Vol. 55, No. 5 (2006)
Lutzky conserved quantities and velocity-dependent symmetries for systems with unilateral holonomic constraints
2006, 55 (5): 2109-2114. doi: 10.7498/aps.55.2109
This paper studies the symmetries and the conserved quantities for systems with unilateral holonomic constraints. The definitions of Lie symmetries for the systems are given, and the Lutzky conserved quantities are directly deduced from the general velocity-dependent Lie symmetries of the systems. The Lutzky conserved quantities of some special cases, for example, the holonomic systems with remainder coordinetes, the non-conservative mechanical systems, and the Lagrangian systems, are given. At the end of the paper, two examples are given to illustrate the application of the results.
2006, 55 (5): 2115-2121. doi: 10.7498/aps.55.2115
The nonlinear Thomson scattering by an electron in a focused circularly polarized laser beam is investigated and a novel scheme for single attosecond pulse generation is proposed.Through computer simulation we find that by using a tightly focused laser beam,the ratio of the highest peak pulse and the second highest peak pulse of the radiation which is defined as the signal-to-noise ratio can be efficiently increased, such that a single attosencond pulse rather than a train of attosecond pulses is obtained. The pulse duration of the radiation decreases and the signal-to-noise ratio increases with increasing laser intensity.Simultaneously,the signal-to-noise ratio can be also improved as the beam-waist decreases.We find that a single attosencond pulse can be generated by using a few-cycle ultrashort laser pulse as well.
The effect of biexciton, wetting layer leakage and Auger capture on Rabi oscillation damping in quantum dots
2006, 55 (5): 2122-2127. doi: 10.7498/aps.55.2122
The decoherence of Rabi oscillation with multi-level processes in semiconductor quantum dots excited by laser pulses is investigated. By using population dynamic equations of multi-level system, the effect of three kinds of multi-level processes on the damping of Rabi oscillation in quantum dots are numerically simulated and discussed. The effect of biexciton can be neglected when the pulse width is larger than 5ps; the population leakage to wetting layer results in the decreasing of the amplitude and average of the population oscillation on the exciton ground state with the increasing of the excitation intensity; the effect of the two kinds of Auger capture processes on the Rabi oscillation and the spectral width of the photoluminescence from the exciton recombination are also discussed.
The time-revolution of the quantum fluctuation and the nonclassical statistical behavior for the polariton system
2006, 55 (5): 2128-2137. doi: 10.7498/aps.55.2128
In this paper, we have considered the polariton system described by the model effective Hamiltonian involving a single-mode photon field interacting with a transverse optical phonon in a more realistic form. On this basis, the dynamical evolution of the polariton system, and the quantum fluctuation and the non-classical properties for this system have been investigated in an analytic form. By the nonlinear interactions of the system, when the radiation field was initially in the coherent state and the polarization wave field in the vacuum state, the radiation field and the phonon field can be developed to the squeezed state with time evolution and their second-order squeezing oscillates with time evolution in a complex periodical form. When k1=0 or k2=0, the squeezed effects do not occur. The non-classical properties are determined by the nonlinear coupling of the system and depend on the existence of the k1 and k2 terms simultaneously. At the same time, it is shown that the new results indicate that the statistical distribution of the photon and phonon can oscillate between super-Poisson and sub-Poisson distribution with time evolution, the nonlinear terms k1(a++) and k2(a+b++ab) have contributions to the non-classical statistical feature.
2006, 55 (5): 2138-2143. doi: 10.7498/aps.55.2138
The relevant aspects of the q-thermostatistical treatment for the free electronic gas which is an ideal Fermi system are discussed.Thermodynamical quantities of the free electronic gas such as total number of particles,total energy,heat capacity,free energy,etc.,are calculated within the Tsallis thermo-statistics where the factorization approach is incorporated,and the ensuing statistics is derived.Special attention is paid to heat capacity.The limiting cases of interest are discussed in some detail,namely,low-temperature regimes,and the approximate results pertaining to the case q→1 (the conventional Fermi-Dirac statistics corresponds to q=1).We also compare our results with previous Tsallis' works.
Investigation of decay characteristics of photoelectrons in nanoparticales of cubic AgBr sensitized by NiS
2006, 55 (5): 2144-2148. doi: 10.7498/aps.55.2144
NiS nanoparticales as a sensitizer is added to cubic silver bromide emulsion for the chemical sensitization. The decay curves of free photoelectrons and shallow-trapped electrons in the cubic silver bromide emulsion sensitized by NiS nanoparticales with the continuous increase of sensitization time are measured by the microwave absorption and dielectric spectrum techniques. The courses of decay of photoelectrons in the cubic silver bromide emulsion sensitized by NiS nanoparticales vis sensitization time is analyzed. The influence of electron traps in silver bromide microcrystals on the behavior of the photoelectrons' movement is discussed. The relationships between the effects of electron traps and the depth of traps with sensitization time are analyzed. By comparing the decay characteristics of the unsensitized and the sensitized samples, the optimum sensitization time under the present experimental conditions is 80 min.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2006, 55 (5): 2149-2151. doi: 10.7498/aps.55.2149
The composite Boson fields are quantizated in FS (Faddeev-Senjanovic) path integral quantizated formalism. The fractional spins and fractional statistics is obtained by using the quantum Noether theorem.
2006, 55 (5): 2152-2157. doi: 10.7498/aps.55.2152
The Floquet form of electromagnetic field in travelling wave tubes with tape helix is analyzed in the cylindrical coordinates frame. The study showed that appropriate coordinate transformation is the sufficient condition for separation of variables of electromagnetic field. The separation of variables under the coordinate transformation is the basis of the theory in which the assumption about the surface current density distribution of the tape is avoided. The dispersion and coupling impedance of a typical structure are calculated. The computed and measured results agree with high accuracy.
On the basis of the generalized Schr?dinger equation， a novel state-vector function that obeys photon motion equation under three quantum conditions has been constructed for a photon in one dimension, which can describe completely the properties of a photon as a relativistic free particle, including its energy and momentum and spin angular momentum. The state-vector function not only defines microscopic parameters such as the probability amplitude and the phase for a photon, but also relates them with the macroscopic polarization of a light beam. As an example, it successfully explains the puzzling polarization problem of a light beam.
2006, 55 (5): 2165-2169. doi: 10.7498/aps.55.2165
Using the 5SDH-2 Accelerator at the Radiation Metrology Center of CIAE, the experiment of neutron energy response of the thin ST-401 scintillator has been performed. Two T(p,n)3 He and D(d,n)3He reactions are chosen for generating the neutrons. The neutron beam is collimated with the multiplex shielding system. The source intensity is monitored by the proportional counter or semiconductor proton recoil telescope. The neutron response curves of ST-401 scintillators with thickness from 0.16mm to 2.00mm are measured versus neutron energy. The uncertainties of experimental results are discussed in detail. The results show that the sensitivity increases with the scintillator thickness, and for the same thickness, the slope of energy response curve does not change greatly with the increase of neutron energy.
2006, 55 (5): 2170-2174. doi: 10.7498/aps.55.2170
Detector grade diamond films have been prepared by DC Arc Plasma Jet chemical vapour deposition from CH4-H2 mixtures.Diamond detectors of pulsed radiation based on the films have been developed and preliminarily studied.The responses of the detector to 1.25MeV γ ray and pulsed ultraviolet laser with picosecond width show that this detector has a very fast time-response and about 70% charge collection efficiency.Its further applications will be found in sub-nanosecond pulse measurements.
ATOMIC AND MOLECULAR PHYSICS
2006, 55 (5): 2175-2179. doi: 10.7498/aps.55.2175
In this paper, the electronic states of the ground state and dissociation limit of XY(H，Li，Na) are correctly determined based on group theory and atomic and molecular reaction statics. The equilibrium geometries, harmonic frequencies and dissociation energies of the ground state of XY(H，Li，Na) are calculated using DFT(B3LYP) method and Quadratic CI method including single and double substitutions (QCISD). The potential curves are scanned using the QCISD/6-311++G(3df,3pd) method for the ground state. The potential energy function and relevant optical constants (Be，αe，ωe and ωeχe) of this state are obtained by least square fitting to the Murrell-Sorbie function. All calculation results are in good agreement with experimental data.
2006, 55 (5): 2180-2184. doi: 10.7498/aps.55.2180
The ground states of plutonium dihydride under electric field ranging from -0.005 to 0.005 a.u. have been optimized using density functional theory DFT/B3LYP with SDD for Pu and 6-311++G** for H. The excitation energies and oscillator strengths have been calculated under the same electric field employing the time-dependent DFT method. The results show that the electronic state, total energy,molecular geometry, dipole moment and excitation energy are strongly dependent on the applied field strength. As the electric field changes from -0.005 to 0.005 a.u., the bond length of Pu-H increases whereas the bond angle of H-Pu-H decreases because of the charge transfer induced by the applied electric field. The dipole moment of the ground state increases linearly with the applied field strength. The total energy of the ground state decreases linearly with the applied field strength. The dependence of the calculated excitation energies on the applied electric field strength fits well to the relationship proposed by Grozema. The excitation energies of the first five excited states of plutonium dihydride decrease as the applied electric field increases because the energy gap between the HOMO and LUMO become narrower with the field, which shows that the molecule is apt to be excited under electric field and hence can be easily dissociated.
Studies on the full vibrational energies and dissociation energies for some states of alkali hydride molecules
2006, 55 (5): 2185-2193. doi: 10.7498/aps.55.2185
The full vibrational energy spectra for the A1Σ+ states of alkali hydride diatomic molecules 6LiH, 7LiH, NaH, KH, RbH and CsH have been studied using algebraic method (AM), and the dissociation energies De of these molecules have also been studied using algebraic energy method (AEM). The results show that the AM produces accurate full vibrational energy spectra for these molecules for the first time so far as we know, and based on these AM spectra, the AEM can give correct molecular dissociation energies De for these systems.
Design and modeling for 1.7—2.7 GHz broad-band left-handed material with miniaturized unit cell and its characterization
2006, 55 (5): 2194-2199. doi: 10.7498/aps.55.2194
An 1.7—2.7 GHz broad-band left-handed (LH) metamaterial with miniaturized unit cell is designed. The relative bandwidth of designed metamaterial is up to 45.5%， the electrical length of its unit cell is less than 0.035，and the transmission loss per unit cell is less than 0.75 dB in the whole operational band of the LH metamaterial. In this paper, the reflection and transmission characteristics of a semi-infinite LH metamaterial plate consisting of LH metamaterial cells is simulated, and the wave number, phase velocity, refraction index, effective permittivity and effective permeability are calculated. The results show the real part of the wave number and the refraction index and the velocity are negative while the real part of the effective permittivity and effective permeability are both negative in the 1.7—2.7 GHz band range.
2006, 55 (5): 2200-2205. doi: 10.7498/aps.55.2200
A planar miniaturized left-handed (LH) metamaterial in C band is designed with broad bandwidth and low loss while the size of the unit cell is small and the structure is simple. In addition, the center of the frequency band where the LH metamaterial is applicable is tunable within the range of 4 to 20 GHz. A semi-infinite slab made of the planar LH metamaterial is simulated and the phase velocity is respectively extracted from the transmission and reflection data at normal incidence and the phase of wave front, results show the LH metamaterials studied exhibit backward wave (BW) properties in the frequency range of interest.
2006, 55 (5): 2206-2209. doi: 10.7498/aps.55.2206
The spectrum of highly ionized titanium was studied by means of the beam-foil technique.Titanium ions were provided by the HI-13 tandem accelerator at China Institute of Atomic Energy.The experimental results are compared with those of laser-produced plasmas. Numerous lines attributed to Ti ⅩⅥ to ⅩⅧ ransitions have been identified,and three of them were newly measured, which were attributed to 2s2p2 4P3/2—2p3 2D3/2, 2s2p2 1S0—2sp3 1P1 and 4p 1P0—5d 1P1 transitions.
An experimental investigation on the resonance enhanced multiphoton ionization and dissociation processes of acetone
2006, 55 (5): 2210-2214. doi: 10.7498/aps.55.2210
Resonance enhanced multiphoton ionization(REMPI) dissociation channels of acetone were studied using REMPI-TOF-MS and mass-selected excitation spectra in the range of 280—286.5nm. The mass-selected excitation spectra of fragment ions were analyzed and identified. It was shown that the main dissociation channel of acetone is the photoionization of parent molecule firstly in the range of 280—286.5nm, then followed by photodissociation of the parent ion.
Theoretical study of the anisotropic interaction potential and partial scattering cross sections for He-HCl system
2006, 55 (5): 2215-2220. doi: 10.7498/aps.55.2215
In this paper, an anisotropic intermolecular potential of the He-HCl complex has been obtained by utilizing the BFW analytic function to fit the intermolecular energy data, which have been calculated at the theoretical level of the single and double excitation coupled-cluster method with noniteractive perturbation treatment of triple excitation CCSD(T). Then the differential and partial scattering cross sections for collisions between He atoms and HCl molecules have been calculated by using CC(Close-Coupling) approximation method. Finally, the law governing the changes of the partial scattering cross sections has been given. This study shows that the fitted anisotropic intermolecular potential not only possesses the advantage of a simpler function form but also offers a better description of the characteristic of interaction in He-HCl system. At the same time, the difficult problem to determine the intermolecular potential parameters can be solved on the basis of the results of ab initio calculation for the collision systems. Therefore, the result obtained may be helpful for probing collision mechanism of atoms and molecules.
2006, 55 (5): 2221-2227. doi: 10.7498/aps.55.2221
This paper studies the X-ray spectra produced by the interaction of highly charged ions of Arq+(q=16,17,18) with metallic surface of Be, Al, Ni, Mo and Au respectively. The experimental results show that the Kα X-ray emerges from under the surface of solid in the interaction of ions with targets. The multi-electron excitation occurred in the process neutralization of the Ar16+ in electronic configuration of 1s2 in metallic surfaces, which produces vacancy in the K shell. Electron from high n state transition to K vacancy gives off X-ray. We find that there is no obvious relation between the shape of X-ray spectra and the different targets. The X-ray yield of incident ions are associated with initial electronic configuration. The X-ray yield of target is related to the kinetic energy of the incidentions.
Total cross sections for scattering of positron from CO, HCl, NH3 and CH4 at intermediate and high energies
2006, 55 (5): 2228-2233. doi: 10.7498/aps.55.2228
A complex optical model potential modified by incorporating the concept of bonded atom, which takes into consideration the overlapping effect of electron clouds between two atoms in a molecule, is firstly employed to calculate the total cross sections for positron scattering from CO, HCl, NH3, SiH4 in the incident energy range 30—3000eV by using additivity rule model at Hartree-Fock level. In the study, the complex optical model potential composed of static, correlation polarization plus absorption contributions firstly uses the bonded-atom concept. The results of quantitative molecular total cross sections are compared with those obtained by experiments and other theories wherever available, and good agreement is obtained. It is shown that the additivity rule model together with the complex optical potential model modified by incorporating the concept of bonded atom can give much better calculation results than the unmodified one.
2006, 55 (5): 2234-2240. doi: 10.7498/aps.55.2234
In the design and fabrication of microelectronic system(MEMS)devices, the process based on silicon is a main technolog which draws great attention of researchers. Bonding technology including silicon to glass and silicon to silicon is fundamental for bulk silicon MEMS devices.For MEMS devices, the bonding area is from the micrometer to millimeter scale, thus traditional methods to test bonding strength are no longer feasible. Measuring the strength at that scale has become the bottle-neck for MEMS development. We first define a new way, with which a series of single crystal cantilever beam was taken to test the maxal shear stress of bonding strength in the micro area.The experiment gives curves of torsional strength versus the bonding area and torsional strength versus the probe movement distance, the designer can use them to determine the bonding area according to the required torque for their MEMS devices.
2006, 55 (5): 2241-2248. doi: 10.7498/aps.55.2241
The possible geometrical and electronic structures of small bimetallic Pd-Pb clusters including Pbn(n=2,3,4,5) and PdnPbm(n+m≤5) have been optimized by density functional theory(B3LYP/LANL2DZ) method with relativistic effective core potential(RECP) using Gaussian 98 code. The shaping regularities of small bimetallic Pd-Pb clusters is analyzed based on structures and vibration spectrum. Finally, the energy level distribution, HOMO-LUMO gaps and the chemical activation are discussed.
Structures and stability of GanN-(n=2—8) and GanN-2 (n=1—7) anionic clusters have been studied using the density functional theory (DFT). Structural optimization and frequency analysis were carried out at the B3LYP/6-31G* level. All ground states of GanN-(n=2—8) and GanN-2 (n=1—7) clusters have been obtained. Our calculations reveal that there exists a transition from planar to spacial structures at the total number of the atoms being 5 with increasing cluster size. Among different GanN-(n=2—8) and GanN-2 (n=1—7) anionic clusters, Ga4N-， Ga6N-, Ga4N-2 and Ga5N-2 are more stable.
CLASSICAL AREA OF PHENOMENOLOGY
2006, 55 (5): 2257-2262. doi: 10.7498/aps.55.2257
Based on the technique of PIV (particle image velocimetry),We calculated the light intensity of the bubble interface and discovered that the intensities of lighr reflected or refracted twice succeedingly abide by the rules of geometric series, and the light will disappear after reflecting or refracting 4 times by the bubbles. When the angle of incidence of 35° is avoided , the intensity of second refraction out of a bubble is independent of the incident beam width, similar to the case of a narrow light ray falling on the bubble interface. The integral formulas for the intensities of reflection or refraction are given for the planar light beam falling on the bubble interface. The numerical solution of the integral is in agreement with the experiment value.
2006, 55 (5): 2263-2268. doi: 10.7498/aps.55.2263
Considering the interaction between two-mode SU(1,1) coherence states and a two-level atom, we find an exact solution of the Milburn equation for the system. We investigate the time evolution of the entanglement of field-atom and one of two modes of field using the quantum-reduced entropy and quantum relative entropy, respectively. The influences of intrinsic decoherence and two-mode photon number difference on the evolution of the entanglement of field-atom and one of two modes of field are discussed. It is shown that the entanglement of field-atom reduces to a stationary value with time evolution and the entanglement between two modes increases to another stationary value with intrinsic decoherence, and the values only depend on the two-mode photon number and mean photon number.
2006, 55 (5): 2269-2274. doi: 10.7498/aps.55.2269
We have studied the effects of quantum measurements on the decay of an excited Λ-type atom embedded in anisotropic photonic crystal. It is found that the decay of the atom can be suppressed os accelerated by the quantum measurements. This depends on the relative positions of the upper levels of the atom from the band edge and the frequency of the quantum measurements. Due to the effects of the anisotropic photonic crystal, the decay suppression effect can be obtained in relatively low frequencies of the quantum measurement.
2006, 55 (5): 2275-2280. doi: 10.7498/aps.55.2275
Teleportation of a general two-mode Gaussian state through double two-mode squeezed state quantum channels is studied. It is found that the quantum channels must be enough entangled for the output state to be inseparable. The required minimum entanglement for the quantum channels to maintain entanglement in the output state depends on the entanglement of the input state when the input state is pure. The minimum entanglement depends not only the on entanglement but also on the global purity of the input state when the input state is mixed.
Based on the experimentally observed phenomena of random laser, a new physical holistic effect model is studied. The random laser is due to the interaction of the local aperiodic quasi-structure with matching pump light. The random laser in ZnO powder is simulated by numerical calculation, and the results are coincident with experimental phenomena qualitatively.
Simulated calculation and analysis of the forbidden band for fabricating two- and three-dimensional photonic crystal structures using holographic lithography
2006, 55 (5): 2286-2292. doi: 10.7498/aps.55.2286
Compared with the semiconductor and micromachining fabrication technology, the holographic lithography method offers a number of advantages, including its ability to create large volume of periodic structures through one irradiation process, the uniformity of period, and more degrees of freedom to control the structures. In this study, a multi-beam interference model is presented for predicting two-and three-dimensional photonic crystal structures through designing the beam intensities, polarization directions and phase delays. Based on plane wave expansion method, complete forbidden bands of three-dimensional photonic crystals with fcc structure are also studied. The calculated results provide a useful guide for choosing proper optical parameters to fabricate two- and three-dimensional photonic crystal structures.
Investigation on reflection tomography imaging with terahertz wave of nonparallel interfaces inside the sample
2006, 55 (5): 2293-2299. doi: 10.7498/aps.55.2293
In previous studies on reflection tomography imaging with terahertz wave, only parallel interfaces were considered. In this article we report the numerical simulation on reflection tomography imaging with terahertz wave of nonparallel interfaces inside the sample. The reconstructing algorithm is given according to the physical model and the correlative reconstructing software is compiled. The results show that the reconstructing method of nonparallel interface is feasible. The analysis indicates that the reconstructing errors result from approximate treatment and accumulative error. The approximate treatment method of the curve contour is discussed. If the curvature radii at all points are large, the curve can be treated by cutting it into several line segmeants. The factors affecting longitudinal resolution are analyzed finally. The results show that the longitudinal resolution becomes worse as the pulse width of input pulse becomes larger. The longitudinal resolution can reach sub-millimeter size usually.
Using the supercell plane wave method, we investigated the effect of the third component material as defects modifying band structure of two-dimension two-component phononic crystal (PC). It is shown that adding the third material as point defect/linear defects in the two-component PC, defect states/bands can appear in the original band gap whose position and width would have only minor changes. The frequency of the defect states/bands are affected by the third material's characteristics; and all the defect states are localized. Therefore, adding proper third material as point/linear defects to the two-component system with wide band gap where defect states/bands will fall in, some special filtered/guided states will be formed. This property has important significance in acoustic /elastic wave propagating and new acoustic applications.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2006, 55 (5): 2306-2312. doi: 10.7498/aps.55.2306
An improvement is made on relaxation time of gas-kinetic traffic model, and the density and velocity are taken into account. A test case validates that the improved gas-kinetic traffic model is more effective on suppressing the perturbation, and more stable at high vehicle densities.
2006, 55 (5): 2313-2321. doi: 10.7498/aps.55.2313
The collision dynamics of the fast ion population in ultra-dense DT plasmas are investigated and analyzed within the framework of the relativistic Fokker-Planck equation. The Green function expansing on the spherical harmonics is used to express the solution of the equation. With the Green function we can calculate the stopping time, the ion range, the straggling range, and the mean transverse dispersion of different kinds of ions in plasmas. Compared to the previous ways of studying the movement of ions in plasmas, we not only calculate the straggling range without the approximation that the energy loss of the ions is much smaller than the energy of the ions, but we also calculate the stopping time and the mean transverse dispersion. This calculation is important for the experimental study of high energy density plasmas and fast ignition of laser fusion.
Photonionization and radiative recombination of Br——Comparison of rate coefficients deduced form the average atom and detailed configuration models
2006, 55 (5): 2322-2327. doi: 10.7498/aps.55.2322
For practical purposes, the ionic stage and populations of hot dense plasma are often described by the average atom(AA) model instead of the detailed average configuration accounting(DCA) model, which is more close to the reality. According the atomic self-consistent field theory, we calculate the DCA reduced photonionization cross-sections of every subshell of Br ions in different ionic stages. Comparing these DCA reduced cross-sections with the reduced photonionization cross-secion of the corresponding average electronic orbital in the AA model, we deduce the inherent rules between the reduced cross-sections. Based on the AA reduced photonionization cross-section, the DCA reduced photonionization cross-sections and detailed configuration rate coefficients of photonionizaiton and radiative recombination can be calculated. This work is a necessary preparation for accurate description of non-local thermal equilibrium(n-LTE) plasma.
The effect of grain boundaries on magnetic and transport properties in colossal magnetoresistance particle film
2006, 55 (5): 2328-2332. doi: 10.7498/aps.55.2328
La0.67Ca0.33MnO3 grain films have been grown by pulsed electron beam deposition technology,and their magnetic and electronic transport properties investigated in detail.The influence of grain boundaries on the physical properties of La0.67Ca0.33MnO3 grain films were studied.Grain boundaries weaken the coupling between grains,resulting in a cluster spin glass state.The metal-insulator transition temperature (Tp) was shifted far below the ferromagnetic-paramagnetic transition temperature (Tc).At low temperatures,the electron transportation indicates a weak localization behavior,and under low magnetic fields,the intrinsic magnetoresisance effect is screened by the influence of grain boundaries.
2006, 55 (5): 2333-2339. doi: 10.7498/aps.55.2333
A linear analysis of the ideal magnetohydrodynamic (MHD) stability of the Z-pinch is presented in which plasma flows are included in the equilibrium. Compressibility is introduced into MHD equations via the acoustic velocity of the plasma. It is found that, compressibility can stabilize the magneto-Rayleigh-Taylor/ Kelvin-Helmholtz (MRT/KH) instability, and this allows the sheared axial flow mitigate MRT instability far more effectively. Therefore, on the early stage of the implosion, because the temperature of the plasma is not high, the compressible model is much more suitable than the incompressible one. Different flow profiles have also been investigated, and it shows that the mitigation effect of the axial flow only depends on the magnitude of the velocity shear where the perturbations concentrate.
2006, 55 (5): 2340-2346. doi: 10.7498/aps.55.2340
The governing equations for the electromagnetic waveguide are derived to Hamiltonian system formulation and symplectic geometric form, and transverse electric and magnetic components are treated as dual vectors to each other. By separation of variables, we arrived at a symplectic eigenvalue problem for Hamiltonian operator matrix, which can be solved by adjoint symplectic orthonormal relationship and the symplectic eigenfunction expansion method. A dual edge element is proposed for electromagnetic waveguide with irregular cross section and inhomogeneous loaded materials. Dual edge element can surmount those difficulties related to node-based finite elements in computational electromagnetics, and our numerical examples show that dual edge element has its own merits when compared with regular edge element.
Vacancy of laser field induced by phase reflection in underdense plasmas and its relation to laser-plasma parameters
2006, 55 (5): 2347-2351. doi: 10.7498/aps.55.2347
It is shown theoretically and by particle- in-cell simulations that the plasma density modulation produced by laser in underdense pre-plasmas can serve as a Bragg reflector, which results in phase reflection occurring at low-density plasma area. The phase reflection can influence the laser propagating in plasmas, such as the vacancy of laser field induced by two laser pulses counter-propagating in plasmas. It is also shown that the period of phase reflection and the reflectivity are related with the plasma density, thickness, intensity and the duration of laser.
2006, 55 (5): 2352-2356. doi: 10.7498/aps.55.2352
The microwave test method of transmission line can be applied to study the characteristics of wave absorption by plasma. With this method, how the parameters of microwave plasma generator, the medium being argon, and the vacuum environment affect the variation trends of plasma return loss, which imply the plasma jet attenuation of reflected electromagnetic wave, is studied in vacuum environment. Experimental results show that when the mass flow rate and output power of plasma generator are 52.5mg/s and 52W, respectively, microwave plasma jet can absorb the energy of reflected electromagnetic wave maximally at the frequency near 5GHz. With the output power increasing and the environment pressure decreasing, the wave attenuation of plasma will be increased. The plasma jet will attenuate reflected wave maximally with appropriate generator parameters.
2006, 55 (5): 2357-2363. doi: 10.7498/aps.55.2357
Hot electrons produced in the ultra-intense laser-plasma interactions play very important roles in the scheme of “fast ignition”. Bremsstrahlung measurements can be an effective method to diagnose the hot electrons accelerated in the forward direction. The transport of the hot electrons and the characteristics of the bremsstrahlung photons were calculated using the Monte Carlo electron-photon transport code MCNP. The feasibility of the bremsstrahlung diagnostic method in laser plasma interactions is also discussed.
2006, 55 (5): 2364-2370. doi: 10.7498/aps.55.2364
The matrix of the point spread function for an X-ray pinhole camera system has been calculated. The computer-simulated pinhole images and real pinhole images measured in laser-plasma experiments have been restored using Lucy-Richardson algorithm. The number of iterations the resolution of the restored imaging and the details of the restore method have been discussed. The reliability of the restoration algorithm is also verified by a simulated optical experiment.
2006, 55 (5): 2371-2380. doi: 10.7498/aps.55.2371
In theoretical simulations and analysis of diagnostic measurements for hot dense plasmas in the inertial confinement fusion(ICF) researches, it is usually necessary to consider thousands of ionic energy states and transition arrays between these energy states. Average atom(AA) models are generally adopted for practical purposes. In order to calculate ionic populations of hot dense plasmas more accurately, we propose a method beyond the AA model, which can calculate the ionic populations of plasma in local thermal equilibrium with satisfactory accuracy, and may also deal effectively with the ionic populations of plasma in non-local thermal equilibrium.
Numerical simulation of acceleration of electrons and ions in the interaction of intense laser pulses with dense gaseous targets
2006, 55 (5): 2381-2388. doi: 10.7498/aps.55.2381
Using a one-dimensional particle-in-cell code, which includes field ionization and electron collisional ionization, as well as elastic binary Coulomb collisions, we study the acceleration of electrons and ions by the interaction of an intense laser pulse (1017—1018W/cm2, 26.7fs) with a helium gas target (1021—1022/cm3, 0.8 μm thick). It is shown that field ionization appears quickly at the target front surface. Collisional ionization is found inside the target by energetic electrons, which are accelerated by the laser fields at the front surface and transported into the target. Part of these electrons transmits through the target rear surface and induces an electrostatic field there. This field further leads to field ionization at the rear surface. Meanwhile, it accelerates new-born ions produced there through the field ionization. Because of the electrostatic fields induced at the two target surfaces, some electrons oscillate between them, resulting in oscillating energy exchange between electrons, ions and the electrostatic fields. Particular attention is also paid to the origins of accelerated ions. Under certain conditions, ions accelerated inside the target are more energetic than those accelerated at the target surfaces.
2006, 55 (5): 2389-2396. doi: 10.7498/aps.55.2389
In the output section of klystrons, microwave energy is extracted from kinetic charged particles through beam-wave interaction process. In this region, the electron velocity will suffer violent changes, and some electrons may become so slow that they are overtaken by other ones. The traditional time integral method cannot track the laggard particles effectively, so its computation result is doubtful in the case of serious surpassing phenomenon existing in electron beam. To solve this problem, we present a new dynamic track method(DTM) that can correctly calculate the space charge force from other particles on the lagged ones. We will briefly explain DTM's principles and introduce a one-dimensional program based on this idea. Further, we apply this new program in output-section calculation of klystrons. The results show better agreement with experimental data than the traditional method.
2006, 55 (5): 2397-2402. doi: 10.7498/aps.55.2397
Analytical theory of electromagnetic field in a helical slow wave structure filled with plasma immersed in a finite axial magnetic field is presented in this paper. By means of helix sheath model and the boundary conditions, the dispersion equation of a magnetized plasma loaded helical slow wave structure is derived. Three particular cases (without plasma, without axial magnetic field and in infinite axial magnetic field) are discussed and the corresponding dispersion equations are deduced. It is found that the simplified results are in accord and with published references.
2006, 55 (5): 2403-2408. doi: 10.7498/aps.55.2403
The real-time visualization system of plasma shape is developed for studying the divertor plasma in HL-2A. The plasma boundary reconstruction code CF(Current Filaments) are written using the current filaments model and the finit current element model. The information of the magnetic field around plasma is collected with USB data acquisition card UA301, having 100kHz/s, 32 channels with simultaneous sampling. A compatible computer with 3GHz E CPU processes the data. This system collects and stores data in every 4ms, and plots the plasma boundary in every 16ms. After discharge, the mgnetic field at the plasma boundary and some plasma parameters which are deduced from it are calculated.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
A simulation study for the effects of cooling rate on evolution of microstructures during solidification of liquid metal Ga
2006, 55 (5): 2409-2417. doi: 10.7498/aps.55.2409
A simulation tracing study has been performed for the effects of different cooling rates on the evolution of microstructures during solidification process of liquid metal Ga by means of molecular dynamics method. The bond-types and basic cluster structures of metal Ga have been analyzed by using HA bond-type index method and cluster-type index method. It has been found that the cooling rate plays an important role to the microstructures in the solidification processes. At the cooling rates of 1.0×1014 K/s, 1.0×1013 K/s and 1.0×1012 K/s, the amorphous structures would be formed in the system with the rhombohedron related to 1311, 1301 bond-types as the main body, and coexist with the fcc, hcp and other structures; at the cooling rate of 1.0×1011K/s, crystallization would take place in the system, the crystalline transition temperature Tc being about 198K, and the lower the cooling rate, the higher the Tc. The crystal would have the orthorhombic structure related to 1421 bond-type(determined by visualized method) as the main body. This gives us a new way for researching the crystalline transition processes of liquid metals.
2006, 55 (5): 2418-2421. doi: 10.7498/aps.55.2418
Base on the embedded-atom method，the enthalpy and the density of Ni2TiAl alloy are calculated by using molecular dynamics simulations.The temperature dependence of the specific heat of normal and undercooled liquid Ni2TiAl alloy is obtained.The results indicate that the specific heat and the density increase linearly with the decrease of temperature，and the Neumann-Kopp rule can be applied to estimate the specific heat of undercooled liquid Ni2TiAl alloy. Meanwhile, the size effect in the simulation domain is indicated to be negligible.
2006, 55 (5): 2422-2426. doi: 10.7498/aps.55.2422
The effect on a particle motion of a deflecting channel is equated to modulation by a weak-periodic potential of similar shape as the channel. The motion equation of a particle is reduced to the nonlinear differential equation with a weak-periodic modulation by sine-squared potential. The critical conditions for the revelation of Smale horseshoe are analyzed using the Melnikov method. It is predicted that the chaotic behaviours may exit in the interaction of a particles between a crystal. Theory is provided to describe PME effects for the semicoductor superlattice.
2006, 55 (5): 2427-2432. doi: 10.7498/aps.55.2427
A composite oscillator model is proposed for studying energy dissipation mechanism of atomic-scale wearless friction. The model consists of the whole macroscopic oscillator and the micro oscillators of interfacial atoms. Different influences of the two oscillators on the energy dissipation process of friction are discussed. It is found that the frequency of the interfacial excitation force is the key to energy conversion in the friction process by analyzing the dynamic characteristics of interfacial atoms. In the equilibrium stage, the interfacial force acts integrally and uniformly on each atom because its frequency is nearly zero. In the non-equilibrium stage, however, the distribution of the energy received by the interfacial atoms is not uniform because the frequency of the interfacial acting force is very high. Therefore, the extra energy may be easily transferred to the adjacent atoms to make the energy dissipate. The results show that the composite oscillator model can more clearly explain the energy dissipation mechanism of friction in detail.
Phase-field simulation of the influence of elastic field on microstructure evolution and equilibrium composition of precipitation
2006, 55 (5): 2433-2438. doi: 10.7498/aps.55.2433
We have simulated the microstructure evolutions of coherent single precipitate and multi-precipitates with inhomogeneous eigen elastic field and applied external strain using phase-field method. The influences of the inhomogeneous eigen elastic field and applied external strain on the morphology pattern of the coherent precipitate are discussed, and the effect of the elastic field on the equilibrium compositions of the precipitate and the matrix phase is also analyzed.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Electronic theoretical study of the influence of impurities on corrosion resistance of magnesium alloy
2006, 55 (5): 2439-2443. doi: 10.7498/aps.55.2439
The atomic structure model of symmetric ［0001］ tilt boundary of α phase in magnesium alloy was set up by using the concept of coincidence-site lattice (CSL). The segregation energies of impurities, the interaction energies between impurities and fermi levels of different systems at grain boundary (GB) were calculated by using recursion method. The segregation behaviour of impurities, the relationship between the interaction between impurities and the ordering tendency, as well as the physical nature of the influence of impurities on corrosion resistance of magnesium alloy were discussed. Calculation results show that impurity atoms segregate to GB, and are apt to occupy the sites in the compressed region of GB. Impurity atoms repel each other, therefore form ordered phases in the region of GB. The relationship between the fermi level of a kind of material and its equilibrium potential is: the higher the fermi level is, the lower the equilibrium potential is; and vice versa. The difference of fermi level of regions with different composition in a system leads to the flow of electrons from the region with high fermi level to the region with low fermi level. It is just the difference of fermi levels that forms the electrodynamic force of the electrochemical corrosion in magnesium alloys.
2006, 55 (5): 2444-2448. doi: 10.7498/aps.55.2444
Effect of iron and cobalt on the electronic structure of (110) martensite twin boundary in Ni2MnGa alloy have been investigated by using density functional theory (DFT), including twin boundary energy, segregation energy, magnetic moment, bond order and electronic density of states. The calculation results demonstrated that cobalt has a stronger doping effect on the twin interface than iron, and has less influence on the magnetic property of the twin boundary than the latter.
The influence of self-trapped exciton on luminescence of Pr3+ in aluminate under vacuum ultraviolet excitation
2006, 55 (5): 2449-2453. doi: 10.7498/aps.55.2449
Spectroscopic properties of SrAl12O19:Pr3+ and Y3Al5O12:Pr3+ have been studied under vacuum ultraviolet excitation at 20, 77 and 300K. Self-trapped exciton(STE) luminescence in these aluminates were observed besides the characteristic emissions of Pr3+. Furthermore, the emission of STE became dominant as the concentration of activators and temperature decreased. The influence of STE on the luminescence of Pr3+ in aluminates was also emphasized.
Interactions of CO molecules adsorbed on metallic nanostructured surfaces in an external electric field
2006, 55 (5): 2454-2458. doi: 10.7498/aps.55.2454
A model is proposed to analyze the interaction energy of CO molecules adsorbed on metallic nanostructured surfaces in an external electric field, including the one between effective dipoles and the local electric field as well as the one between effective dipoles . The local fields near the nanoparticles surfaces are also calculated and discussed. Using Monte-Carlo numerical computation and simulation, the configuration of CO molecules adsorbed on nanoparticle surfaces and the interaction energy are given in detail. The results show that the metallic nanostructures cause the CO molecules to agglomerate and the interaction energy increase may explain the abnormal IR effects (AIREs).
2006, 55 (5): 2459-2463. doi: 10.7498/aps.55.2459
The degradation of thin tunnel gate oxide under constant Fowler-Nordheim current stress was studied using capacitors.The degradation is a function of constant current and time，which depends more on the magnitude of constant current.Thus the degradation is a strong function of injected charge density Qinj. Positive charge trapping is usually dominant at lower Qinj followed by negative charge trapping at higher Qinj，causing a reversal of gate voltage change.
Charge storage characteristics in hybrid electret film consisting of porous PTFE and PP with negative corona charging
2006, 55 (5): 2464-2469. doi: 10.7498/aps.55.2464
The charge storage characteristics of hybrid electret film consisting of porous PTFE and PP, which come from Shanghai Dagong New Materials Co. Ltd., are investigated by means of corona charging, open-circuit thermally stimulating discharge (TSD ) analysis and isothermal surface-potential measurement. The results indicate that there are space charge and polar charge for the hybrid film charged by means of corona discharge method, and the interface between PP and PTFE layers are the main source of charge traps. When charging is carried out through different sides, the difference of TSD current spectra is very large. The electric field generated by space charge can cause polarization of polymer and give rise to polar charge, which exists as compensative charge of space charge, so that the surface potential measurement can't reveal the charge characteristics inside hybrid films.
2006, 55 (5): 2470-2475. doi: 10.7498/aps.55.2470
The simulation of the hole transport properties of wurtzite phase GaN are performed by the full band ensemble Monte Carlo approach. The band structure data of GaN used in the simulation are obtained by the empirical pseudopotential method. The scattering mechanisms included in the simulation are acoustic phonon scattering, optical phonon scattering, polar optical phonon scattering, piezoelectronic scattering, ionized impurity scattering, as well as the intervalley scattering. The dependence of the hole average drift velocity and average energy in three main symmetry directions for wurtzite GaN on the electric field are obtained. At room temperature, the velocity-field relation shows the non-saturated characteristics. The maximum drift velocity is about 6×106 cm s-1, and the maximum average energy is 0.12 eV in the range of the field of this work. These values are much smaller than the corresponding parameters of the electrons in GaN. Furthermore, the dependence of the diffusion mobility on the impurity density is reported.
The GaN films grown on sapphire by metal-organic chemical vapor deposition (MOCVD) have excellent crystalline quality (χmin=2.00%). Combining Rutherford backscattering/channeling (RBS/C) and high-resolution X-ray diffraction (HXRD) measurements, we investigate the radiation damage in GaN films with various doses and angles of Mg+-implantation. The results of experiments reveal that the radiation damage rises with the increasing implantation dose. Under the dose of 1×1015atom/cm2, χmin is less than 4.78%, and when implantation dose exceeds the threshold of 1×1016atom/cm2, χmin will be up to 29.5%. Random implantation causes more serious damage than channeled implantation, and in a definite range the damage level rises with a larger implantation angle. At the implantation angles of 0°,4°,6°and 9°deviating from〈0001〉, χmin(%)is 6.28,8.46,10.06 and 10.85, respectively, at a dose of 4×1015atom/cm2. After annealing at 700℃ for 10min and then 1050℃ for 20s, the damage recovers to some extend. The crystalline quality of the sample with 1×1016atom/cm2 implanted was reduced to 19.08%. In addition, the annealing condition of 1000℃ for 30s is more efficient and the damage recovers better.
Experimental studies on the electronic structure of pyrite FeS2 films prepared by thermally sulfurizing iron films
2006, 55 (5): 2482-2487. doi: 10.7498/aps.55.2482
Pyrite FeS2 films have been prepared by thermally sulfurizing iron films deposited by magnetron sputtering.The electronic structures were studies by X-ray absorption near edge structure and X-ray photoemission spectrum. The results show that an S 3p valence band with relatively higher intensity compared to the calculation exists in 2—10eV range and a high density below the Fermi level of Fe 3d states were detected.A second gap of 2.8eV in the unoccupied density of states was found above the conduction band which was 2.4eV by experimentally calculation.The difference between t2g and eg which were formed in an octahedral crystal field was computed to be 2.1eV.
2006, 55 (5): 2488-2491. doi: 10.7498/aps.55.2488
We report the investigation on the oxidation behavior of Si1-xGex alloys (x=0.05, 0.15, and 0.25). In the PL spectra of Germanium quantum dots formed by the oxidation of Si1-xGex substrate, at high oxidation temperature (800℃—1000℃) an emission band from 550nm to 720nm would originate from the diameter distribution of germanium nanoparticles (Ge clusters diameter: 3nm—4nm); and at low oxidation temperature (400℃—600℃) with Laser beam radiation, there is an emission band from 650nm to 900nm which may have come from the germanium clusters (diameter: 4nm—5nm). It is clearly seen that there are several peaks at 572nm, 620nm, 671nm, 724nm, 769nm, 810nm and 861nm wavelengths along the emission band, which are correlated to the quantum confinement effect with 3.32nm, 3.54nm, 3.76nm, 3.98nm, 4.17nm, 4.35nm and 4.62nm diameters of the germanium clusters, respectively. The simulation result with MC method demonstrates that the germanium clusters of the above diameters are more stable under the above conditions. A quantum confinement model has been set up, and calculations with the UHFR method and the quantum confinement analysis have been proposed to explain the PL spectra.
2006, 55 (5): 2492-2497. doi: 10.7498/aps.55.2492
For the quasi-two-dimensional disordered systems of the form of Nt×Nl, only considering the nearest-neighbor hopping integrals and using a special method to code the sites, we write the Hamiltonians of the systems as precisely symmetric matrixes, which can be transformed into three diagonally symmetric matrixes by the Householder transformation. The densities of states, the localization lengths and the conductance of the systems are calculated numerically using the negative eigenvalue theory and the transfer matrix method. We study mainly the quasi-two-dimensional disordered systems with four and five parallel chains. By comparing the results with that of the disordered systems with one chain, two chains and three chains, we find that the energy band of the system extends slightly and the distribution of the density of states changes obviously with the increase of the effective dimensionality. Especially, for the systems with four or five chains, at the energy band center, there exist extended states whose localization lengths are larger than the size of the systems, accordingly, they have greater conductance. With the increasing of the number of the chains, the correlated ranges expand and the systems present the behaviour similar to that with off-diagonal long-range correlation.
2006, 55 (5): 2498-2503. doi: 10.7498/aps.55.2498
The weak localization and weak antilocalization effects in the coherent scattering of two dimensional electron gas(2DEG) have been observed in Al0.22Ga0.78N/GaN heterostructures by the magneto-transport measurement. The change of magnetoresistance from positive to negative under a perpendicular low magnetic field indicates that electron spin-orbit scattering caused by crystal field exists in Al0.22Ga0.78N/GaN heterojunction. The relation between the spin-orbit scattering time and the temperature is discussed for 2DEG, the inelastic scattering time measured by experiment shows a strong temperature dependence according to T-1 rule, which indicates that the electron-electron scattering with small energy transfer is the dominant inelastic process.
For increasing the efficiency of CdTe/CdS solar cells, it is important to measure the current-voltage and capacitance-voltage characteristics and then carry out a numerical simulation based on electronic measurement of CdTe and CdS thin films. In this paper, the capacitance-voltage characteristics were measured in the frequency range from 50kHz to 1MHz. The carrier concentrations of the absorber layer and the space charge region width were calculated. The dark current-voltage characteristics of the CdTe solar cell were measured in the temperature range from 220K to 300K. The saturated reverse dark current density J0 and the diode ideal factor A of the solar cells were obtained. The relations of J0 and A with temperature were discussed. The results show that the capacitance-voltage curves has two peaks and the intensities and positions of the peaks are dependent on measurement frequency. The results are simulated and explained with the multi-junction model. With decreasing temperature, the saturated reverse dark current decreases from 10-6mAcm-2 at room temperature to 10-7mAcm-2 at 220K, and the diode ideal factor rises from 2.13 at room temperature to 9.95 at 220K.
2006, 55 (5): 2508-2512. doi: 10.7498/aps.55.2508
The NBTI effect is studied in this paper with emphasis on its self-healing phenomenon. The recovery of threshold voltage shift with stress times and recovery time are studied. It is found that the recovery is mainly related to the re-passivation by hydrogen of interface states that occurred after the stress is stopped.
2006, 55 (5): 2513-2517. doi: 10.7498/aps.55.2513
Dielectric breakdown of BST thin films prepared by RF sputtering is studied in this paper. It is found that, the BST thin films will have already broken before the voltage reaches to conventional breakdown strength, which is normally tested by ramping the voltage and recording the voltage at which an abrupt rise in leakage current is observed. Accordingly, another state of BST thin films, i. e. the incipient breakdown, is found instead. The model which describes the films' configuration changes with the increase of voltage is established, and a new method to determine the practical breakdown strength is proposed.
2006, 55 (5): 2518-2522. doi: 10.7498/aps.55.2518
The influences of the applied bias voltage and the pre-irradiation treatment by alpha particles on the electrical properties of sandwich structural diamond film detectors under 5.5 MeV 241Am alpha-particle irradiation were investigated. Results of current- voltage(I-V) and pulse height distribution measurements showed that the dark current of the diamond film detector would increase due to the pre-irradiation by alpha particles. Under the alpha irradiation, the detector under negative bias voltage had a higher response current and a better signal-to-noise ratio than that under a positive bias. Raman scattering studies directly demonstrated that the above phenomenon resulted mainly from the different structural imperfection distributions along the thickness direction. An energy resolution of about 25.0% was obtained for the detector under a negative bias voltage and 38.4% under a positive bias voltage. With increasing alpha-particle irradiation time, both the response current and the charge collection efficiency increased obviously.
2006, 55 (5): 2523-2528. doi: 10.7498/aps.55.2523
Polycrystalline silicon films were prepared from SiCl4 diluted with hydrogen by plasma-enhanced chemical vapor deposition at a low temperature of 250℃. The effect of hydrogen dilution on their structure and optical properties were investigated. It was found that the effect of hydrogen on the growth behavior of the films deposited using SiCl4+H2 are completely different from that of using SiH4/H2. The crystalline fraction increases with decreasing the hydrogen dilution ratio R and reaches a maximum value of 85% at a low hydrogen dilution. However, further decreasing hydrogen dilution ratio leads the crystallinity to deteriorate and a phase transition between microcrystalline silicon and amorphous silicon is observed. The optical band gap gradually decreases from ～1.5eV to ～1.2eV and then increases up to 1.8eV with the decrease of the hydrogen dilution ratio. The deposition rate initially increases and then decreases with decreasing hydrogen dilution ratio, and no film is formed by using pure SiCl4. According to these results, we conclude that under the condition of optimum hydrogen dilution ratio, the enhanced crystalline fraction and the increase in grain size are attributed to the Cl radicals which play important roles in the low-temperature growth of crystalline silicon films.
Experimental study on thermopower of transition metals Fe，Co and Ni in the vicinity of Curie temperatures
2006, 55 (5): 2529-2533. doi: 10.7498/aps.55.2529
Thermopower of pure metals of Fe ，Co and Ni were measured in the vicinity of Curie temperatures.The results show that the curves of thermopower against temperature behave very similarly, being concave first and convex subsequently. Consequently, three special points for each metal can be determined by the corresponding curve, namely the ferromagnetic Curie temperature Tf, Curie temperature TC, and paramagnetic Curie temperature θp. It is obvious that there is a definite transition interval of temperature between ferromagnetic state and paramagnetic state of the metal and the Curie temperature is only an intermediate value. A discussion is made which concludes that the holes in the d band contribute to the electrical conduction in the temperature interval between Tf and TC, which implies that the magnetic carriers are the holes. It is also contemplated that there may exist short range order between TC and θp.
2006, 55 (5): 2534-2538. doi: 10.7498/aps.55.2534
Structures of CoNiZ (Z＝Si, Sn, Sb, Ga) alloys have been investigated. We found that CoNiZ alloys have B2 structure with Z=Sn or Sb, but when Z=Si, the structure is fcc γ phase. When B2 structure coexists with γ phase in CoNiGa alloys, γ phase affects the temperature of martensitic transformation greatly. The structure of the alloy is determined by both the electron concentration and the size effect. The coexistence of two phases in the CoNiGa alloys can be changed by various heat treatment conditions. These results indicate that the martensitic transformation behaviors can be adjusted just by heat treatments, other than by modifying the composition as being done conventionally for other ferromagnetic shape memory materials.
2006, 55 (5): 2539-2542. doi: 10.7498/aps.55.2539
The effect of quasiperiodic interaction on lattice distortion and energy gap in spin-Peierls model is studied by exact diagonalization and self-consistent methods. It is shown that quasiperiodic interaction can result in self-similar characteristics for lattice distortion and energy spectra of spin-Peierls model. Moreover, we find that the average value of lattice distortion will be increased, decreased or increased firstly then decreased as quasiperiodic interaction strength is enhanced. Such kind of behavior strongly depends on the lattice spring constant K. The energy gap also displays similar behavior.
2006, 55 (5): 2543-2547. doi: 10.7498/aps.55.2543
By adjusting the Cr concentration, the effect of texture and grain size on the exchange bias of NiFe/PtMn bilayers with (Ni0.81Fe0.19)1-xCrx seed were systematically investigated. After annealing the bilayers at 270℃ for 5h, it was found that the difference in the texture of PtMn layer is not the cause of the variation in the exchange bias field. The exchange bias field depends strongly on the grain size of PtMn layer. When the grain size is around 11.3nm, the exchange field attains its maximum.
2006, 55 (5): 2548-2552. doi: 10.7498/aps.55.2548
In this paper we discuss the magnetoelectric (ME) effect in bilayers of Nickle Zinc ferrite (NZFO) and lead zirocnate titanate (PZT). The powder of NZFO was prepared by sol-gel technique, then hot-pressed at 900℃, and sintered at high temperatures. In these bilayers we measured strong magnetoelectric interaction and observed that the transverse ME effect was higher than the longitudinal effect by one order of magnitude, and the ME effect tended to be stronger as the sintering temperature of NZFO increased. When the sintering temperature was increased from 950℃ to 1380℃, the maximum voltage coefficient of transverse ME effect could change in the range of 25.6 mV Am-2≤αE≤199.6mV Am-2. Theoretical analysis shows that the ME effect observed in these samples origins from a good magnetoelectric coupling between NZFO and PZT.
Magnetic properties of amorphous Fe74Al4Sn2P10C2B4Si4 powder prepared by water atomization and powder core made from it
2006, 55 (5): 2553-2556. doi: 10.7498/aps.55.2553
Fe74Al4Sn2P10C2B4Si4 powder were prepared by water atomization. It has been shown that this alloy has big glass-forming ability (GFA) and high stability against heating. Amorphous alloy powder can be got in size smaller than 400 mesh. Quality factor of the powder core made from this amorphous powder is remarkably higher than that of MPP in high frequency range, which means the core loss of the amorphous powder core is lower than that of MPP. The electrical resistivity of amorphous powder is much higher than that of MPP powder, that is why the amorphous powder core has lower core loss in the high frequency range.
2006, 55 (5): 2557-2561. doi: 10.7498/aps.55.2557
Co doped ZnO powders were synthesized by the solid-state reaction. The magnetic and optical properties of the powders were investigated. The experimental results indicate that Zn2+ ions were substituted by Co2+ ions randomly. Zn0.95Co0.05O powders prefer paramagnetic state to ferromagnetic state from 3 K to 300 K due to coupling exchange interactions between Co 3d spins, which makes the nearest Co2+ antiparallel.
2006, 55 (5): 2562-2566. doi: 10.7498/aps.55.2562
［FePt/BN］n multilayers were prepared on MgO (100) single crystal substrates at 250℃ with magnetron sputtering. The perpendicular texture of L10-FePt/BN particle films was formed after vacuum annealing. The results of X-ray diffraction and magnetic tests show that the ［FePt(2nm)/BN(0.5nm)］10 and ［FePt(1nm)/BN(0.25nm)］20 multilayers have excellent (001) texture when the films were sputtered on substrates at 250℃ after annealed at 700℃ for 1h. The mean size of L10-FePt particles is roughly 15—20nm and its perpendicular coercive force reaches 522 kA/m. Moreover, the Mr/Ms and switching filed distribution S* reach 0.99 and 0.94, respectively. This particle film is a candidate for future perpendicular magnetic recording media with ultrahigh density.
2006, 55 (5): 2567-2571. doi: 10.7498/aps.55.2567
(FexPt1-x)100-yCuy(x=0.46—0.56,y=0,0.04,0.12) thin films with a fixed thickness of 50nm have been prepared by magnetron sputtering. The atomic ratio of Fe to Pt atoms was accurately controlled by DC co-sputtering. The ordering parameter of the L10 phase is not enhanced for the films with x>0.52. On the contrary, the ordering process is improved by the addition of Cu for the Pt-rich or stoichiometric thin films. In order to get the ordering of FePt films at a low annealing temperature, the lower the Fe content, the more addition of Cu is needed. It is found that the L10 phase can be obtained after annealing at 350℃ for 20 min only for films with the ratio of the (FeCu)/Pt in the range of 1.1—1.2.
Study of fluorinated amorphous carbon films prepared by electron cyclotron resonance chemical vapor deposition
2006, 55 (5): 2572-2577. doi: 10.7498/aps.55.2572
Fluorinated amorphous carbon(a-C:F) films were deposited using C4F8 and CH4 as precursor gases by electron cyclotron resonance chemical vapor deposition (ECR-CVD). Analysis of chemical compositions and bond structures by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) shows that the loss of CF3 and C—C termination bonds are liable for inducing reduction of film thickness after heat treatment through out-gassing process. The dielectric constant of a-C:F films increases due to increased electronic polarization and enhanced film density and the interface trap density decreases from (5—9)×1011eV-1cm-2 to (4—6)×1011eV-1cm-2 after 300℃ annealing in a nitrogen environment. The current-voltage characteristics of α-C:F films was explained by ohmic conduction at low fields and Poole-Frankel conduction at high electric fields. The trap energy of the traps at band tails formed by the delocalized π electrons decreases after annealing, leading to the increase of leakage current due to field-enhanced thermal excitation of trapped electrons into the conduction band.
Influence of heat treatment temperature after the pressure expansion on the electromechanical properties of cellular polypropylene electret
2006, 55 (5): 2578-2583. doi: 10.7498/aps.55.2578
The influence of heat treatment temperature after the pressure expansion on the electromechanical properties of cellular polypropylene (PP) electret is studied. The results point out that over the temperature range form RT to the melting temperature of the sample, the thickness of the sample after pressure expansion increases with the heat treatment temperature. Compared to that mentioned above, obviously different influence of heat treatment temperature on the electromechanical properties of cellular PP is observed. The elastic modulus and electromechanical coupling factor of cellular PP electret reach minimum and maximum at 90℃ heat treatment temperature, respectively. As a result, the piezoelectric d33 coefficient reaches its maximum of 377pC/N. Further, with the increase of heat treatment temperature, the resonance frequency of cellular PP could be regulated from about 794 to 371kHz.
2006, 55 (5): 2584-2589. doi: 10.7498/aps.55.2584
The poled PZT 95/5 ferroelectric ceramic is in ferroelectric (FE) phase, it can be transformed into the antiferroelectric (AFE) phase by shock compression, so the bound charge can be liberated into an external circuit to generate pulses of electrical current. Based on this FE/AFE transition theory, the electrical response of LRC load was discussed, especially for the small resistance (R) and small inductance (L) load in this paper. A set of PZT 95/5 ceramics in parallel connection was devised to generate large pulses of electrical current in two plane-wave tests. The maximum output current reaches above 5 kA, and the corresponding rise time of the front edge is under 500ns.
2006, 55 (5): 2590-2595. doi: 10.7498/aps.55.2590
The polycrystalline perovskite-phase PT/PZT/PT ferroelectric thin film was prepared by sol-gel method. The domain and domain wall structures were investigated by scanning force microscopy. The out-of-plane polarization(OPP), in-plane polarization(IPP), the OPP amplitude and OPP phase images were obtained. The domain of the film has a complex structure with c-domain and deviated-c-domain the direction of which deviated from the surface normal of the film. The complex domain structure is related to the orientation of the crystal grains in the film. For the［111］oriented films, the 180° domain wall is formed when deviated-c-domains have reverse directions in OPP and IPP. The 90° domain wall is formed when the domains have the same direction in OPP and reverse directions in IPP, or when they have reverse directions in OPP and same direction in IPP.
2006, 55 (5): 2596-2600. doi: 10.7498/aps.55.2596
The equal-frequency surface of wavy two-dimensional (2D) photonic crystals (PC) is obtained by virtue of plane wave expansion method. From the equal-frequency surface, we find a iso-frequency contour of rectangular form. A flat dispersion relation exists at this frequency, which is suitable for the self-collimation. The finite-difference time-domain (FDTD) method is used to simulate the self-collimation in wavy 2DPC on which a Gaussian beam is incident at different angles. Subwavelength lensing by self-collimation is also available in wavy 2DPC. The resolution of 0.28λ is achieved when single source is employed in simulation. The resolution will gradually degrade as the source moves beyond the near-field domain.
2006, 55 (5): 2601-2605. doi: 10.7498/aps.55.2601
The effects of Pb, Ga doping on the crystal field, superexchange interaction and magneto-optical effect of Ce:YIG crystal are studied based on the quantum theory. It is found that when the molecular concentration of Ga doping is 12%, the molecular field on the Ce3+ ion is reduced by 51% and the difference of occupation probability between the two lowest energy levels of the Ce3+ is reduced by 49%. The Ga doping affects the crystal field and molecular field at the same time, while the Pb doping affects the crystal field only.
Influence of Si—OH groups on properties and avoidance for SiCOH films prepared by decamethylcyclopentasiloxane electron cyclotron resonance plasma
2006, 55 (5): 2606-2612. doi: 10.7498/aps.55.2606
This paper investigates the effect of Si—OH groups on dielectric property and leakage current of the SiCOH low dielectric constant films deposited by decamethylcyclopentasioxane (D5) electron cyclotron resonance plasma. The results show that the increasing of Si—OH content in the films can lead to the increasing of dielectric constant k, decreasing of leakage current and rise in dielectric dispersion. The increasing of k value is the result of compensation of the decreasing of k value originated from cage by the strong polarization of Si—OH groups. The decreasing of leakage current at high Si—OH content is due to the low connecting probability p of networks because the networks break at the terminal Si—OH groups. In the case of high ionization degree of D5 plasma, more Si—OH groups break and form Si—O—Si linkages by chemical condensation occurring between proximal Si—OH groups. As a result, the k value of SiCOH films can be further reduced.
Spectroscopic properties of Tm3+ ions in heavy metal oxide glasses for S-band optical amplifications
2006, 55 (5): 2613-2617. doi: 10.7498/aps.55.2613
The spectroscopic properties of Tm3+ ions in PbO-Bi2O3-Ga2O3-GeO2 glasses were investigated through the absorption, emission, and lifetime measurements. The JO intensity parameters, the spontaneous emission probability, the fluorescence branching ratio, and the radiative lifetime were calculated using the Judd-Ofelt theory. From the measured 1470nm band emission spectra, we found the FWHM was ～122nm and the peak stimulated emission cross-section was ～2.2×10-21cm2, but the measured lifetimes decreased with the Tm3+ ion concentration increasing and the increase of cross relaxation rates was a function of the square of the Tm3+ ion concentration. As a result, PbO-Bi2O3-Ga2O3-GeO2 glasses were still thought to be a promising host material for potential S-band optical fiber amplifiers.
Studies on the temperature and energy dependence of g factor in GaAs by femtosecond laser absorption quantum beats
2006, 55 (5): 2618-2622. doi: 10.7498/aps.55.2618
Time-resolved elliptically polarized pump-probe spectroscopy developed by us is used to study spin relaxation dynamics of electrons in an intrinsic GaAs under an external magnetic field. Absorption quantum beats are observed and attributed to Larmor precession of electron spin around the magnetic field. The frequency of this quantum beat is used as a new method to measure electronic g factor with high accuracy. In this paper, this new method is utilized to investigate the temperature and energy dependence of electronic g factor in the intrinsic GaAs. It is found that electronic g factor increases with the temperature and energy of electrons, which is guitc different from the dependence predicted by k·p theory. An empirical formula is given to describe the temperature and energy dependence of g factor by fitting experimental data.
2006, 55 (5): 2623-2627. doi: 10.7498/aps.55.2623
Eu3+ doped Y2O3 nanocrystalline and bulk powders with small content of Ag+ were prepared by chemical self-combustion. An interesting phenomenon was observed: with an increase in Ag+ content the luminescent intensity of the nanosized crystal is enhanced at first and then maintains a stable level; however for the bulk powder there is no obvious change in the luminescent intensity when the Ag+ content is varied. The morphology and crystal structure were characterized by transmission electron microscopy (TEM) and X-ray Diffraction (XRD), respectively. Emission spectra were measured for nano- and bulk samples. The fluorescence decay curves for all the samples, including nanosized and bulk ones, either with or without Ag+, were measured. It is found that the lifetime for the nanosized sample with Ag+ is longer than that without Ag+, and the lifetime for the bulk sample has no change. From these experimental results wecome to the conclusion that the Ag+ ion may link with a surfice O2-, which has a free bond, then the surfice unsaturated dangling bonds can be eliminated. The elimination of surfice dangling bonds may cause the increase of luminescence efficiency of Eu3+ doped nanosized Y2O3.
The micro-photoluminescence and micro-Raman study of Zn1－xCdx Se quantum islands (dots) in CdSe/ZnSe heterostructure
2006, 55 (5): 2628-2632. doi: 10.7498/aps.55.2628
We have investigated two types of Zn1-xCdxSe quantum islands in CdSe/ZnSe heterostructure with different size and Cd composition using micro-photoluminescence and micro-Raman spectroscopy. The micro-photoluminescence spectra at 4.2 K indicate that the photoluminescence peaks of the islands have a large red-shift of 166 meV when the thickness of CdSe deposition layer increased from 1.8 ML to 2.3 ML, which can't be explained by considering the change of quantum confinement potential only. We also found two other important mechanisms which may lead to the large red-shift by comparing the micro-photoluminescence and micro-Raman spectra. First, the sheet density of large Zn1-xCdxSe islands which have lower exciton ground state energy will increase and the large islands will dominant gradually the photoluminescence properties of the samples with increasing thickness of the CdSe layer. Second, the increase of Cd composition in the Zn1-xCdxSe quantum islands contributes to the large red-shift as well. This is verified by resonant Raman scattering spectra of the samples.
Calculation of spin and orbital moments of 3d transition metals using X-ray magnetic circular dichroism in absorption
2006, 55 (5): 2633-2638. doi: 10.7498/aps.55.2633
X-ray magnetic circular dichroism(XMCD) in absorption has been extensively used to determine spin and orbital moments by applying the sum rules to the absorption spectra of specified atoms. Due to quench of orbital moments in 3d transition metals, it is necessary to reduce errors in experiments and data analysis. In this article we focas on the data analysis in 4 aspects. 1) The influence of applied magnetic field H on signal intensity. Experiments reveal that, for H-α; for H>200Gs, it changes little with the increase of H. 2) There is a separation between absorption spectra resulting from the iron-core remanence being in the parallel or antiparallel directions. This separation is independent of incident X-ray polarization and can be eliminated by multiplying by a constant. 3) An analytic form of absorption spectrum was obtained by fitting experimental data using XPSPEAK 4.1. It can be used as a criterion to determine which numerical integration method is suitable under certain experimental conditions. And, 4) by choosing an error function as a background to be subtracted from the X-ray absorption spectra, a method of calculation of spin and orbital moments from XMCD absorption spectrum is also developed Finally, the spin moment, of 1.314μB and orbital moment of 0.141μB of cobalt atom in a 20nm thick cobalt film are figured out based on Bode rule numerical integration.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2006, 55 (5): 2639-2643. doi: 10.7498/aps.55.2639
193nm Al2O3/MgF2 HR mirrors were designed and deposited by electron-beam evaporation, and then annealed in air at different temperatures from 250℃ to 400℃. The center wavelength shifted to shorter wavelength after annealing. And the reflectance of HR region increased but the transmittance changed negligibly little with the increasing of the annealing temperature. The total integrated scattering was measured to distinguish the scattering loss and the absorption loss. The results indicated that the total optical loss in the HR region was absorption dominant rather than scattering dominant. To know clearly the loss-reduction mechanism of annealing, the corresponding single-layers of Al2O3 were deposited and annealed in the same way. The change of optical constants was analyzed to explain the reduction of the total optical loss. And the reflectance of the HR mirrors reached was higher than 98%.
2006, 55 (5): 2644-2650. doi: 10.7498/aps.55.2644
The effect of multi-wall nanotubes (MWNTs) treated at different temperature in vacuum on the electrochemical hydrogen storage properties were investigated. The MWNTs were synthesized by chemical vapor deposition. The CNTs(carbon nanotubes)-LaNi5 electrodes were prepared by mixing MWNTs and LaNi5 alloy in a weight ratio of 1∶10. Three-electrode system was introduced. The CNTs-LaNi5 electrodes were used as the working electrode. Ni(OH)2/NiOOH worked as the counter electrode and Hg/HgO as the reference electrode. 6mol/L KOH solution acted as the electrolyte. Results showed that the CNTs-LaNi5 electrodes with CNTs treated at 850℃ in vacuum has the best electrochemical hydrogen storage capacity with a maximum discharging capacity of 503.6mAh/g and a corresponding discharging plateau voltage of 1.18V under the same testing condition. From 500℃ to 850℃, the higher the temperature of heating, the better the electrochemical hydrogen storage property of WMNTs. However, CNTs-LaNi5 electrodes with CNTs treated at 950℃ in vacuum has lower discharging capacity under the same testing condition. This shows that the temperature of CNTs treatment in vacuum is an important factor that influences its electrochemical hydrogen storage performance.
2006, 55 (5): 2651-2656. doi: 10.7498/aps.55.2651
This paper shows that the environmental noise in the HTc rf-SQUID-based quasi-periodic magnetocardiograms (MCGs) can be reduced by singular value decomposition (SVD) and adaptive filtering without the reference channel input. Numerical simulation studies indicate that the method is an effective noise suppresser for one channel MCGs.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Detection of the recent 2000a climatic extreme anomalies based on a new kind of approach-the high-order moment method
2006, 55 (5): 2657-2662. doi: 10.7498/aps.55.2657
Based on the normalized high-resolution climatic proxy data, high-order moment method is used to detect the climate extreme anomalies in recent 2000 years in this paper. Combining with filtering method to extract the dominant components at physical backgrounds from the normalized data, we analyzed the information and examined the contributions of each component of the climate extreme events. The results show that: 1) On the timescale of more than 100 years, climatic oscillation with the period of about 1000 years may exits; besides, the 20th century witnessed the most active climate extreme anomalous phenomenon in recent 2000 years, so it may correspond to an active period of climate extreme anomaly phenomenon. 2) On 20—60year timescale, climate extreme anomaly phenomenon in the period of A.D. 300—1100a is relatively distinct and that in time span of A.D.1100—1980a is comparatively mitigative. Possibly, this level makes no considerable contribution to the climatic anomaly of 20th century. This level and the century scale both reflect that for climatic change in recent 2000 years, the year around A.D. 1100a maybe the key period of climate transition. 3) On the interannual timescale (less than 20years), the years of occurrence of climatic extreme anomalies reflected in Beijing Shihua cave stalagmite proxy records has good correspondence with that of the E1 Ni?o or La Ni?a events (in this paper, only the period of 1960—1980a was considered). 4) high-order moment method has a good prospect in detecting the climatic extreme events.
2006, 55 (5): 2663-2668. doi: 10.7498/aps.55.2663
A new method named “permutation entropy” for detecting dynamical changes in time series was used to analysis the day-to-day temperature time series of north China from 1960a to 2000a. We detected three abrupt climate changes in the mid-1970s and early 1980s. Further analysis using Empirical Mode Decomposition to decompose the permutation entropy shows that the approximate 10a cycle is most important. So we can draw the conclusion that north China suffered three abrupt climate changes in the mid-1970s and early 1980s, and moreover, the the approximate 10a cycle induced the climate changes directly.