Chin. Phys. Lett.

Volume 24 Issue 9

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A New Type of Non-Noether Adiabatic Invariants for Disturbed Lagrangian Systems: Adiabatic Invariants of Generalized Lutzky Type

LUO Shao-Kai

Chin. Phys. Lett. 2007, 24 (9): 2463-2466 .

Abstract

PDF(pc) (200KB) ( 701 )

For a Lagrangian system with the action of small disturbance, the Lie symmetrical perturbation and a new type of non-Noether adiabatic invariant are presented in general infinitesimal transformation groups. On the basis of the invariance of disturbed Lagrangian systems under general infinitesimal transformations, the determining equations of Lie symmetries of the system are constructed. Based on the definition of higher-order adiabatic invariants of a mechanical system, a new type of adiabatic invariant, i.e. generalized Lutzky adiabatic invariants, of a disturbed Lagrangian system are obtained by
investigating the perturbation of Lie symmetries for a Lagrangian system with the action of small disturbance. Finally, an example is given to illustrate the application of the method and results.

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Linearization of Systems of Nonlinear Diffusion Equations

KANG Jing, QU Chang-Zheng,

Chin. Phys. Lett. 2007, 24 (9): 2467-2470 .

Abstract

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We investigate the linearization of systems of n-component nonlinear diffusion equations; such systems have physical applications in soil science, mathematical biology and invariant curve flows. Equivalence transformations of their auxiliary systems are used to identify the systems that can be linearized. We also provide several examples of systems with two-component
equations, and show how to linearize them by nonlocal mappings.

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Realization of Arbitrary Inverse Unitary Transformation of Single Mode Fibre by Using Three Wave Plates

WANG Shao-Kai, REN Ji-Gang, PENG Cheng-Zhi, JIANG Shuo, WANG Xiang-Bin

Chin. Phys. Lett. 2007, 24 (9): 2471-2474 .

Abstract

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We report a method to realize the arbitrary inverse unitary transformation imposed by a single-mode fibre on photon's polarization by the succession of two quarter-wave plates and a half-wave plate. The process of realization is analysed on a Poincare sphere due to the fact that unitary transformation does not change the angle formed by polarization state vector. The method is meaningful in quantum communication experiment such as quantum teleportation, in which an unknown arbitrary quantum state should be kept to be unchanged in the case of using a single-mode fibre for time delay.

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Thermal Entanglement of Anisotropic XY Chains in a Transverse Field

ZHAI Xiao-Yue, TONG Pei-Qing

Chin. Phys. Lett. 2007, 24 (9): 2475-2478 .

Abstract

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By using the concept of concurrence, we numerically investigate the
thermal entanglement between any two nearest-neighbour spins in uniform
and periodic anisotropic XY chains in a transverse field at finite
temperature T. It is found that the entanglement has more than one
critical temperatures on some parameter regions for uniform and periodic
chains. We also discuss the behaviour of the thermal entanglement at the
vicinity of quantum phase transition of periodic anisotropic XY chains
and find that all the derivatives ∂_λ have similar behaviour as that of the uniform chain.

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A Modified Protocol of Quantum Key Distribution Based on Entanglement Swapping

HUANG Xin, LI Shu-Min, WANG An-Min

Chin. Phys. Lett. 2007, 24 (9): 2479-2481 .

Abstract

PDF(pc) (198KB) ( 547 )

By comparing Cabello's addendum to his quantum key distribution protocol [Phys. Rev. A 64(2001)024301], we propose a more convenient modified protocol based on the entanglement swapping which is secure against the eavesdropping strategy addressed by Zhang et al. [Phys. Rev. A 63(2001)036301] and other existing types of attack.

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Entanglement-Assisted Classical Capacity of a Generalized Amplitude Damping Channel

HOU Li-Zhen, FANG Mao-Fa

Chin. Phys. Lett. 2007, 24 (9): 2482-2485 .

Abstract

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The entanglement-assisted capacity of a generalized amplitude damping channel is investigated by using the properties of partial symmetry and concavity of mutual information. The numerical and analytical results of the entanglement-assisted capacity are obtained under certain conditions. It is shown that the entanglement-assisted capacity depends on the channel parameters representing the ambient temperature and dissipation, and the prior entanglement between sender and receiver can approximately double the classical capacity of the generalized amplitude damping channel.

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Three-Party Simultaneous Quantum Secure Direct Communication Scheme with EPR Pairs

WANG Mei-Yu, YAN Feng-Li

Chin. Phys. Lett. 2007, 24 (9): 2486-2488 .

Abstract

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We present a scheme for three-party simultaneous quantum secure direct
communication by using EPR pairs. In the scheme, three legitimate parties can simultaneously exchange their secret messages. The scheme is also proven to be secure against the intercept-and-resend attack, the disturbance attack and the entangled-and-measure attack.

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An Efficient Scheme for Implementing an N-Qubit Toffoli Gate with Superconducting Quantum-Interference Devices in Cavity QED

ZHENG An-Shou, LIU Ji-Bing, XIANG Dong, LIU Cui-Lan, YUAN Hong

Chin. Phys. Lett. 2007, 24 (9): 2489-2492 .

Abstract

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An alternative approach is proposed to realize an n-qubit Toffoli gate with superconducting quantum-interference devices (SQUIDs) in cavity quantum electrodynamics (QED). In the proposal, we represent two logical gates of a qubit with the two lowest levels of a SQUID while a higher-energy intermediate level of each SQUID is utilized for the gate manipulation. During the operating process, because the cavity field is always in vacuum state, the requirement on the cavity is greatly loosened and there is no transfer of quantum information between the cavity and SQUIDs.

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Evolution of Matter Wave Interference of Bose-Condensed Gas in a 1D Optical Lattice

XU Zhi-Jun, ZHANG Dong-Mei

Chin. Phys. Lett. 2007, 24 (9): 2493-2496 .

Abstract

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For a Bose-condensed gas in a combined potential consisting of an axially-symmetric harmonic magnetic trap and one-dimensional (1D) optical lattice, using the mean-field Gross--Pitaevskii (G-P) equation and the propagator method, we obtain the analytical result of the order parameter for matter wave interference at any time. The evolution of the interference pattern under a variation of the relative phase △_ψ between successive subcondensates trapped on an optical lattices is also studied. For △_ψ=π, the interference pattern is symmetric with two sharp peaks, which are symmetrically located on a straight line on both sides of a vacant central peak and moving apart from each other. This work is in agreement with available experimental results.

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Generalized Uncertainty Principle and the Quantum Entropy of Rotating Black Hole

SHU Fu-Wen, , SHEN You-Gen,

Chin. Phys. Lett. 2007, 24 (9): 2497-2500 .

Abstract

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The entropy of rotating Kerr--Newman--Kasuya black hole due to massive
charged fields (bosons and fermions) is calculated by using the new equation of state density motivated by the generalized uncertainty relation. The result shows the entropy does not depend on the mass and the charge but the parameter λ, the area A and the spin of the fields. Moreover, an improved approximation is provided to calculate the scalar entropy.

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Ground-State Properties of Charged Bosons Confined in a One-Dimensional Harmonic Double-Well Trap: Diffusion Monte Carlo Calculations

JIN Jing, TANG Yi

Chin. Phys. Lett. 2007, 24 (9): 2501-2504 .

Abstract

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The diffusion Monte Carlo method is applied to study the ground-state
properties of charged bosons in one dimension confined in a harmonic double-well trap. The particles interact repulsively through a Coulombic 1/r potential. Numerical results show that the well separation has significant influence on the ground-state properties of the system. When the interaction of the system is weak, ground-state energy decreases with the increasing well
separation and has a minimal value. If the well separation increases continually, the ground-state energy increases and approaches to a constant gradually. This effect will be abatable in the strong interacting system. In addition, by calculating the density of the systems for different interaction strengths with various well separations, we find that the density increases abnormally when the well separation is large at the centre of the system.

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System Driven by Correlated Gaussian Noises Related with Disorder

LI Jing-Hui

Chin. Phys. Lett. 2007, 24 (9): 2505-2508 .

Abstract

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A system driven by correlated Gaussian noises related with disorder is investigated. The Fokker--Planck equation (FPE) for the system is derived. Using the FPE derived, some systems driven by correlated Gaussian noises related with disorder can be investigated for Brownian motors, nonequilibrium transition, resonant activation, stochastic resonance, and so on. We only give one example: i.e., using the FPE derived, we study the resonant activation for a single motor protein model with correlated noises related to disorder. Since the correlated noise related to disorder usually exists with the friction, for the
temperature, and so on, our results have generic physical meanings for
physics, chemistry, biology and other sciences.

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Accelerated Stochastic Simulation of Large Chemical Systems

CHEN Xiao, AO Ling

Chin. Phys. Lett. 2007, 24 (9): 2509-2512 .

Abstract

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For efficient simulation of chemical systems with large number of reactions, we report a fast and exact algorithm for direct simulation of chemical discrete Markov processes. The approach adopts the scheme of organizing the reactions into hierarchical groups. By generating a random number, the selection of the next reaction that actually occurs is accomplished by a few successive selections in the hierarchical groups. The algorithm which is suited for simulating systems with large number of reactions is much faster than the direct method or the optimized direct method. For a demonstration of its efficiency, the accelerated algorithm is applied to simulate the reaction-diffusion Brusselator model on a discretized space.

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Nonlinear Dynamics of a Sliding Chain in a Periodic Potential

YUAN Xiao-Ping, ZHENG Zhi-Gang

Chin. Phys. Lett. 2007, 24 (9): 2513-2516 .

Abstract

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Nonlinear dynamics of the sliding process of a chain driven with a constant velocity at one end in a periodic substrate potential is investigated. The driven chain exhibits distinctly different dynamical characteristics at different velocities. In the low velocity region, the chain moves in a stick--slip manner. When the driving velocity is increased, the stick--slip behaviour is replaced by
complicated and regular oscillatory motions. The dependence of the dynamics on the coupling strength is studied and the step-like behaviour is found, where different steps correspond to different dynamical phases.

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Kinetic Phase Transition in A₂+B₂ → 2AB Reaction System with Particle Diffusion

CHEN Ting, HUA Da-Yin, LIN Su

Chin. Phys. Lett. 2007, 24 (9): 2517-2520 .

Abstract

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A lattice gas model for the A₂+B₂ → AB reaction system was studied by Monte Carlo simulation in a two-dimensional triangular lattice surface [Phys. Rev. E 69(2004)046114]. In the model, a reactive window appears and the system exhibits a continuous phase transition to an `A+vacancy' covered state with
infinitely many absorbing states. The critical behaviour was shown to belong to the robust directed percolation (DP) universality class. In this study, we find that as the particle A diffusion is considered, the infinitely many absorbing states for the continuous phase transition change into only two: one is that
in which all sites are occupied by particle A and the other is that in which there is only one vacant site and other sites are occupied by particle A. Furthermore, a parity conserving character appears in the system when the particle A diffusion is included. It is found that the critical behaviour of the continuous phase transition changes from the DP class into the pair contact process with diffusion model (PCPD) class and the parity conserving character has no influence on the critical behaviour in the model.

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Development of the Technique for Fabricating Submicron Moire Gratings on Metal Materials Using Focused Ion Beam Milling

DU Hua, XIE Hui-Min, GUO Zhi-Qiang, LUO Qiang, GU Chang-Zhi, QIANG Hai-Chang, RONG Li-Jian

Chin. Phys. Lett. 2007, 24 (9): 2521-2524 .

Abstract

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A focused gallium ion (Ga+) beam is used to fabricate micro/submicron spacing gratings on the surface of porous NiTi shape memory alloy (SMA). The crossing type of gratings with double-frequency (2500l/mm and 5000l/mm) using the focused ion beam (FIB) milling are successfully produced in a combination mode or superposition mode. Based on the double-frequency gratings, high-quality scanning electron microscopy (SEM) Moire patterns are obtained to study the micro-scale deformation of porous NiTi SMA. The grating fabrication technique is discussed in detail. The experimental results verify the feasibility of fabricating high frequency grating on metal surface using FIB milling.

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Near-Field Fluorescence and Topography Characterization of a Single Nanometre Fluorophore by Apertureless Tip-Enhanced Scanning Near-Field Microscopy

WU Xiao-Bin, WANG Jia, XU Ji-Ying, WANG Rui, TIAN Qian, YU Jian-Yuan

Chin. Phys. Lett. 2007, 24 (9): 2525-2528 .

Abstract

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Tip-enhanced near-field fluorescence and topography characterization of a single nanometre fluorophore is conducted by using an apertureless scanning near-field microscopy system. A fluorophore with size 80nm is mapped with a spatial resolution of 10nm. The corresponding near-field fluorescence data shows significant signal enhancement due to the apertureless tip-enhanced effect. With the nanometre spatial resolution capability and nanometre local tip-enhanced effect, the apertureless tip-enhanced scanning near-field microscopy may be further used to characterize a single molecule by realizing the local near-field spectrum assignment corresponding to topography at nanometre scale.

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Subtraction of Spurious Centre-of-Mass Motion in Quark Delocalization and Colour Screening Model

CHEN Ling-Zhi, PANG Hou-Rong, HUANG Hong-Xia, PING Jia-Lun, WANG Fan

Chin. Phys. Lett. 2007, 24 (9): 2529-2532 .

Abstract

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The quark delocalization colour screening model provides an alternative approach for the NN intermediate range attraction, which is attributed to the σ meson exchange in the meson exchange and chiral quark model. However the quark delocalization induces the spurious centre-of-mass motion (CMM). A method for subtracting the spurious CMM proposed before is applied to the new scattering calculation. The subtraction of the spurious CMM results in an additional NN attraction. The NN scattering data are refitted by a fine tune of the colour screening constant.

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QqQq' States in Chiral SU(3) Quark Model

ZHANG Hai-Xia, ZHANG Min, ZHANG Zong-Ye

Chin. Phys. Lett. 2007, 24 (9): 2533-2536 .

Abstract

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We study the masses of QqQq' states with J^PC=0⁺⁺, 1⁺⁺, 1^+- and 2⁺⁺ in the chiral SU(3) quark model, where Q is the heavy quark (c or b) and q (q') is the light quark (u, d or s). According to our numerical results, it is improbable to make the interpretation of [cncn]₁⁺⁺ and [cncn]₂⁺⁺ (n=u, d) states as X(3872) and Y(3940), respectively. However, it is interesting to find the tetraquarks in the bqbq' system.

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Dispersive Contribution of ρ(1450,1700) Decays and X(1576)

ZHANG Bo, LIU Xiang, ZHU Shi-Lin

Chin. Phys. Lett. 2007, 24 (9): 2537-2539 .

Abstract

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We study whether the broad enhancement X(1576) arises from the final
state interaction (FSI) of ρ(1450,1700)→ρ⁺ρ^-→K⁺K^- decays. Both the absorptive and dispersive contributions of the above amplitudes are considered since the intermediate states are very close to ρ(1450,1700). The same mechanism leads to a similar enhancement around 1580MeV in the π⁺π^- spectrum in the J/ψ→π⁰π⁺π^- channel, which can be used to test whether X(1576) can be ascribed to the FSI effect of ρ(1450,1700)→ρ⁺ρ^-.

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△-Resonance Effective Mass in Medium

LEE Xi-Guo, LI Yong-Qing, LIU Zi-Yu,

Chin. Phys. Lett. 2007, 24 (9): 2540-2543 .

Abstract

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Based on the relativistic chiral effective field theory, we study the effective mass of the △-resonance in medium by investigating the self-energy of the △-resonance related to the πN decay channel in symmetric nuclear matter. We find that the effective mass of △-resonance decreases evidently with increasing nuclear density ρ. In our calculation, we also consider the influence of the shifts of the nucleon mass, pion mass and its decay constant due to the restoration of chiral symmetry in medium. The results are roughly consistent with the data given by Lawrence Berkeley National Laboratory.

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Test of Determination of (p,γ) Astrophysical S-Factors Using the Asymptotic Normalization Coefficients from Neutron Transfer Reactions

GUO Bing, LI Zhi-Hong, LIU Wei-Ping, BAI Xi-Xiang

Chin. Phys. Lett. 2007, 24 (9): 2544-2546 .

Abstract

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The asymptotic normalization coefficients (ANCs) for the virtual decay ¹⁷O→¹⁶O+n are derived from the angular distributions of the ¹⁶O(d,p)¹⁷O reaction leading to the ground and first excited states of ¹⁷O, respectively, using the
distorted wave Born approximation and the adiabatic wave approximation.
The ANCs of ¹⁷F are then extracted according to charge symmetry of mirror nuclei and used to calculate the astrophysical S-factors of ¹⁶O(p,γ)¹⁷F leading to the first two states of ¹⁷F. The present results are in good agreement with those from the direct measurement. This provides a test of this indirect method to determine direct astrophysical S-factors of (p,γ) reaction. In addition, the S-factors at zero energy for the direct captures to the ground and first excited states of ¹⁷F are presented, without the uncertainty associated with the extrapolation from higher energies in direct measurement.

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Hypernuclear Magnetic Moments and λ--N Interaction in ¹⁷_λO

Lü Hong-Feng

Chin. Phys. Lett. 2007, 24 (9): 2547-2551 .

Abstract

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Hypernuclear magnetic moment and λ--N interaction in ¹⁷_λO has been studied within relativistic mean field theory. Without core polarization, the relativistic results are found to fit the Schmidt value well and not be sensitive to λ--N interaction. The relativistic magnetic moment is enhanced with nearly equal contributions of the relativistic and free masses. When λ hyperon occupies the l=0 or l=1 orbit, the effect of λ--N interaction on the magnetic moment of valence proton is different.

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Possible Way to Synthesize Superheavy Element Z =117

FENG Zhao-Qing, JIN Gen-Ming, HUANG Ming-Hui, GAN Zai-Guo, WANG Nan, LI Jun-Qing,

Chin. Phys. Lett. 2007, 24 (9): 2551-2554 .

Abstract

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Within the framework of the dinuclear system model, the production of superheavy element Z=117 in possible projectile--target combinations is analysed systematically. The calculated results show that the production cross sections are strongly dependent on the reaction systems. Optimal combinations, corresponding excitation energies and evaporation channels are proposed, such as the isotopes ^248,249Bk in ⁴⁸Ca induced reactions in 3n
evaporation channels and the reactions ⁴⁵Sc+^246,248Cm in 3n and 4n channels, and the system ⁵¹V+²⁴⁴Pu in 3n channel.

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A Simple Systematical Law of E^₂₁⁺ Values for Heavy Nuclei in the N_pN_n Scheme and the Evolution of the Z=64 Shell Gap

FAN Zhe-Yong, REN Zhong-Zhou, XU Chang

Chin. Phys. Lett. 2007, 24 (9): 2555-2558 .

Abstract

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A systematics of excitation energy of the first 2⁺ state E^₂₁⁺in even--even heavy nuclei (A≥120) is studied in the N_pN_nscheme. It is found that a simple exponential function describes the dependence of E^₂₁⁺ values on N_pN_n values very well. In addition, the Z=64 shell gap is reexamined by investigating the systematics of the 52≤Z≤66 region. It is found that the
Z=64 shell gap is largest at N=82 and becomes smaller with either
increasing or decreasing of neutron numbers. The effects of this shell gap become negligible for counting the valence proton numbers when N≥92 or N≤72.

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Long-Time Dynamical Behaviour of Alternative-Current Stark Effect

WU Wen-Guo, CHEN Jing-Biao

Chin. Phys. Lett. 2007, 24 (9): 2559-2562 .

Abstract

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We report the long time dynamical behaviour of ac Stark effect in a simple quantum model in which two level atoms interact with quantized coherent radiation field. A new phenomenon of periodic quantum collapse and revival of the ac Stark shift of energy level due to ac Stark effect is expressed accurately by analysing the phase of transition probability amplitude. The analytic prediction is confirmed by the numerical results.

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Enhancement of Transfer Efficiency of Cold Atoms Using an Optical Guiding Laser Beam

LI Ke, , WANG Xiao-Rui, , HE Ling-Xiang, ZHAN Ming-Sheng, LUBao-Long,

Chin. Phys. Lett. 2007, 24 (9): 2562-2565 .

Abstract

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We transfer cold ⁸⁷Rb atoms from a vapour cell chamber to a spatially separated UHV magneto-optical trap (MOT) with the assistance of a red-detuned optical guiding beam and a normal push beam. Efficient optical guiding of the cold atoms is observed within a small detuning window. A pulsed optical guiding beam enhances the transfer efficiency and hence allows us to collect more atoms in UHV MOT in a shorter time, which is favourable for our experiment of achieving Bose--Einstein condensates (BEC). Besides the easy operation, another advantage of this optical guiding technique is also demonstrated such that slower atomic beams may be efficiently transferred along horizontal direction. This study is a direct application of the optical guiding technique as a powerful tool.

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Franck--Condon factors and r-Centroids for the A ¹∑⁺_u-X¹∑⁺_g Band System of ^107,109Ag₂: Comparison of the Observed and Calculated Absorption Band Strengths

A. Antic-Jovanovic, M. Kuzmanovic, V. Bojovic, Murtadha A. Khakoo, Russ R. Laher

Chin. Phys. Lett. 2007, 24 (9): 2566-2568 .

Abstract

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Franck--Condon factors and r-centroids for the A ¹∑⁺_u-X¹∑⁺_g band system of ^{107, 109}Ag₂ are computed using Morse and Rydberg--Klein--Rees potentials for both lower and upper electronic states. The differences between the two sets of results are typically in the third decimal place for transitions involving
vibrational levels with v' and v'' up to about 15. Somewhat larger deviations appear for higher vibrational levels, but both sets of results follow the same pattern, which is to match well with the relative absorption band strength distribution in our experimental spectrum. The relative absorption band strengths are calculated by assuming that the electronic transition moment has only a weak dependence on the internuclear distance r. Good agreement between our measured and calculated absorption band strength ratios is found, which provides an excellent test of the calculated Franck-Condon factors and relative absorption band strengths. The r-centroid value for the (v'=0, v''=0) band is found to be approximately equal to the average value of
r_e' and r_e'', indicating that the potentials of both states are not significantly anharmonic around their minimum regions.

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Extended Measurement of the v₂ (1^- ←0⁺) Band of H₃O⁺ by Mid-Infrared Diode Laser Spectroscopy

ZHENG Rui, WANG Rui-Bo, LI Song, HUANG Guang-Ming, DUAN Chuan-Xi

Chin. Phys. Lett. 2007, 24 (9): 2569-2571 .

Abstract

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Twenty-five new R-branch lines of the v₂ (1^- ←0⁺) band of H₃O⁺ are measured using diode laser velocity modulation spectroscopy between 1070 and 1230 cm^-1. The H₃O⁺ ions are produced in a high voltage ac discharge with water diluted in helium. The observed lines together with all the previously published measurements are fit to the standard vibration-rotational Hamiltonian of an oblate symmetric top, yielding a set of improved molecular constants. All the sextic centrifugal distortion constants for both 0⁺ and 1^- states are determined precisely. The observed R(13, 0) transition
is shifted about -0.129cm^-1 from its calculated value, indicating that a near degeneracy exists between the (13, 0)⁺ and (13, 3)^- ground-state rotation-inversion levels.

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Electron--Helium Scattering in Free--Free Transitions in a Bichromatic Laser Field

SUN Jin-Feng, LIANG Ming-Chao, ZHU Zun-Lue

Chin. Phys. Lett. 2007, 24 (9): 2572-2574 .

Abstract

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Theoretical calculation of the differential cross section (DCS) for elastic electron--helium scattering in the presence of a bichromatic CO₂ laser field is carried out in the first Born approximation with a simple screening electric potential. The two components of the laser field have the frequencies ω and 2ω, which are out of phase by an arbitrary scale vp. The variations of the differential cross section as a function of the phase angle vp in the domain
0°≤ψ≤ 360°are presented. We discuss the influence of the number of photons exchanged on the phase-dependence effect. Moreover, for different scattering angles and incident electron energies, the DCS has outstanding differences. These illustrate that the two parameters have important effects on the differential cross section and the screening electric potential is effective.

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Off-Axis Astigmatic Gaussian Beam Combination Beyond the Paraxial Approximation

GAO Zeng-Hui, Lü Bai-Da

Chin. Phys. Lett. 2007, 24 (9): 2575-2578 .

Abstract

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Taking the off-axis astigmatic Gaussian beam combination as an example,
the beam-combination concept is extended to the nonparaxial regime. The
closed-form propagation expressions for coherent and incoherent combinations of nonparaxial off-axis astigmatic Gaussian beams with rectangular geometry are derived and illustrated with numerical examples. It is shown that the intensity distributions of the resulting beam depend on the combination scheme and beam parameters in general, and in the paraxial approximation (i.e., for the small f-parameter) our results reduce to the paraxial ones.

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Local Modulation of Double Electromagnetically Induced Transparency Spectrum

HOU Bang-Pin, WANG Shun-Jin, YU Wan-Lun, SUN Wei-Li

Chin. Phys. Lett. 2007, 24 (9): 2579-2582 .

Abstract

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The double electromagnetically induced transparency induced by two coupling fields can be realized in a four-level tripod-type atom. Such double transparency spectra can be locally modulated by using the weak coherent fields to perturb the coupling transitions. These investigations within this scheme can be independent of Doppler broadening by properly orienting these fields.

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Localization of a Three-Level Cascade Atom via Resonant Absorption

SHEN Wen-Bing, XU Jun, Hu Xiang-Ming

Chin. Phys. Lett. 2007, 24 (9): 2583-2586 .

Abstract

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We show that it is possible to localize a three-level cascade atom under the
resonance condition when it passes through a standing-wave field. The
localization peaks appear at the nodes of the standing-wave field, the detecting probability is 50% in the subwavelength domain, and the peaks
are narrower on the resonance than the off-resonance. The absorption is
the same as that in the usual two-level medium at the nodes and is greatly suppressed outside the nodes due to the Autler--Townes splitting. This is in sharp contrast to the lambda scheme, in which the localization is impossible under the same resonance condition due to the depletion of population of the initial state by the probe field at the nodes and the electromagnetically induced transparency outside the nodes.

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Line-Width Reduction of a Laser Diode Array Using an External Cavity with Two Feedback Mirrors

SU Zhou-Ping, LOU Qi-Hong, DONG Jing-Xing, ZHOU Jun, WEI Yun-Rong

Chin. Phys. Lett. 2007, 24 (9): 2587-2589 .

Abstract

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A novel Littman--Metcalf external cavity laser diode array with two feedback mirrors is introduced. The line-width broadening effect caused by smile can be reduced by the novel external cavity. At the drive current of 16A, the line-width is narrowed to 0.1nm from free-running width of 1.6nm with output efficiency of 84%.

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Orthogonally Linearly Polarized Dual Frequency Nd:YAG Lasers with Tunable Frequency Difference and Its Application in Precision Angle Measurement

TAN Yi-Dong, ZHANG Shu-Lian

Chin. Phys. Lett. 2007, 24 (9): 2590-2593 .

Abstract

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The orthogonally linearly polarized dual frequency Nd:YAG lasers with two quarter wave plates in laser resonator are proposed. The intra-cavity variable birefringence, which is caused by relative rotation of these two wave plates in laser inner cavity, results in the frequency difference of the dual frequency laser also changeable. The theory model based on the Jones matrix is presented, as well as experimental results. The potential application of this phenomenon in precision roll-angle measurement is also discussed.

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A Laser-Diode-Pumped Widely Tunable Single-Longitude-Mode Tm:YAP Laser at Room Temperature

LI Yu-Feng, JU You-Lun, YAO Bao-Quan, WANG Yue-Zhu, Sergii Ubizskii

Chin. Phys. Lett. 2007, 24 (9): 2594-2596 .

Abstract

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A laser-diode-pumped widely tunable single-longitude-mode Tm:YAP laser
in 2μm eye-safe region is built. Continuous tunable range from 1899nm
to 2025nm is achieved with the maximum laser output power of 225mW at
1989nm. In addition, the Tm:YAP laser operating under multimode and
single-mode conditions is discussed.

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Efficient Diode-Pumped Tm:YALO₃ Laser with a Pump Recycling Scheme

YAO Bao-Quan, LI Yu-Feng, WANG Yue-Zhu, DUAN Xiao-Ming, ZHAOGuang-Jun, ZONG Yan-Hua, XU Jun

Chin. Phys. Lett. 2007, 24 (9): 2597-2599 .

Abstract

PDF(pc) (240KB) ( 582 )

By using a pump recycling configuration, the maximum power of 8.1W in the wavelength range 1.935--1.938μm is generated by a 5-mm long Tm:YAlO₃ (4 at.%) laser operating at 18°C with a pump power of 24W. The highest slope efficiency of 42% is attained, and the pump quantum efficiency is up to 100%. The Tm:YAlO₃ laser is employed as a pumping source of singly-doped Ho(1%):GdVO₄laser operating at room temperature, in which continuous wave output power of greater than 0.2W at 2.05μm is achieved with a slope
efficiency of 9%.

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Third-Order Nonlinear Optical Susceptibility of Indium Phosphide Nanocrystals

WANG Hong-Li, WANG Dong, CHEN Guang-De, LIU Hui

Chin. Phys. Lett. 2007, 24 (9): 2600-2602 .

Abstract

PDF(pc) (220KB) ( 1133 )

InP nanocrystals synthesized by refluxing and annealing of organic solvent are determined from XRD measurements to have an average granularity of 25nm. The nonlinear optical properties of the InP nanocrystals studied by using laser Z-scan technique with 50ps pulses at 532nm are found to reveal strong nonlinear optical properties and two-photon absorption phenomenon. Also, the nonlinear absorption coefficient, the nonlinear refractive index and the third-order nonlinear optical susceptibility are determined by experiments, in which the nonlinear refractive index is three orders of magnitude larger than that of bulk InP.

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Synchronously Pumped Femtosecond Optical Parametric Oscillator Based on MgO-Doped Periodically Poled LiNbO₃

ZHU Jiang-Feng, ZHONG Xin, TENG Hao, SUN Jing-Hua, WEI Zhi-Yi

Chin. Phys. Lett. 2007, 24 (9): 2603-2605 .

Abstract

PDF(pc) (228KB) ( 672 )

We report a femtosecond optical parametric oscillator based on MgO-doped PPLN synchronously pumped by a mode-locked Ti:sapphire laser. The wavelengths of the signal and idler are continuously tuned from 1100 to 1300nm and from 2080 to 2930nm, respectively, by changing the pump wavelength and the OPO cavity length. The maximum signal output power of 130mW at the wavelength of 1225nm is obtained, pumped by 900mW of 800nm laser radiation. This corresponds to a total conversion efficiency of 22.1%. The signal pulse duration is measured to be 167fs by intensity autocorrelation with chirped mirrors for intracavity dispersion compensation.

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Effect of Process Parameters on Laser Damage Threshold of TiO₂ Coatings

YAO Jian-Ke, JIN Yun-Xia, ZHAO Yuan-An, HE Hong-Bo, SHAO Jian-Da, FAN Zheng-Xiu

Chin. Phys. Lett. 2007, 24 (9): 2606-2608 .

Abstract

PDF(pc) (229KB) ( 655 )

We investigate the laser damage behaviour of an electron-beam-deposited TiO₂ monolayer at different process parameters. The optical properties, chemical composition, surface defects, absorption and laser-induced damage threshold (LIDT) of films are measured. It is found that TiO₂ films with the minimum absorption and the highest LIDT can be fabricated using a TiO₂ starting material after annealing. LIDT is mainly related to absorption and is influenced by the non-stoichiometric defects for TiO₂films. Surface defects show no evident effects on LIDT in this experiment.

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Heat Conductivity of One-Dimensional Carbon Chain in an External Potential

GE Yong, DONG Jin-Ming

Chin. Phys. Lett. 2007, 24 (9): 2609-2612 .

Abstract

PDF(pc) (253KB) ( 588 )

The heat transport in a one-dimensional (1D) carbon nanowire (CNW) lying in an external potential with different amplitudes and periods is studied by the non-equilibrium molecular dynamics method. It is found that the thermal conductivity of CNW is always anomalous, increasing with the CNW length and obeying the power law k ～ N, in which α decreases with the increasing external potential amplitude. The thermal conductivity could be enhanced by the external potential with rather larger amplitudes, which means that an applied external potential could be an efficient tool to improve the heat conductivity of a real 1D material. In addition, the effect of different periods of
the external potential is studied, finding the external potential with an incommensurate period leads to the smaller α value.

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A Novel Kinetic Model of Liquid Nitrogen's Explosive Boiling at the Initial Stage

HUAI Xiu-Lan, DONG Zhao-Yi, LI Zhi-Gang, YIN Tie-Nan, ZOU Yu,

Chin. Phys. Lett. 2007, 24 (9): 2613-2616 .

Abstract

PDF(pc) (943KB) ( 421 )

The liquid nitrogen's explosive boiling characteristics under transient high heat flux have attracted increasing attentions of researchers over the world due to its wide applications. Although some experiments have been performed, the process and the characteristics at the initial stage, especially within 1μs, have not been described reasonably yet. Based on the related experiments and theoretical analysis, a novel kinetic model combined with quasi-fluid idea is presented to analyse the characteristics of liquid nitrogen's explosive boiling at the initial stage. The results indicate that the model can appropriately describe the liquid nitrogen's explosive boiling. The behaviour and the heat transfer characteristics of a single bubble are very different
from those of the bubble cluster, thus the behaviour of individual bubbles could not be directly applied to describe the explosive boiling process at the initial stage.

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Two Phases of Coherent Structure Motions in Turbulent Boundary Layer

LIU Jian-Hua, JIANG Nan

Chin. Phys. Lett. 2007, 24 (9): 2617-2620 .

Abstract

PDF(pc) (349KB) ( 478 )

Two phases of coherent structure motion are acquired after obtaining conditional phase-averaged waveforms for longitudinal velocity of coherent structures in turbulent boundary layer based on Harr wavelet transfer. The correspondences of the two phases to the two processes (i.e. ejection and sweep) during a burst are determined.

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Observation of Non-Local Transport Phenomena with SMBI in HL-2A

SUN Hong-Juan, DING Xuan-Tong, YAO Liang-Hua, FENG Bei-Bin, LI Wei, PAN Yu-Dong, LIU Ze-Tian, DUAN Xu-Ru, YANG Qing-Wei, LIU Yong

Chin. Phys. Lett. 2007, 24 (9): 2621-2623 .

Abstract

PDF(pc) (264KB) ( 513 )

The non-local transport phenomenon induced by supersonic molecular beam
injection (SMBI) was first observed in the HL-2A tokomak. In comparison with the phenomena induced by other methods in various tokamaks, it has its own feature: the effect induced by SMBI in HL-2A lasts much longer than that induced by pellet injection in other similar size tokomaks. Both the bolometer radiation and H_α emission decrease when the non-local effect appears. This suggests that an electron transport barrier has been formed at the position just outside the q = 1 surface when the non-local effect appears.

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Superconducting State Parameters of Cu_CZr_100-CBinary Amorphous Alloys by Pseudopotential Approach

Aditya M. Vora

Chin. Phys. Lett. 2007, 24 (9): 2624-2627 .

Abstract

PDF(pc) (204KB) ( 765 )

We present the screening dependence theoretical investigations of the
superconducting state parameters, i.e. electron--phonon coupling strength
α, Coulomb pseudopotential μ*, transition temperature T_c, isotope effect exponent α, and effective interaction strength N₀V of some Cu_CZr_100-C (C=25, 30, 33, 35, 40, 45, 50, 55, 57, 60 at.%) binary amorphous alloys by employing the well-known empty core model potential of Ashcroft. Five screening functions proposed by Hartree, Taylor, Ichimaru-Utsumi, Farid et
al. and Sarkar it et al. are used to study the screening influence on the aforesaid superconducting properties. The transition temperature T_c obtained from the Ichimaru-Utsumi screening function is found to be in excellent agreement with available theoretical or experimental data. Also, the present results are found in qualitative agreement with the other earlier reported data, which confirms the superconducting phase in the amorphous alloys.

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Magnetic Properties and Nanostructures of FePtCu:C Thin Films with FePt Underlayers

JIN Ling-Fang, YAN Ming-Lang

Chin. Phys. Lett. 2007, 24 (9): 2628-2630 .

Abstract

PDF(pc) (380KB) ( 566 )

Magnetic properties and nanostructures of FePtCu:C thin films with FePt
underlayers (ULs) are studied. The effect of FePt ULs on the orientation and magnetic properties of the thin films are investigated by adjusting FePt UL thicknesses from 2nm to 14nm. X-ray diffraction (XRD) scans reveal that the orientation of the films is dependent on FePt UL thickness. For a 5-nm FePtCu:C nanocomposite thin film with a 2-nm FePt UL, the coercivity is 6.5KOe, the correlation length is 59nm, the desired face-centred-tetragonal (fct) ordered structure [L1₀ phase] is formed and the c axis normal to the film
plane [(001) texture] is obtained. These results indicate that the better orientation and magnetic properties of the films can be tuned by decreasing the thickness of the FePt UL.

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Interaction of Point Defects with Twin Boundaries in Copper

S. A. Ahmad, Razia Ramzan

Chin. Phys. Lett. 2007, 24 (9): 2631-2634 .

Abstract

PDF(pc) (246KB) ( 580 )

The interaction between small vacancy clusters and twin boundaries in copper is studied by using many-body potential developed by Ackland et al. for fcc metals. The interaction energies of single-, di- and tri-vacancy clusters with (111) and (112) twin boundaries are computed using well established simulation techniques. For (111) twins the vacancy clusters are highly repelled when they are on the adjacent planes, and are attracted when they are away from the boundary. In the case of (112) twins, vacancy clusters are more attracted to the boundary when they are near the boundary as compared to away from it. Vacancy clusters on both the sides of the boundary are also investigated, and it is observed that the clusters energetically prefer
to lie on the off-mirror sites as compared to the mirror position across the twin.

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Elastic Modulus and Hardness of Cr--Nb Nano-Multilayers

YANG Meng-Jin, LAI Wen-Sheng, PAN Feng

Chin. Phys. Lett. 2007, 24 (9): 2635-2638 .

Abstract

PDF(pc) (576KB) ( 712 )

Cr--Nb nano-multilayered films with various modulation wavelengths Λ are prepared by e-gun evaporation and their mechanical properties are investigated. Cr and Nb both have bcc structures with large differences in lattice constants and Young's modulus, which are supposed to favour modulus enhancement. Nevertheless, nano-indention measurements show no enhancement for the modulus and a slight decrease for the hardness with decreasing Λ down to 6nm. This is mainly due to counter-contribution to modulus from adjacent layers subjected to reverse strains, in agreement with recent theoretical study, while the decrease of hardness arises from grain boundary sliding. Interestingly, at Λ= 3nm, the hardness of the film has an
increase of 44% relative to the value of a rule of mixture, owing to the emergence of a new phase for reconciling the structure difference at the interfaces.

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Effect of Quantizing Magnetic Field on Cyclotron Energy and Cyclotron Effective Mass in Size Quantized Films with Non-Parabolic Energy Band

B. .I. GULIYEV, R. F. EMINBEYLI, A. KORKUT

Chin. Phys. Lett. 2007, 24 (9): 2639-2641 .

Abstract

PDF(pc) (212KB) ( 414 )

The Fermi energy, cyclotron energy and cyclotron effective mass of degenerate electron gas in a size-quantized semiconductor thin film with non-parabolic energy bands are studied. The influences of quantizing magnetic field on these quantities in two-band approximation of the Kane model are investigated. It is shown that the Fermi energy oscillates in a magnetic field. The period and positions of these oscillations are found as a function of film thickness and concentration of electrons. Cyclotron energy and cyclotron effective mass are investigated as a function of film thickness in detail. The
results obtained here are compared with experimental data on GaAs quantum wells.

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Elastic Properties of Rutile TiO₂ at High Temperature

WANG Yan-Ju, CHANG Jing, TAN Li-Na, CHEN Xiang-Rong,

Chin. Phys. Lett. 2007, 24 (9): 2642-2645 .

Abstract

PDF(pc) (257KB) ( 1167 )

Dependence of elastic properties on temperature for rutile TiO₂is investigated by the Cambridge Serial Total Energy Package (CASTEP)
program in the frame of density function theory (DFT) and the quasi-harmonic Debye model. The six independent elastic constants of rutile TiO₂ at high temperature are theoretically obtained for the first time. It is found that with increasing temperature, the elastic constants will decrease monotonically. Moreover, we successfully obtain the polycrystalline moduli B_H and G_H, as well as the Debye temperature θ_D.

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Electronic Structure and Optical Properties of Semiconducting Orthorhombic BaSi₂

CHENG Zheng-Ze, CHENG Ze, XU Bin

Chin. Phys. Lett. 2007, 24 (9): 2646-2649 .

Abstract

PDF(pc) (279KB) ( 685 )

Full potential linearized augmented plane wave (FPLAPW) method calculations are carried out for semiconducting orthorhombic BaSi₂. The optical properties and the origin of the different optical transitions are investigated. Our calculated band gap of 1.0918eV is indirect, which is in good agreement with the experimental result. The bonds between Ba and Si are considered to be electrovalent bond. The anisotropy in the imaginary part ε₂(ω) and real part
ε₁ (ω) of the optical dielectric tensor are analysed. The contributions of various transition peaks are explained from the imaginary part of the dielectric function.

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Nonadiabatic Geometric Phase and Induced Persistent Current in Mesoscopic Square Circuit with Tilted Magnetic Field at Edges

ZHONG Yan-Ming, XIONG Shi-Jie

Chin. Phys. Lett. 2007, 24 (9): 2650-2653 .

Abstract

PDF(pc) (258KB) ( 422 )

We investigate the geometric phase produced by nonadiabatic transition of spin states at corners of mesoscopic square circuit with tilted magnetic field at its edges. From the Schrodinger equation, the transitions of electron spin state at corners are described by the transfer matrices. The eigenenergies and eigenstates are obtained from the cyclic condition and the multiplying of the transfer matrices. We show that there exist persistent charge and spin currents in such a system due to the lift of degeneracy between the opposite moving directions in the presence of the tilted magnetic field. The dependences of eigenenergies, geometric phase, charge and spin persistent currents on the tilting angles of magnetic field are analysed.

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Effect of TiO₂ Nanotubes on Polymer-Fullerene Bulk Heterojunction Solar Cells

LI Yan, HOU Yan-Bing, JIN Hui, SHI Quan-Min, ZHANG Xiu-Long

Chin. Phys. Lett. 2007, 24 (9): 2654-2656 .

Abstract

PDF(pc) (368KB) ( 550 )

We investigate the photovoltaic properties of hybrid organic solar cell based on the blend of poly[2-methoxy-5-(2_-ethylhexoxy-1,4-phenylenevinylene) (MEH-PPV), C₆₀ and titanium dioxide (TiO₂) nanotubes. In comparison of the composite devices with different TiO₂:[MEH-PPV +C₆₀] weight ratios of 1wt.% (D_1-1), 2wt.% (D_1-2), 3wt.% (D_1-3), 5wt.% (D_1-4), 10wt.% (D_1-5) and 20wt.% (D_1-6), it is found that the device D_1-3 exhibits the best performance. The
conversion efficiency is improved by a factor of 3 compared with the MEH-PPV:C60 device.

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Hydrogen Passivation Effect on Enhanced Luminescence from Nanocrystalline Si/SiO₂ Multilayers

XIA Zheng-Yue, HAN Pei-Gao, XU Jun, CHEN De-Yuan, WEI De-Yuan, MA Zhong-Yuan, CHEN Kun-Ji, XU Ling, HUANG Xin-Fan

Chin. Phys. Lett. 2007, 24 (9): 2657-2660 .

Abstract

PDF(pc) (490KB) ( 721 )

Nanocrystalline Si/SiO₂ multilayers are prepared by thermally annealing amorphous Si/SiO₂ stacked structures. The photoluminescence intensity is obviously enhanced after hydrogen passivation at various temperatures. It is suggested that the hydrogen trapping and detrapping processes at different temperatures strongly influence the passivation effect. Direct experimental evidence is given by electron spin resonance spectra that hydrogen effectively reduces the nonradiative defect states existing in the Si nanocrystas/SiO₂ system which enhances the radiative recombination probability. The luminescence characteristic shows its stability after hydrogen passivation even after aging eight months.

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Effect of Precursor Molar Ratio of [S^2-]/[Zn²⁺] on Particle Size and Photoluminescence of ZnS:Mn²⁺ Nanocrystals

DONG Dong-Qing, LI Lan, ZHANG Xiao-Song, HAN Xu, AN Hai-Ping

Chin. Phys. Lett. 2007, 24 (9): 2661-2663 .

Abstract

PDF(pc) (490KB) ( 578 )

We investigate the influence of precursor molar ratio of [S^2-]/[Zn²⁺] on particle size and photoluminescence (PL) of ZnS:Mn²⁺ nanocrystals. By changing the [S^2-]/[Zn²⁺] ratio from 0.6 (Zn-rich) to 2.0 (S-rich), the particle size increases from nearly 2.7nm to about 4.0nm. The increase in the ratio of [S^2-]/[Zn²⁺] causes a decrease of PL emission intensity of ZnS host while a distinct increase of Mn²⁺ emission. The maximum intensity for the luminescence of Mn²⁺ emission is observed at the ratio of [S^2-]/[Zn^2+]≈1.5. The possible mechanism for the results is discussed by filling of S^2- vacancies and the increase of Mn²⁺ ions incorporated into ZnS lattices.

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Effects of Magnet Size and Geometry on Magnetic Levitation Force

M. K. Alqadi, H. M. Al-khateeb, F. Y. Alzoubi, N. Y. Ayoub

Chin. Phys. Lett. 2007, 24 (9): 2664-2666 .

Abstract

PDF(pc) (196KB) ( 477 )

We obtain analytical relations for the levitation force as a function of dimensions of the superconductor-magnet system. The force has been
calculated on the basis of the dipole-dipole interaction model. The effect of thickness of the superconductor on the levitation force is investigated. The results show that the influence of geometry and thickness of the magnet becomes significantly large at small levitation distances. Furthermore, approximating the permanent magnet as a point dipole results in an inaccurate estimation of the levitation force.

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X-Ray Magnetic Circular Dichroism Measurement of Fe--Co Alloy Films Prepared by Electrodeposition

LI Zong-Mu, XU Fa-Qiang, WANG Li-Wu, WANG Jie, ZHU Jun-Fa, ZHANG Wen-Hua

Chin. Phys. Lett. 2007, 24 (9): 2667-2670 .

Abstract

PDF(pc) (356KB) ( 848 )

The macro- and micro-magnetic properties of Fe--Co alloy films eletrodeposited on GaAs(100) are studied by synchrotron radiation x-ray
magnetic circular dichroism (XMCD) in combination with the magneto-optical Kerr effect (MOKE) measurements and magnetic force microscopy (MFM). The orbital and spin magnetic moments of each element in the Fe--Co alloy are determined by the sum rules of XMCD. Element-specific hysteresis loops (ESHL) are obtained by recording the L₃ MCD signals as a function of applied magnetic field. MOKE results reveal that the amorphous films are magnetically isotropic in the surface plane. The MFM image shows that the dimension of the magnetic domains is about 1--2μm, which is much larger than that of the grains, indicating that there are intergranular correlations among these grains. Both ESHL and MOKE hysteresis loops indicate the strong ferromagnetic coupling of Fe and Co in the alloy films.

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Structure Stability of LaAlO₃ Thin Films on Si Substrates

HE Meng, LIU Guo-Zhen, XIANG Wen-Feng, Lü Hui-Bin, JIN Kui-Juan, ZHOU Yue-Liang, YANG Guo-Zhen

Chin. Phys. Lett. 2007, 24 (9): 2671-2674 .

Abstract

PDF(pc) (435KB) ( 420 )

A series of amorphous and single-crystalline LaAlO₃ (LAO) thin films are fabricated by laser molecular-beam epitaxy technique on Si substrates under various conditions of deposition. The structure stability of the LAO films annealed in high temperature and various ambients is studied by x-ray diffraction as well as high-resolution transmission electron microscopy. The results show that the epitaxial LAO films have very good stability, and the structures of amorphous LAO thin films depend strongly on the conditions of deposition and post-annealing. The results reveal that the formation of LAO
composition during the deposition is very important for the structure stability of LAO thin films.

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Negative Refraction and Near-Field Imaging of an Elliptical-Rod Photonic Crystal Slab in the Second Band

FENG Shuai, AO Ling, WANG Yi-Quan, LI Zhi-Yuan, ZHANG Dao-Zhong

Chin. Phys. Lett. 2007, 24 (9): 2675-2677 .

Abstract

PDF(pc) (594KB) ( 646 )

Negative refraction and imaging properties of the electromagnetic wave
through a two-dimensional photonic crystal (PC) slab, which consists of a square lattice of elliptical dielectric rods immersed in the air background, is studied by the plane-wave expansion method and the finite-difference time-domain method. A point source placed in the vicinity of the PC slab can form a good-quality image spot through the PC slab for the incident frequencies within the second photonic band. The calculated result also shows that negative refraction occurs in this kind of PC slab.

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Fabrication and Photoluminescence Properties of ZnO Nanorods

ZHONG Hong-Mei, LU Wei, SUN Yan, LI Zhi-Feng

Chin. Phys. Lett. 2007, 24 (9): 2678-2680 .

Abstract

PDF(pc) (372KB) ( 512 )

ZnO nanorods are successfully synthesized by annealing the precursors in argon with the chemical precipitation method. The structural and optical properties of ZnO nanorods are investigated. As annealing temperature increases, the intensity of the green emission increases while the intensity of the yellow emission decreases. The result suggests that the green emission depends strongly on the annealing temperature.

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Modelling and Optimization for Deposition of SiO_xN_yFilms by Radio-Frequency Reactive Sputtering

XU Wen-Bin, DONG Shu-Rong, WANG De-Miao

Chin. Phys. Lett. 2007, 24 (9): 2681-2684 .

Abstract

PDF(pc) (283KB) ( 548 )

SiO_xN_y films are deposited by reactive sputtering from a Si target in Ar/O₂/N₂ atmospheres. In order to achieve the control of film composition and to keep a high deposition rate at the same time, a new sputtering model based on Berg's work is provided for the condition of double reactive gases. Analysis based on this model shows that the deposition process can easily enter the
target-poisoning mode when the preset gas flow (N₂ in this work) is too high, and the film composition will change from nitrogen-rich to SiO₂-like with the increase of oxygen supply while keeping the N₂ supply constant. The modelling results are confirmed in the deposition process of SiO_xN_y. Target self-bias voltages during sputtering are measured to characterize the different sputtering modes. FTIR-spectra and dielectric measurements are used to testify the model prediction of composition. Finally, an optimized sputtering condition is selected with the O₂/N₂ flow ratio varying from 0 to 1 and N₂ supply fixed at 1 sccm. Average deposition rate of 17nm/min is obtained under this selected condition, which has suggested the model validity and potential for industry applications.

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Photoluminescence of ZnO and Mn-Doped ZnO Polycrystalline Films Prepared by Plasma Enhanced Chemical Vapour Deposition

LIN Ying-Bin, YANG Yan-Min, XU Jian-Ping, LIU Xing-Chong, WANGJian-Feng, HUANG Zhi-Gao, ZHANG Feng-Ming, DU You-Wei

Chin. Phys. Lett. 2007, 24 (9): 2685-2688 .

Abstract

PDF(pc) (376KB) ( 708 )

ZnO and Mn-doped ZnO polycrystalline films are prepared by plasma
enhanced chemical vapour deposition at low temperature (220°C), and
room-temperature photoluminescence of the films is systematically investigated. Analysis from x-ray diffraction reveals that all the prepared films
exhibit the wurtzite structure of ZnO, and Mn-doping does not induce the
second phase in the films. X-ray photoelectron spectroscopy confirms the
existence of Mn²⁺ ions in the films rather than metallic Mn or Mn⁴⁺ ions. The emission efficiency of the ZnO film is found to be dependent strongly on the post-treatment and to degrade with increasing temperature either in air or in nitrogen ambient. However, the enhancement of near band edge (NBE) emission is observed after hydrogenation in ammonia plasma, companied with more defect-related emission. Furthermore, the position of NBE shifts towards to high-energy legion with increasing Mn-doped concentration due to Mn incorporation into ZnO lattice.

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Ordered InAs Quantum Dots with Controllable Periods Grown on Stripe-Patterned GaAs Substrates

REN Yun-Yun, XU Bo, WANG Zhan-Guo, LIU-Ming, LONG Shi-Bing

Chin. Phys. Lett. 2007, 24 (9): 2689-2691 .

Abstract

PDF(pc) (595KB) ( 651 )

GaAs (001) substrates are patterned by electron beam lithography and wet chemical etching to control the nucleation of InAs quantum dots (QDs). InAs dots are grown on the stripe-patterned substrates by solid source molecular beam epitaxy. A thick buffer layer is deposited on the strip pattern before the deposition of InAs. To enhance the surface diffusion length of the In atoms, InAs is deposited with low growth rate and low As pressure. The AFM images show that distinct one-dimensionally ordered InAs QDs with homogeneous size distribution are created, and the QDs preferentially nucleate along the trench. With the increasing amount of deposited InAs and the spacing of the trenches, a number of QDs are formed beside the trenches. The distribution of additional QDs is long-range ordered, always along the trenchs rather than across the spacing regions.

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Humidity Effects on Imaging and Nanomanipulation of Individual DNA Molecules on HOPG Surface

ZHOU Xing-Fei, ZHANG Yi, WANG Hua-Bin, NIU Dong-Xiao, WANGPei-Qing, HU Jun,

Chin. Phys. Lett. 2007, 24 (9): 2692-2695 .

Abstract

PDF(pc) (740KB) ( 538 )

Single DNA molecules are aligned on highly ordered pyrolytic graphite (HOPG) surface in ambient air. It is shown that environmental humidity has a remarkable influence on the measured height of DNA by atomic force microscopy (AFM), probably due to the conformation transition of DNA. We also demonstrate that DNA molecules deposited on HOPG surfacecan be `pushed' much more easily by AFM tip at high humidity than at low one.

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Responses of Hodgkin--Huxley Neuronal Systems to Spike-Train Inputs

CHANG Wen-Li, WANG Sheng-Jun, WANG Ying-Hai

Chin. Phys. Lett. 2007, 24 (9): 2696-2699 .

Abstract

PDF(pc) (248KB) ( 581 )

We investigate responses of the Hodgkin--Huxley globally neuronal systems to periodic spike-train inputs. The firing activities of the neuronal networks show different rhythmic patterns for different parameters. These rhythmic patterns can be used to explain cycles of firing in real brain. These activity patterns, average activity and coherence measure are affected by two quantities such as the percentage of excitatory couplings and stimulus intensity, in which the percentage of excitatory couplings is more important than stimulus intensity since the transition phenomenon of average activity comes about.

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Angular Dependence of Lateral and Levitation Forces in Asymmetric Small Magnet/Superconducting Systems

H. M. Al-Khateeb, M. K. Alqadi, F. Y. Alzoubi, N. Y. Ayoub

Chin. Phys. Lett. 2007, 24 (9): 2700-2703 .

Abstract

PDF(pc) (224KB) ( 997 )

The dipole--dipole interaction model is used to calculate the angular dependence of lateral and levitation forces on a small permanent magnet
and a cylindrical superconductor in the Meissner state lying laterally off the symmetric axis of the cylinder. Under the assumption that the lateral displacement of the magnet is small compared with the physical dimensions of the system, we obtain analytical expressions for the lateral and levitation forces as functions of geometrical parameters of the superconductor as well as the height, the lateral displacement and the orientation of magnetic moment of the magnet. The effect of thickness and radius of the superconductor on the levitation force is similar to that for a symmetric magnet/superconducting cylinder system, but within the range of lateral displacement. The splitting in the levitation force increases with the increasing angle of orientation of the magnetic moment of the magnet. For a given lateral displacement of the magnet, the lateral force vanishes when the magnetic moment is perpendicular to the surface of the superconductor and has a maximum value when the moment is parallel to the surface. For a given
orientation of the magnetic moment, the lateral force has a linear relationship with the lateral displacement. The stability of the magnet above the superconducting cylinder is discussed in detail.

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Electrical Characterization of Copper Phthalocyanine Thin-Film Transistors with Fluoride Gate Insulator

YU Shun-Yang, YI Ming-Dong, MA Dong-Ge

Chin. Phys. Lett. 2007, 24 (9): 2704-2706 .

Abstract

PDF(pc) (299KB) ( 678 )

Different fluoride materials are used as gate dielectrics to fabricate copper phthalocyanine (CuPc) thin film transistors (OTFTs). The fabricated devices exhibit good electrical characteristics and the mobility is found to be dependent on the gate voltage from 10^-3to 10^-1cm²V^-1s^-1. The observed noticeable electron injection at the drain electrode is of great significance in
achieving ambipolar OTFTs. The same method for formation of organic
semiconductors and gate dielectric films greatly simplifies the fabrication process. This provides a convenient way to produce high-performance OTFTs on a large scale and should be useful for integration in organic displays.

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Ultraviolet Phototransistors on AlGaN/GaN Heterostructures

CHEN Chen, JIANG Wen-Hai, REN Chun-Jiang, LI Zhong-Hui, JIAO Gang, DONG Xun, CHEN Tang-Sheng

Chin. Phys. Lett. 2007, 24 (9): 2707-2709 .

Abstract

PDF(pc) (297KB) ( 889 )

We report on the fabrication and characterization of phototransistors based on AlGaN/GaN heterostructure grown over 6H-SiC substrates. The device has two functions: as a high electron mobility transistor (HEMT) and an ultraviolet photodetector at the same time. As an HEMT, its maximum transconductance is 170mS/mm, while the minimum cutoff frequency f_T and the maximum oscillation frequency f_m are 19 and 35GHz, respectively. As a photodetector, the device is visible blind, with an ultraviolet/green contrast of three orders of magnitude, and a responsivity as high as 1700A/W at the wavelength of 362nm.

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Effect of the Carbon-Nanotube Length on Water Permeability

LI Jing-Yuan, YANG Zai-Xing, FANG Hai-Ping, ZHOU Ru-Hong, TANGXiao-Wei

Chin. Phys. Lett. 2007, 24 (9): 2710-2713 .

Abstract

PDF(pc) (313KB) ( 846 )

Effect of the carbon nanotube (CNT) channel length on the water flow
through the CNT is studied using molecular dynamics simulations. The water flow is found to decay with the channel length (～1/N^2.3, N is the number of carbon rings along the nanotube axis), much faster than that predicted by a previous continuous-time random walk (CTRW) model (～1/N). This unexpected decay rate of flow is found to result from the weakening of the correlation of the concerted motion of the water molecules inside the CNT. An improved CTRW model is then proposed by taking into account of this effect. Meanwhile, the diffusion constant of water molecules inside CNTs with various
lengths is found to be relatively invariant, which results in a relatively constant hopping rate.

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Measurement of Binding Force between Microtubule-Associated Protein and Microtubule

XU Chun-Hua, GUO Hong-Lian, QU E, LI Zhao-Lin, YUAN Ming, CHENGBing-Ying, ZHANG Dao-Zhong

Chin. Phys. Lett. 2007, 24 (9): 2714-2416 .

Abstract

PDF(pc) (252KB) ( 697 )

Microtubule-associated proteins (MAPs) are important proteins in cells. They can regulate the organization, dynamics and function of microtubules. We measure the binding force between microtubule and a new plant MAP, i.e. AtMAP65-1, by dual-optical tweezers. The force is obtained to be 14.6±3.5pN from the data statistics and analysis. This force measurement is helpful to understand the function and mechanism of MAPs from the mechanical point of view and lays the groundwork for future measurements of the mechanical properties of other biological macro-molecules.

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