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Volume 18 Issue 2
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Original Articles
Classical Coset Hamiltonian for the Electronic Motion and its Application to Anderson Localization and Hammett Equation
XING Guan, WU Guo-Zhen
Chin. Phys. Lett. 2001, 18 (2): 157-159 .  
Abstract   PDF(pc) (365KB) ( 426 )
A classical coset Hamiltonian is introduced for the system of one electron in multi-sites. By this Hamiltonian, the dynamical behaviour of the electronic motion can be readily simulated. The simulation reproduces the retardation of the electron density decay in a lattice with site energies randomly distributed - an analogy with Anderson localization. This algorithm is also applied to reproduce the Hammett equation which relates the reaction rate with the property of the substitutions in the organic chemical reactions. The advantages and shortcomings of this algorithm, as contrasted with traditional quantum methods such as the molecular orbital theory, are also discussed.
Two Theorems on Calculating the Relative Entropy of Entanglement
WU Sheng-Jun, WU Qiang, ZHANG Yong-De
Chin. Phys. Lett. 2001, 18 (2): 160-162 .  
Abstract   PDF(pc) (197KB) ( 737 )
We present two theorems on calculating the relative entropy of entanglement. Theorem 1 is an extension of Vedral and Plenio's theorem (Phys. Rev. A 57 (1998) 1619) for pure states, which is useful for calculating the relative entropy of entanglement for all pure states as well as for a class of mixed states. Theorem 2 gives the relative entropy of entanglement for any bipartite state whose tripartite purification has two separable reduced bipartite states.
Quantum Squeezing Effects of a Non-dissipative Mesoscopic Circuit with Coupled Inductors and Capacitors
LEI Min-Sheng, JI Ying-Hua, XIE Fang-Sen
Chin. Phys. Lett. 2001, 18 (2): 163-165 .  
Abstract   PDF(pc) (267KB) ( 534 )
The quantum fluctuations of the charge and current in a non-dissipative mesoscopic circuit with coupled inductors and capacitors are studied for the squeezed vacuum state. The influence of the phase angle upon the quantum fluctuations is discussed in particular. Our results indicate that when the circuit parameters remain constant and the phases θ of the two circuits are equal, the squeezing of the charge or its conjugate variable increases. When the difference of the two phases equals π, the squeezing will deteriorate. Therefore, if we want to decrease the quantum noise, we should not only control the amplitude r, but also control the phase θ carefully.

Control of Cold Atoms by Slow Waves in a Hollow Fibre
LI Dai-Jun, LIU Xia-Ji, HUANG Hu, LI Shi-Qun, WANG Yu-Zhu
Chin. Phys. Lett. 2001, 18 (2): 169-171 .  
Abstract   PDF(pc) (786KB) ( 457 )
We propose that a slowly-moving standing wave can be used to manipulate the motion of an atomic wave packet in a hollow optical fibre. A cloud of cold atoms can follow the motion of such a slow wave. The evolution of an initial Gaussian atomic wave packet in the moving standing wave is calculated by numerically solving the Schrödinger equation. Our calculation shows that by using a sufficiently strong field and by adjusting the velocity of the standing wave pattern, the motion of the atomic cloud can be controlled in a hollow fibre.
A Fast Melting Release Method in Free-Fall Equivalence Principle Test
WU Zi-Gang, WANG Dian-Hong, LUO Jun, ZHOU Ze-Bing, NIE Yu-Xin, ZHANG Yuan-Zhong
Chin. Phys. Lett. 2001, 18 (2): 172-174 .  
Abstract   PDF(pc) (374KB) ( 554 )
A fast melting release method for the free-fall equivalence principle test using laser interferometry is discussed. The primary experiment result shows that the uncertainty of the differential release time could be controlled at the level of 1ms by this release system, which satisfies the requirement of the expected experimental precision.

Escape Rate in the Bistable Sawtooth System Driven by Correlated Multiplicative and Additive White Noises
YOU Jing-Yun, CAO Li, WU Da-Jin, LIANG Gui-Yun
Chin. Phys. Lett. 2001, 18 (2): 175-177 .  
Abstract   PDF(pc) (424KB) ( 395 )
We study the escape problem of a bistable sawtooth system
driven by correlated multiplicative and additive white noises. Exact analytic representation of the relative escape rate is obtained. Some novel phenomena are found from the calculation result;“suppression platform”appears in the relative escape rate when the noise correlation coefficient λ is positive. Moreover, for small positive values of λ, the relative escape rate exhibits “suppression platform” and then “resonant activation” with the increasing noise strength ratio R.

Transition of One Mode-Phase at the Crisis and Onset of Spatiotemporal Chaos
HE Kai-Fen, ZHANG Hai-Yun
Chin. Phys. Lett. 2001, 18 (2): 178-180 .  
Abstract   PDF(pc) (626KB) ( 545 )
In our previous work [Phys. Rev. Lett. 80 (1998) 696] we investigated a crisis-induced transition from temporal chaos to spatiotemporal chaos (STC) due to a collision between the unstable orbit of the carrier saddle steady wave (SSW) and the attractor of its perturbation wave (PW). In this letter, we find that at the crisis one PW mode-phase experiences a state transition. The variations of PW mode-phases are governed by the linear dispersion as well as two competitive effects arising from the system nonlinearity: (1) interaction between the SSW and the PW (SP); (2) self-nonlinearity of the PW (PP). With the increasing importance of system nonlinearity we find that before the crisis for all PW modes the SP effects dominate and the mode-phases vibrate chaotically; while after the crisis for one PW mode the PP effect surpasses that of SP, in which case the phase transits to a state of combination of chaotic whirling and vibrating, which is different from the case of weak nonlinearity. In the meantime, the PW partial wave is free from the trapping of its carrier, and can be driven or damped chaotically depending on the phase difference relative to the carrier SSW, causing the STC motion after the crisis.

Scanning Tunnelling Microscope Tip-Induced Reconstruction on Si(111)√3 x√3 R30°-Ag Surface
LI Bin, ZENG Chang-Gan, WANG Hai-Qian, WANG Bing, HOU Jian-Guo,
Chin. Phys. Lett. 2001, 18 (2): 181-183 .  
Abstract   PDF(pc) (1201KB) ( 471 )
The reconstruction process of the Si(111) √3 x√3 R30°-Ag surface is studied by using a scanning tunnelling microscope at 78K. By applying a strong interaction between the tip and the surface, a tip-induced reconstruction corresponding to the mergence of two Si(111) √3 x√3 R30°-Ag domains is observed. Based on the inequivalent trimers (IET) model, this reconstruction process is attributed to a transition between the clockwise and counterclockwise IET domains. With this transition, the honeycomb-chained-trimer Si(111) )√3 x√3 R30°-Ag anti-phase boundary disappears and changes to the IET structure.
Effects of Flavour-Dependent qq Annihilation on the Mixing Angle of the Isoscalar Octet--Singlet and Schwinger’s Nonet Mass Formula
LI De-Min, YU Hong, SHEN Qi-Xing,
Chin. Phys. Lett. 2001, 18 (2): 184-186 .  
Abstract   PDF(pc) (210KB) ( 454 )
By incorporating the flavour-dependent quark--antiquark annihilation amplitude into the mass-squared matrix describing the mixing of the isoscalar states of a meson nonet, the new version of Schwinger's nonet mass formula, which holds with a high accuracy for the 0-+, 1--, 2++, 2-+ and 3-- nonets, is derived and the mixing angle of the isoscalar octet--singlet for these nonets is obtained. In particular, the mixing angle of the isoscalar octet--singlet for the pseudoscalar nonet is determined to take the value of -12.92°, which is in agreement with the value range from -13° to -17° deduced from a rather exhaustive and up-to-date analysis of data. It is also pointed out that the omission of the flavour-dependent qq annihilation effect might be a factor resulting in the invalidity of Schwinger’s original nonet mass formula for the pseudoscalar nonet.
A Lattice Study of the Glueball Spectrum
LIU Chuan
Chin. Phys. Lett. 2001, 18 (2): 187-189 .  
Abstract   PDF(pc) (260KB) ( 494 )
The glueball spectrum is studied using an improved gluonic action on asymmetric lattices in the pure SU(3) lattice gauge theory. The smallest spatial lattice spacing is about 0.08fm which makes the extrapolation to the continuum limit more reliable. Converting our lattice results to physical units using the scale set by the static quark potential, We obtain the following results for the glueball masses: MG(0++)=1730(90) MeV for the scalar glueball and MG(2++)=2400(95) MeV for the tensor glueball.

SU(3) Breaking from Baryon-Meson Fluctuation in Baryon Magnetic Moments
CHEN Xiao-Lin, MA Bo-Qiang, ZU Dong-Lin
Chin. Phys. Lett. 2001, 18 (2): 190-192 .  
Abstract   PDF(pc) (204KB) ( 407 )
We investigate the effect of intrinsic sea quark-antiquark pairs on the magnetic moments of the SU(3) octet baryons. The intrinsic quark-antiquark pairs are introduced by a model of energetically-favoured non-perturbative baryon-meson fluctuations. It is shown that the baryon-meson fluctuation model provides a mechanism of SU(3) symmetry breaking in the baryon sea and is related to the violations of Gell-Mann-Okubo and Coleman-Glashow sum rules for the baryon magnetic moments. Such a model is also consistent with its explanation of the violations of Gottfried and Ellis-Jaffe sum rules in deep inelastic scattering.

Analysis of the Yrast Bands with q-Deformed Moment of Inertia
FANG Xiang-Zheng, RUAN Tu-Nan,
Chin. Phys. Lett. 2001, 18 (2): 193-195 .  
Abstract   PDF(pc) (264KB) ( 861 )
The rigid-rotor with a q-deformed moment of inertia is introduced to describe the nuclear rotational spectra. With the representations of quantum algebra, the normal deformed and superdeformed bands are naturally differentiated by softness. The yrast normal deformed bands in rare earth and actinium regions, as well as the yrast superdeformed bands in A-190 and 150 regions are investigated. The calculated results agree with experimental data qualitatively well, and the values of the parameters are physically reasonable. This indicates that the fixed deformation, the stretching effect and the many body statistics effect are three possible dominant factors to govern nuclear rotational bands.
Exploration of Unbound States by Analytical Continuation in the Coupling Constant Method Within Relativistic Mean Field Theory
YANG Si-Chun, MENG Jie, ZHOU Shan-Gui
Chin. Phys. Lett. 2001, 18 (2): 196-198 .  
Abstract   PDF(pc) (424KB) ( 392 )
For the first time, the analytical continuation in the coupling constant method has been combined with the relativistic mean field theory to study the unbound states in spherical nuclei. The 1d3/2 neutron state in 16O and the 2d5/2 and 1g9/2 neutron states in 48Ca are taken as examples. The calculated energies and widths are compared with available data.

Intermediate Mass Dilepton Production in an Expanding Baryon-rich Quark--Gluon Matter
HE Ze-Jun, JIANG Wei-Zhou, ZHOU Wen-Jie, ZHANG Jia-Ju
Chin. Phys. Lett. 2001, 18 (2): 199-201 .  
Abstract   PDF(pc) (311KB) ( 377 )
The intermediate mass dileptons from the quark phase, secondary hadronic processes and background sources have been studied based on a relativistic hydrodynamic model. Due to the effect of the phase boundary on the evolution of the system the contribution from the quark phase is much more important than that from secondary processes and is even comparable with that from background sources. This leads to a spectrum without the obvious humps of the hadronic phase contribution and the yield increasing with the incident energy of colliding nuclei. It is shown that the enhancement of dileptons is a signature for the formation of the quark--gluon plasma.
Alternating Current Zeeman and Stark Interference Effect in Ramsey Separated Oscillating Fields
CHEN Jing-Biao, WANG Feng-Zhi, YANG Dong-Hai, WANG Yi-Qiu
Chin. Phys. Lett. 2001, 18 (2): 202-204 .  
Abstract   PDF(pc) (303KB) ( 1243 )
Analytic expressions have been derived of the alternating current (ac) Zeeman and ac Stark effect in an atomic beam magnetic resonance method using Ramsey separated oscillating fields. An interesting feature which will affect the normal Ramsey pattern is that an interference fringe is superimposed on the dispersion lineshapes of the normal ac Zeeman or ac Stark effect. We point out that this new character of ac Zeeman (ac Stark) effect generally exists in all kinds of Ramsey method, for example, in the optical Ramsey atomic interferometer and atomic beam frequency standard. An important implication is that, particularly in an atomic beam frequency standard using Ramsey method, this effect has an influence on the evaluation of the second-order Doppler frequency shift.
Two-Photon Absorption and Fluorescence of a Novel Compound Trans-1,3,5-tri-(4-N,N-Dimethylaminobenzylstyryl)-Acetylacetone
LI Yun-Jing, ZHAN Chuan-Lang, ZHAO Li-Zeng, WANG Duo-Yuan, LU Zhen-Zhong, NIE Yu-Xin
Chin. Phys. Lett. 2001, 18 (2): 205-207 .  
Abstract   PDF(pc) (339KB) ( 695 )
Large two-photon absorption cross section of a novel compound
trans-1,3,5-tri-(4-N,N-dimethylaminobenzylstyryl)-acetylacetone is measured to be σ2=8.4 x 10-20cm4/GW or σ'2=1.594 x 10-47cm4/(photon.s) by using a 1064 nm Nd:YAG laser with 35 ps pulse width based on the intensity-dependent transmissivity measurement. The frequency upconversion fluorescence is observed by two-photon laser pumping. This material is an ideal candidate for optical limiting and upconversion fluorescence.
Deep Penetration of Charged Particles in Biological Samples
WANG Rui-Jin, XIA Yue-Yuan, MU Yu-Guang, ZHAO Ming-Wen, MA Yu-Chen, LIU Xiang-Dong, ZHANG Jian-Hua LIU Ji-Tian, YU Zeng-Liang
Chin. Phys. Lett. 2001, 18 (2): 208-210 .  
Abstract   PDF(pc) (353KB) ( 513 )
Experimental evidence of abnormally deep penetration in some botanical targets by low-energy ion beams is presented. The energy spectra of 818 keV He+ ions penetrating a 70μm thick seed coat of maize, fruit peel of grape and of tomato all have a common feature. The leading edges of these broad spectra indicate that some of the penetrating ions pass through the thick targets easily and only lose a small fraction of their initial incident energy. Rutherford backscattering spectrometry and electron microprobe measurements are used to determine the argon concentration in multilayer samples of the seed coat of maize implanted by 200 keV Ar+ ions. The results show that about 10% of the Ar+ ions can penetrate deeper than ~ 100μm in these samples.
Measurements of Laser Absorption and Ion Energy in Femtosecond Laser--Cluster Interaction
LIN Jing-Quan, ZHANG Jie, LI Ying-Jun, CHEN Li-Ming, LÜ, Tie-Zheng, TENG Hao, MAN Bao-Yuan, ZHAO Li-Zeng
Chin. Phys. Lett. 2001, 18 (2): 211-213 .  
Abstract   PDF(pc) (344KB) ( 576 )
Laser absorption is measured in Xe, Ar and He clusters irradiated by 5 mJ laser pulses in 150 fs. The measurements show that the absorption efficiency strongly depends on the backing pressure of the gas jets, the laser pulse duration and the atomic number Z of the working gas. The laser absorption for Xe clusters is found to be as high as 45% at a backing pressure of 20 atm and a laser intensity of 1 x 1015 W/cm2. Significant numbers of ions with energies up to 100 keV are detected from Xe cluster explosion. The variation trend of the average ion energy at different backing pressures is consistent with that of the laser energy absorption.

Electromagnetic Scattering from Two-Dimensional Rough Surface Using the Kirchhoff Approximation
GUO Li-Xin, JIAO Li-Cheng, WU Zhen-Sen
Chin. Phys. Lett. 2001, 18 (2): 214-216 .  
Abstract   PDF(pc) (365KB) ( 737 )
A two-dimensional fractional Brown motion (fBm) fractal model is presented, which is suitable for describing the natural rough surface. Using the Kirchhoff approximation, the expressions of the electromagnetic scattering field and the normalized radar cross section from this fractal surface are obtained. Numerical results of the normalized radar cross section as a function of the fractal dimension, characteristic length and incidence frequency are given. Our fBm model is also compared with classical models of Gaussian and exponential correlation.
Computational Generation and the Simulation of the Light Scattering of Self-Affine Fractal Random Surface
LIU Wen-Xian, TENG Shu-Yun, ZHANG Ning-Yu, LIU De-Li, CHENG Chuan-Fu
Chin. Phys. Lett. 2001, 18 (2): 217-219 .  
Abstract   PDF(pc) (344KB) ( 482 )
We propose a method for the generation of self-affine fractal random surfaces, in which we use Fourier transform and its inversion in the algorithm. The light scattering of surfaces of this kind is simulated at different incident angles of illumination. The variation of the full width at half maximum (FWHM) of the intensity profile versus the perpendicular component k of the wave-vector shows clearly the characteristics of the surfaces parameters. The simulation demonstrates how the value of FWHM at k2 w2 ≤ 1 region and the slope of ln wp - ln k curve at k2 w2 ≥ 1 region are used, respectively, to extract the lateral correlation length ξ and the roughness exponent α.

Damped Jaynes--Cummings Model Including Spontaneous Emission
TAN Wei-Han
Chin. Phys. Lett. 2001, 18 (2): 220-222 .  
Abstract   PDF(pc) (396KB) ( 406 )
As a continuation of the previous paper ‘Collapse and revival in the damped Jaynes--Cummings model’ [Chin. Phys. Lett. 16 (1999) 895], at present the problem is solved under the condition that the atomic spontaneous emission is included. In the case of no damping, c=0, γ1 ≠ 0, γ2 ≠ 0, the half atomic inversion operator <σZ> possesses an analytic solution, whereas in the general case c ≠ 0, the problem is reduced to the numerical evaluation of first-order nonlinear differential equations. The final results show that with the increase of γ1 and γ2, the kinetic quantities converge rapidly to stationary solutions.

Atomic Coherence in the Micromaser Injected with Slow V-type Three-State Atoms: Emission Probability
ZHANG Zhi-Ming, LIANG Wen-Qing, XIE Sheng-Wu
Chin. Phys. Lett. 2001, 18 (2): 223-224 .  
Abstract   PDF(pc) (255KB) ( 469 )
The effects of atomic coherence on the single-mode two-photon micromaser injected with slow V-type three-state atoms are studied for the first time. It is shown that the atomic coherence can modify the atomic emission probability. The effects of the atomic centre-of-mass momentum, the cavity length and other parameters are also studied.
Tunable Solid-State Dye Laser with Narrow Linewidth Operation
HONG Zhi, WANG Zhi-Yu, YANG Yu, CHEN Jun, WANG Min-Quan, BAO Cheng-Fang, QIU Wen-Fa
Chin. Phys. Lett. 2001, 18 (2): 225-227 .  
Abstract   PDF(pc) (345KB) ( 485 )
Tunable radiation with a linewidth of 0.014 nm was obtained in Rhodamine B, Rhodamine 6G, Perylene orange and Pyrromethene 567-doped GPTMS solid-state dye materials using intracavity grating dispersive oscillation. The solid dye samples were specially designed in the way in which they are gelling, drying and ageing between two anti-reflection coated disks, thus without any mechanical and optical processing. The conversion efficiency of 4.6% and the tunable range of 40 nm from 589--629 nm have been achieved in Rhodamine B-doped samples. The broadband lasing of 4.4 nm and 54% slope efficiency were also demonstrated in the experiment.
Diode-Pumped Nanosecond Pulsed Laser with Pulse-Transmission-Mode Q-Switch
CHEN Fei, HUO Yu-Jing, HE Shu-Fang, FENG Li-Chun
Chin. Phys. Lett. 2001, 18 (2): 228-229 .  
Abstract   PDF(pc) (494KB) ( 681 )
Q-switched pulses at 1.064μm with a peak power of 5.02 kW and a pulse width of 2.8 ns were obtained which were pumped by a 1 W laser diode on the Nd:YVO4 microchip at the 1 kHz repetition rate. These values were achieved by combining the techniques of acousto-optic Q-switching and electro-optic pulse-transmission-mode Q-switching. The temporal characteristics of the pulses were analysed numerically. The experimental results are shown to be in good agreement with theoretical predictions.
Modelling of the Self Sum-Frequency-Mixing Laser
CHEN Xue-Yuan, LUO Zun-Du, HUANG Yi-Dong
Chin. Phys. Lett. 2001, 18 (2): 230-232 .  
Abstract   PDF(pc) (432KB) ( 415 )
A theoretical model of the self sum-frequency-mixing (SFM) laser generated by a single crystal is proposed, in which spatial distribution of the pump and circulating fundamental lasers with arbitrary beam waists are taken into account. The model is then applied to two kinds of crystals of current interest, Nd:YAI3(BO3)4 and Nd:Ca4GdO(BO3)3. Numerical analyses of the self-SFM laser properties predict and confirm some experimental results. The model proposed is not limited to self-SFM lasers and may be applied to general analyses of the fundamental or nonlinear laser generation with Gaussian beams.
Steady-State Screening-Photovoltaic Spatial Solitons in the Biased Photorefractive-Photovoltaic Crystals
LU Ke-Qing, ZHANG Yan-Peng, TANG Tian-Tong, LU Zhi-Xian, LIU Lin
Chin. Phys. Lett. 2001, 18 (2): 233-235 .  
Abstract   PDF(pc) (292KB) ( 429 )
The theory of the screening-photovoltaic solitons is improved in biased photorefractive-photovoltaic crystals. When the photovoltaic effect is negligible, the screening-photovoltaic solitons are the screening ones, and their space-charge field is the space-charge field of the screening solitons. If the external field is absent, the screening-photovoltaic solitons are the photovoltaic ones on the open- and closed-circuit conditions, and their space-charge field is of the photovoltaic solitons. We also show theoretically that the screening and the photovoltaic solitons on the open- and closed-circuit conditions may be studies together as the screening-photovoltaic solitons.
Classical Dynamics of Harmonic Generation of the Hydrogen Molecular Ion Interacting with Ultrashort Intense Laser Pulses
LI Chao-Hong, DUAN Yi-Wu, Wing-Ki Liu, Jian-Min Yuan
Chin. Phys. Lett. 2001, 18 (2): 236-238 .  
Abstract   PDF(pc) (427KB) ( 525 )
Within Born--Oppenheimer approximation, by using the classical trajectory theory, a description for the high-order harmonic generation of the hydrogen molecular ion interacting with ultrashort laser pulses has been presented. The Coulomb singularities have been remedied by the regularization. The action-angle variables have been used to generate the initial inversion symmetry microcanonical distribution. Within a proper intensity range, a harmonic plateau with only odd harmonics appears. For a larger intensity, because of the existence of chaos, the harmonic spectra become noisier. For a large enough intensity, the ionization takes place and the harmonics disappear. So the chaos causes the noises, the ionization suppresses the harmonic generation, and the onset of the ionization follows the onset of chaos.
Numerical Study of the Apodization Profile Functions, Optimal Profiles and Lengths of a Linearly Chirped Fibre Bragg Grating
QIN Zi-Xiong, ZENG Qing-Ke, FENG De-Jun, XIANG Yang, DING Lei, KAI Gui-Yun, LIU Zhi-Guo, YUAN Shu-Zhong, DONG Xiao-Yi, LIU Ning, LIAO Chang-Jun, LIU Song-Hao
Chin. Phys. Lett. 2001, 18 (2): 239-241 .  
Abstract   PDF(pc) (457KB) ( 548 )
Starting from our definition of apodization profile functions, we discover the optimal profiles and the characteristics of defined apodization functions in sidelobe suppression. It is shown by numerical analysis that the optimal grating length is 45 mm to compensate for dispersion induced by 100 km fibres and the smoothness of the ripples in time-delay characteristics is related to the defined parameters.
Optical Waveguide Formation in LiNbO3 by the 2.6 MeV Nickel Ions Implantation
HU Hui, CHEN Feng, LU Fei, ZHANG Jian-Hua, LIU Ji-Tian, WANG Ke-Ming, SHI Bo-Rong, SHEN Ding-Yu, WANG Xue-Mei
Chin. Phys. Lett. 2001, 18 (2): 242-244 .  
Abstract   PDF(pc) (370KB) ( 542 )
The optical waveguide was formed on an LiNbO3 substrate by 2.6 MeV nickel ions implantation to the dose of 9 x 1014 ions/cm2. Five dark modes were observed by the prism coupling technique. The refractive index profile was obtained by using the reflectivity calculation method. A large index decrease was found in the guiding region and in the optical barrier, which is somewhat different from that of the LiNbO3 waveguide formed by the MeV He+ ions. The position of the optical barrier is deeper than that of the damage peak calculated by TRIM'90 (Transport of Ions in Matter) code. The crystal lattice damage in the guiding region caused by the Ni+ ion implantation was analysed by the Rutherford backscattering/channelling technique.
Silicon-on-Insulator Based 2 x 2 Multimode Interference Coupler with Large Tolerance
WEI Hong-Zhen, YU Jin-Zhong, LIU Zhong-Li, ZHANG Xiao-Feng, SHI Wei, FANG Chang-Shui
Chin. Phys. Lett. 2001, 18 (2): 245-247 .  
Abstract   PDF(pc) (322KB) ( 1454 )
We demonstrate a type of 2 x 2 multimode interference 3 dB coupler based on silicon-on-insulator. The fabrication tolerance was investigated by the effective index method and the guide mode method. The devices with different lengths were fabricated and near-field output images were obtained. Tolerances to width, length and etch depth are 2, 200 and 2μm, respectively. The devices show a uniform power distribution.
Effect of Background Ions on the Selection of the Discharge Path
HE Zheng-Hao, LI Jin
Chin. Phys. Lett. 2001, 18 (2): 248-250 .  
Abstract   PDF(pc) (693KB) ( 433 )
The effects of the background ions on the selection of the discharge path in an air gap have been studied with two different methods. The lightning impulse air discharge experiment is conducted using an independent ion generator, while the air discharge experiment uses a lightning impulse superimposed on a dc high voltage used to produce background ions. The influence of different background ions on the leader development, and thus on the discharge path, is observed. Consistent results have been obtained with the two methods. The probability for the discharge path passing through the negative ion space is much higher than that for the passing through the positive ion space. The mechanism of the effects of background ions is analysed based on the electron avalanche and the electric field.
Analytic Study of Density Limit Mechanism for Tokamak Lower Hybrid Current Drive Experiments
SHI Bing-Ren
Chin. Phys. Lett. 2001, 18 (2): 251-253 .  
Abstract   PDF(pc) (201KB) ( 417 )
The dispersion relation of the lower hybrid wave in the tokamak current drive regime (ω > 2ωLH, where ωLH is the lower hybrid frequency) is analysed. Toroidicity effects can shift the parallel refractive index upward or downward. The downward shift will lead to a mode conversion from the slow wave to the fast wave. It is indicated that this is the main mechanism limiting the effectiveness of tokamak lower hybrid current drive. Accordingly, a sufficient condition is obtained for the wave penetration, corresponding to a critical density, which is proportional to f4/3B2/3(A)4/3, with f being the wave frequency, B the magnetic field strength and A the aspect ratio. Quantitative agreement with experiments is found.
Reynolds Stress and Sheared Poloidal Flow in the Edge Plasma Region of the HT-6M Tokamak
WANG Wen-Hao*, YU Chang-Xuan*, LING Bi-Li, GONG Xian-Zu, LIU Bao-Hua, WAN Bao-Nian
Chin. Phys. Lett. 2001, 18 (2): 254-256 .  
Abstract   PDF(pc) (383KB) ( 531 )
High spatial resolution measurements of the electrostatic Reynolds stress, radial electric field and poloidal phase velocity of fluctuations in the edge region of the HT-6M tokamak are carried out. The Reynolds stress shows a radial gradient in proximity to the poloidal velocity shear. A comparison of the profiles between the Reynolds stress gradient and the poloidal velocity damping reveals some similarity in their magnitude and radial structure. These facts suggest that the turbulence-induced Reynolds stress may play a significant role in generating the poloidal flow in the plasma edge region.
Generation of Radial Electric Field with Electrode Biasing
WANG Cheng, PAN Ge-Sheng, WEN Yi-Zhi, YU Chang-Xuan, WAN Shu-De, LIU Wan-Dong, WANG Zhi-Jiang, SUN Xuan
Chin. Phys. Lett. 2001, 18 (2): 257-259 .  
Abstract   PDF(pc) (424KB) ( 572 )
Time and space resolved measurements of the radial electric field (Er) have been conducted during the electrode biasing experiments on the KT-5C tokamak. The suppression of the turbulent transport with the change of Er induced by the biased electrode is observed. It is found that the poloidal flow contributes to the main part of the Er, and the change of the poloidal flow has a lead of about 20μs to the formation of Er. These observations suggest that a radial current, responding to an induced voltage on the electrode, drives a poloidal flow which in turn drives the radial electric field.
Macro-pyramid in GaN Film
ZHOU Jing, YANG Zhi-Jian, XU Shi-Fa, ZHU Xing, ZHANG Guo-Yi
Chin. Phys. Lett. 2001, 18 (2): 260-262 .  
Abstract   PDF(pc) (1156KB) ( 633 )
A thin film of GaN with the thickness of 1.0μm was grown on α-Al2O3 substrate by metal organic chemical vapour disposition and then a thick GaN film with thickness of 12μm was grown in the halide vapour phase epitaxy system. Some macro-pyramids appeared on the surface of the sample. The macro-pyramids made the surface of the GaN film rough, which was harmful to the devices made by GaN materials. These defects changed the distribution of carrier concentration and affected the optical properties of GaN. The step height of the pyramids was about 30-40 nm measured by atomic force microscopy. A simple model was proposed to explain the macro-pyramid phenomenon compared with the growth spiral. The growth of the macro-pyramid was relative to the physical conditions in the reaction zone. Both increasing growth temperature and low pressure may reduce the pyramid size.
Heterostructure Intervalley Transferred Electron Effects
XUE Fang-Shi
Chin. Phys. Lett. 2001, 18 (2): 263-265 .  
Abstract   PDF(pc) (298KB) ( 687 )
A Gunn active layer is used as an X electron probe to detect the X tunnelling current in the GaAs--AlAs heterostructure, from which a new heterostructure intervalley transferred electron (HITE) device is obtained. In the 8 mm band, the highest pulse output power of these diodes is 2.65 W and the highest conversion efficiency is 18%. The dc and rf performance of the HITE devices was simulated by the band mixing resonant tunnelling theory and Monte Carlo transport simulation. The HITE effect has transformed the transit-time dipole-layer mode in the Gunn diode into a relaxation oscillation mode in the HITE device. From the comparison of simulated results to the measured data, the HITE effect is demonstrated straightforwardly.

A Bound State of a Positively Charged Exciton in Quantum Dots
XIE Wen-Fang
Chin. Phys. Lett. 2001, 18 (2): 266-269 .  
Abstract   PDF(pc) (284KB) ( 383 )
Using the method of few-body physics, we obtain the relation between the correlation energies of the low-lying states of the positively charged exciton X+ and the radius of quantum dots. We also calculate the binding energies of the ground state of a positively charged exciton as a function of the electron-to-hole mass ratio for a few values of the strength of the confinement. We find that there exists a critical mass ratio σc, such that if σ > σc (σ < σc) the X+ configuration is stable (unstable), and σc increases as the strength of the confinement increases.
Dynamic Behaviour of Vortex Matter, Memory Effect and Mittag--Leffler Relaxation
LIU Gang, HAN Ru-Shan
Chin. Phys. Lett. 2001, 18 (2): 269-271 .  
Abstract   PDF(pc) (340KB) ( 449 )
Using fractional Fokker--Planck equation and Langevin equation's Monte Carlo simulation, we find that when current density is less than critical current density, at the very beginning the motion for vortices is mainly within the pinning well with the typical relaxation time Tr. When time t is longer than Tr, the vortex glass will distort between two adjacent pining wells and will lead to memory effect which is lacking in the Mott insulator and pancake vortices. The inertial effect is not dominating as compared with the formation and growth of easy-flow channels. The random quenched disorder will lead to Mittag--Leffler relaxation.
Frequency and Field Dependences of Giant Magneto-Impedance Effect in Sandwiched FeCuCrVSiB Films
DAI You-Yong, XIAO Shu-Qin, LIU Yi-Hua, ZHANG Lin, WU Hou-Zheng, ZHANG Yan-Zhong
Chin. Phys. Lett. 2001, 18 (2): 272-274 .  
Abstract   PDF(pc) (321KB) ( 592 )
The giant magneto-impedance (GMI) effect has been investigated in sandwiched FeCuCrVSiB films annealed at 300°C for 1.5 h. The frequency and field dependences of the GMI have been observed in the frequency range from 50 kHz to 13 MHz. The GMI ratio increases at first with increasing frequency, and reaches its maximum value of 136% at a very low characteristic frequency of about 4 MHz, and then decreases with further increasing frequency. These superior properties are related to the special structure of the sandwiched films.
Experimental and Theoretical Studies on Velocity Field of Buoyancy Convection in KNbO3 Melt
JIN Wei-Qing, Shinichi YODA, JIANG Yuan-Fang, PAN Zhi-Lei, LIANG Xin-An
Chin. Phys. Lett. 2001, 18 (2): 275-277 .  
Abstract   PDF(pc) (934KB) ( 420 )
The Schlieren technique coupling with a differential interference microscope was applied to visualize the KNbO3 melt motion in a loop-shaped Pt wire heater. The natural convection in KNbO3 melt was traced by observing the movement of the tiny KNbO3 crystals (~10μm) and the stream velocities of these tracer crystals were measured. In theoretical analysis, the Navier--Stokes equation was solved as a stable field. The general solution for this system of the differential equation was expressed by an approximate power series of azimuth and radius vector. The expression was substituted in the differential equation; a non-trivial solution was obtained exactly. The velocity distribution in the vertical section was obtained which is in qualitative agreement with the experimental result.
Transmittance and Refractive Index of the Lanthanum Strontium Aluminium Tantalum Oxide Crystal
HU Bo-Qing, WANG Xiao-Ming, ZHOU Tang, ZHAO Zong-Yuan, WU Xing, CHEN Xiao-Long
Chin. Phys. Lett. 2001, 18 (2): 278-279 .  
Abstract   PDF(pc) (229KB) ( 971 )
The lanthanum strontium aluminium tantalum oxide (LSAT) crystal is grown by means of a floating or pulling method. Its optical transmittance, refractive indices in visible and near-infrared areas and its dispersion curve are reported. The transparence range of LSAT is from 0.45 to 4.2μm and its refractive index is 2.0244.
Yb3+ Ion Upconversion Luminescence from YbEr:ZBLAN Glass Excited by 966 nm Laser Light
CHEN Xiao-Bo, NIE Yu-Xin, WEN Ou
Chin. Phys. Lett. 2001, 18 (2): 280-282 .  
Abstract   PDF(pc) (375KB) ( 627 )
The upconversion luminescence of several YbEr co-doped Zr--Ba--La--Al--Na (ZBLAN) glass samples (Er(0.5)Yb(3): ZBLAN, Er(0.5)Yb(1):ZBLAN and Er(0.5):ZBLAN) has been studied. A new kind of upconversion cooperative radiation fluorescence, which comes from coupled cluster states of two Yb3+ ions has been observed. This is significant in that it may lead to a new path to achieve blue upconversion luminescence, which has promising upconversion applications.
Electrical and Optical Properties of InGaN/AlGaN Double Heterostructure Blue Light-Emitting Diodes
SHEN Bo, SHI Hong-Tao, ZHANG Rong, CHEN Zhi-Zhong, ZHENG You-Dou
Chin. Phys. Lett. 2001, 18 (2): 283-285 .  
Abstract   PDF(pc) (381KB) ( 1298 )
Electrical and optical properties of InGaN/AlGaN double heterostructure blue light-emitting diodes were investigated. Measurement of the forward bias current--voltage behaviour of the device demonstrated a departure from the Shockley model of a p--n diode, and it was observed that the dominant mechanism of carrier transport across the junction is associated with carrier tunnelling. Electroluminescence experiments indicated that there was a main emission band around 2.80 eV and a relatively weaker peak at 3.2 eV. A significant blueshift of the optical emission band was observed, which was consistent with the tunnelling character of electrical characteristics. Furthermore, the degradation in I-V characteristics and the low resistance ohmic short of the device were observed.
Synthesis of Nano-Crystalline Diamond Film in Hot Filament Chemical Vapour Deposition by Adding Ar
ZHANG Yu-Feng, ZHANG Fan, GAO Qiao-Jun, YU Da-Peng, PENG Xiao-Fu, LIN Zeng-Dong
Chin. Phys. Lett. 2001, 18 (2): 286-288 .  
Abstract   PDF(pc) (852KB) ( 728 )
Nano-crystalline diamond films are successfully deposited on silicon substrates via the hot filament chemical vapour deposition process using a CH4/H2/Ar gas mixture. The as-grown films are analysed by using field emission scanning electron microscopy, micro-Raman spectroscopy and x-ray diffraction. These results show that the films consist of nano-diamond grains with sizes ranging from 10 to 100 nm, and argon is an important element in the formation of nano-crystalline diamonds.
Shock Tube Studies on Recombination Kinetics of Sodium Ion with Electron
WANG Su, CUI Ji-Ping, HE Yu-Zhong, FAN Bing-Cheng, WANG Jing
Chin. Phys. Lett. 2001, 18 (2): 289-291 .  
Abstract   PDF(pc) (299KB) ( 628 )
The ionization kinetics of sodium diluted in argon is studied in a shock tube, in which the test gas mixture is ionized by a reflected shock wave and subsequently quenched by a strong rarefaction wave. A Langmuir electrostatic probe is used to monitor the variation of the ion number density at the reflection shock wave region. The working state of the probe is in the near free fall region and a correction for reduction of the probe current due to elastic scattering in the probe sheath is introduced. At the temperature range of 800 to 2600 K and in the ambience of argon gas, the three-body recombination rate coefficient of the sodium ion with electron is determined: 3.43 x 10-14T-3.77cm6.s-1.
Frequency Sensitivity in Nervous Systems
LIU Feng, WANG Wei
Chin. Phys. Lett. 2001, 18 (2): 292-294 .  
Abstract   PDF(pc) (399KB) ( 768 )
We first demonstrate how to quantify the information conveyed in temporal firing patterns of neurons. We then show that neurons are more sensitive to signals with frequencies within the range of 20-70 Hz in weak signal detection. Such frequency sensitivity is characterized by both the output signal-to-noise ratio and the information measures.
Generation of Spontaneous Synchronized Rhythm and its Role in Information Processing
YU Yu-Guo, WANG Wei
Chin. Phys. Lett. 2001, 18 (2): 295-297 .  
Abstract   PDF(pc) (416KB) ( 416 )
The generation of spontaneous synchronized rhythm and its role are studied by using a globally coupled excitable stochastic neuronal network. When the coupling strength exceeds a critical value, the neurons with a suitable noise in the network exhibit a strong tendency to synchronize and display a spontaneous rhythm. The coherence of the network can be enhanced by a suitable noise and a coupling of the network. The spontaneous rhythm enhances the ability of the network in processing weak periodic signals.
Interactions Exist in Bacteriorhodopsin?
HU Kun-Sheng, WANG Guang-Yu, HE Jin-An
Chin. Phys. Lett. 2001, 18 (2): 298-300 .  
Abstract   PDF(pc) (220KB) ( 301 )
Metal ions are essential to the structure and physiological functions of bacteriorhodopsin. Experimental evidence suggests the existence of specific cation binding to the negatively charged groups of Asp85 and Asp212 via an electrostatic interaction. However, only using electrostatic force is not enough to explain the role of the metal cations because the carboxylate of Asp85 is well known to be protonated in the M intermediate. Considering the presence of some aromatic amino acid residues in the vicinity of the retinal pocket, the existence of cation-π interactions between the metal cation and aromatic amino acid residues is suggested. Obviously, introduction of this kind of interaction is conducive to understanding the effects of the metal cations and aromatic amino acid residues inside the protein on the structural stability and proton pumping of bacteriorhodopsin.
Wave Propagation in Accretion Disks with Self-Gravity
LIU Xiao-Ci, YANG Lan-Tian, WU Shao-Ping, DING Shi-Xue,
Chin. Phys. Lett. 2001, 18 (2): 301-303 .  
Abstract   PDF(pc) (290KB) ( 330 )
We extend the research by Lubow and Pringle of axisymmetric waves in accretion disks to the case where self-gravity of disks should be considered. We derive and analyse the dispersion relations with the effect of self-gravity. Results show that self-gravity extends the forbidden region of the wave propagation: for high frequency p-modes, self-gravity makes the wavelength shorter and the group velocity larger; for low frequency g-modes, the effect is opposite.
Two-Step Coronal Transport of Solar Flare Particles from Magnetic Multipolarity Sources in a Flare Region
HUANG Yong-Nian, WANG Shi-Jin
Chin. Phys. Lett. 2001, 18 (2): 304-306 .  
Abstract   PDF(pc) (306KB) ( 551 )
The transport of solar flare particles in the corona is studied. Considering the problems in terms of the characteristics of a sunspot group producing solar cosmic rays and solar flare processes, we find that formation of the fast propagation process is associated with annihilation of sunspots in the group with magnetic multipolarity. The slower propagation process depends on magnetic irregularities in the corona, and the evolution of the transport is related to the flare processes. Equations for the coronal transport are proposed and their initial and boundary conditions are given. The predicted results agree with the main observational features.
Generation of High Speed Particles in Herbig--Haro Flow Regions
YAN Hui-Rong, MAO Xin-Jie,
Chin. Phys. Lett. 2001, 18 (2): 307-309 .  
Abstract   PDF(pc) (230KB) ( 365 )
A mechanism is presented for generating high speed particles in Herbig--Haro flow coming from accretion disks associated with
protostars. The disks are threaded with weak magnetic field lines, in which the magnetorotational instability results in magnetohydrodynamic turbulence. Then the turbulent waves accelerate the thermal particles out of the accretion disks to a few hundred kilometres per second, forming the high speed particles of optical jets in star-forming regions.
An Improved Thin Film Brick-Wall Model of Black Hole Entropy
LIU Wen-Biao, ZHAO Zheng
Chin. Phys. Lett. 2001, 18 (2): 310-312 .  
Abstract   PDF(pc) (194KB) ( 684 )
We improve the brick-wall model to take only the contribution of a thin film near the event horizon into account. This improvement not only gives us a satisfactory result, but also avoids some drawbacks in the original brick-wall method such as the little mass approximation, neglecting logarithm term, and taking the term L3 as the contribution of the vacuum surrounding a black hole. It is found that there is an intrinsic relation between the event horizon and the entropy. The event horizon is the characteristic of a black hole, so the entropy calculating of a black hole is also naturally related to its horizon.
52 articles