34 research outputs found
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.
Electronic Properties of Two-Dimensional Carbon
We present a theoretical description of the electronic properties of graphene
in the presence of disorder, electron-electron interactions, and particle-hole
symmetry breaking. We show that while particle-hole asymmetry, long-range
Coulomb interactions, and extended defects lead to the phenomenon of
self-doping, local defects determine the transport and spectroscopic
properties. Our results explain recent experiments in graphitic devices and
predict new electronic behavior.Comment: 4 pages, 5 figures. The paper was originally submitted on May, 12th,
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Adatoms in Graphene
We review the problem of adatoms in graphene under two complementary points
of view, scattering theory and strong correlations. We show that in both cases
impurity atoms on the graphene surface present effects that are absent in the
physics of impurities in ordinary metals. We discuss how to observe these
unusual effects with standard experimental probes such as scanning tunneling
microscopes, and spin susceptibility.Comment: For the Proceedings of the "Graphene Week 2008" at the ICTP in
Trieste, Italy. 8 pages, 8 figure
Transport Properties through Double Barrier Structure in Graphene
The mode-dependent transmission of relativistic ballistic massless Dirac
fermion through a graphene based double barrier structure is being investigated
for various barrier parameters. We compare our results with already published
work and point out the relevance of these findings to a systematic study of the
transport properties in double barrier structures. An interesting situation
arises when we set the potential in the leads to zero, then our 2D problem
reduces effectively to a 1D massive Dirac equation with an effective mass
proportional to the quantized wave number along the transverse direction.
Furthermore we have shown that the minimal conductivity and maximal Fano factor
remain insensitive to the ratio between the two potentials V_2/V_1=\alpha.Comment: 18 pages, 12 figures, clarifications and reference added, misprints
corrected. Version to appear in JLT
Continuous-distribution puddle model for conduction in trilayer graphene
An insulator-to-metal transition is observed in trilayer graphene based on
the temperature dependence of the resistance under different applied gate
voltages. At small gate voltages the resistance decreases with increasing
temperature due to the increase in carrier concentration resulting from thermal
excitation of electron-hole pairs. At large gate voltages excitation of
electron-hole pairs is suppressed, and the resistance increases with increasing
temperature because of the enhanced electron-phonon scattering. We find that
the simple model with overlapping conduction and valence bands, each with
quadratic dispersion relations, is unsatisfactory. Instead, we conclude that
impurities in the substrate that create local puddles of higher electron or
hole densities are responsible for the residual conductivity at low
temperatures. The best fit is obtained using a continuous distribution of
puddles. From the fit the average of the electron and hole effective masses can
be determined.Comment: 18 pages, 5 figure
Quantum field theory approach to the optical conductivity of strained and deformed graphene
The computation of the optical conductivity of strained and deformed graphene is discussed within the framework of quantum field theory in curved spaces. The analytical solutions of the Dirac equation in an arbitrary static background geometry for one dimensional periodic deformations are computed, together with the corresponding Dirac propagator. Analytical expressions are given for the optical conductivity of strained and deformed graphene associated with both intra and interbrand transitions. The special case of small deformations is discussed and the result compared to the prediction of the tight-binding model.The authors acknowledge financial supportfrom the Brazilian agencies FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico)
Finite-Temperature Transport in Finite-Size Hubbard Rings in the Strong-Coupling Limit
We study the current, the curvature of levels, and the finite temperature
charge stiffness, D(T,L), in the strongly correlated limit, U>>t, for Hubbard
rings of L sites, with U the on-site Coulomb repulsion and t the hopping
integral. Our study is done for finite-size systems and any band filling. Up to
order t we derive our results following two independent approaches, namely,
using the solution provided by the Bethe ansatz and the solution provided by an
algebraic method, where the electronic operators are represented in a
slave-fermion picture. We find that, in the U=\infty case, the
finite-temperature charge stiffness is finite for electronic densities, n,
smaller than one. These results are essencially those of spinless fermions in a
lattice of size L, apart from small corrections coming from a statistical flux,
due to the spin degrees of freedom. Up to order t, the Mott-Hubbard gap is
\Delta_{MH}=U-4t, and we find that D(T) is finite for n<1, but is zero at
half-filling. This result comes from the effective flux felt by the holon
excitations, which, due to the presence of doubly occupied sites, is
renormalized to
\Phi^{eff}=\phi(N_h-N_d)/(N_d+N_h), and which is zero at half-filling, with
N_d and N_h being the number of doubly occupied and empty lattice sites,
respectively. Further, for half-filling, the current transported by any
eigenstate of the system is zero and, therefore, D(T) is also zero.Comment: 15 pages and 6 figures; accepted for PR
Exciton swapping in a twisted graphene bilayer as a solid-state realization of a two-brane model
It is shown that exciton swapping between two graphene sheets may occur under
specific conditions. A magnetically tunable optical filter is described to
demonstrate this new effect. Mathematically, it is shown that two turbostratic
graphene layers can be described as a "noncommutative" two-sheeted
(2+1)-spacetime thanks to a formalism previously introduced for the study of
braneworlds in high energy physics. The Hamiltonian of the model contains a
coupling term connecting the two layers which is similar to the coupling
existing between two braneworlds at a quantum level. In the present case, this
term is related to a K-K' intervalley coupling. In addition, the experimental
observation of this effect could be a way to assess the relevance of some
theoretical concepts of the braneworld hypothesis.Comment: 15 pages, 3 figures, final version published in European Physical
Journal
Faraday rotation in graphene
We study magneto--optical properties of monolayer graphene by means of
quantum field theory methods in the framework of the Dirac model. We reveal a
good agreement between the Dirac model and a recent experiment on giant Faraday
rotation in cyclotron resonance. We also predict other regimes when the effects
are well pronounced. The general dependence of the Faraday rotation and
absorption on various parameters of samples is revealed both for suspended and
epitaxial graphene.Comment: 10 pp; v2: typos corrected and references added, v3, v4: small
changes and more reference