116 research outputs found
UV active plasmons in alkali and alkaline earth intercalated graphene
The interband pi and pi+sigma plasmons in pristine graphene and the Dirac
plasmon in doped graphene are not applicable, since they are broad or weak, and
weakly couple to an external longitudinal or electromagnetic probe. Therefore,
the ab initio Density Function Theory is used to demonstrate that the chemical
doping of the graphene by the alkali or alkaline earth atoms dramatically
changes the poor graphene excitation spectrum in the ultra-violet frequency
range (4 - 10 eV). Four prominent modes are detected. Two of them are the
intra-layer plasmons with the square-root dispersion, characteristic for the
two-dimensional modes. The remaining two are the inter-layer plasmons, very
strong in the long-wavelength limit but damped for larger wave-vectors. The
optical absorption calculations show that the inter-layer plasmons are both
optically active, which makes these materials suitable for small organic
molecule sensing. This is particularly intriguing because the optically active
two-dimensional plasmons have not been detected in other materials
Core level spectra in XPS of pristine and doped graphene
Spectra of the C1s core hole, created in XPS and screened by electronic
excitations in pristine and doped graphene, are calculated and discussed. We
find that singular effects in the lineshapes are not possible in the pristine
graphene, and their observation should be connected with the doping. However,
the structure of the low energy excitation spectrum in the region where the
singular behaviour is expected leads to asymmetries in the core hole lineshapes
in pristine graphene similar to those in doped graphene. This makes the
analysis more complex than in the case of metals and may lead to incorrect or
incomplete interpretation of experimental results
Using surface plasmonics to turn on fullerene's dark excitons
Using our recently proposed Bethe-Salpeter formulation, we explore
the optical absorption spectra of fullerene (C) near coinage metal
surfaces (Cu, Ag, and Au). We pay special attention to how the surface plasmon
influences the optical activity of fullerene. We find the lower
energy fullerene excitons at 3.77 and 4.8 eV only weakly interact with the
surface plasmon. However, we find the surface plasmon strongly interacts with
the most intense fullerene exciton, i.e.\ the dipolar mode at
6.5 eV, and the quadrupolar mode at
6.8 eV. When fullerene is close to a copper surface
( 5.3 \AA) the dipolar mode and "localized" surface
plasmons in the molecule/surface interface hybridize to form two coupled modes
which both absorb light. As a result, the molecule gains an additional
optically active mode. Moreover, in resonance, when
, the strong interaction with the surface plasmon
destroys the quadrupolar character and it becomes an optically
active mode. In this case the molecule gains two additional very intense
optically active modes. Further, we find this resonance condition, , is satisfied by silver and gold metal surfaces.Comment: 10 pages, 8 figure
Optical absorption and conductivity in quasi-two-dimensional crystals from first principles: Application to graphene
This paper gives a theoretical formulation of the electromagnetic response of
the quasi-two-dimensional (Q2D) crystals suitable for investigation of optical
activity and polariton modes. The response to external electromagnetic field is
described by current-current response tensor calculated by
solving the Dyson equation in the random phase approximation (RPA), where
current-current interaction is mediated by the photon propagator .
The irreducible current-current response tensor is calculated
from the {\em ab initio} Kohn-Sham (KS) orbitals. The accuracy of
is tested in the long wavelength limit where it gives correct
Drude dielectric function and conductivity. The theory is applied to the
calculation of optical absorption and conductivity in pristine and doped single
layer graphene and successfully compared with previous calculations and
measurements
Changing character of electronic transitions in graphene: From single particle excitations to plasmons
In this paper we clarify the nature of and electron
excitations in pristine graphene. We clearly demonstrate the continuous
transition from single particle to collective character of such excitations and
how screening modifies their dispersion relations. We prove that and
plasmons do exist in graphene, though occurring only for a
particular range of wavevectors and with finite damping rate. The particular
attention is paid to compare the theoretical results with available EELS
measurements in optical () and other ()
limits. The conclusions, based on microscopic numerical results, are confirmed
in an approximate analytical approach
Mechanism of metallization and superconductivity suppression in YBaCuZnO revealed by Zn NQR
We measure the nuclear quadrupole resonance (NQR) signal on the Zn site in
nearly optimally doped YBaCuO, when Cu is substituted by 3\%
of isotopically pure Zn. We observe that Zn creates large insulating
islands, confirming two earlier conjectures: that doping provokes an orbital
transition in the CuO plane, which is locally reversed by Zn substitution,
and that the islands are antiferromagnetic. Also, we find that the Zn impurity
locally induces a breaking of the D symmetry. Cluster and DFT calculations
show that the D symmetry breaking is due to the same partial lifting of
degeneracy of the nearest-neighbor oxygen sites as in the LTT transition in
LaBaCuO, similarly well-known to strongly suppress
superconductivity. These results show that in-plane oxygen orbital
configurations are principally involved in the metallicity and
superconductivity of all high-T cuprates, and provide a qualitative
symmetry-based constraint on the SC mechanism.Comment: extended version, to appear in New Journal of Physic
Quasiparticle spectra and excitons of organic molecules deposited on substrates: G0W0-BSE approach applied to benzene on graphene and metallic substrates
We present an alternative methodology for calculating the quasi-particle
energy, energy loss, and optical spectra of a molecule deposited on graphene or
a metallic substrate. To test the accuracy of the method it is first applied to
the isolated benzene (C6H6) molecule. The quasiparticle energy levels and
especially the energies of the benzene excitons (triplet, singlet, optically
active and inactive) are in very good agreement with available experimental
results. It is shown that the vicinity of the various substrates
(pristine/doped graphene or (jellium) metal surface) reduces the quasiparticle
HOMO-LUMO gap by an amount that slightly depends on the substrate type. This is
consistent with the simple image theory predictions. It is even shown that the
substrate does not change the energy of the excitons in the isolated molecule.
We prove (in terms of simple image theory) that energies of the excitons are
indeed influenced by two mechanisms which cancel each other. We demonstrate
that the benzene singlet optically active (E1u) exciton couples to real
electronic excitations in the substrate. This causes it substantial decay, such
as {\Gamma} = 174 meV for pristine graphene and {\Gamma} = 362 meV for metal
surfaces as the substrate. However, we find that doping graphene does not
influence the E1u exciton decay rate.Comment: 16 pages, 14 figure
Benchmarking van der Waals functionals with noncontact RPA calculations on graphene-Ag(111)
Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).We have benchmarked long range behavior of seven different van der Waals functionals comparing them with our ACF-RPA correlation calculations for graphene on a Ag(111) system. Correlation given by the second version of van der Waals density functional vdW-DF2 agrees remarkably well with our random phase approximation (RPA) calculation in the long range region. In the intermediate and shorter range regions combining vdW-DF2 correlation with proper exchange functional becomes important. We compared the results of the van der Waals functionals in this region to the previous RPA calculations and to some extent to experimental observations, and calculated that the combined vdW-DF2(C09x) or rev-vdW-DF2 functionals show satisfactory behavior.Peer Reviewe
Benchmarking van der Waals functionals with noncontact RPA calculations on graphene-Ag(111)
We have benchmarked long range behavior of seven different van der Waals functionals comparing them with our ACF-RPA correlation calculations for graphene on a Ag(111) system. Correlation given by the second version of van der Waals density functional vdW-DF2 agrees remarkably well with our random phase approximation (RPA) calculation in the long range region. In the intermediate and shorter range regions combining vdW-DF2 correlation with proper exchange functional becomes important. We compared the results of the van der Waals functionals in this region to the previous RPA calculations and to some extent to experimental observations, and calculated that the combined vdW-DF2(C09x) or rev-vdW-DF2 functionals show satisfactory behavior
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