Vol. 53, No. 4 (2004)
Explicit expressions and recurrence formulas of the radial average values for a kind of nonharmonic oscillator model potentials
2004, 53 (4): 973-977. doi: 10.7498/aps.53.973
In this paper, two recurrence formulas are derived for the radial average values of a kind of non-harmonic oscillator model potentials; these non-harmonic oscillator model potentials are ring-shaped non-spherical oscillator, non-spherical oscillator, and ring-shaped oscillator. Explicit expressions for their radial average values are given.
2004, 53 (4): 978-983. doi: 10.7498/aps.53.978
Compared with the spherical quantum dot, the ground-state energies of hydrogen-like impurity in a lens-shaped quantum dot (GaAs/In1-xGaxAs) under vertical magnetic field have been discussed by using effective mass approximation and variational method. It is found that for the spherical quantum dot, the ground-state energy is only related with the deviation of distance of the impurity, but not with the in-plane or perpendicular deviation. For a lens-shaped quantum dot, due to the asymmetry of the bound potential of in-plane and perpendicular to the plane, the electronic ground-state energies are related not only with the deviation of distance but also with the deviation of direction.
2004, 53 (4): 984-990. doi: 10.7498/aps.53.984
The analytical expressions of chemical potential, total energy and heat capacity at a constant volume for a weakly interacting Fermi gas in the absence of external potential are derived by using “pseudopotential” method. Based on the derived expressions, the thermodynamic properties of a weakly interacting Fermi gas trapped in a harmonic potential are studied under the local-density approximation. The effects of interparticle interactions on the properties of the systems are discussed.
2004, 53 (4): 991-995. doi: 10.7498/aps.53.991
The effect of finite number of trapped atoms is discussed in Bose-Einstein condensation without interaction, according to Thomas-Fermi approximation. We obtain the state density by high-temperature expansion and calculate the critical temperature in one-dimensional(1D) and two-dimensional(2D) trap potentials. Then the temperature dependence of specific heat are analyzed. The results show that: in 2D case, the specific heat c is proportional to T2; in 1D case,c increases linearly with temperature.
2004, 53 (4): 999-1002. doi: 10.7498/aps.53.999
This paper develops a modified state observer for chaotic generalized synchronization. An analytic approach is proposed for constructing a response system to implement generalized chaos synchronization with drive system. From the state observer theory, some sufficient conditions of global asymptotic linear-generalized synchronization between the drive system and response system are obtained. Finally, a hyperchaotic R?ssler system is given to illustrate the effectiveness of the proposed synchronization method.
2004, 53 (4): 1003-1007. doi: 10.7498/aps.53.1003
Properties of differently doped Zn0.95Cd0.05Te〈110〉 crystals as terahertz (THz) wave emitter are studied using THz time-domain spectroscopy. It is observed that the THz generation efficiency increases as the DC resistivity increases when the resistivity of crystals is greater than 102 Ω·cm, but the efficiency saturates and even declines when the resistivity goes beyond 106 Ω·cm. It is revealed that the dispersion properties of crystals play an important role, besides the doping-dependent absorption in THz region.
2004, 53 (4): 1008-1014. doi: 10.7498/aps.53.1008
The tapping-mode atomic force microscope (TM-AFM) is a very useful tool to investigate the morphology and the physical properties of sample surface. The tip-sample contact process and phase contrast of TM-AFM is studied by numerical simulation. The cantilever-tip ensemble is simply modeled as a damped harmonic oscillator driven by an actuator. The effects of tip-sample distance zc and the Young modulus of sample Es on the contact time between the tip and sample tc, the deformation of the sample surface Dz and the contrast of phase image have been investigated. The results show that both tc and Dz decrease with increasing Es or zc. A little amount of energy dissipation can occur when the tip is tapping on the sample. Moreover, when zc is fixed, the phase shift of the sample with large Es is found more remarkable than that with small Es. The experimental observation of phase image performed on Au film dispersed with Au clusters supports the simulation results.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
In this paper, the hadronic matrix element of B0→K0π0 decay is calculated systematically, which includes the contribution of factorization in the leading order, the O(αs) correction from hard gluon exchange and the correction from soft gluon exchange. The soft gluon exchange cannot be calculated in quantum chromodynamics(QCD) factorization as well as in perturbed QCD, but it can be calculated in terms of the light cone QCD sum rules,and the calculation shows that the amplitudes of soft gluon exchange, the factorization in the leading order and the O(αs) correction in the decay channel are of the same order of magnitude. So, the soft gluon exchange effect cannot be neglected. Finally, the branch ratio is calculated and it is consistent with the experimental result.
2004, 53 (4): 1020-1022. doi: 10.7498/aps.53.1020
Based on the theory of independent source model, the charged-multiplicity distributions in heavy ion collisions are derived which are then used to analyze the centrality dependence of the charged-multiplicity pseudo-rapidity densities, dnch/dη, in Au+Au collisions at (SNN)1/2=130 GeV in midrapidity region. The theoretical results agree well with experimental facts.
ATOMIC AND MOLECULAR PHYSICS
2004, 53 (4): 1023-1026. doi: 10.7498/aps.53.1023
The effect of pumping laser intensity on electromagnetically induced transparency (EIT) in the cesium atomic three-level system was investigated, and the EIT against double-pumping laser was obtained. The theoretical explanation was put forward using density matrix, and the experimental results is in agreement with theoretical calculation.
2004, 53 (4): 1027-1033. doi: 10.7498/aps.53.1027
The spectra of N2O+ ions in A2Σ+ state have been investigated. Pure parent N2O+ ions, in the X2Π3/2,1/2(000) state, were prepared by (3+1) multiphoton ionization of jet-cooled N2O molecules using a laser beam at 360.55 nm. By introducing another laser, the parent ions were excited to the predissociative A2Σ+ state, and the fragment NO+ was detected by a time-of-flight mass spectrometer. The NO+ photofragment excitation (PHOFEX) spectra were recorded by scanning the dissociation laser in the wavelength range of 278—328 nm. The PHOFEX spectra could be attributed completely to the A2Σ+←X2Π transition of N2O+, in which most vibronic bands were observed for the first time. By considering the Fermi resonance between the ν1 and ν2 modes, the spectra were assigned, and the spectral constants, such as vibrational frequencies, anharmonic constants, and Fermi interaction constant, were obtained with relatively high reliability and precision.
2004, 53 (4): 1034-1038. doi: 10.7498/aps.53.1034
Ground state multiply charged Fen+(n=1—3) ions have been produced by means of two separated crossed laser beams incident upon solid cast ion targets in a Paul ion trap. The decay rate of Fe3+ is measured to be 0.96 s-1 at the ground pressure 3.0×10-7 Pa and the reactive product between Fe3+ and N2 is also obtained under the pressure 1.3×10-5 Pa.
2004, 53 (4): 1039-1043. doi: 10.7498/aps.53.1039
A simplified dynamic model employing a velocity reset method is proposed to describe the dynamic process of nano-cluster growth in vapour-phase at high temperature. As an instance here, this model is applied to simulate the cage-shaping of carbon atoms, providing some interesting information which is found to be in better agreement with relevant experimental observations.
2004, 53 (4): 1044-1049. doi: 10.7498/aps.53.1044
The first principle, all-electron, ab initio calculations have been performed for cluster Ga6N6, based on the density functional theory. Ten possible structures and related electronic structures are obtained. For the most stable structure, the mean binding energy of a pair atoms of GaN is 9.748 eV, so the structure may exist. Compared with the results of clusters Ga3N3 and Ga5N5 calculated by other people, however, the cluster Ga6N6 may not be the “magic number” cluster. The Fermi level of cluster Ga6N6 in the most stable structure is partly occupied with EF=-5.2972 eV, which means “metallicity”. The cluster Ga6N6 has no spin magnetic moment. The electron affinity, ionization energy, and transition energies of cluster Ga6N6 are also calculated. This work should be helpful to the complete study of structures and properties of clusters GanNn.
2004, 53 (4): 1050-1055. doi: 10.7498/aps.53.1050
Based on the first-principle, we optimize all geometric structures of (TM)4 clusters, calculate the energy, frequency and magnetism of (TM)4 clusters by using the local spin density approximation(LSDA) and generalized gradient approximation(GGA) density functional theory. We confirm the ground-state structures of (TM)4 clusters, study their magnetism, binding energy and average interatomic distance systematically, and find that in the 3d series (TM)4 clusters, the local magnetic moment of Mn4 cluster is the largest, that of the V4 cluster is the weakest. Except that the Cr4 cluster in LSDA and GGA shows anti-ferromagnetic coupling，and the V4 cluster in GGA shows also anti-ferromagnetic coupling, other clusters all show ferromagnetic coupling.
2004, 53 (4): 1056-1062. doi: 10.7498/aps.53.1056
Using density functional theory associated with the B3LYP method with 6—31G* basis set, we have carried out the optimizing calculations of the geometry, electronic state and vibrational frequency for GamP-n(m+n≤5) clusters, moreover, discussed the vertical detachment energy of GamP-n and the adiabatic electron affinities of GamPn. The results show that the structures with singlet have higher symmetry, while the one with doublet has lower symmetry.
CLASSICAL AREA OF PHENOMENOLOGY
2004, 53 (4): 1063-1069. doi: 10.7498/aps.53.1063
Based on the purterbative method which was developed by Lax et al, and by using of the Fourier transform, we obtain a vectorial nonparaxial correction method to the propagation of the few-cycle pulsed beam in free space. the first-order of the transverse component of the correction is the same as that of Fu's in the frequency domain, and is more convenient to obtain the analytical nonparaxial solution in the temporal domain. It is shown in the examples and the analysis that the vector corrections must be included in any study that wants to go beyond the scalar paraxial solution. From the examples, we find that the spatial distribution of the vectorial nonparaxial correction would be influenced by the temporal distribution of the pulse.
Incoherent multimode bistable soliton can exist in higher-order nonlinearity media. Using the coherent density method, we obtain the analytical expression of the incoherent multimode bistable soliton, and the higher-order nonlinearity controls the existence and the intensity peak of bistable solitons. We find the cut-off wavelength, the minimum width of the incoherent beam and the nonlinearity condition for the media, and then we study the propagation characteristics of coherent components that consist of incoherent multimode beams. According to the stability criterion, we demonstrate rigorously that the incoherent multimode bistable soliton can propagate stably against small perturbation.
2004, 53 (4): 1076-1080. doi: 10.7498/aps.53.1076
Multi-wavelength photochromic storage is a new optical storage method with high storage capacity based on photochromic principle. The storage mechanism, the system structure and disc structure of multi-wavelength photochromic storage are presented. Three-wavelength photochromic storage experiment on a three-wavelength photochromic storage system is carried out. The results show that three materials used in the experiment can be used in multi-wavelength photochromic storage and the contrast of the readout signal is high and little crosstalk is generated. The contrast of the readout signal remains high after low-speed reading out 50 times.
Statistic properties of photon in the system of two coupling atoms interacting with two-mode squeezed vacuum field in Kerr medium
2004, 53 (4): 1081-1087. doi: 10.7498/aps.53.1081
The statistic properties of photons in the system of two-mode squeezing vacuum field interacting with two coupling atoms in Kerr medium are studied by means of quantum theory.The influences of coupling constant χ, initial squeezed parameter r and the atomic initial state are discussed. The results obtained by means of numerical method show that the photon bunching and anti-bunching appear alternately, and the correlation of the two modes of light is both positive and non-classical. The periodical collapse-revival phenomenon of the time evolution of the second-order coherence degree of the two-mode field appears due to the influence of the Kerr effect. The periods of the collapse-revival become short up to disappearance with increasing Kerr effect.
2004, 53 (4): 1088-1094. doi: 10.7498/aps.53.1088
Propagation stability of non-paraxial beam in nonlinear Kerr media is investigated by using the linear stability method.Both theoretical analysis and numerical simulation show that the modulation instability gain spectrum has three different distribution features, which are determined by ap0, where a and p0 represent the non-paraxial parameter and the incident power, respectively. Furthermore, the corresponding criterion is given to distinguish the three different distributions.
2004, 53 (4): 1095-1098. doi: 10.7498/aps.53.1095
We have studied the optical transmission of one-dimensional photonic crystals with a mirror symmetry. One or more complete transmission peaks exist in the forbidden gap. A simple recursion relation is derived by means of the mathematical induction to calculate the frequencies of the complete transmission peaks. In the center there is always a transmission peak due to the mirror symmetry. Other peaks are dependent on positions of the defects and the refractive index. If the structure is chosen properly, a wider transmission band can be obtained.
2004, 53 (4): 1099-1104. doi: 10.7498/aps.53.1099
The reflectivity of grazing soft x-ray planar mirror is studied.The Beijing Synchrotron Radiation Facility-3W1B beam line with beam current 40—120 mA,the storage ring electron energy 2 GeV,photon energy 50—1500 eV are used.We have improved the calibration method.The x-ray diode detectors of planar mirror facility are replaced by AXUV-100.Therefore,2 to 3 orders of magnitude for the ratio of signal to noise can be increased,and the calibration region from (150—270 eV)to (50—1500 eV)is expanded.The reflectivity calibration curves for C,Si,Ni,and Au planar mirror in different grazing angles are given.Finally,the values obtained in experiment and calculation are compared and analyzed.
A designed model and related theoretical analysis on optical pulse compressor based on a SOA and cascaded NOLM
2004, 53 (4): 1105-1109. doi: 10.7498/aps.53.1105
A designed model of the optical pulse compressor, which is based on a semiconductor optical amplifier and cascaded nonlinear optical loop mirror, has been presented in this paper. The numerical simulations reveal that a ps optical pulse can be compressed into a fs pulse without pedestal.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2004, 53 (4): 1110-1115. doi: 10.7498/aps.53.1110
A simulation model has been developed to calculate the transmission and absorption of electromagnetic waves by inhomogeneous spherically symmetric plasma.In this model,the nonuniform sphere is modeled by numbers of concentric spherical shells,and each has a fixed electron density.Using the geometrical optics approximation and considering that the phase coefficient and attenuation coefficient are vectors,we have studied the property of absorption and transmission of electromagnetic waves by inhomogeneous spherically symmetric plasma for different collision frequency,central plasma density and frequency of electromagnetic waves for three typical density distribution profiles.Some useful results are obtained.
Application of Monte Carlo simulation to the research of ions transport plasma sheaths of glow discharge
2004, 53 (4): 1116-1122. doi: 10.7498/aps.53.1116
In this paper， Monte Carlo method was used to simulate the motion of ions in the cathode fall zone of glow discharge. Energy distributions and angle distributions at different pressures and voltages are presented. The simulations are based on two main scattering processes of charge transfer and elastic scattering. Either the ion energy dependence or independence of collision cross-sections is taken into account. For elastic scattering, the potential interaction and hard-sphere models are considered. The parameter Pdc has a great influence on the energy distribution, but the angle distribution of ions is not sensitive to discharge parameters. Simulation results obtained by different models have been compared and discussed. The model that includes charge transfer and potential interaction scattering processes and is based on the dependence of collision cross-section on the ion energy is obviously the best.
2004, 53 (4): 1123-1128. doi: 10.7498/aps.53.1123
In this paper, a discussion and an optimizing numerical analysis of the penetration and attenuation of the thermonuclear tokamak plasma fueling by the supersonic cluster molecular beam(SMB) has been presented.The cluster formation and dissolution,SMB adiabatic expansion,shielding and cooling effect are all taken into account.An optimized numerical model is applied in the analysis and shown to be in good agreement with the experimental observations.The possibility for fueling large tokamak plasmas with SMB injection is explored.
2004, 53 (4): 1129-1135. doi: 10.7498/aps.53.1129
This paper presents pulse shortening of electron beams drifting through the input cavity and the idler cavity on an L-band long pulse relativistic klystron amplifier. We investigate one of the main mechanism of beam-current-pulse-shortening in which the interaction of asymmetric modes of cavity and the beams leads to the beams' expansion. Selective damping to suppress the asymmetric modes is described. The problem of beam-current-pulse-shortening through the idler cavity gap is removed to a certain extent after parameter adjustment of the input microwave and the idler cavity. In the experiment, the 446 kV,3.0 kA，1.3 μs hollow electron beams generated about 2.0 kA modulated current after two idler-cavity gaps, the width of electron beams' pulse is increased from 0.3 μs to 1.0 μs.
2004, 53 (4): 1136-1144. doi: 10.7498/aps.53.1136
On the basis of the self-consistency equations, the chaotic behavior of electron orbits is studied numerically in a traveling-wave tube amplifier. The results show that motion orbits of electrons in phase space can become chaotic as the current increases, and the chaotic orbits are affected by the detuning. In temporal scale, appearance of chaotic motion of electrons is earlier than one of limit cycle and chaotic oscillation of the field. In comparison with the limit cycle oscillation of the field, the threshold current for the onset of chaotic orbits of electrons is low. In the soft nonlinear regime at which the field exhibits limit cycle oscillation, the chaotic region increases and gradually engulfs the whole phase space as the current increases. In the hard nonlinear regime at which the field exhibits chaotic behavior, the chaotic orbits of electrons occupy almost everywhere in the phase space. The pattern of chaotic motion of electrons is unchanged for a certain current; the characteristic of the limit cycle and chaotic oscillation of the field is certain, but their output power is uncertain in a certain current range.
2004, 53 (4): 1145-1149. doi: 10.7498/aps.53.1145
The relativistic motional plasma model is proposed ane defined. Using the charge fluid perturbation theory, the two-dimensional (2D) self-disturbance of the system and the phenomenon of beam-wave interaction are suudied. From this, the dielectric tensor of 2D relativistic motional plasma is given. We found that the relativistic motional plasma is an electromagnetic dielectric with a complex dispersion relation, and it displays a non-uniform complex electromagnetic space. In this paper, the response of relativistic motional plasma to the wave frequency is obtained by the numerical computation.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
The microstructure and properties of tetrahedral amorphous carbon films deposited by filtered arc with accelerating at different energetic grades
2004, 53 (4): 1150-1156. doi: 10.7498/aps.53.1150
Tetrahedral amorphous carbon (ta-C) films were deposited on single crystalline silicon by accelerating the species with different impinging energetic grades produced by static-electricity pulse substrate bias from 0 to -2000 V. The microstructure of the ta-C films consist of amorphous sp3 hybridization skeleton enchased with sp2 clusters with sizes less than 1 nm by visible Raman measurement. At low energetic grade, sp3-rich energetic window, and sub-high energetic grade, the more the content of sp3 in the film, the smoother the surface of the film. The relationship between the impinging energy of the species and the surface morphology can be illustrated perfectly in the light of subimplantation growth mechanism. Nevertheless, at high energetic grade, the impinging ions with appropriate energy and angle can sputter and smoothen the surface, even the roughness is lower than the surface of the films with the most sp3. The hardness and Young's modulus of the films deposited at high energetic grade (-2000 V) are higher than those of the films prepared on the floating conditions. At the same time, the critical scratching load of the films deposited with the substrate bias of -2000 V is even larger than the one of the sp3-rich films.
2004, 53 (4): 1157-1161. doi: 10.7498/aps.53.1157
The equation of motion of particles has been reduced to the pendulum equation with a damping term and a forced term by using the sine-squared potential.The global bifurcation and chaotic behaviours have been analysed using Melnikov technique, and it is shown that the enhancement of the radiation background and the random phenomenon of the channelling efect are related to chaotic behaviours of the system.
2004, 53 (4): 1162-1165. doi: 10.7498/aps.53.1162
According to Peierls phase transition theory based on the electron-phonon interaction, the Peierls phase transition temperature of a one-dimensional organic conductor tetrathiafulvalene-tetracyanoquinodimethane is computed using the deformation potential model with the data calculated by a semi-empirical crystal orbital method. The mechanism of the metal-insulator phase transition in this conductor is also discussed.
2004, 53 (4): 1166-1170. doi: 10.7498/aps.53.1166
The optical phase retardation method is used in this paper. Based on the experiments on magnetic-field-induced Freedericksz transition of planar nematic liquid crystal, doped with trace bulin, fibre protein and double-azo etc, the trace adulteration effects on the transition of planar nematic liquid crystal are discussed; experimental curves and numerical calculation curves are given. Numerical calculation results are in agreement with measured results. It is shown that different impurities whose magnetic property, molecule shape and size are different, lead to different magnetic-field-induced Freedericksz transition, which is ulteriorly illuminated with the experiment on magnetic-field-induced Freedericksz transition of planar nematic liquid crystal doped with 2-benzothiazolethiol-linked porphyrinatozinc(Ⅱ).
2004, 53 (4): 1171-1176. doi: 10.7498/aps.53.1171
We have obtained the photoelectron diffraction curves from (1010) and (1120) crystal planes on GaN(0001) surface by using a polar scan mode of x-ray photoelectron diffraction (XPD). On the basis of principle of “forward focusing" of XPD, we have determined that its polarity is Ga termination. The polarity of GaN(0001) surface is also studied by using energy dependence photoelectron diffraction called angle-resolved photoemission extended fine structure, as well as the calculation of multiple scattering cluster models, which confirmed that its polarity is Ga termination.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
The contrast on low-temperature physical property of heavy electron compounds CeCu5.8M0.2(M=Ni, Zn, Cd)
2004, 53 (4): 1177-1181. doi: 10.7498/aps.53.1177
In this paper, we report the resistance, specific heat and susceptibility of heavy electron compounds CeCu5.8M0.2(M=Ni，Zn，Cd) as functions of temperature from 1.8 K to 300 K. The effects of magnetism, valence and atom size of different doping elements on Kondo temperature TK, coherent temperature TRmax (e.g. the temperature of maximum resistance) and other parameters have been discussed.
Modulation effect of transverse alternating and direct currents on I-V characteristics in blue bronze K0.3MoO3
2004, 53 (4): 1182-1185. doi: 10.7498/aps.53.1182
We have investigated the transverse alternating and direct current effect on the charge-density wave (CDW) dynamical properties in K0.3MoO3 by measuring I-V characteristics.The results show that,no matter whether DC or AC transverse current is used,the CDW depinning field (Et) along the longitudinal direction is effectively suppressed as the transverse current increases.But the effect of AC current on I-V characteristics is weaker,and probably closer to the intrinsic effect.A frequency dependence of transverse AC modulation effect was observed.
2004, 53 (4): 1186-1190. doi: 10.7498/aps.53.1186
Giant Rashba spin splitting has been found in n-HgTe/HgCdTe quantum wells with an inverted band structure by studying the beating patterns in Shubnikov-de Haas oscillations. Fast Fourier transformation, node positions, as well as numerical simulations, are used to deduce the Rashba spin-orbit splitting and result in an excellent agreement in Rashba spin splitting energy (28-36 meV).
2004, 53 (4): 1191-1194. doi: 10.7498/aps.53.1191
Graphite is a quasi-two-dimensional semimetal. However, for usual graphite the magnetoresistance is not so high due to its small crystal size and no preferred orientation. Huge positive magnetoresistance up to 85300% at 4.2 K and 4950% at 300 K under 8.15 T magnetic field was found in highly oriented pyrolytic graphite. The mechanism of huge positive magnetoresistance is not only due to ordinary magnetoresistance but also due to magnetic-field-driven semimetal-insulator transition.
Study of luminescence due to delocalized exciton recombination in asymmetric-coupled quantum well structure based on Ξ-type configuration
2004, 53 (4): 1195-1200. doi: 10.7498/aps.53.1195
The luminescence properties due to the delocalized exciton recombination in the asymmetric-coupled quantum well (ACQW) structure have been studied based on Ξ-type configuration.We show that the luminescence has mainly two emission peaks:one peak corresponds to the red shift relative to the central transition frequency and the other to the blue shift.Compared with the single quantum well,this quantum-confined Stark shift is very sensitive to electric field.This means that the coupling between two wells for an ACQW structure leads to the enhancement of the quantum-confined Stark effect.From this optical property of ACQW,novel ultrafast optical modulation devices and optical switch fabricated with ACQW structure can be expected in the future optical communication system.
A study of linear and the second nonlinear admittance about the charge polarization around junction-boundaries in a quantum cavity structure
2004, 53 (4): 1201-1206. doi: 10.7498/aps.53.1201
We present explicit expressions for the linear and the second nonlinear imaginary parts of admittanc (emittance) for the charge polarization of accumulation on both sides of the quantum dot (cavity) junctions by using Green function and the coupling parameters in an effective Hamiltonian and the discrete potential model. We found that the emittance and the electrochemical capacitance are equal to the geometric capacitance in the classical limit. In the nonclassical case the emittance is equal to the electrochemical capacitance, but not equal to the geometric capacitance if there is complete reflection. In the case where there is tunneling the emittance and electrochemical capacitance as well as the geometric capacitance are different. The results may be helpful for measurements on capacitance on small-scale structures.
2004, 53 (4): 1207-1210. doi: 10.7498/aps.53.1207
Based on the discreteness of electric charge, the influences of an external magnetic filed on the persistent current of the mesoscopic coupling metallic rings are investigated. The results obtained indicate that there is an additional persistent current in each mesoscopic metallic ring. The additional persistent currents are related to the circuit parameter and coupling coefficient. If the external magnetic flux varies according to sine law, the effect of double-frequency and division-frequency appears in these rings.
2004, 53 (4): 1211-1216. doi: 10.7498/aps.53.1211
Based on analysis of the physical process of hole tunneling, the time characteristics of the writing/erasing and retention in p-channel Ge/Si hetero-nanocrystal-based metal-oxide-semiconductor field-effect transistor(MOSFET) memory have been simulated numerically. Owing to the advantages of a compound potential well and a higher band offset in the valence band, the retention time is increased up to the orders of over 108 and 105, compared with the conventional Si-nanocrystal-based MOSFET memory and the n-channel Ge/Si hetero-nanocrystal-based MOSFET memory, respectively. Moreover, the present device keeps on having high-speed writing/erasing in the direct-tunneling ultrathin oxide regime. It would be expected to solve the contradictory problem between high-speed programming and long retention, therefore, the performance would be substantially improved.
Generation of terahertz radiation via optical rectification and electro-optic detection in ZnTe crystals
2004, 53 (4): 1217-1222. doi: 10.7498/aps.53.1217
We report on the generation of terahertz (THz) radiation via optical rectification and the detection using electro-optic sampling in ZnTe crystal by the pump-probe technique. The pulsewidth of the observed THz radiation is about 0.2 ps, resulting in the wideband THz spectra with a spectral sensitivity beyond 4 THz and the full width at half-maximum of about 2.4 THz. The Jones matrix method has been employed successfully for the calculation of the THz radiation. We study the effects of the optical pulse wavelength, pulsewidth and the ZnTe crystal axis on the generation of THz radiation in the ZnTe crystal. The dependence of the polarization-modulation on the THz detection has also been investigated both experimentally and theoretically by changing the polarization of the probe light.
Characteristics of structure and carrier localization in YBCO systems doped with magnetic ions Fe and Ni
2004, 53 (4): 1223-1231. doi: 10.7498/aps.53.1223
To make clear the physical mechanism of substitution by the magnetic ions Fe and Ni in different sites in YBCO systems, the positron annihilation technology (PAT) and simulated calculations are utilized to systemically investigate the compounds YBa2Cu3-x（Fe，Ni）xO7-δ（x=0.0—0.5）. The results obtained show that the doping ions Fe and Ni form different kinds of ion clusters and enter the crystal lattice. When occupying Cu(2) sites in CuO2 planes, the ions gather into double square and/or other clusters, which results in a strong electronic localization and would directly affect the pairing and transportation of carriers, so the superconductivity is suppressed dramatically. While ions substitution for Cu(1) through gathering hexamer and/or other clusters, this induces the localization of holes and weakens the function of carrier reservoir; thus carriers cannot easily transfer to CuO2 planes. However, in this case, the pairing and transportation of carriers are not affected directly, thus the superconductivity will be suppressed weakly. On the other hand, the present results indicate the suppression of superconductivity has no direct correlation with the magnetism of Fe and Ni ions itself.
Planar Hall effects in exchange biased FeMn/NiFe bilayers were measured under magnetic fields rotated in the film plane. Detailed magnetic anisotropy analysis shows that there is no additional uniaxial anisotropy induced by the interfacial coupling in this system, while the instability of anti-ferromagnetic grains or the formation of many domains in ferromagnetic layer may play an important role responsible for the difference between exchange biasing fields obtained by reversible and irreversible measurements.
2004, 53 (4): 1236-1242. doi: 10.7498/aps.53.1236
Based on the quantum confinement-luminescence center model, the relation between the inner electric field (IEF) and the photoluminescence(PL) character is calculated. Results show that the IEF between nanosilicon and luminescence centers (LCs) can have a strong effect on the carrier recombination rate and the spectrum peak position swinging. In the range from 2 to 5 nm, the carrier recombination rate at the LCs is much bigger than the rates of recombination inside the nanosilicon. And dut to the presence of IEF between nanosilicon and LCs, the PL intensity at the LCs and inside nanosilicon will reduce remarkably.
2004, 53 (4): 1243-1246. doi: 10.7498/aps.53.1243
By analyzing the transmission spectra of hetero-epitaxial GaN films on sapphires, a film thickness measurement method is presented. The method uses the interference effect of the crystal film and considers the effect of the refractive index n on the photon wavelength. Applications of it show that the method is a rapid and precise one for measuring the film thickness of GaN crystals.
Determination of the transition-layer thickness of a crystalline polymer by using small-angle x-ray scattering
2004, 53 (4): 1247-1250. doi: 10.7498/aps.53.1247
The thickness of transition layer of a crystalline polymer is generally calculated by one-dimensional electron density correlation function. If the transition layer of the crystalline polymer is diffusive, the corrected Porod's law is an effective method to deal with the diffuse transition layer of pseudo two-phase material. The method of Porod's law is used to compute the thickness of transition layer of crystalline polymer and compared with that of correlation function. The conclusion is that the two methods obtain identical thickness of the transition layer.
Small angle x-ray scattering study on microstructure evolution of the aging process of Al-Zn-Mg-Cu-Li alloy
2004, 53 (4): 1251-1254. doi: 10.7498/aps.53.1251
Small angle x-ray scattering technique was used to analyze the variation of microstructure parameters of precipitates in an Al-Zn-Mg-Cu-Li alloy aged at 130, 150 and 160℃ for the same duration. Precipitate particles’ radius increased with aging temperatures, but their specific inner surface and integrated intensities decreased with aging temperatures. The study on Porod curves q3J(q)-q2 indicates that there is a sharp boundary between precipitates and the matrix.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2004, 53 (4): 1255-1259. doi: 10.7498/aps.53.1255
Aligned carbon nanotubes were prepared by plasma-enhanced hot filament chemical vapor deposition using CH4, H2, and NH3 as reaction gases. It was investigated how different negative bias affects the growth of aligned carbon nanotubes. The results indicate that the average diameter of the aligned carbon nanotubes is reduced and the average length of the aligned carbon nanotubes is increased with increasing negative bias. Because of the occurrence of glow discharge, a cathode sheath forms near the substrate surface, and a number of ions are produced in it, and a very strong electrical field builds up near the substrate surface. Under the effect of the field, the strong bombardment of ions on the substrate surface will influence the growth of aligned carbon nanotubes. Combined with related theories, we have analyzed and discussed the ion bombardment effects on the growth of the aligned carbon nanotubes.
Nano-graphite synthesized by explosive detonation and its application in preparing diamond under high-pressure and high-temperature
2004, 53 (4): 1260-1264. doi: 10.7498/aps.53.1260
In this article, preparation of nano-graphite powders in a steel chamber by using pure TNT (trinitrotoluene) explosives has been reported. X-ray diffraction results indicated that the black solid powders which were collected from the wall and the bottom of the chamber after detonation have the graphite structure and the average size of the particles of the graphite is 1.86-2.58 nm. Transmission electron microscopy confirms that the black powder is hexagonal-graphite structure with ball or ball-like shape. The grain size of the nano-graphite measured by small angle x-ray scattering lies in the range of 1-50 nm, 92.6wt% of the powder particles is smaller than 16 nm and the average diameter of the grains is 8.7 nm. Specific surface area of the nano-graphite is about 500-650 m2/g. Using nano-graphite as carbon source and choosing Fe powders as the catalyst, the ball-like and lumpy diamonds of 5-15 μm have been synthesized under high-temperature and high-pressure conditions of 1250-1330 K and 5.1 GPa. The synthesized temperature is about 300 K lower than that of bulk graphite synthetic diamond.
2004, 53 (4): 1265-1270. doi: 10.7498/aps.53.1265
High-quality Bi4Ti3O12 ferroelectric thin films on p-Si substrates were prepared by using the sol-gel technique. The growth behavior, ferroelectric properties, C-V characteristic and fatigue characteristic of Bi4Ti3O12 were investigated. The results show that the growth of Bi4Ti3O12 on bare p-Si substrates is c-axis-oriented with the increase of annealing temperature, and the apparent dependence of the ferroelectric properties of Bi4Ti3O12 films on the grain size, growth behavior and carrier concentration stems from the temperature effects. The C-V hysteresis curves with clockwise loops proved that the Ag/Bi4Ti3O12/p-Si heterostructure can realize a memory effect due to the ferroelectric polarization of Bi4Ti3O12 films and the remnant polarization of the Bi4Ti3O12 films reduced by 12% of the initial value after 109 bipolar switching cycles, which indicated that these Bi4Ti3O12 films deposited on p-Si substrates would be qualified for ferroelectric memories.