Cong Son Ho

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. Our full treatment shows the Rashba 2DEG SHE conductivity to be instead of −, and Rashba heavy hole instead of −. This renewed treatment suggests a need to re-derive and re-calculate previously studied SHE conductivity.

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 the 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 Kubos formula. This simple spin force picture gives a clear and intuitive explanation for SHE.

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

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 Bi2Se3 films.

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

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.

Persistent Zitterbewegung of electron wave packet in time-dependent Rashba system

The Zitterbewegung (ZB) of an electron wave packet is theoretically studied in a Rashba spin-orbit coupling system, in the presence of the AC gate voltage modulation of ?, the Rashba coupling strength. In the absence of AC modulation, the ZB dynamics of a wave packet rapidly decays due to interference from the different wave packet components. Under AC modulation of ?, the ZB dynamics may be sustained indefinitely. We quantify two characteristics of this persistent ZB motion as a function of the modulation frequency ?, i.e., the peak amplitude and the regularity RZ of the electron jitter motion. We analytically show that RZ improves with increasing ?. At sufficiently high modulation frequency, the ZB motion becomes totally regular, i.e., with a single characteristic frequency of . On the other hand, at high modulation frequency, the amplitude diminishes asymptotically as . These trends of the ZB dynamics can also be obtained analytically by considering the spin-force picture. Our study provides the reference for achieving sustained large-amplitude and regular ZB oscillations under AC modulation, which is useful for experimental observation of the elusive ZB phenomenon in a Rasbha system.

Effect of surface hybridization on RKKY coupling in ferromagnet/topological insulator/ferromagnet trilayer system

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 amplitude increases at first and reaches its maximum value at some critical thickness, below which the amplitude 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.

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

Spin accumulation in disordered topological insulator ultrathin films

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Role of exchange interaction in nitrogen vacancy center based magnetometry

, which induces an additional electric field

Spin motive force and generation of pure spin current in ferromagnetic ring under time modulation and Rashba spin orbit coupling

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