Quantum Transport of Spin-helical Dirac Fermion Topological Surface States in Topological Insulators

Three-dimensional (3D) topological insulators (TI) are a novel class of electronic materials with topologically-nontrivial band structure such that the bulk is gapped and insulating yet the surface has topologically protected gapless conducting states. Such “topological surface states” (TSS) give helically spin polarized Dirac fermions, and offer a promising platform to realize various other novel physics such as topological magnetoelectric effects and Majorana fermions. However, it is often challenging to unambiguously access and study the transport properties of TSS in many practical TI materials due to non-negligible bulk conducting states. I will discuss our recent experiments on high-quality “intrinsic” TIs with insulating bulk and surface-dominated conduction that allow us to reveal a number of characteristic transport properties of spin-helical Dirac fermion topological surface states. We have observed, for example, a thickness-independent and surface-dominated conductance (even at room temperature) in exfoliated TI thin films [1] and well-developed “half-integer” Dirac fermion quantum Hall effect (QHE) arising from TSS (observed up to ~40K) [1]; fully-tunable “two-species” Dirac fermion QHE and other intriguing states in dual gated devices where both top and bottom surfaces can be independently controlled [2]; current-induced helical spin-polarization detected by spin sensitive transport measurements using magnetic electrodes [3]; and in TI nanoribbons, Shubnikov-de Hass (SdH) oscillations showing gate-tunable Berry phase and ultra-relativistic Dirac mass [4]; and a “half-integer” Aharonov-Bohm effect (ABE) unique to the circumferentially quantized spin helical Dirac fermion surface state modes (sub-bands), with a gate-tunable conductance oscillation and alternation between the “half-integer” ABE and regular ABE periodic in fermi momentum [5]. Such TIs and related devices may enable promising future applications in spintronics, thermoelectrics and various topological quantum devices. References: [1] Y. Xu et al., Nature Physics 10, 956 (2014); [2] Y. Xu et al., arXiv:1511.04597 (2015); [3] J.Tian et al., Scientific Reports 5, 14293 (2015); [4] L. A. Jauregui et al., Scientific Reports 5, 8452 (2015); [5] L. A. Jauregui et al., Nature Nanotechnology, in press, arxiv:1503.00685 (2015).