ORCID Profile
0000-0002-5608-0113
Current Organisations
ARC Centre of Excellence - Future Low -energy Electronics Technologies
,
RMIT University
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Publisher: IOP Publishing
Date: 04-12-2015
Publisher: IOP Publishing
Date: 06-09-2018
Publisher: AIP Publishing
Date: 12-05-2014
DOI: 10.1063/1.4876226
Abstract: The generation of spin current and spin polarization in a two-dimensional electron gas 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 e8π, derived previously via the Berry phase and semi-classical 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.
Publisher: Springer Science and Business Media LLC
Date: 11-04-2017
DOI: 10.1038/S41598-017-00911-4
Abstract: We investigate the thickness optimization for maximum current-induced spin-orbit torque (SOT) generated by topological surface states (TSS’s) in a bilayer system comprising of a ferromagnetic layer coupled to a thin topological insulator (TI) film. We show that by reducing the TI thickness, two competing effects on the SOT are induced: (i) the torque strength is stronger as the bulk contribution is decreased (ii) on the other hand, the torque strength becomes suppressed due to increasing hybridization of the surface states. The latter is attributed to the opposite helicities of the coupled TSS’s. We theoretically model the interplay of these two effects and derive the optimal TI thickness to maximize the spin torque, which is estimated to be about 3–5 nm for typical Bi 2 Se 3 films.
Publisher: IEEE
Date: 05-2015
Publisher: AIP Publishing
Date: 17-02-2017
DOI: 10.1063/1.4977072
Abstract: We theoretically investigate the RKKY exchange coupling between two ferromagnets (FM) separated by a thin topological insulator film (TI). We find an unusual dependence of the RKKY exchange coupling Φex on the TI thickness (tTI). First, when tTI decreases, the coupling litude increases at first and reaches its maximum value at some critical thickness, below which the litude turns to diminish. This trend is attributed to the hybridization between surfaces of the TI film, which opens a gap below critical thickness and thus turns the surfaces into insulating state from semi-metal state. In insulating phase, diamagnetism induced by the gap-opening compensates paramagnetism of Dirac state, resulting in a diminishing magnetic susceptibility and RKKY coupling. For typical parameters, the critical thickness in Bi2Se3 thin film is estimated to be in the range of 3-5 nm.
Publisher: AIP Publishing
Date: 17-08-2017
DOI: 10.1063/1.4985846
Abstract: Topological insulator (TI) ultrathin films differ from the more commonly studied semi-infinite bulk TIs in that the former possess both top and bottom surfaces where the surface states localized at different surfaces can couple to one another across the finite thickness of the film. In the presence of an in-plane magnetization, the 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 Bi2Se3 TI thin film system with an in-plane magnetization and numerically calculate 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 formula 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 around 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.
Publisher: IEEE
Date: 04-2018
Publisher: Elsevier BV
Date: 10-2020
Publisher: IOP Publishing
Date: 31-07-2014
Publisher: Springer Science and Business Media LLC
Date: 22-12-2015
DOI: 10.1038/SREP18409
Abstract: 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. Our full treatment shows the Rashba 2DEG SHE conductivity to be "Equation missing" instead of −"Equation missing" and Rashba heavy hole "Equation missing" instead of −"Equation missing" . This renewed treatment suggests a need to re-derive and re-calculate previously studied SHE conductivity.
Publisher: American Physical Society (APS)
Date: 20-12-2016
Publisher: American Physical Society (APS)
Date: 16-09-2022
Publisher: AIP Publishing
Date: 16-03-2012
DOI: 10.1063/1.3693662
Abstract: The generation of spin current and spin polarization in 2DEG Rashba system is considered, in which the spin-orbital coupling (SOC) is modulated by an ac gate voltage. By using non-Abelian gauge field method, we show the presence of an additional electric field. This field induces a spin current generated even in the presence of impurity scattering and is related to the time-modulation of the Rashba SOC strength. In addition, the spin precession can be controlled by modulating the modulation frequency of the Rashba SOC strength. It is shown that at high modulation frequency, the precessional motion is suppressed so that the electron spin polarization can be sustained in the 2DEG.
Publisher: Korean Physical Society
Date: 15-01-2009
DOI: 10.3938/JKPS.54.361
Publisher: IEEE
Date: 05-2015
Publisher: IOP Publishing
Date: 10-2014
Publisher: IOP Publishing
Date: 10-2019
Abstract: We formulate the effective Hamiltonian of Rashba spin–orbit coupling (RSOC) in LaAlO 3 / SrTiO 3 (LAO/STO) heterostructures. We derive analytical expressions of properties, e.g. Rashba parameter, effective mass, band edge energy and orbital occupancy, as functions of material and tunable heterostructure parameters. While linear RSOC is dominant around the Γ-point, cubic RSOC becomes significant at the higher-energy anti-crossing region. We find that linear RSOC stems from the structural inversion asymmetry (SIA), while the cubic term is induced by both SIA and bulk asymmetry. Furthermore, the SOC strength shows a striking dependence on the tunable heterostructure parameters such as STO thickness and the interfacial electric field which is ascribed to the quantum confinement effect near the LAO/STO interface. The calculated values of the linear and cubic RSOC are in agreement with previous experimental results.
Publisher: IOP Publishing
Date: 07-2020
Abstract: The dynamics of itinerant electrons in topological insulator (TI) thin films is investigated using a multi-band decomposition approach. We show that the electron trajectory in the 2D film is anisotropic and confined within a characteristic region. Remarkably, the confinement and anisotropy of the electron trajectory are associated with the topological phase transition of the TI system, which can be controlled by tuning the film thickness and/or applying an in-plane magnetic field. Moreover, persistent electron wavepacket oscillation can be achieved in the TI thin film system at the phase transition point, which may assist in the experimental detection of the jitter motion (Zitterbewegung). The implications of the microscopic picture of electron motion in explaining other transport-related effects, e.g., electron-mediated RKKY coupling in the TI thin film system, are also discussed.
Publisher: Springer Science and Business Media LLC
Date: 27-03-2023
DOI: 10.1038/S41467-023-37102-X
Abstract: The conducting boundary states of topological insulators appear at an interface where the characteristic invariant ℤ 2 switches from 1 to 0. These states offer prospects for quantum electronics however, a method is needed to spatially-control ℤ 2 to pattern conducting channels. It is shown that modifying Sb 2 Te 3 single-crystal surfaces with an ion beam switches the topological insulator into an amorphous state exhibiting negligible bulk and surface conductivity. This is attributed to a transition from ℤ 2 = 1 → ℤ 2 = 0 at a threshold disorder strength. This observation is supported by density functional theory and model Hamiltonian calculations. Here we show that this ion-beam treatment allows for inverse lithography to pattern arrays of topological surfaces, edges and corners which are the building blocks of topological electronics.
Location: Australia
No related grants have been discovered for Cong Son Ho.