Vol. 60, No. 12 (2011)
2011, 60 (12): 124210. doi: 10.7498/aps.60.124210
According to the theory of chirped pulse amplification (CPA), we analyze numerically the gain characteristic of laser pulses in multi-pass amplifier. A CPA Ti:sapphire laser with 10-pass scheme is designed and built. The experimental result shows that the contrast ratio of the femtosecond pulse is obviously enhanced by two orders from 10-5 to 10-7 under a pump flux of 1.6 J/cm2. This work demonstrates that the contrast ratio of the amplified femtosecond laser pulse could be effectively improved by multi-pass preamplifier.
2011, 60 (12): 124502. doi: 10.7498/aps.60.124502
In this paper, the jamming/unjamming processes of the system composed of 5000 elastic disks with small friction =110-4 are simulated by using the molecular dynamics method. The variations of sidewall pressure P and the height of the first peak of the pair correlation function, g1, with packing fraction are studied. The result shows that the P() curve exhibits an obvious stick-slip-like behavior. The normal force-force correlation function, the tangential force-force correlation function, and the position-position correlation function are found to jump simultaneously during the stick-slip process. By relaxing jammed states obtained as the system undergoes the compression process, we observe that the P is related to -c by the power scaling law P(-c)0.964, although the different sidewall pressures corresponds to different values of c.
2011, 60 (12): 127305. doi: 10.7498/aps.60.127305
The structural stability and the electronic properties of two-dimensional monolayer BC2N are studied by employing the first principles method based on the density functional theory. 16 polymorphic structures of monolayer BC2N are calculated. Analysis of the 16 band structures suggests that the structure with the highest symmetry is of a semimetal which is the same as graphene. All the other structures are of semiconductors with different band gaps, of which the most stable structure is of semiconductor with a direct gap of 1.63 eV. Based on the deformation charge density and the Bader analysis, the bonds CC, CN, CB, and BN in the most stable monolayer BC2N are mainly covalent, however, they present also significant ionic behaviors. Exerting a stress on the monolayer BC2N sheet changes the band gap, showing that the band gap increases during the compression while decreases during the stretch, and the system keeps a direct semiconductor.
2011, 60 (12): 128201. doi: 10.7498/aps.60.128201
We investigate the dynamic evolution of wave packet under the coupled diabatic potentials via employing the time-dependent wave packet approach. The wave packet splits in the crossing region due to the influence of non-adiabatic effects. In our studies, it is found that the non-adiabatic effects are very important to the dissociation of NaI molecule, and it becomes markedly with the increase of evolutionary time. Moreover, our study shows that the mean packet position 〈R(t)〉 and the classical trajectory R(t) are nearly coincident and the dissociation probability is dependent on the laser wavelength.
2011, 60 (12): 120201. doi: 10.7498/aps.60.120201
The far-field model equation is investigated by the approximate homotopy symmetry method. Homotopy series solutions are constructed through summarizing the relevant general formulas for similarity reduction solutions and similarity reduction equations of differernt orders. Similarity reduction equations of different orders are linear variable coefficients ordinary differential equations, and can be solved one by one from zero-order similarity reduction equations. The auxiliary parameter in the homotopy model affects convergence of homotopy series solutions.
A method of constructing peaked soliton solution of nonlinear wave equation by using smooth soliton solution
2011, 60 (12): 120202. doi: 10.7498/aps.60.120202
We propose a method of obtaining peakon solution from bell-shape smooth soliton (or solitary wave)solution, i.e. constructing directly an ansatz solution of peakon according to the well-known bell-shape smooth soliton solution and then determining the coefficients in ansatz solution. The method is verified to be feasible for four nonlinear wave equations and one set of equations. The bell-shape smooth soliton (or solitary wave)solution and peakon solution can exist in the same expression and the expressions of peakon solutions include those of the bell-shape smooth soliton solutions and the latter are the special cases of the former.
Thermal entanglement in the four-qubit Heisenberg XXZ model with the Dzyaloshinskii-Moriya interaction
2011, 60 (12): 120301. doi: 10.7498/aps.60.120301
In this paper, we study the property of thermal entanglement in four-qubit Heisenberg model, where Dzyaloshinskii-Moriya (DM) interaction is considered, and investigate the pairwise concurrences of two nearest-neighbor qubits and two next-neighbor qubits to study this entanglement property. The result shows that for the two-next-neighbor-qubit case, there exists pairwise concurrence neigher in ferromagnetic model nor in antiferromagnetic model; but for the two-nearest-neighbor-qubits case, the DM interaction and the parameter of the anisotropy exchange coupling have a significant influence on the pairwise entanglement and critical temperature Tc. Moreover, the pairwise concurrence will decrease with the increase of temperature. When the temperature execeds its critical value, the pairwise concurrence disappears. Therefore, the pairwise entanglement can be controlled and enhanced by choosing the appropriate parameters of the DM interaction and the anisotropy exchange coupling.
2011, 60 (12): 120302. doi: 10.7498/aps.60.120302
Quantum Fisher information,derived from the classical Fisher information, is closely related to the quantum entanglement in quantum information. The entanglement and the quantum information which are both associated with the classical phase space are investigated in a two-component Bose-Einstein condensate impacted by the impulses. The results reveal that the states regardless of disorder of the phase space after the first impulse are entangled. However, the quantum information is very sensitive to the state centred in the classical phase space, concretely, the value of the quantum information centred in the chaotic region is greater than in the regular region. By employing the good quantum-classical correspondence, we conclude that the quantum information can serve as a signature of the quantum chaos.
The entanglement properties in the system composed of a -type atom and a V-type atom trapped in two distant cavities connected by an optical fiber
2011, 60 (12): 120303. doi: 10.7498/aps.60.120303
We consider a system consisting of a -type atom and a V-type atom, which are individually trapped in two spatially separated cavities that are connected by an optical fiber. The evolution of the state vector of the system is given. We investigate the temporal evolution in the entanglement between atoms and that between cavities. We discuss the influence of cavity-fiber coupling coefficient on entanglement. The results obtained by the numerical method show that the entanglement between atoms and the entanglement between cavities have the same evolution regularities. On the other hand, the entanglement between atoms and that between cavities are strengthened with the increase of cavity-fiber coupling coefficient.
2011, 60 (12): 120304. doi: 10.7498/aps.60.120304
In this paper, we study the spatially chaotic distribution of atoms in a Bose-Einstein condensate system, trapped in an asymmetric periodic potential. For a constant phase of condensate, without atom currents in the system, the space distributed structure of condensated atoms can be described by an undamped Duffing equation with double drivers. Through theoretical analyses, the Mel'nikov chaotic criterion for the system with a repulsive interatomic interaction is presented. Numerical simulations show that an increasing chemical potential can exert considerable suppression on the chaotic distribution of condensated atoms and even completely eliminate chaos. For a system with an attractive interatomic interaction, under some specific parametric conditions, adjusting the ratio between optical lattice potential amplitudes will force the condensated atoms from a periodic state into a spatially chaotic distribution; with the increase of chemical potential, the spatially chaotic distribution is completely suppressed.
2011, 60 (12): 120305. doi: 10.7498/aps.60.120305
Atoms in the Mott insulator state trapped in an optical lattice are incoherence matter wave source. It is not the first-order, but the second-order interference effect (density correlation) that will appear for this incoherence wave source after being released. A density correlation function of the freely expanding ultracold gases is obtained theoretically, which presents sharp peaks of interference, and the stripes structure is similar to the diffraction gratings. It is further pointed out that the peak structure of the density correlation function depends on the relative position between two detectors. The phenomenon of subwavelength interference of matter waves is also proposed in this paper.
2011, 60 (12): 120401. doi: 10.7498/aps.60.120401
Using the 4-simplex and its dual-skeleton spin networks, a quantum transition of pure spacetime is described. The change of vertex volumes between successive steps of the transition of the three-dimensional space woven by spin networks is demonstrated. At the same time, a model of quantum flate fluctuation of the four-dimensional spacetime is given. Employing the excitations created when the loops on the edges of spin network pierce sarfaces, a 2 order symmerric tensor hab is obtained in three-dimensional space. Based on the viewpoint that the spacetime is excitation and transition system, the creation and the change mechanisms of the gravitational field h are investigated.
Mean first-passage time and stochastic resonance in an asymmetric bistable system driven by non-Gaussian noise
2011, 60 (12): 120501. doi: 10.7498/aps.60.120501
In this paper, mean first-passage time (MFPT) and stochastic resonance (SR) are investigated in an asymmetric bistable system driven by multiplicative non-Gaussian noise and additive Gaussian white noise. Using the path integral approach and two-state theory, the expression of MFPT and the signal-to-noise ratio (SNR) are derived. The results show that the influences of the asymmetric coefficient on the MFPTs in two opposite directions are entirely different. SNR is a non-monotonic function of the additive noise intensity and asymmetric coefficient, therefore, an SR is found in this system. Whereas SNR is a monotonic function of the multiplicative noise intensity and no SR appears. This demonstrates that the effect of the multiplicative noise intensity on SNR is different from that of the additive noise intensity in the system.
2011, 60 (12): 120502. doi: 10.7498/aps.60.120502
Memristor with memory function is the fourth fundamental two-terminal circuit element, besides resistor, capacitor and inductor. In this paper, a smooth flux-controlled memristor is described by a monotone-increasing nonlinearity curve in the -q plane, and it has an italic type 8 like voltage current relation curve that looks like a pinched hysteresis loop characteristics. By replacing Chua's diode with an active memristor consisting of a smooth flux-controlled memristor and a negative conductance, a memristor based chaotic oscillation is derived from Chua's circuit. Furthermore, the equivalent circuit implementation form for the active memristor is designed by utilizing conventional components such as operational amplifiers and multipliers. The results from theoretical analysis, numerical simulations and circuit simulations are completely identical with each other, and demonstrate that the dynamical behaviors of the memristor chaotic circuit are dependent on the memristor initial state, showing different orbits such as chaotic oscillation, periodic oscillation and stable sink under different initial states.
2011, 60 (12): 120503. doi: 10.7498/aps.60.120503
Systems with double-loop hysteresis are used increasingly in engineering, but few studies on their dynamics are reported. In this study, the bifurcation characteristics of the primary resonance of a system with double-loop bilinear hysteresis are investigated on the background of a shape memory alloy damper. First, the frequency-amplitude response equation is obtained by using the averaging methods. Then, the influences of the temperature and the amplitude of excitation on amplitude-frequency responses are analyzed by the constrained bifurcation singularity analysis method of non-smooth systems. The calculation results show that the parameter space composed of the temperature and the amplitude of excitation can be divided into 11 regions, which suggest that there are 11 qualitatively different kinds of amplitude-frequency responses to the variation of two parameters. In order to describe and compare the frequency-amplitude response curves conveniently, an encoding rule is proposed to describe their jump phenomena as the frequency sweeps. The above results can guide directly the design of frequency response mode of the system.
2011, 60 (12): 120504. doi: 10.7498/aps.60.120504
Chaos realization in Sprott system depends on the nonlinear function of single absolute term. Under this condition, the chaos in the system is locked with constant Lyapunov exponent spectrum by introducing new control parameters. So the amplitude and the phase of the signal output in the system can be controlled and modulated. Anti-synchronization system is constructed, and anti-synchronization is obtained by introducing only one control term in the response system based on Lyapunov potential function method.
2011, 60 (12): 120505. doi: 10.7498/aps.60.120505
Based on the LaSalle's invariant set theorem, an adaptive controller is constructed to acquire the chaotic control for the permanent magnet synchronous motor. And then a extended adaptive controller is developed by introducing a control strength factor. In comparison with the previous schemes, the present control method is simple, flexible, and is applicable in practice because of reduced control cost. Simulation results are presented to show the effectiveness and the robust of the proposed method.
2011, 60 (12): 120506. doi: 10.7498/aps.60.120506
Synchronization of fractional-order chaotic systems is investigated by designing a novel nonlinear feedback controller. Compared with the previous reports, the controlled synchronization error system does not need to turn into a linear system and the performance is improved effectively. Furthermore, a novel sufficient stabilization criterion for fractional-order chaotic systems is proposed by introducing a stabilization criterion of an interval fractional-order linear time-invariant system and using the linear matrix inequality technique. Consequently, the chaotic synchronization is ensured. The simulation result verifies the effectiveness of the proposed method.
2011, 60 (12): 120507. doi: 10.7498/aps.60.120507
With the help of the symbolic computation system Maple and a projective equation and a linear variable separation method, new exact solutions of the (3+1) -dimensional Jimbo-Miwa equation are derived. Based on the derived solitary wave solution, some novel localized excitations are investigated.
2011, 60 (12): 120508. doi: 10.7498/aps.60.120508
The clustering phenomenon on the solid wall during dropwise condensation is analyzed with reflection spectrum. By the theoretical prediction of reflectivity of thin liquid films with different thicknesses on the stainless steel surface, it is ascertained that the reflectivity value is corresponding to the coacervate character of the steam molecular. Furthermore, by analyzing the experimental data of the reflection spectrum during dropwise condensation, presented in the literature, it is obtained that the reflection character and so the coacervate character lies between liquid and steam after the droplet has fallen off during an actual continuous condensation process. And the clustering model is used to analyze the results, which point out that clusters are formed on the blank surface. And it is found that the different microstructures of the solid wall can lead to different deposition rates of the clusters, which presents an effective way to enhance the heat transfer process of condensation by guickening the deposition rate of clusters with the surface modification.
2011, 60 (12): 120509. doi: 10.7498/aps.60.120509
An adaptive network is a kind of co-evolutionary network in which the behaviors of nodes and the dynamics of the network affect each other. A susceptible-infected-susceptible epidemics spreading model in the adaptive network is proposed using cellular automata. Under different rewriting rules for avoiding the epidemic spreading, the changes in epidemic dynamics and the properties of the network statistic are investigated. The simulation results show that the introduction of rewiring can slow down the spreading speed of epidemic and suppress the infected scale of epidemic. The random rewiring rules make the properties of an original network with any topology tend to those of a random network. Furthermore, the proposed cellular automata model can clearly exhibit the phenomenon of bi-stability in epidemic dynamics.
2011, 60 (12): 122101. doi: 10.7498/aps.60.122101
In this paper we mainly investigate, in the framework of effective mass bag model, how the coupling constant and the bag constant execute their effects on equations of state of strange quark matter, and on the properties of strange stars. Numerical results indicate that with the increase of strong coupling constant and bag constant, equations of state for strange quark matter turn softened, whereas gravitational mass and corresponding radius of strange stars become decreased. For instance the mass of the star decreases from 1.43M⊙(M⊙=1.991030 kg)to 1.25M⊙ and corresponding radius decreases from 8.3 km to 7.7 km while the coupling constant varies from 0.5 to 2.0. As for strange stars, the corresponding values decrease from 1.47M⊙to 1.22M⊙ and 8.6 km and 7.4 km respectively while the bag constant B1/4 increases from 160 MeV to 175 MeV.
ATOMIC AND MOLECULAR PHYSICS
2011, 60 (12): 123101. doi: 10.7498/aps.60.123101
Equilibrium structures of the GeTe and GeSe ground state molecules are obtained by employing the local spin density approximation method with 6-311++G** basis sets for Ge and SDB-cc-pVTZ for Te and Se. Also obtained are the equilibrium geometry, the highest occupied molecular orbital(HOMO) energy level, the lowest unoccupied molecular orbital(LUMO)energy level, the energy gap, the harmonic frequency and the infrared intensity of GeTe and GeSe ground state molecules under different electric fields. On the basis of the above calculation, the excited states of GeTe and GeSe molecules under different electric fields are also investigated by using the single-excitation configuration interaction-local spin density approximation method. The results show that the equilibrium internuclear distance and the intensity of infrared are found to increase, but the total energy and harmonic frequency are proved to decrease with the increase of positive direction electric field. The HOMO energy EH of GeTe molecule is higher than that of GeSe molecule under electric fields ranging from 0 to 2.05691010 V m-1. For GeTe and GeSe molecules, their difference in EH gradually increases with the increase of positive direction electric field. The LUMO energy EL of GeTe molecule is lower than that of GeSe molecule, and their LUMO energies are found to increase with the increase of positive direction electric field. The energy gap of GeTe is low than that of GeSe, and their energy gaps always decrease with the increase the negative direction electric field. The magnitude and the direction of the external electric field have important effects on excitation energy, oscillator strength and wavelength.
2011, 60 (12): 123102. doi: 10.7498/aps.60.123102
The atomic and molecular reaction dynamics for T+OD have been studied basad on the potential energy function of DTO(1 A1) by Monte Carlo quasi-classical trajectory approach. It is shown that the reaction T+ODDTO with a long-lived complex has a threshold energy at low collision energy, which accords with the potential curve. The interchange reaction increases with collision energy increasing, until the DTO molecules decompose into D, T, and O completely, and these reactions have threshold energyies too. The trajectories and the collision cross-sections of T+OD(0, 0) and D+OT(0, 0) are different due to the isotopic effect of D atom and T atom.
Multireference configuration interaction study on spectroscopic parameters and molecular constants of AsN(X1 +) radical
2011, 60 (12): 123103. doi: 10.7498/aps.60.123103
The potential energy curve (PEC) of the AsN(X1 +) radical is investigated by the highly accurate valence internally contracted multireference configuration interaction method in combination with the correlation-consistent basis sets, aug-cc-pV5Z for As and aug-cc-pV6Z for N atom. The PEC is fitted to the Murrell-Sorbie function, which is used to accurately derive the spectroscopic parameters. The parameters De, Re, e, exe, e and Be are obtained to be 4.97 eV, 0.16259 nm, 1061.14, 5.4715, 0.53919, 0.003409 cm-1 respectively; which accord well with the available measurements. With the obtained PEC of AsN(X1 +), a total of 67 vibrational states are predicted when J=0 for the first time by numerically solving the radical Schrdinger equation of nuclear motion. For each vibrational state, the vibrational level, the classical turning points, the inertial rotation and the centrifugal distortion constants are completely reported for the first time.
Single attosecond pulse generated by model helium atom exposed to the combined field of an intense few-cycle chirped laser pulse and a half cycle pulse
2011, 60 (12): 123201. doi: 10.7498/aps.60.123201
We propose an efficient method to generate an ultrashort attosecond pulse when a model He is exposed to the combination of an intense few-cycle chirped laser pulse and a half cycle pulse. By solving the time-dependent Schrdinger equation numerically, we find that the cut-off energy of the harmonics is extended effectively to Ip+21.6Up. By superimposing some high-order harmonics in different regions for the second plateau, the obtained pulses are all single attosecond ones. Minimum pulse achieves 37 as. Especially, by superimposing the lower order harmonics of the second plateau, one can obtain single attosecond pulse, and also the intensity of the single pulse is three order of magnitude higher than the attosecond pulse obtained near cut-off of harmonics.
2011, 60 (12): 123301. doi: 10.7498/aps.60.123301
According to the relationship between Raman intensity and the bond polarizability, we investigate the temporal bond polarizabilities of carbazole molecule from the Raman intensities. We obtain the bond polarizability of the final state and compare it with the electronic density of the ground state by the density functional theory method, then we discuss and analyze the characteristics of carbazole temporal bond polarizabilities. The results show that at the initial stage of exitation, the excited electrons tend to flow toward the bond that we called connecting bond, which connects the two six-member ring, but not toward the molecular periphery. The bond electronic density of the molecule ground state can be mapped out by the temporal bond polarizabilities at the final stage of relaxation, therefore we conclude that the excited electrons flow back to the skeleton bond. Furthermore, we find the relaxation characteristic time of connecting the bonds is longer than that of connecting the other bonds, this further confirms our observations mentioned above. These conclusions will improve our understanding of Raman excited virtual states of the molecule with the bridge bonds.
2011, 60 (12): 123401. doi: 10.7498/aps.60.123401
In this paper, we present a method of determuning the potentials of molecules and dissociation products by shock wave data. The potentials are calculated by the Rosss modification of hard-sphere variation theory at low pressure and the statistical mechanical chemical equilibrium method at high pressure. Our results are in better agreement with the experimental data than the results obtained from the corresponding states theory.
2011, 60 (12): 123402. doi: 10.7498/aps.60.123402
The photoassociation reaction H+D+ induced by two chirped pulses is theoretically studied. The first pulse is employed to accelerate the collision pairs, and the second pulse is used to enhance the yield ratio of HD+. The optimal parameters of the second pulse and the population of product HD+ depend on the chirp rate of the first pulse. The population of the product HD+ can be controlled by the delay time between the two laser pulses.
2011, 60 (12): 123601. doi: 10.7498/aps.60.123601
We optimize the possible geometrical cluster structures and predicte relative stability of (HMgN3)n(n=15) by using the hybrid density functional theory (B3LYP) with 6-311G* basis sets. And the most stable isomers of (HMgN3)n(n=15) clusters, the bond properties, charge distributions, vibrational properties, and stability are analyzed theoretically. The calculated results show that the most stable HMgN3 has a linear structure, the (HMgN)n(n=2,5) clusters have the most stable structures in which an N atom in a sub-system and metal atom in another sub-system constitute an MgNMg structure. And the most stable structures of (HMgN3)n(n=3,4) clusters are the chain structures in which the nitrogen cardinal extremity position N atom and the Mg atom form a ring structure; the metal Mg atoms in the most stable structure show charge positivity, and H atom show charge negativity. The middle N atoms of azido show charge positivity, the N atoms on both sides of azido show charge negativity; what's more, the N atoms influenced by Mg atoms directly show a more charge negativity. MgN bond and MgH bond are the typical ionic bond; the bond between N atoms in azido is the covalent bond. The infrared spectra of the most optimized (HMgN3)n(n=15) clusters have three vibrational sections, the strongest vibrational peak lies in 22582347 cm-1, and the vibrational mode is anti-symmetric stretching vibration of NN bonds in azido. Analysis of stability shows that (HMgN3)3 clusters are more stable than other clusters.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2011, 60 (12): 124101. doi: 10.7498/aps.60.124101
Based on classical electrodynamics theory, the theoretical prediction equations for calculating the thrust of propellanless microwave thruster is deduced according to Maxwell equation and tress tensor. With the calculated electromagnetic fields by using finite element analysis software, the electromagnetic field distribution of a defined thruster cavity operating in a specific resonant mode under different electromagnetic powers is calculated to predict its thrust according to the deduced method. It is found that the thrust increases with the increase of the electromagnetic power and the total thrust is determined by the magnetic thrust. The net thrust will rang from 20 to 250 mN in a power range from 20 to 200 W.
A stochastic functional approach to difference scattering from a three-dimensional perfect electric conductor target on randomly rough surface
2011, 60 (12): 124102. doi: 10.7498/aps.60.124102
A stochastic functional approach (SFA) is developed to calculate electromagnetic composite scattering from a three-dimensional perfectly electric conducting (PEC) target on a randomly rough PEC surface. The stochastic Green's function for randomly rough surface is derived, and a novel four-path model is presented to describe the scattering mechanism of target-surface interactions. The SFA is used to obtain the difference scattering cross section due to the rough surface of target. As a simple example of a spherical target with rough surface, the SFA is numerically compared with other numerical approaches, showing its accuracy and effectiveness. Finally, the difference bistatic scattering from a complex electric large target with rough surface and its functional dependence on the parameters are discussed.
2011, 60 (12): 124201. doi: 10.7498/aps.60.124201
We investigate the interaction between acoustic and optical signals in Bragg gratings. Using the multiple-scale method, we convert the optoacoustic coupled mode equations into a standard nonlinear Schrdinger equation. Thus, we obtain approximate solutions of the optoacoustic coupled mode equations, such as the single-soliton solutions and multiple-soliton solutions. Furthermore, we discuss the properties of slowing light of optoacoustic solitons and the interaction of two-soliton solutions.
Investigation on strong optical activity and negative refractive index in double-layer metallic wires planar chiral structure
2011, 60 (12): 124202. doi: 10.7498/aps.60.124202
A planar chiral structure composed of double-layer metallic wires with fourfold symmetry is proposed. The rotation angle and the dielectric layer thickness between double-layer metallic wires is optimized. The numerical simulation results show that the exceptionally strong polarization rotation and the large chiral parameter are found at the infrared wavelength, and show a negative refraction index in a certain band.
2011, 60 (12): 124203. doi: 10.7498/aps.60.124203
The nonlinear diffusion of image filtering is from the idea of heat equations. Its key point is to choose a proper diffusion coefficient and control the diffusion direction. In the previous models, the diffusivity depends on the gradients of images, thus it is easily affected by noises. Furthermore, many fine structures such as textures are prone to being taken for noise and then will be removed. In order to overcome these shortcomings, first, in this paper we introduce a novel computational technique for diffusivity by using the dual tree complex wavelet transform. Second, we develop a nonlinear diffusion model for image filtering. Finally, an image diffusion filtering method based on the dual tree complex wavelet transform and wave atoms thresholding is presented, and also compared with the previous methods. Experimental results show that many features of image such as edges and textures can be preserved well after filtering via the proposed algorithm.
2011, 60 (12): 124204. doi: 10.7498/aps.60.124204
Axial resolution and traverse resolution in optical coherence tomography (OCT) imaging are determined by different factors, while axial resolution is determined by both the coherence length of light source and the beam-focusing condition, and traverse resolution is determined by the beam-focusing condition of the sample arm. In the main approaches to axial resolution improvement in OCT, a light source with a broaden bandwidth is used and coherence gating is combined with apodization, which cannot improve the traverse resolution. A method is introduced to increase both the axial resolution and traverse resolution simultaneously in an OCT system by the path length code and coherent compounding method. Different effective functions are formed by adding a path length coding lens in to the proposed OCT system, which are corresponding to different path lengths. Owing to the intrinsic ability to differentiate path lengths, we can obtain several images of the same sample, corresponding to the different effective functions simultaneously. By adding these functions through numerically controlling their relative contributions, we can finally obtain a coherent compounding signal with three-dimensional superresolutions of axial resolution and traverse resolution. Compared with the previous approaches, the path length code and coherent compounding method is very easy to operate and its cost is very low, which can not only avoid the high cost and inconvenience in implantation, but also increase both axial and traverse resolutions simultaneously.
Comparison of grating mosaic error tolerance among three types of laser pulse compressor configurations
2011, 60 (12): 124205. doi: 10.7498/aps.60.124205
Theoretical models of three types (two-pass Z-type, X-type and diamond-type) of laser pulse compressors containing grating mosaics are developed. The corresponding grating mosaic error tolerances are calculated and analyzed with a simulation method adopting ray-tracing phase analysis and Fourier transform. The mosaic error tolerances corresponding to 0.9 of ideal far-field intensity maximum (Strehl ratio equivalent to 0.9) in the case of three types of compressor are determined. The simulation shows different sensitivities of respective compressor configurations to grating mosaic errors,which can help to select the configuration and the design parameters of laser pulse compressor.
2011, 60 (12): 124206. doi: 10.7498/aps.60.124206
A model describing optomechanical dynamics via radiation-pressure coupling with a driven optical cavity is investigated by a linearized quantum Langevin equation. The spectrum of the oscillator presents normal mode splitting with the increase of the input laser power in strong coupling regime and our results are in good agreement with the experimental results. The effective mechanical damping and the resonance frequency shift are derived. The redshifted sideband leads to the cooling of the mechanical oscillator, and the blueshifted motional sideband results in amplification. Furthermore, an approximate mechanism is introduced to analyze the cooling of the mechanical oscillator. Since both the normal mode splitting and cooling require working in the resolved sideband regime, whether the normal mode splitting influences the cooling of the mirror is considered. Meanwhile, we give three key factors influencing the cooling of mechanical oscillator, these being initial bath temperature, input laser power and mechanical quality factor.
A 1020 nm fiber laser is designed based on fiber Bragg grating and then its output power is boosted to 590 mW through a two-stage amplifier. A tandem pumping fiber amplifier is constructed with the 1020 nm laser used as a pump source. The output power and the efficiency of the amplifier are measured with different lengths of gain fiber. A 385 mW maximum output power with an 81% slope-efficiency is achieved when the gain fiber is 8.5 m. The output power and the efficiency of a traditional amplifier in which 976 nm diode laser is used as a pump source are also measured. The experimental results indicate that tandem pumping fiber amplifier can achieve a higher efficiency than traditional fiber amplifier.
A fundamental way to improve the ability of lidar to detect the small target far away is to increase the light intensity on it, and the coherent combination of laser beams is an effective way to obtain a large intensity on the target. A parameter named combining effect factor is introduced to evaluate the combining effect on small target. On this basis, the factors influencing combining effect are discussed. The combining effect factor becomes oscillatory as the spacing between the waists of laser beams increases, and its amplitude decreases gradually. The distance at which an ideal combining effect can be achieved increases with the increase of the spacing between the waists of laser beams, which is a reference for the determination of the spacing between the waists of laser beams in the design of coherent combining system. The dependence of combining effect on the phase control accuracy is also studied. The combining effect can reach 80 percent of its ideal value when the phase control accuracy reaches /4, and the combining effect decreases to 50 percent of its ideal value when the accuracy is /2. The influence of unparallel polarization direction between beams on combining effect is not very obvious.
Fluorescence reabsorption analysis on laser cooling of Tm3+ doped ZrF4 -BaF2 -LaF3 -AlF3 -NaF-PbF2 glass
2011, 60 (12): 124209. doi: 10.7498/aps.60.124209
Laser cooling of solid material has become a new developing research area in recent years. Tm3+ doped ZrF4-BaF2-LaF3-AlF3-NaF-PbF2 glass is one of the hot materials in this field. Compared with Yb3+, Tm3+ has better cooling potential. Up to date, one of the main factors restricting the cooling effect is fluorescent reabsorption. In this paper, firstly, using several spectral parameters of Tm3+, the reabsorption effect is calculated by stochastic model which is a semianalytical approach to this problem. The average number of absorption events is obtained. Afterwards, the effect of fluorescence trapping due to total internal reflection is analyzed. The results show that the quantum efficiency will be lowed by 0.5%1% due to reabsorption, that the redshift of the mean fluorescence wavelength is in the range of 210 nm, and that the cooling efficiency and the cooling power decrease. Finally, after discussion, we find that the use of a small size and a long thin geometry will benefit to the fluorescence emission and cooling effect.
2011, 60 (12): 124211. doi: 10.7498/aps.60.124211
The continuous development of femtosecond technique has made it convenient to generate ultrashort pulses with variant structures in different wave ranges. In this paper we optimize the modified-zero-additional-phase spectral phase interferometry for direct electric field reconstruction system, so as to measure femtosecond pulses with different features. Pulses delivered from two femtosecond sources as well as the pulses stretched by an 80 mm-thick BK7 glass block are characterized with this system. The experimental results show sufficiently the versatility of our system.
Chaotic ultra-wideband microwave signal generation utilizing an optical injection chaotic laser diode
2011, 60 (12): 124212. doi: 10.7498/aps.60.124212
The ideal ultra-wideband (UWB) microwave pulses that fully comply with the indoor spectrum mask governed by Federal Communications Commission(FCC Indoor Mask)are generated by using continuous-wave optical injection to a chaotic laser diode. We firstly simulate and demonstrate the photonic generation of the chaotic UWB signal according to the rate equations of laser diode with optical feedback and injection. The simulations display that the -10 dB bandwidth of UWB signal increases with the increases of optical injection strength, frequency detuning, linewidth enhancement factor and with the decrease of bias current of the slave laser, and the UWB signal central frequency changes in a range from 5 to 8 GHz. We further experimentally obtain tunable chaotic UWB microwave signals with a rate up to 500 Mbit/s by tuning optical injection strength when the other parameters are fixed. The experimental results are in accordance with the theoretical analyses.
Three-dimensional tunable colloidal photonic crystal self-assembled on the end face of an optical fiber
2011, 60 (12): 124213. doi: 10.7498/aps.60.124213
High-quality three-dimensional nanostructure colloidal crystal-fiber structure is obtained by the modified vertical deposition method. The morphology of the colloidal crystal is examined by the scanning electron microscope, which illustrates that the (111) plane is parallel to the substrate of the fiber end face. The optical characterization of the colloidal crystal is also analyzed with an all-fiber network system. Reflection spectra show the existence of photonic band gap, which is located at 845 nm. As the liquid refractive index filled in the voids of the sample increases, the tunable wavelength of reflected light which is predicted by Bragg's equation with considering the effect of photonic band gap shows a good agreement with experimental results. Also, as the refractive index is changed because of different concentrations of solution, the colloidal photonic crystal-fiber system can also be distinguished by recording the shift of the maximum of Bragg reflection spectra.
Optimization of electro-optical parameter of optical frequency standard system based on modulation transfer spectroscopy technique
2011, 60 (12): 124214. doi: 10.7498/aps.60.124214
According to the mechanism of nearly degeneration four-wave mixing that occurs in a sufficiently nonlinear resonant gaseous medium caused by phase-modulated laser beam, the electromagnetism excitation and dynamic evolvement of modulation transfer spectroscopy(MTS) technique is studied theoretically, and then mathematic models of dispersion spectrum and absorption spectrum of MTS are established too. The results show that when the frequency of modulation signal source is equal to 0.72 times the linewidth of absorption line approximatively, the intensity of dispersion signal reaches a maximum value; it can obtain a better frequency-stabilization result when modulation factor is set to be nearly 0.3; the instability of demodulation phase has a big effect on the absorption spectrum, but on dispersion spectrum, the effect is tiny when the demodulation phase fluctuates near 90.
2011, 60 (12): 124215. doi: 10.7498/aps.60.124215
The multi-polarization controlled fiber loop mirrors (FLMs) are presented and their characteristics are studied, because of the non-tunable characteristic of the wavelength interval for single-polarization controlled FLM. The multi-polarization controlled FLMs are studied theoretically with the Jones matrix. We experimentally study the single-, double- and triple-polarization FLMs, and the results show that compared with the single-polarization controlled FLM, the multi-polarization controlled FLM possesses the interval tunability and the side-frequency restriction are shown. The experimental results are in good agreement with theoretical results. The side-frequency restriction characteristic is shown in a ring fiber laser of double-polarization FLM and the side-mode suppression ratio is enhanced by 5 dB compared with the one of single-polarization FLM.
2011, 60 (12): 124301. doi: 10.7498/aps.60.124301
A novel composite waveguide is proposed to improve the directional emission of phononic crystal waveguide by introducing a point defect array. The finite element simulations indicate that the point defects attached to the output surface of phononic crystal waveguide produce resonant modes, which act as secondary radiation sources. The interferences of sound wave radiated from these radiation sources and waveguide can enhance the normal radiation intensity by 161.21% and reduce the half-power angular width by 85.35%. In addition, when the number of the point defects is large enough, the directivity approaches a steady value. The investigation provides a new method to improve the directional emission of phononic crystal waveguide.
2011, 60 (12): 124302. doi: 10.7498/aps.60.124302
By the Gaussian expansion approach we investigate fundamental- and second-harmonic generation in practical Bessel beams of several lobes. The analysis is based on the integral solutions of the Khokhlov-Zabolotskaya-Kuznetsov equation under the quasilinear approximation. The influence of the medium attenuation on beam profile is considered. Numerical results show that the absorption parameter has a significant effect on the beam profile of the second harmonic. Under certain circumstances, the second harmonic of a practical Bessel beam with several lobes still has the main properties of an ideal Bessel beam of infinite aperture when it propagates within its depth of field.
2011, 60 (12): 124303. doi: 10.7498/aps.60.124303
An impact energy model (IEM) is proposed for the research of nonlinear rub-impact assessment in this paper. According to the characteristic of the model, IEM index is given to evaluate the severity or probability of rub-impact fault. We investigate the trend of variation of IEM index response to the rotation speed ratio under the condition of different values of damping ratio , and then acquire a joint distribution of IEM response with the parameters of and . Combining the IEM model, a concept of sensitive area is proposed. The sensitive area provides an excellent reference for operating rotor system and a guidance and assessment for designing rotor system.
Simulation of mixing process for size-type binary wet particulate systems in a rotating horizontal drum by discrete element method
2011, 60 (12): 124501. doi: 10.7498/aps.60.124501
Using the four-equation of linear spring-dashpot discrete element method and considering effect of the liquid bridge, the mixing and segregation process of size-type binary wet particulate system in a rotating horizontal drum is simulated. The effect of interstitial liquid on the mixing and segregation process is discussed. To assess the accuracy of the simulation result, some comparisons are made with the experimental date in the literature. The simulation results show that the liquid bridge between particles plays an important role in mixing and segregation process, and that the cohesion force induced by liquid-bridge leads to the formation of agglomerates of particles. As a result, segregation may be mitigated and mixing may be enhanced, and the network distribution of the contact forces is more uniform in wet particulate system.
REVIEW OF FRONTIER AREA
2011, 60 (12): 124216. doi: 10.7498/aps.60.124216
Organic nonlinear optical molecular materials have a number of important applications in frequency transformation, electrooptic modulation, two-photon absorption and so on. In this paper, we introduce the main applications of molecular dynamics simulation in theoretical study on nonlinear optical properties of organic molecules, including the electric field poling effects, the local field factors, the nonlinear susceptibilities, and the two-photon absorption. In addition, the important roles played by molecular dynamics simulation in the study of solvent and aggregation effects are also illustrated in combination with the recent research.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2011, 60 (12): 125201. doi: 10.7498/aps.60.125201
A zero-dimensional model is used for studying the behaviors of NOx in atmosphere plasmas with different ionization degrees. The temporal evolutions of NOx (including NO, NO+, NO2, NO2+, N2O, N2O+, NO3 and N2O5), N and O3, the main important reactants which influence the producing and the consuming of NOx, are obtained in different initial densities for afterglow plasmas. The results show that the removal rates of NO and NO2 are higher when ne0=109 cm-3, and the total nitrogen oxide density is lower, so it is suited for the removal of the pollution of NOx. Some important reactants such as N and O3 varying with the increase of ionization degree are also analyzed.
2011, 60 (12): 125202. doi: 10.7498/aps.60.125202
Plasma pattern formation in 110 GHz microwave air breakdown is investigated by numerical solution of fluid-based plasmas equations coupled with the Maxwell equations. It is found that the filamentary plasmas are observed at high pressure, the filamentary plasmas transit to diffuse plasma at medium pressure, and the diffuse plasma is obtained at low pressure. The plasmas region propagates toward the microwave source. The distribution of initial electrons influences merely plasmas pattern at first stage, but not final plasmas pattern. The movements in the directions parallel and vertical to electric field are different. Due to the strong electric field at the poles of the plasmas region, the plasmas are elongated in the direction of electric field, forming the filamentary plasmas at much low pressure in E plane.
2011, 60 (12): 125203. doi: 10.7498/aps.60.125203
In order to investigate the effects of the inner wall secondary electron emission on the performance of Hall thruster, a hydrodynamic model is used to study the characteristics of plasma sheath considering secondary electron magnetization. The Bohm criterion of the magnetized plasma sheath is obtained. The structures of plasma sheath in Hall thruster with different magnetic field magnitudes and directions, different secondary electron emission coefficients and different plasma species are discussed. Simulation results indicate that both particle density and wall potential increase while the sheath thickness decreases with the augment of secondary electron emission coefficient. The sheath potential and the particle density increase with the magnitude and azimuth angle of magnetic field increasing. Both the sheath thickness and the wall potential are different in different plasma species. These research results provide a theoretical explanation for the magnetism-ampere characteristic of the Hall thruster.
2011, 60 (12): 125204. doi: 10.7498/aps.60.125204
Cold plasma generated by atmospheric air discharge has wide application prospect in industry because it does not need vacuum equipment and mass production is possible. In this paper, a stable uniform discharge is generated in open air by a plasma needle. Discharge mechanism is investigated by optical method, and plasma parameters are given by the spatially resolved measurement of emission spectrum from the discharge. Results show that the discharges have two modes. One is a corona discharge mode and the other is plasma plume mode. In the stable plasma plume mode, a strong emission area and a weak emission one can be distinguished from each other. The development velocity of the weak emission area is much faster than that of the strong emission area. Furthermore, the electron energy and the plasma density in the weak emission area are also bigger than those in the strong emission area. Therefore, the discharge in the strong emission area is dominated by Townsend mechanism, while that in the weak emission area is dominated by streamer discharge. Gas temperature and vibration temperature are also studied in this paper. The experimental results are of great importance to the industrial applications of atmospheric pressure discharge.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2011, 60 (12): 126101. doi: 10.7498/aps.60.126101
The existence of forced and natural melt convections in the liquid in front of dendritic tip will Change the thickness of diffusion layer, which can significantly influence the dendrite morphology. Using the cellular automaton method and considering the influences of melt convection and heat transfer on microstructure evolution, a new numerical model is established by coupling the Navier-Stokes equations, the heat transfer equation and the solute convection and diffusion equation. The influences of forced and natural melt convection on dendrite morphology evolution are investigated by this model. The three-dimensional simulations reproduce the equiaxed dendrite growth, and reveal the influence of convection on dendritic growth rate and tip radius. The effect of natural convection on columnar dendrite growth during directional solidification of NH4Cl-H2O solution is simulated via the model. Experimental validation of this solidification process is performed and compared with simulation results. The simulation results accord well with experimental measurements. Hence, the model is reliable and can be extended to the prediction of the behavior of dendrite growth in real alloys.
Simulation of incipient spallation experiments of high purity aluminum based on a single void growth model
2011, 60 (12): 126201. doi: 10.7498/aps.60.126201
By means of a single void growth model, finite element simulations of incipient spallation experiments of high-purity aluminum under different plate impact velocities are carried out. The relationship between the influence of void growth in the sample on wave propagation and the feature of free surface velocity profile is discussed. By analyzing the change of stress field around the void, the occurrence of pullback signal in free surface velocity profiles is attributed to the local unloading wave effect caused by void growth in the sample, which cannot indicate the whole spallation of materials. Free surface velocity profiles and relative void volumes are simulated for different impact velocities and the simulation results are in good agreement with experimental results, which indicates that the stochastic damage evolution in incipiently spalled sample can be described by a single void growth in cell model.
2011, 60 (12): 126202. doi: 10.7498/aps.60.126202
In this paper, we study the crystallization of water due to fused quartz effect under shock compression by a gas gun and light transmission tests. The experimental results indicate that at 1.28 GPa water rapid crystallizes when the water has come into direct contact with the quartz glass. On the contrary, freezing cannot occur within 2 s, demonstrating that the observed phenomenon of the liquid-solid phase transition of water can be promoted by the fused quartz. The dynamics of the phase transition is also discussed in this paper.
Monte Carlo simulations of the relaxation dynamics of the spatial relaxation modes in the molecule-string model
2011, 60 (12): 126401. doi: 10.7498/aps.60.126401
According to the molecule-string model for glass transition, a more exact Monte Carlo protocol to simulate all the spatial relaxation modes (SRMs) of the string are proposed. The variations of the simulated relaxation times of the SRMs with temperature and string length are consistent with the predictions of the string relaxation equation of the model, i.e. the theretical predictions and the simulation results verify each other. It should be pointed out that the necessary condition of molecule string used as a collective unit in liquid is that the qualitative characteristics of the SRMs cannot be changed when the inter-string interactions are taken into account. This needs to study the coupling between the SRMs, but till now, the corresponding exact solutions have not been achieved, and only the self-consistent relaxation mean-field method is vailable. Therefore, the present simulation protocol will provide a necessary basis to study the coupling between the SRMs of neighboring strings, including the feasibility of the mean-field method.
A Brownian dynamic simulation to verify the effective hard-sphere model criterion for the formation of charged colloidal crystals
2011, 60 (12): 126402. doi: 10.7498/aps.60.126402
The mechanism for the formation of colloidal crystals in charge-stabilized colloids is more complicated than that of hard-sphere colloidal crystals. And there is still lack of available criterion for the formation of charged colloidal crystals. The effective hard-sphere model suggests a criterion in which the effective diameter is used as a crucial parameter. In order to test the validity of this criterion, the characteristics of charged colloidal crystals with different effective diameters are investigated using Brownian dynamics simulations in this study. The crystallization behaviors with different geometric particle diameters and repulsive forces are also studied with some fixed effective diameters. In the simulation, the time evolution of crystallization process and the crystal structure during the simulation are characterized by means of the radial distribution functions and bond-order parameters. The results show that the effective hard-sphere model criterion has its reasonableness to some extent. However, the effective diameter cannot be used as the only parameter that influences the formation of charged colloidal crystals. The influence of other parameters should also be taken into account, which indicates that the criterion is one-sided.
2011, 60 (12): 126501. doi: 10.7498/aps.60.126501
The Si-doped glow discharge polymer (Si-GDP) films are deposited by glow discharge polymerization technology at different tetramethylsilane (TMS) flows. The chemical structure, the composition and the thermal stabilities of Si-GDP films are analyzed by the Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and thermal gravimetric analysis. The results show that the Si content increases from 0 to 16.62%, when the flow of TMS changes from 0 to 0.06 cm3/min. The relative content of SiC, SiH, SiO, SiCH3 increases with TMS flow rate increaseing. As TMS flow increases, the thermal stability of Si-GDP film becomes good.
2011, 60 (12): 126601. doi: 10.7498/aps.60.126601
The Green-Kubo time correlation function is used to predict fluid argon transport properties, such as diffusion coefficient, viscosity and thermal conductivity, through molecular dynamics simulations. The results show that the transport characteristics, especially the viscosity and thermal conductivity, fluctuate intensely during the simulations. The collective stress tensor is separated into two parts, one is due to the kinetic energy and the other is due to the pair virial function, and the collective heat flux vector is contributed from the kinetic energy, the intermolecular potential and the pair virial function. The results show that the transport characteristics, especially the viscosity and the thermal conductivity, fluctuate intensely during the simulations. The most important contribution to the viscosity and the thermal conductivity is from the autocorrelation of the virial term. The calculations indicate that a more compatible integration time step method is needed to reduce instabilities when the Green-Kubo time correlation is used to calculate the fluid transport parameters. Other factors which influence the stability are also discussed in the paper.
2011, 60 (12): 126602. doi: 10.7498/aps.60.126602
The diffusions of He atom and small He clusters in Ti at different temperatures are simulated by molecular dynamics. The prefactors and the activation energies of diffusion coefficients are calculated. It can be concluded that the diffusion is anisotropic. Simulations show that prefactors of diffusion coefficients are different from those of He species diffusing in different directions, but the activation energies are the same. The result demonstrates that it is insufficient to predict the diffusion behavior of He in metal when only the energy barrier in static lattice is considered. Dynamics calculations are necessary to obtain the correct prefactors. Another counterintuitive observation is that He-dimer migrates more quickly than single He atom does at room temperature. The results emphasize the importance of dynamics simulations in predicting diffusion behavior of He in metals.
Pressure threshold and dynamics of nucleation for Si nano-crystal grains prepared by pulsed laser ablation
2011, 60 (12): 126801. doi: 10.7498/aps.60.126801
Si nano-crystal grains are prepared by pulsed laser ablation in low pressure Ar at room temperature through changing the gas pressure and the distance between target and substrate. The morphologies and compositions of samples are characterized by scanning electron microscopy images, Raman scattering spectra and X-ray diffraction spectra.The pressure threshold for Si grain formation is obtained to be 0.6 Pa at a laser fluence of 4 J/cm2, distance between target and substarate of 3 cm, and room temperature.Combining the fluid mechanics model and the nucleation division model, the dynamics process of nucleation is analyzed.The Monte Carlo simulation shows that the nucleation of nano-crystal grains is determined jointly by temperature and supersaturated density.
Acoustic phonon transport and thermal conductance in one-dimensional quantum waveguide modulated with quantum dots
2011, 60 (12): 126802. doi: 10.7498/aps.60.126802
By using scattering matrix method, we compare the propertics of acoustic phonon transport and thermal conductance in one-dimensional quantum waveguide modulated with both convex-shape and concave-shape quantum structures. The results show that the transmission spectra and thermal conductances are sensitive to the geometric structures of quantum dots, and the transmission rate and thermal conductance KCV in the convex-shape quantum structure are bigger than the transmission rate and thermal conductance KCC in the concave-shape quantum structure. The thermal conductance ratio KCV/KCC is dependent on the geometric detail of quantum dot, and the ratio increases with the increase of difference in side-length of the cross section between the quantum dot and the main quantum waveguide. The difference in thermal transport between the convex-shape and the concave-shape quantum structures originates from more excited dilatational acoustic modes in the convex-shape quantum structure than in the concave-shape quantum structure.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
A first principles study on tetragonal distortion, magnetic property and elastic constants of Pd2 CrAl Heusler alloy
2011, 60 (12): 127101. doi: 10.7498/aps.60.127101
Tetragonal distortion, magnetism and elastic constants of Pd2CrAl Heusler alloy are calculated by first principles based on the density function theory with projector augmented wave pseudopotential. Analysis of tetragonal distortion shows that there is a local minimum total energy at c/a1.20, which corresponds to a stable martensitic phase. The magnetic moments of formula unit cells for L21 and tetragonal structure are 3.825B and 3.512B, respectively. There is very strong hybridization between Pd and Cr, and magnetism originates mainly from the 3d-t2g and 3d-eg subbands of Cr atom in the two structures. Elastic constants of both L21 and tetragonal structure of Pd2CrAl satisfy their stability conditions, respectively.
2011, 60 (12): 127102. doi: 10.7498/aps.60.127102
Based on the density functional theory, the electronic properties of N-doped titanates are investigated. It is found that the titanate doped by a nonmagnetic 2p light element N is ferromagnetic. The local magnetic moment is localized mainly on doped N atoms. Our results indicate that the N-doped titanates may be potential candidates for dilute magnetic semiconducting materials and multiferric materials.
2011, 60 (12): 127103. doi: 10.7498/aps.60.127103
Using the full-potential linearized augmented plane wave method based on the density functional theory, spin-polarized calculations of electronic structure for the zinc-blende CrS and CrSe are performed. Zinc-blende CrS and CrSe at their respective equilibrium lattice constants are half-metallic with the same total magnetic moment of 4.00B. The electronic structures of zinc-blende CrS and CrSe are calculated under uniform strains from -10% to +10% relative to the equilibrium lattice constant. The calculated results indicate that zinc-blende CrS and CrSe can maintain half-metallic ferromagnetism and keep the same total magnetic moment of 4.00B from -1% to 10% and from -4% to 10% uniform strain, respectively.
2011, 60 (12): 127201. doi: 10.7498/aps.60.127201
Based on the tight-binding model and the Greens function method, the effects of atomic disorder of lattice configuration and the orientation disorder of side radical spins on the spin polarized transport through a metal/organic-ferromagnet/metal structure are investigated. The results show that the atomic disorder reduces the threshold voltage of the device and suppresses the conducting current. The staircase structure of the current-voltage curve for a molecular device is eliminated when the disorder is enhanced. The current keeps a high spin polarization if the atomic disorder is not strong. The orientation disorder of side radical spins reduces the spin splitting of molecular energy levels, which increases the threshold voltage of the device. The current and its spin polarization are reduced apparently at a low bias when the strength of disorder is enhanced. We further simulate the effect of temperature on the spin polarized transport through the device by taking into account two kinds of disorders.
2011, 60 (12): 127202. doi: 10.7498/aps.60.127202
According to the parameter-free first principles calculations, we investigate the spin polarized transport in antiferromagnetic spin valve (AFSV) based on noncollinear IrMn. The giant magnetoresistance (GMR) in Co/IrMn/Cu(111) with L12-type IrMn shows three-fold rotational symmetry, which is easy to be distinguished from the GMR of normal ferromagnetic spin valves. Moreover, GMR based on -phase IrMn with current-in-plane (CIP) structure shows that GMR is 7.7%, around two time larger than that in current-pendicular-to-plane (CPP) structure (3.4%). Our study demonstrates that the AFSV CIP structure possesses a larger GMR effect than the CPP structure, and the GMR effect in AFSV based on noncollinear antiferromagnetic structure is larger than that based on collinear antiferromagnetic structure such as FeMn.
The effect of variation in pressure-induced electrode position on the measurement accuracy of sample conductivity in a diamond anvil cell
2011, 60 (12): 127203. doi: 10.7498/aps.60.127203
Using the finite element analysis, we study the effect of variation in pressure-induced electrode position on the measurement accuracy of the sample conductivity in diamond anvil cell with the Van der Pauw method. The results show that the electrode contact placement and electrode size play key roles in influencing the conductivity measurement accuracy. Theoretical computation reveals that the Van der Pauw method can provide an accurate result when the spacing between electrode center and sample periphery b is less than or equal to d/9 (d is the sample diameter). Otherwise, the closer to the sample center of the contact location, the more rapidly the sample conductivity accuracy error increases. Such an effect is more significant for the semiconductor sample with high resistivity with the electrode position variation is the same.
The effect of stacked graphene flakes on the electronic transport of zigzag-edged graphene nanoribbons
2011, 60 (12): 127204. doi: 10.7498/aps.60.127204
The effect of stacked graphene flakes (GFs) on the electronic transport property of zigzag-edged graphene nanoribbon (ZGNR) is investigated. By using the Greens function method, we calculate the conductances of ZGNRs with two different stacked-type GFs. It is found that the coupling effect between ZGNRs and GFs can induce dips at the conductance profiles in two different stacked-types. For both stacked-types, the dips far away from the Fermi level are nearly overlapped. However, the position of conductance dip near the Fermi level depends on the stacked-type. In addition, we discuss the effect of geometric size of GF on the electronic transport property. The results show that with the increase of the size of GF, the dips far away the Fermi level in two stacked-types gradually move toward the Fermi level, while the discrepancy of the dips near the Fermi level is much evident. Our results indicate that the stacked GFs can effectively tune the electronic transport of ZGNR.
Influences of the size and breadth of potential barrier on electronic scattering cross-section in spherical nanometer system
2011, 60 (12): 127301. doi: 10.7498/aps.60.127301
Under the condition of the spherical square potential model and the effective mass approximation, the electronic scattering cross-section and the electronic probability distribution are obtained in an open-type spherical nanometer system, and the influences of the size and the width of potential barrier on electronic scattering cross-section, resonance energy and resonance width are discussed. The results show that there exist one maximum and one minimum in the distribution curve of the electronic scattering cross-section versus energy, and the maximum of electronic energy probability distribution curve is between the maximum and the minimum of energy in the scattering cross-section curve; the scattering cross-section increases with the increase of r0, the inner radius, and the scattering cross-section curve will change from smoother to sharper with the increase of r0; the scattering cross-section will enlarge with , the width of potential barrier, but it will become abnormal when is between 1.4aCdS and 1.7aCdS; when =1.6aCdS, the scattering cross-section is extremely small; El, the electronic resonance energy changing with , is related to the electronic state, while l, the electronic resonance width will decrease with the increase of ; no matter what is, both El and l satisfy the uncertainty principle of energy and time.
First principles study of electronic and optical properties of Er-doped silicon nanoparticles with different densities
2011, 60 (12): 127302. doi: 10.7498/aps.60.127302
The structural stability, the electronic and the optical properties of Er-doped silicon nanoparticles are investigated by first principles based on the density functional theory. The results show that the structure is more stable when the doping concentration of Er atoms is smaller in silicon nanoparticles. The doping of Er atom in silicon nanoparticle introduces the impurity levels which result in the narrowing of band gap. A strong absorption peak occurs in the low-energy region of Er-doped silicon nanoparticles, and the intensity of the absorption peak decreases gradually, even disappears with doping concentration decreasing. The study provides the theoretical basis for the design of silicon-based materials.
2011, 60 (12): 127303. doi: 10.7498/aps.60.127303
Based on the energy dispersion relation involving curvature effects for the single-walled carbon nanotube (SWCNT), the electronic velocity and the effective mass of the lowest conduction band for the SWCNT are computed and they are compared with the results without consideration of the curvature effects. The analysis shows that the influences of the curvature effect on the electronic velocity and effective mass depend on type of SWCNT, that is, the metallic zigzag-SWCNT is most sensitive to curvature effect, armchair-SWCNT is second most sensive to curvature effect, and the semiconducting zigzag-SWCNT is poorly sensitive to curvature effect. These findings suggest that curvature effects have the largest effect on the electronic structure and the low-bias transport behaviors of the metallic zigzag-SWCNT,a moderate effect on those of the armchair-SWCNT, and the smallest effect on those of the semiconducting zigzag-SWCNT, which are in good agreement with experimental measurements and the calculated results from the density-functional theory.
2011, 60 (12): 127304. doi: 10.7498/aps.60.127304
An organic field-effect transistor based on pentacene semiconductor with CuI/Al bilayer electrode is investigated. The CuI layer, directly contacting the organic semiconductor layer, serves as the hole-injection layer. The overcoated metal layer is responsible for the reduction in contact barrier. Compared with the device with a single metal (Al, Au) layer used as the source-drain electrode, the device with CuI/Al electrodes considerably improves the hole mobility and the on/off current ratio and greatly reduces the threshold voltage. Results of X-ray photoelectron and ultraviolet/visible absorption studies reveal that the reduction in the contact barrier can be attributed to an electron transfer from pentacene and Al to CuI.
Effect of substrate topography for YBa2 Cu3 O7-x coated conductors on the nucleation mechanism of buffer layer
2011, 60 (12): 127401. doi: 10.7498/aps.60.127401
The surface condition of substrate tape is an important factor to obtain epitaxial buffer layer on biaxially textured Ni tape for YBa2Cu3O7-x coated conductors. We prepare ceria films on Ni single crystal, biaxially textured Ni tape and sulfured Ni substrates by direct current magnetron sputtering. The results show that the ceria films prepared on Ni single crystal and sulfured Ni substrates each have a poor-textured grain structure. However, the ceria film fabricated on rolling assisted biaxially textured substrate (RABiTS) exhibits a good c-axis texture and desirable surface morphology. Reflection high-energy electron diffraction analysis indicates that the c(22) superstructure on the RABiTS Ni surface has a dramatic effect on the heteroepitaxial growth of oxide buffer layer.
Investigation on the relation between the length of magnetic core and the giant magnetoimpedance effect of solenoid with FeCo-based magnetic core
2011, 60 (12): 127501. doi: 10.7498/aps.60.127501
The relation between the length of the magnetic core (l) and the giant magnetoimpedance(GMI) effect of solenoid with magnetic core of Fe36 Co36 Nb4Si4.8 B19.2 ribbon which annealed with current 32 A/mm2 for 10 min, has been investigated. The result shows that there is a sensitive relation between l and GMI. There is a linear relation between the l and the maximum of the GMI (Z/Z)max when l is smaller than the length of the solenoid l0. The linear correlation coefficient can be deduced from the electromagnetism theory. It should deviate from the linear relation when l is greater than l0. There is a maximum value of the (Z/Z)max when the l reaches the best value. There is a similar relation between the height of the tip like part of the GMI profile (Z/Z)T and l. There is a maximum value of the (Z/Z)T when the l is the best value. The tip like part of the GMI profile, which responds to the weak magnetic field sensitively, is influenced by the demagnetizing field. There is a negative exponential relation between the demagnetizing field and the length of the magnetic core l.
Responses of electronic structure and magnetism to tetragonal distortion and their influence on pressure for the Heusler alloy Mn2 NiAl
2011, 60 (12): 127502. doi: 10.7498/aps.60.127502
The change rules of electronic structure and magnetism in the process of transform from a cubic structure to a tetragonal structure and their responses to pressure for Hg2CuTi-type Mn2NiAl are studied by first principles method based on the density functional theory. The study shows that in the process of transform from a cubic austenite phase to a tetragonal martensite phase, the shift of the density of states of occupied state to ward lower energy in the martensitic phase results from the enhanced Ni-Mn(A) hybridization caused by a decrease in the Ni-Mn(A) distance, and it is the reason for the stabilization of the martensitic phase. In the process of transform from the austenite phase to the martensite phase, the bonding interaction becomes stronger, owing to energy band broadening, and it can improve the stabilization in the martensitic phase. In the process of tetragonal distortions, the change of total magnetic moment is determined by the change of magnetic moment of Ni atom. The bulk modulus at zero pressure of Mn2NiAl is calculated to be 125.69 GPa, so that we expect Mn2NiAl to be the more compressible material than the familiar Heusler alloys.
2011, 60 (12): 127503. doi: 10.7498/aps.60.127503
According to the spin-polarized density functional theory, we study the electronic structures, the magnetic and the optical properties of Cr-doped ZnO nanowires. The calculated results show ferromagnetic coupling for Cr atoms substitution for Zn atoms in ZnO nanowires along the  direction, and the antiferromagnetic coupling with Cr-doped in ZnO nanowires along the  and  directions. The results reveal that the magnetic coupling state near the Fermi level gives rise to such a spin splitting phenomenon near the Fermi level, which indicates that Cr 3d and O 2p orbitals have intense hybrid effects. In addition, the spin electronic density results indicate that system magnetic moments are generated mainly by the unpaired 3d electrons of Cr atoms and are also related to the electron configuration. Moreover, the results of optical properties show that the obvious absorption peaks are observed in the far ultraviolet and the near ultraviolet regions and there is a red shift phenomenon in the ultraviolet region. These results indicate that the Cr-doped ZnO nanowires could be a promising dilute magnetic semiconductor material.
2011, 60 (12): 127701. doi: 10.7498/aps.60.127701
Al2O3-Y2O3-ZrO2 ternary composite ceramics are synthesized via the traditional solid state reaction method and sintered at 14001500 ℃. The phase structure, the microstructure and the electrical properties of these samples are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and dielectric spectra. There are not any other impurity phases in this ternary system supported by XRD patterns, and additions of Y2O3 and ZrO2 into Al2O3 make contributions to the lower calcining heat. SEM indicates that the grain sizes of these samples are about 200500 nm. Furthermore, the densities are improved and the grain boundaries are clearer for the samples sintered at higher temperatures. Relaxation peaks are observed in the dielectric loss plots and the relaxation is of non-Debye type according to Cole-Cole complex impedance spectrum.
2011, 60 (12): 127801. doi: 10.7498/aps.60.127801
Eu3+ doped NaLa(MoO4)2 microcrystals with different sizes are synthesized by a mild hydrothermal method, and the morphologies of the microcrystals can be easily controlled by adjusting the volume ratio of ethylene glycol to water and the aging time. Uniform high-quality shuttle-like microcrystals with an average length of 2.0 m are obtained under the hydrothermal condition of 180℃ for 16 h. From the emission spectra of NaLa(MoO4)2∶Eu3+ microcrystals, it is concluded that the dopant Eu3+ ion occupies a La3+ site. It is indicated that the concentration quenching of the emission peak at 613 nm of Eu3+ is ascribed to the electric dipole-electric quadrupole interaction, and the cross relaxation process of Eu3+(5D1 ) + Eu3+(7F0 ) Eu3+( 5D0 ) + Eu3+(7F3) appear.
The effect of Eu2+ doping concentration on luminescence properties of Ca2-xMgSi2O7∶x Eu2+ green phosphor
2011, 60 (12): 127802. doi: 10.7498/aps.60.127802
The Ca2-xMgSi2O7∶xEu2+ green phosphor is synthesized by chemical coprecipitation. X-ray diffraction and the fluorescence spectrophotometry are used to investigate the structural and the luminescent properties of the Ca2-xMgSi2O7∶xEu2+ green phosphor. The result shows that the excitation spectrum of the Ca2-xMgSi2O7∶xEu2+green phosphor extends from 300 nm to 480 nm, and the peaks appear around 389 and 430 nm, thus the phosphor can be excited effectively by InGaN chip in a range of 360480 nm. The peak of the emission spectrum appears around 531 nm. The emission spectrum intensity first increases and then decreases with Eu2+ doping concentration increasing. The strongest emission intensity is obtained when Eu2+doping concentration reaches 0.04. The concentration self-quenching is attributed to the d-d interaction according to the Dexter theory.
Investigations of microstructure and ionic conductivity for (PEO)8-ZnO-LiClO4 polymer nanocomposite electrolytes
2011, 60 (12): 127803. doi: 10.7498/aps.60.127803
The effects of nanosized ZnO on the microstructure, the free volume and the ionic conductivity of poly ethylene oxide (PEO) nanocomposite electrolytes (PEO)8-ZnO-LiClO4 are systematically studied by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and positron annihilation lifetime spectroscopy (PALS), respectively. The experimental results show that the presence of the nanosized ZnO brings about a reduction in the crystallinity of the PEO phase and a very marked increase in ionic conductivity. PALS discrete analysis shows that the free volume size, the free volume concentration and the relative free volume fraction significantly increase with the increase of nano-ZnO. Specially, it is the first time to observe the occurrence of peaks of the free volume distribution split after nano-ZnO has been filled, which indicates that the ZnO added into (PEO)8-LiClO4 has an important effect on the microstructure for nanocomposites due to the interaction between the nano-ZnO and matrix. A direct correlation between the free volume fraction and the ionic conductivity is observed, and the ionic conductivity mechanism is discussed.
2011, 60 (12): 127901. doi: 10.7498/aps.60.127901
GaN photocathode is fully activated by employing a continuous Cs source and an alternate O source. The quantum efficiency curve of transmission-mode photocathode is tested in situ. The quantum efficiency reaches up to 13% in transmission-mode. According to the one-dimensional Schrdinger equation, the electron transmission coefficient formula of GaN vacuum electron source material is deduced. For a certain profile of photocathode surface potential barrier, the electron transmission coefficient relates to the incident electron energy, the height and the width of the surface potential. The energy band of transmission-mode negative electron affinity (NEA) GaN photocathode and the change of surface barrier in the deposit course of Cs,O are given. Using the double dipole layer surface model [CaN(Mg):Cs]:O-Cs, the NEA property formation of GaN vacuum electron source material is analyzed. The results show that the double dipole layer formed in the activation course of Cs,O is conducible to the escape of electrons, and it is the formation of double dipole layer that causes the drop of vacuum energy level of the material surface.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2011, 60 (12): 128101. doi: 10.7498/aps.60.128101
The pressure propagation in high velocity compaction process is simulated based on the discrete element method in this paper. Because the full process is divided into elastic loading, plastic deformation and elastic unloading, the governing equations are established in three stages respectively. With the help of computing software PFC2D, the impacting force through different layers of powders can be obtained, which cannot be observed in experiment. The simulation results show obviously a delay phenomenon and serrate waves with different gradients in loading and unloading processes. Finally, the simulated low-level stress waves are compared with available experimental data, which are consistent with each other.
2011, 60 (12): 128401. doi: 10.7498/aps.60.128401
In a double-gap coupled cavity of klystrons, the electrons exchange energy with the electric field in each gap through beam-wave interaction process, and different beam-loading effects take place in each gap. However in this case the traditional beam-loading model does not hold true. To solve this problem, we present a novel model according to the space-charge-wave theory to calculate the beam-loading conductance in each gap of the coupled-cavity, and also derive the formulations. Moreover, we perform a simulation study using a three-dimensional particle-in-cell code. The results obtained by the model show good agreement with the simulation results. In comparison with the traditional model, the new model can be used to calculate the beam-loading conductances in diffident regions of the coupled-cavity, and then it can be used to study the beam-wave interactions in the gaps and analyze the mode stability in the coupled-cavity in a high accuracy.
2011, 60 (12): 128402. doi: 10.7498/aps.60.128402
In this paper, we analyze the interaction between electron beam and microwave in the coaxial cavity, and obtain the coupling coefficient of the coaxial cavity and the electronic load conductance. The X-band tri-axial relativistic klystron amplifier is designed and simulated. Simulation results show that coaxial cavity can increase the efficiency. With an input microwave power of 70 kW, electron beam voltage of 600 kV, and electron beam of 5 kA, we obtain a microwave with a power of 1.1 GW, frequency of 9.37 GHz, and an efficiency of 37%.
Simulation investigation of the 2 mode operating tunable relativistic magnetron with axial radiation
2011, 60 (12): 128403. doi: 10.7498/aps.60.128403
A tunable relativistic magnetron with axial radiation operating at 2 mode is investigated by three-dimensional particle-in-cell simulation in this paper. The tuning is realized by filling solid dielectric material into the cavities of the interaction region. The effects of changing the relative permittivity and the inner radius of the dielectric material on the operating frequency, the average output power and the efficiency are analyzed. Then the principle of the tuning is demonstrated. The simulation results show that under the unchanged structure parameters and the work point, the relativistic magnetron realizes the tuning from S band to L band by varying the relative permittivity or the inner radius of the solid dielectric material. Furthermore, with inserting the dielectric material, the output capability of the relativistic magnetron is improved. When the relative permittivity is 615 and the radius is 4.184.40 cm, the increase in efficiency can reach 80%, the decreased frequency range is less than 55%.
The study of interface and photoelectric performance of dye-sensitized solar cells in the applied negative bias
2011, 60 (12): 128404. doi: 10.7498/aps.60.128404
Dye-sensitized solar cell (DSC) modules are most likely to work for long time under negative bias due to the mismatch in the outdoor usage, which can obviously influence the cell performance. In this paper, the interface property of DSC under negative bias is investigated by Raman spectroscopy, electrochemical impedance spectroscopy and incident-photon-to-electron conversion efficiency (IPCE). The results of Raman spectroscopy indicate that the decreased peak intensity at 167 cm-1 (the oxidized state of N719 (D+)/ I3-) after 1000 h could be due to Li+ ions diffusing into the TiO2 electrode and partially being intercalated into the TiO2 film. It is also found that the increased recombination resistance in the interface of TiO2/electrolyte resultes in the improved open-circuit voltage and the decreased IPCE values, leading to reduced short-circuit current for DSC with base electrolyte under the long-term negative bias. However, when BI is added into the base electrolyte, the Raman spectrum shows no significant change and that the cell efficiency remains stable after 1000 h. The reason is that BI could prevent Li+ ions from being intercalated into the TiO2 film. It is proven by the further experiments where the DSC with BI exhibits better stability under different negative biases.
2011, 60 (12): 128501. doi: 10.7498/aps.60.128501
An online degradation model is presented, in which is avoided the error in parameter degradation accelerated test, which is caused by temperature shock during parameter measurement. Through the measurement with avoiding the error parameter, the parameter degradation model can be more accurate. To demonstrate the application of the method, a kind of mature product, 3CG120, is tested in a temperature range of 150230 ℃ under online process-stress. Then the error of lifetime is obtained by utilizing the online model to be about 6.5%, much less than that of the old model (23.2%).
Use of the two-point-source -P1 approximation model for recovery of optical parameters in biological tissue
2011, 60 (12): 128701. doi: 10.7498/aps.60.128701
Based on a theoretical model for spatial diffusion reflection with two-point-source -P1 approximation to the radiative transport equation for a semi-infinite homogeneous turbid medium, the optical absorption coefficient a, reduced scattering coefficient ' s, and the second-order parameter of the medium are recovered from the measurement data of the reflectance by the nonlinear least squares method. The results show that the two-point-source -P1 approximation can give more satisfactory results for describing irradiance distribution close to source when the source-detector separation is larger than one transport mean free path. We can also obtain the anisotropy factor g according to the relationship between and ' s. This study is of great significance for the measurement of the optical properties of biological tissues and the application of diffuse reflectance spectroscopy technology.
2011, 60 (12): 128801. doi: 10.7498/aps.60.128801
Molecular dynamics(MD) simulation is used to study the promotion mechanism of store hydrogen via the hydrate formation. The stable structures and the microcosmic properties of pure H2 hydrate, H2+tetrahydrofuran (THF) hydrate, H2+tetra-n-butylammonium bromide (TBAB) and H2+tetraisoamylammonium bromide (TiAAB) semiclathrate hydrates are investigated systematically. The stabilization energy, EGH, between guest and cavity is calculated. It is shown that the large cavity of hydrate plays a main role of stabilizing hydrate. THF in large cavity can promote the stabilization of hydrogen hydrate and reduce the pressure of formation hydrogen hydrate, which are the same as the experimental results. Compared with the EGH between guest and large cavity, the results are in the order of increase as TiAAB,TBAB,THF,H2. It is concluded that the stability of semiclathrate hydrate is better than the structure Ⅱ hydrate, and H2+TiAAB semiclathrate hydrate is stablest. MD simulation provides helpful information for future TiAAB semiclathrate as a new promoter of forming hydrate and a new hydrogen storage material.
2011, 60 (12): 128901. doi: 10.7498/aps.60.128901
Based on the cellular automaton traffic flow model of automatic cruise controlling, the influences of speed maximum, mixing ratio, expectation factor of speed, and mass of vehicle on energy dissipations of the mixed traffic system are studied. Through computer simulation, the energy dissipations of mixed traffic system under the different parameters are obtained and analysed by the mean field theory. The theoretical analyses are consistent with the results of numerical simulation.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2011, 60 (12): 129401. doi: 10.7498/aps.60.129401
The Abel inversion based on a spherical symmetry of the ionospheric electron density distribution is a traditional inversion method of ionospheric occultation. However, the real ionosphere is not strictly spherically symmetric, which can cause error in inversion process of ionospheric occultation data. In this paper, we develop a non-spherically symmetric ionospheric occultation inversion method, in which the gradient information about international reference ionosphere three-dimensional ionospheric model is used to correct total electron content (TEC), thus the ionospheric electron density can be reconstructed from the corrected TEC by the spherical symmetric Abel inversion method. The inversion results retrieved from the measurements data of Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) ionospheric occultation are compared with the ionosonde data, showing that the result obtained from the new method is well coincident with the ionospheric electron density profile.