Vol. 57, No. 12 (2008)
2008, 57 (12): 7415-7418. doi: 10.7498/aps.57.7415
Two new methods for the construction of the Birkhoffian representation of first-order systems are presented. The Birkhoffian functions Rμ and B can be computed from the differential equations. An example is given to illustrate the application of the methods.
2008, 57 (12): 7419-7422. doi: 10.7498/aps.57.7419
Using the homotopic mapping method, a class of nonlinear KdV(Korteweg de Vries) equation is considered. Firstly, by introducing a homotopic transform, the problem of solving soliton for the corresponding equation is changed into a problem of mapping transform. Then on account of the property of the homotopic mapping, the approximate solution of soliton for the original equation is obtained.
Numerical simulation of acceleration and radiation of surface electrons in the interaction of intense laser pulses with a solid target
2008, 57 (12): 7423-7429. doi: 10.7498/aps.57.7423
The movement and radiation of fast electrons emitting along the surface of a target irradiated by intense laser pulses has been investigated theoretically and numerically by use of the classical theory of Thomson scattering by free electrons in static field. The results indicate that the surface electrons are oscillating in the quasistatic field and the laser field at the beginning. The electrons will be accelerated when their oscillation frequency comes close to the laser frequency, and the attosecond pulse trains and high harmonic are emitted along the surface by surface electrons. The temporal and spatial characteristics of the acceleration and radiation of electrons in different initial states has been compared. The frequency characteristic of the coherent radiation of electron beams are also investigated.
2008, 57 (12): 7430-7437. doi: 10.7498/aps.57.7430
In order to solve dynamic problems of double non-identical quantum harmonic oscillators with interaction potential and movement coupling, the new two mode composite coordinate representation |η1，η2〉 is proposed by the technique of integration within an ordered product of operators. The two mode squeezing operator U is constructed, and its characteristics are analyzed. Moreover, its application to double non-identical quantum harmonic oscillators with interaction potential and movement coupling is presented for solving some the dynamic problems. Approach of solving some the dynamic problems involving complicated quantum harmonic oscillators is proposed.
Phase transition to chaos of Bose-Einstein condensates in double-well trap with external periodic modulation is investigated in this paper. Particularly, chaos tunneling is studied. It is found that, when resonance between external periodic frequency and natural frequency is established, this system can turn to instability (chaos). The evolutions of Husimi function in quantum condition are discussed. It is found that, when chaos emerges, chaos region and chaos tunneling varies with the interaction between the particles.
2008, 57 (12): 7447-7451. doi: 10.7498/aps.57.7447
Respectively in two- and high-dimensional systems, we propose schemes for teleportation of a M-particle entangled state using one entangled state as channel. In this scheme, there are many receivers. By controlling the number of the introduced auxilliary particles, these receivers can arbitrarily adjust the distribution of the teleported entangled state's particles among them and the probability of the success is 1.
2008, 57 (12): 7452-7456. doi: 10.7498/aps.57.7452
The security of a multiparty controlled quantum secure direct communication protocol (WCZT protocol) is analyzed and a new attack with teleportation is advanced. Using this attack, the receiver can gain access to the sender's secret message without the permission of any controller; therefore, this protocol is not secure. An improved version of this protocol is proposed and the security analysis shows that the improved protocol can resist the said attack and thus the goal of multiparty controlled quantum secure communication is warranted.
On the properties and controlling of thermal entanglement in a two-qubit Heisenberg XY model with external magnetic fields
2008, 57 (12): 7457-7462. doi: 10.7498/aps.57.7457
We investigate the thermal entanglement of Heisenberg-type coupled two-qubit system in the presence of two controllable magnetic fields (B+b) and (B-b). We find that under the condition of low temperature and uniform magnetic field, the entanglement between the two qubits exhibits platform-like behavior. By controlling the inhomogeneous magnetic field b and selecting suitable material we can optimize the thermal entanglement and enhance the critical temperature Tc of dead entanglement. We also find that the entanglement switch can be realized in a broader region of temperature by modulating the uniform field B.
Taking into consideration the radiation particle reaction with the total energy of spacetime conservation, Hawking radiation from black hole is re-investigated using the Damour-Ruffini method. It is found that the tunneling probability relates to Bekenstein-Hawking entropy, and the radiation spectrum is not exactly thermal. The result is consistent with the works of other researchers， and satisfies the unitary theory of quantum mechanics.
2008, 57 (12): 7467-7476. doi: 10.7498/aps.57.7467
The equilibrium populations and the coherent off-equilibrium spin mixing with dipole-dipole interaction dynamics in an external magnetic field are investigated. By adjusting the external magnetic field, spin-exchange interaction and the aspect ratio of the potential well we can control the tunneling of the hyperfine components, the oscillation of the populations and the relative phases.
The small parameter expansion solution to Fokker-Planck equation for Brownian motion in a periodic potential with internal time derivative Ornstein-Uhlenbeck noise
2008, 57 (12): 7477-7481. doi: 10.7498/aps.57.7477
By introducing suitable variables, the Brownian particles motion in a periodic potential with internal time derivative Ornstein-Uhlenbeck noise can be described by a high-dimensional Fokker-Planck equation. The equation can not be solved by the usual small parameter expansion method and potential valley expansion method. An alternative small parameter expansion method is proposed to deal with the equation, and the result is applicable for small potential barrier height, intermediate correlation time and in a larger phase space domain, the approximation can be improved systematically.
2008, 57 (12): 7482-7486. doi: 10.7498/aps.57.7482
We investigated the stochastic resonance for dichotomous noise in a second derivative linear system, measured the average to get the amplitude gain of the output signal, and studied its relationship with the frequency of the system, the frequency of the signal, the strength and correlation time of noises. The amplitude gain of the output signal shows a nonmonotonic dependence on these factors, and the parameter of the dichotomous noise effects the peak value of stochastic resonance.
2008, 57 (12): 7487-7493. doi: 10.7498/aps.57.7487
To cope with the problem of phase space reconstruction from univariate time series and multivariate time series, the novel approach to phase space reconstruction from multivariate data based on data fusion is presented in this paper. According to Bayes estimation theory, the phase points in the same phase space reconstructed from multivariate data are fused, then an optimal fusion phase space could be determined. The approach is applied to multivariate phase space reconstructions of Lorenz system and Rssler system, respectively. Compared with the figures reconstructed from univariate datas, the information reconstructed from multivariate data includes the main characters of all univariate data and represents the comprehensive information of system attractor, which makes the phase space reconstructed more abundant. At last, the approach is applied to multivariate phase space reconstruction of oil film whirling in the rotor system. The information reconstructed includes all the characters of the system, which improves the veracity for fault diagnosis. So all the analysis further shows that the approach presented here is effective.
2008, 57 (12): 7494-7499. doi: 10.7498/aps.57.7494
The predictability limits of the Logistic map and Lorenz system as functions of initial error are calculated by employing the nonlinear error growth dynamics. It is found that there exists a linear relationship between the predictability limit and the logarithm of initial error. It is revealed by the theoretical analysis under the nonlinear error growth dynamics that the growth of average error will enter the nonlinear growth phase after the error reaches a certain critical magnitude and will finally reach saturation. For a given chaotic system, if the control parameters of the system are given, then the saturation of error growth is determined. Therefore, the predictability limit of the system only depends on the initial error. This is different from the linear error growth dynamics, under which the predictability time scale of chaotic system also depends on the upper limit of forecast error. In the linear expression between the predictability limit and the logarithm of initial error, its linear coefficient is relevant to the largest global Lyapunov exponent of chaotic system. If the largest global Lyapunov exponent and the predictability limit corresponding to a fixed initial error are known, the predictability limit corresponding to other initial errors can be extrapolated by the linear function expression between the predictability limit and initial error.
2008, 57 (12): 7500-7505. doi: 10.7498/aps.57.7500
This paper addresses the adaptive synchronization of two Chua's circuits with uncertain parameters by transmitting one single signal. Based on the Lyapunov stability theory, it is proved that the controller and update rule proposed in this paper can make the states of the driving system and the response system asymptotically synchronized, and with which the uncertain parameters of response system are identified. Numerical simulation results illustrate the effectiveness of proposed method.
2008, 57 (12): 7506-7510. doi: 10.7498/aps.57.7506
Chaos computing is a new scheme which can construct fundamental logical gates by using one chaotic unit. In this paper, we propose a new method to implement full adder based on chaos computing. By setting different thresholds and judgment conditions, the full adder is implemented by using only one chaotic unit. This method reduces the complexness of the structure of full adders and provides certain flexibility compared with the traditional method. At last, the Logistic mapping is taken as an example to show how to implement a full adder by using one chaotic unit.
2008, 57 (12): 7511-7516. doi: 10.7498/aps.57.7511
A method to generate multi-scroll chaotic attractors and its circuit design are proposed. It uses several subsection-linear-functions to construct the nonlinear function of chaotic system. By constructing different numbers of subsection, different numbers of scroll chaotic attractors can be obtained. Numerical simulation and EWB simulation results confirm the availability of the method.
2008, 57 (12): 7517-7521. doi: 10.7498/aps.57.7517
Coupled map lattices are taken as examples to study synchronization of spatiotemporal chaotic systems. Based on Lyapunov stability theory, global synchronization of two unilateral coupled map lattices is realized through appropriate choice of driving functions. Simulation results show the effectiveness of the method. The effect of control parameters on the rate of synchronization is further discussed. Simulation results also show that synchronization can also be realized when there is a systematic bias and system noise, which shows that the method has certain anti-jamming capability.
2008, 57 (12): 7522-7526. doi: 10.7498/aps.57.7522
This paper advances a theory of stability identification based on Lyapunov equation for fractional system whose order is not higher than 1. The theory is successfully applied to synchronize fractional Lorenz chaotic systems with uncertain parameters. Numerical simulation certifies validity of the theory.
2008, 57 (12): 7527-7534. doi: 10.7498/aps.57.7527
In this paper, we studied the amplitude control of the limit cycle in Van der Pol system with time-delay and time-delayed feedback strength. We show the critical stability boundary and the corresponding linearized equation of the Van der Pol system by mean-field approximation method. Mean-field approximate analytical expressions and numerical simulations for the power spectral density and the correlation time were given, and mean-field approximate analytical expressions in dependence upon noise intensity, delay time, and feedback strength are in good agreement with our numerical simulations. The time delayed feedback strength, noise intensity and time delay are taken as variable parameters to investigate their effects on the dynamics of the system, such as the power spectral density and the correlation time.
Chaos prediction in the Duffing-type system with non-smooth periodic perturbation and bounded parametric excitation
2008, 57 (12): 7535-7540. doi: 10.7498/aps.57.7535
In this paper, the dynamics of Duffing-type system with non-smooth periodic perturbation and bounded noise was studied. The theory of Fourier series was used in this system to deal with the non-smooth character for the first time. The analytical condition for the appearance of chaos was given using stochastic Melnikov method. The numerical simulations confirm the validity of this method.
2008, 57 (12): 7541-7546. doi: 10.7498/aps.57.7541
Based on the full velocity difference (FVD) model, with the consideration of the effect of information about the two leading cars to the following car, an improved two_car following model of traffic flow has been deduced. The linear stable judging condition is obtained to the improved model by its stability analysis, which shows the stability range of the improved model is obviously larger than FVD model by comparing them. Numerical simulation shows that our model can avoid the disadvantage of negative velocity occurring at small reaction coefficient in FVD model by adjusting the information of the next-nearest-neighbor leading car. And there is unavoidable effect to traffic flow from the next-nearest-neighbor leading car’s information.
Synchronization of the applause with leaders is investigated. The cases of the presence or absence of the effect of the applause transmission time delay on the part of the spectators are considered rcspectively. Sufficient conditions for the applause synchronization are presented for each case. The obtained theoretical results not only manifest the observation that rhythmic applauses are more frequently heard in a theater with high attendance rate, but also show a surprising conclusion that one leader is enough to make all spectators clap with the same frequency in a very large theater. Furthermore, numerical examples are provided to illustrate the validity of the theoretical results.
2008, 57 (12): 7555-7564. doi: 10.7498/aps.57.7555
Using self-consistent field calculation, the new equilibrium morphology of a diblock copolymer melt confined in a brush_covered cylindrical nanopore can be found. The morphology is formed as a function of brush density and A-monomer fraction f. When f=0.7, due to the wetting effect produced by polymer brush the outside concentric lamellar ring will come up again with increasing φAB. We carefully examined the intermediate phases and discovered a rich variety of new two_dimensional structures that have no analogue in the unconfined system, which include nonhexagonally coordinated cylinder phases and structures intermediate between lamellae and cylinders, like CRCR, when f=0.35. Then we map the stability regions and phase boundaries for all the structures we found.
2008, 57 (12): 7565-7570. doi: 10.7498/aps.57.7565
It is a common method to use the interferential characteristics of light to detect the object in the polarization interference imaging spectrometer system，but the polarized characteristics are usually ignored. In this paper, an advanced way, which combines the basic theories of polarization interference imaging spectroscopy and polarization measurement technology, is studied and used to detect the polarization parameters (e.g. degree of polarization, azimuth angle of polarization), and the error is analyzed, which proves that the system is very stable and precise in theory. Now the polarization interference imaging spectrometer can be used not only as cameras and interferometers but also as polaristrobometers, because the polarized characteristics of the light are made good use of to analyse and distinguish the object in a new way.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2008, 57 (12): 7571-7576. doi: 10.7498/aps.57.7571
According to Faddeev-Popov's rule of path integral quantization, the generating functional of Green function in configuration space for a gauge_invariant constrained system is formulated, transformation properties of a gauge_invariant system at the quantum level are derived and the problems of the conserved laws at the quantum level are discussed. the quantal transformation properties of electromagnetic field near the interface of dielectric media under the Poincar group are studied. It is pointed out that the transverse shift of the reflection and refraction of electromagnetic waves near the interface of dielectric media also occur at the quantum level, but the results of classical theory must be amended in quantum theory.
ATOMIC AND MOLECULAR PHYSICS
Study on the potential energy function for the ground state （X1Σ+）of SiO molecule by density functional theory
2008, 57 (12): 7577-7580. doi: 10.7498/aps.57.7577
The structural optimization for the ground state (X1Σ+) of SiO molecule has been calculated using density functional theory B3P86 method with the basis sets STO-3G, D95**, 6-311G, 6-311++G, 6-311++G** and cc-PVTZ. The conclusion is draun by comparison that the basis set cc-PVTZ is the most suitable for the structural optimization calculation of the ground state (X1Σ+) of SiO molecule. The analytical potential energy curve for the ground state (X1Σ+) of SiO molecule is scanned using B3P86/cc-PVTZ method, and then fitted to the Murrell-Sorbie function using least squares. At last the spectroscopy constants (Be，αe，ωe and ωeχe) related to the analytical potential energy function are calculated and compared with the experimental data.
2008, 57 (12): 7581-7585. doi: 10.7498/aps.57.7581
The equilibrium geometry of N2O, NO and N2 molecules have been calculated on the computational levels of density functional theory. The Murrell-Sorbie analytic potential energy function of NO and N2 molecules have been derived through the least-square fitting to ab initio data. Similarly, the harmonic frequency has been calculated and the analytic potential energy function of linear molecule N2O isomer is derived by many-body expansion theory for the first time.
2008, 57 (12): 7586-7590. doi: 10.7498/aps.57.7586
We present a method of anti-control of chaos in Bose-Einstein condensate by applying periodic signals to convert the periodic state into chaotic state. Numerical simulation shows that there are different chaotic orbits corresponding to different modulation intensity and modulating frequency only if the maximum Lyapunov exponent of the system is positive. The phase difference between the two sinusoidal forces added to the Gross-Pitaevski equation plays an important role in anti_control of chaos.
2008, 57 (12): 7591-7599. doi: 10.7498/aps.57.7591
A novel scheme was proposed to form the double-well optical trap by using shift phase gratings. A few pairs of adjustable double-well optical traps for trapping cold atoms or cold molecules were formed on the focal plane when the gratings were illuminated by plane light wave (or TEM00 mode Gaussian light wave) and focused by positive lens. The intensity distribution was calculated, and then the intensity distribution gradients as well as the analytical relations between geometrical parameters of the traps and the parameters of the optical system were derived. At the same time, three different evolution processes from double-well to unit-well were investigated. Also, optical dipole potential and photon scattering rate of the trapped cold atoms were calculated. The results show that the scheme not only is simple and convenient to realize, but also has wide application prospect in many fields, such as atomic physics, atomic optics, molecular optics and quantum optics.
2008, 57 (12): 7600-7605. doi: 10.7498/aps.57.7600
Based on the existing four-level model of laser-induced collisional energy transfer, the collision process in Ba-Sr system in both weak and strong fields is calculated by immediate integration of the equation of motion, and profiles of laser-induced transition probability and collision cross section under both conditions are obtained. Numerical results for the weak field are in fair to good agreement with former analytical results, while the ones for the strong field show that the effect of laser-induced collision is enhanced when the transfer laser intensity increases. Moreover, the peak of the laser-induced collisional energy transfer spectrum shows an evident shift from resonant frequency to the violet side in strong field, and full width at half maximum (i.e. tuning range) of the spectrum becomes obviously narrower compared with that in weak field.
Measurement of single-electron transfer cross sections in collisions of C3+ with He, Ne, Ar atoms at low to intermediate velocity
2008, 57 (12): 7606-7611. doi: 10.7498/aps.57.7606
Absolute cross sections for single-electron transfer of He， Ne， Ar atoms induced by C3+ at low to intermediate velocity (1.7v0—4.2v0) were measured and calculated using n-body classical trajectory Monte Carlo (nCTMC) method. Though good overall agreements are achieved between experimental and nCTMC results, certain differences are found. Because of the existence of polarization and the nCTMC being unable to calculate the multi-electron correlation capture cross sections, the experimental results are greater than those of nCTMC at low velocities(1.7v0—2.0v0). When the velocity of the projectile is higher(2.2v0—4.2v0), the captured electron’s partial screening of the projectile, the electron-electron dynamic correlation and the effective charge are taken into account in interpreting the qualitative differences between theoretical and experimental results. In addition, the cross section rations of transfer ionization to single electron capture are presented and the mechanism of transfer ionization are discussed.
Total cross sections of electron scattering by large molecules C4H8O, C5H10O2, C6H5CH3 and C4H8O2 at 50—5000 eV
2008, 57 (12): 7612-7618. doi: 10.7498/aps.57.7612
Using the modified additivity rule (AR) approach, the total cross sections (TCSs) for electron scattering by large molecules C4H8O, C5H10O2, C6H5CH3 and C4H8O2 are calculated over a wide energy range from 50 to 5000eV by employing the experimental TCS results of electron scattering from atoms C, H and O. Here the atomic TCSs are derived from the experimental TCS results of simple molecules (H2, O2, CO). The quantitative TCSs are compared with those obtained by measurements and other theories wherever available. It is shown that there is excellent accordance between the theories obtained by the modified AR approach and the measurements even at energies of several tens of eV. And it is also shown that the problem that the log_log TCS curves obtained by the original AR approach are steeper than those obtained by the experiments, is introduced by the AR model itself. The intrinsic reason is that the mutual screening effects are not taken into consideration in the original AR approach at lower energies.
Polarization effect and post-collisional interaction in (e, 2e) reaction process for He and Ar in coplanar asymmetric geometry
2008, 57 (12): 7619-7622. doi: 10.7498/aps.57.7619
The (e, 2e) triple differential cross sections for He(1s2)，Ar(3p6)and Ar(2p6)have been calculated using the modified distorted wave Born approximation (DWBA) in coplanar asymmetric geometry. The kinematics employs large energy transfer and is close to minimum momentum transfer. The theoretical results have been compared with those of the experiment, the Brauner, Briggs and Klar (BBK) method and the standard DWBA calculation. It is shown that the polarization and post-collisional interaction effects are very important in coplanar asymmetric (e, 2e)reaction with special kinematics for He(1s2), Ar(3p6) and Ar(2p6).
2008, 57 (12): 7623-7629. doi: 10.7498/aps.57.7623
With many kinds of initial configurations and spin multiplicities taken in full consideration, the first principile method based on density_functional theory (DFT) has been employed to study the equilibrium geometries, electronic properties and magnetism of BenLa (n=1—18). Our results showed that there are many isomers with extremely close energies near the ground state, thus the cluster is very complex, making the ground state seek quite challenging. The BenLa has magnetism and its stability is far higher than that of Ben+1, indicating that one can acquire magnetic clusters with higher stabilities by choosing the proper doping atom. The Be17La is specially stable and its gap between the highest occupied and the lowest unoccupied molecular orbitals is quite the same as that of the host cluster. The magnetic moment in BenLa cluster system comes mainly from the outer layer unpaired valence electrons and the La element's contribution. The magnetic moment of BenLa is 1μB except that of BeLa and Be3La, which is 3μB.
CLASSICAL AREA OF PHENOMENOLOGY
2008, 57 (12): 7630-7634. doi: 10.7498/aps.57.7630
The electromagnetic interaction between spherical targets is researched. The second scattering field for a target is derived in detail. The complex scattering field and the bistatic radar cross section are presented respectively. The simulation results in Ku wave band show that the size of the principal target affects greatly both the forward and the backward scattering. The influence of the surrounding targets shows definite periodicity. The secondary scattering field diminishes with the increase of distance between the two targets. The nearer to the forward area the surrounding target is, the bigger its affect. The magnitude of the secondary scattering field is about 10-4 that of the first scattering field. The size of surrounding particles can be considered as stable and uniform when researching the electromagnetic interaction. This kind of interaction in three dimensions can be investigated easily by using the obtained result and coordinate transformation.
Application of wavelet-Galerkin time domain method in the composite scattering of target and lossy ground
2008, 57 (12): 7635-7640. doi: 10.7498/aps.57.7635
This paper employs the wavelet-Galerkin time domain (WGTD) technique to study composite scattering of arbitrarily target and a lossy ground, and the incidence is realized through three-wave technology. The far-field radiated by the surface currents is calculated using the reciprocity theorem instead of the more complicated Sommerfeld integration. Numerical radar cross section demonstrate the validity and precision of WGTD. Compared to finite difference time domain method, the WGTD method has the advantages of smaller memory and higher efficiency.
2008, 57 (12): 7641-7649. doi: 10.7498/aps.57.7641
In this paper, we present a novel wide-field fluorescence sectioning microscopy in which speckle patterns are produced on the sample for illumination. The speckle pattern is dynamically changed and a sequence of fluorescence images of the sample is recorded with a CCD camera. Due to a large variation of fluorescence intensity of the in-focus light compared to that of the out-of-focus light, special processing algorithms can be used to reconstruct the sectioned fluorescence image. We calibrated the system and studied the effect of the reconstruction algorithms on the system performance. Fluorescence sectioned images of a few biological samples were obtained. Our experiments showed that this wide-field fluorescence sectioning microscopy can be used to optically section tissue and has potential applications in the clinics.
2008, 57 (12): 7650-7657. doi: 10.7498/aps.57.7650
Based on the quantum dynamics of laser-ion interaction beyond the Lamd_Dicke limit, we discuss how to superpose a series of vibrational number states of a single trapped ion from the motional ground state. It is shown that, by controlling the durations and phases of the sequentially applied laser pulses, these superposed quantum states could well approach to the various target quantum states, e.g., squeezed coherent states, squeezed odd/even coherent states and squeezed vacuum states, etc.Their experimental realizabilities are also discussed.
2008, 57 (12): 7658-7662. doi: 10.7498/aps.57.7658
The electromagnetically induced left-handedness in a four-level atomic system associated with a pair of upper excited hyperfine levels has been investigated. The properties of left-handedness with vacuum-induced coherence (VIC) arising from the cross coupling spontaneous emission pathways have also been discussed. The theoretical analyses show that the left-handedness of medium will be enhanced with the increase of VIC effect.
The self-correlation performance of semiconductor lasers with optical feedback and optical injection
2008, 57 (12): 7663-7667. doi: 10.7498/aps.57.7663
Based on the rate equations describing the semiconductor lasers with optical feedback and optical injection, the influences of the feedback strength and delay time, injection strength and frequency detuning on the self-correlation performance of the output chaotic signal have been investigated. The results show that the full-width at half-maximum (FWHM) and peak sidelobe level of the output self-correlation function is critically dependent on parameters related to injection and feedback. Through reasonably selecting the parameters, the self-correlation function with 0.02ns FWHM can be obtained, which is superior to the reported results.
2008, 57 (12): 7668-7673. doi: 10.7498/aps.57.7668
A self-built self-mode-locked Ti:sapphire laser is operated at the upper edge of the stable region. Tow double-prism systems are used to compensate for thesecond-order dispersion in and out of the laser cavity. Broad band laser pulses with bandwidth of 664—840nm have been generated, and their mode changes from the basic mode to the ring structure. The interesting experimental phenomenon is due to the stimulated Raman scattering and the four wave mixing, which provides the necessary condition to generate the sub-10fs laser pulses.
2008, 57 (12): 7674-7679. doi: 10.7498/aps.57.7674
The influence of turbulence on the coherent and incoherent combinations of off-axis Gaussian beams is studied in detail. The analytical propagation equation of the resulting beam in turbulence for the incoherent combination is derived. The mean-squared beam width, the power in the bucket and the β parameter are taken as the characteristic parameters of beam quality to compare the results of coherent and incoherent combinations of off-axis Gaussian beams propagating through atmospheric turbulence, and the reasonable physical explanations are also given. It is shown that, on one hand, both for the incoherent and coherent combinations the turbulence results in a beam spreading and a degradation of the maximum intensity, and the larger the individual beam numbers is, the less sensitive is the resulting beam to the effect of turbulence. On the other hand, the resulting beam for the incoherent combination is less affected by turbulence than that for the coherent combination.
2008, 57 (12): 7680-7684. doi: 10.7498/aps.57.7680
The model given by Singh for vacuum laser acceleration in a static magnetic field is revisited by using the fifth order corrected laser field equations of a focused laser beam in which the diffraction effect and the longitudinal electric field of a focused laser beam are considered. However, the static magnetic field is replaced with a static electric field in our model. Simulation shows that the electrons can gain much more net energy in this model than that without the added field. Due to the added static electric field, the electrons can be drawn out of deceleration phase from one acceleration stage and directly enter to next acceleration stage.
2008, 57 (12): 7685-7692. doi: 10.7498/aps.57.7685
The concept of atmospheric retrievable index (ARI) is introduced to provide the criterion for channel selection. The definition of ARI is given by 1min the ratio of the determinant of the covariance matrix of the error of the results and the covariance matrix of the error of the background, and the value is within the interval between 0 and 1. For all the probable subsets, the one giving the greatest ARI is the subset to be selected. To illustrate the implementation and effect iveness of the method, two series of channel selection experiments of IASI for retrieval atmospheric temperature and humidity profiles using physical retrieval method are carried out. The results of the experiments show that the method can provide an effective approach to the channel selection for the application of high spectral resolution data.
2008, 57 (12): 7693-7698. doi: 10.7498/aps.57.7693
Different from the conventional model of dispersion-flattened decreasing fiber, a novel dispersion model of dispersion-flattened decreasing fiber is proposed and supercontinuum generation in is simulated and analyzed by utilizing the total field formation in the frequency domain. It is found that the supercontinuum generated in the latter is by far wider than that in the former and the flat bandwidth can be greater than 1000nm. It is further found that both the fiber parameters, such as peak dispersion, differential constant of the dispersion, and decreasing coefficient of the dispersion, and the pump pulse parameters, including pulse width, soliton order and frequency chirp, are crucial to the flat ultra-wideband supercontinuum generation in the fiber.
2008, 57 (12): 7699-7704. doi: 10.7498/aps.57.7699
Multicasting is an important function of wavelength-routing optical communication networks. An all-optical multicasting is presented and experimentally demonstrated based on four-wave mixing (FWM) in highly nonlinear fibers. An input signal with data information together with a pump are injected into a highly nonlinear fiber and two idlers with new frequencies can be generated through two degenerate FWM processes, which carry the same data information as the input signal. The single input signal is copied to two channels and the pump can also be modulated by this data. The advantage of this scheme consists in the simple configuration, where only a single pump is needed and no other injected lights are required, the uniform powers and tunable wavelengths of the multicasting signals, and the capacity to further increase multicasting channel number.
Simulation of the modulation instability in dual-core optical fiber based on complex Ginzburg-Landau equation
2008, 57 (12): 7705-7711. doi: 10.7498/aps.57.7705
Instability of the continuous-wave states in a dual-core optical fiber system is investigated based on the complex Ginzburg-Landau equation. The dual-core optical fiber system consists of an active nonlinear dispersive core and a linear passive core. The modulation instability (MI) conditions are found from linearized equations for small perturbations. Simulations of the full system demonstrate that the development of the MI in the former regime leads to the establishment of a regular or chaotic array of pulses if the MI saturates, or a chain of well-separated peaks with continuously growing amplitudes if the instability does not saturate. It indicates that the peak value of multiple return-to-zero (RZ) pulses or a single RZ pulse will be amplified and the RZ pulse sources emerge in the optical fiber in the anomalous group velocity dispersion regime. This research can be used as the resource of the optical fiber telecommunications and will be useful for the study of the fibre optics and physics and also in some other fields.
2008, 57 (12): 7712-7716. doi: 10.7498/aps.57.7712
Er3+/Yb3+:Gd3Sc2Ga3O12 and Er3+:Gd3Sc2Ga3O12 crystals have been grown by Czochralski method. The absorption spectra from 320 to 1700nm and the up-conversion fluorescence spectra from 500 to 750nm of them were investigated at room temperature, respectively. The possible up-conversion luminescence mechanisms in Er3+/Yb3+-codoped crystals and their influences on the 2.8μm laser emission were discussed. Experimental results show that the absorption of Er3+:Gd3Sc2Ga3O12 around 966nm and its bandwidth are remarkably increased by Yb3+ sensitizing. Under 940nm excitation, the up-conversion luminescence intensity of Er3+/Yb3+:Gd3Sc2Ga3O12 is much stronger than that of Er3+:Gd3Sc2Ga3O12. It demonstrates that there exist effective energy transfer processes between Yb3+ and Er3+, and the dominant mechanism of up-conversion may be the energy transfer processes of Yb3+-Er3+ and Er3+-Er3+.
2008, 57 (12): 7717-7724. doi: 10.7498/aps.57.7717
Coupled defect modes of photonic crystals can be characterized by the transmission spectrum shape that is closely linked to the stripe waveguide connecting the defects. Unlike the direct coupling cases where the phase shift of lightwave depends only on the defect mode frequency, we found that for the waveguide coupling cases the waveguide length and the dispersion relation must be also considered. By changing the waveguide length, the spectrum can exhibit periodically three-peak appearance, two-peak appearance, and flat top appearance. At frequencies close to the defect mode, the results of the coupled mode theory are in good agreement with the simulations using the finite-difference time-domain technique, verifying the validity of the theoretical model even in the complicated cases.
Non-close-packed photonic crystal of TiO2 hollow spheres: Fabrication and photonic bandgap calculation
2008, 57 (12): 7725-7728. doi: 10.7498/aps.57.7725
We have fabricated the non-close-packed photonic crystals of SiO2 microspheres with the help of the modified vertical method, the thermal sintering technology and etching. The face-centered cubic photonic crystals of TiO2 hollow spheres connected by TiO2 cylindrical tubes have been fabricated using silica template. The structure of the non-close-packed opals was characterized by scanning electron microscopy. We performed photonic band-structure calculations of this structure by a plane wave expansion method. The calculations indicate that the photonic bandgaps at all points of the Brillouin zone in non-close-packed structure were opened except point W.
2008, 57 (12): 7729-7736. doi: 10.7498/aps.57.7729
In this paper, we study the backward wave propagation phenomenon in completely anisotropic metamaterials. The formation conditions for backward wave propagation under an arbitrary angle between the metamaterials' intrinsic coordinate axis and transmission axis are obtained. The influences of permittivity and permeability tensors and polarization on the backward wave formation and propagation are analyzed. On this basis, we further analyze the situation of backward wave in anisotropic metamaterials with different dispersion relations, and the angle between wave vector and Poynting vector, and the general conclusion of backward wave propagation are obtained. In addition, we studied the propagation characteristics of backward wave in epsilon-near-zero metamaterials, and discovered that the backward wave in these metamaterials is a perfect backward wave.
2008, 57 (12): 7737-7746. doi: 10.7498/aps.57.7737
Fiber Bragg grating under local transverse force was studied theoretically and experimentally. Based on the transfer matrix method, the consequential changes in reflected spectrum under local transverse force are analyzed, the corresponding mathematical model is established, and the mathematical expressions are deduced. When fiber Bragg grating is pressed by local transverse force, the reflection spectra are independent of the applied length. The wavelength of split point is periodical and linearly proportinal to the applied force. And the reflectivity of split point is approximately hyperbolically tangential versus the applied position.
2008, 57 (12): 7747-7752. doi: 10.7498/aps.57.7747
We applied different shaping factors of Gabor wavelet for the analysis of terahertz time-domain spectroscopy with wavelet_transform. Gabor wavelet analysis allows a convenient switching between time domain and frequency domain by varying shaping factors. The comparison of absorption spectral lines obtained between with wavelet-transform and with other methods demonstrated that wavelet-transform is a reliable technique for the analysis of the absorption properties and the extraction of the characteristic spectral lines in terahertz range.
Carrier-envelope phase measurement and stabilization of amplified Ti:sapphire femtosecond laser pulses by spectral interferometry
2008, 57 (12): 7753-7757. doi: 10.7498/aps.57.7753
Carrier-envelope phase stabilization of amplified femtosecond laser pulses is demonstrated. An octave spanning white-light super-continuum is generated in a sapphire plate. Spectral interference was achieved between the blue component and the second harmonic of the red component of the super-continuum. Carrier-envelope phase information was extracted from the interference through Fourier transform and the phase was stabilized by refined modulation of pumping power of the oscillator through a phase locking loop. The root-mean-square value of the carrier-envelope phase locking error is lower than 80 mrad within the locking time of 3h.
A study on the interval between longitudinal modes of Fabry-Perot cavity composed of fiber Bragg gratings
2008, 57 (12): 7758-7764. doi: 10.7498/aps.57.7758
The characteristics of transmission spectrum of the Fabry-Perot (F-P) cavity composed of two fiber Bragg gratings (FBG) are analyzed detailedly in this paper. The influencing factors of the position of the longitudinal mode are pointedout. The relationship and difference between the FBG F-P cavity and normal F-P cavity are discussed. The phase factor of the amplitude reflection coefficient in the range of the FBG main lobe is simulated linearly using the effective mirror surface model. The effective length of the FBG is defined and the expression of the effective length is obtained. Furthermore， the effective length of the FBG is brought into the equivalent cavity length of the FBG F-P cavity. The interval between longitudinal modes of the FBG F-P cavity is calculated using the equivalent cavity length. Both the numerical simulation and experimental results show that the interval calculated using the equivalent cavity length accords well with the true interval between longitudinal modes of the FBG F-P cavity.
2008, 57 (12): 7765-7769. doi: 10.7498/aps.57.7765
The travelling properties of laser-generated surface acoustic wave with broad bandwidth, which propagated on a sample with surface defects, have been investigated in theory. The numerical results of theoretical models with different defects indicate that the surface defects worked as low-pass filter for the travelling surface acoustic waves. The cut-off frequency of low-pass filter is linearly related to the depth of the defect, and decreased against the increasing depth of the defect. The amplitude of the travelling surface acoustic wave is seriously attenuated with increasing width of surface defect. The numerical results provide a theoretical basis for quantitative detection of the surface defects.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2008, 57 (12): 7770-7775. doi: 10.7498/aps.57.7770
The supersonic jets and the interaction of strong shock waves are ubiquitous features of the nonlinear hydrodynamics of inertial-confinement fusion, astrophysics, and related fields of high energy-density science, so very important value have their studies in laboratory. A laboratories experimental study on jets is reported. The plasma jets were created by an ns laser pulse irradiating a special circular hole target, and the shadow of jets were recorded by using an X-ray laser probe with wavelength of 13.9nm. Legible shadow images of plasma jets were obtained. The experimental results and theoretical simulation were in qualitative agreement.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2008, 57 (12): 7776-7782. doi: 10.7498/aps.57.7776
Monolithic AlN，BN films and AlN/BN multilayers were prepared by reactive magnetic sputtering. The films were characterized by X-ray diffraction, high-resolution transmission electron microscopy and nanoindentation. Results showed that the w-AlN phase and amorphous BN films structure exist in AlN and BN monolithic films. The crystal structure of BN layers in the multilayers depends on the thickness of BN layers. When the thickness of BN is less than 0.55nm, BN layers grew epitaxially with AlN under the influence of the “template effect" of w-AlN layers and form the same structure as AlN. When the thickness of BN is larger than 0.74nm, BN layers are amorphous. The hardness of AlN/BN multilayers also depends on the thickness of BN layers. When the thickness of BN is 1—2 molecular mono_layers, the superhardness enhancement of the multilayer is observed. The superhardness effect disappears when the thickness of BN is larger than 0.74nm. The critical thickness of BN layers marking the change from the crystalline to the amorphous structure has been calculated. The hardening mechanism of AlN/BN multilayers has been discussed. The main reason of the superhardness enhancement of the multilayer is the stress and modulus difference between the layers of BN and AlN.
2008, 57 (12): 7783-7788. doi: 10.7498/aps.57.7783
Thermal stimulated current in Al2O3 single crystal and sandwiched Al2O3 single crystal-Bi2O3-Al2O3 single crystal samples was measured during the heating， soaking and cooling processes. It was found that the thermal stimulated current can be detected in sandwiched samples， while no current was observed in the Al2O3 single crystal sample. With the increase of heating rate， the thermal stimulated current measured during cooling decreases， which implies that the thermal stimulated current is induced by defect ions. The calculation of active energy shows that there are two kinds of defect ions involved in producing the thermal stimulated current.
Experimental study on the shock Hugoniot relationship and the Grüneisen-type equation of state for C30 concrete
2008, 57 (12): 7789-7793. doi: 10.7498/aps.57.7789
The shock properties of C30 concrete with initial porosity is investigated by gas gun planar impact technique. The manganin pressure gauge is used to measure the pressure-time curves of the samples. It is shown by the experimental studies on the shock Hugoniot relationship and the Grüneisen-type equation of state for C30 concrete that the D-u shock Hugoniot relationship is described by the linear relation, and the material parameters in the Grüneisen-type equation of state are determined. Moreover, it is observed from the measured pressure-time curves that the rate-sensitivity of dynamic response for C30 concrete is not negligible, showing marked stress relaxation and dissipation.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2008, 57 (12): 7794-7799. doi: 10.7498/aps.57.7794
The electronic structure of pure and Nb doped TiO2/NiTi interface have been calculated with the first-principle ultrasoft pseudopotential approach of the plane wave based on the density functional theory aiming at examining the effect of Nb on the electronic structure of the TiO2/NiTi interface. The formation energy calculation results show that the structure with Ti_terminated NiTi matrix and O_terminated TiO2 layer (Ti/O interface) is the most stable one among the four possible interface structures. Based on the optimized Ti/O interface model, the calculation results of the partial density of states, charge population and bond order suggest that the introduction of Nb on the interface strengthens the atomic interactions on the interface, as well as the interactions between the matrix and the oxidation layer in the neighborhood of the interface, which results in the improvement in the interfacial adhesion and the oxidation resistance of NiTi intermetallic compound.
2008, 57 (12): 7800-7805. doi: 10.7498/aps.57.7800
The geometrical structures of pure ZnO and Co and Mn codoped 32-atom supercell of ZnO were optimized by using the ultra-soft pseudopotential method of total-energy plane wave based on the density functional theory. Band structure, density of states and optical properties were calculated and discussed in detail. The results revealed that the band gap of Co and Mn codoped ZnO are reduced and the absorption to ultraviolet visible light is obviously enhanced.
2008, 57 (12): 7806-7813. doi: 10.7498/aps.57.7806
Using the density-functional theory combined with the projector augmented wave method, we have investigated the electronic structure of Be-doping wurtzite ZnO. The band structure, total density of states and partial density of states of BexZn1-xO are presented and show that the valence band maximum (VBM) is determined by O 2p electrons and the conduction band minimum (CBM) is occupied by the hybrid Be 2s and Zn 4s electrons. The energies of CBM increase and the energies of VBM decrease with increasing Be-doping concentrations. Both effects lead to broadening of the band gap. Furthermore, it was found that Be-doping can cause compressive strain in the crystal structure, which also widens the band gap.
2008, 57 (12): 7814-7821. doi: 10.7498/aps.57.7814
The electronic structures of pure and Cd-doped wurtzite ZnO have been investigated by using first-principle ultrasoft pseudopotential approach of the plane wave. The calculation indicates that the band gap of ZnO is reduced by Cd doping. With increasing Cd-doping concentration, the lower energy states of Zn 4s orbital can take part in hybridization, so that the conduction band of minimum determined by the antibonding Zn 4s states can shift to the lower energy, and the energy of antibonding pd states which control the valence band of maximum becomes higher.
Effects of additional element on the glass forming ability and corrosion resistance of bulk Zr-based amorphous alloys
2008, 57 (12): 7822-7826. doi: 10.7498/aps.57.7822
By using the real-place recursion method， bulk Zr-based amorphous alloys with effects of additional element Nb，Ta，Y，La on the glass forming ability and corrosion resistance were studied. The atomic structure model of primary precipitation phase Zr2Ni in Zr-based amorphous alloys were constructed by computer programming. The icosahedron cluster in Zr-based amorphous alloys was simulated by using the icosahedral cluster in Zr2Ni primary precipitation phase. The local density of states (LDOS) curves of Zr，Ni (before substitution) and alloying elements (after substitution) at the center or the corner of icosahedron cluster， the total bond order integral between the centered Ni atom and it's nearest neighbor (Zr，Nb，Ta，Y，La) in the icosahedron cluster， and the Fermi energy levels of the icosahedron cluster (before and after alloying elements' substitution) were calculated. The LDOS calculation results showed that the Cu atom occupies the center site of the icosahedral cluster， Nb， Ta， Y， La atoms occupy the site of Zr atom at the corner in Ni-Zr icosahedral clusters. The bond order integer calculation results suggest that Y increases the glass forming ability (GFA) of Zr-base amorphous alloys， Nb and Ta decreased the GFA， and La has little influence on the GFA. The calculated Fermi energy levels suggest that Nb， Y， La make Zr-based amorphous alloys easy to be passivated and improve the Zr-based amorphous alloys corrosion resistance. Ta has little influence on the corrosion resistance of the alloys. Therefore， Y and La are most effective on improving the GFA and corrosion resistance of Zr-based amorphous alloys. By addition of small amounts of Y and La， new bulk amorphous alloys with good corrosion resistance can be prepared.
2008, 57 (12): 7827-7832. doi: 10.7498/aps.57.7827
Geometry optimization and total energy computation of Ag1/4TiSe2 and TiSe2 have been completed by means of plane wave pseudopotential method based on the local density approximation to density functional theory. The calculated lattice parameters accords with the experimental results reported and the negative formation energy shows the stability of ordered Ag1/4TiSe2 system. The atomic populations，bond lengths，band structure and density of states indicate that：Ag atoms are bonded with stronger ionicity in Ag1/4TiSe2 than in TiSe2. The intercalation of Ag makes TiSe2 with semimetal property change into Ag1/4TiSe2 with metal property and the conductivity is remarkably improved.
First-principle study of the effect of boron (nitrogen)-doping on adsorbing characteristics of aluminum on single-walled carbon nanotubes
2008, 57 (12): 7833-7840. doi: 10.7498/aps.57.7833
The geometrical structures of adsorbed aluminum atoms on different sites of undoped and boron (nitrogen)-doped single-walled carbon nanotubes (SWCNTs) were optimized using plane wave pseudopotential method with generalized gradient approximation based upon the density functional theory. The band structures，density of states，electron density difference，Mulliken population， adsorption energies of both undoped and boron (nitrogen)-doped SWCNTs were calculated theoretically. The results revealed that boron-doping enhances the adsorption energy Ea of Al on metallic (5，5) CNTs and semiconducting (8，0) CNTs by forming an electron-deficient structure filled with the electrons offered by Al atom. Meanwhile the results also suggest that nitrogen-doping increases Ea of Al on SWCNTs by forming electron-rich half-filled donor structure around Fermi level filled with the electrons of Al atom. Boron- and nitrogen-doping enhance the adsorption energy Ea by forming ionic-bond-type and covalent-bond-type bonds， respectively. The doping effect may also be beneficial to strengthening contact of interfaces between Al matrix and CNTs.
2008, 57 (12): 7841-7847. doi: 10.7498/aps.57.7841
The influence of electric field and interlamellar effect on the electronic energy and wave function of spherical nanometric system is studied. Taking CdS/HgS/CdS as example， the influence of interlamellar effect， electric field and sample linear size on high-energy level Stark effect， spectral line frequency and intensity are discussed. The results show that for the spherical nanosystem CdS/HgS/CdS, the external electric field， Stark effect and the law of level effect-split-level resemble that of the hydrogen atom， but with different displacement of energy levels. The displacement of spectral lines is always proportional to the square of intensity in the electric field， and most spectral lines' frequency excursion decreased with the increase of linear sample-size， while some of the frequency excursions are on the opposite. The spectral line frequency excursion caused by interlamellar effect will decrease with the increase of linear size. Except a few spectral lines， most spectral lines will weaken with the increase of the linear size. Interlamellar effect will not change the trend of the relative intensity changing with linear size， but will make the intensities of spectral lines change. When the linear size is small， the influence of interlamellar effect on the intensity of spectral line is obvious.
2008, 57 (12): 7848-7854. doi: 10.7498/aps.57.7848
A new method is proposed to generate extreme ultraviolet supercontinuum. The polarization gating at 1600nm is employed to intensify the ellipticity dependence of high harmonic signal， and the addition of a second harmonic field is used for modulating the ionization ratio. Calculations show that a fundamental pulse up to 6 optical cycles (32.4fs) in duration can be applied to confine the efficient high harmonic generation to half an optical cycle. This feature allows us to produce an extreme ultraviolet supercontinuum with the bandwidth of about 280eV which supports generating a Fourier-transform-limited pulse of 10as. By selecting one part of the supercontinuum， a 100as pulse is obtained directly. In addition， the intensity utilizing efficiency of fundamental pulses can be increased by making the delay between the two pulses which compose the polarization gating smaller，when the modulated polarization gating is employed.
2008, 57 (12): 7855-7859. doi: 10.7498/aps.57.7855
Single ZnO nanowire ultraviolet (UV) photodetector has been fabricated， and the enhancement of its UV photoresponse by polystyrene sulfate surface functionalization has been investigated. It is found that， under the same UV light irradiation， UV photoresponse of the detector with surface functionalization increases by three orders of magnitude as compared to that without surface coating. By comparing I-V characteristics of the detectors， we find that the dark conductance of the device can decrease by three orders of magnitude after surface coating while the light conductance shows little variation. The result demonstrates that one can readily enhance the UV photoresponse of the photodetector by surface functionalization.
The investigation on the interference phenomenon in electroluminescence spectrum of vertical structured InGaAlN multiple quantum well light-emitting diodes
2008, 57 (12): 7860-7864. doi: 10.7498/aps.57.7860
The electroluminescence (EL) spectrum of vertical structured InGaN multiple-quantum-well light-emitting diodes were obtained at different space angles. It was found that the multiple EL peak pattern was caused by the interference phenomenon and the distance between the peaks could indicate the thickness uniformity of the p-type layer. The intensity difference between the wave crest and the wave trough of the peak pattern indicates the reflectance ratio of p-type ohmic contact layer. The integral EL intensity in the normal direction of the chip was not so strong， while in the directions of 80° and 75° the intensities were the largest and the interference phenomenon was the sharpest. The intensity of the EL and the interference phenomenon of the side face of the chip was the weakest.
2008, 57 (12): 7865-7871. doi: 10.7498/aps.57.7865
Recently， spin effect in narrow gap semiconductor heterostructures has attracted much attention. However， Rashba spin effect is quite different in Ⅳ-Ⅵ asymmetric quantum wells (QWs) with various growth orientations due to their multivalley and anisotropic band structures. In this work， we calculated Rashba splitting in Pb1-ySryTe/PbTe/Pb1-xSrxTe asymmetric QWs with growth orientations ［100］， ［110］ and ［111］. The results show that Rashba splitting reaches the maximum of 2.2meV when the well width of PbTe QWs with growth orientation ［100］ is 5.0nm， and two groups of Rashba splitting is obtained in PbTe and QWs with growth orientations ［110］ and ［111］， respectively， because the quantum confinement lifts off the fourfold degeneracy of the L-energy valleys. The dependences of Rashba splitting on asymmetry of QWs， well width， temperature and k∥ (the wave vactor in the plane) are also investigated. Large Rashba spin splitting may make Ⅳ-Ⅵ asymmetric QWs as a material candidate for spintronic devices.
Influence of iron doping level upon magnetoelectric coupling in BaTi1-zFezO3+δ-Tb1-xDyxFe2-y bilayer composites
2008, 57 (12): 7872-7877. doi: 10.7498/aps.57.7872
Perovskites BaTi1-zFezO3+δ with doping level from 0.005 to 0.02 have been synthesized with sol-gol method. The X-ray diffraction shows that the doped perovskites retain the original tetragonal structure. Defferential thermal analysis demonstrates that both the transformation temperature of ferroelectric to paraelectric and the latent heat of the transformation decrease with increasing doping level. Bilayer composites BaTi1-zFezO3+δ-Tb1-xDyxFe2-y bilayer composites havebeen fabricated after the piezoelectrics being poled and their magnetoelectric effect has been investigated using a bias magnetic field and an ac magnetic field of 100Hz. The experimental result shows that the transverse magnetoelectric voltage coefficient of the bilayer composites can reach a maximum at doping level of 1.5%. The value of the maximum is 1788mV/A under a bias magnetic field of 60kA/m at room temperature. The analysis of magnetoelectric spectroscopy also shows a higher resonance peak and a higher resonance frequency for the sample containing BaTi0.985Fe0.015O3+δ， suggesting a higher piezoelectric coefficient in the doped BaTiO3 and some difference in interfacial coupling coefficients in the bilayer composites containing the doped samples， respectively.
2008, 57 (12): 7878-7884. doi: 10.7498/aps.57.7878
The absorption， electronic structure and luminescence spectrum of three Sillen bismuth oxyhalides， CaBiO2Cl， SrBiO2Cl and BaBiO2Cl， are reported and analyzed. The calculated electronic structure by the density functional theory shows that both SrBiO2Cl and BaBiO2Cl are semiconductors with direct band gap， which are consistent with the results of ultraviolet (UV)-vis diffuse reflectance experiment. The X-ray and UV excited luminescent spectra show a broad emission band extending from 400 to 550nm at room temperature. The light yield of BaBiO2Cl powders is about 1.4 times that of Bi4Ge3O12 (BGO) powders at room temperature under X-ray excitation， and BaBiO2Cl powders have shorter decay times of 100ns (22%)， 300ns (41%) and 600ns (37%)， compared with those of the BGO powders. The luminescence properties show a correlation between Bi—O(Cl) distance and radius of M2+ ions. Considering that the density of BaBiO2Cl is 6.98g/cm3， it may be a potential practical scintillator.
2008, 57 (12): 7885-7890. doi: 10.7498/aps.57.7885
MgxZn1-xO has been an object of intense study as a novel photo-electronic function material in recent years. Mg0.25Zn0.75O film was prepared on quartz glass substrate by sol-gel method. Structure and optical properties of Mg0.25Zn0.75O film were studied both in theory and by experiment. The result indicates that Mg0.25Zn0.75O film has hexagonal wurtzite structure. The film is homogeneous and the average grain size is about 20nm. Absorption spectrum indicates that the absorption edge starts at 360nm and the corresponding forbidden band width is 3.83eV. The luminescence spectrum of the sample is composed of three peaks at 384.9， 444.8 and 533.6nm respectively and the peak of excitation spectrum is at 378nm. The crystal lattice of Mg0.25Zn0.75O film is increased， the forbidden band width is broadened and the ultraviolet and blue-green luminescence peaks red-shift about 59， 14 and 12.6nm， respectively， due to the interstitial defects introduced by the surplus Mg ions.
Solid state cathodoluminescence of poly (2-methoxy-5-(2-ethyl-hexyloxy)-1，4-phenylene vinylene in devices with Si3N4 as the electronic accelerating layer
2008, 57 (12): 7891-7895. doi: 10.7498/aps.57.7891
A solid state cathodoluminescence (SSCL) device in which Si3N4 acts as the electronic accelerating layer and poly (2-methoxy-5-(2-ethyl-hexyloxy)-1， 4-phenylene vinylene (MEH-PPV) as the luminescent layer is first designed. Under alternating current bias， SSCL of MEH-PPV is realized. The luminescence peak is located at 600 and 417nm， respectively, corresponding to the exciton emission and the recombination luminescence of lowest unoccupied molecular orbital (LUMO) to highest occupied molecular orbital (HOMO). The intensity of the twopeaks changes with the increasing of the driving voltage nonlinearly. Compared with the device with SiO2 as the accelerating layer， the short wavelength emissions are found to be the same and located at 417nm for both devices, although their accelerating layers are different. It confirms the SSCL theory and the short wavelength is caused by the direct recombination of electrons in LUMO and holes in HOMO. Finally， the accelerating abilities of Si3N4 and SiO2 are compared at high driving voltage, which showed that the accelerating ability of SiO2 is superior to that of Si3N4 at the driving voltage involved.
2008, 57 (12): 7896-7899. doi: 10.7498/aps.57.7896
The devices with sandwiched structure are prepared by compounding tris (2-phenylpyridine) iridium (Ir(PPY)3) with SiO2. The green emission of Ir(PPY)3 at 517nm and blue electroluminescence at 435nm are obtained from the devices under AC voltage. Both the green and the blue emission are attributed to the solid-state cathodoluminescence (SSCL) of Ir(PPY)3 excited by hot electrons which are accelerated in SiO2. Phosphorescent SSCL is confirmed after the realization of SSCL on organic polymers and organic small molecules tris-(8-quinolinolato)aluminum (Alq3).
2008, 57 (12): 7900-7905. doi: 10.7498/aps.57.7900
In this work，α-Al2O3：C crystal was grown by temperature gradient technique using α-Al2O3 and graphite powder as raw materials， and the thermoluminescence (TL) and optically stimulated luminescence (OSL) of the crystal were investigated by Ris TL/OSL-DA-15. The as-grown α-Al2O3：C crystal shows a single TL peak at 462K and blue emission peak at 410nm. The TL intensity increases as irradiation dose increases， but no shift is found in the position of the glow peak. The OSL decay curve of α-Al2O3：C crystal consists of two components， namely the fast component and the slow component. With the increasing of the dose，the decay rate of the fast component almost remains unaltered while that of theslow component increases. The TL response of the crystal shows excellent linear characteristic in the dose range of 5×10-6—10Gy， and saturation is observed at 30Gy. The OSL response shows excellent linear characteristic in the dose range of 5×10-6—60Gy， and reaches saturation at 100Gy. Compared with TL response， OSL response shows higher sensitivity and wider linear dose response.
Determination of the optical constants of sol-gel derived ZrO2 films simply form the transmission spectra
2008, 57 (12): 7906-7911. doi: 10.7498/aps.57.7906
By combining Swanepoels theory and the Wemple-DiDomenico dispersion model， a simple method was established to determine the optical constants of the sol-gel derived ZrO2 films directly from the corresponding transmission spectra. The results showed that the sol-gel ZrO2 films exhibite the optical characteristics of high refractive index of 1.63—1.93 (at 632.8nm)， negligible absorption in spectral region of 400—1200nm， as well as the direct band gap of about 4.97—5.63eV. And the specific values of the optical constants strongly depend on the annealing temperature, which is one of the important technologicalparameters for the sol-gel preparation of ZrO2 films. Moreover， in the weakand medium absorption spectral regions， the calculated values of refractive indices are in satisfactory agreement with the results derived from the high-resolution Tek 3000 film-characterization system， indicating the reliability and feasibility of the method in determining the optical constants of ZrO2 films.
2008, 57 (12): 7912-7918. doi: 10.7498/aps.57.7912
Carbon nanotubes (CNTs) cathodes have been made by using screen-printing. Conglutinated CNTs were separated after ablation by laser beam. The spacing between CNTs is increased， the screen effect is diminished. Experiment results show that the field emission properties of CNTs film are highly improved， the relevant turn-on voltage was reduced and the field enhancement factor β was enhanced. From Raman spectra of multi-wall CNT films treated by laser ablation， the defects on CNT surface were increased with increasing laser energy， and became new field emission dots. Compared with diode structure, the flat gate structure showed better field emission performance. So the laser ablation is an effective method that could enhance CNT field emission properties， and has value in the research of CNT cathode materials. At the same time， the flat gate structure has better prospects in the structure design of CNTs field emission flat display.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
A nonlinear elastic analysis of static stress and lateral pressure coefficient for granular Couette systems
2008, 57 (12): 7919-7927. doi: 10.7498/aps.57.7919
Whether or not the classical theory of elasticity is valid for static granular matter is a basic question of granular physics. We present in this work stress distributions inside hollow cylindrical (Couette geometry) specimens， calculated with the nonlinear elastic equations recently reported in literature. Most of these results can be experimentally measured， and are useful for clarifying the basic question by testing the applicability of the nonlinear elastic equations to granular matter. Moreover they can also be used for analyzing the lateral pressure coefficient of interest in soil engineering.
Soot volume fraction and temperature reconstruction model research for asymmetric diffusive C-H flame
2008, 57 (12): 7928-7936. doi: 10.7498/aps.57.7928
Non-intrusive soot volume fraction and temperature distribution reconstruction scheme was proposed based on the approximations of flame emission transfer model according to tomographic principle. Radiative sensor number and arrangement specifications were determined by lease square QR matrix factorization algorithm. Simulations show that the maximum discrepancies of temperature and fraction reconstructions are 0.2% and 2.5% respectively when four 320-pixel line-scan charge-coupled device sensors were arranged non-symmetrically. According to the anti-noise capability assessments， the signal-to-noise ratio of the detectors should be larger than 60 dB in order to get reasonable results when the proposed method is adopted.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Resonant diffusion of radiation belt energetic electrons by field-aligned propagation whistler-mode chorus waves
2008, 57 (12): 7937-7949. doi: 10.7498/aps.57.7937
Adopting the dipole geomagnetic field， Gaussian spectral density for the waves， and semi-empirical latitudinal electron density models obtained from available in situ data， this paper has calculated the local and bounce-averaged quasi-linear resonant electron diffusion coefficients due to chorus and then determined the timescales for electron precipitation loss and stochastic acceleration， in the range of 4≤L≤7 outside the plasmapause. The results indicate that the spatial extent where gyroresonance occurs depends on electron energy， equatorial pitch angle， wave spectrum， and the local electron number density and magnetic field. Besides these five parameters， the actual values of resonant frequency rely on magnetic latitude where resonance occurs. The acceleration of radiation belt energetic electrons occurs predominantly due to equatorial chorus， and the mid-latitude chorus preferably contributes to the precipitation loss of relativistic electrons. The timescales for both electron loss and acceleration due to chorus-driven diffusion have been evaluated to be of hours for lower-energy electrons (about 200keV) and of days for higher-energy electrons (about 1MeV). It is also found that variation of latitudinal density distributions contributes importantly to chorus-driven electron resonant diffusion. In general， an increasing latitudinal electron density increases the loss timescales for untrapped electrons with small equatorial pitch angles， but has negligible effect on the acceleration of trapped electrons with large equatorial pitch angles. The variations of chorus wave amplitude and wave spectrum with magnetic latitude and L-shell also make important contributions to the lifetime and acceleration of radiation belt electrons， which are generally greater than the effects of varying latitudinal distribution of cold plasma density.
Investigation on the surface damage to solar cells by impacts of space micro-debris on low earth orbit
2008, 57 (12): 7950-7954. doi: 10.7498/aps.57.7950
Cumulative damage to the exposed spacecraft materials by the micro-debris on low earth orbit， where the debris is densely populated， is one of the crucial problems calling for due consideration in designing of spacecrafts. The flux of micro-debris encountered by a typical sun-synchronous vehicle was calculated， the associated hypervelocity impact simulation experiment was carried out， and the damage equation was established. Based on the flux result and the damage equation， the surface damage ratio of the solar cell due to micro-impacts was calculated， and the associated optical transmittance decrease of the cell was evaluated according to optical measurement and theoretical model. The result showed that the surface damage ratio of solar cell after 10 a cumulative micro-impacts reaches 0.61% on average， and up to 2.3% in the worst case. The optical transmittance decreases by 0.5% on average， and up to 1.5% in the worst case. In this paper， a quantitative method of evaluation for the issue of solar cells surface damage due to micro-impacts was demonstrated through a concrete example.
2008, 57 (12): 7955-7962. doi: 10.7498/aps.57.7955
The present paper investigates the dimensionless dynamical continuity equation of perfect gas motion in gravitational field. Based on Π axiom of dimensional theory， self-similarity of perfect gas universe with gravity and a series of exact solutions of R(t) are deduced. Based on R(t)，a non-Euclidean homogeneous space-time coordinate system S(t,ξ，θ，φ) can be established. A perfect gas universe solution can be worked out， in which there is a constant density ρ， the velocity u value being zero， and there is a nonzero pressure p. In this solution， the red shift z represents the propagating distance r. When z is much less than 1， it is proportional to r (Hubbles law). The Robertson-Walker (k=-1) metric of normal universe model is obtained from homogeneous expanding coordinates， and the ratio of expanding rate HF to the Hubble constant H0 decreases notably as the value of z rises. It corresponds to the “universal accelerated expansion” observed in the spectrum of a high-red-shift supernova.