arXiv subjects

Seng Ghee Tan

Publications and source records attributed to Seng Ghee Tan.

At least 37 records · Page 2Linked to original sources

arXiv2016

Effect of surface state hybridization on current-induced spin-orbit torque in thin topological insulator films

We investigate the current-induced spin-orbit torque in thin topological insulator (TI) films in the presence of hybridization between the top and bottom surface states. We formulate the relation between spin torque and TI thickness, from which we derived the optimal value of the thickness to maximize the torque. We show numerically that in typical TI thin films made of $\mathrm{Bi_2Se_3}$, the optimal thickness is about 3-5 nm.

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arXiv2016

The role of exchange interaction in nitrogen vacancy centre-based magnetometry

We propose a multilayer device comprising of a thin-film-based ferromagnetic hetero-structure (FMH) deposited on a diamond layer doped with nitrogen vacancy centers (NVC's). We find that when the NVC's are in close proximity (1-2 nm) with the FMH, the exchange energy is comparable to, and may even surpass the magnetostatic interaction energy. This calls for the need to consider and utilize both effects in magnetometry based on NVC's in diamond. As the distance between the FMH and NVC is decreased to the sub-nanometer scale, the exponential increase in the exchange energy suggests spintronic applications of NVC beyond magnetometry, such as detection of spin-Hall effect or spin currents.

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arXiv2016

Spin accumulation in disordered topological insulator thin films

Topological insulator (TI) thin films differ from the more commonly studied semi-infinite bulk TIs in that the former possesses both top and bottom surfaces where the surface states localized at different surfaces can couple to one another due to the finite thickness of the film. In the presence of an in-plane mangnetization TI thin films display two distinct phases depending on which of the inter-surface coupling or the magnetization is stronger. In this work, we consider a TI thin film system with an in-plane magnetization and calculate numerically the resulting spin accumulation on both surfaces of the film due to an in-plane electric field to linear order. We describe a numerical scheme for performing the Kubo calculation calculation in which we include impurity scattering and vertex corrections. We find that the sums of the spin accumulation over the two surfaces in the in-plane direction perpendicular to the magnetization, and in the out of plane direction, are antisymmetric in Fermi energy about the charge neutrality point and are non-vanishing only when the symmetry between the top and bottom TI surfaces is broken. The impurity scattering, in general, diminishes the magnitude of the spin accumulation and can also change the sign of the spin accumulation at some Fermi energies where the accumulation is small.

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arXiv2016

Effective Hamiltonian for surface states of topological insulator nanotubes

In this work we derive an effective Hamiltonian for the surface states of a hollow topological insulator (TI) nanotube with finite width walls. Unlike a solid TI cylinder, a TI nanotube possesses both an inner as well as outer surface on which the states localized at each surface are coupled together. The curvature along the circumference of the nanotube leads to a spatial variation of the spin orbit interaction field experienced by the charge carriers as well as an asymmetry between the inner and outer surfaces of the nanotube. Both of these features result in terms in the effective Hamiltonian for a TI nanotube absent in that of a flat TI thin film of the same thickness. We calculate the numerical values of the parameters for a \ce{Bi2Se3} nanotube as a function of the inner and outer radius, and show that the differing relative magnitudes between the parameters result in qualitatively differing behaviour for the eigenstates of tubes of different dimensions.

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arXiv2016

Influence of Fermi arc states and double Weyl node on tunneling in a Dirac semimetal

Most theoretical studies of tunneling in Dirac and the closely related Weyl semimetals have modeled these materials as single Weyl nodes described by the three-dimensional Dirac equation $H = v_f \vec{p}\cdot\vecσ$. The influence of scattering between the different valleys centered around different Weyl nodes, and the Fermi arc states which connect these nodes are hence not evident from these studies. In this work we study the tunneling in a thin film system of the Dirac semimetal $\text{Na}_3\text{Bi}$ consisting of a central segment with a gate potential, sandwiched between identical semi-infinite source and drain segments. The model Hamiltonian we use for $\text{Na}_3\text{Bi}$ gives, for each spin, two Weyl nodes separated in $k$-space symmetrically about $k_z=0$. The presence of a top and bottom surface in the thin film geometry results in the appearance of Fermi arc states and energy subbands. We show that (for each spin) the presence of two Weyl nodes and the Fermi arc states result in enhanced transmission oscillations, and finite transmission even when the energy falls within the \textit{bulk} band gap in the central segment respectively. These features are not evident in single Weyl node models.

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arXiv2016

Dirac semimetal thin films in in-plane magnetic fields

In this work we study the effects of in-plane magnetic fields on thin films of the Dirac Semimetal (DSM) \ce{Na2Bi} where one of the in-plane directions is perpendicular to the $k$-separation between the two Weyl points for each spin orientation. We show numerically that the states localized near the surface of these thin films are related to the Fermi arc states in semi-infinite slabs. Due to the anisotropy between the two in-plane directions, the application of a magnetic field along these directions have differing effects. A field parallel to the $k$ space separation between the Weyl points leads to a broadening of the surface state band and a formation of an energy plateau, while a perpendicular field shifts the energy where the hole and particle bands meet upwards and sharpens the tips of the bands. We illustrate the effects of these changes to the bandstructure by studying the transmission from a source segment without a magnetic field to a drain segment with a field with the field and interface at various in-plane directions.

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arXiv2016

Klein tunneling in Weyl semimetals under the influence of magnetic field

Klein tunneling refers to the absence of normal backscattering of electrons even under the case of high potential barriers. At the barrier interface, the perfect matching of electron and hole wavefunctions enables a unit transmission probability for normally incident electrons. It is theoretically and experimentally well understood in two-dimensional relativistic materials such as graphene. Here we investigate the Klein tunneling effect in Weyl semimetals under the influence of magnetic field induced by anti-symmetric ferromagnetic stripes placed at barrier boundaries. Our results show that the resonance of Fermi wave vector at specific barrier lengths gives rise to perfect transmission rings, i.e., three-dimensional analogue of the so-called magic transmission angles in two-dimensional Dirac semimetals. Besides, the transmission profile can be shifted by application of magnetic field, a property which may be utilized in electro-optic applications. When the applied potential is close to the Fermi level, a particular incident vector can be selected for transmission by tuning the applied magnetic field, thus enabling highly selective transmission of electrons in the bulk of Weyl semimetals. Our analytical and numerical calculations obtained by considering Dirac electrons in three regions and using experimentally feasible parameters can pave the way for relativistic tunneling applications in Weyl semimetals.

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arXiv2016

Curvature induced out of plane spin accumulation

In this work we show that (real space) curvature in the geometry of curved waveguides with Rashba spin orbit interaction (RSOI) can lead to out of plane spin accumulations. We first derive the RSOI Hamiltonian on arbitrarily curved surfaces. We then analyze the effects of curvature with two distinct methods. We first apply an adiabatic approximation on gently curved, planar waveguides lying flat on the $xy$ plane to show that analogous to the acceleration of the charge carriers by an electric field, the change in momentum direction of the charge carriers as they move along the waveguide leads to an out of plane spin accumulation. We then use the Heisenberg equations of motion to establish the relationships between spin currents and accumulations on non-planar waveguides. These relations predict the existence of out of plane spin accumulation on asymmetrically curved, non-planar waveguides. We finally solve for the eigenstates on such waveguides numerically, and present numerical results to verify our earlier analytic predictions.

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arXiv2016

Perfect valley filter in strained graphene with single barrier region

We present a single barrier system to generate pure valley-polarized current in monolayer graphene. A uniaxial strain is applied within the barrier region, which is delineated by localized magnetic field created by ferromagnetic stripes at the regions boundaries. We show that under the condition of matching magnetic field strength, strain potential, and Fermi energy, the transmitted current is composed of only one valley contribution. The desired valley current can transmit with zero reflection while the electrons from the other valley are totally reflected. Thus, the system generates pure valley-polarized current with maximum conductance. The chosen parameters of uniaxial strain and magnetic field are in the range of experimental feasibility, which suggests that the proposed scheme can be realized with current technology.

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arXiv2016

Efficient Dual Spin-Valley Filter In Strained Silicene

We propose a highly efficient silicene device for dual spin and valley filtering. The device consists of two different barrier regions: the first is a region under uniaxial strain, with an exchange field induced by adjacent top and bottom magnetic insulators, while the second comprises of two ferromagnetic stripes which produces a delta-function fringe magnetic field, and a gate electrode to modify the electrochemical potential. For the first region, we investigated the effect of the uniaxial strain in inducing angular separation of the two valley spins in momentum-space, and further spin separation by the spin dependent electric potential induced by the exchange field. We then evaluated the delta-function magnetic field and electrochemical potential combination in the second region to yield the transverse displacement for the selection of the requisite spin-valley combination. We demonstrated the optimal conditions in the first barrier to induce a highly anisotropic transmission profile, which enables controllable and efficient filtering (> 90% efficiency) by the second region for all four spin-valley combinations. Based on the analytical results, we predict the feasibility of experimental realization of dual spin-valley silicene-based filtering device.

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arXiv2016

Effective Hamiltonian for surface states of \ce{Bi2Te3} nanocylinders with hexagonal warping

The three-dimensional topological insulator \ce{Bi2Te3} differs from other topological insulators in the \ce{Bi2Se3} family in that the effective Hamiltonian of its surface states on a flat semi-infinite slab requires the addition of a cubic momentum hexagonal warping term in order to reproduce the experimentally measured constant energy contours. In this work, we derive the appropriate effective Hamiltonian for the surface states of a \ce{Bi2Te3} \textit{cylinder} incorporating the corresponding hexagonal warping terms in a cylindrical geometry. We show that at the energy range where the surface states dominate, the effective Hamiltonian adequately reproduces the dispersion relation obtained from a full four-band Hamiltonian, which describe both the bulk and surface states. As an example application of our effective Hamiltonian, we study the transmission between two collinear \ce{Bi2Te3} cylinders magnetized in different directions perpendicular to their axes. We show that the hexagonal warping term results in a transmission profile between the cylinders which may be of utility in a multiple state magnetic memory bit.

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arXiv2015

Gate-control of spin-motive force and spin-torque in Rashba SOC systems

The introduction of a strong Rashba spin orbit coupling (SOC) had been predicted to enhance the spin motive force (SMF) [see Phys. Rev. Lett. {\bf 108}, 217202 (2012)]. In this work, we predict further enhancement of the SMF by time modulation of the Rashba coupling $α_R$, which induces an additional electric field $E^R_d={\dot α_R} m_e/e\hbar({\hat z}\times {\mathbf m})$. When the modulation frequency is higher than the magnetization precessing frequency, the amplitude of this field is significantly larger than previously predicted results. Correspondingly, the spin torque on the magnetization is also effectively enhanced. Additionally, the nature of SOC induced spin torque in the system can be transformed from damping to antidamping-like by modulating ${\dot α_R}$. We also suggest a biasing scheme to achieve rectification of SMF, {\it i.e.}, by application of a square wave voltage at the resonant frequency. Finally, we numerically estimate the resulting spin torque field arising from a Gaussian pulse time modulation of $α_R$.

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arXiv2015

Magnified Damping under Rashba Spin Orbit Coupling

The spin orbit coupling spin torque consists of the field-like [REF: S.G. Tan et al., arXiv:0705.3502, (2007).] and the damping-like terms [REF: H. Kurebayashi et al., Nature Nanotechnology 9, 211 (2014).] that have been widely studied for applications in magnetic memory. We focus, in this article, not on the spin orbit effect producing the above spin torques, but on its magnifying the damping constant of all field like spin torques. As first order precession leads to second order damping, the Rashba constant is naturally co-opted, producing a magnified field-like damping effect. The Landau-Liftshitz-Gilbert equations are written separately for the local magnetization and the itinerant spin, allowing the progression of magnetization to be self-consistently locked to the spin.

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arXiv2015

Gauge Physics of Spin Hall Effect

Spin Hall effect (SHE) has been discussed in the context of Kubo formulation, geometric physics, spin orbit force, and numerous semi-classical treatments. It can be confusing if the different pictures have partial or overlapping claims of contribution to the SHE. In this article, we present a gauge-theoretic, time-momentum elucidation, which provides a general SHE equation of motion, that unifies under one theoretical framework, all contributions of SHE conductivity due to the kinetic, the spin orbit force (Yang-Mills), and the geometric (Murakami-Fujita) effects. Our work puts right an ambiguity surrounding previously partial treatments involving the Kubo, semiclassical, Berry curvatures, or the spin orbit force. The full treatment shows the Rashba 2DEG SHE conductivity to be +e/8π instead of -e/8π , and Rashba heavy hole +9e/8π instead of -9e/8π .

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arXiv2014

Spin force and intrinsic spin Hall effect in spintronics systems

We investigate the spin Hall effect (SHE) in a wide class of spin-orbit coupling systems by using spin force picture. We derive the general relation equation between spin force and spin current and show that the longitudinal force component can induce a spin Hall current, from which we reproduce the spin Hall conductivity obtained previously using Kubo's formula. This simple spin force picture gives a clear and intuitive explanation for SHE.

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arXiv2013

Spin force and the generation of sustained spin current in time-dependent Rashba and Dresselhauss systems

The generation of spin current and spin polarization in a 2DEG structure is studied in the presence of Dresselhaus and Rashba spin-orbit couplings (SOC), the strength of the latter being modulated in time by an ac gate voltage. By means of the non-Abelian gauge field approach, we established the relation between the Lorentz spin force and the spin current in the SOC system, and showed that the longitudinal component of the spin force induces a transverse spin current. For a constant (time-invariant) Rashba system, we recover the universal spin Hall conductivity of $\frac e{8π}$, derived previously via the Berry phase and semiclassical methods. In the case of a time-dependent SOC system, the spin current is sustained even under strong impurity scattering. We evaluated the ac spin current generated by a time-modulated Rashba SOC in the absence of any dc electric field. The magnitude of the spin current reaches a maximum when the modulation frequency matches the Larmor frequency of the electrons.

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arXiv2013

Spin Hall effect in a simple classical picture of spin forces

Spin Hall effect (SHE) in a 2D-Rashba system has been treated in the spin-dependent precession [J. Sinova et al., Phys. Rev. Letts. 92, 126603 (2004).] and the time-space gauge [T. Fujita et al., New J. Phys. 12, 013016 (2010).] approaches, both yielding SHE conductivity of σ_y^z=e/8π . Separate studies based on the concept of spin transverse force [S-Q Shen, Phys. Rev. Lett. 95, 187203 (2005).] provide a heuristic but not a quantifiable indication of SHE. In this paper, we provide a more complete description of the SHE using the Heisenberg approach, unifying the Yang-Mills force, the Heisenberg spin force, and the SHE σ_y^z under the classical notion of forces and accelerations. Central to this paper is the spin force equations that are satisfied by both σ_y^z and σ_x^z, the Yang-Mills, and the Heisenberg spin forces. By linking σ_y^z to the spin forces, one sees that the physics of SHE in a 2D-Rashba system is also a simple classical Lorentz force picture.

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arXiv2013

Spin Torque on Magnetic Textures Coupled to the Surface of a Three-Dimensional Topological Insulator

We investigate theoretically the spin torque and magnetization dynamic in a thin ferromagnetic (FM) layer with spatially varying magnetization. The FM layer is deposited on the surface of a topological insulator (TI). In the limit of the adiabatic relaxation of electron spin along the magnetization, the interaction between the exchange interaction and the Rashba-like surface texture of a TI yields a topological gauge field. Under the gauge field and an applied current, spin torque is induced according to the direction of the current. We derived the corresponding effective anisotropy field and hence the modified Landau-Lifshitz-Gilbert equation, which describes the spin torque and the magnetization dynamic. In addition, we study the effective field for exemplary magnetic textures, such as domain wall, skyrmion, and vortex configurations. The estimated strength of the effective field is comparable to the switching fields of typical FM materials, and hence can significantly influence the dynamics of the FM layer.

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