Table of contents

12 October 2002, Volume 11 Issue 10 Previous issue    Next issue

GENERAL

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Resonance modes in optical fibres Yu Shou-Mian (余寿绵), Yu Tian (余恬) Chinese Physics, 2002, 11 (10):  981-987.  DOI: 10.1088/1009-1963/11/10/301 Abstract ( 1026 )   PDF (268KB) ( 419 )   The weakly nonlinear boundary value problem of wave propagation in an optical fibre (for the transverse electric mode, for example) is formulated and a modified linear solution is obtained. It is shown that a self-consistent theory of fibre optics should be weakly nonlinear. The mode of critical refraction that does not exist in the linear theory is obtained, showing that it is a mode consisting of resonance modes. It is shown that the signal carriers in a long fibre are of resonance modes, not normal modes. Some experimental data are given for comparison with the theoretical predictions, and the agreement seems satisfactory.

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Integrating factors and conservation theorem for holonomic nonconservative dynamical systems in generalized classical mechanics Qiao Yong-Fen (乔永芬), Zhang Yao-Liang (张耀良), Han Guang-Cai (韩广才) Chinese Physics, 2002, 11 (10):  988-992.  DOI: 10.1088/1009-1963/11/10/302 Abstract ( 1008 )   PDF (204KB) ( 594 )   In this paper, we present a general approach to the construction of conservation laws for generalized classical dynamical systems. Firstly, we give the definition of integrating factors and, secondly, we study in detail the necessary conditions for the existence of conserved quantities. Then we establish the conservation theorem and its inverse for the Hamilton's canonical equations of motion of holonomic nonconservative dynamical systems in generalized classical mechanics. Finally, we give an example to illustrate the application of the results.

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General explicit solutions of a classical Boussinesq system Zhang Shan-Qing (张善卿), Xu Gui-Qiong (徐桂琼), Li Zhi-Bin (李志斌) Chinese Physics, 2002, 11 (10):  993-995.  DOI: 10.1088/1009-1963/11/10/303 Abstract ( 1167 )   PDF (175KB) ( 579 )   Seeking a travelling wave solution of the classical Boussinesq system and making an ansatz for the solution, we obtain a nonlinear system of algebraic equations. We solve the system using an effective algorithm and then two general explicit solutions are obtained which are of physical interest.

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Probabilistic teleportation of an unknown atomic state Ye Liu (叶柳), Guo Guang-Can (郭光灿) Chinese Physics, 2002, 11 (10):  996-998.  DOI: 10.1088/1009-1963/11/10/304 Abstract ( 1456 )   PDF (166KB) ( 648 )   We propose two schemes for teleporting an unknown state. In our schemes, a three-particle Greenberger－Horne－Zeilinger state is used as a quantum channel. We show that the probabilistic teleportation of an unknown quantum state can be realized.

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Teleportation of an arbitrary three-particle state Chen Li-Bing (陈立冰) Chinese Physics, 2002, 11 (10):  999-1003.  DOI: 10.1088/1009-1963/11/10/305 Abstract ( 1099 )   PDF (188KB) ( 619 )   We propose two schemes for teleporting an arbitrary three-particle state. In the first scheme, a two-particle state and a three-particle entangled state (both non-maximally entangled states) are used as quantum channels, while in the second scheme, three non-maximally entangled particle pairs are employed as quantum channels. We show that teleportation can be successfully realized with certain probability if a receiver adopts some appropriate unitary transformations. Their success probabilities and the classical communication costs are different.

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Bifurcations of a parametrically excited oscillator with strong nonlinearity Tang Jia-Shi (唐驾时), Fu Wen-Bin (符文彬), Li Ke-An (李克安) Chinese Physics, 2002, 11 (10):  1004-1007.  DOI: 10.1088/1009-1963/11/10/306 Abstract ( 1265 )   PDF (209KB) ( 653 )   A parametrically excited oscillator with strong nonlinearity, including van der Pol and Duffing types, is studied for static bifurcations. The applicable range of the modified Lindstedt－Poincaré method is extended to 1/2 subharmonic resonance systems. The bifurcation equation of a strongly nonlinear oscillator, which is transformed into a small parameter system, is determined by the multiple scales method. On the basis of the singularity theory, the transition set and the bifurcation diagram in various regions of the parameter plane are analysed.

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A design of novel type superconducting magnet for super-high field functional magnetic resonance imaging by using the harmonic analysis method of magnetic vector potentials Zu Dong-Lin (俎栋林), Guo Hua (郭华), Song Xiao-Yu (宋枭禹), Bao Shang-Lian (包尚联) Chinese Physics, 2002, 11 (10):  1008-1012.  DOI: 10.1088/1009-1963/11/10/307 Abstract ( 1307 )   PDF (235KB) ( 436 )   The approach of expanding the magnetic scalar potential in a series of Legendre polynomials is suitable for designing a conventional superconducting magnetic resonance imaging magnet of distributed solenoidal configuration. Whereas the approach of expanding the magnetic vector potential in associated Legendre harmonics is suitable for designing a single-solenoid magnet that has multiple tiers, in which each tier may have multiple layers with different winding lengths. A set of three equations to suppress some of the lowest higher-order harmonics is found. As an example, a 4T single-solenoid magnetic resonance imaging magnet with 4×6 layers of superconducting wires is designed. The degree of homogeneity in the 0.5m diameter sphere volume is better than 5.8 ppm. The same degree of homogeneity is retained after optimal integralization of turns in each correction layer. The ratio Bm/B0 in the single-solenoid magnet is 30% lower than that in the conventional six-solenoid magnet. This tolerates higher rated superconducting current in the coil. The Lorentz force of the coil in the single-solenoid system is also much lower than in the six-solenoid system. This novel type of magnet possesses significant advantage over conventional magnets, especially when used as a super-high field functional magnetic resonance imaging magnet.

ATOMIC AND MOLECULAR PHYSICS

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Cross sections for rotational excitation in He－H2 collisions Yang Xiang-Dong (杨向东), Zhou Xiao-Lin (周晓林), Zhang Ji-Yan (张继彦), Sun Gui-Hua (孙桂华), Zhu Jun (朱俊), Chinese Physics, 2002, 11 (10):  1013-1017.  DOI: 10.1088/1009-1963/11/10/308 Abstract ( 1272 )   PDF (211KB) ( 457 )   Close coupling calculations have been carried out for rotational excitations in He－H2 collisions with symmetric isotopic substitution (He－H2, D2, T2) and asymmetric isotopic substitution (He－HD, HT, DT). Cross sections have been obtained at the incidence energy of 0.3eV. Based on the calculations, the effect of isotopic substitution on atom－diatom collisions is discussed.

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The formation mechanism and the binding energy of the body-centred regular tetrahedral structure of He+5 Li Ping (李萍), Xiong Yong (熊勇), Gou Qing-Quan (芶清泉), Zhang Jian-Ping (张建平) Chinese Physics, 2002, 11 (10):  1018-1021.  DOI: 10.1088/1009-1963/11/10/309 Abstract ( 1464 )   PDF (213KB) ( 450 )   We propose the formation mechanism of the body-centred regular tetrahedral structure of the He+5 cluster. The total energy curve for this structure has been calculated by using a modified arrangement channel quantum mechanics method. The result shows that a minimal energy of -13.9106 a.u. occurs at a separation of 1.14a0 between the nucleus at the centre and nuclei at the apexes. Therefore we obtain the binding energy of 0.5202 a.u. for this structure. This means that the He+5 cluster may be stable with a high binding energy in a body-centred regular tetrahedral structure.

CLASSICAL AREAS OF PHENOMENOLOGY

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A virtual optical probe based on evanescent wave interference Sun Li-Qun (孙利群), Wang Jia (王佳), Hong Tao (洪涛), Tian Qian (田芊) Chinese Physics, 2002, 11 (10):  1022-1027.  DOI: 10.1088/1009-1963/11/10/310 Abstract ( 1184 )   PDF (349KB) ( 528 )   A virtual probe is a novel immaterial tip based on the near-field evanescent wave interference and small aperture diffraction, which can be used in near-field high-density optical data storage, nano-lithography, near-field optical imaging and spectral detection, near-field optical manipulation of nano-scale specimen, etc. In this paper, the formation mechanism of the virtual probe is analysed, the evanescent wave interference discussed theoretically, and the sidelobe suppression by small aperture is simulated by the three-dimensional finite-difference time-domain method. The simulation results of the optical distribution of the near-field virtual probe reveal that the transmission efficiency of the virtual probe is 102－104 times higher than that of the nano-aperture metal-coated fibre probe widely used in near-field optical systems. The full width at half maximum of the peak, in other words, the size of virtual probe, is constant whatever the distance in a certain range so that the critical nano-separation control in the near-field system can be relaxed. We give an example of the application of the virtual probe in ultrahigh-density optical data storage.

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Position-dependent dynamics of a trapped ion in a standing wave laser Fang Mao-Fa (方卯发) Chinese Physics, 2002, 11 (10):  1028-1032.  DOI: 10.1088/1009-1963/11/10/311 Abstract ( 1343 )   PDF (234KB) ( 581 )   We have investigated the position-dependent dynamics of a trapped ion in a standing wave laser by transforming it to the Jaynes－Cummings-type system under the Lamb－Dicke limit. A variety of novel phenomena are exhibited, e.g. periodic collapse and revival features and long-time scaled revivals of the ionic inversion, depending on its position in the standing wave. Our result provides a way of producing a system equivalent to the two-photon Jaynes－Cummings model in the trapped ion system, with its exact periodicities.

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New nonlinear dynamics in a semiconductor laser with optical self-feedback Wang Chun-Lin (王春林), Wu Jian (伍剑), Lin Jin-Tong (林金桐) Chinese Physics, 2002, 11 (10):  1033-1036.  DOI: 10.1088/1009-1963/11/10/312 Abstract ( 1139 )   PDF (187KB) ( 548 )   We demonstrate a new kind of instability of the external cavity semiconductor laser. In some parameter regimes, the external cavity system will produce short and strong optical pulses in aperiodic intervals. This instability also shows low-frequency characteristics.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

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Small angle x-ray scattering study on the conformation of polystyrene in toluene during adding anti-solvent CO2 Liu Yi (柳义), Chen Dong-Feng (陈东风), Wang Hong-Li (王洪立), Chen Na (陈娜), Li Dan (李丹), Han Bu-Xing (韩布兴), Rong Li-Xia (荣利霞), Zhao Hui (赵辉), Wang Jun (王俊), Dong Bao-Zhong (董宝中) Chinese Physics, 2002, 11 (10):  1037-1041.  DOI: 10.1088/1009-1963/11/10/313 Abstract ( 1335 )   PDF (195KB) ( 493 )   The conformation of polystyrene in the anti-solvent process of supercritical fluids (compressed CO2 + polystyrene + toluene) has been studied by small angle x-ray scattering with synchrotron radiation as an x-ray source. Coil-to-globule transformation of the polystyrene chain was observed with the increase of the anti-solvent CO2 pressure; i.e. polystyrene coiled at a pressure lower than the cloud point pressure (Pc) and turned into a globule with a uniform density at pressures higher than Pc. Fractal behaviour was also found in the chain contraction, and the mass fractal dimension increased with increasing CO2 pressure.

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Single atomic manipulation and writing with scanning tunnelling microscopy at low temperatures Gu Chang-Zhi (顾长志), K F Braun, K H Rieder Chinese Physics, 2002, 11 (10):  1042-1046.  DOI: 10.1088/1009-1963/11/10/314 Abstract ( 1052 )   PDF (541KB) ( 500 )   In the work reported in this paper, we have used a low-temperature scanning tunnelling microscope (LT-STM) system to manipulate accurately single atoms. We show how we can use a LT-STM to image and modify a bulk Ag(111) surface and manipulate Ag atoms from substrate and evaporated adsorbates on Ag(111) substrates. We present a synergistic combination of STM-induced modification and ordered arrays of nanometre-scale structures. In particular, we demonstrate the ability to modify Ag atomic nanometre structures on the Ag(111) substrate, and some English letters and a Chinese character can be written by single Ag atoms coming from the substrate and evaporated adsorbates on Ag(111). In this way, we supply an effective basis to explore the fundamental physical properties of a nanometre structure and to develop nanotechnology with a `bottom-up' approach.

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Investigation of the growth process of Si nanowires using the vapour－liquid－solid mechanism Xing Ying-Jie (邢英杰), Yu Da-Peng (俞大鹏), Xi Zhong-He (奚中和), Xue Zeng-Quan (薛增泉) Chinese Physics, 2002, 11 (10):  1047-1050.  DOI: 10.1088/1009-1963/11/10/315 Abstract ( 1511 )   PDF (568KB) ( 620 )   Silicon nanowires have been grown by the thermal decomposition of silane via the vapour－liquid－solid (VLS) mechanism. Three different stages of VLS growth (eutectic alloy formation, crystal nucleation and unidirectional growth) were studied separately using a scanning electron microscope and a high-resolution transmission electron microscope. Very short silicon nanowires prepared under particular conditions provide direct evidence of the VLS mechanism on a nanometre scale. Our results will be very helpful for the controllable synthesis of Si nanowires.

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Universality principle and the development of classical density functional theory Zhou Shi-Qi (周世琦), Zhang Xiao-Qi (张晓琪) Chinese Physics, 2002, 11 (10):  1051-1059.  DOI: 10.1088/1009-1963/11/10/316 Abstract ( 1186 )   PDF (232KB) ( 549 )   The universality principle of the free energy density functional and the ‘test particle' trick by Percus are combined to construct the approximate free energy density functional or its functional derivative. Information about the bulk fluid radial distribution function is integrated into the density functional approximation directly for the first time in the present methodology. The physical foundation of the present methodology also applies to the quantum density functional theory.

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

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The effect of the ferromagnetic metal layer on tunnelling conductance and magnetoresistance in double magnetic planar junctions Xie Zheng-Wei (谢征微), Li Bo-Zang (李伯臧), Li Yu-Xian (李玉现) Chinese Physics, 2002, 11 (10):  1060-1065.  DOI: 10.1088/1009-1963/11/10/317 Abstract ( 1025 )   PDF (210KB) ( 472 )   Based on the free-electron approximation, we investigate the effect of the ferromagnetic metal layer on the tunnelling magnetoresistance (TMR) and tunnelling conductance (TC) in the double magnetic tunnel junctions (DMTJs) of the structure NM/FM/I(S)/NM/I(S)/FM/NM, where FM, NM and I(S) represent the ferromagnetic metal, nonmagnetic metal and insulator (semiconductor), respectively. The FM, I(S) and inner NM layers are of finite thickness, while the thickness of the outer NM layer is infinite. The calculated results show that, due to the spin-dependent interfacial potential barriers caused by electronic band mismatch between the various magnetic and nonmagnetic layers, the dependences of the TMR and TC on the thicknesses of the FM layers exhibit oscillations, and a much higher TMR can be obtained for suitable thicknesses of FM layers.

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Magnetic ordering of the bond-diluted two-dimensional mixed spin Ising system Yan Shi-Lei (晏世雷) Chinese Physics, 2002, 11 (10):  1066-1072.  DOI: 10.1088/1009-1963/11/10/318 Abstract ( 1207 )   PDF (221KB) ( 817 )   Numerical and analytical results are presented for the magnetic ordering in a bond-diluted spin-1/2 and spin-1 mixed transverse Ising system with a single-ion anisotropy on a honeycomb lattice. Special emphasis is placed on the magnetic ordering under the bond dilution and percolation threshold. We discuss in detail the influence of transverse fields of different sublattices on the normal magnetic ordering and on the magnetic ordering induced by single-ion anisotropy. We find that the magnetic ordering of a system exhibits an explicit difference when receiving the transverse field. This phenomenon has not been revealed in previous reports.

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Read-out of a read-only super-resolution optical disc with a Si mask Wei Jin-Song (魏劲松), Ruan Hao (阮昊), Shi Hong-Ren (施宏仁), Gan Fu-Xi (干福熹) Chinese Physics, 2002, 11 (10):  1073-1075.  DOI: 10.1088/1009-1963/11/10/319 Abstract ( 1122 )   PDF (191KB) ( 497 )   A novel read-only super-resolution optical disc structure (substrate/mask layer/dielectric layer) is proposed in this paper. By using a Si thin film as the mask layer, the recording pits with a diameter 380nm and a depth 50nm are read out on the dynamic measuring equipment; the laser wavelength α is 632.8nm and the numerical aperture is 0.40. In the course of reproduction, the laser power is 5mW and the rotation velocity of the disc is 4m·s-1. The optimum thickness of the Si thin film is 18nm and the signal-to-noise ratio is 32dB.

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Numerical analysis of the mechanism of carrier transport in organic light-emitting devices Peng Ying-Quan (彭应全), Zhang Fu-Jia (张福甲), Tai Xi-Shi (台夕市), He Xi-Yuan (何锡源), Zhang Xu (张旭) Chinese Physics, 2002, 11 (10):  1076-1081.  DOI: 10.1088/1009-1963/11/10/320 Abstract ( 1262 )   PDF (230KB) ( 580 )   The mechanism of carrier transport in organic light-emitting devices is numerically studied, on the basis of trapped-charge-limited conduction with an exponential trap distribution. The spatial distributions of the electrical potential, field and carrier density in the organic layer are calculated and analysed. Most carriers are distributed near the two electrodes, only a few of them are distributed over the remaining part of the organic layer. The carriers are accumulated near the electrodes, and the remaining region is almost exhausted of carriers. When the characteristic energy of trap distribution is greater than 0.3 eV, it leads to a reduction of current density. In order to improve the device efficiency, organic materials with minor traps and low characteristic energy should be chosen. The diffusion current is the dominant component near the injection electrode, whereas the drift current dominates the remaining region of the organic layer.