Searcharxiv⌕ Search

arXiv subjects

Didier Mayou

Publications and source records attributed to Didier Mayou.

At least 37 records · Page 2Linked to original sources
arXiv2014

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.

cond-mat.mes-hall↗
arXiv2013

Electronic structure and transport in approximants of the Penrose tiling

We present numerical calculations of electronic structure and transport in 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 confirmed 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).

cond-mat.mtrl-sci↗
arXiv2013

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.

cond-mat.mes-hall↗
arXiv2013

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.

cond-mat.mtrl-sci↗
arXiv2011

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.

cond-mat.mes-hall↗
arXiv2010

Breakdown of the semi-classical conduction theory in approximants of the octagonal tiling

We present numerical calculations of quantum transport in perfect octagonal approximants. These calculations include a Boltzmann (intra-band) contribution and a non-Boltzmann (inter-band) contribution. When the unit cell size of the approximant increases, the magnitude of Boltzmann terms decreases, whereas the magnitude of non-Boltzmann terms increases. It shows that, in large approximants, the non-Boltzmann contributions should dominate the transport properties of electrons. This confirms the break-down of the Bloch-Boltzmann theory to understand the transport properties in approximants with very large unit cells, and then in quasicrystals, as found in actual Al-based approximants.

cond-mat.mtrl-sci↗
arXiv2010

Quantum transport through resistive nanocontacts: Effective one-dimensional theory and conductance formulas for non-ballistic leads

We introduce a new quantum transport formalism based on a map of a real 3-dimensional lead-conductor-lead system into an effective 1-dimensional system. The resulting effective 1D theory is an in principle exact formalism to calculate the conductance. Besides being more efficient than the principal layers approach, it naturally leads to a 5-partitioned workbench (instead of 3) where each part of the device (the true central device, the ballistic and the non-ballistic leads) is explicitely treated, allowing better physical insight into the contact resistance mechanisms. Independently, we derive a generalized Fisher-Lee formula and a generalized Meir-Wingreen formula for the correlated and uncorrelated conductance and current of the system where the initial restrictions to ballistic leads are generalized to the case of resistive contacts. We present an application to graphene nanoribbons.

cond-mat.other↗
arXiv2009

Localization of Dirac electrons by Moire patterns in graphene bilayers

We study the electronic structure of two Dirac electron gazes coupled by a periodic Hamiltonian such as it appears in rotated graphene bilayers. Ab initio and tight-binding approaches are combined and show that the spatially periodic coupling between the two Dirac electron gazes can renormalize strongly their velocity. We investigate in particular small angles of rotation and show that the velocity tends to zero in this limit. The localization is confirmed by an analysis of the eigenstates which are localized essentially in the AA zones of the Moire patterns.

cond-mat.mes-hall↗
arXiv2008

Coherent electronic transport through graphene constrictions: sub-wavelength regime and optical analogies

Graphene two-dimensional nature combined with today lithography allows to achieve nanoelectronics devices smaller than the Dirac electrons wavelength. Here we show that in these graphene subwavelength nanodevices the electronic quantum transport properties present deep analogies with classical phenomena of subwavelength optics. By introducing the concept of electronic diffraction barrier to represent the effect of constrictions, we can easily describe the rich transport physics in a wealth of nanodevices: from Bethe and Kirchhoff diffraction in graphene slits, to Fabry-Perot interference oscillations in nanoribbons. The same concept applies to graphene quantum dots and gives new insigth into recent experiments on these systems.

cond-mat.mes-hall↗
arXiv2008

Graphene nanodevices: bridging nanoelectronics and subwavelength optics

The unconventional properties of graphene, with a massless Dirac band dispersion and large coherence properties, have raised a large interest for applications in nanoelectronics. In this work, we emphasize that graphene two dimensional character combined with current standard lithography processes allow to achieve devices smaller than the Dirac electrons wavelength. In this regime, we demonstrate that the electronic properties present deep analogies with subwavelength optics phenomena. We describe the rich transport physics in graphene-based nanodevices through optical analogies: From the Bethe and Kirchhoff-like diffraction patterns in the conductance of graphene slits to the Fabry-Perot oscillations of the conductance in nanoribbons. We introduce the concept of {\it electronic diffraction barriers}, which transmission cancels at the Dirac point. This gives central insight in the properties of Graphene subwavelength devices including nanoelectronics standard systems, such as quantum dots. As an application we propose a new type of quantum dots, namely functionalized subwavelength quantum dots, which could be used as molecular spin valves.

cond-mat.mes-hall↗
arXiv2008

Quenching of quantum Hall effect and the role of undoped planes in multilayered epitaxial graphene

We propose a mechanism for the quenching of the Shubnikov de Haas oscillations and the quantum Hall effect observed in epitaxial graphene. Experimental data show that the scattering time of the conduction electron is magnetic field dependent and of the order of the cyclotron orbit period, \textit{i.e.} can be much smaller than the zero field scattering time. Our scenario involves the extraordinary graphene $n=0$ Landau level of the uncharged layers that produces a high density of states at the Fermi level. We find that the coupling between this $n=0$ Landau level and the conducting states of the doped plane leads to a scattering mechanism having the right magnitude to explain the experimental data.

cond-mat.mes-hall↗
arXiv2007

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.

cond-mat.mtrl-sci↗
arXiv2006

Ab initio GW electron-electron interaction effects in Quantum Transport

We present an ab initio approach to electronic transport in nanoscale systems which includes electronic correlations through the GW approximation. With respect to Landauer approaches based on density-functional theory (DFT), we introduce a physical quasiparticle electronic-structure into a non-equilibrium Green's function theory framework. We use an equilibrium non-selfconsistent $G^0W^0$ self-energy considering both full non-hermiticity and dynamical effects. The method is applied to a real system, a gold mono-atomic chain. With respect to DFT results, the conductance profile is modified and reduced by to the introduction of diffusion and loss-of-coherence effects. The linear response conductance characteristic appear to be in agreement with experimental results.

cond-mat.mes-hall↗
arXiv2006

Quantum transport of slow charge carriers in quasicrystals and correlated systems

We show that the semi-classical model of conduction breaks down if the mean free path of charge carriers is smaller than a typical extension of their wavefunction. This situation is realized for sufficiently slow charge carriers and leads to a transition from a metallic like to an insulating like regime when scattering by defects increases. This explains the unconventional conduction properties of quasicrystals and related alloys. The conduction properties of some heavy fermions or polaronic systems, where charge carriers are also slow, present a deep analogy.

cond-mat.mtrl-sci↗
arXiv2005

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.

cond-mat.mtrl-sci↗
arXiv1999

Anomalous Quantum Diffusion and Conductivity of Quasicrystals

A phenomenological diffusion law $L(t)\propto t^β$, where L(t) measures the spreading of a wave-packet in a time t, is assumed for perfect quasicrystals. We show that it affects their conductivity with striking differences compared to the case of periodic metals. In the absence of defects the dissipative part of conductivity is non zero even at low frequencies contrary to the case of crystals. Also if $β<1/2$ the d.c. conductivity increases when disorder increases and the so-called Drude peak, characteristic of metals, is replaced by a dip . Experimental results are briefly discussed.

cond-mat↗