9,065 research outputs found
Renormalization of the Optical Response of Semiconductors by Electron-Phonon Interaction
In the past five years enormous progress has been made in the ab initio
calculations of the optical response of electrons in semiconductors. The
calculations include the Coulomb interaction between the excited electron and
the hole left behind, as well as local field effects. However, they are
performed under the assumption that the atoms occupy fixed equilibrium
positions and do not include effects of the interaction of the lattice
vibrations with the electronic states (electron-phonon interaction). This
interaction shifts and broadens the energies at which structure in the optical
spectra is observed, the corresponding shifts being of the order of the
accuracy claimed for the ab initio calculations. These shifts and broadenings
can be calculated with various degrees of reliability using a number of
semiempirical and ab initio techniques, but no full calculations of the optical
spectra including electron-phonon interaction are available to date.
This article discusses experimental and theoretical aspects of the
renormalization of optical response functions by electron-phonon interaction,
including both temperature and isotopic mass effects. Some of the theoretical
techniques used can also be applied to analyze the renormalization of other
response functions, such as the phonon spectral functions, the lattice
parameters, and the elastic constants.Comment: Latex 2.09, 28 pages, 13 Figs., 2 Tables, submitted to Phys. Stat.
Sol.
Prediction of new sp3 silicon and germanium allotropes from the topology-based multiscale method
This article continues our recent publication [I.A. Baburin and D.M.
Proserpio and V.A. Saleev and A.V. Shipilova, Phys. Chem. Chem. Phys.17, 1332
(2015)] where we have presented a comprehensive computational study of sp3
carbon allotropes based on the topologies proposed for zeolites. Here we
predict six new silicon and six new germanium allotropes which have the same
space group symmetries and topologies as those predicted earlier for the carbon
allotropes, and study their structural, elastic, vibrational, electronic and
optical properties.Comment: 16 pages, 16 figures, 5 tables, supplementary fil
Nonorthogonal Tight-Binding Molecular Dynamics for Si(1-x)Ge(x) Alloys
We present a theoretical study of Si(1-x)Ge(x) alloys based on tight-binding molecular dynamics (TBMD) calculations. First, we introduce a new set of nonorthogonal tight-binding parameters for silicon and germanium based on the previous work by Menon and Subbaswamy [Phys. Rev. B 55, 9231 (1997); J. Phys: Condens. Matter 10, 10991 (1998)]. We then apply the method to structural analyses of Si(1-x)Ge(x) alloys. The equilibrium volume and atomic structure for a given x are obtained by the TBMD method. We also calculate the bulk modulus B, elastic constants C(11), C(12) and C(44) as a function of x. The results show that the moduli vary monotonically, but nonlinearly, between the values of Si crystal and Ge crystal. The validity of the results is also discussed
Strain and correlation of self-organized Ge_(1-x)Mn_x nanocolumns embedded in Ge (001)
We report on the structural properties of Ge_(1-x)Mn_x layers grown by
molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are
embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray
scattering, atomic force and transmission electron microscopy to study the
structural properties of the columns. We demonstrate how the elastic
deformation of the matrix (as calculated using atomistic simulations) around
the columns, as well as the average inter-column distance can account for the
shape of the diffusion around Bragg peaks.Comment: 9 pages, 7 figure
Tunability and Losses of Mid-infrared Plasmonics in Heavily Doped Germanium Thin Films
Heavily-doped semiconductor films are very promising for application in
mid-infrared plasmonic devices because the real part of their dielectric
function is negative and broadly tunable in this wavelength range. In this work
we investigate heavily n-type doped germanium epilayers grown on different
substrates, in-situ doped in the to cm range, by
infrared spectroscopy, first principle calculations, pump-probe spectroscopy
and dc transport measurements to determine the relation between plasma edge and
carrier density and to quantify mid-infrared plasmon losses. We demonstrate
that the unscreened plasma frequency can be tuned in the 400 - 4800 cm
range and that the average electron scattering rate, dominated by scattering
with optical phonons and charged impurities, increases almost linearly with
frequency. We also found weak dependence of losses and tunability on the
crystal defect density, on the inactivated dopant density and on the
temperature down to 10 K. In films where the plasma was optically activated by
pumping in the near-infrared, we found weak but significant dependence of
relaxation times on the static doping level of the film. Our results suggest
that plasmon decay times in the several-picosecond range can be obtained in
n-type germanium thin films grown on silicon substrates hence allowing for
underdamped mid-infrared plasma oscillations at room temperature.Comment: 18 pages, 10 figure
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