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.     

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.     

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.     

A study of Fermi surfaces of the α-phase Cu-Ge and Cu-Si alloys by positron annihilation

Angular correlation measurements on the fcc solid solutions of Cu?Ge and Cu?Si alloys have been carried out on single crystals with four crystallographic orientations by a crossed-slit geometry. The results on both the alloys are nearly the same. The 〈111〉-neck radius and the 〈100〉-radius of the Fermi surfaces increase almost linearly with increasing electron concentration in accord with previous results on Cu?Al and Cu?Zn alloys. The Fermi surface does not touch the square faces of the Brillouin zone at the solubility limit. The results agree fairly well with a calculation based on the sinking-conduction band model.     

Positron annihilation in copper: Ak-space analysis

A new approach to the analysis of positron-annihilation long-slit angular distributions which can provide a more direct reflection of Fermi surface profile has been described in a recent publication. The analysis involves the periodic superposition of angular distributions which has the effect of converting a momentum distribution into a distribution in reduced Bloch wave vectorsk. When applied to data for a Cu crystal, having the resolved momentum component parallel to the [110] direction, the analysis results in excellent agreement with the computed surface of Halse and also provides a guide to the form and intensity of the angular distribution for core electron annihilation. A similar treatment of the results for a [100] orientation, at first sight, appears less encouraging. However, a more careful appraisal supports the general value of the approach, the validity of the analysis for the [110] crystal orientation, and gives further clues to the form and anisotropy of the core distributions for Cu single crystals.     

Electronic properties of the amorphous FeCuNbSiB alloy

Variations in the electronic properties of amorphous Fe-Cu-Nb-Si-B alloys during crystallization have been studied by methods of positron annihilation spectroscopy. The measurements of the positron lifetime in samples and the angular distribution of annihilation photons have demonstrated substantial variations during crystallization. There is a correlation of the results with the performed measurements of the thermopower. The experimental results allow the assumption that degenerate electron gas is not formed in the amorphous alloys under study, and localized electrons participate mainly in the conduction. It can be suggested that the main role in formation of the amorphous alloy structure is played by the covalent bond. The spectrum of momentum distribution of the amorphous alloy differs little from the pure iron spectrum. Irradiation induces vacancy-copper complexes in the amorphous alloy.     

Studies of the Fermi surface of V3Si by 2D ACAR measurements

We report two-dimensional angular correlation of annihilation radiation (2D ACAR) measurements using positrons in single crystals of V₃Si. Anisotropic structure commensurate with the reciprocal lattice is observed and attributed to the complex Fermi surface (FS). A simple geometrical model for the FS is developed and compared with folded ACAR distributions. The results of a 3D reconstruction are compared with recent band calculations.     

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.     

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].     

Contribution to the determination of the Fermi surface of V3Si by positron annihilation

The angular correlation of the positron annihilation radiation has been measured with a bi-dimensional apparatus in the (100) and (110) planes of V₃Si. The main features of the Fermi Surface are determined.     

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.     

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。     

A study of the fermi surface of α-brass

Fermi surface of α-brass was investigated up to 24.6 at.% Zn by the rotatingspecimen method in the annihilation radiation. The anisotropy in the Fermi surface of α-brass was found to be similar to that of pure copper. It was also noted that the 〈111〉-neck radius of the alloys increase monotonically with increasing content of zinc.     

Positron annihilation in solid and liquid metals

The angular correlation of the gamma rays resulting from the annihilation of positrons in 15 solid and liquid metals and semiconductors has been studied. Experiments have been done on each material at room temperature and at temperatures above and below the melting point.

The elements investigated fall into three categories according to the way the angular correlation distribution changes as the melting point is reached. To within the experimental angular resolution (0.5 milliradians) no change in the angular correlation distribution is observed for Li, Se, Na, and Tl upon heating from room temperature to beyond the melting point. The elements Sb, Bi, Ga, Hg, Sn and Te exhibit changes in their angular correlation distributions only upon being melted, whereas for the metals Al, Cd, In, Pb and Zn changes occur when the specimen is heated from room temperature to temperatures below the melting point.

Changes in the angular correlation distribution upon heating or melting are generally manifested as (a) a narrowing of the central part of the curve, (b) a rounding-off of the parts of the curve near the Fermi cut-off angle and (c) a change in the area of the broad background curve as compared with the area under the central peak.     

Interaction of positrons with niobium atom chains in Nb3Al

The angular correlation of annihilation in Nb₃Al is investigated. The results indicate that positrons may be trapped by deformed sections of Nb atomic chains. It is concluded that the Fermi level for d-electrons of such chains is located near the boundary of the Brillouin zone.     

Positron annihilation in silicon crystals with mechanically processed surfaces

The angular correlation curves of the annihilation photons and the probability of three-quantum annihilation in silicon single crystals with mechanically processed surfaces were measured. The narrowing of the angular correlation curves increases and the three-quantum yield decreases with the depth of the surface abrasion. This is tentatively interpreted in terms of formation of positronium states on the surface of the silicon crystals.     

Chlorine adsorption on copper: III. Angle-resolved electron energy loss spectroscopy

Electronic excitations on Cu(001) and Cu(001)c(2 × 2)-Cl have been investigated by angle-resolved electron energy loss spectroscopy at an angular resolution of Δθ = ±1° and an energy resolution of ΔE = 60 meV. Primary energies in the 50–100 eV range were chosen and the specular reflection was studied for angles 35° ? θ ? 71° with respect to the surface normal. The results are summarized as follows: The specular Cu(001) spectra are compared to optical data and good agreement is found for the energetic position of direct transitions. The electronic losses observed for the ordered overlayer system may be interpreted by one-electron excitations from occupied surface bands (known from angle-resolved photoemission results) into an empty band with a minimum energy at 0.4 eV above the Fermi level.     

Hydrogen behaviour in electron-irradiated titanium alloys studied by positron annihilation method

Measurements of angular distribution of annihilation photons are applied to investigations of hydrogen behavior in annealed, plastically deformed, and irradiated titanium alloys. It is shown that hydrogen interaction with materials becomes more intensive in presence of structural imperfections of deformational and radiation origin. Probable mechanisms of hydrogen interaction with deformational and radiation defects are discussed.