arXiv subjects

Ahmed Jellal

Publications and source records attributed to Ahmed Jellal.

At least 91 records · Page 5Linked to original sources

arXiv2022

Effect of magnetic field on fermions in graphene through time-oscillating potential

We study the effect of a magnetic field on Dirac fermions in graphene subject to a scalar potential oscillating in time. Using the Floquet theory and resonance approximation, we show that the energy spectrum exhibits extra subbands resulted from the oscillating potential in addition to quantized Landau levels. It is found that a current density can be generated in $ x $ and $ y $-directions that is strongly dependent on the magnetic field and potential. Our numerical analysis show that the energy spectrum possesses a symmetry and the current density oscillates with different amplitudes under various conditions.

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arXiv2022

Electron scattering of mass-inverted in graphene quantum dots

We study the scattering of Dirac electrons of circular graphene quantum dot with mass-inverted subject to electrostatic potential. The obtained solutions of the energy spectrum are used to determine the scattering coefficients at the interface of the two regions. Using the asymptotic solutions at large arguments, we explicitly determine the radial component of reflected current density and the scattering efficiency. It is found that the presence of a mass term outside in addition to another one inside the quantum dot strongly affects the scattering of electrons. In particular, a non-null square modulus of the scattering coefficient is found at zero energy.

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arXiv2022

Quantum tunneling in graphene Corbino disk in a solenoid magnetic potential with wedge disclination

We investigate the wedge disclination effect on quantum tunneling of a Corbino disk in gapped-graphene of inner $R_1$ and outer $R_2$ radii in the presence of magnetic flux $Φ_i$. We solve Dirac equation for different regions and obtain the solutions of energy spectrum in terms of Hankel functions. The asymptotic behaviors for large arguments allow us to determine the transmission, Fano factor and conductance. We establish the case where the crystal symmetry is modified locally by replacing a hexagon by pentagon, square, heptagon or octagon. We show that the wedge disclination $n$ modifies the amplitude of transmission oscillations. We find that the period of Fano factor oscillations is of the Aharonov-Bohm type, which strongly depends on $n$ where intense peaks are observed. As another result, $n$ changes the minimum and period of conductance oscillations of the Aharonov-Bohm type. We show that $n$ minimizes the effect of resonance and decreases the amplitude of conductance magnitude $ΔG$ oscillations.

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arXiv2022

Strain effect on Goos-Hänchen shifts and group delay time in gapped graphene barrier

We investigate the strain effect on the Goos-Hänchen (GH) shifts and group delay time for transmitted Dirac fermions in gapped graphene through a single barrier potential. The solutions of energy spectrum are used to compute the transmission probabilities together with the GH shifts and group delay time. Our results show that the two last quantities are strongly depending to weather the strain is applied along armchair or zigzag directions. In particular it found that both of quantities can be enhanced with the applied strain.

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arXiv2021

Strain Effect on Transmission in Graphene Laser Barrier

We investigate the strain effect along armchair and zigzag directions on the tunneling transport of Dirac fermions in graphene laser barrier through a time dependent potential along y-axis. Our system is composed of three regions and the central one is subjected to a deformation of strength S. Based on Dirac equation and the Floquet approach, we determine the eigenvalues and eigenspinors for each region. Using the boundary conditions at interfaces together with the transfer matrix method we identify the transmission in the different Floquet sideband states as function of the physical parameters. In the strainless case, we show that the transmisson of central band decreases for smaller values of the barrier width and rapidly oscillates with different amplitude for larger ones. Whereas the transmission for the first sidebands increases from zero and shows a damped oscillatory profile. It is found that the number of oscillations in all transmission channels reduces with increasing the strength of armchair strain but becomes more important by switching the deformation to zigzag. Moreover, it is observed the appearance of Fano type resonance peaks by altering the amplitude and the frequency of the laser field.

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arXiv2021

Tunneling Effect in Gapped Phosphorene through Double Barriers

We study the transport properties of charge carriers in phosphorene with a mass term through double barriers. The solutions of the energy spectrum are obtained and the dependence of the eigenvalues on the barrier potentials and wave vectors in the $x$-direction is numerically computed. Using the boundary conditions together with the matrix transfer method, we determine transmission and the conductance of our system. These two quantities are analyzed by studying their main characteristics as a function of the physical parameters along the armchair direction. Our results show the highly anisotropic character of phosphorene and the no signature of Klein tunneling at normal incidence contrary to graphene. Moreover, it is found that the transmission and conductance display oscillatory behaviors in terms of the barrier width under suitable conditions.

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arXiv2021

Klein tunneling through double barrier in ABC-trilayer graphene

Klein tunneling and conductance for Dirac fermions in ABC-stacked trilayer graphene (ABC-TLG) through symmetric and asymmetric double potential barriers are investigated using the two and six-band continuum model. Numerical results for our system show that the transport is sensitive to the height, the width and the distance between the two barriers. Klein paradox at normal incidence and resonant features at non-normal incidence in the transmission result from resonant electron states in the wells or hole states in the barriers and strongly influence the ballistic conductance of the structures.

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arXiv2021

Dynamics of Non-Gaussian Entanglement of Two Magnetically Coupled Modes

This paper surveys the quantum entanglement of two coupled harmonic oscillators via angular momentum generating a magnetic coupling $ω_{c}$. The corresponding Hamiltonian is diagonalized by using three canonical transformations and then the stationary wave function is obtained. Based on the Schmidt decomposition, we explicitly determine the Schmidt modes $λ_{k}$ with $k\in\left\lbrace 0,1,\cdots,n+m\right\rbrace$, $n$ and $m$ being two quantum numbers associated to the two oscillators. By studying the effect of the anisotropy $ R=ω_{1}^{2}/ω_{2}^{2} $, $ω_{c}$, asymmetry $ |n-m| $ and dynamics on the entanglement, we summarize our results as follows. $ (i)- $ The entanglement becomes very large with the increase of $ (n,m) $. $ (ii)- $ The sensistivity to $ω_c$ depends on $ (n,m) $ and $R$. $ (iii)- $ The periodic revival of entanglement strongly depends on the physical parameters and quantum numbers.

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arXiv2021

Band strutures of hybrid graphene quantum dots with magnetic flux

We study the band structures of hybrid graphene quantum dots subject to a magnetic flux and electrostatic potential. The system is consisting of a circular single layer graphene surrounded by an infinite bilayer graphene. By solving the Dirac equation we obtain the solution of the energy spectrum in two regions. For the valley $K$, it is found that the magnetic flux strongly acts by decreasing the gap and shifting energy levels away from zero radius with some oscillations, which are note observed for null flux case. As for the valley $K'$, the energy levels rapidly increase when the radius increases. A number of oscillations appeared that is strongly dependent on the values taken by the magnetic flux.

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arXiv2021

Tunneling of Electrons in Graphene via Double Triangular Barrier in External Fields

We study the transmission probability of Dirac fermions in graphene scattered by a triangular double barrier potential in the presence of an external magnetic field. Our system made of two triangular potential barrier regions separated by a well region characterized by an energy gap. Solving our Dirac-like equation and matching the solutions at the boundaries allowed us to express our transmission and reflection coefficients in terms of transfer matrix. We show in particular that the transmission exhibits oscillation resonances that are manifestations of the Klein tunneling effect. The triangular barrier electrostatic field was found to play a key role in controlling the peak of tunneling resistance. However, it only slightly modifies the resonances at oblique incidence and leaves Klein paradox unaffected at normal incidence.

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arXiv2021

Tunneling in an anisotropic cubic Dirac semi-metal

Motivated by a recent first principles prediction of an anisotropic cubic Dirac semi-metal in a real material Tl(TeMo)$_3$, we study the behavior of electrons tunneling through a potential barrier in such systems. To clearly investigate effects from different contributions to the Hamiltonian we study the model in various limits. First, in the limit of a very thin material where the linearly dispersive $z$-direction is frozen out at zero momentum and the dispersion in the $x$-$y$ plane is rotationally symmetric. In this limit we find a Klein tunneling reminiscent of what is observed in single layer graphene and linearly dispersive Dirac semi-metals. Second, an increase in thickness of the material leads to the possibility of a non-zero momentum eigenvalue $k_z$ that acts as an effective mass term in the Hamiltonian. We find that these lead to a suppression of Klein tunneling. Third, the inclusion of an anisotropy parameter $λ\neq 1$ leads to a breaking of rotational invariance. Furthermore, we observed that for different values of incident angle $θ$ and anisotropy parameter $λ$ the Hamiltonian supports different numbers of modes propagating to infinity. We display this effect in form of a diagram that is similar to a phase diagram of a distant detector. Fourth, we consider coexistence of both anisotropy and non-zero $k_z$ but do not find any effect that is unique to the interplay between non-zero momentum $k_z$ and anisotropy parameter $λ$. Last, we studied the case of a barrier that was placed in the linearly dispersive direction and found Klein tunneling $T-1\propto θ^6+\mathcal{O}(θ^8)$ that is enhanced when compared to the Klein tunneling in linear Dirac semi-metals or graphene where $T-1\propto θ^2+\mathcal{O}(θ^4)$.

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arXiv2021

Effect of Strain on Band Engineering in Gapped Graphene

We study the effect of strain on the band engineering in gapped graphene subject to external sources. By applying the Floquet theory, we determine the effective Hamiltonian of electron dressed by a linearly, circularly and an elliptically polarized dressing field in the presence of strain along armchair and zigzag directions. Our results show that the energy spectrum exhibits different symmetries and for the strainless case it takes an isotropic and anisotropic forms whatever the values of irradiation intensity, whereas it is linear as in the case of pristine graphene. It increases slowly when strain is applied along the armchair direction but rapidly for the zigzag case. Moreover, it is found that the renormalized band gap changes along different strain magnitudes and does not change for the polarization phase $θ$ compared to linear and circular polarizations where its values change oppositely.

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arXiv2021

Dynamics and Redistribution of Entanglement and Coherence in Three Time-Dependent Coupled Harmonic Oscillators

We study the dynamics and redistribution of entanglement and coherence in three time-dependent coupled harmonic oscillators. We resolve the Schrödinger equation by using time-dependent Euler rotation together with a linear quench model to obtain the state of vacuum solution. Such state can be translated to the phase space picture to determine the Wigner distribution. We show that its Gaussian matrix $\mathbb{G}(t)$ can be used to directly cast the covariance matrix $σ(t)$. To quantify the mixedness and entanglement of the state one uses respectively linear and von Neumann entropies for three cases: fully symmetric, bi-symmetric and fully non symmetric. Then we determine the coherence, tripartite entanglement and local uncertainties and derive their dynamics. We show that the dynamics of all quantum information quantities are driven by the Ermakov modes. Finally, we use an homodyne detection to redistribute both resources of entanglement and coherence.

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arXiv2021

Gap-tunable of Tunneling Time in Graphene Magnetic Barrier

We study the tunneling time of Dirac fermions in graphene magnetic barrier through an electrostatic potential and a mass term. This latter generates an energy gap in the spectrum and therefore affects the proprieties of tunneling of the system. For clarification, we first start by deriving the eigenspinors solutions of Dirac equation and second connect them to the incident, reflected and transmitted beam waves. This connection allows us to obtain the corresponding phases shifts and consequently compute the group delay time in transmission and reflection. Our numerical results show that the group delay time depends strongly on the energy gap in the tunneling process through single barrier. Moreover, we find that the group approaches unity at some critical value of the energy gap and becomes independent to the strengths of involved physical parameters.

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arXiv2021

Classical Instability Effects on Photon Excitations and Entanglement

The Schrödinger dynamics of photon excitation numbers together with entanglement in two non-resonant time-dependent coupled oscillators is investigated. By considering $ π-$periodically pumped parameters and using suitable transformations, we obtain the coupled Meissner oscillators. Consequently, our analytical study shows two interesting results, which can be summarized as follows. (i): Classical instability of classical analog of quantum oscillators and photon excitation {averages $\left\langle N_{j}\right\rangle $} are strongly correlated. (ii): Photon excitation's and entanglement are connected to each other. These results can be used to shed light on the link between quantum systems and their classical counterparts. Also it allow to control entanglement by engineering only classical systems where the experiments are less expensive.

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arXiv2021

Transport Properties in Gapped Bilayer Graphene

We investigate transport properties through a rectangular potential barrier in AB-stacked bilayer graphene (AB-BLG) gapped by dielectric layers. Using the Dirac-like Hamiltonian with a transfer matrix approach we obtain transmission and reflection probabilities as well as the associated conductance. For two-band model and at normal incidence, we find extra resonances appearing in transmission compared to biased AB-BLG, which are Fabry-Pérot resonance type. Now by taking into account the inter-layer bias, we show that both of transmission and anti-Klein tunneling are diminished. Regarding four band model, we find that the gap suppresses transmission in an energy range by showing some behaviors look like "Mexican hats". We examine the total conductance and show that it is affected by the gap compared to AA-stacked bilayer graphene. In addition, we find that the suppression in conductance is more important than that for biased AB-BLG.

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arXiv2020

Energy Levels of Gapped Graphene Quantum Dot in Magnetic Field

We study the energy levels of carriers confined in a magnetic quantum dot of graphene surrounded by a infinite graphene sheet in the presence of energy gap. The eigenspinors are derived for the valleys $K$ and $K'$, while the associated energy levels are obtained by using the boundary condition at interface of the quantum dot. We numerically investigate our results and show that the energy levels exhibit the symmetric and antisymmetric behaviors under suitable conditions of the physical parameters. We find that the radial probability can be symmetric or antisymmeric according to the angular momentum is null or no-null. Finally, we show that the application of an energy gap decreases the electron density in the quantum dot, which indicates a temporary trapping of electrons.

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arXiv2020

Density of States Analysis of Electrostatic Confinement in Gapped Graphene

We investigate the electrostatic confinement of charge carriers in a gapped graphene quantum dot in the presence of a magnetic flux. The circular quantum dot is defined by an electrostatic gate potential delimited in an infinite graphene sheet which is then connected to a two terminal setup. Considering different regions composing our system, we explicitly determine the solutions of the energy spectrum in terms of Hankel functions. Using the scattering matrix together with the asymptotic behavior of the Hankel functions for large arguments, we calculate the density of states and show that it has an oscillatory behavior with the appearance of resonant peaks. It is found that the energy gap can controls the amplitude and width of these resonances and affect their location in the density of states profile.

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