Localized positronium atoms in porous structures studied by 2D-ACAR

The localization of positronium (Ps) atoms was studied by measurements of two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) for porous glasses. Annihilations of positron-electron pairs with a large anisotropy were found in 2D-ACAR spectra. This fact can be attributed to the momentum uncertainty due to the localization of Ps in a finite dimension of pores. The present investigation showed the possibility of the detection of microstructures in amorphous materials by the 2D-ACAR technique.     

Image of electron densities from line and plane projections

We compare Fourier transforms with orthogonal polynomials techniques applied in reconstructing three-dimensional electron–positron momentum densities from two-dimensional angular correlation of annihilation radiation (2D-ACAR) spectra and electron momentum densities from one-dimensional Compton profiles (1D-CP). In the case of Fourier transforms, we show results for two different algorithms: filtered back projection and Fourier–Bessel method. These techniques are presented for 2D-ACAR spectra in Y, ErGa₃ and model profiles. PACS 78.70.Bj; 87.59.Fm; 71.18.+y     

Study on distribution of electron momentum density and Fermi surface in lead by positron annihilation

The momentum density of electrons and the Fermi surface are measured in metal lead by the technique of positron annihilation. The three-dimensional electron-positron momentum distribution is reconstructed from measurements of the two-dimensional angular correlation of positron annihilation radiation (2D-ACAR), followed by the image reconstruction technique based on a direct Fourier transformation. On the basis of the obtained experimental spectrum, the Fermi surface topology, the high momentum components (HMC), the enhancement effect, and the autocorrelation function are discussed. The obtained experimental results are in good agreement with theoretical calculations.     

Defects in semiconductors observed by positron annihilation

The origin of the effect of defects on positron annihilation in semiconductors has been studied. The electron-positron momentum densities in elemental semiconductors (Si and Ge), III-V compound semiconductors (GaAs, InP and GaSb), diamond and the proton irradiated Si were investigated by a full-scale use of the two-dimensional angular correlation of positron annihilation radiations (2D-ACAR). The obtained results showed, as a whole, good agreement with the electron momentum distribution of the fully occupied Jones zone with a small exception for the fact that the low density channels are running along the three principal axes. This anisotropy was strong in elemental semiconductors, while it was weakened in compound semiconductors. This anisotropy and its dependence on the material were found to be generally understood by the incorporation of crystal symmetry. The anisotropy will be discussed by group theory in conjunction to the effect of defects on positron annihilation.     

碳同素异形体中的正电子理论

在密度函数理论的基础上,采用中性原子叠加模型和有限差分方法(SNA-FD)计算了石墨,金刚石和C60这三种碳的同素异形体中的正电子分布和湮没情况. 计算表明,在片层结构的石墨晶体中,正电子主要在石墨层间的空隙中湮没,计算出的石墨中的正电子寿命为208ps,与文献中的实验结果210ps符合很好. 在金刚石单晶中,正电子主要在碳原子之间的空隙中存在并发生湮没,计算出的金刚石中的正电子寿命为1159ps,与文献中的实验结果110ps相符合;在面心立方结构的C60晶体中,正电子主要在C60分子球壳内外侧及分子之 关键词： 石墨 金刚石 C60 正电子寿命     

Self-energy correction to momentum-density distribution of positron-electron pairs

Positron two-dimensional angular correlation of annihilation radiation (2D ACAR), i.e., the 2D projection of the electron momentum densities sampled by positron, in Si is employed to verify the prediction of the density functional theory within the local-density approximation (LDA). Carefully conducted test shows that the LDA introduces small but definite discrepancies to the 2D-ACAR anisotropies. Self-energy calculation using the GW method indicates that density-fluctuation contributes anisotropic momentum-density correction and thus improves the agreement between theory and experiment. These results provide valuable annotations to the arguments concerning the accuracy and validity of the LDA and GW schemes.     

Fermi surface and magnetic structure of TmGa3

We carry out measurements of the two-dimensional angular correlation of the positron annihilation radiation (2D-ACAR) to reconstruct the complex multisheet Fermi surface (FS) of the cubic rare-earth (RE) compound TmGa3. We discover a correlation between the antiferromagnetic structures and the nesting of the FS along the [110] directions. Moreover, we propose methods to estimate the density of states at the Fermi energy ( EF) and the electronic contribution to the specific heat [we obtain N(EF) = 13.6 states/Ryd cell and gamma = 2.4 mJ/mole K2].     

1D-ACAR Spectra of YBa2－xSrxCu3Oy

The measurements of one-dimensional angular correlation of positron annihilation radiation (1D-ACAR) spectra,superconducting critical temperature,lattice parameters and oxygen contents have been made on YBa_2－xSr_xCu₃O_y high temperature supercnductor system.It was found that the substitution of Sr for Ba weakens the orthorhombic symmery,suppresses T_C,hardly changes oxygen content and does not disturb the electronic structure of Cu-O chains.Based on the above results.the authors propose that the localized electronic structure of Cu-O plane is a dominating factor for high temperature superconductivity in 1:2:3 phase superconductors.     

Local structure reconstruction in hydrogenated amorphous silicon from angular correlation and synchrotron diffraction studies

Hydrogenated amorphous silicon (a-Si:H) is a widely used thin film semiconductor material which is still incompletely understood. It is generally assumed to form a continuous random network, with a high concentration of coordination defects (dangling bonds), which are hydrogen terminated. Neither the exact nature of these sites nor the degree of medium range order has been fully determined. In this paper, we present the first results for the local structure, from a combined study using angular correlation of positron annihilation radiation (ACAR) and synchrotron radiation diffraction. Reciprocal space information is obtained directly, for the mesoscale structure and the local defect structure, from the orientation dependent diffraction and 2D-ACAR patterns, respectively. Furthermore, inversion of both patterns yields a comparison of real space information through maps of the silicon-silicon pair correlation function and the electron-positron autocorrelation function B^2γ(r). From this information, it is possible to identify the dominant structural defect as a vacancy-size dangling bond cluster, around which the network strain is fully relaxed.     

Effect of magnetic field on triplet excitons annihilation in Langmuir-Blodgett films

We study the effect of an external magnetic field on annihilation of triplet excitons of aromatic molecules in Langmuir-Blodgett films. Differences in the magnetic effect in relation to the number of layers, temperature, and recording time of annihilation delayed fluorescence are determined.     

Positron lifetime and 2D-ACAR studies of divacancies in Si

We have measured positron lifetime and Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) distributions of Floating-Zone grown (FZ) Si specimens containing divacancies (V₂) with the definite charge states, V ₂ ⁰ , V ₂ ^–1 or V ₂ ^–2 from room temperature to about 10 K. These charge states are accomplished by an appropriate combination of dopant species, their concentration and irradiation doses of 15 MeV electrons. with reference to the currently accepted ionization level of divacancies. The positron lifetime of the negatively charged divacancy increases with temperature, while that of the neutral divacancy shows little change with temperature. The positron trapping rate, obtained from lifetime and 2D-ACAR measurements, increases markedly with decreasing temperature. This is found not only for the negative divacancies but also for the neutral divacancy. We need a model which explains this temperature dependence. The 2D-ACAR distribution from positrons trapped at divacancies shows nearly the same distribution for the different charge states, which differs considerably from the case of As vacancies in GaAs studied by Ambigapathy et al. We have observed a small but definite anisotropy in the distribution of trapped positrons in V ₂ ^– using a specimen containing oriented divacancies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Positron annihilation study for cadmium (electronic structure and enhancement effect)

The three dimensional electron density in momentum space ρ(p) and in wave vector space n(k) was reconstructed for cadmium (Cd). The measurements were performed using the two dimensional angular correlation of annihilation radiation (2D-ACAR) technique. Enhanced contributions in the spectra were observed around 5.5 mrad, discussed in terms of a Kahana-like enhancement effect. From another viewpoint, Fermi radii were analyzed in the (λM K), (ALM) and (AHK) planes, and they showed a maximum deviation of about 4% from the free electron Fermi radius. Moreover, comparisons to a radio-frequency size effect (RFSE) experiment and theoretical band structure calculations (using augmented plane wave (APW), linear combination of atomic orbital (LCAO) and linear muffin tin orbital (LMTO) methods) were examined. The results showed a qualitative agreement with both APW and LCAO calculations. However, a favorable agreement with the APW method was determined via Fermi surface dimensions. The differences of bands' occupation of n(k) between the current work and the APW method were argued in view of positron wave function in Cd.     

Self-consistent band structures of higher stage graphite intercalation compounds

Self-consistent non-empirical band structures of third and fourth stage graphite intercalation compounds have been calculated by using the numerical-basis-set LCAO method within the local density functional formalism. The calculations are carried out for a thin film model consisting of n contiguous graphite layers bounded by two partially ionized intercalant layers. The calculated band structures show that most of electrons transferred from the intercalant layers occupy the states in the lowest two conduction π bands mainly localized on the bounding graphite layers. The low-energy optical transitions of higher stage GICs are discussed in terms of the obtained band structures.     

Modeling the graphitization of amorphous carbon

Three-dimensional ordering in carbon materials during graphitization has been studied by numerical simulation. At small values of the diameter of the graphite layers (1 nm), there is a region of values of the relative displacement of neighboring layers in which the interaction energy reaches a minimum at each point. There are no potential barriers between the points in this region. If the diameter of the neighboring graphite layers increases to infinity, only two fixed displacements with a minimum binding energy of the layers are possible. These particular displacements are characteristic of the graphite structure. The relative positions of neighboring graphite layers are thus determined by the size of these layers. The process which limits the formation of the threedimensional graphite structure is the increase in the diameter of the graphite layers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 67–69, October, 1991.     

STS observations of Landau levels at graphite surfaces

Scanning tunneling spectroscopy (STS) measurements were made on surfaces of two different kinds of graphite samples, Kish graphite and highly oriented pyrolytic graphite (HOPG), at very low temperatures and in high magnetic fields. We observed a series of peaks in the tunnel spectra associated with Landau quantization of the quasi-two-dimensional electrons and holes. A comparison with the calculated local density of states at the surface layers allows us to identify Kish graphite as bulk graphite and HOPG as graphite with a finite thickness of 40 layers. This explains the qualitative difference between the two graphites reported in the recent transport measurements which suggested the quantum-Hall effect in HOPG. This work demonstrates how powerful the combined approach between the high quality STS measurement and the first-principles calculation is in material science.     

Infrared and Raman spectroscopy of graphite-ferric chloride

We present the first detailed study of the stage dependence of the IR- and Raman-active optic graphitic modes in a graphite acceptor intercalation compound. The general frequency upshift observed with increasing FeCl₃ concentration for all optic modes is interpreted to indicate an in-plane compression within the graphitic layers. An identification of the IR-active modes with bounding and interior graphite layers is made. A lineshape analysis of the IR spectra implies IR dipole moments corresponding to ～70% of the effective charge in the graphite bounding layers, independent of stage, and ～30% distributed among the graphite interior layers for stage n?3 compounds.     

Magnetic-state tuning of the rhombohedral graphite film by interlayer spacing and thickness

Based on first-principles total-energy calculations, we systematically investigate how the electronic and magnetic properties of rhombohedral graphite thin films depend on the interlayer spacing and number of layers. Our calculations show that the magnetic ordering of the thin films depends on the interlayer spacing. Thin films under compression normal to the layers possess finite magnetic moments, indicating parallel spin coupling between the two surfaces. We also find that thin graphite films with seven or more atomic layers exhibit magnetic ordering while films with six or less atomic layers are metallic with no magnetic ordering.     

Cs/C(0001)-(2×2)表面结构分析低能电子衍射动力学计算

采用背散射微扰(RSP)、重整化前向散射(RFS)等低能电子衍射(LEED)动力学计算法,计算了Cs/C(0001)-2×2表面的各种可能模型的I-V曲线。将这些曲线与实验曲线拟合,用可靠性因子在这一百多条I-V曲线中搜寻最佳结构。结果排除了Cs原子插入石墨层中和吸附在碳原子顶位的可能。该表面的最可能结构为Cs原子吸附在石墨面上芯位,Cs原子层与C原子层的层间距为2.80?,衬底石墨的结构与其体结构相同。 关键词：     

石墨和纳米碳中缺陷和电子动量的研究

用正电子湮没技术研究了石墨和纳米碳中的缺陷和电子动量. 结果表明, 纳米碳中缺陷的开空间和缺陷浓度分别大于和高于石墨晶体. 纳米碳中存在开空间小于单空位的自由体积以及开空间相当于约10个空位聚集体的微孔洞. 石墨晶体中的自由电子动量分布表现出显著的各向异性: 沿石墨晶体的\[0 001\]晶向的自由电子(即2Pz电子)的动量最大; 偏离该方向越大, 自由电子的动量越小; 垂直于\[0 001\]晶向的自由电子的动量最小. 而纳米碳中自由电子动量的分布表现出各向同性. The defects and electronic momenta in graphite and nanocrystalline carbon have been studied by positron annihilation techniques. The results show that the concentration and open volume of defects in nanocrystalline carbon are higher/larger than that in graphite. Two kinds of microdefects were found in the nanocrystalline carbon: free volume (with a size of smaller than that of a monovacancy) and microvoids (with a size of about ten monovacancies). The anisotropic distribution of electronic momentum was found in single crystalline graphite, the momentum of free electron shows a maximum value in \[0001\] direction, and decreases with the increase of the angle deviation from \[0001\] direction and then reaches a minimum value in the direction perpendicular to \[0001\]. However, this phenomenon was not found in nanocrystalline carbon since the distribution of electronic momentum is isotropic.     

石墨狭缝中甲烷吸附的分子动力学模拟

用分子动力学模拟研究石墨狭缝中甲烷的吸附,考察狭缝宽度和温度对甲烷吸附的影响.模拟发现甲烷在石墨狭缝中出现分层现象,吸附层中甲烷具有类液特征,第一吸附层内甲烷中总有两个氢原子的连线与另外两个氢原子的连线分别位于平行于狭缝壁的两个平面内,游离层中甲烷呈现气体的特征;碳原子间的平均作用势说明吸附层中甲烷分子间结合能力大于游离层,吸附态是甲烷在石墨狭缝中的主要赋存形式之一;伦敦力以及由吸附层净电荷产生的电场力是甲烷吸附和分层的主要原因;甲烷的吸附量随狭缝宽度增大或温度升高而减少,当狭缝宽度小于16.46Å时,甲烷仅以吸附形态存在.甲烷在第一吸附层中的扩散能力最弱、游离层中最强,甲烷扩散系数随狭缝宽度的增大或温度的升高而增大.