216 research outputs found
The phonon dispersion of graphite revisited
We review calculations and measurements of the phonon-dispersion relation of
graphite. First-principles calculations using density-functional theory are
generally in good agreement with the experimental data since the long-range
character of the dynamical matrix is properly taken into account. Calculations
with a plane-wave basis demonstrate that for the in-plane optical modes, the
generalized-gradient approximation (GGA) yields frequencies lower by 2% than
the local-density approximation (LDA) and is thus in better agreement with
experiment. The long-range character of the dynamical matrix limits the
validity of force-constant approaches that take only interaction with few
neighboring atoms into account. However, by fitting the force-constants to the
ab-initio dispersion relation, we show that the popular 4th-nearest-neighbor
force-constant approach yields an excellent fit for the low frequency modes and
a moderately good fit (with a maximum deviation of 6%) for the high-frequency
modes. If, in addition, the non-diagonal force-constant for the second-nearest
neighbor interaction is taken into account, all the qualitative features of the
high-frequency dispersion can be reproduced and the maximum deviation reduces
to 4%. We present the new parameters as a reliable basis for empirical model
calculations of phonons in graphitic nanostructures, in particular carbon
nanotubes.Comment: 26 pages, 7 figures, to appear in Solid State Com
Ab initio calculation of the peak intensity of graphene: Combined study of the laser and Fermi energy dependence and importance of quantum interference effects
We present the results of a diagrammatic, fully ab initio calculation of the
peak intensity of graphene. The flexibility and generality of our approach
enables us to go beyond the previous analytical calculations in the low-energy
regime. We study the laser and Fermi energy dependence of the peak
intensity and analyze the contributions from resonant and non-resonant
electronic transitions. In particular, we explicitly demonstrate the importance
of quantum interference and non-resonant states for the peak process. Our
method of analysis and computational concept is completely general and can
easily be applied to study other materials as well.Comment: 10 pages, 5 figure
Band structure of boron doped carbon nanotubes
We present {\it ab initio} and self-consistent tight-binding calculations on
the band structure of single wall semiconducting carbon nanotubes with high
degrees (up to 25 %) of boron substitution. Besides a lowering of the Fermi
energy into the valence band, a regular, periodic distribution of the p-dopants
leads to the formation of a dispersive ``acceptor''-like band in the band gap
of the undoped tube. This comes from the superposition of acceptor levels at
the boron atoms with the delocalized carbon -orbitals. Irregular (random)
boron-doping leads to a high concentration of hybrids of acceptor and
unoccupied carbon states above the Fermi edge.Comment: 4 pages, 2 figure
Non-adiabatic exciton-phonon coupling in Raman spectroscopy of layered materials
We present an ab initio computational approach for the calculation of
resonant Raman intensities, including both excitonic and non-adiabatic effects.
Our diagrammatic approach, which we apply to two prototype, semiconducting
layered materials, allows a detailed analysis of the impact of phonon-mediated
exciton-exciton scattering on the intensities. In the case of bulk hexagonal
boron nitride, this scattering leads to strong quantum interference between
different excitonic resonances, strongly redistributing oscillator strength
with respect to optical absorption spectra. In the case of MoS, we observe
that quantum interference effects are suppressed by the spin-orbit splitting of
the excitons.Comment: In press at Sci. Adv. Main text: 23 pages, 6 figures; Supplementary
Material: 6 pages, 2 figure
Vibrational and optical properties of MoS: from monolayer to bulk
Molybdenum disulfide, MoS2, has recently gained considerable attention as a
layered material where neighboring layers are only weakly interacting and can
easily slide against each other. Therefore, mechanical exfoliation allows the
fabrication of single and multi-layers and opens the possibility to generate
atomically thin crystals with outstanding properties. In contrast to graphene,
it has an optical gap of 1.9 eV. This makes it a prominent candidate for
transistor and opto-electronic applications. Single-layer MoS exhibits
remarkably different physical properties compared to bulk MoS due to the
absence of interlayer hybridization. For instance, while the band gap of bulk
and multi-layer MoS is indirect, it becomes direct with decreasing number
of layers. In this review, we analyze from a theoretical point of view the
electronic, optical, and vibrational properties of single-layer, few-layer and
bulk MoS. In particular, we focus on the effects of spin-orbit interaction,
number of layers, and applied tensile strain on the vibrational and optical
properties. We examine the results obtained by different methodologies, mainly
ab initio approaches. We also discuss which approximations are suitable for
MoS and layered materials. The effect of external strain on the band gap of
single-layer MoS and the crossover from indirect to direct band gap is
investigated. We analyze the excitonic effects on the absorption spectra. The
main features, such as the double peak at the absorption threshold and the
high-energy exciton are presented. Furthermore, we report on the phonon
dispersion relations of single-layer, few-layer and bulk MoS. Based on the
latter, we explain the behavior of the Raman-active and
modes as a function of the number of layers
Excitonic effects in optical absorption and electron-energy loss spectra of hexagonal boron nitride
A new interpretation of the optical and energy-loss spectra of hexagonal
boron nitride is provided based on first-principle calculations. We show that
both spectra cannot be explained by independent-particle transitions but are
strongly dominated by excitonic effects. The lowest direct and indirect gaps
are much larger than previously reported. The direct gap amounts to 6.8 eV. The
first absorption peak at 6.1 eV is due to an exciton with a binding energy of
0.7 eV. We show that this strongly bound Frenkel exciton is also responsible
for the low frequency shoulder of the pi plasmon in the energy-loss function.
Implications for nanotube studies are discussed.Comment: 5 pages, 5 figure
Raman spectra of BN-nanotubes: Ab-initio and bond-polarizability model calculations
We present it ab-initio calculations of the non-resonant Raman spectra of
zigzag and armchair BN nanotubes. In comparison, we implement a generalized
bond-polarizability model where the parameters are extracted from
first-principles calculations of the polarizability tensor of a BN sheet. For
light-polarization along the tube-axis, the agreement between model and it
ab-initio spectra is almost perfect. For perpendicular polarization,
depolarization effects have to be included in the model in order to reproduce
the it ab-initio Raman intensities.Comment: 4 pages, submitted to Phys. Rev. B rapid com
Theory of resonant Raman scattering: Toward a comprehensive \textit{ab initio} description
We develop a general, fully quantum mechanical theory of Raman scattering
from first principles in terms of many-body correlation functions. In order to
arrive at expressions that are practically useful in the context of condensed
matter physics, we adopt the Lehmann-Symanzik-Zimmermann reduction formula from
high-energy physics and formulate in the modern language of many-body
perturbation theory. This enables us to derive a general and practically useful
expression for the Raman scattering rate in terms of quantities that can be
computed \textit{ab initio}. Our work paves the way toward a comprehensive
computational approach to the calculation of Raman spectra that goes beyond the
current state of the art by capturing both excitonic and non-adiabatic effects.Comment: 19 page
- …