Defects, Electrons, and a Long-Standing Controversy

topological insulators
Samarium hexaboride (SmB6) is a topological insulator—a material that carries electrical charge on its surface but the interior is an electrical insulator. Understanding the intricacies of topological insulators is key to taking these materials from the lab to applications. Neutron diffraction (above image, aluminum (Al) reflections shown with the white arrow) and the corresponding X-ray micro-computed tomography image (top right image) show that aluminum impurities were introduced when SmB6 single crystals were produced. The inclusions explain the different measurements reported for the low-temperature electrical properties. Image courtesy of Tyrel McQueen, Johns Hopkins University

Faster, more energy-efficient electronics could be created with topological insulators, which carry an electrical charge on just the surface, while the interior acts as an insulator. Scientists are delving into how the material’s structure and chemistry correlate with its unusual electronic properties. By carefully controlling and monitoring chemical changes in the topological insulator bulk samarium hexaboride (SmB6 ), scientists reconciled a long-standing controversy on the diverse results on its low-temperature electrical properties and established a systematic relationship between the bulk chemistry and surface topological properties. Using chemical structures and electrical properties measured under extremely cold conditions, the team showed that electron dopants enhance the topological behavior of SmB6 while aluminum defects and samarium vacancies suppress it.

The Impact

Understanding of the mechanism paves a way to control the properties of topological insulators, as they hold great promise for next-generation energy-efficient electronics and a platform for scalable quantum computers, if their exotic surface electronic states can be selectively harnessed.

Summary

Departures from regularity—defects and imperfections—in materials are the key to their novel properties, and control over the type, number, and distribution of defects is essential to design materials for future technologies. For over four decades, samarium hexaboride (SmB6) has been extensively investigated for its range of electronic properties. Recently, it serves as a paradigm material for topological insulators—materials with charge on their surfaces that are protected against disorder within the bulk. A major and controversial issue has been the large variation in its resistivity data at low temperatures. Addressing the long-standing controversy on this important material requires high-quality single crystal samples with controlled defects within the bulk, coupled with advanced characterization of their structure and physical properties. Applying two well-known crystal growth techniques—floating zone and aluminum flux synthesis—scientists from The Johns Hopkins University examined a series of high-quality SmB6 single crystals with aluminum impurities (defects) as well as systematically controlled levels of electrons by replacing boron with carbon and holes by creating samarium atom vacancies. Chemical structures derived from high-resolution neutron and X-ray diffraction and electrical properties measured at cryogenic conditions revealed a correlation between the defects and electrical properties; electron dopants enhance the topological behavior while aluminum defects and samarium vacancies suppress it. This finding reconciles a long-standing controversy of disparate reports on the behavior of this fascinating class of materials and demonstrates that it is possible to chemically control exotic topological surface states by tuning the bulk chemistry. The understanding and control of these materials is of great interest for the creation of viable quantum computers and more energy-efficient electronics.

Funding

The work at the Institute for Quantum Matter (IQM) at The Johns Hopkins University was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Grant No. DE-FG02-08ER46544. D.P.Y. acknowledges support from the National Science Foundation under Grant No. NSF-DMR1306392. Use of the advanced-photon-source/” title=”View all articles about Advanced Photon Source here”>Advanced Photon Source at Argonne National Laboratory was supported by the DOE, Office of Science, Office of sciences/” title=”View all articles about Basic Energy Sciences here”>Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The neutron diffraction data were collected at the Oak Ridge National Laboratory’s Spallation Neutron Source, supported by the Division of Scientific User Facilities, Office of sciences/” title=”View all articles about Basic Energy Sciences here”>Basic Energy Sciences, DOE Office of Science, under Contract No. DE-AC05 00OR22725 with UT Battelle, LLC.

Publications

W.A. Phelan, S.M. Koohpayeh, P. Cottingham, J.A. Tutmaher, J.C. Leiner, M.D. Lumsden, C.M. Lavelle, X.P. Wang, C. Hoffmann, M.A. Siegler, N. Haldolaarachchige, D.P. Young, and T.M. McQueen, “ On the chemistry and physical properties of flux and floating zone grown SmB6 single crystalsExternal link.” Scientific Reports 6 , 20860 (2016). [DOI: 10.1038/srep20860]

W.A. Phelan, S.M. Koohpayeh, P. Cottingham, J.W. Freeland, J.C. Leiner, C.L. Broholm, and T.M. McQueen, “ Correlation between bulk thermodynamic measurements and the low temperature resistance plateau in SmB6External link.” Physical Review X 4, 031012 (2014). [DOI: 10.1103/PhysRevX.4.031012]