Vol. 55, No. 4 (2006)
2006, 55 (4): 1539-1542. doi: 10.7498/aps.55.1539
The spacetime structure and properties of the solution of dust particle are discussed. We derive the discrete structure of interior space of dust particle, and prove that the matter ball in the interior of dust particle is a flat ball without coordinates and has minimal volume and unitariness. Then, we derive the radial geodesics in the interior and exterior spacetime of dust particle and prove its continuity. Finally, we expound the inherent links between matter, gravitation and spacetime which were brought to light in the solution of dust particle.
2006, 55 (4): 1543-1547. doi: 10.7498/aps.55.1543
Starting from the Мещерский equations, the higher order D'Alembert-Lagrange principle for mechanical system of variable mass is obtained, and different kinds of higher order differential equations of motion for holonomic mechanical system of variable mass are derived. The result shows that these equations extend and optimize the correlative theory of holonomic mechanics.
2006, 55 (4): 1548-1554. doi: 10.7498/aps.55.1548
By using Mathematica and multi-linear variable separation(MLVS) approach which is based on the Bcklund transformations, a new exact solution which include low dimensional arbitrary functions of the (2+1)-dimensional modified Veselov-Novikov system is obtained. Two new foldons are constructed and their entirely elastic interactions are considered. In additon, foldon and ghoston interactions are derived. MLVS approach is also extended to solve a new (1+1)-dimensional nonlinear system.
2006, 55 (4): 1555-1560. doi: 10.7498/aps.55.1555
We obtained a class of approximate periodic solutions for the (2+1)-dimensional modified Zakharov-Kuznetsov equation by using the homotopy analysis method (HAM). The solutions we got agree with the exact solutions. The results indicate that the HAM is still valid for solving a class of higher dimensional evolution equations. We also made some efforts to extend the HAM to find the analytical solutions for more nonlinear evolution equations in an easier way.
2006, 55 (4): 1561-1564. doi: 10.7498/aps.55.1561
The shock problems for a class of the nonlinear singularly perturbed equations is studied. Using Sinc-Galerkin method, the shock solutions of boundary value problem are constructed, the approximate solution by Newton's method is obtained.
A simple method for constructing elliptic function solutions to the nonlinear evolution equations and its applications
2006, 55 (4): 1565-1570. doi: 10.7498/aps.55.1565
Many travelling wave solutions of nonlinear evolution differential equations can be written as a polynomial of two elementary functions which satisfy a projective Riccati equation. From that property, we deduce a method for building these solutions by establishing the relation between the solutions of the general elliptic equation and the projective Riccati equation. The method is effective for both type Ⅰ and type Ⅱ equtions. At the same time, we also answer the question of how to construct the elliptic function solutions in fraction form to the nonlinear evolution equations.
We present and prove the exact quantization rules both for the one-dimensional Schrdinger equation and for the three-dimensional Schrdinger equation with a spherically symmetric potential. In the exact quantization rule, in addition to the usual term Nπ, there is an integral term, called the correction term. For the quantum systems with a so-called shape invariant potential in the supersymmetric quantum mechanics, we find that the correction term is an invariant,independent of the number of nodes in the wave functions. In those systems, the invariant can be determined with the help of the energy and the wave function of the ground state, and then, the energy levels of all the bound states can be easily calculated from the exact quantization rule. Conversely, the validity of the calculated energy levels shows that the correction term is an invariant in those quantum system with a shape invariant potential. The systems with a shape invariant potential we calculated in this paper are the one-dimensional systems with a finite square well, with the harmonic oscillator potential, with the Morse potential and its generalizations, with the Rosen-Morse potentials, with the Pschl-Teller potentials, with the Eckart potential, and with the Hulthen potential, and the three-dimensional systems of harmonic oscillators and the hydrogen atom.
2006, 55 (4): 1580-1584. doi: 10.7498/aps.55.1580
We investigate time evolution of the entanglement between charge qubit initially in mixed states and quantized field. By calculating the concurrence lower bound, we study the effect of initial mixedness λ and detuning Δ on the entanglement of the system. In the case of weak field, the degree of entanglement for charge qubit being initially in excited state is much larger than that for charge qubit being initially in ground state, and detuning Δ can efficiently reduce the entanglement. But in the case of strong field, the time evolution of the entanglement is about the same for charge qubit being initially in excited and ground states, the more mixed the initial state of charge qubit is prepared, the weaker the entanglement is, and Δ has less effect on the entanglement than in the case of weak field.
2006, 55 (4): 1585-1590. doi: 10.7498/aps.55.1585
We investigate the entanglement time evolution of two entangled two-level atoms that interact resonantly with a single-mode field in the Fock state. The results show that the two-atom entanglement state appears with periodicity. The influence of the two-atom initial state, the dipole-dipole coupling intensity between two atoms, and the field in the Fock state on the entanglement degree of two atoms are revealed. Meanwhile the two-atom quantum state will forever stay in the maximum entangled state when the initial state is proper.
Single qubit and its universal logic gate made of an annular spin cluster with anisotropic Heisenberg-chain
2006, 55 (4): 1591-1595. doi: 10.7498/aps.55.1591
We investigate an annular spin cluster made of three particles with anisotropic Heisenberg-chain controlled by a time-dependent magnetic field. The system has an SU(2) algebraic structure. Using algebraic dynamical method, we have obtained the exact analytical solution to the time dependent Schrdinger equation. Based on the analytical solution, it is shown that the system can be used as a universal single qubit logic gate controlled by the strength and frequency of the magnetic field, and consequently six special single qubit logic gates can be realized.
2006, 55 (4): 1596-1601. doi: 10.7498/aps.55.1596
According to the LR invariant theory, the mesoscopic RLC circuit driven by a alternating current source was discussed. The mutual coupling of electron wave functions between the two plates of capacitor and the resistance of the circuit were taken into account. In our conclusion, the quantum states which dominate the dynamic property of the system and the time evolution operator of the mesoscopic circuit were obtained. The wave function of the mesoscopic RLC system will evolve from an arbitrary initial state to a generalized squeezed state.
2006, 55 (4): 1602-1606. doi: 10.7498/aps.55.1602
Using the four-leg metric tensor λ(α)μ, a gravitational field 4-vector potential for index μ is defined as ω(α)μ≡-cλ(α)μ, and a covariant gravitational field equation that includes the gravitational field contribution is proposed as Rμν-gμνR/2+Λgμν=8πG(T(Ⅰ)μν+T(Ⅱ)μν)/c4, where Λ is Einstein's cosmic constant， T(Ⅰ)μν and T(Ⅱ)μν are energy-momentum tensor of pure matter part and pure gravitational field part, respectively. The covariant energy-momentum tensor of gravitational field that belongs to the part of the gravitational source can be constructed as T(Ⅱ)μν=c2(D(α)μρDρν(α)-gμνD(α)τγDτγ(α)/4)/4πG, where D(α)μν≡ω(α)μ;ν-ω(α)ν;μ. The static spherically symmetric gravitational field, the missing mass and the gravitational field quantization are discussed.
2006, 55 (4): 1607-1615. doi: 10.7498/aps.55.1607
By using the entanglement entropy method, in the Gibbons-Maeda(G-M) dilaton space-time, the statistical entropy of the quantum field in a thin film is calculated and the Bekenstein-Hawking entropy of the G-M dilaton black hole is obtained.Here, the quantum field is entangled with the quantum states in the black hole and the thin film sticks to the event horizon from outskirt of the black hole.Taking into account the effect of the generalized uncertainty principle on the quantum state density, the difficulty of the divergence of the state density near the event horizon in the brick-wall model is removed.Calculating the statistical entropy of the degrees of freedom entangling to the quantum states in the black hole in the quantum field outside the brick-wall and comparing the result to the entropy from the degrees of freedom inside brick-wall, we see that the two results are consistent but the latter may embody preferably the essence of black hole entropy.Using the residue theorem, the integration difficulty in he calculation is overcome and the result of the paper is founded quantitatively. These calculation and discussion imply that the high density quantum states near the event horizon are strongly correlated with the quantum states in black hole and the ultraviolet cut-off in the brick-wall model is not reasonable. The quantization of gravity field should be considered in the high energy quantum field near the event horizon and the ultraviolet cut-off is not necessary. In the quantum field inside and outside the brick-wall, the degrees of freedom contributing to the black hole entropy are just those correlating with the degrees of freedom in the black hole.
2006, 55 (4): 1616-1621. doi: 10.7498/aps.55.1616
The presupposition of deriving the fluctuation from equilibrium thermodynamics is that the fluctuation must be small, yet for some systems the derived temperature fluctuations are divergent as the temperature approaches 0K. For such fluctuations, the statistical physics has to be utilized to yield the correct results. Our results show that the fluctuations go to zero as the temperature approaches 0K.
2006, 55 (4): 1622-1627. doi: 10.7498/aps.55.1622
The analytical expressions of internal energy, chemical potential and heat capacity of the little parameter r for a weakly interacting Fermi gas in weak magnetic field are derived by using “pseudopotential" method and ensemble theory. Based on the derived expressions, the thermodynamic properties of a weakly interacting Fermi gas in weak magnetic field at both high and low temperatures are given. The effects of magnetic field and interparticle interactions on the thermodynamic properties are discussed. The difference in the effects of magnetic trap and three-dimensional harmonic trap on the properties of the system and their reasons are analyzed.
2006, 55 (4): 1628-1633. doi: 10.7498/aps.55.1628
By specializing the Hamiltonian of Nose's extended system in rigid multibody systems, we develop rigorous motion equations for rigid multibody molecular dynamics in canonical ensemble. The equations are proved to generate correct canonical equilibrium distributions. The corresponding reversible and symplectic integrator is also given in this paper.
The influence of weak interaction on thermodynamic properties and the stability of imperfect Bose gas
2006, 55 (4): 1634-1638. doi: 10.7498/aps.55.1634
Using the free energy and the equation of state for an imperfect Bose gas, the analytical expression between the critical temperature and the length of dispersion wave and the pressure for the imperfect Bose gas is derived. The unusual properties of the specific heat at constant pressure and the expansion coefficient at constant pressure and isothermal compressibility by the attractive force for the imperfect Bose gas are revealed. The instability arising from the attractive force is pointed out, and the temperature criterion and particle density criterion of instability is given for the imperfect Bose gas under the action of attraction.
Entropy flux and entropy production of a stochastic system with double singularities driven by colored noise
2006, 55 (4): 1639-1643. doi: 10.7498/aps.55.1639
This paper shows the Fokker-Planck(FP) equation of a stochastic system with double singularities driven by Gaussian colored noise. Based on the FP equation and the definition of Shannon's information entropy, the exact time dependence of entropy flux and entropy production of the system is calculated both in the absence and in the presence of non-equilibrium constraint. The relationship between the properties of double singularities, noise correlation time and dissipative parameter and their effects on entropy flux and entropy production are discussed.
Proportional-integral-derivative chaotic system control algorithm based on particle swarm optimization
2006, 55 (4): 1644-1650. doi: 10.7498/aps.55.1644
Proportional-integral-derivative(PID) control algorithm has the property of simplicity and practicability, but it is difficult to be tuned to control a complex nonlinear system. An advanced particle swarm optimization (PSO) was used to optimize the PID controller for controlling chaotic systems. Computer simulation of its application to several chaotic systems has been done. The controlled chaotic systems are the Hénon chaos, Duffing chaos, six-roll UC mill chaos, Nagumo-sato neuron chaos, Chen's chaos, and permanent magnet synchronous motor chaos. Th chaos control simulation results show that: 1) The output feedback based PID control for chaotic systems is effective; 2) using PID to control complex chaotic systems is feasible; and 3) using PSO to optimize the PID parameters for chaos control is effective and simple in programming.
2006, 55 (4): 1651-1658. doi: 10.7498/aps.55.1651
Based on a certain characteristic of cellular neural network hyper-chaotic system, an error system for the drive system and response system is got, and the problem of synchronization of hyper-chaotic system is put forward thereupon. The problem is to ensure the global approximate stability of the synchronization error system. A kind of passivity method is adopted to design stable feedback controller to ensure global approximate stability of the error system. The simulation result of the hyper-chaotic system of four order cellular neural network testifies the validity of the method.
2006, 55 (4): 1659-1665. doi: 10.7498/aps.55.1659
Support vector regression (SVR) is an effective method for the predication of chaotic time-series, which is a fundamental topic of nonlinear dynamics. Through analyzing the possible variation of support vector sets after new samples are inserted to the training set, a novel SVR algorithm is proposed; thus an online learning algorithm is set up. In connection with the specific characteristics of chaotic signals, a wavelet kernel satisfying wavelet frames is also presented. The wavelet kernel can approximate arbitrary functions, and is especially suitable for local processing; hence the generalization ability of SVR is improved. To illustrate the good performance of the online wavelet SVR, a benchmark problem, i.e. the online prediction of chaotic Mackey-Glass time-series, is considered. The simulation results indicate that the online wavelet SVR algorithm outperforms the existing algorithms in higher efficiency of learning as well as better accuracy of prediction.
2006, 55 (4): 1666-1671. doi: 10.7498/aps.55.1666
Based on the short-term predictability of chaotic time series and the adaptive tracking chaotic trajectory of adaptive algorithm, a novel multi-step adaptive prediction method is proposed in this paper to resolve the problem of adjusting the filter parameters of the local adaptive prediction method during multi-step prediction. Simulation results show that this multi-step adaptive prediction method can be successfully used to make multi-step predictions, and its performance is better than that of the local adaptive prediction algorithm.
Applying a special kind of two coupled Duffing oscillator system to detect periodic signals under the background of strong colored noise
2006, 55 (4): 1672-1677. doi: 10.7498/aps.55.1672
The paper shows that the coupled oscillator systems are more complicated in the dynamic behaviors, more stable in periodic phases and stronger in the anti-noise capacity than the same kind of single oscillator systems by analyzing the function of coupled parameters, the complexity of the large-scale periodic phase states of the system and the comparison with the states of the system under the condition of no noise. Moreover, the harmonious wave and square wave signals are detected under the background of colored noise, applying a kind of constructed middle-intensity two coupled Duffing oscillator system including a special resilience force item. The signal-to-noise ratio reaches to -111.0 and -108.45dB, respectively.
Influence of harmonic and bounded noise excitations on chaotic motion of Duffing oscillator with homoclinic and heteroclinic orbits
2006, 55 (4): 1678-1686. doi: 10.7498/aps.55.1678
In this paper, the influence of harmonic and bounded noise excitations on the chaotic motion of a double well Duffing oscillator possessing both homoclinic and heteroclinic orbits is investigated. The criteria for occurrence of transverse intersection on the surface of homoclinic and heteroclinic orbits are derived by Melnikov theory, and are complemented by numerical calculations which display the bifurcation surfaces and the fractality of the basins of attraction. The results imply that the threshold amplitude of bounded noise for the onset of chaos moves upwards as the noise intensity increases beyond a critical value, which is further verified by numerically calculating the top Lyapunov exponents of the original system. Then we come to the conclusion that larger noise intensity results in smaller possible chaotic domain in the parameter space. The influence of bounded noise on Poincaré maps of the system response is also discussed, which indicates that when the noise intensity is less than some critical value, larger noise intensity results in larger area which the map occupies in the phase plane.
2006, 55 (4): 1687-1695. doi: 10.7498/aps.55.1687
The chaotic repeater is proposed for long-haul chaotic communications. The physical model of all optical chaotic semiconductor laser repeater is presented. The synchronization of chaos repeater system is theoretically demonstrated. Chaos synchronization of a transmitter, a repeater and a recoverer is numerically achieved. Relationships of synchronization error and synchronization time with the system feedback coefficient are analyzed. Application of the repeater system is numerically simulated in chaotic laser communications. Chaotic communications and synchronization error analysis with a sinusoidal modulation signal of frequency 0.2GHz and a digital signal of 0.2Gb/s rate are given respectively to illustrate that chaotic repeater can improve the decoding quality in chaotic laser communications. The repeater bandwidth, chaotic communications with a high rate digital signal of 20Gb/s and synchronization error are analyzed respectively. Numerical results of a sinusoidal modulation signal of bandwidth from 0.1 to 20GHz, and a digital signal of modulation bandwidth from 0.1 to 20Gb/s are obtained, and numerical relationship of synchronization error to the modulation frequency is also shown. The system parameter mismatch is also numerically analyzed.
2006, 55 (4): 1696-1705. doi: 10.7498/aps.55.1696
Based on the approximate entropy theory and related algorithm, approximate entropy analysis of current in short-circuiting arc welding in CO2 shielding gas are performed from the point of view of nonlinear time sequences. Calculation and comparison of approximate entropy of current under different wire-feeding speeds, setting voltages and different gas flow rates show that approximate entropy of current can be taken as a creterion of stability of short-circuiting transfer. The results show that greater approximate entropy and less fluctuation indicate a more stable welding process.
2006, 55 (4): 1706-1710. doi: 10.7498/aps.55.1706
Based on the NaSch traffic model, we propose a cellular automaton traffic model for moving-like block system to simulate the railway traffic. We simulate the traffic phenomenon of delay propagation. We also investigate how the main facts including the length of location unit, train time interval and initial delay affect the delay propagation of trains.
2006, 55 (4): 1711-1717. doi: 10.7498/aps.55.1711
The weaving sections often turn into the bottleneck on the elevated roads. On the basis of the NS cellular automaton traffic model, the weaving section with one-lane main road was simulated and analyzed. For the free traffic flow, weaving almost has no influence on the system, even with increased weaving length. On the other hand, when the traffic flow is in congested state, weaving conflicts have negative effects on the system. The traffic condition will be improved with the increase of weaving length. Our simulation results suggest that the length of weaving sections need not to be inappropriately increased, and a proper moderate value should be chosen to get an optimal traffic condition.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2006, 55 (4): 1718-1723. doi: 10.7498/aps.55.1718
Based on the modal expansion and field matching technique, the S-matrix of a abrupt change is derived. The open resonator with a series of abrupt changes in radius is studied, and the S-matrix of the open cavity is derived by matrix cascading. A computer program was deueloped to calculate the scattering matrix of the cavity and the result shows good agreement with experiment and HFSS simulation.
2006, 55 (4): 1724-1730. doi: 10.7498/aps.55.1724
If electron cyclotron wave is adopted for heating of D-３Ｈe advanced fuel fusion plasma, it makes the latter operate at elevated electron temperature. Under these circumstances, some new physical processes, such as nuclear-nuclear elastic scattering, nuclear plus Coulomb interference scattering, secondary reaction propagation between the fusion products and bulk Maxwellian ions, become relatively more pronounced. However the Coulomb-interactions between fusion produced energetic ions and electrons turn to be weakening, a fraction of bulk Maxwellian fuel ions will be shifted to super-hot tail ions in their distribution functions, hence D-３Ｈe fusion reactivity may be enhanced. The energy transfer of these new mechanisms is highlighted in this paper.
2006, 55 (4): 1731-1735. doi: 10.7498/aps.55.1731
The general characteristics of the synchrotron radiation of the permanent magnetic wiggler, and the angular distribution of power and total power radiated for the permanent magnetic wiggler are introduced briefly. After briefly describing the basic structure of the permanent magnetic wiggler, we report the procedure of applying the finite element analysis to the C-shape frame and the integral structure which are the main bearer of the load,in order to determine the displacement of structure of magnet under different conditions. The displacement of the C-shape frame is calculated by the theoretical formula to provide a theoretical basis for further structure design and improvement of the permanent magnetic wiggler.
ATOMIC AND MOLECULAR PHYSICS
2006, 55 (4): 1736-1739. doi: 10.7498/aps.55.1736
The dissociation process of the sulfur trimer S３ in the low-lying excited states is investigated using ab initio method in this work. The dissociation limits of S３ are constructed. The calculation of transition moment between certain states indicates that the nonadiabatic process of conversion between different excited states occurs in the crossing regions of potential surfaces. Thus, we conclude that the nonadiabatic predissociation is important in the dissociation paths of S３ into S２+S.
2006, 55 (4): 1740-1750. doi: 10.7498/aps.55.1740
In this paper, we propose a novel scheme to build a controllable four-well optical trap for cold atoms or molecules by using an optical system of a binary π-phase plate and a lens illuminated by a plane light wave. We calculate the intensity distribution of the four-well optical trap, and discuss the evolution process of the optical trap from four wells to two wells or form four wells to a single well, and derive some analytical relations between the characteristic parameters of the four-well trap (including geometric parameters, intensity distribution, intensity gradients and their curvatures) and the parameters of the optical lens system. Our study shows that our four-well trap can be continuously changed into a double-well one or a single-well one by moving the π-phase plate, and the relation between the distance of the trapping centers of the four wells or two wells and the moving distance of the π-phase plate is obtained by data fitting. So our four-well trap scheme has important and wide potential aplications in the fields of ultracold atomic physics, cold molecular physics, atom optics, molecule optics and quantum optics,and even in the fields of quantum calculation and information processing, and so on.
CLASSICAL AREA OF PHENOMENOLOGY
Padé-finite-difference time-domain analysis of electromagnetic scattering in magnetic anisotropic medium
2006, 55 (4): 1751-1758. doi: 10.7498/aps.55.1751
Based on the matrix Padé approximation theory, the complex relative permeability tensor of magnetized dispersive media is described by a matrix function expansion with respect to jω in frequency domain. By substituting the operator of /t into jω, the expansion is then transferred into the time-domain. In order to derive the formulation of the matrix expansion of complex relative permeability tensor in discretised time domain, a shifted operator in discretised time domain is introduced as a replacement of time differential operator /t. Therefore the dispersion relation between B and H in discretised time domain can be obtained, which is then implemented to the gyrotropic medium, yielding the time iterative formulation for finite-difference time-domain computation. To verify the feasibility of the presented scheme, we apply the above-mentioned method to the electromagnetic scattering by a magnetized ferrite sphere. The computed result is in good agreement with the one obtained by recursive convolution technique. The analysis and example show the feasibility of the proposed scheme.
2006, 55 (4): 1759-1763. doi: 10.7498/aps.55.1759
Multi-level run-length limited (ML-RLL) storage is a novel method to increase the recording density and data transfer rate without making changes on optical pickup head. The storage mechanism and experimental system of ML-RLL photo-chromic storage are presented. The mathematical model of ML-RLL storage using photo-chromic materials is proposed. The nonlinear relation between the reflectivity of marks and the exposure power and time is reflected in this model. Based on the model, a proper write strategy used for 4-level run-length limited (4L-RLL) photo-chromic storage is obtained. Then a 4L-RLL photo-chromic dynamic experiment is carried out on 650nm photo-chromic material. Experimental results show that the 650nm photo-chromic materials used in the experiment can be used for ML-RLL storage and the recording channel can be effectively linearized by using proper write strategy, so the conventional signal processing techniques used for linear system can be used.
2006, 55 (4): 1764-1768. doi: 10.7498/aps.55.1764
The entropy evolution of the field interacting with two entangled-atoms are studied by means of quantum theory.Using the numerical method,the influence of the entanglement of atoms and the intensity of field on the entropy evolution of the field is discussed. The results show that the entanglement of atoms affects the mean-value of field entropy and the intensity of field affects the oscillation properties of field entropy evolution.
Phonon mediated electromagnetically induced transparency and ultraslow light in strongly coupled exciton-phonon systems
2006, 55 (4): 1769-1773. doi: 10.7498/aps.55.1769
Phonon mediated electromagnetically induced transparency(EIT)and ultraslow light in strongly coupled exciton-phonon systems are investigated theoretically without making any approximation for excitons. It is shown that electromagnetically induced transparency mediated by phonons and the ultraslow light can occur in such systems as the signal field detuning from the exciton frequency is exactly equal to the phonon frequency. It is further shown that the phonon mediated EIT and ultraslow light calculated by using the exact exciton model is essentially the same as the numerical results obtained using the Bose approximation or the Dyson-Maleev approximation.
Amplitude-squared squeezing of Roy-type even and odd nonlinear coherent states in a finite-dimensional Hilbert space
2006, 55 (4): 1774-1780. doi: 10.7498/aps.55.1774
The Roy-type even and odd nonlinear coherent states in a finite-dimensional Hilbert space are constructed. Their amplitude-squared squeezing effect, orthonormalized property, unitary property and completeness relations are discussed. The results reveal the existence of unitary property, completeness relations and non-orthonormalized property. There exists the amplitude-squared squeezing effect for the Roy-type even and odd nonlinear coherent states when the phase θ of parameter β meets the fixed condition. The relations between conditions of squeezing effect and parameters s,r and function f(n) are given. Finally using the numerical method, it is found that in some different ranges of r, the amplitude-squared squeezing effect exists in Roy-type even and odd nonlinear coherent states field in a finite-dimensional Hilbert space when the parameters s,θ and Lamb-Dike parameter η are given as the fixed value.
Efficiency of single photon emission in three-level system of semiconductor quantum dots with pulsed excitation
2006, 55 (4): 1781-1786. doi: 10.7498/aps.55.1781
The statistic characteristics of single photon emission in single semiconductor quantum dot with pulse excitation have been investigated.With rotating-wave approximation,we deduce the dynamics equations including second-order correlation function from the master equations of the populations and discuss the correlation function of photon emission from the exciton combination under different input pulse areas.There will appear an apparant split peak around zero time delay under strong excitation.For short pulse width,the probability of single photon emission p and the efficiency of single photon emission η oscillate as the excitation intensity increases.It is revealed that high efficient single photon emissions can be realized in this system when excited with input pulse area about π and short pulse width.
2006, 55 (4): 1787-1792. doi: 10.7498/aps.55.1787
The laser properties of laser diode end-pumped Tm and Ho co-doped lithium yttrium fluoride (YLF) laser are studied. The experiment of Tm,Ho:YLF laser is conducted for different output couplings, and the optimum output coupler transmission is 2% when the crystal is kept at room temperature. The distribution of far field intensity is measured by pin-hole scanning, the output laser is proved to be operating in the TEM00 mode, and the relation between thermal focal length and pump power is obtained. To achieve single frequency operation, two solid, uncoated fused silica etalons are used. The laser threshold pump power is 250 mW, and the maximum continuous wave single frequency output power is 118 mW at the pump power of 2.8 W. The single frequency laser may be used as a seed laser for either a larger oscillator or an amplifier.
2006, 55 (4): 1793-1797. doi: 10.7498/aps.55.1793
The resonance radiation trapping in alkaline-earth metal atomic lasers were investigated according to Holstein's theory. The threshold temperature, ground-state density and resonant level effective lifetime of the resonance radiation trapping were calculated for Ca, Sr, and Ba atomic vapor lasers with different tube radii. The calculated threshold temperature is found to be in good agreement with the experimental laser-starting temperature, which indicates the resonance radiation trapping is a significant factor in establishing the population inversion of the resonance to metastable transition lasers.
2006, 55 (4): 1798-1802. doi: 10.7498/aps.55.1798
The acceleration of electrons by a Laguerre-Gaussian(LG) beam in vacuum is studied. It is shown that only the longitudinal electric field of the LG beam with mode indices p and l=1 can be used to accelerate electrons. The linearly- and circularly-polarized LG beams with mode indices p and l=1 effectively play the role in laser electron acceleration. Some physical characteristics, such as phase and group velocities of the axial optical field, and energy gain of electrons etc., are discussed. The analytical expressions for the phase and group velocities of the axial optical field, accelerating potential and energy gain etc. are derived and are used to make numerical analysis.
2006, 55 (4): 1803-1808. doi: 10.7498/aps.55.1803
We study the interaction between the ultra-short laser pulses and the one-dimensional symmetrical π conjugated molecular material (4,4′-bis (dimethylamino) stilbene) by solving Maxwell-Bloch equations. This kind of molecular material has strong nonlinear optical properties, and its electronic structures and dipole moments are calculated by use of density functional theory on ab initio level. The numerical results show that the slowly varying envelope approximation and the rotating wave approximation cannot accurately describe the propagation properties of the ultra-short pulse in the molecular medium. In the case of single photon resonance, the two-level model can well describe the interaction between the small area pulse and the molecular system. For large area pulse, due to the existence of strongly secondary excitation to the higher-lying levels, the three-level model should be used. When the amplitude of the incident electric field is kept constant, the third level's population increases with the increase of the pulse area.
Experimental study and numerical analysis of femtosecond pulse propagation and supercontinuum generation in highly nonlinear photonic crystal fiber
2006, 55 (4): 1809-1814. doi: 10.7498/aps.55.1809
Femtosecond laser pulse propagation and supercontinuum generation in a highly nonlinear photonic crystal fiber is investigated experimentally and numerically. Consistent results of continuum generation measured and calculated for 790, 800, and 820 nm pump wavelengths are presented. It is shown that when the pump wavelength overlaps with the zero-dispersion wavelength self-phase modulation and third-order dispersion play an important role, and the oscillation structure appears in the temporal waveform of the propagating pulses, whereas in anomalous dispersion region, the formation and the red-shift of solitons are evident, and the self phase modulation plays a dominant role only in the initial stage. Further spectral broadening is due to solitons self-frequency shift, fission of higher-order solitons and four-wave mixing. It is also found that four-wave mixing occurs more easily in anomalous dispersion region than at the zero-dispersion wavelength.
Effect of initial chirp on supercontinuum generation by femtosecond pulse in photonic crystal fibers
2006, 55 (4): 1815-1820. doi: 10.7498/aps.55.1815
The numerical simulations with the method of split-step Fourior were presented to investigate the effect of initial chirp on supercontinuum generation in photonic crystal fibers. According to the fiber length, the evolution of pulse in photonic crystal fibers was divided into three stages: initial broadening stage, dramatic broadening stage and saturation broadening stage. By discussing the differences in evolution of the chirp pulse and non-chirp pulse in each stage, we find that the initial chirp only affects the spectrum at the initial broadening stage and the dramatic broadening stage, and the chirp is profitable for spectrum broadening when β2C2C>0. In the saturation broadening stage, chirp does not affect the spectrum anymore. If we want to broaden the spectrum by using chirp pulse we must choose a right fiber length, and output the pulse in the dramatic broadening stage. These conclusions provide significant reference for supercontinuum generation in photonic crystal fibers with chirp pulse.
2006, 55 (4): 1821-1826. doi: 10.7498/aps.55.1821
We experimentally studied the temporal and spectral characteristics of the output pulses after the negatively chirped pulses passed through normally dispersive media. The spectrum of the pulse is narrowed and reshaped due to strong self-actions. The pulse is self-compressed, instead of broadening, accompanied with the spectrum narrowing in the experiment. For the same spectral bandwidth, shorter than sech2 transform-limited self-compressed femtosecond pulses are obtained. The simulation result is in agreement with the experiment.
Theoretical investigation on dispersion effect and two-photon resonance enhancement of molecular first hyperpolarizability
2006, 55 (4): 1827-1831. doi: 10.7498/aps.55.1827
The calculations of the first hyperpolarizability frequency dispersion effect of hemicyanine derivative chromophore have been performed employing time-dependent Hartree-Fock and sum-over-states method respectively. Two-photon resonance enhancement characteristics of second harmonic generation β(-2ω；ω，ω) have also been investigated using the two-level model (TLM), which was originated by Ouder and Chemla. The results indicate that the first excited state dominates the nonlinear optical properties of hemicyanine derivatives. The first hyperpolarizability β(-2ω；ω，ω) increases with the decrease of fundamental wavelength and gives rise to two-photon resonance near the fundamental wavelength of 950 nm, in accordance with the known experimental facts. In addition, the relaxation effect of the excited states should be considered for TLM in the resonance regime. These researches provide theoretical references for experimental measurements.
2006, 55 (4): 1832-1839. doi: 10.7498/aps.55.1832
Based on the (1+1)-dimensional Snyder-Mitchell model,discussed is the interactions of a pair of optical beams and the three optical beams with some frequencies and polarizations co-propagating in (1+1)-dimensional strong nonlocal nonlinear media.The exact analytical solutions of the interactions are obtained for the arbitrarily oblique incident beams.The interactions of strongly nonlocal spatial optical solitons are the specific cases of such interactions.Through the collisional behavior of the solitons,the photonic switching/logic are presented.The optimal designs of these photonic devices are discussed.
2006, 55 (4): 1840-1845. doi: 10.7498/aps.55.1840
Propagation of optical lattice soliton in nonlinear Kerr medium with harmonic modulation of refractive index is investigated analytically and numerically. The equations governing evolution of the soliton parameters and the conditions for soliton formation and stable propagation are obtained. It is shown that the beam is finally trapped in a guide-like channel and propagates stably when the launching angle is smaller than a critical value. The critical angle increases as the depth and period of modulation of refractive index increase, and increases as the beam width decreases. Furthermore, linear spatial chirp upsets the balance between diffraction and nonlinearity, thus affects the formation and stable propagation of the soliton, although it doesn't affect the central position of the beam. In order to maintain the soliton propagation one can offset the effect of the chirp by using proper beam power.
2006, 55 (4): 1846-1850. doi: 10.7498/aps.55.1846
Based on the electromagnetic theory and by adopting the method of direct calculation, this paper proves that the modes of left-handed materials slab waveguide are orthogonal among each other, i.e. the fast waves, the slow waves, and the radiation modes are orthogonal among each other. This work provides a basis to study modes conversion in left-handed materials slab waveguide.
Coupling characteristics of electromagnetic waves in parallel three photonic crystal waveguides and its application
2006, 55 (4): 1851-1856. doi: 10.7498/aps.55.1851
We consider the coupling between three photonic crystal waveguides as a multimode interference system and show that the dispersion curves of the eigenmodes intersect or almost intersect. Degenerate modes appear in the system. Due to the strong coupling between the degenerate modes, the modal patterns of the guided modes are changed obviously. We find that the power distribution of output light waves of different frequencies is different by selecting the coherence length. At the crossing point, the multimode interference is deprived and power is confined to its input direction without observable transferring to other photonic crystal waveguides. On the basis of these, a wavelength de-multiplexer or multiplexer is designed.
Horn waveguide couplers for interfacing between two-dimensional photonic crystal and single mode planar dielectric waveguides
2006, 55 (4): 1857-1861. doi: 10.7498/aps.55.1857
We propose and optimize the horn waveguide couplers based on distributed Bragg reflector waveguide for coupling between two-dimensional dielectric cylinder photonic crystal waveguide and single mode planar dielectric waveguide and evaluate their transmission efficiency using the finite difference time domain method. The simulation results show that the power transmission efficiency is over 98% in a large part of the frequency range of the guided mode of photonic crystal waveguide. The highest transmission efficiency is 99.85%.
The two-dimensional metal photonic band gap structure consisting of a skew lattice in a nonorthogonal coordinate system
2006, 55 (4): 1862-1867. doi: 10.7498/aps.55.1862
The metal photonic band gap structure has potentialities in the areas of high-energy accelerators,microwave vacuum electron devices, and terahertz radiation sources etc. The real space transfer matrix method in a nonorthogonal coordinate system is used to study the two-dimensional metal photonic band gap structure consisting of a skew lattice. A general formula for calculating the complete band gap structure is derived for transverse-electric and transverse magnetic modes, and the influences of the filling fraction, skew angle and cross section of the metal rod on band gap are analyzed. The results are in good agreement with results of other methods in some special cases.
Optical-stopping effect of As2S8 channel waveguide fabricated by method of photoinduced refractive index changes
2006, 55 (4): 1868-1872. doi: 10.7498/aps.55.1868
The photorefractive phenomenon of As2S8 film is reported in this paper. An ultraviolet irradiation technique is presented and applied successfully to As2S8 channel waveguide preparation on the basis of analyzing these experimental data. This channel waveguide of As2S8 displays good characteristics of a waveguide using a guided mode excitation. It is found that the optical stopping effect of As2S8 channel waveguide has the rotential application owing to its switching function with the photo-optical effect.
2006, 55 (4): 1873-1882. doi: 10.7498/aps.55.1873
The optimization of pumping schemes of distributed Raman amplifiers employed in long-haul optical transmission systems is investigated in detail. Considering the received optical signal to noise ratio (OSNR) as well as nonlinear distortion and the optimal signal power distribution, i.e. uniform distribution, is achieved by using variational method. But the analytical optimal distribution does not exist when taking double Rayleigh backscattering (DRB) into account. Comparison between three Raman amplification schemes, i.e. bidirectional dual-order, first-order pumping and Raman-plus-Erbium doped fiber amplifier hybrid amplification, is carried out at identical nonlinear phase shifts. The results show that symmetric bidirectional dual-order pumping can achieve the best OSNR performance through appropriate choice of the second-order pump wavelength and second-to-first-order pump power ratio in most cases for both short- and long-span applications. However, hybrid amplification will be a better choice when the DRnoise is dominant. These results will be helpful for designing long-haul transmission systems.
Comparison of accuracy between two spectral phase interferometric methods in the characterization of complex pulses
2006, 55 (4): 1883-1888. doi: 10.7498/aps.55.1883
By means of numerical simulation, a comparison is made between conventional spectral phase interferometry for direct electric -field reconstruction (SPIDER) and delay-controlled fringe-free SPIDER (DCFF-SPIDER) in the accuracy of characterization of complex pulses. The results show that errors occur likely when complex pulses with abrupt variations in spectra or/and phases are characterized with conventional SPIDER, while DCFF-SPIDER can give high accuracy. The underlying reasons for this difference are discussed.
2006, 55 (4): 1889-1893. doi: 10.7498/aps.55.1889
A novel fiber strain sensor based on the fiber laser's transient regime is reported. A Sagnac fiber loop and a fiber Bragg grating(FBG) that also acts as the sensing element form the linear Fabry-Perot cavity, in which a long-period grating(LPG) is inserted. The original peak wavelength of the FBG, whose bandwidth is 0.23nm, is 1557.98nm and located on the left side of the LPG dip (1557.4nm). The light reflected from the Sagnac loop and the FBG is transmitted through the LPG twice in each round trip, and the transmission loss increases with the strain-induced wavelength shift of the FBG, which in turn modifies the laser build-up time. The strain value can be obtained by measuring the build-up time of the laser. In the present experiment, the sensor's sensitivity and resolution can be adjusted by simply controlling the pumping level. When the pump pulse's high level and low level are 32 and 6mW, respectively, a sensitivity of 7×10-6ε/μs and resolution of 7×10-7ε are achieved.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
Effects of target thickness on emission direction of hot electrons generated from subrelativistic intensity laser pulses interacting with foil targets
2006, 55 (4): 1894-1899. doi: 10.7498/aps.55.1894
The effects of target thickness on emission directions of hot electrons are investigated in the interaction of subrelativistic intensity laser pulses with foil targets. The results show that the emission direction of the hot electrons at the rear side of the target shifts towards the target normal direction as the target thickness increases. It can be seen that the electrostatic field at the rear side of the target surface is the main cause affecting the emission direction of the hot electrons.
2006, 55 (4): 1900-1906. doi: 10.7498/aps.55.1900
A preliminary experiment for studying the argon-puff Z-pinch implosion process has been performed on the Yang accelerator. The ten-frame nanosecond temporal and spatial gated camera，visible high-speed scanning camera, differential laser interferometer, X-ray time integration pinhole camera and X-ray power system have been used to investigate the evolution of the argon-puff Z-pinch. Some typical results of argon-puff Z-pinch during implosion and pinch phase, including the “zipper” effect, necking phenomenon, sausage instability，temperature changes and the effect of load current rise time, are given and analyzed as examples, and some relevant conclusions are drawn.
2006, 55 (4): 1907-1913. doi: 10.7498/aps.55.1907
A radio frequency flat plasma sheath model in an oblique magnetic field is built, which has one-dimensional coordinate space and three-dimensional speed space. The effect of magnetic field on the structure of radio frequency sheath and characteristic parameters are discussed. The numerical simulation results show that the effect of magnetic field on the sheath structure cannot be ignored. In particular, it brings about obvious changes in the ion density distribution and velocity distribution. Furthermore, the magnetic field cannot change the total energy density distribution of ions, but the Lorentz force does influence its movement state, and at the same time affects the ion energy distribution in every directions, as well as the incidence departure angle of ions.
2006, 55 (4): 1914-1920. doi: 10.7498/aps.55.1914
Half-doped manganese oxides are very important on clarifying physical mechanism of strong correlation electron system and colossal magnetoresistance (CMR) effec t due to their rich physical content. The structure, electrical and magnetic tra nsport properties of Nd0.5Ca0.5MnO3 manganite w as systemically studied. The results show that Nd0.5Ca0.5 MnO3 compound displays the O′-orthorhombic structure and indicate th e existence of the typical Jahn-Teller distortion under 300 K. The transport pro perties show that the system undergoes paramagnetic insulator-ferromagnetic meta l transition under a magnetic field of 8 T and is accompanied by CMR effect. Mea nwhile, it is found that the temperature of antiferromagnetic transition and cha rge ordering is around 150 and 240K, respectively. There appears a typical reent rant spin glass behavior around 41K and the negative magnetization is also obser ved in the compound. All these phenomena indicate that there exist several comp lex magnetic interactions in the ground state of this half-doped Nd0.5Ca0.5MnO3 compound at low temperature. The present result s provide experimental data for strongly electronic correlated system of manga nites.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
In order to meet the requirements of the design of radiation detectors, CdZnTe (or Cd1-xZnxTe) wafers grown by vertical Bridgman method w ere annealed in the vapor of In. The nature of this treatment is a diffusion pro cess, thus it is meaningful to relate the change of resistivity to the diffusion parameters. A transformation model correlating resistivity and conduction type of CdZnTe with the main diffusion parameter —— diffusion coefficient——is put forward in this paper. Combining the model with the analysis of our experimenta l data (namely DIn = 5.17 10-9, 2.625×10-10 an d 3.455×10-11cm2·s-1), the values of the diff usion coefficient of In in Cd0.9Zn0.1Te at 1073, 973 and 873K have been given for the first time, which coincide closely with those in Cd Te given by different authors. With the data, the effect of annealing time on th e change of resistivity and conduction type for Cd0.9Zn0.1 Te slice, which is annealed in saturated In vapor at 1073, 973 and 873K, have be en simulated and good consistency acquired. This work suggests an alternative wa y to determine the diffusion coefficient in semiconductor material, and also ena bles us to analyze the diffusion process quantitatively and predict the annealin g result.
2006, 55 (4): 1930-1935. doi: 10.7498/aps.55.1930
Zr65-xCu17.5Al7.5Ni10Nbx bulk metallic glasses (BMGs) with x=0, 2 and 5 were prepared by water-cooled cop per mold casting. X-ray diffraction and differential scanning calorimetry reveal ed that the addition of Nb changes crystallization pathway of the base alloy and promotes an initial formation of icosahedral quasicrystals. The mechanical prop erties of the BMGs prepared were measured with MTS in a mode of quasi-static com pression. It was found that the addition of an appropriate amount of Nb signific antly enhances the compression strength and plastic strain with the maximum valu e of σb =1741MPa and εp= 5.0% for the BMG containing 5at % Nb. In addition, the corrosion behavior of the BMGs was investigated in 3% NaC l aqueous solution by electrochemical measurements. It showed that the BMGs with different compositions are all spontaneously passivated with passive current de nsity of 10-6—10-5 A/cm2, which is three order of magnitude lower than that of 1Cr18Ni9Ti stainless steel. The corrosion resis tance of the BMGs can be further enhanced with the addition of Nb, as indicated by the increase in pitting potential with the increase of Nb content.
Hydrogen bonding configurations and energy band structures of hydrogenated nanocrystalline silicon films
2006, 55 (4): 1936-1941. doi: 10.7498/aps.55.1936
The correlation between the hydrogen bonding configurations and the energy band structures of hydrogenated nanocrystalline (NC) silicon films has been investiga ted. The samples were prepared by helicon wave plasma chemical vapor deposition technique. Raman scattering, infrared absorption and optical absorption measurem ents were used to analyze the microstructure, hydrogen bonding configurations an d energy band structures of the deposited films. Raman results indicate that dis tinct different structures appear in films deposited at various substrate temper atures and there is a threshold at about 200℃ for the amorphous silicon to be t ransformed to the NC phase. The energy band structures of the films are closely related to their hydrogen bonding configurations. The chemical shift of the vala nce band to the lower state induced by high hydrogen content and the structural disorder related to low substrate temperature makes the films have wide band gap s and large band tail widths. With increasing the substrate temperature, the dec rease trend of optical band and tail width is mainly related to the enrichment o f the hydrogen located in the grain boundary, which perform the function of term inating the dandling bonds at NC silicon grain surface. Further increasing the s ubstrate temperature broadens the band tail width of the films because the hydro gen content is too low to passivate the dangling bonds at the NC silicon grain s urface.
2006, 55 (4): 1942-1946. doi: 10.7498/aps.55.1942
The relation of Debye temperature with the interaction potential of atoms, the r elation of the mean coordination number with the shape and atomicity are deduce d. Through applying the theory of statistical physics, the variation law of the surface energy of rectangular nanocrystals along with the temperature, atomicit y and shape is obtained, and taking Ar nanocrystal as an example, the influence of shape and atomicity on the surface energy of the nanocrystal is discussed.
2006, 55 (4): 1947-1952. doi: 10.7498/aps.55.1947
The influence of F,Y co-doping on the luminescence properties and defects of PbW O4 (PWO) have been investigated by correlated measurements of transmi ssion spectra, light yield, photoluminescence spectra, thermoluminescence and po sitron annihilation lifetime. F doping results in a new luminescence band peaked around 350nm, which gives a significant improvement in luminescence properties of PWO. Meanwhile, Y-dopant efficiently reduces trapping state and accelerates t he scintillation decay process. Compared with the undoped PWO samples, the light yield (100ns) of F,Y co-doped PWO crystal was increased by a factor of 2.7. Th e main defects (WO3)- and (WO4)3- in PWO crystals for lack of oxygen during growth were investigated. Thermoluminesc ence results show that the defects in PWO crystals increase when annealed in vac uum and these defects can be also suppressed by annealing in air at 600℃.
A new model for calculating the critical cooling rate of bulk metallic glass under non-isothermal condition
2006, 55 (4): 1953-1958. doi: 10.7498/aps.55.1953
Based on the non-isothermal transformation theory and non-steady state nucleatio n theory, a new method for calculating the continuous cooling transformation cur ve and the critical cooling rate of bulk metallic glass alloys is put forward in this paper. The continuous cooling transformation curves of Zr- and Pd-based al loys are calculated and the results are in good agreement with the experimental values. It is suggested that the method can be used to evaluate glass forming ab ility of alloy systems more effectively. In addition, the calculation results sh ow that the main factors effecting the critical cooling rate are the viscosity o f melt, nucleation barrier and critical volume fraction of the crystal, the crit ical cooling rate is reduced with the increase of the viscosity, nucleation barr ier and critical volume fraction of crystal.
2006, 55 (4): 1959-1964. doi: 10.7498/aps.55.1959
The thermal diffusivities of BaTiO3 materials are measured by phototh ermal technique at different pressures, sintering temperatures and tantalum dopi ng quantities. The effect of tantalum doping on thermal conduction of BaTiO3 materials is studied in this paper. When tantalum doping quantity is l ess than 1.5mol%, the diffusivity of BaTiO3 increase with the incre ase of the tantalum doping. When tantalum doping quantity is greater than 1.5mol %, the diffusivity of BaTiO3 decreases with the increase of the tan talum doping. The experimental results of doped BaTiO3 materials are analyzed and discussed.
Study on nucleation and dynamic scaling of morphological evolution of ZnO film deposition by reactive magnetron sputtering
2006, 55 (4): 1965-1973. doi: 10.7498/aps.55.1965
Atomic force microscopy has been applied to study the morphological evolution o f ZnO film on Si(100) and Si(111) substrates with a native oxide layer. With dyn amic scaling of the film morphology at different growth stages, the nucleation a nd growth behavior have been studied for ZnO films deposited by radio-frequency reactive magnetron sputtering. It is found that ZnO film has three nucleation st ages, namely the stages of initial nucleation, low-rate nucleation, and seconda ry nucleation. The growth exponents of the three stages are β1=1.04 ，β2=0.25±0.01 and β3=0.74 for ZnO film on Si(100) and β1=0.51，β2=0.08±0.02 and β3=0.63 for ZnO f ilm on Si(111), respectively. In the initial nucleation stage, the intrinsic def ects on Si substrates may be responsible for the surface roughening and the dens ity of surface defects determines the nucleation density of ZnO films. The growt h behavior of ZnO film is dominated by the diffusion effect and oriented growth mechanism, as well as the coarsening mechanism induced by the lattice mismatch s tress. In the low-rate nucleation stage, few new ZnO islands are detected and t he films are roughened slowly in morphology. The deposition rate plays a role of controlling the morphological evolution and the lattice mismatch stress may be released in this stage. The secondary nucleation of ZnO films may result from th e bombardment of energetic ions or atoms on the surface of Si substrates. In the secondary nucleation stage, shadowing effect may influence the roughening of Z nO films. In the stage of steady growth, ZnO films have a roughness value much l ower than the ones in nucleation stages and grow in the form of columnar grains.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2006, 55 (4): 1974-1978. doi: 10.7498/aps.55.1974
By applying the elastic scattering Green's function theory in combination with t he hybrid density function theory, the electronic transport properties of molecu lar junctions constructed by the six-membered heterocyclic molecules, pyridazine -2,5-dithiol, pyrazine-2,5-dithiol, and pyrimidine-2,5-dithiol, have been studie d. For the heterocyclic molecule pyridine, the influence of the terminal atoms o n the current-voltage characteristics has also been obtained. We have accurately determined the coupling constant between the molecule and electrodes by using t he frontier molecular orbital theory and the perturbation method. The numerical results show that pyridazine-2,5-dithiol has higher current and conductance, whi le the conductance of pyrimidine-2,5-dithiol is very small when the bias voltage is lower than 4V. The conductivity of the molecule pyridine with selenium atom as the terminal atom is higher than that with oxygen atom or sulfur atom.
2006, 55 (4): 1979-1982. doi: 10.7498/aps.55.1979
Using the first principle calculations based on density function theory with the generalized gradient approximation and local density approximation，we studied electronic structure and mechanical stability of the recently synthesized nitrid e compound PtN with cubic structure. The results show that the PtN with zinc-ble nde structure is unstable. We further find that the rock-salt structure has a b igger cohesive energy than the zinc-blende structure, it is possible that a pha se transition from zinc-blende to rock-salt occurs under certain pressure.
2006, 55 (4): 1983-1986. doi: 10.7498/aps.55.1983
In order to reveal the nature of the stress corrosion of Ti theoretically, the a tomic cluster model of α-Ti and the crack formed by dislocation accumulation wa s set up. The electronic structure parameters (Fermi energy level, structure ene rgy, surface energy, cluster energy and environment-sensitive embedding energy) of α-Ti and the crack were calculated by using the recursion method. The calcul ated results show that the environment-sensitive embedding energy of H atoms in the region of crack is smaller than that in the perfect area of α-Ti, so H atom s are apt to segregate at the crack. The positive cluster energy of H atoms in t he region of the crack in Ti metal shows that H atoms can not form cluster and h ave the tendency of forming ordered phases (hydrides). When H atoms accumulate o n the surface of the crack, they reduce the surface energy of the crack, making the crack propagation easier. The Fermi energy of the tip of crack is higher th an that in other areas, so electrons move to the perfect region of α-Ti from th e tip of crack, leading to the formation of the potential difference between the crack tip and the perfect region of α-Ti, then the dissolution-corrosion proce ss of crack tip as anode occurs under the action of the electrolyte.
Negative-ion element impurities breakdown model of HfO2 optical thin film is reported. We believe the main impurity element in thin film comes from t he coating material. The weak absorption and laser induced damage threshold (LID T) of HfO2 thin film are measured to testify the negative-ion element impurities breakdown model. These results indicate that the LIDT would decrease and the absorption of the films would increase with the increase of the content of negative-ion element. The main reason is the negative-ion elements become th e center of volatile gas source and form defects, which in turn become the cente r of absorption during laser irradiation. So negative-ion elements are harmful i mpurities, their existence will speed up the damage of the thin film.
2006, 55 (4): 1991-1996. doi: 10.7498/aps.55.1991
In the weak field scheme, the complete diagonalizing Hamiltonian matrices and th e constants of crystal structure and g factor of electron paramagnetic resonanc e are derived for 3d4/3d6 ions configuration in the trigon al symmetry in terms of Racahs irreducible tensor operator method. The ground- state energy levels and the constants of crystal structure and g factor of elec tron paramagnetic resonance of the Co3+ in LiCoO2 and LiCo O2:Ni crystals are studied. The spin singlets and triplet of the grou nd-state energy levels of the Co3+ in LiCoO2 and LiCoO2:Ni crystals are taken into consideration in the calculation and applica tion of the crystal-field energy of 3d4/3d6. The changes o f the constants of local structure, which are the causes of the changes of the g round-state energy levels of the Co3+, and the values of g factor of electron paramagnetic resonance of Co3+ in LiCoO2 and LiCo O2:Ni crystals are also studied. The theoretical results are in good agreement with the experimental observations.
2006, 55 (4): 1997-2002. doi: 10.7498/aps.55.1997
By fitting the empirical pseudopotential band structure data using piecewise pol ynomials, an analytical band model of ZnS is presented for thin-film electrolumi nescent devices. The density of states and scattering rates are calculated using the above model. As compared with the results from the full band model, we have shown that our model, which takes less time, has the same precision as that obt ained from the full band model. Using Monte Carlo method, we simulated the field -dependent electron occupation functions of 1st and 2nd bands, electron energy d istribution functions under four-electron fields and the dependence of electron energy on time with or without impact ionization. This shows that the inter-vall ey scattering, inter-band scattering and impact ionization are important for tra nsporting electrons among valleys. Another important result is that the effect o f impact ionization on current multiplication and electron energy distribution i s also discussed.
2006, 55 (4): 2003-2007. doi: 10.7498/aps.55.2003
Single-phase Ti1-x(Hf0.919Zr0.081)xN iSn (x＝0.00—0.15) compounds were synthesized by solid-state reaction and high- density polycrystalline bulk material was prepared by spark plasma sintering (SP S). The effect of Hf and Zr substitution for Ti on the thermoelectric propertie s of TiNiSn half-Heusler compounds were investigated. It was shown that the subs titution of a small amount Hf and trace Zr for Ti resulted in significant reduct ion of the thermal conductivity and remarkable enhancement of the Seebeck coeff icient. As a result, the dimensionless figure of merit ZT of Ti0.85(H f0.919Zr0.081)0.15NiSn reached a high maximum v alue of 0.56 at 700K and the enhancement of ZT was 190%—310% at the same temper ature compared with ternary TiNiSn compounds.
2006, 55 (4): 2008-2013. doi: 10.7498/aps.55.2008
In this paper the electroluminescene of highly efficient blue dopant EBDP was in vestigated. The blue and white organic light emitting diodes with the structure: indium-tin oxide(ITO)/ copper phthalocyanine(CuPc)/N,N′-bis-(1-naphenyl)-N,N′ -biphenyl -1,1′-bipheny1-4-4′-diamine (NPB)/2-t-butyl-9,10-di-(2-naphthyl)anth racene(TBADN):EBDP/tris(8-hydroxyquinoline) aluminum(Alq3)/LiF/Al and ITO/CuPc/NPB/TBADN:EBDP: 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethylj ulolidyl-9-enyl)-4H-pyran/Alq3/LiF/Al were studied by using EBDP as b lue dopant. For the blue device, the maximum luminance was 26961cd/m2 , and its maximum efficiency was 8.29cd/A. The luminance at a current density 20 mA/cm2 was 1597cd/m2. For the white device, the maximum lu minance 32291cd/m2 and maximum efficiency 8.31cd/A,the luminance of 1 413cd/m2 at a current density 20mA/cm2 were obtained. The slow decrease of efficiency with the increase of current density indicates weak exciton-exciton annihilation, which results from the large steric hindrance due to a non-planar structure of the fluorescence dye EBDP.
2006, 55 (4): 2014-2020. doi: 10.7498/aps.55.2014
The current-density distribution and giant magnetoimpedance (GMI) properties are theoretically described for the homogenous ferromagnetic wires and composite wi res consisting of an inner conductive core covered by an outer ferromagnetic coa ting. The numerical simulation shows that the current is distributed much close r to the surface for the Cu/FeCoNi composite wires than for FeCoNi homogenous f erromagnetic wires at the same frequency and with the same geometric size and m agnetic structure. When obvious MI effect can be observed, the skin effect has a lready been strong in its ferromagnetic coating. The skin effect is still an imp ortant factor for the GMI effect in composite wires. Above a certain frequency o f about 4MHz, the driving current will flow through the ferromagnetic coating ma inly and the electromagnetic interactions become weaker, the MI behavior in the composite wire becomes similar to that in the homogenous ferromagnetic wire.
2006, 55 (4): 2021-2026. doi: 10.7498/aps.55.2021
High quality single crystalline PbTe thin films have been grown on BaF2(111) by using molecular beam epitaxy(MBE).Raman scattering measurements have revealed the vibration modes of oxide on the surface，longitudinal optical(LO) p honon at Brillouin center q≈0，and coupled plasmon-phonon.With the variation of the focusing-depth(FD) of the pumping laser beam，the integrated intensities，p eak positions and line widths of the observed Raman peaks showed different behav iors.When FD changed from 3μm above the top-surface to 3μm below the bottom-su rface, the PbTe LO phonon frequency shifted from 119 to 124cm-1.Diffe rent Raman scattering results were also observed by comparison of the freshly su rface-etched and the as grown PbTe samples. These phenomena originated from the different vibration modes. The frequency shift of LO phonon is explained by the fact that the strain relaxation caused by the lattice mismatch between PbTe and BaF2 is as high as 4.2％.
Based on the non-equilibrium Green function formalism, we studied the coherent t ransport through a quantum dot ring. Compared with a one-dimensional quantum dot array, a quantum dot ring presents a number of new quasi-bounding-states in its electronic spectra. Moreover, the enhancement of the inter-dot coupling would l ead to the prominent decoherence of Coulomb-blockade conductance oscillation.
2006, 55 (4): 2033-2037. doi: 10.7498/aps.55.2033
To overcome the difficulties in the fabrication of the nanoimprint mold with lin ewidth smaller than 50nm, we deposited a-Si/SiNx multilayer films in plasma enhanced chemical vapor deposition system and then prepared the relieo-n anomold on the cleaved section of the multilayer films by selectively etching or reactive ion etching process. Due to the slow deposition rate, the thickness of the sublayer, and therefore the size of the strips and grooves can be controlle d on the nanometer scale by altering deposition time. The smallest width we get by now is the 20nm strips and 20nm pitches，which is better than that fabricated by electron beam lithography.
Influence of light exposure and applied bias on the conductivity of microcrystalline silicon films at room temperature
2006, 55 (4): 2038-2043. doi: 10.7498/aps.55.2038
Microcrystalline silicon films were deposited using the plasma enhanced chemical vapor phase deposition system and SiCl4/H2 gas source. We investigate the evolution of the dark conductivity of the fabricated films afte r a brief exposure to light or under DC bias. Raman spectra measurements indicat e that the volume fractions of crystalline phase for all samples are over 70%. T he measurement of dark resistance reveals the existence of weak persistent photo conductivity in our samples. Under DC electric field, it is found that the dark conductivity increases slowly with time and a restoration process occurs once th e field is reversed. This behavior depends on the DC bias level and the measurem ent temperature and can be explained by a heterojunction barrier model. It is su ggested that the spatial separation and redistribution of charge carriers under applied conditions and the potential barrier associated with inhomogeneity are r esponsible for the observed phenomena.
Magneto-transport characteristics of two-dimensional electron gas for Si δ-doped InAlAs/InGaAs single quantum well
2006, 55 (4): 2044-2048. doi: 10.7498/aps.55.2044
Magneto-transport measurements have been carried out on a Si heavily δ-doped In 0.52Al0.48As/In0.53Ga0.47As single q uantum well in the temperature range between 1.5 and 60K under magnetic field up to 10T. We studied the Shubnikov-de Haas(SdH) effect and the Hall effect for th e In0.52Al0.48As/In0.53Ga0.47As sing le quantum well occupied by two subbands，and have obtained the electron concent ration, mobility, effective mass and energy levels respectively. The electron co ncentrations of the two subbands derived from mobility spectrum combined with mu lti-carrier fitting analysis are well consistent with the result from the SdH os cillation. From fast Fourier transform analysis for d2ρ/dB2-1/B，it is observed that there is a frequency of f1-f2 insensitive to the temperature, besides the frequencies f1, f2 for the two subbands and the frequency doubling 2f1, both depen dent on the temperature. This is because that the electrons occupying the two di fferent subbands almost have the same effective mass in the quantum well and the magneto-intersubband scattering between the two subbands is strong.
2006, 55 (4): 2049-2056. doi: 10.7498/aps.55.2049
A finite-temperature Landau theory is proposed to describe competing orders and interlayer tunneling in multilayered cuprate superconductors. For mono- and do uble-layered superconductors, the zero temperature order parameters and transiti on temperatures are calculated as functions of doping concentration. By comparin g such phase diagrams with relevant experiments, the phenomenolgical parameters in the theory are determined. Then we apply the theory to multilayered supercond uctors and determine the superconducting transition temperature as a function of layer number N in the underdoped, nearly optimally doped and overdoped region, respectively. In a reasonable parameter space the result of optimally doped regi on turns out to be in agreement with experiment, and as theoretical predictions, in the very underdoped and very overdoped case, Tc increases monoton ically with N.
2006, 55 (4): 2057-2063. doi: 10.7498/aps.55.2057
We studied within the framework of a mean-field approach the nonequilibrium dyn amic phase transition of a kinetic Ising spin system subject to a perturbative f ield and temperature simultaneously by comparison between time-dependent Ginzbur g-Landau and Glauber dynamics models. The dynamic phase transition (DPT) boundar ies, separating a symmetry-breaking dynamic ordered phase from its symmetric dyn amic disordered counterpart, were identified through a systematic simulation of the above two models. The dependence of the dynamic order parameter Q and the fo urth order cumulant ratio U upon the temperature t(r0), the frequency ω and amplitude h0 of driving field were also investigated in detai l. A discussion was presented concerning the current controversies on whether bo th a discontinuous dynamic phase transition occurs possibly below a specific low temperature and a tri-critical point exists on the DPT boundary in a kinetic Is ing spin system.
2006, 55 (4): 2064-2068. doi: 10.7498/aps.55.2064
In this paper, we report the observation of exchange bias in (Pt/Co)n /FeMn multilayer with perpendicular magnetic anisotropy. The samples were deposi ted on glass substrates by magnetron sputtering and capped with 2nm thick Pt lay er. Our results show that the relation between perpendicular exchange bias Hex of (Pt/Co)n/FeMn multilayers and the thickness of antiferro magnetic layer is similar to that in the case of in-plane exchange bias. He x is inversely proportional to the number of bilayer repeats. In the case of (Pt/Co)3/FeMn multilayers, perpendicular exchange bias Hex reachs 22.3kA/m. With insertion of 0.4nm Pt layer between Co layer and FeMn layer, Hex can reach 39.8kA/m.
Dielectric properties and leakage current of MgO/(Ba0.8Sr0.2)TiO3 heterostructured films prepared by sol-gel technique
2006, 55 (4): 2069-2072. doi: 10.7498/aps.55.2069
Ferroelectric MgO/(Ba0.8Sr0.2)TiO3 heterostruct ured films have been grown on LaNiO3/Si(100) substrates by an improve d sol-gel technique. The dielectric constant and the leakage current of (Ba 0.8Sr0.2)TiO3 films have been modified by MgO insert ion. The dramatic reduction in the leakage current has been attributed to the mi nute solid solubility of MgO in the (Ba0.8Sr0.2)TiO3 lattice and the high resistance of the MgO layers.
2006, 55 (4): 2073-2077. doi: 10.7498/aps.55.2073
This paper reports the fabrication of ZnMnO semiconductor by high-dose Mn impla ntion. We studied the influence of implantation dose and annealing on its optica l properties. The broad band at 575cm-1 in Raman spectrum is attribut ed to defects related to high-dose Mn implantion. The vibration modes at 528cm-1 are considered to be associated with Mn impurities. Room temperature photoluminescence spectra show that the high-dose Mn implantion can enhance the intensity in visible band.
2006, 55 (4): 2078-2083. doi: 10.7498/aps.55.2078
Au/SiO2 nano-composite multilayer thin films were prepared by magnetr on plasma sputtering. The microstructure, morphology and optical properties were investigated using transmission electron microscopy and absorption spectra. The Au particles dispersed in the SiO2 matrix of one single Au layer gre w with increasing deposition time in the initial time interval of less than 10s. No obvious change in the size of Au particles was observed in the films deposit ed for more than 10s, however, the shape of Au particles changed. The optical ab sorption peaks due to the surface plasma resonance appeared at a wavelength of 5 40—560nm for ［Au(t1)SiO2(600)］×5 thin films . The intensity of the absorption peak increased with increasing deposition time . Influence of the Au particle shape on the position and intensity of plasma res onance absorption peak was also discussed. The optical absorption spectra of Au/ SiO2 thin films are well in agreement with the theoretical optical ab sorption spectra calculated from reformed Maxwell-Garnett effective medium theor y.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2006, 55 (4): 2084-2091. doi: 10.7498/aps.55.2084
The ZnSe/SiO2 composite thin films were prepared by sol-gel process a nd in-situ growth technique. X-ray diffraction results showed that the phase str ucture of ZnSe particles embedded in ZnSe/SiO2 composite thin films i s the sphalerite (cubic ZnS). X-ray fluorescence results revealed that the molar ratio ofZn/Se is about 1∶1.01—1∶1.19. Scanning electron microscopy results r evealed that the size of ZnSe crystal particles is about 400nm, while some parti cles are less than 100nm in size. The dependence of ellipsometric angle Ψ, Δ w ith wavelength λ of ZnSe/SiO2 composite thin films was investigated with spectroscopic ellipsometers. The optical constant, thickness, porosity and the concentration of ZnSe in ZnSe/SiO2 composite thin films were fitt ed according to Maxwell-Garnett effective medium theory. The photoluminescence p roperties of ZnSe/SiO2 composite thin films were investigated with fl uorescence spectrometer. The photoluminescence results indicated that the emissi on peak at 487nm under 395nm excitation corresponds to the band-to-band emission of sphalerite ZnSe crystal. The strong free exciton emission and other emission peaks correlated with ZnSe lattice defect were also observed.
2006, 55 (4): 2092-2098. doi: 10.7498/aps.55.2092
Evaporation self-assembly method, which is used to assemble sub-micrometer polys tyrene microspheres, is presented in this paper. In this process the polystyrene microspheres are effectively assembled at the gas-liquid interface of suspensio n as moisture evaporates at fixed temperature. The scanning electron microscope shows that the three-dimensional colloidal assemblies have regular close-packed face-centered cubic structure. It was shown that the evaporation was blocked by the polystyrene colloid crystals growing at the gas-liquid interface, and the am ount of the polystyrene microspheres transferred to the colloid crystals from th e suspension in unit time increased with the area of the assemblies. High-qualit y polystyrene colloid crystals can be assembled when not changing or decreasing the phase concentration, whose changes can be controlled by adjusting the assemb ly area and the evaporation temperature.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2006, 55 (4): 2099-2108. doi: 10.7498/aps.55.2099
Numerical experiments are performed on the chaotic movement of a simplified cli mate model. Considering various factors related to the predictability, we numeri cally compare the harmonized multi-time difference scheme (HMTDS) with the varia tional assimilation. It is shown that the HMTDS corrects remarkably the model bi as, especially the differential equation bias, which is just the drawback of the variational assimilation.