229 research outputs found
Excitons and Many-Electron Effects in the Optical Response of Single-Walled Boron Nitride Nanotubes
We report first-principles calculations of the effects of quasiparticle
self-energy and electron-hole interaction on the optical properties of
single-walled BN nanotubes. Excitonic effects are shown to be even more
important in BN nanotubes than in carbon nanotubes. Electron-hole interactions
give rise to complexes of bright (and dark) excitons, which qualitatively alter
the optical response. Excitons with binding energy larger than 2 eV are found
in the (8,0) BN nanotubes. Moreover, unlike the carbon nanotubes, theory
predicts that these exciton states are comprised of coherent supposition of
transitions from several different subband pairs, giving rise to novel
behaviors.Comment: 4 pages, 4 figure
Scaling of excitons in carbon nanotubes
Light emission from carbon nanotubes is expected to be dominated by excitonic
recombination. Here we calculate the properties of excitons in nanotubes
embedded in a dielectric, for a wide range of tube radii and dielectric
environments. We find that simple scaling relationships give a good description
of the binding energy, exciton size, and oscillator strength.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. Let
Temperature Dependence of the Band Gap of Semiconducting Carbon Nanotubes
The temperature dependence of the band gap of semiconducting single-wall
carbon nanotubes (SWNTs) is calculated by direct evaluation of electron-phonon
couplings within a ``frozen-phonon'' scheme. An interesting diameter and
chirality dependence of is obtained, including non-monotonic behavior
for certain tubes and distinct ``family'' behavior. These results are traced to
a strong and complex coupling between band-edge states and the lowest-energy
optical phonon modes in SWNTs. The curves are modeled by an analytic
function with diameter and chirality dependent parameters; these provide a
valuable guide for systematic estimates of for any given SWNT.
Magnitudes of the temperature shifts at 300 K are smaller than 12 meV and
should not affect assignments based on optical measurements.Comment: To appear in Phys. Rev. Let
Exciton-plasmon states in nanoscale materials: breakdown of the Tamm-Dancoff approximation
Within the Tamm-Dancoff approximation ab initio approaches describe excitons
as packets of electron-hole pairs propagating only forward in time. However, we
show that in nanoscale materials excitons and plasmons hybridize, creating
exciton--plasmon states where the electron-hole pairs oscillate back and forth
in time. Then, as exemplified by the trans-azobenzene molecule and carbon
nanotubes, the Tamm-Dancoff approximation yields errors as large as the
accuracy claimed in ab initio calculations. Instead, we propose a general and
efficient approach that avoids the Tamm--Dancoff approximation, and correctly
describes excitons, plasmons and exciton-plasmon states
First-Principles Study of Electron Linewidths in Graphene
We present first-principles calculations of the linewidths of low-energy
quasiparticles in n-doped graphene arising from both the electron-electron and
the electron-phonon interactions. The contribution to the electron linewidth
arising from the electron-electron interactions vary significantly with
wavevector at fixed energy; in contrast, the electron-phonon contribution is
virtually wavevector-independent. These two contributions are comparable in
magnitude at a binding energy of ~0.2 eV, corresponding to the optical phonon
energy. The calculated linewidths, with both electron-electron and
electron-phonon interactions included, explain to a large extent the linewidths
seen in recent photoemission experiments.Comment: 5 pages, 3 figure
Fano resonances in a three-terminal nanodevice
The electron transport through a quantum sphere with three one-dimensional
wires attached to it is investigated. An explicit form for the transmission
coefficient as a function of the electron energy is found from the first
principles. The asymmetric Fano resonances are detected in transmission of the
system. The collapse of the resonances is shown to appear under certain
conditions. A two-terminal nanodevice with an additional gate lead is studied
using the developed approach. Additional resonances and minima of transmission
are indicated in the device.Comment: 11 pages, 5 figures, 2 equations are added, misprints in 5 equations
are removed, published in Journal of Physics: Condensed Matte
Excitonic Effects on Optical Absorption Spectra of Doped Graphene
We have performed first-principles calculations to study optical absorption
spectra of doped graphene with many-electron effects included. Both self-energy
corrections and electron-hole interactions are reduced due to the enhanced
screening in doped graphene. However, self-energy corrections and excitonic
effects nearly cancel each other, making the prominent optical absorption peak
fixed around 4.5 eV under different doping conditions. On the other hand, an
unexpected increase of the optical absorbance is observed within the infrared
and visible-light frequency regime (1 ~ 3 eV). Our analysis shows that a
combining effect from the band filling and electron-hole interactions results
in such an enhanced excitonic effect on the optical absorption. These unique
variations of the optical absorption of doped graphene are of importance to
understand relevant experiments and design optoelectronic applications.Comment: 15 pages, 5 figures; Nano Lett., Article ASAP (2011
On Quantum Markov Chains on Cayley tree II: Phase transitions for the associated chain with XY-model on the Cayley tree of order three
In the present paper we study forward Quantum Markov Chains (QMC) defined on
a Cayley tree. Using the tree structure of graphs, we give a construction of
quantum Markov chains on a Cayley tree. By means of such constructions we prove
the existence of a phase transition for the XY-model on a Cayley tree of order
three in QMC scheme. By the phase transition we mean the existence of two now
quasi equivalent QMC for the given family of interaction operators
.Comment: 34 pages, 1 figur
Anomalous Quasiparticle Lifetime in Graphite: Band Structure Effects
We report ab initio calculation of quasiparticle lifetimes in graphite, as
determined from the imaginary part of the self-energy operator within the GW
aproximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above
the Fermi level presents significant deviations from the quadratic behavior
naively expected from Fermi liquid theory. The deviations are explained in
terms of the unique features of the band structure of this material. We also
discuss the experimental results from different groups and make some
predictions for future experiments.Comment: 4 pages, 4 figures, submitted PR
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