Vol. 54, No. 12 (2005)

Momentum-dependent symmetries and non-Noether conserved quantities for mechanico-electrical systems
Li Xian-Hui, Zheng Shi-Wang, Fu Jing-Li
2005, 54 (12): 5511-5516. doi: 10.7498/aps.54.5511
Abstract +
The Hamiltonian canonical equation of the systems, the definition, criterion, structure equation and conserved quantities of momentum-dependent symmetries for Lagrange-Maxwell mechanico-electrical systems were presented. This work shows that the function ψ in the structure equation is only an invariant on the symmetry group. A new method to deduce conserved quantities of mechanico-electrical systems is obtained. An example is designed to illustrate these results.
d'Alembert-Lagrange principle on Riemann-Cartan space
Wang Yong, Guo Yong-Xin
2005, 54 (12): 5517-5520. doi: 10.7498/aps.54.5517
Abstract +
The nonholonomic mapping put forward by Kleinert is generalized to a first-order linear mapping. By means of this method, Riemann-Cartan space is embedded into Euclidean space. Based on this construction, the d'Alembert-Lagrange principle of nonholonomic constrained systems in Euclidean space is reduced to an “unconstrained” representation on Riemann-Cartan space.
Unified symmetry of the holonomic system in terms of quasi-coordinates
Mei Feng-Xiang, Xu Xue-Jun
2005, 54 (12): 5521-5524. doi: 10.7498/aps.54.5521
Abstract +
This paper discusses the unified symmetry of a holonomic mechanical system in terms of quasi-coordinates under special infinitesimal transformations in which time is not varied. The differential equations of motion of the system are given. The definition and the criterion of the unified symmetry for the system are presented. A new conserved quantity, besides the Noether conserved quantity and the Hojman conserved quantity, is deduced from the unified symmetry. An example is iven to illustrate the application of the results.
New soliton structures in the (2+1)-dimensional nonlinear KdV equations
Zhang Jie-Fang, He Bao-Gang, Xu Chang-Zhi
2005, 54 (12): 5525-5529. doi: 10.7498/aps.54.5525
Abstract +
A new variable separation approach for the (2+1)-dimensional nonlinear KdV equations is obtained by using variable separation technique and selecting a class of new seed solutions. Some new kinds of periodic soliton structutres, ring form soliton structures and curvilinear soliton structures are revealed by selecting the arbitrary functions appropriately. These structures, which can not be obtained from the formula commonly used in literature, are first reported.
Stability and approximate solution of nonlinear dynamic system of a cylinder with two end faces in relative rotation
Wang Kun
2005, 54 (12): 5530-5533. doi: 10.7498/aps.54.5530
Abstract +
For the nonlinear dynamics system of a cylinder with two end faces in relative rotation, a qualitative analysis of the autonomous system dynamics equation is performed. The stability of the solution of the equation is established, and approximate solution of the equation under special conditions is obtained by averaging method.
Basic differential equation of self-similar motion of one-dimensional nonsteady flow of ideal gas
Bian Bao-Min, He An-Zhi, Li Zhen-Hua, Yang Ling, Zhang Ping, Shen Zhong-Hua, Ni Xiao-Wu
2005, 54 (12): 5534-5539. doi: 10.7498/aps.54.5534
Abstract +
Self-similar function of one-dimensional nonsteady flow is extended to a general form. With total energy kept constant, basic differential equation of self-similar motion of one-dimensional nonsteady flow of ideal gas is derived using dimension theory in combination with the basic motion equations of hydromechanics. When a non-dimensional natural parameter L, which is the ratio of velocity of fluid (u) and self-similar surface (r·), serves as the independent variable, the theoretical model reveals that self-similar law of non-dimensional nonsteady flow of ideal gas has the simplest mathematical form. The model overcomes the difficulty of divergence at the origin of self-similar function of Taylor and thus has significant importance.
A new method to construct Weierstrass elliptic function solutions for soliton equations
Zhang Hong-Qing, Li De-Sheng
2005, 54 (12): 5540-5543. doi: 10.7498/aps.54.5540
Abstract +
Utilizing the equation that has Weierstrass elliptic function solutions, we have obtained two groups of new solutions of projection Riccati equation. Because the projection Riccati equation can be used to solve nearly all kinds of non-linear evolution equations, thus we have given a method for constructing. Weierstrass elliptic function solutions for nearly all equations with soliton solutions.
Multi-party multi-level quantum key distribution protocol based on entanglement swapping
Yang Yu-Guang, Wen Qiao-Yan, Zhu Fu-Chen
2005, 54 (12): 5544-5548. doi: 10.7498/aps.54.5544
Abstract +
A multi-party multi-level quantum key distribution protocol based on entanglement swapping is proposed. A perfect complete orthonormal basis of the bi-party three-level systems is first constructed. By using this basis and swapping the entanglement, bi-party quantum key distribution is realized. In addition, three-level can be generalized to multi-level and bi-party to multi-party,i.e., multi-party multi-level quantum key distribution based on entanglement swapping can be achieved. Using entanglement swapping and multi-level key distribution can significantly increase the efficiency of the detection of eavesdropping, the key generation rate and the information flux.
A novel quantum key distribution scheme with unextendible product basis and exact entanglement basis
Yang Yu-Guang, Wen Qiao-Yan, Zhu Fu-Chen
2005, 54 (12): 5549-5553. doi: 10.7498/aps.54.5549
Abstract +
A novel quantum key distribution scheme with unextendible product basis and exact entanglement basis in the 33 Hilbert space is proposed. The probability of successful eavesdropping by the eavesdropper is also analysed. This scheme has many distinct features such as great capacity, high efficiency.
Dynamics of many particles in the urn model
Tong Pei-Qing, Chen Bo
2005, 54 (12): 5554-5558. doi: 10.7498/aps.54.5554
Abstract +
We study the behavior of many particles moving in the 2-urn and 3-urn systems driven by a stochastic heat bath. The velocities and the positions of the particles are calculated through noise activated 1-D Langevin function. We find the time in which the system is in equilibrium becomes longer than the time in which the system is in non-equilibrium as the bath temperature increases. The velocity distribution of the particles is always Gaussian. The effective temperature T2 characterizing non-equilibrium state obeys the power law with the restitution coefficient r.
Stochastic stability and control of coupled Duffing-van der Pol systems
Li Wei, Xu Wei, Zhao Jun-Feng, Jin Yan-Fei
2005, 54 (12): 5559-5565. doi: 10.7498/aps.54.5559
Abstract +
By using the stochastic averaging method of quasi-Hamilton system and stochastic stbilization strategy, the stochastic stability and stabilization control of a coupled Duffing-van der Pol system under Gaussian white noise excitation with in semi-infinite time interval are investigated. The results show that, with suitable controlled parameters, effective stochastic stabilization can be realized.
Keyed Hash function based on composite nonlinear autoregressive filter
Zhang Wen-Fang, Wang Xiao-Min, Zhang Jia-Shu
2005, 54 (12): 5566-5573. doi: 10.7498/aps.54.5566
Abstract +
In recent years, chaotic dynamic systems are widely applied in information security because of its characteristic that the trajectory is sensitive to initial conditions and seems to be random though it is really a determinate process. The nonlinear autoregressive digital filters satisfying the Kelber conditions can also produce chaotic sequences just like chaotic maps do. The composite filter system consists of several above-mentioned filters, and the filter used for iteration is completely decided by a predetermined sequence called composite sequence. Consequently, the trajectory of the composite filter system is not only sensitive to its initial conditions, but also related with the composite sequence, which determines the choice of iterated sub-filter system in the iterating process. o the trajectory is more complex than that of general chaotic systems or single filter system. After analyzing some properties of composite filter systems such as N-dimensional uniform distribution and invariant distribution density function,a new keyed Hash algorithm based on composite system is presented. The approach selects the sub-filter system with the composite sequence obtained from message to be hashed, uses the initial iteration value of composite system as the secret key, and the coarse-grained trajectory as Hash value. Because of the sensitivity to initial value and randomicity of the iteration process, there is a very complex nonlinear relation between Hash value and the corresponding message and secret key, and then every bit of the Hash value derived from the message M is related with every bit of M. Furthermore, the filter-based algorithm is simple enough without complex operations, so it can be realized easily.
A stochastic digital oscillating system induced by the truncation error
Sheng Li-Yuan, Jia Wei-Yao
2005, 54 (12): 5574-5580. doi: 10.7498/aps.54.5574
Abstract +
It is generally believed that chaotic systems will collapse due to computer truncation error. In this paper we introduce a contrary example, in which a simple system is induced to complication or chaos completely by computer truncation. The system is defined as a cross-focus system in elliptical reflecting cavity map system. The theoretical solution is a limiting sequence and the computer solution is a stochastic digital oscillating system induced from the theoretical solution by truncation error. There is only a turn-round mechanism in the system theoretically. On the other hand, round-off induces the nonhyperbolic fixed points in the system to produce a new escape mechanism of intermittent chaos.
Perturbation theory of soliton solution for the generalized Landau-Ginzburg-Higgs equation
Mo Jia-Qi, Lin Yi-Hua, Wang Hui
2005, 54 (12): 5581-5584. doi: 10.7498/aps.54.5581
Abstract +
Using the perturbed method, a class of generalized Landau-Ginzburg-Higgs equation is studied. Introducing a homotopic mapping, the solution of original equation is expressed as an asymptotic expansion, then it is expressed approximately by corresponding solutions of linear equations. Finally, the solution of equation in relation to the obtained approximate solution is considered.
Hyperchaos synchronization and control using intermittent feedback
Mo Xiao-Hua, Zhang Ping-Wei, Liao Gao-Hua, Ma Jun, Li Wei-Xue
2005, 54 (12): 5585-5590. doi: 10.7498/aps.54.5585
Abstract +
Based on the stability theory,intermittent feedback scheme is proposed to synchronize and control the four-dimensional continuous hyperchaotic Rssler and LC systems. The systems are directed to reach arbitrary desired target within about 8 to 35 time units and realize complete synchronization within about 15 to 50 time units under this scheme by selecting right autonomous period Ta, controlling period Ts and gain coefficient k.The advantage of this scheme is that the controller is designed analytically in detail,not only relying on the common numerical simulation.The controller works only on certain period and the system develops freely in other periods. The numerical simulation results confirmed its effectiveness and gave evidence that shorter transient period before reaching desired target requires bigger feedback coefficient k,and it is consistent with the theoretical analysis.
Tracking control and synchronization of the R?ssler's chaotic system
Wang Xing-Yuan, Shi Qi-Jiang
2005, 54 (12): 5591-5596. doi: 10.7498/aps.54.5591
Abstract +
This paper presents a strategy of the Rssler's chaotic system tracking control based on the theory of stability of the linear system. This strategy can track all kinds of reference signals. It is also proved that the strategy can make the system approach to any desired smooth orbit at an exponential rate. Numerical simulations have shown the proposed strategy can not only trace the given reference signals, but also synchronize structures, including self structures and diverse ones.
Cellular automaton model of traffic flow based on safety driving
Mou Yong-Biao, Zhong Cheng-Wen
2005, 54 (12): 5597-5601. doi: 10.7498/aps.54.5597
Abstract +
Considering the condition of having a stationary vehicle ahead, a new one-dimensional cellular automaton model emphasizing high safety is proposed to reduce the probability of rear-end collision in the Nagel-Schreckenberg model. The simulation results reveal the presence of metastable state, nonequilibrium phase transition and hysteresis effects, which have been observed in real traffic. Due to the introduction of the safety probability, traffic flow near the critical density will turn to a synchronized flow at low speed, rather than the jam state. So the probability of rear-end collision will be reduced and the traffic capacity may be increased at high traffic density.
Spectrum analysis on dissipative motion in finite systems
Yuan Chang-Qing, Zhao Tong-Jun, Wang Yong-Hong, Zhan Yong
2005, 54 (12): 5602-5608. doi: 10.7498/aps.54.5602
Abstract +
Dynamical problems about energy dissipation in a finite multi-degree of freedom Hamiltonian system are studied. The energy dissipation in the relevant system is realized by numerical simulation. By employing power spectrum analysis on q(t) or p(t) of the motion in the relevant system, it is found that during the dissipative process, distribution of the power spectral density about the motion extends to both lower and higher frequencies. The more irrelevant degrees of freedom are, the broader the distribution extends. Higher frequencies decrease as time evolves but there are no remarkable changes for lower frequency components.
Fabrication process and power and lifetime characteristics of very-small-aperture laser
Xu Jun, Kang Xiang-Ning, Qu Xin, Fang Pei-Yuan, Song Guo-Feng, Chen Liang-Hui, Gan Qiao-Qiang, Gao Jian-Xia, Xu Yun, Zhong Yuan, Yang Guo-Hua, Cao Qing
2005, 54 (12): 5609-5613. doi: 10.7498/aps.54.5609
Abstract +
High output power very-small-aperture laser has been created on 650nm edge emitting laser diodes.The far-field output power is 0.4mW at the 25mA driving current,and the highest output power exceeds 1mW. The special fabrication process is described and the failure mechanism leading to the short lifetime of the devices is discussed.
Structure and potential energy function of the ground state (X2Π) of OH
Fan Xiao-Wei, Geng Zhen-Duo, Zhang Yan-Song
2005, 54 (12): 5614-5617. doi: 10.7498/aps.54.5614
Abstract +
The equilibrium structure, harmonic frequency and dissociation energy of the ground state of OH are calculated using density functional theory (B3LYP) method and quadratic CI method including single, double and triple substitutions (QCISD(T)). Based on the theory of atomic and molecular statics, the reasonable dissociation limit for the ground state (X2Π) of OH is derived. The potential energy curve and relevant optical constants of this state are obtained by least square fitting to the Murrell-Sorbie function. All calculation results are in good agreement with the experimental data.
Spin-orbit ab initio study of the S2(B3Σ-u→X3Σ-g(ν′≥18,0)) diffuse bands
Yu Jun-Hua, Yan Bing, Pan Shou-Fu, Wang Zhi-Gang
2005, 54 (12): 5618-5621. doi: 10.7498/aps.54.5618
Abstract +
The diffuse bands B3Σ-u→X3Σ-g(ν′≥18,0) of diatomic molecule S2 observed in the experiment are investigated. The electronic potential curves,including the spin-orbit coupling (SOC) effect of B3Σ-u and repulsive 15Πu, 23Σ+u states are calculated. For the diffuse bands beginning at (18,0), a point of view different from others' results is presented in this work. Our results indicate that the SOC induced predissociation between B3Σ-u and 15Πu, 23Σ+u plays the key role in the diffusion of spectra. Comparison with experimental results shows good agreement.
Propagation of matter waves through the joint between two atomic waveguides
Zhang Wei-Ping, Zhou Zhao-Ying, Feng Yan-Ying, Zhu Rong, Duan Zheng-Lu, Li Shi-Qun
2005, 54 (12): 5622-5628. doi: 10.7498/aps.54.5622
Abstract +
In this paper, the propagation properties of matter waves through the joint between two atomic waveguides are investigated. A multimode coupling model is developed to describe the excitation of the matter waves through the waveguides. We analyze the dependence of the reflection and excitation on the parameters of the atomic waveguides. The condition to minimize the excitation of the atomic waves is given.
Research on parameters of higher-order transverse magnetic modes in cylindrical coaxial cavity resonator
Ding Yao-Gen, Xiao Liu, Dong Yu-He
2005, 54 (12): 5629-5636. doi: 10.7498/aps.54.5629
Abstract +
The inter-related parameters of higher order transverse magnetic (TM) modes in coaxial cavity are calculated and analyzed. It is found that TMn10 modes have large mode intervals. There exists a maximum characteristic impedance while keeping the specific cavity outer radius constant. Large cross-section cavity can be employed in higher frequency range. The solutions show good agreement with the results simulated by ISFEL 3-demention and HFSS, softwares used for electromagnetic simulation. The results calculated can serve as a database from which the optimum seeking in design can be gotten instantly instead of the time-consuming simulation by trial and error.
Influence of upconversion effect on Er3+, Yb3+co-doped phosphate glass lasers
Wu Zhao-Hui, Song Feng, Liu Shu-Jing, Qin Bin, Su Jing, Tian Jian-Guo, Zhang Guang-Yin
2005, 54 (12): 5637-5641. doi: 10.7498/aps.54.5637
Abstract +
An analytical model of cw longitudinally diode-pumped Er3+,Yb3+ co-doped phosphate glass lasers that includes the influence of cooperative upconversion and cumulative energy transfer effects has been developed. The results of general output modeling were applied to a laser with Gaussian beams, and numerical calculation has been made to study the influence of cooperative upconversion and cumulative energy transfer effects, where the dependence of laser output on the influence of cooperative upconversion and cumulative energy transfer effects was investigated in particular. Especially, we find that the influence of cooperative upconversion on the threshold can be up to 22%. The model is in good agreement with measurements of the laser in a plane-plane cavity. Further numerical analysis will be developed.Our results are useful for optimizing the parameters of this kind of lasers.
The far-field properties of ultrashort pulsed beams with constant waist width in free space
Zou Qi-Hui, Lü Bai-Da
2005, 54 (12): 5642-5647. doi: 10.7498/aps.54.5642
Abstract +
By using the angular spectrum and complex analytic signal representations, the analytic propagation expression and its Fourier spectrum for ultrashort Gaussian pulsed beams with constant waist width in the far field are derived. The far-field properties of few- and single-cycle ultrashort Gaussian pulsed beams with constant waist width are studied, and compared with those of ultrashort Gaussian pulsed beams with constant diffraction length. The properties of pulse broadening, spectrum narrowing, off-axis spectrum redshift and on-axis spectrum blueshift are found for ultrashort Gaussian pulsed beams with constant waist width in the far field.
Experimental and theoretical study of heat generation in Ti:sapphire end-pumped Nd:YAG ceramic lasers
Zhu Hong-Tao, Lou Qi-Hong, Qi Yun-Feng, Ma Hai-Xia, Dong Jing-Xing, Wei Yun-Rong
2005, 54 (12): 5648-5653. doi: 10.7498/aps.54.5648
Abstract +
The heat generation in Nd:YAG ceramic lasers under the conditions of laser extraction was investigated through the heat generation coefficient, which was defined as the ratio of heat generation power to laser output power. The heat generation coefficient of the 1.0% neodymium-doped YAG ceramic laser was calculated to be 0.63. The main factors which influences the heat generation coefficient were discussed in this paper, and the results showed that the heat generation coefficient was very sensitive to the changes in quantum emission efficiency, overlap efficiency and laser extraction efficiency. This work provides a reference for further research of thermal effect of Nd:YAG ceramic lasers.
Influences of optical breakdown on stimulated Brillouin scattering
Lü Zhi-Wei, He Wei-Ming, Li Qiang, Ba De-Xin, Hasi Wu-Li-Ji
2005, 54 (12): 5654-5658. doi: 10.7498/aps.54.5654
Abstract +
Avalanche ionization model is introduced to the coupled stimulated Brillouin scattering (SBS) equations. Numerical simulation of SBS energy reflectivity as a function of pump intensity is made with and without the optical breakdown factor in the equations. In the experiment, purified and unpurified CCl4 media are studied with an Nd:YAG laser system with Q switch, the results are found to agree well with the numerical solutions. It is observed that under conditions that the optical breakdown does not occur, the energy reflectivity increases nonlinearly with pump intensity until reaching a certain value above which it tends to grow slowly. In cases with the breakdown, the energy reflectivity first increases nonlinearly with the pump intensity to a peak value and then decreases dramatically as optical breakdown occurs. The stability of energy reflectivity in the cases without optical breakdown is obviously better than cases otherwise.
Fabrication of straight waveguide in two-dimensional photonic crystal slab and its light propagation characteristics
Han Shou-Zhen, Tian Jie, Feng Shuai, Ren Cheng, Li Zhi-Yuan, Cheng Bing-Ying, Zhang Dao-Zhong
2005, 54 (12): 5659-5662. doi: 10.7498/aps.54.5659
Abstract +
Using focused ion beam lithograph, we fabricated W3 straight waveguide in two-dimensional photonic crystal slab on SOI (silicon on insulator) wafer. The light propagation was observed on top and from the end of the sample. The transmission dip and mode distribution at the outlet of ridged waveguide were measured. The results show that light can propagate in the photonic crystal waveguide.
Y-splitting of dark stripe in photoisomerization polymers
Li Yang-Gang, Wang Xiao-Sheng, She Wei-Long, Jiang De-Sheng
2005, 54 (12): 5663-5670. doi: 10.7498/aps.54.5663
Abstract +
In photoisomerization polymer,the evolvement and some related properties of the dark stripe,in which the light field has a phase jump on both sides of the summetrical axis,are simulated by numerical method.The numerical results show that if the phase jump differs from π,the incident dark stripe will split into two grey stripes called Y-splitting, and the larger the phase jump (from 0 to π),the smaller the Y-splitting.The numerical results also show that the angle between the phase-lead-branch and the direction of incidence is almost a constant for different phase jumps,while that of the phase-lag-branch varies with the phase jump.The angle between the two branches is unchanged under the condition of the same phase jump and different incident intensity.Besides,the numerical results also show that the refractive index change induced by Y-splitting in the material can act as a wave-guide,which leads an incident Gauss beam to form a Y-splitting.
Unilateral coupling synchronization of spatiotemporal chaos in the Bragg acousto-optic bistable system
Shen Ke, Yue Li-Juan
2005, 54 (12): 5671-5676. doi: 10.7498/aps.54.5671
Abstract +
The one- and two-dimensional coupled map lattices in nonlinear dynamic system are used as the system model. We report a method with which the spatiotemporal chaos can be synchronized using unilateral coupling in two Bragg acousto-optic bistable spatial extended systems. The synchronization can be realized by appropriately selecting the coupling strength and the equilibrium coefficient. By calculating the largest conditional Lyapunov exponent, we obtain the minimum coupling strength for achieving the synchronization and the functional relationship between the minimum coupling strength and the system parameters. Our simulation shows that the synchronization can also be realized under the influence of small random noise, so this method is robust.
Absorbing boundary conditions for two-dimensional photonic crystal waveguides
Zhang Bo
2005, 54 (12): 5677-5682. doi: 10.7498/aps.54.5677
Abstract +
We designed a distributed Bragg reflector waveguide for terminating the square lattice dielectric cylinder photonic crystal waveguide and evaluated the reflection of their interface numerically. The reflection coefficient was reduced to below 1% in a large part of the frequency range of the guided mode.
Reflection and negative refraction of left-handed metamaterials at microwave frequencies
Zheng Qing, Zhao Xiao-Peng, Fu Quan-Hong, Zhao Qian, Kang Lei, Li Ming-Ming
2005, 54 (12): 5683-5687. doi: 10.7498/aps.54.5683
Abstract +
We experimentally investigated the microwave reflection of the negative permeability material composed of periodical arrays of the copper split-ring resonators (SRRs).The negative dielectric materials are composed of periodical arrays of copper wires and left-handed materials(LHMs) in a planar slab waveguide, and the microwave is incident to the surface in different directions.We also investigated the negative refraction through a prism fabricated from the LHMs.The experimental results show that for the negative permeability materials,there is a reflection peak at the resonance frequencies of the SRRs, and the reflectivity of the negative dielectric material is closed to 0dB.A single smaller reflection peak occurs in the reflected curve of the LHMs,the reflectivity of which increases with the incident direction,that is,the reflection increases.The reflection peak has a shift with respect to the transmission peak. It is demonstrated that negative refractive index of -0.796 occurs in the LHMs at the frequency o 800MHz.
Talbot effect of a grating under chirped ultrashort pulsed laser illumination
Wang Huai-Sheng
2005, 54 (12): 5688-5691. doi: 10.7498/aps.54.5688
Abstract +
The Fresnel diffraction equation of a grating under chirped ultrashort pulsed laser illumination is given through Fourier spectrum analysis, and the Talbot effect of the grating is studied using the equation. Numerical calculation shows that the intensity distribution of the grating at Talbot distances is related to both the width and the chirp parameter of the ultrashort pulse. According to the intensity change of the grating, a simple method is put forward to check whether an ultrashort pulsed laser contains chirp information.
Period doubling motion in vertically vibrated granular beds
Jiang Ze-Hui, Liu Xin-Ying, Peng Ya-Jing, Li Jian-Wei
2005, 54 (12): 5692-5698. doi: 10.7498/aps.54.5692
Abstract +
Experiments are performed to investigate the impact interaction between granular bed and container bottom, where the container is vertically vibrated. It is observed that the impact strength, controlled by the normalized vibration acceleration, undergoes a series of period doubling bifurcations, typically in the sequence of period-doubling, period-quadrupling, chaos, period-tripling, period-sextupling, chaos, period-quadrupling, period-octupling, and chaos. A condensed state of particles at the bottom of the container is observed, in which the particles are densely compacted and move as a bulk block in the vertical direction. Based on a completely inelastic bouncing ball model, a possible explanation for the bifurcation phenomena is presented.
Second-order Stokes solutions for internal waves in three-layer density-stratified fluid
Song Jin-Bao, Sun Qun, Chen Xiao-Gang
2005, 54 (12): 5699-5706. doi: 10.7498/aps.54.5699
Abstract +
In this paper, internal waves in three-layer stratified fluid are investigated by using a perturbation method, and the second-order asymptotic solutions of the velocity potentials and the second-order Stokes solutions of the associated elevations of the interfacial waves are presented based on the small amplitude wave theory. As expected, the first-order solutions are consistent with ordinary linear theoretical results, and the second-order solutions describe the second-order modification on the linear theory and the interactions between the two interfacial waves. Both the first-order and second-order solutions derived depend on the depths and densities of the three-layer fluid. It is also noted that the solutions obtained from the present work include the theoretical results derived by Umeyama as special cases.
Structural transition of Fe80Cu20 alloy induced by annealing
Yin Shi-Long, Yan Wen-Sheng, Fan Jiang-Wei, Pan Zhi-Yun, Wei Shi-Qiang, Hao Lu-Yuan, Li Yu-Zhi, Liu Wen-Han
2005, 54 (12): 5707-5712. doi: 10.7498/aps.54.5707
Abstract +
The extended x-ray absorption fine structure (EXAFS) and x-ray diffraction (XRD) techniques have been used to study the dependence of structural changes on annealing temperature for metastable body centered cubic (bcc) Fe80Cu20 alloy synthesized by mechanical alloying. In the temperature range from 300 to 873 K, the lattice constant of the bcc-structured phase exhibits a nearly linear decrease with the increase of annealing temperature. This is mainly due to the segregation of Cu atoms from the bcc Fe80Cu20 alloy to form the face centered cubic (fcc) Cu phase. Annealing at 603 K results in an increase of the average Cu—Cu distance RCu—Cu by about 0.003 nm, and about 50% Cu atoms are segregated from the bcc Fe80Cu20 alloy. After annealing at 773K, two phases of bcc α-Fe and fcc-Cu are almost completely separated from the as-milled bcc Fe80Cu20 alloy.
Structural, transport and magnetic properties of KxCoO2·yH2O(x<0.2, y≤0.8)
Li Ming-Xing, Fu Guang-Cai, Dong Cheng, Guo Juan, Yang Li-Hong
2005, 54 (12): 5713-5716. doi: 10.7498/aps.54.5713
Abstract +
Layered potassium cobaltate K0.36CoO2 has been successfully synthesized from KOH fluxes at 480℃. The K0.36CoO2 sample can be oxidized and intercalated with water by treatment in KMnO4 and K2S2O8 solutions. K0.12CoO2·0.8H2O and K0.16CoO·0.6H2O have been obtained after the KMnO4 and K2S2O8 treatment, respectively. The diffraction peaks of K0.12CoO2·0.8H2O and K0.16CoO2·0.6H2O can be well indexed by a hexagonal cell similar to the monolayer hydrate NaxCoO2·yH2O. After dehydration, the major phases have an orthorhombic structure similar to Na0.5CoO2 and show semiconductor behavior. Both K0.12CoO2·0.8H2O and K0.16CoO2·0.6H2O are primari paramagnetic and show metallic behavior. K0.16CoO2·0.6H2O has a spin-glass-like transition or other magnetic fluctuations around 56K. The spin-glass-like transition or the regions of magnetic phase separation are reduced in K0.12CoO2·0.8H2O due to the increasing of the intercalated water. We also discussed similarities and differences between the structural and physical properties of KxCoO2 and NaxCoO2.
Surface analysis of LiBq4/ITO and LiBq4/CuPc/ITO using atomic force microscopy and x-ray photoelectron spectroscopy
Song Zhen, Ou Gu-Ping, Gui Wen-Ming, Zhang Fu-Jia
2005, 54 (12): 5717-5722. doi: 10.7498/aps.54.5717
Abstract +
We have investigated the surfaces of the samples LiBq4/ITO and LiBq4/CuPc/ITO. The atomic force microscopy (AFM) observations indicate that different surface morphologies are formed on different substrates, and what is more, x-ray photoelectron spectroscopy is also utilized to further demonstrate the AFM results. It is concluded that the introduction of a CuPc buffer layer under the LiBq4 layer can improve the film quality of LiBq4, and the improvement should be attributed to the differences in molecular structure and electron affinity.
Simulation study of effects of cooling rate on microstructure of liquid metal Na during solidification processes
Hou Zhao-Yang, Liu Rang-Su, Li Chen-Shan, Zhou Qun-Yi, Zheng Cai-Xing
2005, 54 (12): 5723-5729. doi: 10.7498/aps.54.5723
Abstract +
A tracing simulation study has been performed for the solidification process of liquid metal Ni with four different cooling rates by means of molecular dynamics method. The pair distribution function g(r) curves, the bond-type index method of Honeycutt-Andersen (HA) and the cluster-type index method have been used to analyze the variations of microstructures during the solidification processes. The results show that the cooling rate plays a critical role in the transitions of microstructures. When the cooling rates are 1.0×1014 K/s and 1.0×1013 K/s, the amorphous structures are formed mainly with the 1551 and 1541 bond-types or polyhedron basic cluster (13 1 10 2) and icosahedron cluster (12 0 12 0) in the system. When the cooling rates are 1.0×1012 K/s and 1.0×1011 K/s, the crystal structures are formed mainly with 1441 and 1661 bond-types or the bcc basic cluster (14 6 0 8) in the system. At the same time, it has been found that the effects of different cooing rates on the microstructures of metal Na are very small in liquid and supercooled states, however, they are very remarkable in solid (amorphous and crystal) states, and can be fully displayed only near the liquid-solid transition points,i.e., the glass transition temperature Tg and the crystallization temperature Tc, respectively. Accordingly,it possibly provides a new method to determine the Tg and Tc of liquid metals.The cluster-type index method would be more favorable than the bond-type index method for investigating the concrete structural characteristics of the disordering liquid, the amorphous and some crystallized systems.
Theoretical study on the influence of the microstructure of Fe73.5Cu1Nb3Si13.5B9 on its giant magneto-impedance effect
Wang Ling-Ling, Sun De-Cheng, Yang Quan-Min
2005, 54 (12): 5730-5737. doi: 10.7498/aps.54.5730
Abstract +
Based on the experimental results of atomic force microscope observation of the microstructure and the giant magneto-impedance (GMI) effect of Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy, we propose a model for exploring the influence of the microstructure on the GMI effect in this kind of Fe-based nanocrystalline soft magntic alloy which explains successfully the GMI behavior at low frequency. The model possesses the main features of the sandwiched model while eliminating its deficiencies. This model is called the mesoscopic model. We also point out the influence of the electrical conductivity σ and the permeability μ of nanocrystal grains on the GMI effect of the Fe-based nanocrystalline alloys.
The laser ablated deposition of Si nanocrystalline film with narrow photoluminescence peak
Peng Ying-Cai, Wang Ying-Long, Lu Li-Fang, Yan Chang-Yu, Chu Li-Zhi, Zhou Yang, Fu Guang-Sheng
2005, 54 (12): 5738-5742. doi: 10.7498/aps.54.5738
Abstract +
The single crystalline Si target with high resistivity was ablated by a XeCl excimer laser (laser fluence 4J/cm2, repetition rate 1Hz), and at the ambient pressure of 10Pa of pure Ar gas, the nanocrystalline silicon film was deposited on a glass or single crystalline (111) Si substrate located at a distance of 3cm from the Si target in 30 and 10min, respectively. The Raman and x-ray diffraction spectra of the film deposited on the glass substrate indicate the film is nanocrystalline, which means that it is composed of Si nanoparticles. Scanning electron microscopy of the film on the Si substrate shows that the film has the mosaic structure of Si nano-crystallites of uniform size. The photoluminescence peak wavelength is 599nm with full width at half maximum of 56nm, which is blue-shifted and narrower than that obtained in He gas.
Effect of current density on physical and chemical properties of microarc oxidation coatings of aluminium alloy
Bai Yi-Zhen, Wang Jian-Bo, Lü Xian-Yi, Long Bei-Hong, Jin Zeng-Sun, Long Bei-Yu, Bi Dong-Mei, Wu Han-Hua
2005, 54 (12): 5743-5749. doi: 10.7498/aps.54.5743
Abstract +
Ultra-hard ceramic coatings with microhardness of 42.14GPa have been synthesized on the Al alloy substrate by microarc oxidation (MAO) technique. The effects of anodic current density ja and the ratio of cathodic to anodic current density jc/ja on the mechanical property and phase composition of MAO coatings have been studied by microhardness test and x-ray diffraction, respectively. In addition, the microstructure and corrosion resistance of the coatings were analyzed by scanning electron microscopy and pitting corrosion test, respectively. The results show that the samples prepared at high anodic current density consist mainly of α-Al2O3, while the samples fabricated at low anodic current density are composed almost entirely of γ-Al2O3. Microhardness test shows that these kinds of samples have high hardness, and the microhardness of the coating prepared at ja=15A/dm2 and jc/ja=0.7 is t highest of all coatings prepared. Pitting corrosion test shows that the structure of coatings is strongly influenced by the variation in jc/ja.
Shear modulus of aluminum under shock loading
Hu Jian-Bo, Yu Yu-Ying, Dai Cheng-Da, Tan Hua
2005, 54 (12): 5750-5754. doi: 10.7498/aps.54.5750
Abstract +
Steinberg-Cochran-Guinan (SCG) model, the corrected SCG model and the finite strain theory were reviewed with respect to the basic assumptions and applicability, and the shear modulus data of aluminum under shock compression predicted by these models were compared with the available data obtained in one-dimensional plate impact experiments. The comparison shows that the corrected SCG model is the best one,being capable of describing the variation of the measured shear modulus of aluminum with shock pressure, although the other two are applicable at lower pressure. It is found that the shear modulus increases gradually with the increase of shock pressure in the range of 10—80GPa due to the work-hardening effects. As the shock pressure goes above ~80GPa, the shear modulus of aluminum falls quickly due to the dominating effect of high-temperature softening, and when the shock pressure reaches ~125GPa, corresponding to the onset of melting on Hugoniot, the shear modulus of aluminum decreases to zero.
Investigation of phase instability in the binary Gaussian core model
Guo Yuan-Yuan, Chen Xiao-Song
2005, 54 (12): 5755-5762. doi: 10.7498/aps.54.5755
Abstract +
By considering the fluctuation of grand potential Ω around equilibrium with respect to small density fluctuations δρα(r), the phase instability of binary Gaussian core model has been investigated and characterized. The spinodal of this model is obtained by using its direct correlation functions calculated in the hypernetted chain(HNC) approximation. Our analysis shows that this phase transition is predominately demixing. The results in HNC approximation have been compared with that in the random phase approximation.
Effect of grain size on thermoelectric properties of CoSb3 compound
Yu Bo-Lin, Qi Qiong, Tang Xin-Feng, Zhang Qing-Jie
2005, 54 (12): 5763-5768. doi: 10.7498/aps.54.5763
Abstract +
The effects of grain size on thermoelectric properties of CoSb3 compound was investigated systematically. The results indicate that the lattice thermal conductivity κp of CoSb3 compound decreases significantly, the Seebeck coefficient α increases remarkably, and the band gap Eg increases and electrical conductivity σ decreases, when the grain size decrease from μm-scale to nano-scale. The dimensionless thermoelectric figure of merit ZT of CoSb3 compound with grain size of 200nm reaches 0.43 at 700K, about 4 times that of sample with grain size of 5000nm.
Theoretical study on the transport in metal/organic bi-layer thin films
Jiang Yi-Ming, Xie Heng-Bo, Guo Feng, Liu Ping, Li Jin
2005, 54 (12): 5769-5773. doi: 10.7498/aps.54.5769
Abstract +
Ag/TCNQ (7,7,8,8-tetracyanoquinodimethane) bi-layer films with different thickness were prepared by physical vapor deposition method. The metal-organic complex Ag-TCNQ is formed through transport. The transport behavior in the film was studied using transmission spectrum to monitor the process. Tunneling model and non-tunneling model were established respectively based on whether the tunneling condition was met. Theoretical analysis was carried out in detail. The results of our theoretical models are in good agreement with the experiment data.
Theory and experiment of surface thermal lens technique used in absorption measurement of thin films
He Hong-Bo, Fan Zheng-Xiu, Shao Jian-Da, Fan Shu-Hai, Zhao Yuan-An
2005, 54 (12): 5774-5777. doi: 10.7498/aps.54.5774
Abstract +
The expression of surface thermal lens(STL) signal is deduced from photothermal deformation theory of thin film surface and STL diffraction theory. The linearity of STL signal to absorption of thin film can be seen from the expression. A measurement apparatus for thin film absorption based on STL technique is constructed. The measuring results prove that the sensitivity and accuracy of this instrument reach 10-6 magnitude.
The dipolar interaction model of the molecular aggregation in Langmuir-Blodgett films
Han Kui, Li Hai-Peng, Wu Yu-Xi, Shen Xiao-Peng, Huang Zhi-Min
2005, 54 (12): 5778-5783. doi: 10.7498/aps.54.5778
Abstract +
Based on the classical electrostatics theory, the model of dipole-dipole interaction in the aggregation of rod-like amphiphilic molecules in Langmuir-Blodgett(LB)films has been presented in the paper. The relations between LB films structure and spectral absorption peak were obtained by using this model. For Z-type films, the dependence of molecular aggregation state on the structure parameters, i.e. separation distance a, separation between two adjacent layers d, molecular orientation θ, and layer number g, were studied by applying our model. The theoretical results agree well with experimental data.
Adsorption geometry of C2H4 and C2H on Ni(110) surface
Li Bo, Bao Shi-Ning, Cao Pei-Lin
2005, 54 (12): 5784-5790. doi: 10.7498/aps.54.5784
Abstract +
The favorite adsorption sites of ethylene (C2H4) and acetylide (CCH) on the Ni(110) surface are investigated by the theoretical calculation of the pseudo-potential plane-wave method. At low coverage the ethylene molecule occupies approximately the midway between the short-bridge and atop sites. The C-C axes of molecules are almost parallel to the ridges of the Ni atoms. The tilt angle of C-C axis relative to the Ni surface is about 12°. The C—C bond length is 0.147nm. At high coverage(0.5ML), ethylene molecules form a c(2×4) phase. The local adsorption sites of two ethylene molecules in a surface unit mesh are found to be quite similar to that at low coverage, but the C—C bond lengths are shorter than that in the case of the low coverage. The adsorption site of cetylide, decomposed from ethylene, is similar to that of the ethylene molecule on the surface, but there is a small displacement towards the atop site, the C—C bond length is 0.131nm, and the hydrogen atom bonds with the carbon a an atop site, the tilt angle of C-H axis relative to the Ni surface is about 45°.
Study on the influence of substitutional impurity on the stability of noble metal (111) surfaces by molecular dynamics simulation
Zhang Chao, Tang Xin, Wang Yong-Liang, Zhang Qing-Yu
2005, 54 (12): 5791-5796. doi: 10.7498/aps.54.5791
Abstract +
The stability of noble metal(111) surfaces doped with substitutional impurities were studied by molecular dynamics simulation. The atomic interaction potentials with the embedded atom method were applied in the simulation. The change of surface energy and the formation energy of surface vacancy induced by substitutional impurity were calculated. We found that the main factors effecting the surface energy of noble metal(111) surfaces are the cohesive energy and atomic radius of the substitutional impurity. The heat of solution of the binary alloy also has an important role in the influence of substitutional impurity on the formation energy of surface vacancy, along with the cohesive energy and atomic radius of substitutional impurity. Furthermore, the change of surface energy induced by substitutional impurity was used to predict the segregation of the binary alloy in the surface theoretically, yielding good agreement with the experimental results obtained by others.
Study on the influence of three-dimensional Cu(111) islands on the diffusion of adatom by molecular dynamics simulation
Tang Xin, Zhang Chao, Zhang Qing-Yu
2005, 54 (12): 5797-5803. doi: 10.7498/aps.54.5797
Abstract +
The adatom diffusion barriers on homoepitaxial three-dimensional (3D) Cu(111) islands with different numbers of monolayers have been studied by using molecular dynamics simulation combined with a molecular statics method. The parameter of atomic interaction potentials, which has been introduced in the embedded atom method, was applied in the simulation. The influence of 3D islands with different numbers of monolayers on the hopping diffusion and the exchange diffusion has been discussed in the paper. The results show that the two-dimensional (2D) Enrilich-Schwoebel (ES) barriers are lower than the 3D-ES barriers and the 3D-ES barrier is independent of the atomic layers in 3D islands. For the 3D island with (100) side-facet, the adatom diffusion along 〈011〉 will be affected by the number of monolayers in the 3D island. The adatom diffusion on the 3D islands is dominated by downward diffusion mechanism. For the 3D islands with the (111) side-facet, however, upward diffusion on the 3D islands is possible.
Study on the evolution of Au heteroepitaxial islands on Cu(001) by molecular dynamics simulation
Meng Yang, Zhang Qing-Yu
2005, 54 (12): 5804-5813. doi: 10.7498/aps.54.5804
Abstract +
Molecular dynamics simulation was used to relax the heteroepitaxial Au islands in monolayer on Cu(001) substrate in the initial stage of film growth. The evolution of the morphology and local pressure of the heteroepitaxial Au islands as well as the adhesive energy to the substrate have been analyzed. The simulation results show that a pseudo-morphologically strained Au island is formed when the heteroepitaxial Au island is not larger than 7×7. When the heteroepitaxial Au island is as large as 8×8, however, a misfit dislocation induced by the stress, which results from the mismatch in the interface, appears. The number of the misfit dislocations increases with increasing scale of the heteroepitaxial Au island, and moreover, crossing dislocations and vacancies can be observed in large heteroepitaxial Au islands. The analysis results of the local pressure map show that the difference of the neighborhood results in the difference of the stress of the atoms in the heteroepitaxial Au islands. Furthermore, the psudo-morphologically strained Au islands are the results of the different local pressure of Au atoms in the heteroepitaxial Au islands. On the other hand, we found that the misfit dislocations lead to the decrease of the adhesive energy of the mismatched atoms to the substrate, and the adhesion of other atoms increases relatively.
Theoretical research on critical thickness of HgCdTe epitaxial layers
Wang Qing-Xue, Yang Jian-Rong, Wei Yan-Feng
2005, 54 (12): 5814-5819. doi: 10.7498/aps.54.5814
Abstract +
Based on the relationships between stress and strain in an arbitrary coordinate system, the elastic theory of crystal and the dislocation gliding theory, the critical thicknesses of HgCdTe/CdZnTe oriented in the [111] and [211] directions are calculated, and the dependence of the critical thickness of HgCdTe on substrate composition and film composition are studied. The results show that the critical thickness of HgCdTe depends sensitively on substrate composition and film composition. For 10μm films oriented in the [111] direction prepared by liquid phase epitaxy, the substrate composition and the films composition must match to within ±0.225‰ and ±5‰, respectively, to prevent the occurence of misfit dislocations. In addition, for 10μm films oriented in the [211] direction prepared by molecular beam epitaxy, the actual ranges of zinc composition and the films composition are ±0.2‰ and ±4‰ for the films to remain below the critical thickness, respectively.
Pulsed laser deposition of epitaxial CeO2 thin films on biaxially textured Ni substrate
Wang Shu-Fang, Liu Zhen, Zhao Song-Qing, Zhou Yue-Liang
2005, 54 (12): 5820-5823. doi: 10.7498/aps.54.5820
Abstract +
High-quality epitaxial CeO2 thin films were fabricated on the biaxially textured Ni substrates by pulsed laser deposition using a gas mixture of 4%H2 and 96%Ar. X-ray diffraction θ—2θ scan and ω-scan indicate the CeO2 thin films are c-axis textured, and φ-scan and pole figure of the films reveal the presence of good in-plane texture. The biaxially textured CeO2 films provide a good template for the fabrication of YBCO conductors.
First principle study on β-SiC(001)-(2×1) surface structure
Xu Peng-Shou, Li Yong-Hua, Pan Hai-Bin
2005, 54 (12): 5824-5829. doi: 10.7498/aps.54.5824
Abstract +
We calculate the atomic and electronic structure of β-SiC(001)-(2×1) using augmented plane wave and local orbital (APW+LO) method. The calculated results show that the atomic structure of β-SiC(001)-(2×1) surface can be described by symmetrical Si dimmer model, which is different from the asymmetrical Si dimmer model of Si(001)-(2×1) surface. The bond length of Si dimmer of β-SiC(001)-(2×1) surface is 0.269 nm, which is larger than that of Si(001)-(2×1) surface. The calculated results of electronic structure show that a prominent density of states exists at the Fermi level, so β-SiC(001)-(2×1) surface has the characteristics of metal. There are four surface state bands in the gap. Among them, two occupied surface state bands have been confirmed by valence band spectra of synchrotron radiation photoelectron spectroscopy.
Molecular dynamics study on the structural and thermodynamic properties of the zinc-blende phase of GaN at high pressures and high temperatures
Sun Xiao-Wei, Chu Yan-Dong, Liu Zi-Jiang, Liu Yu-Xiao, Wang Cheng-Wei, Liu Wei-Min
2005, 54 (12): 5830-5836. doi: 10.7498/aps.54.5830
Abstract +
The structural and thermodynamic properties of zinc-blende phase of GaN have been investigated systematically using the molecular dynamics method with Buckingham potential model. These properties including lattice constant, phase transition pressure (from the zinc-blende to the rock-salt structure), thermal expansion, isothermal bulk modulus, and heat capacity at constant pressure were calculated in a wide range of temperatures (300—3000K) and pressures (0—65GPa). The structural and thermodynamic parameters obtained under normal state are found to be in good agreement with the existing experimental and theoretical results. Meanwhile, some thermodynamic parameters such as isothermal bulk modulus and heat capacity at constant pressure have been predicted at elevated temperatures and high pressures based on the reliable potential model. The simulated results are of fundamental importance in material science.
Near edge x-ray absorption fine structure studies of local structure of dihexyldisulfide multilayer
Tang Jing-Chang, Zhu Ping, Li Hai-Yang, Wu Tai-Quan
2005, 54 (12): 5837-5844. doi: 10.7498/aps.54.5837
Abstract +
The multiple-scattering cluster (MSC) method is employed to calculate the S 1s near edge x-ray absorption fine structure (NEXAFS) of single dihexyldisulfide molecule and dihexyldisulfide multilayer. The local structure model of the dihexyldisulfide multilayer is proposed. The MSC calculation shows that the neighboring dihexyldisulfide molecules are parallelly arranged in an ordered square cross section with side of 0.47 nm. The discrete variational Xα method is employed to calculate the electronic structure of the single molecule and multilayer, which confirms the MSC results, and reveals the physical origin of all the features in the S 1s NEXAFS spectrum. The analysis of the interaction between the dihexyldisulfide molecules shows that the local structure of the multilayer has the property of self-assemblage.
Correlation between optical spectrum, electron paramagnetic resonance spectrum and local structure of MnF2:Mn2+
Xie Lin-Hua, Qiu Min
2005, 54 (12): 5845-5848. doi: 10.7498/aps.54.5845
Abstract +
The optical luminescence spetrum and electron paramagnetic resonance hyperfine constant of MnF2:Mn2+ are given a unified explaination on the basis of semi-self consistent field d-orbit theory and a new method of determination of bond length from hyperfine constant. The bond length of Mn—F in MnF2:Mn2+ crystal is determined as 0.2124±0.0010nm.
Magnetism of 3d transition metal monolayers on Pd(001) surface: density functional theory study
Zhao Xin-Xin, Tao Xiang-Ming, Chen Wen-Bin, Cai Jian-Qiu, Tan Ming-Qiu
2005, 54 (12): 5849-5854. doi: 10.7498/aps.54.5849
Abstract +
In this article we present a report on the total energy calculations for the series of 3d transition metals as monolayers on Pd(001) surface using the density functional theory simulation. Analysis of total energy and magnetic moment shows that the first four atoms, i.e., Sc, Ti, V and Cr, favor p(1×1) ferro-magnetic configuration. Manganese prefers c(2×2) anti-ferromagnetic configuration on this surface. For the next three 3d transition metals, iron, cobalt and nickel, both p(1×1) (ferro-magnetic) and c(2×2) (antiferro-magnetic) configurations exist with a little difference in total energy. Copper and zinc monolayers do not show any magnetic ordering on the Pd(001) surface. Magnetic moment of vanadium in p(1×1) configuration is 2.41μB per atom, which is in contrast to the previously reported 0.51μB using full-potential linearized augmented-plane-wave method. For the rest 3d atoms, the present results are very close to the available research results both quantitatively and qualitatively.
The effect of size distribution on photoluminescence of excited states from InAs/GaAs quantum dots
Tang Nai-Yun, Chen Xiao-Shuang, Lu Wei
2005, 54 (12): 5855-5860. doi: 10.7498/aps.54.5855
Abstract +
We have used the effective mass approximation to calculate the effect of size non-uniformity on the width of photoluminescence (PL) emission peaks of the quantum dots (QDs). In order to investigate the variation in PL inhomogeneity of the energy levels, we have calculated the effects of small changes of the structural parameters on the energy spectrum. Theoretical calculations have shown that different size distributions effect differently on the width of the ground and excited states of QD. The distribution of height, as well as the diameter and the volume, appears to be the key parameter that controls the effective potentials in the vertical and lateral directions, and these two potentials change the sharpness of all PL peaks. This causes the line width of the higher energy levels to be either broader or sharper than that of the ground state, or in certain cases to be equal to the line width of the ground state.
Electron holography investigation on the barrier structures of Co based magnetic tunnel junctions
Zhang Zhe, Zhu Tao, Feng Yu-Qing, Zhang Ze
2005, 54 (12): 5861-5866. doi: 10.7498/aps.54.5861
Abstract +
The barrier potential profiles and microstructure of Co based magnetic tunnel junctions (MTJs) annealed up to 340℃ have been studied by electron holography (EH) and high resolution electron microscopy. The EH results reveal that the annealing process can well improve the quality of interfaces between barrier and ferromagnetic electrodes, and of AlOx barrier itself, which are responsible for the improvement of tunneling magnetoresistance in MTJs after anneal at 280℃.
Generation mechanism of stress induced leakage current in flash memory cell
Liu Hong-Xia, Zheng Xue-Feng, Hao Yue
2005, 54 (12): 5867-5871. doi: 10.7498/aps.54.5867
Abstract +
The generation mechanism of stress induced leakage current (SILC) in flash memory cell is studied by experiments. The result shows that the reliability problem under low electronic field stress is mainly caused by carriers charging and discharging inside the oxide, while under high electronic field, the trap-assisted tunneling and positive charge assisted tunneling induced charge variation of floating-gate is the major cause of flash memory cell failure. For both high and low electronic field stresses, the transient current and the steady-state current in SILC are calculated, respectively.
Modeling of resistance changes based on the free volume in VLSI interconnection electromigration
Zong Zhao-Xiang, Du Lei, He Liang, Zhuang Yi-Qi, Wu Yong
2005, 54 (12): 5872-5878. doi: 10.7498/aps.54.5872
Abstract +
A well-known model for the rate of heterogeneous nucleation and growth of a second-phase precipitate in a solid matrix has successfully been applied to analyze the electromigration process in Al thin-film interconnects. The change of resistance is considered resulting mainly from the scattering of electrons by atoms around the vacancies or voids at grain boundaries. Free volume is introduced to describe the behavior of these atoms and to simplify the complicated scattering process by means of the effective scattering cross section of the free volume. The quantitative relation between the scattering cross section and the resistance change is established, and from which the resistance revolution can be characterized during different electromigration stages. Comparison of our simulation results with experimental data shows good agreement and our model can explain the previously unexplained abrupt resistance change phenomena.
Magnetocrystalline anisotropy and spin-reorientation transition of intermetallic compound HoMn6Sn6
Guo Guang-Hua, Zhang Hai-Bei
2005, 54 (12): 5879-5883. doi: 10.7498/aps.54.5879
Abstract +
The molecular field theory of exchange interaction has been applied to investigate the spin-reorientation transition in intermetallic compound HoMn6Sn6. The theoretically calculated temperature dependence of easy magnetization direction is in good agreement with experimental data. The temperature dependence of magnetocrystalline anistropic constants of Ho ion K1R and K2R have been calcullated in the framework of single ion model. It is shown that the fourth-order crystal field parameter B04 and the corresponding second-order magnetocrystalline anistropic constant K2R of Ho ion have to be taken into account in order to explain the spin-reorientation transition. The competition between K2R and K1R plays an important role in the spin-reorientation transition in HoMn6Sn6.
Magnetic properties and magnetocaloric effects of Mn5Ge2.7M0.3 (M=Ga, Al, Sn) compounds
Shen Bao-Gen, Liu Xi-Bin
2005, 54 (12): 5884-5889. doi: 10.7498/aps.54.5884
Abstract +
The magnetic properties and magnetocaloric effects of Mn5Ge2.7M0.3(M=Ga, Al, Sn) compounds have been studied. All samples crystallize in the hexagonal Mn5Si3-type structure with space group P63/mcm. The average Mn magnetic moment decreases and the Curie temperature remains almost unchanged by the substitution of M for Ge. The magnetic entropy changes in these compounds are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation. The substitution of M for Ge reduces the magnitude of the magnetic entropy change owing to the decrease of the magnetic moment but broadens the magnetic entropy change peak. The maximum magnetic entropy changes for a field changes of 4.0×106A·m-1 are 6.1, 6.3, and 5.3J·kg-1K-1 for M=Ga, Al, and Sn, respectively. In addition, anomalies are found in the Arrott curves of Mn5Ge2.7M0.3(M=Ga Al, Sn) compounds under a critical field Hc when the temperature is higher than the Curie temperature TC, which indicates that the paramagnetic susceptibilities of these compounds change sharply. The value of Hc increases almost linearly with increasing temperature. This phenomenon is probably due to the change of Fermi energy by the applied magnetic field, which reduces the number of the effective charges.
Magnetization behavior and magnetic viscosity in nanocomposite Pr9Fe74Co12B5Snx(x=0, 0.5) ribbons
He Shu-Li, Zhang Hong-Wei, Chen Ren-Jie, Rong Chuan-Bing, Sun Ji-Rong, Shen Bao-Gen, Chen Yun-Zhong
2005, 54 (12): 5890-5894. doi: 10.7498/aps.54.5890
Abstract +
Nanocomposite Pr9Fe74Co12B5 and Pr9Fe74Co12B5Sn0.5 ribbons were directly prepared by melt-spinning. Measurements of reversible and irreversible magnetization and magnetic viscosity were performed on the ribbons. It is found that the demagnetization curves of the both samples show a single hard phase behavior at room temperature, while a two-phase behavior at low temperature. The sample with Sn addition, because of its more homogeneous microstructure, shows a more obvious two-phase behavior than the Sn free one at low temperature. Furthermore, the increase of the volume fraction and grain sizes of the soft phase, in the Sn-doped ribbons, increases the portions of reversible magnetization and decreases the nucleation field Hn. The investigation of the magnetic viscosity shows that the activation volume is related to the grain sizes of the soft phase.
Mechanism of low magnetic aging of Nd-Fe-B permanent magnet
Zhu Ming-Gang, Li Xiu-Mei, Guo Zhao-Hui, Li Wei
2005, 54 (12): 5895-5900. doi: 10.7498/aps.54.5895
Abstract +
The irreversible flux loss (FL) of sintered Nd-Fe-B magnets has been investigated after aging for about 920 d. Exposed at room temperature (RT) and 353K, the values of FL are about 1.4% and 13%, respectively. However, with a pre-aging the FL are about 6‰ and 6.9% for the magnets exposed at RT and 353K, respectively. Therefore, the stability of flux has been improved obviously by the pre-aging. A phenomenological formula, based on the Boltzmann statistics, has been used to discuss the effect of microstructure on FL. It is found that the experimental results can be well explained by the formula.
Fabrication and properties of the Y-doped Al2O3 high-k gate dielectric films
Geng Wei-Gang, Lan Wei, Huang Chun-Ming, Wang Yin-Yue, Guo De-Feng
2005, 54 (12): 5901-5906. doi: 10.7498/aps.54.5901
Abstract +
Y-doped Al2O3 dielectric films have been fabricated by reactive radio frequency co-sputtering method. Grazing angle incidence x-ray diffraction results show that the as-deposited and annealed films are amorphous. High resolution scanning electron microscope and atomic force microscope have been applied to observe the cross-section and the surface morphology of the thin films. The electric C-V and I-V characteristics were measured at high and variable frequency, respectively. It was found that the dielectric constant k of the films increases remarkably (from 8.14 to 11.8) with increasing Y-doping concentration. The Y—O bond is stronger than Al—O due to the obvious difference in electro-negativity between the two bond members, which enhanced the ionic polarization in the thin films leading to an increase of the dielectric constant. It was supposed that the presence of Y ions changed the structure and atomic coordination of Al2O3. The films were very smooth which meet the requirements of the device.
Temperature effect on the adsorption, diffusion and initial growth mode of ZnO/Al2O3(0001) from first principles
Li Yan-Rong, Yu Yi, Yang Chun, Liu Yong-Hua
2005, 54 (12): 5907-5913. doi: 10.7498/aps.54.5907
Abstract +
A heteroepitaxial growth model of the ZnO film on sapphire(0001) is simulated using the plane wave ultrasoft pseudo-potential method based on density functional theory. It is found that interfacial atoms have different diffusivity at 400,600 and 800℃. The temperature has a decisive effect on the surface and interface structures of ZnO/α-Al2O3(0001) and on the growth mode of ZnO thin films. In the whole process of the adsorption and growth of ZnO, the diffusivity of O atoms is higher than that of Zn, and the interlayer diffusion has an important role in the homogeneous growth of the thin films. There exist two growth modes of ZnO on sapphire (0001), which is further demonstrated by theoretical calculation. The growth mode at about 400℃ has the character of mainly spiral-twisted growth with Zn hexagonal symmetry structure, and it is favorable for forming the Zn-terminated surface. In the case of 600℃, a regular in-plane growth is observed, which facilitates the O-terminated surface of the nO thin film. It can be concluded from the calculation that the number of Zn vacancies is greater than that of O in the atomic layer of ZnO film near the α-Al2O3(0001) substrate surface.
Surface study of zirconium implanted with yttrium and lanthanum
Peng De-Quan, Bai Xin-De, Pan Feng, Sun Hui
2005, 54 (12): 5914-5919. doi: 10.7498/aps.54.5914
Abstract +
Zirconium substrates were implanted with yttrium and lanthanum ions with a fluence ranging from 1016/cm2 to 1017/cm2 at approx 130℃, using a metal vapor vacuum arc source at an acceleration voltage of 40kV. The surfaces of the implanted samples were then analysed. The valence states of elements in the implanted surface layer were analysed using x-ray photoelectron spectroscopy, showing that yttrium exists in the form of Y2O3, and lanthanum exists in the form of La2O3. Depth distributions of elements in the implanted surface of samples were obtained by Auger electron spectroscopy, showing that the thickness of oxide film on zirconium substrate increases with increasing implantation fluence, and when the fluence of 1017/cm2 was used, the maximum thickness of the oxide film was obtained. Rutherford backscattering indicates that a profile distribution depth of ~30nm of La appears in Zr, which also indicates that a serious sputtering occurred during the (La+Y) 1017/cm2 implantation.
Determination of optical parameters of ordered Ag/AAO nano-array composite structure
Shigeaki Zaima, Wang Jian, Wang Cheng-Wei, Li Yan, Liu Wei-Min
2005, 54 (12): 5920-5925. doi: 10.7498/aps.54.5920
Abstract +
The optical transmission function is modified by taking account of the influence of thickness variation Δd on transmission of thin films. Meanwhile, we introduce an optimization technique and define an evaluating function, such that a simple method is developed based on the envelope curves of the optical transmission spectrum over the wavelength range 500—2700nm at normal incidence, which is used for the determination of the equivalent refractive index n, equivalent extinction coefficient k, average equivalent thickness d and thickness variation Δd of the ordered Ag/AAO nano-array composite structure. The relative error is about 0.3% in the calculation of average equivalent thickness . The simulated transmission of Ag/AAO nano-array composite structure is in satisfactory agreement with the measuremental values. All these results show that with this simple method the optical parameters of Ag/AAO nano-array composite structure can be calculated of accurately and the results are consistent with the experiment.
Fabrication of CNx nanotubes films using different nitrogen sources and their low field emission properties
Guo Xin-Yong, Chao Ming-Ju, Liang Er-Jun, Ding Pei
2005, 54 (12): 5926-5930. doi: 10.7498/aps.54.5926
Abstract +
The CNx nanotube films have been fabricated on Si substrates by thermal decomposition when NH4Cl and C2H8N2 were used as nitrogen sources. The scanning electron microscopy, transmission electron microscopy and Raman spectroscopy were used to characterize the CNx nanotubes. It is found that not only the degrees of purity and ordering, but also the morphology of the CNx nanotubes are quite different when different nitrogen sources are used. The CNx nanotubes fabricated by pyrolyzing C2H8N2/C10H10Fe have “bamboo-like” structure and lower crystallinity,and grow horizontally in good order on silicon substrates. Low field emission measurement gives a turn-on field of 1.0V/μm and 860μA/cm2 current density at 2.89V/μm.
Comparison of field emission of carbon, carbon nitride and boron carbon nitride nanotubes
Li Qiang, Liang Er-Jun
2005, 54 (12): 5931-5936. doi: 10.7498/aps.54.5931
Abstract +
Carbon,CNx and BCN nanotubes were synthesized by pyrolysis at 860℃ and purified. Thin films of the purified samples were fabricated by a screen-printing method. The field emission effect of these films was compared. It was found that the turn-on field was 2.22,1.1 and 4.4V/μm,and the current density at 5.7V/μm was 1400,3000 and <50μA/cm2 for C, CNx and BCN nanotubes,respectively. The electron field emission property of the CNx nanotubes was better than the carbon nanotubes while that of BCN nanotubes was the worst. The origin of the differences in field emission of these films was explained.
Effective interaction between charged plane and charged colloidal particle in electrolyte
Yang Tao, He Dong-Hui, Zhang Qing-Lan, Ma Hong-Ru
2005, 54 (12): 5937-5942. doi: 10.7498/aps.54.5937
Abstract +
The effective interaction between two infinite, uniformly charged planes immersed in electrolyte solution having only monovalent counterions is calculated using Poisson-Boltzmann equation. Meanwhile, the influence of the boundary (a infinite, uniform charged plane) on a charged colloid sphere is investigated using the Derjaguin approximation. It is found that there is only repulsive force between two charged planes in the absence of microion correlation. Furthermore, the variation in the boundary condition has little effect on their effective interaction. The conclusions are similar with that of the charged-plane and charged-colloid-sphere system.
Optimization of photoelectrode introduced to dye-sensitized solar cells by anodic oxidative hydrolysis
Xu Wei-Wei, Dai Song-Yuan, Fang Xia-Qin, Hu Lin-Hua, Kong Fan-Tai, Pan Xu, Wang Kong-Jia
2005, 54 (12): 5943-5948. doi: 10.7498/aps.54.5943
Abstract +
The influence of three different modification methods on the photoelectrode introduced to dye-sensitized solar cells by anodic oxidative hydrolysis of acidic aqueous TiCl3 solutions was investigated. X-ray diffraction analysis shows that the nanocrystalline TiO2 prepared by sol-gel and electrodeposition is anatase. The average nanocrystalline size of TiO2 prepared by electrodeposition is 19nm and that by sol-gel method is 23.7nm. The surface structure of the nanoporous TiO2 films and TCO glass are observed by field-emission scanning electron microscopy before and after electrodeposition. It is found that the short-current density and the photoelectric conversion efficiency of dye-sensitized solar cells are improved a lot through these different methods.