Vol. 47, No. 7 (1998)
1998-04-05
GENERAL
1998, 47 (7): 1057-1063.
doi: 10.7498/aps.47.1057
Abstract +
We have studied the motion law of spiral waves in two-dimensional excitable media. The motion of the wave front of spiral waves is divided into two parts, i.e., those along the tangent and the normal directions, and the motion equations in the two directions are obtained with the help of the perburbation method. When these equations are used to analyse the spiral tip, the basic motion law of the spiral tip and the structures of twisted rings and long twisted chains are obtained. When these equations are used to analyses the wave front and its disturbance, the result not only shows ordinary spiral shapes but also explains the super-spiral and the double-spiral structures.
1998, 47 (7): 1064-1070.
doi: 10.7498/aps.47.1064
Abstract +
It is well known that three kinds of soliton solutions have been found to the nonlinear coupled scatar field equations. In this paper we continue to present five other kinds of exact solutions by using some appropriate function transformations. Applications of these results to other well-known equations in physics are also discussed.
1998, 47 (7): 1071-1077.
doi: 10.7498/aps.47.1071
Abstract +
In this paper are discussed the time evolution and squeezed states of a time-depedent oscillator system. We obtain closed formulas for the time evolution of quantum states and the evolution operator of the system by selecting proper Hermitian invariant operator. We derive the condition generating squeezed states and obtain that the drive part does not influence the squeezed states of the system.
1998, 47 (7): 1078-1083.
doi: 10.7498/aps.47.1078
Abstract +
Finite impulse response digital filter was used to stabilize the unstable periodic orbits (UPO) embedded in a chaotic attractor of differential dynamic system and two-dimensional map; and thus high periodic orbits was controlled. As examples we studied the Lorenz system and Henon map. A preliminary analysis and results of numerical analogy are presented. The advantages of this method are that it does not need to know the linear properities and the periodicity about a UPO or that of the UPO extracted from the chaotic attractor. The choice of control parameter is independent of the controlled chaotic system.
1998, 47 (7): 1084-1089.
doi: 10.7498/aps.47.1084
Abstract +
This paper reports the experimental evidence for the scaling law of Lyapunov exponent in type V intermittency obtained in an electronic relaxation oscillator. The results are in very good agreement with the numerivcal computations conducted with the theoretical model of the oscillator.
1998, 47 (7): 1090-1100.
doi: 10.7498/aps.47.1090
Abstract +
By applying the definitions of curvilinear coordinate in differential geometry and using the main electron trajectory as the curve axis, the first order and second order paraxial electron trajectory equations in the three-dimensional coordinate system are obtained. Some numerical results of these equations applied to the analysis of an electron-beam scanning flat cathode ray tube are given.
CLASSICAL AREA OF PHENOMENOLOGY
1998, 47 (7): 1101-1109.
doi: 10.7498/aps.47.1101
Abstract +
To avoid the poor performance of the Hopfield model for unequally distributed neuron states and to alleviate the dynamic range constraint of optical system, a clipped model with asymmetric clipping points is proposed. Apart from its easiness for optical implementation, the capacity and capablilty of noisy tolerance are improved greatly, compared with the former clipped model. In addition, a practical encoding method-beam-direction encoding method is proposed to implement the above clipped model. The preliminary experimental results are given.
1998, 47 (7): 1110-1117.
doi: 10.7498/aps.47.1110
Abstract +
We study the quantum correlation between the signal and pump modes in forward three-wave mixing in media with an intensity-dependent index of refraction, by adopting the semiclassical and the full-quantum theories. Strong positive correlation exists between the two signal modes, but there is strong anti-correlation between the signal modes and the pump. In the semiclassical model, the phase mismatched cases are investigated. In the full-quantum analysis, the effects of the pump quantization and losses are considered.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
1998, 47 (7): 1118-1122.
doi: 10.7498/aps.47.1118
Abstract +
A new mass resolving neutral particle analyzer was installed on the HT-7 tokamak.The analyzer is constructed with electric and magnetic fields and is calibrated over an energy range of 0.2—50keV.The new analyzer provides a wide energy range,mass resolving,perpendicular scanning charge-exchange diagnostic.In Ohmic-heated deuterium discharges,the analyzer provides the measurement of deuterium ion temperature over a range of viewing angles,and radial ion temperature profiles can be obtained.
1998, 47 (7): 1123-1129.
doi: 10.7498/aps.47.1123
Abstract +
The propagation of ultra-intense short laser pulse in uniform underdense plasma has been investigated.The evolution equation and the nonlinear envelope equation were derived.Based on these derived equations,the characteristics of the relativistic harmonics saturation and the laser envelope evolution are discussed.
1998, 47 (7): 1130-1136.
doi: 10.7498/aps.47.1130
Abstract +
The electromagnetic field distribution of ion cyclotron wave in tokamak geometry is discussed in detail in this paper.As for the problem that conventional antenna coupling slab model gives larger error to the calculation of large aperture antenna,an improved model is presented that can upgrade the calculation precision greatly.The theoretically computed results are in good agreement with the experimental observation.
1998, 47 (7): 1137-1142.
doi: 10.7498/aps.47.1137
Abstract +
The standard normal distribution for particles in tokamak geometry is usually assumed in fast wave heating.In fact,due to the quasi-linear diffusion effect,the parallel and vertical temperatures of the resonant particles are not equal to each other,so,this will bring some error.For this case,the Fokker-Planck equation is introduced in this paper,and iteration algorithm is adopted to solve the problem well.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
1998, 47 (7): 1143-1148.
doi: 10.7498/aps.47.1143
Abstract +
A new approach for quantitative X-ray diffraction phase analysis of a powder mixture has been developed using the doping method proposed by Popovic.Two cases of known and unknown mass absorption coefficients of amorphous materials are discussed.The weight fraction of amorphous material or unidentified phases can be obtained by this method.For the determination of multicomponent system containing n phases,in which (n-2) pure phases are deliberately added,the weight fractions of all n phases can be determined.The procedure has been tested by using four-to five-component samples,the analysis results agree well with the theory.
1998, 47 (7): 1149-1154.
doi: 10.7498/aps.47.1149
Abstract +
Solid state amorphization reaction process in Ni/amorphous Si multilayers has been quanlitatively studied by using in situ X-ray diffraction. An amorphous formation and growth model is suggested for elucidating the solid state reaction in the Ni/amorphous Si multilayers. Thermodynamic and kinetic interpretations for the amorphization reaction at grain boundaries in Ni sublayers are presented.
1998, 47 (7): 1155-1160.
doi: 10.7498/aps.47.1155
Abstract +
The wake potential and energy loss of a swift ion moving parallel to a heavily doped semiconductor surface in vacuum are studied with the specular-reflection model and the dielectric response theory. The phonon-electron coupling effect on the surface wake potential and the energy loss is investigated by using a frequency-dependent dielectric function. We find that there are two branches of phonon-electron coupling excitations which contribute to the wake potential and the energy loss. Numerical results show that the oscillation form of the wake potential is obviously influenced by the phonon-electron coupling effect.
1998, 47 (7): 1161-1170.
doi: 10.7498/aps.47.1161
Abstract +
Internal friction measurements have been performed at the different frequencies, temperatures and strain amplitudes to examine the damping characteristics of foamed aluminum. It is found that the pores contribute to the damping by raising the internal friction background, particularly the high temperature one. The other significant feature is that the internal friction is strain-amplitude-dependent, which becomes more obvious as the pore size decreases. There are inhomogeneous pores and high density of defects in the structure, leading to nonuniformly distributed stress and strain. This state will be intensified under the disturbance of applied load, making the defect density increase. The defect effects are therefore responsible for the damping characteristics of foamed aluminum.
1998, 47 (7): 1171-1179.
doi: 10.7498/aps.47.1171
Abstract +
The admittance spectroscopy technique has been used to study the hole confinement in Si/Si1-xGex/Si and boron highly doped superthin Si quantum wells. Based on the carrier thermal emission medel of the carriers in the well, the value of activation energy derived from the admittance spectra was considered as the distance from the position of the heavy hole ground state in the well to the top of it . For Si/Si1-xGex/Si quantum wells we found the value of activation energy increased with increasing annealing time at 800℃. The phenomena could be explained the annealing-induced interdiffusion of Si and Ge, the interface broadening, the weakening of quantum confinement, the decrease of the position of the heavy hole ground state, and so the increase of the activation energy. At the annealing temperature of 900℃, we only see the decrease of the value of activation energy with increasing annealing time. For boron highly doped superthin Si quantum wells due to quantum confinement effect, we observed the activation energy are different for samples with the same doping density but different well widths.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
1998, 47 (7): 1180-1186.
doi: 10.7498/aps.47.1180
Abstract +
We present a theoretical study of quasi-classical transport of two-dimensional Bloch electrons under the influence of an electric and a magnetic fields, starting respectively from the quasi-classical equation of motion of electrons and from the balance equations proposed by one of the authors. The electron longitudinal velocity vx and the Hall drift velocity vy are calculated as functions of the magnetic field under different strengths of the applied electric field. It is shown that both vx and vy always change continuously with variation of magnetic field. No sudden jump in the vx-B or the vy-B curve has been found at any given electric field, in contradiction with the claim made in a recent paper published in Acta Physica Sinica (vol. 46). The possible cause for this disagreement is discussed. Furthermore, we point out that the validity of the naive way to study electron transport by directly introducing a damping force into the quasi-classical equation of motion of electron is quite limited.
1998, 47 (7): 1187-1192.
doi: 10.7498/aps.47.1187
Abstract +
Starting from the equation for the classical motion of a nondissipative mesoscopic capacitance coupled circuits with source,we have studied the quantum fluctuations of the charge and current in the eigenstate of the coupled circuits and in the squeezed vacuum state,respectively.The results show that the quantum fluctuations in the two circuits are correlated with each other.
1998, 47 (7): 1193-1200.
doi: 10.7498/aps.47.1193
Abstract +
A series of La0.67Ca0.33MnO3/(La0.35Nd0.65)2/3Ca1/3MnO3/La0.67Ca0.33MnO3(termed as LCMO/LNCMO/LCMO) epitaxial trilayered films, in which LNCMO under zero magnetic field exhibits semiconductor-like conducting behavior down to 4.2K, have been fabricated on SrTiO3(001) substrates by d.c. magnetron sputtering technique. The measurements of resistance show that transport properties of the trilayered films are clearly different from that of the pure LNCMO or LCMO films. In the LCMO(75nm)/LNCMO(450nm)/LCMO(75nm)film, two resistance peaks have been observed. With the measurements of magnetic hysteresis loop, it show that there may exist a magnetic exchange-coupling interaction and an intra-magnetic field in the trilayered film. When the external magnetic field is applied, the resistance decreases and the resistance peak shifts to higher temperature, therefore it appears colossal magnetoresistance effect. Compared with the magnetoresistance response in pure LCMO film, the enhanced magnetoresistance effect have been observed in these trilayed films, and it maintains a stronger magnetoresistance effect in a wide temperature range. We suggest it results from the middle layer LNCMO in the trilayered film.
1998, 47 (7): 1201-1206.
doi: 10.7498/aps.47.1201
Abstract +
Rare earth element of erbium implanted into silicon was studied by photoluminescence,and nine spectral lines were observed.The analysis of the symmetry of crystal field shows that the nine spectral lines come from the crystal-field splitting of the transition 4I13/2→4I15/2.We concludethat the symmetry of the two low-energy excited (initial) state levels,in order of increasing energy,are Γ8 and Γ6, from which to the splitting state levels of 4I15/2 nine radiative transitions are allowed.Oxygen codopants can increase greatly the PL intensity of Er3+, but the spectral features are similar,indicating oxygen codopants can enhance the intensity of Td crystal field rather than change the Td symmetry of the crystal field on the Er3+ sites.
1998, 47 (7): 1207-1212.
doi: 10.7498/aps.47.1207
Abstract +
Nanometric Scale X1-Y2SiO5∶Eu was prepared by sol-gel method with particle size about 50nm.The optical properties of the sample were investigated.Its luminescence spectra were recorded at room temperature and 15K using site selective excitation by a dye laser in addition to conventional UV excitation.Site selective excitation gave two sites for the Eu3+ ion in X1-Y2SiO5.These two sites differ significantly from each other as stated by their emission spectra.Wavelengths of the 5D0→7F0—4 transitions of the two sites were tabulated.Lifetimes of the 5D0 energy level were also measured. They are 3.00ms (site 1) and 2.18ms (site 2).
1998, 47 (7): 1213-1221.
doi: 10.7498/aps.47.1213
Abstract +
A theoretical Treatment is presented on the propagation characteristics of magnetostatic forward volume waves and related guided-wave magnetooptic (MO) Bragg diffraction in an YIG-GGG waveguide under an inclined bias magnetic field.It is found that,in contrast to the usual situation of a perpendicular bias magnetic field,an inclined bias magnetic field leads to the modification to the phase matching condition and,as a result of increased Faraday effect etc.,facilitates enhancement of MO Bragg diffraction efficiency.Such an efficiency enhancement enables utilization of low-propagation loss materials such as the pure YIG for construction of viable guided-wave MO Bragg devices.The theoretical predictions are in good agreement with the experimental results.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
1998, 47 (7): 1222-1228.
doi: 10.7498/aps.47.1222
Abstract +
The characterization of Nb-doped layered perovskite SrBi2Ta2O9 was studied.The mechanism of modification of properties was analyzed.Photoacoustic spectroscopy has been used to study the light absorption of SrBi2Ta2O9 with various Nb contents.The results show that the absorption band at 580nm in the photoacoustic spectroscopy exhibits a red shift with increasing niobium content.The remanent polarization of Nb-doped SrBi2Ta2O9 films depends on the concentration ratio of Ta/Nb.The remanent polarization of the films arose an maximum at the area of Ta/Nb≈1.This may be related to the existence of morphotropic phase boundary.
LETTERS
1998, 47 (7): 1229-1232.
doi: 10.7498/aps.47.1229
Abstract +
Crystalline organic complex thin films have been performed by vacuum evaporation method on HOPG substrates.Using Scanning Tunneling Microscope storage,array has been made on the film by applying voltage pulses between the tip and the substrate.The recording marks are 1.3nm in diameter.Voltage-current characterization shows that the recorded region has a conductor behavior while the unrecorded regions shows an insulator behavior.It is confirmed from experiments that the distance between two recording marks can be as short as 2nm,which corresponds to the storage density of 2.5×1013bit/cm2.The mechanism about the recording is briefly discussed.
