Alpha Cluster in Hartree-Fock Calculations

Hartree-Fock (HF) calculations have been performed fcr ¹²C nucleus assuming a trigonal D_3h equilibrium configuration. Different aspects including symmetry, kinds of two-body interactions are investigated. The HF calculations have been carried out using a velocity dependent effective potential of s-wave interaction. The results are compared with previous calculations. In all HF calculations no deviation from axial symmetry is obtained.     

The origin of non-planarity in ketyls

A series of one-electron properties, i.e. dipole, quadrupole, octupole moments, diamagnetic susceptibility, second moments and second moment anisotropies of charge distribution, electric field, electric field gradient, diamagnetic shielding, nuclear quadrupole coupling constant, have been obtained by ab initio S.C.F. calculations for the molecules COF₂, SO₂F₂ and SOF₂. The effect of the size of the gaussian basis sets, including d-type polarization functions, as well as the contraction schemes on the latter properties have been studied for the COF₂ molecule. This study enables us to determine the basis giving the best results as compared to experiment. With this basis the calculations have been extended to the properties of the molecules SO₂F₂ and SOF₂. Good overall agreement with experiment is obtained.     

The preparation and characterization of the cobalt skutterudites CoP3, CoAs3 and CoSb3

Single crystals of the cobalt skutterudites CoP₃, CoAs₃ and CoSb₃ have been prepared by the chemical vapor transport technique using chlorine as the transport agent. Chemical analysis and density measurements were used to determine accurately the stoichiometry of these crystals. Physical properties of chemically pure single crystals were obtained by X-ray diffraction, magnetic susceptibility and electrical measurements. CoP₃ is a diamagnetic semiconductor with an optical band gap of 0.45 eV. CoAs₃ and CoSb₃ are also diamagnetic and show an increase in their resistivity as a function of increasing temperature. In addition, they do not give any measurable absorption edge but absorb radiation at all frequencies from 0.4 to 4.3 eV.     

The diamagnetic susceptibility of hydrogenic donor in two-dimensional semiconductors with anisotropic effective mass of carriers

In this study, the effects of quantum confinement and effective mass anisotropy parameter on the diamagnetic susceptibility of a hydrogenic donor placed in GaAs, Si, and Ge quantum wells with infinite confinement potential are investigated in the effective mass and parabolic band approximations by using two and one parameter trial wave functions. It is observed that the diamagnetic susceptibility of a hydrogenic donor in anisotropic quantum wells is essentially equal to the transverse diamagnetic susceptibility part when well widths are larger than L > 100 Å, and the impurity is located at center. Moreover, a two parameter trial wave function model gives higher values of diamagnetic susceptibility, except for χ_z (GaAs).     

Level crossing resonance in ice: Identification of two diamagnetic muon states

Level crossing resonance studies of ice, doped with HF and NH₃ to enhance and suppress the concentration of orientational defects, support the hypothesis that muons can trap at such defects to form complexes of the type H₂O-Mu⁺-OH₂. The doping alters the partition of the total diamagnetic fraction between this state, which is stable against charge migrationvia proton rearrangement at low temperature, and the neutral HMuO state, the isotopically substituted water molecule.     

Modification of several physicochemical properties of a high-temperature superconducting yttrium ceramic upon doping with gold

The influence of additions of gold (5 and 10 at. %) on the lattice parameters, the superconducting transition temperature, and the diamagnetic susceptibility of the high-temperature superconducting ceramic YBa₂Cu₃O_7−δ is investigated. The influence of gold on the kinetics of the release of weakly bound oxygen during heating of the ceramic in a vacuum is also studied. It is shown that doping the ceramic with gold causes uniaxial expansion of the lattice of the material, a significant decrease in the diamagnetic susceptibility, and nonmonotonic variation of the kinetic parameters of the oxygen release, while the superconducting transition temperature remains nearly unchanged. Zh. Tekh. Fiz. 67, 31–34 (February 1997)     

Die magnetische Suszeptibilität der Defektelektronen in Silizium und Germanium

For semiconductors it is possible, to determine the contribution of the free carriers to the magnetic susceptibility. Holes in silicon (density 10¹⁸ to 10¹⁹cm^?3) have at 297 and 141° K a small paramagnetic susceptibility. In contrast, one computes from cyclotron mass parameters for 4° K a strong diamagnetic susceptibility. Holes in germanium (density 4 · 10¹⁷ and 4 · 10¹⁹ cm^?3) have a diamagnetic susceptibility, which is much smaller, than one would expect from cyclotron resonance masses. — Neutral boron atoms contribute in small concentration at 141 °K to the susceptibility.     

Diamagnetic susceptibility near the surface or twinning plane at the transition to the superconducting state

Besides the well-known bulk fluctuation contribution to the diamagnetic susceptibility there is an additional surface contribution with the singularity at T_SH exceeding the superconducting bulk transition temperature T_c(H). The results obtained allow to explain the anomalous behavior of the diamagnetic susceptibility found by Khaikin and Khlyustikov.     

Simultaneous effects of temperature and pressure on diamagnetic susceptibility of a shallow donor in a quantum antidot

The effect of temperature and pressure, simultaneously, on the diamagnetic susceptibility and binding energy of a hydrogenic donor impurity at the center of a GaAs/Ga_1−xAl_xAs quantum antidot is studied within the effective mass approximation. For this goal, we first analytically solve the Schrödinger equation to obtain wavefunctions and energy levels. Then, using the electronic states, we can calculate the diamagnetic susceptibility. The results obtained from the present work reveals that (i) the diamagnetic susceptibility increases with increasing pressure, (ii) the diamagnetic susceptibility decreases by increasing temperature, (iii) the value of 〈r²〉 decreases with increasing pressure due to the quantum confinement, and (iv) an increase in the pressure enhances the binding energy for a constant temperature.     

Virial expansion for the radial distribution function of a fluid using the 6 : 12 potential

The radial distribution function can be expressed in a virial expansion. Using the 6 : 12 potential the second-order density coefficient, g ₂(r), is numerically calculated for a wide range of temperatures and intermolecular separations. These results are used to calculate the second-order density coefficient, c ₂(r), in the expansion of the direct correlation function and to calculate the fourth virial coefficient, B ₄. In addition, approximate results for g ₂(r), c ₂(r), and B ₄ are calculated on the basis of the Percus-Yevick, hypernetted chain, and the self-consistent approximations of Hurst and Rowlinson. These approximate results are compared with the exact results. The Percus-Yevick theory is in good agreement with the exact results at high temperatures but is unsatisfactory at low temperatures. The hyper-netted-chain approximation is in fair agreement with the exact results at high temperatures, is in poor agreement at intermediate temperatures, but is in good agreement at low temperatures. The self-consistent approximations are in reasonably good agreement with the exact calculations at all temperatures.     

Nuclear magnetic shielding and diamagnetic susceptibility of interacting hydrogen atoms

A quantum-mechanical study is made of the changes of the nuclear magnetic screening constant σ and the diamagnetic susceptibility X of two interacting hydrogen atoms due to van der Waals and overlap interatomic forces (effects of electron spin being neglected). At large distances the calculations show that van der Waals forces decrease the nuclear screening but increase the diamagnetic susceptibility (in magnitude). As the internuclear distance is reduced the first effect of overlap forces is to increase the screening in the repulsive (electronic triplet) state but this is followed by a further reduction. Attractive overlap forces (as in the ground state of H₂) ultimately lead to an increase in screening.     

高空核爆炸下大气的X射线电离及演化过程数值模拟

利用数值模拟程序模拟了不同高度核爆炸下大气的X射线电离及演化过程.结果表明: X射线电离产生的电子数密度在射线到达后约100 ns时刻达到峰值, 峰值数密度随着到裸核区距离的增加而减小;电子具有较长的寿命, 电子寿命随着到裸核区距离的增加而增大; X射线电离空气产生正离子O⁺, O₂⁺, N₂⁺,爆高为120 km情况下 O⁺的峰值数密度与O₂⁺的相近,能维持约1 s. X射线对空气的电离影响范围在数十千米以内,在距裸核区较近的区域, 爆高为80 km时产生的电子峰值数密度比爆高为120 km时的电子峰值数密度高, 在距裸核区较远的区域则相反.     

Die magnetische Suszeptibilität von Bi und Bi-Sb-Legierungen

We have measured and discussed the field independent magnetic properties of Bi_1-xSb_x alloys for the concentration range 0≦x≦0,4. The observed transitions from the semimetal to semiconductor phase at about 7 At.-% Sb and from the semiconductor to semimetal phase at about 22 At.-% Sb are very well defined. Measurements on the semiconductor phase at low temperatures allowed us to separate the lattice componentχ _G of the susceptibility from that due to the charge carriersχ _L. It is therefore possible to estimate these two components for pure Bi. The results show thatχ _G is strongly diamagnetic in all crystall directions and thatχ _L is paramagnetic. An estimate of the charge carrier susceptibility based on a modified Pauli Landau model is found to give an order of magnitude agreement with the experimental results. The deviation can be attributed to a nonparabolic conduction band. The dependence of the energy gapΔE on the Sb concentration has also been determined.     

External electric field, hydrostatic pressure and conduction band non-parabolicity effects on the binding energy and the diamagnetic susceptibility of a hydrogenic impurity quantum dot

Electric field, hydrostatic pressure and conduction band non-parabolicity effects on the binding energies of the lower-lying states and the diamagnetic susceptibility of an on-center hydrogenic impurity confined in a typical GaAs/Al_xGa_1−xAs spherical quantum dot is theoretically investigated, by direct diagonalization of the Hamiltonian. To this end, the effect of band non-parabolicity has been performed, by means of the Luttinger-Kohn effective mass equation. Binding energies and diamagnetic susceptibility of the hydrogenic impurity are computed as a function of the dot size, external electric field strength and hydrostatic pressure, with considering the edge-band non-parabolicity. Results show that the external electric field and the hydrostatic pressure have an obvious influence on the binding energies and the diamagnetic susceptibility of the impurity.     

Do electron scattering and muonic x-ray data really require a central depression in the charge distribution of208Pb?

The charge distribution of²⁰⁸pb calculated in the Hartree-Fock (HF) approach using the density dependent nucleon-nucleon interaction of Ehlers and Moszkowski is tested by simultaneous comparison with experimental data from 502 MeV elastic electron scattering and muonic atoms. In both cases the agreement is very good and nearly as good as the best fits with a phenomenological charge distribution of Fermi type, if the effect of the polarization of the nucleus due to the presence of the muon is properly taken into account. In contradiction to the Fermi fits the HF distribution shows a hump at the center of the nucleus.     

Diamagnetic susceptibility of hydrogenic donor impurity in low-dimensional semiconducting systems

The binding energies of a hydrogenic donor both in the parabolic and non-parabolic conduction band model within the effective mass approximation have been computed for the low-dimensional semiconducting systems (LDSS) like quantum well, quantum well wire and quantum dot taking GaAs/Al_xGa_1−xAs systems as an example. It is observed that the effect of non-parabolicity is not effective when the system goes to lower dimensionality. The diamagnetic susceptibility of a hydrogenic donor impurity has also been computed in these LDSS in the infinite barrier model. Since no theoretical or experimental works on the diamagnetic susceptibility of LDSS are available in the literature, as a realistic case the diamagnetic susceptibility has been computed in the finite barrier model (x=0.3) for a quantum well and the results are discussed in the light of semiconductor-metal transition.     

Magnetic and structural anomalies of Na<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>C<Subscript>60</Subscript> (n = 2, 3)

Sodium fullerides Na _n C60 (n = 2, 3) have been synthesized by a liquid phase reaction and investigated with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), electron paramagnetic resonance, and differential thermal analysis. XRD data indicate that the crystal structure of Na₂C₆₀ at 300 K is face centered cubic (FCC). A phase transition from primitive cubic to FCC crystal structure has been observed in this work in Na₂C₆₀ fulleride at 290 K. The transition is accompanied by the step-like change of paramagnetic susceptibility. The crystal structure of Na₃C₆₀ is more complicated than, and different from, what has been reported in the literature. A nearly seven-fold increase of paramagnetic susceptibility with increasing temperature has been observed in the Na₃C₆₀ fulleride at 240–260 K. In the same temperature range, a new line at about 255 ppm appears in the ²³Na NMR spectrum, indicating a significant increase of electron density near the Na nucleus. The observed effect can be explained by a metal-insulator transition caused by a structural transition.     

Diamagnetic properties of tetrahedrally coordinated Ge1-xSx and Ge1-xSex amorphous alloys

Measurements of the diamagnetic susceptibility χ at room temperature are reported for Ge_1-xS_x and Ge_1-xSe_x amorphous alloys as a function of composition. The composition dependence of χ is shown to reflect the corresponding changes in the average gap E_g and it is concluded that the dia diamagnetic enhancement observed in tetrahedrally bonded amorphous semiconductors is caused by a decrease in the interband matrix element in the Van-Vleck paramagnetic susceptibility term with bond angle distortion rather than changes in E_g.     

Calculation of electric hyperfine interaction parameters in solids

A theoretical approach for the calculation of hyperfine parameters in solids from first principles is presented. These calculations are based on the full potential linearized-augmented-plane-wave (LAPW) band structure method, which is currently one of the most accurate schemes to determine the electronic structure in ordered solids. Exchange and correlation is treated within density functional theory using the generalized gradient approximation. Once the electron density is calculated self-consistently with high accuracy, quantities like electric field gradients (EFG), isomer shifts or hyperfine fields can easily be obtained from this density without further approximations. Using this approach we have studied various systems including metals, insulators, ionic compounds or the highT _c superconductors. In general we find good agreement between theory and experiment, which proves that our method is very accurate. Having these results in mind we are confident that this method is accurate enough to determine the value of the nuclear quadrupole momentQ, provided experimental measurements of the quadrupole coupling constant are available. This procedure is demonstrated forQ of⁷⁷Se and¹⁰⁰Rh, two nuclei recently used in PAC measurements. An extensive study of EFGs at Fe sites in various Fe-compounds has been performed leading to a very reliable quadrupole moment ofQ(⁵⁷Fe)=0.16 b, a value twice as large as that deduced from recent HF calculations but back to older estimates.