arXiv subjects

M. B. A. Jalil

Publications and source records attributed to M. B. A. Jalil.

At least 19 recordsLinked to original sources

arXiv2022

Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in Weyl semimetals with tilted energy dispersion

Ruderman-Kittel-Kasuya-Yosida (RKKY) is an essential long range magnetic interaction between magnetic impurities or magnetic layered structures, the magnitude of which oscillates with the distance ($\mathrm{R}$) between them. We have investigated the RKKY interaction between two magnetic impurities in both time-reversal and inversion symmetry broken Weyl semimetals (WSMs) where the energy dispersion is tilted in momentum space and the momentum of the conduction electron is locked with the pseudo-spin. Two important features are revealed, firstly, at the small tilt limit, we show that the RKKY coupling varies quadratically with the tilt parameter and strikingly, at large separation distance $\mathrm{R}$, the coupling decays as $1/\mathrm{R}$ compared to the conventional of $1/\mathrm{R}^{3}$ dependence exhibited by WSMs with non-tilted dispersion. The slower decay by two orders i.e. ($1/\mathrm{R}$ as opposed to $1/\mathrm{R}^{3}$) of the RKKY coupling is significant for maintaining long range RKKY coupling. Secondly, the RKKY coupling exhibits an anisotropy with respect to the angle between the tilt direction ($\mathbf{w}$) and the separation direction $\mathbf{R}$ unlike the case of non-tilted WSMs which exhibit isotropic RKKY coupling. Consequently, the RKKY coupling in tilted WSMs alternately favours ferromagnetic and anti-ferromagnetic orders and vice-versa with the change of the angle. Our results are derived analytically and verified by numerical calculations based on realistic parameter values.

cond-mat.mes-hall↗

arXiv2019

Magnetotransport of Weyl semimetals with tilted Dirac cones

Weyl semimetals (WSM) exhibit chiral anomaly in their magnetotransport due to broken conservation laws. Here, we analyze the magnetotransport of WSM in the presence of the time-reversal symmetry-breaking tilt parameter. The analytical expression for the magnetoconductivity is derived in the small tilt limit using the semiclassical Boltzmann equation. We predict a planar Hall current which flows transverse to the electric field and in the plane containing magnetic and electric fields and scales linearly with the tilt parameter. A tilt-induced transverse conductivity is also present in the case where the electric and magnetic fields are parallel to each other, a scenario where the conventional Hall current completely vanishes.

cond-mat.mes-hall↗

arXiv2018

Strain-controlled valley and spin separation in silicene heterojunctions

We adopt the tight-binding mode-matching method to study the strain effect on silicene heterojunctions. It is found that valley- and spin-dependent separation of electrons cannot be achieved by the electric field only. When a strain and an electric field are simultaneously applied to the central scattering region, not only are the electrons of valleys K and K' separated into two distinct transmission lobes in opposite transverse directions, but the up-spin and down-spin electrons will also move in the two opposite transverse directions. Therefore, one can realize an effective modulation of valley- and spin-dependent transport by changing the amplitude and the stretch direction of the strain. The phenomenon of the strain-induced valley and spin deflection can be exploited for silicene-based valleytronics devices.

cond-mat.mes-hall↗

arXiv2015

Nondegenerate and almost hexagonal skyrmion lattices

We obtain the lowest energy solutions for the skymion field equations and their corresponding vortex structures. Two nondegenerate solutions emerge with their vortex swirls in opposite directions. The solutions are associated with an extremum property, which favors an array of almost hexagonal shape. We predict that a regular hexagonal lattice must have a mix of skyrmions of both swirls. Although our solutions could not keep the norm of the magnetization constant at unity, their greatest deviation from unity occurred in regions where the spins are far from planar; we show how to improve this situation.

cond-mat.mes-hall↗

arXiv2015

Topological dynamics and current-induced motion in a skyrmion lattice

We study the Thiele equation for current-induced motion in a skyrmion lattice through two soluble models of the pinning potential. Comprised by a Magnus term, a dissipative term and a pinning force, Thiele's equation resembles Newton's law but in virtue of the topological character of the first two, it differs significantly from Newtonian mechanics and because the Magnus force is dominant, unlike its mechanical counterpart, the Coriolis force, skyrmion trajectories do not necessarily have mechanical counterparts. This is important if we are to understand skykrmion dynamics and tap into its potential for data-storage technology. We identify a pinning threshold velocity for the one-dimensional potential and for a two-dimensional potential we find a pinning point and the skyrmion trajectories toward the point are spirals whose frequency (compare Kepler's second law) and amplitude decay depends only on the Gilbert constant and potential at the pinning point.

cond-mat.mes-hall↗

arXiv2012

High magnetoresistance in graphene nanoribbon heterojunction

We show a large magnetoresistance(MR) effect in a graphene heterostructure consisting of an metallic(M) and semiconductor(SC)-type armchair-graphene-nanoribbon(aGNR). In the heterostructure, the transmission across the first subband of the SC-aGNR and M-aGNR is forbidden under zero magnetic-field, due to the orthogonality of the wavefunctions. A finite magnetic-field introduces the quantum hall-like effect, which distorts the wavefunctions. Thus, a finite transmission occurs across the heterojunction, giving rise to a large MR effect. We study the dependence of this MR on temperature and electron energy. Finally, we design a magnetic-field-effect-transistor which yields a MR of close to 100%(85%) at low(room) temperature.

cond-mat.mes-hall↗

arXiv2012

Electrical modulation of the edge channel transport in topological insulators coupled to ferromagnetic leads

The counterpropagating edge states of a two-dimensional topological insulator (TI) carry electrons of opposite spins. We investigate the transport properties of edge states in a two-dimensional TI which is contacted to ferromagnetic leads. The application of a side-gate voltage induces a constriction or quantum point contact (QPC) which couples the two edge channels. The transport properties of the system is calculated via the Keldysh nonequilibrium Green's function method. We found that inter-edge spin-flip coupling can significantly enhance (suppress) the charge current when the magnetization of the leads are anti-parallel (parallel) to one another. On the other hand, spin-conserving inter-edge coupling generally reduces the current by backscattering regardless of the magnetization configuration. The charge current and the conductance as a function of the bias voltage, also exhibit similar trends with respect to spin-flip coupling strength, for both parallel and anti-parallel configurations. Hence, gate voltage modulation of edge states via a QPC can provide a means of modulating the spin or charge current flow in TI-based spintronics devices.

cond-mat.mes-hall↗

arXiv2012

Inverse Spin Hall Effect in Ferromagnetic Metal with Rashba Spin Orbit Coupling

We report an intrinsic form of the inverse spin Hall effect (ISHE) in ferromagnetic (FM) metal with Rashba spin orbit coupling (RSOC), which is driven by a normal charge current. Unlike the conventional form, the ISHE can be induced without the need for spin current injection from an external source. Our theoretical results show that Hall voltage is generated when the FM moment is perpendicular to the ferromagnetic layer. The polarity of the Hall voltage is reversed upon switching the FM moment to the opposite direction, thus promising a useful readback mechanism for memory or logic applications.

cond-mat.mes-hall↗

arXiv2012

Non-equilibrium spatial distribution of Rashba spin torque in ferromagnetic metal layer

We study the spatial distribution of spin torque induced by a strong Rashba spin-orbit coupling (RSOC) in a ferromagnetic (FM) metal layer, using the Keldysh non-equilibrium Green's function method. In the presence of the s-d interaction between the non-equilibrium conduction electrons and the local magnetic moments, the RSOC effect induces a torque on the moments, which we term as the Rashba spin torque. A correlation between the Rashba spin torque and the spatial spin current is presented in this work, clearly mapping the spatial distribution of Rashba Spin torque in a nano-sized ferromagnetic device. When local magnetism is turned on, the out-of-plane (Sz) Spin Hall effect (SHE) is disrupted, but rather unexpectedly an in-plane (Sy) SHE is detected. We also study the effect of Rashba strength (α_R) and splitting exchange (Δ) on the non-equilibrium Rashba spin torque averaged over the device. Rashba spin torque allows an efficient transfer of spin momentum such that a typical switching field of 20 mT can be attained with a low current density of less than 10^6 A/cm^2.

cond-mat.mes-hall↗

arXiv2011

High magnetoresistance at room temperature in p-i-n graphene nanoribbons due to band-to-band tunneling effects

A large magnetoresistance effect is obtained at room-temperature by using p-i-n armchair-graphene-nanoribbon (GNR) heterostructures. The key advantage is the virtual elimination of thermal currents due to the presence of band gaps in the contacts. The current at B=0T is greatly decreased while the current at B>0T is relatively large due to the band-to-band tunneling effects, resulting in a high magnetoresistance ratio, even at room-temperature. Moreover, we explore the effects of edge-roughness, length, and width of GNR channels on device performance. An increase in edge-roughness and channel length enhances the magnetoresistance ratio while increased channel width can reduce the operating bias.

cond-mat.mes-hall↗

arXiv2011

Topological Insulator Cell for Memory and Magnetic Sensor Applications

We propose a memory device based on magnetically doped surfaces of 3D topological insulators. Magnetic information stored on the surface is read out via the quantized Hall effect, which is characterized by a topological invariant. Consequently, the read out process is insensitive to disorder, variations in device geometry, and imperfections in the writing process.

cond-mat.mes-hall↗

arXiv2010

The effect of magnetic field and disorders on the electronic transport in graphene nanoribbons

We developed a unified mesoscopic transport model for graphene nanoribbons, which combines the non-equilibrium Green's function (NEGF) formalism with the real-space π-orbital model. Based on this model, we probe the spatial distributions of electrons under a magnetic field, in order to obtain insights into the various signature Hall effects in disordered armchair graphene nanoribbons (AGNR). In the presence of a uniform perpendicular magnetic field (B\perp-field), a perfect AGNR shows three distinct spatial current profiles at equilibrium, depending on its width. Under non-equilibrium conditions (i.e. in the presence of an applied bias), the net electron flow is restricted to the edges and occurs in opposite directions depending on whether the Fermi level lies within the valence or conduction band. For electrons at energy level below the conduction window, the B\perp-field gives rise to local electron flux circulation, although the global flux is zero. Our study also reveals the suppression of electron backscattering as a result of the edge transport which is induced by the B\perp-field. This phenomenon can potentially mitigate the undesired effects of disorders, such as the bulk and edge vacancies, on the transport properties of AGNR. Lastly, we show that the effect of B\perp-field on electronic transport is less significant in the multimode compared to the single mode electron transport.

cond-mat.mes-hall↗

arXiv2010

Charge fractionalization in biased bilayer graphene

We study charge fractionalization in bilayer graphene which is intimately related to its zero modes. In the unbiased case, the valley zero modes occur in pairs rendering it unsuitable for charge fractionalization. A bias plays the role of a bosonic field with nontrivial topology allowing for the exploration of Dirac-like dynamics at higher particle momenta. It also induces an odd number of zero modes, which, together with the conjugation symmetry between positive and negative energy states, are the requisite conditions for charge fractionalization. A self-conjugate, localized zero mode is constructed for a semi-infinite graphene sheet with zigzag edge; scenarios can occur readily where one sublattice component (pseudospin) dominates. While the other valley also has a similar zero mode, a layer asymmetry can be invoked to lift this degeneracy allowing for detection of distinguishable charge-1/2 edge states per valley.

cond-mat.mes-hall↗

arXiv2010

Magnetoresistive effect in graphene nanoribbon due to magnetic field induced band gap modulation

The electronic properties of armchair graphene nanoribbons (AGNRs) can be significantly modified from semiconducting to metallic states, by applying a uniform perpendicular magnetic field (B-field). Here, we theoretically study the bandgap modulation induced by a perpendicular B-field. The applied B-field causes the lowest conduction subband and the top-most valence subband to move closer to one another to form the n=0 Landau level. We exploit this effect to realize a device relevant MR modulation. Unlike in conventional spin-valves, this intrinsic MR effect is realized without the use of any ferromagnetic leads. The AGNRs with number of dimers, Na=3p+1 [p=1,2,3,...] show the most promising behavior for MR applications, with large conductance modulation and hence, high MR ratio at the optimal source-drain bias. However, the MR is suppressed at higher temperature due to the spread of the Fermi function distribution. We also investigate the importance of the source-drain bias in optimizing the MR. Lastly, we show that edge roughness of AGNRs has the unexpected effect of improving the magnetic sensitivity of the device and thus increasing the MR ratio.

cond-mat.mes-hall↗

arXiv2010

Local spin polarization of Landau levels under Rashba spin-orbit coupling

We investigate the local spin polarization texture of Landau levels under Rashba spin-orbit coupling in bulk two-dimensional electron gas (2DEG) systems. In order to analyze the spin polarization as a function of two-dimensional coordinates within the 2DEG, we first solve the system eigenstates in the symmetric gauge. Our exact analytical wavefunction solutions are shown to be gauge invariant with solutions obtained in the commonly used Landau gauge. We illustrate the two-dimensional spatial spin profile for a single Landau level and suggest means to measure and utilize the local polarization in practice.

math-ph↗

arXiv2010

Valley filter in strain engineered graphene

We propose a simple, yet highly efficient and robust device for producing valley polarized current in graphene. The device comprises of two distinct components; a region of uniform uniaxial strain, adjacent to an out-of-plane magnetic barrier configuration formed by patterned ferromagnetic gates. We show that when the amount of strain, magnetic field strength, and Fermi level are properly tuned, the output current can be made to consist of only a single valley contribution. Perfect valley filtering is achievable within experimentally accessible parameters.

cond-mat.mes-hall↗

arXiv2009

Unified Description of the Intrinsic Spin-Hall Effects

The intrinsic spin-Hall effects (SHE) in $p$-doped semiconductors [S. Murakami et al., Science 301, 1348 (2003)] and two-dimensional electron gases with Rashba spin-orbit coupling [J. Sinova et al., Phys. Rev. Lett. 92, 126603 (2004)] have been the subject of many theoretical studies, but their driving mechanisms have yet to be described in a unified manner. The former effect arises from the adiabatic topological curvature of momentum space, from which holes acquire a spin-dependent anomalous velocity. The SHE in the Rashba system, on the other hand, results from the momentum-dependent spin dynamics in the presence of an external electric field. The two effects clearly appear to originate from distinct mechanisms. Our motivation for this article is to address this apparent disparity and, in particular, to seek a unifying description of the effects. In this endeavor, we consider the explicit time-dependence of SHE systems starting with a general spin-orbit model. We find that by performing a gauge transformation of the general model with respect to time, a well-defined gauge field appears in time space which has the physical significance of an effective magnetic field. This magnetic field is shown to precisely account for the SHE in the Rashba system in the adiabatic limit. Remarkably, by carefully analyzing the equation of motion of the general model, this field component is also found to be the origin of the anomalous velocity due to the momentum space curvature. Our study therefore unifies the two seemingly disparate intrinsic SHEs under a common adiabatic framework.

cond-mat.mes-hall↗

arXiv2009

Unified Semi-Classical Description of Intrinsic Spin-Hall Effect in Spintronic, Optical, and Graphene Systems

A semi-classical description of the intrinsic spin-Hall effect (SHE) is presented which is relevant for a wide class of systems. A heuristic model for the SHE is developed, starting with a fully quantum mechanical treatment, from which we construct an intuitive expression for the spin-Hall current and conductivity. Our method makes transparent the physical mechanism which drives the effect, and unifies the SHE across several spintronic and optical systems. Finally, we propose an analogous effect in bilayer graphene.

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