Table of contents

20 August 1993, Volume 2 Issue 8 Previous issue    Next issue

GENERAL

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THE SECOND-ORDER APPROXIMATION OF THRESHOLD VALUE OF CHAOS SU JING-HUI (苏景辉) Acta Physica Sinica (Overseas Edition), 1993, 2 (8):  561-568.  DOI: 10.1088/1004-423X/2/8/001 Abstract ( 1125 )   PDF (173KB) ( 393 )   For the Melnikov theory of chaos, the second-order approximation is given. Applying the result to the dynamics system with quadratic nonlinear term, it is shown that the threshold of chaos depends on initial condition and can be greater than that of the Melnikov method. The first order variational equations of some nonlinear dynamical systems are all the second-order ordinary differential equations with hyperbolic cosine function, its solution is given.

ATOMIC AND MOLECULAR PHYSICS

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INTENSE LASER-INDUCED AUTOIONIZATION OF A “$\Lambda$” TYPE ATOM LI GAO-XIANG (李高翔), PENG JIN-SHENG (彭金生) Acta Physica Sinica (Overseas Edition), 1993, 2 (8):  569-576.  DOI: 10.1088/1004-423X/2/8/002 Abstract ( 1246 )   PDF (199KB) ( 416 )   In this paper, we investigate the intense laser-induced autoionization about a "$\Lambda$" type atom by taking into account the second-order ionization processes. The effects of the second-order ionization processes and the second discrete state of the atom on the photoelectron spectrum are discussed.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

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POSITRON ANNIHILATION WITH VACANCIES IN THIN SURFACE LAYER OF As HEAVILY DOPED Si LU ZHI-HENG (卢志恒), WANG DA-CHUN (王大椿), G.K·gel Acta Physica Sinica (Overseas Edition), 1993, 2 (8):  577-582.  DOI: 10.1088/1004-423X/2/8/003 Abstract ( 1124 )   PDF (170KB) ( 408 )   The lifetime spectroscopy of slow positron accelerated with linear accelerator and pulse punch system was first used to analyze the vacancies in the thin surface layer of silicon heavily doped with arsenic. The results demonstrated that no mono-vacancy was detected to support the arsenic-vacancy complex models for explaining the electrical deactivation mechanism of arsenic-heavily-doped silicon.

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THERMAL STABILITY AND GRAIN GROWTH OF NANOCRYSTALLINE METAL SILVER ZHANG HONG-FEI (张鸿飞), WU XI-JUN (吴希俊) Acta Physica Sinica (Overseas Edition), 1993, 2 (8):  583-590.  DOI: 10.1088/1004-423X/2/8/004 Abstract ( 1263 )   PDF (217KB) ( 592 )   The nanocrystalline metal silver (n-Ag) with average grain size of 10 nm was synthesized by using an inert gas evaporation and in situ compacting technique. The thermal stability of grain size and grain growth caused by isothermally annealing heat treatment, as well as the thermal behavior during grain growth, have been studied. The results indicated that the thermal stability temperature of grain size is about 200℃. The grain growth depends upon the annealing temperature and exhibits threedifferent stages, i.e., slow, fast, and rapid growth stages, corresponding to the temperature ranges from 200℃ to 300℃, from 300℃ to 400℃ and above 400℃, respectively. An exothermal peak and an endothermal peak occur on the differential scanning calorimetriy (DSC) curve of n-Ag. The exothermal peak and the endothermal peak appear in the temperature range from 200℃ to 400℃, and from 400℃ to 660℃, respectively. The enthalpies calculated from the above two peaks depend on the compacting pressure. Further analyses indicated that the grain growth of n-Ag is related to the release of the surface energy of grains and the interracial energy, as well as the strain energy stored in the bulk samples induced by compacting process.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

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STRUCTURE AND PROPERTIES OF THE La-DOPED Bi-2201 SYSTEM MAO ZHI-QIANG (毛志强), YANG LI (杨丽), FAN CHENG-GAO (范成高), WANG NAN-LIN (王楠林), YAO ZHEN (姚震), WANG YU (王瑜), JI MING-RONG (季明荣), ZHANG YU-HENG (张裕恒) Acta Physica Sinica (Overseas Edition), 1993, 2 (8):  591-603.  DOI: 10.1088/1004-423X/2/8/005 Abstract ( 1483 )   PDF (421KB) ( 544 )   In this article, the influence of La substitution for Sr on structure and physi-cal properties of the 2201 phase is studied. First, the crystal microstructure of the Bi2.1Sr1.9-xLaxCuOy (0≤x≤1.0) is characterized by means of X-ray diffraction and electron diffraction analyses, it is discovered that in the La-doped 2201 system with increasing La content the 2201 phase undergoes tetragnoal-orthorhombic-monoclinic structural transition. With this phase transition, the modulation vector of the 2201 phase transforms from incommensurate to commensurate, and the period of the mod- ulation wave decreases. Secondly, the superconductivity of this system is analyzed systematically. The results of the resistivity measurement show that the appropriate amount of La doping can raise the superconducting transition temperature (Tc) of the 2201 phase, with x=0.2, Tc reaches as high as 25K. However the excessive doping of La (x≥0.3) leads the 2201 phase to transit from metal to insulator and the su-perconductivity disappears. The measurement of thermoelectric power (TEP) of this system shows that with increasing La content the TEP value also increases gradu-ally. But the interesting point is that the transition from positive to negative in TEP occurs for x=0.2 sample. In addition, the electronic structure and oxygen content of this system are also studied. Based on these, we analyze the relation between the microstructure, electronic structure of the La-doped 2201 phase and its superconduc-tivity, indicating that the type of element substitution with higher valence, together with the microstructural distortion induced by the substitution affects the electronic states of Cu 3d seriously, that is, changes the degree of coulomb correlation of Cu 3d electrons, thus resulting in the transition of the type of carrier of this system and the decrease of hole carrier concentration, as well as the corresponding variation of the superconductivity of La-doped 2201 phase.

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57Fe M$\ddot{\rm O}$SSBAUER STUDY ON (NdDy)TiFe11Ny COMPOUNDS CAO LEI (曹蕾), KONG LIN-SHU (孔麟书), SHEN BAO-GEN (沈保根), ZHAO JIAN-CAO (赵见高), GONG HUA-YANG (宫华扬), CHEN YI-LONG (陈义龙) Acta Physica Sinica (Overseas Edition), 1993, 2 (8):  604-609.  DOI: 10.1088/1004-423X/2/8/006 Abstract ( 1084 )   PDF (171KB) ( 397 )   57Fe M$\ddot{o}$ssbauer spectra have been obtained at room temperature for (NdDy)TiFe11, and at 11 K and room temperature for the corresponding nitrides (NdDy)TiFe11Ny. The magnetic behaviors of Fe atoms at different sites have been studied. We have found a larger increase of the hyperfine fields upon nitrogenation due to the higher nitrogen content in these compounds and got a bigger enhancement of the isomer shift in 8j site because of the nearest nitrogen environment.

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AN INVESTIGATION FOR THE Cd2+-VACANCIES IN RbCdF3 and CsCdF3 CRYSTALS DOPED WITH Fe3+ IMPURITIES YU WAN-LUN (余万伦) Acta Physica Sinica (Overseas Edition), 1993, 2 (8):  610-618.  DOI: 10.1088/1004-423X/2/8/007 Abstract ( 1289 )   PDF (251KB) ( 402 )   This paper presents high-order perturbation calculations for the zero-field splitting parameters D, F, and a of Fe3+-VCd pairs in RbCdF3 and CsCdF3 crystals doped with Fe3+ impurities. In comparison with experimental data, the present results are better than those obtained in previous work by using the phenomenological su-perposition model of spin-Hamiltonian. Moreover, the crystal fields arising from the Cd2+-vacancy and their induced crystal lattice distortion are analysed and calculated. It is found that the contribution from the Cd2+-vacancies cannot be ignored. Mena-while, we obtain that a ligand F- along the [001]-axis moves towards the central Fe3+ ion by $\Delta$= 0.0020nm in RbCdF3 and 0.0028nm in CsCdF3. These results are quite different from $\Delta$ = 0.0104nm and $\Delta$ = 0.0112nm, respectively, obtained in previous work by using the superposition model of spin-Hamiltonian and ignoring the effect of the vacancy.

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FRANZ-KELDYSH OSCILLATION FROM THE SPACE CHARGE REGION OF MBE GaAs FILMS PAN SHI-HONG (潘士宏), WANG ZHONG-HE (王忠和), LIU YI (刘毅), ZHANG CUN-ZHOU (张存洲), ZHOU XIAO-CHUAN (周小川), JIANG JIAN (蒋健), XU GUI-CHANG (徐贵昌) Acta Physica Sinica (Overseas Edition), 1993, 2 (8):  619-629.  DOI: 10.1088/1004-423X/2/8/008 Abstract ( 1290 )   PDF (282KB) ( 438 )   We have investigated doped MBE GaAs films using photoreflectance (PR) spec-troscopy. Special spectral structures have been observed in the vicinity of the funda-mental band gap, which are quite different from the Franz-Keldysh oscillation (FKO) from uniform electric fields under flatband modulations. Numerical analysis has been performed for FKO from electric fields in the space charge region under non-flatband modulations. Some typical FKO line shapes are illustrated. For moderately doped samples the calculated line shapes are basically consistent with experiments. The surface electric field and the Fermi level pinning have also been deduced from exper-iments.