R.N. West

Bio: R.N. West is an academic researcher from University of Texas at Arlington. The author has contributed to research in topics: Fermi surface & Positron. The author has an hindex of 12, co-authored 34 publications receiving 441 citations.

Direct observation of Fermi surface in YBa2Cu3O7- delta.

Abstract: We have performed a high-precision measurement (5{times}10{sup 8} coincidence counts) of the basal-plane electron-positron momentum density in well oxygenated, twin-free, single crystals of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. The raw, processed, and {bold 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 {Gamma}-{ital X} electron ridge section associated with states in the CuO chains.

Direct observation and calipering of the 'Webbing' Fermi surface of Yttrium

Abstract: The first measurement of both the size and shape of the region of the Fermi surface of yttrium known as the {open_quotes}webbing{close_quotes} is reported. This particular Fermi surface feature is of considerable interest because it is very similar to that found in a number of the heavier rare earth metals, where it is believed to play a vital role in driving the exotic magnetic structures found therein. In this positron study, two-dimensional angular correlation measurements combined with three-dimensional reconstruction provide a direct image of this part of the yttrium Fermi surface. {copyright} {ital 1997} {ital The American Physical Society}

Angular correlation studies of positron annihilation in the noble gases

Abstract: 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.

Evidence for phonon-assisted positronium emission from graphite.

Abstract: We observe positronium (Ps) emission from a graphite surface being bombarded by slow positrons. The band structure of graphite does not support the usual one-electron-hole process for Ps formation because of conservation of momentum parallel to the surface. A large temperature coefficient for the emission of energetic Ps suggests a new mechanism in which momentum conservation is satisfied by the emission and absorption of phonons. A simple theory including one-phonon processes explains the temperature dependence as well as the angular distribution of the Ps.

Fermi surface of the colossal magnetoresistance perovskite La0.7Sr0.3MnO3

Abstract: Materials that exhibit colossal magnetoresistance (CMR) are currently the focus of an intense research effort, driven by the technological applications that their properties suggest. Using the angular correlation of photons from electron-positron annihilation, we present a first view of the Fermi surface of a material that exhibits CMR, supported by `virtual crystal' electronic structure calculations. The Fermi surface is shown to be sufficiently cubic in nature that it is likely to support nesting.

Extrinsic magnetotransport phenomena in ferromagnetic oxides

Abstract: Magnetic oxides show a variety of extrinsic magnetotransport phenomena: grain-boundary, tunnelling and domain-wall magnetoresistance. In view of these phenomena the role of some magnetic oxides is outstanding: these are believed to be half-metallic having only one spin-subband at the Fermi level. These fully spin-polarized oxides have great potential for applications in spin-electronic devices and have, accordingly, attracted intense research activity in recent years. This review is focused on extrinsic magnetotransport effects in ferromagnetic oxides. It consists of two parts; the second part is devoted to an overview of experimental data and theoretical models for extrinsic magnetotransport phenomena. Here a critical discussion of domain-wall scattering is given. Results on surface and interfacial magnetism in oxides are presented. Spin-polarized tunnelling in ferromagnetic junctions is reviewed and grain-boundary magnetoresistance is interpreted within a model of spin-polarized tunnelling through natural oxide barriers. The situation in ferromagnetic oxides is compared with data and models for conventional ferromagnets. The first part of the review summarizes basic material properties, especially data on the spin polarization and evidence for half-metallicity. Furthermore, intrinsic conduction mechanisms are discussed. An outlook on the further development of oxide spin-electronics concludes this review.

Stress focusing in elastic sheets

Abstract: This paper reviews recent progress in understanding phenomena such as crumpling, in which elastic membranes or sheets subject to structureless forces develop sharply curved structure over a small fraction of their surface. In the limit of zero thickness, these structures become singular. After reviewing several related phenomena, the paper recalls the physical elements that give rise to the singular behavior: elasticity and the nearly inextensible behavior of thin sheets. This singular behavior has counterparts in higher dimensions. Then the paper discusses the most basic of these singularities, the vertex. The paper recounts mathematical progress in describing the $d$-cone, a simple realization of a vertex. After discussing the size of the core that governs departure from singularity, the paper concludes that fundamental understanding is lacking. It points out further mysterious behavior at the region where a $d$-cone is supported. Next comes a discussion of an emergent singularity that appears when two or more vertices are present: the stretching ridge. The paper offers several explanations of the scale of this singularity, ranging from qualitative scaling arguments to a formal asymptotic analysis. It discusses recent experiments and theories about the interaction of ridges and vertices and reviews evidence that these ridges dominate the mechanics of crumpled sheets.

Low-energy positron interactions with atoms and molecules

Abstract: This paper is a review of low-energy positron interactions with atoms and molecules. Processes of interest include elastic scattering, electronic and vibrational excitation, ionization, positronium formation and annihilation. An overview is presented of the currently available theoretical and experimental techniques to study these phenomena, including the use of trap-based positron beam sources to study collision processes with improved energy resolution. State-resolved measurements of electronic and vibrational excitation cross sections and measurement of annihilation rates in atoms and molecules as a function of incident positron energy are discussed. Where data are available, comparisons are made with analogous electron scattering cross sections. Resonance phenomena, common in electron scattering, appear to be less common in positron scattering. Possible exceptions include the sharp onsets of positron-impact electronic and vibrational excitation of selected molecules. Recent energy-resolved studies of positron annihilation in hydrocarbons containing more than a few carbon atoms provide direct evidence that vibrational Feshbach resonances underpin the anomalously large annihilation rates observed for many polyatomic species. We discuss open questions regarding this process in larger molecules, as well as positron annihilation in smaller molecules where the theoretical picture is less clear.