178 research outputs found
Optical absorption in small BN and C nanotubes
We present a theoretical study of the optical absorption spectrum of small
boron-nitride and carbon nanotubes using time-dependent density-functional
theory and the random phase approximation. Both for C and BN tubes, the
absorption of light polarized perpendicular to the tube-axis is strongly
suppressed due to local field effects. Since BN-tubes are wide band-gap
insulators, they only absorb in the ultra-violet energy regime, independently
of chirality and diameter. In comparison with the spectra of the single C and
BN-sheets, the tubes display additional fine-structure which stems from the
(quasi-) one-dimensionality of the tubes and sensitively depends on the
chirality and tube diameter. This fine structure can provide additional
information for the assignment of tube indices in high resolution optical
absorption spectroscopy.Comment: 5 pages, 3 figure
Reduced Density-Matrix Functional Theory: correlation and spectroscopy
In this work we explore the performance of approximations to electron
correlation in reduced density-matrix functional theory (RDMFT) and of
approximations to the observables calculated within this theory. Our analysis
focuses on the calculation of total energies, occupation numbers,
removal/addition energies, and spectral functions. We use the exactly solvable
Hubbard molecule at 1/4 and 1/2 filling as test systems. This allows us to
analyze the underlying physics and to elucidate the origin of the observed
trends. For comparison we also report the results of the approximation,
where the self-energy functional is approximated, but no further hypothesis are
made concerning the approximations of the observables. In particular we focus
on the atomic limit, where the two sites of the molecule are pulled apart and
electrons localize on either site with equal probability, unless a small
perturbation is present: this is the regime of strong electron correlation. In
this limit, using the Hubbard molecule at 1/2 filling with or without a
spin-symmetry-broken ground state, allows us to explore how degeneracies and
spin-symmetry breaking are treated in RDMFT. We find that, within the used
approximations, neither in RDMFT nor in the signature of strong
correlation are present in the spin-singlet ground state, whereas both give the
exact result for the spin-symmetry broken case. Moreover we show how the
spectroscopic properties change from one spin structure to the other. Our
findings can be generalized to other situations, which allows us to make
connections to real materials and experiment
Ab initio GW many-body effects in graphene
We present an {\it ab initio} many-body GW calculation of the self-energy,
the quasiparticle band plot and the spectral functions in free-standing undoped
graphene. With respect to other approaches, we numerically take into account
the full ionic and electronic structure of real graphene and we introduce
electron-electron interaction and correlation effects from first principles.
Both non-hermitian and also dynamical components of the self-energy are fully
taken into account. With respect to DFT-LDA, the Fermi velocity is
substantially renormalized and raised by a 17%, in better agreement with
magnetotransport experiments. Furthermore, close to the Dirac point the linear
dispersion is modified by the presence of a kink, as observed in ARPES
experiments. Our calculations show that the kink is due to low-energy single-particle excitations and to the plasmon. Finally, the GW
self-energy does not open the band gap.Comment: 5 pages, 4 figures, 1 tabl
Enhancements to the GW space-time method
We describe the following new features which significantly enhance the power
of the recently developed real-space imaginary-time GW scheme (Rieger et al.,
Comp. Phys. Commun. 117, 211 (1999)) for the calculation of self-energies and
related quantities of solids: (i) to fit the smoothly decaying time/energy
tails of the dynamically screened Coulomb interaction and other quantities to
model functions, treating only the remaining time/energy region close to zero
numerically and performing the Fourier transformation from time to energy and
vice versa by a combination of analytic integration of the tails and
Gauss-Legendre quadrature of the remaining part and (ii) to accelerate the
convergence of the band sum in the calculation of the Green's function by
replacing higher unoccupied eigenstates by free electron states (plane waves).
These improvements make the calculation of larger systems (surfaces, clusters,
defects etc.) accessible.Comment: 10 pages, 6 figure
Transforming nonlocality into frequency dependence: a shortcut to spectroscopy
Measurable spectra are theoretically very often derived from complicated
many-body Green's functions. In this way, one calculates much more information
than actually needed. Here we present an in principle exact approach to
construct effective potentials and kernels for the direct calculation of
electronic spectra. In particular, the potential that yields the spectral
function needed to describe photoemission turns out to be dynamical but {\it
local} and {\it real}. As example we illustrate this ``photoemission
potential'' for sodium and aluminium, modelled as homogeneous electron gas, and
discuss in particular its frequency dependence stemming from the nonlocality of
the corresponding self-energy. We also show that our approach leads to a very
short derivation of a kernel that is known to well describe absorption and
energy-loss spectra of a wide range of materials
Density-based mixing parameter for hybrid functionals
A very popular ab-initio scheme to calculate electronic properties in solids
is the use of hybrid functionals in density functional theory (DFT) that mixes
a portion of Fock exchange with DFT functionals. In spite of their success, a
major problem still remains, related to the use of one single mixing parameter
for all materials. Guided by physical arguments that connect the mixing
parameter to the dielectric properties of the solid, and ultimately to its band
gap, we propose a method to calculate this parameter from the electronic
density alone. This method is able to cut significantly the error of
traditional hybrid functionals for large and small gap materials, while
retaining a good description of structural properties. Moreover, its
implementation is simple and leads to a negligible increase of the
computational time.Comment: submitte
Robustness of electronic screening effects in electron spectroscopies: example of VO
In bulk and low-dimensional extended systems, the screening of excitations by
the electron cloud is a key feature governing spectroscopic properties. Widely
used computational approaches, especially in the framework of many-body
perturbation theory, such as the GW approximation and the resulting approximate
Bethe-Salpeter equation, are explicitly formulated in terms of the screened
Coulomb interaction. In the present work we explore the effect of screening in
absorption and electron energy loss spectroscopy, concentrating on the effect
of local distortions on the screening and elucidating the resulting changes in
the various spectra. Using the layered bulk oxide VO as prototype
material, we show in which way local distortions affect the screening, and in
which way changes in the screening impact electron energy loss and absorption
spectra including excitons. We highlight cancellations that make many-body
effects in the spectra very robust with respect to structural modifications,
while the band structure undergoes significant changes and the nature of the
excitations may also be affected. This yields insight concerning the
structure-properties relations that are crucial for the use of VO as
energy storage material, and more generally, that may be used to optimize the
analysis and the calculation of electronic spectra in complex materials
Approximations for many-body Green's functions: insights from the fundamental equations
Several widely used methods for the calculation of band structures and photo
emission spectra, such as the GW approximation, rely on Many-Body Perturbation
Theory. They can be obtained by iterating a set of functional differential
equations relating the one-particle Green's function to its functional
derivative with respect to an external perturbing potential. In the present
work we apply a linear response expansion in order to obtain insights in
various approximations for Green's functions calculations. The expansion leads
to an effective screening, while keeping the effects of the interaction to all
orders. In order to study various aspects of the resulting equations we
discretize them, and retain only one point in space, spin, and time for all
variables. Within this one-point model we obtain an explicit solution for the
Green's function, which allows us to explore the structure of the general
family of solutions, and to determine the specific solution that corresponds to
the physical one. Moreover we analyze the performances of established
approaches like over the whole range of interaction strength, and we
explore alternative approximations. Finally we link certain approximations for
the exact solution to the corresponding manipulations for the differential
equation which produce them. This link is crucial in view of a generalization
of our findings to the real (multidimensional functional) case where only the
differential equation is known.Comment: 17 pages, 7 figure
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