382 research outputs found
Electronic Transport in Graphene: Quantum Effects and Role of Local Defects
In this paper we present generic properties of quantum transport in
mono-layer graphene. In the scheme of the Kubo-Geenwood formula, we compute the
square spreading of wave packets of a given energy with is directly related to
conductivity. As a first result, we compute analytically the time dependent
diffusion for pure graphene. In addition to the semi-classical term a second
term exists that is due to matrix elements of the velocity operator between
electron and hole bands. This term is related to velocity fluctuations i.e.
Zitterbewegung effect. Secondly, we study numerically the quantum diffusion in
graphene with simple vacancies and pair of neighboring vacancies (divacancies),
that simulate schematically oxidation, hydrogenation and other
functionalisations of graphene. We analyze in particular the time dependence of
the diffusion and its dependence on energy in relation with the electronic
structure. We compute also the mean free path and the semi-classical value of
the conductivity as a function of energy in the limit of small concentration of
defects.Comment: 10 pages, 5 figure
Electronic transport in AlMn(Si) and AlCuFe quasicrystals: Break-down of the semiclassical model
The semi-classical Bloch-Boltzmann theory is at the heart of our
understanding of conduction in solids, ranging from metals to semi-conductors.
Physical systems that are beyond the range of applicability of this theory are
thus of fundamental interest. It appears that in quasicrystals and related
complex metallic alloys, a new type of break-down of this theory operates. This
phenomenon is related to the specific propagation of electrons. We develop a
theory of quantum transport that applies to a normal ballistic law but also to
these specific diffusion laws. As we show phenomenological models based on this
theory describe correctly the anomalous conductivity in quasicrystals.
Ab-initio calculations performed on approximants confirm also the validity of
this anomalous quantum diffusion scheme. This provides us with an ab-initio
model of transport in approximants such as alpha-AlMnSi and AlCuFe 1/1 cubic
approximant.Comment: 11 pages, 5 figure
Numerical studies of confined states in rotated bilayers of graphene
Rotated graphene multilayers form a new class of graphene related systems
with electronic properties that drastically depend on the rotation angles. It
has been shown that bilayers behave like two isolated graphene planes for large
rotation angles. For smaller angles, states in the Dirac cones belonging to the
two layers interact resulting in the appearance of two van Hove singularities.
States become localised as the rotation angle decreases and the two van Hove
singularities merge into one peak at the Dirac energy. Here we go further and
consider bilayers with very small rotation angles. In this case, well defined
regions of AA stacking exist in the bilayer supercell and we show that states
are confined in these regions for energies in the [-\gamma_t, +\gamma_t] range
with \gamma_t the interplane mean interaction. As a consequence, the local
densities of states show discrete peaks for energies different from the Dirac
energy.Comment: 8 page
Transient localization in crystalline organic semiconductors
A relation derived from the Kubo formula shows that optical conductivity
measurements below the gap frequency in doped semiconductors can be used to
probe directly the time-dependent quantum dynamics of charge carriers. This
allows to extract fundamental quantities such as the elastic and inelastic
scattering rates, as well as the localization length in disordered systems.
When applied to crystalline organic semiconductors, an incipient electron
localization caused by large dynamical lattice disorder is unveiled, implying a
breakdown of semiclassical transport.Comment: Revised version, to appear in Phys. Rev. B Rapid Communication
Conduction mechanism and magnetotransport in multi-walled carbon nanotubes
We report on a numerical study of quantum diffusion over micron lengths in
defect-free multi-walled nanotubes. The intershell coupling allows the electron
spreading over several shells, and when their periodicities along the nanotube
axis are incommensurate, which is likely in real materials, the electronic
propagation is shown to be non ballistic. This results in magnetotransport
properties which are exceptional for a disorder free system, and provides a new
scenario to understand the experiments (A. Bachtold et al. Nature 397, 673
(1999)).Comment: 4 page
Quantum transport in flat bands and super-metallicity
Quantum physics in flat-band (FB) systems embodies a variety of exotic
phenomenon and even counter intuitive features. The quantum transport in
several graphene based compounds that exhibit a flat band and a tunable gap is
investigated. Despite the localized nature of the FB states and a zero group
velocity, a super-metallic (SM) phase at the FB energy is revealed. The SM
phase is robust against the inelastic scattering strength and controlled only
by the inter-band transitions between the FB and the dispersive bands. The SM
phase appears insensitive and quasi independent of the gap amplitude and nature
of the lattice (disordered or nano-patterned). The universal nature of the
unconventional FB transport is illustrated with the case of electrons in the
Lieb lattice
Two-dimensional electronic transport in rubrene: the impact of inter-chain coupling
Organic semi-conductors have unique electronic properties and are important
systems both at the fundamental level and also for their applications in
electronic devices. In this article we focus on the particular case of rubrene
which has one of the best electronic transport properties for application
purposes. We show that this system can be well simulated by simple
tight-binding systems representing one-dimensional (1D) chains that are weakly
coupled to their neighboring chains in the same plane. This makes in principle
this rubrene system somehow intermediate between 1D and isotropic 2D models. We
analyse in detail the dc-transport and terahertz conductivity in the 1D and in
the anisotropic 2D models. The transient localisation scenario allows us to
reproduce satisfactorily some basics results such as mobility anisotropy and
orders of magnitude as well as ac-conductivity in the terahertz range. This
model shows in particular that even a weak inter-chain coupling is able to
improve notably the propagation along the chains. This suggest also that a
strong inter-chain coupling is important to get organic semi-conductors with
the best possible transport properties for applicative purposes.Comment: 21 pages, 17 figure
Electronic transport properties of quasicrystals: a Review
We present a review of some results concerning electronic transport
properties of quasicrystals. After a short introduction to the basic concepts
of quasiperiodicity, we consider the experimental transport properties of
electrical conductivity with particular focus on the effect of temperature,
magnetic field and defects. Then, we present some heuristic approaches that
tend to give a coherent view of different, and to some extent complementary,
transport mechanisms in quasicrystals. Numerical results are also presented and
in particular the evaluation of the linear response Kubo-Greenwood formula of
conductivity in quasiperiodic systems in presence of disorder.Comment: Latex, 28 pages, Journ. of Math. Phys., Vol38 April 199
- …