arXiv subjects

Ahmed Jellal

Publications and source records attributed to Ahmed Jellal.

At least 73 records · Page 4Linked to original sources

arXiv2023

Rashba contribution of 2D Dirac-Weyl fermions: Beyond ordinary quantum regime

We study the energy levels of Dirac-Weyl fermions in graphene subject to a magnetic field with Rashba contribution in the minimal length situation. The exact solution for the energy dispersion of Dirac-like charge carriers coupled to the magnetic moments in a (2+1)-dimension is obtained by the use of the momentum space representation. Moreover, as it comes to applications for 2D Dirac-like quasiparticles, we also extend our theory and results in some special cases, showing that the emerging energy spectrum at the high magnetic field limit becomes independent of the Rashba coupling, $λ_{R}$, and the band index of Landau levels.

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arXiv2023

Spin-dependent transmission in curved graphene superlattice

We investigate spin-dependent transmission in a curved graphene superlattice of $N$ cells where each one is made up of four regions. The first is concave, and the third is convex, two arcs of circles separated by a distance $d$ from flat graphene sheets. The tunneling analysis allows us to determine all transmission and reflection channels associated with our system. As a result, we show that the number of cells acts by decreasing the transmissions with the same spin. We predict a solid spin-filtering effect when $d$ and $N$ are sufficiently large. Finally, it is determined that the degree and duration of suppression of the transmissions with the same spin over a range of energy are controllable using $d$.

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arXiv2022

Virtual excitations and entanglement dynamics and polygamy in three ultra-strongly coupled systems

The Milburn dynamics of three nonresonant ultra-strongly coupled oscillators are resolved by using symplectic geometry. We look at the Milburn dynamics of virtual excitations and how they affect pairwise entanglement. It is found that the dynamics of excitations and entanglement experience similar profiles against time, physical parameters, and decoherence rate. Furthermore, we show that the extinction of excitations entails separability, which demonstrates the hierarchy between entanglement and virtual excitations. Additionally, we analyze the effects of physical parameters on the redistribution of virtual excitations among the three bi-partitions. As a result, we show the violation of the monogamy of excitations as in quantum discord. This implies that excitations can be considered as signatures of quantum correlations beyond entanglement. Besides, we emphasize that our treatment can be used to model coupled quantum circuits in real situations (with decoherence).

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arXiv2022

Floquet Hofstadter butterfly in trilayer graphene with a twisted top layer

The magnetic field generated Hofstadter butterfly in single-twist trilayer graphene (TLG) is investigated using circularly polarized light (CPL) and longitudinal light emanating from a waveguide. We show that single-twist TLG has two distinct chiral limits in the equilibrium state, and the central branch of the butterfly splits into two precisely degenerate components. The Hofstadter butterfly appears to be more discernible. We also discovered that CPL causes a large gap opening at the central branch of the Hofstadter butterfly energy spectrum and between the Landau levels, with a clear asymmetry corresponding to energy $E = 0$. We point out that for right-handed CPL, the central band shifts downward, in stark contrast to left-handed CPL, where the central band shifts upward. Finally, we investigated the effect of longitudinally polarized light, which originates from a waveguide. Interestingly, we observed that the chiral symmetries of the Hofstadter butterfly energy spectrum are broken for small driving strengths and get restored at large ones, contrary to what was observed in twisted bilayer graphene.

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arXiv2022

Energy levels of graphene magnetic quantum dot in inhomogeneous gap

We investigate the energy levels of charge carriers confined in a magnetic quantum dot in graphene with an inhomogeneous gap through an electrical potential. We solve the eigenvalue equation for two regions. We explicitly determine the eigenspinors for both valleys $ K $, $ K' $ and use the boundary condition at the quantum dot interface to obtain the energy levels. We show that the energy levels exhibit symmetric and asymmetric behavior under appropriate conditions of the physical parameters. It has been found that changing the energy levels by introducing an energy gap outside the quantum dot changes the electrical properties.

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arXiv2022

Tunneling conductance through gapped bilayer graphene junctions

The conductance through single-layer graphene (SLG) and AA/AB-stacked bilayer graphene (BLG) junctions is obtained by taking into account band gap and bias voltage terms. First, we consider gapped SLG, while in between, they are connected into pristine BLG. For Fermi energy larger than the interlayer hopping, the conductance as a function of the bilayer region length $d$ reveals two different models of anti-resonances with the same period. As a function of the band gap, with AA-BLG stacking, the results show that the conductance has the same minima whatever the value of $d$, and for AB-BLG, $d$ remains relevant such that the system creates a global energy gap. Second, we consider pristine SLG, and in between, they are connected to gapped-biased BLG. We observe the appearance of peaks in the conductance profile with different periods and shapes, and also the presence of Klein tunneling with zero conductance in contrast to the first configuration. When $ d $ is less than 10, $G(E)$ vanishes and exhibits anti-Klein tunneling as a function of the Fermi energy $E$. We also investigate the conductance as a function of the bias. For AA-BLG, the results show antiresonances and diminish for a large value of the bias, independently of the bilayer region of length. In contrast, the conductance in AB-BLG has distinct characteristics in that it begins conducting with maxima for small $E$ and with minima for large $E$.

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arXiv2022

Effect of parallel magnetic field on Klein tunneling in $pn$ and $pnp$ graphene trilayer junctions

The effect of a parallel magnetic field $B$ and a potential substrate on the transmission, conductivity, and Fano factor of a biased ABC-stacked $pn$ and $pnp$ trilayer graphene junction (ABC-TLG) is investigated theoretically at low energy. We discovered that, in the presence of a high magnetic field $B=1400$T, the ABC-TLG can exhibit new Klein tunneling at two new incidence $k_y$ values, in addition to the usual value of $k_y=0$ and whatever the energy value is. Indeed, the transmission of ABC-TLG via $ pn $ and $ pnp $ junctions is transformed into a three-separate transmission of single-like graphene (SLG), with Klein tunneling appearing at $k_y=0$ and $k_y=\pmκ$ ($κ$ is highly dependent on $B$). Furthermore, the conductivity $σ$ and Fano factor $F$ behave similarly to SLG, with ABC-TLG having the lowest $σ=3g_0/π$ and highest $F=1/3$ at $B=1400$T and $σ_{\text{TLG}}=3σ_{\text{SLG}}$.

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arXiv2022

Effect of strain on tunneling time in graphene magnetic barrier

We solve the Dirac equation in three regions of graphene to get the solutions of the energy spectrum in connection to the strain, energy gap, and magnetic field. The Goos-Hänchen shifts and group delay time will be obtained by applying the stationary phase approximation after the wave functions at the interfaces have been matched. Our results suggest that the group delay time is influenced by the presence of strain along the armchair and zigzag directions. We show that the gate voltage and strain have the ability to change the group delay from subluminality to superluminality. This may have significant uses in high-speed graphene-based nanoelectronics.

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arXiv2022

Transmission gaps in phosphorene superlattice

Our research focuses on the transmission gaps of charge carriers passing through phosphorene superlattice, which are made up of a series of barriers and wells generating $n$ identical cells. We determine the solutions of the energy spectrum and then transmission using Bloch's theorem and the transfer-matrix approach. The analysis will be done on the impact of incident energy, barrier height, potential widths, period number, and transverse wave vector on transmission. We show that pseudo-gaps appear and turn into real transmission gaps by increasing the number of cells. Their number, width, and position can be tuned by changing the physical parameters of the structure. At normal incidence, a forbidden gap is found, meaning that there is no Klein tunneling effect, in contrast to the case of graphene. Our findings can be used to create a variety of phosphorene-based electronic devices.

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arXiv2022

Tuning Gap in Corrugated Graphene with Spin Dependence

We study transmission in a system consisting of a curved graphene surface as an arc (ripple) of circle connected to two flat graphene sheets on the left and right sides. We introduce a mass term in the curved part and study the effect of a generated band gap in spectrum on transport properties for spin-up/-down. The tunneling analysis allows us to find all transmission and reflections channels modeled by the band gap. This later acts by decreasing the transmissions with spin-up/-down but increasing with spin opposite, which exhibit some behaviors look like bell-shaped curve. We find resonances appearing in reflection with the same spin, thus backscattering with a spin-up/-down is not null in ripple. We observe huge spatial shifts for the total conduction in our model and the magnitudes of these shifts can be efficiently controlled by adjusting the band gap. This high order tunability of the tunneling effect can be used to design highly accurate devises based on graphene.

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arXiv2022

Magnetic Field Effect on Dynamics of Entanglement for Time-dependent Harmonic Oscillator

We investigate the dynamics of entanglement, uncertainty and mixedness by solving time dependent Schrödinger equation for two-dimensional harmonic oscillator with time dependent frequency and coupling parameter subject to a static magnetic field. We compute the purities (global/marginal) and then calculate explicitly the linear entropy $S_{L}$ as well as logarithmic negativity $\mathcal{N}$ using the symplectic parametrization of vacuum state. We introduce the spectral decomposition to diagonalize the marginal state and get the expression of von Neumann entropy $S_{von}$ and establish its link with $S_{L}$. We use the Wigner formalism to derive the Heisenberg uncertainties and {show their dependencies on both $S_{L}$ and the coupling parameters $γ_{i}$ $ (i=1,2)$ of the quadrature term $x_{i}p_{i}$.} We graphically study the dynamics of the three features (entanglement, uncertainty, mixedness) and present the similar topology with respect to time. We show the effects of the magnetic field and quenched values of $J(t)$ and $ω_{2}(t)$ on these three dynamics, which lead eventually to control and handle them.

quant-ph↗

arXiv2022

Zero-energy states in graphene quantum dot with wedge disclination

We investigate the effects of wedge disclination on charge carriers in circular graphene quantum dots subjected to a magnetic flux. Using the asymptotic solutions of the energy spectrum for large arguments, we approximate the scattering matrix elements, and then study the density of states. It is found that the density of states shows several resonance peaks under various conditions. In particular, it is shown that the wedge disclination is able to change the amplitude, width, and positions of resonance peaks.

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arXiv2022

Klein tunneling through triple barrier in AB bilayer graphene

We investigate the transport properties of charge carriers in AB bilayer graphene through a triple electrostatic barrier. We calculate the transmission and reflection using the continuity conditions at the interfaces of the triple barrier together with the transfer matrix method. First, we consider the case where the energy is less than the interlayer coupling $γ_1$ and show that, at normal incidence, transmission is completely suppressed in the gap for a large barrier width while it appears in the gap for a small barrier width. For energies greater than $γ_1$, we show that in the absence of an interlayer potential difference, transmission is less than that of a single barrier, but in its presence, transmission in the gap region is suppressed, as opposed to a double barrier. It is found that one, two, or three gaps can be created depending on the number of interlayer potential differences applied. Resonance in the $T_-^+$ transmission channel is observed that is not seen in the single and double barrier cases. Finally, we compute the conductance and show that the number of peaks is greater than the double barrier case.

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arXiv2022

Tunneling in rippled graphene superlattice with spin dependence and a mass term

The insertion of the band gap $Δ$ in the rippled graphene superlattice leads to new outcomes, as demonstrated. The essential thing is the appearance of opposite-spin transmissions, which increase with $Δ$ and vanish without it. Furthermore, compared to the $Δ=0$ scenario, the duration of the suppression of the transmission with the same spin is longer, with many peaks. The maximum value of transmissions with the same spin declines and remains around unity. Furthermore, for particular energy values, a shift in the behavior of the transmission channels is found. As a result, we demonstrate that with $Δ$, transmission filtering becomes crucial. Finally, as a result of the band gap, distinct variations in total conductance are discovered.

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arXiv2022

Virtual excitations and quantum correlations in ultra-strongly coupled harmonic oscillators under intrinsic decoherence

We study the intrinsic decoherence of coupled harmonic oscillators. The Milburn master equation is solved exactly, and the dynamics of virtual ground state excitations are investigated. The interaction of quantum correlations and virtual excitation was then studied. The following is a summary of our major findings. (i) The damped oscillatory profile of all three quantities is the same. (ii) Ultra-strong coupling combined with huge anisotropy values results in the reemergence of entanglement and steering. (iii) To sustain entanglement and steering, virtual excitations are required. (iv) The quantum correlations are amplified in the quantum synchronous regime. (v) Ultra-strong couplings cause inherent decoherence to be avoided.

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arXiv2022

Group delay time of fermions in graphene through tilted potential barrier

The group delay time of Dirac fermions subjected to a tilting barrier potential along the $ x $-axis is investigated in graphene. We start by finding the eigenspinor solution of the Dirac equation and then relating it to incident, reflected, and transmitted beam waves. This relationship allows us to compute the group delay time in transmission and reflection by obtaining the corresponding phase shifts. We discovered that the barrier width, incident energy, and incident angle can all be used to modify the group delay time, and that the particles travel through the barrier at the Fermi velocity $ v_F $. Our findings also show that the transmission group delay might be controlled, and that gate voltage control could be useful in graphene-based tilting barriers.

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arXiv2022

Tunneling phase diagrams in anisotropic Multi-Weyl semimetals

Motivated by the exciting prediction of Multi-Weyl topological semimetals that are stabilized by point group symmetries [Phys. Rev. Lett. 108 (2012) 266802], we study tunneling phenomena for a class of anisotropic Multi-Weyl semimetals. We find that a distant detector for different ranges of an anisotropy parameter $λ$ and incident angle $θ$ will measure a different number of propagating transmitted modes. We present these findings in terms of phase diagrams that is valid for an incoming wave with fixed wavenumber $k$--energy is not fixed. To gain a deeper understanding of this phenomenon we then focus on the simplest case of an anisotropic quadratic Weyl-semimetal and analyze tunneling coefficients analytically and numerically to confirm the observations from the phase diagram. Our results show non-analytical behavior, which is the hallmark of a phase transition. This serves as a motivation to make a formal analogy with phase transitions that are known from statistical mechanics. Specifically, we argue that the long distance limit in our tunneling problem takes the place of the thermodynamic limit in statistical mechanics. More precisely, find a direct formal connection to the recently developed formalism for dynamical phase transitions [Reports on Progress in Physics 81 (5) (2018) 054001]. We propose that this analogy to phase transitions can help classify transport properties in exotic semimetals.

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arXiv2022

Chiral limits and effect of light on the Hofstadter butterfly in twisted bilayer graphene

We study the magnetic field induced Hofstadter butterfly in twisted bilayer graphene (TBG) in various kinds of situations. First, we study the equilibrium case and identify the interlayer hopping processes that are most crucial for the appearance of a Hofstadter butterfly. Surprisingly, the hopping processes that are important for the appearance of the Hofstadter butterfly can be categorized as AA stacking type - that is interlayer hoppings between equivalent sublattices. This is in contrast to AB/BA-type hoppings that are important for the appearance of flat bands in magic angle TBG and were discussed in [Phys. Rev. Lett. 122, 106405 (2019)]. We also find that if AB-type interlayer-hopping processes are turned off the resulting model is chiral but differs from the model discussed in \cite{Tarnopolsky}. Therefore, TBG has two separate chiral limits - one of them is important to understand the formation of flat bands and the other for the Hofstadter butterfly. Taking this as motivation we discuss how the role of AA-type hoppings in combination with lattice relaxation effects can make individual Landau levels slightly harder to resolve in an experimental setting than one would expect from a non-relaxed lattice setting. Finally, we consider the impact of different forms of light on the fractal structure of the butterfly. Particularly, we study the impact of circularly polarized light and longitudinal light originating from a waveguide. As the system is exposed to circularly polarized light we find butterflies with increasingly pronounced asymmetry with respect to energy $E=0$. This is due to the introduction of a gap term that breaks the chiral symmetries for both of the two chiral limits mentioned above. Lastly, we study the effect of longitudinal light that can be produced at the exit of a waveguide, in a slightly simplified model. Here, we find ...

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