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Volume 17 Issue 8
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Original Articles
Simulating Strongly Correlated Electron Systems with Hybrid Monte Carlo
LIU Chuan
Chin. Phys. Lett. 2000, 17 (8): 547-548 .  
Abstract   PDF(pc) (136KB) ( 393 )
Using the path integral representation, the Hubbard and the periodic Anderson model on D-dimensional cubic lattice are transformed into field theories of fermions in D + 1 dimensions. These theories at half-filling possess a positive definite real symmetry fermion matrix and can be simulated using the hybrid Monte Carlo method.

Correlation Function of Potts Model on Bethe Lattice
HAN Bing, CHEN Shu
Chin. Phys. Lett. 2000, 17 (8): 549-551 .  
Abstract   PDF(pc) (195KB) ( 394 )
We exactly calculate the partition function and correlation function of Potts model on Bethe type lattice and discuss its phase transition. The calculation shows that the thermodynamic properties of Potts model on those Bethe lattices which is constructed via connecting the generators by single joint points has the similar character of one dimensional system. The phase transition exists only at zero temperature and no finite temperature phase transition occurs.
Phase Transition of Spin-Peierls Systems with Impurities
XU Bo-Wei, DING Guo-Hui, YE Fei
Chin. Phys. Lett. 2000, 17 (8): 552-554 .  
Abstract   PDF(pc) (163KB) ( 406 )
The quasi-one-dimensional spin-Peierls(SP) systems with impurities are studied in their bosonized form. The spins of the dimerized state are bounded into singlets with an SP gap, while the impurities of doped systems will induce fluctuations of the coupling strength between the spins at different sites and break some pairs of spin singlets. The doping suppresses the dimerized SP state and induces a Kosterlitz-Thouless phase transition from the dimerized state into the undimerized one.
Measurement of R Line Fluorescence in Ruby Using the Diamond Anvil Cell at Low Temperature
XU Li-Wen, CHE Rong-Zheng, JIN Chang-Qing
Chin. Phys. Lett. 2000, 17 (8): 555-557 .  
Abstract   PDF(pc) (252KB) ( 450 )
Using our custom-built high-pressure cryostat we have performed the measurements of the R line fluorescence of ruby in the diamond anvil cell from room temperature down to 90K. The maximum pressure was 11.1 GPa. By computer curve fitting, different equations have been found for functions of wavenumber versus temperature between ambient pressure and 2.7GPa. The changes of the shape and linewidth of R lines were observed.
Is f1(1420) the Partner of f1(1285) in the 3P1 qq Nonet?
LI De-Min, YU Hong, SHEN Qi-Xing
Chin. Phys. Lett. 2000, 17 (8): 558-559 .  
Abstract   PDF(pc) (151KB) ( 277 )
Based on a 2 x 2 mass matrix, the mixing angle of the axial vector states f1(1420) and f1(1285) is determined to be 51.5o, and the theoretical results about the decay and production of the two states are presented. The theoretical results are in good agreement with the present experimental results, which suggests that f1(1420) can be assigned as the partner of f1(1285) in the 3P1 qq nonet. We also suggest that the existence of f1(1510) needs further experimental confirmation.
Rotational State g-Factors in 84Zr
ZHU Sheng-Yun, LUO Qi, LI Guang-Sheng, FAN Zhi-Guo, XU Yong-Jun, GAO Zao-Chun, WEN Shu-Xian, WU Xiao-Guang, LIU Xiang-An, XU Guo-Ji, ZHU Jia-Zheng, CHEN Yong-Shou, K. Matsuta*, M. Fukuda*, M. Mihara*, T. Minamisono*
Chin. Phys. Lett. 2000, 17 (8): 560-561 .  
Abstract   PDF(pc) (165KB) ( 371 )
The g-factors of the rotational states along the positive parity yrast band in even-even nuclei 84Zr were measured up to a spin I = 16+ by a transient magnetic field-ion implanted perturbed angular distribution method and calculated by a cranking shell model. A peak structure of the g-factors has been observed for the first time. The measured g-factors confirm the mixed configuration of proton and neutron alignments.

An Understanding of the Meaning Behind Boson-Description in the Collective Motion of Nuclei
LU Da-Hai, YANG Li-Ming, DENG Wei-Zhen, CHEN Xiao-Lin, ZHANG Jian-wei
Chin. Phys. Lett. 2000, 17 (8): 562-564 .  
Abstract   PDF(pc) (188KB) ( 369 )
A possible meaning of boson-description for the collective motion of nuclei as a fermion system is discussed. It is shown that the use of boson-description comes out naturally from the requirement of independent separation of the collective degrees of freedom of nuclei.
Study on Excitation Function and Isospin Dependencies of Total Reaction Cross Section via the Boltzmann-Uehling-Uhlenbeck Model
CAI Xiang-Zhou, SHEN Wen-Qing, FENG Jun, FANG De-Qing, MA Yu-Gang, SU Qian-Min, ZHANG Hu-Yong, HU Peng-Yun
Chin. Phys. Lett. 2000, 17 (8): 565-567 .  
Abstract   PDF(pc) (269KB) ( 437 )
The excitation function and isospin dependencies of σR have been investigated by using the Boltzmann-Uehling-Uhlenbeck (BUU) model with a square-type density distribution. When the width parameter of the square distribution is obtained by fitting σR at relativistic energies, the BUU-model can reproduce the experimental data at intermediate energies better than Glauber model. The systematical underestimation of σR at intermediate energy by Glauber model was removed out now by BUU calculation framework. It is also found that σR is sensitive to nuclear equation of state and σNNin-medium . The difference factor d defined in text is sensitive to the nuclear structure such as neutron halo and neutron skin, etc.

Spatial and Temporal Evolution of Ultra-Wide-Band Optical Pulses in Propagation
XU Jing-Zhou, WANG Li, YANG Guo-Zhen
Chin. Phys. Lett. 2000, 17 (8): 568-570 .  
Abstract   PDF(pc) (240KB) ( 416 )
The propagation of ultrashort coherent electromagnetic pulses with broad spectral bandwidth in free space is studied by using scalar diffraction theory. It is confirmed and experimentally demonstrated that the diffraction not only affects the spatial structure but also changes the temporal waveform of an ultra-wide-band pulse during propagation. The terahertz pulse travelling as basic mode of Gaussian beam is discussed in detail.
Photorefractive Holographic Dynamics in Doubly Doped LiNbO3:Fe:Mn
LIU You-Wen, LIU Li-Ren, ZHOU Chang-He, XU Liang-Ying
Chin. Phys. Lett. 2000, 17 (8): 571-573 .  
Abstract   PDF(pc) (254KB) ( 368 )
The Kukhtarev two-center model for photochromic doubly doped LiNbO3:Fe:Mn crystals is both numerically and analytically solved for the recording sensitivity and the fixed nonvolatile space-charge field. An optimal prescription for material doping and oxidation/reduction processing is found, which requires that the crystal should be strongly oxidized together with a suitable Mn density with respect to the Fe density. Experiments performed with an oxidized crystal and a reduced crystal prove the prediction.

Nonlinear Absorption and Refraction in Multilevel Organic Molecular System
LI Chun-Fei, DENG Xiao-Xu, WANG Yu-Xiao
Chin. Phys. Lett. 2000, 17 (8): 574-576 .  
Abstract   PDF(pc) (208KB) ( 495 )
The nonlinear absorption and refraction in a multilevel organic molecular system is described by using the density matrix theory. The total absorptive coefficient of the system in the low-density case is equal to a linear sum of contributions from each energy level. Similarly, the total refractive index is equal to a linear sum of contributions from each energy level plus the refractive index of the vacuum. The absorption coefficient or refractive index due to each level is proportional to the population of that level, where the constant of proportionality is called the absorption cross-section or the refraction volume, respectively. The relation between the absorption cross-section and the refraction volume for each level is also given.

Limitation of the Capacity due to Amplified Spontaneous Emission in a Subpicosecond Soliton Communication System
ZHONG Wei-Ping, LU0 Hong-Ji
Chin. Phys. Lett. 2000, 17 (8): 577-579 .  
Abstract   PDF(pc) (218KB) ( 401 )
Subpicosecond pulse soliton solutions of the modified nonlinear Schrödinger equation are obtained by applying Hirota's method. It is shown that the accumulation of arrival time jitter due to amplified spontaneous emission in a subpicosecond soliton communication system is less than that for a picosecond one. The former has a greater maximum capacity and so it has greater superiority as an information carrier.

Internal Subharmonic Resonance in Faraday Experiment
WANG Xin-Long, CHEN Yi-Huang, WEI Rong-Jue,
Chin. Phys. Lett. 2000, 17 (8): 580-582 .  
Abstract   PDF(pc) (227KB) ( 403 )
The classical Faraday experiment is a tractable problem in nonlinear physics, and its rich spectrum of nonlinear phenomena has made it a prototype in studying nonlinear wave dynamics in general. Here we report the observation of a new type of subharmonic resonances which occurs at some smaller fractional frequencies, such as ν/3 , ν/4, and ν/6, where ν is the forcing frequency. As a result of these resonances, the lower-order surface-wave modes whose eigenfrequencies are close to the fractional frequencies participate into the wave motions. Our theory involving quadratically nonlinear mode couplings shows that an already excited mode itself can act as a parametric source on the lower-order primary modes, thus leading to the cascading subharmonic resonances.

Formation of the Chain of Ring-Like Vortices in a Transitional Boundary Layer
LI Cun-Biao, FU Song
Chin. Phys. Lett. 2000, 17 (8): 583-585 .  
Abstract   PDF(pc) (420KB) ( 649 )
One mechanism by which laminar flow becomes turbulence is through the instability of vortex interaction. The nonlinear evolution of the interactions between the A-vortex and the secondary closed vortex has been investigated. This nonlinear instability process leads to some fundamental changes in the topology of flows. Experimental observations are presented here, showing the formation of a chain of ring-like vortices, which generate high-frequency spikes on time traces in a transitional boundary layer.
A New Method for Thin Film Deposition-Faced Microwave Electron Cyclotron Resonance Plasma Sources Enhanced Direct-Current Magnetron Sputtering
XU Jun, MA Teng-Cai, LU Wen-Qi, XIA Yuan-Liang, DENG Xin-Lu
Chin. Phys. Lett. 2000, 17 (8): 586-588 .  
Abstract   PDF(pc) (306KB) ( 423 )
A new reactive magnetron sputtering system enhanced by the faced microwave electron cyclotron resonance plasma source was designed and amorphous CNx films has been prepared by using this system. The characterization of the films by interference microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy shows that the deposition rate is strongly affected by the direct-current bias, and the films are composed by a single carbon nitride phase and the N/C ratio is 4:3.2, which is close to that of C3N4 (4:3).
Atomic Aggregation Processes in the Early Stages of Pt/Pt (111) Growth
ZHUANG Guo-Ce, ZHU Xiao-Bin, WANG Wei
Chin. Phys. Lett. 2000, 17 (8): 589-591 .  
Abstract   PDF(pc) (316KB) ( 312 )
The atomic aggregation processes in the earth stages of Pt/Pt (111) growth are studied by using kinetic Monte Carlo simulations. Our results show that the average neighbor coordination number of the atoms in a cluster is a function of temperature, agreeing well the experiment observations of scanning tunneling microscopy. The influence of diffusion barriers of various atomic processes on the morphology of islands is also studied. Different morphologies of the islands (dendritic, fractal, or compact islands ) are found.
Preparation and Characterization of InAs/Si Composite Film
YANG Lin, LI Guang-Hai, ZHENG Mao-Jun, ZHANG Li-De
Chin. Phys. Lett. 2000, 17 (8): 592-594 .  
Abstract   PDF(pc) (295KB) ( 409 )
Composite thin films consisting of nanosized InAs particles embedded in amorphous Si matrices were prepared by radio frequency co-sputtering of InAs and Si. X-ray diffraction spectra show that the particle size of InAs increases with the increasing annealing temperature, while the particle sizes of In and As reach their maximum values at the temperature of 200oC, and decrease with the further increase of the annealing temperature. In and As can not exist in the 500oC sample due to the sublimation of In and As the reaction In + As →InAS. The composition of the film in different levels was analyzed. We found that only in the deep level, the mole contents of As and In conform to the stoichiometric ratio and the oxidation occurs only a few nanometers from the surface. We believe that the scarcity of In and As near the surface is due to the sublimation of In and the oxide of As.
Pb Surfactant-Assisted Co Film Growth on Cu (111)
XU Ming-Chun QIAN Hai-Jie, LIU Feng-Qin, KRASH Ibrahim, LAI Wu-Yan, WU Si-Cheng
Chin. Phys. Lett. 2000, 17 (8): 595-597 .  
Abstract   PDF(pc) (324KB) ( 402 )
Surfactant-assisted Co film epitaxy growth on Cu (111) using Pb as a surfactant was studied by means of Auger electron spectra and synchrotron radiation photoemission spectra. The results reveal that with increasing the Co thickness most of the Pb atoms float on the surface. Compared with 0.7ML(monolayer) Pb, the Co film with 1.5ML Pb surfactant has more layer-by-layer growth on Cu (111). The predeposited Pb layer can suppress the intralayer diffusion on the Cu (111) surface and effectively increase the Co island density at the initial stage of Co growth. On the contrary, a Pb-Co surface alloy was found during the Co film growth; this may hinder the interlayer diffusion of the deposited Co atoms, which is unfavorable to the layer-by-layer growth. The Pb-Co is also considered to be the main reason why some Pb atoms have been buried in the Co films.
Resonant Peak Splitting for Ballistic Conductance in Two-Dimensional Electron Gas Under Electromagnetic Modulation
WANG Ru-Zhi, YAN Xiao-Hong
Chin. Phys. Lett. 2000, 17 (8): 598-600 .  
Abstract   PDF(pc) (211KB) ( 425 )
By developing a transfer-matrix method, the resonant peaks splitting of ballistic conductance are investigated into the two-dimensional electron gas system with both electric and magnetic modulations of nanoscale periods. It is found that there exists the n-fold resonant peak splitting for ballistic conductance through n perpendicular magnetic barriers to n electric barriers. With a combination of m magnetic barriers and n electric barriers by increasing the amplitude of electric field, the folds of the splitting wound shift from m -1 to -1.

Superconductivity of Nd1.85Ce0.15CuO4-y by Electrochemical Oxidation
CHEN Ling, CHE Guang-Can, LI Hong, DONG Cheng, Zhou Fang, Huang Yu-Zhen, ZHAO Zhong-Xian
Chin. Phys. Lett. 2000, 17 (8): 601-602 .  
Abstract   PDF(pc) (182KB) ( 325 )
Superconductivity of n-type superconductor Nd1.85Ce0.15CuO4-y(NCCO) by electrochemical oxidation is reported for the first time and the cyclic voltammogram curve for NCCO as working electrode is presented. Investifations indicate that after oxidation, superconductivity of NCCO is improved: Tc is raised from 20K up to 26K, the superconducting volume fraction is increased also. The valence electronic states of oxygen and copper in NCCO are studied by x-ray photoelectron spectroscopy. It is found that the Cu-2p3/2 peak shifts to the higher binding energy side about 0.4eV, which indicates that the valence-state of copper ions is risen up after oxidation. These results imply that hole-carriers exist in n-type superconductor.
Spin-Polarized Electron Injection in Co/Cu/Fe Sandwich Structure
WANG Shou-Guo, CHEN Yan-Xue, WANG Zhi-He, CHEN Qiang, CHEN Jing-Lin, SHEN Hong-Lie, LIU Yi-Hua, XIE Shi-Jie, MEI Liang-Mo
Chin. Phys. Lett. 2000, 17 (8): 603-605 .  
Abstract   PDF(pc) (306KB) ( 351 )
A material asymmetry Co/Cu/Fe junction structure has been prepared for studying the spin-polarized electron injection at 77K. The sample performance was demonstrated to be analogous to that of a bipolar transistor. The maximal value of the output pulse voltage between Cu and Fe layers could reach the order of several μV when the bias current between Co and Cu layers was 10μA. The interface roughness, photograph of material, magnetic loop and injection characteristic curves have been measured. Some important points on this topic have been discussed.

Raman Scattering Study of PbSe Grown on (111) BaF2 Substrate
YANG Ai-Ling, WU Hui-Zhen, LI Zhi-Feng, QIU Dong-Jiang, CHANG Yong, LI Jian-Feng, P. J. McCann, X. M. Fang
Chin. Phys. Lett. 2000, 17 (8): 606-608 .  
Abstract   PDF(pc) (254KB) ( 736 )
PbSe films were grown on (111)-oriented BaF2 substrates by using molecular beam epitaxy. High resolution x-ray diffraction characterization showed good crystalline quality of PbSe films. Both longitudinal optical phonon at 135cm-1 and transverse optical phonon at 47.6cm-1 were observed by Raman scattering measurements. The Raman tensor calculation demonstrates that both transverse-optical and longitudinal-optical (LO) phonons in PbSe crystal are Raman active on (111)-oriented surface. Furthermore, 2LO phonon a t about 270cm-1 and polaron at about 800cm-1 in PbSe, were also observed. The observed Raman frequencies are in good agreement with theoretical calculations using point ion model.

Photoluminescence of Eu2+ Doped ZnS Nanocrystals
LIU Shu-Man, GUO Hai-Qing, ZHANG Zhi-Hua, LIU Feng-Qi, WANG Zhan-Guo
Chin. Phys. Lett. 2000, 17 (8): 609-611 .  
Abstract   PDF(pc) (274KB) ( 523 )
Eu2+ doped ZnS nanocrystals exhibit new luminescence properties because of the enlarged energy gap of nanocrys-talline ZnS host due to quantum confinement effects. Photoluminescence emission at about 520nm from Eu2+ doped ZnS nanocrystals at room temperature is investigated by using photoluminescence emission and excitation spectroscopy Such green emission with long lifetime (ms) is proposed to be a result of excitation, ionization, carriers recapture and recombination via Eu2+ centers in nanocrystalline ZnS host.

Optical Transitions in Cubic GaN Grown by Metalorganic Chemical Vapor Deposition on GaAs (100) Substrate
CHEN Ye, LI Guo-Hua, HAN He-Xiang, WANG Zhao-Ping, XU Da-Peng, YANG Hui
Chin. Phys. Lett. 2000, 17 (8): 612-614 .  
Abstract   PDF(pc) (283KB) ( 483 )
Photoluminescence measurements have been performed on cubic GaN films with carrier concentration as low as 3x1013 cm-3. From the temperature and excitation intensity dependence, the emission lines at 3.268, 3.150 and 3.081 eV were assigned to the excitonic, donor-acceptor pair, and free-to-acceptor transitions, respectively. Additionallx we observed two additional emission lines at 2.926 and 2.821 eV, and suggested that they belong to donor-acceptor pair transitions. Furthermore, from the temperature dependence of integral intensities, we confirmed that three donor-acceptor pair transitions (3.150, 2.926, and 2.821 eV) are from a common shallow donor to three different acceptors. The excitonic emission at 3.216 eV has a full-width-at-half-maximum value of 41 meV at room temperature, which indicates a good optical quality of our sample.

Excitation Transfer in Vertically Self-organized Pairs of Unequal-Sized InAs/GaAs Quantum Dots
WANG Hai-Long, FENG Song-Lin, YANG Fu-Hua, SUN Bao-Quan, JIANG De-Sheng
Chin. Phys. Lett. 2000, 17 (8): 615-616 .  
Abstract   PDF(pc) (213KB) ( 334 )
The excitation transfer processes in vertically self-organized pairs of unequal-sized quantum dots (QD’s), which are created in InAs/GaAs bilayers with different InAs deposition amounts in the first and second layers, have been investigated experimentally by photoluminescence technique. The distance between the two dot layers is varied from 3 to 12nm. The optical properties of the formed pairs of unequal-sized QD’s with clearly discernible ground-state transition energy depend on the spacer thickness. When the spacer layer of GaAs is thin enough, only one photoluminescence peak related to the large QD ensemble has been observed as a result of strong electronic coupling in the InAs QD pairs. The results provide evidence for nonresonant energy transfer from the smaller QDs in the second layer to the larger QD’s in the first layer in such an asymmetric QD pair.
Preparation of AlGaN/GaN Heterostructures on Sapphire Using Light Radiation Heating Metal-Organic Chemical Vapor Deposition at Low Pressure
ZHOU Yu-Gang, SHEN Bo, ZHANG Rong, LI Wei-Ping, CHEN Peng, CHEN Zhi-Zhong, GU Shu-Lin, SHI Yi, Z. C. Huang, ZHENG You-Dou
Chin. Phys. Lett. 2000, 17 (8): 617-618 .  
Abstract   PDF(pc) (175KB) ( 468 )
AlGaN/GaN heterostructures on sapphire substrate were fabricated by using light radiation heating metalor-ganic chemical vapor deposition. Photoluminescence excitation spectra show that there are two abrupt slopes corresponding to the absorption edges of AlGaN and GaN, respectively. X-ray diffraction spectra clearly exhibit the GaN (0002), (0004), and AlGaN (0002), (0004) diffraction peaks, and no diffraction peak other than those from the GaN {0001} and AlGaN {0001} planes is found. Reciprocal space mapping indicates that there is notilt between the AlGaN layer and the GaN layer. All results also indicate that the sample is of sound quality and the A1 composition in the AlGaN layer is of high uniformity.
Enhancement-Mode InAlAs/InGaAs/InP High Electron Mobility Transistor with Strained InAlAs Barrier Layer
AO Jin-Ping, ZENG Qing-Ming, ZHAO Yong-Lin, LI Xian-Jie, LIU Wei-Ji, LIU Shi-Yong, LIANG Chun-Guang
Chin. Phys. Lett. 2000, 17 (8): 619-620 .  
Abstract   PDF(pc) (190KB) ( 426 )
The relatively low Schottky barrier height on In0.52Al0.48As lattice-matched to InP has hampered the achievement of enhancement-mode InAlAs/InGaAs/InP high electron mobility transistors (E-HEMTs). By introducing lattice-mismatched strained aluminum-rich In0.45Al0.55As as the Schottky contact material to enhance the barrier height, we have developed InP-based InAlAs/InGaAs/InP E-HEMT with threshold voltage of about 150mV. A maximum extrinsic transconductance of 660mS/mm and output conductance of 15 mS/mm are measured for 1 μm-gate-length devices at room temperature. The devices also show excellent radio-frequency performance with cutoff frequency of 50 GHz and maximum oscillation frequency of 54 GHz.
Mapping Analysis of Solar Radio Burst at 17GHz on 1992 October 27
YU Xing-Feng, YAO Jin-Xing
Chin. Phys. Lett. 2000, 17 (8): 621-622 .  
Abstract   PDF(pc) (155KB) ( 357 )
The contour maps at 17 GHz on the 1992 October 27 solar burst are analysed. Using a nonthermal inhomogeneous model of the radio source, the variations of calculated source size with time are basically consistent with the observation of maps. The variations of calculated brightness temperature with a distance ρ displaced from the project center are in better agreement with the observation of map at 0 1 : 45 : 29 : 889 UT. The calculation shows that the characterizing magnetic field gradient αβ at the peak of 17GHz is about 2, magnetic strength B is about 737G with the magnetic field Bm of the core being 2000G. However in order to measure the coronal magnetic field accurately, it is necessary to obtain radio-imaging data at more frequencies.

Gamma-Ray Bursts Subset and Supernova Remnants Low Radio-Frequency Turnover
LIU Xiang
Chin. Phys. Lett. 2000, 17 (8): 623-624 .  
Abstract   PDF(pc) (134KB) ( 326 )
Durations of gamma-ray bursts (GRB’s) are featured by >2s subset and <2s one, with initial corresponding energy ratio being 20:1. It is found that supernova remants(SNR’s) turnover frequencies peak at 100 and 500 MHz. After assuming that GRB’s originate from hypernova and making an analysis on the evolution of GRB’s, we find that the initial energy of two GRB subsets leads to a different radio-frequency turnover of their remnant spectra, which accords positively with the turnover-frequency ratio of SNR’s.
29 articles