Defects, Electrons, and a Long-Standing Controversy

Scientists explain diverse results around a material that is both insulator and conductor and offer chemical roadmap to harness it.

Image courtesy of Tyrel McQueen, Johns Hopkins University

The Science

Topological insulators, which carry an electrical charge on just the surface, while the interior acts as an insulator, have been the subject of intense curiosity. Recent research explored how the material’s structure and chemistry correlate with its unusual electronic properties. By carefully controlling and monitoring chemical changes in a sample, 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.

The Impact

Understanding the mechanism of topological insulators may pave a way to control the properties of these exotic materials, which hold great promise for next-generation energy-efficient electronics and as a potential platform for scalable quantum computers.

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. 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. 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.

Contact

C.L. Broholm
The Johns Hopkins University
[email protected]

T.M. McQueen
The Johns Hopkins University
[email protected] (https://occamy.chemistry.jhu.edu)

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 at Argonne National Laboratory was supported by the DOE, Office of Science, Office of 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 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 crystals.” 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 SmB₆.” Physical Review X 4, 031012 (2014). [DOI: 10.1103/PhysRevX.4.031012]