746 research outputs found
Spectral gap induced by structural corrugation in armchair graphene nanoribbons
We study the effects of the structural corrugation or rippling on the
electronic properties of undoped armchair graphene nanoribbons (AGNR). First,
reanalyzing the single corrugated graphene layer we find that the two
inequivalent Dirac points (DP), move away one from the other. Otherwise, the
Fermi velocity decrease by increasing rippling. Regarding the AGNRs, whose
metallic behavior depends on their width, we analyze in particular the case of
the zero gap band-structure AGNRs. By solving the Dirac equation with the
adequate boundary condition we show that due to the shifting of the DP a gap
opens in the spectra. This gap scale with the square of the rate between the
high and the wavelength of the deformation. We confirm this prediction by exact
numerical solution of the finite width rippled AGNR. Moreover, we find that the
quantum conductance, calculated by the non equilibrium Green's function
technique vanish when the gap open. The main conclusion of our results is that
a conductance gap should appear for all undoped corrugated AGNR independent of
their width.Comment: 7 pages, 5 figure
Tuning the polarized quantum phonon transmission in graphene nanoribbons
We propose systems that allow a tuning of the phonon transmission function
T() in graphene nanoribbons by using C isotope barriers, antidot
structures, and distinct boundary conditions. Phonon modes are obtained by an
interatomic fifth-nearest neighbor force-constant model (5NNFCM) and
T() is calculated using the non-equilibrium Green's function formalism.
We show that by imposing partial fixed boundary conditions it is possible to
restrict contributions of the in-plane phonon modes to T() at low
energy. On the contrary, the transmission functions of out-of-plane phonon
modes can be diminished by proper antidot or isotope arrangements. In
particular, we show that a periodic array of them leads to sharp dips in the
transmission function at certain frequencies which can be
pre-defined as desired by controlling their relative distance and size. With
this, we demonstrated that by adequate engineering it is possible to govern the
magnitude of the ballistic transmission functions T in graphene
nanoribbons. We discuss the implications of these results in the design of
controlled thermal transport at the nanoscale as well as in the enhancement of
thermo-electric features of graphene-based materials
Partially unzipped carbon nanotubes as magnetic field sensors
The conductance, , through graphene nanoribbons (GNR) connected to a
partially unzipped carbon nanotube (CNT) is studied in the presence of an
external magnetic field applied parallel to the long axis of the tube by means
of non-equilibrium Green's function technique. We consider (z)igzag and
(a)rmchair CNTs that are partially unzipped to form aGNR/zCNT/aGNR or
zGNR/aCNT/zGNR junctions. We find that the inclusion of a longitudinal magnetic
field affects the electronic states only in the CNT region, leading to the
suppression of the conductance at low energies. Unlike previous studies, for
the zGNR/aCNT/zGNR junction in zero field, we find a sharp dip in the
conductance as the energy approaches the Dirac point and we attribute this
non-trivial behavior to the peculiar band dispersion of the constituent
subsystems. We demonstrate that both types of junctions can be used as magnetic
field sensors.Comment: final version to appear in Applied Physics Letter
The role of atomic vacancies and boundary conditions on ballistic thermal transport in graphene nanoribbons
Quantum thermal transport in armchair and zig-zag graphene nanoribbons are
investigated in the presence of single atomic vacancies and subject to
different boundary conditions. We start with a full comparison of the phonon
polarizations and energy dispersions as given by a fifth-nearest-neighbor
force-constant model (5NNFCM) and by elasticity theory of continuum membranes
(ETCM). For free-edges ribbons we discuss the behavior of an additional
acoustic edge-localized flexural mode, known as fourth acoustic branch (4ZA),
which has a small gap when it is obtained by the 5NNFCM. Then, we show that
ribbons with supported-edges have a sample-size dependent energy gap in the
phonon spectrum which is particularly large for in-plane modes. Irrespective to
the calculation method and the boundary condition, the dependence of the energy
gap for the low-energy optical phonon modes against the ribbon width W is found
to be proportional to 1/W for in-plane, and 1/W for out-of-plane phonon
modes. Using the 5NNFCM, the ballistic thermal conductance and its
contributions from every single phonon mode are then obtained by the non
equilibrium Green's function technique. We found that, while edge and central
localized single atomic vacancies do not affect the low-energy transmission
function of in-plane phonon modes, they reduce considerably the contributions
of the flexural modes. On the other hand, in-plane modes contributions are
strongly dependent on the boundary conditions and at low temperatures can be
highly reduced in supported-edges samples. These findings could open a route to
engineer graphene based devices where it is possible to discriminate the
relative contribution of polarized phonons and to tune the thermal transport on
the nanoscale
Thermomechanical properties of a single hexagonal boron nitride sheet
Using atomistic simulations we investigate the thermodynamical properties of
a single atomic layer of hexagonal boron nitride (h-BN). The thermal induced
ripples, heat capacity, and thermal lattice expansion of large scale h-BN
sheets are determined and compared to those found for graphene (GE) for
temperatures up to 1000 K. By analyzing the mean square height fluctuations and the height-height correlation function we found that the h-BN
sheet is a less stiff material as compared to graphene. The bending rigidity of
h-BN: i) is about 16% smaller than the one of GE at room temperature (300 K),
and ii) increases with temperature as in GE. The difference in stiffness
between h-BN and GE results in unequal responses to external uniaxial and shear
stress and different buckling transitions. In contrast to a GE sheet, the
buckling transition of a h-BN sheet depends strongly on the direction of the
applied compression. The molar heat capacity, thermal expansion coefficient and
the Gruneisen parameter are estimated to be 25.2 J\,mol\,K,
7.2K and 0.89, respectively
Social Integration in EU Law: Content, Limits and Functions of an Elusive Notion. Introduction
Anderson impurity in the one-dimensional Hubbard model on finite size systems
An Anderson impurity in a Hubbard model on chains with finite length is
studied using the density-matrix renormalization group (DMRG) technique. In the
first place, we analyzed how the reduction of electron density from
half-filling to quarter-filling affects the Kondo resonance in the limit of
Hubbard repulsion U=0. In general, a weak dependence with the electron density
was found for the local density of states (LDOS) at the impurity except when
the impurity, at half-filling, is close to a mixed valence regime. Next, in the
central part of this paper, we studied the effects of finite Hubbard
interaction on the chain at quarter-filling. Our main result is that this
interaction drives the impurity into a more defined Kondo regime although
accompanied in most cases by a reduction of the spectral weight of the impurity
LDOS. Again, for the impurity in the mixed valence regime, we observed an
interesting nonmonotonic behavior. We also concluded that the conductance,
computed for a small finite bias applied to the leads, follows the behavior of
the impurity LDOS, as in the case of non-interacting chains. Finally, we
analyzed how the Hubbard interaction and the finite chain length affect the
spin compensation cloud both at zero and at finite temperature, in this case
using quantum Monte Carlo techniques.Comment: 9 pages, 9 figures, final version to be published in Phys. Rev.
Thermal properties of fluorinated graphene
Large scale atomistic simulations using the reactive force field approach
(ReaxFF) are implemented to investigate the thermomechanical properties of
fluorinated graphene (FG). A new set of parameters for the reactive force field
potential (ReaxFF) optimized to reproduce key quantum mechanical properties of
relevant carbon-fluor cluster systems are presented. Molecular dynamics (MD)
simulations are used to investigate the thermal rippling behavior of FG and its
mechanical properties and compare them with graphene (GE), graphane (GA) and a
sheet of BN. The mean square value of the height fluctuations and the
height-height correlation function for different system sizes and
temperatures show that FG is an un-rippled system in contrast to the thermal
rippling behavior of graphene (GE). The effective Young's modulus of a flake of
fluorinated graphene is obtained to be 273 N/m and 250 N/m for a flake of FG
under uniaxial strain along arm-chair and zig-zag direction, respectively.Comment: To appear in Phys. Rev.
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