Two-dimensional Angular Correlation Research Articles

Momentum density and Fermi surface of Nd2-xCexCuO4- delta.

High-temperature positron two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurements have recently been succesfully applied to map parts of the Fermi surface of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. Using the same principle, we have been able to observe with a bulk sensitive method, the Fermi surface of ${\mathrm{Nd}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. Although positron trapping by defects and correlation effects are strong, positron 2D-ACAR measurements provide a signal from the Fermi surface which agrees with band-structure calculations, confirming earlier surface sensitive photoemission experiments. \textcopyright{} 1996 The American Physical Society.

A SYMMETRY BASED STUDY OF POSITRON ANNIHILATION SPECTRA

We describe a method for off-line analysis of spectra measured by two-dimensional angular correlation of annihilation radiation [Formula: see text] positron spectroscopy. The method takes into account, at all its stages, two salient data features: the piecewise constant discretization of the [Formula: see text] physical momentum distribution into square pixels, performed by the setup, and the occurrence of a characteristic [Formula: see text] projected symmetry of the positron–electron pair momentum distribution. Several validating criteria are derived which secure significantly increased reliability of the output. The method is tested on [Formula: see text] spectra measured on (R)Ba 2 Cu 3 O 7−δ( R123 ; R= Y, Dy) single crystals. It resolves ridge Fermi surfaces [Formula: see text] up to 3rd Umklapp components on both kinds of R123 spectra. Moreover, on a c-axis-projected Y123 spectrum, measured at 300 K, it resolves a small but clear signature of the pillbox [Formula: see text] at the S point of the first Brillouin zone as well.

Characterization of diamond single crystals by means of double-crystal X-ray diffraction and positron annihilation

The crystallinity of synthesized and natural crystals of diamond was characterized by double-crystal X-ray diffraction and positron annihilation. The two-dimensional angular correlation of annihilation radiation and positron lifetime measurements revealed that in natural crystals positroniums are formed in a high fraction. The synthesized crystal Ib showed both an extremely small width for the diffraction and a positron lifetime spectrum with a single component of the lifetime of 115 ps. In contrast, the natural diamonds contain a long-lived component of lifetime longer than 2 ns. The diffusion length of positrons was also measured by a variable-energy positron beam. In the synthesized crystal IIa, a diffusion length of about 100.8 nm was observed.

2D Angular Correlation of Annihilation Radiation: Current Trends and Future Possibilities

Recent improvements in two-dimensional angular correlation measurement and analysis techniques have greatly increased the power of the method to image the Fermi surfaces of electronically complex systems. That, and the further and dramatic improvement that must result when the present radioisotope positron sources are replaced by intense monoenergetic positron beams are discussed and illustrated. PACS numbers: 78.70.B.1, 71.28.+d, 71.25.Ηc

Positron annihilation study of the electronic structure of CeCu6 and LaCu6

The results of a prelimianry study of the electronic structure of the heavy-fermion system CeCu6 via the two-dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) technique were compared with those obtained for the isostructural reference non-f-electron material LaCu6. The spectra relating to the 2 compounds displayed extremely similar anisotropic structures, with features related to the symmetry of the Brillouin zone. At present, these structures cannot however be attributed to Fermi-surface or positron wave function effects or to anisotropies of some full valence band. In the case of LaCu6, data processing prior to the LCW procedure via new filtering techniques revealed a reasonably good agreement between the LCW data and LAPW Fermi-surface calculations while in the case of CeCu6 it appeared difficult to relate the results, still of controversial interpretation, to any theoretical model.

Positron trapping in Y1-xPrxBa2Cu3O7- delta and the Fermi surface of YBa2Cu3O7- delta.

Temperature-dependent positron lifetime measurements in ceramic Y1−xPrxBa2Cu3O7−δ samples reveal positron trapping, in particular at low temperature and for small x. Positrons appear to be completely delocalized for T∼400 K and higher. At high temperatures the lifetime for YBa2Cu3O7−δ and PrBa2Cu3O7−δ is identical (∼165 ps) and close to the theoretical value. For these reasons a two-dimensional angular correlation of annihilation radiation (2D-ACAR) spectrum was measured in YBa2Cu3O7 at T=400 K. The spectrum width confirms the delocalization of the positron and the 2D-ACAR shows, apart from the one-dimensional Fermi surface due to CuO chains, a smaller Fermi surface sheet centered around the S point, in the first Brillouin zone.

Positron annihilation study of the electronic structure of CeB 6, CeCu 6 and LaCu 6

The two-dimensional angular correlation of electron-positron annihilation eradiation (2D-ACAR) from the heavy-fermion dense Kondo system CeB 6 was measured. The use of a recently developed filter procedure, designed to remove characteristic positron-induced long range distortions that are frequently superimposed on the Fermi breaks, provided a good description of the Fermi surface features. Further, the results of a preliminary 2D ACAR study of the electronic structure of the heavy-fermion system CeCu 6 are shown and compared with those obtained for the reference non-f-electron material LaCu 6.

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.

Intense low energy positron beams

Intense positron beams are under development or being considered at several laboratories. Already today a few accelerator based high intensity, low brightness e+ beams exist producing of the order of 108–109 e+/s. Several laboratories are aiming at high intensity, high brightness e+ beams with intensities greater than 109 e+/s and current densities of the order of 1013–1014 e+ s−1 cm−2. Intense e+ beams can be realized in two ways (or in a combination thereof) either through a development of more efficient Β+ moderators or by increasing the available activity of Β+ particles. In this review we shall mainly concentrate on the latter approach. In atomic physics the main trust for these developments is to be able to measure differential and high energy cross-sections in e+ collisions with atoms and molecules. Within solid state physics high intensity, high brightness e+ beams are in demand in areas such as the re-emission e+ microscope, two-dimensional angular correlation of annihilation radiation, low energy e+ diffraction and other fields. Intense e+ beams are also important for the development of positronium beams, as well as exotic experiments such as Bose condensation and Ps liquid studies.

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.

Positron Annihilation Study of the Electronic Structure of CeCu<sub>6</sub> and LaCu<sub>6</sub>

The results of a prelimianry study of the electronic structure of the heavy-fermion system CeCu6 via the two-dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) technique were compared with those obtained for the isostructural reference non-f-electron material LaCu6. The spectra relating to the 2 compounds displayed extremely similar anisotropic structures, with features related to the symmetry of the Brillouin zone. At present, these structures cannot however be attributed to Fermi-surface or positron wave function effects or to anisotropies of some full valence band. In the case of LaCu6, data processing prior to the LCW procedure via new filtering techniques revealed a reasonably good agreement between the LCW data and LAPW Fermi-surface calculations while in the case of CeCu6 it appeared difficult to relate the results, still of controversial interpretation, to any theoretical model.

Vacancy defects in photoexcited GaAs studied by positron two-dimensional angular correlation of annihilation radiation.

The positron two-dimensional angular correlation of annihilation radiation (2D-ACAR) technique has been coupled with optical excitation to study the native point defects in semi-insulating GaAs. The As vacancy was observed below \ensuremath{\sim}170 K when illuminated with 1.41\ifmmode\pm\else\textpm\fi{}0.07 eV photons. The temperature dependence of the 2D-ACAR spectra, with and without illumination, was studied. Data were also collected at 25 K as a function of the intensity of infrared light. The 2D-ACAR spectra reflect the ${\mathit{e}}^{+}$-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ pair momentum distribution at or near the vacancy and provides symmetry and electronic structure information, which can be used as a unique defect signature for the vacancy.

A positron-annihilation study of the electronic structure of CeB6

The two-dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) from the heavy-fermion dense Kondo system CeB6 was measured. The anisotropies of the measured momentum spectra were in reasonable agreement with those of model spectra based on the Fermi-surface topology suggested by band-structure calculations and de Haas-van Alphen measurements. However, the related k-space densities, obtained by an LCW analysis, were far removed from those expected from the Fermi-surface topology. The use of a recently developed filter procedure, designed to remove the characteristic long-range distortions that are frequently encountered in such LCW spectra, provided a much improved image of the Fermi-surface features.

A-axis-projected electron-positron-momentum density and positron-annihilation spectra in YBa2Cu3O7-x.

We present and discuss theoretical and experimental a-axis-projected two-dimensional angular correlation of annihilation radiation (2D-ACAR) spectra from ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$. The experiment involved an untwinned metallic single crystal of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.9}$ at room temperature. The calculations have been carried out within the conventional first-principles band-theory framework based on the local-density approximation; the positron state is treated in the independent-particle model. After the as-observed 2D-ACAR spectrum is corrected for an isotropic background, a good accord is found between not only the overall shape but also the fine structure in the measured and computed spectra, some discrepancies notwithstanding. Extensive analysis shows that the experimental spectrum clearly contains the signature of the electron ridge Fermi surface (FS) arising from Cu-O chains in the first Brillouin zone. Also the first and second Umklapp images of the ridge FS at higher momenta are in substantial accord with theoretical predictions. These results suggest that the electron states in Y123 are not localized, at least not strongly, along the c direction.

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.

Angular correlation studies of positron annihilation in the noble gases

Two-dimensional angular correlation distributions have been measured for the first time for positron and positronium annihilation in low-density helium, neon, argon, krypton and xenon, and in a range of helium-xenon mixtures. The full widths of the (cylindrically averaged) free positron components are in reasonable agreement with theoretical values. The intensity of the mixed-state positronium (Ps) component (mean vacuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consistent with the Ps fraction previously determined in lifetime measurements; the widths of the Ps component in helium, neon and argon are consistent with a Ps-atom scattering cross section below 6.8 eV in all three gases of approximately (9-0.5 E) pi a02, where E is the Ps energy in eV.

Surface acidity by positron annihilation lineshapes

An alternative new method to measure the specific acidity of catalytic materials with large internal surface areas by using positron annihilation spectroscopy was introduced. Results of lineshape parameter evaluations from two-dimensional angular correlation and Doppler broadened annihilation radiation measurements for a zeolite ZSM-5 were presented.

Window least squares method applied to statistical noise smoothing of positron annihilation data

The paper deals with the off-line processing of experimental data obtained by two-dimensional angular correlation of the electron-positron annihilation radiation (2D-ACAR) technique on high-temperature superconductors. A piecewise continuous window least squares (WLS) method devoted to the statistical noise smoothing of 2D-ACAR data, under close control of the crystal reciprocal lattice periodicity, is derived. Reliability evaluation of the constant local weight WLS smoothing formula (CW-WLSF) shows that consistent processing 2D-ACAR data by CW-WLSF is possible. CW-WLSF analysis of 2D-ACAR data collected on untwinned YBa 2Cu 3O 7 − δ single crystals yields significantly improved signature of the Fermi surface ridge at second Umklapp processes and resolves, for the first time, the ridge signature at third Umklapp processes.

Fermi surface ridge at second and third UMKLAPP positron annihilations in YBa 2Cu 3O 7− δ

Results of statistical noise smoothing of the electron momentum distribution got by two-dimensional angular correlation of the electron-positron annihilation radiation technique on untwinned YBa 2Cu 3O 7− δ single crystals are reported. Two distinct signatures of the sheet of Fermi surface related to the CuO chains (the ridge) are resolved. The first occurs at second Umklapp processes, in agreement with previous evidence. The second one, identified for the first time, occurs at third Umklapp processes. Comparison with FLAPW calculations confirms this result.

The electronic structure of alkali metals: experiment and theory

The high-momentum components of the electron and positron Bloch waves in the three alkali metals Li, Na, and K have been studied by comparing calculations employing the linear muffin tin orbital method and the independent particle model with accurate measurements of the two-dimensional angular correlation of annihilation radiation. It is found that such an analysis provides a sensitive test of electronic structure, and that electron-positron correlation effects do not distort the high-momentum components in this example in any significant way. It is shown that thermal effects due to Debye-Waller damping are important.