arXiv subjects

Ahmed Jellal

Publications and source records attributed to Ahmed Jellal.

At least 145 records · Page 8Linked to original sources

arXiv2015

Goos-Hänchen Shifts in AA-Stacked Bilayer Graphene Superlattices

The quantum Goos-Hänchen shifts of the transmitted electron beam through an AA-stacked bilayer graphene superlattices is investigated. We found that the band structures of graphene superlattices can have more than one Dirac point, their locations do not depend on the number of barriers. It was revealed that any $n$-barrier structure is perfectly transparent at normal incidence around the Dirac points created in the superlattices. We showed that the Goos-Hänchen shifts display sharp peaks inside the transmission gap around two Dirac points ($E= V_B + τ$, $E= V_W + τ$), which are equal to those of transmission resonances. The obtained Goos-Hänchen shifts are exhibiting negative as well as positive behaviors and strongly depending on the location of Dirac points. It is observed that the maximum absolute values of the shifts increase as long as the number of barriers is increased. Our analysis is done by considering four cases: single, double barriers, superlattices without and with defect.

cond-mat.mes-hall↗

arXiv2015

Periodic Barrier Structure in AA-Stacked Bilayer Graphene

We study the charge carriers transport in an AA-stacked bilayer graphene modulated by a lateral one-dimensional multibarrier structure. We investigate the band structures of our system, that is made up of two shifted Dirac cones, for finite and zero gap. We use the boundary conditions to explicitly determine the transmission probability of each individual cone ($τ=\pm 1$) for single, double and finite periodic barrier structure. We find that the Klein tunneling is only possible when the band structure is gapless and can occur at normal incidence as a result of the Dirac nature of the quasiparticles. We observe that the band structure of the {barriers} can have more than one Dirac points for finite periodic barrier. The resonance peaks appear in the transmission probability, which correspond to the positions of new cones index like associated with $τ=\pm 1$. Two conductance channels through different cones ($τ=\pm 1$) are found where the total conductance has been studied and compared to the cases of single layer and AB-stacked bilayer graphene.

cond-mat.mes-hall↗

arXiv2015

Transmission and Goos-Hänchen like Shifts through a Graphene Double Barrier in an Inhomogeneous Magnetic Field

We studied the transport properties of electrons in graphene as they are scattered by a double barrier potential in the presence of an inhomogeneous magnetic field. We computed the transmission coefficient and Goos-Hänchen like shifts for our system and noticed that transmission is not allowed for certain range of energies. In particular, we found that, in contrast to the electrostatic barriers, the magnetic barriers are able to confine Dirac fermions. We also established some correlation between the electronic transmission properties of Dirac fermions with the Goos-Hänchen like shifts, as reflected in the numerical data.

cond-mat.mes-hall↗

arXiv2015

Gate-Tunable Graphene Quantum Dot and Dirac Oscillator

We obtain the solution of the Dirac equation in (2+1) dimensions in the presence of a constant magnetic field normal to the plane together with a two-dimensional Dirac-oscillator potential coupling. We study the energy spectrum of graphene quantum dot (QD) defined by electrostatic gates. We give discussions of our results based on different physical settings, whether the cyclotron frequency is similar or larger/smaller compared to the oscillator frequency. This defines an effective magnetic field that produces the effective quantized Landau levels. We study analytically such field in gate-tunable graphene QD and show that our structure allow us to control the valley degeneracy. Finally, we compare our results with already published work and also discuss the possible applications of such QD.

cond-mat.mes-hall↗

arXiv2015

Double Barriers and Magnetic Field in Bilayer Graphene

We study the transmission probability in an AB-stacked bilayer graphene of Dirac fermions scattered by a double barrier structure in the presence of a magnetic field. We take into account the full four bands of the energy spectrum and use the boundary conditions to determine the transmission probability. Our numerical results show that for energies higher than the interlayer coupling, four ways for transmission probabilities are possible while for energies less than the height of the barrier, Dirac fermions exhibits transmission resonances and only one transmission channel is available. We show that, for AB-stacked bilayer graphene, there is no Klein tunneling at normal incident. We find that the transmission displays sharp peaks inside the transmission gap around the Dirac point within the barrier regions while they are absent around the Dirac point in the well region. The effect of the magnetic field, interlayer electrostatic potential and various barrier geometry parameters on the transmission probabilities are also discussed.

cond-mat.mes-hall↗

arXiv2015

Integer Quantum Hall Effect in Graphene

We study the quantum Hall effect in a monolayer graphene by using an approach based on thermodynamical properties. This can be done by considering a system of Dirac particles in an electromagnetic field and taking into account of the edges effect as a pseudo-potential varying continuously along the $x$ direction. At low temperature and in the weak electric field limit, we explicitly determine the thermodynamical potential. With this, we derive the particle numbers in terms of the quantized flux and therefore the Hall conductivity immediately follows.

cond-mat.mes-hall↗

arXiv2015

Transmissions in Graphene through Double Barriers and Periodic Potential

Transmission of Dirac fermions through a chip of graphene under the effect of magnetic field and a time vibrating double barrier with frequency $w$ is investigated. Quantum interference within the oscillating barrier has an important effect on quasi-particles tunneling. A combination of both a time dependent potential and a magnetic field generate physical states whose energy is double quantified by the pair of integers $(n, l)$ with high degeneracy. The large number of modes that exist in the energy spectrum presents a colossal difficulty in numerical computations. Thus we were obliged to make a truncation and limit ourselves to the central $(n = 0)$ and two adjacent side band ($n=\pm 1$).

cond-mat.mes-hall↗

arXiv2014

Factorization of Dirac Equation and Graphene Quantum Dot

We consider a quantum dot described by a cylindrically symmetric 2D Dirac equation. The potentials representing the quantum dot are taken to be of different types of potential configuration, scalar, vector and pseudo-scalar to enable us to enrich our study. Using various potential configurations, we found that in the presence of a mass term an electrostatically confined quantum dot can accommodate true bound states, which is in agreement with previous work. The differential cross section associated with one specific potential configuration has been computed and discussed as function of the various potential parameters.

cond-mat.mes-hall↗

arXiv2014

Goos-Hänchen like Shifts for Graphene Barrier in Constant Magnetic Field

We consider a system of Dirac fermions in graphene submitted to a constant perpendicular magnetic field and scattered by a barrier potential. We show that our system can be used to establish a link with quantum optics through the Goos-Hänchen shifts. This can be done by evaluating the corresponding transmission probability and shift phase. We obtain Goos-Hänchen like shifts in terms of different physical parameters such as energy, electrostatic potential strength and magnetic field. On the light of this relation, we discuss the obtained results and make comparison with literature.

cond-mat.mes-hall↗

arXiv2014

Tunneling of Massive Dirac Fermions in Graphene through Time-periodic Potential

The energy spectrum of graphene sheet with a single barrier structure having a time periodic oscillating height and subjected to magnetic field is analyzed. The corresponding transmission is studied as function of the obtained energy and the potential parameters. Quantum interference within the oscillating barrier has an important effect on quasiparticles tunneling. In particular the time-periodic electromagnetic field generates additional sidebands at energies ε+ l\hbar ω(l=0,\pm 1, \cdots) in the transmission probability originating from the photon absorption or emission within the oscillating barrier. Due to numerical difficulties in truncating the resulting coupled channel equations we limited ourselves to low quantum channels, i.e. l=0,\pm 1.

cond-mat.mes-hall↗

arXiv2014

Band Tunneling through Double Barrier in Bilayer Graphene

By taking into account the full four band energy spectrum, we calculate the transmission probability and conductance of electrons across symmetric and asymmetric double potential barrier with a confined interlayer potential difference in bilayer graphene. For energies less than the interlayer coupling γ_{1}, E<γ_1, we have one channel for transmission which exhibits resonances, even for incident particles with energies less than the strength of barriers, E < U_j, depending on the double barrier geometry. In contrast, for higher energies E > γ_{1}, we obtain four possible ways for transmission resulting from the two propagating modes. We compute the associated transmission probabilities as well as their contribution to the conductance, study the effect of the double barrier geometry.

cond-mat.mes-hall↗

arXiv2013

Transport Properties for Triangular Barriers in Graphene

We theoretically study the electronic transport properties of Dirac fermions through one and double triangular barriers in graphene. Using the transfer matrix method, we determine the transmission, conductance and Fano factor. They are obtained to be various parameters dependent such as well width, barrier height and barrier width. Therefore, different discussions are given and comparison with the previous significant works is done. In particular, it is shown that at Dirac point the Dirac fermions always own a minimum conductance associated with a maximum Fano factor and change their behaviors in an oscillatory way (irregularly periodical tunneling peaks) when the potential of applied voltage is increased.

cond-mat.mes-hall↗

arXiv2013

Goos-Hanchen like Shifts in Graphene Double Barriers

We study the Goos-Hanchen like shifts for Dirac fermions in graphene scattered by double barrier structures. After obtaining the solution for the energy spectrum, we use the boundary conditions to explicitly determine the Goos-Hanchen like shifts and the associated transmission probability. We analyze these two quantities at resonances by studying their {main} characteristics as a function of the energy and electrostatic potential parameters. To check the validity of our computations we recover previous results obtained for a single barrier under appropriate limits.

cond-mat.mes-hall↗

arXiv2013

Factorization of Dirac Equation in Two Space Dimensions

We present a systematic approach for the separation of variables for the two-dimensional Dirac equation in polar coordinates. The three vector potential, which couple to the Dirac spinor via minimal coupling, along with the scalar potential are chosen to have angular dependence which emanate the Dirac equation to complete separation of variables. Exact solutions are obtained for a class of solvable potentials along with their relativistic spinor wavefunctions. Particular attention is paid to the situation where the potentials are confined to a quantum dot region and are of scalar, vector and pseudo-scalar type. The study of a single charged impurity embedded in a 2D Dirac equation in the presence of a uniform magnetic field was treated as a particular case of our general study.

math-ph↗

arXiv2013

Hall Conductivities for Confined System in Noncommutative Plane

We propose an approach based on the generalized quantum mechanics to deal with the basic features of the spin Hall effect. We begin by considering two decoupled harmonic oscillators on the noncommutative plane and determine the solutions of the energy spectrum. We realize two algebras in terms of the quadratic observables and show their importance in filling the shells with fermions. Under some transformation we show that our system is submitted to an effective Lorentz force similar to that acting on one particle in an external magnetic field. From equation of motions, we end up with the charge and spin Hall conductivities as function of the noncommutative parameter θ. By switching off θwe recover standard results developed on the subject and in the limit θ--> 0 we show that our approach can reproduce the Laughin wavefunctions.

hep-th↗

arXiv2013

Graphene Nanoribbon in Sharply Localized Magnetic Fields

We study the effect of a sharply localized magnetic field on the electron transport in a strip (ribbon) of graphene sheet, which allows to give results for the transmission and reflection probability through magnetic barriers. The magnetic field is taken as a single and double delta type localized functions, which are treated later as the zero width limit of gaussian fields. For both field configurations, we evaluate analytically and numerically their transmission and reflection coefficients. The possibility of spacial confinement due to the inhomogeneous field configuration is also investigated.

cond-mat.mes-hall↗

arXiv2012

Confined Dirac Particles in Constant and Tilted Magnetic Field

We study the confinement of charged Dirac particles in 3+1 space-time due to the presence of a constant and tilted magnetic field. We focus on the nature of the solutions of the Dirac equation and on how they depend on the choice of vector potential that gives rise to the magnetic field. In particular, we select a "Landau gauge" such that the momentum is conserved along the direction of the vector potential yielding spinor wavefunctions, which are localized in the plane containing the magnetic field and normal to the vector potential. These wave functions are expressed in terms of the Hermite polynomials. We point out the relevance of these findings to the relativistic quantum Hall effect and compare with the results obtained for a constant magnetic field normal to the plane in 2+1 dimensions.

hep-th↗

arXiv2012

Transport Properties through Double Barrier Structure in Graphene

The mode-dependent transmission of relativistic ballistic massless Dirac fermion through a graphene based double barrier structure is being investigated for various barrier parameters. We compare our results with already published work and point out the relevance of these findings to a systematic study of the transport properties in double barrier structures. An interesting situation arises when we set the potential in the leads to zero, then our 2D problem reduces effectively to a 1D massive Dirac equation with an effective mass proportional to the quantized wave number along the transverse direction. Furthermore we have shown that the minimal conductivity and maximal Fano factor remain insensitive to the ratio between the two potentials V_2/V_1=α.

cond-mat.mes-hall↗