Vol. 50, No. 2 (2001)
2001-01-20
NUCLEAR PHYSICS
2001, 50 (2): 189-192.
doi: 10.7498/aps.50.189
Abstract +
The dose would be enhanced on the low- Z material side when X-ray enters the interface constructed with two different materials.The mechanism of dose enhancement has been discussed and the Dose Enhancement Factors of W-Si,W-SiO2,Ta-Si and Ta-SiO2 interfaces are calculated by the Monte-Carlo method.
2001, 50 (2): 193-197.
doi: 10.7498/aps.50.193
Abstract +
Within the framework of an isospin-dependent quantum molecular dynamics (IQMD) model,the emission features of light particles in 112Sn+112Sn and 124Sn+124Sn are investigated at different beam energy,different impact parameter,different EOS and different nucleon-nucleon cross section.Calculations show that the ratios of light particles are consistent with the systematics of entrance channel isospin and the midrapidity region can emit more neutron-rich particles than projectiles-like rapidity.In addition,it is found that the light particle ratio is sensitive to the nuclear EOS,but not to the nucleon-nucleon cross section,which makes it possible to extract the isospin-dependent EOS.
2001, 50 (2): 198-203.
doi: 10.7498/aps.50.198
Abstract +
We have solved the Shr?dinger equation of three Fermi ions with coplanar configuration in a Paul trap,when the whole spin angular momentum of the system is S=1/2 or 3/2.The structure and quantum dynamics of this system are investigated by inspecting the wave function nodal structure and the shape-density.At the same time,we compare our result with that of three Bose ions system and three ions system in classical condition.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2001, 50 (2): 204-208.
doi: 10.7498/aps.50.204
Abstract +
The local density fluctuation in femtosecond laser plasmas has been studied using optical shadowgraphy and scattering imaging techniques.The instability of filamentation in the laser plasma has been observed,and the local density fluctuation studied.The result shows that this instability of filamentation is caused by the ponderomotive force.
2001, 50 (2): 209-216.
doi: 10.7498/aps.50.209
Abstract +
The surface border of weldpool in MIG welding is irregular and the temperature in the pool is very high and is distributed extremely nonuniformly.The violent motion occur under the action of the electromagnetic force,the buoyancy force and the surface tension,etc.According to the bimodal distribution mode for arc current density with a large deformation of the pool surface,the calculation mode for electromagnetic force is developed and described in this paper.Numerical analysis is used to study the hydrodynamics behavior in the weldpool and the effects of welding technical parameters on fluid flow.The experiments show that the predicted results are in good agreement with experimental measurements.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2001, 50 (2): 217-222.
doi: 10.7498/aps.50.217
Abstract +
Using the projection and Fourier transform methods,we have derived the diffraction formulas and discussed the rules of diffraction peaks and systematic absences for the one-dimensional Fibonacci-class quasilattices.Moreover,we have calculated and labeled the positions of the diffraction peaks of FC(1) and FC(3).In order to check our diffraction formulas,we extend the formulas to the FC(n) quasiperiodic superlattices.Comparing the present diffraction results of the FC(1) superlattice with the experimental data,we have found that most of them agree with each other but one of the spectra is not consistent.Particularily,we doubt that the peak (4,2) in the experiment should be a stronger peak.
2001, 50 (2): 223-226.
doi: 10.7498/aps.50.223
Abstract +
We have investigated the growth of gold nanoclusters on the surface of highly oriented pyrolytic graphite in ultrahigh vacuum.Studies by ultrahigh vacuum scanning tunneling microscopy indicated that the size distribution of gold clusters was very narrow and quasi-one-dimensional chains of gold nanoclusters of approximately 2 nm diameter were produced after annealed at 74℃.And gold nanowires made of different sized gold clusters could also be formed after annealed at 122℃.These results suggest the viability of a new route to the creation of ordered nanoscale structures.
2001, 50 (2): 227-232.
doi: 10.7498/aps.50.227
Abstract +
We have investigated the polarization dependence of FT-IR of the stretching of CD2 in chiral alkyl chain of TFMHxPOCBC-D2(4-(trifluoromethylhexy-3-d2 carbonyl) phenyl 4′-octyloxybiphenyl caboxylate) with a homeotropically aligned cell.It is found that the polarization dependence of Sm-C and that of Sm-CA are different.Simulation calculation indicates that the rotation of CD2 around the alkyl chain is hindered.
2001, 50 (2): 233-238.
doi: 10.7498/aps.50.233
Abstract +
The martensitic phase transformation temperature,Tm and the Curie temperature, TC, of polycrystalline samples Ni2+xMn1-x Ga(x=-0.1,0,0.08,0.13,0.18,0.2) and Ni2-xMn1+x/2Ga1+x/2(x=-0.1,0,0.04,0.06,0.1) have been investigated by measuring the temperature dependence of alternating-current susceptibility. It was found that, with the increase of Ni content,TC decreases and Tm increases for Ni2+xMn1-xGa, and both TC and Tm increase first and decrease afterwards for Ni2-xMn1+x/2Ga1+x/2. Stress-free and two-way thermoelastic shape memory, with 1.2% strain, has been found in single crystal Ni52Mn24Ga24. The deformation can be enhanced more than three times, up to 4.0% shrinkage with a bias field 1.2T applied along the measurement direction. The origin of large recoverable strain should be attributed to the low level of internal stress and the field-induced rearrangement of martensitic variants through twin boundary motion rather than the phase boundary motion.
2001, 50 (2): 239-243.
doi: 10.7498/aps.50.239
Abstract +
The rigid-ion model with only eight or nine meaningful parameters is applied to calculate the dispersion relations of both ferroelectric and paraelectric phases in PbTiO3. Phonon frequencies and vibration mode at the zone center were obtained. The good agreement between our calculation and the previously mode with eighteen parameters is shown. A simple model for investigation of the soft modes is suggested.
2001, 50 (2): 244-250.
doi: 10.7498/aps.50.244
Abstract +
The structure of diamond/silicon interface, which was formed by the contact and the subsequent relaxation of the unreconstructed (001) surfaces of diamond film and silicon substrate, has been investigated by molecular dynamics simulation.The interaction among atoms of the silicon/carbon binary system was described by Tersoff many-body empirical potential. Before relaxation, the ratios of silicon atoms to carbon atoms along [110] and [110] directions are both 3∶2 and the ratio of the total number of interfacial silicon atoms to that of carbon is 9∶4. After relaxation, the matching of diamond and silicon lattices has been changed: along[110] direction, a 3∶2 coincidence relation is roughly remained, but along [110] direction, a 1∶1 coincidence relation is approximately adopted. Accordingly, the ratio of the total number of interfacial silicon atoms to that of carbon is near 3∶2. The migration of partial silicon atoms, which were originally in the second layer, upward to the interface is responsible for such changes. The silicon lattice near the interface shows the tendency of disordering along [001] direction. The strong bonding between interfacial silicon and carbon atoms was found and Si-C bonds with an average length of 0.189 nm were formed. This study has confirmed that the main feature of lattice matching is the meeting of silicon atoms at and near the interface to the arrangement of interfacial carbon atoms.
2001, 50 (2): 251-255.
doi: 10.7498/aps.50.251
Abstract +
The enhanced nucleation process of diamond by negative substrate bias was investigated by scanning electron microscopy and atomic force microscopy. The mechanism of enhanced joint adhesion of diamond nuclei on Si substrate by ion bombardment was theoretically approached, and the relationship between negative substrate bias and adhesion of diamond nuclei on Si substrate was given.
2001, 50 (2): 256-261.
doi: 10.7498/aps.50.256
Abstract +
SiC films were prepared on Si(111) substrates by annealing polystyrene gel films at temperature 950℃ in vacuum(10-3 Pa).Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD), Transmission electron microscopy(TEM).Raman scattering and X-ray photoelectron spectroscopy(XPS) were used to study the morphology of the surface, crystal structure. composition and chemical state of the element of the SiC films. It revealed that the films consisted of preferentially oriented crystalline 6H-SiC epilayer which grew along(0001) planes parallel to Si(111) planes. The ratio of Si to C was about 1 and there were some carbon and oxide contaminant species in the form of CH, CO and Si2O3 along with a small amount of adsorptive oxide at the surface of the films. The polycrystalline grains in the films grew cylindrically along c axis.Their maximum size was about 150nm. The film was smooth. dense and uniform with a thickness of about 0.3 μm. It was found that covering the polystyrene film with a Si plate could increase the quantity of SiC during anncaling.
2001, 50 (2): 262-267.
doi: 10.7498/aps.50.262
Abstract +
We have designed a new type metal-semiconductor-metal structure with InAs self-assembled quantum dots. Hystereses loops were observed in DC current transport. With quantum dots(QDs) directly embedded beneath the GaAs-metal interface, the charge and discharge of electrons in the dots modulate the current to form hysteresis. For a single quantum dot, the charge and discharge can be thought as single-electron processes controlled by applied voltage. In this paper we analyzed the characteristics of charging and discharging processes for the ensemble of QDs as a whole. The discharging process was dominated by the change of tunneling rate with gate voltage. The charging process was controlled by forward current flowing though the diode. The calculated results indicate that the electrons trapped to the ground states of QDs give rise to charging effect. The excited states capture fewer electrons and influence the charging effect weakly.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2001, 50 (2): 268-272.
doi: 10.7498/aps.50.268
Abstract +
We have developed a new Si-H tight-binding potential through introducing hydrogen atom into the previous silicon tight-binding potential model,in which the correction of environment around a Si-H bond is considered for the interaction between silicon and hydrogen.The testing results show good transferability,hence this new model can be used to do research on complicated silicon-hydrogen systems.
2001, 50 (2): 273-277.
doi: 10.7498/aps.50.273
Abstract +
In free electron band model of three different crystal structures,face-centered cubic (fcc),body-centered cubic (bcc),and hexagonal close-packed (hcp) structures,we fine that the average energy of the four lowest band eigenvalues and the five sub-low band eigenvalues (called as average bond energy Em) is rather close to Fermi level EF.Meanwhile,we also confirm that this conclusion still holds for the practical band in some metals,such as Ti,Zr and Hf with hcp structure as well as Fe with bcc structure etc.,using ab initio pseudopotential and average bond energy methods.Thereby one can further understand the physical connotation of average bond energy Em.
2001, 50 (2): 278-286.
doi: 10.7498/aps.50.278
Abstract +
We calculated the electronic structure of double quantum-dot molecule with a generalized LCAO approach and the finite element method.Compared with real molecule,the quantum dot molecule can be in the form of covalent bonding or ionic bonding between the two same (or different) quantum dots under different conditions.The condition is related to the distance between the two quantum dots and the energy of each quantum dot.The effects of the potential and radius of the quantum dots on the electronic structure have also been studied with the finite element method.
2001, 50 (2): 287-292.
doi: 10.7498/aps.50.287
Abstract +
By employing the concept of photon flux,the evolution of the phase and photon flux in the eigenmode state in three-wave mixing,and the requirements for the initial phase and photon flux in this state,are studied in the paper.A special state in which there are only exchanges of the photon flux but no phase variations in the three-wave mixing process is investigated.An all-optical switch based on the eigenmode pushpull nonlinear-optical-mirror is presented,and the properties of this all-optical switch are numerically calculated.The numerical results show that the performance stability is extremely good,and the intensity and phase of the output signal beams can be easily realized and controlled.
CHARGE STORAGE AND TRANSPORTATION IN DOUBLE LAYERS OF Si3N4/SiO2 ELECTRET FILM BASED ON Si SUBSTRATE
2001, 50 (2): 293-298.
doi: 10.7498/aps.50.293
Abstract +
The characteristics of charge storage in APCVD Si3N4/thermal-grown SiO2 double layers electret films, charged by coroma were investigated by measurements of isothermal surface potential decay and thermally stimulated discharge(TSD).The results show that all of the samples have high charge stability at room temperature, at 300 and 60℃ under 95% of the relative humidity. The transport of the trapped charges due to external excitation for the negatively charged samples is basically controlled by the slow retrapping effect. However, the transport of detrapped positive charges due to external excitation can be described by a model of thermal ion emission.
2001, 50 (2): 299-303.
doi: 10.7498/aps.50.299
Abstract +
Based on the fact that the charge is quantized in the mesoscopic circuit,the quantum theory of the mesoscopic inductance-coupling circuits and the condition for Coulomb blockade have been given in this paper,and the quantum fluctuations in the mesoscopic inductance coupling circuits have also been shown.
2001, 50 (2): 304-309.
doi: 10.7498/aps.50.304
Abstract +
Raman spectra at different temperatures are reported for K3Ba3C60.A systematic change of the Raman spectra with temperature is observed.The development of the linewidth and intensity for all modes with temperature is in sharp contrast to the case for pure C60.An anomalously large up-shift of 9 cm-1 between 20 K and room temperature for the pinch Ag(2) mode suggests the existence of hybridization between the C60 molecules and the intercalants.The relative intensity of the doublet of Ag(1) systemtically changes with decreasing temperature.
2001, 50 (2): 310-312.
doi: 10.7498/aps.50.310
Abstract +
We consider a massless fermion interacting with the gauge field by Pauli interaction in which the coupling constant is dimensionless. The model is equivalent to the s=1/2 antiferromagnetic XXZ spin chain. The two flavours massless fermion of such model can describe the two-leg spin ladder system qualitatively, and there is no gapless exitation in such a system.
2001, 50 (2): 313-318.
doi: 10.7498/aps.50.313
Abstract +
The temperature dependence of lattice constants a and c of intermetallic compund DyMn2Ge2 is measured in the temperature range 10-800K by using the X-ray method. The magnetoelastic anomaly of lattice constants a is observed at spontaneous magnetic phase transition from paramagnetic state to antiferromagnetic state. The ac magnetic susceptibility of DyMn2Ge2 is measured in the temperature range 4.2-200K. Low temperature magnetic phase transitions in DyMn2Ge2 are discussed in the molecular field approximation by taking into account of next nearest Mn-Mn interlayer exchange interaction. The temperature dependence of magnetization and magnetic curves at different temperatures of DyMn2Ge2 single crystal are calculated. Theoretical calculation shows that the observed field-induced magnetic phase transition in DyMn2Ge2 single crystal at temperature lower than 33K is the transition from ferrimagnetic state into intermediate state.
2001, 50 (2): 319-323.
doi: 10.7498/aps.50.319
Abstract +
Polycrystalline samples of La0.8-xCa0.2MnO3(x=0,0.01,0.03,0.05,0.08,0.1) were synthesized using a sol-gel technique. Structure analysis, morphology observation and magnetic measurement have been used to study the effect of vacuum concentration of La3+ on the Curie temperature and magnetic entropy change of La0.8-xCa0.2MnO3. The experimental results show that by tuning the vacuum concentration of La\+\{3+\}, the materials could exhibit very different Curie temperatures (from 180 to 260K). All the samples in this paper have large magnetic entropy change, especially for La0.77Ca0.2MnO3 sintered at 1100℃, which has a very large magnetic entropy change ΔSM=3.76J/kg·K under the magnetic field H=1.0T. The phenomenon of large magnetic entropy change, high Curie temperature, high electrical resistivity and high chemical stability make this material a suitable candidates for refrigerants at high temperature.
2001, 50 (2): 324-328.
doi: 10.7498/aps.50.324
Abstract +
A model of a chain of oblate ellipsoids is proposed for magnetization reversal of barium ferrite particles. The angular dependences of coercivity and critical field were calculated with the consideration of the uniaxial magnetocrystalline anisotropy, the shape anisotropy, the interaction anisotropy of the ellipsoids, as well as the number of ellipsoids. This model could be used to explain the magnetization reversal mechanism of the oriented Ba ferrite particulate media.
2001, 50 (2): 329-334.
doi: 10.7498/aps.50.329
Abstract +
By Application of the V-F-B model which is obtained by combining the Vasquez-Flint (V-F) model for donor-acceptor energy transfer and the model of Burshtein for donor-donor energy migration, the experimental luminescence decay curves of 4S3/2 state of Er3+ in LaF3 at 295K for four different concentrations are re-simulated successfully.It is found that in this system the donor-acceptor interaction is dipole-dipole interaction,and that the donor-acceptor interaction constant CDA is 4.75×10-41cm6/s. The results are all in agreement with the previous results of Okamoto. However, the hopping time τ0 obtained from the above simulation in proportional to xD-1.237 (xD is the donor concentration), which is different from the theoretical dependence on the donor concentration (that is τ0∝xD-2). In addition, by considering donor-acceptor energy transfer and donor-donor energy migration within the V-F model at the same time, we obtain the estimated values of the hopping time τ0′ for four different concentrations, which are in reasonable agreement with the values of τ0 obtained from the above simulation.It seems that the V-F-B model is rather self-consistent.
2001, 50 (2): 335-340.
doi: 10.7498/aps.50.335
Abstract +
Upconversion fluorescence in Nd:YVO4 crystal under a laser diode (LD) pump was investigated. It is proved that the predominant mechanism for excitation of 4G7/2 level is an energy transfer upconversion. Then,a theoretical approach to upconversion effect on LD pumped Nd:YVO4 laser is given.In addition,the role of upconversion in thermal effect is discussed in detail.
2001, 50 (2): 341-346.
doi: 10.7498/aps.50.341
Abstract +
C-nanoparticles with a diameter smaller than 60 nm in xAl2O3-xP2O5-100SiO2(x=0.25—2) gel-glasses have been prepared by sol-gel process. The gels synthesized through partial hydrolysis of PO(OC2H5)3,Al(NO3)3·9H2O and Si(OC2H5)4 were heated at 400℃ or 450℃ in air atmosphere or from 300 to 700℃ in N2 atmosphere, in which —OC2H5 were carbonized to form nanosized C particles. These C-nanoparticles-doped gel-glasses were characterized by optical absorption and TEM.The optical absorption edge shifts to the higher-energy side as the size of the C particle decreases. This phenomenon is interpreted in terms of a quantum confinement effect of electron and hole in the C-nanoparticles. The C-nanoparticles structures were found to be amorphous carbon in the gel-glasses by electron diffraction pattern.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2001, 50 (2): 347-353.
doi: 10.7498/aps.50.347
Abstract +
This paper mainly deals with problems of optimal field pattern control to the field conjugate direct synthesis of phased-array ultrasound hyperthermia. First,the rational optimal objective function for acoustic field pattern is put forward. Then, two optimal algorithms for global optimization are proposed. One is the genetic algorithm for constructing acoustic field pattern control and the other is the eigenvector algorithm for the same purpose. The simulation is performed with these two algorithms, genetic algorithm and eigenvector algorithm. Eventually, the objective of optimal field pattern control is achieved successfully. The genetic algorithm has better characteristics as compared with the eigenvector algorithm.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2001, 50 (2): 354-360.
doi: 10.7498/aps.50.354
Abstract +
We have collected 29 gamma-ray-loud blazars (16 BL Lac objects and 13 flat-spectrum radio quasars) with both observed near-IR and γ-ray flux densitees, with the following main results:(1)there is a very strong correlation between Fγ and FIR in the low state, and a weaker but also significant correlation between Fγ and FIR in the high state for 23 objects with both high and low state fluxes; (2) there is a very significant correlation between Fγ and FIR in the low state, and a weak correlation between Fγ and FIR in the high state for 29 sources; (3) there is a correlation between Fγ and Fx, but not between Fγ and F0, and FR for both the low and high state of 26 sources; (4) there is a strong correlation between Fγ and FIR in the low and high states for 15 BL Lac objects, but not for 11 flat-spectrum radio quasars. Possible constraints on the γ-ray emission mechanism are discussed.We suggest that the main gamma-ray radiation mechanism is probably the synchrotron self-Compton process.The inverse Compton scattering of the radiation from hot circumnuclear dust with Tg=2000K, which is located within the region of r≈3pc, by beamed ultrarelativistic electrons is likely to be an important complementary mechanism.In addition, the γ-ray emission may be somewhat different for BL Lac objects and flat-spectrum radio quasars.
LETTERS
2001, 50 (2): 361-364.
doi: 10.7498/aps.50.361
Abstract +
Ultrahigh-density data storage on a 3-phenyl-1-ureidonitrile (PUN) thin film was performed using a scanning tunneling microscope. The recorded marks of 0.8 nm in diameter were obtained when voltage pulses of 4 V for 10ms were applied between the STM tip and highly ordered pyrolytic graphite substrate. The current-voltage relations at the local regions of the films indicate that the recorded region is conductive and the unrecorded region is in a high impedance state. A possible mechanism of this data storage was suggested and discussed.
2001, 50 (2): 365-368.
doi: 10.7498/aps.50.365
Abstract +
High temperature, high density and strong magnetic fields in plasmas produced by ultra-high intensity and ultrashort laser pulses are similar to the main characteristics of astrophysical plasmas. This makes it possible to simulate some astrophysical processes at laboratories. This paper presents our theoretic simulation of aluminum emission spectra in astrophysical plasmas. It can be concluded that using laser produced plasmas, we can obtain rich information on astrophysical spectroscopy, which is unobservable for astronomer.
