Monopole Current and Unconventional Hall Response on Topological Insulator

We study theoretically the charged current above a topological insulator (TI) separated by a ferromagnetic insulating layer. An unconventional Hall response occurs in the conducting layer on top of the TI which approaches to a constant value independent of R for R>l, where R is the separation between TI and conducting layer and l is the screening length. In the comoving frame, it can be interpreted as a monopole current attached to the TI surface. The same mechanism gives the Hall response and deflection of the electron beam injected to the surface of insulating ferromagnet. A realistic estimate of an order of magnitude shows that both effects give reasonably large signal experimentally accessible.Comment: 5 pages, 3 figure

Dynamics of magnetization on the topological surface

We investigate theoretically the dynamics of magnetization coupled to the surface Dirac fermions of a three dimensional topological insulator, by deriving the Landau-Lifshitz-Gilbert (LLG) equation in the presence of charge current. Both the inverse spin-Galvanic effect and the Gilbert damping coefficient $\alpha$ are related to the two-dimensional diagonal conductivity $\sigma_{xx}$ of the Dirac fermion, while the Berry phase of the ferromagnetic moment to the Hall conductivity $\sigma_{xy}$. The spin transfer torque and the so-called $\beta$-terms are shown to be negligibly small. Anomalous behaviors in various phenomena including the ferromagnetic resonance are predicted in terms of this LLG equation.Comment: 4+ pages, 1 figur

Spin Josephson effects in Exchange coupled Anti-ferromagnets

The energy of exchange coupled antiferromagnetic insulators (AFMIs) is a periodic function of the relative in-plane orientation of the N\'{e}el vector fields. We show that this leads to oscillations in the relative magnetization of exchange coupled AFMIs separated by a thin metallic barrier. These oscillations pump a spin current ($I_{S}$) through the metallic spacer that is proportional to the rate of change of the relative in-plane orientation of the N\'{e}el vector fields. By considering spin-transfer torque induced by a spin chemical potential ($V_{S}$) at one of the interfaces, we predict non-Ohmic $I_{S}$-$V_{S}$ characteristics of AFMI exchange coupled hetero-structures, which leads to a non-local voltage across a spin-orbit coupled metallic spacer