This chapter provides an overview of quasiparticle calculations in solids, and, in particular, the GW approximation (GWA). A successful approximation for the determination of excited states of solids is based on the quasiparticle concept and the Green function method. The Coulomb repulsion between electrons leads to a depletion of negative charge around a given electron, and the ensemble of this electron and its surrounding positive screening charge forms a quasiparticle. The mathematical description of quasiparticles is based on the single-particle Green function (G), whose exact determination requires complete knowledge of the quasiparticle self-energy ∑. The self-energy ∑ is a non-Hermitian, energy-dependent, nonlocal operator that describes exchange and correlation effects beyond the Hartree approximation. A determination of the self-energy can only be approximate, and a working scheme for the quantitative calculation of excitation energies in metals, semiconductors, and insulators is the so-called dynamically screened interaction or the GWA. The chapter also discusses (1) the physics and extensions of the GWA; (2) numerical aspects of GWA calculations; (3) applications of the GWA to semiconductors, insulators and metals; and (4) the relevance of GWA calculations to optical response. Finally, the chapter presents parallel algorithms both for reciprocal and real-space/imaginary-time GWA calculations and several alternative methods to determine excited states of solids within density functional theory.

Quasiparticle calculations in solids

Good thermoelectric materials are often complex compounds. Here, the authors reveal that elemental tellurium has a high thermoelectric figure of merit between 300 and 700 K when doped with As, with the potential advantages of easy preparation and relative isotropy.

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Tellurium as a high-performance elemental thermoelectric.

We report on an inelastic (Raman) light scattering study of the local structure of amorphous GeTe (a-GeTe) films. A detailed analysis of the temperature-reduced Raman spectra has shown that appreciable structural changes occur as a function of temperature. These changes involve modifications of atomic arrangements such as to facilitate the rapid amorphous to crystal transformation, which is the major advantage of phase-change materials used in optical data storage media. A particular structural model, supported by polarization analysis, is proposed which is compatible with the experimental data as regards both the structure of a-GeTe and the crystallization transition. The remarkable difference between the Raman spectrum of the crystal and the glass can thus naturally be accounted for.

https://iopscience.iop.org/article/10.1088/0953-8984/18/3/014/pdf

Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous to crystal transition

Single crystalline nanorods and nanowires of t-Te have been prepared by a simple solution route. The procedure involves the disproportionation of NaHTe, prepared by the reduction of Te with NaBH4. By carefully controlling the reaction conditions, the diameter of the nanorods could be varied in the 20–300 nm range. Nanowires of 10 nm diameter were obtained in the presence of sodium dodecylbenzenesulfonate. Te nanobelts and nano junctions were obtained by employing hydrothermal and solvothermal conditions. The nanorods have been characterized by a variety of microscopic and spectroscopic techniques. UV-Visible spectra reveal two absorption bands, one around 300 nm which is size-sensitive and the other at 600 nm insensitive to size.

Controlled synthesis of crystalline tellurium nanorods, nanowires, nanobelts and related structures by a self-seeding solution process

Recently developed direct conversion flat-panel X-ray image detectors provide not only potentially superior images but also enable a simple and convenient means of achieving digital radiography. The present flat-panel sensors use stabilized a-Se as an X-ray photoconductor to convert absorbed X-ray photons to collectable charge carriers. The present paper outlines the basics of the direct conversion flat-panel X-ray detector, formulates and reviews the required X-ray photoconductor properties and examines to what extent stabilized amorphous selenium (a-Se) fulfills these requirements and how it compares with other potential X-ray photoconductors that have been proposed. Charge transport and the electron–hole pair creation energy in stabilized a-Se are reviewed within the context of our present knowledge. Current materials problems and future potential developments are also critically discussed.

P. Grosse

Papers

The Physics of Selenium and Tellurium