Topological Spintronics*

Tetradymite narrow bandgap semiconductors (Bi₂Te₃, Bi₂Se₃, Sb₂Te₃, and their alloys) are known to support topologically protected, two dimensional (2D) helical Dirac fermion surface states characterized by a spin-texture in momentum space [1]. The 'spin-momentum locking' of the 2D surface states in these three dimensional (3D) 'topological insulators' lends itself naturally to 'topological spintronics,' device applications that might exploit efficient spin-charge interconversion. We present an introductory overview of the emergence of 'topological spintronics' and then focus on recent experiments that probe spin-charge interconversion at the interface between a 3D topological insulator and an insulating ferrimagnet [2], with a view toward understanding how the spin Hall conductivity in topological insulators varies with chemical potential [3]. An important issue examined in this context is the relative contribution to spin-charge conversion of the surface and bulk states, both of which have large spin-orbit coupling in the tetradymites. Finally, we address emerging demonstrations of spin-orbit torque switching using topological insulator/ferromagnetic metal heterostructures [4].

1. J. P. Heremans, R. J. Cava, and N. Samarth, Nature Reviews Materials 2, 17049 (2017).
2. Hailong Wang, James Kally, J. S. Lee, T. Liu, H. Chang, D. Reifsnyder-Hickey, K. Andre Mkhoyan, M.-Z. Wu, A. Richardella, and N. Samarth, Phys. Rev. Lett. 117, 076601 (2016).
3. Hailong Wang, James Kally, C. Sahin, T. Liu, A. Richardella, M.-Z. Wu, M. E. Flatté, N. Samarth, submitted.
4. Jiahao Han, A. Richardella, S. A. Siddiqui, J. Finley, N. Samarth and Luqiao Liu, Phys. Rev. Lett. 119, 077702 (2017).