444 research outputs found
Wannier interpolation of the electron-phonon matrix elements in polar semiconductors: Polar-optical coupling in GaAs
We generalize the Wannier interpolation of the electron-phonon matrix
elements to the case of polar-optical coupling in polar semiconductors. We
verify our methodological developments against experiments, by calculating the
widths of the electronic bands due to electron-phonon scattering in GaAs, the
prototype polar semiconductor. The calculated widths are then used to estimate
the broadenings of excitons at critical points in GaAs and the electron-phonon
relaxation times of hot electrons. Our findings are in good agreement with
available experimental data. Finally, we demonstrate that while the Fr\"ohlich
interaction is the dominant scattering process for electrons/holes close to the
valley minima, in agreement with low-field transport results, at higher
energies, the intervalley scattering dominates the relaxation dynamics of hot
electrons or holes. The capability of interpolating the polar-optical coupling
opens new perspectives in the calculation of optical absorption and transport
properties in semiconductors and thermoelectrics.Comment: To appear on Phys. Rev.
Role of Dimensionality and Size in Controlloing the Drag Seebeck Coefficient of Doped Silicon Nanostructures: A Fundamental Understanding
In this theoretical study, we examine the influence of dimensionality, size
reduction, and heattransport direction on the phonon-drag contribution to the
Seebeck coefficient of silicon nanostructures. Phonon-drag contribution arises
from the momentum transfer between out-of-equilibrium phonon populations and
charge carriers, and significantly enhances the thermoelectric coefficient. Our
implementation of the phonon drag term accounts for the anisotropy of
nanostructures such as thin films and nanowires through the boundary- and
momentum-resolved phonon lifetime. Our approach also takes into acconout the
spin-orbit coupling, which turns out to be crucial for hole transport. We
reliably quantify the phonon drag contribution at various doping levels,
temperatures, and nanostructure geometries for both electrons and holes in
silicon nanstructures. Our results support the recent experimental findings,
showing that a part of phonon drag contribution survives in 100 nm silicon
nanostructures
Catherine Lacour-Astol, Le genre de la Résistance. La Résistance féminine dans le nord de la France
L'entrée au Panthéon, le 27 mai 2015, de quatre personnalités de la Seconde Guerre mondiale, à parité entre les hommes et les femmes, devait signifier, aux yeux du Président de la République François Hollande, un changement notable dans la mise en scène de ce rituel républicain. À bien y regarder, cette forme d'innovation semble se heurter aux réalités de parcours bien plus complexes... Difficilement réductibles, en tout cas, à la trilogie traditionnelle, à laquelle il conviendrait d'ajouter ..
Disorder-induced phonon self-energy of semiconductors with binary isotopic composition
Self-energy effects of Raman phonons in isotopically disordered
semiconductors are deduced by perturbation theory and compared to experimental
data. In contrast to the acoustic frequency region, higher-order terms
contribute significantly to the self-energy at optical phonon frequencies. The
asymmetric dependence of the self-energy of a binary isotope system on the concentration of the heavier isotope mass x can be explained by
taking into account second- and third-order perturbation terms. For elemental
semiconductors, the maximum of the self-energy occurs at concentrations with
, depending on the strength of the third-order term. Reasonable
approximations are imposed that allow us to derive explicit expressions for the
ratio of successive perturbation terms of the real and the imaginary part of
the self-energy. This basic theoretical approach is compatible with Raman
spectroscopic results on diamond and silicon, with calculations based on the
coherent potential approximation, and with theoretical results obtained using
{\it ab initio} electronic theory. The extension of the formalism to binary
compounds, by taking into account the eigenvectors at the individual
sublattices, is straightforward. In this manner, we interpret recent
experimental results on the disorder-induced broadening of the TO (folded)
modes of SiC with a -enriched carbon sublattice.
\cite{Rohmfeld00,Rohmfeld01}Comment: 29 pages, 9 figures, 2 tables, submitted to PR
Atomic structure and vibrational properties of icosahedral BC boron carbide
The atomic structure of icosahedral BC boron carbide is determined by
comparing existing infra-red absorption and Raman diffusion measurements with
the predictions of accurate {\it ab initio} lattice-dynamical calculations
performed for different structural models. This allows us to unambiguously
determine the location of the carbon atom within the boron icosahedron, a task
presently beyond X-ray and neutron diffraction ability. By examining the inter-
and intra-icosahedral contributions to the stiffness we show that, contrary to
recent conjectures, intra-icosahedral bonds are harder.Comment: 9 pages including 3 figures, accepted in Physical Review Letter
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