Vol. 53, No. 1 (2004)
2004, 53 (1): 1-4. doi: 10.7498/aps.53.1
The Collin principle has been used to estimate the upper bound and lower bound of practical capacitance,which gave out many simple and applicable results.The principle is generalized to the three-dimensional isolated and multi-conductor systems in this paper.Some applicable examples are given by using the approximate method of equal surface area.
Mei symmetry,Noether symmetry and Lie symmetry of Hamiltonian canonical equations in a singular system
2004, 53 (1): 5-10. doi: 10.7498/aps.53.5
The Mei symmetry,i.e.the form invariance,of the Hamiltonian canonical equations in a singular system is studied.The definition,the determining equations,the restriction equations and the additional restriction equations of Mei symmetry of the system are given.The relations among the Mei symmetry,the Noether symmetry and the Lie symmetry are studied,and the conserved quantities of the singular system are obtained.An example is given to illustrate the application of the result.
2004, 53 (1): 11-14. doi: 10.7498/aps.53.11
A new function-transformation method is proposed and four kinds of explicit exact solitary wave solutions of the Lienard equation, the generalized PC equation, the important nonlinear wave equation in mechanics, which have a fifth-order stronger nonlinear term, are obtained by a combination of the function-transformation method and the direct integral method. The method proposed in this paper can also be applied to other nonlinear equations with higher-order nonlinear terms.
2004, 53 (1): 15-20. doi: 10.7498/aps.53.15
A method of sampled-data and prediction -feedback control for chaotic systems is put forward. By this method,the chaotic Lorenz system and Chua's circuit system can be controlled in assigned equilibrium points and the periodical trajectories. The results of theoretic and numerical calculations indicate that this controlling method is strongly robust.
2004, 53 (1): 21-24. doi: 10.7498/aps.53.21
We consider the fuzzy control problem of a chaotic system, based on genetic algorithm. We apply the genetic algorithm to optimize the parameters of membership function of fuzzy controller, which integrates the global searching and high precision of genetic algorithm and the celerity of the fuzzy controller. The results of simulation show the effectiveness of this method.
Based on the lattice model of traffic flow, the lattice models which consider the next-nearest-neighbour flow on traffic and pay attention to the following vehicle as well as the preceding vehicle are presented, respectively. The stability criterion is derived by the linear stability analysis. The evolution of density wave has been investigated analytically with the perturbation method. The results show that the occurrence of traffic jamming transitions is described by the kink solution of the modified KdV equation. The theoretical results are in good agreement with the simulation. Moreover, the simulation indicates that the lattice models considering the next-nearest-neighbour interaction can stabilize the traffic flow, but the other model is just in opposition to the former model.
2004, 53 (1): 31-36. doi: 10.7498/aps.53.31
In order to increase the resolution and realize the versatility of cantilever, U-shaped piezoresistive cantilever array has been designed and fabricated in this research. The optimized geometries of cantilever and piezoresistor were determined by analyzing the stress, noise and sensitivity of piezoresistive cantilever from theory. Based on silicon micromachining technology, the piezoresistive cantilevers were fabricated by using polysilicon as the piezoresistive materials. With the measurement results of noise and sensitivity, the Hooge factor was calculated to be 3×10-3, the gauge factor is 27, and the minimum detectable deflection of piezoresistive cantilever was calculated to be 0.5nm at a 6V bias voltage and a 1000Hz measurement bandwidth. The noise sources of polysilicon pizeoresistor are also discussed in this paper.
ATOMIC AND MOLECULAR PHYSICS
2004, 53 (1): 37-41. doi: 10.7498/aps.53.37
A theoretical study on Cu2n±(n=0,1,2) using density functional method (B3LYP/LANL2DZ) shows that Cu2, Cu+2,Cu-2,Cu2-2 are stable . Electronic ground states are 1Σg＋（Cu2），2Σg（Cu＋2），2Σu（Cu-2）and 1Σg（Cu22-）and their force constants and spectroscopic data have been worked out .The potential energy curves of Cu-2 and Cu2-2 have both minimum and maximum, which are the so-called“energy trapped”molecules，while Cu2+2 is unstable. At the same time, the effect of charge on the potential energy function and energy levels for Cu2n±(n=0,1,2) are discussed.
Theoretical calculation of ionization potential and term energy of the ground states 1s22s of lithium-like systems from Z=21 to 30
2004, 53 (1): 42-47. doi: 10.7498/aps.53.42
In the paper, the full-core plus correlation (FCPC) and the minimizing the expectation value of the Hamiltonian method is extended to calculate the non-relativistic energies and the wave functions of 1s22s states for the lithium-like systems from Z=21 to 30. The mass-polarization and the relativistic correction included the kinetic-energy correction, the Darwin term, the electron-electron contact term, and the orbit-orbit interaction are calculated perturbatively as first-order correction. The contribution from quantum electrodynamic (QED) is also included by using the effective nuclear charge formula. The ionization potential and term energies of the ground states 1s22s are given and compared with other's theoretical calculation and experimental results. It is shown that the FCPC methods are also effective to theoretical calculation of the ionic structure for high nuclear ion of lithium-like systems.
2004, 53 (1): 48-53. doi: 10.7498/aps.53.48
The Rayleigh-Ritz variational method is used with a multiconfiguration-interaction function to study the energies of singly excited and doubly excited Rydberg series in helium atom, including the mass polarization and relativistic corrections. The oscillator strengths, the transition rates, and the wavelengths are also calculated. The results are compared with other theoretical and experimental data in the literatures.
2004, 53 (1): 54-57. doi: 10.7498/aps.53.54
The influence of the interaction between atoms in Bose-Einstein condensate (BEC) on the squeezing properties of atomic lasers is studied under Bogoliubov approximation. The results show that the fluctuations of two quadrature components of the atomic laser can be squeezed and the interaction between atoms in BEC is disadvantageous to the squeeze of atomic lasers.
CLASSICAL AREA OF PHENOMENOLOGY
2004, 53 (1): 58-61. doi: 10.7498/aps.53.58
By upgrading the cell's structure and other conditions in experiment,we have prepared the large diameter (1μm and 700nm)artificial opal crystals with the operation of the nitrogen pressure and gently rock,and measured the samples' band gap property.This experimental result will be useful in preparation of the photonic crystal with infrared band gap.
2004, 53 (1): 62-65. doi: 10.7498/aps.53.62
In this letter energy transfer between continuous channels is analysed both theoretically and experimentally. Energy transference has been realized by changing the parameters of combined wave-guides. In the experiment, we have also measured the transmission spectrum.
2004, 53 (1): 66-69. doi: 10.7498/aps.53.66
Co-polarized and cross-polarized holographic gratings were respectively recorded on the indolylfulgimide/PMMA film and their diffracted images were obtained. The experiment indicates that the indolylfulgimide possesses the characteristic of photo-induced anisotropy, therefore it can be used for cross-polarized holographic recording. In polarization holography, the polarization of diffraction beam is dependent on the polarizations of reference beam and object beam. The noise of the diffraction image mainly comes from the diffusion of the readout beam on the sample. Cross-polarized holography can obtain higher signal-to-noise ratio of reconstructed image than that of co-polarized holography. Holographic gratings recorded on the film can be kept for at least five months in darkness at room temperature without decrease of diffraction efficiency. The results show that indolylfulgimide is an erase-rewritable optical storage material that can be used in polarization holography.
2004, 53 (1): 70-74. doi: 10.7498/aps.53.70
A scheme for reconstructing the Wigner function of cavity fields is proposed, and this scheme can be realized with the micromaser. We find that there is a simple relation between the Wigner function of cavity fields and the atomic populations. Therefore, we can reconstruct the Wigner function of cavity fields by measuring the atomic populations in the micromaser experiments. We test our theory with the one-photon Fock state and the Schr?dinger cat state, and find that the results are rather good.
Entanglement and disentanglement in the nonlinear interaction between squeezing vacuum state field and atom
2004, 53 (1): 75-81. doi: 10.7498/aps.53.75
The properties of the entanglement in the intensity-dependent coupling interaction between squeezing vacuum state field and atom with an additional Kerr medium are investigated by using the von Neumann entropy. The influences of nonlinear interaction intensity of Kerr medium and the squeezing degree of initial squeezing vacuum state field on the entanglement of the quantum system are discussed. The results show that the nonlinear interaction of Kerr medium can result in the generation of the maximally entangled state, disentangled-free state or disentangled state, and the degree of entanglement is stationary.
2004, 53 (1): 82-86. doi: 10.7498/aps.53.82
The propagation equations of second-order moments of truncated circularly symmetric beams are derived, which obey the propagation laws similar to those for the non-truncated case. Based on the method of generalized truncated second-order moments, the second-order characteristic parameters of two-dimensional truncated beams in the rectangular coordinate system are extended to the truncated circularly symmetric beams in the cylindrical coordinate system, and the generalized M2 factor (M2G factor) can be defined in a similar way and remains invariant upon propagation. The consistency of the theory is interpreted physically. The second-order moments parameters and M2G factor of truncated super-Gaussian beams in the cylindrical coordinate system are derived and some interesting special cases are discussed and illustrated with numerical examples.
Generation of high repetition rate femtosecond Ti:sapphire regenerative amplified pulse of high beam quality for ultra-precision machining
2004, 53 (1): 87-92. doi: 10.7498/aps.53.87
On the basis of calculation and analysis on femtosecond amplification gain saturation and femtosecond pulse stretching, we believe it is possible to achieve moderate energy femtosecond regenerative amplification without stretcher if the amplifying cavity parameters is optimized and the pumping pulse energy is changed. According to the conclusion, applying low pumping energy and Ti:sapphire off-focusing technique to the amplifier, we do the experiment about high repetition rate femtosecond Ti:sapphire regenerative amplification and compressing without the stretcher. The femtosecond amplified pulse of perfect beam quality is obtained. The energy of amplified pulse is more than 100μJ.The amplified pulse repetition is 1kHz. The spectrum FWHM is 7.7nm.After the compressing,the pulsewidth is 500fs.
2004, 53 (1): 93-98. doi: 10.7498/aps.53.93
In this paper we have theoretically investigated the time telescope that offers a function of pulse expanding. Time telescope, with features of high efficiency and near Fourier-transform limit, is constructed mainly by using nonlinear parametric conversions of chirped pulses. Based on both analytical and numerical studies, it is found that group-velocity mismatch of the nonlinear crystal plays an important role in temporal aberration and conversion efficiency, and the criteria for designing a near aberration-free telescope are summarized. Finally, it is experimentally demonstrated that a 70fs short pulse is expanded to a 60ps long pulse near Fourier-transform limit.
2004, 53 (1): 99-104. doi: 10.7498/aps.53.99
Theoretical model is established to study the Stimulated Brillouin Scattering (SBS) process with both noise initiation and seeding Stokes laser assisted initiation.Multi-steps finite difference method is adopted to solve coupled wave equations numerically, and the temporal and spatial profiles of scattered light intensity and density variations of the medium are obtained. The dependence of Stokes laser on SBS reflectivity and initiation thresholds is investigated.It is found that the time needed for SBS initiation decreases sharply since the seeding Stokes laser is introduced. Finally, the damage potential of the SBS process in optical materials is evaluated and future work is outlined.
2004, 53 (1): 105-113. doi: 10.7498/aps.53.105
The effects of a series of parameters such as injected signal intensity , pump intensity, crystal length have been analyzed in Optical parametric amplification(OPA). The gain saturation and pump depletion were taken into account. The stability of an OPA system was investigated in both the small signal injection and the large signal injection. Experimental investigations of OPA based on two noncollinear phase-matched LBO OPA under the small signal injection and the large signal injection have been carried out，and the experimental data are in agreement with the theoretical results. The theoretical and experimental results all demonstrated that there exists a strong gain saturation region where parametric light with high efficiency and high quality and high stability can be obtained by properly choosing relevant system parameters.
The correlations among three important parameters in SPIDER, the time delay τ between the test pulse replicas, the frequency shear Ω and the second-order dispersion φ determined from a stretcher, are investigated. The results show that there are different optimum time delays τ for different test pulses with a fixed φ. The optimum time delays τ will increase as the duration of test pulse gets longer. However, the measurement may fail if the frequency shear Ω, which is direct proportional to the time delay τ, becomes too large to meet the condition of the sum-frequency. Therefore, for different widths of pulses, the magnitude of φ should be properly adjusted for selecting the optimum τ.
“The quasi invariant V” analysis for the chaotic behavior of Lorenz map in the nearest neighborhood of the second threshold of laser
2004, 53 (1): 119-124. doi: 10.7498/aps.53.119
In order to study the laser oscillation instability, in this paper we present the “quasi invariant V” method for analysis of the chaotic behavior of Lorenz map in the nearest neighborhood of the second threshold of laser, and demonstrate the relation between initial value V0 and the time tc for the occurrence of chaos, i.e. V0tc equals a constant C0 depending on the parameter σ,b,r of the system.
Enhancement and suppression of the spontaneous emission of a two-level atom in a photonic crystal with a state-conservative photonic pseudogap
2004, 53 (1): 125-131. doi: 10.7498/aps.53.125
It is shown that there exist a conservation rule of the number of states in a photonic crystal with pseudogaps in the whole frequency regime, and a local conservation rule of the number of states in a photonic crystal with completely photonic band gaps in every band regime. These conservation rules state that, relative to the photonic density of states (DOS) in the effective medium of a photonic crystal, if a valley of the DOS in the photonic crystal appears in some range of frequencies, it must be compensated for by the increases over some other ranges. In this paper, using a model DOS with a state-conservative photonic pseudogap, we obtain naturally the enhancement and suppression of spontaneous emission of a two-level atom in a photonic crystal, and a DOS-induced suppression, enhancement, narrowing and redshift or blueshift of spontaneous emission spectra. Our theory will be especially appropriate in the problem of the spontaneous emission of a collection of dependently emitting atoms or molecules with essentially random dipole orientations in a photonic crystal with a photonic pseudogap.
Probing the total density of states in a photonic crystal with different number density distributions of luminescent molecules
2004, 53 (1): 132-136. doi: 10.7498/aps.53.132
The dynamical characteristics of the spontaneous decay of a single excited atom or molecule depend strongly on its position within a photonic crystal (PC) and the relative orientation of its dipole and the electric field there. Measuring its dynamical characteristics of the spontaneous decay in a PC can only obtain the local density of states (DOS) in the PC, but cannot reflect the total DOS in the PC. It is found, theoretically, that using different number density distributions of luminescent molecules within the PC, one can probe the partial information or even all of the total DOS in the PC. In this paper, we separate, for the first time to our knowledge, the total DOS into two parts, DOS in the scatterer and DOS in the matrix of the Wigner-Seitz unit cell, according to their origin, and demonstrate that a special number density distribution of fluorescent molecules can reflect the total scatterers DOS or total matrix DOS or even both of them, i.e. the total DOS in the PC. This is meaningful in explaining and designing the experiments of prohibiting and enhancing the spontaneous emission of excited molecules.
2004, 53 (1): 137-144. doi: 10.7498/aps.53.137
In this paper, principles for monocyclic pulse generation by crossed phase modulation in silicon fibers are introduced. The effect of pulse parameters on the spectra broadening is analyzed in detail. For an actual example, split-step Fourier method is performed to verify the theoretic analytic results. Finally, an ultrabroad-band spectrum ranging from 200 to 1000THz and its ideally compressed pulse is achieved.
2004, 53 (1): 145-150. doi: 10.7498/aps.53.145
The characteristics of transmission spectrum of Fabry-Perot(F-P) cavity composed of two fiber Bragg gratings(FBG) are analysed in this paper. As the influence factors, bandwidth of FBG, phase factor of reflection coefficient of FBG and length of cavity are included. The changes of its characteristics caused by temperature and strain are dicussed. Then the method of how to design this kind of F-P cavity with single mode at the Bragg wavelength of FBG is provided. The numerical simulation spectrum and experimental result are also given.
Annealing effect on the photoluminescence and dangling bonddensity in erbium-doped hydrogenated amorphous silicon
2004, 53 (1): 151-155. doi: 10.7498/aps.53.151
In this paper, Er photoluminescence(PL) of Er-doped a-Si:H films, prepared by rf magnetron co-sputtering with various densities of Si dangling bonds(DBs), was studied with structure modification resulting from post annealing at a temperature range of 200-500℃. It was unexpectedly found that, the intensity of the Er-PL continually increases with annealing temperature below 350℃, meanwhile the density of Si-DBs decreases up to 250℃ due to the structural relaxation but then increases up to 500℃ probably due to Si-H being broken. Since the Er3+ ion transition depends on the symmetry of the Er3+ site in the host. The increase in the Er-PL intensity against the decrease of the density of Si-DBs is probably attributed to a change in the environment of Er3+ ions during annealing.
2004, 53 (1): 156-159. doi: 10.7498/aps.53.156
LiNbO3 crystals doped with different amount of MgO have been produced using vapor transport equilibration technique. The poling characteristics of the crystals with different MgO doping levels have been investigated. The switching field and the spontaneous polarization Ps of the crystals were expressed as functions of crystal stoichiometry and Mg content. The switching field for the domain reversal in the near stoichiometric crystal with ［Li］/［Nb］=0.973 doped with 2.0mol.% MgO was 1.8kV/mm. This is about one twelfth of the switching field for the congruent crystal, and the pattern of the periodic domain in the crystal is better than in the congruent and stoichiometric crystal.
2004, 53 (1): 160-164. doi: 10.7498/aps.53.160
Water molecular gel is a kind of soft condensed systems. It was formed by the aggregation and self-assembly of gelator 4,4′-bisstearylamide diphenyl ether (BSADE) which makes water into gelation at very low concentration in water. Its microcosmic morphology, thin fibre-like aggregate, was observed by transmission electron microscope(TEM) of the gels. By digitalized treatment of TEM photos, the structure of three-dimensional networks of gels shows a typical fractal characteristic. The fractal dimension D of the gels was calculated by the methods of Sandbox and density-density correlation function under the C++ program. The fractal dimension D calculated by the two methods is in the range of 1.814—1.977. Through the fractal theory, the aggregation process of gelator and the fractal characteristic of the gels was discussed. Further investigation by small angle x-ray scattering indicated that the fractal scale of the gels is between α=1nm and ε=40nm. and fractal characteristic was shown in a larger area. Its fractal dimension is 1.9, which is in agreement with the results from TEM method.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2004, 53 (1): 165-170. doi: 10.7498/aps.53.165
Ponderomotive-force driven acceleration of an electron at the focus of a high-intensity short-pulse laser is considered using a model that the electrons accelerated are near the focus but decelerated far away the focus. For intensities above 1019Wμm2/cm2, the electron's energy gain in the range of MeV can be realized when the electron leaves the laser pulse. Final energy gain of the electron as a function of its initial position has also been discussed. We find that an electron initially near the focus can be accelerated well.
2004, 53 (1): 171-175. doi: 10.7498/aps.53.171
When two ultrashort and ultrahigh laser pulses spacing an appropriate distance propagate through tenuous plasma, the second laser pulse will be compressed and shift to higher frequency because the anterior part of the pulse experiences the higher plasma density and propagates with lower group velocity while the rear part of the pulse experiences the lower plasma density and propagates with higher group velocity. The frequency shift of the second pulse in our PIC simulation is in good agreement with the theoretical predication. Furthermore, the optimum intervals between the two pulses is larger than that estimated with linear theory.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2004, 53 (1): 176-181. doi: 10.7498/aps.53.176
Based on elastic theory and yield strength formula of polycrystalline films, the strain-energy densities in differently oriented grains have been calculated for a bcc-polycrystalline film on a substrate. The results show that, (1) prior to yielding, the Fe and Ta are basically similar and with (100), (510), (410) and (511) oriented grains having the lowest strain-energy densities. For Cr, Mo, Nb and V, however, the four lowest strain-energy densities correspond to the grains with (111), (332), (322) and (211) planes oriented parallel to the film surface. For isotropic W, the strain energy density is independent of the grain orientation. (2) In all of yielded bcc-polycrystalline films, the four lowest strain energy densities correspond to the grains with (100), (111), (110) and (411) planes oriented parallel to the film surface. Considering strain energy minimization, we predicate that the grains with these orientations should be favorable in crystal growth successively.
2004, 53 (1): 182-188. doi: 10.7498/aps.53.182
The microstructure of TiN coatings and Ti1-xSixNy nanocomposite coatings was studied by means of x-ray diffrection, x-ray photoelection spectroscopy, high resolution transmission electron microscopy and atomic force microscopy. The results showed that the Ti1-xSixNy coatings consisted of TiN nanocrystallites with grain size of 3-5nm and amorphous Si3N4 phase. The roughness of Ti1-xSixNy coatings was less than that of TiN coatings prepared under the same conditions. The orientation of TiN crystals in the Ti1-xSixNy coatings was discussed on the basis of interface energy.
The effect of Co substitution and stabilizing element on the structure and magnetic properties of Nd3(Fe,Co,M)29(M=Ti,V,Cr) compounds
2004, 53 (1): 189-193. doi: 10.7498/aps.53.189
The effect of Co substitution and stabilizing elements on the structure and magnetic properties of Nd3Fe29-x-yCoxMy(M=Ti,V,Cr) compounds was investigated by means of x-ray diffraction and magnetic measurements. The results show that all the synthesized compounds have been crystallized in Nd3(Fe,Ti)29type structure with monoclinic symmetry and space group A2/m. The solution of Co element in the compounds is different for the different stabilizing elements. For Cr as the stabilizing element, the pure Co-based 3:29 phase could be obtained and the higher Cr content is required to stabilize the phase with increasing Co content. For Ti and V, the solution of Co in the compounds could be only 6.63 and 12 atoms, respectively. All the Nd3(Fe, Co, M)29compounds show easy-planar type anisotropy at room temperature. For the Nd3(Fe,Co,V)29compounds, the Curie temperature and saturation magnetization increase first and then decrease with further increasing Co content. The spin reorientation temperature shows and increasing tendency except the minimum for x=6 due to the preferential occupation of Co atoms. For the Nd3(Fe, Co, Cr)29compounds, the Curie temperature increases first and then decreases with increasing Co content, but the magnetization is diffcult to be saturated and only shows a decreasing tendency. The spin reorientation transition was only observed for the samples with x=0 and 6.
2004, 53 (1): 194-199. doi: 10.7498/aps.53.194
A kind of test method of total dose effects (TDE) is presented for very large scale integrated circuits (VLSI). The consumption current of devices is measured while function parameters of devices (or circuits) are measured. Then the relation between data errors and consumption current can be analyzed and mechanism of TDE in VLSI can be proposed. Experimental results of 60Co γ TDEs are given for SRAMs, EEPROMs, FLASH ROMs and a kind of CPU.
2004, 53 (1): 200-203. doi: 10.7498/aps.53.200
Diffusion of implanted Mo in the Zr57Nb5Cu15.4Ni12.6Al10 bulk non-crystalline alloy has been researched using secondary ion mass spectroscopy (SIMS). Activation energy Q and pre-exponential factor D0 were obtained to be 1.95 eV and 1.13×10-5m2s-1, respectively. Based on the Stokes-Einstern relation, the viscosity of the Zr57Nb5Cu15.4Ni12.6Al10 bulk non-crystalline alloy was investigated at temperatures below the glass transition, it decreases with the increase of temperature from 1.07×1010 Poise at 593 K to 1.42×108 Poise at 673 K.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2004, 53 (1): 204-209. doi: 10.7498/aps.53.204
The photoluminescence (PL) spectra of Mg-doped GaN on sapphire have been measured at 12K. The residual strain induced by the mismatch of the lattice constants and thermal expansion between GaN epilayers and substrates have been investigated by micro-Raman scattering and x-ray diffraction. Redshifts of the ultraviolet PL bands were observed in Mg-doped GaN after annealing in nitrogen ambient, which may be related to the carrier concentration and the strain in the epilayers. For heavily Mg-doped GaN films, the hole concentration in GaN, which shifts the PL peaks, is the dominant influence, whereas the strain is comparatively secondary.
2004, 53 (1): 210-213. doi: 10.7498/aps.53.210
In order to investigate the effect of Mg, Al doping on the electronic structure of LiCoO2, and to find the mechanism of high electronic conductivity in Mg-doped LiCoO2, we have carried out first-principles calculations based on density-functional theory (DFT) for LiCoO2, Li(Co, Al)O2 and Li(Co, Mg)O2 systems. Studies of band structures and densities of states show that hole states, which enhance the conductivity of semiconductor, appear in the valence bands of Mg-doped material, and the effect of charge disproportionation changes the distribution of Co-3d states in different energy levels. The ionicity of O2- increases with Mg and Al doping.
2004, 53 (1): 214-217. doi: 10.7498/aps.53.214
We have calculated the optical properties (the imaginary part of dielectric function ε2(ω), the optical absorption coefficient I(ω), the reflectivity R(ω) of SrTiO3 and SrHfO3 by the full potential linearized augmented plane wave (FLAPW)method. Through comparing their optical properties, we obtained the ifference between SrTiO3 and SrHfO3 and explained such a difference. Our calculated optical spectra have mainly four peaks around 4.4, 7.4, 8.3 and 23.6 eV, and the peak at about 4.4eV is sharp and high. Our calculated results are in good agreement with the experimental data.
2004, 53 (1): 218-225. doi: 10.7498/aps.53.218
We present a theoretical study of the optical gain of InGaAsSb/AlGaAsSb type-I quantum well lasers, whose lasing wavelength is designed to be 2.7μm. A self-consistent solution, which solves the Schrdinger equations and Poisson equation simultaneously, is used to calculate the band structure and gain spectra of the quantum wells. By studying the influence of strain and width of the well material, we find that the main factor limiting the optical gain is not the optical matrix element, but the population inversion, especially the probability to find a hole in the first valence subband. Increasing the compressive strain or (and) decreasing the well width will enlarge the optical gain. The former lowers the in-plane effective mass of the hole. Although the latter slightly increases the in-plane effective mass of holes, it does enlarge the energy separation of the valence subbands. Both effects lower the total state density near the valence edge, and finally enlarge the optical gain. Our theoretical results can explain qualitatively the reported experimental results, and are useful for the design of InGaAsSb/AlGaAsSb long wavelength quantum well lasers.
2004, 53 (1): 226-228. doi: 10.7498/aps.53.226
A series of FexSn100-x granular alloy films with different atomic fractions were fabricated by ion beam sputtering. The highest value ever observed for ferromagnetic metal/non-magnetic metal systems has been obtained. The extraordinary Hall effect was studied systemically by varying the atomic fractions and the thickness of the samples. With the power law between the saturated Hall resistivity and the resistivity the mechanism of the extraordinary Hall effect was also discussed.
2004, 53 (1): 229-234. doi: 10.7498/aps.53.229
Absorbing material is used to reduce the Q factors of resonant cavities, enhance the bandwidth and restrain the competitive modes in high-power microwave devices. In this paper, solving field in absorbing material and field matching method were used to derive the dispersion equation in waveguides and field matching formulas of hybrid modes (HEM) in an abrupt cavity with absorber. Based on the theoretical analysis, numerical simulations of the cavities in gyroklystrons were done.The influence of absorber on transmission and attenuation in waveguides, the loss and resonance characteristic and quality factors in cavities has been studied. The main results are presented.
2004, 53 (1): 235-238. doi: 10.7498/aps.53.235
A pn heterojunction C-V technique used to determine the bandgap of SiGe strained layers is presented in this paper. The SiGe bandgap is analyzed and calculated by acguiring the built-in potential and discontinuities of valence and conduction bands, according to the C-V profile of the stained SiGe/Si pn heterojunction. This technique is much more convenient and the experimental results agree very well with the theoretical and published calculations, indicating that the method is correct. This method is suitable for not only the bandgap of the single SiGe/Si pn heterojunctions, but also that of the SiGe/Si devices with SiGe/Si pn heterojunctions.
2004, 53 (1): 239-243. doi: 10.7498/aps.53.239
A gold thin film percolation system, deposited on glass surfaces by the vapor deposition method, has been fabricated. By using the expansive and movable properties of silicone oil drop, a characteristic wedge - shaped film system with a slope of ～ 10-5 naturally forms during deposition. The electrical conductivity of the band-like gold film, i.e., the uniform part with a fixed thickness, is measured with the four-probe method. It is found that hopping and tunneling effects of the films are stronger than those of the other films.
Valence-band electronic density of states of Yb2.75C60 thin films was measured by the ultraviolet photoemission spectrum technique. The phase-pure Yb2.75C60 sample was characterized by the C 1s XPS measurements. The result indicates Yb2.75C60 has no Fermi edge and thus is semiconductor. The hybridization between 6s state of Yb and the LUMO band of C60 cannot be considered to be negligible. Some Yb 6s electrons occupy the Yb-C60 covalent band in Yb2.75C60.
2004, 53 (1): 248-253. doi: 10.7498/aps.53.248
Rb-intercalated C60 thin film was prepared on the cleaved (111) surface of C60 single crystal under the ultra-high-vacuum condition. Phase evolution was studied by the synchrotron radiation photoemission technique. The energy distribution curves for the solid solution phase, Rb1C60 and Rb3C60 phases were observed. After the Rb3C60 thin film with a thickness of nanometers has formed on the surface layers of the C60 single crystal, the excess depositions of Rb do not induce the bulk-like fcc to bct or bcc structure transitions at room temperature. The large size of C60 molecule offers the surface vacancies for the formations of Rb4C60 and Rb5C60 adsorption phases that are further verified by Rb 3d core-level photoemission measurements. Valence band photoemission results exhibit that the surface phases are metallic.
2004, 53 (1): 254-259. doi: 10.7498/aps.53.254
The Xα method is applied to study the spin-polarized electronic structures and atomic magnetic moments of clusters La8-xBaxCuO6 in the compound La2-yBayCuO4.This paper shows that Cu magnetic moment in La2CuO4 is 0.37μB, which is in better agreement with the experimental value(0.48±0.15μB). Ba substitution lowers the structure symmetry, removes molecular orbital degeneracy, and hence strengthens orbital hybridization and weakens quasi-two-dimensional feature, which enhance the coupling between Cu-O layer and the neighbormg layers and influence the charge transfer. These changes strongly affect the properties of carriers. Holes are introduced due to Ba substitution and they mainly enter into O sites, a few d-electronic holes enter into Cu sites, which causes important effects on the density of states, atomic magnetic moment and valence states of Cu and O in Cu-O layers. The results confirm that the holes are not always beneficial to the superconductivity. At the beginning the holes are beneficial to the superconductivity; they can decrease the superconductivity when more holes are introduced. This paper can explain that the transition temperature of high-temperature superconductive oxides changes with the variation of the doping level.
Transport properties and magnetoresistance effect of polycrystalline La0.7Sr0.3MnO3 at low temperatures
2004, 53 (1): 260-264. doi: 10.7498/aps.53.260
The resistivity and magnetoresistance of bulk polycrystalline La0.7Sr0.3MnO3(LSM), deliberately fabricated to produce nanoscale grains, has been investigated as functions of temperature. With decreasing temperature below room temperature the resistivity (ρ) exhibits a maximum near 250K， below which it displays a “metallic" behaviour. However, ρ exhibits subsequently a minimum near 50K, below which it is well fit by the predictions for the tunneling of conduction electrons through insulating interfacial layers viz. lnρ∝T-1/2, a result interpreted as conduction-electron tunneling between adjacent LSM granules. The minimum of ρ occurs in this particular sample may result from the competition of the tunneling effect and the intrinsic transport mechanism of LSM. The temperature dependence of the associated tunneling magnetoresistance is also reported in this low temperature regime.
2004, 53 (1): 265-271. doi: 10.7498/aps.53.265
In this paper we present our numerical study on short-time critical dynamics for the q-state random-bond Potts model with self-dual quenched disorders. By exploring the universal power-law scaling behavior, the results of magnetic exponent η and dynamic exponent z are estimated for the q=3 and q=8 cases with two specific disorder distribution functions. Our Monte Carlo simulations show evidence that the results of magnetic exponent η are independent of distribution forms, which verifies the existence of universality for the general quenched random-bond models numerically.
2004, 53 (1): 272-275. doi: 10.7498/aps.53.272
The establishment and thermal stability of exchange bias between the ferromagnetic NiFe layer and antiferromagnetic FeMn layer have been studied. We focused on the relationship between FeMn grain size and exchange bias with the size of FeMn grains controlled by varying Cr content in Ni-Fe-Cr alloy buffer layer. The results indicate that, smaller FeMn grains lead to a larger exchange bias field, but the thickness with the exchange bias setting on is larger, which is consistent with Mauri's model. Thermal stability of the exchange bias indicates that, the films with bigger FeMn grains have a higher blocking temperature, which is in agreement with Noshioka's conclusion: bigger antiferromagnetic grains possess higher activation energies and longer relaxation times.
Dielectric study on relaxor-like phase transition of lanthanum doped bismuth layer-structured ferroelectrics
2004, 53 (1): 276-281. doi: 10.7498/aps.53.276
Lanthanum doped bismuth layer-structured ferroelectrics and its intergrowth type, SrBi4-xLaxTi4O15, Sr2LaxBi4-xTi5O18, and (Bi,La)4Ti3O12-Sr(Bi,La)4Ti4O15, have been prepared by solid-state reaction method. The dielectric properties of those materials are measured. The results show that SrBi4-xLaxTi4O15 (x=1.0), Sr2LaxBi4-xTi5O18(x≥0.5)， (Bi,La)4Ti3O12-Sr(Bi,La)4Ti4O15(x=1.50) exhibit typical relaxational characteristics. The degree of diffuseness and relaxation increased with increasing La3+ content. The origin of relaxation is the microdomain induced by La3+ doing. The micromechanism corresponding to the transition between normal ferroelectrics and relaxors is explained as the transition from the macrodomain of normal ferroelectrics to the microdomain of relaxors.
2004, 53 (1): 282-285. doi: 10.7498/aps.53.282
Nanocrystalline zirconia doped with 1mol%Eu3+ was prepared by co-precipitation method, Its structure was characterized with x-ray diffraction(XRD), the grain size of the nanocrystals was about 10nm obtained by XRD and Scherrer' equation. The luminescence of nanosized zirconia powders doped with Eu3+ ions was investigated under 394nm excitation, the emission of 5D0→7F2 transition peaked at 604nm. Furthermore, an enhanced emission at 604nm induced by ultraviolet irradiation was observed for the nanosized zirconia powders doped with Eu3+ ions.
2004, 53 (1): 286-290. doi: 10.7498/aps.53.286
A disorder hopping model for carriers recombination at organic/organic interface (OOI) in double-layer organic light emitting diodes (OLEDs) was presented. According to the structure of an OOI as well as spatial and energetic disorder of hopping states, it was more reasonable to use the disorder hopping model than to use the Fowler-Nordheim formalism. It was shown that the carriers hopping distance, the effective barrier height and the electric field contribution had heavy effects on recombination efficiency. Firstly, when the applied voltage was less than 19.5V (in double-layer OLEDs ITO/α-NPD/Alq3/Al), recombination efficiency increased with the increase of hopping distance, while the applied voltage was larger than 19.5V, it decreased with the increase of hopping distance; secondly, it also increased with the increase of effective barrier height at OOI; Finally, it increased with the increase of differences of electric field at OOI, while it decreased when the value of differences of electric field reached 24×105V/cm. These results were consistent with some experiments.
2004, 53 (1): 291-295. doi: 10.7498/aps.53.291
The thermoluminescence (TL) glow curves, three-dimensional emission spectra and optical-stimulated luminescence (OSL) of undoped α-Al2O3 single crystals were investigated. Two glow peaks at 76 and 207 ℃ are observed after β-rays irradiation of Sr-90 source. After γ-rays irradiation the emission spectra of α-Al2O3 show that the emission peak of 416 nm wavelength at about 207 ℃ is related to the relaxation of an electron from the excited 3P state to the ground state 1S of the F center. The first order kinetic equation can fit the 207 ℃ glow peaks well. Besides, the measured shape and peak temperature of the glow curves are almost not related to the absorbed dose. The dose response of the peak heights is linear-sublinear. Therefore, it is concluded that the glow peak at 207 ℃ at 416 nm can be described by the first-order kinetics. The dose responses of OSL and TL are linear-sublinear.
2004, 53 (1): 296-300. doi: 10.7498/aps.53.296
In this paper, the light-modulated characteristics of the gene-variant bacteriorhodopsin-D96N molecule film are studied by spectrum response experiment. The absorption enhancement phenomena of BRD96N films at 550—600nm modulated light wavelength are gained, and there are different absorption changes at different monochromatic wavelengths. Using curve-fitting method, it is found that the process of the absorption change includes fast and slow responses with time constants of 30 s and 5 min respectively. Applying the relations between the absorption change and gray-scale image to the write-readout image experiment, the reverse image phenomena in a wavelength range of 550—600nm are analyzed.
2004, 53 (1): 301-305. doi: 10.7498/aps.53.301
A columnal islands system, which was composed of three layers of self-assembled InAs/GaAs quantum dots (QDs), has been fabricated by solid-source molecular beam epitaxy (MBE) through S-K mode on a (100) semi-insulating GaAs substrate. The effects of the thickness of GaAs space layer, the growth interruption time and the amount of InAs deposition on the emission wavelength of columnal islands were presented. The image of atomic force microscopy (AFM) indicated the columnal islands with high uniformity in size and shape. At room temperature, the emission wavelength of columnal islands with different effective heights was achieved 1.32 and 1.4μm; however, the emission wavelength of single-layer QDs with normal height was just 1.1μm. It provides a useful and intuitive approach to artificially control the emission wavelength of a QD material system.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
The influence of the magnetic field magnetron under the substrate on the sputtering glow and film thickness gradient
2004, 53 (1): 306-310. doi: 10.7498/aps.53.306
A new magnetron sputtering method is tried by placing a magnet under the substrate during sputtering. Compared with the case without applied magnetic filed, we have found that the glow appearances and the film thickness distribution are changed greatly. The aspect of the glow above the substrate face vary with the magnet diameter. We make the glow appearances mechanism clear using the simulation of the magnetic field, and interpret the film thickness distribution as a result of the motion of the particles sputtered from the target in the special-distributed magnetic field.
2004, 53 (1): 311-315. doi: 10.7498/aps.53.311
Low-temperature epitaxy of MgxZn1-xO thin films is achieved on sapphire substrates by the reactive electron beam evaporation. X-ray diffraction (XRD) and energy dispersive x-ray (EDX) analyses of the films demonstrate that the structure of MgxZn1-xO depends upon the concentration of Mg in the films. With the increase of Mg concentration, the structure of MgxZn1-xO changes from the wurtzite crystal structure of ZnO to the cubic one with lattice constant similar to that of MgO. By the measurements of UV-visible transmission and ultraviolet photoluminescence spectra of the MgxZn1-xO films, it is found that the absorption band edge of MgxZn1-xO is blue shifted obviously with the increase of Mg concentration, indicating the absorption band edge of MgxZn1-xO becomes wider. However, the lattice mismatch between cubic MgxZn1-xO film and MgO is small (0.16%).
2004, 53 (1): 316-319. doi: 10.7498/aps.53.316
A new method to fabricate vertically aligned films for liquid crystals was proposed and studied experimentally. Photosensitive monomers of hexafluorobiphenol A dicinnamic esters were crosslinked by exposure under ultraviolet light. The obtained films could induce vertical alignment of liquid crystals. The process of the film making was very simple and did not need annealing at high temperatures. UV-visible spectra and polarized FT-IR spectra revealed that the photo-crosslinking reaction happened and reaction degree was rather high after the exposure under ultraviolet light. The stability of the aligned film could be kept to 120℃.
2004, 53 (1): 320-324. doi: 10.7498/aps.53.320
This paper describes the recent development of an instrument designed to allow real-time measurement of the size and chemical composition of individual aerosol particles. We focused on the principle and features of aerodynamic sizing, then combine it with laser desorption ionization time-of-flight mass spectrometry, and the real-time measurement of wood smoke particles was performed.