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ABSTRACT: Polarized electromagnetic waves passing through (reflected from) a dielectric medium parallel to a magnetic field undergo Faraday (Kerr) rotation of their polarization. Recently, Faraday rotation angles as much as 0.1 rad were observed for terahertz waves propagating through graphene over a SiC substrate. We show that the same effect is observable with the magnetic field replaced by an in-plane strain field which induces a pseudomagnetic field in graphene. With two such sheets a rotation of π/4 can be achieved, which is the required rotation for an optical diode. Similarly a Kerr rotation of 1/4 rad is predicted from a single reflection from a strained graphene sheet.
Optics Letters 08/2012; 37(15):3237-9. · 3.40 Impact Factor
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ABSTRACT: Fractional charge may arise when fermionic zero modes exist in a topological background field. In biased bilayer graphene (BBLG), the bias plays the role of the nontrivial background field. When semi-infinite BBLG with a zigzag edge is used, the dynamics induces an odd number of zero-energy modes, which, together with the conjugation symmetry between positive- and negative-energy states, are the requisite conditions for fractionalization. Exploiting the trigonal interaction to isolate a given zero-energy mode on the zigzag edge, we consider extended and localized modes (the latter being obtained from a localized wavepacket generated by prior irradiation of the sample with an electromagnetic vortex). The valley degeneracy is lifted by a layer asymmetry, while an edge-induced spin polarization breaks the spin degeneracy. We describe scenarios for the detection of charge-1/2 edge states.
Journal of Physics Condensed Matter 07/2012; 24(33):335302. · 2.55 Impact Factor
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ABSTRACT: 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.
07/2011;
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ABSTRACT: 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.
06/2011;
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03/2011; , ISBN: 978-953-307-152-7
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ABSTRACT: The Klein tunneling of charge pairs in an electrostatically created p-n junction of monolayer graphene is shown to occur at an observable rate for moderate fields. The pairs undergo zitterbewegung (ZBW) in opposite directions leading to their separation and transverse dipole moment, since the valleys contribute constructively. The dipole moment depends critically on the exponential collimation characteristic of Klein tunneling and serves as a diagnostic signature of ZBW.
Applied Physics Letters 09/2010; · 3.84 Impact Factor
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ABSTRACT: We developed a unified mesoscopic transport model for graphene nanoribbons, which combines the nonequilibrium Green's function (NEGF) formalism with the real-space π-orbital model. Based on this model, we probe the spatial distribution 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[Symbol: see text]-field), a perfect AGNR shows three distinct spatial current profiles at equilibrium, depending on its width. Under nonequilibrium 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 an energy level below the conduction window, the B[Symbol: see text]-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[Symbol: see text]-field. This phenomenon can potentially mitigate the undesired effects of disorder, such as bulk and edge vacancies, on the transport properties of AGNR. Lastly, we show that the effect of [Formula: see text]-field on electronic transport is less significant in the multimode compared to the single-mode electron transport.
Journal of Physics Condensed Matter 09/2010; 22(37):375303. · 2.55 Impact Factor
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ABSTRACT: 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. Comment: 13 pages, 5 figures
06/2010;
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ABSTRACT: 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. Comment: 4 pages, 3 figures; minor corrections, updated Figs. 2 and 3, added references
05/2010;
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ABSTRACT: 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.
New Journal of Physics 01/2010; 12(1):013016. · 4.18 Impact Factor
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ABSTRACT: When bilayer graphene is gated with a kink potential, pair particle localization occurs at the kink where a particle (electron) and its chiral partner (hole) are held in balance by electrostatic coupling. Zero-energy states (zero modes) are always present in pairs and occur at the same point in the dispersion graph, regardless of kink strength. The robust and binary nature of the kink-induced modes, which are topologically protected against disorder, and the ease with which a kink is created suggest applications in switching devices or information storage.
Applied Physics Letters 12/2009; · 3.84 Impact Factor
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ABSTRACT: We study graphene monolayer charge carriers irradiated by an electromagnetic vortex. From this, two scenarios are envisaged: canonical oscillator coherent states, which form for large particle numbers and from which a sublattice filter can be constructed, and pair-coherent states, which emerge when the carrier velocity is much less than the Fermi velocity and which can exhibit nonclassical properties. The first should be useful in the control (e.g., confinement and guided transport) of graphene electrons, while the second provides a physical system for examining nonclassical properties of wave packets.
Applied Physics Letters 10/2009; · 3.84 Impact Factor
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ABSTRACT: We proposed that a viable form of spin current transistor is one to be made from a single-mode device which passes electrons through a series of magnetic-electric barriers built into the device. The barriers assume a wavy spatial profile across the conduction path due to the inevitable broadening of the magnetic fields. Field broadening results in a linearly increasing vector potential across the conduction channel, which increases spin polarization. We have identified that the important factors for generating high spin polarization and conductance modulation are the low source-drain bias, the broadened magnetic fields, and the high number of FM gates within a fixed channel length.
Nanotechnology 10/2009; 20(36):365204. · 3.98 Impact Factor
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ABSTRACT: 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. Comment: 5 pages, 2 figures, 1 table
03/2009;
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ABSTRACT: 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. Comment: 20 pages, 1 figure
03/2009;
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ABSTRACT: A charged particle whose energy is less than the electric potential step it is incident upon, is expected to undergo partial reflection and transmission. In bilayer graphene, however, a potential step in the form of an antisymmetric kink results in particle localization due to the interaction between the particle and its chiral partner. It is found that when the potential step exceeds a threshold, zero-energy modes of the system emerge, and causes the kink to acquire a charge. The Hall-effect plateaus in the vicinity of the zero modes correspond, unexpectedly, to those of the monolayer. The topological nature of these kink-induced effects and the ease with which a kink can be generated in practice, suggest possible applications in e.g. storage of information or switching devices.
12/2008;
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ABSTRACT: We show that a moderately strong constant electric field in the plane of a monolayer graphene sheet can create particle-hole pairs at an observable rate. The pairs undergo zitterbewegung in opposite directions leading to a Hall-like separation of the charge carriers and a measurable transverse dipole moment is predicted which serves as the signature of the zitterbewegung. In contrast with the created pairs, the zero modes of the excitation induce a current transverse to the electric field but do not result in separated charges. For bilayer graphene a similar effect by the electric field is shown not to be possible.
07/2008;
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ABSTRACT: We present a general method for evaluating the maximum transmitted spin polarization and optimal spin axis for an arbitrary spin-orbit coupling (SOC) barrier system, in which the spins lie in the azimuthal plane and finite spin polarization is achieved by wavevector filtering of electrons. Besides momentum filtering, another prerequisite for finite spin polarization is asymmetric occupation or transmission probabilities of the eigenstates of the SOC Hamiltonian. This is achieved most efficiently by resonant tunneling through multiple SOC barriers. We apply our analysis to common SOC mechanisms in semiconductors: pure bulk Dresselhaus SOC, heterostructures with mixed Dresselhaus and Rashba SOC and strain-induced SOC. In particular, we find that the interplay between Dresselhaus and Rashba SOC effects can yield several advantageous features for spin filter and spin injector functions, such as increased robustness to wavevector spread of electrons.
Journal of Physics Condensed Matter 03/2008; 20(11):115206. · 2.55 Impact Factor
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ABSTRACT: We introduce a spin tunneling theory across the interfaces of multilayer spintronic devices, e.g., spin valves and magnetic random access memories, which integrates the microscopic Green’s function formalism for electron propagation within the interfacial barrier with the macroscale spin-dependent Boltzmann theory, which governs the spin accumulation in the adjacent contacts. This multiscale approach makes possible the detailed studies of interfacial properties (e.g., height, shape, and spin asymmetry) required to achieve high spin injection via tunneling. Based on the calculated results, the optimal interfacial properties have been identified for possible experimental verification.
Phys. Rev. B. 02/2008; 77(8).
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ABSTRACT: We describe an intrinsic spin-Hall effect in $n$-type bulk zinc-blende semiconductors with topological origin. When electron transport is confined to a waveguide structure, and the applied electric field is such that the spins of electrons remain as eigenstates of the Dresselhaus spin-orbit field with negligible subband mixing, a gauge structure appears in the momentum space of the system. In particular, the momentum space exhibits a non-trivial Berry curvature which affects the transverse motion of electrons anisotropically in spin, thereby producing a finite spin-Hall effect. The effect should be detectable using standard techniques in the literature such as Kerr rotation, and be readily distinguishable from other mechanisms of the spin-Hall effect. Comment: 6 pages, 3 figures
09/2007;
