Alfred A. Manuel

HOLE DEPLETION AND LOCALIZATION DUE TO DISORDER IN INSULATING PRBA2CU3O7-DELTA : A COMPTON SCATTERING STUDY

The (mostly) insulating behaviour of PrBa2Cu3O7-d is still unexplained and even more interesting since the occasional appearance of superconductivity in this material. Since YBa2Cu3O7-d is nominally iso-structural and always superconducting, we have measured the electron momentum density in these materials. We find that they differ in a striking way, the wavefunction coherence length in PrBa2Cu3O7-d being strongly suppressed. We conclude that Pr on Ba-site substitution disorder is responsible for the metal-insulator transition. Preliminary efforts at growth with a method to prevent disorder yield 90K superconducting PrBa2Cu3O7-d crystallites.

Experimental evidence for fast cluster formation of chain oxygen vacancies in YBa2Cu3O7−δ as the origin of the fishtail anomaly

Abstract We report on three different and complementary measurements, namely magnetization measurements, positron annihilation and NMR spectroscopy, which give evidence that the formation of oxygen vacancy clusters is the origin of the fishtail anomaly in YBa 2 Cu 3 O 7− δ . In contrast to YBa 2 Cu 3 O 7.0 , where the anomaly is intrinsically absent, it can be suppressed in the optimally doped state where vacancies are present. Thus, clusters of oxygen vacancies rather than single vacancies or point defects are responsible for this anomaly.

An application of proportional chambers to the measurement of the electronic properties of solids by positron annihilation

Abstract A system has been developed to measure the two gamma ray angular correlation resulting from positron annihilation in solids using two proportional chambers equipped with high density converters. These detectors allow two coordinates of the electron momentum distribution to be resolved. This constitutes a significant progress compared to conventional techniques. The application of the determination of the Fermi surface is described and illustrated with results for two different metals (Al, Cu) and one intermetallic compound (V 3 Si).

Theoretical two-particle momentum density in YBa2Cu3O7−δ

Abstract We present calculations of the electron-positron momentum density for YBa 2 Cu 3 O 7−δ for δ = 0 and 1 respectively and for an ordered YBa 2 Cu 3 O 7 /YBa 2 Cu 3 O 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.

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

Abstract 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.

Electron momentum density of TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane) studied by Compton scattering

We have investigated the electronic structure of the quasi-one-dimensional organic metal TTF-TCNQ by measuring Compton scattering from single crystals. The measured profiles are significantly anisotropic. The directional anisotropies were compared with those derived by two different theoretical approaches. The first is the molecular orbital approximation: Compton profiles were calculated for the TTF and TCNQ molecules, separately, and then these were superposed. In spite of the simplicity of this approach, the agreement is reasonably good, implying that the electronic wave functions of TTF and TCNQ in the crystal are not very different from those of isolated molecules. The second approach is an ab initio pseudopotential band-structure calculation. The agreement is better, presumably due to the more accurate description of the crystallinity (including, for example, the inter-molecule charge transfer and band formation).

Ab initio pseudopotential calculation for (TMTSF)2ClO4

We have investigated the electronic structure of the quasi-one-dimensional organic conductor (TMTSF)2ClO4 by ab initio band calculation for the first time. Very anisotropic band dispersions and sheet-like Fermi surfaces are obtained. The present results are compared with previous tight-binding calculations. The overall agreement is good but some quantitative differences are observed. This work provides more accurate wave functions, which will be useful for estimating various physical properties and comparing with experimental results.

Anisotropy of electronic distribution in a cobalt disilicide single crystal

An experimental and theoretical analysis of cobalt disilicide Compton profiles is presented. The measurements along [100], [110] and [111] directions of single crystal CoSi2 are performed with use of synchrotron radiation (59.38 keV). There is a good overall agreement between experimental and theoretical profile anisotropies, obtained from a self consistent linear-muffin-tin-orbitals band structure calculation. The slight difference of observed structures is discussed in terms of d-type bonds.

Spin polarized positron annihilation in nickel

Abstract The two photon angular correlation of the positron annihilation radiation (ACPAR) has been studied in nickel, using polarized positrons and two-dimensional detectors (2-D). The relative spin dependent momentum density distribution in the (110) plane obtained from the measurements in two opposite directions of the magnetic field is compared with that obtained from a ferromagnetic LMTO band structure calculation.

Fermi surface ridge at second and third UMKLAPP positron annihilations in YBa2Cu3O7−δ

Abstract 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 2 Cu 3 O 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.