arXiv subjects

M. Tahir

Publications and source records attributed to M. Tahir.

47 records · Page 3Linked to original sources

arXiv2009

The Magnetoplasmon spectrum of a weakly modulated two-dimensional electron gas system

The magnetoplasmon spectrum of a magnetically modulated two-dimensional electron gas (MM2DEG) is investigated. We derive the inter and intra Landau band magnetoplasmon spectrum within the self consistent field approach. The derivation is performed at zero temperature as well as at finite temperature. Results are presented for the inter and intra Landau band magnetoplasmon spectrum as a function of the inverse magnetic field. As a result of magnetic modulation, magnetic Weiss oscillations are found to occur in the magnetoplasmon spectrum. Furthermore, our finite temperature theory facilitates analysis of effects of temperature on the magnetoplasmon spectrum. The results are compared with those obtained for an electrically modulated 2DEG system. In addition, we derive and discuss the effects of simultaneous electric and magnetic modulations on the magnetoplasmon spectrum of 2DEG when the modulations are in phase as well as when they are out of phase. Magnetic oscillations are affected by the relative phase of the two modulations and position of the oscillations depends on the relative strength of the two modulations in the former case while we find complete suppression of Weiss oscillations for particular relative strength of the modulations in the latter case.

cond-mat.mes-hall↗

arXiv2009

Magnetotransport of a periodically modulated graphene monolayer

We have performed a detailed investigation of the electrical properties of a graphene monolayer which is modulated by a weak one dimensional periodic potential in the presence of a perpendicular magnetic field. The periodic modulation broadens the Landau Levels into bands which oscillate with B. The electronic conduction in this system can take place through either diffusive scattering or collisional scattering off impurities. Both these contributions to electronic transport are taken into account in this work. In addition to the appearance of commensurability oscillations in both the collisional and diffusive contributions, we find that Hall resistance also exhibits commensurability oscillations. Furthermore, the period and amplitude of these commensurability oscillations in the transport parameters and how they are affected by temperature are also discussed in this work.

cond-mat.mes-hall↗

arXiv2009

Suppression of Weiss oscillations in the magnetoconductivity of modulated graphene monolayer

We have investigated the electrical transport properties of Dirac electrons in a monolayer graphene sheet in the presence of both electric and magnetic modulations. The effects of the modulations on quantum transport when they are in phase and out of phase are considered. We present the energy spectrum and the bandwidth of the Dirac electrons in the presence of both the modulations. We determine the $σ_{yy}$ component of the magnetoconductivity tensor for this system which is shown to exhibit Weiss oscillations.Asymptotic expressions for $σ_{yy}$ are also calculated to better illustrate the effects of in-phase and out-of-phase modulations.We find that the position of the oscillations in magnetoconductivity depends on the relative strength of the two modulations. When the two modulations are out-of-phase there is complete suppression of Weiss oscillations for particular relative strength of the modulations.

cond-mat.mes-hall↗

arXiv2008

Inter-band magnetoplasmons in mono- and bi-layer graphene

Collective excitations spectrum of Dirac electrons in mono and bilayer graphene in the presence of a uniform magnetic field is investigated. Analytical results for inter-Landau band plasmon spectrum within the self-consistent-field approach are obtained. SdH type oscillations that are a monotonic function of the magnetic field are observed in the plasmon spectrum of both mono- and bi-layer graphene systems. The results presented are also compared with those obtained in conventional 2DEG. The chiral nature of the quasiparticles in mono and bilayer graphene system results in the observation of $π$ and $2π$ Berry's phase in the SdH- type oscillations in the plasmon spectrum.

cond-mat.mes-hall↗

arXiv2008

Chiral tunneling through time-periodic potential in graphene

Chiral tunneling through a harmonically driven potential barrier in graphene monolayer is considered in this work. Since the quasiparticles in this system are chiral in nature, tunneling is highly anisotropic, we determine the transmission probabilities for the central and sidebands as the incident angle of the electron beam is changed . Furthermore, we investigate how the transmission probabilities change as the width, amplitude and frequency of the oscillating barrier is changed. An interesting result of our study is that perfect transmission for normal incidence that has been reported for a static barrier persists for the oscillating barrier, manifestation of Klein tunneling in a time harmonic potential.

cond-mat.mes-hall↗

arXiv2008

Energy loss of charged particles in a two-dimensional Dirac plasma

The stopping power and energy loss rate of charged particles traversing a two-dimensional Dirac plasma is investigated. The Dirac plasma considered here models a solid state system, recently realized graphene monolayer, where the conduction electrons obey the Dirac-like equation and exhibit a linear in momentum dispersion relation. Theoretical work presented here is based on the the dielectric response function and the dynamical structure function within the Random-Phase-Approximation (RPA).

cond-mat.mes-hall↗

arXiv2008

Weiss oscillations in the magnetoconductivity of modulated graphene bilayer

We present a theoretical study of Weiss oscillations in magnetoconductivity of bilayer graphene. Bilayer graphene in the presence of a perpendicular magnetic field and a unidirectional weak electric modulation is considered.We determine the $σ_{yy}$ component of the magnetoconductivity tensor for this system which is shown to exhibit Weiss oscillations. We show that Weiss oscillations in the magnetoconductivity of bilayer graphene are enhanced and more robust with temperature compared to those in conventional two-dimensional electron gas systems whereas they are less robust with temperature compared to monolayer graphene. In addition, we also find phase differences of $π$ and $2π$ in the magnetoconductivity oscillations compared to monolayer graphene and conventional 2DEG system which arises due to the chiral nature of quasiparticles in graphene.

cond-mat.mes-hall↗

arXiv2008

Thermodynamic Properties of electrically modulated monolayer Graphene

Theoretical investigation of thermodynamic properties of electrically modulated monolayer graphene in the presence of a perpendicular magnetic field B is presented.This work is aimed at determining the modulation induced effects on the thermodynamic properties of electrically modulated graphene.The results obtained are compared with those of the conventional 2DEG. The one-dimensional periodic potential due to electric modulation lifts the degeneracy of the Landau Levels and converts them into bands whose width oscillates as a function of B. We find Weiss type oscillations for small values of B and dHvA type oscillations at larger values values of $B$. We find that the modulation induced effects on the thermodynamic properties are enhanced and less damped with temperature in graphene compared with conventional 2DEG system.

cond-mat.mes-hall↗

arXiv2008

Quantum transport of Dirac electrons in graphene in the presence of a spatially modulated magnetic field

We have investigated the electrical transport properties of Dirac electrons in a monolayer graphene sheet in the presence of a perpendicular magnetic field that is modulated weakly and periodically along one direction.We find that the Landau levels broaden into bands and their width oscillates as a function of the band index and the magnetic field.We determine the $σ_{yy}$ component of the magnetoconductivity tensor for this system which is shown to exhibit Weiss oscillations.We also determine analytically the asymptotic expressions for $σ_{yy}$.We compare these results with recently obtained results for electrically modulated graphene as well as those for magnetically modulated conventional two-dimensional electron gas (2DEG) system.We find that in the magnetically modulated graphene system cosidered in this work,Weiss oscillations in $σ_{yy}$ have a reduced amplitude compared to the 2DEG but are less damped by temperature while they have a higher amplitude than in the electrically modulated graphene system. We also find that these oscillations are out of phase by $π$ with those of the electrically modulated system while they are in phase with those in the 2DEG system.

cond-mat.mes-hall↗

arXiv2007

Theory of Weiss oscillations in the magnetoplasmon spectrum of Dirac electrons in graphene

We present the collective excitations spectrum (magnetoplasmon spectrum) of Dirac electrons in a weakly modulated single graphene layer in the presence of a uniform magnetic field. We consider electric modulation in one-dimension and the magnetic field applied perpendicular to graphene.We derive analytical results for the intra-Landau band plasmon spectrum within the self-consistent-field approach. We find Weiss oscillations in the magnetoplasmon spectrum which is the primary focus of this work. Results are presented for the intra-Landau band magnetoplasmon spectrum as a function of inverse magnetic field. These results are also compared with those of conventional 2DEG. We have found that the Weiss oscillations in the magnetoplasmon spectrum are larger in amplitude compared to those in conventional 2DEG for the same modulation strength, period of modulation and electron density.

cond-mat.mes-hall↗

arXiv2007

Collective Excitations Spectrum in Density Modulated One-Dimensional Electron Gas in a Magnetic Field

We determine the collective excitations spectrum and discuss the numerical results for a parabolically confined density modulated quasi-one dimensional electron gas (1DEG) in the presence of an external magnetic field. We derive the inter-and intra-band magnetoplasmon spectrum within the Self Consistent Field (SCF) approach. In this work we focus on magnetoplasmon oscillations in this system and as such results are presented for the intra-Landau-band magnetoplasmon spectrum that exhibits oscillatory behavior, these oscillations are not with constant period in 1/B and are significantly effected at low B and corresponding high 1/B.

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