcc basic cluster (12 0 0 0 12 0) and the hcp basics cluster (12 0 0 0 6 6) coexisting in a certain proportion . At the same time, it has been found that there are obvious effects of the cooling rate on the relative proportion of the fcc basic clusters to the hcp basic clusters, the smaller the cooling rate is, the bigger relative proportion of the fcc basic cluster, and the system tends to form highly perfect fcc crystal structure.
2008, 57 (6): 3661-3665.
doi: 10.7498/aps.57.3661
Abstract +
The Si-in-SiNx:H films were prepared by helicon wave plasma chemical vapor deposition technique with active gas N2/SiH4/H2, and the tuning photoluminescence of the films from red to blue-green is achieved by changing the N2 flow rate. The analysis of Fourier transform infrared spectroscopy and ultraviolet-visible absorption demonstrate that the deposited films have higher hydrogen contents. Increasing of N2 flow rate makes the hydrogen bonding configurations change and the size of amorphous silicon particles decrease, to which corresponds the increase of band gap and also the decrease of the microstructure order. The tuning photoluminescence is mainly related to the quantum confinement of amorphous silicon particles. The broadening of the spectral widths of main photoluminescence and the enhancement of their intensities are obtained for the films deposited with increasing N2 flow rates.
2008, 57 (6): 3666-3673.
doi: 10.7498/aps.57.3666
Abstract +
The molecular dynamics method was used to investigate the elastic properties of armchair and zigzag single-walled carbon nanotubes grafted by hydroxyl on their ports. The results showed that the Young's moduli of ungrafted armchair (5, 5)，(10,10) and zigzag (9, 0) , (18,0) single-walled carbon nanotubes were 948，901 and 804, 860GPa, respectively. When the single-walled carbon nanotubes were grafted by 2 to 8 hydroxyl functional groups, the Young's modulus of the zigzag single-walled carbon nanotubes was little changed, while the Young's modulus of the armchair single-walled nanotubes first decreased significantly due to the grafting, then increased appreciably to some steady value with increasing of the grafts after having been grafted to certain numbers. The reasons were analyzed in terms of the isoline structure of deformation electron density, C—C bond-length and the system binding energy variation of the carbon nanotubes with different graft numbers.
2008, 57 (6): 3674-3678.
doi: 10.7498/aps.57.3674
Abstract +
The electron field emission characteristics of nanocrystalline Si thin films prepared by KrF excimer laser crystallization of ultrathin amorphous Si films and subsequent thermal annealing is reported. Stable and reproducible field emissionbehavior can be observed for the crystallized Si films. The turn-on electric field is reduced from 17V/μm for the as-deposited sample to 85V/μm for the crystallized one, and the emission current density can reach as high as 01mA/cm2 The improvement in field emission characteristics is attributed to both the change of film surface and the formation of high-density nanocrystalline Si, which induces the enhancement of internal local electric field.
2008, 57 (6): 3679-3683.
doi: 10.7498/aps.57.3679
Abstract +
We report results of molecular dynamics simulations of melting and premelting of single-walled carbon nanotubes (SWNTs) with different types of point defects, namely, the Stone-Wales defect, vacancy defect, and impurity defect. Using the Lindemann parameter, we found that the melting temperature of perfect SWNTs is around 4800K. We further show that premelting occurs in the regions close to the defects with premelting temperatures of 2600K for the Stone-Wales defect and3200K for the cavity defect. The SWNTs with Si impurity defects are thermally stable up to 3800K.
2008, 57 (6): 3684-3689.
doi: 10.7498/aps.57.3684
Abstract +
Geometric and electronic properties of the endohedral fullerene H2＠C60 and its dimmer have been studied using generalized gradient approximation of density functional theory. It is found that the two hydrogen atoms exist in the form of a molecule in the center of the cage for both H2＠C60 and its dimmer, and the hydrogen molecule can rotate freely at room temperature. The electronic structures of both H2＠C60 and its dimmer show that the formation of hydrogen molecule only affects the energy levels below the Fermi level from -8 to -5eV, while the other energy levels and the energy gap between HOMO and LUMO hardly have any change.
2008, 57 (6): 3690-3697.
doi: 10.7498/aps.57.3690
Abstract +
Geometric and electronic properties of fullerene-barbituric acid and its dimmer have been studied using density functional theory (DFT) at BLYP level with DNP basis sets. The calculation results indicate that the only one stable geometry of the fullerene-barbituric acid is that with ［6,6］ close-bond structure, and the effects of the barbituric acid on the cage structure is mainly in the local region neighboring the barbituric acid. Among the three isomers of the fullerene-barbituric acid dimmers, one can come to the conclusion both from the energy gap and from relative energy that the ［6,6］—［6,6］ structure is the most stable one. With respect to the electronic structures, the donor-acceptor system corresponds to C60 fullerene-barbituric acid, ie. electron transfers from the fullerene cage to the barbituric acid. The frontier orbitals and spin population show that C60 fullerene-barbituric acid has similar electromagnetic properties of C60, but its stability is reduced, so that the addition reaction would easily take place to form its dimmer. For the C60 fullerene-barbituric acid dimmer, the Mulliken analysis shows there are 0104e and 0106e charge transfers to the four carbons from their neighboring carbons, and the farther the atoms is, the less charge it loses. Compared with those of the C60 fullerene-barbituric acid, the energy gap is changed from 152eV to 1.45eV, while the distribution of highest occupied molecular orbital (HOMO) is little changed but that of lowest unoccupied molecular orbital (LUMO) is changed a lot.
2008, 57 (6): 3698-3702.
doi: 10.7498/aps.57.3698
Abstract +
Titanium carbide with low work function was formed on the surface of CNTs after deposition of a thin layer of titanium film and subseguest vacuum annealing at high temperature. The CNTs modified by titanium carbide was characterized by XPS, and the field emission characteristics of the CNTs film before and after modification were investigated. The characterization results indicated that carbon atoms reacted with the coated titanium atoms to form titanium carbide during annealing. The field emission measurements showed that, compared to that of unmodified sample, the turn on electric field of the modified CNTs film decreased from 1.21V/μm to 1.04V/μm and the emission current density at applied electric field of 2.34 V/μm increased from 2.3mA/cm2 to 13.5mA/cm2 Also, the emission stability of the titanium carbide-modified CNTs was improved. The mechanism of the CNTs film modified by titanium carbide has been discussed.
2008, 57 (6): 3703-3708.
doi: 10.7498/aps.57.3703
Abstract +
The elastic properties of helium-bubble embedded aluminum have been investigated by molecular dynamics simulation. First, the interaction potentials between aluminum and helium atoms are parametrized with the help of first-principle method. Secondly, we studied the influence of helium bubble on the elastic properties of aluminum from the following two aspects: the size of helium bubble (the diameters being 16, 20, 25, 30 and 3.5nm, respectively) and the internal pressure of helium bubble (the ratios of the number of helium atoms to the vacancies being about 5%, 15%, 45% and 85%, respectively). The results show that the elastic constants c11, c12 and c44 decrease with increasing diameter of helium bubble, however, for a fixed size they are almost independent of the internal pressure of helium bubble. Using the elastic composite system model, the influence of the size and internal pressure of helium bubble on aluminum is explained qualitatively based on the analytical results and is consistent with our calculated results.
2008, 57 (6): 3709-3713.
doi: 10.7498/aps.57.3709
Abstract +
The lattice dynamics of hexagonal perovskite BaVS3 compound is studied by first-principles calculations based on the ab initio pseudopotential and plane wave basis. Phonon spectra throughout the Brillouin zone are obtained using the density-functional perturbation method within the framework of linear response theory. The calculated frequencies of the Raman active modes are compared with the experimental measurements. For E22g and A1g modes, our calculated values are in good agreement with the experiment. But for E32g mode, the value of the calculated frequency differs significantly from the experiment. After further calculation using the frozen-in phonon method, we suggest that this difference is caused by the large anharmonicity of E32g mode. The most prominent feature of the phonon spectrum is that there appears the imaginary frequency for E12u mode, which involves the planar vibration of the vanadium atoms. The appearance of imaginary frequency for E12u mode indicates the planar instability of hexagonal BaVS3 crystal structure. We believe that this soft mode is responsible for the observed hexagonal-orthorhombic phase transition.
2008, 57 (6): 3714-3719.
doi: 10.7498/aps.57.3714
Abstract +
Based on lattice dynamics theory and force constant model, we calculated phonon dispersion relation, density of states, and specific heat of graphene ribbons.It is shown that the characteristics of phonon spectra behaves as those of a system between one_dimensional carbon nanotube and two-dimensional graphene. The high and intermediate phonon branches of armchair and zigzag nanoribbons resemble those of zigzag and armchair carbon nanotubes, respectively. With the ribbon width increasing, interestingly, it is found that the low frequency peaks in density of states shifted to lower frequency, indicating a phonon confinement effect. At both low and high temperature,specific heat decreased with the increase of the band width,showing up as a singular quantum size effect.The specific heat at 300K can be fitted by CV=CVg+A/n, where CVg is specific heat of graphene and the A/n term represents the effect of edge on specific heat of ribbons.
2008, 57 (6): 3720-3729.
doi: 10.7498/aps.57.3720
Abstract +
Based on the static interaction potential and the (electrical) dipole-dipole interaction between neighboring molecules, the Hamiltonian model of the exciton system in one-dimensional molecular crystal can be expressed by means of the molecular projection operators. In the case of approximate resonance, according to our findings with respect to the kinematics and dynamics effects of the exciton motion, the Klein-Gordon-type equation set for the time evolution of the exciton_soliton and lattice motion has been obtained. With regard to the solitary wave function , it was found that the kinematics and dynamics effects of exciton motion not only greatly contributed to its nonlinear terms 3 and 2{2}\{ξ2}, but also led to its important high-order nonlinear terms. We solved its 5 and 7 nonlinear equation in an analytical form. In particular the solitary wave solution for the 5-nonlinear equation has been studied in detail under the Bell-type and kink-type boundary conditions. Then we found that the kinematics and dynamics effects of exciton motion contributed remarkably to the increase of exciton effective mass, and makes further negative correction to the exciton-soliton energy Ω. As for the Bell-type soliton movement in supersonic speed v＞cs and the Kink-type soliton movement in subsonic speed v＜cs, and the types of the soliton bound states appear to be situated at the bottom and the top of the exciton energy band respectively.
2008, 57 (6): 3730-3734.
doi: 10.7498/aps.57.3730
Abstract +
Accelerated stress-migration testing under 200℃ of Cu (M1/M2) interconnects has been done for 700h. Finite element analysis and focused-ion beam techniques have been used to study the stress-induced voiding in the Cu interconnects with vias of 500 and 350nm in diameter. The voiding mechanism and the effect of via size on the stress migration have been studied. The results show that peak values of stress and stress gradient in M1 lines are reached underneath the edge of vias. The stress gradient shows crucial effect on the voiding process. The vacancies introduced by thermo-mechanical stress diffuse along Cu M1/SiN interfaces under the force of stress gradient and nucleate at the peak values of the stress gradient. The void grows faster along the length direction because the stress in M1 lines changes faster horizontally. The stress and stress gradient increase with increasing via diameter, leading to a faster voiding rate.
2008, 57 (6): 3735-3739.
doi: 10.7498/aps.57.3735
Abstract +
Highly c-axis oriented Mg0.05Zn0.95O thin films were fabricated on Si(111) substrates at different temperature by pulsed laser deposition. The influence of the substrate temperature on the structure and luminescent properties of Mg0.05Zn0.95O thin films were studied by XRD and photoluminescence (PL) spectra and the relationship between the crystalline quality and PL properties of thin films was discussed. The results indicate that the Mg0.05Zn0.95O thin films grown at substrate temperature of 450℃ show good c-axis orientation and strong PL peaks. Different PL spectra are acquired at room temperature using xenon lamps as excitation light sources，with excitation wavelengths of 240, 300 and 325nm, respectively. The UV peaks and violet peaks were found to originate from free excition radiation recombination and the PL intensity was related to crystalline quality, and the blue-green peaks may be related to the oxygen vacancies. We also discuss the physical mechanism throngh which the substrate temperature affects the red shift and blue shift of UV PL peaks.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2008, 57 (6): 3740-3746.
doi: 10.7498/aps.57.3740
Abstract +
The electronic structure and optical properties of pure and Al_and Mg-doped cubic-blende GaN have been studied based on the density functional theory and the generalized gradient approximation method. The relationship between the distribution of the electronic states and their structures has been analyzed.The dielectric function and the complex refractive index function on the doping GaN system have been obtained. The calculation results indicate that the hole concentration of Mg-doped GaN is increased and the Mg-doping markedly enhanced the conductivity of the material.But the carrier concentration of Al-doped GaN is not changed and optical bandgap is broadened.The mechanism of luminescence has been explained in terms of an analysis of the dielectric function and the complex refractive index function. It offers theoretical data for the design and application of GaN optoelectronic materials. By comparison, our results are in good agreement with that published in the literatures.
2008, 57 (6): 3747-3752.
doi: 10.7498/aps.57.3747
Abstract +
Based on ab initio methods and nonequilibrium Green's function theory, we have investigated the photoswitching mechanism of diarylethenes with a phenyl linker and the effect of F-substituent on the photoswitchable performance. Numerical results show that the current through the closed-state system is larger than that through the open-state system under small bias, indicating the switching behavior is stable under small bias, and the photoswitchable performance can be improved by F-substituent significantly.
2008, 57 (6): 3753-3759.
doi: 10.7498/aps.57.3753
Abstract +
We have calculated the electronic structure and optical properties of zinc blende structure ZnSe under different high pressure by means of plane wave pseudo-potential method (PWP) with generalized gradient approximation (GGA) in this paper. The total energy, density of state, energy band structure, and optical absorption and reflection properties under high pressure are calculated. Furthermore, we analyzed the change of electron structure, band structure and optical properties under high pressure using the results of experiment and theory.
2008, 57 (6): 3760-3768.
doi: 10.7498/aps.57.3760
Abstract +
The crystal structure, impurity formation energy, electronic structure and optical properties of anatase TiO2 doped with sulfur in different valence states were studied by using the plane-wave ultrasoft pseudopotential method based on first_principles. The calculations shown that the existence form of sulfur element in anatase TiO2 was related to the experimental conditions; the octahedral dipole moments were increased due to the changes of lattice parameters, bond length and charges on atoms; the band gap narrowed and the curves of light absorption red-shifted to visible-light region due to the position of conduction band downward shifting, the position of valence band upper shifting and the width of valence band broadening result from hybridizing S 3p states with O 2p states and Ti 3d states. All of these results can explain the better photocatalytic properties of S-doped anatase TiO2 in visible-light region. According to these results, the effects on electronic structures and photocatalytic properties of anatase TiO2 doping with different valence states of sulfur were compared and analyzed.
2008, 57 (6): 3769-3774.
doi: 10.7498/aps.57.3769
Abstract +
First principle calculations based on DFT have been applied to study various properties of rutile (110) surface. We used a nine_layer slab model with stoichiometric formula of TiO2 and the vacuum length of the cell fixed at 15nm. The interactions between ionic core and valence electrons were represented by pesudopotentials. After analyzing the results, we found that an admixture bonding states existed in titanium dioxide: a stronger ionic bond accompanied by a weaker covalent bond between Ti and O. STM(Scanning tunneling microscope)image of Rutile (110) surface has been successfully simulated by DFT calculations, which was in accord with the Tersoff-Hamann's theory. A positive basis voltage of 2V was added to simulate STM experimentd conditions in imaging process. The distance between tip and surface were also changed step by step. By analyzing the electron density map and DOS plot, it is clear that electron factors dominated the STM image of rutile (110) surface and the jut in the image corresponded to 5_coordinated Ti atom rather than the bridge oxygen on the top.
2008, 57 (6): 3775-3779.
doi: 10.7498/aps.57.3775
Abstract +
Under effective-mass approximation, the Kubo formula is used to calculate the minimal conductivity of graphene. It is found that the minimal conductivity depends on the spectral function of quasiparticle. If the distribution of the spectral function is taken to be Lorentz distribution, the minimal conductivity is 4e2/πh. If the distribution of the spectral function satisfies Gaussian distribution, however, the minimal conductivity is 2e2/h.
2008, 57 (6): 3780-3785.
doi: 10.7498/aps.57.3780
Abstract +
The martensitic transition and magnetocaloric property in Ni50Mn25+xSn25-x (x=11, 12) Heusler alloys were investigated by means of structure and magnetism measurements. The results show that the antiferromagnetic interaction strengthens in the martensitic state, leading to the coexistence of ferromagnetic and antiferromagnetic interaction in this state, which is different from the magnetic property of austenitic state in these alloys. Furthermore, based on Maxwell equation, the inverse magnetocaloric property near martensitic transition temperature of this system was studied. It was found that large positive magnetic entropy change (ΔSM) depends on both magnetic jumping (ΔM) and temperature region during martensitic transition.
2008, 57 (6): 3786-3790.
doi: 10.7498/aps.57.3786
Abstract +
GaN:Er thin film has been prepared by direct current (DC) planar magnetron reactive sputtering deposition. The film was characterized by X-ray diffraction (XRD), ultraviolet—visible (UV—Vis) spectroscopy, transmission electron microscope (TEM) and photoluminescence (PL) spectroscopy, XRD shows the structure of the film was nano/poly-crystalline. The average grain size of the film was derived from XRD peaks using the Scherrer formula and the value was 58nm. TEM result indicated that the film was GaN nano-particles embed in amorphous matrix and the particle size was between 6—8nm. UV—Vis transmission spectrum shows that the transmission ratio of the film exceeds 80% in the visible zone from 500nm to 700nm. From UV-Vis spectrum, the optical band gap of the film was calculated using Tauc plot and the value was 322eV. Last, PL was measured by fluorescence spectrometer at room temperature and green emission of Er3+ ions was clearly observed for the transition: 4S3/2(2H11/2)—4I15/2(554nm).
2008, 57 (6): 3791-3797.
doi: 10.7498/aps.57.3791
Abstract +
Ni and Se co-doped skutterudite compounds Co1-xNixSb3-ySey with fine grains were fabricated by combining mechanical alloying and spark plasma sintering. The coupling effect between grain boundary scattering and point defect scattering on the thermoelectric transport properties was focused.The enhanced power factor by co_doping is twice that of the sample without Ni and achieved a maximum value of 1750μWm-1K-2 at 450℃ for x=0.1. Due to the coupling scattering effect, the lattice thermal conductivity at room temperature decreased to 1.67Wm-1K-1, which approach the reported minimum of 1.60Wm-1K-1, whareas the improved ZT value reached 0.53 at 450℃ for the Co0.9Ni0.1Sb2.85Se0.15. A suggested model shows that the decreased thermal conductivity is duc to the coupling effect between grain boundary scattering and point defect scattering, The calculated values are in good agreement with experimental results. The simulated results also suggest that the lattice thermal conductivity could be further decreased to 0.8Wm-1K-1 when the grain size was reduced to 50nm.
2008, 57 (6): 3798-3802.
doi: 10.7498/aps.57.3798
Abstract +
Nd0.7Sr0.3MnO3 ceramics of perovskite-type were respectively prepared by solid-state reaction and high-energy ball milling with post heating-treatment. The samples obtained using these methods show a great of difference in electrical transport despite having the same crystallographic structure, similar grain size and chemical composition. For the sample prepared by solid-state reaction, the electrical resistance keeps unchanged with current loads and no electroresistance effect takes place. However, for the sample prepared by high-energy ball milling with post heating treatment, the electrical resistance decreases remarkably with increasing current loads and shows colossal electroresistance effect. The microstructure of materials, in particular the screw dislocation-like structure, is supposed to be responsible for the difference in electrical transport.
2008, 57 (6): 3803-3806.
doi: 10.7498/aps.57.3803
Abstract +
Based on the Y element, a novel element for the Frequency selective surface (FSS) is presented in this paper. The transmission characteristics of Y element FSS and the novel element FSS is calculated using Galerkin's method in the spectral domain, and the optimum parameters of the FSS is obtained. The corresponding equivalent FSS sample plate was fabricated. It's transmission characteristics were measured by microwave network analyzer at 8—12GHz. The agreement between the calculated and measured values was good. The results show that, the novel element FSS has excellent transmission characteristic at large-angle incidence, the transmission coefficient is -052dB at TE 66° incidence. From 0° to 66° TE incidence, the drift of the resonant frequency is 200MHz. Meanwhile the transmission coefficient of Y element FSS is -082dB at TE 66° incidence, and from 0° to 66° TE incidence, the drift of the resonant frequency is 400MHz. Therefore, the novel element FSS is more suitable than Y element FSS for the application in large-angle incidence curved streamlined radome.
2008, 57 (6): 3807-3812.
doi: 10.7498/aps.57.3807
Abstract +
A 2-D analytical model for the surface potential and threshold voltage in fully depleted dual-material gate(DMG) SOI MOSFETs with high-k dielectric is developed to investigate the short-channel effects(SCEs). Our model takes into account the effects of the length of the gate metals and their work functions, the applied drain biase,and the gate dielectric constant. We demonstrate that the surface potential in the channel region exhibits a stepped potential variation by the gate near the drain, resulting in suppressed SCEs. With dielectric constants increasing, this novel device shows inverse SCEs. The derived analytical models are in good agreement with the resafts of the two-dimensional device simulator ISE.
2008, 57 (6): 3813-3817.
doi: 10.7498/aps.57.3813
Abstract +
A 375nm thick LaMg2Ni alloy thin film was prepared by electron beam evaporation using LaNi, Mg and Ni targets, and followed by vacuum annealing at 350℃. Upon hydrogen loading/unloading, the reversible conversion from a metallic, reflecting state to a semiconducting, color-neutral transparent state was observed in the as-prepared film. The contrast ratios in both the transmission (400—900nm) and the resistivity between the reflecting state and the transparent state are over four orders of magnitude. The surface and the cross-section of the alloy films before and after hydrogenation were observed by scanning electron microscope and atomic force microscope, which showed that the lower reflection observed in the dehydrogenated film is due to the rough surface originating from the first hydrogenation.
2008, 57 (6): 3818-3822.
doi: 10.7498/aps.57.3818
Abstract +
The magnetic field dependence of filling factors has been investigated on InP based In053Ga047As/In052Al048As quantum well samples with two occupied subbands by means of magnetotransport measurements at the temperature of 15K in a magnetic field range of 0 to 13T. Under the condiction that Laundau-level broadening is larger than the spin splitting of each subband, filling factors are even when the splitting energy of two subbands is an integer multiple of the cyclotron energy, i.e.ΔE21=kωc. If the splitting energy of two subbands is half of an odd interger multiple of the cyclotron erergy, i.e.ΔE21=(2k+1)ωc/2, the filling factor is odd.
2008, 57 (6): 3823-3827.
doi: 10.7498/aps.57.3823
Abstract +
The (Nd,Dy)-(Fe,Al)-B alloys with different rare earth (RE) contents were prepared using melt-spun and strip cast techniques, and the dependence of their microstructures and magnetic properties on RE contents were investigate. It is found that an appropriate RE in magnets is beneficial to the formation of (Nd,Dy)2Fe14B single phase and uniform distribution of the Nd-rich phase on grain boundaries. When the rare earth content of casting strip treated as main phase is close to the stoichiometric concentration, the coercivity and energy product of magnet are enhanced significantly. Moreover, heat treatment of casting strips can improve effectively the magnetic performance of final sintered magnets.
2008, 57 (6): 3828-3833.
doi: 10.7498/aps.57.3828
Abstract +
In this paper we investigated the structures and propagation characteristics of electrical trees observed in the high volatage cross-linked polyethylene (XLPE) cable insulation samples under high volatage of 10kV at frequency of 1000—2000Hz. Based on the specific tree growth mechanism and the fractal nature of tree structures, a dynamic model has been developed to qualitatively predict the electrical tree growth characteristics in XLPE cable insulation when subjected to an electrical stress under high frequency voltage, and then the tree growth rate equation and life formula of tree growth to breakdown are derived.We performed electrical tree growth tests in XLPE cable insulation samples at high frequency and compared the results, which showed that the proposed model was in good agreement with the expreriment.So,it seems that the proposeed model can be applied to the qualitative analysis of aging law of electrical tree in XLPE cable insulation.
2008, 57 (6): 3834-3839.
doi: 10.7498/aps.57.3834
Abstract +
Space charge formation and evolution, charge trap distribution, and depolarization process in linear low density polyethylene (LLDPE) samples with semiconducting electrodes subjected to dc electric stress were systematically studied using the techniques of laser induced pressure pulse and thermally stimulated discharge. Space charge distribution and evolution in the samples showed obviously the characteristics of rapid and symmetrical injection of homo-charges from the two electrodes. The interface between the semiconducting electrode and LLDPE showed approximately ohmic contact properties. The charge trap distribution in LLDPE was characterized by shallow bulk traps and deep surface layer traps. Hot-pressing conditions between the semiconducting electrode and LLDPE or contamination of the surface layer of LLDPE by the semiconducting electrode material has a remarkable effect on energy distribution of the surface layer traps, resulting in depth and density decrease of the deeper traps and density increase of the shallower traps in the surface layer. During the whole short-circuit depolarization process, the centers of positive charges and negative charges in the sample shifted to their nearer electrodes, respectively, whereas behaviors different from the short-circuit depolarization were seen in the later stage of the open-circuit depolarization, that is, the positive charges captured in the deep surface layer traps were detrapped and shifted to the rearer electrode.
2008, 57 (6): 3840-3845.
doi: 10.7498/aps.57.3840
Abstract +
A dc poling model is developed for the 0-3 ferroelectric composite by employing Laplace equation, combined with certain boundary conditions and taken account of the initial conditions of the polymer/ferroelectric ceramic interfacial charges. Two kinds of 0-3 composites with different matrixes, namely the PZT/epoxy and PZT/P(VDF-TrFE) composites, were prepared. The influences of matrix electrical conductivity on the poling behaviors were studied experimentally and theoretically. The results showed that the experimental dates are fitted well with the theoretical results：with increased electric conductivity of the matrix, the poling time is shortened, and the polarization of ferroelectric ceramic and the total electric properties of composites are improved.Howerer,the leakage current and dielectric loss are increased,too.
2008, 57 (6): 3846-3852.
doi: 10.7498/aps.57.3846
Abstract +
The enhanced transmission of terahertzs radiation through the metal subwavelength fractal structures is studied by means of the terahertz time domain spectroscopy (THz-TDS). In view of experiment and theory, the physical mechanism of enhanced transmission of THz wave through each level of fractal lines on the copper film is studied. It is found that THz transmission enhancement in low frequency region mainly resulted from the electron resonance in low level lines, and that in high frequecy region mainly resulted from the electron resonance in high level lines. Thus the transmission enhancement can be attributed to the electron resonant radiation in the fractal structure, namely,the localized resonance efficiency of the frqctal structure.
2008, 57 (6): 3853-3856.
doi: 10.7498/aps.57.3853
Abstract +
The injection and relaxation of electron spins in In0.1Ga0.9N film were studied by femtosecond time-resolved circularly polarized pump-probe spectroscopy. An initial degree of spin polarization of 02 was obtained, which agrees with 3∶1 ratio of heavy- to light-hole valence bands in transition strength, but not with the 1∶1 or 1∶0.94 ratios. A spin relaxation lifetime of 490±70ps was obtained at room temperature. The spin relaxation mechanism is discussed qualitatively, and is thought to be dominated by BAP mechanism here.
2008, 57 (6): 3857-3861.
doi: 10.7498/aps.57.3857
Abstract +
In this paper, the excitation energy density dependence of carrier spin relaxation is studied at room temperature for the as-grown and annealed (Ga,Mn)As samples using femtosecond time-resolved pump-probe Kerr spectroscopy. It is found that spin relaxation lifetime of electrons lengthens with increasing excitation energy density for both samples, and the annealed (Ga,Mn)As has shorter carrier recombination and electron spin relaxation lifetimes as well as larger Kerr rotation angle than the as-grown (Ga,Mn)As under the same excitation condition, which shows that DP mechanism is dominant in the spin relaxation process for (Ga,Mn)As at room temperature. The enhanced ultrafast Kerr effect in the annealed (Ga,Mn)As shows the potential application of the annealed (Ga,Mn)As in ultrafast all-optical spin switches, and also provides a further evidence for the p-d exchange mechanism of the ferromagnetic origin of (Ga,Mn)As.
EDITOR'S SUGGESTION
2008, 57 (6): 3862-3867.
doi: 10.7498/aps.57.3862
Abstract +
Using nanosphere lithography and pulsed laser deposition, Au nanoparticles with different height have been fabricated on quartz substrates. The surface images of Au particles were observed by scanning electron microscopy, and the results showed that the prism-shaped Au particles form a periodical array. In the U-V-visible absorption spectrum, strong absorption peaks due to the surface plasmon resonance of Au nanoparticles were observed. Furthermore, theoretical calculation based on the discrete dipole approximation was performed. The experimental and theoretical results show that the absorption peaks of the Au nanoprism shifted to the blue side as the out-of-plane height increases.
2008, 57 (6): 3868-3874.
doi: 10.7498/aps.57.3868
Abstract +
The Fe2P, Ni2P and (Fe1-xNix)2P compounds (x=01,025,05) are studied by X-ray absorption near- edge structure (XANES) spectroscopy at the Fe, Ni and P K-edges. Combined with full multiple-scattering approach calculations of XANES, different contributions of 3f, 3g crystallographic positions to XANES spectra for metal atoms are discussed. It was found that the Ni atom prefers to substitute Fe (3f) sites when introducing Ni atoms. The first-principle density of states calculations were performed and the results show that unoccupied P p-projected DOS of paramagnetic Fe2P and Ni2P are matched with their K-edges XANES spectroscopy. Comparison with DOS calculations for ferromagnetic Fe2P, indicates that the magnetism of Fe2P is mostly due to Fe(3g) sites. The magnetic moments calculated for Ni(3f) and Ni(3g) sites of ferromagnetic Ni2P approach zero, which is in accordance with the conclusion that Ni2P generally appears paramagnetic.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2008, 57 (6): 3875-3879.
doi: 10.7498/aps.57.3875
Abstract +
A magnetic field generated by a magic ring was applied to the space containing the silicon melt, and the effective viscosities of silicon melt were measured by the method of rotary vibration. The magic ring was constructed by permanent magnets made of NdFeB permanent-magnet material. The results showed that, at certain temperature, the effective viscosity increased with the increase of magnetic field intensity, and they are parabolically related. The magnet field had stronger effect when the silicon melt temperature increased, which was indicated by a more warped up parabola. In the range of 1490 to 1610℃, the viscosity had an extraordinary fluctuation which indicated a structure change in the silicon-melt. The value of silicon melt viscosity increased by 2—3 orders of magnitude when the magnetic field reached 0.068T. The results indicated that applying a magnetic field to the silicon melt space is an effective way to suppress thermal convection during the growth of large diameter single crystal silicon.
2008, 57 (6): 3880-3886.
doi: 10.7498/aps.57.3880
Abstract +
Cd-doped CdTe polycrystalline films were deposited on ITO/glass substrates by close-spaced-sublimation (CSS)method. The microstructural, morphological, electrical and optical properties of the films were characterized by X-ray diffraction, scanning electron microscopy, Hall effect, and optical absorption. It is found that the crystallinity is improved, the electrical resistivity drops 4—5 orders of magnitude and the conductivity changes from p-type to n-type when pure CdTe film is doped with a proper amount of Cd. There is no obvious influence on the optical band gaps of the films by the doping of Cd.
2008, 57 (6): 3887-3891.
doi: 10.7498/aps.57.3887
Abstract +
The micro-nano multi-scale ZnO structure was synthesized by the vapor phase transport process onto Si (100) substrate. A typical scanning electron microscope (SEM) image of the sample clearly showed the surface has the complex structure of hexagonal micro and nano- rods. Photoluminescence (PL) spectra at room temperature showed an intense near-band-gap ultraviolet (UV) emission peak but no deep level emission related to impurities or defects, indicating that the sample has good optical quality. The temperature (83—307K) dependence of the PL spectra showed that the room-temperature UV emission resulted from two optical transitions, one related to the ZnO free exciton and the other related to the free-to-bound (FB) transition of the free cawier with the binding energy of about 124.6meV.
2008, 57 (6): 3892-3897.
doi: 10.7498/aps.57.3892
Abstract +
The intrinsic layers with different thickness were deposited on amorphous n-type layers and microcrystalline n-type layers respectively, and all samples were prepared by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). The effect of different n-type layers on structural properties of i-layer was studied. It is concluded that the highly microcrystalline n-layer can reduce the thickness of incubation layer, promote structural uniformity of i-layer and improve the performance of n-i-p μc-Si:H thin film solar cells.
2008, 57 (6): 3898-3904.
doi: 10.7498/aps.57.3898
Abstract +
The solar cells based on the blend of MEH-PPV (poly(2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylene vinylene) as the donor and PCBM(1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61) as the acceptor are fabricated. We analyzed the influence of composition on the performance of the device by measurements of atomic force microscopy, photoluminescence spectroscopy, dark current density-voltage(J-V) curve of charge-only device, wavelength response of photoconductiveity and J-V curve. The highest efficiency is achieved at the 1∶4 (MEHPPV:PCBM) weight ratio. The good performance is attributed to reasons such as the optimized interpenetrating network, effective charge separaption and rapid charges transport in bicontinuous path. We obtained an open circuit voltage of 0.8V, short circuit current density of 3.47mA/cm2 and fill factor of 55.9% under 100mW/cm2 Air-Mass 1.5 solar simulator illumination, yielding a 1.55% power conversion efficiency.
2008, 57 (6): 3905-3911.
doi: 10.7498/aps.57.3905
Abstract +
We report an improvement to the lateral-magnetic-tweezers which can be used to stretch and twist DNA molecules, and to probe the interaction between DNA and proteins. The apparatus enables not only direct measurement of the extension and rotation of DNA but also easy refreshing of the buffer in the flow chamber. With this apparatus, the experimental efficiency can be greatly improved by manipulating several DNA molecules simultaneously. The force exerted on DNA molecules ranges from 0.1 to 40pN, satisfying the requirement of most single molecule experiments. To demonstrate the applicability of the apparatus, we measured the extension-force curve and the extension-rotation curve of DNA and studied the translocation of DNA by a motor protein. The results agree well with those reported in the literature.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2008, 57 (6): 3912-3919.
doi: 10.7498/aps.57.3912
Abstract +
It is well known that it often leads to large error to calculate the first order partial derivatives of physical quantities by the difference method from observation data with error. Especially in meteorology, observational wind has large error, and calculating vorticity from observational wind is very inaccurate. Therefore, one-dimensional numerical differentiation algorithm is applied to calculate vorticity from observational wind, and the result is compared with that of the central difference method. It turns out that the one-dimensional numerical differentiation algorithm is stable and feasible. Its accuracy is superior to the central difference method, and it has stronger recognizing ability for the smaller-scale weather systems.
2008, 57 (6): 3920-3931.
doi: 10.7498/aps.57.3920
Abstract +
Using dry/wet index series of China during the past 531 years and 1041 years, by defining the dry (wet) grade and making running calculation of the appearance times of each dry (wet) grade in the original index series, it was shown that there is a exponential distribution P(x)=A*e-γx of the appearance times of each grade with respect to the window size. We defined 1/γ as the scale factor to characterize the dry (wet) durative and used it to analyze the dry (wet) durative of north China and the area between Yangtze and Huai rivers. Results show that the dry factor has the semi-belt wave distribution from north to south, the dry durative in north China is somewhat longer than that between Yangtze and Huai rivers, the average dry factors of area A, B and C are 1.87, 1.62 and 1.82, respectively. The dry (wet) durative is rather obvious in those intervals in which dry or wet are comparatively concentrated, so the large spatial scale and synchronized extreme dry accidents are easier to happen during the end of 12th century, the beginning of 13th century, the beginning of 17th century and the end of 20th century, which verifies the existence of cluster phenomena in the dry/wet index. The influence to dry durative in north China caused by the dry/wet index during 1260—1280 (the climate back ground being the end of the middle ages warm) was much lavger than that caused by the index during 1980—2000 (the climate background being the global warming). Furthermore, the number of dry years during 1260—1280 was also larger than that of 1980—2000. Obviously, serious, larger spatial scale and synchronized extreme dry accidents are easy to happen in warm periods, and the drought since 1970s in north China might be caused by co-operation of human and natural factors, in which the natural factor might have played the more important role.
2008, 57 (6): 3932-3940.
doi: 10.7498/aps.57.3932
Abstract +
This paper investigates the probability of extreme drought in four regions of China during the 1470—2000 A.D., and analyzes the δ18O record in Guliya ice core. By wavelet analysis, we find that the probability of drought in the four regions has the quasi-cycle of 350a, like the δ18O record in Guliya ice core. When the δ18O record is high, the temperature of Qinghai-Xizang Plateau is high too, and the probability of drought is low, and vice versa. This paper also analyzes the Pacific decadal oscillation and finds the quasi-cycle of 80a is weakening but the quasi-cycle of 50a is enhancing. When either of them is strong, the probability of drought is high, and the probability is low when the two quasi-cycles are alternating.
2008, 57 (6): 3941-3947.
doi: 10.7498/aps.57.3941
Abstract +
A new pure rotational Raman-lidar system for monitoring atmospheric temperature is presented. Instead of using the double grating monochromator, the system employs two high resolution spectral resolving channels,each of which is composed of a three-stage Fabry-Perot etalon combined with a filter with band width of 7nm. The transmitting light source is an injection-seeded Nd:YAG laser. This configuration can achieve a high rejection rate (10-10) to Mie-Rayleigh elastic scattering (at 532nm wavelength)，and has an extremely narrow receiving bandwidth (less than 10pm) for pure rotation Raman scattering of N2 with quantum numbers of ±6 and ±12 respectively. Therefore, the system effectively rejects the interference of background radiation and pure rotational Raman scattering of O2, and the measurement of daytime tropospheric temperature from a single Raman spectral line intensity ratio using the lidar system is made possible because of the narrow receiving bandwidth. The temperature measurement precision of this Raman-lidar is improved at the same time. At last, two_channels signals were simulated by using balloon-based sounding temperature profile. The simulation demonstrated the feasibility of this lidar for atmospheric temperature profile measurement.
