46 research outputs found
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
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
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
Electronic structure of complex spd Hume-Rothery phases in transition-metal aluminides
The discovery of quasicrystals phases and approximants in Al(rich)-Mn system
has revived the interest for complex aluminides containing transition-metal
atoms. On one hand, it is now accepted that the Hume-Rothery stabilization
plays a crucial role. On the other hand, transition-metal atoms have also a
very important effect on their stability and their physical properties. In this
paper, we review studies that unifies the classical Hume-Rothery stabilization
for sp electron phases with the virtual bound state model for transition-metal
atoms embedded in the aluminum matrix. These studies lead to a new theory for
\"spd electron phases\". It is applied successfully to Al(Si)--transition-metal
alloys and it gives a coherent picture of their stability and physical
properties. These works are based on first-principles calculations of the
electronic structure and simplified models, compared to experimental results. A
more detailed review article is published in Prog. Mater. Sci. 50 (2005) p.
679-788
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 Structure and Transport in Approximants of the Penrose Tiling
Proceedings of the 12th International Conference on Quasicrystal. 4 pagesInternational audienceWe present numerical calculations of electronic structure and transport in the Penrose approximants. The electronic structure of perfect approximants shows a spiky density of states and a tendency to localization that is more pronounced in the middle of the band. Near the band edges the behavior is more similar to that of free electrons. These calculations of band structure and in particular the band scaling suggest an anomalous quantum diffusion when compared to normal ballistic crystals. This is con firmed by a numerical calculation of quantum diffusion which shows a crossover from normal ballistic propagation at long times to anomalous, possibly insulator-like, behavior at short times. The time scale t∗(E) for this crossover is computed for several approximants and is detailed. The consequences for electronic conductivity are discussed in the context of the relaxation time approximation. The metallic-like or non-metallic-like behavior of the conductivity is dictated by the comparison between the scattering time due to defects and the time scale t∗(E)
Electronic localization in twisted bilayer MoS with small rotation angle
Moir\'e patterns are known to confine electronic states in transition metal
dichalcogenide bilayers, thus generalizing the notion of magic angles
discovered in twisted bilayer graphene to semiconductors. Here, we present a
revised Slater-Koster tight-binding model that facilitates the first reliable
and systematic studies of such states in twisted bilayer MoS for the whole
range of rotation angles . We show that isolated bands appear at low
energy for . Moreover, these bands become
"flatbands", characterized by a vanishing average velocity, for the smallest
angles .Comment: 13 pages, 14 figure
Atomic relaxation and electronic structure in twisted bilayer MoS2 with rotation angle of 5.09 degrees
It is now well established theoretically and experimentally that a moir\'e
pattern, due to a rotation of two atomic layers with respect to each other,
creates low-energy flat bands. First discovered in twisted bilayer graphene,
these new electronic states are at the origin of strong electronic correlations
and even of unconventional superconductivity. Twisted bilayers (tb) of
transition metal dichalcogenides (TMDs) also exhibit flat bands around their
semiconductor gap at small rotation angles. In this paper, we present a DFT
study to analyze the effect of the atomic relaxation on the low-energy bands of
tb-MoS2 with a rotation angle of 5.09 degrees. We show that in-plane atomic
relaxation is not essential here, while out-of-plane relaxation dominates the
electronic structure. We propose a simple and efficient atomic model to predict
this relaxation.Comment: 5 pages, 4 figures. arXiv admin note: text overlap with
arXiv:2005.1305
Quantum localization and electronic transport in covalently functionalized carbon nanotubes
Carbon nanotubes are of central importance for applications in
nano-electronics thanks to their exceptional transport properties. They can be
used as sensors, for example in biological applications, provided that they are
functionalized to detect specific molecules. Due to their one-dimensional
geometry the carbon nanotubes are very sensitive to the phenomenon of Anderson
localization and it is therefore essential to know how the functionalization
modifies their conduction properties and if they remain good conductors. Here
we present a study of the quantum localization induced by functionalization in
metallic single walled carbon nanotubes (SWCNT) with circumferences up to . We consider resonant and non-resonant adsorbates that represent two
types of covalently functionalized groups with moderate and strong scattering
properties. The present study provides a detailed analysis of the localization
behaviour and shows that the localization length can decrease down to at concentrations of about 1 percent of adsorbates. On this basis we
discuss the possible electronic transport mechanisms which can be either
metallic like or insulating like with variable range hopping