514 research outputs found
Luttinger liquids with boundaries: Power-laws and energy scales
We present a study of the one-particle spectral properties for a variety of
models of Luttinger liquids with open boundaries. We first consider the
Tomonaga-Luttinger model using bosonization. For weak interactions the boundary
exponent of the power-law suppression of the spectral weight close to the
chemical potential is dominated by a term linear in the interaction. This
motivates us to study the spectral properties also within the Hartree-Fock
approximation. It already gives power-law behavior and qualitative agreement
with the exact spectral function. For the lattice model of spinless fermions
and the Hubbard model we present numerically exact results obtained using the
density-matrix renormalization-group algorithm. We show that many aspects of
the behavior of the spectral function close to the boundary can again be
understood within the Hartree-Fock approximation. For the repulsive Hubbard
model with interaction U the spectral weight is enhanced in a large energy
range around the chemical potential. At smaller energies a power-law
suppression, as predicted by bosonization, sets in. We present an analytical
discussion of the crossover and show that for small U it occurs at energies
exponentially (in -1/U) close to the chemical potential, i.e. that bosonization
only holds on exponentially small energy scales. We show that such a crossover
can also be found in other models.Comment: 16 pages, 9 figures included, submitted for publicatio
Universal asymptotic behavior in flow equations of dissipative systems
Based on two dissipative models, universal asymptotic behavior of flow
equations for Hamiltonians is found and discussed. Universal asymptotic
behavior only depends on fundamental bath properties but not on initial system
parameters, and the integro-differential equations possess an universal
attractor. The asymptotic flow of the Hamiltonian can be characterized by a
non-local differential equation which only depends on one parameter -
independent of the dissipative system or truncation scheme. Since the fixed
point Hamiltonian is trivial, the physical information is completely
transferred to the transformation of the observables. This yields a more stable
flow which is crucial for the numerical evaluation of correlation functions.
Furthermore, the low energy behavior of correlation functions is determined
analytically. The presented procedure can also be applied if relevant
perturbations are present as is demonstrated by evaluating dynamical
correlation functions for sub-Ohmic environments. It can further be generalized
to other dissipative systems.Comment: 15 pages, 9 figures; to appear in Phys. Rev.
Efficient graphene-based photodetector with two cavities
We present an efficient graphene-based photodetector with two Fabri-P\'erot
cavities. It is shown that the absorption can reach almost 100% around a given
frequency, which is determined by the two-cavity lengths. It is also shown that
hysteresis in the absorbance is possible, with the transmittance amplitude of
the mirrors working as an external driving field. The role of non-linear
contributions to the optical susceptibility of graphene is discussed.Comment: 10 pages, 8 figures. published version: minor revisio
Effect of Holstein phonons on the optical conductivity of gapped graphene
We study the optical conductivity of a doped graphene when a sublattice
symmetry breaking is occurred in the presence of the electron-phonon
interaction. Our study is based on the Kubo formula that is established upon
the retarded self-energy. We report new features of both the real and imaginary
parts of the quasiparticle self-energy in the presence of a gap opening. We
find an analytical expression for the renormalized Fermi velocity of massive
Dirac Fermions over broad ranges of electron densities, gap values and the
electron-phonon coupling constants. Finally we conclude that the inclusion of
the renormalized Fermi energy and the band gap effects are indeed crucial to
get reasonable feature for the optical conductivity.Comment: 12 pages, 4 figures. To appear in Eur. Phys. J.
Dynamical polarization of graphene at finite doping
The polarization of graphene is calculated exactly within the random phase
approximation for arbitrary frequency, wave vector, and doping. At finite
doping, the static susceptibility saturates to a constant value for low
momenta. At it has a discontinuity only in the second derivative.
In the presence of a charged impurity this results in Friedel oscillations
which decay with the same power law as the Thomas Fermi contribution, the
latter being always dominant. The spin density oscillations in the presence of
a magnetic impurity are also calculated. The dynamical polarization for low
and arbitrary is employed to calculate the dispersion relation and
the decay rate of plasmons and acoustic phonons as a function of doping. The
low screening of graphene, combined with the absence of a gap, leads to a
significant stiffening of the longitudinal acoustic lattice vibrations.Comment: 17 pages, 6 figures, 1 tabl
Plasmons in layered structures including graphene
We investigate the optical properties of layered structures with graphene at
the interface for arbitrary linear polarization at finite temperature including
full retardation by working in the Weyl gauge. As a special case, we obtain the
full response and the related dielectric function of a layered structure with
two interfaces. We apply our results to discuss the longitudinal plasmon
spectrum of several single and double layer devices such as systems with finite
and zero electronic densities. We further show that a nonhomogeneous dielectric
background can shift the relative weight of the in-phase and out-of-phase mode
and discuss how the plasmonic mode of the upper layer can be tuned into an
acoustic mode with specific sound velocity.Comment: 18 pages, 6 figure
Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2
The linear dispersion relation in graphene[1,2] gives rise to a surprising
prediction: the resistivity due to isotropic scatterers (e.g. white-noise
disorder[3] or phonons[4-8]) is independent of carrier density n. Here we show
that acoustic phonon scattering[4-6] is indeed independent of n, and places an
intrinsic limit on the resistivity in graphene of only 30 Ohm at room
temperature (RT). At a technologically-relevant carrier density of 10^12 cm^-2,
the mean free path for electron-acoustic phonon scattering is >2 microns, and
the intrinsic mobility limit is 2x10^5 cm^2/Vs, exceeding the highest known
inorganic semiconductor (InSb, ~7.7x10^4 cm^2/Vs[9]) and semiconducting carbon
nanotubes (~1x10^5 cm^2/Vs[10]). We also show that extrinsic scattering by
surface phonons of the SiO2 substrate[11,12] adds a strong temperature
dependent resistivity above ~200 K[8], limiting the RT mobility to ~4x10^4
cm^2/Vs, pointing out the importance of substrate choice for graphene
devices[13].Comment: 16 pages, 3 figure
Dirac electrons in graphene-based quantum wires and quantum dots
In this paper we analyse the electronic properties of Dirac electrons in
finite-size ribbons and in circular and hexagonal quantum dots made of
graphene.Comment: Contribution for J. Phys.: Cond. Mat. special issue on graphene
physic
Entanglement in Many-Body Systems
The recent interest in aspects common to quantum information and condensed
matter has prompted a prosperous activity at the border of these disciplines
that were far distant until few years ago. Numerous interesting questions have
been addressed so far. Here we review an important part of this field, the
properties of the entanglement in many-body systems. We discuss the zero and
finite temperature properties of entanglement in interacting spin, fermionic
and bosonic model systems. Both bipartite and multipartite entanglement will be
considered. At equilibrium we emphasize on how entanglement is connected to the
phase diagram of the underlying model. The behavior of entanglement can be
related, via certain witnesses, to thermodynamic quantities thus offering
interesting possibilities for an experimental test. Out of equilibrium we
discuss how to generate and manipulate entangled states by means of many-body
Hamiltonians.Comment: 61 pages, 29 figure
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
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