If you’re familiar with topology, you will know that a doughnut and a coffee cup are the same–topologically speaking–but differ from a soccer ball. Researchers here at Northeastern University are taking these ideas a step further.

Theoretical physicist Prof. Arun Bansil and his research team have found a way to vary the sulfur to selenium ratio of a Thalium-Bismuth-Sulfur-Selinium compound to show how an ordinary insulator transitions into an exotic topological insulator.

Why is this so exotic? “This amounts to seeing a soccer ball turn into a doughnut in that these two objects possess completely different topologies,” said Bansil, whose research on this topic was featured in the April 29 issue of Science.

This discovery brings researchers a step closer to exploiting topological insulators in wide-ranging applications in efficient thermoelectrics for harvesting electricity from sustainable energy sources without carbon emissions, quantum computing, next generation electronics or spintronics, and tackling problems in fundamental physics. Topological insulators could play a key role in these applications by providing new material platforms with design flexibility and electronic, magnetic and superconducting tunability.

“The topological insulators are a recently discovered novel form of quantum matter in which — due to powerful symmetry reasons — the material can harbor conducting states on the surface, even though it is an insulator in the bulk,” said Bansil.

Read: How Do You Want That Insulator?

Experimental teams at Princeton University, Lawrence Berkeley National Laboratory in Berkeley, California and the Paul Scherrer Institute in Zurich, Switzerland collaborated with Northeastern University for this study.

Bansil and his team at Northeastern University have pioneered the discovery of many new classes of topological insulators, including Bi2Se3 series, half-Heuslers, thallium-based III-V-VI2 compounds (TlBiSe2), Li-based ternary compounds (Li2AgSb), and the GemBi2nTem+3n series of compounds.

Other members of Bansil’s group working on the problem of topological insulators include Associate Research Scientist Hsin Lin, Prof. Robert Markiewicz, Research Associate Tanmoy Das, and graduate students Susmita Basak, Wael Al-Sawai and Ray Wang.

The results of their research have been featured several times in the Nature group of journals, and in the journals Science and Physical Review Letters.

Prof. Arun Bansil
Bansil is the founding director of Northeastern’s Advanced Scientific Computation Center. During his 35-year career at the University, Bansil’s research has focused on understanding how electrons behave in complex novel materials and how these electrons can be probed by modern spectroscopic techniques.

Bansil founded the ELMO Laboratory for science education at Northeastern and the PASTEL program for informal science education in collaboration with the major art and science museums of Boston. He is the US editor of the International Journal of Physics and Chemistry of Solids since 1994, and served as the program manager of the Theoretical Condensed Matter Physics Program at the United States Department of Energy from 2008 – 2010.

Below are some of the most recent articles featuring work done by Bansil’s group on topological insulators.

Topological Insulators: The Dirt on Topology featured in Nature Physics (2011)
Observation of Topological Order in a Superconducting Doped Topological Insulator featured in Nature Physics (2010)
Single-Dirac-Cone Topological Surface States in the TlBiSe2 Class of Topological Semiconductors featured in Physical Review Letters (2010)
Half-Heusler Ternary Compounds as New Multifunctional Experimental Platforms for Topological Quantum Phenomena featured in Natural Materials and Nature Physics (2010)
Image Caption: Evolution of the surface electronic structure as the Se content δ is varied in a BiTl(S1-δSeδ)2 alloy. The material transitions from being an ordinary insulator (leftmost panel) to a topological insulator (rightmost panel) around δ=0.4 (middle panel).