Vol. 58, No. 8 (2009)
2009-04-20
GENERAL
2009, 58 (8): 5133-5141.
doi: 10.7498/aps.58.5133
Abstract +
In this paper, space approach of moving object in a kind of Markov stochastic process is studied. A state transfer function of Markov stochastic model in space-time network is deduced by mathematic method firstly. The space-time network is defined as a three-dimensional space which is formed from moving objects and their trajectories, and the corresponding distance space is constructed. Then the fixed point theorem is presented. A self-mapping operator in the distance space is attained by the analysis of state transfer function. On the basis of the above theory, the moving object can be mapped from the former node to the target one by themselves. All of these are proved by theorems and emulational experiments. Moreover, we attempt to use the method of space decompounds with granularity in moving object. The moving object can be approached better through the mapping of fixed point, and the requirements of moving objects are satisfied in real time. The relevanr experiments also validate the feasibility and validity of space approach idea.
2009, 58 (8): 5142-5149.
doi: 10.7498/aps.58.5142
Abstract +
The method of nonholonomic mapping is adopted to construct a Riemann-Cartan space embedded in a known Riemann space. As a special case, Weitzenbock space is embedded in an Euclidean pace. By means of the nonholonomic mapping and d’Alembert-Lagrange principle a geodesic in a Riemann space is mapped to an autoparallel in a Riemann-Cartan space. The mapping theory is applied to the problem of rotation of a rigid body with a fixed point. It is proved that Euler equations for the rigid body are equations of geodesic in the Riemann configuration space described by Euler angles, whereas the equations in the pseudo-coordinate space corresponding to angular velocities of the rigid body are equations of autoparallel in the Riemann-Cartan space with constant torsion.
2009, 58 (8): 5150-5154.
doi: 10.7498/aps.58.5150
Abstract +
In this paper, the conformal invariance of holonomic mechanical systems with variable mass is studied. The necessary and the sufficient conditions of conformal invariance of the system are obtained and shown to be also those of Lie symmetry simultaneously. Finally the Noether conserved quantities of conformal invariance are presented.
2009, 58 (8): 5155-5161.
doi: 10.7498/aps.58.5155
Abstract +
In the discrete element method, the damping coefficient, which is a key parameter for rational simulation, is uncertain and often chosen based on researchers experience. To make the physical model reflect the reality better, in this paper acoustic technology is used to calibrate the damping coefficient. Oscillogram of the acoustic wave generated by the collisions is analyzed and the calibrated damping coefficient is obtained to be 0.5. The calibrated parameters are used to simulate the random packing process of 2000 monosize spherical particles. The final packing density is 0.625, which is in accordance with the classic measurements quite well. Besides the obtained correct damping coefficient, in this paper both the usefulness of the physical model and the practicability of acoustic technology for detecting the collision time are also demonstrated precisely.
2009, 58 (8): 5162-5168.
doi: 10.7498/aps.58.5162
Abstract +
The simple approach to improve the computational precision of Melnikov method is presented by using the undetermined fundamental frequency and normal form method. We construct the improved Melnikov expression for a triple-well nonlinear oscillator subject to principal parametric resonance and external excitation. For the occurrence of chaos, the approxime threshold values of chaotic motion are obtained from the Homoclinicity and Heteroclinicity points of view. It depends on the introduction of undetermined fundamental frequency, and adopting new time transformation for fulfilling the homoclinic and heteroclinic orbits, so that the effect of disturbing parameter can be easily detected and embodied in the Melnikov operation. As is illustrated, the explicit applications show that the improved results coincide very well with the results of numerical simulation.
EDITOR'S SUGGESTION
2009, 58 (8): 5169-5173.
doi: 10.7498/aps.58.5169
Abstract +
Entangled states are key resource for quantum information processing. In linear optics quantum computing，the most widely used entanglement resource is polarization entangled photon pairs generated via type-Ⅱ spontaneous parametric down-conversion. However，the brightness is not sufficient to perform quantum communication experiment between ground and satellite. In this paper we have developed a high-brightness entangled resource based on periodic polarization and quasi-phase marched periodically poled KTiOPO4 crystal. The number of generated entangled pairs is about 1.6×104s-1mW-1 and the visibility is 27∶1. Our source is with about one order of magnitude brighter than previous sources based on β-barium borate crystal. This leads to an immediate possibility for applications in the field of quantum key distribution，quantum teleportation and quantum computation. Particularly it would form a solid basis for future global quantum network.
2009, 58 (8): 5174-5178.
doi: 10.7498/aps.58.5174
Abstract +
The movement of small particles in a vortex flow is simulated by lattice Boltzmann method. The vortex flow is produced by a flow in a cave, and the trajectory, velocity and anglular velocity of the particles are evalnated with the hydrodynamic force calculated by the momentum-exchange and stress-integration, respectively. At last, multiple particles with different radii moving in a vortex flow are also simulated with the hydrodynamic force calculated by stress-integration.
2009, 58 (8): 5179-5185.
doi: 10.7498/aps.58.5179
Abstract +
A stochastic system with double singularities driven by non-Gaussian noise is investigated. The Fokker-Plank equation of the system is obtained through the path-integral approach and the method of transformation. Based on the definition of Shannon’s information entropy, the time dependence of entropy flux and entropy production of the system is calculated both in the absence and in the presence of non-equilibrium constraint. The influences of the system dissipative parameter, the system singularity strength parameter, the noise correlation time and the noise deviation parameter on entropy flux and entropy production are discussed.
2009, 58 (8): 5186-5190.
doi: 10.7498/aps.58.5186
Abstract +
We employ an analytical approach introduced by López to determine the anomalous scaling exponent of the growth equation with a generalized conservation law in both the weak-and strong-coupling regimes, which included the Kardar-Parisi-Zhang（KPZ）, Sun-Guo-Grant（SGG）, and molecular beam epitaxy （MBE）equations as special cases and allows for a unified treatment of growth equations. Analysis shows that KPZ equation and SGG equation exhibit normal Family-Vicsek scaling behavior, whether in the weak-coupling or strong-coupling regime. Differently, MBE equation exists intrinsic anomalous scaling in the strong-coupling regime and normal Family-Vicsek scaling behavior in weak coupling regime. All the conclusions obtain here are well consistent with the corresponding results derived by the dynamic renormalization group theory, numerical simulation and experiment.
2009, 58 (8): 5191-5195.
doi: 10.7498/aps.58.5191
Abstract +
The effects of multiplicative colored noise and additive white noise on the mean first-passage time （MFPT） T+（xs1→xs2） and T-（xs2→xs1） of an optical bistable system are investigated. The approximate Fokker-Planck equation is obtained based on the Novikov theorem and the Fox approach. The expression of MFPT is obtained in the steepest-descent approximation. Numerical results show that （1） the multiplicative noise intensity Q and additive noise intensity D play the same roles in T+（xs1→xs2） but different roles in T-（xs1→xs2）;（2）T+（xs1→xs2）increases with the increase of the self-correlation time τ but decreases with the increase of the correlation intensity λ；（3）T-（xs2→xs1）decreases with τ increasing， but increases with λ increasing.
2009, 58 (8): 5196-5200.
doi: 10.7498/aps.58.5196
Abstract +
Kramers rate restricts the stochastic resonance （SR） frequency within the scope of half of its limit value. This restriction is the cause of SR failure of high frequency signal. By revealing the mechanism of frequency transformation of the twice sampling SR, we prove that the frequency scale ratio of the twice sampling SR can map or transform any signal frequency to the SR frequency scale. Compared with the method of twice sampling SR, the approach of adjusting system parameters to realize SR can only be used within the small SR frequency range, since in tuning bistable system parameters it is hard to make half the Kramers rate equal to the high frequency of the real signal.
2009, 58 (8): 5201-5208.
doi: 10.7498/aps.58.5201
Abstract +
A novel four-dimensiond hyperchaos system with only one nonlinear term is reported. The Lyapunov exponents spectrum, periodic orbit, quasi- periodic orbit, and the chaos and hyperchaos attractor of this system are obtained. A circuit is designed to realize this system by electronic workbench. A synchronization method for the new hyperchaotic systems is established and the mathematical proof of the method is provided. The nonlinear terms in the response system are not dropped. Theoretical analysis and simulation results show that the method is effective.
2009, 58 (8): 5209-5213.
doi: 10.7498/aps.58.5209
Abstract +
A kind of generalized predictive control fast algorithm with predictive function for Hénon chaotic system is proposed. Firstly, we identify the chaotic system by recursive least square parameter of time varying forget factor. Then one kind of predictive function is added into this algorithm and makes full use of compensation of predictive information, overcoming the disadvantage of calculating inverse matrix to increase the response speed of the system. The ability of tracking the reference signal and restraining the parameter perturbation and stochastic disturbance are improved. Simulated results show the effectiveness of the algorithm.
2009, 58 (8): 5214-5217.
doi: 10.7498/aps.58.5214
Abstract +
Based on wavelet transformation and chaos theory, low frequency noise in semiconductor laser is studied. The feasibility of chaotic noise model is discussed and prescribed in detail based on techniques of phase locus, power spectrum，Lyapunov exponents and correlation dimension. And the reasons for this chaos phenomenon it also explained in theory. The rigorous chaotic character of low-frequency noise in semiconductor laser is found, which loys a theoretical basis for the correlative investigation based on semiconductor laser reliability.
2009, 58 (8): 5218-5223.
doi: 10.7498/aps.58.5218
Abstract +
The usual linear variable feedback control method is extended to the speed feedback approach in the study of controlling spatiotemporal chaos in the one-dimensional complex Ginzburg-Landau equation. The controllabilities in diverse systems with different sizes and target periodic states are investigated by theoretical analysis and numerical simulation.
2009, 58 (8): 5224-5237.
doi: 10.7498/aps.58.5224
Abstract +
Various types of border collision bifurcations occurring in a discrete feedback transmission control protocol-random early detection （TCP-RED） system are analyzed based on normal form method. As the system parameters change, the system experiences the border collision and a sudden jump from a periodic attractor to a chaotic attractor leading to the intense vibration of the system and serious degradation of its performance. According to the linear stability condition of fixed point, a method of chaotic control is presented. With this method, the system can be stabilized to the fixed point by perturbing the state variable in the neighborhood of the fixed point. This method can significantly improve the performance and the resource utilization of TCP-RED system, and simulation results show that it is effective.
2009, 58 (8): 5238-5243.
doi: 10.7498/aps.58.5238
Abstract +
The nonlinear dynamic characteristics of metal rubber isolator are studied and a dynamic model with parameters with physical meaning is developed in this paper. According to the experimental results from a setup with single degree of freedom, the identification method for related parameters is researched. The nonlinear function that describes the dynamic behavior of the system is converted to the series of linear functions by using energy method and least square method. The identification results show good agreement with the experimental results and explain the effect of parameters on the performance of isolators.
2009, 58 (8): 5244-5250.
doi: 10.7498/aps.58.5244
Abstract +
Based on the Nagel-Schreckenberg model, a traffic model for an arterial road with equal-distance-distributed intersections is established. The traffic flow is numerically simulated under the open boundary condition and the control of red and green lights. It is found from the results that the capacity of the arterial road is determined by the capacity of the intersection. For small densities, the flow has no relation with the number of intersections. Moreover, the rule of variation of the maximum point of average velocity with the green-light period is obtained with the mean-field method. On the contrary, the number of intersections plays a crucial role in the flow at larger densities. The arterial road can achieve optimal traffic state via regulating the period of traffic lights.
2009, 58 (8): 5251-5258.
doi: 10.7498/aps.58.5251
Abstract +
In order to find factors which influences on short message spreading, we compare short message propagation on three networks: scale-free, small-world and true-life short messages networks. The topology of short message network and the behaviors of mobile phone users play important roles in determining the behavior of a short message spreading. The impact of the scale-free network on the spread of short messages is twofold: messages spread more quickly than on small-world or true-life short messages networks, but longevity of short message is less than the other networks. Some behaviors of mobile phone users obviously influence the process of short message spreading. Meanwhile, we found that the longevity of short message relate to network diameter.
ATOMIC AND MOLECULAR PHYSICS
2009, 58 (8): 5259-5265.
doi: 10.7498/aps.58.5259
Abstract +
Geometry optimization of target complexes were performed at the level of B3LYP/ LanL2DZ/6-31G*, and their absorption spectra and the second nonlinear optical properties were calculated at the TD-B3LYP/LANL2DZ/6-31++G** level and B3LYP/LANL2DZ/6-31++G** level, respectively. The results indicate that the introduction of conjugated substituting groups of electron-donor and conjugated substituting groups of strong electron-accepter makes maximal absorption wavelength red-shifted and blue-shifted, respectively. The introduction of conjugated substituting groups resultes in obvious increase of second nonlinear optical properties for IrQ3 complexes. The electronic transfer belongs to intra-ligand charge transfer （LLCT） for AgQ complexes, and LLCT mixed with partially metal-to-ligand charge transfer for PtQ2 and IrQ3 complexes. There is no effect of substituting groups on transition properties for AgQ, PtQ2 and IrQ3 complexes.
2009, 58 (8): 5266-5269.
doi: 10.7498/aps.58.5266
Abstract +
In this paper,the structure and dissociation energy of the ground state of BiH （D,T） are investigated by quantum mechanical ab initio method in the level QCISD/ LanL2DZ methods. Based on the theory of atomic and molecular statics, the reasonable dissociation limit for the ground state （X3Σ-） of BiH （D,T） is derived. The potential energy curve and relevant optical constants of this state are obtained by least square fitting to the Murrell-Sorbie function. All calculation results are in good agreement with the experimental data.
2009, 58 (8): 5270-5273.
doi: 10.7498/aps.58.5270
Abstract +
In this paper,the structure and dissociation energy of the ground state of GaH （D,T） is investigated by quantum mechanical ab initio method in the QCISD/6-311++G（3df,3pd） and QCISD（T）/6-311++G（3df,3pd） level. Based on the theory of atomic and molecular statics, the dissociation limit for the ground state （X1Σ+） of GaH （D,T） is derived. The potential energy curve and relevant optical constants of this state are obtained by least square fitting to the Murrell-Sorbie function. All calculation results are in good agreement with the experimental data.
2009, 58 (8): 5274-5279.
doi: 10.7498/aps.58.5274
Abstract +
The effect of plasma screening has been investigated by incorporating the Debye potential into the framework of the relativistic Dirac-Fock method， and the GRASP92 code has been modified to perform the calculations. As an example, we have studied the plasma effecting on energy levels and transition probabilities of H-like ions embedded in dense and hot plasma. The following three important results have been obtained: （1） all energy levels are shifted away from their unscreened values toward the continuum, and the shift of energy increases as screening strength decreasing. Furthermore, for each eigenstate with given quantum numbers n and l, there is a finite value of λc （critical length） which corresponds a zero energy. （2） For a given transition, the oscillator strength and transition probability decrease with increase of the screening strength. （3） The degenerate fine-structure levels with the same n, j but different l can be destroyed by the plasma screening effect, even without the consideration of the Lamb shift, which has received little attention in the past.
2009, 58 (8): 5280-5284.
doi: 10.7498/aps.58.5280
Abstract +
The density function （B3P86） method has been used to optimize the possible ground-state structure of SiF2 molecule. The results show that the ground state of SiF2 molecule has C2v symmetry and its electronic ground state is X1A1. The equilibrium parameters of the structure are RSi—F=0.1061 nm and De=13.8 eV. On the base of atomic and molecular reaction statics, the reasonable dissociation limit for the ground state of SiF2 molecule is determined. Then the analytic potential energy function of SiF2 molecule is derived by many-body expansion theory. The potential curves describe correctly the configurations and the dissociation energy for the SiF2 molecule.
2009, 58 (8): 5285-5290.
doi: 10.7498/aps.58.5285
Abstract +
The hyperspherical method is used to study the electron correlation energy，radiative transition rate and Auger transition rate of the highly-doubly-excited states of He-like oxygen.All these doubly-excited Rydberg states are designated by quantum numbers K，T，A.The results show that properties such as the energy，radiative and Auger transition rate regularly change along with the quantum numbers K，T，A.
2009, 58 (8): 5291-5296.
doi: 10.7498/aps.58.5291
Abstract +
By introducing a concept of condensing potential, a theoretical method is established to predict the geometric structure of two-dimensional （2D） adatom islands on crystal surface. Calculations based on semi-empirical potentials （surface-embedded-atom method potential and O. Johnson potential） show that on Cu（111） and Ag（111） surfaces the homoepitaxial 2D islands take on hexagons, while on Pt（111） surface the islands are truncated triangles, and on Cu（100） and Ag（100） surfaces the islands have square shape. These theoretical predictions are in good accordance with the experimental observations . Since calculation of the condensing potentials is independent of the size of islands, this theoretical model could be widely used to predict the shapes of 2D islands on various surfaces.
2009, 58 (8): 5297-5303.
doi: 10.7498/aps.58.5297
Abstract +
In this paper, the Ne 1s photoelectron spectrum and the corresponding Auger decay processes have been investigated theoretically by using the GRASP92, RATIP and the recently developed RERR06 codes based on the multi-configuration Dirac-Fock method. Good overall agreement was found between the present calculations and the previous theoretical and experimental results. By analyzing the Auger decay processes of the photoionization satellite state Ne+1s-12p-1（1,3Ｐ）3p, it was found that the spectator Auger decay is the dominant decay mode and the Auger intensity of the Ne+1s-12p-1（1Ｐ）3p is greater than that of the Ne+1s-12p-1（3Ｐ）3p as a whole. Meanwhile, the relative populations of the final ions Ne+, Ne2+and Ne3+produced by the photoionization satellite state Ne+1s-12p-1（1,3Ｐ）3p were predicted to be 0.02, 0.58 and 0.41, respectively.
2009, 58 (8): 5304-5310.
doi: 10.7498/aps.58.5304
Abstract +
We have studied the one-photon absorption （OPA） and two-photon absorption （TPA） properties of the one-, two- and three-dimensional compounds with the same unit of （4-{2-［4-（2-pyridin-4-yl-vinyl）-phenyl］-vinyl}-phenyl）-amine moieties, using the response theory approach at the hybrid density functional theory level. The numerical simulation shows that the three organic molecules exhibit strong OPA and TPA capabilities. All chromophores present two linear absorption peaks in the ultra UV-visible region, which is in agreement with the experimental measurements. Strong broadband TPA properties from 780 to 1020 nm have been demonstrated in the near infrared region. The ratio of the largest TPA cross sections for three molecules DPVPA, BPVPA and TPVPA is 1.0∶2.3∶4.0, and the molecule TPVPA has the maximum TPA cross section 101.73×10-48 cm4·s. It is substantiated that the TPA cross section can be effectively enhanced with increasing molecular dimensionality. The charge-transfer process for the charge-transfer state is visualized in order to understand the enhancement of the TPA cross sections.
2009, 58 (8): 5311-5317.
doi: 10.7498/aps.58.5311
Abstract +
From the non-U（1） invariance of the atom-dimer conversion system, we generalized the definition of fidelity to this nonlinear system. By making use of the adiabatic fidelity, we investigated the dynamics and adiabaticity of the atom-dimer conversion system in a stimulated Raman adiabatic passage （STIRAP）. We found that the adiabatic fidelity for the coherent population trapping state or dark state, as the function of the adiabatic parameter, approaches to unity following a power law. The power exponent, however, is much smaller than that predicted by the linear adiabatic theorem. We further discuss how to achieve higher adiabatic fidelity for the dark state through optimizing the external parameters of STIRAP and hence to optimize the adiabaticity of the system and obtain high conversion efficiency.
2009, 58 (8): 5318-5322.
doi: 10.7498/aps.58.5318
Abstract +
Based on Bohr-Sommerfeld quantum theory, the formation mechanism of characteristic X-ray has been studied. A formula of the X-ray wavelength based upon the calculation of atomic number is derived. Error analysis is carried out systematically for the calculated values of wavelength, and the rules of relative error are obtained. It is shown that the results of the calculation are very close to the experimental values. The formula is relatively simple in application.
2009, 58 (8): 5323-5328.
doi: 10.7498/aps.58.5323
Abstract +
The equilibrium geometries for the ground state of PH2 radical are calculated by using the coupled_cluster single_double model with a perturbative triple excitations （CCSD（T）） method in combination with the series of correlation-consistent basis sets of Dunning and co-workers. It can be found that the cc-pV5Z basis set is the most suitable basis set by comparing the calculations with the experiment. The results obtained at CCSD（T）/ cc-pV5Z level of theory are 0.14185 nm for the equilibrium bond length RP—H, 91.8624° for the bond angle αHPH, 3.483 eV for the dissociation energy De and 2399.9781, 1128.4213 and 2407.8374 cm-1 for the normal of vibrational frequencies（ν（a1）, ν2（a1） and ν3（b2）, respectively. The potential energy function of PH2 （C2v, X2B1） is derived using many-body expansion theory. The characters of structure and the change of energy are reflected correctly by the function.
2009, 58 (8): 5329-5334.
doi: 10.7498/aps.58.5329
Abstract +
The coupled-cluster single-double withy a perturbative triple excitations ［CCSD（T）］ theory in combination with the quintuple correlation-consistent basis set （cc-pV5Z） of Dunning and co-workers is employed to determine the equilibrium geometry, dissociation energy and vibrational frequencies of the SiH2（C2v, X1A1） radical. By comparison, excellent agreement can be found between the present results and the experiments. The values obtained at cc-pV5Z are 0.15163 nm for the equilibrium bond length RSi—H, 92.363° for the bond angle α of H—Si—H, 3.2735 eV for the dissociation energy De（HSi—H） and 1020.0095, 2074.8742 and 2076.4762 cm-1 for the vibrational frequencies ν1（a1）, ν2（a1） and ν3（a1）, respectively. The equilibrium geometry, harmonic frequency and potential energy curve of the SiH（X2Π） radical are calculated at the CCSD（T）/aug-cc-pV5Z level of theory. The ab initio points are fitted to the Murrell-Sorbie function with the least-squares method. The spectroscopic parameters, whether directly determined by the Gaussian03 program package or they are derived from the analytic potential energy function， conform almost perfectly with the available experimental results. The analytic potential energy function of the SiH2（C2v, X1A1） radical is derived by using the many-body expansion theory. This function correctly describes the configuration and dissociation energy of the SiH2（C2v, X1A1） radical. Two symmetrical saddle points have been found at （0.312, 0.160 nm） and （0.160, 0.312 nm）, respectively. And the barrier height is found to be 0.5084 eV.
2009, 58 (8): 5335-5341.
doi: 10.7498/aps.58.5335
Abstract +
For the first time, the elastic, inelastic and total differential cross sections for collision between He atom and the ground state of BH molecule have been calculated by using accepted exact close-coupling approximation method. The calculation is performed at the incident energies from 25 to 150 meV. Further, the change tendency and characteristics of the differential cross sections have been discussed. The calculated results show that the total differential cross section is the general rule and characteristics of collision between an atom and a diatomic molecule, the phenomenon of the scattering oscillation at large angles is more evident along with increase of the incident energy for low-lying rotational excitation state-to-state differential cross sections in He-BH collision system.
EDITOR'S SUGGESTION
2009, 58 (8): 5342-5349.
doi: 10.7498/aps.58.5342
Abstract +
We measured the cross-section ratios of helium induced by Cq+ and Oq+ （q=1—4） produced by the 2×1.7 MV tandem accelerator of Lanzhou University in the energy range from 20 to 500 keV. We obtained the two-dimensional spectrum by employing coincidence method combined with the MPA-3 data acquisition system. Thus, we obtained the ratios of direct ionization, projectile capture electrons and projectile loss electrons to the total cross-section which are denoted by of Rdirect， Rcapture and Rloss in this paper. A qualitative explanation of the competitive relation of the three reaction-channels and the emperical laws of the three reaction channels are separately presented.
2009, 58 (8): 5350-5354.
doi: 10.7498/aps.58.5350
Abstract +
Helium-radon clusters will migrate upwards provided the clusters are made up of one radon atom and more than 8 helium atoms. With less than 8 the atoms, the specific gravity of the helium-radon clusters will be heavier than their buoyancy and the clusters will migrate downwards. Hence the average number of atoms will be greater 1a upwards-migrating clusters. The mathematical calculation has been done to obtain the average atomicity of clusters from experimental radon data. The results show that the number of α particles provided by uranium ore is insufficient, if not considering the diffusion of radon, because one radon atom needs to combine with 7.5 helium atoms. But if the diffusion of radon is taken into account, there will have enough number of α particles because one radon atom needs to combine with just 4.26—5.57 helium atoms. Therefore, the diffusion of radon cannot be ignored when we apply the migration mechanism of helium-radon clusters to explain the vertical migration experiment data of radon and its daughters.
2009, 58 (8): 5355-5361.
doi: 10.7498/aps.58.5355
Abstract +
The geometry and electronic properties of Bn （n=2—15） clusters with different growth pattern were calculated and analyzed by using the density functional theory at B3LYP level. The energy gap, first ionization potential and binding energy per atom of boron clusters were also discussed. The results show that linear structures are unstable and are strongly metallic, especially for n=8, which has an energy gap as low as 0.061 eV, indicating the metallic characteristic. Planar and quasi-planar structures are most stable and weak in metallicity. The stability and metallicity of tree-dimensional structures are intermediate between those of the linear and the planar or quasi-planar structures. Furthermore, we also analyzed the electronic properties of ground-state clusters, including the binding energy per atom, second difference in energy, energy gap and the first ionization potential. The results show that B12 and B14 are magic-number clusters.
2009, 58 (8): 5362-5369.
doi: 10.7498/aps.58.5362
Abstract +
The possible models of the silicon oxide clusters on Si（001） surface, including the regular symmetric structure （A）, the periodic asymmetric structure （B）, the periodic asymmetric structure （C） and the irregular structure （D）, have been fully optimized using the first-principles general gradient approximation method based on density-functional theory. The results show that all the optimized surface structures are amorphous. The optimized surface structures of the B, C and D models have the similar geometric character as the tetrahedron structure of SiO2. Furthermore, the coalescence between the Si and O atoms of the silicon oxide clusters includes significant ionic bond and certain covalent bond as shown by employing the Mulliken population analysis and the graphics of electron localization function.
2009, 58 (8): 5370-5375.
doi: 10.7498/aps.58.5370
Abstract +
The geometric and magnetic properties of M13 and M13-doped Au20 （M = Fe, Ti） clusters have been studied using the generalized gradient approximation based on the density functional theory. The optimized geometries of the clusters are close to the Ih structure within 0.006—0.05 nm tolerance. The lowest-energy spin states of the Fe13 and Fe13-doped Au20 clusters are 44 μB and 38 μB, respectively, while there is week ferromagnetic interaction between the Fe and Au atoms for the Fe13-doped Au20 cluster. On the other hand, the lowest energy spin states of the Ti13 and Ti13-doped Au20 clusters are 6 μB and 4 μB, respectively. The magnetic ordering is in a week ferromagnetic arrangement between the 12 surface Ti atoms and Au atoms, while in a week antiferromagnetic arrangement between the 12 surface Ti atoms and the core Ti atom. Comparing with the bare Fe13 and Ti13 clusters, the magnetic moments of Fe13 and Ti13 in Fe13-doped Au20 and Ti13-doped Au20 clusters are reduced by 6.81 μB and 2.88 μB, respectively.
CLASSICAL AREA OF PHENOMENOLOGY
2009, 58 (8): 5376-5384.
doi: 10.7498/aps.58.5376
Abstract +
The two-dimensional speckle field and the phase produced by the Gaussian correlation random surfaces on the Fraunhofer plane were simulated. It was found that the zero-contour of the real and imaginary parts can be in the tangent and superposition situations besides the traditional intersection situation. The tangential points and the superposition-lines can also form phase singularities, around which the phase distribution shows the characteristics of discontinuity and symmetry and differs from the spiral distribution around the traditional singular points formed by the zero crossings of the real and imaginary parts. With the propagation of the optical wave, the relative positions of the zero-contour of the real and imaginary parts change from tangent to superposition, and then to intersection on the different observation plane with the simultaneously changes of the the phase singularities.
2009, 58 (8): 5385-5391.
doi: 10.7498/aps.58.5385
Abstract +
The system of digital holographic wave-front reconstruction is studied based on Abbe imaging principle. The results show that the high-frequency angular spectral component of object light can arrive at charge-coupled device（CCD） detector through using imaging system to transform the object light field. To evaluate the quality of reconstructed object light field, the calculation formula to reconstruct wave-front at arbitrary position after the optical system on the CCD is derived, and pulse response of wave-front reconstruction system is researched when the object light arrives at CCD through paraxial optical system. Theoretical and experimental researches show that the object light field can be reconstructed with high quality when the CCD plane is close to the image surface.
2009, 58 (8): 5392-5398.
doi: 10.7498/aps.58.5392
Abstract +
A fast algorithm for chirp Z-transforms is improved form chirp Z-transform, which is developed by using two fast Fourier transforms and an analytical Gaussian kernel. Its computational complexity is less than a fast convolution algorithm. However, there are some problems when the algorithm is implemented, such as the discarding of the data, the smallness of the response domain, the bigness of the computational complexity and so on. To avoid the problems mentioned above, we make a change on the implementing of the algorithm in this paper. Then we compare the numerical results of some chirp systems with the analytical ones. The accuracy of Fourier transforms of Gaussian function is higher than the 10-15 order for most cases, and the accuracy of Fourier transforms of rectangle function is about the 10-3 order, which is essentially limited by the accuracy of the fast Fourier transform. Finially this algorithm is used to calculate some typical systems of scalar diffraction and fractional-order Fourier transforms, and the results are in good agreement with other published results in the literatures.
2009, 58 (8): 5399-5405.
doi: 10.7498/aps.58.5399
Abstract +
The spectral transfer function of the imaging spectrometer is defined on the basis of the linear system theorem. The spectral transfer functions including spectral modulation transfer functions and the spectral phase transfer functions of the temporally modulated Fourier transform spectral imager based on Michelson interferometer and those of the spatially modulated Fourier transform spectral imagers based on Sagnac interferometer,Fresnel interferometer, and Lloyd interferometer are deduced. These spectral transfer functions could be regarded as an objective criteria in the spectral domain of spectral imagers. Together with the well known optical transfer function, the performance of the imaging spectrometer could be described comprehensively.
2009, 58 (8): 5406-5411.
doi: 10.7498/aps.58.5406
Abstract +
Applying the method of coherent-state orthogonalization expansion，the energy spectra and the dynamical properties of the Jaynes-Cummings model with non-rotating wave approximation are calculated exactly. By comparison with the approximate analytical solutions, it is found that the approximate analytical solutions agree with our numerical exact solutions well both in weak coupling regime and in strong coupling regime, but not so well in intermediate strength coupling regime.
2009, 58 (8): 5412-5422.
doi: 10.7498/aps.58.5412
Abstract +
We study the co-existence of multi-wave mixing processes in an open five-level atomic system affected by strong coupling fields. Competition among four-wave mixing （FWM） with two dressing fields, the six-wave mixing （SWM） with one dressing field as well as the eight-wave mixing （EWM） propagating along the same direction can be evoked by adjusting laser beams because these multi-wave mixing signals are induced by atomic coherence between common levels. With the dressing fields properly controlled, the FWM and SWM signals can be suppressed while the EWM signals enhanced in the beam. We also amply analyze the difference between the parallel-cascade and sequentical-cascade doubly dressing mechanisms in doubly-dressed four-wave mixing （DDFWM） processes.
2009, 58 (8): 5423-5428.
doi: 10.7498/aps.58.5423
Abstract +
The characteristics of the transverse modes of a sealed-off He-N2-CO2 laser are measured on-line by using a laser beam analyzer. The influence of the transverse modes on discharge current is observed. It is shown that this laser tube can easily operate on lower order modes under optimum discharge current，but hardly operate on the TEM00 mode. Under the condition of improved symmetry of laser tube and increasing the speed of cooling water, the laser can operate on the TEM00 mode. The variety of laser modes is analyzed through the kinetic process theoretically, which is helpful to the control and improvement of laser mode for high power lasers.
2009, 58 (8): 5429-5435.
doi: 10.7498/aps.58.5429
Abstract +
In this paper, we investigate laser ablation of silicon wafers in air and water through using Ti:sapphire femtosecond laser with central wavelength of 800 nm and pulse duration of 50 fs—24 ps at a repetition rate of 1 kHz. Unique concentric rings are observed on the sample surfaces during the laser ablation in ambient air. Formation mechanisms of the inner and outer ablative structures are attributed to the thermal melting and Coulomb explosion, respectively. The influence of laser parameters, such as the laser energy, the number of pulses and the pulse duration on the radius of the ablation ring is observed experimentally. It is found that drilling a blind hole with large aspect ratio usually requires multiple laser shots with relatively small pulse energy. In the water surroundings, femtosecond laser ablation results in porous microstructure formation in the irradiated area. However, laser pulses with picosecond duration lead to large material removal by evidently nonthermal processes. Theoretical analysis suggests that this phenomenon could be explained by the photo-mechanical stress and bubble cavitation, which are strengthened with increasing pulse duration. The critical pulse width separating the two different ablation processes in the water ambience is determined experimentally for the first time.
2009, 58 (8): 5436-5442.
doi: 10.7498/aps.58.5436
Abstract +
A simplified two-dimensional plasma hydrodynamic modal of X-ray laser on slab target is proposed on the basis of self-similarity method. The properties of two-dimensional hydrodynamics of X-ray laser plasma are studied using the model. It is shown that after the action of driving laser, the electron temperature and the electron density along the focus line will tend to equibrium with elapse of time. This result has an important meaning in the optimized design of the experiment on the X-ray laser.
2009, 58 (8): 5443-5449.
doi: 10.7498/aps.58.5443
Abstract +
A diode-end-pumped Nd:YAG intracavity frequency-tripled quasi-continuous 355 nm laser has been demonstrated. The efficient third-harmonic generation with high peak power and stable operation is generated by a lithium triborate （LBO） crystal successfully. The laser cavity is a simple plane-to-plane linear cavity, which is short and compact. An average 355 nm laser output power of 163 mW is obtained at a pump power of 5.73 W and a pulse repetition rate of 9 kHz, with the highest optical-to-optical efficiency of 2.84%. The highest average 355 nm laser output power is up to 174 mW at a pump power of 6.7 W and a pulse repetition rate of 5 kHz. The instability of the output power of 174 mW is less than 5% and the beam quality M2 factor is 3.79. An average 355 nm laser output power of 112 mW is achieved at a pump power of 5.73 W and a pulse repetition rate of 2 kHz and the peak power reaches the highest value of 9.15 kW. Compared with the technology of external cavity frequency conversion, the technology of intracavity frequency conversion can cut down some of the optical elements in the laser cavity, reduce the size of the laser, and enhance the handiness of the laser. These advantages will benefit the wide applications of the ultraviolet lasers.
2009, 58 (8): 5450-5454.
doi: 10.7498/aps.58.5450
Abstract +
Optical bistability（OB） and optical multistability（OM） behaviour of ladder-type four-level atom in a unidirectional ring cavity driven by three laser fields is studied. Three paths of photon transition are coupled with the laser fields. In this case, the laser field ΩB coupling with two middle levels is very important. When the ΩB is increased, optical bistability threshold significantly decreases and optical multistability appears. The effects of the laser field coupling with two upper levels on optical bistability are also discussed. Furthermore, we analyze the effects of the detuning of probe field and atomic cooperation parameter on OB and OM.
2009, 58 (8): 5455-5461.
doi: 10.7498/aps.58.5455
Abstract +
Taking the one-dimensional asymmetrical π conjugation molecule material （DBASVP molecules） as medium, with the two-photon resonance, we studied the propagation of ultrashort laser pulses in the medium based on the two-photon area theorem and the numerical solutions of the coupled Bloch and Maxwell's equations. The evolution of two-photon area is thus investigated and the applicability of the two-photon area is then discussed. A theoretical method for simulating the optical limiting properties of molecular medium is pointed out. The molecular electronic structures and dipole moments are calculated by use of density functional theory at ab initio level. The numerical results show that the two-photon area theorem based on the slowly varying envelope and phase approximations cannot accurately describe the evolvution of the two-photon area in the molecular medium. The optical limiting behavior based on the two-photon absorption is related to the width of the molecular medium.
2009, 58 (8): 5462-5466.
doi: 10.7498/aps.58.5462
Abstract +
The nonlinear propagation of 100 fs Ti:sapphire laser pulses in a bi-refringent photonic crystal fiber is experimentally investigated. Super-continuum generation with spectra extending from 450—1050 nm was obtained and the spectra consist of distinct peaks. The spectral broadening is shown to be caused by the mechanism of spectral broadening through fission of higher-order solitons into red-shifted fundamental solitons and blue-shifted dispersion waves. The simulation of light propagation property in the waveguide can explain well the experimental results.
2009, 58 (8): 5467-5472.
doi: 10.7498/aps.58.5467
Abstract +
The formation of defect modes in optically induced photonic lattices with one_and two_dimensional defects are analyzed by combining the diffraction relationship of uniform lattices and the Brillouin zone spectra. The defect modes associated with different high symmetry points in Brillouin zone are obtained, including the positive and negative defect modes bifurcating from the X1 point, which is “embedded” in the photonic bandgap. It is revealed that, for two_dimensional defects, the positive （negative） defect modes exist only in the regions corresponding to normal （anomalous） diffraction along both principal axes in k-space, whereas for one_dimensional defects, the positive （negative） defect modes can exist in the regions corresponding to normal （anomalous） diffraction along a single direction. The positive defect mode at X1 point predicates the existence of in-band solitons under self-focusing nonlinearity. The conclusions obtained above could be helpful to understanding the formation of lattice solitons （especially in_band solitons） in photonic lattices.
2009, 58 (8): 5473-5478.
doi: 10.7498/aps.58.5473
Abstract +
Multi-beam laser heterodyne measurement of glass thiclcness is based on laser heterodyne measurement technology and laser Doppler technology. We deduced the harmonic expression of photodetector output current, discussed the feasibility and theory of the method which make use of the non-contact multi-beam laser heterodyne to increase the precision of the glass thickness measurement, and made simulation in different situation with Matlab. The results showed that the maximum error is 0.3% at different incidence angles in flat glass thickness measurement, which has an obvious advantage in accuracy over the other methods.
2009, 58 (8): 5479-5483.
doi: 10.7498/aps.58.5479
Abstract +
Based on the nonreciprocal property of Faraday effect, we investigate the light output from optical reflection cavity filled with Faraday optical rotation medium. It is shown that such a reflection cavity has the enhancement effect of megneto-optical rotation （MOR）. Base on this phenomenon, a detection method for small rotation angle is proposed, and the characteristics of MOR enhancment device is analyzed. The relations between the detection sensitivity and the factors of reflection, absorption and rotation angle are discussed analytically and simulated. The problems discussed include the angular working point, the measuring range, and the theoretical limitation of detection sensitivity. Under the condition of small angle approximation, the output intensity is proportional to the square of the rotation angle, and increses with the nise of device reflectivity. The angular working point and measuring range decrease monotoniccally with the increase of reflectivity and the drop of the absorption. The limiting value of relative sensitivity is 78.49. The results indicate that the MOR enhancement device can be applied to MOR detection of microfluidic system and miniaturization of MOR instruments.
2009, 58 (8): 5484-5490.
doi: 10.7498/aps.58.5484
Abstract +
A comparison of the existing sound source model for calculating transfer matrix of two-dimensional rectangular acoustic cavity is performed in this paper. Based on the comparison, a new square linar sound source model is developed to calculate the pressure response function. Results show that this new source model can not only overcome the singularity problem of the point source model but also get more uniform pressure distribution than that of the surface source model provided that the size of the new model’s geometry is reasonably controlled. On the other hand, the mathematical model of the new square line-source is simpler than that of the circular line-source model. Therefore, it can improve the computational efficiency significantly. Finally, the feasibility of the new source model is substantiated by performing some numerical experiments.
2009, 58 (8): 5491-5500.
doi: 10.7498/aps.58.5491
Abstract +
In researching long-distance interface bistatic reverberation, the scattering area is taken as an elliptical ring. In researching short-distance bistatic reverberation, the model should be a three-dimensional ellipsoid. The ellipse and ellipsoid models can work without wireless communication, only using GPS clock to make sure the systems work synchronously so as to realize ranging and orienting. It is convenient for the integration of bistatic sonar system. The two-dimensional and three-dimensional prediction formulas of bistatic pressure reverberation are deduced, the vector-sensor three-dimensional reverberation is simulated and the anti-reverberation performance of vector-sensor directivity composite is discussed.
2009, 58 (8): 5501-5506.
doi: 10.7498/aps.58.5501
Abstract +
The analytical approximate solution for Marangoni convection is which is induced by variation of surface tension along the liquid of finite thickness presented. The surface tension changes with the variety of temperature. The Marangoni convection boundary layer problem is solved by an efficient transformation and asymptotic expansion technique. The associated transfer mechanism is analyzed in detail.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2009, 58 (8): 5507-5512.
doi: 10.7498/aps.58.5507
Abstract +
The dielectric tensor for the weakly ionized dust plasma is deduced by solving the Boltzmann’s equation and equation for charging of dust particles. It was proved that longitudinal and transverse dielectric susceptibility are equal for “cold” dust plasma, the theory of electromagnetic property for the weakly ionized dust plasma has been revised.
2009, 58 (8): 5513-5519.
doi: 10.7498/aps.58.5513
Abstract +
In order to distinguish whether thermal mechanism or ionization mechanism dominates shock wave control process by plasma aerodynamic actuation, the transformation rules of wedge oblique shock wave control by arc discharge plasma were theoretically deduced using thermal choking model and ionic sound wave model respectively. If the thermal mechanism dominates shock wave control process, the shock wave start point will shift upward, its shape will not bend and its angle will decrease. However, if the ionization mechanism dominates shock wave control process, the shock wave start point will maintain at the wedge forward apex, its shape will bend as two subsections and its angle of first subsection will increase. Then the wedge oblique shock wave control by arc discharge plasma was studied in a small-scale short-duration supersonic wind tunnel, and the test results showed that the shock wave start point shifted upward 4 mm, its angle decreased 8.6% and its shape did not bend, which demonstrated that the thermal mechanism dominated the shock wave control process by plasma aerodynamic actuation.
2009, 58 (8): 5520-5525.
doi: 10.7498/aps.58.5520
Abstract +
The Plasma in cylindrical Hall anode layer plasma accelerator is simulated by the two-dimensional particle-in-cell method. Monte Carlo collision method is used to simulate the collision among particles. The distribution of electrons an ions, the movement of ion beam and the energy distribution of ions near the exit plane are studied. The result shows that the magnetic field effectively confines the electrons. Electrons are collected in a small region. Due to the special electromagnetic field, the ion beam has a two-peaked distribution. Near the exit plane, the ion energy ranges from 20% to 100% of the anode voltage. The average energy of ions near the exit plane is about 40%—50% of the anode voltage.
2009, 58 (8): 5526-5531.
doi: 10.7498/aps.58.5526
Abstract +
The theoretical and computational model is presented to study the interaction between electrons and microwave in electron cyclotron resonance discharge. The interaction between charged particles and microwave is described by particle-in-cell method, the collisions and the interaction between charged particles and boundaries of system are described by Monte Carlo collision method. The quasi-three-dimensional simulation code is original. And the spatio-temporal evolvements of electron energy and microwave field in ECR discharge is diagnosed.
2009, 58 (8): 5532-5538.
doi: 10.7498/aps.58.5532
Abstract +
The experimental investigation of the current-voltage relationship of homogeneous dielectric barrier discharge （HDBD） was performed in helium and nitrogen at atmospheric pressure, respectively. The experiment results agree with the simulation results. The glow diacharge mode （GDM） and Townsend discharge mode （TDM） can coexist in one multiple current peak （MCP） discharge sequence in atmospheric He MCP discharge. Moreover, the discharge modes of atmospheric HDBD can be discriminated according to the differential conductivity of the current-voltage relationship in the ascent stage of discharge current, in which the current pulse is GDM if the differential conductivity of the current-voltage relationship is negative,and it is TDM if the differential conductivity is positive. Thus we concluded that, the atmospheric N2 HDBD is TDM.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2009, 58 (8): 5539-5545.
doi: 10.7498/aps.58.5539
Abstract +
Ultrafast electron diffraction is a powerful technique for the measurement of transient structure at atomistic level and on ultrafast timescale. Here we report the development of the first such an ultrafast electron diffraction system in China. With careful design and after performing optimization of its spatiotemporal resolution, we are able to detect a sub-milli-ngstrom lattice spacing change with a sub-picosecond temporal resolution in an ultrafast heated Al film. This achievement offers great promise to conduct real-time measurements of transient structures in many dynamical processes that can be initiated by ultrafast optical excitation.
2009, 58 (8): 5546-5551.
doi: 10.7498/aps.58.5546
Abstract +
Ultrafast electron diffraction （UED） is a powerful technique for the measurement of transient structures. Employing our UED system, with its capability of detecting a sub-milli-ngstrom lattice spacing change with a sub-picosecond temporal resolution, we performed the real-time observation of both the coherent phonon and thermal lattice motions of polycrystalline 20 nm Al film, which was excited by femtosecond laser pulses. The obtained coherent oscillation period of ～7.4 ps and temperature rise of ～77 K agree well with those from theoretical calculation. This achievement offers great promise to conduct measurements of transient structures with atomic spatiotemporal resolution in many dynamical processes that can be initiated by ultrafast optical excitation.
2009, 58 (8): 5552-5559.
doi: 10.7498/aps.58.5552
Abstract +
Molecular dynamics simulation was performed to study the diffusion mechanism of n-alkanes C1—C4 in copper（Ⅱ） benzene-1,3,5-tricarboxylate（Cu-BTC）. The self-diffusivities of n-alkanes were calculated, and the diffusion paths of different types in the main and second channels were discussed by means of center of mass probabilities and trajectories. The simulation results indicate that the diffusion of various n-alkanes in Cu-BTC follows their preferential paths, which have distinct diffusion resistance, leading to the difference in diffusivity. The information obtained provides a better understanding of the diffusion of n-alkanes in metal-organic frameworks（MOFs）, as well as to guide the practical applications and future design of new MOFs.
2009, 58 (8): 5560-5566.
doi: 10.7498/aps.58.5560
Abstract +
Based on a full-atomic force field model, molecular dynamics （MD） studies of nCB serials（4-n-alkyl-4′-cyanobiphenyls, n=5—8）were performed under the isothermal-isobaric NPT ensemble. The second and forth rank order parameters and the orientational time correlation function （TCF） were obtained by analyzing the trajectory of MD simulation. Then the correlation time was obtained from the fitting of TCF by a single exponential approximation and the rotational diffusion coefficient （RDC） was computed. The rotational viscosity coefficients （RVC） of nCB（n=5—8） were calculated by Nemtsov-Zakharov and Fialkowski equations, which are based on the statistical-mechanical approach. The odd-even effect of RVC, RDC and the correlation time were discussed separately. Reasonable agreement between the calculated results and the measured data published in literature is obtained, including identical odd-even effect.
2009, 58 (8): 5567-5571.
doi: 10.7498/aps.58.5567
Abstract +
Zinc films with three different thicknesses in the range of 1—3 μm were deposited on a glass substrate under a high magnetic field of 3 T, and compared with the samples prepared without magnetic field. It’s indicated that the all samples in 3 T magnetic field tend to align in the c-axis direction of zinc crystal and those prepared under in 0 T have lower intensity in c-axis with the thickness increasing, as shown by the X-ray diffraction results. From the scanning electron microscope photographs we found that crystals of zinc films prepared in 3 T magnetic field have refined appearance compared to the ones without magnetic field. Thermodynamic theory was used to analyze the coacervation of zinc aggregates. Preliminary analysis shows that the critical nucleation radius r*M and critical nucleation Gibbs free energy ΔG*M in the magnetic field is less than r* and ΔG* in normal state. Thereby, the nucleation intensity of zinc crystals in 3 T magnetic field is larger than that in 0 T and accordingly the refine ment phenomenon ensues.
2009, 58 (8): 5572-5577.
doi: 10.7498/aps.58.5572
Abstract +
There are many factors such as process technologies, dose rates and biased conditions which can affect radiation damage in npn transistors. High- and low-dose-rate radiation response of domestic npn transistors with three kinds of emitter areas were investigated in this article. The influence of emitter area on radiation damage was analyzed. The results show that the degradation of current gain was more severe at low dose rate, i.e. enhanced low-dose-rate sensitivity. Furthermore, radiation damage was more apparent at low current injection. Through the comparison of radiation damage for different emitter areas, it was found that greater perimeter-to-area ratio （P/A） would cause greater normalized excess base current （IB/IB0）. The damage mechanism for npn transistors is explained in detail, and the radiation hardness assurance is explored with respect to the emitter area and operating voltage of npn transistors.
2009, 58 (8): 5578-5584.
doi: 10.7498/aps.58.5578
Abstract +
N-type GaN films bombarded with different highly charged 126Xeq+-ions（9≤q≤30） at room temperature was studied by atomic force microscopy. The experimental results show that when q exceeds the threshold value 18, remarkable swelling turns into obvious erosion in the irradiated area. On the other hand, surface disorder of GaN films strongly depends on the charge state q of ions, incident angle and ion influence, and the damage behavior of films is unrelated to the kinetic energy within the scope of experimental parameters（180 keV≤Ek≤600 keV）.For q=18, the surface morphology of the films almost does hot change at normal incidence, and at incidence angle of 30° relative to the film surface, there appears small-scale swelling in irradiated region and a low step forms between the irradiated and un-irradiated regions. For qq>18, film surface is etched, forming a deep dump with a high step with the increase of ions influence. Unambiguous indentations relevant to the ion influence on the step appear. Furthermore, the step height is proportional to the ion influence approximately and is much higher for tilted incidence than normal incidence.
2009, 58 (8): 5585-5590.
doi: 10.7498/aps.58.5585
Abstract +
The stacked, self-assembled and vertically aligned quantum dot superlattices are fabricated by alternating growth of substrate and epitaxial materials, the stress/strain fields in the buried quantum dots can influence their optical and piezoelectric properties and mechanical stability. The distributions of stresses, strains, hydrostatic strains and biaxial strains in buried strain self-assembled Ge/Si semiconductor quantum dot are investigated based on the theory of anisotropy elasticity and also compared with those of free-standing quantum dot. The sameness and difference of the stresses/strains between the buried and the free-standing quantum dots, and the influence of cap layer on the stress/strain fields in quantum dots are given.
2009, 58 (8): 5591-5597.
doi: 10.7498/aps.58.5591
Abstract +
Plastic deformation is accompanied by the conversion of mechanical energy into heat. The deformation heterogeneity related to the Portevin-Le Chatelier effect （PLC） in Al-Mg alloy during a tensile test at ambient temperature is investigated through temperature field measurement by infrared pyrometry. Under different applied strain rates, three types of stress-strain curves （types A—C） are obtained respectively. Correspondingly, the similarities and differences of temperature-strain curves are compared and interpreted qualitatively. And the spatial propagating characteristics of the three types of PLC deformation bands are observed and analyzed. The temperature rise on the surface of the specimen increases with the increase of strain rate. And the obliquity of PLC band changes either at the ends of the specimen or at the highest temperature area outside the band.
2009, 58 (8): 5598-5603.
doi: 10.7498/aps.58.5598
Abstract +
We have investigated the phase transition, the structural and the thermodynamic properties of AlAs in three crystallographic structures, i.e. B3 （zinc blende）, B1 （rocksalt）, and B8 （nickel arsenide）, at high pressures by using the full-potential linearized muffin-tin orbital scheme in the framework of density functional theory. By taking the common tangent to the energy-volume curves of B3-B8 and B3-B1 structures, we obtain a pressure of about 5.44 GPa for the transition from B3 to B8 and forecast a transition pressure of about 6.46 GPa for the transition from B3 to B1. We have also investigated the structural properties of B8 under high pressures, the results show that c/a ratio keeps nearly constant （～0.2% fluctuation） when V/V0≈0.7—1.05; c/a ratio increases linearly with the value of V/V0 decreasing when V/V0≈0.4—0.7. The relation between relative volume V/V0 and pressure P is obtained by fitting equation of state （EOS）, and the EOS of B8 accords well with measurements. Finally according to quasi-harmonic Debye model the dependences of bulk module B on pressure P and heat capacity CV on temperature T under different pressures are successfully obtained.
2009, 58 (8): 5604-5609.
doi: 10.7498/aps.58.5604
Abstract +
Molecular dynamics simulation is performed to study the phase separation and the glass transition of a binary liquid mixture when the external pressure is increased from 0 to 2.75 GPa. The structure and dynamic characteristics in the glass transition process are calculated. We find that external pressure will promote the phase separation, and make the glass transition temperature increase. The external pressure will make the transition temperature become higher, at which the β relaxation emerge, the time of β relaxation becomes longer, and the diffusibility of the system becomes lower. We first found that in the phase separation liquid microscopic heterogeneity exists during the glass transition.
2009, 58 (8): 5610-5617.
doi: 10.7498/aps.58.5610
Abstract +
Molecular dynamics simulation employing an embedded-atom-method potential is performed to investigate the body-centered cubic（bcc）-hexagonal close-packed（hcp） structural transition in single crystal iron induced by isothermal compression along ［001］ divection. Above the critical strain of transition, homogeneous nucleation of hcp phase appears and grows into flakes along the （011） face. The elastic constants C31 and C32 harden during the compression in bcc phase, while C33 undergoes a softening prior to the transition；all the elastic constants increase rapidly with compression after the system entering the hcp phase. Increasing temperature can weaken the hardening and softening process of C33, but affect the stress threshold of transition only weakly. Hcp twins are formed at 300 K, leading to the shear of crystal latlice. For the mixed phase, the potential of hcp phase is greater than that of bcc phase, hcp phase shows an over-relaxation of the stress, and the longitudinal partial stress keeps linearly decreasing with hcp mass fraction throughout the transition.
2009, 58 (8): 5618-5623.
doi: 10.7498/aps.58.5618
Abstract +
The distribution of strain energy and the strain relaxation degree as functions of the aspect ratio in different-shaped quantum dots were studied using finite element method. The impact on strain relaxation, originating from the shape and the inter-island distance, was also quantitatively analyzed. The results indicate that when ignoring the surface energy, the relaxation degree increases with the increase of the aspect ratio regardless of the shape, among which the truncated pyramid quantum dots tend to become steady earlier than others. With the increase of the inter-island distance, the strain energy in the dots decreases, especially in the cubic dots. A good reference is provided by the relaxation degree for controlling the shape of quantum dots.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2009, 58 (8): 5624-5631.
doi: 10.7498/aps.58.5624
Abstract +
The lattice structure, band structure, density of state and optical properties of pure ZnO, Al, Ni doped ZnO and Al-Ni codoped ZnO were calculated by first-principles method respectively. The results reveal that the imaginary part of the dielectric function of pure ZnO and Al-ZnO, Ni-ZnO, （Al,Ni）-ZnO have an obvious difference in the low energy range, but are similar in the high energy range. As to the optical properties, the absorption coefficient and reflectivity of Al-ZnO are much smaller than Ni-ZnO, which means a higher transmissivity in the visible light range. Especially, compared with Al, Ni doped ZnO, （Al,Ni）-ZnO has a significant change in the optical properties.
2009, 58 (8): 5632-5639.
doi: 10.7498/aps.58.5632
Abstract +
Using the full-potential linearized augmented plane wave method, we studied the electronic energy band structures of β-pyrochlore superconductors AOs2O6（A=K, Rb, Cs）. The values of electronic density of states at the Fermi level have been significant enhanced for the three compounds due to taking to consideration of the spin-orbit coupling and the on-site Coulomb interactions. The electronic Coulomb correlation constants λc=1.55, 1.12 and 0.73 were obtained. Analyzing the electronic mass enhancement parameters obtained from the experimental data and the calculated electronic density of states, we deduced the electron-phonon coupling constants λep=1.56, 0.78 and 1.08 for the three compounds, respectively. It is suggested that KOs2O6 is a strongly-correlated and strongly electron-phonon coupled system, while RbOs2O6 and CsOs2O6 are medium in these properties.
2009, 58 (8): 5640-5644.
doi: 10.7498/aps.58.5640
Abstract +
By using first-principles electronic structure and transport calculations based upon the density functional theory, we have studied the divacancy-defect effect of armchair graphene nanoribbons with zigzag edges. It is shown that the existence of 585 divacancy defects do not change the metallic characteristics of graphene nanoribbon, while changing the energy band structures near the Fermi energy level. Moreover, the spatial orientations of the divacancy defects have obvious effect on the transport properties of armchair graphene nanoribbons: for armchair graphene nanoribbons with odd width, slanting divacancies defects weaken conducting performace of graphene nanoribbons, while graphene nanoribbons with vertical divacancies defects basically remain linear I-V characteristics and little decrease in conducting capacity；For armchair graphene nanoribbons with even width, inclined divacancy defects increase the conducting property of graphene nanoribbons, while graphene nanoribbons with vertical divacancy defects have I-V characteristics of the perfect graphene nanoribbons.
2009, 58 (8): 5645-5652.
doi: 10.7498/aps.58.5645
Abstract +
Tc and its nitrides TcN, TcN2, TcN3 and TcN4 are studied using the projector augmented wave method based on the density functional theory. The elastic properties, electronic structure and atomic populations, theoretical hardness of Tc nitrides are calculated and analyzed. The calculation results show that with increasing N content by incorporated into transition-metal Tc, the solid structures of the material change into laminar, and its elastic moduli do not monotonically increase. It reveals that TcN with solid structure has the largest bulk modulus and the smallest shear modulus, while TcN3 with layered structure has the largest shear modulus and the smallest bulk modulus, and TcN4 is found to have the largest theoretical hardness by means of chemical bonding analysis. It can be seen that the bulk modulus depends mainly on electrons per atomic volume and mass density, while covalent structure of chemical bonding and N content make more contribution to theoretical hardness, and appropriate content N—N covalent bonding in the same direction tends to increase the shear modulus of nitrides.
2009, 58 (8): 5653-5660.
doi: 10.7498/aps.58.5653
Abstract +
The electronic properties of multiferroic BaCoF4 with ferroelectric antiferromagnetic phase and paraelectric phase are calculated using density functional theory with spin-polarized generalized-gradient approximation and plane wave pseudopotentials. It is found that anti-ferromagneticism probably favors to the ferroelectric stability at low temperature and the ion bond interaction is the main interation in the BaCoF4 system due to the strong electronegativity of F atoms. As to the CoF6 octahedron, there is entirely ionic bond between Co ion and F（2） or F（3） ions （bc plane）, but weak covalent bond between Co ion and F（1） ion and still weaker for F（4） ion. The ferroelectric distortion is only induced by relative displacement of Ba ion and F ion along c axis, and F（4） displacement contributes least to ferroelectric phase transformation . In addition, the energy of s and p orbitals of F（2） or F（3） ion is lower in the ferroelectric phase than in the centor-symmetric phase and the covalence character of F（1） ion, whose contribution to the displacement is the largest, is almost lost, which stabilizes the structure of the ferroelectric system.
2009, 58 (8): 5661-5666.
doi: 10.7498/aps.58.5661
Abstract +
With a large-scale Monte Carlo simulation, non-equilibrium dynamics of the two-dimensional fully frustrated XY model is investigated. We tackle the Kosterlitz-Thouless phase transition. Starting from an ordered initial state, we study the dynamic evolution of the magnetization as well as a specifically defined Binder cumulant. From the dynamic scaling ansatz, we extract the correlating time of the dynamics and the spatial correlation length of the equilibrium state. The dynamic exponent z is determined with relatively high accuracy. Especially, we suggest and demonstrate how one may directly measure the dynamic exponent z above TKT from the scaling fit of the Binder cumulant. These results indicate that the dynamic exponent z fluctuates around z=2, and this is consistent with that observed at temperatures below the transition temperature TKT.
2009, 58 (8): 5667-5672.
doi: 10.7498/aps.58.5667
Abstract +
Molecular dynamics simulations were used to study the plastic behavior of copper monocrystal under shock loading and unloading. We obtained the Hugoniot relation for both ［001］ and ［111］ orientations and the melting pressure, which turns out to be in agreement with experiment. The results showed that higher initial temperature increases the possibility for the dislocation loop generate and expansion. The release behavior after the shock wave reaching the back of the sample was also analyzed. We found that the release behavior was quasi-elastic and most of the stacking-fault networks disappear behind the release wave along ［001］ orientation while few stacking faults disappear along ［111］ orientation.
2009, 58 (8): 5673-5678.
doi: 10.7498/aps.58.5673
Abstract +
Using Sherrington-Kirkpatric （SK） model we investigated the nonequilibrum properties of spin glass by dynamical Monte Carlo simulation. Our results revealed that the susceptibility in the weak field shows a cusp near the glass transition, while the specific heat shows a more broad transition. The aging and zero-field colding memory effect is also reproduced in our simulation, and the phenomena can be explained by energy relaxation. Our simulation also reveals that homogeneous SK model can not reproduce the exchange bias effect observed in the experiment.
2009, 58 (8): 5679-5684.
doi: 10.7498/aps.58.5679
Abstract +
The propagation of high-order harmonics with attosecond ionization gate is simulated with three-dimensional propagation equation. After 0.5 mm propagation distance， the short trajectories in the harmonics， which are generated by two 5 fs laser pulses centered at 800 and 400 nm， are selected and a highly efficient smooth supercontinuum with the bandwidth of about 80 eV is obtained in the plateau， because of the phase matching. Simulation results of different propagation distances show the broadband supercontinuum can last steadily. A highly efficient pure isolated attosecond pulse with the duration of about 135 as can be generated by directly filtering out of the harmonics from 60—90 eV in the supercontinuum， and a single 50 as pulse can be generated， which facilitates the experimental implementation for the isolated attosecond pulse generation.
2009, 58 (8): 5685-5692.
doi: 10.7498/aps.58.5685
Abstract +
When the Rashba spin orbit interaction is available， the spin polarization along the spin quantization axis is derived form the spin density matrix， with the help of current and shot noise formula based on the scattering theory. Charge shot noise and spin polarization are derived in the case of single channel， and there exists no relation between them. Due to the difficulty in derivation in multichannel， at least two channels， the nonequilibrium Greens function technique is used to numerically compute the scattering matrix， the current and the shot noise of clean two-dimensional electron gas governed by the spin orbit coupling effect. The relation between charge shot noise and spin polarization is investigated separately in conditions of variable bias， spin orbit interaction coefficient and conductor length， which demonstrates that the establishment of quantitive relation may provide a new way to realize all-electrical detection of spin polarization.
2009, 58 (8): 5693-5699.
doi: 10.7498/aps.58.5693
Abstract +
Si（001） chips were implanted by Zn ions of 40 keV with different ion dosages and are annealed in air at different temperatures. Atomic force microcopy， transmission electron microcopy， X-ray diffraction and electron probe are applied to study the microstructure， morphology and chemical composition of the chips, either as-implanted or annealed at different temperatures. It was found that the as-implanted Zn atoms aggregate into clusters scattering about 35 nm beneath the surface of the chips. During the annealing process， Zn atoms are found to migrate towards the surface of the chips and aggregate into nanoparticles at the interface between the amorphous SiO2 layer and polycrystal line Si layer. Annealing temperature was found to be the crucial factor controlling the formation of ZnO nanoparticles. ZnO nanoparticles begin to appear at about 400 ℃ and the diffraction intensity of ZnO becomes strong while the diffraction intensity of metallic Zn weakens with increasing annealing temperature. At the annealing temperature of 800 ℃， Zn2SiO4 phase was observed due to the reaction between ZnO and SiO2 or Si.
2009, 58 (8): 5700-5704.
doi: 10.7498/aps.58.5700
Abstract +
A study of the capacitance-voltage characteristics of GaN light emitting diode（LED） with a view to reveal its degradation mechanism is presented in this article by forward alternating current small-signal method. Combined with series resistance， ideality factor and tunneling current measurement， the properties of negative capacitance and conductance are discussed. The threshold voltage of negative capacitance during degradation is qualitatively analysed. The decreased negative capacitance during aging may be due to the decrease in the effective acceptor concentration and radiative recombination rate， and the increase of defects and non-radiative recombination centers resulting in the enhanced capture effect of carriers. The tunneling effect leads to the obvious increase of conductance under reversed and low-forward bias voltage. The decreased conductance during aging may be due to the series resistance. On the basis of the capacitance-voltage characteristic， light output， current-voltage curves with the degradation mechanism of LED， the characteristics of negative capacitance and conductance can be used as important evidence for the degradation analysis of LED as proved by experiment and theory.
2009, 58 (8): 5705-5708.
doi: 10.7498/aps.58.5705
Abstract +
Nonpolar a-plane （1120） GaN has been grown on r-plane （1102） sapphire by metal-orgamic chemical vapor deposition. The crystal quality has been greatly improved by using the AlGaN multiple-quantum-well interlayers. The surface morphology and the crystal quality were investigated by high resolution X-ray diffraction and atomic force microscopy.The triangular pits were eliminated completely. The precession of the X-ray diffraction symmetric reflection peak full with width at half maximum of （1120） is 680″.
2009, 58 (8): 5709-5715.
doi: 10.7498/aps.58.5709
Abstract +
The electron transport properties of a TaSi3 cluster sandwiched between two Al electrodes are investigated by using the first-principle-analysis. The results show that transport properties under low bias are determined by the 8th and 9th state of MPSH. Equilibrium conductance of TaSi3 cluster is very sensitive to the cluster-electrode distance. The oscillation form of conductance becomes obviously when the distance is greater than 0.35 nm. While the equilibrium conductance decreases quickly when the distance is less than 0.35 nm. The asymmetric current-voltage characteristics occur when the bias is in the range of -1 to 1 V and negative differential resistance is observed when the bias changes from 0.3 to 0.4 V.
2009, 58 (8): 5716-5720.
doi: 10.7498/aps.58.5716
Abstract +
The density of states （DOS） above Fermi level of hydrogenated microcrystalline silicon （μc-Si:H） films is correlated to the material microstructure. We use Raman scattering and infrared absorption spectra to characterize the structure of the films made with different hydrogen dilution ratios. The DOS of the films is examined by modulated photocurrent measurement. The results have been accounted for in the framework of a three-phase model comprised of amorphous and crystalline components， with the grain boundary as the third phase. We observed that the DOS increases monotonically as the grain boundary volume fractions fgb is increased， which indicates a positive correlation between the DOS and the grain boundary volume fraction.
2009, 58 (8): 5721-5725.
doi: 10.7498/aps.58.5721
Abstract +
In this paper， the dielectric spectra of ZnO varistor ceramic were measured by Novocontrol wide band dielectric spectrometer in the temperature range of -100 ℃—20 ℃ and frequency range of 10-2—106 Hz. It is found that the activation energy is 0.26 and 0.36 eV， respectively. After careful analysis of the measurement conditions， quantitative calculation results and other phenomena， three relaxations of cathode electron injection， dipole polarization and interface polarization are discarded. Thermally stimulated current（TSC） has three peaks， of which the higher temperature peak comes from thermionic polarization introduced by TSC itself， and other two peaks correspond to the characteristic loss peaks in dielectric spectra. Therefore， it is concluded that the characteristic loss peaks of ZnO ceramics originate from electronic relaxation of intrinsic defects.
2009, 58 (8): 5726-5729.
doi: 10.7498/aps.58.5726
Abstract +
The electron transport property of a graphene nanoribbon （GNR） quantum dot in a dilution refrigerator at temperature 20 mK is studied. The Coulomb blockade diamonds and excited energy levels of the quantum dot are clearly observed. It is found that the statistical distribution of the spacing between the nearest-neighboring Coulomb blockade peaks and that of the individual peak height are consistent respectively with the typical Wigner-Dyson distribution and Porter-Thomas distribution as prescribed by the random matrix theory for a quantum chaotic system. Thus， our results demonstrate that the GNR quantum dot is a quantum chaotic system at low temperatures. The possible origin of this phenomenon is also discussed.
2009, 58 (8): 5730-5735.
doi: 10.7498/aps.58.5730
Abstract +
Current-voltage （I-V） and the electrical pulse induced resistive switching （EPIR）characteristics of metal/Pr0.7Ca0.3MnO3/Pt sandwich structures, where the metal is a point-contact Pt or W electrode, are investigated by using a home-made conductive AFM. The structures with both Pt and W tips behaves as rather reproducible bipolar resistive switching （RS） with an RS ratio larger than 100. However W/Pr0.7Ca0.3MnO3/Pt structures shows EPIR under a compliance current of 10 nA and reproducible bipolar RS under a compliance current of 100 pA，which is about three orders lower than the published values and shows the potential applications in low power memory device. Further analysis of I-V characteristics at different positions of sample，under different current compliances， with different contact areas of device suggests that an enhanced oxygen vacancy migration due to point contact induced local intense electrical field， gives rise to a stable RS for Pt/Pr0.7Ca0.3MnO3/Pt devices.
2009, 58 (8): 5736-5743.
doi: 10.7498/aps.58.5736
Abstract +
ZnO thin films with strong c-axis preferred orientation have been deposited on Ti buffered Si （111） substrate by reactive ratio-frequency magnetron sputtering. With X-ray diffraction analysis and photoluminescence （PL） measurement， the structure， residual stress and PL emission properties of Ti-buffered ZnO films are studied and effects of Ti-layer thickness and annealing temperature are discussed. It is found that the PL properties of the films are improved after the introduction of Ti buffer layer and an optimized thickness of Ti buffer layer is given. Annealing treatment can improve the properties of Si（111）/Ti-buffer/ZnO films further. The residual stress is found to be an important factor influencing the emission properties of ultraviolet light. It is helpful for the emission of ultraviolet light if the films have a small residual stress. The residual stress can also change the transition energy of excitons. With the increase of annealing temperature， the increase of tension strain in films reduces the band gap of ZnO， hence results in the shift of the excitonic peak to lower energy.
2009, 58 (8): 5744-5749.
doi: 10.7498/aps.58.5744
Abstract +
We have carried out nuclear magnetic resonance experiments in microtesla range.The free induction signal was recorded using a high Tc dc-superconducting quahtum interference device.The measurement was performed in a home-made magnetic shield room.Resonance spectra of 1H from a sample of 15 ml tap water was obtained in the field range from 7—70 μT，corresponding to resonance frequency of 300—3000 Hz，The signal to noise ratio in a single-shot measurement is around 4，which could be increased to about 40 after 100 times averaging.The effect of residual magnetic field in the shield room， prepolarization time and data acquisition time was investigated.
2009, 58 (8): 5750-5756.
doi: 10.7498/aps.58.5750
Abstract +
Using the resistively shunted junction model，we study magnetic-field-induced phase transitions of the vortex glass states in a disordered two-dimensional Josephson junction array. From calculated results of the vorticity of the vortex system and the voltage noise generated by current-driven vortices as a function of the magnetic field， the following conclusions can be drawn. 1） With decreasing the magnetic field， the vortex system exhibits a reentrant transition from a vortex glass phase to a pinned dilute vortex liquid. 2） There is a peak of voltage noise driven by the current in the vortex glass state. It follows that the system is in a sub-stable state as a result of the competition between ordered and disordered interactions， which results in peak effects of the critical current. The calculated results are consistent with recent experimental reports by Okuma and Kamada on disordered and superconducting MoxSi1-x films.
2009, 58 (8): 5757-5762.
doi: 10.7498/aps.58.5757
Abstract +
The dynamical reversal of rectangular CoFe nanodot with initial C-state subject to square magnetic field pulse has been studied by using the micromagnetic simulations. It was found that the reversals proceeded through different modes as the pulse amplitude is increased. For the weak magnetic field pulse， the magnetization reversal is realized through the motion of the end domain walls toward the central part of magnet and， subsequently， the formation and motion of a single vortex. As the strength of magnetic field pulse become stronger， the reversal is accomplished through the motion of the end domain walls plus the formation and motion of two vortex structure. For much stronger field pulse the reversal process is the motion of the end domain walls and subsequent formation and annihilation of several vortices. The dependence of reversal time on the field pulse amplitude displays oscillating characteristic due to the transitions of reversal modes with the field strength.
2009, 58 (8): 5763-5767.
doi: 10.7498/aps.58.5763
Abstract +
Mn doped ZnO films with and without N codoping have been fabricated by oxidative annealing of sputtered Zn:Mn and Zn2N3:Mn films on silicon substrates in flowing O2 ambient. It was found that the ZnO:Mn films show very weak ferromagnetic behavior, while for those with N codoping, significant ferromagnetism with a moderate saturation magnetization of about 0.23—0.61 μB per Mn2+ ion was observed at room temperature. It suggests that significant ferromagnetism in ZnO:Mn films could be activated by N codoping. The results indicate that holes are favorable for ferromagnetic ordering of the doped Mn2+ ions in ZnO, in agreement with the recent theoretical studies.
2009, 58 (8): 5768-5772.
doi: 10.7498/aps.58.5768
Abstract +
The effects of Al substitution for Fe on the structure， the magnetism， the magnetostriction， the anisotropy and the spin reorientation of a series of Tb0.3Dy0.6Pr0.1（Fe1-xAlx）1.95 alloys （x=0.05，0.1，0.15，0.2，0.25，0.3） at room temperature are investigated. It is found that the compositions of Tb0.3Dy0.6Pr0.1（Fe1-xAlx）1.95 substantially retain MgCu2-type C-15 cubic Laves phase structure when xx=0.2 and the mixture phase increases with x increasing. The lattice constants increase linearly， Curie temperature decreases gradually and the coercive force decreases sharply with x increasing. The vibrating sample magnetormeter measurements show that the relationship between magnetization and substitution x is rather ambiguous. The magnetostriction of the Tb0.3Dy0.6Pr0.1（Fe1-xAlx）1.95 alloys decreases drastically with x increasing and the magnetostrictive effect disappears when x>0.15. However， a small amount of Al substitution is beneficial to a decrease in magnetocrystalline anisotropy. The analysis of the Mssbauer spectra indicates that the easy magnetization direction in the {110} plane deviates slightly from the main axis of symmetry with the increase of Al concentration x，namely spin reorientation takes place, thereby leading to the change of macroscopical magnetism and magetostriction.
2009, 58 (8): 5773-5777.
doi: 10.7498/aps.58.5773
Abstract +
The effect of aging temperature on the magnetic properties has been studied. The magnetic measurement results indicate that the remanence decreases with increasing aging temperature Ta，and the coercivity has a maximum value at Ta=690℃. After the first aging， the degree of ordering increases with Ta， which shows that the grain size of ordered phase also increases. This is the reason for the change of the coercivity. After the second aging， the microstructure does not change and the anisotropy field of ordered phase increases， which leads to an increase in the coercivity.
2009, 58 (8): 5778-5783.
doi: 10.7498/aps.58.5778
Abstract +
We first calculated the negative refractive index on the solid Er3+：YAlO3 crystal with a Λ-type four-level scheme proposed for atomic vapour by Thommen and Mandel and Kastel using quantum interference and electromagnetically induced transparency （EIT）. It is shown that the frequency band with the negative index is wider （～1 MHz） than that reported previously. Usually，Im［n］ is always positive，corresponding to absorption，and the ratio of refraction to absorption，|Re［n］/Im［n］|，is only of the order of unity. In our case the ratio |Re［n］/Im［n］|=4.6，which indicates that the absorption has been well suppressed by EIT approach. The negative Im［n］ may be related to the stimulated emission of 4I13/2→4I15/2 of rare earth ion Er3+. It indicates that the rare earth ion doped material with abundant energy levels and various electric and magnetic transitions is an outstanding and practical candidate for electromagnetically induced negative refractive index material.
2009, 58 (8): 5784-5789.
doi: 10.7498/aps.58.5784
Abstract +
By successfully fabricating dense Ba0.70Sr0.30TiO3 ceramics in the size range of 0.1—10 μm，the grain size dependence of the Curie temperature TC，dielectric constant in ferroelectric phase εF and in peak εM were systematically investigated. The results show that TC will not decrease considerably until the grain size becomes small enough. This can be explained by Buesseums internal stress model. As the grain size increases，εF increases first and decreases afterward，which can be interpreted by Shaikhs series-parallel model. The main active factors are internal stress，domain and grain boundary. On the other hand，if the grain size is small，εM increases first and decreases afterward as the grain size increases. If the grain size is bigger，εM increases slightly. This can be explained by the diffuse phase transition theory and series-paralled model. The main active factors are grain boundary，internal stress，for small grain sizes, and domain and grain boundary for bigger grain sizes，respectively.
2009, 58 (8): 5790-5797.
doi: 10.7498/aps.58.5790
Abstract +
（Sr1-3x/2Ax/2Ndx）Bi2Nb2O9 （x=0，0.05，0.1 and 0.2） ceramics are prepared by traditional solid-reaction method， and effects of substitutions of Nd3+ for Sr2+ on the properties of SrBi2Nb2O9 ceramics and their mechanism are investigated. The results show that the temperature spectra of dielectric constant and loss for these Nd-doped samples are obviously characteristic of ion relaxor polarization. The partial substitution of Nd3+ for Sr2+ leads to the slight reduction in remanent polarization （Pr） and the increase in piezoelectric coefficient d33， which can be attributed to the increase in dielectric constant of samples according to ferroelectric thermodynamics theory. Raman spectrum shows that no change in the Curie temperature （TC） of （Sr1-3x/2Ax/2Ndx） Bi2Nb2O9 samples is observed， which might result from no appreciable variation in NbO6 octahedron distortion. For （Sr1-3x/2Ax/2Ndx） Bi2Nb2O9 ceramics， the substitution of Nd3+ for Sr2+ increases permittivity εr， piezoelectric coefficient d33，and electromechanical coupling coefficient Kp and reduces mechanical quality factor Qm, but it does not change temperature coefficient of harmonic frequency value.
2009, 58 (8): 5798-5804.
doi: 10.7498/aps.58.5798
Abstract +
Based on the energy-level structure of quasi-three-level Yb ion，an amplified spontaneous emission model in pumping process is set up. The stored energy density distribution and the energy that can be extracted in Yb：YAG crystal are calculated. Our calculation，which is based on the laser rate equations and the theory of anglular spectrum propagation，can simulate time-resolved evolution of the spatial intensity distribution and M2 factor of the laser pulses. At the same time，an experiment in laser diode-pumped Q-switched rep-rated Yb：YAG disk laser was performed. This model was proved to be correct by comparing the results of the experiment and calculation，which will be helpful in designing active Q-switched solid laser.
2009, 58 (8): 5805-5809.
doi: 10.7498/aps.58.5805
Abstract +
Using terahertz wave time-domain spectroscopy system，we measured the terahertz wave time-domain spectroscopy of the corn oil between 0.2 and 1.4 THz. Taking care of the effect of the utensil，we employed a novel iterative algorithm to analyze the optical properties of the corn oil. In contrast to most of the published experiments，the iterative algorithm further improves the accuracy of the parameter extraction, and we are able to reliably investigate the samples. The results show that the novel calculating method provides a guidance for detecting the quality of seed oils.
2009, 58 (8): 5810-5815.
doi: 10.7498/aps.58.5810
Abstract +
The transient thermally induced nonlinearity of a novel metal cluster polymer solution was investigated by using the phase object （PO） Z-scan technique which can distinguish the transient thermal effect from the third-order nonlinear refraction easily in the nanosecond regime. We studied the optical nonlinearity of the ［Tp*W（μ3-S）3Cu3Py3（μ3-Br）］（PF6）/DMF solution using both PO Z-scan and conventional Z-scan techniques with the laser pulse width of 8 ns under different input pulses energe. Results of numerical simulations and solution of simultaneously acoustic wave equation and the diffusion equation of conduction of heat agree well with the experimental results，which shows that the nonlinear refraction of the sample is due to transient thermally induced nonlinear refraction.
2009, 58 (8): 5816-5820.
doi: 10.7498/aps.58.5816
Abstract +
Ag-Au alloys/SiO2 composite thin films （nAg/nAu=1∶0，2∶1，1∶2，0∶1） were prepared with AgNO3，HAuCl4 and tetraethyl orthosilicate by means of sol-gel method and deoxidized using ultraviolet radiation. The results of scanning electron microscope and X-ray diffraction indicate that the composite thin films are very homogeneous and the size of the nanoparticles is about 10 nm. The results of the optical absorption spectra show that, with the increasing of nAg/nAu, the absorption peaks close to 430 nm, which belongs to the surface plasmon resonance absorption peak of Ag nanoparticles, are red-shifted to the viscinity of 605 and 880 nm, which belong to the surface plasmon resonance absorption peaks of Au nanoparticles. The surface plasma resonance absorption peaks around 515 and 730, 550 and 730 nm, belong to the samples （nAg∶nAu＝2∶1，1∶2），respectively，showing the formation of Ag-Au alloys dispersed in the SiO2.
2009, 58 (8): 5821-5825.
doi: 10.7498/aps.58.5821
Abstract +
As a wide band-gap semiconductor，SnO2 films have attracted much attention due to their novel optical and electronic properties. It has been reported that the physical properties can be quite different when the size of SnO2 is reduced to nanometer scale due to the large surface-to-volume ratio and the quantum size effects，which may be applied in many kinds of devices, such as solar cells sensors. etc. It is interesting to study the synthesis of SnO2 nanoparticles and their physics properties. In the present work，a soft chemical technique was used to prepare SnO2 nanoparticles with uniform size and good crystallization in alkalescent solution. The surfactant was added during the preparation process to control the growth and agglomeration of crystal precipitates in the solution. X-ray diffraction spectra and transmission electron microscopy were used to characterize the structures of SnO2 nanoparticles before and after thermal annealing. It was found that the nanocrystalline SnO2 particles can be formed by the present technique and the size is about 4 nm with good crystallinity. With changing the preparation parameters，the size of nanocrystalline SnO2 particles is changed. Post thermal annealing at various temperatures （400—1000 ℃） can promote the crystallization and the size of formed particles was increased with increasing annealing temperature. Optical absorption spectra were used to see the change of the optical properties for samples prepared under different conditions. It was found that the optical band gap is enlarged in nanocrystalline SnO2 particles compared with its bulk counterpart, which can be attributed to the quantum confinement effect. The red-shift of the optical band gap with the particle size supported the quantum size effect. A broad photoluminescence band in the range of 350—750 nm can be detected in the annealed samples and the intensity was significantly enhanced after the thermal annealing. The luminescence peak energy was kept at 390 nm which was independent of the particle size. This luminescence band can be ascribed to the luminescence center associated with the oxygen vacancies on the SnO2 particle surface.
2009, 58 (8): 5826-5830.
doi: 10.7498/aps.58.5826
Abstract +
The BaAl2Si2O8：Eu2+ blue phosphor was synthesized by one-step calcination of the precursor prepared by chemical coprecipitation. X-ray diffraction and fluorescence spectrophotometry were used to investigate the structural and luminescent properties of the BaAl2Si2O8：Eu2+ blue phosphor. The result showed the BaAl2Si2O8：Eu2+ synthesized by the process was pure. The excitation spectrum of the BaAl2Si2O8：Eu2+ blue phosphor extended from 240 to 410 nm， and the peak appeared around 320 nm，thus the phosphor can be excited effectively by InGaN chip in the range of 350—410 nm. The emission spectrum excited by 365 nm showed a characteristic wide band with the peak at about 465 nm. The emission spectrum intensity firstly increased with Eu2+ doping concentration increasing，then decreased. The strongest emission intensity was obtained when Eu2+ doping concentration reached 3.5mol%. The concentration quenching mechanism was caused by the exchange interaction of Eu2+ ions according to the Dexter theory.
2009, 58 (8): 5831-5835.
doi: 10.7498/aps.58.5831
Abstract +
LiBaBO3：Ce3+ phosphor was prepared by solid-state method，and its luminescence characteristics were investigated. LiBaBO3：Ce3+ phosphor shows one asymmetrical band at 440 nm. Monitored at 440 nm，the excitation spectrum has a broad band at 370 nm. The crystallographic sites of Ce3+ in LiBaBO3 were calculated by van Uitert formula，the results show that the emission band centered at 438 nm originates from the Ce3+ center of the nine compounds，and at 469 nm originates from the Ce3+ center of the eight compounds. The effect of Ce3+ concentration on luminescent intensity of LiBaBO3：Ce3+ phosphor was investigated，the result shows that the luminescent intensity firstly increases with increasing Ce3+ concentration，and reaches the maximal value at 3mol% Ce3+, then decreases. The concentration quenching mechanism is the dipole-dipole interaction by Dexter theory. Under the condition of doping charge compensator Li+，Na+ or K+，the emission intensity of LiBaBO3：Ce3+ was all heightened. The relative emission spectrum of the InGaN-based LiBaBO3：Ce3+ light emitting diode（LED） was investigated，and the CIE chromaticity of InGaN-based LiBaBO3：Ce3+ LED is （x=0.291，y=0.297），and shows the blue white emission.
2009, 58 (8): 5836-5841.
doi: 10.7498/aps.58.5836
Abstract +
Single-crystalline Zn1-xMgxO thin films with c-axis orientation have been deposited on Si（100） substrates by pulsed laser deposition. The photoluminescence characteristics of the films are studied by fluorescence spectrometer. The results show that the ultraviolet emission peak has a blue shift and the intensity weakens with the increasing content of Mg. At the same time，the intensity of defect emission increases with increasing content of Mg. Some local bound states were introduced by Mg-doping. For the samples grown in oxygen atmosphere，the results show that both the ultraviolet emission peak and green emission peak are enhanced，but the value of R reduced and the ultraviolet emission peak has a red shift. The study of green-emitting mechanism indicated that the green emission band mainly depends on zinc vacancy，substitutional O on the zinc site （OZn） and interstitial oxygen vacancies （Oi）. Green emission band is composed of many defect-peaks，and its movement as a whole mobile is due to the change of relative intensities of individual defect-peaks.
2009, 58 (8): 5842-5846.
doi: 10.7498/aps.58.5842
Abstract +
The modulation transfer function （MTF） of transmission-mode exponential-doping photocathodes has been solved from the 2-dimensional continuity equations. According to the MTF equation，we calculated and analyzed the theoretical resolution characteristic of the exponential-doping photocathodes. The calculated results show that，compared with the uniform-doping photocathodes，the exponential-doping structure can increase the resolution of photocathodes evidently. For the spatial frequency franges between 100 and 400 lp/mm，the increase of resolution is more pronounced. Let f=200 lp/mm，the resolution of exponential-doping photocathodes generally increases by 20%—50%. The resolution improvement of exponential-doping photocathodes，as well as the quantum efficiency improvement，is also attributed to the built-in electric field.
2009, 58 (8): 5847-5851.
doi: 10.7498/aps.58.5847
Abstract +
The photocurrent curves during either Cs or Cs/O activation of GaN photocathode were tested by using dedicated experimental system for activation and evaluation of negative electron affinity （NEA） photocathode. Aiming at explaining the formation of NEA property of GaN photocathode and according to the rule of photocurrent change during activation period and the surface model of a fully activated photocathode，the activation mechanism for NEA GaN photocathode was studied. The experiment results show that：the obvious NEA property is be induced in GaN photocathode mainly due to the activation by Cs. The increase extent of photocurrent is not large after introducing O during Cs/O activation process for GaN photocathode. The NEA property formation reasons of GaN photocathode after being activated successfully can be well explained using the double dipole layer model ［GaN（Mg）：Cs］：O—Cs.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2009, 58 (8): 5852-5856.
doi: 10.7498/aps.58.5852
Abstract +
A pseudo 2-2 type multilayered magnetoelectric composite of Pb（Zr，Ti）O3 slice array （with base） and Terfenol-D/epoxy medium was prepared by the dice-and-fill technique. Obvious magnetoelectric anisotropy was observed in the pseudo 2-2 type composite，e.g.，the magnetoelectric coefficients at low frequency were about （1.62—1.75）×105 V·m-1T-1 in the in-plane directions，but only 1.3×104 V·m-1T-1 in the out-of-plane direction. The pseudo 2-2 type composite can detect AC magnetic field as small as 10-9 T under resonance drive. The magnetoelectric anisotropy and high magnetic field sensitivity make such pseudo 2-2 type multilayered magnetoelectric composites detect both direction and magnitude of magnetic field，which is of technological importance for applications as magnetic sensors.
2009, 58 (8): 5857-5863.
doi: 10.7498/aps.58.5857
Abstract +
Vibration driven compartmentalized granular gas is capable of clustering in one of the compartments，which is an effect called Maxwell demon phenomenon. One of the keys for understanding the Maxwell demon phenomenon is establishing a flux model. Based on granular hydrodynamics，Eggers deduced a simplified flux model which has been successfully used in describing the spontaneous symmetry breaking of granular gas in the compartmentalized system，but so far has not been able to predict the oscillation phenomenon of bidisperse granular gas. Experimentally studying the flux of mono-disperse granular gas in one compartment，we modify the flux model of Eggers to fit the experimental results. The modified flux model can be extended to the bidisperse granular case，and the oscillatory state can be well obtained by this modified flux model.
2009, 58 (8): 5864-5870.
doi: 10.7498/aps.58.5864
Abstract +
The colloidal system consisting of two large spheres confined in a cylindrical pipeline and immersed in the sea of small spheres is considered in this paper. In this system，the depletion interactions between the large spheres and the wall of cylindrical pipeline are studied by Monte Carlo simulation and acceptance ratio method. The numerical results show that the depletion forces are coupled，i.e.，the depletion force between the two large spheres is strengthened by the depletion force between one of the large spheres and the wall of the cylindrical，meanwhile，the depletion force between one large sphere and the wall of the cylindrical is also strengthened by the depletion force of the two large spheres. Furthermore，through the numerical results，we found that the coupling effect is increased with the decrease of the diameter of the cylindrical pipeline.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2009, 58 (8): 5871-5878.
doi: 10.7498/aps.58.5871
Abstract +
According to the previous theory of the motion of charged particles in the Earths magnetic field， we derive the spatial region to which energetic particles around the Earth can extend. Using an empirical model for radiative debris produced by the high altitude nuclear detonation （HAND），we investigate the primary region where HAND-induced artificial radiation belt can form.Finally，in terms of the fission property of HAND and the characteristic features of energetic electron distributions in the natural belts，the electron flux within the artificial radiation belt is calculated and the preliminary study of its dependences on explosion latitude，altitude，and equivalence are carried out quantitatively. The numerical results show that，under certain circumstances，the HAND with 0.1—1 Mt TNT explosion equivalence can be expected to produce an artificial radiation belt near the Earth with the flux 3—4 orders of magnitude higher than the natural ones. The central location of the artificial belt largely relies on the magnetic latitude where the detonation takes place while the thickness and electron flux of the artificial radiation belt are affected by the explosion altitude and equivalence.
2009, 58 (8): 5879-5885.
doi: 10.7498/aps.58.5879
Abstract +
TeV energy emission from gamma ray bursts （GRB） is very important for studying the origin and the radiation mechanism of GRB. Searching for TeV GRBs is carried out by using the Tibet ASγ data. A GRB candidate is a shower cluster appearing in a given small sky window and a given time interval. An equi-zenith-angle method is used to estimate the background. In this analysis，two methods are applied in searching for possible GRBs signals，one considers the coincidence with satellite GRB data and the other doesnt. No significant TeV GRBs is detected in either case. The upper flux limit on top of the atmosphere at the 95% confidence level is estimated to be about 3.32×10-9—1.24×10-7 cm-2·s-1 accordingly.
