218 research outputs found
Excitonic and Quasiparticle Life Time Effects on Silicon Electron Energy Loss Spectrum from First Principles
The quasiparticle decays due to electron-electron interaction in silicon are
studied by means of first-principles all-electron GW approximation. The
spectral function as well as the dominant relaxation mechanisms giving rise to
the finite life time of quasiparticles are analyzed. It is then shown that
these life times and quasiparticle energies can be used to compute the complex
dielectric function including many-body effects without resorting to empirical
broadening to mimic the decay of excited states. This method is applied for the
computation of the electron energy loss spectrum of silicon. The location and
line shape of the plasmon peak are discussed in detail.Comment: 4 pages, 3 figures, submitted to PR
Anisotropic thermal expansion of bismuth from first principles
Some anisotropy in both mechanical and thermodynamical properties of bismuth
is expected. A combination of density functional theory total energy
calculations and density functional perturbation theory in the local density
approximation is used to compute the elastic constants at 0 K using a finite
strain approach and the thermal expansion tensor in the quasiharmonic
approximation. The overall agreement with experiment is good. Furthermore, the
anisotropy in the thermal expansion is found to arise from the anisotropy in
both the directional compressibilities and the directional Gr\"uneisen
functions.Comment: accepted for publication in PR
Dispersion corrections in graphenic systems: a simple and effective model of binding
We combine high-level theoretical and \emph{ab initio} understanding of
graphite to develop a simple, parametrised force-field model of interlayer
binding in graphite, including the difficult non-pairwise-additive
coupled-fluctuation dispersion interactions. The model is given as a simple
additive correction to standard density functional theory (DFT) calculations,
of form where is the interlayer
distance. The functions are parametrised by matching contact properties, and
long-range dispersion to known values, and the model is found to accurately
match high-level \emph{ab initio} results for graphite across a wide range of
values. We employ the correction on the difficult bigraphene binding and
graphite exfoliation problems, as well as lithium intercalated graphite
LiC. We predict the binding energy of bigraphene to be 0.27 J/m^2, and the
exfoliation energy of graphite to be 0.31 J/m^2, respectively slightly less and
slightly more than the bulk layer binding energy 0.295 J/m^2/layer. Material
properties of LiC are found to be essentially unchanged compared to the
local density approximation. This is appropriate in view of the relative
unimportance of dispersion interactions for LiC layer binding
Simulation of hydrogenated graphene Field-Effect Transistors through a multiscale approach
In this work, we present a performance analysis of Field Effect Transistors
based on recently fabricated 100% hydrogenated graphene (the so-called
graphane) and theoretically predicted semi-hydrogenated graphene (i.e.
graphone). The approach is based on accurate calculations of the energy bands
by means of GW approximation, subsequently fitted with a three-nearest neighbor
(3NN) sp3 tight-binding Hamiltonian, and finally used to compute ballistic
transport in transistors based on functionalized graphene. Due to the large
energy gap, the proposed devices have many of the advantages provided by
one-dimensional graphene nanoribbon FETs, such as large Ion and Ion/Ioff
ratios, reduced band-to-band tunneling, without the corresponding disadvantages
in terms of prohibitive lithography and patterning requirements for circuit
integration
Pressure-Induced Simultaneous Metal-Insulator and Structural-Phase Transitions in LiH: a Quasiparticle Study
A pressure-induced simultaneous metal-insulator transition (MIT) and
structural-phase transformation in lithium hydride with about 1% volume
collapse has been predicted by means of the local density approximation (LDA)
in conjunction with an all-electron GW approximation method. The LDA wrongly
predicts that the MIT occurs before the structural phase transition. As a
byproduct, it is shown that only the use of the generalized-gradient
approximation together with the zero-point vibration produces an equilibrium
lattice parameter, bulk modulus, and an equation of state that are in excellent
agreement with experimental results.Comment: 7 pages, 4 figures, submitted to Europhysics Letter
Binding and interlayer force in the near-contact region of two graphite slabs: experiment and theory
Via a novel experiment, Liu \emph{et al.} [Phys. Rev. B, {\bf 85}, 205418
(2012)] estimated the graphite binding energy, specifically the cleavage
energy, an important physical property of bulk graphite. We re-examine the data
analysis and note that within the standard Lennard-Jones model employed, there
are difficulties in achieving internal consistency in the reproduction of the
graphite elastic properties. By employing similar models which guarantee
consistency with the elastic constant, we find a wide range of model dependent
binding energy values from the same experimental data. We attribute some of the
difficulty in the determination of the binding energy to: i) limited
theoretical understanding of the van der Waals dispersion of graphite cleavage,
ii) the mis-match between the strong bending stiffness of the graphite-SiO
cantilever and the weak asymptotic inter-layer forces that are integrated over
to produce the binding energy. We find, however, that the data does support
determination of a maximum inter-layer force that is relatively model
independent. We conclude that the peak force per unit area is GPa
for cleavage, and occurs at an inter-layer spacing of nm
Huge excitonic effects in layered hexagonal boron nitride
The calculated quasiparticle band structure of bulk hexagonal boron nitride
using the all-electron GW approximation shows that this compound is an
indirect-band-gap semiconductor. The solution of the Bethe-Salpeter equation
for the electron-hole two-particle Green function has been used to compute its
optical spectra and the results are found in excellent agreement with available
experimental data. A detailed analysis is made for the excitonic structures
within the band gap and found that the excitons belong to the Frenkel class and
are tightly confined within the layers. The calculated exciton binding energy
is much larger than that obtained by Watanabe {\it et al} using a Wannier model
to interpret their experimental results and assuming that h-BN is a
direct-band-gap semiconductor.Comment: 4 pages, 3 figure
A theoretical analysis of the chemical bonding and electronic structure of graphene interacting with Group IA and Group VIIA elements
We propose a new class of materials, which can be viewed as graphene
derivatives involving Group IA or Group VIIA elements, forming what we refer to
as graphXene. We show that in several cases large band gaps can be found to
open up, whereas in other cases a semimetallic behavior is found. Formation
energies indicate that under ambient conditions, sp and mixed sp/sp
systems will form. The results presented allow us to propose that by careful
tuning of the relative concentration of the adsorbed atoms, it should be
possible to tune the band gap of graphXene to take any value between 0 and 6.4
eV.Comment: 5 pages, 4 figures. Transferred to PR
Electron correlations in MnGaAs as seen by resonant electron spectroscopy and dynamical mean field theory
After two decades from the discovery of ferromagnetism in Mn-doped GaAs, its
origin is still debated, and many doubts are related to the electronic
structure. Here we report an experimental and theoretical study of the valence
electron spectrum of Mn-doped GaAs. The experimental data are obtained through
the differences between off- and on-resonance photo-emission data. The
theoretical spectrum is calculated by means of a combination of
density-functional theory in the local density approximation and dynamical
mean-field theory (LDA+DMFT), using exact diagonalisation as impurity solver.
Theory is found to accurately reproduce measured data, and illustrates the
importance of correlation effects. Our results demonstrate that the Mn states
extend over a broad range of energy, including the top of the valence band, and
that no impurity band splits off from the valence band edge, while the induced
holes seem located primarily around the Mn impurity.Comment: 5 pages, 4 figure
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