Vol. 57, No. 7 (2008)
2008-04-05
GENERAL
2008, 57 (7): 3949-3953.
doi: 10.7498/aps.57.3949
Abstract +
The RF field in the RF cavity can be expressed by the modes of the cavity. And the characters of the cavity is determined by such modes as the frequency and field. In the paper，an actual cavity is divided into two parts: the normal part (such as a normal cylindrical cavity) and the abnormal part (including a coupling hole). According to Maxwell theory，there exists a coupling equation between the modes of normal part and the modes of the cavity. So the influence of the coupling hole can be studied. Utilizing the method，an L-band cylindrical cavity with a coupling hole is studied，the calculated frequency of the working mode is very close to the simulated value.
2008, 57 (7): 3954-3961.
doi: 10.7498/aps.57.3954
Abstract +
This paper presents a modified dissipative particle dynamics (DPD) method，which employs an interaction potential with short-range repulsion and long-distance attraction. The modified DPD method is capable of simulating liquids with free surfaces，the behavior of bubbly liquids，drop dynamics and other important multiphase fluid flow processes. The modified DPD method was used to investigate the formation of micro-drops，and the large-amplitude oscillations of an initially oblate liquid drop. The numerical results clearly demonstrate that the modified DPD method involving the interaction potential of short-range repulsion and long-distance attraction is capable of capturing the physics of multiphase micro-drop dynamics，and is of significant importance for investigating complex multiphase systems at micro-meso-scales.
2008, 57 (7): 3962-3968.
doi: 10.7498/aps.57.3962
Abstract +
The ground state solutions of two-component Bose-Einstein condensates (BECs) in Bessel optical lattices (BLs) are studied by means of the balance condition between BLs strength and inter-atomic interactions. We consider a quasi-two-dimensional (2D) BECs system，strongly confined in longitudinal direction and weakly trapped in the radial direction in the transverse plane，which obeys 2D Gross-Pitaevskii equation (GPE) derived from its 3D counterpart. Analytically we obtained the atom number distribution，atom number and energy of ground state and give the parameter ranges. Compared to single-component BEC，two-component BECs exhibit a rich variety of ground state structures. These structures depend upon various parameters，in particular the inter-atomic interactions and the BLs strength. Neglecting the inter-component atomic interaction，the corresponding results of single-component situation are given.
2008, 57 (7): 3969-3972.
doi: 10.7498/aps.57.3969
Abstract +
The coordinate state vector 〈f(x)| is expressed with 〈x| in virtue of particle number states and the eigen equations of coordinate operator functions，accordingly we can convert non-symmetric projective operators into symmetric，the integrations over non-symmetric projective operators are obtained by the completeness relation of coordinate eigenstates.
Coherent state of hydrogen-like atoms: an application of the pseudo-angular-momentum operator method
2008, 57 (7): 3973-3977.
doi: 10.7498/aps.57.3973
Abstract +
Using the pseudo-angular-momentum operator method，the radial equation for bound state of the hydrogen-like atom is solved and the analytical expression for the eigenstate is derived. The result shows that the normalization of eigenfunction should be done carefully owing to its peculiarity. The corresponding coherent state is also discussed.
2008, 57 (7): 3978-3982.
doi: 10.7498/aps.57.3978
Abstract +
A 2-D dark energy star model with the action of strange matter is studied in this article. Equation set of the field and balance equation of star have been deduced，and some analytical solutions are obtained，which can be used to calculate the mass of the star. Moreover，we also investigated the influence of temperature on the mass of star and find that the gross mass of star has a upper limit in each case.
2008, 57 (7): 3983-3988.
doi: 10.7498/aps.57.3983
Abstract +
The transient properties (e.g., the mean first passage time T) of the gene transcriptional regulatory system driven by colored cross-correlated noise are investigated. The approximate Fokker-Planck equation is obtained based on the Novikov theorem and the Fox approach and the explicit expression of the mean first passage time is derived. The numerical computation results show that for the case of strong correlation intensity and short correlation time，a successive switch process (i.e.，“on”→“off”→“on”) occurs; for tha case of weak correlation intensity and long correlation time，only one switch process occurs (i.e.，“on”→“off”). The reentrance phenomena appear in the gene transcriptional regulatory system.
2008, 57 (7): 3989-3995.
doi: 10.7498/aps.57.3989
Abstract +
By using of Gegenbauer orthogonal polynomials，an effective numerical method for the reliability analysis of double random Duffing system with large coefficient of variation is presented. On the basis of the first passage model，the fourth-moment technique，the Edgeworth series approximation theory and the orthogonal polynomial expansion method are employed to develop a reliability analysis method systematically. The dynamic reliability and mean reliability of the system is discussed. The results obtained by this method are verified by Monte-Carlo simulations.
2008, 57 (7): 3996-4006.
doi: 10.7498/aps.57.3996
Abstract +
For nonlinear channel distortions of chaos-based communication systems，based on the analysis of the theory of chaotic signal reconstruction and the characteristics of transversal filter and functional link neural network (FLNN)，a novel nonlinear adaptive equalizer with the architecture of combination of transversal filter and functional link neural network (CFFLNN) is proposed in this paper. The novel nonlinear equalizer fully utilizes faster convergence characteristics of transversal filter and the nonlinear approximation capability of FLNN by function expansion due to enhanced input space. Furthermore，the performance of the novel nonlinear adaptive equalizer is also improved. Finally，the proposed equalizer is designed and its adaptive algorithm is deduced by the low complexity normalized least mean square (NLMS) method. And an analysis of stability and convergence for the derived algorithm is provided. To illustrate the analysis，results obtained from the computer simulation are also provided for both linear and nonlinear channels in chaos-based communication system.
2008, 57 (7): 4007-4013.
doi: 10.7498/aps.57.4007
Abstract +
We present a universal algorithm for transforming chaotic sequences of either chaotic map systems or chaotic differential dynamic systems into uniform pseudo-random sequences. Theoretically，the algorithm is based on bit-operations represented by floating-point algorithm，not aiming at any definite physical chaotic systems. It has been proved that，any real random variable generally has a type of natural tendency of homogenization which exponentially increases bitwise with random variable. As a result，any real chaotic sequence can be completely transformed into the pseudo-random sequence having uniform identical independent distribution. Adopting logistic map，Hénon map and Lorenz system as examples to test the universal validity of the algorithm，respectively，the experiments demonstrate that the algorithm is correct. We can reasonably expect that the universally valid algorithm should become the technological basis of standardized modular design of chaotic pseudo-random sequence generator in hardware implementation.
2008, 57 (7): 4014-4020.
doi: 10.7498/aps.57.4014
Abstract +
Two novel methods for generating spatiotemporal chaotic pseudorandom-bit sequences (PRBS) based on the one way coupled map lattices (OCML) model are proposed. In the first one，the symbolic sequences are generated by a partition，which is based on the statistic property of the state of the one way coupled map lattice. And in the other，the sequences are obtained by comparing the two consecutive state values of a single lattice of the OCML. We compareed the performance of these two types of sequences with the conventional ones. The numerical experiments demonstrated that the sequences have ideal randomicity, excellent correlation and run-length distribution，and they are good candidates for information security，cryptography and spread-spectrum communications.
2008, 57 (7): 4021-4028.
doi: 10.7498/aps.57.4021
Abstract +
In order to improve the accuracy and efficiency，an iterative digraph cell mapping method is presented through introducing the notion of composite cell space. This iterative method can refine any interesting region of cell state space. Because the refining process is performed by algebraic operation，the computer memory does not be increased additionally. Furthermore，the digraph dynamical properties can be preserved in the refining process. In order to implement it easily，an effective algorithm is designed. As an example，the Henon map is taken to demonstrate the validity and efficiency of this method.
2008, 57 (7): 4029-4033.
doi: 10.7498/aps.57.4029
Abstract +
The complex phase tracks of Canard have caused concern of the scientists in the fields of physics，biology and chemistry. In order to find the accurate function describing the phase track of Canard，a nonlinear model involving consecutive segments is suggested，from which the analytical solution of the phase track function of a nonlinear second order autonomous function can be readily obtained，therefore the function describing the phase track of a Canard and its parametric conditions can be obtained analytically. It follows that all the phase tracks can be simply described by two kinds of functions.
2008, 57 (7): 4034-4041.
doi: 10.7498/aps.57.4034
Abstract +
The goal gratings of five frequencies with different mean luminance were displayed on a CRT display，the contrast detection thresholds under different condition were measured for 9 young observers，and the curve reflecting the relationship between luminance and contrast detection threshold was obtained. Thus，human vision characteristic for transferring luminance was discussed in the range of luminance of CRT. The experimental results showed that the contrast thresholds of different observers were very similar for gratings with frequencies of 3.79 and 4.97cpd，while they were quite different for gratings with frequencies of 1.23，1.97 and 9.86cpd，and the contrast detection threshold could be lowered by increasing the luminance of grating，but the resolving power went to a limit with the luminance increasing，and effect of the CRT display matched with human vision characteristic very well when the luminance was 60cd/m2. The results would make a good foundation for research and application in image manipulation，display manufacture，photography，printing，color matching and so on.
ATOMIC AND MOLECULAR PHYSICS
2008, 57 (7): 4042-4048.
doi: 10.7498/aps.57.4042
Abstract +
The influence of plasma screening on the energy levels and oscillator strength for ions in a high-temporary plasma is investigated by using the Debye-model. The 2s2—［2s1/2,2p1/2］1 and 2s2—［2s1/2,2p3/2］1 transitions of 11 beryllium-like ions from MnXXII to BrXXXII are calculated by including the effect of plasma-screening. The results show that the effect of plasma-screening increases the 2s2—［2s1/2,2p1/2］1 transition energy of beryllium-like ions, which leads to a blue-shift; it also shows that the blue-shift becomes more and more obvious as the plasma-screening increases. The overall trend for the oscillator strength of the 2s2—［2s1/2,2p3/2］1 transition is similar to that of the transition energy.
2008, 57 (7): 4049-4058.
doi: 10.7498/aps.57.4049
Abstract +
On the basis of the improved averaged atomic model,the distribution of the free electrons is dealt with by partial wave method in central field approximation to improve the precision of energy level and electron populations, The broadening of high electronic levels into bands is taken into account in a uniform approximation. Therefore,the precision of atomic inner energy and pressure is improved. As samples, the parameter of EOS of Mo, Au are calculated, respectively.
2008, 57 (7): 4059-4065.
doi: 10.7498/aps.57.4059
Abstract +
The atomic data such as energy levels and transition probabilities of Xe10+ have been obtained basing on multi-configuration Dirac-Fock (MCDF) method. The electron correlation effects on transition probabilities of 4d8—4d75p have been investigated by adding the electron correlation of 4dn—5pn(n=1, 2, 3) step by step, which shows the effects are so obvious that one should include at least up to the correlation of 4d2—45p2 to get a convergent result. The calculated wavelengths are in good accordance with the experimental ones. The agreement between the length and velocity forms of transition probabilities is excellent, which shows the atomic wavefunction is accurate and the present result is reliable.
2008, 57 (7): 4066-4069.
doi: 10.7498/aps.57.4066
Abstract +
According to the relative intensities of lines and transition parameters, through linear fit of linear function, two fit parameter are obtained. And by adopting these parameters, transition probabilities of spectral lines for special configuration or highly excited states can be calculated. In the present work, on the basis of spectroscopic analysis for N+ spectra, the transition probability whose upper states are pair coupling is obtained. The present work gives a simple and feasible new semi-classical method to calculate transition probabilities, especially for special configurations or highly excited states.
2008, 57 (7): 4070-4075.
doi: 10.7498/aps.57.4070
Abstract +
In order to certificate the electromagnetic characteristic of left-handed materias (LHM), we adopt unsplit finite-difference time domain method (FDTD) to the ideal Drude model of LHM. This method does not need to split the electric and magnetic fields and does not need the use of special treatment of PML. PML and working space can be converted with the parameters. The penetrating wave will be attenuated in the PML which is set up by lossy dielectric. So it is an effective method to analyze dispersion and anisotropic material. Thus we can confirm the LHM and its unique characteristics such as negative refraction, convergent and phase compensation and verify the effectiveness of analyzing LHM using unsplit FDTD.
2008, 57 (7): 4076-4082.
doi: 10.7498/aps.57.4076
Abstract +
Ba 6p3/2nd (J=1,3) autoionizing states, especially those with lower principal quantum numbers, have been experimentally studied using the isolated core excitation scheme with three dye lasers. Level energies and widths of 6p1/2nd and 6p3/2nd (J=1,3)autoionizing states, which are excited from 6snd1D2 Rydberg states with n=7—15 and 6snd3D2 Rydberg states with n=7—12, are reported. Examination of autoionizing states with lower principal quantum number (n=7—9) exhibited the admixture between states converging to 6p3/2 and 6p1/2 ionization threshold. Moreover, some discussion is made based on comparison between different autoionizing series, so that one may explain the complex spectral structures of these states.
2008, 57 (7): 4083-4090.
doi: 10.7498/aps.57.4083
Abstract +
We present the analytic expressions for different types of quantum interference effects in coherent instantaneous state quantum system, and discuss the behaviors of the optics and quantum interference, respectively. The important role that the geometrical phase plays in quantum interference is analyzed in detail for the first time. It is demonstrated theoretically that the measurement of the geometrical phase can be realized by appropriately choosing the pulse area of the pump field. The valid control of the quantum interference is achieved by manipulating the chirped pump field.
2008, 57 (7): 4091-4098.
doi: 10.7498/aps.57.4091
Abstract +
Reported are the emission properties (energy-time distributions) of high-order harmonic generation (HHG) calculated for lasers of different pulse durations (τL, in units of radian) in full width at half maximum (FWHM) and different carrier-envelope phases (CEPs,Φ). Calculations demonstrate that the cutoff energy of HHG spectrum produced by a few-cycle laser is less than that by an infinite duration laser (ωmax=3.17Up+Ip, where ωmax is the angular frequency of the photon, Up the ponderomotive potential of the laser field, and Ip the atomic ionization potential). A τL＝4π rad, Φ＝15° laser can produce a single distribution pulse peaked at 0.94 rad and spanning 1.29 rad with a cutoff energy ωmax=2.90Up+Ip and a bandwidth 0.86Up. For the same laser but with Φ＝-75°, double distribution pulses occur with equal amplitudes, cutoff energy ωmax=2.70Up+Ip and bandwidth 0.70Up. These two pulses are peaked at -0.58 rad and 2.43 rad，span 1.22 rad and 1.33 rad, respectively. Calculations also demonstrate that the bandwidth of a selected distribution pulse decreases much faster than its duration as the laser duration grows. The CEP dependence of the single distribution pulse parameters (its energy and time positions) show interesting 180° periodic structures. These features enable us to select attosecond pulse parameters and temporally control them by adjusting CEP. Theoretical analysis shows that the influences of CEP instability on the measured photoelectron spectra and the experimental results may be weakened to the maximum extent or even eliminated by choosing appropriate energy bandwidths of X-rays in attosecond measurements. All these results are helpful in realizing the HHG dynamic process. They can be used as new references in optimizing and selecting single and double attosecond pulses.
2008, 57 (7): 4099-4103.
doi: 10.7498/aps.57.4099
Abstract +
The method of enhancing the alignment of N2 molecules by controlling pulse duration of two-pulse laser is proposed. The time-dependent Schrdinger equation of the rigid rotor model of N2 molecule in the two-pulse laser is solved numerically by using the pseudospectral method. The degree of alignment for N2 molecules by laser pulses with different durations is calculated. The results show that the alignment can be optimized by selecting the pulse duration and the delay time between the two pulses.
2008, 57 (7): 4104-4109.
doi: 10.7498/aps.57.4104
Abstract +
We numerically calculated the high-order harmonic generation (HHG) power spectra from a one-dimensional model atom irradiated by linearly polarized ultra-short laser pulses whose duration is two optical cycles. It was found that, the cut-off positions of the HHG spectra change when the carrier phases of the ultra-short laser pulses are altered, and the distinct double plateau structure appears in some special carrier phases. The above issues are reasonably explaned by the three-step model. Finally, the wavelet time-frequency analysis verifies that the three-step model can accurately exactly predict the cut-off frequencies of the ultra-short pulse HHG spectra.
2008, 57 (7): 4110-4118.
doi: 10.7498/aps.57.4110
Abstract +
Based on the algebraic method (AM) presented in our previous study, a new algebraic approach (AM2) is suggested to evaluate accurate rovibrational energies and corresponding rotational constants using known experimental data of rovibrational energies of limited accuracy of a given rovibrational band for a diatomic electronic state. The results of reseach on rovibrational energies of the B1Σ electronic state of HF molecule shows that Algebraic Method 2 gives rovibrational energies in excellent agreement with experimental data, and the method generates reliable energies of high-lying rovibrational excited states which may be difficult to obtain experimentally or theoretically.
2008, 57 (7): 4119-4124.
doi: 10.7498/aps.57.4119
Abstract +
In this paper, the composition structures of SiC epilayer surface are characterized by high resolution X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectrum. The results of XPS wide scan spectroscopy, infrared glancing reflection absorption spectrum and infrared specular reflection spectroscopy show that the amorphous phase SiCxOy:H is made up of Si-O-Si and Si-CH2-Si polymers. The chemical state structures are composed of Si(CH2)4, SiO(CH2)3, SiO2(CH3)2, SiO3(CH3), Si-Si, dissociative H2O, combined OH，Si-OH, O and O2. The order of the atom core electron binding energy values is determined by chemical state structures and element electronegativity. Compared with the XPS narrow scan spectrum fitting results, the corresponding relation between chemical states and its binding energies is established, and all chemical state binding energies are obtained using the calibration of C 1s binding energy of Si(CH2)4. The result shows that the C 1s and O 1s binding energy values of SiCxO4-x(x=1,2,3) are different from each other, similar as their Si 2p binding energies, of which the C 1s of SiO2(CH3)2 and SiO3(CH3) are respectively close to those of CHm and C—O. The reasonable explanation is presented in terms of chemical state structures, electronegativity and neighbouring site effects.
2008, 57 (7): 4125-4129.
doi: 10.7498/aps.57.4125
Abstract +
X-ray photoelectron spectroscopy (XPS)is highly adapt to characterize the chemical states of the surface of SiC samples. However, there were notable discrepancies in results for C ls spectra from fitting XPS data reported by using the method of fixed number and peak values (or full width half maximum) of peaks in different previous papers. In this paper, a novel XPS fitting method is proposed by adopting variable peak values (full width half maximum) and peak areas of peaks, and then the effects of number of peaks, functional types of peak and background on fitting results of the C 1s spectroscopy for SiC surface have been investigated. Comparing the XPS wide-scan spectroscopy and infrared glancing reflection absorption spectroscopy, optimal fitting parameters and fitting result for C 1s spectroscopy of SiC surface are determined. The binding energies of C 1s photoelectron spectroscopy obtained in this paper have the same values as that reported. All this establishes the basis for fitting narrow scan spectroscopy of elements and the investigation of chemical state structure of the surface of SiC and other materials.
2008, 57 (7): 4130-4133.
doi: 10.7498/aps.57.4130
Abstract +
The effective potential energy curves correlated to 2I03/2 and 2I01/2* atom limits are calculated using second order spin-obit multiconfigurational quasidegenerate perturbation theory (SO-MCQDPT). The absorption spectra of CH3I molecule are calculated, photodissociation processes of CH3I molecule are analyzed and the quantum yields of 2I01/2* atom are estimated. The present calculations can be used to interpret the experimental results.
2008, 57 (7): 4134-4137.
doi: 10.7498/aps.57.4134
Abstract +
Density function method (B3LYP) has been used to optimize the possible structures of Pb2 and PdPb2 molecules by basis set (LANL2DZ) for Pd and Pb atoms. The Murrell-Sorbie potential energy function of Pb2 molecule has been fitted through the least square fitting, and the potential energy function of PdPb2is given by many-body expansion theory. The potential energy contours describe the structure character of PdPb2 ground state molecule rightly. Furthermore, molecular static reaction pathway based on this potential energy function is investigated.
2008, 57 (7): 4138-4142.
doi: 10.7498/aps.57.4138
Abstract +
The total differential cross section, elastic differential cross section and inelastic differential cross section for collision between He isotopic atoms and NO molecule have been calculated by using close-coupling approximation method. The influence of He isotope on the differential cross section for He-NO collision system has been given. The calculated result shows that the total differential cross section at 0 degree increases, and rainbow phenomenon of collision of He isotopic atoms with NO molecule is more evident along with increase of incident He isotope mass at identical incident energy. At the same time, the effect of increased reduced mass is larger than the effect of diminished relative velocity of collision between He atom and NO molecule, and enables the scattering spacing of the oscillation to gradually decrease.
2008, 57 (7): 4143-4147.
doi: 10.7498/aps.57.4143
Abstract +
In this paper, momentum transfer cross sections are obtained using body frame vibrational close-couple method (BFVCC) modified by the authors. Using the static potential, correlation-polarization potential and exchange potential calculated ab-initio, the results accord with experiment very well.
2008, 57 (7): 4148-4152.
doi: 10.7498/aps.57.4148
Abstract +
Employing the multiconfiguration self-consistent field method and the multiconfiguration quasi-degenerate perturbation method, the adiabatic potential energy curves and vertical excitation energies of alkyl iodides CF3I and C2H2F3I are calculated for the low-lying states， respectively. It is found that the low-lying excited states of the two molecules are repulsive, and the calculated dissociation energies for their ground states are 2.473eV and 2.835eV, respectively, in which the former one agrees well with the experimental results.
2008, 57 (7): 4153-4158.
doi: 10.7498/aps.57.4153
Abstract +
The possible geometrical and electronic structures of small TinMg (n=1—10) clusters have been optimized by using B3LYP method of density functional theory (DFT). The binding energies, energy level distribution, energy gaps, electron affinities, dissociation energy and second difference in energy are calculated and discussed. The resulting geometries show that the magnesium atom remains on the surface of clusters. The geometric effect and electronic effect clearly indicate the Ti5Mg cluster to be endowed with special stability.
2008, 57 (7): 4159-4165.
doi: 10.7498/aps.57.4159
Abstract +
Supersonic molecular beam injection (SMBI) can penetrate the separatrix of the HL-2A plasma and result in the increase of important plasma parameters, such as the plasma storage energy, the βp, the line averaged electron density, the central electron density and temperature. During deuterium plasma ECRH and deuterium SMBI, the neutron account rapidly increases along with the electron density and temperature increments. SMBI was considered as an alternate fuelling technique, which may provide a higher particle source at the pedestal top for ITER. A mean velocity for hydrogen SMBI of about 1.7km/s was detected in HL-2A torus and the measured result was compared with that for the effusion in vacuum.
2008, 57 (7): 4166-4173.
doi: 10.7498/aps.57.4166
Abstract +
The geometrical structure, frequency and electronic properties of the NiMgn(n=1—12) clusters have been studied with the generalized gradient approximation (GGA) based on the density functional theory (DFT) with the consideration of spin multiplicity. The results indicate that: when n is 1 or 2, the spin multiplicity of the ground state structures of the clusters is triplet while it is singlet from n≥3. The ground state structures of the host clusters are changed obviously due to the encapsulation of Ni atom for n≤8, the growth patterns of the ground state structures of the NiMgn clusters are dominated by the trigonal bipyramidal, as well as the octahedron structures. The evolution behaviors of the ground state structures based on the trigonal prism of the host clusters are partly modified from n≥9. The Ni atom completely falls into the center of the host clusters as n≥6. The doping of Ni atoms increases the average binding energy, but reduces the energy gap of the host clusters. n=4, 6 and 10 are the magic numbers. The 3d and 4p orbitals of the Ni atom for different sized clusters play distinct roles in the s-p-d orbital hybridization. The NiMg6 cluster with higher symmetry Oh not only possesses improved stability, but also has the smallest energy gap (just about 0.25eV) of all of the NiMgn clusters.
2008, 57 (7): 4174-4181.
doi: 10.7498/aps.57.4174
Abstract +
Possible geometrical structures and relative stabilities of (Li3N)n(n=1—5) clusters are studied by using the hybrid density functional theory(B3LYP) with 6-31G* basis sets. For the most stable isomers of (Li3N)n(n=1—5) clusters, the electronic structure, vibrational properties, bonds properties, ionization potentials are analyzed. The calculated results showed the following tendencies: the coordination number of N are usually 4 or 5; the bond lengths for the most of the optimized (Li3N)n(n=1—5) clusters are about 0.210—0.259nm for Li—Li and 0.185—0.204nm for the bridging Li—N bonds, while the terminal Li—N bonds span 0.172—0.178nm. The outcome of population analysis suggests that the natural charge of N atoms are about -2.01e and that of Li atoms are about +0.67e; the dynamic stabilities of Li3N and (Li3N)5 clusters are higher than that of other clusters.
CLASSICAL AREA OF PHENOMENOLOGY
2008, 57 (7): 4182-4188.
doi: 10.7498/aps.57.4182
Abstract +
The continuation of magnetic field of magnetic object is studied, and a novel method based on integral equation method and singular value decomposition is presented. In the implementation of the new method, the magnetic object's structure only needs to be coarsely partitioned as some elements by integral equation method, thus a linear system of equations can be obtained with the magnetic field measurements at a large plane below the magnetic object. The system of equations can be solved regularly by the truncated singular value decomposition technique and the modified singular value decomposition technique based on singular value decomposition, then the 3D magnetic fields reconstruction the upward and downward continuation of the magnetic object's magnetic field are realized. The precision and reliability of the new method are higher than the conventional ones; especially, the difficulty of the upward continuation in certain ranges is surmounted.
2008, 57 (7): 4189-4194.
doi: 10.7498/aps.57.4189
Abstract +
The left-handed material, whose permittivity and permeability are simultaneously negative, has some exotic electromagnetic properties, and is one of the most absorbing subjects in recent physical academic domain. This paper suggests that a tunable left-handed metamaterial can be realized by using ferrite as the substrate of metallic wire structure, owing to the field dependence of ferrite's permeability. Finite element method was used to simulate the effect of substrate properties on the effective properties of a metamaterial slab. With the rise of substrate permittivity or permeability, the resonance frequency drops significantly. Ferroxplana has been prepared, and the field dependence of its permeability was studied.
2008, 57 (7): 4195-4201.
doi: 10.7498/aps.57.4195
Abstract +
Guide-mode resonances of double-layer gratings are analyzed with the Fourier modal method and the relationship between spectral characteristics of resonant gratings such as peak diffraction efficiency, linewidth and side-band reflectance and depth, period and fill ratio of grating are illustrated in detail. It is shown that the diffraction efficiency of resonant gratings will be higher than 99.5 percent when the groove depth tolerance is in the range of 15 percent, and mean while the relative error of line width will reach to 7 and 60 percent respectively for the negative and positive conditions. Compared with the position of resonant wavelength, the line width of resonant grating filters is relatively sensitive to the error of groove depth and residual thickness of waveguide layer when they deviate from designed values. Moveover, the resonant wavelength and width of spectrum do vary as a function of period and fill ratio.
2008, 57 (7): 4202-4207.
doi: 10.7498/aps.57.4202
Abstract +
The diffraction field will show aberration if an axicon has astigmatism for beams incident at an angle. In this paper, the diffractional optical field expression of an axicon with astigmatism was derived based on the diffraction integral theory. The influence of the astigmatism on the diffraction property of the axicon was analyzed through the numerical simulation of the diffraction pattern with different astigmatism and the optical intensity distribution along the propagation direction for a given astigmatism. The method of correcting the beam aberration caused by the astigmatism is proposed and realized by the experiment using a precise tunable rotator. Experimental results agree with the theoretical analysis. The result extends the field of applications of the axicon.
2008, 57 (7): 4208-4213.
doi: 10.7498/aps.57.4208
Abstract +
For the grazing incidence of a narrow beam, a part of plane wave components can not reach the reflecting interface directly, so they can not be reflected and transmitted in the traditional way. The inverse mode of reflection and transmission is proposed in this paper to describe the interaction between the non-incident plane wave components and the reflection region. Based on the two modes, the plane wave expansion method is utilized to derive exact distribution of the output optical field. Theoretical calculation agrees well with the simulation result which employs the BPM algorithm.
2008, 57 (7): 4214-4218.
doi: 10.7498/aps.57.4214
Abstract +
A free-space quantum key distribution system is demonstrated experimentally which makes use of polarized coherent short laser pulses and the differential phase shift scheme with a Faraday-Michelson interferometer. The system features low noise, high stability and low bit-error rate. In the proposed experimental scheme, polarizated short laser pulse of several nanoseconds duration is made to transmit through the free space to decrease noise and a low bit-error rate less than 6% is achieved.
2008, 57 (7): 4219-4223.
doi: 10.7498/aps.57.4219
Abstract +
The system of inverted Y-type four-level atoms interacting with multi-mode light fields is discussed. Through the mechanism of quantum interference, the relative dielectric permittivity and the relative magnetic permeability will changed remarkably. Using appropriately chosen parameters, the real parts of both the relative dielectric permittivity and the relative magnetic permeability will be negative, and then the left handedness effect happens and the left-handed material is realized. The properties of left-handed materials such as the range of frequencies of the negative refractive index can be manipulated via changing certain parameters of the system.
2008, 57 (7): 4224-4229.
doi: 10.7498/aps.57.4224
Abstract +
There are abundant nonlinear effects in atomic vapor. In this paper, the phenomenon of laser breaking up into filaments is studied in the near-resonant rubidium atomic vapor，which is induced by the strong nonlinear Kerr effect. When the continuous-wave laser beam with transverse Gaussian distribution passes through the atomic vapor, the pattern arising from the coherent superposition of the diffraction patterns of filaments is observed. We also investigated the influence on the mode pattern by input power of laser, the temperature of the vapor cell and the frequency detuning of pump light. Due to the hyperfine structure of rubidium atoms, the four-wave mixing effect exists in hot rubidium atomic vapor, which is related to the third-order nonlinear effect closely. The Raman gain of the Stokes and anti-Stokes photons is observed by scanning the frequency of the probe beam.
2008, 57 (7): 4230-4237.
doi: 10.7498/aps.57.4230
Abstract +
Based on full-vector plane-wave method (FVPWM), the hollow-core photonic band-gap fiber (HC-PBGF) fabricated using the improved twice stack-and-draw technique has been simulated. Under given values of propagation constant β, some valid photonic band-gaps (PBGs) with different sizes will emerge within the wave band of 500—1000 nm. For different values of the effective refractive index, the air-guided modes can exist in different forms in part of PBGs. Comparing the experimental data with the simulation result, we have discovered that the measured positions of PBGs are shifted to shorter wavelengths distinctly. The primary reasons are considered to be the structure asymmetry along longitudinal direction and the existence of interstitial holes at nodes in the cladding region.
2008, 57 (7): 4238-4243.
doi: 10.7498/aps.57.4238
Abstract +
An ultraviolet single photon imaging system was built and its structure,working principle and spatial resolution are reported. Ultraviolet single photon can be obtained by ultraviolet lighr emitted by a mercury lamp and is attenuated by filters. Single photon irradiates on the microchannel plate directly, producing the multiplied electrons. The multiplied electrons are collected by wedge and strip anode. The charge signal coming from the anode is transformed to voltage signal by a charge_sensitive preamplifier. Then the voltage signal is filtered and shaped by a main amplifier. The output wave signal is collected by high speed data collection card continuously and processed with software. Finally, a ten-minute counting image of ultraviolet single photon is acquired and its revised image is obtained by image processing. The spatial resolution of this system can reach 150 micrometers as tested with the self-designed resolution board. The system can be used widely for very faint luminescence detecting/imaging, biological luminescence and space environment weak light detection.
2008, 57 (7): 4244-4250.
doi: 10.7498/aps.57.4244
Abstract +
Sustained vowel analysis is an important method in pathological voice evaluation system. In view of the fact that conventional methods generally analyzes the hand-cut samples in FFT frequency domain, an auditory model is proposed as an accurate and robust method for use in pathological vowel analysis. An automatic sample cutting method increases the stability of analysis. Expertiment showed this method is more effective and robust.
2008, 57 (7): 4251-4257.
doi: 10.7498/aps.57.4251
Abstract +
This paper simulates and analyzes numerically the dispersion characteristics of the waves propagating in the shallow-ocean overlying a semi-infinite elastic solid. The analytical solution of the acoustic field in the shallow-ocean, excited by an explosive point source in the solid bottom, is derived by using the theories for sound propagation in liquid and solid. The time-domain waveforms of transient acoustic field are simulated with the above analytical solution. The dispersion characteristics of the transient acoustic field are analyzed by using the smoothed pseudo Wigner-Ville distribution.
2008, 57 (7): 4258-4264.
doi: 10.7498/aps.57.4258
Abstract +
Wave superposition is a numerical method for calculating sound radiation based on the principle of equivalent source. However, the principle does not indicate definitely how to collocate these equivalent sources. So, it is necessary to evaluate its reconstruction accuracy and analyze the impact factors when wave superposition is applied to sound field reconstruction. The purpose of this study is to find some rules for improving the reconstruction accuracy. A number of impact factors on the precision of sound field reconstruction have been analyzed through theoretical analysis and numerical simulation. It has been proved that the equivalent sources have to be collocated inside of a radiator; moreover, it is best for them to conform to the radiator's shape. However, the measurement plane needs not to conform to the radiator's shape, and even does not need to measure in near-field. In addition, it has been shown that Tikhonov and TSVD regularization method can work well with wave superposition when applied to sound field reconstruction. On the basis of these rules, wave superposition can be used to perform better sound field reconstruction; and also its application range can be extended.
2008, 57 (7): 4265-4272.
doi: 10.7498/aps.57.4265
Abstract +
Dispersion is the intrinsic characteristic of Lamb waves, which influences the effective applications of Lamb waves in plate-like structures. The continuous wavelet transform (CWT) was applied to the Lamb wave signals with narrow frequency band captured from aluminium structures. The CWT-based contour and ridge were therefore searched in the time-scale domain. The unavoidable dispersion characteristic was used in this paper to distinguish the mode of each wave package and pinpoint these packages at each scale for determining the actual group velocities of dispersion curves. Analysis results demonstrate that the proposed approach is effective in dispersion analysis and wave mode differentiation.
EDITOR'S SUGGESTION
2008, 57 (7): 4273-4281.
doi: 10.7498/aps.57.4273
Abstract +
Based on the mass-energy relation in Einstein's relativity theory, thermal energy has equivalent mass, which is referred to as thermal mass. The concepts of phonon gas mass in solids and thermon gas mass in gases are then introduced. Based on these concepts, the momentum conservation equation, including driving force, resistance and inertial force, for the thermal mass motion is established using Newtonian mechanics. Since the heat conduction is just the motion of the thermal mass (phonon gas or thermon gas) in a medium, the momentum conservation equation for thermal mass is a general heat conduction law which can unify the description of heat conduction under various conditions. The momentum conservation equation reduces to Fourier's heat conduction law when the heat flux is not very high so that the inertial force of the thermal mass can be ignored. For micro-/nano_scale heat conduction, the heat flux may be very high and the inertial force due to the spatial velocity variation can not be ignored. Therefore, the heat conduction deviates from Fourier's law, i.e. non-Fourier phenomenon takes place even under steady state conditions. In such cases, the thermal conductivity can not be calculated by the ratio of the heat flux to the temperature gradient. Under ultra fast heat conduction conditions, the inertial force of the thermal mass must be taken into consideration and the momentum conservation equation for the thermal mass motion leads to a damped wave equation. Compared with the CV model, the general heat conduction law includes the inertial force due to the spatial velocity variation. Thus the physically impossible phenomenon of negative temperatures induced by the thermal wave superposition described by CV model, is elliminated, which demonstrates that the present general law of heat conduction based on thermal mass motion is more reasonable.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2008, 57 (7): 4282-4291.
doi: 10.7498/aps.57.4282
Abstract +
A reconstruction algorithm that synthesizes the relaxation attenuation spectrum in a wide frequency range is presented. And the effective relaxation time of gas can be estimated by two approaches, which are base on SSH theory and experimental data, respectively. The acoustic attenuation spectra of various polyatomic gas mixtures, consisting of nitrogen, methane, oxygen, carbon dioxide and water, were estimated by the reconstruction algorithm. The sound frequency rang of interest is from 1 Hz to 10 GHz. Comparing with the results of the relaxation attenuation based on the DL model, the estimations of relaxation attenuation of the reconstruction algorithm agreed with the latter. The precision of the reconstruction algorithm depends upon the knowledge of the physical mechanism of molecular relaxation processes. In addition, the reconstruction algorithm was applied to quantitatively determining the concentrations of water and carbon dioxide in some polyatomic gas mixtures. The results show that the relaxation attenuation can be used as a quantitative probe of the composition of polyatomic gas mixtures. And this could lead to the development of smart acoustic gas sensors capable of determining the precise compositions of gas mixtures.
2008, 57 (7): 4292-4297.
doi: 10.7498/aps.57.4292
Abstract +
The frequency dependence of plasma-column antenna input impedance has been measured by a vector network analyzer with different applied exciting power. The experimental results show obvious resonance behavior of the impedance. The effect of plasma parameters on the input impedance was established by means of an equivalent circuit model and the characteristics of input current. This method is useful for the research of plasma antenna dynamic reconfiguration and the realization of fast impedance matching.
EDITOR'S SUGGESTION
2008, 57 (7): 4298-4303.
doi: 10.7498/aps.57.4298
Abstract +
Homogenous discharges were produced in a 2-mm air gap at atmospheric pressure by using 325# fine wire mesh as electrodes covered with 0.1 mm PET films and 50Hz line power source paralleled with a discharge-stabilizing capacitor. For obtaining such a homogeneous discharge it is important to attach tightly the wire mesh together with the PET film on the surface of a Rogowski electrode. The experiments show that the commonly used method to distinguish a homogenous discharge from a non-homogeneous one by the waveforms of discharge current and transferred charge is not always reliable since the waveforms could tell only the discharge simultaneity rather than the discharge homogeneity. A fast image of discharge with an exposure time not longer than 100ns should be used instead to judge the discharge homogeneity. It is the first time in the experiments of atmospheric pressure discharge using wire mesh electrode covered by PET film such fast images were obtained, which indicates that the homogeneous discharge is a Townsend discharge rather than a glow discharge.
2008, 57 (7): 4304-4315.
doi: 10.7498/aps.57.4304
Abstract +
In this paper, Z-scan, current and voltage probe were used to measure rf discharge power, coupling coil rf current, voltage and dc plasma floating voltage in RF inductive dischargr, and the effects of impedance matching network, coil coupling intensity and the source ground area on the properties of E,H mode and the mode transition are investigated. The positive and negative feedback regions are put forth on the basis of the influence of the series capacitance in a Γ-type impedance matching network on RF output power of the RF power source. It is experimentally founded that under identical experimental parameters, the discharge mode evolution is apt to be discontinuous and continuous in the positive and negative feedback regions, respectively. In addition, the threshold coil RF current (or plasma absorbed power) corresponding to the mode transition, the hysteresis loop width, the difference in plasma absorbed power during the discharge mode transition vary evidently with the series capacitance of the impedance matching network. The threshold coil RF current of E-H mode transition is not necessarily higher than that of H-E mode transition. In combination with the influences of impedance matching network and discharge pressure, the modes of transition are diversified for different ground areas.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2008, 57 (7): 4316-4321.
doi: 10.7498/aps.57.4316
Abstract +
Films deposited at high temperature are subjected to thermal stresses in cooling down, duc to the mismatch of thermal expansion between substrate and film materials, which tends to peel off the film from the substrate. The purpose of the present work is to search the effect of thickness, Young's modulus and thermal expansion of the films on the delamination of the films. The results are helpful in reducing the possibility of the peeling of the films.
2008, 57 (7): 4322-4327.
doi: 10.7498/aps.57.4322
Abstract +
The structure of MnxSi1-x magnetic semiconductor thin films prepared by molecular beam epitaxy(MBE) on Si(100) substrate at 600 ℃ has been studied by X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) technique. The XRD results show that in the MnxSi1-x thin films with high Mn doping concentrations (x=0.08 and 0.17), only diffraction peaks of crystalline Mn4Si7 are observed. XANES results indicate that all the Mn K-edge XANES spectra of MnxSi1-x thin films with different Mn doping concentrations (x=0.007, 0.03, 0.08 and 0.17) show the similar feature. XANES calculation based on multiple-scattering theory further reveals that the experimental spectra for samples with different Mn doping concentrations are reproduced by the calculated Mn4Si7 spectrum. These results reveal that for the MnxSi1-x magnetic semiconductor thin films, Mn atoms mainly exist in the Si thin film substrate in the form of Mn4Si7 nanocrystalline grains, the substitutional or interstitial Mn atoms scarcely exist.
2008, 57 (7): 4328-4333.
doi: 10.7498/aps.57.4328
Abstract +
Zn1-xCoxO (x=0.01, 0.02) dilute magnetic semiconductor thin films deposited on Si (001) substrates at 650℃ by pulsed laser deposition method were studied by X-ray absorption fine structure, X-ray diffraction and magnetic measurement. The typical ferromagnetic hysteresis curves were obtained by superconducting quantum interference device magnetometry at room temperature. The X-ray diffraction results showed that Zn1-xCoxO films were of the wurtzite structure. The X-ray absorption fine structure results revealed that the Co atoms were incorporated into the ZnO lattice and located at the substitutional Zn sites, and a homogeneous phase of Zn1-xCoxO was formed. Comparing the experimental curves with the theoretical calculation results, the additional peak C was assigned to the oxygen vacancies, which indicated that the ferromagnetism of Zn1-xCoxO films was strongly correlated with the existence of oxygen vacancies.
2008, 57 (7): 4334-4340.
doi: 10.7498/aps.57.4334
Abstract +
Three samples of spin ladder compounds (Sr1-xCax)14Cu24O41-δ (x=0, 0.25, 0.43) were prepared by conventional solid-state reaction in air. The EDS results revealed that all three samples are in the state of oxygen deficiency (δ＝7.64，6.99 and 6.67, respectively). X-ray diffraction showed that the samples have single phase, and the lattice parameters a, b, c decrease continuously with the increase of oxygen content. For the sample withoutca Ca doping, the measurement of magnetic susceptibility showed that oxygen deficiency reduces the number of holes and Z-R singlets, and leads to the increase of free spins and the decrease of dimer spins. For Ca doped samples, the oxygen deficiency decreases with the increase of Ca content, but Ca doping enhances the transfer of holes from spin chains to spin ladders.
2008, 57 (7): 4341-4346.
doi: 10.7498/aps.57.4341
Abstract +
Rapid solidification of bulk Cu60Sn30Pb10 monotectic alloy was investigated. The obtained undercooling varied from 37 to 173 K. Solidification microstructures showed obvious macroscopic segregation. X-ray diffraction analysis revealed that the top of sample is a ternary_phase section composed of Sn and Pb solid solution phases and ε(Cu3Sn) intermetallic compound phase, the bottom is a Pb-rich solid solution phase section. With the increase of undercooling, ε(Cu3Sn) intermetallic compound phase in the ternary phase section showed a transition from bulk dendrites to refined lamellar microstructure; in addition, while the lamellar spacing was also decreasing. The experiment indicted that the Sn and Pb phases exist as seperated eutectoids. There is a small amount of ε(Cu3Sn) dendrites in the Pb-rich phase. The ε(Cu3Sn) dendrites increase in length with the increase of undercooling and break into pieces under the condition of great undercooling. The Sn solid solution phase always accretes the circumference of ε(Cu3Sn) intermetallic compound phase. Their shape resembles the peritectic microstructure.
2008, 57 (7): 4347-4355.
doi: 10.7498/aps.57.4347
Abstract +
The structure and thermal stability of gold nanowire encapsulated in single-walled carbon nanotube (SWCNT) were studied by molecular dynamics simulations. A (8,8) SWCNT was used as a model system and the simulated annealing method was employed to find the stable structure of the gold nanowire at room temperature. Ourresults show that the gold nanowire encapsulated in a (8,8) SWCNT has a cylindrical shape and a helical shell structure. The helical-shell nanowire is thermally stable at temperatures much higher than the melting temperature of its bulk counterpart and is different from the gold nanowire formed in free space without the nanotube envelope. We also found that the spiral nanowire undergoes structural changes at higher temperatures.
2008, 57 (7): 4356-4363.
doi: 10.7498/aps.57.4356
Abstract +
In this paper，we have studied formation processes of silicon nanostructures on edges of graphene ribbons by molecular dynamics simulation.We found that，Si atoms in a gas state can be absorbed on the edges of zigzag graphene ribbons，forming different types of silicon nanostructures under different temperature T，namely，amorphous clusters，chain structures and defective chain structures in the intervals 300K≤TT≤2800K and 2800KTT≥3900K，carbon atoms on the edges of graphene ribbons are gradually replaced by Si atoms.For the graphene ribbons with armchair edges，existence of multiple adsorption sites does not allow growth of chain-like Si nanostructures.
2008, 57 (7): 4364-4370.
doi: 10.7498/aps.57.4364
Abstract +
In this paper, we performed the first-principles calculations of the effects of the uniaxial pressure on electronic structures of the (6, 6) single-walled carbon nanotube crystal (SWNTC). The applied pressure is perpendicular to the axis of carbon nanotubes. The calculated results show that the (6, 6) SWNTC with tetragonal structure (t phase) is metallic, and its electrons can move along the walls of carbon nanotubes. With the increasing of the uniaxial pressure, the (6, 6) SWNTC undergoes the structural phase transitions, and transforms from the t phase to the unbonded phase, and then to the bonded phase. The uniaxial pressure mainly affects the π and π* bands of the band structure of the (6, 6) SWNTC, and these effects on the bands are not only in kxky plane but also in the direction of kz. The electronic properties of the crystal can alter from metal to semiconductor, then to metal again as the uniaxial pressure increases. The distributions of electronic charge densities show that electrons of the unbonded phase are localized near the carbon nanotubes. And electrons of the bonded phase can move not only along the walls of carbon nanotubes, but also between the two nearest nanotubes through the chemical bonds.
2008, 57 (7): 4371-4378.
doi: 10.7498/aps.57.4371
Abstract +
By using pulsed-laser deposition technique, a series of multilayers of NdBa2Cu3O7-δ/YBa2Cu3O7-δ, namely with the structures of p×［NdBa2Cu3O7-δ(m)/YBa2Cu3O7-δ(n)］, are prepared on single-crystal SrTiO3, where m and n denotes the number of laser pulses and p is the number of repetetions. XRD patterns indicate that all samples are highly c-axis oriented, and the superconducting transition temperatures of these samples are in the range of 87—91 K, depending on different modulating structures. The larger the modulation period p and consequently the more the interfacial surfaces, the lower the superconducting transition temperature. Magneto-transport measurements reveal that the quasi-multilayers such as NY3［40×(75/5)］ (island-like YBa2Cu3O7-δ distributed in NdBa2Cu3O7-δ films) exhibit not only the highest Tc, but also the best magnetic properties. The critical current density of such a quasi-multilayers can be as high as 4×106 A/cm2 under self fields, showing the promising potential application for coated conductors.
2008, 57 (7): 4379-4385.
doi: 10.7498/aps.57.4379
Abstract +
Based on the generalized gradient approximation (GGA) of density function and the full-potential linearized augmented plane wave (FLAPW) method, the micro-arrangement of He atoms after tritium decay in LaNi5He have been modeled for theoretical study. The energy curves between 1b-6m-2d, 4h-2d-4h, 6m-12o-12n and 12n-3f-12n interstitial sites for LaNi5He are worked out theoretically. The results indicate that 1b site is most stable for He occupation in LaNi5He among the seven nonequivalent interstices. The potential barrier doesn't exist when He atoms migrate from 6m site to 1b site, but there is a potential barrier of height 1.55eV between 2d site and 1b site. And He atoms which lie in the 12n sites can easily go throngh the 12o sites to get to the 6m sites finally. Helium atoms can freely migrate between the 4h sites. By getting through a 13.6eV potential barrier, He atoms are able to directly migrate between the 12n sites. Finally, the density of states (DOS), charge density (CD) and potential distribution for characteristic interstitial LaNi5He and LaNi5H are plotted and compared with each other in detail.
2008, 57 (7): 4386-4390.
doi: 10.7498/aps.57.4386
Abstract +
The potential that a hydrogen atom experiences in a metallic niobium crystal is calculated using the pseudopotential plane-wave method based on density functional theory. The quantum states of the hydrogen atom in such a potential are obtained by solving Schrdinger equation directly. The ground state and the first excited state are shown to be local, while the others are delocalized. A hydrogen atom in high excited states may move on a 4T or 6T ring.
2008, 57 (7): 4391-4396.
doi: 10.7498/aps.57.4391
Abstract +
Molecular dynamics simulations are employed to simulate the axial compression of both perfect and defective armchair single-walled carbon nanotubes (SWCNTs). Three temperature conditions are considered. It is revealed that the vacancy defects in SWCNTs have remarkable influences on their buckling properties, especially at low temperature. It is noticed that the single vacancy defect leads to excessive stress concentration, as a result of which the local buckling occurs easily at the defective position. Thus, the (7, 7) tube with single vacancies seems inferior to that with double vacancies.
2008, 57 (7): 4397-4401.
doi: 10.7498/aps.57.4397
Abstract +
Based on the condensing vapor model, the expression of sound speed close to critical point is deduced,which shows the relationship between the sound speed and the density fluctuating index and thermal capacity at constant volume. Close to critical point, the sound speed of liquid carbon dioxide is inversely proportional to the density fluctuation index. The bigger the density fluctuation index, the smaller the sound speed. The sound speed is maximal at minimal density fluctuation index, where small fluctuations of density induces large fluctuations of sound speed. When the pressure increases and approaches to critical point, the rapid increase of thermal capacity induces decrease of sound speed. When the pressure increases and departs from critical point, the rapid decrease of thermal capacity at constant volume induces increase of sound speed. The calculated values of sound speed from the expression are in good agreement with the reference values from NIST.
2008, 57 (7): 4402-4411.
doi: 10.7498/aps.57.4402
Abstract +
On the basis of the Hamiltonian of the Holstein one_dimensional molecular crystals, using the squeezed_coherent state expansion method, the influence of the electron_two phonon interaction on the properties of the ground state and quantum fluctuation for the polaron_soliton system were investigated by including the quantum correlation between the polarons and the squeezed phonons, and the renormalized displacement correction.The nonlinear Schringer equation for one_dimensional polaron_soliton state has been found in a closed form.By the use of the nonlinear expansion, we have given the analytical solution of the corresponding nonlinear equation so as to obtain the ground state energy,the quantum fluctuation and the polaron energy of the polaron_soliton system in analytical form. We have found that, when the electron_two phonon coupling strength g1g12n〉 and 〈Δ2p〉〈Δ2q〉 for the two-phonon effect are larger than the one_phonon one and the polaron energy for the two_phonons effect is more negative compared with the one_phonon one.
2008, 57 (7): 4412-4416.
doi: 10.7498/aps.57.4412
Abstract +
When the cohesive energy of LiH is calculated, the many-body correlation effect between ions and the contraction of ionic orbital are included, as two separate physical mechanisms in the total energy formula. The exchange-energy between electronic orbitals is directly computed without any adjustable parameters introduced. The results showed that many-body exchange-correlation effects markedly influence the properties at zero pressure and under high pressures. When the contributions of the nearest and next-nearest ions were taken into account, the results of the isothermal equation of state is in good agreement with experimental data in the pressure range of 0—90 GPa.
2008, 57 (7): 4417-4423.
doi: 10.7498/aps.57.4417
Abstract +
The Sm0.9Sr0.1AlO3-δ samples were prepared by solid-state reaction. Their microstructure, morphology, electrical properties and transport mechanism were studied by X-ray diffraction(XRD), scanning electron microscopy(SEM), alternating current impedance technology and oxygen concentration cell method, respectively. The relationship between sintering technology, relative density and conductivity was studied. The experimental results show that the Sm0.9Sr0.1AlO3-δ perovskite oxides with single tetragonal phase can be obtained when they are sintered at 1650℃. Among all the samples, Sm0.9Sr0.1AlO3-δ sample sintered at 1650℃ for 16h has the highest relative density of 96.7% and electrical conductivity of 1.3×10-2 S/cm at 900℃. The curves of ln(σT) against 1000/T reveal two straight lines intersecting at 670 ℃, showing the activation energy of conduction in the high temperature range(T>670℃) is lower than that in the low temperature range(T0.9Sr0.1AlO3-δ is a mixed conductor of oxygen ion and hole in air. Its oxygen ionic transference number is around 0.7 in the measuring temperature region, and slightly increases with increasing temperature. By means of oxygen ionic transference numbers measured in air, the relative contributions of ionic and electronic conduction are separated, and the results reveal that activation energy for oxygen ionic conduction of 0.95eV is lager than that for hole conduction of 0.84eV, and Sm0.9Sr0.1AlO3-δ has an oxygen ionic conductivity of 9.65×10-3 S/cm at 900℃.
2008, 57 (7): 4424-4427.
doi: 10.7498/aps.57.4424
Abstract +
SrTiO3 and SrNb0.2Ti0.8O3 thin films were prepared by pulsed laser deposition technique on LaAlO3 (100) single crystal substrates. X-ray diffraction shows that all the thin films have［001］ orientation. With the oxygen pressure increasing, the lattice parameter c of SrTiO3 thin films decreases, while the lattice parameter c of SrNb0.2Ti0.8O3 thin films decreases and then increases. Meanwhile, the optimal oxygen pressure to prepare (SrTiO3/SrNb0.2Ti0.8O3)L multilayer of two-dimensional electron gas was found to be 1.0×10-2Pa. In addition, laser-induced thermoelectric voltage effect was observed on SrNb0.2Ti0.8O3 thin film prepared on vicinal-cut LaAlO3 (100) substrate for the first time.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2008, 57 (7): 4428-4433.
doi: 10.7498/aps.57.4428
Abstract +
The atomic and electronic structures of an Al grain boundary with segregated Sr (substitutional) impurity atoms have been calculated by the first-principles pseudopotential method based on the local density functional theory. The results show that the boundary expands and the charge density decreases significantly over the whole boundary due to Sr segregation. This suggests grain boundary weakening, which should be responsible for the experimentally observed Sr-induced Al intergranular embrittlement.
2008, 57 (7): 4434-4440.
doi: 10.7498/aps.57.4434
Abstract +
Electronic structures of the (001) surface of cubic phase barium titanate have been calculated by using plane-wave pseudopotential method within the density functional theory. Geometry optimization indicates that the largest atomic relaxation occurs to metal atoms in surface layers where all atoms are displaced inward, and the interlayer distances vary alternately. Total energy calculation shows that the TiO2-terminated surface is unstable compared with that with BaO termination. One reason is that surface state of O-2p orbital observed in band structure of TiO2 termination helps the electron state shift towards higher energy region in both valence band and conduction band. Redistribution of electrons near surface region in TiO2 termination also provides evidence that intensity of covalent bonding between Ti and O atoms differs and hence TiO2 termination is more suitable for surface adsorption. In contrast, such difference was eliminated in BaO termination, accounting for its superiority in surface stability.
2008, 57 (7): 4441-4445.
doi: 10.7498/aps.57.4441
Abstract +
The atomic cluster models of the crack in αTi formed by dislocation accumulation were set up. The electronic structure of Ti was calculated by using recursion method, and the influence of O, Cl, and Pd on the electronic structure of Ti was studied. The calculated results show that, the total density of states near Fermi level is lowerd due to the existence of O, which leads to the decrease of the chemical activity of Ti. The binding energy of Ti is reduced by oxygen. The affinity between O and Ti is large, so O is easy to react with Ti to form the oxide film. The stability of Cl in Ti and the affinity with Ti are not as good as O. Cl is difficult to substitute the O atom on the surface of Ti, so the oxide film of Ti is very stable, the phenomenon of over passivation can not occur. The environment-sensitive embedding energy of Pd in the crack is smaller than in αTi grain, so Pd is easy to diffuse to the crack and makes the environment sensitive embedding energy of H rise obviously, which leads to the weakening of H diffusion to the crack and the improvement of the stress-corrosion resistance of Ti.
2008, 57 (7): 4446-4449.
doi: 10.7498/aps.57.4446
Abstract +
Electronic states of quantum ring are obtained by solving precisely the energy eigen-equation and a quantum bit is formed by both the ground state and the first excited state of the electron in the quantum ring. The numerical results for InAs/GaAs quantum ring indicate that when the size of quantum ring has been determined, the probability density distribution of an electron is related to its position in the quantum ring and time, there is a maximum of probability density for the given angle and time, and the probability density distribution of electron makes periodical oscillations in time and space.
2008, 57 (7): 4450-4455.
doi: 10.7498/aps.57.4450
Abstract +
Polycrystalline Ni50Mn35In15 alloy was fabricated by arc-melting, and its martensitic transformation and magnetocaloric effect were systematically investigated with magnetic property measurement. The experimental results indicated that, with decrease of temperature, the second-order magnetic transition and the martensitic transition with characteristic of the first-order structural transition happened successively near room temperature, leading to an abrupt change of magnetization. At the same time, exchange bias was found by means of hysteresis loop measurement at low temperature, which clearly suggested coexistence of ferromagnetic and anti-ferromagnetic phases at low temperatures. In addition, a large magnetic entropy change during martensitic transition was calculated from Maxwell equations. Its value can reach 22.3 J/kg K at 309K in applied field of 5T.
2008, 57 (7): 4456-4458.
doi: 10.7498/aps.57.4456
Abstract +
The Green-band luminescence (GL) from an Al-doped 4H-SiC Homoepitaxial layer prepared by using chemical vapur deposition (CVD) has been observed and studied. The deep pattern obtained by SEM and the buffer layer can be found in the interface between SiC substrate and epilayer. The deep profiles of Si atom in 4H-SiC epilayers obtained by using the secondary ion mass spectrometry (SIMS) and the X-ray photoelectron spectroscopy (XPS) show the relative distribution of Si and C. The results strongly suggest that the vacancy of C and the extended defects (point defects, threading dislocations, et al) from buffer layer are responsible for the GL emission. The full width at half maximum (FWHW) of GL indicates the distributions of VC and its complex defects. The quality of crystals and the distribution of defects in epitaxial layers can be characterized by the intensity and the wavelength of GL obtained from samples at room temperature.
2008, 57 (7): 4459-4465.
doi: 10.7498/aps.57.4459
Abstract +
A modified variational method is adopted to investigate the binding energies of the shallow impurity states near the interface of a strained GaN/AlxGa1-xN heterojunction by using a simplified coherent potential approximation. The relations between the impurity binding energies and pressure, the impurity position, electric field strength and the Al components are calculated for strained zinc-blende GaN/AlxGa1-xN heterojunctions with (001) and (111) orientations respectively. The result indicates that the binding energy of impurity state nearly linearly increases with pressure. The Stark effect on the binding energy as a function of electric field shows blue or red shift in the spectra for different impurity positions. The results corresponding to (001) and (111) orientations are compared. It is found that the effect on the binding energy of Al component is obvious. For an impurity located in the channel side and comparatively far from the interface, a monotonic increase of the binding energies will appear with increasing Al content due to the stronger two-dimensional properties of the electrons, and pressure enhances the increase of amplitude. Whereas for an impurity located in the barrier side, the increase of Al content will weaken the bound between the impurity and the electron to decrease the binding energy.
2008, 57 (7): 4466-4470.
doi: 10.7498/aps.57.4466
Abstract +
The powders of n-type (Bi1-xAgx)2(Te1-ySey)3 thermoelectric materials were synthesized by mechanical alloying and consolidated by spark plasma sintering. The milled powders were investigated by XRD and SEM, which shows that the pure Bi, Te, Ag and Se particles were alloyed after milling for 2h. With the increase of the time of mechanical alloying, the size of the powder grains decreases to about 1—0.5 μm. The effects of the doped elements on the thermoelectric properties have been studied. It was found that Ag can improve the power factor and decrease the lattice thermal conductivity, the compound (Bi0.99Ag0.01)2(Te0.96Se0.04)3 after milling for 10h possesses the highest value of the power factor and minimum thermal conductivity, which results in the highest value of ZT of 0.52 at 323K.
2008, 57 (7): 4471-4475.
doi: 10.7498/aps.57.4471
Abstract +
ZnO:Al films have been prepared on p-type Si substrates by DC reactive sputtering. Hall measurement showed that the ZnO:Al films exhibited apparent n type conductivity. I-V characteristics in darkness and under illumination have been performed. It was found that the illumination reverse current increased rapidly with the applied voltage and was saturated at -1 V, which was obviously different from that in darkness. This phenomenon resulted from the photovoltaic effect of heterojunction. Furthermore, in order to investigate the mechanism of photovoltaic conversion, photoconductivity and photovoltage response spectra have been studied. It was found that the photocurrent decreased sharply when the wavelength was 380 nm, and the direction of photovoltage response also changed at this point. It was presumed that this phenomenon has a close relationship with the energy band structure of the heterojunction.
2008, 57 (7): 4476-4481.
doi: 10.7498/aps.57.4476
Abstract +
The impact of high-k dielectrics on the performance of Schottky barrier source/drain (SBSD) ultra-thin body (UTB) SOI is investigated in this paper. With the dielectric constants increasing, the on-state currents of SBSD UTB SOI MOSFET decrease, which suggests that the fringing induced barrier lowering (FIBL) is not the major mechanism for the variation of Schottky barrier height. This phenomenon can be understood in terms of the fringing induced barrier shielding (FIBS). It is also shown that the influence of high-k dielectrics on the performance is quite different in the cases that source/drain and gate electrode have an offset or overlap. For the device with an overlap， the structure with a low-k interfacial layer between high-k gate dielectric and substrate is quite effective in suppressing the degradation of drive current due to FIBS. However, for the device with an offset, the combination of high-k dielectric spacer with stack gate can significantly improve the on-state current. This fact can be explained in terms of the refraction of lines of electric force from gate electrode at interfaces of two materials with different dielectric constants. These lines of electric force with refraction can concentrate at the source region, thus lowering the barrier heights and improving the drive currents. Besides, it is shown that on-state current has a local maximum in the case that source/drain and gate electrode has an offset. The structure parameters can be optimized to improve the drive current.
2008, 57 (7): 4482-4486.
doi: 10.7498/aps.57.4482
Abstract +
The silicon dioxide (SiO2) film was fabricated from layer-by-layer depositing amorphous silicon (a-Si) film combined with step-by-step plasma oxidation in the plasma-enhanced chemical vapor deposition (PECVD) system. The capacitance-voltage(C-V) and conductance-voltage(G-V) characteristics show that the fixed charge and interface state densities of the SiO2 film are 9×1011 cm-2 and 2×1011 cm-2·eV-1, respectively. Furthermore, the breakdown field strength is as high as 4.6 MV/cm, which is comparable to that formed by hot oxidation. The prepared SiO2 is employed as control oxide in nc-Si based double-barrier floating gate memory structure and is found to be an effective way to prevent the charge exchange between the gate electrode and nc-Si, which also lead to an enhancement in the retention time. The improved performance of the memory is discussed and is ascribe to the moderate-thickness of SiO2 as well as its excellent electrical properties.
2008, 57 (7): 4487-4491.
doi: 10.7498/aps.57.4487
Abstract +
Under different stress, the current collapse, gate current leakage and breakdown voltages of AlGaN/GaN high electron mobility transistors change before and after high temperature annealing. The results show that characteristics of devices are greatly improved for AlGaN/GaN high electron mobility transistor after Schottky high temperature annealing. Interface of Schottky contacts is studied by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) before and after high temperature annealing. The analysis indicates that eliminating the medium between Ni and AlGaN and reducing of traps near the surface of AlGaN can improve the effective Schottky barrier, which can enhance the electric characteristics of the devices.
2008, 57 (7): 4492-4496.
doi: 10.7498/aps.57.4492
Abstract +
A 30-volt double diffused drain MOS (DDDMOS) is fabricated with standard 0.15μm CMOS process. The substrate current of this DDDMOS is investigated and the two-humps of Ib-Vg curves is observed. The origin of these two humps of substrate current is demonstrated by experiments and TCAD simulation. The cause of first peak is the same as that in conventional MOS device; the second hump is caused by the impact ionization under high electric field in the drift region far away from the channel edge. The correlation between the electric field and the device parameters is studied through the Poisson's Equation and Current continuity equation. Based on the mechanism of the second hump of Ib, its impaction on device reliability is studied.
2008, 57 (7): 4497-4507.
doi: 10.7498/aps.57.4497
Abstract +
With the continuous downscaling of CMOS technology, process-induced mechanical stress effects become remarkable with the shrinkage of active region. Many processing steps individually or collectively contribute to mechanical stress development. The stress results in not only the layout dependency of device performances, but also diverse reliability issues, which would shorten the chip lifetime. In many cases, stress-related problems are determinative of IC yield. Here, based on the summary of mechanical stress sources, we review the achievements to date in observing and understanding these stress problems, and propose the prospective considerations when analyzing stress-related phenomenon.
2008, 57 (7): 4508-4511.
doi: 10.7498/aps.57.4508
Abstract +
Two non-centrosymmetric superconductors Mg9.3Ir19B16.7 and Mg11Ir19B15 are synthesized. The normal state electronic coefficient of specific heat γn, the Debye temperature ΘD and the superconducting upper critical field Hc2 are found to be sample-dependent, while the electron-phonon interaction constant λ=0.58 and the normalized specific heat jump ΔC/γnTC=1.66 are sample-independent. The higher superconducting transition temperature (TC=5.7K) in Mg9.3Ir19B16.7 is found to be due to the higher density of states and higher Debye temperature ΘD compared with that of Mg11Ir19B15. We also obtained other characteristic parameters of both superconductors, including the Ginzburg-London coherence ξGL, penetration λGL, lower critical field HC1 and thermodynamic critical field HC.
2008, 57 (7): 4512-4520.
doi: 10.7498/aps.57.4512
Abstract +
The magnetic interactions as well as the microscopic origins of the spin-Hamiltonian(SH) parameters including a, g, and Δg for 6S(3d5) state ion in cubic symmetry crystal field, taking into account the spin-spin (SS), the spin-other-orbit (SOO), the orbit-orbit (OO) magnetic interactions besides the well-known spin-orbit (SO) magnetic interaction, have been investigated using the complete diagonalization method (CDM). It is shown that the SH parameters arise from the five microscopic mechanisms of SO coupling, SS coupling, SOO coupling, OO coupling, and SO-SS-SOO-OO combined coupling. The relative importance of the contributions of the mechanisms to the SH parameters are investigated. It is shown that the SO coupling mechanism and the SO-SS-SOO-OO combined coupling mechanism are the most important of these coupling mechanisms. The contributions from SS coupling mechanism, SOO coupling mechanism, OO coupling mechanism to the SH parameter are quite small but their combined effects (SO-SS-SOO-OO combined coupling mechanism) is appreciable. The zero-field splitting (ZFS) arises from the net spin quartet states as well as the combined effects between the spin doublet states and the spin quartets states whereas the Zeeman g (or Δg)factor arises from the net spin quartet states. The contributions to the SH parameters from the net spin doublet states are zero. It is found that the relations: a>0，a(-|Dq|)a(|Dq|)，g(-Dq)=g(Dq)，a(-Dq,-ξd, B, C)=a(Dq,ξd, B, C) and Δg(-Dq, -ξd, B, C)=Δg(Dq, ξd, B, C) always hold for the selected crystal field area.Illustrative evaluations are performed for the four typical materials. Mn2+: KZnF3, Mn2+: RbCdF3, Mn2+: MgO and Mn2+: CaO. Good agreement between the theoretical values and the experimental results are obtained.
2008, 57 (7): 4521-4526.
doi: 10.7498/aps.57.4521
Abstract +
Using the Landau-Lifshitz equation, the propagating of spin wave in metallic magnetic stripe has been investigated, in which the effective boundary condition is applied for the dynamic magnetization of the metallic magnetic strip. The relationship between spin wave spectrum and width of the strip has analytically been derived. Numerical calculations show that in-plane confinement results in the obvious kinks in the threshold curves. And with the increase of strip width, the kink numbers almost rise exponentially, while the threshold jump at the first kink is nearly in inverse proportion to its width.
2008, 57 (7): 4527-4533.
doi: 10.7498/aps.57.4527
Abstract +
A series of composite structured CoxCu1-x(x=0.38—0.87) nanowire arrays have been successfully deposited in the same Co2+/Cu2+=10∶1 solution by applying various depositing potentials. We found that the structures of CoxCu1-x composite nanowires regularly changed with increase of Cu content, thus causing regular change of the magnetic properties of CoCu nanowires. With the percent content of Cu increasing, part of Cu and Co form metastable face-centred cubic (fcc) CoCu solid solution which decreases the competition between the crystallographic anisotropy and shape anisotropy and improves the squareness of CoCu nanowires; the remaining part of Cu exists in the nanowires as fcc Cu crystals which destroy the crystallographic anisotropy and increases the pinning of domain walls, thus improves the squareness and coercivity of CoCu nanowires. Comparing CoxCu1-x nanowires of different compositions, we found that the nanowires have their maximum of the squareness (Mr/Ms) and coercivity (Hc) values for Co75Cu25 and Co60Cu40, respectively. Because of the special composite structures, the values exceed those of single phase CoCu nanowire with the same diameter.
2008, 57 (7): 4534-4538.
doi: 10.7498/aps.57.4534
Abstract +
High Co doping concentration Ti1-xCoxO2 magnetic semiconductor films were prepared by rf co-sputtering and then annealed for 2 hours at 200℃, 300℃ and 400℃ respectively. Microstructure and composition analysis by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) indicated that the films were in the metastable state and the annealing has large effect on their microstructure, composition and magnetism.
2008, 57 (7): 4539-4544.
doi: 10.7498/aps.57.4539
Abstract +
Co-precipitation method was employed to synthesize Mn doped ZnO dilution semiconductors Zn1-xMnxO with the nominal composition of x=0.001, 0.005, 0.007 and 0.01. The annealing temperature (Ts＝400,600,700 and 800℃)dependent structure as well as the magnetic property of the produced samples were studied. The results indicated that samples sintered in air under the temperatures of 600℃ has single wurtzite ZnO structure with prominent ferromagnetism at room temperature; while in samples with the Mn nominal content of 0.007 and 0.01 sintered in air at 800℃, a second phase, namely the ZnMnO3 was observed. The sample with x=0.007 shows largest magnetization and coercivity. Increasing the annealing temperature to 800℃ results in decreasing of both magnetization and coercively，and the paramagnetism was enhanced. Combining with the analysis of the PL spectrum, it is reasonably concluded that the ferromagnetism observed in the studied samples originates from the doping of Mn in the lattice of ZnO crystallites.
2008, 57 (7): 4545-4551.
doi: 10.7498/aps.57.4545
Abstract +
Strain in a laminated structure is transferred by the interlayer adhesive layer. However, the analysis of the adhesive layer behaviors was rarely carried out. The Hamilton's principle is used to derive the governing motion equation of the GMM/elastic plate/PZT three-layered structure. The effects of the adhesive layer resulting from the shear and longitudinal deformation are considered in the analysis. From the motion equation of the GMM/elastic plate/PZT structure and the structural boundary conditions, the equation predicting the system natural frequencies is derived. Then, using the constitutive equations of magnetostrictive and piezoelectric effects, the magneto-electric response over frequency of underlying structure is obtained. By comparing the theoretical magneto-electric response over frequency with the experimental results，it can be seen that the frequency deviation is below 9.42%，and the values of the theoretical magneto-electric voltage coefficient are in agreement with the experiment ones. The dependence of the resonance frequency of the structure on the length of the elastic plate is also discussed.
2008, 57 (7): 4552-4557.
doi: 10.7498/aps.57.4552
Abstract +
The electrical properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-0.3PT) single crystal/polymer 1-3 composites were simulated under different volume fraction of the piezoelectric phase. It was concluded that the composites exhibit optimal properties when the PMN-0.3PT volume fraction is around 64%. Under this volume fraction, the composites with different polymer phases were fabricated using dice-and-fill method. Experiment shows that the stiffness coefficient c, density ρ, as well as the bonding strength have great influence on the obtained properties of the composites. The composites exhibit excellent properties, with electromechanical coupling factor kt as high as 90.1%, piezoelectric d33 larger than 1000 pC/N, mechanical quality factor Qm only 10.39, and acoustic impedance Z less than 20 MRayls. The results indicate that these piezoelectric composites are promising candidates for transducer and sensor applications.
2008, 57 (7): 4558-4563.
doi: 10.7498/aps.57.4558
Abstract +
A new device of two parallel distributed feedback (DFB) lasers integrated monolithically with Y-branch waveguide coupler was fabricated by means of quantum well intermixing. Optical microwave signal was generated in the Y-branch waveguide coupler through frequency beating of the two laser modes coming from two DFB lasers in parallel, which had a small difference in frequency. Continuous rapidly tunable optical microwave signals from 13 GHz to 42 GHz were realized by adjusting independently the driving currents injected into the two DFB lasers.
2008, 57 (7): 4564-4569.
doi: 10.7498/aps.57.4564
Abstract +
The property of the near field subwavelength imaging in a metal thin-film structure is investigated using finite difference time domain method based on Drude model. Using the transfer matrix method，the transmission of this structure for the evanescent wave component is given，which illustrates the influence on the subwavelength imaging quality. In practice a Cr grating mask with 1μm period is imaged onto a photosensitive material through a silver thin film structure by utilizing the optical near-field lithography techniques.
Effect of p-GaN layer thickness on the performance of p-i-n structure GaN ultraviolet photodetectors
2008, 57 (7): 4570-4574.
doi: 10.7498/aps.57.4570
Abstract +
We investigated the influence of thickness of p-GaN layer on the performance of p-i-n structure GaN ultraviolet photodetector. Through the simulation calculation，it was found that both the quantum efficiency and dark current of device decrease when employing thicker p-GaN layer，while both the quantum efficiency and dark current increase with decreasing thickness of p-GaN layer. It is suggested that the Schottky contact junction between the metal and p-GaN may be responsible for the incompatible effect. We has to make a suitable choice of the thickness of p-GaN in the device design according to the application requirement.
2008, 57 (7): 4575-4579.
doi: 10.7498/aps.57.4575
Abstract +
The Ca2SiO4:Dy3+ phosphor was synthesized by high temperature solid-state method. The emission spectrum of Ca2SiO4:Dy3+ shows bands at 486nm，575nm and 665nm under the 365nm excitation. The excitation spectrum for 575nm emission has excitation bands at 331nm，361nm，371nm，397nm，435nm，461nm and 478nm. The effect of Dy3+ concentration on the emission spectrum and luminescent intensity of Ca2SiO4:Dy3+ was investigated.The result shows that the ratio (Y/B)of yellow emission (575nm)to blue emission (486nm)increases with increasing Dy3+ concentration，and the reason was explained by Judd-Offelt theory. The luminescent intensity firstly increases with the increasing Dy3+ concentration，then decreases，and the concentration self-quenching mechanisms are the d-d interaction according to the Dexter theory. The effect of Li+，Na+ and K+ on the emission spectra of Ca2SiO4:Dy3+ phosphor was studied. The results show that the emission spectrum intensity of Ca2SiO4:Dy3+ phosphor is greatly influenced by Li+，Na+ and K+，and the evolvement trend is the same for different charge compensations，i.e.，the emission spectrum intensity firstly increases with increasing charge compensation concentration，then decreases. However, the charge compensation concentration corresponding to the maximum emission intensity is different for different charge compensations，and the concentrations are 4，4 and 3 mol% for the Li+，Na+ and K+，respectively.
2008, 57 (7): 4580-4584.
doi: 10.7498/aps.57.4580
Abstract +
Nanocrystals of up-conversion phosphor，rare earth-doped sodium yttrium fluoride were prepared by hydro-thermal method for the application in thin film solar cells. Effect of Yb3+ and/or Er3+ doped materials on the crystal structure and optical properties were studied. Up-conversion materials with Yb3+/Er3+ co-doped absorb the infrared near 980 nm and 1530nm and emit red (653nm)and green (520nm，540nm)light，which is well adapted for use in thin film solar cells. Nanoparticels with efficient near-infrared to visible up-conversion fluorescence with appropriate doping concentrations of 18% Yb3+ and 2% Er3+ were prepared.
2008, 57 (7): 4585-4589.
doi: 10.7498/aps.57.4585
Abstract +
The phase-field model based on the Tong and Beckermann model coupled with the flow field is used to simulate two-dimensional microstructural dendrite growth of metal by way of finite difference method. The effect of different convection velocities on metal dendritic crystal growth is studied. The result shows that the upstream and downstream dendritic crystal present asymetric pattern: the growth velocity of upstream is much fasfer than downstream. As the velocity of convection increases，actual supercooling of upstream dendritic crystal increases，the growth velocity of dendritic crystalis also speeded up. This is because of the bigger convection velocity leads to stronger flushing to the upstream dendritic crystal and the greater actual supercooling of upstream dendritic crystal，making the dendritic crystal to grow faster.
2008, 57 (7): 4590-4595.
doi: 10.7498/aps.57.4590
Abstract +
The electron emission of a novel antiferroelectric cathode material La-doped Pb(Zr，Sn，Ti)O3(PLZST)has been studied. For driving voltage of 800V and accelerating voltage of 0V，the emission current density was 1.27A/cm2. For driving voltage of 800V and accelerating voltage of 4kV，a strong emission current density with 1700A/cm2 was obtained. The dependence of emission current on accelerating voltage was analyzed and the mechanism of antiferroelectric electron emission was discussed. It was found that strong electron emission from antiferroelectric material can be realized under lower driving voltage and the emission current was much larger than that predicted by the Child-Langmuir law. Local antiferroelectric-ferroelectric phase transition in the vicinity of the triple junction leads to initial electron emission，and these initial electrons then cause desorption of gas which had been absorbed at the ceramic surface.The desorbed gas is then ionized，which leads to plasma generation. The formation of surface plasma enhances the emission current.
2008, 57 (7): 4596-4601.
doi: 10.7498/aps.57.4596
Abstract +
A single multi-walled carbon nanotube (MWCNT)was assembled onto a W tip and transferred to an ultrahigh vacuum field-emission/field-ion microscope (UHV-FEM/FIM)for the study of field evaporation and field emission. The results showed that the field evaporation lowered the work function of the MWCNT and thus enhanced its field emission. The evaporation field of the MWCNT was estimated to be lower than 1.3×108V·cm-1 and the evaporation rate under this field was measured to be 9.4nm·min-1. The physical origin of the fact that the evaporation field of the MWCNT was much lower than the theoretical value for carbon is also qualitatively explained. Firstly，the clean end of the MWCNT，which resulted from field desorption，contained a large number of dangling bonds and the C atoms on it had small coordination number，thus the heat of sublimation of these C atoms was low. Secondly，H atoms that possibly existed in the MWCNT could collide with the C atoms under strong electric field and make the latter evaporate easier. These results indicated an effective approach to obtaining better field emission performance of a carbon nanotube by cutting it short using field evaporation.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2008, 57 (7): 4602-4606.
doi: 10.7498/aps.57.4602
Abstract +
Carbon nanotubes (CNTs) were grown on 25 nm Ni coated Si substrate by dielectric barrier discharge (DBD)-type plasma enhanced chemical vapor deposition (PECVD) in downstream. The temperature was 973 K and the mixture gases were methane, hydrogen and water. The contribution of water concentration on the growth of carbon nanotubes was investigated. Scanning electron microscopy (SEM) results showed that the grown CNTs in CH4/H2 gas mixure had chain-shaped structure and the diameter was about 40—90nm. With the addition of proper amount of water, the diameter of CNTs became uniform, and the surfaces of nanotubes were fully covered with nano-sized node-like structures. When more water was added, branched carbon nanotubes were synthesized. Optical emission spectra of the plasma during synthesis of carbon nanotubes were in-situ detected for the purpose of understanding the water effect.
2008, 57 (7): 4607-4612.
doi: 10.7498/aps.57.4607
Abstract +
Na2O-B2O3-SiO2 glasses doped with semiconductor PbS quantum dots were fabricated via a sol-gel routine in this paper. The influence of different heat treatment on microstructure of glass were made, and characterization of the micro-crystals doped in glass matrix were made by varies methods. The result of N2 adsorption-desorption shows that the diameter of pores in the interior of glass diminishes, with increasing temperature, and the pores totally disappear at last. IR spectra indicate that network structure of glass is form at low temperature, and becomes compact at higher temperature. The PbS crystals are found in the glass matrix by both X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM), and the average size of the crystallites is 3.5nm in diameter. Comparing with the bulk PbS, the absorption edge in the optical absorption spectra of glass containing PbS quantum dots exhibits a blue shift and shows obvious quantum size effect. The nonlinear refractive index (γ) of the glass doped with PbS micro-crystals was determined by Z-scan technology to be -1.6×10-11cm2/GW.
2008, 57 (7): 4613-4622.
doi: 10.7498/aps.57.4613
Abstract +
This paper discusses the basic theory of PIC-MCC method and its implementation in the software UNIPIC. Simulation of relativistic backward-wave oscillator (RBWO)filled with different kinds of neutral gas of different pressures is made by using UNIPIC. The effects of plasma on the propagation of electron beam and wave-beam interaction are discussed. The dependence of the output power，frequency，and the start-oscillation time of the RBWO on the kinds and pressures of the filled gases are presented. The simulation results show that the improvement of RBWO is mainly attributed to the produced ions，and that the low-energy electrons remaining in the tube is the key factor of oscillation-failure and pulse-shortening.
2008, 57 (7): 4623-4628.
doi: 10.7498/aps.57.4623
Abstract +
A focused transducer，featuring extended focal zone，is employed for scanning photoacoustic mammography.Using the transducer with extended focal zone，a large tissue sample can be imaged conveniently.Phantom experiments demonstrate that the location，size and optical energy deposition of absorbers can be imaged correctly.The depth and lateral resolution of image is 0.3mm and 2mm，respectively.And the transducer can be applied in conventional ultrasonic technology to achieve multimode diagnosis as well.Preliminary study shows that this method could be potential in early diagnosis of breast cancer.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2008, 57 (7): 4629-4633.
doi: 10.7498/aps.57.4629
Abstract +
Based on the daily precipitation observations of 740 stations in China from 1960 to 2000，the abrupt change point of power-law exponents of daily precipitationof 0—7mm in north China，northeast China and northwest China is located exactly on 1979. This indicates that the large-scale climate background of these regions has changed at that time，and it has close relation with the index MEI.This maybe one of the main reasons for the drying trend of north China.The 0—29mm daily precipitation is to some extent a stationary process，and the daily precipitation being≥30mm is obviously non-stationary，which makes the long-term prediction of heavy rainfall very difficult.
2008, 57 (7): 4634-4640.
doi: 10.7498/aps.57.4634
Abstract +
Based on daily and monthly temperature records from 1957 to 2001 of China，using climate extreme detecting method based on average and median，we have analyzed the transition and distribution of frequency of temperature extremes.The results indicate that while the average number of extreme high temperature events per year has been increased in the last 50 years,the extreme low temperature events decreased.Except for some stations located in southwest China，the frequency of extreme low temperature events in most regions over China decreased.The spatial pattern of transition of frequency of extreme high temperature events showed decreased in southeast coastal regions and increase in inland regions.
