50 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.
Analytical study of non-linear transport across a semiconductor-metal junction
In this paper we study analytically a one-dimensional model for a
semiconductor-metal junction. We study the formation of Tamm states and how
they evolve when the semi-infinite semiconductor and metal are coupled
together. The non-linear current, as a function of the bias voltage, is studied
using the non-equilibrium Green's function method and the density matrix of the
interface is given. The electronic occupation of the sites defining the
interface has strong non-linearities as function of the bias voltage due to
strong resonances present in the Green's functions of the junction sites. The
surface Green's function is computed analytically by solving a quadratic matrix
equation, which does not require adding a small imaginary constant to the
energy. The wave function for the surface states is given
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
Spin-wave spectrum in La2CuO4 -- double occupancy and competing interaction effects
The recently observed spin-wave energy dispersion along the AF zone boundary
in La2CuO4 is discussed in terms of double occupancy and competing interaction
effects in the Hubbard model on a square lattice.Comment: 4 pages, 2 figure
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
Thermodynamic properties of the periodic Anderson model:X-boson treatment
We study the specific dependence of the periodic Anderson Model (PAM) in the
limit of employing the X-boson treatment in two fifferent regimes of
the PAM: the heavy fermion Kondo (HF-K) and the heavy fermion local magnetic
regime (HF-LMM). We obtain a multiple peak structure for the specific heat in
agreement with experimental results as well as the increase of the electronic
effective mass at low temperatures associated with the HF-K regime. The entropy
per site at low T tends to zero in the HF-K regime, corresponding to a singlet
ground state, and it tends to in the HF-LMM, corresponding to a
doublet ground state at each site. The linear coefficient
of the specific heat qualitatively agrees with the experimental results
obtained for differents materials in the two regimes considered here.Comment: 9 pages, 14 figure
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
Graphene based superconducting quantum point contacts
We investigate the Josephson effect in the graphene nanoribbons of length
smaller than the superconducting coherence length and an arbitrary width .
We find that in contrast to an ordinary superconducting quantum point contact
(SQPC) the critical supercurrent is not quantized for the nanoribbons
with smooth and armchair edges. For a low concentration of the carriers
decreases monotonically with lowering and tends to a constant minimum for
a narrow nanoribbon with . The minimum is zero for the
smooth edges but for the armchair edges. At higher
concentrations of the carriers this monotonic variation acquires a series of
peaks. Further analysis of the current-phase relation and the Josephson
coupling strength in terms of and the concentration of carriers
revels significant differences with those of an ordinary SQPC. On the other
hand for a zigzag nanoribbon we find that, similar to an ordinary SQPC,
is quantized but to the half-integer values .Comment: 8 pages, 5 figure