Vol. 58, No. 5 (2009)
2009, 58 (5): 2873-2877. doi: 10.7498/aps.58.2873
This paper puts forward the finite precision function theory. Based on the function theory, a photon solution of finite precision Einstein-Maxwell equations is founded, and the important properties of electromagnetism, gravitation and space-time geometry of the classical photon are revealed.
2009, 58 (5): 2878-2883. doi: 10.7498/aps.58.2878
Taking the gross-pitaevskii（G-P） mean-field energy functional of Bose condensed gas as the objective function, we study the properties of Bose condensed gas in 1D optical lattice system by using the genetic algorithm, and present a new method to find the ground-state wave function of the system. Through the optimized computation, the currently commonly used Thomas-Fermi approximation and Gaussian approximation model are revised and discussed, and the optimal ground state wave function is given.
2009, 58 (5): 2884-2888. doi: 10.7498/aps.58.2884
The Pancharatnam phase in simple polariton model is investigated, and the influences of temperature, coupling strength, frequency difference of photon and phonon and average photon number on the evolution dynamics of Pancharatnam phase are also discussed. The results show that the Pancharatnam phase oscillates with time, the oscillating frequency and wave-form vary with time, and the variation increases with the enhancement of temperature, coupling strength and frequency difference of photon and phonon. The system Pancharatnam phase evolutes regularly when the average photon number of coherent light is small, while it becomes more chaotic as the average photon number increases.
2009, 58 (5): 2889-2894. doi: 10.7498/aps.58.2889
Stochastic resonance （SR） is studied in linear systems under modulated noise when the modulated noise is a linear combination of multiplicative and additive noise.The exact expression of the first two moments and signal-to-noise ratio（SNR） are obtained.We find the bona fide SR, conventional SR and SR in the broad sense. Both resonance and suppression appear in the bona fide SR. Moreover,choosing mutilplicative and additive noise as modulated noise can enhance SNR for different degrees of cross correlation.
2009, 58 (5): 2895-2901. doi: 10.7498/aps.58.2895
The stochastic resonance is studied for a damped linear oscillator subject to both parametric excitation of random noise and external excitation of periodically modulated random noise. By means of the Shapiro-Loginov formula, the expressions of the first-order and the second-order moments are obtained for the system response. It is found that there exist conventional stochastic resonance, bona fide stochastic resonance and stochastic resonance in a broad sense in the system. When the noise intensity ratio R≥1, the stochastic multi_resonance is found in the system. Moreover, the numerical results of power spectrum density of system response are presented to verify the analytic results.
2009, 58 (5): 2902-2906. doi: 10.7498/aps.58.2902
A dynamic renormalization-group method is generalized to explore the anomalously dynamic scaling property of kinetic roughening growth equation and the general conclusion on the anomalous exponents of the growth equation with spatially and temporally correlated noise is drawn. The results of the anomalous exponents are employed in several typical local growth equations，which include the Kardar-Parisi-Zhang（KPZ）equation，linear equation and Lai-Das Sarma-Villain（LDV） equation, to judge the condition of anomalous scaling behaviors. Analysis shows that within the long wavelength limit the dynamic scaling property of a growth equation is related to the most relevant term, the dimension of the system and noise; and if the anomalous scaling of the equation exists, super_roughening instead of intrinsic anomalous roughening will be displayed in local growth models.
Effects of time period modulation of the noise correlation intensity and bias signal modulation of the noise on stochastic resonance of a single-mode laser
2009, 58 (5): 2907-2913. doi: 10.7498/aps.58.2907
In the case of the noises modulated by the bias signal, the stochastic resonance of a single-mode laser driven by two colored noises with time period modulation correlation intensity is investigated. By means of the linear approximation method, we calculate the output signal to-noise ratio. The evolution of the output signal to-noise ratio with the self-correlation time of the noises and noise intensity and bias signal is discussed. It is found that the evolution of the output signal to-noise ratio with the pump noise intensity and self-correlation time presents the stochastic resonance. The peak of the stochastic resonance is decreased by the bias signal. Thus we should control the intensity of the bias signal in practice.
2009, 58 (5): 2914-2920. doi: 10.7498/aps.58.2914
The question about the control of hyper chaotic Lorenz system has been studied, and the method of hybrid control by constant impulse and adaptation impulse has been proposed according to the impulse control principle and the adaptive control policy. The method can extend the range of control parameter that can control the system to periodical orbit. The simulation results showed the effectiveness of this method.
2009, 58 (5): 2921-2929. doi: 10.7498/aps.58.2921
The initial condition estimate of local coupled map lattice system based on given symbolic sequence is researched. The relationships be tween the parameter of coupling strength, the map function and the performance of estimating initial condition are analyzed. It is proved that any value taken from the phase-space IM does not necessarily converge to the initial vector. There is a direct relationship between convergence property and coupling strength. Moreover, the performances of estimating initial vector are different according to different map functions. The simulation results are completely consistent with theoretical analysis, which shows that the proposed arithmetic for estimating initial vector of coupled map lattice system is correct. The inverse process of coupled map lattice system is fully investigated and the simulation results in this paper provide a theoretical and factual basis for better analysis and description of the physical process of the actual model.
2009, 58 (5): 2930-2933. doi: 10.7498/aps.58.2930
Using the homotopic mapping method, a class of generalized Sine-Gordon equation is obtained. Firstly, by introducing a homotopic mapping, the homotopic mapping solution for original equation is constructed. The analytic solution for the problem is finally obtained.
2009, 58 (5): 2934-2938. doi: 10.7498/aps.58.2934
Based on the characteristic of the electrical-optical bistable system under the condition of long delay, we propose a new scheme of realizing chaos control and synchronization. The results of numerical calculation show that by adjusting the driving intensity and the state of the driving system, not only the driven system can achieve stable control of different periodic states, but also the general chaos synchronization between the driving system and driven system can be realized. Using the maximal Lyapunov exponent （MCLE） as the criterion, the parameter range for realizing chaos synchronization is given.
Fuzzy impulsive control of chaos in permanent magnet synchronous motors with parameter uncertainties
2009, 58 (5): 2939-2948. doi: 10.7498/aps.58.2939
In this paper, the Takagi-Sugeno（T-S） fuzzy impulsive control model for PMSMs with parameter uncertainties is established via the T-S modeling technique and impulsive technique. Based on the new model, the control conditions of asymptotical and exponential stability for PMSMs with parameter uncertainties have been derived by Lyapunov method and matrix analysis. An illustrative example is also given to show the effectiveness of our results. Compared with the existing results, the obtained results exhibit certain advantage.
2009, 58 (5): 2949-2956. doi: 10.7498/aps.58.2949
Current mode controlled Boost converter has two boundaries in a wide range of circuit parameters. This paper establishes the piecewise smooth iterative map of the converter by utilizing ramp compensation current and derives borderline equations of the orbit state shifting. The reverse bifurcation diagrams and dynamic behavior distribution diagrams with input voltage and compensation slope as parameters are obtained by numerical simulation. The results indicate that with the input voltage reduction, Boost converter enters into robust chaos under continuous conduction mode （CCM） through a border-collision bifurcation on border 1 from stable period-one, and enters into weak chaos and strong intermittency under discontinuous conduction mode （DCM） via a border-collision bifurcation on border 2 By using ramp compensation, the Boost converter can shift from DCM to CCM, and can effectively be controlled to operate at stable period-one region.
2009, 58 (5): 2957-2962. doi: 10.7498/aps.58.2957
This paper studies the chaotic behaviors of the fractional-order conjugate Chen system. The necessary condition for the existence of chaotic attractors in the fractional-order conjugate Chen chaotic system is obtained. Furthermore, a new circuit unit is proposed to realize the fractional-order chaotic system. The results between numerical emulation and circuit experimental simulation are in agreement with each other and prove that chaos actually exits in the fractional-order conjugate Chen system.
2009, 58 (5): 2963-2970. doi: 10.7498/aps.58.2963
The dynamical evolution process of the coupled system connecting two nonlinear electrical circuits with suitable circuit is investigated. The bifurcation behavior as well as the ways to chaos of the two subsystems is presented. It is pointed out when both of the two subsystems behave as periodic, the coupled system may also be led to chaos via cascading of period-doubling bifurcations. Meanwhile, in the chaotic region, critical increase of period as well as period-adding bifurcation can be observed. As to the interaction between periodic movement and the chaotic oscillation, the original periodic subsystem may chaotically oscillate around the original orbit. The amplitude associated with the oscillation increases rapidly, resulting in the obvious chaotic characteristics. On the contrary, the periodic subsystem may not only cause the instability of the chaotic subsystem, but also lead to change of the chaotic structures.
2009, 58 (5): 2971-2976. doi: 10.7498/aps.58.2971
Chaotic behaviors in loss-modulated erbium-doped fiber-ring lasers are reported. In a single-frequency loss-modulated erbium-doped fiber-ring laser, the coexistence of period-doubling bifurcation and intermittency routes to chaos is observed by changing the modulation frequency or the modulation voltage. The results are in agreement with those in an erbium-doped fiber-ring laser subjected to pump modulation. Dual-frequency loss-modulated erbium-doped fiber-ring lasers are proposed. When the value of the frequency ratio is approximately equal to the gold mean and the silver mean, respectively, the transition to chaos via quasiperiodicity is found when the modulation voltage is increased. The modulation voltages for the transition are different for the varied values of the frequency ratio.
2009, 58 (5): 2977-2982. doi: 10.7498/aps.58.2977
The control problems of chaotic dynamical patterns of single Hindmarsh-Rose neuron model are studied by using delayed feedback self-control method. Taking gain factor and time delay as controlling parameters respectively, in some ranges of the combination of gain factor and time-delay, we find that the chaotic burst pattern of inter-spike interval sequences of H-R neuron can be controlled to a single_spike period, double_spikes period, 3_or 4_spikes period pattern or multi-period of these patterns for inter-spike interval as the results of numerical simulation analysis. Choice of delay is in-dependent and doesn't rely on the period of unstable periodic orbits embedded within chaotic attractor. The chaotic burst orbit will be controlled to a certain type of periodic patterns of inter-spike interval automatically.
p-moment stability of stochastic impulsive differential equations and impulsive synchronization of Lorenz system excited by parameter white-noise
2009, 58 (5): 2983-2988. doi: 10.7498/aps.58.2983
In this paper, the p-moment stability of impulsive differential equations is considered. A theory about this problem under a weak condition and an assumption which is more familiar in impulsive system is proved. As an application, the impulsive synchronization of Lorenz system excited by white-noise is considered, the p-moment stability of error system is proved, so the synchronization can be realized using impulsive method under p-moment stability. Numerical simulation verify the feasibility of this synchronization method.
2009, 58 (5): 2989-2995. doi: 10.7498/aps.58.2989
Spring pendulum under single-frequency excitation can be described by a group of very complicated nonlinear equations. The paper studies the amplitude of the spring pendulum, and feedback controllers are designed. Analytic method is used to obtain the control equation of amplitude, the relationship function between control parameter and amplitude is also abtained,which may act as an implementation of feedback control method for saddle-node bifurcation control, and also brings multi-scale method to numerical calculation of multiple-degree-of-freedom nonlinear systems.
2009, 58 (5): 2996-3000. doi: 10.7498/aps.58.2996
The effect of diffusion on the refraction of plane wave in two-dimensional reaction-diffusion system described by complex Ginzburg-Landau equation are numerically investigated. We derive the refractive index of plane wave from Snell's law. The numerical and theoretic results show that the refraction of plane wave in a simple purely diffusive system obeys Snell's law, i.e, diffusion only impacts the refractive index of plane wave. However, the refraction of plane wave in reaction-diffusion system obeys Snell's law only for proper diffusion coefficients and system parameters. These results show that diffusion impacts the refraction law and refractive index.
2009, 58 (5): 3001-3007. doi: 10.7498/aps.58.3001
Gathered movement follow with the group is a common behavior among familiar pedestrians. In order to study such behavior acts on the impact of the evacuation process, we proposed a new cellular automaton evacuation model which taking into account side-by-side in paired, front-behind in paired, and mixed paired, studied the influence of evacuation among the three paired pattern and compared each other under different parameters.
2009, 58 (5): 3008-3013. doi: 10.7498/aps.58.3008
The conception of mass subset is proposed for aerosol which also has been extended to the equivalent mass subset corresponding to counting signal channel. According to statistics, the fractal measurement model of aerosol's equivalent volume for counting signal is established. Due to the aerosols' shape distribution of different mass subsets showing the statistical self-similar character, the calculating formula is given for aerosol equivalent mass of signal channels in the process of aerosol mass measurement. Based on this, the theoretical basis is founded for aerosol mass measurement by using counting method.
2009, 58 (5): 3014-3021. doi: 10.7498/aps.58.3014
In this paper, a cellular automaton model is proposed to study the traffic of urban expressway systems with on-off-ramps and accessory roads. Three lane changing rules are defined for different road sections. Simulation results show that higher on-ramp rate easily produces traffic jams on main roads, on-ramps and their upstream sections. Higher off-ramp rate easily leads to conflict with the inflow of accessory road. The system having two lanes on main and accessory roads can alleviate the jam degree and decrease the vehicles' running time when on-ramp rates are high.
Investigation of phase equilibria and nucleation for supercritical carbon dioxide and model copolymer mixtures
2009, 58 (5): 3022-3027. doi: 10.7498/aps.58.3022
In this work, an equation of state based on the Weeks-Chandler-Andersen （WCA） perturbation theory was established for the mixture of SC-CO2 and model copolymer. The influences of temperature, pressure and chain length of model copolymer molecules on phase equilibria and critical micelle concentration （CMC） were investigated.
2009, 58 (5): 3028-3033. doi: 10.7498/aps.58.3028
In this paper, we propose a spatial network model based on the optimal expected traffic. The model maximizes the total expected traffic of the network. By changing two parameters α and γ which represent the dependence of the network on the fitness and geographical constraints respectively, the model can vary its topology from heterogeneous star-like networks to homogeneous road-like networks. We use our model to simulate the Chinese city airline network and compare it with the real data. In the end of the paper, we discuss the relationship of the expected and the real traffic （namely the real weight on the network）.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2009, 58 (5): 3034-3040. doi: 10.7498/aps.58.3034
Most recently, both BABAR and Belle experiments found evidences of neutral D mixing. In this paper, we discuss the constraints on the strong phase difference in D0→Kπ decay from the measurements of the mixing parameters y′, yCP, and x at the B-factories. With CP tag technique at ψ（3770） peak, the extraction of the strong phase difference at BES-Ⅲ is discussed. The sensitivity of the measurement of the mixing parameter y is estimated in BES-Ⅲ experiment at ψ（3770） peak. Finally, we make an estimate on the measurements of mixing rate RM.
2009, 58 (5): 3041-3046. doi: 10.7498/aps.58.3041
Gaussian pulse shaping filter of exponential decay signal based on wavelet analysis can be more described by using linear combination of both Gaussian function and its first order derivative. On the basis of the recursive approximate algorithm of Gaussian function and its derivative, the recursive approximate algorithm of Gaussian pulse shaping of exponential decay signal based on wavelet analysis is studied. The experiment results with simulating and sampling signals show that the recursive approximate Gaussian pulse shaping is well accorded with the direct convolution of the Gaussian pulse shaping. The characteristic of the Gaussian pulse shaping based on wavelet analysis is well exhibited by the recursive approximate Gaussian pulse shaping. The obtained result can serve as the digital algorithmic base for studying the realization of the Gaussian pulse shaping in digital chip.
ATOMIC AND MOLECULAR PHYSICS
2009, 58 (5): 3047-3051. doi: 10.7498/aps.58.3047
The structure and dissociation energy of the ground state of OH and SH are calculated using QCISD（T）/6-311++G（3df,2pd） method and QCISD（T）/ 6-311++G（3df,2pd）. 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.
2009, 58 (5): 3052-3057. doi: 10.7498/aps.58.3052
The influence of external electric field ranging from -003 to 003 a.u. on the equilibrium geometry, HOMO energy level, LUMO energy level, energy gap, harmonic frequency and infrared intensity of MgO ground state molecule is investigated by employing the density functional methods （B3LYP） with basis sets 6-311+G（2DF）. The results show that the LUMO energy level, energy gap and harmonic frequency decrease, but the total energy increases with increasing external field. The bond length is proved firstly decreasing, then increasing, however, the HOMO energy level is proved firstly increasing, then decreasing with increasing external field. The maximum of HOMO energy level and minimum of bond length are -021765 a.u. at F=001 a.u. and 017397 nm at F=002 a.u, respectively. The external electric field has effect on excitation energies, oscillator strengths and the position and intensity of infrared spectrum.
2009, 58 (5): 3058-3063. doi: 10.7498/aps.58.3058
The transition wavelengths, oscillator strength, Einstein An0 and B0n coefficients of excitation of silicon dioxide molecule from ground state to the first five different excited states are calculated by employing density function theory B3P86 and single substitute configuration interaction approach with basis set 6-311G**. The excited states of silicon dioxide molecule under different external electric fields are investigated. It is shown that the HOMO-LUMO gaps become smaller and the electrons of the occupied orbital are more apt be exited to the virtual orbital as the external electric field intensity becomes stronger. Thus the application of the external electric field facilitates the exitation of the SiO2 molecules.
First-principles calculation of the transport properties of silicon-carbon （Si-C） and alumium-nitrogen （Al-N） nanowires
2009, 58 (5): 3064-3070. doi: 10.7498/aps.58.3064
We investigate the transport properties of oue dimensional silicon-carbon ［（SiC）n］ and alumium- nitrogen ［（Al-N）n］ nano-wires coupled to two Al（100） electrodes based on a recently developed ab-initio nonequilibrium Green function formalism.The equilibrium conductance of silicon-carbon and alumium-nitrogen nano-wires decrease with the length of wires. The charge transfer of silicon-carbon increases monotonically with the the length of wires. On the contrary, the charge transfer of alumium-nitrogen nano-wire decreases monotonically with the increasing length of wires. The charge transfer changes almost linearly with increasing gate-voltage for both nano-wires and the variation of the equilibrium conductance is different for the wires. With the increase of the gate-voltage, both nano-wires might be good candidates for molecular switch, especially the （AlN）5 nano-wires.
Investigation of odd-parity excited states of europium atoms by resonant photoionization spectroscopy
2009, 58 (5): 3071-3077. doi: 10.7498/aps.58.3071
In this paper, two different detection methods, photoionization and autoionization, are combined to study the spectrum of highly excited states of Eu atom. On the one hand, the bound Eu spectrum with odd-parity in the region of 43927—45010 cm-1 has been measured. The spectral information, such as position and intensity, of 32 transitions can be deduced from the spectra with calibration and the error estimation. The information of excitation process and the selection rules enables us to determine their total angular momenta. Comparison with the related literature show that 12 of the states are newly discovered. On the other hand, we design an effective method to identify different types of highly excited states. The spectra of odd-parity states in the same region are measured by both photoionization and autoionization methods, from which we may separate the Rydberg states from the valence states. of the 32 transitions only 14 can be identified as Rydberg states, and the others are valence states. Furthermore, the effective quantum numbers of the Rydberg states are also discussed in detail with the quantum theory.
Numerical study of the enhancement of efficiency of high-order harmonic generation in two-color laser field
2009, 58 (5): 3078-3083. doi: 10.7498/aps.58.3078
The time-dependent Schrdinger equations （TDSE） of the interaction of two-color laser pulse with Pschl-Teller potential and He+ are solved by using the asymptotic boundary condition and symplectic algorithm. The population of ionization, average distance, high-order harmonic generation （HHG） and population of transition are calculated. The numerical results show that the conversion efficiency of HHG is enhanced in the two-color laser fields, then the qualitative and quantitative analysis for the enhancement of conversion efficiency of HHG are given.
2009, 58 (5): 3084-3093. doi: 10.7498/aps.58.3084
The interaction potentials of the He-HF（DF,TF） van der Waals complexes have been obtained by center of mass transformation and then employing Murrell-Sorbie potential function to fit the accurate interaction energy data, which have been computed at symmetry-adapted perturbation theory （SAPT） level. The close coupling calculation of the partial cross sections for collision of He with HF（DF,TF） is performed by employing the fitted interaction potential. This calculation is performed for incident energies from 30 meV to 120 meV, and the information of the elastic, inelastic and total partial cross sections has been obtained. Further, the change tendency and character of the partial cross sections are discussed, and the ranges of effective interaction of the beginning of elastic and inelastic scattering have been determined for He-HF（DF,TF）collision system.
2009, 58 (5): 3094-3103. doi: 10.7498/aps.58.3094
The energy properties and the differential cross-sections of laser-electron-Compton scattering are quantitatively investigated. The calculations show simple structures of the scattered photon energy and the angular distribution of the differential cross-section. At high incident electron energy, Compton scattering X-ray source has features of easily tunable photon energy and small forward emission angle. Although the dispersion of Compton scattering X-ray source is large, narrow bandwidth X-ray emission can be obtained experimentally by using specific monochromatic filter. The total cross-section and the cross-section within the forward emission angle （γ, where γ= E/m0c2 is the total electron energy （E） in the unit of its static energy （m0c2）） change slightly for different laser wave lengths and different incident electron energies in a broad range from 1 MeV to 10 GeV. They have values of 0067 mb and 0033 mb, respectively. The total cross-section of the visible lights is very small. These results may be useful for building new generation ultra-short X-ray source based on Compton scattering technique.
2009, 58 (5): 3104-3111. doi: 10.7498/aps.58.3104
Possible geometrical structures and relative stability of SimCn （m+n≤7） clusters are studied by using the hybrid density functional theory （B3LYP） with 6-31G*basis sets in this article. For the most stable isomers of SimCn （m+n≤7） clusters, the binding energy per atom （Eb）, second difference in energy （Δ2E） and HOMO-LUMO gaps （Eg） et al. are analyzed. The calculated results show that: with increasing of the number of atoms the structure of SiC binary clusters transform linear into planar, and then into a three-dimensional structure. When the atomic number is less than 5, all clusters have planer structure except for Si5 and Si4C. With the increase of C atom, the average binding energy of SimCn （m+n≤7） clusters increases, which means that clusters of “rich C" are more stable than clusters of “rich Si", and Sin clusters with C doping can increase the stability. Cn, SiCn and Si2Cn clusters show clearly “odd-even" oscillation and the “magic number" effect, and Si2C,Si3C, Si5C,SiC2,Si3C2, Si4C2 and SiC4 clusters are more stable than other clusters.
Density functional calculation of the geometric and magnetic properties of MPb10（M=Ti，V，Cr，Cu，Pd） clusters
2009, 58 (5): 3112-3117. doi: 10.7498/aps.58.3112
The geometric and magnetic properties of MPb10 （M=Ti，V，Cr，Cu，Pd）clusters with four probable isomers have been studied using the generalized grandient approximation based on density functional theory. It was found that the D4d structures of MPb10 （M=Ti，V，Cr，Cu，Pd） have the highest binding energy and largest energy gaps among the four possible isomers, indicating that the D4d structure is the ground statess of MPb10 cluster with high kinetical stability. The magnetism study shows that the ground states of TiPb10，VPb10 and CuPb10 clusters have 2 μB, 1 μB and 1 μB magnetic moments, respectively. For M=Ti and Cu, the magnetic ordering of MPb10 clusters is in a weak ferromagnetic arrangement between M and Pb atoms, while there is both weak ferromagnetic and weak antiferromagnetic arrangements between Cu and Pb atoms for the CuPb10 cluster. On the other hand, there is no magnetic moment in the CrPb10 and PdPb10 clusters. Thus, the magnetic properties of MPb10 clusters could be tuned by doping different transition metal atoms into Pb10 cage.
CLASSICAL AREA OF PHENOMENOLOGY
Calculations of three-dimensional frequency-domain nonlinear beam-wave reaction for helix traveling wave tubes
2009, 58 (5): 3118-3124. doi: 10.7498/aps.58.3118
Considering the attenuation, severing, taper and magnetic admittance, the self-consistent working equations are presented, which describe the operation of 3D nonlinear beam-wave interaction for helix traveling wave tube based on periodic structure characteristic and Poynting's theorem. Using Fourier transform from time-dependent current to ac current consisting of a series of wave frequencies, the space fields are obtained through weighing charge particles to fixed meshes with the help of particle_in_cell technique. The model also computes 3D electronic tracks and helix interception current, the electronic energy spectrum of spent beam are represented, which is helpful for designing multistage depressed collector （MDC）. Calculation of Ku-band TWT in IECAS was shown to be in better agreement with experiment, including output power and analysis of MDC.
Geometric optics analysis on self-reconstruction of the nondiffracting beam generated from an axicon
2009, 58 (5): 3125-3129. doi: 10.7498/aps.58.3125
Self-reconstruction of the nondiffracting beam generated from an axicon was analyzed for the first time based on the geometrical optics and the formation principle of self-reconstruction was well explained. In the Fresnel approximation, beam transmission characteristics were described well using the theory of Fresnel diffraction. But the approximation condition will not satisfy in the quite little distance after beam passing through the obstacle and the diffraction theory is no longer suitable. In such case, geometrical optics can be used to describe the beam transmission characteristics. In this paper, self-reconstruction of the nondiffracting beam generated from an axicon was analyzed in detail based on the geometrical optics. Simulation and experimental observation of the self-reconstruction characteristic are presented. The experimental results agree with the theoretical analysis.
2009, 58 (5): 3130-3134. doi: 10.7498/aps.58.3130
Optimal placement for continuous phase plate （CPP） in terminal optical system of the Inertial Confinement Fusion （ICF） driver has been studied. Based on non-linear perturbation and transmitting theory of high power laser, the optical characteristics in near and far fields as well as third harmonic conversion efficiency after passing throngh the frequency conversion system has been calculated. As a result, the third harmonic conversion efficiency and optical characteristics of the emergent light beam are influenced by the CPP which is placed before the frequency conversion system. But if the diameter of the round focal spot in far field is smaller than 05 mm, the decline of the third harmonic conversion efficiency and the rise of contrast ratio are within permitted range, the shape of focal spot in far field and the encircled energy of focal spot also accord with the design requirement at the same time. The CPP which enables small focal spot in far field, when located in the beam path of fundamental frequency of ICF system to smooth and shape the light beam, will not exert influence on the normal running of ICF system.
A method to analyze insertion loss of electroabsorption-modulator using photocurrent and power transmission vs.wavelength
2009, 58 (5): 3135-3139. doi: 10.7498/aps.58.3135
A novel method to ascertain diversified factors contributing to total insertion loss in an electroabsorption modulator is presented.Only the measurement of photocurrent and power transmission vs.wavelength is needed.It's an accurate method, as shown by both theory analysis and experimental results.
Statistics of the instantaneous polarization in Weibull——distributed fields-the same shape parameter case
2009, 58 (5): 3140-3153. doi: 10.7498/aps.58.3140
The statistical properties of partially polarized light in Gaussian stochastic plane wave fields have been studied. In this paper, the statistics of the stochastic plane electromagnetic （EM） wave fields with Weibull distributed is examined. The polarimetric Weibull_distributed wave is characterized by three parameters: the sum of the amplitudes of the two electric vector components, the angle which the major axis makes with the reference coordinate system, and the ratio of the minor to the major axis. The joint and marginal probability densities of these random variables are determined as a function of the covariance matrix. The main properties of this important distribution are shown. Then the statistical properties of the normalized Stokes parameters are described in detail in Weibull_distributed stochastic fields. The joint and marginal probability density functions （PDF） of the three components of the normalized Stokes parameters are presented. Results of some numerical calculation are obtained. The description of the Weibull polarimetric wave field may be useful to random medium scattering and speckle filtering.
2009, 58 (5): 3154-3160. doi: 10.7498/aps.58.3154
The utilization efficiency of light energy is affected by the multi-order images during the reconstruction of holograms based on liquid crystal spatial light modulator （LC-SLM）, which leads to the low intensity of single image. A novel method is proposed to improve the diffraction efficiency of single reconstructed image by synthesizing phase hologram and digital blazed grating. The property and principle of wave-front modulation are analyzed by use of digital blazed grating as diffractive optical element （DOE）. The effect of digital blazed grating on the reconstructed image of phase hologram by use of LC-SLM is also described. Holographic optoelectronic display system based on LC-SLM is set up for phase hologram reconstruction. Theoretical analysis shows that, the intensity of single reconstructed image is increased by 1302 per cent by adding digital blazed grating with period of 2 pixels in vertical or horizontal direction, and 4297 per cent with the same period of digital blazed grating in vertical & horizontal directions. The experimental result also verifies that, by the proposed method, the energy intensity of single reconstructed image is increased and the utilization efficiency of light energy is improved.
Effect of spontaneously generated coherence on transient evolution of LWI gain in an open Ladder type system
2009, 58 (5): 3161-3165. doi: 10.7498/aps.58.3161
The effect of spontaneously generated coherence （SGC） on transient evolution of gain of lasing without inversion （LWI） and modulation roles of the injection rate ratio and exit rate on SGC-dependent gain in an open Ladder type three level system are studied by using the numerical solution of the density matrix motion equation of the system. It was shown that: variation of SGC strength will make the transient and final stationary gains having a considerable changing; when incoherent pumping presents, LWI gain increases with the ratio of injection rates and exit rate decreasing, however, when incoherent pumping is absent, the case is just the opposite; by choosing suitable values of SGC strength, the injection rate ratio and exit rate, we can obtain the largest transient and final stationary gain.
2009, 58 (5): 3166-3172. doi: 10.7498/aps.58.3166
A basic quantum voting protocol using two-mode squeezed states is proposed firstly in this paper. This protocol makes use of the uncertainty principle by using random-selection, possible attack modes are then analyzed. The mode-mode correlation of two-mode squeezed states guarantees the security of the protocol.
2009, 58 (5): 3173-3178. doi: 10.7498/aps.58.3173
A numerical model for self-similar pulse nonlinear polarization rotation mode-locked fiber laser is built by utilizing coupled nonlinear Schrdinger equations （CNLSE）. In this model, the propagation of the pulse within single mode fiber is described by CNLSE. In the gain fiber, gain bandwidth and saturable gain are both considered. Other optical components which implement NPE mode-locking are modeled by transfer matrixes. According to the optimized net cavity dispersion and length of the cavity, the evolution of the pulse along the cavity is simulated. At the same time, characteristics and the typical regime of the self-similar pulse are found. The simulation results show that pulse with 7 nJ energy, 11 ps duration, linear chirped, and of parabolic shape occurs within the optimized self-similar regime. The characteristics of the laser with different dispersion are compared. The influence of the third order dispersion on the laser is simulated.
2009, 58 (5): 3179-3184. doi: 10.7498/aps.58.3179
The lasing threshold of evanescent-wave pumped whispering_gallery_mode （WGM） fiber lasers has been studied. Quartz fibers of different diameters were immersed in a mixed solution of ethanol and ethylene glycol doped by rhodamine 6G dye molecules, the refractive index of the solution was variable and lower than that of quartz fiber. We find that the dependence of lasing threshold on the refractive index of the mixed solution differs for different diameter of quartz fiber. With the increase of refractive index of the mixed solution from 1362 to 1432, the threshold energy decreases slowly and monotonicly for fibers of diameters larger than 296 μm, but increases for fibers of diameters smaller than 93 μm; for a fiber of diameter 197 μm, the threshold energy decreases slowly at first, and then increases sharply, there is an optimum refractive index of the mixed solution which matches a minimum threshold energy. Based on the theory of evanescent-wave pumped WGM fiber laser, we deduce a formula of lasing threshold, the calculated curves of lasing threshold are well consistent with the experimental data.
2009, 58 (5): 3185-3192. doi: 10.7498/aps.58.3185
We proposed a novel extended semiconductor optical amplifier （SOA） model for applications in femtosecond pulse transmission，in which various ultrafast nonlinear effects，such as gain dispersion and group velocity dispersion were taken into account. We analyzed the refractive index in response to variation of the input current and the length of SOA. Furthermore，the impact of carrier density pulsation and carrier heating on the refractive index dynamics of SOA was discussed respectively. The simulation model can be applied in parameter and operation condition optimization of SOAs in order to improve their femtosecond dynamic characteristics.
Study of acousto-optic Q-switched green laser with high efficiency，high stability and high-quality coherent beam generation
2009, 58 (5): 3193-3197. doi: 10.7498/aps.58.3193
High-power and high efficient coherent beam generation with good green-beam quality has been achieved by optimizing the diode-side-pumped linear cavity geometry with two modules in tandem, acousto-optic Q-switching and with S-KTP of Ⅱ-type phase-matching for intracavity frequency doubling of a Nd：YAG laser. When the pump current is 45A, the maximum average green output power of 132 W is obtained with pulse width of 120 ns at pulse repetition rate of 15 kHz, corresponding to the conversion efficiency of 132% from diode pump power to green output power. The M2 beam quality factor is 67 at output power of 130 W, and the output power instability is less than 05% at the same output power during an hour measurement time. The thermo-lensing effect of the laser medium and the stable region of the laser cavity under high power pumping are also theoretically analyzed and experimentally researched.
2009, 58 (5): 3198-3202. doi: 10.7498/aps.58.3198
By using multiple-scale method，we analytically study the nonlinear dynamical properties of an electromagnetically induced transparency medium with three-level atoms and a strong coupling light field. It is shown that a weak probe light beam can form spatial weak-light dark solitons. When it propagates along axial direction，the dark soliton will transforms into a steady spatial weak-light ring dark soliton，which results from the balance of the interplay between beam diffraction and refractive nonlinearity.
2009, 58 (5): 3203-3207. doi: 10.7498/aps.58.3203
The chaotic carrier with smooth spectrum and broad bandwidth is generated by a distributed feedback （DFB） semiconductor laser subject to unidirectional fiber ring optical feedback. The smooth spectrum of the obtained chaotic carrier hides the external cavity resonance frequency peak，therefore the security of the system is enhanced. By unidirectional coupling，we implement the stable chaos synchronization when the chaotic light is injected into a solitary DFB laser diode. The correlation coefficient of synchronization is 0.84. The effects of injection strength and optical frequency detuning on synchronization are investigated experimentally. The results show that in conditions of strong injected locking the synchronization can be achieved in a large range of frequency detuning，and the stronger injection corresponds to the larger range of frequency detuning in which chaos synchronization maintains. The synchronization quality improves with the injection strength increasing.
2009, 58 (5): 3208-3213. doi: 10.7498/aps.58.3208
The effect of nonlinear exposure on the characteristics of photonic crystal fabricated by holography is investigated theoretically and experimentally. A kind of materials with low refractive index （n=152） is used to fabricate diamond structure. By controlling the exposure in strong nonlinear range, the property of photonic bandgap is improved. Also，some proposals are made to achieve complete band gaps by materials with low refractive index.
Tunable effects of connective dendritic left-handed metamaterials based on electrorheological fluids
2009, 58 (5): 3214-3219. doi: 10.7498/aps.58.3214
We experimentally studied the tunable effects of connective dendritic left-handed metamaterials based on electrorheological fluids. Three influencing factors-the distance of electrodes，the permittivity of embedding media，and the intensity of applied electric fields-are discussed respectively. The experimental results show that the distance of electrodes exhibits an important effect on the transmission peak of the samples. The left-handed transmission peak shifts to lower frequency when the electrorheological fluids infiltrate. Moreover，by changing the intensity of electric field，the left-handed transmission peak can be modulated actively，and the maximum shift is up to 140 MHz.
2009, 58 (5): 3220-3223. doi: 10.7498/aps.58.3220
Based on the fishnet negative index metamaterials （NIMs） of circular-shaped holes operating at optical frequencies，the influence of modifying the hole shape is studied using the commercial software package CST Microwave studio. Numerical simulations show that，even a tiny modification may lead to broadening of the negative refractive band. So we designed two fishnet NIMs of the gradient holes and half-global holes. Compared with the fishnet NIMs of circular-shaped holes，these two structures can realize negative refraction at multi-frequency bands，and the resonance frequencies exhibit an obvious red-shift. We present a chemical technique for preparing the fishnet NIMs of half-global holes，which is a simple and feasible way to realize multiband negative index metamaterials at optical frequencies.
2009, 58 (5): 3224-3229. doi: 10.7498/aps.58.3224
By theoretical analysis and experimental simulation，planar left-handed metamaterials using double split-ring resonator pairs are studied in this paper. Simultaneous negative permittivity and permeability are described and explained by using equivalent circuit theory. Moreover，double split-ring resonator pairs used in the design of 3-dimensional left-handed metamaterials are also investigated. The results show that double split-ring resonator pairs can be used as planar left-handed metamaterials with small sizes and that double split-ring resonator pairs can be also used as 3-dimensional anisotropic magnetic metamaterials to realized negative permeability in 3-dimensional left-handed metamaterials.
2009, 58 (5): 3230-3237. doi: 10.7498/aps.58.3230
Based on rigorous coupling theory，the influence of the dispersion of the core，cladding，film and surrounding material on the dual-peak resonance effect is studied. The result shows that the theoretical model which considers both the core and cladding material dispersion is necessary and feasible. Further，the influence of the material dispersion on the transmission characteristics of LPFG is analyzed for different film parameters. The deviation value between the dual resonant peaks is calculated before and after considering the material dispersion. The result shows that the deviation value is 1.5—2 nm and 6.5—7.5 nm, respectively. Finally，the influence of the material dispersion on the sensor sensitivity is studied for the dual-peak LPFG sensor. The result indicates that the minimum resolution for the film refractive index is 10-8，and the contour line figure after considering the material dispersion supplies accurate parameter combinations in the design of this kind of sensor.
Photoinduced refractive index change effect of amorphous Sn-doped As2S8 film and its application in stripe waveguide fabrication
2009, 58 (5): 3238-3242. doi: 10.7498/aps.58.3238
Photoinduced changes in refractive index and film thickness of amorphous Sn1As20S79 semiconductor film are studied experimentally. The emperical rules in as-evaporated，annealed and well-illuminated states are obtained respectively. An ultraviolet irradiation technique is presented and employed successfully to fabricate a Sn1As20S79 stripe waveguide, which shaws good characteristics of a waveguide under the 632.8nm guided mode excitation.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2009, 58 (5): 3243-3247. doi: 10.7498/aps.58.3243
The temporal and spatial distributions of the plasma temperature during the dissipation process of lightning are calculated theoretically by applying the equation of motion，the equation of continuity and the equation of energy conservation of hydrostatics. The result shows that the temperature decay at a given radius is quicker in the earlier stage and slower in the later stage; Furthermore，the higher the original temperature，the quicker the decay. The temperature decays slower when the radius becomes larger. At the same time，the larger the radius of lightning channel，the greater the temperature gradient, and vice versa. According to the time and position of freezing temperature of NO，we can conclude primarily that NOx are produced in the channel of less than 9 mm radius in the time within 50 ms after the lightning return stroke.
Atom-orbital close-coupling calculation of charge exchange processes in collisions of H+ with Li（5d）
2009, 58 (5): 3248-3254. doi: 10.7498/aps.58.3248
The charge exchange process in collisions of H+ with Li （5d） is investigated using the two-center atomic-orbital close-coupling method. The state-selective cross-sections are obtained in the energy range of 0.5—10 keV. It is found that the processes for capture to n=4—7 are the dominant reactions，and the n-distribution of state-selective cross-sections is weakly dependent on the collision energy. But the l-distribution of state-selective cross-sections strongly depends on the collision energy. For the lower collision energy around 1 keV，the l-distribution increases with increasing l. However，the l-distribution moves to the lower quantum number as the collision energy increases，and the maximum value appears around l=1.
Experimental study of radiation temperature for gold hohlraum heated with 1 ns，0.35 μm lasers on SG-Ⅲ prototype laser facility
2009, 58 (5): 3255-3261. doi: 10.7498/aps.58.3255
Experimental measurement of radiation temperature by a streaked optical pyrometer and a soft X-ray spectrometer viewing through the laser entrance hole are performed on SG-Ⅲ prototype laser facility. It was found that the two methods compares well. Using the power balance relation，the laser-hohlraum coupling efficiency for 1.0 mm×1.7 mm hohlraum and 1.0 mm×2.1 mm hohlraum is around 50%—55%.
2009, 58 (5): 3262-3267. doi: 10.7498/aps.58.3262
Fusion neutron and photon neutron producing behavior in the deuterium plasma discharge have been studied on HT-7 superconductive tokamak device by BF3 and 3He proportional ionization chamber implementing the fast time respond neutron flux measurement system. Typical time resolved neutron flux signals have been analyzed in connection with gamma and hard X-ray diagnostic data in the cases of ohmic discharge and lower hybrid wave discharge. Data analysis show that many neutrons are produced by D-D fusion reaction when high-power lower hybrid wave was employed to heat the plasma.
2009, 58 (5): 3268-3273. doi: 10.7498/aps.58.3268
This paper presents the electrostatic PIC-MCC model for simulating the breakdown of dielectric surfaces illuminated by high-power microwave，and the processes of multipactor and collisional ionization in the case of vacuum and different pressures are simulated by using the code developed by ourselves. The numerical results show that the multipactor effect is the main source of electrons，and the number of the electrons oscillates in the time domain at twice the rf frequency in the case of vacuum and low pressures. In the case of high pressure，the collisional ionization is the main source of electrons.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2009, 58 (5): 3274-3279. doi: 10.7498/aps.58.3274
Zn0.95Co0.05O Bulk samples were synthesized by solid state reaction method and annealed by different processes. The Zn treated samples show ferromagnetism，whereas the vacuum annealed samples do not show ferromagnetism. The XRD patterns show that there is Co impurity phase in the sample after Zn treatment at 1100℃，which is the reason of the ferromagnetism observed. At low temperature，both the positive and negative magnetoresistance （MR） effect can be observed for all the samples. We conclude that the s-d exchange_interaction_induced spin-splitting gives rise to positive MR， while the formation of bound magnetic polarons and the magnetic field suppressed spin-disorder scattering are responsible for the negative MR behavior under high field.
2009, 58 (5): 3280-3286. doi: 10.7498/aps.58.3280
A thermodynamic model that describes the thermal properties of the compound phases in the nanocrystalline （NC） alloy systems has been developed，which provides quantitative predictions on the stabilities and transition features of different phases. By using the NC Sm-Co alloy system as an example，the temperature dependence of the mole Gibbs free energy of the NC alloy phases with different excess volumes in the nano-grain boundary regions were provided. Based on the model calculations，the relative stability of different phases and the phase transition rules in the NC Sm-Co system were analyzed. It was shown by the calculation results that near the room temperature，some of the NC alloy phases，whose mole Gibbs free energy values change from negative to positive due to the decrease of the nano-grain size，will transform into stable NC alloy phases. This is distinctly different from the phase transformation features in the coarse-grained alloy systems，whose thermodynamic properties are only dependent on the temperature. The experimental results on the phase configuration and the phase stability obtained from the prepared NC Sm-Co alloy confirmed the theoretical predictions of the present thermodynamic model.
2009, 58 (5): 3287-3292. doi: 10.7498/aps.58.3287
On the basis of the tight-binding theory，we derived the band structure formula of carbon nanotube field-effect transistors with/without a magnetic field. The results show that the band gap becomes zero periodically with increasing strength of the magnetic field，which indicates that the magnetic field can change the nature of the conductivity of carbon between the metal-type and semiconductor-type with 0.50 for the period. Furthermore，by using Natori theory on the field-effect transistor, we studied the influence of applied magnetic field on the current-voltage characteristics of carbon nanotube field-effect transistors. The results show that in the two types of zigzag and armchair carbon tubes the conductivity oscillates with the voltage and the magnetic field and the oscillation behaviors are obviously different for these two types of carbon nanotubes.
2009, 58 (5): 3293-3301. doi: 10.7498/aps.58.3293
In the present paper，two concepts，“defect degree" and “crystallization potential"，are introduced for evaluating the defects in crystal and describing the potential field near the crystal surfaces，respectively. Based on vast MD simulations of Ni，Cu，Al and Ar crystal growth from different faces at various temperatures，it is shown that the defect degree decreases with increasing of the crystallization potential，indicating that the crystallization ability is determined uniquely by the crystallization potential，which can be obtained easily even with ab initio calculations. This result suggests a convenient way to evaluate the ability of materials to form perfect single crystals.
Optical properties revealing annealing behavior of high-temperature He-implantation induced defects in silicon carbide
2009, 58 (5): 3302-3308. doi: 10.7498/aps.58.3302
6H-SiC single crystal specimens were implanted at 600 K with 100 KeV He ions to three successively increasing fluences and subsequently annealed at different temperatures ranging from 600℃ to 1200℃ in vacuum. After the annealing, the samples were investigated by using Raman scattering spectroscopy and photoluminescence spectrometry，respectively. Both of the two methods showed that the damage induced by helium-ion-implantation in the lattice is closely related to the dose. The thermal annealing brings about recovery of the damage，and different levels of damage require different annealing temperature to recover efficiently. It is indicated that different annealing stages involve different mechanisms，corresponding to recombination of point defects，formation of He-vacancy complexes，and nucleation and coarsening of bubbles，respectively. The experimental results indicate that high temperature implantation is an effective way to avoid amorphization of the implanted layer due to damage accumulation. Helium implantation can be used to introduce buried nanoscale cavities as the nucleation site for the buried oxide in a well defined region proposed for an alternative and more economical method of manufacturing SiC-on-Insulator （SiCOI）.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Magnatic field induced phase transition and transport properties in phase separated Eu0.5Sr0.5MnO3 system
2009, 58 (5): 3309-3314. doi: 10.7498/aps.58.3309
The structure，electrical and magnetic properties of the half doping phase separated Eu0.5Sr0.5MnO3 manganite have been systematically studied. The results show that Eu0.5Sr0.5MnO3 compound has the O′ orthorhombic structure and shows typical Jahn-Teller distortion. It is found that ferromagnetic phase turn up near 75 K and evident split of field cooling （FC） and zero field cooling （ZFC） under 4000 A/m is observed around 42 K. The ac susceptibility curve shows also a sharp peak around 42 K，indicating cluster-glass state rather than a spin glass state at 42 K. Meanwhile，throughout the measuring temperature range，the electric transport shows insulating behavior and there is no insulator-metal （I-M） transition. However，an applied magnetic field of 1.6×105 A/m can induce I-M transition. The conducting behavior of Eu0.5Sr0.5MnO3 is well fitted by the Mott variable range hopping （VRH） model. All these phenomena indicate that for the ground state of Eu0.5Sr0.5MnO3，there exists the competition mechanism of several complex magnetic interactions. The study provides abundant experimental information to underdtand the mechanism of the strongly electron correlated system.
2009, 58 (5): 3315-3318. doi: 10.7498/aps.58.3315
The atomic cluster models of α-Mg，liquid Mg and the interface between liquid/solid have been founded. The environment-sensitive embedding energy of Ca and Be in α-Mg，liquid Mg，liquid/solid interface has been calculated by recursion method. The atomic affinity energy between Mg，Ca，Be with O has been defined and calculated. The calculated results show that the solid solubility of Ca and Be is very small in α-Mg，because of their higher environment-sensitive embedding energy leads to instability in α-Mg crystal. The Ca and Be diffuse in to the liquid Mg, which has lower environment-sensitive embedding energy than the solid，and congregate on the surface of liquid Mg as the alloys solidify. Because the atomic affinity energy of Ca-O and Be-O is lower than Mg-O，The Ca and Be aggregating on the surface of liquid Mg will priorly combined with O，forming compact oxides of Ca，Be and alloys of elements，which prevent Mg alloys from burning.
2009, 58 (5): 3319-3323. doi: 10.7498/aps.58.3319
The electronic parameters，such as the structural energy and atom binding energy of both α-Mg and α-Zr，the Mg/Zr interface energy and surface energy of Mg，as well as the interaction energies between Zr atoms and between Zr and impurity atoms，were calculated by means of recursion method. The calculated results showed that the structural energy and atom binding energy of α-Zr are lower than those of α-Mg，and the Mg/Zr interface energy is lower than the surface energy of Mg，which，in an energetical point，explains the experimental phenomenon. Specifically，Zr particles first crystallize out of the Mg liquid and then act as heterogeneous nucleation sites to refine the Mg grains. The atom interaction energies indicate that the Zr atoms can attract each other to form Zr atom clusters in Mg，and combine with impurity atoms to form compounds，thus weakening the Mg grain refining effect.
2009, 58 (5): 3324-3330. doi: 10.7498/aps.58.3324
Calculation of electronic structures and magnetic properties of Mg （or Zn＼Si＼O） and Mn co-doped GaN were carried out by means of first-principle plane-wave pesudopotential （PWP） based on density functional theory. Estimation of Curie temperature was achieved by using Heisenberg model in the mean-field approximation and Zener theory，respectively. The spin polarized impurity bands of deep energy levels are found for several co-doped systems, which are half metallic and suitable for spin injectors. Compared with GaN：Mn，p-type co-doped （GaN：Mn-Mg＼Zn） systems exhibit more stable ferromagnetic state and a significant increase in TC. Nevertheless，n-type co-doped （GaN：Mn-Si＼O） systems fail to increase the TC and stability of ferromagnetic state.
2009, 58 (5): 3331-3337. doi: 10.7498/aps.58.3331
Rhodium adsorption on （6, 6） single-walled carbon nanotubes （SWCNTs） was systematically investigated by density functional theory （DFT）. According to the analysis of adsorption configurations and adsorption energies, it is found that the most stable configurations are I-H1, I-H2, O-H1, O-H2 and O-B1. The adsorption energy of O-H2 is -2.29?eV, which is 0.49?eV higher than that of I-H2. This might be attributed to the graphite's winding effect, which the well-proportioned charge density on the graphite redistributes during the formation process of SWCNTs and then induces more charge to the outside of SWCNTs than the inside. Based on the partial density of states （PDOS） and Mulliken charge analysis, it is found that 5s electrons of Rh transfer to 4d orbit, while 4d electrons transfer to SWCNTs. Therefore, the Rh atom is positively charged, while the （6, 6） SWCNTs is negatively charged. Combining the PDOS calculations with the band structure results, the magnetism of Rh adsorbed outside of SWCNTs is higher than that inside SWCNTs.
The geometrical structure, electronic structure and magnetism of bimetallic AunM2 （n=1,2; M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni） clusters
2009, 58 (5): 3338-3351. doi: 10.7498/aps.58.3338
The geometrical structure, stability, electronic structure and magnetism of bimetallic clusters AuM2 and Au2M2, where M is 3d transition metal element, are investigated systematically by using the first-principles method based on density functional theory. In contrast to semiconductor clusters, the bimetallic clusters consisting of Au and transition metal elements usually form a large number of low-energy isomers, some of which are very similar in structure. Similar to the pure transition metal cluster, AuM2 and Au2M2 clusters also display dramatic magnetism. The magnetic moment of transition metal element in AuM2 and Au2M2 clusters is either enhanced or weakened with respect to the bulk value, which is closely dependent on the orbital splitting. For the ground state, the magnetic moments of two transition metal elements in AuCr2, Au2Cr2 and Au2Mn2 clusters are anti-parallel, and those in other clusters are parallel.
2009, 58 (5): 3352-3358. doi: 10.7498/aps.58.3352
The adsorption of H2O on Fe（100）, Fe（110） and Fe（111） crystal surfaces were studied by first principles. The calculation results indicate that H2O monomers bind preferentially at top sites and lie nearly flat on the three crystal surfaces. The results of adsorption energy and geometry structure show that the effect of H2O adsorbed on three iron crystal surfaces is different. The strongest interaction occurred between H2O and Fe（111） crystal surface and the weakest interaction is between H2O and Fe（110） crystal surface. Such relationship is related to the different surface atoms density. The similar conclusion can be drawn from the calculation results of electronic structure and Mulliken analysis. The Mulliken analysis also indicates that when H2O is adsorbed on the iron surface, charge exchange between O atom and Fe atom make the iron surface negatively charged and reduce the surface potential, which promote the electrochemical corrosion of iron surfaces.
2009, 58 (5): 3359-3363. doi: 10.7498/aps.58.3359
The electronic structure parameters of pure Ti and TiPt alloy, such as density of states, Fermi energy level, environment-sensitive embedding energy and order energy have been calculated by recursion method. The environment-sensitive embedding energy induced by Pt in Ti crystal is greater than that on Ti surface, so Pt is prone to gather on Ti surface. The order energy between Pt atoms is positive, which leads Pt atoms to form intermetallics with bulk Ti atoms. The electric potential of intermetallics is lower than that of Ti atom cluster at Ti alloy surface, so cells will form between intermetallics and Ti atom clusters, then the intermetallic will decompose in corrosive medium. With the deposition of Pt, a rough electrocatalytic Pt layer would form on the surface of Ti alloy. This electrocatalytic Pt layer enhances the passivation of Ti alloy and improves the corrosion resistance of Ti alloy.
2009, 58 (5): 3364-3369. doi: 10.7498/aps.58.3364
The atomic cluster models of Mg/CNT（carbon nanotube or nickel carbon nanotube） interface have been built in magnesium composite reinforced by （nickel） carbon nanotube. The interface electronic structures have been calculated by recursion method. The calculated results show that nickel enhances the stability of nanotube/matrix interface, improves the interface bond strength, since nickel makes the structure energy and atomic binding energy of nanotube/matrix interface lowered. The positive interaction energy of magnesium atoms in nickel film of interface causes the magnesium atoms in nickel film to repel each other and can not form magnesium atomic cluster, so they prefer to form ordering phase which consolidate the interface. The carbon and magnesium atoms have higher energy of position, which causes interface instability and flimsiness at carbon nanotube without nickel plating and magnesium interface. But when the carbon nanotubes have been plated by nickel, the energy of position of carbon and magnesium atoms are made lower and the interface gets more stable and tough.
2009, 58 (5): 3370-3376. doi: 10.7498/aps.58.3370
The generalized gradient approximation based on density functional theory is used to analyze the structural and electronic properties of the unconventional exohedral fullerene C64Si. It is found that among four possible stable isomers, the structure with the single Si atom situated at the vertex of the directly fused pentagons, i.e., C64Si-1, is the most stable, which agrees well with the stable position of Si outside the C28 cage proposed by Ge et al. By analyzing the energy levels, orbital wave functions and density of states of both C64 and C64Si-1, Si atom is found to have greater contribution to the lowest unoccupied molecular orbital, but contribute less to the highest occupied molecular orbital. In addition, the effect of Si on the total density of states of C64 is very small. It is found from the vertical ionization potential and the vertical affinity that the ability for both detaching and obtaining electrons is decreased after the exohedral absorbing Si atom.
2009, 58 (5): 3377-3382. doi: 10.7498/aps.58.3377
The amorphous GaN（a-GaN） films are deposited by direct current planar magnetron sputtering in different compositions of argon-nitrogen mixtures. X-ray diffraction patterns and the Raman spectra indicate that the films have amorphous structures. Spectroscopic ellipsometry shows that the refractive index and the film thickness increase with increasing argon content. The UV-Vis spectra indicate that the band gap of the film deposited without argon is 3.90eV, which is much larger than that of crystalline GaN （c-GaN）, owing to the structural disorder. When the films are deposited at higher argon content, the band gap becomes much smaller （2.80—3.30eV）, probably because of more excessive Ga in the films. The band tails extending to lower energies are modeled. Two parameters representing the energy broadening of the electronic transitions and the width or slope of the exponential tail are 0.257—0.338eV and 1.44—1.89?eV, respectively, which are higher than that of c-GaN films, indicating that a-GaN films have wider absorption tails than the c-GaN films. Photoluminescence peak at 360 nm observed at room temperature comes from band_to_band emission.
2009, 58 (5): 3383-3389. doi: 10.7498/aps.58.3383
To estimate the field enhancement factor of the gated nanowire, the image charge model of floating sphere between parallel gate and cathode plates is proposed. The field enhancement factor of the gated nanowire is expressed by β=1/2（3.5+L/r0+W）, where L and r0 are the length and tip radius of nanowire, respectively, and W is a function of the gate-hole radius R, gate-cathode distance d and the geometrical parameters of the nanowire. The calculation results show that the influence of the aspect ratio of the nanowire on the enhancement factor is remarkable, i.e., the enhancement factor increases rapidly with the increase of the length and top curvature of the nanowire. Furthermore, the enhancement factor decreases with the increase of the gate-cathode distance and is equal to β0=3.5+L/r0 when the gate-cathode distance tends to infinite. The smaller the gate-hole radius, the larger the enhancement factor, and the enhancement factor will be equal to β=β0+1.202（L/d）3 when the gate-hole radius tends to zero.
2009, 58 (5): 3390-3396. doi: 10.7498/aps.58.3390
The multi-terminal quantum transport in a dot-array through an AB ring is studied via single-band tight-binding Hamiltonian. It is shown that the output spin current is a periodic function of the magnetic flux in the quantum unit Φ0. Moreover, we can get the spin current with contrary spin polarization in the same energy range in different terminals.
Spin polarized current transport and charge polarization effect in ferromagnetic GaMnN resonant tunneling diode
2009, 58 (5): 3397-3401. doi: 10.7498/aps.58.3397
The spin-polarized tunneling current transport through a ferromagnetic GaMnN resonant tunneling diode is investigated theoretically. Two distinct spin splitting peaks can be observed in the current-voltage characteristic. Spin splitting peaks and the spin polarization decrease and then disappear with increasing temperature. When charge polarization effect is considered for the GaN heterostructure, the spin-down resonant current peak becomes enhanced significantly and spin polarization is also increased accordingly. A highly spin polarized current can be obtained at a certain polarization charge.
Preparation and electrical properties of the novel mixed ionic-electronic conductors Sm0.9Ca0.1Al1-xMnxO3
2009, 58 (5): 3402-3408. doi: 10.7498/aps.58.3402
The novel mixed ionic-electronic conductors Sm0.9Ca0.1Al1-xMnxO3 （SCAM, x= 0.1—0.5） were prepared by the organic gel method combined with solid state sintering technology. The thermal decomposition and phase inversion process of the gel precursors, crystal strucutres and phase stability of the sinters were studied by using thermogravimetric and differential thermal analysis （TG-DTA） and X-ray diffraction. The electrical conductivities of sintered ceramics in both air and 5%H2/Ar were measured by the direct current four-wire method. The experimental results show that the well-crystallized nanopowders with tetragonal perovskite structure can be obtained after calcining the gel precursors at 900℃ for 5h, the phase stability of SCAM ceramics decreases with increasing Mn content under reduction atmosphere. The electrical conductivity of SCAM ceramics is dominated by p-type electronic conduction and increases with Mn content and temperature. The mechanism of electronic transport is the hopping of p-type small polaron. With increasing calcination temperature and prolonging holding time, both the electrical conductivity and relative density of SCAM9155 become large initially and then decrease. The SCAM9155 sample sintered at 1600℃ for 10h has the highest relative density of 98.2% and electrical conductivity reaching 7.30S/cm in air and 1.91S/cm in 5%H2/Ar at 850℃. The apparent activation energies for SCAM9155 in air and in 5%H2/Ar were calculated to be 0.273 and 0.371eV, respectively. The Ca- and Mn-substituted SmAlO3 with relatively high conductivity may be used as a potential anode material for solid oxide full cells （SOFCs）.
The effect of back-barrier layer on the carrier distribution in the AlGaN/GaN double-heterostructure
2009, 58 (5): 3409-3415. doi: 10.7498/aps.58.3409
The one-dimensional self-consistent simulation of the band diagram and carrier distribution of the AlGaN/GaN double hetero-structure is firstly carried out to research the effect of the thickness and Al content of the AlGaN back-barrier layer on the carrier distribution. Then the AlGaN/GaN double hetero-structure materials with different back-barrier layers were grown by low-pressure MOCVD method on c-plane sapphire substrate. The mercury probe CV measurement was carried out to verify the results of theoretical simulation. The results of theoretical simulation and experiment both indicate that with the increase of Al content and thickness of the AlGaN back-barrier layer, the two-dimensional electron Gas density becomes low in the main channel and high in the parasitic channel gradually.The increase of Al content and thickness of the AlGaN back-barrier layer effectively enhances the two-dimensional electron Gas confinement but simultaneity produces higher-density parasitic channel. So a compromise has to be made between the improvement of the two-dimensional electron Gas confinement and the restraint of the carrier density in parasitic channel in designing the double heterostructure.
2009, 58 (5): 3416-3420. doi: 10.7498/aps.58.3416
InN thin films with different thicknesses are grown by metal organic chemical vapor deposition, and the dislocations, electrical and optical properties are investigated. Based on the model of mosaic crystal, by means of X-ray diffraction skew geometry scan, the edge dislocation densities of 4.2×1010cm-2 and 6.3×1010cm-2 are fitted, and the decrease of twist angle and dislocation density in thicker films are observed. The carrier concentrations of 9×1018cm-3 and 1.2×1018cm-3 are obtained by room temperature Hall effect measurement. VN is shown to be the origin of background carriers, and the dependence of concentration and mobility on film thickness is explained. By the analysis of S-shape temperature dependence of photoluminescence peak, the defects induced carrier localization is suggested be involved in the photoluminescence. Taking both the localization and energy band shrinkage effect into account, the localization energies of 5.05meV and 5.58meV for samples of different thicknesses are calculated, and the decrease of the carrier localization effect in the thicker sample can be attributed to the reduction of defects.
Experimental and theoretical study of blue InGaN/GaN multiple quantum well blue light-emitting diodes
2009, 58 (5): 3421-3426. doi: 10.7498/aps.58.3421
A 2D simulation of electrical and optical characteristics of InGaN/GaN multiple quantum well blue light-emitting diodes by APSYS software with a dot-well model and well model are investigated. It shows that I-V and electrical luminescence simulation results based on the quantum dot model are in better agreement with the experimental data than that based purely on quantum well model. Moreover, simulation result also suggest that the non-equilibrium quantum transport plays an important role in the InGaN/GaN multiple quantum well light-emitting diodes.
2009, 58 (5): 3427-3432. doi: 10.7498/aps.58.3427
Vertical light-emitting transistor is a kind of optoelectronic devices combining characteristics of light-emitting and the “on/off” function of transistor, and its channel length can be reduced to nanometers to make it operate at low voltages and high speeds. Holes and electrons injected from source and drain electrodes form excitons in the emissive layer and radiate light. The amount of charge carriers can be controlled by the gate voltage, consequently to control the intensity of light emission. In this paper, by inserting a thin hole-transporting BCP layer at different locations in the device ITO/NPB（40nm）/Al（30nm）/NPB（20nm）/Alq3（55nm）/Al, we explored the reasons of large drain current and the detailed carriers processes in the device by studying the variation of their optoelectronic characteristics. Our experimental data confirmed that holes injected from central Al gate contribute to the current in the channel. Further, LiF thin film was used as a buffer layer to modify the drain electrode, in this way to improve electron injection and reduce drain current. As a result, relatively stable light-emitting transistors were obtained with significant enhancement in light emission, and the emission intensity can be controlled by the gate voltage. The color of the emission light can easily be tuned by using different luminescent materials.
Influence of Ta and TaN bottom electrodes on electrical performances of MIM capacitors with atomic-layer-deposited HfO2 dielectric
2009, 58 (5): 3433-3436. doi: 10.7498/aps.58.3433
Based on atomic-layer-deposited high permittivity HfO2 films on both Ta and TaN substrates, we compare the influence of different bottom electrodes on electrical performance of metal-insulator-metal （MIM） capacitors. The experimental results indicate that the TaN bottom electrode can give higher capacitance density （7.47?fF/μm2） and smaller voltage coefficients of capacitance （356ppm/V2 and 493ppm/V）, which are attributed to the high quality interface between TaN bottom electrode and HfO2 dielectric films. Moreover, a low leakage current of ～5×10-8A/cm2 at 3V is achieved for both types of capacitors, and TaN bottom electrode-based MIM capacitors exhibit higher breakdown field. Finally, the conduction mechanism of the TaN-based capacitor is studied, showing a Schottky emission at room temperature.
2009, 58 (5): 3437-3442. doi: 10.7498/aps.58.3437
By solving electron tunneling problem in semiconductor superlattice, the magnetic-electric controllable spin transport is theoretically investigated.The results show that, with magnetic modulation only,the spin transmission will separate, and with the magnetic filed increasing,the conductance polarization is enhanced and its peaks are widened. By both magnetic and single interval electric barrier modulation,the conductance polarization will be evidently improved; and at the same time,there are two distinct transport regions for different electric modulation, in which the down-spin electron obeys different change rules with different electric filed.However,applying electric modulation at intervals of two periods on the magnetic superlattic,it is noticed that the critical behavior of electron spin transport disappears,and the resonant peak of the conductance polarization also degenerates in the high energy region.These results show that the symmetry is an important factor for spin transport in the semiconductor superlattice.
2009, 58 (5): 3443-3447. doi: 10.7498/aps.58.3443
The effect of hydrofluoric acid （HF） etching time on Ni/6H-SiC ohmic contacts was investigated． The as-deposited Ni/6H-SiC contacts prepared by 6H-SiC substrates which have been subjected to different HF etching time have different I-V characteristics． For SiC substrates etched for less than 12 hours, the contacts were rectifying, and excellent linear curves were observed after high temperature thermal annealing．X_ray diffraction, Auger electronic spectroscopy and low_energy reflection electron energy loss spectroscopy showed that Ni2Si and amorphous C were the main reaction products after annealing.For SiC substrate etched for 24 hours, the as-deposited Ni/6H-SiC contact was ohmic． The carbon-enriched layer （CEL） on the SiC surface plays an important role in the formation of ohmic contact．
Properties of spin transport of double quantum rings with structures of ferromagnet/ semiconductor/ ferromagnet
2009, 58 (5): 3448-3455. doi: 10.7498/aps.58.3448
Spin-dependent transport through ferromagnetic/semiconductor/ferromagnetic double quantum rings is studied in this paper. It is found that the average value of the spin-dependent electron transmission coefficient of the double quantum ring is larger than that of the single quantum ring under the condition of zero magnetic flux and antiparallel configration of the ferromagnetic electrodes. When the magnetization directions of the ferromagnetic electrodes are parallel, the average tunneling coefficient of the spin-down electrons in double quantum rings increases distinctly. When the Rashba spin-orbit coupling is considered, the average tunneling coefficient of the spin electrons in double quantum rings is bigger than that in single quantum ring. The applied magnetic field enhences the tunneling coefficient. The δ barrier of the double quantum rings suspresses the tunneling of the electron. The tunneling coefficient decreases monotonically and nonlinearly with the δ barrier strength Z increasing.
2009, 58 (5): 3456-3460. doi: 10.7498/aps.58.3456
Hole only devices were fabricated with the structure of ITO/PEDOT/P3HT （poly（3-hexylthiophene））/Ag. The capacitance-frequency characteristics of samples annealed at different temperatures were investigated by admittance spectroscopy technique. Hole mobilities were calculated and it was found that the hole mobility could be pronouncedly influenced by annealing. The hole mobility was enhanced to the 10-3 cm2/Vs order after annealing, while the hole mobility of the unannealed sample was just of 10-4 cm2/Vs order. The hole mobility of the annealed sample is almost unchanged under different electric field. In contrast, the hole mobility of the annealed sample showed relatively significant change with the electric field.
2009, 58 (5): 3461-3467. doi: 10.7498/aps.58.3461
Single-crystalline Zn1-xMgxO thin films with c-axis orientation have been deposited on Si（100） substrate by pulsed lser deposition. The effect of the thickness, Mg content, annealing temperature and oxygen atmosphere on the structure, morphology and photoluminescence of the Zn1-xMgxO thin films are studied by X-ray diffraction,atomic force microscopy,scanning electron microscopy and photoluminescence spectra. The results indicate that the hexagonal wurtzite type of Zn1-xMgxO can be stabilized up to Mg content xx≤0.35.The grain size of the samples increased by the post_annealing, and the structure of the Zn0.75Mg0.25O film changed from cubic to hexagonal wurtzite type when annealed at 600℃. An appropriate oxygen pressure can reduce both the number of defects and the c-axis stress. But superfluous oxygen is apt to combine with Mg and hinders the growth of hexagonal wurtzite type ZnO. Photoluminescence spectrum indicates that the defect_level peak is mainly related with the zinc vacancy, substitutional O on the zinc site （OZn） and interstitial oxygen vacancies （Oi）, and the ultraviolet emission peak has a blue shift due to annealing.
Influence of pillar-and hole-patterned sapphire substrates on MOVPE grown GaN bulk and LED structures
2009, 58 (5): 3468-3473. doi: 10.7498/aps.58.3468
Bulk GaN material and LED structures on pillar-patterned sapphire substrates （PSS-p） and hole-patterned sapphire substrates （PSS-h） were grown by MOCVD and the characteristic was compared in detail. X-ray diffraction and atomic force microscope measurements show a better crystal quality and surface morphology of GaN on PSS-h than that of GaN on PSS-p, which is due to the lateral growth of GaN on PSS-h observed from cross-sectional scanning electron microscopy. Furthermore, the output power of LED on PSS-p and PSS-h with 20 mA injection current are 46% and 33% higher than LED on conventional sapphire substrate, respectively. The temperature-dependent photoluminesence measurements indicate that the internal quantum efficiencies of all samples are quite close. Therefore, the airgaps between GaN and PSS-h act against the improvement of light extraction efficiency.
2009, 58 (5): 3474-3478. doi: 10.7498/aps.58.3474
A numerical model of organic devices，which includes charge injection, transport, space charge effect and trap influence，was discussed in this paper. Both thermionic emission over the barrier and tunneling through the barrier are considered in charge injection into the bulk. The result is in good agreement with the experimental data. Several parameters have been simulated to study the change of J-V characteristics. The current decreases with the increases of the length of device. The current density becomes higher when the hole injecting barrier is smaller. However, when the electronic injecting barrier decreases from 1.7 eV to 0.5 eV, the current density becomes smaller. This is because the electronic mobility is too small, and the electronic injecting current is negligible compared with the hole current. When the electronic injecting barrier is smaller, the built-in potential becomes bigger, under the same applied voltage, the field diminishes, and the hole current becomes smaller. When the barrier is reduced to 0.1 eV, the current density increases. This is because the electronic mobility is too small, and electrons accumulate near the cathode, the electric field around the anode increases and the current increases.
Structure and magnetic properties of stress-induced martensites in ferromagnetic shape memory alloy Mn2NiGa
2009, 58 (5): 3479-3484. doi: 10.7498/aps.58.3479
In-situ high pressure X-ray diffraction experiments on Heusler-type ferromagnetic shape memory alloy Mn2NiGa under pressure up to 40 GPa have been carried out by using diamond anvil cell device with synchrotron radiation source， and magnetic measurements have also been performed on uninstalled samples by using vibration sample magnetometer. At ambient pressure Mn2NiGa has body centered cubic structure. Two phase transitions in Mn2NiGa were observed during high pressure experiments. The first at about 0.77 GPa is belong to martensitic transformation, and the second at about 20 GPa is isostructural transformation between two different martensites. Meanwhile, a lot of defects and distortion of lattice were generated in martensites due to pressurization. Consequently, coercive force of stress-induced martensites rises up to 204 kA/m and it is almost ten times larger than that of thermo-induced martensites. Pressure processes also make saturation magnetization of martensites decrease rapidly, which reveals the obvious defect effect.
2009, 58 (5): 3485-3490. doi: 10.7498/aps.58.3485
In connection with local structure, the g factors of 2E state are studied by using complete diagonalization method （CDM）, with the spinel type crystals ZnAl2Ｏ4:Cr3＋, ZnGa2Ｏ4:Cr3＋ and MgAl2Ｏ4:Cr3＋ as the cases. The slight magnetic interactions, including spin-spin （SS）, spin-other-orbit （SOO） and orbit-orbit （OO） interactions are taken into account in our investigations. Theoretical explanations of g factors of 2E state for ZnAl2Ｏ4:Cr3＋ have been given. The results show that the sign of g‖（E′） for ZnGa2Ｏ4:Cr3＋ should be negative. And the value of g factors of 2E state for MgAl2Ｏ4:Cr3＋ have been predicted. It is shown that, the contributions to g factors of 2E state from slight magnetic interactions are appreciable. Compared with the g factors of ground state, the g factors of 2E state are sensitive to the local structure.
Magnetoelectric coupling and magnetoelectric properties of single-phase ABO3 type multiferroic materials
2009, 58 (5): 3491-3496. doi: 10.7498/aps.58.3491
The anomalies in both the dielectric constant and loss tangent occur at fixed temperature close to the magnetic ordering transition, which have been observed experimentally in multiferroic pervoskite YMnO3 and BiMnO3, indicating coupling between the magnetism and dielectric properties. In this paper, by analyzing the magnetoelectric coupling between magnetic and ferroelectric subsystems, an appropriate coupling term related to a combination of electric polarization and spin correlation is added to system Hamilton expression, Then apply the soft-mode theory for electric subsystem and mean-field approximation under Heisenberg model for magnetic subsystem,magnetoelectric properties have been investigated, including the change of polarization, dielectric constant induced by external magnetic field, and the change of magnetization induced by external electric field. We also make some qualitative comparison and analysis between our results and the experimented ones and give a reasonable interpretation of magnetoelectricity phenomenon in some multiferroic pervoskite materials.
Preparation and laser induced thermoelectric voltage effect of ［（SrTiO3）n/（SrTi0.8Nb0.2O3）m］20/LaAlO3 superlattices
2009, 58 (5): 3497-3502. doi: 10.7498/aps.58.3497
The （SrTiO3）n/（SrTi0.8Nb0.2O3）m superlattices on vicinal-cut LaAlO3 （100） single crystal substrate with tilting angle 10° have been prepared using pulsed laser deposition. The superlattices were characterized by X-ray diffraction, in which regularly distributed satellite peak structure was observed. According to the position of satellite peaks, the superperiod of superlattices was calculated and the deposition rates of SrTiO3 and SrTi0.8Nb0．2Ｏ3 were deduced as 0.78 /pulse for SrTiO3 and 0.57 /pulse for SrTi0．8Nb0．2Ｏ3. In addition, laser induced thermoelectric voltage （LITV） effect was measured for the first time in these superlattice thin films. This phenomenon indieales that this kind of artificial atomic layer thermopile structure possesses Seebeck anisotropy. It was found that when the thickness of dielectric SrTiO3 layer n=46.8 nm, for conductive SrTi0．8Nb0．2Ｏ3 layer with m=19.0 nm and m=11.4 nm, the average peak voltage of LITV signals reaches the maximum U—P=0.7 V/mJ·mm, and the minimum average response time is τ—=124 ns, respectively.
2009, 58 (5): 3503-3508. doi: 10.7498/aps.58.3503
FePt/Au mutilayers were deposited on 100℃ MgO（001） single crystal substrate by magnetron sputtering. The influence of the FePt/Au multilayer structure on the ordering temperature, coercivity （ＨC）, perpendicular magnetic anisotropy （PMA）, FePt grain size and interparticle exchange coupling （IEC） of films were studied. Results show that the film possesses high ＨC, excellent PMA, small grain size and weak interparticle exchange coupling after annealing. The results of HRTEM reveal that Au can relieve mismatch between MgO and FePt lattices, leading to excellent PMA. Moreover, the FePt/Au interface energy, the stress energy due to the mismatch between FePt and Au lattices and the diffusion of Au atoms promote the ordering of FePt film. This leads to the decrease of ordering temperature and considerable increase of ＨC. Au atoms partly diffuse to the boundaries of FePt phase and function as grain finer and particle isolator, which decrease the FePt grain size and IEC.
2009, 58 (5): 3509-3514. doi: 10.7498/aps.58.3509
The full magnetization reversal process of perpendicular orientated Nd2Fe14B/α-Fe trilayer has been investigated by using a micromagnetic method. Both nucleation and pinning fields as well as hysteresis loops have been calculated reliably as functions of Ｌs （soft layer thickness）. In addition, detailed comparison between our results and experimental data has been done. With the demagnetization energy considered, the nucleation field of perpendicular by orientated case is lower than the parallel one and the nucleation occus before the reversal of the applied field. For small Ｌs, the theoretical coercivity equals to the nucleation field （it equals to pinning filed also）. As the thickness of the soft phase increases, the coercivity mechanism changes from nucleation to pinning. For large Ｌs, however, the coercivity is slightly smaller than the pinning field.
2009, 58 (5): 3515-3519. doi: 10.7498/aps.58.3515
Based on the elasticity mechanical model, we employ the free_state bilayer magnetoelectric nano_thin film model which introduces the magnetoelectric （ME） theoretical voltage coefficient formula simply. By this way, the ME voltage coefficient of CoFe2O4/Pb （Zr0．52Ti0．48）Ｏ3 bilayer has been calculated and analyzed under the corresponding material parameters. The results show that the ME voltage coefficient is reduced significantly when the thickness of substrate increases due to the gripping effect. The optimum volume ratio of piezoelectric film and magnetoelectric film increase when the gripping effect increasing.
2009, 58 (5): 3520-3524. doi: 10.7498/aps.58.3520
The polarization and depolarization of antiferroelectric Pb（Nb,Zr,Sn,Ti）O3 ceramics under dc bias were studied under hydraulic pressure ranging from zero to 250 MPa. It was found that the antiferroelectric ceramic can be induced to a metastable ferroelectric state and that the ceramic at this ferroelectric state can be switched to the antiferroelectric state by using hydraulic pressure. The hydraulic pressure that induces the transition from the ferroelectric to the antiferroelectric states increases with the positive and decreases with the negative dc bias. The electrical energy stored in the poled ceramics at its ferroelectric state releases during the transition from the ferroelectric to antiferroelectric state, which results in a strong current pulse a cross the ceramic. Based on the results, the pressure-electric field phase diagram for polarized PNZST ceramics was established.
Piezoelectricity of laminated polymer films made of nonporous fluoroethylene and porous polytetrafluoroethylene layers
2009, 58 (5): 3525-3531. doi: 10.7498/aps.58.3525
The piezoelectrets made of porous polytetrafluoroethylene （PTFE） films and nonporous fluoroethylenepropylene （FEP） films are prepared by using a hot-pressing method. The quasi-static piezoelectric d33-coefficients of such piezoelectret films are measured. The thermal stability of d33-coefficients and dynamic characteristics of charges in the piezoelectret films are investigated. The dynamic d33-coefficients are obtained by analyzing the dielectric resonance spectra of the piezoelectret films. The results show that the piezoelectric d33-coefficients up to 300 pC/N are achieved for the laminated PTFE/FEP films. The d33 value retains 40% of the initial value when the sample was annealed at the temperature of 90℃ for 20 h. Pre-ageing is an effective way to further improve the thermal stability of d33-coefficients. The dominant drift path of the detrapped charges in the shallow traps is most likely along the surface of the PTFE fibers, while charge drift through the solid layer of FEP is possibly prevailing for the charges in the deeper traps. The d33 value determined from the dielectric resonance spectrum is smaller than the quasi-static d33 value, which is mainly due to the enhanced Young's modulus with the increase of frequency.
2009, 58 (5): 3532-3537. doi: 10.7498/aps.58.3532
The Faraday rotation of two-dimensional magnetophotonic crystals （including those with fourfold-symmetry and non-fourfold-symmetry） with structural defects and the ellipticity of outgoing optical waves are analyzed by using finite-difference frequency-domain （FDFD） method. The numerical simulation results show that the localization of the light wave in the vicinity of central defect occurs in these types of the structure. With increasing Faraday rotation of fourfold-symmetry structures, the change of the ellipticity of outgoing optical waves from these structures is unobvious. In contrast, the increases of the ellipticity of outgoing optical waves are obvious with the raise of Faraday rotation in non-fourfold-symmetry structures. It is concluded that introducing defects to the fourfold-symmetry structure constrains the change of the state of polarization of optical waves. This kind of two-dimensional magnetophotonic crystal with structural defects may be used to manufacture magnetophotonic crystal fiber.
2009, 58 (5): 3538-3542. doi: 10.7498/aps.58.3538
The Raman spectra of CS2 in C6H6 is messured and the Fermi resonance of v1—2v2 in solution and in pure CS2 is obvionsly different.The characteristic parameters are calculated by Berttern equation.It was found that with the concentration of CS2 declining,the ratio of the spectral intensity （R=Ｉ655/I796） decreases and the coupling coefficient （Ｗ） raises, and the other parameters also change correspondingly. The analysis shows that dissipation coefficient of v01 of CS2 is affected by the concentration of C6H6. Asymmetric frequency shift of v01 and 2v02 are observed in experiment,which is explained by amending the Bertran theory. This report has reference value for the molecule spectrum certification and the understanding and use of Fermi resonance theory.
Study of electron spin diffusion transport in intrinsic GaAs quantum wells by time- and space-resolved absorbtion spectroscopy
2009, 58 (5): 3543-3547. doi: 10.7498/aps.58.3543
A time- and space-resolved circularly polarized pump-probe spectroscopy and its theory are developed, and are used experimentally to study electron spin diffusion transport in intrinsic GaAs quantum wells. The ambipolar spin diffusion coefficient of Ｄas=37.5±15 cm2/s is obtained at room temperature, and is smaller than that in a doped GaAs quantum well obtained by spin grating. The difference is explained based on “hole Coulomb drag" effect which slows the diffusion transport of electron spin packet.
The time resolved pump-probe spectroscopy was used to study the ultrafast carrier dynamics of semimetal Sb films at different wavelengths and pump fluences. The profiles of differential transmission versus delayed time showed sharp oscillation near the zero-time_delayed point. A positive absorption saturation peak turned into a negative absorption peak in subpicosecond time. Then the latter began to attenuate and again showed absorption saturation eventually in several picoseconds. The amplitudes of the positive and negative peaks were proportional to the pump fluence and wavelength. These phenomena are analyzed in this article and explained reasonably and semi-quantitatively by introducing the “defect model" and considering fast capture and release by “defect states" of hot electrons.
2009, 58 (5): 3553-3559. doi: 10.7498/aps.58.3553
The research of temperature effect on the surface adsorption and diffusion is an important problem for the initial growth mechanism of AlN/α-Al2Ｏ3（0001） thin films. Using the ab initio molecular dynamics method based on the first principles, the adsorption process, system energy, dynamic track and the diffusion coefficient are calculated at temperatures from 300℃ to 800℃. The results indicate that the adsorption process can be divided into three stages, the physical adsorption, the chemical adsorption and the stable surface growth state. With the temperature increasing, the average diffusivity of the particles at α-Al2Ｏ3（0001） surface is enhanced in the bonding process. The diffusion coefficient of N atom is higher than that of the Al atom, especially in the physical adsorption stage. At higher temperatures （over 700℃）, the role of desorption of the N atom is markedly strengthened, so it is unfavorable to the stable absorption of AlN. The temperature between 500℃ and 700℃ is beneficial to the stable adsorption and the growth of the AlN on α-Al2Ｏ3（0001） surface.
Mid-infrared photoluminescence of PbSe/PbSrSe multiple quantum wells grown by molecular beam epitaxy
2009, 58 (5): 3560-3564. doi: 10.7498/aps.58.3560
Mid-Infrared photoluminescence of PbSe/PbSrSe multiple quantum wells （MQWs） grown by molecular beam epitaxy is studied. High-order satellite peaks are observed by high resolution X-ray diffraction, which indicates the quantum well structure has sharp interfaces. Temperature dependence of photoluminescence spectra shows that the MQWs make good confinement of electrons and holes. At the same measurement temperature, the photoluminescence peaks of the MQW sample show a clear blue shift compared with that of the bulk PbSe material. We find that the intensity of photoluminescence is dependent on the measurement temperature. When temperature increases from 150 K to 230 K, the PL intensity reaches its maximum at 230 K, and with further increasing the intensity decreases slowly. We can still observe strong PL intensity above room temperature, which indicates MQWs have the potential to be used in room-temperature_operating mid-infrared optoelectronic devices.
2009, 58 (5): 3565-3571. doi: 10.7498/aps.58.3565
The red long afterglow phosphor Sr3Al2O6:Eu2+,Dy3+ was prepared by sol-gel method. The crystal structure was characterized by X-ray diffraction. The results reveal that the samples are composed of single Sr3Al2Ｏ6 phase at 1200℃, and the small amount of doped rare earth ions （Eu2＋ and Dy3＋） has no effect on the Sr3Al2Ｏ6 phase composition. The excitation, emission and afterglow spectra were investigated by fluorescence spectrophotometer technique. The broad excitation spectra in the range of 400—550 nm were found in the Sr3Al2Ｏ6:Eu2＋ and Sr3Al2Ｏ6:Eu2＋,Dy3＋ phosphors. The broad band spectrum is associated with defects and vacancies of host material and the peak at 530.1 nm can be assigned to the recombination of donor-acceptor pairs of Ｄy·Sr-V″Sr from dysprosium doping in the Sr3Al2O6. With the increasing concentration of Dy below 0.03, the emission intensity of Sr3-0．02-yAl2Ｏ6:0.02Eu2＋,yDy3＋ phosphors increases, and the samples exhibit the red light emission spectra with the main emission peak at 612 nm under an excitation of 473 nm. The red emission has a broad-band feature, belonging to the emission of the 4f65d1→4f7 of Eu2＋ions. With the increasing concentration of Dy within the range of 0.03—0.06, there was a new emission peak in the green region, and the emission intensities of the two peaks at 530 nm and 612 nm decrease. The afterglow time of the Sr2．95Al2Ｏ6:0.02Eu2＋,0.03Dy3＋ phosphors lasts 540s（≥1 mcd/m2） after removal of excitation. The Dy3＋ ions are auxiliary activator in the Sr3Al2O6:Eu2+,Dy3+ phosphors.
The fluorescence spectral characteristics and the emission mechanism of ethanol excimer in water solution
2009, 58 (5): 3572-3577. doi: 10.7498/aps.58.3572
The fluorescence spectra of ethanol-water solution excited by UV-light have been studied, in which there is a broad and featureless fluorescence spectrum with the peak at 410 nm. It is considered that when the clustered molecules with a special long chain structure approach each other, the excimer is formed between the excited state and ground state cluster monomers, which can emit fluorescence. According to the experimental result and the energy transfer theory, we know that an electron transfer complex is formed by the excimer and the cluster monomer in the excited state. Based on Mulliken theory and the quantum mechanics theory, the energy of ground state EbN and excited state ＥbE, as well as the electrostatic interaction energy Ｅs which is caused by electron migration from donor to acceptor, have been calculated, respectively. Based on the theory of one-dimensional harmonic vibrator model, the energy valley in the potential energy curve when the excimer is formed has been simplified. Thus the potential energy function and the energy value of the excimer formed by the clusters in the critical distance can be calculated, and then the electron migration electrostatic interaction energy can be gotten too. Accordingly the fluorescence emission mechanism of the broad and featureless fluorescence peak has been explained theoretically.
2009, 58 (5): 3578-3583. doi: 10.7498/aps.58.3578
A novel side-chain conjugated polythiophene derivative poly［3-（5′-octyl-thienylene-vinyl）-thiophene］ （POTVTh） was synthesized. The polymer films showed a broad absorption band and a narrow bandgap of about 1.82 eV. The third order nonlinear optical properties of the film were measured by Z-scan technique at 800 nm, which showed the nonlinear absoption coefficient, the nonlinear optical refractive index and third order nonlinear optical susceptibility as 5.63×10-7cm/W, -6.38×10-11cm2/W and 4.32×10-9esu, respectively. The results are one to two orders of magnitude larger than those of normal conjugated polymers without conjugated side-chain, indicating the good nonlinear optical properties of the polymer.
2009, 58 (5): 3584-3591. doi: 10.7498/aps.58.3584
Monte Carlo simulation is an essential tool to assist in the design of new medical imaging devices for emission tomography. This paper modeled the full 3D whole body PET scanner and 2D animal PET scanner with septa using Monte Carlo simulation code GATE based on Geant4, analyzed the scatter fraction, scatter distribution, multiple scatter events, and scatter events from outside of FOV under 3D acquisition mode and scatter events from the septa under 2D acquisition mode. For the critical problems of 3D scatter correction: multiple scatter events and scatter events from outside of FOV, we designed additional experiments, got the fitted relationship between the percentage of multiple scatter events and the cross sectional area of the phantom, and the effects of the scatter events from outside of FOV at different ring positions; for the 2D scatter correction, we separated the scatter events from septa for analysis, and provided a quantitative reference for the elimination of these events or introduction into the procedure of reconstruction.
2009, 58 (5): 3592-3596. doi: 10.7498/aps.58.3592
To utilize the interaction between the field and the electron beam in a traveling wave tube （TWT）, the velocity of electrons and the longitudinal phase velocity of field should be made approximately equal. But when the electromagnetic energy increases, the electrons are slowed down and the synchronization condition is no longer satisfied in periodic structures. However, the varying-period slow-wave structure can be used to maintain the synchronization in the whole interaction section. This structure also has the advantages of space harmonic selectivity and wide bandwidth. The space harmonic in varying-period slow-wave structures is analyzed here. The synchronization condition of one space harmonic with the electron beam during the whole interaction process is presented in this paper. And the folded waveguide with varying period is analyzed as an example.