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CN 11-1958/O4
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The amplitude and phase measurements of femtosecond pulses using fringe-free SPIDER
Lei Liang, Wen Jin-Hui, Jiao Zhong-Xing, Lai Tian-Shu, Lin Wei-Zhu
Acta Physica Sinica, 2008, 57 (1): 312
The light source, optical waveguide and light enhancement of nano-integrated optical circuit
Liu Dan, Ma Ren-Min, Wang Fei-Fei, Zhang Zeng-Xing, Zhang Zhen-Sheng, Zhang Xue-Jin, Wang Xiao, Bai Yong-Qiang, Zhu Xing, Dai Lun, Zhang Bei
Acta Physica Sinica, 2008, 57 (1): 381
Acta Physica Sinica  
  Acta Physica Sinica--2008, 57 (1)   Published: 15 January 2008
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Influence of dispersive magnetic permeability on modulation instability in metamaterials

Dai Xiao-Yu, Wen Shuang-Chun, Xiang Yuan-Jiang
Acta Physica Sinica. 2008, 57 (1): 0186 doi: 10.7498/aps.57.186
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The influence of dispersive magnetic permeability on propagation of untrashort pulses in metamaterials is mainly in that it leads to the appearance of the pseudo-χ(5), self-steepening (SS) and second-order nonlinear dispersion terms in the propagation equations. In this paper, the role of dispersive magnetic permeability in modulation instability (MI) in metamaterials is identified based on the Drude model. It is found that in the anomalous dispersion regime, the pseudo-χ(5) nonlinear parameter, which is always negative, increases the MI frequency and growth rate, which is opposite to that in ordinary positive-index materials; the SS tends to suppress MI regardless of its sign, while the second-order nonlinear dispersion effect tends to stimulate MI in the positive-index region and suppress MI in the negative-index region. In the normal dispersion regime, in which MI cannot occur in the ordinary materials, MI can occur due to the role of the second-order nonlinear dispersion, suggesting a new way of generating solitons or ultrashort pulse trains in the normal dispersion regime.

Transverse superresolution and axial extended focal depth realized by the tunable phase pupil filter

Yun Mao-Jin, Wan Yong, Kong Wei-Jin, Wang Mei, Liu Jun-Hai, Liang Wei
Acta Physica Sinica. 2008, 57 (1): 0194 doi: 10.7498/aps.57.194
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In order to overcome the drawback of the conventional pupil filters and realize the tunable superresolution of the optical system, a new type of tunable pure phase filter composed of a half-wave plate located between two quarter-wave plates is presented in this paper, in which the half-wave plate is made of two zones. The simulation results show that with such a pupil filter the tunable transverse superresolution as well as the axial extended focal depth of the optical system can be realized by rotating any zone of the half-wave plate.

The extension effect of point spread function in wavefront coding system due to off-axis illumination

Zhao Ting-Yu, Ye Zi, Zhang Wen-Zi, Yu Fei-Hong
Acta Physica Sinica. 2008, 57 (1): 0200 doi: 10.7498/aps.57.200
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The off-axis illumination pupil function of the wavefront coding system with cubic phase mask is analyzed by coordinate-rotation for the first time. The approximate expression of the pupil function shows that off-axis illumination brings about the extension effect of cubic phase and defocusing. The extension effect is independent of the sign of the incident angle; however, the extension effect will get larger as the absolute value of the angle increases. This causes a decrease in modulation transfer function (MTF) and the extension effects on both sides of the point spread function (PSF) envelope. When the off-axis illumination is in the tangential plane, the extension effects of cubic phase and defocusing in y direction is larger than that in x direction, which results in larger extension of PSF and lower MTF in the y direction. When off-axis illumination is in the sagittal plane, the extension effects of cubic phase and defocusing in x direction is larger than that in the y direction, which results in larger extension of PSF and lower MTF in x direction.

Phase control of inversionless gain in a closed atomic system with Doppler broadening

Ma Hui, Bu Fan-Ge, Qiao Hong-Xia, Tong Dian-Min, Fan Xi-Jun
Acta Physica Sinica. 2008, 57 (1): 0206 doi: 10.7498/aps.57.206
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We studied the controlling role played by the relative phase between the probe and driving fields on inversionless gain of the probe field in a closed and Doppler broadened Λ-type three-level system with spontaneously generated coherence (SGC). It was found that:(1) Regardless of the driving field being in or off resonance, and regardless of the probe and driving fields being co- or counter- propagating, the gain maximum value always varies periodically with variation of the relative phase with the period 2π. (2) When the driving field is in resonance, the gain maximum value decreases monotonicly with Doppler width increasing, and moreover, the decreasing in the counter-propagation case is faster than that in the co-propagation case. When the driving field is off resonance, the gain maximum value does not monotonicly decrease or increase with Doppler width increasing. In both above cases, the largest inversionless gain can be gotten by adjusting the relative phase. (3) The contribution of SGC to the inversionless gain is much larger than that of the dynamically induced coherence.

Localization of the relative positions of two atoms in the presence of optical feedback

Cheng Gui-Ping, Zheng Jun, Deng Wen-Wu, Li Gao-Xiang
Acta Physica Sinica. 2008, 57 (1): 0212 doi: 10.7498/aps.57.212
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We have investigated the localization of the two atoms passing through a standing wave in an optical cavity by measuring the field quadrature. It was found that the localization of the two atoms can be better preserved in the absence of the damping of cavity. If the damping of cavity is taken into account, the compromise of atomic-position localization resulting from the damping of cavity can be decreased by feeding back into the cavity part of the output signal.

Characteristics of a cataphoresis He-Sr+ recombination laser

Wang Yu-Bo, Mao Bang-Ning, Chen Li, Wang Li-Min, Pan Bai-Liang
Acta Physica Sinica. 2008, 57 (1): 0219 doi: 10.7498/aps.57.219
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A cataphoresis discharge tube of 7mm inner diameter and 38cm active length was made for the He-Sr+ laser. The recombination laser at 430.5nm and the R-M transition laser at 1.03μm were obtained with the modified Blumlein circuit by high-frequency longitudinal pulsed discharge. The laser components are concentrated on the 430.5nm wavelength. Dependence of working parameters, such as the pulse frequency, the supply voltage and the helium pressure on laser output characteristics were measured and discussed. The maximum laser output power of 819mW and specific power of 56mW/cm3 were obtained.

Cystal end temperature distribution under different pumping light in solid state laser

Yang Yong-Ming, Xu Qi-Ming, Guo Zhen
Acta Physica Sinica. 2008, 57 (1): 0223 doi: 10.7498/aps.57.223
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In order to study how different pumping light profiles affect the crystal end temperature field, a classification creterion is suggested, according to which the pumping light can be classified as column light, Gaussian light and plane top light. The end temperature under different pumping power and different pumping light focus diameter are studied.It was found that the end temperature is the highest under the column pumping light, intermediate plane top pumping light and lowesl under Gaussian pumping light when the pumping light has the same power and the same focus diameter.It shows that the pumping light profile can seriously affect the crystal end temperature field. Finally the effect of the thermal effect caused by the end temperature was studied, which was found to affect the laser quality seriously. When the equivalent curvature radius of the thermal lens changes from 85 to 150mm, the far field walk-off angle and waist diameter become larger and larger and the laser works unsteadily.

A dual-wavelength diode-end-pumped Nd:YAG laser operating simultaneously at 1319 and 1338nm

Liu Huan, Yao Jian-Quan, Zheng Fang-Hua, Lu Yang, Wang Peng
Acta Physica Sinica. 2008, 57 (1): 0230 doi: 10.7498/aps.57.230
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Based on the lasing threshold equation of a diode-end-pumped Nd:YAG laser, the threshold condition of simultaneous dual-wavelength lasing is determined and the relationship between the transmission ratios of the output mirror corresponding to the two wavelengths is calculated. A simultaneous dual-wavelength continuous wave (cw) or quasi-cw diode-end-pumped Nd:YAG laser operating at 1319nm and 1338nm has been demonstrated. A total cw output power of 6W was achieved at a pump power of 21.6W with a slope efficiency of 30%. And a total quasi-cw output power of 4.75W was obtained at a pump power of 21.6W and a pulse repetition rate of 50kHz, with a slope efficiency of 24.73%, and the corresponding pulse width was 55.05ns. Inserting a Brewster plate in the cavity and the highest linearly polarized wave output power was boosted up to 2.22W at a pulse repetition rate of 50kHz. The instability of the output power was less than 0.52% and the beam quality M2 factor was as low as 1.16 at the linearly polarized wave output power of 2.17W. The simultaneous dual-wavelength laser provides an experimental basis for generating highly coherent terahertz wave radiation of 3.23THz by nonlinear optical difference frequency method.

Franz-Keldysh effect in two-photon absorption

Cui Hao-Yang, Li Zhi-Feng, Li Ya-Jun, Liu Zhao-Lin, Chen Xiao-Shuang, Lu Wei, Ye Zhen-Hua, Hu Xiao-Ning, Wang Chong
Acta Physica Sinica. 2008, 57 (1): 0238 doi: 10.7498/aps.57.238
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The Franz-Keldysh effect in the two-photon absorption process in the space charge region of an Hg0.695Cd0.305Te photodiode is demonstrated. By employing as the exciting light source a pulsed laser beam at λ0=7.92μm from an optical parametric generator and difference frequency generator pumped by a pico-second Nd:YAG laser, the photo-response has been measured as a function of the excitation intensity. The peak intensity of the pulsed photo-response shows a quadratic dependence on the incident intensity. A relationship between the pulsed photo-voltage and the incident optical intensity has been established with an equivalent RC circuit model to derive the degenerate single beam two-photon absorption coefficient. The results show that the two-photon absorption coefficient within the space charge region of the photodiode is about a factor of 2.7 higher than that outside the space charge region, implying an electric field induced enhancement of the two-photon absorption. This indicates that the Franz-Keldysh effect exists not only in the one-photon absorption process but also in the two-photon absorption process.

Three-photon absorption and dispersion of sub-femtosecond polarization beast in reverse V-type four-level

Nie Zhi-Qiang, Li Ling, Jiang Tong, Shen Lei-Jian, Li Pei-Zhe, Gan Chen-Li, Song Jian-Ping, Zhang Yan-Peng, Lu Ke-Qing
Acta Physica Sinica. 2008, 57 (1): 0243 doi: 10.7498/aps.57.243
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We investigate theoretieally the color-locked twin noisy field_correlation effects in fifth_order nonlinear absorption and dispersion of ultrafast polarization beats in the RV type system.We demonstrate a phase_sensitive method for studying the three_photon six_wave mixing due to atomic coherence in this multi_level system.In the limit of the narrowband and tail approximation,the fifth_order nonlinear coefficients are independent of the color_locked noisy lights.In broadband FWM and SWM signals show time asymmetry due to field_correlation,and RDO effect of frequency_time domain.Moreover,the polarization presents RDO oscillation in frequency_domain.The reference signal is a two_photon non_degenerate four_wave mixing signal,which propagates in the direction a little different from the optical path of the SWM signal.This method is used for studying the phase dispersion of the fifth_order dispersion and absorption and for the optical heterodyne detection of the three_photon SWM signal.The fifth_order nonlinear response can be controlled and modified through the color_locked correlation of twin noisy fields.The method of phase sensitive detection can be used to detect the nonlinear absorption and dispersion of the three_photon SWM.

High-speed data format conversion from non-return-to-zero to return-to-zero based on periodically poled lithium niobate waveguides

Wang Da-Lin, Sun Jun-Qiang, Wang Jian
Acta Physica Sinica. 2008, 57 (1): 0252 doi: 10.7498/aps.57.252
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Based on the cascaded second-order nonlinear interactions in a periodically poled lithium niobate waveguide, a novel scheme of all-optical data format conversion from non-return-to-zero to return-to-zero is proposed, using a Mach-Zehnder interferometer structure. The conversion mechanism relies on the amplification effect induced on the signal field via cascaded second-harmonic generation and difference-frequency generation, which makes the Mach-Zehnder interferometer unbalanced and leads to the output of return-to-zero due to destructive interference. Firstly, the non-return-to-zero to return-to-zero conversion process is numerically simulated based on the coupled-wave equations. Secondly, the dependence of the extinction ratio on the waveguide length, optical power, and time-delay is analyzed and optimized. Finally, the conversion bandwidth is also discussed. It is found that the signal wavelength can be tuned in a wide wavelength range of approximately 90nm, thus simultaneous multi-channel non-return-to-zero to return-to-zero data format conversion can potentially be implemented with the proposed scheme.

Attenuation characteristics of laser beam in water

Liu Juan, Bai Jian-Hui, Ni Kai, Jing Hong-Mei, He Xing-Dao, Liu Da-He
Acta Physica Sinica. 2008, 57 (1): 0260 doi: 10.7498/aps.57.260
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Through different sample lengths the attenuation coefficient of laser beam with different pulse energy was measured experimentally in water. And the effect of stimulated Brillouin scattering on the attenuation characteristics of water was studied. It was found that the attenuation coefficient is dependent on the pulse energy and the line width of the laser, but not a constant, as usually assumed to be. Theoretical analysis is also given.

Second-harmonic power generated in the absorbing and birefringent nonlinear medium

Yan Guo-Jun, Chen Guang-De, Wu Ye-Long, Yang Jian-Qing
Acta Physica Sinica. 2008, 57 (1): 0265 doi: 10.7498/aps.57.265
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Considering the absorption of the nonlinear medium to the second-harmonic (SH) wave and using the complete boundary conditions for the reflections of the fundamental and SH waves at the input and output faces of the nonlinear medium, a formula for calculating the second-harmonic power generated in the absorbing and birefringent nonlinear medium was derived from the Maxwell's equation that the SH field E2 satisfies. The result shows that the formula for calculating the second-harmonic power generated in the nonzero absorption medium can be used for calculating the second-harmonic power generated in the absorptive medium if the complex refractive index, instead of the ordinary refractive index is used.

Ultra-broadband optical parametric amplification based on pre-chirp controlling

Song Xiao-Zhong, Liu Hong-Jun, Wang Yi-Shan, Zhao Wei
Acta Physica Sinica. 2008, 57 (1): 0271 doi: 10.7498/aps.57.271
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For optical parametric amplification with broadband pump coming from the second harmonic of an amplified Ti:sapphire laser syetem, a novel ultra-broad bandwidth phase-matching approach is investigated. The results show that an approximately 400nm phase-matching bandwidth can be obtained by using a 10nm pump bandwidth. Moreover, the phase-matching bandwidth can reach 600nm when the pump bandwidth attains 20nm. In order to obtain temporal overlap of phase-matched spectral components, a linear chirp in the pump requires a quadratic chirp in signal. The photonic crystal fiber with a zero-dispersion wavelength of 800nm generates a supercontinuum with a quadratic chirp. The broadband pump pulse is stretched to a linear chirped pulse by a prism pairs. Calculations concerning the appropriate length of fiber and prism sequence to be inserted in optical path for the pre-chirp controlling were performed, the results of which provide a theoretical basis for implementing the ultra-broadband optical parametric amplification.

Study on intrinsic optical bistability in Tm-doped laser crystal pumped at 648nm avalanche wavelength

Li Li, Zhang Xin-Lu, Chen Li-Xue
Acta Physica Sinica. 2008, 57 (1): 0278 doi: 10.7498/aps.57.278
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Intrinsic optical bistability (IOB) in Tm-doped laser crystal pumped at 648nm avalanche wavelength is predicted theoretically and studied numerically. Based on the theory of nonlinear rate equations, the analytical formula of avalanche threshold condition is deduced in the steady-state approximation. Transient response of photon avalanche, IOB and influence of system parameters on IOB are studied numerically. The numerical simulation shows that IOB of 2μm fluorescence emission in Tm-doped laser crystal can be experimentally observed. Controllable bistable hysteresis loop can be achieved by adjusting experimentally adjustable parameters.

Interactions between strong nonlocal optical spatial solitons in nematic liquid crystals

Yang Ping-Bao, Cao Long-Gui, Hu Wei, Zhu Ye-Qing, Guo Qi, Yang Xiang-Bo
Acta Physica Sinica. 2008, 57 (1): 0285 doi: 10.7498/aps.57.285
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In this paper, we investigate the interactions between optical spatial solitons in nematic liquid crystals (NLC) with numerical simulations and experiments. The response function is exponential-decay function in NLC, which is different from the Snyder-Mitchell model. Through numerical simulations, we discovered that the crosspoint of soliton interaction is in inverse proportion to the square root of injection power in NLC . We also found that the position of crosspoint is independent of the distance between solitons in strong nonlocal Gauss function response, but it is not the same with exponential-decay function in NLC. Through experiments,we found the positions of crosspoint is in inverse proportion to the square root of inject power,it is also related to the critical power of solition formation and the distance between solitons.

A novel method for fabricating sampled fiber grating amplitude mask

Liu Yan, Li Bin, Lu Shao-Hua, Xu Ou, Ren Wen-Hua, Wang Yan-Hua, Feng Su-Chun, Jian Shui-Sheng
Acta Physica Sinica. 2008, 57 (1): 0291 doi: 10.7498/aps.57.291
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A novel method for fabricating sampled amplitude mask is presented, the featureof which is the varying sampling period and fixed sampling duty ratio. By usingsuch an amplitude mask, sampled fiber gratings with different wavelength spacings of reflectivity peaks can be easily fabricated. So the novel method has the advantages of reduced fabrication cost and improved versatility of sampled grating. The possible disadvantages of the method are also discussed. Wavelength selectable fiber laser with vernier wavelength tuning mechanism is also demonstrated by the sampled fiber grating.

Ultrafast internal conversion and vibrational relaxation in singlet excited-state all-trans-β-carotene as revealed by femtosecond time-resolved stimulated Raman spectroscopy

Zhao Xiao-Hui, Ma Fei, Wu Yi-Shi, Ai Xi-Cheng, Zhang Jian-Ping
Acta Physica Sinica. 2008, 57 (1): 0298 doi: 10.7498/aps.57.298
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A femtosecond time-resolved stimulated Raman scattering (FSRS) apparatus was built, and was used to study the ultrafast internal conversion and vibrational relaxation processes between the singlet excited states of all-trans-β-carotene in solution. The FSRS apparatus, constructed on the basis of a three-pulse pump-probe scheme, achieved a time resolution of 150fs, a spectral resolution of 23.7cm-1, and a detection spectral window of 300—4000cm-1. The FSRS results are interpreted in terms of a serial internal-conversion scheme between the low-lying singlet excited states, i.e. S2→SX→S1→S0. Spectral dynamics analysis and kinetics analysis proved that the intra-molecular vibrational relaxation in the S1 state proceeds on a time scale of 0.3—0.6ps, while the ‘hot’ S1 state decays with a time constant similar to the lifetime of this state (~11ps). Furthermore, the time scale of intra-molecular vibrational relaxation and vibrational cooling processes in the S0 state are shown to be ~0.3 and ~15ps, respectively. These results are useful for a deeper insight into the light-harvesting function of carotenoids in photosynthetic organisms.

The amplitude and phase measurements of femtosecond pulses using fringe-free SPIDER Hot!

Lei Liang, Wen Jin-Hui, Jiao Zhong-Xing, Lai Tian-Shu, Lin Wei-Zhu
Acta Physica Sinica. 2008, 57 (1): 0307 doi: 10.7498/aps.57.307
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A developed method of fringe free spectral phase interferometry reconstruction (FF-SPIDER) for femtosecond pulse measurement is demonstrated with theoretical simulation and experimental measurements. FF-SPIDER overcomes the disadvantages of the traditional SPIDER in which the time delay measured pulse replica results in fringes in the interference spectrum and complicates the spectral phase reconstruction. The measurement results show that FF-SPIDER reconstructs the amplitude and phase of the measured pulses with higher efficiency, higher accurate and lower requirements on measurement apparatus.

Nearfield acoustic holography based on wave superposition algorithm for cyclostationary sound filed

Zhang Hai-Bin, Jiang Wei-Kang, Wan Quan
Acta Physica Sinica. 2008, 57 (1): 0313 doi: 10.7498/aps.57.313
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The seriously modulated radiation sound waves, as a kind of special nonstationary signal in the engineering, can be considered as cyclostationary. The modulating feature can not be understood clearly via the reconstructed sound field by conventional nearfield acoustic holography (NAH). A cyclostationary NAH (CYNAH) technique based on wave superposition algorithm (WSA) is suggested for reconstructing the cyclostationary sound field, which can be applied to the cyclostationary acoustic sources with complicated profiles. The information of modulating wave can be understood by the reconstructed hologram of cyclic spectrum density (CSD), in which the distribution of modulating wave is visualized and the location of modulating wave can be determined. The simulation and the experiment verified that the modulating characteristic can be understood by CYNAH based on WSA. The main advantages of WSA, including the high calculation accuracy and calculation efficiency due to the exclusion of uniqueness and singularity problems inherent in the boundary element method, are retained in the presented CYNAH procedure.

Numerical simulation of radial segregation patterns of binary granular systems in a rotating horizontal drum

Zhao Yong-Zhi, Cheng Yi
Acta Physica Sinica. 2008, 57 (1): 0322 doi: 10.7498/aps.57.322
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A 3D three-equation linear spring-dashpot model considering the normal contact force, the tangential contact force, and the rolling friction torque, and the corresponding programs for granular dynamics were developed. The flow and the segregation of the S-type binary granular mixtures in a thin rotating horizontal drum were simulated by using the new model. The influences of the rotation speed and the load of the drum on the pattern formation were discussed. The simulation results show that the pattern formation of the S-type binary granular mixtures in the rotating drum can be predicted accurately. As the rotation speed is high, the moon pattern appears,in which the small particles concentrate in a central core and the large particles distribute in the periphery. As the rotation speed is lowered, the petal pattern is found in thin drums, and the number of petals increases as the rotation speed decreases. The process of pattern formations was obtained by the simulation. As the petal pattern forms, the wavy flow of granules in the drum appears. Finally, the necessary condition and mechanisms of the pattern formation in the thin drums are discussed in the paper.

Numerical simulation of nonlinear mechanical behaviour of single-walled carbon nanotubes

Yao Xiao-Hu, Han Qiang, Xin Hao
Acta Physica Sinica. 2008, 57 (1): 0329 doi: 10.7498/aps.57.329
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The mechanical behavior of single-walled carbon nanotubes (SCWNTs) is one of the basic properties of the nanotube composite and nano machinery. The axial and bending buckling behavior of SWCNTs is systematically investigated by finite element mothod(FEM) in this paper. The results of molecular dynamics and FEM simulations are compared. Simulation results show that the axial buckling loads are dependent strongly on the diameter, and the buckling and postbuckling behavior of SWCNTs under bending depends strongly on the length and radii of the tube. With appropriate choice of Young's modulus and tube thickness, the FEM simulation provides some useful insight into the buckling mechanism of carbon nanotubes. The finite element method turns out to be a new, forthright and precise method for the simulation in solving large-scale nano-mechanics problems.

The law of the underwater explosion bubble motion near free surface

Zhang A-Man, Yao Xiong-Liang
Acta Physica Sinica. 2008, 57 (1): 0339 doi: 10.7498/aps.57.339
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The numerical model of underwater explosion bubble coupling with free surface is created based on boundary integral method and the corresponding program is exploited to get the calculated result which presents satisfactory precision with experiment data. From the basic phenomenon of the interaction between bubble and free surface, the dynamic behavior of bubble near free surface is studied systematically employing the exploited program including the ring rebound of bubble near free surface and spike of free surface as well as the relation between distance parameter buoyancy parameter strength parameter and Bjerknes effect near free surface to summarize relative laws that the dynamic behavior near free surface has close relation with these characteristic parameters. In the process of study, scope of applicability of Blake rule on the basis of Kelvin_impulse theory is discussed and the reason for inactivation of Blake rule is interpreted to present references to research on dynamic behavior of underwater explosion bubble near free surface.

The construction and application of the many-body potential for the Fe-Cr-V-Ni-Si-C system multi-component alloy

Liu Yan-Xia, Wang Xun, Ma Yong-Qing, Zhang Cheng-Hua
Acta Physica Sinica. 2008, 57 (1): 0358 doi: 10.7498/aps.57.358
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The interatomic potential function for the Fe-Cr-V-Ni-Si-C system multi-component alloys was constructed by fitting to the data of this alloy system via quantum chemistry ab initio calculation , first-principlespseudopotentials calculation and least squares method. The stability of the austenite matrixs of Fe-Cr-V-Ni-Si-C system alloys was examined by the acquired interatomic potential functions. The effect of Ni element on the stability of the alloy matrix was studied. The results of calculation show that the stability of austenite matrix with nickel content in the range of 1.02—2.03wt% increases with the increase of nickel content. When the nickel content in the alloys increases to 2.88 wt%, the austenite matrix energy increases and the stability of the austenite matrix decreases accordingly. The austenite matrix energy of N5—N8 alloy is lower than the energy of its martensite phase. Therefore, the microstructure of N5—N8 alloy tends to be of austenite-type. Analysis by XRD indicates that the matrix of N5—N8 alloy is mainly the austenite phase, complemented by martensite. The calculation results agree with the results of XRD.

Investigation of interfacial structure and property of CoFe/TiZr multilayers by polarized neutron reflectometry

Huang Chao-Qiang, Chen Bo, Li Xin-Xi, V. G. Syromyatnikov, N. K. Pleshanov
Acta Physica Sinica. 2008, 57 (1): 0364 doi: 10.7498/aps.57.364
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Polarized neutron reflectivity has been measured for the CoFe/TiZr alloy multilayers. The results show that: (a) The reflectivity behavior of the sample with un-equal-thickness bilayer (type B) is better than that of the sample with equal-thickness bilayer (type A). The former performs better in producing neutron guide. (b) The best annealing temperature is about 250℃ for the sample of type A, however, the structures of both type A and type B samples change apparently after 350℃ annealing. (c) Different from the type B sample, the first Bragg peak position for the type A sample moves along Q axis, indicating that the period of the multilayer varies in a way of increase-decrease-increase with the increase of annealing temperature.

The light source, optical waveguide and light enhancement of nano-integrated optical circuit Hot!

Liu Dan, Ma Ren-Min, Wang Fei-Fei, Zhang Zeng-Xing, Zhang Zhen-Sheng, Zhang Xue-Jin, Wang Xiao, Bai Yong-Qiang, Zhu Xing, Dai Lun, Zhang Bei
Acta Physica Sinica. 2008, 57 (1): 0371 doi: 10.7498/aps.57.371
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Scanning near-field microscopy (SNOM) was used to investigate the low-dimensional semiconductor fluorescence divices, which can work as micro-nano-light source, nano-optical waveguide and light enhancement component in nano-integrated optical circuit. It was found that the quality of semiconductor wire end emission was mainly determined by the sample radius. By designing the wire radius and topography reasonably, it can be used as the light source and optical waveguide in nano-integrated optical circuit. ZnO tetrapod, comb, and CdS multi-branch nano-ribbon also presented quite good adequacy for optical waveguide application, which can function well in light division, congregation, coupling and filtering. 2-D photonic crystal can enhance the light intensity of GaN based LED by as large as 5.2 times. About 30% fluorescence is confined to the surface of photonic crystal. This conclusion is helpful for the optimization of LED light emission. When the resonance excitation condition is fulfilled, Surface plasmm polaritons (SPP) will be excited at the interface between the semiconductor and the silver thin film, which will bring about strong enhancement of electromagnetic field at the interface. Photonic crystal and SPP can realize light enhancement under low_dimensional and nano-scale conditions. The experimental results show that both of them have the potential application as enhancement components in nano-integrated optical circuit.

Formation and structure transition of gold nanoparticles on surface of carbon nanotube

Li Jun, Zhang Kai-Wang, Meng Li-Jun, Liu Wen-Liang, Zhong Jian-Xin
Acta Physica Sinica. 2008, 57 (1): 0382 doi: 10.7498/aps.57.382
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Formation and structure of gold nanoparticles at room temperature on the surface of carbon nanotube have been studied by molecular dynamics simulation.It was shown that,different from isolated gold nanoparticles,a gold nanoparticle AuN sitting on the surface of carbon nanotube undergoes a structural transition.Small gold nanoparticles with N≤130 have disordered structures while larger particles with N≥140 have the face centered cubic structure of bulk crystalline gold.The binding energy per atom of AuN decreases as its size decreases.It was also shown that the strength of the interaction between a small gold nanoparticle and the carbon nanotube is proportional to their contact surface area,indicating a close contact between the two structures.

A quasi-dynamics method for searching for cluster isomers

Zhou Shi-Yun, Wang Yin, Ning Xi-Jing
Acta Physica Sinica. 2008, 57 (1): 0387 doi: 10.7498/aps.57.387
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A quasi-dynamics method is developed to search for cluster isomers and can quickly produce isomers which are probable in gas-phase growth under common experimental conditions. We studied the spectrum of C21 isomers by the method and performed molecular dynamics simulations on the clustering processes of 21 isolated carbon atoms in helium buffer, showing that the isomers formed in the dynamical process at higher probabilities are all contained in the isomer spectrum obtained by the method. The potential energy of the most stable configuration in our spectrum is much lower than that obtained by the genetic algorithm [Chem. Phys. Lett. 364 213,2002].

Stacking fault tetrahedra in single-crystal aluminum induced by high-current pulsed electron beam

Guan Qing-Feng, Chen Bo, Zhang Qing-Yu, Dong Chuang, Zou Guang-Tian
Acta Physica Sinica. 2008, 57 (1): 0392 doi: 10.7498/aps.57.392
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The specimens of single-crystal aluminum were irradiated with high current pulsed electron beam (HCPEB). The vacancy cluster defect microstructure has been investigated by using transmission electron microscopy (TEM). The results show that three types of vacancy clusters including dislocation loop, void and even stacking fault tetrahedron (SFT), which are not normally formed in metals with high SFT formation energy, can be formed in single-crystal aluminum specimens irradiated with HCPEB. The nucleation process of three types of vacancy clusters do not appear at the same time. There is a close relationship between the three types of vacancy clusters. Based on the experimental results, a possible mechanism of SFT formation and evolution was presented.

Ab initio structural simulation and electronic structure of amorphous carbon

Gao Wei, Zhu Jia-Qi, Niu Li, Han Jie-Cai, Dai Bao-Chang
Acta Physica Sinica. 2008, 57 (1): 0398 doi: 10.7498/aps.57.398
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Within the framework of density-functional theory, a new and more accurate exchange correlation functional OLYP(OPTX+LYP)was used to simulate the structures of amorphous carbon at densities from 2.0 to 3.2 g/cm3. The radial distribution function and sp3 fraction of the five simulated carbon networks are in good agreement with experimental results. The calculated electronic structures indicate that the states near Fermi level are mainly contributed by sp2 carbon atoms. The states near Fermi level have a decreasing trend with increasing density and increasing sp3 content. The small ring structure increases the density of states near Fermi level. The defects will give a peak at Fermi level, which strongly reduces the band gap.

Measurements of dynamic yield strength of aluminum alloy and mechanism analysis of elastic precursor during reloading

Hu Jian-Bo, Tan Hua, Yu Yu-Ying, Dai Cheng-Da, Ran Xian-Wen
Acta Physica Sinica. 2008, 57 (1): 0405 doi: 10.7498/aps.57.405
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Dynamic yield strength is one of the key variables in construction of the constitutive relationship for solids under shock loading. This paper reports the yield strength of LY12 aluminum alloy which was measured experimentally on the basis of the method proposed by J. R. Asay and L. C. Chhibildas, in which the test sample underwent a load-reload process and a load-unload process, and the particle velocity history at the LY12 Al sample/LiF transparent window interface was observed by velocity interferometer system for any reflecting surface (VISAR). Improvement in fabrication of the layered flyer plate made these experiments successful. The measured yield strength data at shock stresses of 22GPa and 34GPa are consistent with the reported data at relatively lower pressures. Moreover, physical mechanism of the elastic precursor emerging in the reloading-process was discussed briefly. It is concluded that dislocations is the principal factor resulting in the elastic precursor.

Experimental studies of spallation in nanocrystalline copper film by laser irradiation

Wang Yong-Gang, He Hong-Liang, Boustie Michel, Sekine Toshimori
Acta Physica Sinica. 2008, 57 (1): 0411 doi: 10.7498/aps.57.411
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The spall behavior of nanocrystalline copper film synthesized by electrodeposition technique is investigated by laser irradiation. The push-pull type VISAR with sub-nanosecond time resolution is used to measure the rear free surface velocity profile of the sample. The spall strength in nanocrystalline copper film is calculated from the measured free surface velocity profile as a function of tensile strain rate. Results show that the dynamic tensile strength of nanocrystalline copper film is about 3 GPa, which is much higher than that of polycrystalline bulk copper, but lower than that of single crystal copper. This difference may be attributed to the effective obstacles to dislocation motion by the presence or more grain boundaries in nanocrystalline copper.

Influences of temperature and polaron effect on the ground state of quasi-two-dimensional strong-coupling exciton

Acta Physica Sinica. 2008, 57 (1): 0416 doi: 10.7498/aps.57.416
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The influences of temperature and polaron effect on the ground state of the system,for which the exciton is strongly coupled with interface-optical (IO) phonons but weakly coupled with bulk-longitudinal-optical (LO) phonons in a quantum well, are studied by using the Huybrechts' linear-combination operator and Lee-Low-Pines (LLP) transformation method. The expressions for the induced potential and energy shift of the ground state of the exciton were derived. Numerical calculations for AgCl/AgBr QW, as an example, are performed. The result indicates that the induced potential and the energy shift increases for strong exciton-IO-phonon coupling but decreases for weak exciton-LO-phonon coupling with temperature.

A study based on diffusion theory for exchange-reaction of RLA model

Zheng Xiao-Ping, Zhang Pei-Feng, Fan Duo-Wang
Acta Physica Sinica. 2008, 57 (1): 0425 doi: 10.7498/aps.57.425
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In the research work, on the basis of the classic diffusion theory we have advanced a novel model of the reaction limited aggregation (RLA), which involues the basic micro-processes. We firstly introduced the concept of exchange rate and have also simulated the growing process of surfactant-mediated epitaxial thin film growth with the kinetic Monte Carlo (KMC) method. By simulating, it was found that both surfactant atoms and adatoms would diffuse from one layer to another. This kind of diffusion mainly happens beween single atoms and the number of this diffusion increases with increasing growth temperature or increasing deposition depth. The exchange reaction of RLA model is the combination of several basic micro-processes. The majority of exchanges are not complete position exchange and the exchange ratio is not always equal to unity.

Research on spectral-properties of nonpolar ZnO film on γ-LiAlO2

Zhou Jian-Hua, Zhou Sheng-Ming, Huang Tao-Hua, Lin Hui, Li Shu-Zhi, Zou Jun, Wang Jun, Han Ping, Zhang Rong
Acta Physica Sinica. 2008, 57 (1): 0430 doi: 10.7498/aps.57.430
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Nonpolar a-plane ZnO film was successfully grown on (302)γ-LiAlO2 substrate by pulsed laser deposition. The temperature of substrate was 500℃, the film was pure nonpolar a-plane ZnO. The FWHM of the peak of near band emission in the PL spectrum was found to be only 115 meV. Both polarized transmission spectra and polarized Raman spectra revealed the in-plane anisotropy of the nonpolar ZnO film by the shift of absorption edge and the change of intensity of Raman E2 mode.

Theoretical study of ignition-proof mechanism of RE element in Mg alloys

Liu Gui-Li
Acta Physica Sinica. 2008, 57 (1): 0434 doi: 10.7498/aps.57.434
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The atomic cluster models of α-Mg, liquid Mg and the interface between liquid/solid have been founded by computer program. The environment-sensitive embedding energy of RE elements in α-Mg, liquid Mg, liquid/solid interface has been calculated by recursion method. The atomic affinity energy between Mg, La, Y with O has been defined and calculated. The calculated results show that the solid solubility of La and Y is very small in α-Mg, because their higher environment-sensitive embedding energy leads to instability in α-Mg crystal. The RE elements diffuse to the liquid Mg, which has lower environment-sensitive embedding energy than the solid, and congregate on the surface of liquid Mg as the alloy solidifies. Because the atomic affinity energy of RE-O is lower than Mg-O(The atomic affinity energy between Mg, La, Y with O are Mg-O:-14.9338eV,La-O:-19.0608 eV,Y-O:-19.5050 eV, respectively), the RE elements congregating on the surface of liquid Mg will priorily combined with O, forming compact RE oxides, which prevent Mg alloys from burning.

Effects of the concentration of oxygen vacancy of anatase on electric conducting performance studied by frist principles calculations

Hou Qing-Yu, Zhang Yue, Chen Yue, Shang Jia-Xiang, Gu Jing-Hua
Acta Physica Sinica. 2008, 57 (1): 0438 doi: 10.7498/aps.57.438
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Based on the density of stales calculation of the concentration of oxygen vacancy in anatase semi-conductor, the model of anatase with different sizes and proper concentrations of oxygen vacancy were studied under the same condition.It was found that the anatase with relatively low oxygen vacancy shows better electric conducting performance by comparison of the relative average number of electrons in the conduction band , mobility and conductivity. So we arive at the conclusion that the lower the concentration of oxygen the better anatase can be prepared with heavy doping, and under the conditions of low tempeature and heavy oxygen vacancy, the conductivity of anatase semi-conductor is closely related to the concentration of oxygen vacancies, average number of electrons in the conduction band and the conductivity caused by scattering of oxygen vacancies.

Effect of impurity P on electronic structure of kink in the edge dislocation of α-iron

Chen Li-Qun, Yu Tao, Wang Chong-Yu, Qiu Zheng-Chen
Acta Physica Sinica. 2008, 57 (1): 0443 doi: 10.7498/aps.57.443
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Using the first-principles self-consistent discrete variational method based on density functional theory, we have investigated the effect of impurity P on the electronic structure of the kink on the [100](010) edge dislocations in bcc iron. Furthermore,we have calculated the segregation energy, the interatomic energy, the local density of states and the charge density. Our results show that the P atom proeures electrons from its neighboring Fe atoms when trace P is introduced into interstitial sites in the kink. It is found that the interactions between impurity P atom and neighboring Fe atoms are strengthened due to the strong hybridizations between P-p with Fe-3d4s4p states. The migration of kink and dislocation motion are impeded by P, which may be beneficial to increase the strength of material. Meanwhile, the bonding between the impurity P and the host Fe atoms are mainly ascribed to contribution of p and d states, which leads to the bonding between them to have strong orientation. This is not beneficial to the toughness of materials.

Investigation on the quantum coherent transport in magnetic tunnel junctions

Yang Jun, Wu Wen-Yuan, Gong Yan-Chun
Acta Physica Sinica. 2008, 57 (1): 0448 doi: 10.7498/aps.57.448
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Using the method of scattering matrix, we investigated the quantum coherent transport in ferromagnet/insulating barrier/semiconductor/insulating barrier/ferromagnet (FM/I/SM/I/FM) magnetic double tunnel junctions. When the mean free path of the tunneling electrons(lp) is comparable to the thickness of the middle semiconductor(L), the tunneling magneto-resistance(TMR) shows quantum oscillations with the variation of the L. At the inflection point of the oscillations the cut-off vector will appear if lp is much longer than L. The analysis reveals that it is the mismatch of the electron momentum in ferromagnet and semiconductor layer that leads to the appearance of the cut-off vector. The decay of oscillation amplitude caused by phase loss restrains the occurrence of the cut-off vector. We arrive at the conclusion that the value of TMR is the square of the value of spin injection efficiency(SIE) η1 in a single junction; and the value of TMR is also the square of the value of SIE η2 in double junctions(parallel configuration)if L is much longer than lp. For this reason, the value of SIE in a single junction or double junctions can be obtained by measuring the value of TMR of double junctions. Numerical calculations indicate that the ratio TMR/η22 will also show quantum oscillations with the variation of the L if the L is comparable to the lp.

Abnormal transport behavior in the mult-spin Co-doped La2/3Ca1/3MnO3 manganites

Yu Qian-Ying, Zhang Jin-Cang, Jia Rong-Rong, Jing Chao, Cao Shi-Xun
Acta Physica Sinica. 2008, 57 (1): 0453 doi: 10.7498/aps.57.453
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The spin structure and polarization characters are the dominant aspects in the physical properties of manganite systems with colossal magnetoresistance effect. In this paper, a series of Mn-site doping samples La2/3Ca1/3Mn1-xCoxO3 (0≤x≤0.15) have been prepared with Co ion which has mult-spin states. The structure and transport properties of this system have been systematically investigated. The results showed that all the samples show good single phase and the lattice parameters decrease with the increasing of doping concentration x. The abnormal transport properties are induced by Co doping. To be specific, the resistivity exhibits two metal_insulator (M—I) transitions (double peak effect). Both high temperature peak (TMIH) and low temperature peaks (TMIL) shift to low temperature end with increasing x. While the TMIH shifts to high temperature, the TMIL is almost unchanged under the external field. The corresponding peak values of resistivity show strong dependence on the external field and Co doping concentration x. All the peak values of resistivity increase with the increase of x, and the low temperature peak value seems more sensitive than the high temperature peak. Compared with the magnetization properties, it could be concluded that, the high temperature peak is related with the undoped M-I transition, and the abnormal behavior of low temperature peak has certain relationship with the oxygen vacancy and high spin state induced by Co3+ substituting Mn4+ in the present system.

Small-signal theoretical analysis and design of Kα band dielectric loaded gyro-TWT

Yan Ran, Lou Yong, Li Jia-Ying, Pu You-Lei, Wang Jian-Xun, Lei Chao-Jun, Liu Ying-Hui
Acta Physica Sinica. 2008, 57 (1): 0460 doi: 10.7498/aps.57.460
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Using the small-signal theory of the gyro-TWT, a pinch-point analysis based on the theory of Briggs and Bers in combination with the cold field analysis of the dielectric-loaded waveguide is employed to analysis the threshold current of the operation modes and critical oscillation lengths of the spurious mode under different load condition. The analysis concludes that the improvement in the dielectric loss, through altering the load condition can enhance the threshold current and critical oscillation length. The cold field analysis of the dielectric-loaded waveguide and a smooth waveguide dispersion relation is used to analyse the small signal gain of the dielectric-loaded gyro-TWT. The analysis gives the gain-frequency response of a dielectric-loaded gyro-TWT with the normalized background magnetic flux density B0/Bg, the thickness of the dielectric and beam current as parameters. Through the analysis of the self-oscillation and the small signal gain, the optimum operating parameters of a Kα-band dielectric-loaded gyro-TWT can be got to support the nonlinear analysis.

Current collapse mechanism of field-plated AlGaN/GaN HEMTs

Wei Wei, Lin Ruo-Bing, Feng Qian, Hao Yue
Acta Physica Sinica. 2008, 57 (1): 0467 doi: 10.7498/aps.57.467
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Current collapse restrain ability of passivated AlGaN/GaN HEMTs and AlGaN/GaN HEMTs with varying field-plate(FP) length is investigated under different drain bias. The results show that, the passivated HEMTs suffer no current collapse at relatively low but not at higher drain bias, while the HEMTs with optimal field-plate suffer no current collapse for all drain bias used in our tests. Under high drain bias, the FP length plays a crucial role in the current collapse removal. After a thoroughly analysis, it can be concluded that FP structure not only restrains the trapping of virtual gate, but also discharge the virtual gate. Finally, a discharging model of dielectric under FP is presented to explain the effect of FP length on current collapse removal.

Research on effects of current spreading and optimized contact scheme for high-power GaN-based light-emitting diodes

Shen Guang-Di, Zhang Jian-Ming, Zou De-Shu, Xu Chen, Gu Xiao-Ling
Acta Physica Sinica. 2008, 57 (1): 0472 doi: 10.7498/aps.57.472
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GaN-based light-emitting diodes (LEDs) grown on sapphire substrate have current transport along the lateral direction due to the insulating nature of the substrate, and the anode and cathode contacts are in a side-by-side configuration. The resistance of the n-type material of the GaN and the lower confinement layer is not negligible, which causes the current to crowd near the edge of the n-contact pad. The current crowding problems will become more severe for large area and high power devices. In this paper, the current spreading effect is qualitatively analyzed. The applied current density and the effective length of the lateral current transport are found to have a considerable effect on the uniform current spreading. Based on these findings, an optimized contact scheme of high-power GaN-based LEDs is proposed to alleviate current crowding effect and reduce the series resistance of the devices. It is clearly shown that the high-power GaN-based flip-chip LEDs with optimized ring-shape interdigitated contact scheme have a relatively uniform temperature distribution by measuring the surface temperature distribution of the device with infrared thermal imaging system. It is confirmed that the current distributes more uniformly over the contact, and current densities in a localized region of the device are reduced, which can decrease the joule heat generated and improve the reliability of the GaN-based LED.

The power dissipation of equivalent series resistance and its influence on lumen efficiency of GaN based high power light-emitting diodes

Li Bing-Qian, Liu Yu-Hua, Feng Yu-Chun
Acta Physica Sinica. 2008, 57 (1): 0477 doi: 10.7498/aps.57.477
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Because of the self-heating effect of InGaN based high power light-emitting diodes (LEDs), the temperature of the LED chip is much higher than the case temperature (TC). The maximum Tj in our experiment is 148K higher above the case temperature. The voltage drops on p-n junction and equivalent series resistance are extracted from the measured I-V curve, and then the values of equivalent series resistance at different forward power are obtained. Due to the self-heating effect, the values of equivalent series resistance show a very strong forward power-dependent characteristics, the maximum equivalent series resistance reported in this paper is about 1.9Ω at high forward power, being more than twice the minimum value. The power dissipation of equivalent series resistance is increased sharply when the forward power is increased, nearly half of the forward electrical power was dissipated on equivalent series resistance at high power, and it becomes the an important factor restraining the lumen efficiency of high power GaN based light emitting diodes.

Electronic structure and optical properties of fluorite-structure TiO2

Guan Li, Liu Bao-Ting, Li Xu, Zhao Qing-Xun, Wang Ying-Long, Guo Jian-Xin, Wang Shu-Biao
Acta Physica Sinica. 2008, 57 (1): 0482 doi: 10.7498/aps.57.482
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We calculated the band structure and density of states of fluorite type TiO2 using density functional theory. The results show that fluorite type TiO2 is an indirect-band-gap semiconductor, and its indirect band gap Eg=2.07eV is smaller than that of rutile or anatase TiO2. To make optical properties clear, we calculated the complex dielectric constant of fluorite-structured TiO2 and compared it with the rutile TiO2. The results illustrate that the fluorite TiO2 has a large static dielectric constant of 8.31, and ε1xy(0)=6.01 and ε1z(0)=7.07 for rutile TiO2 show excellent agreement with experimental data. Also, two new absorption peaks located at 51nm and 153nm respectively were observed and the absorption range of fluorite TiO2 extends to the visible region.

A density-functional theory study on the chemisorption and STM images of Ag(100)/O surface

Chen Wen-Bin, Tao Xiang-Ming, Chen Xin, Tan Ming-Qiu
Acta Physica Sinica. 2008, 57 (1): 0488 doi: 10.7498/aps.57.488
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In this article we have investigated the influence of oxygen adsorption on the surface geometry and electronic properties of Ag(100) surface by using the density-functional theory calculations. The total energy calculations based on projector-augmented wave (PAW) method have been preformed to describe the adsorption geometry at several coverages of oxygen adsorption including p(1×1), c(2×2), and (21/2×221/2)R45°, and a series of main physical properties at these coverages on Ag(100) surface such as the surface relaxation, adsorption energy, work function and so on. The results presented in this work show that for an Ag(100) (21/2×221/2)R45°-2O geometry, the most stable atomic structure is a type of missing-row reconstruction. Eventually, this structural change causes various displacements of surface atoms which have been calculated in this work. The calculations on the local density of states reveal that in the Ag(100) (21/2×221/2)R45°-2O geometry the cohesive effect between the adsorbed oxygen atoms and the substrate Ag layer is essentially due to the sufficient Ag4d-O2p orbital hybridization. Finally we have simulated the STM images for several bias-voltages and tip heights, providing experiments with abundant data and theoretical support.

Study on the influence of quantum state for phonon bath on the quantum behavior of mesoscopic circuit

Hu Ju-Ju, Cai Shi-Hua, Wang Jian-Qiu, Ji Ying-Hua
Acta Physica Sinica. 2008, 57 (1): 0496 doi: 10.7498/aps.57.496
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Taking into account the interaction between electrons and phonons, we study the dynamical behavior of a mesoscopic dissipative circuit for two classes of pure initial states (grand canonical ensemble and number state) of phonon bath modes. In the long time limit t→∞: when the environment is initially at thermal equilibrium, the average values of current and charge in the circuit system will only depend on the average values of the circuit in initial quantum state, and they are not related with the environment; when the environment is initially in number state, the time evolution of the average values of current and charge are just the same as that in thermal equilibrium, which indicates that the average values of current and charge are related to certain set of occupied numbers. The quantum fluctuations of current and charge in the circuit not only depend on the initial state of the system but also depend on the quantum state and temperature of the environment. Generally, the entanglement between the circuit system and environment will lead to the reduction of the purity of the quantum state. The mesoscopic circuit will remain in a pure quantum state only when the environment is at very low temperature and at thermal equilibrium.

The study on the rectifying properties of oxygen non-stoichiometric La0.9Ba0.1MnO3-δ/SrTiO3:Nb p-n heterojunction

Wang Chong, Liu Zhao-Lin, Liu Jun-Ming, Chen Xue-Mei, Cui Hao-Yang, Xia Chang-Sheng, Yang Yu, Lu Wei
Acta Physica Sinica. 2008, 57 (1): 0502 doi: 10.7498/aps.57.502
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The La0.9Ba0.1MnO3-δ/SrTiO3:Nb p-n heterojunctions have been fabricated by growing the oxygen nonstoichiometric La0.9Ba0.1MnO3-δ thin film on the Nb doped SrTiO3 substrate with pulsed laser deposition. The good rectifying curves have been demonstrated in the temperature range of 20—300K. The diffusion voltage VD exhibits a maximum near the transition temperature of La0.9Ba0.1MnO3-δ thin film, which is different to the observation in the oxygen stoichiometric La0.9Ba0.1MnO3/SrTiO3:Nb p-n heterojunction. The interesting phenomenon has been well explained with the resistance-temperature experiment of La0.9Ba0.1MnO3-δ thin film and energy band calculations.

Analysis of diluted magnetic semiconductor GaMnN grown by electron cyclotron resonance-plasma enhanced metal organic chemical vapor deposition

Wang Ye-An, Qin Fu-Wen, Wu Dong-Jiang, Wu Ai-Min, Xu Yin, Gu Biao
Acta Physica Sinica. 2008, 57 (1): 0508 doi: 10.7498/aps.57.508
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Diluted magnetic semiconductor film GaMnN was grown on sapphire (α-Al2O3) substrate using biscyclopentyldienyl manganese (Cp2Mn), N2 and TEGa by electron cyclotron resonance-plasma enhanced metal organic chemical vapor deposition(ECR-PEMOCVD) . The crystal structure and surface topography of the GaMnN films were characterized by RHEED, XRD and AFM. GaMnN films exhibit good (0002) preferred orientation, showing the films are inclined to c-axis growth and retain good wurtzite structure. The surface topography of GaMnN film is composed of many submicron grains piled in the consistent orientation. The magnetism of films is characterized by SQUID. SQUID shows that the film is ferromagnetic, which comes probably only from the ternary phase GaMnN and the Curie temperature of GaMnN film is higher than 350 K. Moreover, higher Mn concentration can enhance the Curie temperature of the film.

“Black silicon" antireflection thin film prepared by electrochemical etching

Liu Guang-You, Tan Xing-Wen, Yao Jin-Cai, Wang Zhen, Xiong Zu-Hong
Acta Physica Sinica. 2008, 57 (1): 0514 doi: 10.7498/aps.57.514
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Solar cells and optical detection devices are often covered with antireflective surfaces to enhance their performance. An economical, fast, and easily operational electrochemical etching technique has been developed to realize a continuous uniform variation in the refractive index of porous silicon layer as a function of the etching depth. By using this technique a black silicon sample was fabricated, which has a reflectance below 5% over a broad band and a thickness below 1μm. The depression mechanism of the optical reflectance is analyzed by simulating the structure with the transfer matrix method. The simulated results give a good agreement with the experimental measurements.

Effects of annealing of r-cut sapphire substrate on its surface morphology and the growth of CeO2 buffer layers and the Tl-2212 superconducting films

Xie Qing-Lian, Yan Shao-Lin, Zhao Xin-Jie, Fang Lan, Ji Lu, Zhang Yu-Ting, You Shi-Tou, Li Jia-Lei, Zhang Xu, Zhou Tie-Ge, Zuo Tao, Yue Hong-Wei
Acta Physica Sinica. 2008, 57 (1): 0519 doi: 10.7498/aps.57.519
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In this study, the surface morphology evolution and the change of the phase structure of r-cut sapphire substrates annealed at different temperatures for different time in O2, and the effects of annealing conditions on the growth of CeO2 and Tl-2212 films, were investigated by AFM and XRD. The results of AFM show that the local steps on the substrate annealed at 1000℃ is formed firstly, and then the multilayer terrace-and-step structure, yielding from prolonging annealing time, evolves into wide terrace-and-step structure with ultrasmooth terrace through the coalition of initial localized steps, which slightly tilts to the surface. XRD measurements show that the CeO2 films prepared on r-cut sapphire annealed at the optimized conditions and the 500 nm thick Tl-2212 films grown on the CeO2 buffer layers subsequently possess excellent in-plane and out-of plane orientation, and the annealing temperature and annealing time have strong effect on the crystalline quality of substrates and CeO2 films. The Tl-2212 films have a high transition temperature (Tc=104.7 K), a high critical current density (Jc=3.5 MA/cm2 at 77.3K and zero applied magnetic field) and a low surface resistance (Rs=390μΩ at 10GHz and 77K).

Role of Dy addition in rapid-quenched NdFeB permanent magnets

Zhang Ran, Liu Ying, Gao Sheng-Ji, Xie Zhi, Tu Ming-Jing
Acta Physica Sinica. 2008, 57 (1): 0526 doi: 10.7498/aps.57.526
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Following the previous study on the effect of Nb addition in rapid-quenched (Nd,Dy)11.5Fe82.4-mNbmB6.1 permanent magnets on the magnetic properties, temperature characteristics and microstructure, the role of Dy addition in rapid-quenched Nd11.5-nDynFe81.4Nb1B6.1 (n=0,0.5,1,1.5,2) permanent magnets has been investigated. It was found that Hci increases with increasing Dy content and the irreversible loss of magnetic flux decreases. Dy addition results in smaller grains and more homogeneous and regular grain structure. It is considered that Dy addition may enhance exchange coupling pining field and reduce the internal stray magnetic field. As a result, the irreversible loss of magnetic flux decreases notably and the magnetic properties at higher temperature are improved. Dy atoms on entering the main lattice of Nd9.5Dy2Fe81.4Nb1B6.1 alloy cause crystal lattice distorsion and affect the local structure of Fe atoms; however, Dy atoms have little influence of the local structure of Nd atoms.

Quantum phase transition as a result of magnetic frustrations in a ferrimagnetic Heisenberg system

Zhang Song-Jun, Jiang Jian-Jun, Liu Yong-Jun
Acta Physica Sinica. 2008, 57 (1): 0531 doi: 10.7498/aps.57.531
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Using density-matrix renormalization group (DMRG) method, we studied the effect of magnetic frustrations due to next-nearest- neighbor bonds in a system of quasi-one-dimensional Heisenberg antiferromagnetic chain but with ferrimagnetic properties. In this paper, the ground state energy per unit cell, spin correlation function and spin gap are calculated. As a result of the frustrations, the quantum disordered phase with the spin gap appears at the spin order-disorder phase transition point αC≈0.412 to α=1. And in the region of magnetic-order phase, the increasing magnetic frustrations reduce the magnetic order.

Effects of eddy current on surface retarded modes of a lateral antiferromagnetic/nonmagnetic superlattice

Lei Wei-Ping, Jia Cheng, Wang Xuan-Zhang
Acta Physica Sinica. 2008, 57 (1): 0535 doi: 10.7498/aps.57.535
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Using the equivalent medium theory, we have investigate the surface retarded modes of a semi-infinite lateral superlattices composed of semiconductor antiferromagnetic layers and insulating nonmagnetic layers in the Voigt geometry. We studied the effect of eddy current on surface retarded modes of a lateral antiferromagnetic/nonmagnetic superlattice when the external field is zero or nonzero. Particularly, we have considered the effect of conduction current and displacement current on the retarded modes, and have obtained for the first time the curves for different cases of the frequency and the spin wave damping versus the wave vector k.

Magnetic and electrical properties of layered perovskite La1.3Sr1.7Mn1-xCuxO7

Han Li-An, Chen Chang-Le, Dong Hui-Ying, Wang Jian-Yuan, Gao Guo-Mian, Luo Bing-Cheng
Acta Physica Sinica. 2008, 57 (1): 0541 doi: 10.7498/aps.57.541
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Magnetic and electrical properties of polycrystalline La1.3Sr1.7Mn2-xCuxO7 prepared by solid state reaction were investigated. The X-ray diffraction pattern shows that the two samples have Sr3Ti2O7 type perovskite structure. The two samples undergo complex transition with lowering temperature. The x=0 samples, have the two-dimensional short-range ferromagnetic order at T*=231 K and then transform to the three-dimensional long-rangs ferromagnetic state at TC=114 K, at last they enter the canted antiferromagnetic state at TN=56K. The T*, TC and TN are all reduced at the 5% Cu doping level. Additionally, the two samples exhibited two distinct resistivity peaks, which is due to coexistence of the two phases. The metal-insulator transition temperature was decreased but the magnetoresistance was enhanced at the 5% Cu doping level.

Comparison of magnetic properties of Fe/Co multilayers prepared by DC magnetron sputtering on glass and Si substrates

Xia Ai-Lin, Han Bao-Shan
Acta Physica Sinica. 2008, 57 (1): 0545 doi: 10.7498/aps.57.545
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We presented a comparison of magnetic properties of [Cot/Fe3t5 (t=2, 3, 4, 5nm) multilayers prepared by DC magnetron sputtering on glass and Si substrates. For the multilayers with the same t but prepared on different substrates, it was found that the saturated magnetization differed little, but the coercivity differed greatly. Moreover, the difference in coercivity mechanism was analyzed theoretically to explain this difference.

Theoretical computation and numerical simulation of the relaxation of sphere-capillary model saturated with oil and water

Xu Feng, Liu Tang-Yan, Huang Yong-Ren
Acta Physica Sinica. 2008, 57 (1): 0550 doi: 10.7498/aps.57.550
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According to Brownstein-Tarr theory and using the series expansion method, the Bloch equations controlled by diffusion effect are solved for double-phase pore space of sphere-capillary model saturated with oil and water, and the theoretical relaxation computational formulas for the sphere-capillary model are obtained. The result shows that the relaxation process of oil is only related to oil saturation and isn't associated with pore structure on the condition of wetness. A great deal of computation simulations were carried out based on the theoretical computational formulas, and the numerical simulation results show that the main relaxation process of sphere or capillary is a single exponential function and the other relaxation processes are negligible. In other words, the relaxation of sphere-capillary model is approximatively a double exponential descending process.

Experimental research on the degree of clustering in Bi3+-Ga3+ co-doped high concentration Er3+-doped silica-based fiber

Chang De-Yuan, Zheng Kai, Wei Yan, Li Bin, Fu Yong-Jun, Wei Huai, Jian Shui-Sheng
Acta Physica Sinica. 2008, 57 (1): 0556 doi: 10.7498/aps.57.556
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In order to obtain high-concentration doped erbium ions in silica-based fiber, we manufactured Er3+-Bi3+-Ga3+ co-doped fiber using MCVD and solution doping technique. The concentration of Er3+ in the fiber was about 5.24×1025/m3. We described a new method to measure the degree of clustering in the Er3+-Bi3+-Ga3+ co-doped fiber using fiber transmission experiment. Compared with other methods, the new method is simpler and more accurate. By the new method, the degree of clustering in Er3+-Bi3+-Ga3+ co-doped fiber was determined. It was shown that co-doped Bi3+-Ga3+ strongly increased the Er3+ solubility in the silica-based fiber.

Growth kinetics and microstructure characterization of oxide film formed on La-implanted Co-Cr alloy

Jin Hui-Ming, Felix Adriana, Aroyave Majorri
Acta Physica Sinica. 2008, 57 (1): 0561 doi: 10.7498/aps.57.561
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The isothermal and cyclic oxidizing kinetics of Co-40Cr alloy and its lanthanum ion-implanted samples were studied at 1000℃ in air by thermal-gravimetric analysis (TGA). Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) were used to examine the morphology and structure of the oxide film after oxidation. Secondary ion mass spectrum (SIMS) method was used to examine the binding energy change of chromium caused by La-doping and its influence on formation of Cr2O3 film. Laser Raman spectrum was used to examine the stress changes within oxide films. It was found that lanthanum implantation remarkably reduces the isothermal oxidizing rate of Co-40Cr and improves the anti-cracking and anti-spalling properties of Cr2O3 oxide film. The reasons for the improvement were mainly that the implanted lanthanum reduces the grain size and internal stress of Cr2O3 oxide and increases the high temperature plasticity of oxide film. Lanthanum mainly exists in the outer surface of Cr2O3 oxide film in the forms of fine La2O3 and LaCrO3 spinel particles.

Resistivity of PZT 95/5 ferroelectric ceramic under shock wave compression

Jiang Dong-Dong, Du Jin-Mei, Gu Yan, Feng Yu-Jun
Acta Physica Sinica. 2008, 57 (1): 0566 doi: 10.7498/aps.57.566
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Resistivity of the shocked PbZr0.95Ti0.05O3(PZT 95/5) ferroelectric ceramic has been determined under shock wave compression. A physical model has been developed which takes into account the finite resistivity of ferroelectric ceramic compressed by shock wave propagating in a direction perpendicular to the remanent polarization vector. The results reveal that under the pressure of about 2.0 GPa under short circuit, the resistivity of the shocked PZT 95/5 ceramic drops very quickly from 107—1011Ωcm to a minimum of 40Ωcm and then rises to values between 120 and 140Ωcm until the end of the shock depoling.

Slow wave effect of 2-D photonic crystal coupled cavity array

Du Xiao-Yu, Zheng Wan-Hua, Ren Gang, Wang Ke, Xing Ming-Xin, Chen Liang-Hui
Acta Physica Sinica. 2008, 57 (1): 0571 doi: 10.7498/aps.57.571
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We designed a two-dimensional coupled photonic crystal resonator array with hexagonal lattice. The calculation by plane-wave-expansion method shows that the dispersion curve of coupled cavity modes in the bandgap are much flattened in all directions in the reciprocal space. We simulated the transmission spectra of transverse electric(TE) wave along the ΓK direction. Compared with the PC single cavity structure, the transmission ratio of the coupled cavity array increases more than three orders of magnitude, while the group velocity decreases to below 1/10, reaching 0.007c. The slow wave effect has potential application in the field of miniaturized tunable optical delay components and low-threshold photonic crystal lasers.

Fabrication of high voltage thyristor based on silicon organic compounds and metal oxides type isolation protective material

Liu Xiu-Xi, Wang Gong-Tang
Acta Physica Sinica. 2008, 57 (1): 0576 doi: 10.7498/aps.57.576
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In this paper, we present some experimental results of the KP 500 thyristor which are fabricated by using silicon organic compounds and metal oxides type isolation material. Investigation on the principles of quality control of the semiconductor devices is also reported. The technique was based on coating the device surfaces with the mixture of high purity silicon organic compounds and metal oxides at a proper mixing ratio to protect the semiconductor p-n junction surface aswell as to control the characteristic properties. At room temperature, the resistivity of the solid isolation protective material ρ>7.5×1015Ω·cm, its dielectric constant is equal to 4.7, and the breakdown voltage V>16kV/mm. We demonstrated the advantages of using this material on KP500 thyristor by significantly improving its surface properties, reducing the leakage current and increasing its voltage breakdown level.

Luminescence and energy transfer of Eu2+, Mn2+ in BaZnP2O7

Yang Zhi-Ping, Yang Guang-Wei, Wang Shao-Li, Tian Jing, Li Pan-Lai, Li Xu
Acta Physica Sinica. 2008, 57 (1): 0581 doi: 10.7498/aps.57.581
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The BaZnP2O7:Eu2+, Mn2+ phosphor was synthesized by high temperature solid state reaction. The luminescence properties and the energy transfer between Eu2+ and Mn2+ were investigated. The emission bands of Eu2+ and Mn2+ peaking at 380nm and 670nm were originated from 5d—4f transition of Eu2+ which partly substituted the Ba2+ sites and 4T1(4G)—6A1(6S) transition of Mn2+ occupying Zn2+ sites, respectively. The dependence of Eu2+ and Mn2+ luminescence properties on their concentration was investigated. With the increasing Mn2+ dopant content, the intensity of Eu2+ was observed to decrease, but the intensity of Mn2+ was simultaneously found to increase. It is suggested from this phenomenon that Mn2+ ion is sensitized by Eu2+ ion, in other words, there is an energy transfer from Eu2+ to Mn2+ in BaZnP2O7 host. According to the Dexter's energy transfer formula of multipolar interaction, we have demonstrated that the energy transfer between Eu2+ and Mn2+ is due to the electric dipole-quadripole interaction of the resonance transfer.

Investigation of the UV absorption of porous TiO2/Al/SiO2 nanostructures

Ma Bao-Hong, Li Yan, Wang Cheng-Wei, Wang Jian, Chen Jian-Biao, Liu Wei-Min
Acta Physica Sinica. 2008, 57 (1): 0586 doi: 10.7498/aps.57.586
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The porous TiO2/SiO2 and TiO2/Al/SiO2 nanostructures are successfully fabricated by anodic oxidation of titanium or titanium/aluminum thin films deposited on transparent SiO2 glass substrates with pulsed laser deposition (PLD) technique. The optically transparent titania films, with highly ordered and uniformly arrayed pores are characterized by field emission scanning electron microscope (FESEM). The influence of Al buffer layer on the optical absorption of the porous TiO2 thin films was investigated. The results show that without Al buffer layer the UV absorption peak of the porous TiO2 films was fixed at 270nm and the intensity of absorption peak was not modulated by the anodic potential. However, with an Al buffer layer, the UV absorption peak will shift to 293nm, and the intensity of absorption peak was not only modulated by anodic potential, but also sensitively influenced by the thickness of Al buffer layer. Moreover, the optical transition property of the two nanostructures at the absorption edge was analysed.

Study on the optical property of quartz glass and Al film reflector under charged particles irradiation

Zhou Zhong-Xiang, Wang Hong-Li, Shen Yan-Qing, Liu Da-Jun, Liu Hai, He Shi-Yu, Yang De-Zhuang
Acta Physica Sinica. 2008, 57 (1): 0592 doi: 10.7498/aps.57.592
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The damage model and the rate equation of quartz glass were set up under charged particles irradiation, and the concentration of ions and electrons produced by ionization were also given. The mathematical fitted results of the change in optical density are in accordance with the experimental results. The model was also applied to optical Al film reflector, and the mathematical fitted results of the integral reflectance of Al film reflector were given. The proposed damage model can be used to describe quantitatively the change in optical property of quartz glass with increasing absorption dose under charged particles irradiation.

Fabrication, microstructure and UPS spectra of Co: BaTiO3/Si(100) nano-composite films

Wu Wei-Dong, He Ying-Jie, Wang Feng, Zhan Yong-Jun, Bai Li, Ju Xin, Chen Zheng-Hao, Tang Yong-Jian, Sun Wei-Guo, Pan Hai-Bin
Acta Physica Sinica. 2008, 57 (1): 0600 doi: 10.7498/aps.57.600
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Co: BaTiO3 nano-composite films were successfully fabricated by pulsed laser deposition on single crystal Si(100) surface. The micro-structure of the Co: BaTiO3 nano-composite films with about 30 nm and 100 nm thicknesses were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The Co: BaTiO3 nano-composition films with the above thicknesses were unidirectionally prefer-oriented and polycrystalline, respectively. The atomic force microscopic (AFM) analysis indicated that there were tetragonal crystal grains in the films of about 30 nm thickness. The valence band of the films were investigated by ultraviolet photoelectron spectroscopy (UPS) in detail. The UPS spectra showed that density of electronic states (DOS) of the films with high concentration of Co grains were obviously different from that of bulk BaTiO3 single crystal.

Preparation and structure of NaREO2 (RE=Lu,Y)

Guo Ru-Wang, Guo Chang-Xin, Wu Di
Acta Physica Sinica. 2008, 57 (1): 0607 doi: 10.7498/aps.57.607
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NaLuO2 and NaYO2 powder crystals were prepared by the conventional solid state reaction method. It is more difficult to synthesize pure NaLuO2 powder crystals than NaYO2. To monitor the quantity of Lu2O3 in the NaLuO2 sample, we calculated the relation between weight ratio and XRD peak absolute intensity ratio of Lu2O3 to NaLuO2 by X-ray diffraction method, and obtained the constant (kLu2O3NaLuO2=3.8). Pure NaLuO2 powder crystals were obtained by optimizing reaction conditions (sintering temperature, reaction time, raw material molar ratio and fluxes) in a two-step method. The best reaction condition was found to be that the mixture of Na2O2, Lu2O3 and Na3PO4 at a molar ratio of 200∶100∶7 is filled into a small inner ceramic crucible which is placed inside a larger outer ceramic crucible and sintered at 850℃ for 3 h. The sample has pure hexagonal structure (space group R3m, lattice constant a=0.3335 nm, c=1.6545 nm). XRD pattern of NaLuO2 in this experiment indicated that there is an additional diffraction peak (006) compared with JCPDS23-0684, and the labeled (007), (111) and (117) peaks should be corrected to (104), (110) and (024) according to the Bragg reflection conditions of hexagonal system. Eu3+ doped NaLuO2 and NaYO2 have different emission spectra, which indicates that Eu3+ occupy different symmetric sites. The main peak of the former (having inversion centre i) is at about 590nm, corresponding to the magnetic dipole transition of Eu3+ (5D07F1), and the later (no inversion centre i) is at about 610nm, corresponding to the electric dipole transition of Eu3+ (5D07F2).

Study on the stability of carbon nanotubes

Ouyang Yu, Peng Jing-Cui, Wang Hui, Yi Shuang-Ping
Acta Physica Sinica. 2008, 57 (1): 0615 doi: 10.7498/aps.57.615
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When the a graphite crystal is crushed, it is breaks into micro-scale laminar flakes. There are many dangling bonds on the edge of an isolated graphite micro-sheet which elevates its energy and makes it unstable. When a graphite micro-sheet curles into a nano-tube, the number of dangling bonds decrease, and the system energy is reduced accordingly. Meanwhile, the curled sheet obtains deformation energy, which can more than counteract the energy reduction due to the diminished dangling bonds and make the CNT even less favorable energetically than the sheet, and becomes unstable. On the basis of advancing the mechanical model of the formation of CNTs, a thorough theoretical analysis was made, which lead to the conclusion that the CNTs can exist stably at a minimum diameter of approximate 0.32nm.

Molecular noise induced circadian oscillation and coherence resonance in the gene network of biological clock

Yi Ming, Jia Ya, Liu Quan, Zhan Xuan
Acta Physica Sinica. 2008, 57 (1): 0621 doi: 10.7498/aps.57.621
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The molecular noise in the gene regulation network of biological clock in Drosophila was studied. Especially, we investigated the effect of molecular noise on the circadian rhythmic oscillation when the system is in the steady state region but only slightly away from the oscillating region. We show that (i) Though the prodution and the decay of the protein are stochastic with the stochastic time interval, the obvious circadian oscillation can be induced depending on the system intrinsic regulation mechanism. (ii) The coherence of molecular noise-induced oscillation is optimal under an appropriate molecular noise level, leading to the occurrence of coherence resonance.

UPS study of electronic states of CuPc/Ag(110)

Dou Wei-Dong, Song Fei, Huang Han, Bao Shi-Ning, Chen Qiao
Acta Physica Sinica. 2008, 57 (1): 0628 doi: 10.7498/aps.57.628
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The electronic structure of copper phthalocyanine (CuPc) adsorbed on Ag (110) surface has been investigated by using the ultra-violet photoemission spectra (UPS). The signals from the 3d band of the substrate Ag atoms faded, while two new peaks grew at 4.45 and 6.36eV below the Fermi level with increasing coverage of CuPc molecules. With further increasing the CuPc coverage, other two peaks can be detected at 1.51 and 9.20 eV below the Fermi level. All four peaks originate from CuPc molecules. With increasing CuPc coverage, the four peaks increase in intensity and shift in binding energy. The results of angular resolved UPS suggest a “flat-lying" mode for CuPc molecules adsorbed on Ag (110) surface. And this has been supported by the theoretical calculation based on the density functional theory (DFT). The theoretical study shows that the CuPc molecule prefers to lie on the hollow position of Ag substrate.

Reproducing kernel particle method with complex variables for elasticity

Chen Li, Cheng Yu-Min
Acta Physica Sinica. 2008, 57 (1): 01 doi: 10.7498/aps.57.1
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The reproducing kernel particle method with complex variables is developed in this paper. The advantages of the developed method is that the correction function of a 2-D problem is formed with 1-D basis function. Then,we apply the method to two-dimensional elasticity,and the application to two-dimensional elasticity is presented, and the corresponding formulae are obtained. Compared with the conventional reproducing kernel particle method,the reproducing kernel particle with complex variables developed in this paper has greater precision and computational efficiency. Some examples given in this paper demonstrated the efficiency of the present method.

New exact solutions and special soliton structures for the (3+1)-dimensional Burgers system

Ma Song-Hua, Wu Xiao-Hong, Fang Jian-Ping, Zheng Chun-Long
Acta Physica Sinica. 2008, 57 (1): 011 doi: 10.7498/aps.57.11
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Applying the improved mapping approach and variable separation method to the (3+1)-dimensional Burgers system,the new exact solutions of the system is derived. Based on the derived solitary wave solution,we obtained some special soliton structures,such as cylinder-like soliton,tapered soliton and embeded-solitons,and the interactions between the solitons are discuessed.

Integral of differential equations of three-particle Toda lattice

He Guang, Mei Feng-Xiang
Acta Physica Sinica. 2008, 57 (1): 018 doi: 10.7498/aps.57.18
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The differential equations of three-particle Toda lattice are a Hamiltonian system. The integrals of the equations can be obtained by using the Noether theory and the Poisson theory.

The entanglement character of two entangled atoms in multiphoton Tavis-Cummings model

Zhang Ying-Jie, Zhou Yuan, Xia Yun-Jie
Acta Physica Sinica. 2008, 57 (1): 021 doi: 10.7498/aps.57.21
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We investigated the entanglement time evolution of two entangled two-level atoms. The two atoms can undergo multiphoton transition when they interact resonantly with a single-mode field in the coherent state. The results show that the two-atom entangled state evolves periodicity. The influence of the two-atom initial state,the dipole-dipole coupling intensity between two atoms,the parameter of the coherent state and the number of the transitional photons on the entanglement degree of two atoms are revealed. When the initial state is a maximum entangled state,the state of the two entangled atoms will forever stay in this maximum entangled state. So this kind of entanglement state can be used for quantum information storage.

Quantum Generalized Reed-Solomon codes

Li Zhuo, Xing Li-Juan
Acta Physica Sinica. 2008, 57 (1): 028 doi: 10.7498/aps.57.28
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We construct a family of quantum error-correcting codes with parameters [[nn-2kk+1]]q which are defined in q-dimensional quantum systems,where q is an arbitrary prime power. These codes are optimal in the sense that the minimum distance is maximal. It is shown that codes exist for all n satisfying 2≤nq or q2-q+2≤nq2.

Generalized projective synchronization and tracking control of complex Dynamos systems

Min Fu-Hong, Wang Zhi-Quan
Acta Physica Sinica. 2008, 57 (1): 031 doi: 10.7498/aps.57.31
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Based on the system stability theory,a new tracking control method is proposed to acquire asymptotically generalized projective synchronization of full states of a complex dynamos system and arbitrary trajectories. It also allows us to drive a complex dynamos system to arbitrary periodic orbits or fixed point. The technique can direct the scaling factor to a desired value. Sinusoidal waves,chaotic systems,hyper-chaotic systems and fixed points are taken as examples respectively. Numerical simulation results are presented to demonstrate the effectiveness of the proposed method.

An improved block cryptosystem based on iterating chaotic map

Xu Shu-Jiang, Wang Ji-Zhi
Acta Physica Sinica. 2008, 57 (1): 037 doi: 10.7498/aps.57.37
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Recently,a large number of chaotic cryptosystems have been proposed,yet many of them have the drawbacks of lack of robustness and security. In this paper,we point out the weakness of a very recent block cipher algorithm which is based on the chaotic map and give the improved scheme of it. We provide the chosen plaintext attack to recover the permuted plaintext string. It is shown that the generation mechanism of binary sequences which depends on the key but not on the plaintext facilitates leakage of information and is vulnerable to attacks. Based on such a fact,we give the improved scheme to achieve higher security.

Generalized synchronization by coupling of restoring force and determination of parameters in vibration systems

Qin Wei-Yang, Wang Hong-Jin, Gao Hang-Shan
Acta Physica Sinica. 2008, 57 (1): 042 doi: 10.7498/aps.57.42
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For a class of vibration systems excited by external harmonic force,generally the restoring force cannot be given an accurate analytical expression. For such system, a derived system of generalized synchronization (GS) is set up. The GS between the two systems is proved. Since there is no time element in the equations of the derived system,the derived system can reach GS with the original system automatically,even if the frequency and amplitude of external force are varied. By using GS,a novel method to determine parameters in a vibration system,e.g.,the damping coefficient and the amplitude and frequency of external exciting force,is presented and proved. From the simulation of Duffing system,it can be seen that the presented method is effective and can give a result of high accuracy.

A novel parameter estimation method of signal in chaotic background

Chen Zheng, Zeng Yi-Cheng, Fu Zhi-Jian
Acta Physica Sinica. 2008, 57 (1): 046 doi: 10.7498/aps.57.46
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To estimate the parameters of signal in chaotic background is very important. A novel method named minimization of relative singular value(MRSV) is proposed in this paper, which is based both on the fact that points reconstructed from the mixed signal time series typically lie off the embedded image of the manifold for the dynamics underlying the chaotic time series, and also on the characteristic of the signal in chaotic background. The parameter estimation can be achieved by minimizing the relative singular value of the output of an inverse filter of the received signal in a reconstructed phase space. Several experiments of estimating the AR model parameters and sinusoidal signal frequency are carried out to confirm the effectiveness of the method.

Chaos anti-control of nonlinear system with uncertainties

Li Wen-Lin, Song Yun-Zhong
Acta Physica Sinica. 2008, 57 (1): 051 doi: 10.7498/aps.57.51
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The model for realizing chaos anti-control of nonlinear system is studied based on sliding mode variable structure control,and on account of which the identity of the controlled system model and chaos reference model is guaranteed in finite time. Both its speed and robustness are discussed theoretically. As an example,Lorenz chaotic system is employed as the chaos reference model for the simulation verification. Results show that the suggested method is valid.

A traffic flow model considering signal light influence and its numerical simulation

Tang Tie-Qiao, Huang Hai-Jun, Xu Gang, Xue Yu
Acta Physica Sinica. 2008, 57 (1): 056 doi: 10.7498/aps.57.56
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Based on the characteristics of car-following behavior in a road traffic system containing signal light,we improve the car-following equation and then present a traffic flow model which considers the signal light influence. Numerical results show that the model can reproduce such phenomena as flow clustering and dissipating,and capture the propagation of stopping and starting waves. It is concluded that the model proposed in this paper is reasonable.

Free energy method and the melting point of aluminum under the zero pressure

Lin Jie, Liu Shao-Jun, Li Rong-Wu, Zhu Wen-Jun
Acta Physica Sinica. 2008, 57 (1): 061 doi: 10.7498/aps.57.61
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In this paper,free energy method is used to calculate the melting poing of Al. We adopt the Lennard-Jones (LJ) liquid as the reference system and compare the calculated results with that of Mei,Davenport,and Morris et al. The calculated results show: (1) Using the LJ reference system,the CPU-time consumed for molecular dynamics simulation is reduced to one half of that of Mei,Davenport,and Morris et al while the results are the same; (2) Errors exist in calculated values of melting point in all molecular dynamics simulations using different versions of empirical embedded-atom-model potentials (EAM). For the metal Al,the calculated result using Cai's EAM potential is improved a little comparing with that using EAM potentials in the simulations of Mei,Davenport,and Morris et al.

Error analysis of four-intensity algorithm used for the measurement of upper atmosphere

Ye Jian-Yong, Zhang Chun-Min, Zhao Bao-Chang, Li Ying-Cai
Acta Physica Sinica. 2008, 57 (1): 067 doi: 10.7498/aps.57.67
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With imaging interferometry and Doppler Effect,the information of the upper atmosphere including velocity and temperature can be obtained by measuring four interference intensities of the aurora in the upper atmosphere (80—300km). Theoretical analysis of the measurement error when using four-intensity algorithm in the moving mirror model and divided mirror model is presented respectively. The error of velocity and temperature of the upper atmosphere caused by random phase error is given. By computer simulation,we get the distribution map of velocity error and temperature error in the two models. The result shows that the error of the velocity with the divided mirror model is about half that with the moving mirror model,while the moving mirror model has appreciably higher accuracy for temperature. The research can help us to improve the detection precision of upper atmospheric wind in theory. It has important theoretical significance and practical value both for the research of the detect mode and manufacture of the equipment.

Microscopic optical potentials in 6Li scattering by nuclei

Ma Yin-Qun, Ma Zhong-Yu
Acta Physica Sinica. 2008, 57 (1): 074 doi: 10.7498/aps.57.74
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A parameter-independent microscopic optical potential of nucleus-nucleus interaction is presented by a folding model with the isospin dependent complex nucleon-nuclear potential, which is calculated in the framework of the Dirac-Bruecker-Hartree-Fock approach. Investigations on 6Li scattering by 12C, 28Si, 40Ca, 58Ni, 90Zr, and 208Pb over a wide range of incident energy and scattering angle with the microscopic nucleus-nucleus optical potential is presented. To take account of the breakup effect of 6Li and the high order dynamic effect in the reaction a modification factor NR in the real part and an enhancing factor NI in the imaginary part of the microscopic optical potential are introduced. We take the imaginary part enhancing factor NI=3.0, which has been obtained in the previous study on 6He scattering by 12C. The modification factor NR is found to be almost constant with respect to the incident energy and target mass number. The calculations with NR≈0.5—0.6 and NI=3.0 well reproduce the experimental elastic scattering data for all targets and incident energies investigated. Our parameter-independent model should be of value in the description of the nucleus-nucleus scattering of many-body systems, especially unstable nucleus-nucleus systems.

A composite Si-PIN 14MeV neutron detector

Zhang Xian-Peng, Ouyang Xiao-Ping, Zhang Zhong-Bing, Tian Geng, Chen Yan-Li, Li Da-Hai, Zhang Xiao-Dong
Acta Physica Sinica. 2008, 57 (1): 082 doi: 10.7498/aps.57.82
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This paper presents the principle of a new composite (C2H4)n-PIN sandwiched fusion neutron detector, which uses 2mm (C2H4)n as sensitivity enhancement medium, and add circuit to merge the output signals. The main merits of the detector are: (1) greatly improved neutron sensitivity and better statistics; (2) improved n/γ discrimination; (3) implementing the signal summation without obviously affecting the time response. Characteristics of the detector, including the sensitivity to 14MeV neutron and 1.25MeV γ ray, n/γ discrimination, time response, and statistics are studied theoretically and experimentally.

Theoretical study on the dielectronic recombination of highly charged mercury and uranium ions

Shi Ying-Long, Dong Chen-Zhong, Zhang Deng-Hong, Fu Yan-Biao
Acta Physica Sinica. 2008, 57 (1): 088 doi: 10.7498/aps.57.88
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Based on the multiconfiguration Dirac-Fock method, with the recently developed program for the calculation of cross section of dielectronic recombination (DR), the KLL DR process of highly charged mercury and uranium ions from helium-like to boron-like have been systematically studied. For Hg ions, the contributions to the level of doubly excited state from Breit effect and quantum electrodynamic (QED) effect, KLL DR resonant energies, total line width and corresponding DR resonant strength have been calculated and analyzed. A good overall agreement was found between the present calculations and the previous theoretical and experimental result. Based on the calculation of Hg ions, the KLL DR process of highly charged uranium ions was studied theoretically. The comparison of present results with the other theoretical and experimental results has been made.

The solvent effects on the complexing reaction of nickel dithiolene and ethylene

Han Qing-Zhen, Geng Chun-Yu, Zhao Yue-Hong, Qi Chuan-Song, Wen Hao
Acta Physica Sinica. 2008, 57 (1): 096 doi: 10.7498/aps.57.96
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Theoretical studies of solvent effects on the reaction of complexing ethylene and nickel dithiolene have been carried out by calculating the molecular geometry, electron distribution, and frequency of all the stagnation points existing in the reaction potential profiles by means of density functional theory methods at the B3LYP/6-31G(d) level. On the basis of frequency analysis and statistical thermodynamic theory, some thermodynamic parameters for the titled compound have been obtained for different solvents. It is shown that the bond energy of ethylene and nickel dithiolene enhances with the increase of the solvent polarity. In addition, the frontier orbital energy gaps of the two transition states will increase, while those of the product and the intermediate as well as the corresponding solvation stabilization energy will decrease. Furthermore, these results demonstrate that in polar solvents the reaction of complexing ethylene and nickel dithiolene may become easier and faster to occur and yield a more stable product. Notably, an obvious solvent effect can be observed when the value of the solvent dielectric constant lies between 1.0 and 7.58, while the solvent effect will tend to be maximal when the value of solvent dielectric constant exceeds 7.58.

Micro-Raman spectroscopic investigation of the thermal conductivity of oxidized meso-porous silicon

Fang Zhen-Qian, Hu Ming, Zhang Wei, Zhang Xu-Rui
Acta Physica Sinica. 2008, 57 (1): 0103 doi: 10.7498/aps.57.103
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A theoretical model describing the thermal conductivity of oxidized meso-porous silicon (meso-PS) was brought forward, which was based on the effective medium theory for the mechanisms of heat transfer in oxidized meso-PS. The factors affecting effective thermal conductivity (ETC) of oxidized meso-PS were analyzed theoretically, and a calculating formula of TC of oxidized meso-PS was given. Meso-PS samples were prepared by double-tank electrochemical corrosion method. Using Micro-Raman spectroscopic technique, the dependence of TC values of oxidized meso-PS samples on porosities of as-prepared meso-PS was studied and the thermal property of oxidized meso-PS with different oxidized temperatures was compared. TC values yielded by Micro-Raman spectroscopic technique of as-prepared meso-PS with porosities of 60%,73.4% and 78.8% oxidized at 300℃ were 8.625,3.846 and 1.817W/(m·K), respectively, while that of as prepared meso-PS with a porosity of 73.4% oxidized at 450℃ and 600℃ were 2.466 and 2.100 W/(m·K), respectively. And their corresponding theoretical ETC values were 4.549,2.432,1.792,2.105 and 2.096 W/(m·K), respectively. It was shown that the theoretical values were in good agreement with experimental data. Oxidized meso-PS with lower TC and desirable mechanical stability is well suited for thermal insulation material, which is attractive to the application in thermal-effect-micro-systems (TEMS).

Investigation on Kα X-ray emission from ultraintense laser-produced hot electrons

Huang Wen-Zhong, Li Yu-Tong, Xiong Yong, Zhang Shuang-Gen, Wen Xian-Lun, Hong Wei, Gu Yu-Qiu, Wen Tian-Shu, He Ying-Ling
Acta Physica Sinica. 2008, 57 (1): 0111 doi: 10.7498/aps.57.111
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A dispersionless X-ray Kα spectrometer is constructed. The experiments were performed using the SILEX-I laser facility. The Kα X-ray emission from ultraintense laser-plasma interactions is measured with the spectrometer in front of and behind the foil targets. The conversion efficiency from laser energy to Kα X-ray is obtained. The experimental results show that the Kα yield increases with the laser energy. With a 100μm thick Mo foil, the conversion efficiency is 10-5.

Study of Rydberg states of lithium in a static electric field using B-spline functions

He Yong-Lin, Zhou Xiao-Xin, Li Xiao-Yong
Acta Physica Sinica. 2008, 57 (1): 0116 doi: 10.7498/aps.57.116
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The B-spline expansion technique and the method of model potential of atom have been applied to study the properties of Stark states of lithium. Our results for Stark maps, oscillator strength distributions, avoided crossing positions and widths for Li are in good agreement with other reported theoretical calculations and experimental measurement. It shows that the method we used can be applied to investigate the properties of alkali-metal atoms in external static electric field. Finally, for the first time, we obtained the evolution of the oscillator strength spectrum of Li with varying static electric field.

The effect of static electric field on nonsequential double ionization

Li Hong-Yun, Wang Bing-Bing, Jiang Hong-Bing, Chen Jing, Li Xiao-Feng, Liu Jie, Gong Qi-Huang, Fu Pan-Ming
Acta Physica Sinica. 2008, 57 (1): 0124 doi: 10.7498/aps.57.124
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Nonsequential double ionization of He atom in the combined field of a laser and a static electric field has been investigated. It is found that the presence of the static electric field breaks the inversion and reflection symmetry, the symmetric double-peak structure of the He2+ ion momentum parallel to the polarization of the laser field is broken. Since the ultrashort laser pulse has the similar asymmetric character, this study is helpful to understand the mechanism of the double ionization process in ultrashort laser field.

Optical-optical double-color and double-resonance multiphoton ionization spectrum of NO2

Zhang Gui-Yin, Jin Yi-Dong
Acta Physica Sinica. 2008, 57 (1): 0132 doi: 10.7498/aps.57.132
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The optical-optical double resonance multiphoton ionization (OODR-MPI) technique has been applied to the study of the Rydberg states of nitrogen dioxide. The results show that NO2 molecule is ionized though a 1+3+1 double resonance multiphoton ionization process in 605—675nm probe wavelength region. The ionization pathway is: NO2(X2A1)1NO2(A2B2)32NO2(3pσu)2NO22+e. The OODR-MPI spectrum corresponds to the excitation of NO2 3pσu Rydberg state by a three photon process. The value ω1=(1422.5±23.5) cm-1 and ω2=(590.5±4.9) cm-1 of symmetric stretching and bending vibration frequencies of this Rydberg state are presented. The position of the band origin and the quantum defect of 3pσu Rydberg state are obtained to be (58331±71) cm-1 and 0.69, respectively.

The coulomb potential energy effect on the intensity of the characteristic lines at highly charged ion incendence on Al surface

Wang Li, Zhang Xiao-An, Yang Zhi-Hu, Chen Xi-Meng, Zhang Hong-Qiang, Cui Ying, Shao Jian-Xiong, Xu Xu
Acta Physica Sinica. 2008, 57 (1): 0137 doi: 10.7498/aps.57.137
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The Al atomic characteristic spectral lines were induced by the impact of 40Arq+ ions (8≤q≤16; kinetic energy 150keV) on Al surface. The result shows that by Penning impinging and resonant capture, the ion energy is deposited on the Al surface to excite the target atom, which is different from light excitation. Not only are the transitions betweem electronic configurations of the atomic complex excited, but the enhancing tendency of the characteristic spectral line intensity is consistent with the enhancing tendency of the coulomb potential energy of the incident ions with increasing charged states.

Studies on vibrational excitation differential cross sections of low-energy electron scattering from N2 molecule by vibrational close-coupling method

Dai Wei, Feng Hao, Sun Wei-Guo, Tang Yong-Jian, Shen Li, Yu Jiang-Zhou
Acta Physica Sinica. 2008, 57 (1): 0143 doi: 10.7498/aps.57.143
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The differential cross sections (DCS) of low-energy electron-N2 scattering are studied using vibrational close-coupling (VCC) method and vibrational scattering potentials which include the static, exchange and polarization contributions, where the polarization is obtained on the basis of “better than adiabatic dipole" (BTAD) and “distributed spherical Gaussian" (DSG) respectively. The converged (0→2, 0→3) DCS are obtained for the impact energy of 2.40eV. Calculations using both polarization potentials agree with experimental results. The physical factors that possibly affect the accuracy of DCS values are also discussed.

The structure and potential energy function of PdCO molecule

Kong Fan-Jie, Du Ji-Guang, Jiang Gang
Acta Physica Sinica. 2008, 57 (1): 0149 doi: 10.7498/aps.57.149
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Density functional method(B3LPY) has been used to optimize the possible structures of PdC,PdO and PdCO molecules with contracted valence basis set (LANL2DZ) for Pd atom and the AUG-cc-pVTZ basis set for C and O atoms respectively.It was found that the ground state of PdC molecule is 1Σ,whose equilibrium nuclear distance and dissociation energy are RPdC=0.17285nm and 4.919eV, respectively.The ground state of PdO molecule is 3Π with equilibrium geometry RPdO=0.18546nm and dissociation energy De=2.455eV. The ground state of the linear Pd—C≡O (Cv) is 1Σ+ and the configuration and dissociation energy are RPdC=0.18721nm,RCO=0.11427nm and 12.563eV, respectively. At the same time, another metastable structure Pd—O≡C(Cv) was found. Its equilibrium geometry and dissociation energy are RCO=0.11336nm, RPdO=0.23001nm and 10.937eV, respectively.The possible dissociation limit of PdCO molecule is determined.The analytical potential energy function for PdCO molecule has been obtained from the many-body expansion theory.The contour of the potential energy surface sheds light on the accurate structure and dissociative energy for PdCO molecule.Furthermore, the molecular static reaction pathway based on this potential energy function is investigated.

Theoretical calculation of the partial cross section in He-H2(D2,T2) collisions

Shen Guang-Xian, Wang Rong-Kai, Linghu Rong-Feng, Yang Xiang-Dong
Acta Physica Sinica. 2008, 57 (1): 0155 doi: 10.7498/aps.57.155
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Tang-Toennies potential model and close-coupling method were applied to the He-H2(D2,T2) system,and the vibrational and rotational excitation cross sections at different incidence energy have been calculated.By analyzing the differences of these partial wave cross sections, the change rules of the partial wave cross sections with increase of quantum number, and with change of reduced mass of symmetric isotopic substituted system have been obtained. Based on the calculation, influence on the cross sections exerted by the variations in the reduced mass of systems and in the relative incoming energy of incident atom is discussed.

Density functional study of CO adsorption on Pu (100) surface

Luo Wen-Hua, Meng Da-Qiao, Li Gan, Chen Hu-Chi
Acta Physica Sinica. 2008, 57 (1): 0160 doi: 10.7498/aps.57.160
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The adsorption of CO on Pu (100) surface were studied by the density functional theory (DFT). The calculation results show that the O-down adsorption is less stable than the C-down adsorption which is found to be a strong chemisorption. The stability of adsorption configuration of CO is hollow tilted > hollow vertical > bridge > on-top. The interaction between Pu atom and CO molecule results mainly from the contribution of hybridized molecular orbital of CO molecule and hybridized orbital of surface Pu atom. A small activation barrier of 0.280eV is found for the dissociation of CO molecule in hollow tilted configuration, which indicates the dissociative adsorption of CO on Pu (100) surface at the lower temperature with dissociated C and O atoms favoring the hollow site of lowest energy.

Investigation on vibrational levels, inertial rotation and centrifugal distortion constants of 7Li2(X1Σ+g)

Shi De-Heng, Sun Jin-Feng, Zhu Zun-Lüe, Ma Heng, Yang Xiang-Dong
Acta Physica Sinica. 2008, 57 (1): 0165 doi: 10.7498/aps.57.165
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The density-functional theory (B3LYP, B3P86) and the configuration-interaction method (CCSD(T), QCISD) presented in Gaussian03 program package are employed to calculate the equilibrium internuclear distance Re, the harmonic frequency ωe and the dissociation energy De of 7Li2(X1Σ+g) under a number of basis sets. By comparison with the measurement, the conclusion is that the most accurate Re, De and ωe results (0.2698nm, 1.0487eV and 346.82 cm-1, respectively) can be obtained at the CCSD(T)/cc-PVQZ level of theory. The potential energy curve at this level is calculated over a wide internuclear separation range of 0.15 to 2.7nm at a 0.03nm step size and is fitted to the analytic Murrell-Sorbie function. With the potential obtained at the CCSD(T)/cc-PVQZ level of theory, the rest spectroscopic parameters (ωeχe, αe and Be) are calculated and the values are 2.648cm-1, 0.00702cm-1 and 0.6601cm-1, respectively, which are in excellent agreement with the experimental ones. By solving the radial Schrdinger equation of nuclear motion, the total number of 41 vibrational states has been found when J=0 for this X1Σ+g state. For each vibrational state, the vibrational level, the classical turning point and the inertial rotation constant have been computed, which are in good agreement with the experimental findings. The complete centrifugal distortion constants (Dν, Hν, Lν, Mν, Nν and Oν) are reported for the first time for the first 31 vibrational states when J=0.

Progress in yo-yo process to produce polarized electrons from GaAs photocathode

Li Qian, Hao Liang, Pang Wen-Ning
Acta Physica Sinica. 2008, 57 (1): 0172 doi: 10.7498/aps.57.172
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By studying the process of producing polarized electrons by the photoemission of GaAs, different experimental parameters on the generation and stability of the electron beam is discussed. A feasible way to generate stable beam of long duration is provided. There is also a discussion on producing polarized electron beam with controlled intensity.

Diagnosis and investigation of the formation of low density and large sized clusters

Yang Ming, Liu Jian-Sheng, Cai Yi, Wang Wen-Tao, Wang Cheng, Ni Guo-Quan, Li Ru-Xin, Xu Zhi-Zhan
Acta Physica Sinica. 2008, 57 (1): 0176 doi: 10.7498/aps.57.176
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The laser-cluster interaction has been an important research field for decades, which would play an important role in understanding the laser-matter interaction. In order to generate satisfactory cluster sources, all factors that would affect the generation of the clusters should be understood. In this paper, linearly chirped spectral scattering is employed to study the size of the clusters during the process. And also, low density and large sized clusters is observed, which could be important clean targets in future experiments.

First-principles calculations on the structures of Wn (n=3—27) clusters

Lin Qiu-Bao, Li Ren-Quan, Wen Yu-Hua, Zhu Zi-Zhong
Acta Physica Sinica. 2008, 57 (1): 0181 doi: 10.7498/aps.57.181
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The structural properties of Wn clusters (n=3—27) has been studied by employing the first-principles calculations based on the density functional theory. The most stable structures of clusters (n=3—7) with global energy minimum and optimized structures of clusters (n=8—27) with local energy minimum are determined. Based on the jellium model, the electronic configuration 1s21p61d102s21f142p63s23p62d104s22f143d101g181h223f14 is proposed which can explain well the electronic magic numbers and the relative stabilities of W clusters. The binding energy, the first and second differences of binding energies and the HOMO-LUMO gaps versus the number of atoms in the cluster are also analyzed, showing that W clusters become metallic very quickly with the increase of cluster size. This may also imply a quick change of bonding characters in the W clusters.

Determination of air ionization threshold with femtosecond laser pulses

Wang Xiao-Lei, Zhang Nan, Zhao You-Bo, Li Zhi-Lei, Zhai Hong-Chen, Zhu Xiao-Nong
Acta Physica Sinica. 2008, 57 (1): 0354 doi: 10.7498/aps.57.354
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The intensity thresholds of air ionization induced by 800nm ultrashort laser pulses are investigated for laser pulse durations from 50fs to 22ps. The intensity threshold was determined by measuring the fluorescence energy of the stimulated air ionization plasma. It is shown that as the pulse duration increases from 50fs to 22ps, the threshold intensity of air ionization decreases from 8.7×1014 to 2.7×1013W/cm2 and undergoes the transition from rapid decrease to slow decrease. For pulse duration shorter than 1ps, the theoretical calculation was done based on the multiphoton ionization and for pulse duration longer than 1ps, the calculation was done based on the impact ionization initiated by the multiphoton ionization. For both above cases the calculated results are in good agreement with the experiment.

Effect of atmospheric turbulence on the spatial coherence of quasi-monochromatic Gaussian Schell-model beams propagating in the slant path

Wang Hua, Wang Xiang-Zhao, Zeng Ai-Jun, Yang Kun
Acta Physica Sinica. 2008, 57 (1): 0634 doi: 10.7498/aps.57.634
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By using the mutual coherence function of quasi-monochromatic Gaussian Schell-model (GSM) beams propagating in a slant path through the turbulent atmosphere, the complex degree of coherence of those beams is derived analytically. By employing the lateral coherence length (that can be derived from the complex degree of coherence ) of quasi-monochromatic GSM beams to characterize their spatial coherence, the effect of atmospheric turbulence on the spatial coherence of the beams was studied. The result shows that:(1) When θ≤88°, the lateral coherence length increases firstly and then keeps unchanged as the beam propagates in the turbulent atmosphere. (2) When θ≥89°, the lateral coherence length increases firstly and then reaches a maximum value after the beam propagates acertain distance in the turbulent atmosphere, then begins degrading and keeps decreasing with increasing distance. (3) Three beams with different lateral coherence lengths at the source plane (z=0) will have almost the same lateral coherence length after they propagate a certain distance in the turbulent atmosphere. Lastly, a physical explanation was given to the results.
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