arXiv subjects

Youness Zahidi

Publications and source records attributed to Youness Zahidi.

17 recordsLinked to original sources

arXiv2026

Quantum tunneling and defect-induced transport modulation in twisted bilayer graphene superlattices

We investigate quantum tunneling of charge carriers through a periodic superlattice in twisted bilayer graphene (TBG) with rectangular potential barriers, including the presence of a defect, using a low-energy continuum model. Transmission probabilities are numerically analyzed depending on the parameters of the problem, highlighting the roles of twist angle, number of barriers, barrier geometry, and the presence of a defect barrier within the superlattice. Our numerical results reveal that transmission is highly sensitive to these parameters: reducing the twist angle changes the number, depth, and position of transmission gaps and resonance peaks. The presence of defect affects the transmission, leading to the appearance of tunneling states inside transmission gaps with energy position can be tuned by the well width. At low incident energy, the transmission for normally incident electrons is perfect or nearly perfect, independent of the twist angle and the number of barriers. However, at large incident energy, the transmission becomes distinctly anisotropic, reflecting the separation of Dirac cones induced by twist angle variations. The presence of defects, particularly at smaller twist angles, provides additional control of tunneling behavior, allowing complete suppression of Klein tunneling under certain conditions. These findings extend the established understanding of miniband transport in periodic graphene systems and open new possibilities for twist-tunable nanoelectronic and quantum devices.

cond-mat.mes-hall↗

arXiv2025

Klein Tunneling and Fabry-Pérot Resonances in Twisted Bilayer Graphene

The paper discusses the Klein tunneling and Fabry-Pérot resonances of charge carriers through a rectangular potential barrier in twisted bilayer graphene. Within the framework of the low-energy excitations, the transmission probability and the conductance are obtained depending on the parameters of the problem. Owing to the different chirality in twisted bilayer graphene, the propagation of charge carriers exhibits an anisotropic behavior in transmission probability and Fabry-Pérot resonances. Moreover, we show that the anisotropy of the charge carriers induces asymmetry and deflection in the Fabry-Pérot resonances and Klein tunneling, and they are extremely sensitive to the height of the potential applied. Additionally, we found that the conductance is strongly sensitive to the barrier height but weakly sensitive to the barrier width. Therefore, it is possible to control the maxima and minima of the conductance of charge carriers in twisted bilayer graphene. With our results, we gain an in-depth understanding of tunneling properties in twisted bilayer graphene, which may help in the development and designing of novel electronic nanodevices based on anisotropic 2D materials.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

arXiv2019

Magnetic Field Effect on Strained Graphene Junctions

We investigate the spin-dependent transport properties of a ferromagnetic/strained/normal graphene junctions with central region subjected to a magnetic field $B$. An analytical approach, based on Dirac equation, is implemented to obtain the eigenstates and eigenvalues of the charge carrier in three regions. Using the transfer matrix method, we determine the spin-dependent transmission in the presence of an applied strain along the armchair and zigzag directions of the graphene sample. We find that the strain remarkably modifies the Landau levels (LLs) originating from the applied $B$. It is shown that the spin up/down energy bands, in the first region, are shifted by the exchange $H_{ex}$ and left the whole spectrum linear as in the case of pristine graphene. In the central region, the position of the Dirac point changes due to the uniaxial strain and $B$. It is also found that the uniaxial strain in graphene induces a contraction of the LLs spectra. Moreover, the strain and $B$ modify the shape and position of some peaks in the transmission probabilities.

cond-mat.mes-hall↗

arXiv2018

Electron Scattering in Gapped Graphene Quantum Dots

Due to Klein tunneling in graphene only quasi-bound states are realized in graphene quantum dots by electrostatic gating. Particles in the quasi-bound states are trapped inside the dot for a finite time and they keep bouncing back and forth till they find their way out. Here we study the effect of an induced gap on the scattering problem of Dirac electrons on a circular electrostatically confined quantum dot. Introducing an energy gap inside the quantum dot enables us to distinguish three scattering regimes instead of two in the case of gapless graphene quantum dot. We will focus on these regimes and analyze the scattering efficiency as a function of the electron energy, the dot radius and the energy gap. Moreover, we will discuss how the system parameters can affect the scattering resonances inside the dot.

cond-mat.mes-hall↗

arXiv2017

Energy Levels of Quantum Ring in ABA-Stacked Trilayer Graphene

We present the solutions of the energy spectrum of charge carriers confined in quantum ring in ABA-stacked trilayer graphene subjected to a perpendicular magnetic field. The calculations were performed in the context of the continuum model by solving the Dirac equation for a zero width ring geometry, i.e. freezing out the carrier radial motion. We show that the obtained energy spectrum exhibits different symmetries with respect to the magnetic field and other parameters. The application of a potential shift the energy spectrum vertically while the application of a magnetic field breaks all symmetries. We compare our results with those of of the ideal quantum ring in monolayer and bilayer graphene.

cond-mat.mes-hall↗

arXiv2016

Energy Levels of an Ideal Quantum Ring in AA-Stacked Bilayer Graphene

We theoretically analyze the energy spectrum of a quantum ring in AA-stacked bilayer graphene with radius $R$ for a zero width subjected to a perpendicular magnetic field $B$. An analytical approach, using the Dirac equation, is implemented to obtain the energy spectrum by freezing out the carrier radial motion. The obtained spectrum exhibits different symmetries and for a fixed total angular momentum $m$, it has a hyperbolic dependence of the magnetic field. In particular, the energy spectra are not invariant under the transformation $B \longrightarrow -B$. The application of a potential, on the upper and lower layer, allows to open a gap in the energy spectrum and the application of a non zero magnetic field breaks all symmetries. We also analyze the basics features of the energy spectrum to show the main similarities and differences with respect to ideal quantum ring in monolayer, AB-stacked bilayer graphene and a quantum ring with finite width in AB-stacked bilayer graphene.

cond-mat.mes-hall↗

arXiv2016

AA-stacked Bilayer Graphene Quantum Dots in Magnetic Field

By applying the infinite-mass boundary condition, we analytically calculate the confined states and the corresponding wave functions of AA-stacked bilayer graphene quantum {dots} in the presence of an uniform magnetic field $B$. It is found that the energy spectrum shows two set of levels, which are the double copies of the energy spectrum for single layer graphene, shifted up-down by $+γ$ and $-γ$, respectively. However, the obtained spectrum exhibits different symmetries between the electron and hole states as well as the intervalley symmetries. It is noticed that, the applied magnetic field breaks all symmetries, except one related to the intervalley electron-hole symmetry, i.e. $E^e(τ,m)=-E^h(τ,m)$. Two different regimes of confinement are found: the first one is due to the infinite-mass barrier at weak $B$ and the second is dominated by the magnetic field as long as $B$ is large. We numerically investigated the basics features of the energy spectrum to show the main similarities and differences with respect to monolayer graphene, AB-stacked bilayer graphene and semiconductor quantum dots.

cond-mat.mes-hall↗

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

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↗

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↗

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↗