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DE-SC0018994: Room-Temperature Topological Insulator alpha-Sn Thin Films - From Fundamental Physics to Applications

Award Status: Active
  • Institution: Colorado State University, Fort Collins, CO
  • DUNS: 785979618
  • PM: Pechan, Michael
  • Most Recent Award Date: 05/23/2019
  • Number of Support Periods: 2
  • PI: Wu, Mingzhong
  • Current Budget Period: 08/01/2019 - 07/31/2020
  • Current Project Period: 08/01/2018 - 07/31/2021
  • Supplement Budget Period: N/A

Public Abstract


Room-Temperature Topological Insulator Alpha-Sn Thin Films

- From Fundamental Physics to Applications

Mingzhong Wu (Principal Investigator)

Department of Physics, Colorado State University

Topological insulators are materials that are electrically insulating in their interior but are conducting on their surfaces.  There has been a rapidly growing interest in topological insulators in recent years.  The focus so far has been on bismuth-based compounds such as Bi2Se3 and Bi2Te3 as well as on Kondo insulators such as SmB6, but these “conventional” topological insulators share a common, vital drawback – they do not host bona fide topological surface states (TSS) at room temperature (RT).  The bismuth-based topological insulators cannot have a truly insulating bulk at RT, mainly due to the presence of statistically unavoidable crystal defects in the materials.  The topological Kondo insulators, on the other hand, rely on the hybridization of conduction electrons and localized electrons, and such hybridization is strong only at low temperatures (<150 K).   The fact that the TSS in these materials is present only at low temperatures not only sets limitations on fundamental research, but also bottlenecks the practical applications of topological insulator materials.  This project will explore a new topological insulator – “alpha-Sn thin films.”  Tin (Sn) exists in two major crystalline forms: alpha and beta. Very recent work shows that alpha-Sn thin films can host TSS at RT, but many fundamental questions regarding the surface states in alpha-Sn thin films remain open.  One of the major characteristics of the topological surface states is spin-momentum locking, thanks to which the electrons moving along opposite directions (forward or backward) spin in opposite manners (clockwise or counterclockwise).  The project will use different approaches to study the strength and helicity of such spin-momentum locking.  Work is also planned to examine how the spin-momentum locking varies with the film thickness and how to manipulate it via different approaches, such as doping of different elements to the films or voltage gating.  Previous theoretical work shows that, in addition to the “usual” surface states, there are also “unusual” sub-surface states in alpha-Sn thin films that are present not at the surface but at some depth of the films.  Work is planned to explore the features of such sub-surface states and possible approaches, such as voltage gating, to tune them.  Further, this project will examine the effects of the spatial extension of TSS in alpha-Sn to an adjacent magnetic thin film on the properties of the magnetic film, including the magnetic damping and the anisotropy, and also explore the possibility of using the TSS in alpha-Sn thin films to control magnetization dynamics in adjacent magnetic films, such as driving domain wall motion and exciting magnetization precession.

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