A. A. Manuel

Electronic Structure of YBa2Cu3O7-x by Positron Annihilation

We report the first 2D-ACPAR measurement performed on single crystals of the high-Tc superconductor YBa2Cu3O7-x. The main features of the electronic structure have been experimentally determined. The electronic occupation number in the Brillouin zone supports the results of band structure calculations. Occupation numbers do not change drastically at Tc. The 2D shape parameter is anisotropic. Its temperature dependence exhibits an anomaly around the superconducting transition and has a larger slope in the normal state.

Positron annihilation and superconductivity

Abstract Positron annihilation in solids can be used as a powerful diagnostic technique both for probing the local density distribution (lifetimes) and for measuring the electronic momentum-density distribution in powders and single crystals. In particular, this technique allows the study of Fermi surfaces, and we have used this technique on alkali metals, transition metals and compounds including the superconducting A15 phases, and recently on superconducting YBa2Cu3O7−δ. In all these cases we have obtained good correlation with calculated band structures. While our investigation of the high-Tc compound requires further confirmation, it is fair to say that there too a Fermi surface seems to emerge which corresponds to the one expected from the band structure calculated by the group of Freeman (1). Such results tend to confirm our previous experience as to the reliability of calculations based on the density functional formalism.

Reconstruction of 3D Electron-Positron Momentum Densities from 2D Projections: Role of Maximum-Entropy Deconvolution Prior to Reconstruction

The benefit of using a Maximum-Entropy deconvolution technique prior to reconstructing the three-dimensional momentum density from two-dimensional angular correlation of positron annihilation radiation (2DACAR) is highlighted. The technique is applied to experimental data for a single crystal of chromium in its paramagnetic phase. The resulting reconstructed momentum density features are significantly enhanced when the 2DACAR spectra are first deconvoluted, although there are artefacts present around p ~ 0. It is also shown to agree well with the theoretical momentum density.

Fermi surface of YBa2Cu3O7−x by positron annihilation

Abstract We have studied the Fermi surface (FS) of YBa 2 Cu 3 O 7−x by measuring the two dimensional angular correlation of the positron annihilation radiation (2D - ACPAR) from single crystals, in the three crystalline directions [100], [110] and [210]. The topology of the FS is presented, and compared with results from structure calculations. The main features of the experimentally obtained topology are in good agreement with those of the theoretical model.

A high-resolution Compton scattering study of the Fermi surface of chromium

The first high-resolution (<0.2 a.u.) Compton scattering measurements on chromium (Cr) are presented, and represent the first 1D to 2D reconstruction of the electron occupancy. The results are compared with Korringa–Kohn–Rostoker calculations of the electronic structure.

A study on the Fermi surface of Cr by high-resolution Compton scattering

Abstract A direct Fourier transform method is employed for the first time to reconstruct the electron momentum density projected onto the (110) plane from seven high-resolution Compton profiles. The topological features of the projected Fermi surface obtained by the LCW-folding is compared with the most recent corresponding theoretical results of the all electron charge self-consistent KKR computations.

Electron-positron momentum density in Tl2Ba2CuO6

Abstract We present calculations of the electron-positron momentum density for the high-Tc superconductor Tl2Ba2CuO6, 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 CuOo2 plane electrons. Therefore, this compound may be well suited for investigating the relevant CuO2 Fermi surface by 2D-ACAR.

A Compton scattering study of insulating PrBa2Cu3O7−δ and superconducting YBa2Cu3O7−δ

Abstract We have measured the electron momentum density in both insulating PrBa 2 Cu 3 O 7− δ and superconducting YBa 2 Cu 3 O 7− δ . We find a remarkable difference in the anisotropy of the momentum density, even though LDA based calculations predict similar behaviour for both. Our results can be explained assuming Pr on Ba-site substitution disorder and we propose that this phenomenon is responsible for the observed effects.

Defect analysis using 2D-ACAR: GaAs as a test case

Abstract We describe how the nature of defects may be investigated by positron 2D-ACAR measurements. We show that it is possible to identify a specific distribution for each defect. Focusing on GaAs, we characterize negative ions, identified as antisites, as well as As and Ga vacancies in various charge states. We find that each 2D-ACAR distribution is different, hence providing 2D “fingerprints” of the defects which might be used for the characterization of real materials. We also show that 2D-ACAR measurements provide information on the atomic relaxations around vacancies: we infer that VAs− has a smaller open volume than VAs0. This finding is in agreement with results from positron lifetime experiments and molecular dynamics calculations.

Differences in the Fermi surface topology of paramagnetic and antiferromagnetic Cr studied by positron annihilation

Two-dimensional angular correlation of the positron annihilation radiation has been measured from Cr in its paramagnetic and antiferromagnetic phases, as well as from paramagnetic Cr0.95V0.05 alloy. We report the first observation of a difference of the Fermi surface topology between the two phases. Using available band structure calculations, we interpret our results as the appearance of electron pockets at the point X of the Brillouin zone, when Cr goes to the paramagnetic phase.