# Topological Insulator in Junction with Ferromagnets: Quantum Hall Effects

**When:**Wednesday, July 16, 2014 at 1:00 pm

**Where:**DA 114

**Speaker**: Victor Kagalovsky

**Sponsor**: Condensed Matter Seminar

The ferromagnet-topological insulator-ferromagnet (FM-TI-FM) junction exhibits thermal and electrical quantum Hall effects. The generated Hall voltage and transverse temperature gradient can be controlled by the directions of magnetizations in the FM leads, which inspires the use of FM-TI-FM junctions as electrical and as heat switches in spintronic devices. Thermal and electrical Hall coefficients are calculated as functions of the magnetization directions in ferromagnets and the spin-scattering time in TI. Both the Hall voltage and the transverse temperature gradient decrease but are not completely suppressed even at very short spin-scattering times. The Hall coefficients turn out to be independent of the spin-scattering time for symmetric configuration of FM leads.

The discovery and experimental realization of topological insulators opened a new and vividly developing field of theoretical and experimental investigations [1,2]. Two-dimensional TI belong to the class of quantum spin Hall systems [1,2], that are distinguished by the existence of chiral spin-polarized edge states. There are two chiral states with opposite spin-projections at each edge that propagate in the opposite directions. The existence of the edge states implies strong similarity between the properties of TI and of a quantum Hall system, although no external magnetic field is applied. It has been suggested in the early papers on quantum spin Hall effect that the properties of spin-polarized edge states can be probed by injecting spin-polarized currents in TI [1,2].

[1] C. L. Kane, E. J. Mele, *Phys. Rev. Lett.* **95**, , (2005) 146802; C. L. Kane, E. J. Mele, *ibid*., **95** (2005) 226801.

[2] B. Andrei Bernevig and Shou-Cheng Zhang, *Phys. Rev. Lett*. **96** (2006) 106802.

Host: Professor Sergey Kravchenko