60 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
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
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
Plasmon channels in the electronic relaxation of diamond under high-order harmonics femtosecond irradiation
We used high order harmonics of a femtosecond titanium-doped sapphire system
(pulse duration 25 fs) to realise Ultraviolet Photoelectron Spectroscopy (UPS)
measurements on diamond. The UPS spectra were measured for harmonics in the
range 13 to 27. We also made ab initio calculations of the electronic lifetime
of conduction electrons in the energy range produced in the UPS experiment.
Such calculations show that the lifetime suddenly diminishes when the
conduction electron energy reaches the plasmon energy, whereas the UPS spectra
show evidence in this range of a strong relaxation mechanism with an increased
production of low energy secondary electrons. We propose that in this case the
electronic relaxation proceeds in two steps : excitation of a plasmon by the
high energy electron, the latter decaying into individual electron-hole pairs,
as in the case of metals. This process is observed for the first time in an
insulator and, on account of its high efficiency, should be introduced in the
models of laser breakdown under high intensity
First-principles GW calculations for DNA and RNA nucleobases
On the basis of first-principles GW calculations, we study the quasiparticle
properties of the guanine, adenine, cytosine, thymine, and uracil DNA and RNA
nucleobases. Beyond standard G0W0 calculations, starting from Kohn-Sham
eigenstates obtained with (semi)local functionals, a simple self-consistency on
the eigenvalues allows to obtain vertical ionization energies and electron
affinities within an average 0.11 eV and 0.18 eV error respectively as compared
to state-of-the-art coupled-cluster and multi-configurational perturbative
quantum chemistry approaches. Further, GW calculations predict the correct \pi
-character of the highest occupied state, thanks to several level crossings
between density functional and GW calculations. Our study is based on a recent
gaussian-basis implementation of GW with explicit treatment of dynamical
screening through contour deformation techniques.Comment: 5 pages, 3 figure
Infinite-layer fluoro-nickelates as model materials
We study theoretically the fluoro-nickelate series NiF ( Li, Na,
K, Rb, Cs) in the tetragonal infinite-layer structure. We use density
functional theory to determine the structural parameters and the electronic
band structure of these unprecedented compounds. Thus, we predict these
materials as model systems where the Ni oxidation is realized and
the low-energy physics is completely determined by the Ni-3 bands only.
Fluoro-nickelates of this class thus offer an ideal platform for the study of
intriguing physics that emerges out of the special electronic
configuration, notably high-temperature unconventional superconductivity.Comment: 6 pages, 4 tables, and 6 figure
On aspects of self-consistency in the Dyson-Schwinger approach to QED and \lambda (\phi^\star \phi)^2 theories
We investigate some aspects of the self-consistency in the Dyson-Schwinger
approach to both the QED and the self-interacting scalar field theories. We
prove that the set of the Dyson-Schwinger equations, together with the
Green-Ward-Takahashi identity, is equivalent to the analogous set of integral
equations studied in condensed matter, namely many-body perturbation theory,
where it is solved self-consistently and iteratively. In this framework, we
compute the non-perturbative solution of the gap equation for the
self-interacting scalar field theory.Comment: 9 pages, to appear on Phys. Rev.
The bandstructure of gold from many-body perturbation theory
The bandstructure of gold is calculated using many-body perturbation theory
(MBPT). Different approximations within the GW approach are considered.
Standard single shot G0W0 corrections shift the unoccupied bands up by ~0.2 eV
and the first sp-like occupied band down by ~0.4 eV, while leaving unchanged
the 5d occupied bands. Beyond G0W0, quasiparticle self-consistency on the
wavefunctions lowers the occupied 5d bands by 0.35 eV. Globally, many-body
effects achieve an opening of the interband gap (5d-6sp gap) of 0.35 to 0.75 eV
approaching the experimental results. Finally, the quasiparticle bandstructure
is compared to the one obtained by the widely used HSE (Heyd, Scuseria, and
Ernzerhof) hybrid functional
Transport properties of molecular junctions from many-body perturbation theory
The conductance of single molecule junctions is calculated using a Landauer
approach combined to many-body perturbation theory MBPT) to account for
electron correlation. The mere correction of the density-functional theory
eigenvalues, which is the standard procedure for quasiparticle calculations
within MBPT, is found not to affect noticeably the zero-bias conductance. To
reduce it and so improve the agreement with the experiments, the wavefunctions
also need to be updated by including the non-diagonal elements of the
self-energy operator
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