arXiv subjects

Ahmed Jellal

Publications and source records attributed to Ahmed Jellal.

At least 55 records · Page 3Linked to original sources

arXiv2024

Boosting energy levels in graphene magnetic quantum dots through magnetic flux and inhomogeneous gap

We study the effects of a magnetic flux and an inhomogeneous gap on the energy spectrum of graphene magnetic quantum dots (GMQDs). By considering the Dirac equation in the infinite mass framework, we can analytically obtain eigenspinor expressions. By applying boundary conditions, we obtain an energy spectrum equation in terms of system parameters such as radius, magnetic field, energy, flux, and gap. In the infinite limit, we recover Landau levels for graphene in a magnetic field. We show that the energy spectrum increases significantly in the presence of flux and a gap inside the GMQDs, which prolongs the lifetime of the trapped electron states. We show that higher flux also produces new Landau levels of negative angular momentum. Meanwhile, we find that the gap increases the separation between the electron and hole energy bands. As shown in the radial probability analysis, flux and gap emerge as influential factors in controlling electron mobility, affecting confinement, and prolonging the presence of quasi-bound states.

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arXiv2024

Transmission in graphene through a double laser barrier

We study the tunneling behavior of Dirac fermions in graphene subjected to a double barrier potential profile created by spatially overlapping laser fields. By modulating the graphene sheet with an oscillating structure formed from two laser barriers, we aim to understand how the transmission of Dirac fermions is influenced by such a light-induced electric potential landscape. Using the Floquet method, we determine the eigenspinors of the five regions defined by the barriers applied to the graphene sheet. Applying the continuity of the eigenspinors at barrier edges and using the transfer matrix method, we establish the transmission coefficients. These allow us to show that oscillating laser fields generate multiple transmission modes, including zero-photon transmission aligned with the central band $\varepsilon$ and photon-assisted transmission at sidebands $\varepsilon+ l\varpi$, with $l=0,\pm1, \cdots$ and frequency $\varpi$. For numerical purposes, our attention is specifically directed towards transmissions related to zero-photon processes ($l=0$), along with processes involving photon emission ($l=1$) and absorption ($l=-1$). We find that transmission occurs only when the incident energy is above the threshold energy $\varepsilon>k_y+2\varpi$, {with transverse wave vector $k_y$}. We find that the variation in distance {$d_1$ separating two barriers of widths $d_2-d_1$} suppresses one transmission mode. Additionally, we show that an increase in laser intensity modifies transmission sharpness and amplitude.

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arXiv2024

Transport properties in ABC-ABA-ABC trilayer graphene junctions

Trilayer graphene {(TLG)} consists of three layers of graphene arranged in a particular stacking order. In the case of ABC-ABA-ABC stacking, the layers are arranged in an A-B-C sequence, followed by an A-B-A sequence, and again an A-B-C sequence. This stacking arrangement introduces specific electronic properties and band structures due to the different stacking configurations. We focus on elucidating the transport properties of a p-n-p junction formed with ABC-ABA-ABC stacking TLG. Employing the transfer matrix method and considering continuity conditions at the junction boundaries, we establish transmission and reflection probabilities, along with conductance. Notably, electron transport through the ABC-ABA-ABC junction exhibits Klein tunneling, resulting in substantial conductance even in the absence of a potential barrier $V_0$. This effect arises from the effective barrier induced by our specific stacking, facilitating the passage of a maximal number of electrons. However, the presence of $V_0$ diminishes Klein tunneling, leading to conductance minima. Furthermore, our findings highlight that interlayer bias $δ$ induces a hybridization of the linear and parabolic bands of ABA-TLG within the junction, reducing resonances. In cases where $δ\neq0$ and $V_0\neq0$, we observe a suppression of the gap, contrary to the results obtained in ABC tunneling studies where a gap exists.

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arXiv2024

Semiclassical perspective on Landau levels and Hall conductivity in an anisotropic Cubic Dirac Semi-Metal and the peculiar case of star-shaped classical orbits

We study an anisotropic cubic Dirac semi-metal subjected to a constant magnetic field. In the case of an isotropic dispersion in the $x$-$y$ plane, with parameters $v_{x}=v_{y}$, it is possible to find exact Landau levels, indexed by the quantum number $n$, using the typical ladder operator approach. Interestingly, we find that the lowest energy level (the zero energy state in the case $k_z=0$) has a degeneracy that is three times that of other states. This degeneracy manifests in the Hall conductivity as a step at zero chemical potential that is 3/2 the size of other steps. Moreover, as $n\to\infty$ we find energies $E_n\propto n^{3/2}$, which means the $n$-th step as a function of chemical potential roughly occurs at a value $μ\propto n^{3/2}$. We propose that these exciting features could be used to identify cubic Dirac semi-metals experimentally. Subsequently, we analyze the anisotropic case $v_{y}=λv_{x}$ with $λ\neq 1$. First, we consider a perturbative treatment around $λ\approx 1$ and find that energies $E_n\propto n^{3/2}$ still holds as $n\to\infty$. To gain further insight into the Landau level structure for a maximum anisotropy, we turn to a semi-classical treatment that reveals interesting star-shaped orbits in phase space that close at infinity. This property is a manifestation of weakly localized states. Despite being infinite in length, these orbits enclose a finite phase space volume and permit finding a simple semi-classical formula for the energy, which again has the form as above. Our findings suggest that both isotropic and anisotropic cubic Dirac semi-metals should leave similar experimental imprints.

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arXiv2024

Tunneling effect of fermions in silicene through potential barrier

The influence of a rectangular potential barrier on the quantum transport of fermions in silicene is explored. Specifically, analytical solutions are presented to derive transmission and reflection probabilities together with conductance. It is shown that the transmission is highly sensitive to both the barrier height and incident energy. As a result, the occurrence of Klein and resonant tunnelings is observed, with a significant dependence on the barrier width. Notably, it is found that perfect transmission extends beyond normal incidence, occurring at various oblique angles. Moreover, the transmission pattern exhibits a more fragmented structure with increasing barrier width, reminiscent of Fabry-Pérot resonances. In contrast, the conductance displays a non-monotonic dependence on incident energy and features rapid oscillations with a rising barrier height. However, at a constant barrier height, there is a minimal disparity among conductance profiles for high incident energy values. When incident energy equals the barrier height, the conductance experiences a local minimum. For a thin barrier, a substantial reduction in conductance is observed, unlike the oscillatory behavior seen with a thicker barrier. These findings underscore the progress in silicene research and offer a fresh perspective on the relativistic applications of tunneling in this material.

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arXiv2024

Electrons trapped in graphene magnetic quantum dots with mass term

Owing to the Klein tunneling phenomenon, the permanent confinement or localization of electrons within a graphene quantum dot is unattainable. Nonetheless, a constant magnetic field can transiently ensnare an electron within the quantum dot, giving rise to what are known as quasi-bound states characterized by finite lifetimes. To prolong the retention of electrons within the quantum dot, we introduce a mass term into the Hamiltonian, thereby inducing an energy gap. We resolve the Dirac equation to ascertain the eigenspinors, and by ensuring their continuity at the boundaries, we investigate the scattering behavior. Our findings indicate that the presence of an energy gap can extend the lifetimes of these quasi-bound states within the quantum dot. In particular, we demonstrate that even in the absence of a magnetic field, the scattering efficiency attains significant levels when the energy gap gets closed to the incident energy of an electron traversing the quantum dot. It is found that an augmentation in the electron density within the quantum dot results in an enhancement of the electron-trapping time.

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arXiv2024

Magnetic field effect on tunneling through triple barrier in AB bilayer graphene

We investigate electron tunneling in AB bilayer graphene through a triple electrostatic barrier of heights $U_i (i=2,3,4)$ subjected to a perpendicular magnetic field. By way of the transfer matrix method and using the continuity conditions at the different interfaces, the transmission probability is determined. Additional resonances appear for two-band tunneling at normal incidence, and their number is proportional to the value of $U_4$ in the case of $U_2 U_4$, anti-Klein tunneling increases with $U_2$. The transmission probability exhibits an interesting oscillatory behavior when $U_3>U_2=U_4$ and $U_3 U_2=U_4$. In the four-band tunneling case, the transmission decreases in $T^+_+$, $T^-_+$ and $T^-_-$ channels in comparison with the single barrier case. It does, however, increase for $T^+_-$ when compared to the single barrier case. Transmission is suppressed in the gap region when an interlayer bias is introduced. This is reflected in the total conductance $G_{\text{tot}}$ in the region of zero conductance. Our results are relevant for electron confinement in AB bilayer graphene and for the development of graphene-based transistors.

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arXiv2024

Effects of AB-flux and gap on magnetic graphene quantum dots

We consider magnetic graphene quantum dots (MGQDs) and study the impact of the Aharonov-Bohm (AB) flux and gap on the scattering process of electrons. Our emphasis is on the finite lifetimes of quasi-bound states arising from the interaction between electrons and the magnetic field within the dot. Initially, we calculate the scattering coefficients, scattering efficiency, and probability density by ensuring the continuity of eigenspinors at the boundary of MGQD. The results indicate that as the gap increases, the quasi-bound states reach higher maxima. We show that an increase in AB-flux leads to a generation of quasi-bound states requiring less magnetic field, and the scattering efficiency starts to take non-zero values at smaller MGQD sizes. The analysis of probability density shows that the quasi-bound states, corresponding to non-resonantly excited scattering modes, exhibit a significant improvement in the concentrated density at MGQD. The improvement is a result of reducing the diffraction phenomenon and suppressing the Klein effect through an increase in AB-flux and gap. This increases the probability of retaining the electron for a longer period of time.

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arXiv2024

Effect of a perpendicular magnetic field on bilayer graphene under dual gating

By studying the impact of a perpendicular magnetic field $B$ on AB-bilayer graphene (AB-BLG) under dual gating, we yield several key findings for the ballistic transport of gate $U_\infty$. Firstly, we discover that the presence of $B$ leads to a decrease in transmission. At a high value of $B$, we notice the occurrence of anti-Klein tunneling over a significant area. Secondly, in contrast to the results reported in the literature, where high peaks were found with an increasing in-plane pseudomagnetic field applied to AB-BLG, we find a decrease in conductivity as $B$ increases. However, it is worth noting that in both cases, the number of oscillations decreases compared to the result in the study where no magnetic field was present $(B = 0)$. Thirdly, at the neutrality point, we demonstrate that the conductivity decreases and eventually reaches zero for a high value of $B$, which contrasts with the result that the conductivity remains unchanged regardless of the value taken by the in-plane field. Finally, we consider the diffusive transport with gate $U_\infty = 0.2 γ_1$ and observe two scenarios. The amplitude of conductivity oscillations increases with $B$ for energy $E$ less than $U_\infty$ but decreases in the opposite case $E>U_\infty$.

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arXiv2023

Tuned gap in graphene through laser barrier

We study the effect of the energy gap on the transmission of fermions in graphene exposed to linearly polarized light as a laser barrier. We determine the energy spectrum, apply boundary conditions at interfaces, and use the transfer matrix approach to obtain transmissions for all energy modes. We show that when the energy gap increases, the oscillations of transmissions decrease dramatically until they vanish entirely. However, when the barrier width varies, the oscillations become more significant and exhibit sharp peaks. By increasing the incident energy, the laser field suppresses the Fabry-Pérot resonance, and the transmissions move to the right when the energy gap is tuned.

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arXiv2023

Transport properties in gapped graphene through magnetic barrier in a laser field

We study the transport properties of Dirac fermions through gapped graphene through a magnetic barrier irradiated by a laser field oscillating in time. We use Floquet theory and the solution of Weber's differential equation to determine the energy spectrum corresponding to the three regions composing the system. The boundary conditions and the transfer matrix approach {are} employed to explicitly determine the transmission probabilities for multi-energy bands and the associated conductance. As an illustration, we focus only on the three first bands: the central band $T_0$ (zero photon exchange) and the two first side bands $T_{\pm1}$ (photon emission or absorption). It is found that the laser field activates the process of translation through photon exchange. Furthermore, we show that varying the incident angle and energy gap strongly affects the transmission process. The conductance increases when the number of electrons that cross the barrier increases, namely when there is a significant transmission.

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arXiv2023

Transport properties of hybrid single-bilayer graphene interfaces in magnetic field

We investigate the electronic properties of a hybrid system that comprises single-bilayer graphene structures subjected to a perpendicular magnetic field. Specifically, our focus is on the behavior exhibited by the zigzag boundaries of the junction, namely Zigzag-1 (ZZ1) and Zigzag-2 (ZZ2), using the continuum Dirac model for rigorous analysis. Our findings reveal a striking dependence of conductance on the width of the bilayer graphene at ZZ1, providing essential insights into the transport behavior of this boundary. Moreover, we observe a captivating phenomenon where the conductance at ZZ2 exhibits prominent maxima, demonstrating a robust correlation with the applied magnetic field. Additionally, our investigation uncovers the profound impact of interfaces on transmission probability, with ZZ1 being notably more affected compared to ZZ2. The variation of the Fermi energy further highlights the significant influence of magnetic field strength on the system's conductive properties, resulting in distinct conductance characteristics between the two regions. The combined results of ZZ1 and ZZ2 provide valuable insights into the system's transport properties. Notably, a clear exponential-like trend in conductance variation with the applied magnetic field underscores the system's strong sensitivity to magnetic changes.

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arXiv2023

Energy levels of gapped graphene quantum dots in external fields

We investigate the energy levels of fermions within a circular graphene quantum dot (GQD) subjected to external magnetic and Aharonov-Bohm fields. Solving the eigenvalue equation for two distinct regions allows us to determine the eigenspinors for the valleys $K$ and $K^\prime$. By establishing the continuity of eigenspinors at the GQD interface, we derive an equation that reveals the reliance of energy levels on external physical parameters. Our observations suggest that the symmetry of energy levels hinges on the selected physical parameters. We observe that at low magnetic fields, the energy levels display degeneracy, which diminishes as the field strength increases, coinciding with the convergence of energy levels toward the Landau levels. We illustrate that the introduction of a magnetic flux into the GQD leads to the creation of an energy gap, extending the trapping time of electrons without perturbing the system. Conversely, the addition of gap energy widens the band gap, disrupting the system's symmetry by introducing new energy levels.

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arXiv2023

Electron trapping in graphene quantum dots with magnetic flux

It is known that the appearance of Klein tunneling in graphene makes it hard to keep or localize electrons in a graphene-based quantum dot (GQD). However, a magnetic field can be used to temporarily confine an electron that is traveling into a GQD. The electronic states investigated here are resonances with a finite trapping time, also referred to as quasi-bound states. By subjecting the GDQ to a magnetic flux, we study the scattering phenomenon and the Aharonov-Bohm effect on the lifetime of quasi-bound states existing in a GQD. We demonstrate that the trapping time increases with the magnetic flux sustaining the trapped states for a long time even after the flux is turned off. Furthermore, we discover that the probability density within the GQD is also clearly improved. We demonstrate that the trapping time of an electron inside a GQD can be successfully extended by adjusting the magnetic flux parameters.

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arXiv2023

Implementing quantum Fourier transform using three qubits

Using the circulant symmetry of a Hamiltonian describing three qubits, we realize the quantum Fourier transform. This symmetry allows us to construct a set of eigenvectors independently on the magnitude of physical parameters involved in the Hamiltonian and as a result, the entanglement will be maintained. The realization will be leaned on trapped ions and the gate implementation requires an adiabatic transition from each spin product state to Fourier modes. The fidelity was numerically calculated and the results show important values. Finally, we discuss the acceleration of the gate by using the counter-driving field.

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arXiv2023

Transmission in graphene through tilted barrier in laser field

We study the transmission of Dirac fermions in graphene through a tilted barrier potential in the presence of a laser field of frequency $ω$. By using Floquet theory, we solve the Dirac equation and then obtain the energy spectrum. The boundary conditions together with the transfer matrix method allow us to determine the transmission probabilities corresponding to all energy bands $E+l\hbarω$ $(l=0,\pm1, \cdots)$. By limiting to the central band $l=0$ and the two first side bands $l=\pm 1$, we show that the transmissions are strongly affected by the laser field and barrier. Indeed, it is found that the Klein effect is still present, a variety of oscillations are inside the barrier, and there is essentially no transmission across all bands.

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arXiv2023

Band structures and contact points in phosphorene superlattice

We study the band structures and the associated contact points for a phosphorene superlattice made up of two periodic areas. We use the boundary conditions to extract an equation describing the dispersion relation after obtaining the eigen-wavefunctions. We show that energy transforms into linear behavior near contact points, and fermions move at different speeds along $x$- and $y$- directions. It was discovered that the periodic potential caused additional Dirac points, which we located in $k$-space by establishing their positions. We demonstrate that the barrier height and width can be used to adjust the energy gap and modify the contact points. It might be that our findings will be useful in the development of phosphorene-based electronic devices.

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arXiv2023

Transmission in strained graphene subjected to laser and magnetic fields

We investigate the effect of strain along armchair and zigzag directions on electrical transport in graphene through a magnetic barrier and a linearly polarized electromagnetic wave. In the context of Floquet theory, the eigenvalues and related eigenspinors are calculated analytically. The transmission probabilities are expressed as a function of different parameters using the transfer matrix approach and boundary conditions at two interfaces with current densities. We see that as the barrier width and incident energy change, the transmission via the center band oscillates less at zero strain. The transmission across the first sidebands begins at 0 and follows the pattern of a sinusoidal function that grows with increasing barrier width and becomes nearly linear for larger incident energy. When the strain magnitude is activated, the number of oscillations in all transmission channels drops marginally in the armchair direction but increases dramatically in the zigzag direction. The behavior of the total transmission is found to be comparable to that of the central band, with the exception that it exhibits a translation to the up. The suppression of Klein tunneling at normal incidence is another result seen in all strain settings.

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