Vol. 53, No. 7 (2004)
2004, 53 (7): 2029-2034. doi: 10.7498/aps.53.2029
The stochastic averaging method for a quasi-integrable Hamiltonian system has been applied to the study of the motion of active Brownian particles. Firstly, the stochastic averaging method is introduced. Then, the stationary solution of the dynamic equations governing an active Brownian particle moving in a plane with Schienbein-Gruler velocity-dependent friction model subject to Guassian white noise excitation is obtained by using this method. The solution is proved validated by comparison with the results from digital simulation and those from experiment. Finally, the solutions for the Rayleigh and Erdamnn model are also given.
Non Noether conserved quantity of the holonomic mechanical systems in terms of quasi-coordinates ——An extension of Hojman theorem
2004, 53 (7): 2035-2039. doi: 10.7498/aps.53.2035
Using the Lie symmetry under infinitesimal transformations in which the time in not variable,a new conserved quantity of holonomic mechanical systems in terms of quasi-coordinates is studied.The differential equations of motion of the systems are established.Determining equations of Lie symmetry under infinitesimal transformations are given.The Hojman theorem is generalized.Finally,an example is given to illustrate the application of the results.
2004, 53 (7): 2040-2045. doi: 10.7498/aps.53.2040
Nonlinear mechanics of thin elastic rod, as a model of DNA, aroused extensive interest as a joint research subject of mechanics and molecular biology. The study of the equilibrium of a thin elastic rod constrained on a surface found an important application in industry, especially in molecular biology. In the present paper the constraint equations and constraint forces of the elastic rod are analyzed, and the differential/algebraic equations of equilibrium are established with the arc-coordinate of the central line as the independent variable. In a special case when the constraint surface is a cylinder the dimensionless differential equations contain only one physical parameter, the ratio of the bending and torsional stiffness of the cross section. A special solution of helical equilibrium can be derived and is corresponding to a regular precession of the Lagrange heavy rigid body about a fixed point. The numerical analysis shows that the geometrical form of the central line is dependent on the initial conditions of the rod more than its physical parameters.
2004, 53 (7): 2046-2049. doi: 10.7498/aps.53.2046
In this paper, the form invariance of second-order linear nonholonomic systems in the phase space is studied. The definition and criterion of the form invariance of secondorder linear nonholonomic systems in the phase space is given. The structure equations and the conserved quantities of the form invariance are obtained. and an example is given to illustrate the application of the results.
2004, 53 (7): 2050-2055. doi: 10.7498/aps.53.2050
Non-uniform microchannels are used in the micro space propulsions and micro gas turbine generators. The performances of these applications are determined primarily by the performance of the microchannel used. Based on the micro-fabrication technology, three-dimensional rectangular cross-section straight-convergent-divergent-straight (SCDS) microchannel was fabricated. Its width in the throat cross-section was 16μm, the depth was 20μm and the contraction ratio was 1.625∶1 For different inlet-to-outlet pressure ratios, the volume flow rate characteristics of nitrogen were measured experimentally. The pressure ratios ranged from 1.0 to 4.0, outlet volume flow rate ranged from 0 to 0.12 mL/s and outlet Reynolds number was less than 350 The gas flow characteristics were analyzed by numerical simulation based on finite-volume method. Good agreements between computational results and experimental data were found. The simulation results indicate that two novel behaviors are different from that at conventional scales. One is the first sonic point position, the other is the critical pressure ratio（the critical pressure ratio is defined as the local-to-inlet pressure ratio , here the local pressure corresponding to the pressure at the first sonic point in the internal flow of SCDS microchannel）. These two novel behaviors should be attributed to the surface effects. Due to the larger surface-to-volume ratio (S/V) for microchannels, the factors related to the surface effects have more impact on the microflow. So the relationships between novel behaviors and the value of S/V are discussed further.
The envelope solutions to the coupled nonlinear Schr?dinger equation of Manakov type─with Jacobi elliptic functions
2004, 53 (7): 2056-2059. doi: 10.7498/aps.53.2056
We have simplified the extended Jacobi elliptic function expansion method, namely, improved the modified Jacobi elliptic function expansion method. We applied this method to solve the coupled nonlinear Schr?dinger equation of Manakov type and obtained some Jacobi elliptic envelope solutions and corresponding solitary wave solution, shock wave solution.
2004, 53 (7): 2060-2064. doi: 10.7498/aps.53.2060
Group theory is introduced into the calculation of photonic band gap with the plane wave expansion method, and improved calculation formulae are derived. Both the analysis and the practical band gap calculation for an exemplary twodimensional case have shown that, as compared with the conventional plane wave method, the group theory based improved algorithm can substantially release memoryspace requirement and reduce computational time.
2004, 53 (7): 2065-2068. doi: 10.7498/aps.53.2065
Divergence structure for the quantum entropy of a black hole in Anti-de Sitter spacetime due to the electromagnetic and gravitational fields is investigated by using the brick-wall model. The entropy has both linearly and logarithmically divergent terms, where the linearly divergent term can reduce to the form proportional to the event horizon area if the temperature is replaced by the Hawking temperature and the coordinate cutoff is replaced by the proper cutoff; while the logarithmic divergences not only depend on the characteristics of the black hole but also on the spin of the fields. Since this term exists, the contribution of a spin field in general is not proportional to the scalar one.
2004, 53 (7): 2069-2073. doi: 10.7498/aps.53.2069
An approach of the harmonic signals extraction from chaotic interference is proposed. Based on empirical mode decomposition and the concept that any signals are composed of a series of the simple intrinsic modes, and the chaoic interference signals are decomposed to a series of intrinsic mode functions, the harmonic signals can be extracted. Simulation results show the approach is satisfactory.
2004, 53 (7): 2074-2079. doi: 10.7498/aps.53.2074
A general framework algorithm is proposed for energy minimization control for a discrete chaotic system. A quadratic performance function is first given and the chaotic system is decomposed into a linear and a nonlinear parts. Then, the twolevel algorithm is presented to solve the nonlinear optimal control problem: The first level predicates the nonlinear part of the chaos system; the second level solves a nonlinear quadratic optimization control problem by dynamic programming. The solution is fed back into the first level. The first level re-estimates the nonlinear part according to the solution from the second level. The information has been exchanged between the two levels by this means such that the optimal control law is obtained eventually. This method not only can make the control of chaos system be realized but also makes the energy consumed minimal during the whole control process.Simulations show the effectiveness of this algorithms.
Study on phase synchronization of chaotic oscillators with many rotational centers based on amplitude coupling
2004, 53 (7): 2080-2083. doi: 10.7498/aps.53.2080
In order to find some relation between phase synchronization and dynamical topological variation in chaotic systems with many rotational centers,we propose a method of linear amplitude coupling to study the phase synchronization of Lorenz system and Duffing system.First,we convert the original Lorenz system and Duffing system into the dynamics of amplitude and phase.Based on linear amplitude coupling,we calculate the average winding number and Lyapunov exponents.We find that the phase synchronization comes along with the transition of Lyapunov exponents by increasing coupling strength.The results obtained indicate that phase synchronization is definitely related to dynamical topological variation in chaotic systems with many rotational centers.
2004, 53 (7): 2084-2088. doi: 10.7498/aps.53.2084
By constructing an odd function with multiple piecewise-linear segments in the double-folded torus chaotic attractor equation and using recurrence algorithm, the recurrence formula capable of producing a family of multi-folded torus chaotic attractors is derived. Utilizing the recurrence formula thus derived and selecting properly the slope values of piecewise-linear segments, the values for equilibrium points and breakpoints are calculated. Eventually, a family of multiple-folded torus chaotic attractors can be obtained. Results of numeral simulations by using computer for generating this kind of chaotic attractors are also given.
2004, 53 (7): 2089-2093. doi: 10.7498/aps.53.2089
The statistical performance of detecting optically rough surfaces with pulsed laser radar using energy detection techniques has been investigated in this paper. Under the far field condition, the analytical expressions for computing the speckle number integrated by the receiving aperture is presented, and the speckle number subtended by the telescope is found to be dependent only on the relative size of the laser beam waist and the area of the entrance pupil. A new statistics for photoelectron counts is proposed when the receiving telescope collects a large number of speckle cells and photoelectrons. The detection probabilities for airborne and spacebased laser radar have been derived. The results indicate that the laser radar performance is closely related to the speckle number observed by the receiving aperture and the influence of the speckle noise is abated with the increase of integrated speckle cells.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
Equivalence of the Dirac method and the Faddeev-Jackiw method in the quantization of constrained systems
2004, 53 (7): 2094-2099. doi: 10.7498/aps.53.2094
We have discussed the Dirac method and Faddeev-Jackiw method in the quantization of constrained systems. The equivalence of motion equation and canonical quantization in the two methods has been proved, and also the correspondence relation of constraints found.
Canonical Ward identity and dynamical mass generation for Abelian gauge theory in higher-order derivative systems
2004, 53 (7): 2100-2105. doi: 10.7498/aps.53.2100
Based on the canonical Ward-Takahashi identities to include composite fields to study gauge symmetry dynamical breaking for Abelian gauge theory in higher-order derivative systems, the mass spectra of both fermion and bound states are obtained. The effect of higher-order derivative terms on mass generation is discussed.
Dependence of isospin fractionation process on the neutron-proton ratio of a colliding system in intermediate energy heavy-ion collisions
2004, 53 (7): 2106-2111. doi: 10.7498/aps.53.2106
The degree of isospin fractionation is measured by the ratio of saturated neutron-proton:i.e. the ratio of gas phase (nucleon emission) to that of liquid phase (fragment emission) in heavy ion collisions. We have studied the dependence of the degree of isospin fractionation on the neutron-proton ratio in the colliding system by using isospin-dependent quantum molecular dynamical model. The calculated results show that the degree of isospin fractionation depends sensitively on the symmetry potential and weakly on the isospin effect of nucleon-nucleon cross section. In particular, the degree of isospin fractionation increases with increasing neutron-proton ratio in the colliding system for the neutron-rich system, in this process the neutron-rich gas phase and neutron-poor liquid phase are produced. The degree of isospin fractionation is very sensitive to the degree of symmetry potential. On the contrary, for the neutron-poor system the neutron-poor gas phase and neutron-rich liquid phase are produced. In this case, the degree of isospin fractionation is not sensitive to the symmetry potential. We also find that the role of momentum dependent interaction in the isospin fractionation process is not obvious. We propose that our calculated results can compare directly with the experimental data to get the information about the symmetry potential in the intermediate energy heavy-ion collisions.
ATOMIC AND MOLECULAR PHYSICS
The influence of symmetries of the substituted donor and acceptor on two-photon absorption cross sections of trans-stilbene derivatives
2004, 53 (7): 2112-2117. doi: 10.7498/aps.53.2112
We have designed a series of trans-stilbene derivatives with different arrangement of the substituents and studied their one-photon and two-photon absorption properties by using the response theory methods at HF level. The calculations show that these molecules have strong two-photon absorption cross sections. In the visible light region, the maximum two-photon absorption cross section for molecules with asymmentric substitution of the donor and acceptor occurs in the first excited state, while for the symmetric substitution cases it occurs in the fourth excited state. The asymmetry-substituted molecules have a larger two-photon absorption cross section than the symmetry substituted molecules, indicating that the effect of the different kinds of substituents on the two-photon absorption of molecules is related to the property of theiry π-centers. The charge-transfer process for the charge-transfer states was visualized in order to understand their maximal two-photon absorption cross sections.
CLASSICAL AREA OF PHENOMENOLOGY
2004, 53 (7): 2118-2122. doi: 10.7498/aps.53.2118
The first and second kind formulas of angular dispersion, corresponding respectively to the polar angle and azimuthal angle of diffraction grating, are presented based on the generalized grating equation produced on the basis of vector diffraction theory under the condition of conical diffraction mounting; at the same time, by making theoretical and numerical analysis of the condition in which the second kind formula of angular dispersion changes evidently and the corresponding mathematical expression is obtained.
2004, 53 (7): 2133-2138. doi: 10.7498/aps.53.2133
In this paper, the quantum properties of light in a system of two coupling atoms interacting with a single-model squeezed vacuum field at the two-photon transition in Kerr medium are studied by means of quantum theory.The influences of the Kerr medium and the dipole-dipole interaction between atoms are discussed. The results show that when the Kerr effect and the dipole-dipole interaction between atoms may be neglected, the fluctuation of U2 can be squeezed periodically. With increasing the Kerr effect and the dipole-dipole interaction between atoms, the squeezing effect of light gradually becomes weak and the squeezing time of light gradually decreases. The periodical collapse-revival phenomenon of the time evolution of the second-order coherence degree of the field appears, which is caused by the influence of the Kerr effect and the dipole-dipole interaction between atoms. No matter how large are the coupling constants, the photon bunching always appears.
2004, 53 (7): 2139-2144. doi: 10.7498/aps.53.2139
In this paper, the interactions between a two-level atom and a coherent field with a time-varying frequency have been studied. The two typical cases, in which the frequency of the field varies with time in the form of sine or impulse, have been considered. It is found that the collapse and revival phenomena of the atomic population inversion will disappear as the frequency changing with time in sine form; and the frequency varying in impulse form can lead to some new collapses and new revivals in the evolution of the atomic population inversion.
The generalized M2 factor of rotationally symmetric truncated beams is studied in detail by using the asymptotic analysis, generalized truncated second-order moments method and self-convergent beam width approach. The physical relation between the three methods and how to determine the generalized M2 factor in a unified way are analyzed and illustrated with numerical results.
2004, 53 (7): 2150-2153. doi: 10.7498/aps.53.2150
A novel coupled large optical cavity cascaded by tunnel-junction semiconductor lasers is put forward to resolve the major difficulties of ordinary laser diodes. In this structure several active regions are cascaded by tunnel junctions to couple a large optical cavity. This structure can solve the problem of catastrophic optical damage of facet and large vertical divergence caused by thin emitting area in ordinary laser diodes. The near-field facalur size reaches 1μm. Low-pressure metal-organic chemical vapor deposition is adopted to grow the novel semiconductor lasers. Slope efficiency as high as 0.80W/A per facet and vertical divergence angle of 20°and threshold current density of 277 A/cm2 are achieved from an uncoated novel laser device.
2004, 53 (7): 2154-2158. doi: 10.7498/aps.53.2154
Based on the linear dependence of laser pulse energy on the ablation area, the ablation threshold fluence of sapphire is calculated in this paper. Meanwhile the threshold fluence of sapphire as a function of pulse duration is studied with an ultra-short pulse laser. The experimental results agree well with the theory of avalanche mode. Finally we discussed the photon absorption mechanism of the conduction band electrons in Al2O3.
2004, 53 (7): 2159-2164. doi: 10.7498/aps.53.2159
The design uses a composite Yb:YAG/YAG disc laser configuration with the pump light incident from the disc edge. The crucial advantages of the thin-disc design is that the thermal gradients are aligned with the beam propagation direction, the transverse temperature gradients are reduced, previous difficulties with high average power pumping/cooling and the rejection of amplified spontaneous emission are resolved. In contrast to high power rods or slabs, the one-dimensional nature of the cooling geometry and the edge-pump geometry scale gracefully to very high average power. Two point designs illustrating power scaling are also given as examples.
2004, 53 (7): 2165-2169. doi: 10.7498/aps.53.2165
Tunable wavelength converters are key components in intelligent all-optical network. In this paper, a novel scheme for tunable wavelength conversion based on semiconductor-fiber ring laser is proposed. Probe laser in the conventional scheme could be left out and self-started tunable wavelength conversion could be achieved. Theoretical model for this novel scheme is also established and the output characteristics are calculated. Experimental results show good agreement with theoretical results. In experimental study, 2.5Gb/s tunable XGM wavelengthconversion with 40nm tuning range was demonstrated. Experimental and theoretical studies show that this novel scheme is worthy of further investigation.
Nonlinear propagation and power limiting of nanosecond laser pulse by stimulated Brillouin scattering in CCl4
2004, 53 (7): 2170-2174. doi: 10.7498/aps.53.2170
Based on the quasi-steady stimulated Brillouin scattering theory, the nonlinear propagation of 1.064μm, YAG nanosecond pulsed light was analyzed numerically and treated with a theoretical model that includes the spontaneous nature of the initiation of stimulated Brillouin scattering(SBS). Then the pulse propagation and power limiting effects (including both power limiting and energy limiting) were obtained in the SBS process. The temporal pulse reshaping and power limiting have been given graphically on SBS mechanism with a nonlinear medium CCl4 for various pump energies and pulse-width (10ns). The power transmission properties and delay time of the power limiting effect depending on pump energy were given theoretically. And the influence of pump energy on the transmitted pulse profiles and delay time were investigated experimentally. The numerical simulations of transmitted pulse characteristics were in good agreement with the experimental results.
2004, 53 (7): 2175-2179. doi: 10.7498/aps.53.2175
The nonlinear hot image effect from phase scatterer in a high-power laser system has been investigated theoretically and numerically. According to the Fresnel-Kirchhoff diffraction integral and nonlinear paraxial wave equation, we derive the functional relationship of the intensity of hot image and its location. It is shown that the image intensity is likely several times larger than that from amplitude scatterer, and the location is situated in the equidistant downstream plane of the scatterer in an optical component located upstream from a nonlinear element. Good agreements are shown between the theoretical prediction and the computer codes developed for ns-pulse propagation simulations. The results indicate that the formation of hot image is akin to the real-time, in-line and volume-phase holographic imaging.
2004, 53 (7): 2180-2183. doi: 10.7498/aps.53.2180
Using the one dimensional particle in cell simulation code, we have studied the generation and redshift of harmonics from overcritical plasma layer irradiated by ultrashort intense laser pulses. Except for the strong oscillations of target surface driven by the laser field, the light pressure of the intense laser can ablate and push the target suface, which is the reason for the redshift of harmonics.
Spatial chirp and angular dispersion of the nonlinear crystal for a femtosecond optical parametric oscillation
2004, 53 (7): 2184-2190. doi: 10.7498/aps.53.2184
In this paper the spatial chirp and the angular dispersion for femtosecond optical parametric oscillation(OPO) are calculated theoretically and analysed. The analytical expression of the spatial chirp in the nonlinear crystal(BBO) is presented. And the analytical expressions of second-and third-order dispersions including angular dispersion of the nonlinear crystal for the noncollinearly phase-matched femtosecond BBO OPO in type-Ⅰ configuration with angle tuning. Moreover, variations of second- and third-order dispersions with the angular dispersion are calculated exactly.
2004, 53 (7): 2191-2196. doi: 10.7498/aps.53.2191
Considering the atomic coherences and injected classical field, we derived the n-Onphase-locking Lorenz-Haken equation by using the technique of stochastic differential equations. The effects of detuning, injected field and the atomic coherence on the dynamical characteristics of this equation is then numerically studied. The results show that in laser working situations, the detuning can result in chaotic behaviors of the light phase. Under different conditions, the system can generate four attractors, double attractors and single attractor, and the fractal dimension of the system is larger than that of the locking-phase system. The optical detuning, injected field and atomic coherence can inhibit the chaos of the cavity field. In the optical bistability regions, we obtain symmetric bistability pair, but never find the chaotic attractors.
2004, 53 (7): 2197-2200. doi: 10.7498/aps.53.2197
By coupling two doped one-dimensional photonic crystals, a novel photonic crystal film heterostructure with both functions of frequency and spatial filters is proposed. In a broad frequency range, this optical film filter only transmits light of a selective frequency at normal direction and reflects light of this selective frequency at all other directions; while all other frequencies within the broad frequency range are reflected in any directions.
2004, 53 (7): 2201-2205. doi: 10.7498/aps.53.2201
The transmission properties of the one-dimensional photonic crystals with defects were studied using the eigen matrix method, and a new kind of tunable optical filter is proposed in this paper. When the one-dimensional photonic crystals (L1H1)m have defects such as LcH2(L2H2)n, there will be a narrow transmission band in the gap of the photonic crystals, meanwhile the other transmission peak will weaken and disappear with the increase of the periodic number of the cover layer. When the thickness of the couple layer changes, there will be two regions with high transmittivity besides the edges of the photonic crystals gap. The properties of the high transmission band depend on the photonic crystals and the cover layers. When the photonic crystals gap is narrowed, the two regions will come near, which results in a wide transmission region about 150 nm. A new kind of tunable optical filter thus can be fabricated based on this phenomenon by the surface micro-mechanical technology.
2004, 53 (7): 2206-2211. doi: 10.7498/aps.53.2206
It was shown theoretically and experimentally that a metamaterial composed of periodically positioned split ring resonators (SRR) can lead to a negative effective permeability μeff close to its resonance frequency. In this paper, the nearest neighbor interactions between hexagon SRR were investigated in experiment. The experimental results show that there is only one resonance frequency for a one-dimensional negative permeability μeff material. The main and defect resonance frequencies have a shift when the defect SRR is introduced into the materials, and the shifts of which increase with the resonance frequency difference between defect and based SRR. It is thought that the interaction between SRR becomes stronger when the resonance frequency of the defect SRR is close to that of the based SRR.
2004, 53 (7): 2212-2215. doi: 10.7498/aps.53.2212
We introduce a technique, chirped pulse cross-correlation, for single shot THz detection. The time-resolution limitation of previously demonstrated single-shot amplitude modulation spectral encoding is analyzed. The new technique is free from the limitation.
2004, 53 (7): 2216-2220. doi: 10.7498/aps.53.2216
In this paper we report the measurements of the damage threshold and ablation depth for fused silica by laser pulses at the wavelength of 800 nm, with the pulse duration ranging from 45 to 800fs. These results can be well understood by using an improved avalanche model. The model includes the production of conduction band electrons(CBE), laser energy deposition, and CBE diffusion. The evolution of micro-explosion is investigated based on this model.
2004, 53 (7): 2221-2225. doi: 10.7498/aps.53.2221
A novel method for multiplexing fiber-optic Fizeau strain sensors based on optical amplification is proposed and demonstrated for the first time to our knowledge. This method overcomes the two intrinsic disadvantages of fiber-optic Fabry-Perot strain sensors, i.e. weak signal and difficult multiplexing. Also, the amplified spontaneous emission and optical amplification of the Erbiumdoped fiber pumped by a 980nm laser diode are used simultaneously. The structure, principle and experimental results of the sensor system are described in this paper. A strain accuracy of ～±10με has been achieved, which can meet the practical requirement.
2004, 53 (7): 2226-2232. doi: 10.7498/aps.53.2226
The sweep down flow and related flow structures in a turbulent boundary layer were visualized with hydrogen bubble method. The streamwise and transverse vortices induced by sweep down flow near the wall of a vessel were observed. A new kind of vortical structure, whose features are contrary to those of the hairpin vortex, was observed in the present experiments. It is termed “contrahairpin vortex” here. Its head points upstream, while the head of a hairpin vortex points downstream; lift-up flow is induced between the legs of a hairpin vortex, while between the legs of this new vortical structure, sweep down flow is induced.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2004, 53 (7): 2233-2236. doi: 10.7498/aps.53.2233
An auxiliary differential equation finite-difference time-domain (ADE-FDTD) methodology for anisotropic magnetized plasmas is derived. The method is based on a difference approximation of the auxiliary differential equation. A comparison with the JEC method is included. The CPU time saving by several times and accuracy of the method are confirmed by computing the reflection and transmission through a magnetized plasma layer, with the direction of propagation parallel to the direction of the biasing field.
2004, 53 (7): 2237-2243. doi: 10.7498/aps.53.2237
The whole ablation process of target heated by pulsed lasers is studied in this paper. Under the appropriate dynamic boundary conditions, have been investigated in detail the heat flow equation with heat generation term, and the temperature distribution of target before and after the melting. In particular, after target melting, the temperature distribution and the interface position between the solid and liquid phases as functions of time are presented in the solution of analytical expressions. Additionally, under the energy balance conditions, the dependence of the ablating surface position on the time is investigated. Consequently, the ablation of target Si has been calculated. This research shows a positive result compared with the relevant experiments.
2004, 53 (7): 2244-2249. doi: 10.7498/aps.53.2244
In the form of joint experiment between China and Russia, the experimental studies of Zpinches of mixed wire array of aluminum (A1) and tungsten (W) were carried out on S-300 generator,which was located on Kurchatov Institute of Russia. The experimental results were compared with those of single A1 array and single W array, respectively. There are obvious difference between mixed one and single one in their photon spectral distributions. The intensity of K-series emission lines from the mixed wire array Z-pinch is lower than that from single A1 array. The radiated lines with wavelengths less than 1.6nm were not found in single W array Z-pinches. In the Z-pinch processes, the area radiating x-arys in mixed wire array is smaller than that of single A1 array, but is larger than that of single W array. The radiated energy from mixed wire array Z-pinch is higher than that from single A1 array, but is slightly lower than that from single W array. The FWHM of x-ray pulse with a maximal power 0.3—0.5TW and total energy 10—20kJ is about 25ns, which radiated from Z-pinches with a radial convergence of 4—5 on S-300 generator. The shadow photograph of the mixed wire-array Z-pinch plasma by laser probe shows that the core-corona configuration was formed and the corona was moving toward the center axis during the wire-array plasma formation, that the interface of the plasma is not clear, and that therer are a number of structures inside. They also suggests that there was an obvious development of Magneto Rayleigh-Taylor instability in the Z-pinch process as well.
Theoretical study on the nuclear fusion mechanism of deuterium clusters aroused by Coulomb explosions with femtosecond intense laser
2004, 53 (7): 2250-2253. doi: 10.7498/aps.53.2250
Considering the Coulomb explosion induced by the interaction of a deuterium cluster target with an ultra-intensie femtosecond laser, the mechanism which generates energetic deuterium nuclei for the fusion has been analyzed．The formulas for expansions of deuterium ion clusters, which are driven by Coulomb explosion, are proposed; and hence the kinetic energies of deuterium nuclei and the expansion time of deuterium ion clusters have been estimated．
The study on the efficient method for producing large amount of single-walled carbon nanotubes with recirculated arc discharge
2004, 53 (7): 2254-2257. doi: 10.7498/aps.53.2254
By using Y-Ni-C composite rod as cathode, large amount of single-walled carbon nanotubes was produced by an improved arc discharge method, in which a tungsten rod was used as an electrode and the electrodes were exchanged after each discharge. The raw soot was characterized by high resdution electron microscope and Raman spectrum, and the results show that this is an efficient way to produce large amount of single-walled carbon nanotubes.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2004, 53 (7): 2258-2262. doi: 10.7498/aps.53.2258
Individual high-aspectratio carbon nanofibers(CNFs) were grown on W wire by plasma-enhanced hot filament chemical vapor deposition method, using a gas mixture of methane and hydrogen. The average diameter and length of carbon nanofibers are 60—100 nm and 6—30 μm, respectively, and the density of carbon nanofibers is less than 10.6cm-2. The field emission properties from individual carbon nanofibers have been measured using a movable W probe with a low-curvature radius. The results indicated that the CNFs showed a turn on field of about 5 V/μm and the field-emission current density of 20μA/cm2 at 5 V/μm.The experimental data also indicated that the length of CNFs and the position of a CNF are responsible for the properties of field emission related to the parameters such as the work function of materials, applied field and the field amplification factor. Furthermore, the electron scattering induced by defects in CNFs is also a key factor on the field-emission current.
Investigation of a-Si:H film characteristics influenced by magnetic field gradient in MWECR CVD plasma system
2004, 53 (7): 2263-2269. doi: 10.7498/aps.53.2263
In this paper the magnetic field profiles produced by coil currents of I=115.2A，I=137.7A and I=137.7A with a SmCo permanent magnet under the substrate holder in the deposition chamber and plasma chamber in the single-coil divergent field MWECR(micro-wave electron cyclotron resonance) CVD system was investigated. Then the magnetic field gradient of these magnetic field profiles was obtained quantitatively using Lorentz fit. The results indicated that the gradient value nearby the substrate, which was produced under the condition that the magnetic field profiles are produced by the coil current 137.7A with a SmCo permanent magnet under the substrate holder, is the largest, the larger one being produced by the coil current of 137.7A without the permanent magnet and the smallest one being produced by coil current of 115.2A. The effect of magnetic field gradient on characteristics of a-Si:H film has been also analyzed. It was found that high deposition rate was observed nearby the substrate with a high magnetic field gradient and that the better photosensitivity of a-Si:H film could be obtained at the lower substrate temperature when magnetic field gradient is high.
2004, 53 (7): 2270-2274. doi: 10.7498/aps.53.2270
The laser-micro-Raman spectra of near-stoichiometric LiNbO3 crystals were measured and analyzed. The crystals were grown by flux pulling method. Compared with the congruent LiNbO3 crystal, the number and frequency of Raman spectral lines of A1(TO) and E(TO) modes have no change. It was validated that the LiNbO3 crystal is substitutional solid solution. The relative intensity of 738cm-1 Raman peak(A1(LO)) exhibited obvious change when the Li2O contents increase in pure LiNbO3 crystal, the 738cm-1 Raman peak disappeared in the Mg-doped and Zn-doped LiNbO3 crystals. These Raman peaks were discussed and explained from the occupied site and structure according to the Li vacancy model. The full width at half maximum (FWHM) of the 152 and 872cm-1 Raman peaks narrows with increasing the Li/Nb in the crystals. Their relationships were fitted linearlly, using the fitting equation, and the Li/Nb ratio can be calculated according to the FWHM of Raman peaks.
2004, 53 (7): 2275-2280. doi: 10.7498/aps.53.2275
Using the renormalization-group transformation and cumulant expansion technique, the Gaussian model with two-spin interactions and triplet-spin interactions on the Sierpinski gasket lattice is studied, and its fixed points and critical exponents are obtained. Compared with the case of only two-spin interactions, the fixed points and critical exponents have changed. The results show that the fixed points and critical exponents are all dependent on triplet-spin interactions in the cases with or without external field.
Morphological characterization and nanoindentation hardness scatter evaluation for Cu-W thin films based on wavelet transform
2004, 53 (7): 2281-2286. doi: 10.7498/aps.53.2281
A strategy based on wavelet transform to describe surface morphology of thin films is presented in this paper.The evolution of surface morphology of Cu-W thin films with deposition time on silicon wafers was investigated by discrete wavelet transform (DWT) method. The results show that the surface morphology of the thin films is unstable until the sputtering time exceeds 10 min. The surface morphology variation of different thin films can be distinguished primarily by high frequency signals.A scattering of the nanoindentation hardness values, which results from the roughness of the thin films surface, can be characterized by the roughness defined by the surface texture based on wavelet transform.
Influence of hydrogen-enhanced etching on the quality of diamond films and the existing form of sp2 bonding carbon atoms
2004, 53 (7): 2287-2291. doi: 10.7498/aps.53.2287
Diamond thin films were prepared by using chemical vapor deposition. It is enhanced to etch the sp2 bonding carbon atoms of the film by intermittently closing the methane gas during the deposition. These films were characterized by Raman spectroscopy and microscopy. The results show that the enhanced etching of sp2 bonding carbon atoms has no obvious effect on quality and microstructure of the diamond films. It reveals that the sp2 bonding carbon atoms do not mainly exist in graphitic or amorphous carbon grain form in the films but mainly among the carbon atoms of diamond grain surface or grain boundary.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2004, 53 (7): 2292-2298. doi: 10.7498/aps.53.2292
In C60 molecules, the lowest unoccupied molecular orbital has T1u symmetry. The orbital is easily occupied by an electron through molecular inner excitation or doping and becomes an active Jahn-Teller (JT) electronic state. The T1u-hg- JT system is formed by the coupling of such an electronic state to a five-fold degenerate hg mode. The linear vibronic coupling in the system forms a trough on the adiabatic potential surface of the system, but D5d and D3d minima will be warped while considering the quadratic vibronic coupling. Phonon states in minima will have anisotropic effects and the vibrational frequencies should be different in different directions. This phenomenon will further affect the energy distribution, phonon overlap and tunneling effect in the system. The anisotropic effect is carefully studied in detail for D3d-minima. The energies and vibrational frequencies are calculated using unitary shift transformation, pick-Pryce method, scale transformation and energy minimization procedure. The energy gap and the variation of perturbation energy in minima are obtained in this paper and the same physical quantities in trough are derived through changing the values of the quadratic coupling constants.
2004, 53 (7): 2299-2304. doi: 10.7498/aps.53.2299
The structure and transport properties of perovskite (La1-xYx)2/3Ca1/3 MnO3 (0≤x≤0.3) systems are systematically investigated. It is found that all the compounds show a single phase structure. With the increase of Y3+ doping content the metalinsulator transition temperature TMI(M—I) shifts to lower temperatures. While the relevant resistance peak ρp is sharply increased, for the sample of x=0.3, it has been enhanced by eight orders of magnitude larger than the non-doped sample(x=0.0). In these materials a large magnetoresistance effect has been observed under an external magnetic field. At the same time, it is also directly shown that the correlation between the transport properties and the variation of crystal structure from the experiment result. Based on the double exchange model and the variable-range-hopping(VRH) theory, the mechanism of the influence of Y doping for La in La2/3Ca1/3MnO3 systems is also discussed.
2004, 53 (7): 2305-2309. doi: 10.7498/aps.53.2305
Lattice vibration of one-dimensional copolymer chains was studied and the dispersion relation was obtained numerically. The character of the vibration modes was recognized. It was found that the interface modes may appear if the homopolymer blocks have a strong interaction with each other, whose frequencies lie in the band gap between the acoustic and optical branches or between two optical branches. The character of the organic quantum well or superlattice of a copolymer chain was discussed from the view point of vibration modes.
Study on the high-resolution thermoelectric power of metallic state quasi-one-dimensional organic superconductor of (TMTSF)2ClO4 salt
2004, 53 (7): 2310-2315. doi: 10.7498/aps.53.2310
We investigate the high-resolution thermoelectric power of Bechgaard salt (TMTSF)2ClO4 in temperatures between 6 and 280K for metallic state with different cooling rates through the anion-ordering transition Ta=24K. There is a linear temperature-dependent thermoelectric power at high temperatures, which cannot be explained by conventional one-dimensional(1D) electron band theory. A sudden change of the slope at about 240K is observed for the first time. Below 140K, the curve deviates from linear behavior and goes down all the way through, which is attributed to a 1D—2D crossover. Comparing with the results of specific heat and resistivity with different cooling rates through the anion-ordering transition at 24K, no clear trace of anion-ordering transition can be spotted in thermoelectric power by the slowest cooling process unless we take differentiation of the data and we found a gradual rise in thermoelectric power with increasing cooling rate through the Ta for the first time. The influence of anion-ordering transition is discussed.
2004, 53 (7): 2316-2319. doi: 10.7498/aps.53.2316
The scheme of attosecond high-order harmonic generation driven by a two-pulse laser field is proposed. Calculated results show that, not only a single attosecond x-ray pulse can be produced with two pulses, which may break the energy limitation of the few-cycle pulse, but also the intensity is even higher than that generated by a single laser pulse with the same intensity.
Influence of polarizations and doping in AlGaN barrier on the two-dimensional electron-gas in AlGaN/GaN heterostruture
2004, 53 (7): 2320-2324. doi: 10.7498/aps.53.2320
The two-dimensional electron-gas (2DEG) distribution, sheet density and subband occupations in AlxGa1-xN/GaN heterostructure are calculated by solving the coupled Schrdinger and Poisson equations selfconsistently. By involving degree of relaxation to distinguish the spontaneous and piezoelectric polarizations, the influence of the polarizations and the doping in the AlGaN barrier on the 2DEG properties are investigated separately. It is found that the 2DEG properties depend much stronger on the polarizations than on the doping, and that the contribution of the spontaneous polarization is dominant. The 2DEG sheet density is calculated to be 1.6×10-13cm-2 with an Al-content of x=0.3 in the strained AlGaN barrier, the contributions of spontaneous and piezoelectric polarizations are 0.9×10-13 and 0.7×10-13cm-2 respectively. When the doping of the AlGaN barrier is increased from 1×10-17 to 1×10-18cm-3, the 2DEG sheet density is increased by 0.2×10-13cm-2
2004, 53 (7): 2325-2329. doi: 10.7498/aps.53.2325
Multicomponent excited-state emissions from monomer, exciplex and electroplex were observed in a bilayer organic electroluminescent device with a structure of ITO/TPD(60nm)/PBD(60nm)/Al(where TPD and PBD stand for N,N′diphenylN,N′bis(3-methylphenyl)1,1′biphenyl4,4′diamine) and (2(4′biphenyl)5(4″tertbutylphenyl)1,3,4oxadiazole, respectively). The difference between the photoluminescence of this device and the TPD∶PBD equimolar evaporated films suggests that the exciplex takes place only at the interface of TPD/PBD. The Variation of electroluminescence spectra with bias indicates the different formation mechanisms and the different population of each excited state component, as well as the different dynamic recombination process of carriers in this device.
2004, 53 (7): 2330-2335. doi: 10.7498/aps.53.2330
A theoretical study, within the effective-mass approximation, on the coupling effects of applied magnetic fields along the growth direction of the superlattice on the surface electron states in a semi-infinite semiconductor superlattice is presented by using an effective-barrier height method. The coupling effect is necessarily considered when the difference of the electron effective masses between different materials cannot be neglected. Our numerical results show that the magneto-coupling effect brings about not only the splitting of the surface electron levels but also the definite dependence of the surface levels and its localization degree on magnetic fields and Landau indices. Our results also indicate that the imaginary component of Bloch wave number can serve as a measure of the localization degree of the surface electron states.
2004, 53 (7): 2336-2341. doi: 10.7498/aps.53.2336
We study theoretically the properties of the ground state of a mescopic ring with a seriescoupled double quantum dot in the Kondo regime by means of the two-inpurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. It is shown that two dots can be coupled coherently, which is reflected in the appearance of parity effects and the complex current-phase relation in this system. This system might be a possile candidate for future device applications.
Magnetic properties and colossal magnetoresistance effect in perovskite-type manganite Ca(Mn2Cu1)Mn4O12
2004, 53 (7): 2342-2346. doi: 10.7498/aps.53.2342
The perovskitetype manganites Ca(Mn2Cu1)Mn4O12 were prepared using the conventional solid state reaction method. X-ray diffraction patterns show that the annealing in air at 1073 K after the first sintering at higher temperatures can reduce the volumetric percentage of impurity phases in samples. Magnetic measurements indicate that the samples exhibit ferromagnetic antiferromagnetic transitions at low temperatures. Magnetization of the samples at 4.5 K is greatly decreased due to the existence of the antiferromagnetic phase, and shows no sign of saturation at 4.5 K and a magnetic field up to 8 T. The samples show semiconducting behavior. Their MR ratio reaches -35%—-46% at the temperature of 85 K and the magnetic field of 6 T.
2004, 53 (7): 2352-2356. doi: 10.7498/aps.53.2352
Fe-R-O (R=Hf, Nd and Dy) films were made by reactive sputtering technique and annealed at 400℃. X-ray diffraction and electron diffraction show that nanometer scale Fe grains are embedded in amorphous Roxide matrix. Inplaneferromagnetic resonance was systematically carried out to measure the inplaneresonance absorption spectra and utilized to analyze the relationship between the local magnetization and grain size. The asdeposited samples show obvious uniaxial anisotropy, while annealed samples display weak magnetic crystalline anisotropy at the expense of weakening the uni-axial anisotropy. The local magnetization was calculated based on the formula. (ω/γ)2=(Hres+HK)(Hres+HK+4πMs).It decreases with decreasing grain size, being as only about 30% of the saturation magnetization of bulk Fe. It is also smaller than the bulk magnetization calculated according to the volume ratio of Fe. The linear dependence on the inverse of the grain size indicates a strong surface moment pinning effect.
2004, 53 (7): 2357-2362. doi: 10.7498/aps.53.2357
The dielectric, piezoelectric, ferroelectric and pyroelectric properties of (Bi0.5Na0.5)1-xBaxTiO3 (x=0.00，0.04，0.06，0.08, 0.12) ceramics prepared by sol-gel method were investigated. It was found that all the samples in this experiment show improved piezoelectric properties,obtained by using this chemical method. For(Bi0.5Na0.5)0.94Ba0.06TiO3 ceramic, as its composition is near the morphotropic phase boundary (MPB), it shows the largest piezoelectric constant, d33=173×10-12C/N, which is about 40% improvement compared to the conventional processed ceramics in this system. In addition, from the ferroelectric and pyroelectric measurements, the remanent polarization Pr, coercive field Ec and depolarization temperature Td of the (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramic were found to be 25μC/cm2, 28kV/cm, and 80℃, respectively.
2004, 53 (7): 2363-2367. doi: 10.7498/aps.53.2363
Ferroelectric SrxBa1-xNb2O6 (0.2＜x＜0.8, SBN100 x＝ thin films of highly preferred c axis orientation have been grown on Si (100) substrate by the sol-gel method with post annealing at 1000°C. Investigated by x-ray diffraction and second ion mass spectrum,we observed that the SBN thin films prepared using NbCl5 precursor solution contained K+ ions, compared with Nb(OC2H5)5 precursored SBN films. By choosing appropriate contents of K+ there exists the optimum preferred orientation in SBN thin film.K ions dissolving in SBN cell and entering Si substrate can make SBN cell and Si cell a small twist simultaneously,which improves the matching of the film and the substrate to promote the high c axis superior growth.Finally,the optic characteristics of thin films have been tested.
2004, 53 (7): 2368-2372. doi: 10.7498/aps.53.2368
In this paper we present a solution that addresses the reflectance loss due to oxidation. The solution is a capping layer (CL) that acts as an effective oxidation barrier when the multilayer (ML) is exposed to the extreme ultraviolet (EUV) light in the presence of water vapor, so that the optical element of multilayer can be used as long as possible. The theoretical reflectivity of ML Mo/Si EUV mirrors is calculated at 13.9nm under the different CL materials. The thicknesses of the CL and standard ML are optimized by the simplex method, meanwhile the reflectivity of ML can be further enhanced when the layer by layer theory is used. Finally the electric-field intensity distribution in the top layers of the ML is analyzed when the CL is added.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
The effect of baking temperature on the crystal structure and ferroelectric properties of Bi3.25La0.75Ti3O12 thin films prepared by sol-gel processing
2004, 53 (7): 2373-2377. doi: 10.7498/aps.53.2373
The crystal structures, surface morphology and ferroelectric properties of Bi3.25La0.75Ti3O12 (BLT) thin films were found to be greatly affected by the baking temperature during sol-gel processing. At lower baking temperatures, the (117) orientation in the BLT thin film was preferred. However, with the increase of baking temperature, the (117) diffraction peak became weaker. In addition, the surface morphology of the films changed from rodlike to platelike. The ferroelectric properties were measured and it was found that the BLT thin film has the largest remnant polarization (2Pr) 284μC/cm2 prepared at a baking temperature of 250℃.
Influence of pressure on solidification microstructure of FeSi2 alloy is investigated. Different from the eutectic structure of the sample solidified at normal pressure, the microstructure of the sample solidified at high pressure is composed of dendritic primary phase and anomalous eutectic. The significant change in microstructure results from the change of phase diagram and solute atom diffusion induced by high pressure. By introducing the pressure parameter, the constitution undercooling criterion is attained. Formation mechanism of dendritic primary phase under high pressure is analysed according to the constitution criterion.
Experimental study on the hydrogen storage and release in nano-graphite synthesized by explosive detonation
2004, 53 (7): 2384-2388. doi: 10.7498/aps.53.2384
In this article, synthesis of nanographite powders using pure TNT ( trinitrotoluene )explosives is reported. X-ray Diffraction (XRD) results indicated that the detonation product has a hexagonal-graphite structure and the average size of the particles of graphite is 1.86—2.61nm. Specific surface area of nano-graphite is about 500—650m2/g measured by BET gas sorption instrument.Crystal grain size of the nano-graphite calculated is 4.41—6.85nm by its specific surface area. A hydrogen storage and release capacity of 0.33—0.37 weight at room temperature under a modestly high pressure (12MPa) for untreated nano-graphte samples synthesized under different conditions. Under the same condition, the capacity of hydrogen storage and release of untreated carbon nanofibers、multi walled carbon nanotubes and superactive carbon is 0.15wt%—0.35wt％，0.15wt%—0.20wt％，0.92wt%—0.98wt％，respectively.The specific surface area of carbon nanostructured materials is the most important factor in its hydrogen storage and release.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2004, 53 (7): 2389-2395. doi: 10.7498/aps.53.2389
In light of irreversibility of atmospheric motion, the memory function obtained by utilizing the previous information fully obtained from the observational data is introduced. The selfmemorization equation of the heat conduction equation is induced, and the stability and characteristics of which are studied, taking respectively Richardson scheme (RS) and DuFort-Frankel scheme (DS) as the retrospective order p=1. The calculation results indicate that the numerical integral is diffused sometimes in the multi-time model, due to the fact that the memory function is determined by the observational data via the special mathematics arithmetic, which makes the difference scheme designed previously change, and is a smooth time factor in itself.
2004, 53 (7): 2396-2400. doi: 10.7498/aps.53.2396
Dirac equation near horizons of Vaidya-Bonner-de Sitter black hole is discussed. Its Hawking temperature and radiation spectrum are calculated accurately. At the same time, the equation of event horizon is given, which is consistent with the result calculated by zero-curve equation.
The entropies of quantum fields on event horizon of a toroidal black hole are given using the improved brick-wall model. According to the view of the improved brick-wall model, these entropies are nothing but the entropy of the black hole. So the entropy of the toroidal black hole is proportional to the area of event horizon, which satisfies the Bekenstein-Hawking formula.