M. B. A. Jalil

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 stripes. 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.

Gauge fields in spintronics

We present an overview of gauge fields in spintronics, focusing on their origin and physical consequences. Important topics, such as the Berry gauge field associated with adiabatic quantum evolution as well as gauge fields arising from other non-adiabatic considerations, are discussed. We examine the appearance and effects of gauge fields across three spaces, namely real-space, momentum-space, and time, taking on a largely semiclassical approach. We seize the opportunity to study other “spin-like” systems, including graphene, topological insulators, magnonics, and photonics, which emphasize the ubiquity and importance of gauge fields. We aim to provide an intuitive and pedagogical insight into the role played by gauge fields in spin transport.

Spin filtering in a two-dimensional electron gas device with asymmetric spatially spread magnetic-electric barriers

We calculate, based on the effective mass Hamiltonian, the spin dependence of ballistic transport through a two-dimensional electron gas (2DEG) under the influence of an electric U and a pair of magnetic B barriers with finite spatial width. The spin polarization of electron transmission P is strongly dependent on the degree of asymmetry in the magnetic barriers. An asymmetry in the B barrier widths and heights which conserves the magnetic vector potential A along the conduction path leads to a low P of less than 1% in the high transmission region. If conservation of A is broken, much higher P values of up to 30% is achievable but at the cost of increasing the energy threshold Eth. Finally, we suggest a scheme which utilizes a thin break in the 2DEG conduction path to achieve a comparable modulation of P of 25%, while conserving A and maintaining Eth at moderate electron energies.

High spin filtering using multiple magnetoelectric barriers

A periodic array of magnetoelectric barriers is modeled to achieve maximum spin polarization (P) at high transmission probability (T). Each double-pair unit of the array consists of four magnetic barriers designed in several ways, such that an electron passing through, in the Landau gauge A=(0,Ay(x),0), acquires zero gain in kinetic energy. This enables multiple double-pairs to be used to enhance P without sacrificing T. By tuning the magnetoelectric barrier heights, a high P of 75%–100% is obtained at 0.8–1.0EF, for a 27 unit array. For antisymmetrical arrays, electrical barriers act as a switch to the polarization capability.

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.

Mapping the Monte Carlo scheme to Langevin dynamics: a Fokker-Planck approach.

We propose a general method of using the Fokker-Planck equation (FPE) to link the Monte Carlo (MC) and the Langevin micromagnetic schemes. We derive the drift and diffusion FPE terms corresponding to the MC method and show that it is analytically equivalent to the stochastic Landau-Lifshitz-Gilbert (LLG) equation of Langevin-based micromagnetics. Subsequent results such as the time-quantification factor for the Metropolis MC method can be rigorously derived from this mapping equivalence. The validity of the mapping is shown by the close numerical convergence between the MC method and the LLG equation for the case of a single magnetic particle as well as interacting arrays of particles. We also find that our Metropolis MC method is accurate for a large range of damping factors alpha, unlike previous time-quantified MC methods which break down at low alpha, where precessional motion dominates.

Spin transverse separation in a two-dimensional electron-gas using an external magnetic field with a topological chirality

We propose a two-dimensional electron gas (2DEG) system in which an external magnetic (B) field with a small chirality is applied to provide a topological U(1) U(1) gauge field that separates conduction electrons of opposite spins in the transverse direction. Additionally, the vertical electric (E) field in the 2DEG, together with spin-orbit coupling, produces a SU(2) gauge field which reinforces / opposes the effect of the topological gauge. The system thus provides a tunable spin Hall effect, where an applied gate voltage on the 2DEG can be used to modulate the transverse spin current. As this method leads to the enhancement or cancellation of intrinsic spin Hall, it naturally distinguishes the extrinsic from the intrinsic effect.

ANALYSIS OF MULTIPLE-TUNNEL JUNCTIONS AND THEIR APPLICATION TO BIDIRECTIONAL ELECTRON PUMPS

A quantitative analysis is performed of a bidirectional electron pump circuit that incorporates multiple-tunnel junctions (MTJs), with special emphasis on stability diagrams. The number of electrons transferred per cycle is calculated to be in general agreement with experimental data. The frequency and temperature conditions under which the circuit deviates from ideal pumping operation are evaluated, which suggests that the prospect for a metrological application is not improved by the presence of ultrasmall capacitances in the MTJs.

Unified description of intrinsic spin-Hall effect mechanisms

The intrinsic spin-Hall effects (SHEs) in p-doped semiconductors (Murakami et al Science 301 1348) and two-dimensional electron gases with Rashba spin–orbit coupling (Sinova et al 2004 Phys. Rev. Lett. 92 126603) 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 Rashba systems, on the other hand, results from momentum-dependent spin dynamics in the presence of an external electric field. Our motivation in this paper is to address the disparity between the two mechanisms and, in particular, to clarify whether there is any underlying link between the two effects. In this endeavor, we consider the explicit time dependence of SHE systems starting with a general spin–orbit model in the presence of an electric field. 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 applying the same limit to the equations of motion of the general model, this magnetic field is also found to be the underlying origin of the anomalous velocity due to the momentum-space curvature. Thus, our study unifies the two seemingly disparate intrinsic SHEs under a common adiabatic framework.

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 band gap modulation induced by a perpendicular B-field. The applied B-field causes the lowest conduction subband and the topmost valence subband to move closer to one another to form the n=0 Landau level. We exploit this effect to realize a device relevant magnetoresistive (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 th...