Session Y48: Invited Session: Spin Transport in Novel 2d Electronic Systems

Topological Electronic Structures and Spintronics Applications for Silicene and Other Spin-Orbit Thin Films

While spin-orbit coupling plays a critical role in generating topologically insulating phases, it also provides a novel route for realizing spin-split states in nonmagnetic materials without the need for exchange coupling. Two-dimensional thin films with significant spin-orbit coupling strength enable potential applications for spintronics devices because the spin-splitting energy can be controlled by an external field (gating). Moreover, spin-orbit coupling can induce nontrivial topological phases, i.e. quantum spin Hall phases, which could harbor back-scattering-free spin-polarized current at the edge. Recently, we have shown via first-principles calculations that field-gated silicene possesses two gapped Dirac cones exhibiting nearly 100\% spin-polarization, situated at the corners of the Brillouin zone. Band gaps as well as the band topology can be tuned with an external electric field perpendicular to the plane, which breaks the inversion symmetry of the system due to the presence of buckling in the honeycomb structure. Using this fact, we propose a design for a silicene-based spin-filter that would enable the spin-polarization of an output current to be switched electrically, without the need to switch external magnetic fields. Our quantum transport calculations indicate that the proposed designs will be highly efficient (nearly 100\% spin polarization) and robust against weak disorder and edge imperfections. We also propose a Y-shaped spin/valley separator that produces spin-polarized current at two output terminals with opposite spins. Ge, Sn, and Pb counterparts of silicene are shown to have similar properties, but their larger spin-orbit coupling results in larger energy differences between the spin-split states making these materials better suited for room temperature applications. Other spin-orbit thin films will be discussed. Our investigations demonstrate that spin-orbit thin films present great potential for manipulating spin/valley degrees of freedom efficiently, moving us a step closer to realizing the dream of spintronics applications.   

Direct electrical detection of spin-momentum locking in the topological insulator Bi$_{2}$Se$_{3}$

Topological insulators (TIs) are a new quantum state of matter [1] characterized by metallic surface states populated by massless Dirac fermions. TIs are expected to exhibit new behaviors and open horizons for science previously inaccessible with ``conventional'' materials. One of the most striking properties is that \textit{of spin-momentum locking} -- the spin of the TI surface state lies in-plane, and is locked at right angle to the carrier momentum. An unpolarized charge current should thus create a net spin polarization whose amplitude and orientation are controlled by the charge current. This remarkable property has been anticipated by theory [2], but never accessed in a simple transport structure. Here we show that a bias current indeed produces a net surface state spin polarization \textit{via} spin-momentum locking in molecular beam epitaxially grown Bi$_{2}$Se$_{3}$ films, and this polarization is directly manifested as a voltage on a ferromagnetic metal contact. This voltage is proportional to the projection of the TI spin polarization onto the contact magnetization, is determined by the direction and magnitude of the bias current, scales inversely with Bi$_{2}$Se$_{3}$ film thickness, and its sign is that expected from spin-momentum locking rather than a Rashba effect [3]. Similar data are obtained for structures with two different ferromagnet/tunnel barrier contacts, demonstrating that these behaviors are independent of the details of the detector contact. These results demonstrate direct electrical access to the TI surface state spin system and enable utilization of its remarkable properties for future technological applications.\\[4pt] [1] J. E. Moore, Nature \textbf{464}, 194 (2010); M. Z. Hasan et. al., Rev. Mod. Phys. \textbf{82,} 3045 (2010); L. Fu et. al., PRL \textbf{98}, 106803 (2007); D. Hsieh et. al., Nature \textbf{452}, 970 (2008).\\[0pt] [2] A. A. Burkov et. al. PRL \textbf{105}, 066802 (2010); D. Culcer et. al., PRB \textbf{82}, 155457 (2010); V. Yazyev et. al., PRL \textbf{105}, 266806 (2010).\\[0pt] [3] S. Hong et. al., PRB \textbf{86}, 085131 (2012).   

Intrinsic limitations of spin transport in 2D membranes

Two dimensional membranes have become the playground for both theorists and experimentalists due to their unique intrinsic properties emerged from simple lattice structures. They are the new focus of spintronic applications. Therefore, it is important that we have a clear view of the relaxation processes in spin transport, limited by their intrinsic and symmetry structures. In this talk, we present our findings [1,2] by systematically applying group theory to the coupling of phonons and transport carriers in spin-dependent scattering. Scattering by phonon is amplified in 2D membranes due to its unique and populous flexural mode. Opposite spin coupling by one flexural phonon is allowed by symmetry, unlike the momentum scattering by higher-order two flexural phonons. Furthermore, we specifically discuss the ultrafast electron spin relaxation in single-layer transition metal dichalcogenides (SL-TMDs) [1]. The additional factor stems from the decoupling of tiny conduction band spin splitting and the large spin scattering constant. The former results from conduction band orbital orientation, while the latter comes from inter-band coupling and reflects the atomic SOC strength. We will present that the essential use of group theory (invariant quantities) elucidates various spin-dependent selection rules of electron/hole-phonon interaction, within and between all relevant band-valley edges. Multiple potential applications of the derived results can be explored in transport problems, such as the strain effects [2], spin Gunn effect, hot exciton dynamics [1], and the scattering angle and spin anisotropy dependence. We compare different 2D membranes (graphene, SL-TMD, silicene and germanene) from general consideration of the lattice and band-edge symmetries. \\[4pt] [1] Y. Song and H. Dery, Phys. Rev. Lett., 111, 026601 (2013).\\[0pt] [2] T. Cheiwchanchamnangij, W. Lambrecht, Y. Song and H. Dery, Phys. Rev. B, 88, 155404 (2013).   

Layered Chalcogenides beyond Graphene: from Electronic Structure Evolution to the Spin Transport

Recent efforts on graphene-like atomic layer materials, aiming at novel electronic properties and quantum phenomena beyond graphene, have attracted much attention for potential electronics/spintronics applications. Compared to the weak spin-orbit-interaction (SOI) in graphene, metal chalcogenides MX$_{\mathrm{2}}$ have heavy 4d/5d elements with strong atomic SOI, providing a unique way for generating spin polarization based on valleytronics physics. Indeed, such a spin-polarized band structure has been demonstrated theoretically and supported by optical investigations. However, despite these exciting progresses, following two important issues in MX$_{\mathrm{2}}$ community remain elusive: 1. the quantitative band structure of MX$_{\mathrm{2}}$ compounds (where are the valleys -band maxima/minima- locating in the BZ) have not been experimentally confirmed. Especially for those cleaved ultrathin mono- and bi-layer flakes hosting most of recently-reported exotic phenomena at the 2D limit, the direct detection for band dispersion becomes of great importance for valleytronics. 2. Spin transports have seldom been reported even though such a strong SOI system can serve as an ideal platform for the spin polarization and spin transport. In this work, we started from the basic electronic structures of representative MX$_{\mathrm{2}}$, obtained by ARPES, and investigated both the band variation between these compounds and their band evolution from bulk to the monolayer limit. After having a systematic understanding on band structures, we reported a giant Zeeman-type spin-polarization generated and modulated by an external electric field in WSe$_{\mathrm{2}}$ electric-double-layer transistors. The non-magnetic approach for realizing such an intriguing spin splitting not only keeps the system time-reversally invariant but also suggests a new paradigm for manipulating the spin-degrees of freedom of electrons. Acknowledge the support from DoE, BES, Division of MSE under contract DE-AC02-76SF00515.