Topological insulators are a newly discovered class of materials with insulating bulk states and metallic surface states. The surface states are chiral and spin-polarized, and therefore protected from backscattering. The spin structure and topological nature of the surface states in these materials make them a novel playground for studying magnetism, topological superconductivity and exotic particles on the one hand, and enabling dissipationless spin transport and topological quantum computing on the other.

This award will support low temperature scanning tunneling microscopy (STM) and spectroscopy (STS) studies on topological insulators (Bi2X3 class). The spin texture of the topological surface states will be studied using a sub-Kelvin STM and spin-polarized STM in conjunction with two-axis high-field magnets. Quasiparticle interference, an established STS technique, will be used to determine physical quantities of interest, e.g. the Fermi velocity and g-factor of surface states. Bulk and surface doping with magnetic impurities will be used to study the interplay of these surface states with magnetism. Bulk intercalation and epitaxial film growth will be used to look for topological interactions with superconductivity.

The award will support the education and training of two graduate students and will create a rich intellectual environment through collaborations with theory and complementary experimental techniques.


Nontechnical Abstract

Every few years a new class of materials is discovered and takes the scientific world by storm with novel physics and technological promise. In 2009, the discovery of topological insulators created one such ripple. A topological insulator (TI) is an electrical insulator in its interior. On its surface, however, it is not only a metal with charges free to move, but a special metal where those charges are prevented from backscattering, removing the primary source of dissipation and heating in microelectronics. These properties make TIs promising candidates for enabling a couple of futuristic technologies: spintronics, a very low power replacement for conventional electronics, and quantum computing, a transformational method of rapidly solving computationally intensive problems.

This award will support studies of topological insulators using scanning tunneling microscopes - instruments that map electronic behavior at the atomic scale. Surface-sensitive probes like these are ideally suited to study the novel surface properties of these materials. In collaboration with theory and other experimental techniques, the path toward these and other new technologies will be assessed.

Project Report

The 2008 discovery of three-dimensional topological materials has prompted worldwide excitement about fundamental physics exploration and revolutionary applications. The hallmark of a topological insulator is that electrons flow on the surface without resistance with their spin locked to their momentum, while the interior of the material is insulating. The conducting surface electrons can mimic high-energy relativistic particles, and serve as a model system for understanding fundamental physics. Topological materials also hold great application potential in both room-temperature spintronics and quantum computing. Spin-polarized topological surface states could enable high-speed data processing, massive storage capacity, and low energy consumption devices. The utility of real topological materials for spin transport applications requires a robust insulating interior and well-characterized spin and motion properties for the topological surface electrons. The existing topological materials, e.g. Bi2Se3, always have excess conducting electrons in the interior, which overwhelm the surface states and foil their use. Furthermore, it has been difficult to achieve a comprehensive characterization of topological surface states at the nanometer length scale relevant to modern devices. Under the support of NSF grant DMR-1106023, we accomplished nanoscale spectroscopic studies on topological materials that have advanced both the fundamental physics exploration and new material discovery. There are two major outcomes from our projects. One is the establishment of momentum-resolved scanning tunneling microscopy – the first synergistic use of Landau level spectroscopy and quasiparticle interference imaging – to reconstruct the multi-component surface state band structure at the nanoscale, and to fully quantify the crucial topological utility metrics for a topological semimetal antimony (Sb). The second outcome is the nanoscale spectroscopic study of the first strongly correlated topological insulator, SmB6. With our experimental breakthrough in establishing the synergistic use of Landau levels and quasiparticle interference for nanoscale band structure measurements, we have achieved the determination of three crucial metrics for topological utility at atomic length scale: strong spin-momentum locking, long mean free path, and small g-factor, minimizing vulnerability to magnetic fields. Our observation of long mean free path and a record 27 Landau levels on Sb suggests a new strategy to exploit topological semimetal heterostructures in nanoscale spintronics devices. The imperfection of known topological materials with excess conducting bulk electrons seriously hampers their application. Our scanning tunneling microscopy and spectroscopy work on SmB6 was the first nanoscale study on the newly predicted family of topological Kondo insulators, a family of materials with robust insulating bulk due the interactions between the conduction and localized electrons. The significance of our work is the quantitative estimation of the hybridization gap size, essentially a measure of how insulating the interior is. The NSF support also enabled the training of three graduate students and one postdoctoral fellow. During the grant period, they have developed the skills to use the low temperature scanning tunneling microscope, perform data analysis, write scientific papers, and present talks in seminars and conferences. These experiences position them well to achieve their ambitions in academic and industrial scientific careers.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Guebre X. Tessema
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Harvard University
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