Vol. 37, No. 12 (1988)
1988, 37 (12): 1915-1924. doi: 10.7498/aps.37.1915
Hole subbands and optical transitions of superlattices in magnetic field are calculated on the basis of the Luttinger-Kohn effective mass theory. The method of calculation is described in detail, and the dependences of Landau energy levels on magnetic fields and well widths are presented. It is found that when magnetic field or well width increases the intermixing of heavy and light holes becomes obvious, only the three lowest states still keep their original characters of heavy holes. It is also found that the third light hole subband is necessary for the convergence of solutions. The magnetic-optical transition probabilities and the effective masses of cyclotron resonance are discussed.
1988, 37 (12): 1925-1931. doi: 10.7498/aps.37.1925
The photoluminescence of GaAs0.15P0.85: N has been investigated under hydrostatic pressure at 77 K. The NN1 emission is clearly seen when P>10kbar. Meanwhile, luminescence quenching and band narrowing of Nx line have been observed under pressure. The results show that the pressure effectively enhances the Nx→NN1 thermally assisted exciton transfer processes. The pressure behaviors of Nx and NN1 levels have been analysed and fitted to a theoretical model. The pressure coefficients of the levels and some parameters related to their wavefunctions have been determined by fitting calculations.
A SIMPLE APPROACH FOR THE CALCULATION OF FORCE CONSTANTS OF SEMICONDUCTORS——APPLICATION TO THE STUDY OF PHONONS AT THE GaAs(111)-2×2 SURFACE
1988, 37 (12): 1932-1939. doi: 10.7498/aps.37.1932
An analytical expression for the force constants of semiconductors with arbitrary hybridization is derived by using the bond orbital approximation to simplify the calculation of electronic band structure energy of the system. This method is shown to be simple and effective by the comparison between the theoretical calculation and experimental results of phonon dispersion curves of several materials. Applying this method to the study of phonon at GaAs (111)-2×2 sarface, the dispersion curves of phonons that are strongly localized at the surface are obtained.
1988, 37 (12): 1940-1948. doi: 10.7498/aps.37.1940
The nonstoichiometric compound Ba1+xFe2S4(0.062≤x≤0.143) and the rare earth-transition rnetal tetraborides R1+εFe4B4(ε≈ 0.1, R = Ce, Pr, Nd, Sm, Gd, Tb), both composed of two interpenetrating substructures, are examples of the vernier structures. In present paper, the symmetry of the crystal structure in these two systems is characterized by the superspace group approach developed by de Wolff et al. The superspace group for Ba1+xFe2S4 compound is P1IssI4/mmm and the one for all R1+εFe4B4 compounds is P1Iss(P42/ncm). The diffraction selection rules in the commensurate models for the vernier structures, those often occur and are not related to the systematic absences of the three-dimensional space groups, are explained according to the results of the systematic absences discussed in the superspace. Such diffraction selection rules are very useful to predict where the measurable intensities can be found in the case of crystal structure analysis for the vernier structures.
1988, 37 (12): 1949-1955. doi: 10.7498/aps.37.1949
A calculation scheme for the maximum entropy evaluation of the distribution of hyper-fine parameters from overlapped M?ssbauer spectra is proposed, which is subjected to the weak contraint condition. Maximum entropy evaluation is consistent with all given informations and has little correlation with the indefinite informations. The fitted distribution is characteristic of its positivity and bulk smoothness. Its local smoothness can be improved with relaxation of the contraint. The maximum entropy evaluation proved a more reliable method for fitting overlapped M?ssbauer spectra.
1988, 37 (12): 1956-1964. doi: 10.7498/aps.37.1956
An exact solution for a two-level atom system interacting with a bichromatic field of a symmetrical detuning parameter has been derived using fully quantum theory. By numerical method, we analyse the dynamic behavior of the atom system in the case of the initial two mode fields with the same or different statistical properties. We have found some new kinds of collapserevival phenomena and investigated in detail the factors on which the phenomena depend.
1988, 37 (12): 1965-1971. doi: 10.7498/aps.37.1965
We studied excitation processes of the targets in collisions of electron with helium and argon by optical method. Using the optical multichannel analysis system (OMA), we measured the emission spectrum of the collision systems. And the absolute emission cross sections are obtained in the electronic energy range of 200-500 eV.
1988, 37 (12): 1972-1980. doi: 10.7498/aps.37.1972
Applying the Hooper's first-order theory, we have calculated the static micro-electric field distributions and have examined the validity of the first-order theoretical calculations. The Lyman line profiles (n→1) from hydrogenic ions have been calculated. As the principle quantum number n is odd, there is no central peak and two splitted peaks exhibit for the Lyman line profile. Electron-impact broadening has negligible effects on such sepera-rions of the splitted peaks, which are suitable for plasma density diagnostics. The Lyman line proifles in the wing regions, which are formed dominantly by the static micro-electric field distributions, can also be applied for plasma density diagnostics.
1988, 37 (12): 1981-1986. doi: 10.7498/aps.37.1981
In this paper, an entire new approach for quantum mechanics calculation, the non-degenerate ground state steepest descent perturbation theory (SDPT) suggested by Cioslowski is developed and extended for dealing with degenerate ground state problem. Using sysmme-try adapted trial wave ket, the SDPT iteration for different sublevel is independent from each other and improves its eigen energy and wave function step by step. Therefore, the steepest descent perturbation theory can be used to calculate the low symmetry splitting of degenerate ground state and the correlation energy to arbitrary precison. Unlike other perturbation theory, this does not require either infinite summation overlstates or the solution of a set of differential equations, it could be expected wide adaptability in the calculation of energy level perturbation splitting of atoms, molecules and orther many body quantum system.
THE SCALING EXPONENTS OF MASS DISTRIBUTION IN INHOMOGENEOUS COMPOSITES WITH SELF- SIMILAR STRUCTURES
1988, 37 (12): 1987-1992. doi: 10.7498/aps.37.1987
The scaling low of mass distribution in composite media which have the same topology as that of the corresponding fractals but have more inhomogeneity in mass distributions is studied, and the scaling exponents (dimensions) are calculated. We suggest that a set of dimensions may be introduced to describe the details of the inhomogeneous distribution of mass.
1988, 37 (12): 1993-2003. doi: 10.7498/aps.37.1993
The effects of adding warm plasmas on the kinetic DCLC mode in high βloss cone plasmas are investigated in detail. It is found that when the fluid DCLC mode is stabilized by a small amount of warm plasma, the kinetic excitation still remains due to two different mechanisms, namely, (1) magnetic drift resonance dissipation excites the negative energy wave; (2) a new type of positive energy wave can become unstable as the resonance condition is met. Comparing with fluid approximation theory, more warm plasmas are needed to suppress the kinetic DCLC instabilities.
1988, 37 (12): 2004-2010. doi: 10.7498/aps.37.2004
The absorption and fluorescence spectra of Tb3+ and Eu3+ in LnP5O14 crystals at room temperture were observed. Based on absorption spectra and Judd-Ofelt theory, experimental and theoretical oscillator strengths were calculated. Series phenomenolo-gical intensity parameters were determined according to a least-square fitting. Then a calculation of the radiative transition probabilities and lifetimes of 5D3→7F5, 5D4→7F4 and 5D4→7F6 in TbP5O14, as well as 5D0→7F2 and 5D0→7F4 in EuP5O14 was carried out. We also obtain corresponding fluorescence lifetime by using time-resolved spectroscopy at various temperatures. The experimental results are in agreement with theoretical results. Our results confirmed that the lifetime of Tb3+ 5D3 depends mainly on the electric dipole-dipole cross relaxation between 5D3→5D4 and 7F6 →7F0.
1988, 37 (12): 2011-2016. doi: 10.7498/aps.37.2011
The OLCAO method is extended to treat f-electron system, band structure and density of states for metal gadonium are calculated and compared with APW method and experiments of photoemission spectroscopy.
1988, 37 (12): 2017-2022. doi: 10.7498/aps.37.2017
This paper reports a new method of measuring the work function of meterials. The method is based on accurate measuring of energy distribution of secondary electrons and contact potential difference of meterials. We call it the method of secondary electron emission-contact potential difference (SEE-CPD method). The work function of pure metals of Ni, Al, Au and Mo were measured by this method. The results are well consistent with that obtained by using photo-electron and Kelvin's method. The fundamentals and applications of the SEE-CPD method are proved to be correct and reliable.
1988, 37 (12): 2023-2027. doi: 10.7498/aps.37.2023
The growth of large single crystal LnBa2Cu3O7-δ and their superconductivity is reported in this paper. Identification methods for single crystals e.g., Laue pattern, precession pattern and the OOL multiple-order diffraction with X-ray, are discussed.
1988, 37 (12): 2028-2033. doi: 10.7498/aps.37.2028
The self-consistent band structure, total and partial density of states for the stable compounds hcp-WC and MoC are calculated by the LMTO-ASA method based on Kohn-Sham's LDF theory. On the basis of theoretical parameters obtained from our ab initio calculations, the total and partial electronic pressure, the electron-ion interaction constant η, electron- ph-onon coupling constant λ and superconducting transion temperature Tc are studied. The theoretical results have been verified by both the experimental photoemission spectra and ground-state properties, as well as the superconductivity data.
1988, 37 (12): 2034-2037. doi: 10.7498/aps.37.2034
Thermal expansion coefficient of YBa2Cu3O7-x with the tetragonal and orthorhombic structural phases were measured by the method of differential parallel-plate capacitance. There exist anomalies near 205 K for orthorhombic phase and between 92 K-130 K for tetragonal phase respectively. We suggest that the former may be related to the lattice instability, indicating that there is a strong electronphonon interaction; the latter may result from the structure phase transition which destroys the high Tc superconductivity. A small jump of the thermal expansion coefficient at Tc was observed in the orthorhombic phase and from this the pressure effect was estimated.
1988, 37 (12): 2038-2043. doi: 10.7498/aps.37.2038
The crystal structure of the ideal composition compound Bi2Sr2CaCu2O8 with zero resistance temperature Tc(0) = 81.4K had been determined by X-ray powder diffraction method. The basic structure of this compound exhibits tetragonal symmetry with lattice constants a = 3.825 ?, c = 30.82 ?. The most probable space group is D4h17-14/mmm, Each unit cell contains two formula units. The arrangements of atoms in the unit cell are as follows:2Ca cations occupy 2(a) equivalent point position; 4Sr, 4Bi and 4Cu cations occupy the same equivalent point position 4(e) with respective parameters z= 0.110, 0.302, 0.445; 16O anions respectively occupy 8(g) (z = 0.445) and two 4(e) equivalent point positions (z= 0.210 and z = 0.380), The crystal structure of Bi2Sr2CaCu2O8 can be derived from Aurivillius type structure. The main feature of the structure is alternate arrangement of cations along (00z) and (1/2, 1/2, z) of tetragonal system. The other possible superconducting phases in the Bi-Sr-Ca-Cu-O system can be considered as the structure having different stacking layers along z axis.
1988, 37 (12): 2044-2047. doi: 10.7498/aps.37.2044
The dependence of σs of ultrafine iron particles on their average diameter d is measured. It is found that the smaller the particles, the smaller their σs. The variation of σ with temperature, σ(T), which is irreversible, is also measured both in temperature rising and dropping processes. The experimental results can be explained by spin pinning in the oxide layers.
1988, 37 (12): 2048-2052. doi: 10.7498/aps.37.2048
Using interferometric diagnostics by visible probing beam, we have studied the electron density profiles in coronal plasmas of solid targets irradiatied by Nd glass laser at an intensity of 1014 W/cm2. We have observed not only the phenomena such as density steepening and hollowing due to light pressure, but also different characters of density hollowing in various configuration targets.
Based on atomic self-consistent-field theory (i.e., independent electron model), we have performed both nonrelativistic and relativistic calculations for the energy levels of exited Ry-dberg francium atoms. Our relativistic theoretical results agree well with recent experimental measurments. Relativistic effect of high Z atoms is also discussed. A relativistic calculation is necessary to calculate the energy levels for high Z atoms.
1988, 37 (12): 2057-2061. doi: 10.7498/aps.37.2057
In this work, the relations between the diffraction efficiency of recorded holograms in photorefractive crystals and the ratio of writing beam intensities are analyzed, the condition corresponds to the maximum diffraction efficiency is obtained and an experiment is carried out to prove the conclusions.
1988, 37 (12): 2062-2067. doi: 10.7498/aps.37.2062
The OH- infrared absorption band in the LiNbO3 crystal with various Li/Nb ratios and MgO contents have been measured at room temperature. The structure of the absorption band of nearly stoichiometric crystals consists of three sub-bands with slightly different peak position, their relative heights change with Li/Nb ratios. The OH- absorption band in heavily Mg-doped crystal has a structure consisting of two peaks, the shift of peak position to shorter wavelength is about 54 cm-1. Basing on the crystal structure and the model of the defect structure of the LiNbO3 crystal suggested by Abrahams and Smyth (1-2), these observations are discussed for three different structures, i.e., stroichiometric, congruent and Mg-doped crystals, respectively. The shift of the peak position of the OH- absorption band in heavily Mg-doped crystal to shorter wavelength is suggested to be caused by the Mg2+ ions getting into Nb5+ sites when the Mg-doping level was higher than the threshold level in Mg-doped LiNbO3 crystals.
1988, 37 (12): 2068-2072. doi: 10.7498/aps.37.2068
The I-V curves of a series of possible models of Cs/C(0001)—(2×2) surface are calculated by dynamical LEED theory with RSP and RFS methods. The dalculated curves are selected with experimental results and the best structure is chosen by R-factor calculation. The model of Cs intercalating graphite layers forming intercalate structure and the model of Cs atoms adsorbed on top position of graphite surface are ruled out by R-factor examination. The most probable structure is that Cs atoms adsorbed on hollow position of graphite layer with a interlayer distance of 2.80 ? between Cs laver and graphite, and substrate has the same, structure as that of graphite bulk structure.