Ion Garate

Inverse spin-galvanic effect in the interface between a topological insulator and a ferromagnet.

When a ferromagnet is deposited on the surface of a topological insulator (TI) the topologically protected surface state develops a gap and becomes a 2-dimensional quantum Hall liquid. We demonstrate that the Hall current in such a liquid, induced by an external electric field, can have a large effect on the magnetization dynamics of the ferromagnet by changing the effective anisotropy field. This change is dissipationless and may be substantial even in weakly spin-orbit coupled ferromagnets. We study the possibility of dissipationless current-induced magnetization reversal in monolayer-thin, insulating ferromagnets with a soft perpendicular anisotropy and discuss possible applications of this effect.

Weak localization and antilocalization in topological insulator thin films with coherent bulk-surface coupling

We evaluate quantum corrections to conductivity in an electrically gated thin film of a three-dimensional (3D) topological insulator (TI). We derive approximate analytical expressions for the low-field magnetoresistance as a function of bulk doping and bulk-surface tunneling rate. Our results reveal parameter regimes for both weak localization and weak antilocalization, and include diffusive Weyl semimetals as a special case.

Spin-polarized tunneling study of spin-momentum locking in topological insulators

We demonstrate that the charge-spin conversion efficiency of topological insulators (TI) can be experimentally determined by injecting spin-polarized tunneling electrons into a TI. Through a comparative study between bismuth selenide and bismuth antimony telluride, we verified the topological-surface-state origin of the observed giant spin signals. By injecting energetic electrons into bismuth selenide, we further studied the energy dependence of the effective spin polarization at the TI surface. The experimentally verified large spin polarization, as well as our calculations, provides new insights into optimizing TI materials for near room-temperature spintronic applications.

Influence of a transport current on magnetic anisotropy in gyrotropic ferromagnets

because the applied current breaks time reversal invariance. At a practical level, we provide a concise analytical expression for the current-induced change in the magnetic anisotropy. This expression is suitable for evaluation from first principles because it requires the knowledge of only the band structure of the ferromagnet and the lifetime of the Bloch states. At a technical level, our theory allows for the spatial inhomogeneities that inevitably occur in the magnitude of the ferromagnet’s exchange field at atomic lenghtscales.

Nonadiabatic spin-transfer torque in real materials

The motion of simple domain walls and of more complex magnetic textures in the presence of a transport current is described by the Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations depend sensitively on the ratio between the dimensionless material parameter β which characterizes non-adiabatic spin-transfer torques and the Gilbert damping parameter α. This ratio has been variously estimated to be close to 0, close to 1, and large compared to 1. By identifying β as the influence of a transport current on α, we derive a concise, explicit and relatively simple expression which relates β to the band structure and Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate that intrinsic spin-orbit interactions lead to intraband contributions to β which are often dominant and can be (i) estimated with some confidence and (ii) interpreted using the “breathing Fermi surface” model. PACS numbers:

Experimental evidence and control of the bulk-mediated intersurface coupling in topological insulatorBi2Te2Senanoribbons

Nearly a decade after the discovery of topological insulators (TIs), the important task of identifying and characterizing their topological surface states through electrical transport experiments remains incomplete. The interpretation of these experiments is made difficult by the presence of residual bulk carriers and their coupling to surface states, which is not yet well understood. In this work, we present the first evidence for the existence and control of bulk-surface coupling in Bi2Te2Se nanoribbons, which are promising platforms for future TI-based devices. Our magnetoresistance measurements reveal that the number of coherent channels contributing to quantum interference in the nanoribbons changes abruptly when the film thickness exceeds the bulk phase relaxation length. We interpret this observation as an evidence for bulk-mediated coupling between metallic states located on opposite surfaces. This hypothesis is supported by additional magnetoresistance measurements conducted under a set of gate voltages and in a parallel magnetic field, the latter of which alters the intersurface coupling in a controllable way.

Phonon-Induced Topological Transitions and Crossovers in Dirac Materials

We show that electron-phonon interactions can alter the topological properties of Dirac insulators and semimetals, at both zero and nonzero temperature. Contrary to the common belief that increasing temperature always destabilizes topological phases, our results highlight instances in which phonons may lead to the appearance of topological surface states above a crossover temperature in a material that has a topologically trivial ground state.

Surface-to-bulk scattering in topological insulator films

We present a quantitative microscopic theory of the disorderand phonon-induced coupling between surface and bulk states in topological insulator (TI) films. We find a simple mathematical structure for the surface-to-bulk scattering matrix elements and confirm the importance of bulksurface coupling in transport and photoemission experiments, assessing its dependence on temperature, carrier density, film thickness and particle-hole asymmetry.

Magnetoelectric response of the time-reversal invariant helical metal

We derive compact analytical expressions for the coupled spin-charge susceptibility of a clean helical metal at the surface of a three-dimensional topological insulator (TI). These expressions lead to unconventional noncollinear Ruderman-Kittel-Kasuya-Yoshida interactions between two impurity magnetic moments placed on the surface of a TI and predict the generation of electric currents by time-dependent magnetic moments. We determine the influence of gate and bias voltages on the interlayer exchange coupling between two single-domain ferromagnetic monolayers deposited on top of a TI.

Excitons and optical absorption on the surface of a strong topological insulator with a magnetic energy gap

We present a theoretical study of interacting electron-hole pairs located on a magnetized surface of a strong topological insulator (TI). The excitonic energy levels and the optical absorption on such a surface display unique and potentially measurable features such as (i) an enhanced binding energy for excitons whose total angular momentum is aligned with the magnetic exchange field, (ii) a stark dependence of the optical absorption on the direction of the magnetic exchange field as well as on the chirality of the incident light, and (iii) a tunable center-of-mass motion of spinful excitons induced by particle-hole asymmetry in the exchange field. Our predictions are relevant to surfaces of magnetically doped TIs or surfaces coated with magnetic films, in addition to TI nanowires placed under longitudinal magnetic fields.