80 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
Influence of static disorder and polaronic band formation on interfacial electron transfer in organic photovoltaic devices
Understanding the interfacial charge-separation mechanism in organic
photovoltaics requires, due to its high level of complexity, bridging between
chemistry and physics. To elucidate the charge separation mechanism, we present
a fully quantum dynamical simulation of a generic one-dimensional Hamiltonian,
which physical parameters model prototypical PCBM or acceptor
systems. We then provide microscopic evidence of the influence random static
and dynamic potentials have on the interfacial charge-injection rate. In
particular, we unveil that dynamic potentials, due to strong electron-vibration
interactions, can lead to the formation of polaronic bands. Such dynamical
potentials, when compared to random static potentials, can provide the main
detrimental influence on the efficiency of the process of interfacial
charge-separation
Conductivity of Graphene with Resonant Adsorbates: Beyond the Nearest Neighbor Hopping Model
Adsorbates on graphene can create resonances that lead to efficient electron
scattering and strongly affect the electronic conductivity. Therefore a proper
description of these resonances is important to get a good insight of their
effect on conductivity. The characteristics of the resonance and in particular
its T-matrix depend on the adsorbate itself but also on the electronic
structure of graphene. Here we show that a proper tight-binding model of
graphene which includes hopping beyond the nearest-neighbor lead to sizable
modifications of the scattering properties with respect to the mostly used
nearest neighbor hopping model. We compare results obtained with hopping beyond
the nearest-neighbor to those of our recent work Phys. Rev. Lett. 113, 146601
(2013). We conclude that the universal properties discussed in our recent work
are unchanged but that a detailed comparison with experiments require a
sufficiently precise tight-binding model of the graphene layer.Comment: 8 pages, 5 figure
Conductivity of graphene with resonant and non-resonant adsorbates
We propose a unified description of transport in graphene with adsorbates
that fully takes into account localization effects and loss of electronic
coherence due to inelastic processes. We focus in particular on the role of the
scattering properties of the adsorbates and analyze in detail cases with
resonant or non resonant scattering. For both models we identify several
regimes of conduction depending on the value of the Fermi energy. Sufficiently
far from the Dirac energy and at sufficiently small concentrations the
semi-classical theory can be a good approximation. Near the Dirac energy we
identify different quantum regimes, where the conductivity presents universal
behaviors.Comment: 6 page
Quantum transport in quasicrystals and complex metallic alloys
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. This is the case of disordered systems which
present quantum interferences in the diffusive regime, i.e. Anderson
localization effects. It appears that in quasicrystals and related complex
metallic alloys another type of breakdown of the semi-classical Bloch-Boltzmann
theory operates. This type of quantum transport is related to the specific
propagation mode of electrons in these systems. 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 experimental transport properties. Ab-initio
calculations performed on approximants confirm also the validity of this
anomalous quantum diffusion scheme. Although the present chapter focuses on
electrons in quasicrystals and complex metallic alloys, the concept that are
developed here can be useful for phonons in these systems. There is also a deep
analogy between the type of quantum transport described here and the conduction
properties of other systems where charge carriers are also slow, such as some
heavy fermions or polaronic systems.Comment: review article. 65 page
Phenomenological model for charge dynamics and optical response of disordered systems: application to organic semiconductors
We provide a phenomenological formula which describes the low-frequency
optical absorption of charge carriers in disordered systems with localization.
This allows to extract, from experimental data on the optical conductivity, the
relevant microscopic parameters determining the transport properties, such as
the carrier localization length and the elastic and inelastic scattering times.
This general formula is tested and applied here to organic semiconductors,
where dynamical molecular disorder is known to play a key role in the transport
properties. The present treatment captures the basic ideas underlying the
recently proposed transient localization scenario for charge transport,
extending it from the d.c. mobility to the frequency domain. When applied to
existing optical measurements in rubrene FETs, our analysis provides
quantitative evidence for the transient localization phenomenon. Possible
applications to other disordered electronic systems are briefly discussed.Comment: extended version with optical conductivity formulas for both
non-degenerate and degenerate electron system
Anomalous electronic transport in Quasicrystals and related Complex Metallic Alloys
We analyze the transport properties in approximants of quasicrystals
alpha-AlMnSi, 1/1-AlCuFe and for the complex metallic phase lambda-AlMn. These
phases presents strong analogies in their local atomic structures and are
related to existing quasicrystalline phases. Experimentally they present
unusual transport properties with low conductivities and a mix of metallic-like
and insulating-like characteristics. We compute the band structure and the
quantum diffusion in the perfect structure without disorder and introduce
simple approximations that allow to treat the effect of disorder. Our results
demonstrate that the standard Bloch-Boltzmann theory is not applicable to these
intermetallic phases. Indeed their dispersion relation are flat indicating
small band velocities and corrections to quantum diffusion that are not taken
into account in the semi-classical Bloch-Boltzmann scheme become dominant. We
call this regime the small velocity regime. A simple Relaxation Time
Approximation to treat the effect of disorder allows us to reproduce the main
experimental facts on conductivity qualitatively and even quantitatively.Comment: 14 page
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