arXiv subjects

Omid Faizy Namarvar

Publications and source records attributed to Omid Faizy Namarvar.

6 recordsLinked to original sources

arXiv2025

Electronic structure and transport in materials with flat bands: 2D materials and quasicrystals

In this review, we present recent works on materials whose common point is the presence of electronic bands of very low dispersion, called "flat bands", which are due to specific atomic order effects without electron interactions. These states are always indicative of some form of confinement and have consequences on the electronic properties. A first part is devoted to the cases where this confinement is due to the long-range geometry of the defect-free structure. We have thus studied periodic approximant structures of quasiperiodic Penrose and octagonal tilings, and twisted bilayers of graphene (TBG) or transition metal dichalcogenides (TMDs) whose rotation angle between the two layers assumes a special value, called "magic angle". In these materials, the flat bands correspond to electronic states distributed over a very large number of atoms (several hundreds or even thousands of atoms). We have shown that their electronic transport properties cannot be described by usual Bloch-Boltzmann theories, because the interband terms of the velocity operator dominate the intraband terms as far as quantum diffusion is concerned. In the case of TBG, flat bands can induce a magnetic state and other electron-electron correlation effects. The second part focuses on two-dimensional nanomaterials in the presence of local point defects that cause resonant electronic states (vacancies, adsorbed atoms or molecules). We present studies on monolayer graphene, twisted or Bernal bilayer graphene, carbon nanotubes, monolayer and multilayer black phosphorene, and monolayer TMDs. A recent result is the discovery that the selective functionalization of a Bernal bilayer graphene sublattice leads to a metallic or insulating behavior depending on the functionalized sublattice type. This result suggests that functionalization can be a key parameter to control the electronic properties of 2D materials.

cond-mat.mes-hall↗

arXiv2020

Electronic structure and quantum transport in twisted bilayer graphene with resonant scatterers

Staking layered materials revealed to be a very powerful method to tailor their electronic properties. It has indeed been theoretically and experimentally shown that twisted bilayers of graphene (tBLG) with a rotation angle $θ$, forming Moiré pattern, confine electrons in a tunable way as a function of $θ$. Here, we study electronic structure and transport in tBLG using tight-binding numerical calculations in commensurate twisted bilayer structures and a pertubative continuous theory, which is valid for not too small angles ($θ> \sim 2^\circ $). This two approaches allow to understand the effect of $θ$ on the local density of states, the electron lifetime due to disorder, the dc-conducitivity and the quantum correction of the conductivity due to multiple scattering effects. We distinguished the cases where disorder is equaly distributed in the two layer or only in one layer. When only one layer is disordered, diffusion properties depends strongly on $θ$, showing thus the effect of Moiré electronic localisation at intermediate angles, $\sim 2^\circ < θ< \sim 20^\circ$.

cond-mat.mes-hall↗

arXiv2018

Quantum Hamiltonian Computing protocols for molecular electronics Boolean logic gates

Quantum Hamiltonian Computing is a recent approach that uses quantum systems, in particular a single molecule, to perform computational tasks. Within this approach, we present explicit methods to construct logic gates using two different designs, where the logical outputs are encoded either at fixed energy and spatial positioning of the quantum states, or at different energies. We use these results to construct quantum Boolean adders involving a minimal number of quantum states with the two designs. We also establish a matrix algebra giving an analogy between classical Boolean logic gates and quantum ones, and assess the possibilities of both designs for more complex gates.

quant-ph↗

arXiv2016

Electronic properties of asymmetrically doped twisted graphene bilayers

Rotated graphene bilayers form an exotic class of nanomaterials with fascinating electronic properties governed by the rotation angle theta. For large rotation angles, the electron eigenstates are restricted to one layer and the bilayer behaves like two decoupled graphene layer. At intermediate angles, Dirac cones are preserved but with a lower velocity and van Hove singularities are induced at energies where the two Dirac cones intersect. At very small angles, eigenstates become localized in peculiar moire zones. We analyse here the effect of an asymmetric doping for a series of commensurate rotated bilayers on the basis of tight binding calculations of their band dispersions, density of states, participation ratio and diffusive properties. While a small doping level preserves the theta dependence of the rotated bilayer electronic structure, larger doping induces a further reduction of the band velocity in the same way of to a further reduction of the rotation angle.

cond-mat.mes-hall↗

arXiv2016

Parallel Quantum Circuit in a Tunnel Junction

The spectrum of 1-state and 2-states per line quantum buses is used to determine the effective $V_{ab}(N)$ electronic coupling between emitter and receiver states through the bus as a function of the number $N$ of parallel lines in the bus. When the calculation of $V_{ab}(N)$ is spectrally difficult, an Heisenberg-Rabi time dependent quantum exchange process can be triggered through the bus by preparing a specific initial non-stationanry state and identifying a target state to capture the effective oscillation frequency $Ω_{ab}(N)$ between those. For $Ω_{ab}(N)$ (for $V_{ab}(N)$), two different regimes are observed as a function of $N$: linear and $\sqrt{N}$ more moderate increases. This state preparation was remplaced by electronically coupling the quantum bus to two semi-infinite electrodes. The native quantum transduction process at work in this tunnel junction is not faithfully following the $Ω_{ab}(N)$ variations with $N$. Due to normalisation to unity of the electronic transparency of the quantum bus and to the low pass filter character of the transduction, large $Ω_{ab}(N)$ cannot be followed by the tunnel junction. At low coupling and when $N$ is small enough not to compensate the small through line coupling, an $N^2$ power law is preserved for $Ω_{ab}(N)$. The limitations of the quantum transduction in a tunnel junction is pointing how the broadly used concept of electrical contact between a metallic nanopad and a molecular wire can be better described as a quantum transduction process.

cond-mat.mes-hall↗

arXiv2014

Fano Interference between a STM Tip and Mid Gap States in Graphene

We analyze the STM current through electronic resonances on a substrate as a function of tip-surface distance. We show that when the tip approaches the surface a Fano hybridization can occur between the electronic resonance on the substrate and the continuum of conduction states in the STM tip. A maximum of the density of states of the electronic resonance at some energy can then lead to a dip of the STM signal $dI/dV$. Resonances in graphene, known as mid gap states, are good candidates to produce this type of Fano interference. The mid gap states can be produced by local defects or adsorbates and we analyze the cases of top and hollow configurations of adsorbates.

cond-mat.mes-hall↗