5,059 research outputs found
Theory of double-resonant Raman spectra in graphene: intensity and line shape of defect-induced and two-phonon bands
We calculate the double resonant (DR) Raman spectrum of graphene, and
determine the lines associated to both phonon-defect processes, and two-phonons
ones. Phonon and electronic dispersions reproduce calculations based on density
functional theory corrected with GW. Electron-light, -phonon, and -defect
scattering matrix elements and the electronic linewidth are explicitly
calculated. Defect-induced processes are simulated by considering different
kind of idealized defects. For an excitation energy of eV, the
agreement with measurements is very good and calculations reproduce: the
relative intensities among phonon-defect or among two-phonon lines; the
measured small widths of the D, , 2D and lines; the line shapes; the
presence of small intensity lines in the 1800, 2000 cm range. We
determine how the spectra depend on the excitation energy, on the light
polarization, on the electronic linewidth, on the kind of defects and on their
concentration. According to the present findings, the intensity ratio between
the and 2D lines can be used to determine experimentally the electronic
linewidth. The intensity ratio between the and lines depends on the
kind of model defect, suggesting that this ratio could possibly be used to
identify the kind of defects present in actual samples. Charged impurities
outside the graphene plane provide an almost undetectable contribution to the
Raman signal
Electron Transport and Hot Phonons in Carbon Nanotubes
We demonstrate the key role of phonon occupation in limiting the high-field
ballistic transport in metallic carbon nanotubes. In particular, we provide a
simple analytic formula for the electron transport scattering length, that we
validate by accurate first principles calculations on (6,6) and (11,11)
nanotubes. The comparison of our results with the scattering lengths fitted
from experimental I-V curves indicates the presence of a non-equilibrium
optical phonon heating induced by electron transport. We predict an effective
temperature for optical phonons of thousands Kelvin.Comment: 4 pages, 1 figur
Kohn Anomalies and Electron-Phonon Interaction in Graphite
We demonstrate that graphite phonon dispersions have two Kohn anomalies at
the Gamma-E_2g and K-A'1 modes. The anomalies are revealed by two sharp kinks.
By an exact analytic derivation, we show that the slope of these kinks is
proportional to the square of the electron-phonon coupling (EPC). Thus, we can
directly measure the EPC from the experimental dispersions. The Gamma-E_2g and
K-A'1 EPCs are particularly large, whilst they are negligible for all the other
modes at Gamma and K.Comment: 4 pages, 2 figure
Total energy global optimizations using non orthogonal localized orbitals
An energy functional for orbital based calculations is proposed, which
depends on a number of non orthogonal, localized orbitals larger than the
number of occupied states in the system, and on a parameter, the electronic
chemical potential, determining the number of electrons. We show that the
minimization of the functional with respect to overlapping localized orbitals
can be performed so as to attain directly the ground state energy, without
being trapped at local minima. The present approach overcomes the multiple
minima problem present within the original formulation of orbital based
methods; it therefore makes it possible to perform calculations for an
arbitrary system, without including any information about the system bonding
properties in the construction of the input wavefunctions. Furthermore, while
retaining the same computational cost as the original approach, our formulation
allows one to improve the variational estimate of the ground state energy, and
the energy conservation during a molecular dynamics run. Several numerical
examples for surfaces, bulk systems and clusters are presented and discussed.Comment: 24 pages, RevTex file, 5 figures available upon reques
Phonon Linewidths and Electron Phonon Coupling in Nanotubes
We prove that Electron-phonon coupling (EPC) is the major source of
broadening for the Raman G and G- peaks in graphite and metallic nanotubes.
This allows us to directly measure the optical-phonon EPCs from the G and G-
linewidths. The experimental EPCs compare extremely well with those from
density functional theory. We show that the EPC explains the difference in the
Raman spectra of metallic and semiconducting nanotubes and their dependence on
tube diameter. We dismiss the common assignment of the G- peak in metallic
nanotubes to a Fano resonance between phonons and plasmons. We assign the G+
and G- peaks to TO (tangential) and LO (axial) modes.Comment: 5 pages, 4 figures (correction in label of fig 3
The Raman Fingerprint of Graphene
Graphene is the two-dimensional (2d) building block for carbon allotropes of
every other dimensionality. It can be stacked into 3d graphite, rolled into 1d
nanotubes, or wrapped into 0d fullerenes. Its recent discovery in free state
has finally provided the possibility to study experimentally its electronic and
phonon properties. Here we show that graphene's electronic structure is
uniquely captured in its Raman spectrum that clearly evolves with increasing
number of layers. Raman fingerprints for single-, bi- and few-layer graphene
reflect changes in the electronic structure and electron-phonon interactions
and allow unambiguous, high-throughput, non-destructive identification of
graphene layers, which is critically lacking in this emerging research area
Pulsating Strings in Deformed Backgrounds
This is a brief summary on pulsating strings in beta deformed backgrounds
found recently.Comment: 8 pages. Talk presented at Quantum Theory and Symmetries 7, Prague,
August 7-13, 201
Phonon surface mapping of graphite: disentangling quasi--degenerate phonon dispersions
The two-dimensional mapping of the phonon dispersions around the point of
graphite by inelastic x-ray scattering is provided. The present work resolves
the longstanding issue related to the correct assignment of transverse and
longitudinal phonon branches at . We observe an almost degeneracy of the
three TO, LA and LO derived phonon branches and a strong phonon trigonal
warping. Correlation effects renormalize the Kohn anomaly of the TO mode, which
exhibits a trigonal warping effect opposite to that of the electronic band
structure. We determined the electron--phonon coupling constant to be
166 in excellent agreement to calculations. These results
are fundamental for understanding angle-resolved photoemission,
double--resonance Raman and transport measurements of graphene based systems
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