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Volume 19 Issue 10
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Original Articles
General Solution and Fractal Localized Structures for the (2+1)-Dimensional Generalized Ablowitz-Kaup-Newell-Segur System
ZHENG Chun-Long, , ZHANG Jie-Fang,
Chin. Phys. Lett. 2002, 19 (10): 1399-1402 .  
Abstract   PDF(pc) (256KB) ( 655 )
Using the standard truncated Painlevé expansions, we derive a quite general solution of the (2+1)-dimensional generalized Ablowitz-Kaup-Newell-Segur (AKNS) system. Except for the usual localized solutions, such as dromions, lumps, and ring soliton solutions, etc., some special localized excitations with fractal behaviour, i.e., the fractal dromion and fractal lump excitations, are obtained by some types of lower-dimensional fractal patterns.
New Three-Mode Einstein-Podolsky-Rosen Entangled State Representation and Its Application in Squeezing Theory
FAN Hongyi, JIANG Nian-Quan
Chin. Phys. Lett. 2002, 19 (10): 1403-1406 .  
Abstract   PDF(pc) (203KB) ( 433 )
By extending the Einstein-Podolsky-Rosen bipartite entanglement to the tripartite case, we construct the common eigenvector of the tripartite center-of-mass coordinate and two mass-weighted relative momenta, which is a new entangled state of continuum variables. The classical dilation transform of variables in such a state induces a new three-mode squeezing operator related to a three-mode bosonic operator realization of SU(1,1) Lie algebra.
Entanglement Concentration of Entangled Squeezed Vacuum States
CAI Xin-Hua, KUANG Le-Man
Chin. Phys. Lett. 2002, 19 (10): 1407-1409 .  
Abstract   PDF(pc) (182KB) ( 735 )
By analysing the action of a beamsplitter on squeezed vacuum states (SVSs), it is found that, if the two input light beams of the beamsplitter have the same squeezing amplitude and phase, the output state is a two-mode SVS. If the two input light beams of the beam splitter have the same squeezing amplitude but a phase difference of π, the output state is a direct product state of two single-mode SVSs. Based on these results, we propose to distill maximally entangled SVS from partially entangled SVS via using 50/50 beam splitter and photodetectors.
Dynamics of Coherent Two-Colour Photoassociation of a Bose Einstein Condensate
WU Ying, YANG Xiao-Xue
Chin. Phys. Lett. 2002, 19 (10): 1410-1412 .  
Abstract   PDF(pc) (337KB) ( 505 )
It is shown that the semiclassical description of the coherent two-colour photoassociation of a Bose-Einstein condensate can be put into the canonical Hamiltonian formalism. This considerably simplifies the investigation of the corresponding steady states and their quantum fluctuations, and corresponding dynamics.

Phonon Spectrum and Modulational Instability in a Bose-Einstein Condensate Array
YANG Xiao-Xue, WU Ying,
Chin. Phys. Lett. 2002, 19 (10): 1413-1415 .  
Abstract   PDF(pc) (183KB) ( 375 )
We derive the phonon spectrum and the corresponding modulational instability conditions of an array of traps containing Bose-Einstein mcondensates with each trap linked to adjacent traps by tunneling. It is shown that modulational instability regimes always exist regardless of the sign of the two-body interaction.

Stochastic Resonance in a Bistable Sawtooth Potential Driven by Correlated Multiplicative and Additive Noise
WANG Jun, CAO Li, , WU Da-Jin,
Chin. Phys. Lett. 2002, 19 (10): 1416-1419 .  
Abstract   PDF(pc) (364KB) ( 520 )
We present an analytic investigation of the signal-to-noise ratio (SNR) by studying the bistable sawtooth system driven by correlated Gaussian white noises. The analytic expression of SNR is obtained. We detect some novel results, as follows. (1) Stochastic resonance (SR) always occurs in the SNR-Q (the multiplicative noise intensity) curve for the whole range of correlation coefficient. (2) There are not only resonance but also suppression and monotony in the dependence of SNR upon the additive noise intensity. (3) There are two extrema (one is minimum, the other maximum) in the SNR-b (the barrier height) curve for different multiplicative noise intensity.
Interaction Strength and a Generalized Bak-Sneppen Evolution Model
LI Wei, CAI Xu
Chin. Phys. Lett. 2002, 19 (10): 1420-1423 .  
Abstract   PDF(pc) (344KB) ( 430 )
The Bak-Sneppen evolution model is generalized in terms of a new concept and quantity, interaction strength. Based on a quantitative definition, the interaction strength describes the strength of the interaction between the nearest neighbouring individuals in the model. Self-organized criticality is observed for the generalized model with ten different values of interaction strength. The gap equation governing the self-organization is derived. It is also found that the self-organized threshold depends on the value of the interaction strength.

Femtosecond Laser Induced Optical Waveguides and Micro-mirrors Inside Glasses
GAO Ren-Xi, ZHANG Jia-Hua, ZHANG Li-Gong, SUN Jiang-Ting, KONG Xiang-Gui, SONG Hong-Wei, ZHENG Jie
Chin. Phys. Lett. 2002, 19 (10): 1424-1426 .  
Abstract   PDF(pc) (257KB) ( 492 )
Optical waveguides and micro-mirrors have been successfully induced inside fused silica glass and k9 glass, respectively, by focusing a 800 nm femtosecond (fs) pulsed laser with a repetition rate of 1kHz. The change of refractive index was determined to be 0.001-0.008 in the fused silica glass and 0.006 in the k9 glass. The refractive index change is dependent on both the dose of irradiation and the power density of the fs pulsed laser. Photolumines-cence was observed in the irradiated region. and was attributed to the defects induced by fs laser irradiation. We discuss the relationship between the optical property and the luminescent property of the irradiated region.
Lifetime of Polarized Fermions in Flight
SHI Zhi-Qiang, NI Guang-Jiong,
Chin. Phys. Lett. 2002, 19 (10): 1427-1429 .  
Abstract   PDF(pc) (209KB) ( 348 )
Based on the parity violation in the standard model, we study the dependence of the lifetime on the helicity of an initial-state fermion in weak interactions. It is pointed out that if initial fermions in decays are longitudinally polarized, then the lifetime of right-handed polarized fermions is greater than that of left-handed polarized fermions in flight with the same speed in any one of inertial systems.
Quantum Dynamics of One-Dimensional Nanocrystalline Solids
DING Jian-Wen, YAN Xiao-Hong, CAO Jue-Xian, WANG Deng-Long,
Chin. Phys. Lett. 2002, 19 (10): 1430-1431 .  
Abstract   PDF(pc) (289KB) ( 445 )
A novel ballistic-nonballistic dynamic transition in one-demensional nanocrystalline solids is found upon varying the strength of the composition modulation and the grain-boundary effect. This can contribute to the understanding of the strange electronic transport properties of nanostructured systems.
Proton Fraction in Hot Neutron Star Matter with a Chiral Hadronic Model
GUO Hua, ZHOU Ran, LIU Yu-Xin, ,
Chin. Phys. Lett. 2002, 19 (10): 1432-1435 .  
Abstract   PDF(pc) (561KB) ( 372 )
In the framework of a chiral hadronic model it has been numerically proven that the parabolic expansion with respect to δ2 at finite density and temperature is a good approximation in the relativistic mean field theory. It is also shown that the proton fraction in the modified Urca process is considerably restrained in comparison with that of the direct Urca process. Then, we discuss the influences of the density-dependent and temperature-dependent asymmetry energy on the proton fraction in hot neutron star matter.
Non-statistical Fluctuation of Compound Multiplicity in Nucleus-Nucleus Interactions: Evidence of Strong Intermittency
Dipak Ghosh, Argha Deb, Pasupati Mandal, Subrata Biswas, Keya Chattopadhyay, Rinku Sarkar, Ishita Dutta, Biswanath Biswas, Jayanta Roychowdhury, Jayita Ghosh, Mitali Mondal, Swarnapratim Bhattacharyya, Kanchan Kr. Patra, Swarup Ranjan Sahoo, Prabir Kr. Haldar
Chin. Phys. Lett. 2002, 19 (10): 1436-1438 .  
Abstract   PDF(pc) (250KB) ( 453 )
We present a study of the ‘compound multiplicity’distribution in terms of scaled factorial moments of orders 2, 3, 4, and 5 in 24Mg-AgBr interactions at 4.5 AGeV. The study reveals a strong signal of the intermittent pattern of fluctuation in the compound multiplicity distribution.
Calculations of 2s2S1/2-2p2P1/2,3/2 Transition Energies for Lithium-Like Systems from Na IX to Ca XVIII
CHEN Chao, WANG Zhi-Wen,
Chin. Phys. Lett. 2002, 19 (10): 1439-1442 .  
Abstract   PDF(pc) (252KB) ( 433 )
The transition energies (2s2S1/2-2p2P1/2,3/2) of lithium-like systems with nuclear charge from Z = 11-20 are calculated by using a full-core plus correlation method. Relativistic and mass-polarization effects on the energies are included as the first-order perturbation corrections. The quantum-electrodynamics contributions to the transition energy are evaluated by using effective nuclear charge. Our results are in excellent agreement with previous theoretical and experimental data available in the literature.
Coupled-Channel Investigation of Collisions of Protons and Antiprotons with Hydrogen Atoms in the 2p States
Reda S. Tantawi
Chin. Phys. Lett. 2002, 19 (10): 1443-1446 .  
Abstract   PDF(pc) (509KB) ( 354 )
The impact parameter method is used to calculate the inelastic
scattering cross sections of protons and antiprotons, which raise hydrogen atoms from the 2p states to the n = 3 states. The calculation involves the n = 1,2,3 states of the target and covers the energy range from 3 keV to 2500 keV. The study of the influence of the sign of the projectile charge is the main target of this investigation. Therefore, the exchange effects in the case of proton-induced reactions are ignored. The results of the calculations are in reasonable agreement with a previous work.
Three-Colour Single-Mode Electroluminescence from Alq3 Tuned by Microcavities
ZHAO Jia-Min, MA Feng-Ying, LIU Xing-Yuan, LIU Yun, CHU Guo-Qiang, NING Yong-Qiang, WANG Li-Jun,
Chin. Phys. Lett. 2002, 19 (10): 1447-1449 .  
Abstract   PDF(pc) (310KB) ( 542 )
Organic metal microcavities were fabricated by using full-reflectivity aluminum film and semi-transparent silver film as cavity mirrors. Unlike conventional organic microcavities, such as the typical structure of glass/DBR/ITO/organic layers/metal mirror, a microcavity with a shorter cavity length was obtained by using two metal mirrors, where DBR is the distributed Bragg reflector consisted of alternate quarter-wave layers of high and low refractive index material. It is realized that red, green and blue single-mode electroluminescence (EL) from the microcavities with the structure, glass/Ag/TPD/Alq3/Al, are electrically-driven when the thickness of Alq3 layer changes. Compared to a non-cavity reference sample whose EL spectrum peak is located at 520 nm with a full width at half maximum (FWHM) of 93 nm, the microcavity devices show apparent cavity effects. The EL spectra of red, green and blue microcavities are peaked at 604 nm, 540 nm, and 491 nm, whose FWHM are 43 nm, 38 nm, and 47 nm, respectively.
Self-Mode-Locking in a Diode-Pumped Self-Q-Switched Nd3+, Cr4+: YAG Laser
YANG Lin, FENG Bao-Hua, ZHANG Zhi-Guo, Volker Gaebler, LIU Bai-Ning, Hans J. Eichler, ZHANG Shi-Wen
Chin. Phys. Lett. 2002, 19 (10): 1450-1452 .  
Abstract   PDF(pc) (394KB) ( 532 )
We report for the first time on the observation of self-mode-locking in a diode pumped self-Q-switched (SQS) Nd3+, Cr4+:YAG laser. This phenomenon results from significant excited state absorption of the Cr4+ ions in the co-doped host during the SQS laser pulses. The self-mode-locking occurs already slightly above the SQS laser threshold. Experiments using relatively low saturable intensity achieved a modulation depth of more than 40%.
A TEM00 Mode Lamp-Pumped Nd:YAG Laser Operated at 1318.8 nm in the Absence of Intracavity Etalons and Additional Apertures
LIN Wen-Xiong, HUANG Jian-Hong, LIN Shou-Qun
Chin. Phys. Lett. 2002, 19 (10): 1453-1455 .  
Abstract   PDF(pc) (329KB) ( 559 )
We present a lamp-pumped high-efficiency continuous-wave (cw) TEM00 mode Nd:YAG laser operated at 1318.8 nm single line in the absence of intracavity etalons and additional apertures. A 15-W laser with beam quality M2 < 2 has been achieved, which is, to our knowledge, the highest power in the cw TEM00 mode at 1318.8 nm within China. From the experimental results of laser behaviour at lines 1318.8 nm and 1338.2 nm, we conclude that the emission cross section of transition line at 1318.8 nm is larger than that at 1338.8 nm.
Enhancement of Optical Coherence Tomography Axial Resolution by Spectral Shaping
SUN Shan, GUO Ji-Hua, GAO Jian-Song, XUE Ping
Chin. Phys. Lett. 2002, 19 (10): 1456-1458 .  
Abstract   PDF(pc) (344KB) ( 488 )
We propose a new method of changing the spectrum shape to improve the axial resolution of optical coherence tomography (OCT). Theoretical analysis shows that certain spectral shaping can shorten the coherence length. Comparisons of the simulation and experimental measurement of spectral shape and axial resolution of OCT are given, showing that the axial resolution of OCT is enhanced by a factor of 1.4.

Opacity of Hot and Dense Plasmas of a Mixture using an Average-Atom Approach
YUAN Jian-Min
Chin. Phys. Lett. 2002, 19 (10): 1459-1462 .  
Abstract   PDF(pc) (297KB) ( 505 )
An average-atom model is proposed to calculate the opacities of hot and dense plasmas of mixture. A self-consistent scheme is used to reach the requirements of the same temperature and chemical potential for all kinds of atoms in the mixtures, the same electron density at the boundaries between the atoms, and the electrical neutrality within each atomic sphere. The orbital energies and wave functions for the bound electrons are calculated with the Dirac-Slater equations. The occupation numbers at each orbital of each kind of atom are determined by the Fermi-Dirac distribution with the same chemical potential for all kinds of atom. As an example, the opacity of the mixture of Au and Cd is calculated at a few temperatures and densities.
Longitudinal Waves in Strongly Coupled Magnetized Dusty Plasma with Dust Charging Relaxation
XIE Bai-Song
Chin. Phys. Lett. 2002, 19 (10): 1463-1466 .  
Abstract   PDF(pc) (203KB) ( 487 )
Low-frequency longitudinal dust waves in strongly coupled magnetized dusty plasmas are investigated. The dust charging relaxation is taken into account. It is found that the frequency and damping of dust waves are modified significantly due to the existence of the magnetic field as well as the effect of dust charging.
Experimental Phenomena of Improved Ohmic Confinement Induced by Modulated Toroidal Current on the HT-7 Tokamak
MAO Jian-Shan, LUO Jia-Rong, P. Phillips, ZHAO Jun-Yu, JIE Yin-Xian, WU Zhen-Wei, HU Li-Qun, LI Jian-Gang
Chin. Phys. Lett. 2002, 19 (10): 1467-1469 .  
Abstract   PDF(pc) (573KB) ( 436 )
The phenomena of improved Ohmic confinement have been observed during modulation of toroidal current on the Hefei superconducting Tokamak-7 (HT-7). In the experiment, the programming ohmic heating field was modulated. A toroidal frequency-modulated current induced by modulated loop voltage was added on the plasma equilibrium current. The ratio of ac amplitude of the plasma current to the main plasma current is about 12%--30%. These improved plasma confinement phenomena include the facts that the average electron density and the central electron temperature both increase, the Dα radiation from the edge is reduced, the magnetichydrodynamics is obviously suppressed by oscillating plasma current, and the global energy confinement time increases by 27%-45%. It is found that the faster the modulation is, the more effective the improved ohmic confinement phase.

Tripolar Vortices of Dust-Drift Waves in Dusty Plasma with Shear Flow
Chin. Phys. Lett. 2002, 19 (10): 1470-1472 .  
Abstract   PDF(pc) (385KB) ( 388 )
Nonlinear equations governing dust-drift waves in magnetized dusty plasma with transverse shear flow are derived. For the specific profiles of flow and the plasma equilibrium density, a new type of solutions in the form of tripolar vortices is found. The results show that the peak magnitude of tripolar vortices increases with increasing shear intensity and dust content.
Pulsed Ion-Sheath Dynamics in a Cylindrical Bore for Inner Surface Grid-Enhanced Plasma Source Ion Implantation
WANG Jiu-Li, ZHANG Gu-Ling, FAN Song-Hua, YANG Wu-Bao, YANG Si-Ze,
Chin. Phys. Lett. 2002, 19 (10): 1473-1475 .  
Abstract   PDF(pc) (392KB) ( 359 )
Based on our recently proposed grid-enhanced plasma source
ion implantation (GEPSII) technique for inner surface modification of materials with cylindrical geometry, we present the corresponding theoretical studies of the temporal evolution of the plasma ion sheath between the grid electrode and the target in a cylindrical bore. Typical results such as the ion sheath evolution, time-dependent ion density and time-integrated ion energy distribution at the target are calculated
by solving Poisson's equation coupled with fluid equations for
collisionless ions and Boltzmann assumption for electrons using finite difference methods. The calculated results can further verify the feasibility and superiority of this new technique.
Temporal Behaviour of Micro-discharge in Dielectric Barrier Discharges
YIN Zeng-Qian, DONG Li-Fang, CHAI Zhi-Fang, LI Xue-Chen, WANG Long
Chin. Phys. Lett. 2002, 19 (10): 1476-1479 .  
Abstract   PDF(pc) (416KB) ( 747 )
A special dielectric barrier discharge (DBD) device consisting of two water electrodes has been designed. The temporal behaviour of the micro-discharge (filament) in the DBD in air at atmospheric pressure is measured by using an optical method. The nonsymmetrical character of the filament is discovered for the first time. We propose an equation regarding the discharge moment by considering the memory effect of accumulated charge and the influence of the fluctuation. The results deduced from the equation give a good explanation to the experimental phenomenon, which show that the decay time constant of the accumulated charges is much larger than the order of 100μs. The relative intensity of fluctuation amplitude is in the range from 2% to 4% under the present experimental conditions.
Determination of the Potential Barrier at the Metal/Oxide Interface in a Specular Spin Valve Structure with Nano-oxide Layers by Using Electron Holography
WANG Yan-Guo, SHEN Feng, ZHANG Ze, CAI Jian-Wang, LAI Wu-Yan
Chin. Phys. Lett. 2002, 19 (10): 1480-1482 .  
Abstract   PDF(pc) (290KB) ( 494 )
The local potential distribution in a specular spin valve structure with nano-oxide layers has been mapped by using off-axis electron holography in a field emission gun transmission electron microscope. A potential jump of 3-4V across the metal/oxide interface was detected for the first time. The presence of the potential barrier confirms the formation of the metal/insulator/metal structure, which contributes to increasing mean free path of spin-polarized electrons via the specular reflection of spin-polarized electrons at the metal/oxide interface. It leads to nearly double enhancement of the magnetoresistance ratio from 8% to 15%.
A Kinetic Study of the Non-isothermal Crystallization of a Zr-Based Bulk Metallic Glass
LIU Lin, WU Zhi-Fang, CHEN Li
Chin. Phys. Lett. 2002, 19 (10): 1483-1486 .  
Abstract   PDF(pc) (501KB) ( 385 )
The kinetics of the non-isothermal crystallization of Zr55Cu30Al10Ni5 bulk metallic glass was studied by differential scanning calorimetry in the mode of continuos heating. It is found that both glass transition and crystallization of the amorphous alloy behave in a marked kinetic nature. The activation energy E and frequency factor k0 for crystallization were determined by the Kissinger method, which yields E = 230kJ/mol, and k0 = 4.2 x 1012/s. The value of k0 is much smaller than that for conventional amorphous alloys, demonstrating that the atomic mobility in bulk metallic glasses is quite sluggish. The crystallization mechanism in the non-isothermal process was analysed in terms of the kinetic transformation equation for the solid-state phase transformation. This reveals that the crystallization kinetic function f(x) for Zr55Cu30Al10Ni5 can be described by the Johnson-Mehl-Avrami equation, i.e. f(x) = (1-x) [-\ln (1-x)](n-1)/n, with n = 1.75. This indicates that the non-isothermal crystallization of Zr55Cu30Al10Ni5 bulk metallic glass is governed by diffusion-controlled one-dimensional growth. Finally, the different crystallization mechanisms in the isothermal and non-isothermal processes are discussed.
Structure Stability of I-Type Carbon Nanotube Junctions
XIA Dan, YUAN Zhe, LI Jia-Ming,
Chin. Phys. Lett. 2002, 19 (10): 1487-1489 .  
Abstract   PDF(pc) (224KB) ( 340 )
Carbon nanotubes with junctions may play an important role in
future‘nanoelectronics’and future ‘nano devices’. In particular, junctions constructed with metal and semiconducting nanotubes have potential applications. Based on the orthogonal tight-binding molecular dynamics method, we present our study of the structure stability of I-type carbon nanotube junctions.
Calculation of the Griffith Cohesive Energy of the Ni3AlBx Symmetrical Grain Boundary
ZHENG Li-Ping, ZHANG Hu-Yong, LI Dou-Xing, CUI Fu-Zhai
Chin. Phys. Lett. 2002, 19 (10): 1490-1493 .  
Abstract   PDF(pc) (336KB) ( 330 )
A Monte Carlo simulation, with the energetics described by the embedded atom method, has been employed to study the physical behaviour of boron atoms during relaxation of the Ni3AlBx grain boundary, It has also been used to calculate not only the peak concentrations of Ni and B and the valley concentration of Al at the grain boundary, but also the dependence of the grain boundary cohesion on the B bulk concentration. During relaxation of impure Ni3Al grain boundaries, we suggest that, as the segregating species, the B atoms either insert into interstices in the grain boundary or substitute Ni atoms. Meanwhile, as the inducing species, they induce Ni atoms to substitute for Al atoms. Calculations show that in the equilibrium, when the B bulk concentration x increases from 0.1 to 0.9, the peak concentration of B increases, the peak concentration of Ni maximizes while the valley concentration of Al minimizes at x = 0.5. The calculations also show the best cohesion of the grain boundary at x = 0.5.
Preparation and Properties of N-Doped p-Type ZnO Films by Solid-Source Chemical Vapor Deposition with the c-Axis Parallel to the Substrate
LÜ, Jian-Guo, YE Zhi-Zhen, WANG Lei, ZHAO Bing-Hui, HUANG Jing-Yun
Chin. Phys. Lett. 2002, 19 (10): 1494-1497 .  
Abstract   PDF(pc) (417KB) ( 693 )
We report on N-doped p-type ZnO films with the c-axis parallel to the substrate. ZnO films were prepared on an α-Al2O3 (0001) substrate by solid-source chemical vapor deposition (CVD). Zn(CH3COO)2.2H2O was used as the precursor and CH3COONH4 as the nitrogen source. Growth temperature is varied from 300°C to 600°C. The as-grown ZnO film deposited at 500°C showed p-type conduction with its resistivity of 42 Ωcm, carrier density 3.7 x 1017cm-3 and Hall mobility 1.26cm2V-1.s-1 at room temperature, which are the best properties for p-type ZnO deposited by CVD. The p-type ZnO film possesses a transmittance about 85% in the visible region and a band gap of 3.21 eV at room temperature.
Tunable Adsorption and Desorption of Hydrogen Atoms on Single-Walled Carbon Nanotubes
ZHAO Ming-Wen, XIA Yue-Yuan, MA Yu-Chen, YING Min-Ju, LIU Xiang-Dong, MEI Liang-Mo
Chin. Phys. Lett. 2002, 19 (10): 1498-1500 .  
Abstract   PDF(pc) (381KB) ( 521 )
Chemical adsorption and desorption of hydrogen atoms on single-walled carbon nanotubes (SWNTs) are investigated by using molecular dynamics simulations. It is found that the adsorption and desorption energy of hydrogen atoms depend on the hydrogen coverage and the diameter of the SWNTs. Hydrogen-adsorption geometry at the coverage of 1.0 is more energetically stable. The adsorption energy decreases with the increasing diameter of the armchair tubes. The adsorption and desorption energy of hydrogen atoms can be modified reversibly by externally radial deformation. The averaged C-H bond energy on the high curvature sites of the deformed tube increases with increasing radial deformation, while that on the low curvature sites decreases.
Electrical Conductivity of Aluminum Alloy Foams
FENG Yi, ZHENG Hai-Wu, ZHU Zhen-Gang, ZU Fang-Qiu
Chin. Phys. Lett. 2002, 19 (10): 1501-1504 .  
Abstract   PDF(pc) (284KB) ( 815 )
Closed-cell aluminum alloy foams were produced using the powder metallurgical technique. The effect of porosity and cell diameter on the electrical conductivity of foams was investigated and the results were compared with a number of models. It was found that the percolation theory can be successfully applied to describe the dependence of the electrical conductivity of aluminum alloy foams on the relative density. The cell diameter has a negligible effect on the electrical conductivity of foams.
Coherent Transport Through a Quantum Dot Embedded in a Double-Slit-Like Aharonov-Bohm Ring
HUANG Li, YOU Jian-Qiang, YAN Xiao-Hong, WEI Shi-Hao
Chin. Phys. Lett. 2002, 19 (10): 1505-1508 .  
Abstract   PDF(pc) (378KB) ( 379 )
Coherent transport through a quantum dot embedded in one arm of a double-slit-like Aharonov-Bohm (AB) ring is studied using the Green's function approach. We obtain experimental observations such as continuous phase shift along a single resonance peak and sharp inter-resonance phase drop. The AB oscillations of the differential conductance of the whole device are calculated by using the nonequilibrium Keldysh formalism. It is shown that the oscillating conductance has a continuous bias-voltage-dependent phase shift and is asymmetric in both linear and nonlinear response regime.
Occupation Modulation of Higher Subbands in a Three-Barrier Tunneling Structure with a Magnetic Field
AN Long, TANG Yan, ZHENG Hou-Zhi, ZHANG Ji-Dong, TAN Ping-Heng, YANG Fu-Hua, JI Yang, CHEN Yuan-Zhen
Chin. Phys. Lett. 2002, 19 (10): 1509-1512 .  
Abstract   PDF(pc) (416KB) ( 412 )
We verify that the magnetic suppression of intersubband LO or LA phonon scattering can give rise to a noticeable nonthermal occupation in higher-lying subbands. This is clearly determined by the relative intensity ratio of the interband photoluminescence spectra for the E2-HH1 and E1-HH1 transitions. The observed phenomenon may provide an effective method to control the intersubband scattering rate, which is a key factor of the so-called quantum cascade lasers. This is helpful for the population inversion between both the subbands in quantum wells.
Preparation of p-n Junction Diode by B-Doped Diamond Film Grown on Si-Doped c-BN
WANG Cheng-Xin, GAO Chun-Xiao, ZHANG Tie-Chen, LIU Hong-Wu, LI Xun, HAN Yong-Hao, LUO Ji-Feng, SHEN Cai-Xia
Chin. Phys. Lett. 2002, 19 (10): 1513-1515 .  
Abstract   PDF(pc) (310KB) ( 570 )
A Heterojunction diode has been fabricated by boron-doped p-type diamond thin film grown epitaxially on a silicon-doped n-type cubic boron nitride bulk crystal using the conventional hot filament chemical vapor deposition method. The ohmic electrode of Ti (50 nm)/Mo (100 nm)/Au (300 nm) for the p-type diamond film and the bulk crystal of the c-BN were deposited by the rf planar magnetron method. Then the device was annealed at 410°C in air for 1 h in order to form ohmic metal alloy. The I-V characteristics of the hetero-junction diode were measured and the result indicated that the rectification ratio reached 105, and the turn-on voltage and the highest current were 7 V and 0.35 mA, respectively.
Mixed Former Effect: A Kind of Composition Adjusting Methods of Er-Doped Glass for Broadband Amplification
YANG Jian-Hu, DAI Shi-Xun, WEN Lei, LIU Zhu-Ping, HU Li-Li, JIANG Zhong-Hong
Chin. Phys. Lett. 2002, 19 (10): 1516-1518 .  
Abstract   PDF(pc) (361KB) ( 498 )
A compositional adjusting method called the mixed former effect is proposed to improve effectively optical properties such as emission cross section, the fluorescence full width at half maximum (FWHM) and the lifetime of the 4I13/2 level of Er-doped glass. A kind of Er-doped bismuth-based glass illustrated high emission cross section (σpe = 0.66-0.90pm2), large fluorescence FWHM (68-85 nm), and relative long lifetime of the 4I13/2 level (τm = 1.6-4.3ms) by this method. Comparison of spectroscopic parameters shows that bismuth-based glass is much better for broadband amplifier than other glass hosts.
Current Self-Oscillations in Negative Effective Mass Terahertz Oscillators
CAO Jun-Cheng
Chin. Phys. Lett. 2002, 19 (10): 1519-1521 .  
Abstract   PDF(pc) (237KB) ( 353 )
We theoretically study current self-oscillations and spatiotemporal current patterns in quantum-well negative-effective mass (NEM) p+pp+ diodes by considering scattering contributions from impurity, acoustic phonon, and optic phonon. It is indicated that both the applied bias and the doping concentration strongly influence the patterns and self-oscillating frequencies. The NEM p+pp+ diode presented here may be used as an electrically tunable terahertz source.
Band Structure of the Non-Oxide Perovskite MgCNi3
WAN Xian-Gang, WENG Hong-Ming, DONG Jin-Ming
Chin. Phys. Lett. 2002, 19 (10): 1522-1524 .  
Abstract   PDF(pc) (261KB) ( 687 )
Using the self-consistent full-potential linearized augmented plane wave method, we carry out the electronic structure calculations of MgCNi3 in the local spin density approximation (LSDA) of the density-functional theory. The LSDA solution is metal. The magnetic moment on Ni ion is only 0.014μB. There is a peak in the density of state just below the Fermi energy (Ef), which can be strongly correlated with various instabilities. Afonter including the strong electron-electron correlation effects the Ni 3d state by the on-site Coulomb interaction correction, the density of state is greatly redistributed, and the peak just below Ef disappears. The magnetic moment on the Ni ion becomes 0.66μB. Both LSDA and LSDA+U have shown that the electronic state nearby the Fermi surface are dominated by the 3d orbital of Ni.
An Extended Effective Potential Method with Topological Phase of Spin Tunneling
KOU Su-Peng, LÜ, Rong, LIANG Jiu-Qing
Chin. Phys. Lett. 2002, 19 (10): 1525-1527 .  
Abstract   PDF(pc) (244KB) ( 405 )
An extended effective potential (EEP) method is proposed to calculate the spin tunneling splitting of a single-domain ferromagnetic grain. By adding a phase factor to replace the boundary condition, the new method is not only accurate but also shows topological effects. In the large spin limit, the results from spin-coherent-state path integral have systematic disparity with numerical studies, while those from the EEP method agree well with numerical studies.
Charge-Orbital Ordering in Ferromagnetic and Charge-Exchange-Type Antiferromagnetic Phases for Half-Doped Manganites
SHU Zheng-Huang, LÜ, Wen-Gang, DONG Jin-Ming
Chin. Phys. Lett. 2002, 19 (10): 1528-1530 .  
Abstract   PDF(pc) (248KB) ( 701 )
Using a two-orbital DE model including the Coulomb interaction, we show that the charge-orbital ordering usually found in the charge-exchange phase may exist in the ferromagnetic phase for half-doped manganites. Thus, two types of phase transitions observed experimentally can be explained by the obtained phase diagram.
Effect of A Cr Underlayer on the Magnetic Properties of TbCo Amorphous Films with Perpendicular Anisotropy
HUANG Zhi-Xin, LI Zuo-Yi, JIN Fang, CHENG Xiao-Min, XU Xiao-Hong, LI Zhen, WANG Xian-Ran, LIN Geng-Qi
Chin. Phys. Lett. 2002, 19 (10): 1531-1533 .  
Abstract   PDF(pc) (307KB) ( 447 )
The effect of the Cr underlayer on the magnetic properties of TbCo amorphous films with perpendicular anisotropy has been investigated. It was found that the Cr underlayer thickness can influence the coercivity of TbCo films. The coercivity as high as 6.3 kOe was obtained in 120 nm Tb31Co69 films with 180 nm Cr underlayer. It was only 4.4 kOe for the same thickness Tb31Co69 films without the Cr underlayer. Cross-sectional scanning electron microscopy indicated that the TbCo film with the Cr underlayer consisted of the column structure. It is considered that this heterogeneous structure gives rise to the coercivity enhancement of TbCo films in the presence of the Cr underlayer.

Enhanced Electron Injection Efficiency and Electroluminescence in Organic Light-Emitting Diodes by Using an Sn/Al Cathode
LI Feng, CHENG Gang, FENG Jing, LIU Shi-Yong
Chin. Phys. Lett. 2002, 19 (10): 1534-1536 .  
Abstract   PDF(pc) (431KB) ( 439 )
A series of organic light-emitting diodes (OLEDs) have been fabricated with different thickness of the tin (Sn) layer. The structure of the devices is indium-tin oxide (ITO)/copper phthalocyanine (CuPc)(12 nm)/N, N'-diphenyl-N, N'-bis(1-naphthyl)-(1,1'-biphenyl)-4, 4'-diamine (NPB) (60 nm)/tris-(8-hydroxyquinoline)aluminum (Alq3) (60 nm)/Sn/aluminum (Al)(120 nm). It is found that compared to OLEDs with only Al cathode, both the electron injection efficiency and electroluminescence are improved when the thickness of the Sn layer is properly chosen. The maximum efficiency and brightness of the devices with Sn (2.1 nm)/Al and Al cathode are 0.54lm/W and 9800cd/m2, 0.26lm/W and 3000cd/m2, respectively. One possible explanation to this phenomenon is that the enhancement of the electron density of the Sn layer due to the electron transfer from the Al to the Sn layer leads to the improvement of the electron injection efficiency and electroluminescence.
Iron Kα Emission Lines in Seyfert(-Like) Active Galactic Nuclei: Revelation of a Rapidly Spinning Central Black Hole
MA Zhen-Guo
Chin. Phys. Lett. 2002, 19 (10): 1537-1539 .  
Abstract   PDF(pc) (384KB) ( 410 )
Fe Kα lines are superimposed upon the x-ray continuum in most Seyfert(-like) active galactic nuclei (AGNs). By a data-fitting study, previous authors have claimed that the central black hole (BH) is either rotating or non-rotating according to the thin disk model. We develop the disk model to the torus model to determine the real spin of the BH. With formulations of the motion of both torus particles and photons near a BH in Kerr metric, we simulate iron emission lines from a thin luminous torus. It is found that only spinning BH galaxies can radiate observable profiles. The data-fitting to Fe lines of four AGNs observed by ASCA predicts that the central BH is spinning rapidly with the dimensionless specific angular momentum approaching the maximal value 1.
Storage of Photoexcited Electron-Hole Pairs in an AlAs/GaAs Heterosturcture Created by Electron Transfer in Real and k Spaces
PENG Jin, HU Bing, HU Cheng-Yong, BIAN Song-Bao, YANG Fu-Hua, ZHENG Hou-Zhi
Chin. Phys. Lett. 2002, 19 (10): 1540-1542 .  
Abstract   PDF(pc) (403KB) ( 491 )
The storage of photoexcited electron-hole pairs is experimentally carried out and theoretically realized by transfering electrons in both real and k spaces through resonant Γ - X in an AlAs/GaAs heterostructure. This is proven by the peculiar capacitance jump and hysteresis in the measured capacitance-voltage curves. Our structure may be used as a photonic memory cell with a long storage time and a fast retrieval of photons as well.
Rapid Growth of Ice Dendrite in Acoustically Levitated and Highly Undercooled Water
LÜ, Yong-Jun, XIE Wen-Jun, WEI Bing-Bo
Chin. Phys. Lett. 2002, 19 (10): 1543-1546 .  
Abstract   PDF(pc) (422KB) ( 580 )
Water drops with diameter ranging from 2.5 to 4 mm are highly
undercooled by up to 24 K with acoustic levitation technique. Compared to the case of water contained in tube, acoustic levitation has efficiently avoided the heterogeneous nucleation from container walls and consequently increased the undercooling level. However, the cavitation effect induced by ultrasound may prematurely catalyze nucleation, which hinders the further achievement of bulk undercooling. The growth velocity of ice dendrite determined experimentally in highly undercooled water is characteristic of rapid dendritic growth, which reaches 0.17m/s at the undercooling of 24 K. The Lipton-Kurz-Trivedi dendritic growth model is used to predict the kinetic characteristics of rapid growth of ice dendrite under high undercooling conditions, which shows good agreement with the experimental results.

Mechanical and Magnetostrictive Properties of Fe-Doped Ni52Mn24Ga24 Single Crystals
FENG Xue, FANG Dai-Ning, HWANG Keh-Chih
Chin. Phys. Lett. 2002, 19 (10): 1547-1549 .  
Abstract   PDF(pc) (507KB) ( 424 )
To improve the magnetic and mechanical properties of a Heusler alloy of Ni52Mn24Ga24, iron was doped with some contents. Single crystals of the pseudoquaternary Heusler alloy of Ni52Mn9Fe15Ga24 have been synthesized for mechanical and magnetostrictive measurements. The magnetostriction loops and stress-strain curves were measured under different coupled magnetic mechanical loads. The experimental results show that the brittleness of the sample is clearly improved and Young's modulus of 13.7 GPa is obtained in the [001] direction due to the part substitution of Fe for Mn. Furthermore, the toughness and Vickers-hardness of the sample are also given by use of the indention technique.
Low-Frequency Dielectric Dispersion of Highly Concentrated Spherical Particles in an Electrolyte Solution
NI Fu-Sheng, GU Guo-Qing, CHEN Kang-Min
Chin. Phys. Lett. 2002, 19 (10): 1550-1552 .  
Abstract   PDF(pc) (284KB) ( 518 )
We deal with the problem of calculating the effective dielectric dispersion and electrical conductivity of colloidal dispersions. A comparison of the theoretical calculation of first principles with the experimental data of Schwan shows that our technique proposed here is no longer restricted to dilute solutions and is very effective to studying the dielectric properties of colloids with highly concentrated charged spherical particles in an electrolyte solution.

Photocurrent Property of GaN on the Si Photodetector with a Nearly Polycrystalline α-Al2O3 Buffer Layer
JIANG Ruo-Lian, WANG Jun-Zhuan, CHEN Peng, ZHAO Zuo-Ming, MEI Yong-Feng, SHEN Bo, ZHANG Rong, WU Xing-Long, ZHENG You-Dou
Chin. Phys. Lett. 2002, 19 (10): 1553-1555 .  
Abstract   PDF(pc) (300KB) ( 346 )
Using nearly polycrystalline α-Al2O3 as the buffer layer, GaN epilayers were grown on Si (111) substrates by low pressure metalorgnic chemical-vapor deposition. The nearly polycrystalline α-Al2O3 was formed by anodic porous alumina annealed at high temperature. Prototype photoconductive detectors were fabricated with these materials. The spectral response of these detectors exhibits a relatively sharp cutoff near the wavelength of 360 nm and a peak at 340 nm with a shoulder near 360 nm. Under 5 V bias, the responsivities at 340 nm and 360 nm were measured to be 3.3 A/W and 2.4 A/W, respectively. The relationship between the responsivity and the bias voltage shows that the responsivity is saturated when the bias voltage reaches 5 V.
Excitation of Electromagnetic Waves by an Electron Ring Beam in a Magnetized Plasma Waveguide
ZHOU Guo-Cheng, WU Ching-Sheng, WANG De-Ju, CHEN Yan-Ping,
Chin. Phys. Lett. 2002, 19 (10): 1556-1559 .  
Abstract   PDF(pc) (312KB) ( 316 )
We study the resonant interactions between an electron ring eam and plasma waveguide modes. This is motivated by the research of radio emission in low solar corona. We consider a density-depleted duct (above an active region near a flare site) that may be treated as a magnetized plasma waveguide. The electromagnetic waves excited in the waveguide are classified into the so-called E- and B-type waves. The results show that there are two unstable modes of B-type waves propagating parallelly and anti-parallelly to the direction of the electron beam, and the effect of the finite radius and boundary conditions of the waveguide on the excitation of waveguide modes is important. For a given B-type mode, the smaller the radius R, the larger the temporal and spatial amplification rate. We suggest that these excited waveguide modes could be one of the processes responsible for the observed solar radio emission.

Magnetic Fields in Active Late-Type Stars
SHI Jian-Rong, ZHAO Gang, ZHAO Yong-Heng
Chin. Phys. Lett. 2002, 19 (10): 1560-1561 .  
Abstract   PDF(pc) (207KB) ( 357 )
The very high resolution and high signal-to-noise ratio spectra of ten x-ray selected stars are obtained by the NAOC Coudé spectrograph. Magnetic field measurements of these stars have been performed using the profile-addition technique. The magnetic field strength and filling factor are obtained, and we find that there exist strong magnetic fields on very active stars.
Energetic Argument for Bimodal Black Hole Accretion Disks
LIN Yi-Qing, LU Ju-Fu, GU Wei-Min
Chin. Phys. Lett. 2002, 19 (10): 1562-1564 .  
Abstract   PDF(pc) (296KB) ( 404 )
Based on simple energetic considerations, we show that two crucial ingredients of bimodal black hole accretion discs, namely the sonic point and the transition radius, can be determined from the disc constant parameters. Thus, we can further justify the model of bimodal discs containing thermal instability triggered transition.
Modelling Neutral Hydrogen Discs of Spiral Galaxies
LIN Wei-Peng, , HONG Bi-Hai
Chin. Phys. Lett. 2002, 19 (10): 1565-1568 .  
Abstract   PDF(pc) (432KB) ( 373 )
We present an analytical model of a neutral hydrogen disc in a spiral galaxy. The gas disc of the spiral galaxy is assumed to have an exponential surface density profile and be ionized by the cosmic ultraviolet background. To compare with observations, we consider the disc position angle and inclination angle for a sight line going through the galaxy disc. The HI column densities depend on the strength of ionizing field and disk position and inclination. The model was applied to NGC 3198 and the results were compared with observational data. The HI disc profile at large disc radii which can be tested by further HI observations using radio telescopes with larger aperture than present facilities. This HI disc model can be used to predict quasar absorption line systems by galaxy disks if quasar sight lines go through the discs.
Violation of the Weak Energy Condition by the Inflationary Yang-Mills Condensate
Chin. Phys. Lett. 2002, 19 (10): 1569-1572 .  
Abstract   PDF(pc) (270KB) ( 427 )
Recent cosmic microwave background data favours a blue-tilted primeval perturbation spectrum with an index n > 1, which implies that the equation of state of dominant matter should be such that ρ+p < 0 during the inflationary expansion of the early universe. We study the quantum Yang-Mills condensate inflationary model and find that this requirement can be satisfied by the effective quantum Yang-Mills condensate naturally in the states with the negative dielectric constant.
52 articles