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Volume 24 Issue 3
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Original Articles
An Integrable Decomposition of the Derivative Nonlinear Schrodinger Equation
ZHOU Ru-Guang
Chin. Phys. Lett. 2007, 24 (3): 589-591 .  
Abstract   PDF(pc) (190KB) ( 712 )
The nonlinearization method of spectral problem is developed and applied to the derivative nonlinear Schrodinger equation (DNLS). As a result, an integrable decomposition of the DNLS equation is obtained.
Modified Form of Wigner Functions for Non-Hamiltonian Systems
HENG Tai-Hua, LI Ping, JING Si-Cong
Chin. Phys. Lett. 2007, 24 (3): 592-595 .  
Abstract   PDF(pc) (215KB) ( 404 )
Quantization of non-Hamiltonian systems (such as damped systems) often gives rise to complex spectra and corresponding resonant states, therefore a standard form calculating Wigner functions cannot lead to static quasi-probability distribution functions. We show that a modified form of the Wigner functions satisfies a *-genvalue equation and can be derived from deformation quantization for such systems.
Establishment of Entanglement for Two Atoms Trapped in Two Distant Bad Cavities
LU Dao-Ming, ZHENG Shi-Biao
Chin. Phys. Lett. 2007, 24 (3): 596-598 .  
Abstract   PDF(pc) (201KB) ( 435 )
A scheme is presented for generation of entangled states for two atoms trapped in two distant bad cavities. The scheme can work with bad cavities with the coupling strength smaller than the cavity decay rate, which is important from the viewpoint of experiment. In the scheme the atoms have no probability of being populated in the excited state and thus the atomic spontaneous emission is suppressed, which increases the probability of success. The fidelity of the entangled state is not affected by the detection efficiency. Furthermore, the scheme does not require the detection of the left-polarized photon and right-polarized photon at the same time.
Implementation of a Two-Atom (swap)1/2 Gate in Cavity QED
LIU Qi, YE Liu
Chin. Phys. Lett. 2007, 24 (3): 599-601 .  
Abstract   PDF(pc) (198KB) ( 353 )
We implement a two-atom (swap)1/2 gate via cavity QED. During the preparation, a cavity-assisted collision between atoms is required, and this does not need any auxiliary atom. The cavity is only virtually excited, thus our scheme is insensitive to the cavity field states and to the cavity decay. The scheme can be implemented by the present cavity QED techniques.
Scheme for Concentration of Unknown Greeberger--Horne--Zeilinger Entangled States via Cavity Decay
ZHANG Xiao-Wei, LIU Xiang, TANG Xiao-Fang, TANG You-Liang
Chin. Phys. Lett. 2007, 24 (3): 602-605 .  
Abstract   PDF(pc) (220KB) ( 491 )
We present a scheme for entanglement concentration of an unknown atomic
non-maximally entangled GHZ state via cavity decay. In the scheme, the atom trapped in a cavity is manipulated by laser field, so the maximally entangled GHZ state can be obtained by performing certain operation, which can be realized by illuminating the atom in a cavity. Our method is robust against spontaneous atomic decay.
Remote State Preparation with Genuine Multipartite Entanglement
MA Yi-Cong, ZHANG Yong-Sheng, GUO Guang-Can
Chin. Phys. Lett. 2007, 24 (3): 606-608 .  
Abstract   PDF(pc) (193KB) ( 366 )
Enlightened by the work of Yeo and Chua [Phys. Rev. Lett. 96(2006)060502] for teleportation and dense coding with genuine multipartite entanglement, we present an explicit protocol for faithful remote state preparation in a real coefficient case by using the same four-particle entangled state which is not reducible to pair of Bell states. It is shown that any complex coefficient case can be changed to a real coefficient case. With this protocol, the state can play an analogous role to Einstein--Podolsky--Rosen pairs in the theory of multipartite entanglement.
Continuous Variable Entanglement and Violation of Bell Inequality for Two Modes in a Three-Level Cascade Atomic System
PING Yun-Xia, CHENG Ze
Chin. Phys. Lett. 2007, 24 (3): 609-612 .  
Abstract   PDF(pc) (213KB) ( 441 )
Continuous variable entanglement and violation of Bell inequality for two modes are investigated in a three-level cascade atomic system. Entanglement of the system is demonstrated according to the entanglement criterion [Phys. Rev. Lett. 84(2000)2722]. Violation of Bell inequality is studied within the framework of a quantum theory of multiwave mixing. It is shown that there are some states that are entangled but do not violate the Bell inequality.
Generation of Multiple-Photon GHZ State via Cavity-Assisted Interaction
DU Qian-Hua, CHEN Li-Bo, LIN Gong-Wei, CHEN Mei-Ying, CHEN Zhi-Hua, LIN Xiu-Min
Chin. Phys. Lett. 2007, 24 (3): 613-615 .  
Abstract   PDF(pc) (219KB) ( 485 )
We propose a scheme for preparing multiple-photon GHZ state via cavity-assisted interaction. There are n-pair single-photon pulses successively injected and reflected from two sides of the cavity, which traps one atom. After the atomic state is measured, a 2n-photon GHZ state is produced. In the ideal case, the successful probability of the scheme is close to unity.
Landau Damping of Collective Modes in a Disc-Shaped Bose--Einstein Condensate
MA Xiao-Dong, MA Yong-Li, HUANG Guo-Xiang
Chin. Phys. Lett. 2007, 24 (3): 616-619 .  
Abstract   PDF(pc) (269KB) ( 333 )
We investigate the Landau damping of collective modes in an anisotropic Bose--Einstein condensate (BEC). Based on divergence-free analytical solutions for the ground state wavefunction of the condensate and all eigenvalues and eigenfunctions for thermal excited quasiparticles, we make a detailed analytical calculation on coupling matrix elements. We evaluate the Landau damping of a quadrupole collective mode in the BEC with a disc-shaped trap and discuss its dependence on temperature and particle number of the system.
Josephson Dynamics of a Bose--Einstein Condensate Trapped in a Double-Well Potential
YANG Hong-Wei, ZUO Wei,
Chin. Phys. Lett. 2007, 24 (3): 620-623 .  
Abstract   PDF(pc) (315KB) ( 367 )
The Josephson equations for a Bose--Einstein Condensate gas trapped in a
double-well potential are derived with the two-mode approximation by the
Gross--Pitaevskii equation. The dynamical characteristics of the equations are obtained by the numerical phase diagrams. The nonlinear self-trapping effect appeared in the phase diagrams are emphatically discussed, and the condition EcN>4EJ is presented.
Negative and Superluminal Group Velocity Propagation with Narrow Pulse in a Coaxial Photonic Crystal
OU Xiao-Juan, ZHOU Wei, LI Lin, TENG Li-Hu, FENG Bao-Ying, ZHENG Sheng-Feng, WANG Feng-Wei
Chin. Phys. Lett. 2007, 24 (3): 624-627 .  
Abstract   PDF(pc) (381KB) ( 428 )
We investigate the propagation of electric signal along a spatially periodic impedance mismatched transmission line group. Anomalous dispersion is caused by the periodically mismatched impedance structure and a forbidden band appears near 8MHz in transmission. The group velocity of the amplitude-modulated signal is augmented up to infinity, even -3.89c (c the speed of light in vacuum) in the forbidden region with the amplitude of the modulated signal increasing. When the carrier sinusoid signal is modulated in amplitude by the modulating sinusoid signal, of which the peak is superimposed with a narrow pulse at fivefold frequency, the superluminal group velocity also occurs. The experiment failed to show whether the propagation velocity of narrow pulse exceeds c or not.
Properties of Neutron Stars Rotating at Kepler Frequency with Uniform Strong Magnetic Field
WEN De-Hua, CHEN Wei, LU Yi-Gang, LIU Liang-Gang
Chin. Phys. Lett. 2007, 24 (3): 628-631 .  
Abstract   PDF(pc) (249KB) ( 372 )
A uniform strong magnetic field is considered in calculating the properties of neutron star rotating at the Kepler frequency. The results show that the effect of the magnetic field on the properties of neutron star is evident, and the properties of the neutron stars rotating at the Kepler frequency can be used as a criterion to the equations of states of the neutron star matters.
Influence of Coloured Correlated Noises on Probability Distribution and Mean of Tumour Cell Number in the Logistic Growth Model
HAN Li-Bo, GONG Xiao-Long, CAO Li, WU Da-Jin
Chin. Phys. Lett. 2007, 24 (3): 632-635 .  
Abstract   PDF(pc) (249KB) ( 360 )
An approximate Fokker--Planck equation for the logistic growth model which is driven by coloured correlated noises is derived by applying the Novikov theorem and the Fox approximation. The steady-state probability distribution (SPD) and the mean of the tumour cell number are analysed. It is found that the SPD is the single extremum configuration when the degree of correlation between the multiplicative and additive noises, λ, is in -1<λ≤ 0 and can be the double extrema in 0<λ<1. A configuration transition occurs because of the variation of noise parameters. A minimum appears in the curve of the mean of the steady-state tumour cell number, <x>, versus λ. The position and the value of the minimum are controlled by the noise-correlated times.
Rhythm Synchronization of Coupled Neurons with Temporal Coding Scheme
SHI Xia, LU Qi-Shao
Chin. Phys. Lett. 2007, 24 (3): 636-639 .  
Abstract   PDF(pc) (265KB) ( 531 )
Encoding information by firing patterns is one of the basic neural functions, and synchronization is important collective behaviour of a group of coupled neurons. Taking account of two schemes for encoding information (that is, rate coding and temporal coding), rhythm synchronization of coupled neurons is studied. There are two types of rhythm synchronization of neurons: spike and burst synchronizations. Firstly, it is shown that the spike synchronization is equivalent to the phase synchronization for coupled neurons. Secondly, the similarity function of the slow variables of neurons, which have relevant to the
bursting process, is proposed to judge the burst synchronization. It is also found that the burst synchronization can be achieved more easily than the spike synchronization, whatever the firing patterns of the neurons are. Hence the temporal encoding scheme, which is closely related to both the spike and burst synchronizations, is more comprehensive than the rate coding scheme in essence.
Chaotic Dynamics of a Periodically Modulated Josephson Junction
WU Qin, LI Fei
Chin. Phys. Lett. 2007, 24 (3): 640-643 .  
Abstract   PDF(pc) (287KB) ( 330 )
We study the chaotic dynamics of a periodically modulated Josephson junction with damping. The general solution of the first-order perturbed equation is constructed by using the direct perturbation technique. It is theoretically found that the boundedness conditions of the general solution contain the
Melnikov chaotic criterion. When the perturbation conditions cannot be satisfied, numerical simulations demonstrate that the system can step into chaos through a period doubling route with the increase of the amplitude of the modulating term. Regulating specific parameters can effectively suppress the chaos.
Effects of Substrate Hydrophobicity/Hydrophilicity on Height Measurement of Individual DNA Molecules
WANG Hua-Bin, ZHOU Xing-Fei, AN Hong-Jie, GUO Yun-Chang, SUNJie-Lin, ZHANG Yi, HU Jun,
Chin. Phys. Lett. 2007, 24 (3): 644-647 .  
Abstract   PDF(pc) (465KB) ( 525 )
Effects of substrate hydrophobicity/hydrophilicity on height measurement
of individual ds-DNA molecules are investigated with tapping mode atomic
force microscopy (TMAFM) and vibrating mode scanning polarization force
microscopy (VSPFM). The measured heights of ds-DNA on hydrophobic highly
oriented pyrolytic graphite (HOPG) are remarkably less than those on
hydrophilic bare mica and Ni 2+ treated mica in both TMAFM and VSPFM. By analysing the results, we propose that the hydrophobicity/hydrophilicity of substrate can greatly influence the height measurement of DNA molecules.
Chiral Lagrangian Treatment of 0++ Mesons and 0++ Glueballs
XIA Zheng-Tong, WANG Shun-Jin
Chin. Phys. Lett. 2007, 24 (3): 648-651 .  
Abstract   PDF(pc) (255KB) ( 372 )
Within the framework of a nonlinear chiral Lagrangian the mass spectra and the decay properties of 0++ states below 2GeV are studied. Assuming that f0(980), a0(980), K*0(1430), and f0(1500) comprise an SU(3) nonet, we make a detailed prediction about the static properties of the 0++ mesons. The substructure analysis of these states in terms of two- and four-quark components as well as a glueball component is carried out. We also consider the interaction Lagrangian and provide a preliminary study of the strong and radiative decays of the 0++ mesons. The scalar glueball masses and partial widths are also presented. In view of the fact that few data of 0++
mesons are clearly given in the present PDG (Particle Data Group) list and that the four-quark content of mesons is a hot issue both experimentally and
theoretically, the predicted results of the paper may be helpful for upcoming experimental and theoretical studies of these mesons.
Structures of Light Neutron-Rich Nuclei with Orbit--Orbit Coupling
GUO Yan-Qing, REN Zhong-Zhou,
Chin. Phys. Lett. 2007, 24 (3): 652-655 .  
Abstract   PDF(pc) (224KB) ( 383 )
The properties of N=7, 8, 9 isotones with Z=4-8 are studied in the framework of the single-particle shell model. A tentative orbit--orbit coupling is introduced in the average nuclear potential. Calculations give a unified description of the
structures of N=7,8,9 isotones. The neutron level inversion in N=7 and N=9 isotones is discussed. The ground-state level inversion in 11Be and 15C is reproduced. The inversion between 2s 1/2 and 1d 5/2 neutron levels in 14B and 13Be is predicted. The possible halo structures in N=7 and N=9 isotones are analysed. The numerical results confirm the one-neutron halo structures in 11Be(2s 1/2), 11Be(1p 1/2), 12B(2s 1/2), 13C(2s1/2),14B(2s1/2) and 15C(2s1/2). The study implies that the attempt of considering orbit--orbit interaction in the shell model may be a feasible way to explain the anomalous
properties of exotic light nuclei.
Effects of E2 and E1--E2 Interference on Coulomb Dissociation of 19C
Rajesh Kharab, Pardeep Singh, Ravinder Kumar
Chin. Phys. Lett. 2007, 24 (3): 656-659 .  
Abstract   PDF(pc) (137KB) ( 530 )
We investigate the effects of higher order multipole transitions, in particular electric quadrupole (E2) and E1--E2 interference, on the Coulomb dissociation of 19C within the framework of the first order eikonal approximation. The sensitivity of the total Coulomb breakup cross section and the longitudinal momentum distribution of the core fragment to these effects are checked. The breakup occurs predominately through the dipole transition and the contribution of E2 transition to the total cross section is found to be within the range from 1 to 3% of that of E1. It is further observed that the E1--E2 interference term contributes nothing to the integrated cross section. On the other hand, the longitudinal momentum distribution is observed to be insensitive to the E2 transition while the E1--E2 interference introduces a small asymmetry in its shape.
Sensitive Probe for Symmetry Potential
LIU Jian-Ye, , XIAO Guo-Qing, GUO Wen-Jun, REN Zhong-Zhou, ZUO Wei, LEE Xi-Guo,
Chin. Phys. Lett. 2007, 24 (3): 660-663 .  
Abstract   PDF(pc) (250KB) ( 790 )
Based on both very obvious isospin effect of the neutron--proton number ratio of nucleon emissions (n/p) nucl on symmetry potential and (n/p) nucl's sensitive dependence on symmetry potential in the nuclear reactions induced by halo-neutron projectiles, compared to the same mass stable projectile, probing symmetry potential is investigated within the isospin-dependent quantum molecular dynamics with isospin and momentum-dependent interactions for different symmetry potentials U1sym and U2 sym. It is found that the neutron-halo projectile induces very obvious increase of (n/p) nucl and strengthens the dependence of (n/p) nucl on the symmetry potential for all the beam energies and impact parameters, compared to the same mass stable projectile under the same incident channel condition. Therefore (n/p) nucl induced by the neutron-halo projectile is a more favourable probe than the normal neutron-rich and neutron-poor projectiles for extracting the symmetry potential.
Examining the Chiral Geometry in 104Rh and 106Rh
WANG Shou-Yu, ZHANG Shuang-Quan, QI Bin, MENG Jie,
Chin. Phys. Lett. 2007, 24 (3): 664-667 .  
Abstract   PDF(pc) (465KB) ( 350 )
The criteria for chiral doublet bands based on one particle and one hole coupled to a triaxial rotor have been summarized. Two representative cases in A~100 odd--odd nuclei, nearly degenerate △I =1 doublet bands in 104Rh and 106Rh, are checked against these chiral criteria. It is shown that 106Rh possesses better chiral geometry than 104Rh, although the energy near degeneracy is achieved in 104Rh in comparison with the constant energy separation of doublet bands in 106Rh.
Measurement of L X-Ray Intensity Ratios for 92U and 90Th Elements Using Photoionization in an External Magnetic Field
D. Demir, Y. Sahin
Chin. Phys. Lett. 2007, 24 (3): 668-671 .  
Abstract   PDF(pc) (132KB) ( 545 )
L x-ray intensity ratios Ll/Lγ, Lα/Lγ and Lβ/Lγ for 92U and 90Th are measured by using 59.5keV incident photon energy at 110° and 125° scattering angles. The samples are located in the external magnetic field of intensities ± 0.15 T, ±0.30 T, ± 0.45 T, ± 0.60 T and ± 0.75 T. The experimental results obtained for B=0 are compared with the theoretical values calculated using Scofield's tables based on the Hartree--Slater theory. The contribution to the alignment of the external magnetic field is discussed. It is observed that the L x-ray intensity ratios decrease with the increasing magnetic field intensity.
Motional Stark Effect and Its Active Cancellation in Diamagnetic Spectrum of Barium
QUAN Wei, , LIU Hong-Ping, SHEN Li, , ZHAN Ming-Sheng,
Chin. Phys. Lett. 2007, 24 (3): 672-674 .  
Abstract   PDF(pc) (230KB) ( 962 )
With time-of-flight and electric field ionization detection technique, we investigate the motional Stark effect for highly excited Rydberg barium in high magnetic field and its active cancellation experimentally. In the experiment, the atom beam is aligned at a small angle of 15° with respect to the magnetic field. The motional Stark effect cancellation is demonstrated on two sets of circularly polarized spectra in static magnetic field B=1.00000 Tesla and B=1.70000 Tesla, respectively, although the effect is very small
(~3.5Vcm-1) in our apparatus configuration.
Josephson Effect of Dilute Bose--Einstein Condensates in Laser-Modulated Double Wells
LI Yan, , LIU Min, , ZHAN Ming-Sheng,
Chin. Phys. Lett. 2007, 24 (3): 675-678 .  
Abstract   PDF(pc) (372KB) ( 354 )
The Josephson effect of Bose condensates with a weak link created by superposing a far-off-resonant red-detuned laser beam on a double-well potential is theoretically considered. The numerical simulations show that there would be clear Josephson effect for this sort of three-well system. The present work gives a feasible scheme to study a new type of weak link which is crucial to investigate experimentally the Josephson effect.
An Enriched 40K Source for Atomic Cooling
WEI Dong, XIONG De-Zhi, CHEN Hai-Xia, ZHANG Jing
Chin. Phys. Lett. 2007, 24 (3): 679-682 .  
Abstract   PDF(pc) (520KB) ( 622 )
We have developed an enriched 40K source used in 40K--87Rb atomic mixture cooling experiment. The enriched 40K source is a home-made dispenser which releases 40K atoms by the redox reaction between 40K enriched KCl and calcium. It is efficient and easy to be made and used. We collect 107 ~108 40K atoms in collection magneto-optical trap. With this dispenser, we perform a quantum degenerate Fermi gas experiment.
Strengthened Hyper-Raman Lines with the Coherent Superposition State
ZHOU Zhao-Yan, YUAN Jian-Min
Chin. Phys. Lett. 2007, 24 (3): 683-686 .  
Abstract   PDF(pc) (241KB) ( 442 )
High-order harmonic generations from a one-dimensional Coulomb potential atom are calculated with the initial state prepared as a coherent superposition between its ground and first excited states. When the energy difference of the two states is small, we can choose proper laser pulse such that the first excited state can be excited only to other bound states instead of being ionized. We show that only the hyper-Raman lines are observable instead of the harmonics. The energy difference of the ground and the first excited state can be deduced from the highest peak of the hyper-Raman lines. We further show that the similar results can be obtained by using a
combination of two laser pulses with different frequencies interacting with the atom initially at the ground state.
Theoretical Study of Photoabsorption Spectra near Si 2p Edges of Silanes: to Determine Orientations of Adsorbed Silanes
ZHANG Wei-Hua, LI Jia-Ming,
Chin. Phys. Lett. 2007, 24 (3): 687-690 .  
Abstract   PDF(pc) (881KB) ( 313 )
In the frame work of quantum defect theory, photoabsorption spectra near
Si 2p edges of silane have been studied. When silanes are adsorbed on a physical surface and excited by polarized x-ray photons, relative intensities of the spectra will be different from that of free molecules. Such features can be used to determine orientations of adsorbed silanes based on selection rules.
Relativistic Distorted-Wave Calculations of Electron Impact Excitation Cross Sections of Be-Like C 2+ Ions
JIANG Jun, DONG Chen-Zhong, XIE Lu-You, WANG Jian-Guo, YAN Jun, Stephan Fritzsche
Chin. Phys. Lett. 2007, 24 (3): 691-694 .  
Abstract   PDF(pc) (264KB) ( 553 )
A fully relativistic distorted-wave program is developed based on the Grasp92 and Ratip packages to calculate electron impact excitation (EIE) cross sections. As a first application of the program, the EIE cross sections of Be-like C 2+ ions from the metastable 1s22s2p 3P to 1s22p2 3P excitation and the inner-shell excitations are calculated systematically. Meanwhile, the correlation effects of target states are discussed. It is found that the correlation effects play an important role in the low energy EIE cross sections. An excellent agreement is found when the results are compared with previous
calculations and recent measurements.
Resonances in Positron--Mg Scattering
PENG Yue, CHENG Cheng, ZHOU Ya-Jun
Chin. Phys. Lett. 2007, 24 (3): 695-697 .  
Abstract   PDF(pc) (204KB) ( 381 )
We present the results for resonances in positron--Mg scattering at low impact energy (0--5.0eV) by using the momentum space coupled-channel optical (CCO) method in momentum space. The S-partial wave resonance at 3.880eV, P-partial wave resonance at 4.020eV, and D-partial wave resonance at 4.267eV are found.
Re-excited States in Conjugated Polymers
ZHANG Da-Cheng, LIU De-Sheng, MEI Liang-Mo, XIE Shi-Jie, SUN Xin
Chin. Phys. Lett. 2007, 24 (3): 698-701 .  
Abstract   PDF(pc) (305KB) ( 332 )
We investigate the re-exciting process of a soliton, a polaron and a bipolaron in conjugated polymers. For a soliton case, the processes S0+ hυ=>S±+Pμ and S±+ hυ=>S0+P± will take place, where S0, S± and P±express a neutral soliton, charged solitons and charged polarons,respectively. For a polaron case, the processes P±+ hυ=> P^μ + BP±2 will occur, where BP±2 expresses a charged bipolaron with two electronic units. For a bipolaron case, the processes BP±2+ hυ=>P±+ P±and BP±2+hυ=> BP±2 +E0 are obtained.
A Time Domain Algorithm on the Reconstruction of Rough Surfaces
REN Yu-Chao, GUO Li-Xin, WU Zhen-Sen
Chin. Phys. Lett. 2007, 24 (3): 702-705 .  
Abstract   PDF(pc) (237KB) ( 432 )
We consider the inverse scattering problem of a perfectly conducting one-dimensional rough surface in the case that the incident field is unnecessary to be time harmonic. Based on our previous investigation of the frequency domain algorithm, a new time domain algorithm is proposed, in which we approximate the incident pulse by a finite sum of time harmonic fields and then apply the frequency domain algorithm for time harmonic waves. Numerical experiments indicate that the time domain algorithm shows great accuracy of reconstruction of the surface profile and yields significant improvement than the frequency domain algorithm.
All-Optical RZ-to-NRZ Format Conversion with a Tunable Fibre Based Delay Interferometer
YU Yu, ZHANG Xin-Liang, HUANG De-Xiu
Chin. Phys. Lett. 2007, 24 (3): 706-709 .  
Abstract   PDF(pc) (389KB) ( 415 )
All-optical format conversion from return-to-zero (RZ) to non-return-to-zero (NRZ) is demonstrated with temperature-controlled all-fibre delay interferometer (DI) at 20Gb/s. The operation principle is theoretical analysed with the help of numerical simulation and spectra analysis. Theoretical analysis results are consistent well with the experimental results. The format conversion can be achieved with power penalty of 0.54dB and with output extinction ratio 20dB.
Optimal Recording Wavelength for Nonvolatile Photorefractive Holograms in Ce:Cu:LiNbO
DAI Cui-Xia, YU Ying-Jie, LIU De-An, LIU Li-Ren
Chin. Phys. Lett. 2007, 24 (3): 710-712 .  
Abstract   PDF(pc) (227KB) ( 309 )
Three wavelengths of red, green and blue of recording beams are systemically tested for the UV-assistant recording and optical fixing
of holograms in a strongly oxidized Ce:Cu:LiNbO3 crystal. Three different hotorefractive phenomena are observed. It is shown that the green beams will optimally generate a critical strong nonvolatile hologram with quick sensitivity and the optimal switching technique could be jointly used to obtain a nearly 100% high diffraction. Theoretical verification is given, and a prescription on the doping densities and on the oxidation/reduction states of the material to match a defined recording wavelength for high diffraction is suggested.
Influence of Feedback Levels on Polarized Optical Feedback Characteristics in Zeeman-Birefringence Dual Frequency Lasers
MAO Wei, ZHANG Shu-Lian, ZHOU Lu-Fei, LIU Xiao-Yan, WANG Ming-Ming
Chin. Phys. Lett. 2007, 24 (3): 713-716 .  
Abstract   PDF(pc) (325KB) ( 352 )
The influence of feedback levels on the intensity and polarization properties of polarized optical feedback in a Zeeman-birefringence dual frequency laser is systematically investigated. By changing the feedback power ratio, different feedback levels are obtained. Three distinct regimes of polarized optical feedback effects are found and defined as regimes I, II and III. The feedback level boundaries among the regimes are acquired experimentally. The theoretical analysis is presented to be in good agreement with the experimental results.
Low Threshold Current Density Operation of Strain-Compensated Quantum Cascade Laser
SHAO Ye, LI Lu, LIU Jun-Qi, LIU Feng-Qi, WANG Zhan-Guo
Chin. Phys. Lett. 2007, 24 (3): 717-720 .  
Abstract   PDF(pc) (258KB) ( 646 )
We report the low threshold current density operation of strain-compensated
In 0.64 Ga 0.36 As/In 0.38 Al 0.62 As quantum cascade lasers emitting near 4.94μm. By employing an enlarged strain-compensated structure and optimizing the injector doping density, a rather low threshold current density of 0.57kA/cm2 at 80K is achieved for an uncoated 20-μm-wide and 2.5-mm-long laser.
Single Mode Er-Doped π-Phase-Shifted Distributed Feedback Fibre Grating Laser
WANG Li, CHEN Bai, CHEN Jia-Lin, LI Guo-Yang, CHANG Li-Ping, SUN An, LINZun-Qi
Chin. Phys. Lett. 2007, 24 (3): 721-723 .  
Abstract   PDF(pc) (214KB) ( 371 )
A novel method incorporating the shielded method and the post-processing method has been proposed to fabricate the π-phase-shifted fibre grating. Then an Er-doped π-phase-shifted distributed feedback fibre grating laser has been fabricated using the grating. The laser threshold is 20mW. When pumped with 90mW light at 980nm, the laser gives an output of 1.1mW. Its signal-to-noise ratio is better than 60dB. It is demonstrated that the laser is single mode operation by means of a Fabry--Perot scanning interferometer.
Widely Tunable cw Diode-Pumped 1.9-μm Tm:GdVO4 Laser at Room Temperature
LI Yu-Feng, YAO Bao-Quan, LIU Yan-Mei, WANG Yue-Zhu, JU You-Lun
Chin. Phys. Lett. 2007, 24 (3): 724-726 .  
Abstract   PDF(pc) (230KB) ( 514 )
A widely tunable cw diode-pumped room-temperature Tm:GdVO4 laser is built. Output power of 2.8W and a slope efficiency of 22% pumped by a 18W Fibre-coupled diode laser at 795nm have been obtained. Continuous tunability from 1820nm to 1946nm is achieved. In addition, the factors that contribute to the efficiency of oscillation are studied.
Large and Ultrafast Third-Order Nonlinear Optical Properties of Ge-S Based Chalcogenide Glasses
CHU Sai-Sai, WANG Shu-Feng, TAO Hai-Zheng, WANG Zhen-Wei, YANG Hong, LIN Chang-Gui, GONG Qi-Huang, ZHAO Xiu-Jian
Chin. Phys. Lett. 2007, 24 (3): 727-729 .  
Abstract   PDF(pc) (243KB) ( 477 )

We report ultrafast third-order nonlinear optical (NLO) properties of several chalcogenide glasses GeSx (x=1.8, 2.0, 2.5) measured by femtosecond time-resolved optical Kerr gate technique at 820nm. The third-order nonlinear susceptibility of GeS 1.8 glass is determined to be as large as 1.41×10-12 esu, which is the maximum value of the third order nonlinear susceptibility
χ(3) for the three compositions investigated. The symmetric Gauss profiles of
optical Kerr signals reveal the nature of ultrafast nonlinear response of these samples, which are originated from the ultrafast polarization of the electron clouds. By detailed microstructural analysis of these glasses based on the chain-crossing model (CCM) and the random-covalent-network model (RCNM), it can be concluded that χ(3) value of GeSx glasses can be enhanced greatly by S--S covalent bonds or S3Ge--GeS3 ethane-like units.

Femtosecond Third-Order Optical Nonlinearity of Au:Bi2O3 Nanocomposite Films
YOU Guan-Jun, ZHOU Peng, DONG Zhi-Wei, ZHANG Chun-Feng, CHEN Liang-Yao, QIAN Shi-Xiong
Chin. Phys. Lett. 2007, 24 (3): 730-733 .  
Abstract   PDF(pc) (446KB) ( 501 )
Ultrafast third-order optical nonlinearities of the as-deposited and annealed Au:Bi2O3 nanocomposite films deposited by magnetron cosputtering are investigated by using femtosecond time-resolved optical Kerr effect (OKE) and pump--probe techniques. The third-order optical nonlinear susceptibility is estimated to be 2.6×10-10 esu and 1.8×10-9 esu at wavelength of 800nm, for the as-deposited and the annealed film, respectively. The OKE signal of the as-deposited film is nearly temporally symmetrical with a peak centred at zero delay time, which indicates the dominant contribution from intraband transition of conduction electrons. For the annealed film, the existence of a decay process in OKE signal implies the important contribution of hot electrons. These characteristics are in agreement with the hot electron dynamics observed in pump--probe measurement.
Flat Supercontinuum Generation at 1550nm in a Dispersion-Flattened Microstructure Fibre Using Picosecond Pulse
XU Yong-Zhao, REN Xiao-Min, WANG Zi-Nan, ZHANG Xia, HUANG Yong-Qing
Chin. Phys. Lett. 2007, 24 (3): 734-737 .  
Abstract   PDF(pc) (255KB) ( 476 )
The generation of a flat supercontinuum of over 80nm in the 1550nm region by injecting 1.6 ps 10 GHz repetition rate optical pulses into an 80-m-long dispersion-flattened microstructure fibre is demonstrated. The fibre has small normal dispersion with a variation smaller than 1.5 (ps.nm-1.km-1) between 1500 and 1650nm. The generated supercontinuum ranging from 1513 to 1591nm has the flatness of ±1.5dB and it is not so flat in the range of several
nanometres around the pump wavelength 1552nm. Numerical simulation is
also used to study the effect of optical loss, fibre parameters and pumping conditions on supercontinuum generation in the dispersion-flattened microstructure fibre, and can be used for further optimization to generate flat broad spectra.
Structure Tuning of Line-Defect Waveguides Based on Silicon-on-Insulator Photonic Crystal Slabs
WANG Chun-Xia, XU Xing-Sheng, XIONG Gui-Guang, HU Hai-Yang, SONG Qian, DU Wei, CHEN Hong-Da
Chin. Phys. Lett. 2007, 24 (3): 738-740 .  
Abstract   PDF(pc) (489KB) ( 317 )
We present fabrication and experimental measurement of a series of photonic crystal waveguides. The complete devices consist of an injector taper down from 3μm into a triangular-lattice air-hole single-line-defect waveguide with lattice constant from 410nm to 470nm and normalized radius 0.31. We fabricate these devices on a silicon-on-insulator substrate and characterize them using a tunable laser source over a wavelength range from 1510nm to 1640nm. A sharp attenuation at photonic crystal waveguide mode edge is observed for most structures. The edge of guided band is shifted about 30nm with the 10nm increase of the lattice constant. We obtain high-efficiency light propagation and broad flat spectrum response of the photonic crystal waveguides.
Optimization of Mechanically Induced Long-Period Fibre Grating Transmission Spectrum
WANG Xin-Wei, LI Zhi-Lei, SHENG Qiu-Qin
Chin. Phys. Lett. 2007, 24 (3): 741-743 .  
Abstract   PDF(pc) (338KB) ( 508 )
A new double-layer grating template is designed to reduce the out-of-band loss as much as 1.8dB when the loss of LP03 reaches 10.2dB. Meanwhile, we propose a method to remove the sidelobes in the transmission spectra by the adjustment of the thickness of pressure plates. The plate-thickness-induced shift of resonant wavelength and the attenuation of loss peak intensity when removing sidelobes can be modified by the fibre distance and contact point on the pressure plates.
Geometry Parameter Optimization of Large Cross-Sectional S-Shaped Bent Rib Waveguides for Silicon Based Optical Switches
LI Zhi-Yong, LIU Jing-Wei, YU Jin-Zhong
Chin. Phys. Lett. 2007, 24 (3): 744-746 .  
Abstract   PDF(pc) (216KB) ( 695 )
By using a genetic algorithm, geometry parameters of large cross-sectional S-bend rib waveguides are optimized aiming at the least total loss when the propagation loss is considered. Optimized results are presented as an example of S-bend rib waveguides based on silicon-on-insulator (SOI) 4×4 optical switches. The value of 2dB/cm is given to the propagation loss according to the experimental results. The simulation results indicate that the total loss drops from 1.0002dB down to 0.4375dB without considering a lateral offset. If the offset is adopted, the total loss reduces from 0.5463dB to 0.2365dB. In addition, the effect of the rib height ratio on the loss is analysed, and the optimal ratio is obtained to be 0.55.
Homogenization of Three-Dimensional Periodic Solid--Solid Elastic Composites
NI Qing, CHENG Jian-Chun
Chin. Phys. Lett. 2007, 24 (3): 747-750 .  
Abstract   PDF(pc) (267KB) ( 467 )
We develop an approach to homogenize three-dimensional periodic solid--solid elastic composites with cubic lattice at low frequencies, by using plane wave expansion and perturbation theory with respect to the long wavelength limit. Based on the fact that the two shear waves propagating along lattice axis are degenerated, we derive formulae for effective velocities parallel and normal to the lattice axis, from which three independent effective elastic moduli are calculated, respectively. Theoretical results, which take into account the multiple scattering and the structure of the periodic medium, are in good agreement with the previous isotropic theory at high-symmetry
directions.
In Vivo Monitoring of Neovascularization in Tumour Angiogenesis by Photoacoustic Tomography
XIANG Liang-Zhong, XING Da, GU Huai-Min, ZHOU Fei-Fan, YANG Di-Wu, ZENGLv-Ming, YANG Si-Hua
Chin. Phys. Lett. 2007, 24 (3): 751-754 .  
Abstract   PDF(pc) (717KB) ( 416 )
Photoacoustic tomography (PAT) is presented to in vivo monitor neovascularization in tumour angiogenesis with high resolution and high
contrast images in a rat. With a circular scan system, the photoacoustic signal, generated by laser pulses at a wavelength of 532nm from a Q-switched Nd:YAG laser, is captured by a hydrophone with a diameter of 1mm and a sensitivity of 850nV/Pa. The vascular structure around the rat tumour is imaged clearly, with optimal contrast, because blood has strong absorption near this wavelength. Serial noninvasive photoacoustic images of neovascularization in tumour angiogenesis are also obtained consecutively from a growing tumour implanted under the skin of a rat over a period of two weeks. This work demonstrates that PAT can potentially provide a powerful tool for tumour angiogenesis detection in cancer research. It will bring us closer to clinical applications for tumour diagnosis and treatment monitoring.
Thickness Determination for a Two-Layered Composite of a Film and a Plate by Low-Frequency Ultrasound
MAO Jie, LI Ming-Xuan, WANG Xiao-Min
Chin. Phys. Lett. 2007, 24 (3): 755-758 .  
Abstract   PDF(pc) (260KB) ( 576 )
We present an ultrasonic method for determining the thickness of a composite consisting of a soft thin film attached to a hard plate substrate, by resonance spectra in the low frequency region. The interrogating waves can be incident only to the two-layered composite from the substrate side. The reflection spectra are obtained by FFT analysis of the compressive pulsed echoes from the composite, and the thicknesses of the film and the substrate are simultaneously inversed by the simulated annealing method from the resonant frequencies knowing other acoustical parameters in prior. The sensitivity of the method to individual thickness, its convergence and stability against experimental noises are studied. Experiment with interrogating wavelength 4 times larger than the film thickness in a sample of a polymer film (0.054mm) on an aluminium plate (6.24mm) verifies the validity of the method. The average relative errors in the measurement of the thicknesses of the film and the substrate are found to be -4.1% and -0.62%, respectively.
Influence of Orbital Motion of Inner Cylinder on Eccentric Taylor Vortex Flow of Newtonian and Power-Law Fluids
FENG Shun-Xin, FU Song
Chin. Phys. Lett. 2007, 24 (3): 759-762 .  
Abstract   PDF(pc) (274KB) ( 309 )
The effects of inner cylinder orbital motion on Taylor vortex flow of Newtonian and power-law fluid are studied numerically. The results demonstrate that when the eccentricity is not small, the orbital motion influences the stability of the flow in a non-monotonic manner. The variations of the flow-induced forces on the inner cylinder versus orbital motion are also different from the cases in which the flow is two-dimensional and laminar.
Supersonic Turbulent Boundary Layer: DNS and RANS
XU Jing-Lei, MA Hui-Yang
Chin. Phys. Lett. 2007, 24 (3): 763-766 .  
Abstract   PDF(pc) (256KB) ( 526 )
We assess the performance of a few turbulence models for Reynolds averaged Navier--Stokes (RANS) simulation of supersonic boundary layers,
compared to the direct numerical simulations (DNS) of supersonic flat-plate turbulent boundary layers, carried out by Gao et al. [Chin. Phys. Lett. 22(2005)1709] and Huang et al. [Sci. Chin.48(2005)614], as well as some available experimental data. The assessment is made for two test cases, with incoming Mach numbers and Reynolds numbers M = 2.25, Re = 365,000/in, and M = 4.5, Re =1.7×107/m, respectively. It is found that in the first case the prediction of RANS models agrees well with the DNS and the experimental data, while for the second case the agreement of the DNS models with
experiment is less satisfactory. The compressibility effect on the RANS models is discussed.
Effect of Thermionic Emission on Dust-Acoustic Solitons in a Dust-Electron Plasma
REN Li-Wen, WANG Zheng-Xiong, LIU Yue, WANG Xiao-Gang
Chin. Phys. Lett. 2007, 24 (3): 767-770 .  
Abstract   PDF(pc) (274KB) ( 519 )
The effects of thermionic emission on dust-acoustic solitons with a very small but finite amplitude in a dust--electron plasma are studied using the reductive perturbation technique. The self-consistent variation of dust charge is taken into account. It is shown that the thermionic emission could significantly increase the dust positive charge. The dependences of the phase velocity, amplitude, and width of such solitons on the dust temperature and the dust work function of dust material are plotted and discussed.
Dust Lattice Waves of Dusty Plasma Chain with an External Magnetic Field
SUN Xiao-Xia, WANG Chun-Hua, WANG Xiao-Gang,
Chin. Phys. Lett. 2007, 24 (3): 771-773 .  
Abstract   PDF(pc) (247KB) ( 404 )
Dust lattice waves of a one-dimensional plasma crystal chain with an external magnetic field are investigated. When the magnetic field is in the vertical direction (θ=0), perpendicular to the chain, the vertical transverse mode is not affected, while the horizontal transverse mode is coupled with the longitudinal mode. In the high frequency range, we obtain an `upper-hybrid' dust lattice mode and in the low frequency range, we obtain a `lower-hybrid' dust lattice mode. Between the two modes, a `gap' is formed. When the
magnetic field is oriented to the chain (θ=π/2), the longitudinal mode is not affected while both the horizontal and vertical transverse modes are shifted due to the effect of the magnetic field.
Growth of AlGaN Epitaxial Film with High Al Content by Metalorganic Chemical Vapour Deposition
WANG Xiao-Lan, ZHAO De-Gang, YANG Hui, LIANG Jun-Wu
Chin. Phys. Lett. 2007, 24 (3): 774-777 .  
Abstract   PDF(pc) (638KB) ( 981 )
A high-Al-content AlGaN epilayer is grown on a low-temperature-deposited AlN buffer on (0001) sapphire by low pressure metalorganic chemical vapour deposition. The dependence of surface roughness, tilted mosaicity, and twisted mosaicity on the conditions of the AlGaN epilayer deposition is evaluated. An AlGaN epilayer with favourable surface morphology and crystal quality is deposited on a 20nm low-temperature-deposited AlN buffer at a low V/III flow ratio of 783 and at a low reactor pressure of 100Torr, and the adduct reaction between trimethylaluminium and NH3 is considered.
Molecular Dynamics Simulation of Myoglobin Collision with Low Energy Ions
CHENG Wei, ZHANG Feng-Shou, ZHOU Hong-Yu
Chin. Phys. Lett. 2007, 24 (3): 778-780 .  
Abstract   PDF(pc) (218KB) ( 430 )
Collisions of a low energy heavy ion with a myoglobin in water are simulated by molecular dynamics model. The increase of total energy is very small. The mean squared fluctuation decreases at 300K and increases at 250K. This is an important novel cooling effect that protects the protein from ion damage. The possible collision side effect is the change of tertiary structure that blocks the normal functions of the myoglobin.
Nitrogen and Silicon Co-Doping of Ge2Sb2Te5 Thin Films for Improving Phase Change Memory Performance
CAI Yan-Fei, ZHOU Peng, LIN Yin-Yin, TANG Ting-Ao, CHEN Liang-Yao, LI Jing, QIAO Bao-Wei, LAI Yun-Feng, FENG Jie, CAI Bing-Chu, CHEN Bomy
Chin. Phys. Lett. 2007, 24 (3): 781-783 .  
Abstract   PDF(pc) (380KB) ( 663 )
Electrical properties and phase structures of (Si+N)-codoped Ge2Sb2Te5 (GST) for phase change memory are investigated to improve the memory erformance. Compared to the films with N or Si dopants only in previous reports, the (Si+N)-doped GST has a remarkable improvement of crystalline resistivity of about 104m Ωcm. The Fourier-transform infrared spectroscopy
spectrum reveals the Si--N bonds formation in the film. X-ray diffraction patterns show that the grain size is reduced due to the crystallization inhibition of the amorphous GST by SiNx, which results in higher crystalline resistivity. This is very useful to reduce writing current for phase change memory applications.
Effects of Shock Pressure on Transition Pressure in Zr
LI Ying-Hua, ZHANG Lin, CAI Ling-Cang
Chin. Phys. Lett. 2007, 24 (3): 784-785 .  
Abstract   PDF(pc) (228KB) ( 443 )
Measurements of free surface velocity profiles of high-purity Zr samples under shock-wave loading are performed to study the dynamic strength and phase transition parameters. The peak pressure of the compression waves is within the range from 9 to 14GPa, and the Hugoniot elastic limit is 0.5GPa. An anomalous structure of shock waves is observed due to the αl-ω phase transition in Zr. Shock pressure has effects on transition pressure which increases with increasing compression strength, and the stronger shocks have a lower transit time.
Mechanical Yielding and Strength Behaviour of OFHC Copper in Planar Shock Waves
CHEN Da-Nian, FAN Chun-Lei, HU Jin-Wei, WU Shan-Xing, WANG Huan-Ran, TAN Hua, YU Yu-Ying
Chin. Phys. Lett. 2007, 24 (3): 786-789 .  
Abstract   PDF(pc) (321KB) ( 1295 )
It is necessary to study the validation of strength models under planar shock loading in view of the fact that strength models for metals obtained at moderate strain rates are often used in the numerical simulations of shock wave phenomena. The variations of longitudinal stress, transverse stress and yield strength of oxygen-free high conductance (OFHC) copper with time under planar shock loading are obtained by using the manganin stress gauges and compared with the predicted results by the constructed seven constitutive models based on Y/G=constant and on G/B=constant (Y the yield strength, G the shear modulus, B the bulk modulus), respectively. It seems that the pressure, density, temperature and plastic strain dependence of the yield strength for OFHC copper under planar shock loading is essential to the
constitutive description.
Ge1Sb2Te4 Based Chalcogenide Random Access Memory Array Fabricated by 0.18-μm CMOS Technology
ZHANG Ting, SONG Zhi-Tang, FENG Gao-Ming, LIU Bo, WU Liang-Cai, FENG Song-Lin, CHEN Bomy
Chin. Phys. Lett. 2007, 24 (3): 790-792 .  
Abstract   PDF(pc) (280KB) ( 473 )
Ge1Sb2Te4-based chalcogenide random access memory array, with a tungsten heating electrode of 260nm in diameter, is fabricated by 0.18-μm CMOS technology. Electrical performance of the device, as well as physical and electrical properties of Ge1Sb2Te4 thin film, is characterized. SET and RESET programming currents are 1.6 and 4.1mA, respectively, when pulse width is 100ns. Both the values are larger than those of the Ge2Sb2Te5-based ones with the same structure and contact size. Endurance up to 106 cycles with a resistance ratio of about 100 has been achieved.
Phonon Transport and Thermal Conductivity in an Acoustic Filter
LU Jian-Duo, SHAO Liang, HOU Yang-Lai, YI Lin
Chin. Phys. Lett. 2007, 24 (3): 793-796 .  
Abstract   PDF(pc) (315KB) ( 351 )
We investigate the phonon ballistic transmission and the thermal conductivity in a dielectric quantum structure. It is found that these observable quantities sensitively depend on geometric parameters, and are of quantum character. The total transmission coefficient as a function of the reduced waveguide-length exhibits periodical behaviour and the reduced thermal conductance decreases below the ideal universal value for the low temperature. Our results show that one can control the thermal conductivity of the structure and make all kinds of acoustic filters to match practical requirements in devices by adjusting the geometric parameters.
Superfluid-Mott-Insulator Phase Transition and Collective Fluctuations in both Phases of Bosons in an Optical Lattice
ZHU Rui
Chin. Phys. Lett. 2007, 24 (3): 797-799 .  
Abstract   PDF(pc) (221KB) ( 415 )
The Bose Hubbard model describing interacting bosons in an optical lattice is reduced to a simple spin-1 XY model with single-ion anisotropy in the vicinity of the Mott phase. In the strong coupling Mott insulating regime, we propose a mean field theory based on a constraint SU(3) pseudo-boson representation on the effective model and discuss the excitation spectra and the phase transition to the superfluid state. Further to the superfluid phase, we use the coherent-state approach to derive the collective excitation modes. It is found that the Mott phase has two degenerate gapped quadratic excitation spectra which graduate into two degenerate gapless linear ones at the transition point, and one gapless linear mode with one gapped quadratic
mode in the superfluid phase.
Surfactant Effects of Au on Polar ZnO Surfaces
WEI Shu-Yi, WANG Zhi-Guo, YANG Zong-Xian
Chin. Phys. Lett. 2007, 24 (3): 800-802 .  
Abstract   PDF(pc) (210KB) ( 644 )
The electronic properties of one monolayer of Au atoms on polar ZnO surfaces are examined by first-principles slab calculations. It is found that an Au ad-layer on top of the surface is energetically more favourable than other gold diffused cases, and Au capping layer on the ZnO polar surfaces may modify the growing properties of ZnO nanostructures by enhancing the binding energy.
Effect of Bias on Content of GeC in Ge 1-x Cx Films
ZHAN Chang-Yong, WANG Li-Wu, HUANG Ning-Kang
Chin. Phys. Lett. 2007, 24 (3): 803-806 .  
Abstract   PDF(pc) (225KB) ( 502 )
Ge 1-x Cx films with GeC content up to 11.6% can be prepared by using a medium frequency magnetron sputtering technique in our study. X-ray photoelectron analysis for these Ge 1-x Cx films shows that the Ge 1-x Cx films consist of C, Ge, GeC and GeOy. The content of GeC increases from 10.7% at 0V to 11.6% at 250V, and decreases to 9.6% at 350V, and then increases again to 10.4% at 450V. The Raman analysis confirms the result of XPS for checking GeC in the deposited Ge 1-x Cx films. The related mechanism is discussed.
Non-Local Density Functional Description of Poly-Para-Phenylene Vinylene
ZHENG Guang, Clark S. J., Brand S., Abram R. A.
Chin. Phys. Lett. 2007, 24 (3): 807-810 .  
Abstract   PDF(pc) (384KB) ( 489 )
A fully non-local exchange-correlation formalism within the framework of density functional theory, known as the weighted density approximation (WDA), has been applied to the conjugated polymer poly-para-phenylene vinylene (PPV) and is shown to lead to a marked improvement in the agreement of theory and experiment for the electronic band structure of the conjugated polymer. In particular, some new model WDA functions are developed, which substantially increase the electronic band gap of the polymer relative to those obtained with the local density approximation and generalized gradient approximation. The calculated band gap of PPV is quantitatively or at least semiquantitatively in agreement with the experimental data.
Electronic Structure of Si 1-x IVx/Si Superlattices on Si (001)
CHEN Jie, LÜ, Tie-Yu, HUANG Mei-Chun,
Chin. Phys. Lett. 2007, 24 (3): 811-813 .  
Abstract   PDF(pc) (430KB) ( 290 )
We have preformed systematical ab initio studies of the structural and electronic properties of short-period Si 1-x IVx/Si/Si (x=0.125, 0.25, 0.5, IV=Ge, Sn) superlattices (SLs) grown along the [001] direction on bulk Si. The present calculations reveal that the Si 0.875 Ge 0.125/Si, Si 0.75 Ge 0.25/Si and
Si 0.875 Ge 0.125/Si are the Γ-point direct bandgap semiconductors. The technological importance lies in the expectation that the direct gap Si 1-x IVx/Si/Si SLs may be used as components in integrated optoelectronic devices, in conjunction with the already well-established and highly advanced silicon technology.
Structural and Thermodynamic Properties of Gallium Arsenide with Hexagonal Wurtzite Structure from First-Principles Analysis
CUI Hong-Ling, ZHANG Wei, CHENG Yan, CHEN Xiang-Rong,
Chin. Phys. Lett. 2007, 24 (3): 814-817 .  
Abstract   PDF(pc) (255KB) ( 1202 )
A first-principles plane wave method with the ultrasoft pseudopotential
scheme in the frame of the generalized gradient approximation (GGA) is
performed to calculate the lattice parameters, the bulk modulus B 0 and its pressure derivative B0' of the hexagonal wurtzite GaAs (w-GaAs) by the Cambridge serial total energy package (CASTEP). Our calculations show that the most stable structure of the w-GaAs corresponds to the axial ratio c/a=1.651 and the internal parameter u= 0.374, consistent with other theoretical results. Also, the thermodynamic properties of the w-GaAs are investigated from the quasi-harmonic Debye model. The dependences of the normalized lattice parameters a/a0, c/c0, the axial ratio c/a, the normalized
volume V/V0, the heat capacity Cv and the thermal expansion α on pressure P and temperature T are also obtained successfully.
Enhanced Efficiency of Polymer: Fullerene Bulk Heterojunction Solar Cells with the Insertion of Thin TiO2 Layer near the LiF/Al Electrode
LI Yan, HOU Yan-Bing, JIN Hui, SHI Quan-Min, LIU Jun, SUN Xin, CHANG Xiao-Wei
Chin. Phys. Lett. 2007, 24 (3): 818-821 .  
Abstract   PDF(pc) (258KB) ( 502 )
The insertion layer of TiO2 between polymer-fullerene blend and LiF/Al electrode is used to enhance the short-circuit current I sc and fill factor (FF). The solar cell based on the blend of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and C60 with the modifying layer of TiO2 (about 20nm) shows the open-circuit Voc of about 0.62V, short circuit current Isc of about 2.35mA/cm2, filling factor FF of about 0.284, and the power conversion efficiency (PCE) of about 2.4% under monochromatic light (500nm) photoexcitation of about 17mW/cm 2. Compared to cells without the TiO2
layer, the power conversion efficiency increases by about 17.5%. Similar effect is also obtained in cells with the undoped MEH-PPV structure of ITO/PEDOT:PASS/MEH-PPV/(TiO2)LiF/Al. The improved solar cell performance can be attributed to enhanced carrier extraction efficiency at the active layer/electrode interfaces when TiO2 is inserted.
Columnar Structures and Stress Relaxation in Thick GaN Films Grown on Sapphire by HVPE
WEI Tong-Bo, MA Ping, DUAN Rui-Fei, WANG Jun-Xi, LI Jin-Min, ZENG Yi-Ping
Chin. Phys. Lett. 2007, 24 (3): 822-824 .  
Abstract   PDF(pc) (412KB) ( 739 )
Thick GaN films with high quality are directly grown on sapphire in a home-built vertical hydride vapour phase epitaxy (HVPE) reactor. The optical and structural properties of large scale columnar domains near the interface are studied using cathodoluminescence and micro-Raman scattering. These columnar domains show a strong emission intensity due to extremely high free carrier concentration up to 2×1019cm-3, which are related with impurities trapped in structural defects. The compressive stress in GaN film clearly decreases with increasing distance from interface. The quasi-continuous columnar domains play an important role in the stress relaxation for the upper high quality layer.
Structural, Optical and Electrical Properties of ZnS/Porous Silicon Heterostructures
WANG Cai-Feng, LI Qing-Shan, LV Lei, ZHANG Li-Chun, QI Hong-Xia, CHEN Hou
Chin. Phys. Lett. 2007, 24 (3): 825-827 .  
Abstract   PDF(pc) (372KB) ( 1094 )
ZnS films are deposited by pulsed laser deposition on porous silicon (PS) substrates formed by electrochemical anodization of p-type (100) silicon wafer. Scanning electron microscope images reveal that the surface of ZnS films is unsmoothed, and there are some cracks in the ZnS films due to the roughness of the PS surface. The x-ray diffraction patterns show that the ZnS films on PS surface are grown in preferring orientation along cubic phase β-ZnS (111) direction. White light emission is obtained by combining the blue-green emission from ZnS films with the orange--red emission from PS layers. Based on the I--V characteristic, the ZnS/PS heterojunction exhibits the rectifying junction behaviour, and an ideality factor n is calculated to be 77 from the I--V plot.
Organic Light-Emitting Devices with a LiF Hole Blocking Layer
LIAN Jia-Rong, YUAN Yong-Bo, ZHOU Xiang
Chin. Phys. Lett. 2007, 24 (3): 828-830 .  
Abstract   PDF(pc) (225KB) ( 1512 )
We introduce a thin LiF layer into tris-8-hydroxyquinoline aluminium (Alq3) based bilayer organic light-emitting devices to block hole transport. By varying the thickness and position of this LiF layer in Alq3, we obtain an lectroluminescent efficiency increase by a factor of two with respect to the control devices without a LiF blocking layer. By using a 10nm dye doped Alq3 sensor layer, we prove that LiF can block holes and excitons effectively. Experimental results suggest that the thin LiF layer may be a good hole and exciton blocking layer.
Tetragonal Distortion of GaN Epilayer with Multiple Buffer Layers on Si (111) Studied by RBS/Channelling and HRXRD
DING Zhi-Bo, WANG Kun, ZHOU Sheng-Qiang, CHEN Tian-Xiang, YAO Shu-De
Chin. Phys. Lett. 2007, 24 (3): 831-834 .  
Abstract   PDF(pc) (287KB) ( 476 )
Rutherford backscattering (RBS)/channelling and high resolution x-ray diffraction (HRXRD) have been used to characterize the tetragonal distortion of a GaN epilayer with four AlxGa 1-x N and single AlN buffer layers grown on a Si (111) substrate by metal-organic vapour phase epitaxy (MOVPE). The results show that a 1000nm GaN epilayer with a perfect crystal quality (χmin=1.54%) can be grown on the Si (111) substrate in virtue of multiple buffer layers. Using the RBS/channelling angular scan around an off-normal <1 213> axis in the (1010) plane and the conventional HRXRD θ-2θ scans normal to GaN (0002) and (1122) planes at the 0° and 180° azimuth angles, the tetragonal distortion eT, which is caused by the elastic strain in the epilayer and different buffer layers, can be obtained respectively. The two experiments are testified at one result, the tetragonal distortion of GaN epilayer is nearly to a fully relaxed (eT=0).
Stochastic Resonance in Neural Systems with Small-World Connections
YUAN Wu-Jie, LUO Xiao-Shu, YANG Ren-Huan
Chin. Phys. Lett. 2007, 24 (3): 835-838 .  
Abstract   PDF(pc) (276KB) ( 300 )
We study the stochastic resonance (SR) in Hodgkin--Huxley (HH) neural ystems with small-world (SW) connections under the noise synaptic current and periodic stimulus, focusing on the dependence of properties of SR on coupling strength c. It is found that there exists a critical coupling strength c* such that if c<c*, then the SR can appear on the SW neural network. specially, dependence of the critical coupling strength c* on the number of neurons N shows the monotonic even almost linear increase of c* as N increases and c* on the SW network is smaller than that on the random network. For the effect of the SW network on the phenomenon of SR, we show that decreasing the connection-rewiring probability p of the network topology leads to an enhancement of SR. This indicates that the SR on the SW
network is more prominent than that on the random network (p=1.0). In addition, it is noted that the effect becomes remarkable as coupling strength increases. Moreover, it is found that the SR weakens but resonance range becomes wider with the increase of c on the SW neural network.
Navigation on Power-Law Small World Network with Incomplete Information
CHEN Jian-Zhen, ZHU Jian-Yang
Chin. Phys. Lett. 2007, 24 (3): 839-842 .  
Abstract   PDF(pc) (270KB) ( 527 )

We investigate the navigation process on a variant of the Watts--Strogatz small-world network model with local information. In the network construction, each vertex of an N×N square lattice sends out a long-range link with probability p. The other end of the link falls on a randomly chosen vertex with probability proportional to r-α, where r is the lattice distance
between the two vertices, and α≥0. The average actual path length, i.e. the expected number of steps for passing messages between randomly chosen vertex pairs, is found to scale as a power-law function of the network size Nβ, except when α is close to a specific value αmin, which gives the highest efficiency of message navigation. For a finite network, the exponent β epends on both α and p, and αmin drops to zero at a critical value of p which depends on N. When the network size goes to infinity, β depends only on α, and αmin is equal to the network dimensionality.

Observational Constraints on the Unified Dark-Energy--Dark-Matter Model
WU Pu-Xun, YU Hong-Wei
Chin. Phys. Lett. 2007, 24 (3): 843-845 .  
Abstract   PDF(pc) (209KB) ( 336 )
We investigate the constraints on a generalized Chaplygin gas (GCG) model using the gold sample type-Ia supernovae (Sne Ia) data, the new Supernova Legacy Survey (SNLS) Sne Ia data and the size of baryonic acoustic oscillation peak found in Sloan Digital Sky Survey (SDSS). In a spatially flat universe case we obtain, at a 95.4% confidence level, As=0.76-0.07+0.07 and α=0.028-0.238+0.322. Our results are consistent with the ΛCDM model (α=0), but rule out the standard Chaplygin gas model (α=1).
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