Momentum density measurements by positron annihilation in metals and alloys; Recent experiments with a multicounter two-dimensional angular correlation apparatus

The technique of positron annihilation as applied to the study of momentum densities and Fermi surfaces is reviewed. The angular correlation of the two annihilation photons is directly related to the momentum distribution of the positron-electron system; breaks in this distribution reveal the size and shape of the Fermi surface. After a general introduction to the theory and the experimental techniques used, the results yielding various features of the fermi surface in high concentration disordered alloys are reviewed, and are compared with theoretical predictions. A new multicounter two-dimensional correlation apparatus is described and results in several solids are presented.     

Fermi surface studies of non-dilute disordered alloys by positron experiments

The technique of positron annihilation experiments as applied to the study of Fermi surfaces in metals and alloys is briefly reviewed. The angular correlation of the two annihilation photons is directly related to the momentum distribution of the positron-electron system; breaks in this distribution reflect the size and shape of the Fermi surface. Recent two-dimensional angular correlation measurements designed to study the Fermi surface of Cu?Al and Cu?Zn alloys are presented, and are compared with theoretical predictions.     

The Fermi surface of [Formula: see text

The heavy-fermion compound [Formula: see text] has been studied using the fully relativistic spin-polarized mean muffin-tin orbital method within the local density approximation. Two separate calculations, one where the f electron is treated as a valence electron and the other where it is treated as part of the core, have been performed and the Fermi surface is obtained. The angular-dependent de Haas - van Alphen (dHvA) frequencies are calculated in both cases and they are compared with the experimental dHvA frequencies. We also calculated the electron momentum densities and compared them with the electron - positron momentum densities measured from the two-dimensional angular correlation of electron - positron annihilation radiation. The spin polarization of the Fermi surface is analysed and we present a new interpretation of the experimental data of Harrison et al.     

On positron annihilation in zinc

The purpose of this work is to understand Mogensen's and Petersen's positron annihilation curves for zinc. Mijnarends' approach is used as an auxiliary method of localizing inhomogeneities of the electronic density in momentum space, as defined in the paper. Evidence is found for a new effect consisting of a strong enhancement of the annihilation probability in the lenses obtained by the intersection of the Fermi surface with HMC surfaces. This effect, not the anisotropy of the Fermi surface, is the main reason for the anisotropy of the annihilation curves. Paper presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

Study of the electron momentum distribution in vanadium by positron annihilation

The angular correlation of the positron annihilation radiation has been measured with a bidimensional apparatus in four planes of V single crystals. The results are compared with calculations of the electron- positron momentum distribution and with theoretical Fermi surface.     

正电子湮没参量及其所反映物质信息的探讨

本根据正电子在凝聚态物质中的湮没机制及捕获模型,讨论了正电子湮没参量——正电子寿命谱和多普勒线形等参量所反映的物质信息,给出了各正电子湮没参量与所反映的物质信息之间的定量关系。分析了在该体系中正电子寿命参量与局域电子密度、多普勒线形参量和角关联参量与电子动量密度分布和费米面之关联。     

慢正电子束流技术在金属/合金微观缺陷研究中的应用

正电子湮没谱学技术是研究材料微观结构非常有效的一种核谱学分析方法, 主要用于获取材料内部微观结构的分布信息, 特别是微观缺陷结构及其特性等传统表征方法难以获取的微观结构信息. 近年来, 在慢正电子束流技术快速发展的基础上, 正电子湮没谱学技术在薄膜材料表面和界面微观结构的研究中得到了广泛应用. 特别是该技术对空位型缺陷的高灵敏表征能力, 使其在金属/合金材料表面微观缺陷的形成机理、缺陷结构特性及其演化行为等研究方面具有独特的优势. 针对材料内部微观缺陷的形成、演化机理以及缺陷特性的研究, 如缺陷的微观结构、化学环境、电子密度和动量分布等, 正电子湮没谱学测量方法和表征分析技术已经发展成熟. 而能量连续可调的低能正电子束流, 进一步实现了薄膜材料表面微观结构深度分布信息的实验表征. 本文综述了慢正电子束流技术应用研究的最新进展, 主要围绕北京慢正电子束流装置在金属/合金材料微观缺陷的研究中对微观缺陷特性的表征和表面微观缺陷演化行为的应用研究成果展开论述.     

Refining Fermi surface topologies from ab initio calculations through momentum density spectroscopies

It is well known that the agreement between the Fermi surface topologies predicted by ab initio electronic structure calculations and experiment can often be brought into much better agreement through small rigid-band-like shifts. A new method for refining these calculations using experimental data containing Fermi surface information, based on a rigid-band-like fitting approach is presented. In this method, experimental data from different methods can be combined to refine and deliver a ‘tuned’ bandstructure, allowing an investigation of FS nesting properties, a quantitative comparison between experiment and calculation, and highlighting the origin of inconsistencies. Results of the application of this method to positron annihilation experiments in vanadium are presented, showing significant improvement over the ab initio calculation. In order to demonstrate the versatility of this fitting method, it has been applied to a combination of positron annihilation measurements and magnetic Compton scattering experiments in ferromagnetic nickel.     

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.     

Tunable monoenergy positron annihilation spectroscopy of polyethylene glycol thin films

Doppler broadening and coincidence Doppler broadening of annihilation radiation experiments have been performed in three kinds of polyethylene glycol(PEG) membrane formed with different average molecular weight using the tunable monoenergy slow positron probe as a function of implantion energy. The obtained positron annihilation parameters are interpreted from two aspects: surface effect and differences in micro-structure or chemical environment of positron annihilation. The experimental results show that the regulation of densification of PEG molecular packing and distribution uniformity from the near surface layer to the bulk region in the film forming process can be well realized by changing its molecular weight. Combining a variable monoenergetic slow positron beam and these two positron annihilation spectroscopy methods is a powerful tool to study positron annihilation characteristics and for polymeric thin-film fine structure analysis.     

Is positron annihilation a suitable tool for investigating high-T c superconductors?

Summary The effectiveness of the positron annihilation method for gaining information on electron momentum distribution in high-T _c superconductors is discussed critically. The few available experimental results are considered. The possibility of making substantial improvements in the experimental technique is examined in some detail.     

Direct observation of the multisheet Fermi surface in the strongly correlated transition metal compound ZrZn2

The existence of flat areas of a Fermi surface (FS), predicted by electronic structure calculations and used in models of both magnetically mediated and phonon-mediated Fulde-Ferrell-Larkin-Ovchinnikov superconducting states, is reported in the paramagnetic phase of the ferromagnetic superconductor ZrZn2 using positron annihilation. The strongly mass-renormalized FS sheet, dominating the Fermi level density of states, is seen for the first time. The delocalization of the magnetization is studied using measured and calculated magnetic Compton profiles.     

A positron annihilation study of the Fermi surface in concentrated α-phase CuGa alloys

High resolution positron annihilation studies upon a sequence of copper- gallium alloys have determined the variation of the important Fermi surface features with gallium concentration. The Hume-Rothery rules do not arise from contact between the Fermi surface at the [200] Brillouin zone boundary.     

Vacancy-solute complexes in aluminum

Several vacancy-solute complexes in the Al matrix are examined theoretically. In particular, these are V-Cu, V-Cd, V-In, V-Sn, V-Si and V-Fe. We concentrate on coincidence Doppler broadening (high momentum) profiles and positron lifetimes that bring complementary information about these defects. Positron calculations are carried out utilizing the atomic superposition method employing realistic atomic configurations obtained using an ab initio pseudopotential method. In this study we inspect to what extent such defects are detectable and differentiable using positron annihilation techniques. The influence of lattice relaxations around defects on the positron properties turns out to be important and is also debated. The obtained results are discussed in connection with experimental data published in literature.     

The exact solution of the diffusion trapping model of defect profiling with variable energy positrons

We report an exact analytical solution of so-called positron diffusion trapping model. This model have been widely used for the treatment of the experimental data for defect profiling of the adjoin surface layer using the variable energy positron (VEP) beam technique. However, up to now this model could be treated only numerically with so-called VEPFIT program. The explicit form of the solutions is obtained for the realistic cases when defect profile is described by a discreet step-like function and continuous exponential-like function. Our solutions allow to derive the analytical expressions for typical positron annihilation characteristics including the positron lifetime spectrum. Latter quantity could be measured using the pulsed, slow positron beam. Our analytical results are in good coincidence with both the VEPFIT numerics and experimental data. The presented solutions are easily generalizable for defect profiles of other shapes and can be well used for much more precise treatment of above experimental data.     

Positron angular correlation in copper

The two photon pair momentum density and the angular correlation curves for positron annihilation radiation from copper have been obtained by a band structure calculation, employing Hubbard’s approximation scheme. A comparison of the calculated curve for the long slit geometry with the experimental results shows that the theory rather overestimates the contributions in the high momentum region. An angular correlation curve for a point slit geometry is also calculated.     

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.     

Study on the Fermi surface and the spin-dependent momentum space density of ferromagnetic Gd by positron annihilation experiments

The Fermi surface (FS) and spin-dependent momentum space density distribution of ferromagnetic Gd was studied via longitudinally polarised positrons. The measurements were performed using a 2D angular correlation of the annihilation radiation experiments with the reversal magnetic field direction parallel and anti-parallel to the polarisation direction of the positron. It was found that the minority-spin states were concentrated in the basal plane and majority-spin states were concentrated around the A, L and H points. The analysis confirmed that the main contributions to the FS of Gd were influenced by the mixing of both the 5d–6s and the 4f–5d hybrid bands. The general layout of this FS was observed as two hole-like surfaces running along the [ΓA] axis and one electron-like surface running along the [MK] direction. In general, the experimental results showed good agreement with earlier investigations.     

Electron–positron momentum density distribution of Gd from 2D ACAR data via Maximum Entropy and Cormack’s methods

A successful application of the Maximum Entropy Method (MEM) to the reconstruction of electron–positron momentum density distribution in gadolinium out of the experimental of 2D ACAR data is presented. Formally, the algorithm used was prepared for two-dimensional reconstructions from line integrals. For the first time the results of MEM, applied to such data, are compared in detail with the ones obtained by means of Cormack’s method. It is also shown how the experimental uncertainties may influence the results of the latter analysis. Preliminary calculations, using WIEN2k code, of band structure and Fermi surface have been done as well.     

Hg1-xCdxTe晶体缺陷的正电子湮没寿命

利用正电子(e⁺)湮没寿命谱实验研究了Hg_1-xCd_xTe晶体样品的空位缺陷.碲溶剂法生长的样品，不论是n型导电还是p型导电都存在大量的Hg空位.经过合适的退火工艺，p型材料转为n型，同时对正电子的俘获效应减小，表现为正电子湮没平均寿命值减小14—17ps.若退火温度高于350℃，正电子湮没寿命值又增大，表明Hg空位浓度增加.得到HgCdTe中正电子的体寿命为τ_b=272ps.根据正电子湮没寿命和电参数的测量结果，得出 关键词：