Electron-positron Momentum Density Research Articles

Positron annihilation and positron-electron correlation effects in high- Tc oxides

We have calculated electron and positron states in the Nd 2 − x Ce x CuO 4 compound, for the perfect bulk lattice as well as when vacancy defects are included. We study the sensitivity of the results to different approximations in the positron potential. We also compute the annihilation rate and the electron-positron momentum density using the local density approximation and the newly developed gradient correction for the electron-positron contact term. Our main goal is to give predictions for the Fermi surface signals.

Direct observation of the Fermi surface in Ba1-xKxBiO3.

Positron angular correlation measurements have been used to measure electron-positron momentum density distributions in high-quality single crystals of the 32 K superconductor Ba[sub 1[minus][ital x]]K[sub [ital x]]BiO[sub 3]. Using first-principles calculations we are able to separate those features due to the strong overlap of the electron and positron wave functions from those associated with Fermi surface breaks. We have identified a roughly cubic Fermi surface centered around [Gamma], whose location is in excellent correspondence with that predicted by the band structure calculations.

Electron-positron momentum density in Tl 2Ba 2CuO 6

We present calculations of the electron-positron momentum density for the high- T c superconductor Tl 2Ba 2CuO 6, together with some preliminary two-dimensional angular correlation of the annihilation radiation (2D-ACAR) measurements. The calculations are based on the first-principles electronic structure obtained using the full-potential linearized augmented plane wave (FLAPW) and the linear muffin-tin orbital (LMTO) methods. We also use a linear combination of the atomic orbitals-molecular orbital method (LCAO-MO) to discuss orbital contributions to the anisotropies. Some agreement between calculated and measured 2D-ACAR anisotropies encourage sample improvement for further Fermi surface investigations. Indeed, our results indicate a non-negligle overlap of the positron wave function with the CuOo 2 plane electrons. Therefore, this compound may be well suited for investigating the relevant CuO 2 Fermi surface by 2D-ACAR.

Influence of doping on the electronic structure of (La, Sr)2CuO4

High-statistics (>4 × 108 counts), room-temperature measurements of the electron-positron momentum density of La2−x Sr x CuO4 have been performed for samples with Sr concentrations of x=0.0, 0.1, 0.13, and 0.2. These spectra have been analyzed in conjunction with theoretical calculations of the electron-positron momentum density. The metallic samples show features consistent with the presence of a Fermi surface, but its evolution with increasing Sr concentration does not follow the predictions of band theory. These results may indicate the effects of electron-electron correlation on the electron momentum distribution in the Cu-O plane.

Positron annihilation study of Sr doping in La 2− xSr xCuO 4

We present a combined experimental and theoretical study of the effects of Sr doping on the electronic structure of La 2− x Sr x CuO 4. Electron-positron momentum distributions have been measured to high statistical precision (> 4 × 10 8 counts) at room temperature for samples with Sr concentrations of x = 0.0, 0.1, 0.13 and 0.2. Analysis of all tour spectra reveal strong features due to electron-positron wavefunction overlap, in quantitative agreement with theoretical calculations. The Sr doped samples show discontinuities consistent with the presence of a Fermi surface. The form and position of these features are in general agreement with the predictions of band theory. Correspondence between theory and experiment, as well as some differences, are revealed by a detailed study of the changes in electron-positron momentum density with increasing Sr doping.

Pressure dependence of positron annihilation in InP

We have obtained the valence electron and positron charge densities in InP from wavefunctions derived in a model pseudopotential bandstructure calculation at normal and under hydrostatic pressure. We find that the positron density is maximum in the open interstices and is excluded not only, as usual, from the ion cores but also to a considerable degree from the valence bonds. Electron-positron momentum densities are calculated for the (001–110) plane. The results are used to analyze the positron effect in InP.

Interpretation of positron-annihilation data with respect to the electron-positron enhancement factors. II. Applications.

The theoretical considerations of the properties of positron-annihilation characteristics in simple metals, presented in the previous paper, are verified for the alkali metals and Mg. Results are based on a local-density approach to electron-positron correlations and band-structure calculations are performed within the linear-muffin-tin-orbitals--average-spheres approximation. The validity of the average electron density approximation is discussed and the effective densities, determined by electron-positron correlation effects, are presented for valence electrons in simple metals. Methods of extracting the full shape of the electron-positron momentum density from experimental curves are recommended. The proposed analysis of experimental data applied to simple metals allows more reliable verificaton of the shape of electron-positron enhancement factors near the Fermi surface.

Modern Band Theory of Disordered Alloys: Basic Concepts Including a Discussion of Momentum Densities

Abstract An overview of some of the basic concepts in the modern first-principles band theory of disordered alloys is given. The question of how the notion of Bloch energy bands and Fermi surfaces generalizes to the case of the disordered system, insofar as the average electronic spectrum is concerned, is stressed. The theory is illustrated with examples chosen from the work on binary alloys; a few examples of the very recent studies of disordered phases of the high-Tc superconductors are also included. The application of the alloy theory to obtain electron and electron-positron momentum densities involved in the analysis of Compton scattering and positron annihilation (angular correlation) experiments in alloys is discussed, with a selection of relevant theoretical and experimental studies.

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.

The three-dimensional positron-electron momentum density study in Ge by means of a full-scale use of two-dimensional angular correlation of positron annihilation radiations

The positron-electron momentum density in germanium was studied by a full-scale use of the two-dimensional angular correlation of positron annihilation radiations (2D-ACAR). The term, a full-scale use of 2D-ACAR, means that the three-dimensional momentum-space density rho (p) of the annihilation positron-electron item is directly determined from a set of experimental 2D-ACAR spectra, followed by the reconstruction technique. The obtained electron momentum density, rho (p), showed, as a whole, good agreement with the electron momentum distribution of the fully filled Jones zone with the exception that a deep dip in the origin appears, which has been observed in the 1D-ACAR or semi-2D-ACAR integrated spectra in germanium. In the present work, the electron momentum density distribution shows a ridge along the (110) line and a vary deep valley exists along the (100) line, which is also the deviation from the Jones zone. In germanium, the valence 4p electrons are mainly participating in bonding sigma bonds, and the distribution in space is prominently anisotropic. This anisotropy is reflected on the electron momentum density distribution in the present work. The analysis based on group theory was performed by Saito et al. in order to investigate the origin of the anisotropy in the present work.

Electronic states in RB 6 (R [sbnd]La, Ce, Pr and Nd)

The experimentally obtained electron-positron k-space momentum densities of the paramagnetic phases of RB 6 (R La, Ce, Pr and Nd) are studied by both band models on the basis of the local density approximation and local models with no f-electron hybridization. For LaB 6, the experimental results are reasonably reproduced by both the band and local models within the experimental statistical accuracy. For RB 6 (R Ce, Pr and Nd), it is indicated that the experimental results are consistent with those of the local model. Furthermore, the measured Fermi surface of antiferromagnetic NdB 6 is studied by both the band and local models. It is found that the observed Fermi surface can be well explained by the local model.

Pair momentum distribution in Bi2Sr2CaCu2O8+ delta measured by positron annihilation: Existence and nature of the Fermi surface.

We report the first measurement of the positron-electron momentum density in superconducting single-crystal ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}90 K). The observed anisotropy exhibits a twofold (rather than fourfold) symmetry, which is attributed to the superlattice modulation along the b axis of the ${\mathrm{BiO}}_{2}$ layers. Subtraction of the superlattice contribution also reveals a pair momentum distribution consistent with the ${\mathrm{CuO}}_{2}$ and ${\mathrm{BiO}}_{2}$ Fermi surfaces, and in reasonable agreement with the theoretical pair momentum density derived from band theory.

The Effects of Lattice Constant Variations on Electron and Positron Charge Densities in Grey Tin

AbstractElectron and positron charge densities are calculated as a function of the lattice constant variations in grey tin. The calculations of the electron–positron momentum densities are carried out by using the independent particule approximation coupled with the Gaussian nonlocal pseudopotential method. The results are correlated to the effects of pressure.

Direct observation of Fermi surface in YBa2Cu3O7- delta.

We have performed a high-precision measurement (5\ifmmode\times\else\texttimes\fi{}${10}^{8}$ coincidence counts) of the basal-plane electron-positron momentum density in well oxygenated, twin-free, single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. The raw, processed, and K-space reduced spectra unambiguously show a clear image of a major Fermi surface sheet. The form and profile of that image are in substantial quantitative agreement with theoretical predictions of a \ensuremath{\Gamma}-X electron ridge section associated with states in the CuO chains.

Fermi surface and electron-positron momentum density in YBa2Cu3O7.

By folding the local-density-approximation (LDA) band-theory-based electron-positron momentum density into the first Brillouin zone, we find that signatures from all the Fermi-surface sheets can be obtained in YBa{sub 2}Cu{sub 3}O{sub 7}, including in particular the Cu-O plane sheets. Our theoretical predictions appear to be in reasonable overall accord with the corresponding measurements by positron-annihilation two-dimensional angular correlation of annihilation radiation (2D-ACAR) of Smedskjaer {ital et} {ital al}., which have been interpreted in terms of Fermi surfaces. The Cu-O chain-related (electron) states appear to be substantially modified from LDA predictions. However, no firm conclusions concerning the existence of the Fermi surface in YBa{sub 2}Cu{sub 3}O{sub 7} can at the moment be drawn, since the experimental 2D-ACAR situation in this regard is still controversial.

Momentum density in Y-123: LDA predictions

We discuss theoretical results concerning the electron-positron momentum density in the normal state YBa 2 Cu 3 O 7 based on the LDA-band-theory framework. Our focus is on the c-axis projected two-dimensional(2D)-momentum distribution relevant for interpreting the 2D-angular- correlation-of-annihilation-radiation (2D-ACAR) positron experiments, paying particular attention to identifying Fermi surface signatures in the spectra, and related issues. The theoretical results from various groups using different band structure methodologies are in reasonable accord, relatively smaller differences between various first principles calculations notwithstanding, and thus a robust picture of the LDA-based predictions for the 2D-ACAR spectra has emerged. Although this article is devoted mainly to discussing theoretical results, we note that the latest high-statistics 2D-ACAR. experiments on untwinned single crystals of YBa 2 Cu 3 O 7 appear to be in remarkable accord with a number of detailed theoretical predictions, including some Fermi surface features, and indicate that the conventional band theory framework is able to provide a reasonable description of the rather complex positron annihilation spectra at least in the normal state YBa 2Cu 3O 7.

Theoretical two-particle momentum density in YBa 2Cu 3O 7−δ

We present calculations of the electron-positron momentum density for YBa 2Cu 3O 7−δ for δ = 0 and 1 respectively and for an ordered YBa 2Cu 3O 7/YBa 2Cu 3O 6 (100) superlattice, based on first-prindples electronic structure calculations performed within the local density approximation (LDA) using the full potential linearized augmented plane wave (FLAPW) method. Our present results account for the trends observed in the experimental two-dimensional angular correlation of annihilation radiation (2D-ACAR) of the metallic and insulating compounds, as measured in Geneva. The Lock, Crisp, and West folding is applied in order to emphasize Fermi surface structures. While for δ = 0 the chain related breaks are large, they are predicted to be smaller by a factor of two for δ = 0.5, and to be hardly visible in experiments.

Electron-positron P- and K-space densities in YBa2Cu 3O 7-δ

We have performed an ultra-high precision measurement (5 × 10 8 coincidence counts) of the basal-plane electron-positron momentum density (MD) in well oxygenated, twin-free, single crystals of YBa 2Cu 3O 7δ. The anisotropies of the raw and processed MD spectra and LCW-transformed spectra (k-space densities) not only show the D 2 symmetry appropriate to the untwinned crystals but, more importantly, unambiguously show a dear image of a major Fermi surface sheet. The form of the FS image is in substantial agreement with theoretical predictions of a Γ-X electron ridge section associated with states in the Cu-O chains.

Positron 2D-Acar Experiments and Electron-Positron Momentum Density in YBa2Cu307−x

ABSTRACTWe discuss positron annihilation (2D-ACAR) measurements in the c-projection on an untwinned metallic single crystal of YBa2Cu3O7-x as a function of temperature, for five temperatures ranging from 30K to 300K. The measured 2D-ACAR intensities are interpreted in terms of the electron-positron momentum density obtained within the KKR-band theory framework. The temperature dependence of the 2D-ACAR spectra is used to extract a ‘background corrected’ experimental spectrum which is in remarkable accord with the corresponding band theory predictions, and displaysin particular clear signatures of the electron ridge Fermi surface.

Study on Momentum Density of Electrons and Fermi Surface in Niobium by Positron Annihilation

The three dimensional electron-positron momentum density in niobium has been reconstructed from measurements of two dimensional angular correlation of positron annihilation radiations (2D-ACAR) followed by the image reconstruction technique based on a direct Fourier transformation. We determined the position of the Fermi surface sheets; Γ-centered hole octahedron, multiply connected jungle-gym arms and N -centered hole ellipsoids. The Fermi surface topology is in good agreement with the theory.