1,808 research outputs found
A library of ab initio Raman spectra for automated identification of 2D materials
Raman spectroscopy is frequently used to identify composition, structure and
layer thickness of 2D materials. Here, we describe an efficient
first-principles workflow for calculating resonant first-order Raman spectra of
solids within third-order perturbation theory employing a localized atomic
orbital basis set. The method is used to obtain the Raman spectra of 733
different monolayers selected from the computational 2D materials database
(C2DB). We benchmark the computational scheme against available experimental
data for 15 known monolayers. Furthermore, we propose an automatic procedure
for identifying a material based on an input experimental Raman spectrum and
illustrate it for the cases of MoS (H-phase) and WTe
(T-phase). The Raman spectra of all materials at different excitation
frequencies and polarization configurations are freely available from the C2DB.
Our comprehensive and easily accessible library of \textit{ab initio} Raman
spectra should be valuable for both theoreticians and experimentalists in the
field of 2D materialsComment: 17 pages, 7 figure
Renormalization of Optical Excitations in Molecules near a Metal Surface
The lowest electronic excitations of benzene and a set of donor-acceptor
molecular complexes are calculated for the gas phase and on the Al(111) surface
using the many-body Bethe-Salpeter equation (BSE). The energy of the
charge-transfer excitations obtained for the gas phase complexes are found to
be around 10% lower than the experimental values. When the molecules are placed
outside the surface, the enhanced screening from the metal reduces the exciton
binding energies by several eVs and the transition energies by up to 1 eV
depending on the size of the transition-generated dipole. As a striking
consequence we find that close to the metal surface the optical gap of benzene
can exceed its quasiparticle gap. A classical image charge model for the
screened Coulomb interaction can account for all these effects which, on the
other hand, are completely missed by standard time-dependent density functional
theory.Comment: 4 pages, 3 figures; revised versio
The age of 47Tuc from self-consistent isochrone fits to colour-magnitude diagrams and the eclipsing member V69
Our aim is to derive a self-consistent age, distance and composition for the
globular cluster Tucanae (Tuc; NGC104). First, we reevaluate the
reddening towards the cluster resulting in a nominal as
the best estimate. The of the components of the eclipsing binary
member V69 is found to be K from both photometric and spectroscopic
evidence. This yields a true distance modulus (random)(systematic) to Tuc when combined with existing measurements of
V69 radii and luminosity ratio. We then present a new completely
self-consistent isochrone fitting method to ground based and
cluster colour-magnitude diagrams and the eclipsing binary member V69. The
analysis suggests that the composition of V69, and by extension one of the
populations of Tuc, is given by [Fe/H], [O/Fe], and
on the solar abundance scale of Asplund, Grevesse & Sauval.
However, this depends on the accuracy of the model scale which is
50-75 K cooler than our best estimate but within measurement uncertainties. Our
best estimate of the age of Tuc is 11.8 Gyr, with firm () lower
and upper limits of 10.4 and 13.4 Gyr, respectively, in satisfactory agreement
with the age derived from the white dwarf cooling sequence if our determination
of the distance modulus is adopted.Comment: 19 pages, 8 figures, accepted for publication in MNRA
Benchmarking GW against exact diagonalization for semi-empirical models
We calculate groundstate total energies and single-particle excitation
energies of seven pi conjugated molecules described with the semi-empirical
Pariser-Parr-Pople (PPP) model using self-consistent many-body perturbation
theory at the GW level and exact diagonalization. For the total energies GW
captures around 65% of the groundstate correlation energy. The lowest lying
excitations are overscreened by GW leading to an underestimation of electron
affinities and ionization potentials by approximately 0.15 eV corresponding to
2.5%. One-shot G_0W_0 calculations starting from Hartree-Fock reduce the
screening and improve the low-lying excitation energies. The effect of the GW
self-energy on the molecular excitation energies is shown to be similar to the
inclusion of final state relaxations in Hartree-Fock theory. We discuss the
break down of the GW approximation in systems with short range interactions
(Hubbard models) where correlation effects dominate over screening/relaxation
effects. Finally we illustrate the important role of the derivative
discontinuity of the true exchange-correlation functional by computing the
exact Kohn-Sham levels of benzene.Comment: 9 pages, 5 figures, accepted for publication in Phys. Rev.
Electron transport through an interacting region: The case of a nonorthogonal basis set
The formula derived by Meir and Wingreen [Phys. Rev. Lett. {\bf 68}, 2512
(1992)] for the electron current through a confined, central region containing
interactions is generalized to the case of a nonorthogonal basis set. As in the
original work, the present derivation is based on the nonequilibrium Keldysh
formalism. By replacing the basis functions of the central region by the
corresponding elements of the dual basis, the lead- and central
region-subspaces become mutually orthogonal. The current formula is then
derived in the new basis, using a generalized version of second quantization
and Green's function theory to handle the nonorthogonality within each of the
regions. Finally, the appropriate nonorthogonal form of the perturbation series
for the Green's function is established for the case of electron-electron and
electron-phonon interactions in the central region.Comment: Added references. 8 pages, 1 figur
Spin coherence times of point defects in two-dimensional materials from first principles
The spin coherence times of 69 triplet defect centers in 45 different 2D host
materials are calculated using the cluster correlation expansion (CCE) method
with parameters of the spin Hamiltonian obtained from density functional theory
(DFT). Several of the triplets are found to exhibit extraordinarily large spin
coherence times making them interesting for quantum information processing. The
dependence of the spin coherence time on various factors, including the
hyperfine coupling strength, the dipole-dipole coupling, and the nuclear
g-factors, are systematically investigated. The analysis shows that the spin
coherence time is insensitive to the atomistic details of the defect center and
rather is dictated by the nuclear spin properties of the host material.
Symbolic regression is then used to derive a simple expression for spin
coherence time, which is validated on a test set of 55 doublet defects unseen
by the regression model. The simple expression permits order-of-magnitude
estimates of the spin coherence time without expensive first principles
calculations
Defect Tolerant Monolayer Transition Metal Dichalcogenides
Localized electronic states formed inside the band gap of a semiconductor due
to crystal defects can be detrimental to the material's optoelectronic
properties. Semiconductors with lower tendency to form defect induced deep gap
states are termed defect tolerant. Here we provide a systematic first
principles investigation of defect tolerance in 29 monolayer transition metal
dichalcogenides (TMDs) of interest for nanoscale optoelectronics. We find that
the TMDs based on group VI and X metals form deep gap states upon creation of a
chalcogen (S, Se, Te) vacancy while the TMDs based on group IV metals form only
shallow defect levels and are thus predicted to be defect tolerant.
Interestingly, all the defect sensitive TMDs have valence and conduction bands
with very similar orbital composition. This indicates a bonding/anti-bonding
nature of the gap which in turn suggests that dangling bonds will fall inside
the gap. These ideas are made quantitative by introducing a descriptor that
measures the degree of similarity of the conduction and valence band manifolds.
Finally, the study is generalized to non-polar nanoribbons of the TMDs where we
find that only the defect sensitive materials form edge states within the band
gap
'Rapid fire' spectroscopy of Kepler solar-like oscillators
The NASA Kepler mission has been continuously monitoring the same field of
the sky since the successful launch in March 2009, providing high-quality
stellar lightcurves that are excellent data for asteroseismology, far superior
to any other observations available at the present. In order to make a
meaningful analysis and interpretation of the asteroseismic data, accurate
fundamental parameters for the observed stars are needed. The currently
available parameters are quite uncertain as illustrated by e.g. Thygesen et al.
(A&A 543, A160, 2012), who found deviations as extreme as 2.0 dex in [Fe/H] and
log g, compared to catalogue values. Thus, additional follow-up observations
for these targets are needed in order to put firm limits on the parameter space
investigated by the asteroseismic modellers. Here, we propose a metod for
deriving accurate metallicities of main sequence and subgiant solar-like
oscillators from medium resolution spectra with a moderate S/N. The method
takes advantage of the additional constraints on the fundamental parameters,
available from asteroseismology and multi-color photometry. The approach
enables us to reduce the analysis overhead significantly when doing spectral
synthesis, which in turn will increases the efficiency of follow-up
observations.Comment: 3 pages, 2 figures. Proceedings from Asteroseismology of Stellar
Populations in the Milky Way 2013 to appear in 'Astrophysics and Space
Science Proceedings
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