Novel insulators with conducting edges

Article courtesy of the University of Zurich

An international team including scientists at Princeton University is researching a new class of materials: higher-order topological insulators. The edges of these crystalline solids conduct electric current without dissipation, while the rest of the crystal remains insulating. These materials could be useful for applications in semiconductor technology and for building quantum computers. The study was published in the journal Sciences Advances.

Topology examines the properties of objects and solids that are protected against perturbations and deformations. Topological materials known so far include topological insulators, which are crystals that insulate on the inside but conduct electrical current on their surface. The conducting surfaces are topologically protected, which means that they cannot easily be brought into an insulating state.

A new class of materials: Higher-order topological insulators

Theoretical physicists have now predicted a new class of topological insulators that have conducting properties on the edges of crystals rather than on the surface. The research team, made up of scientists from University of Zurich (UZH), Princeton University, the Donostia International Physics Center, and the Max Planck Institute of Microstructure Physics in Halle, dubbed the new material class “higher-order topological insulators.” The extraordinary robustness of the conducting edges makes them particularly interesting: The current of topological electrons cannot be stopped by disorder or impurities. If an imperfection gets in the way of the current, it simply flows around the impurity.

Like a highway for electrons

In addition, the crystal edges do not have to be specially prepared to conduct electrical current. If the crystal breaks, the new edges also conduct current. “The most exciting aspect is that electricity can at least in theory be conducted without any dissipation,” said Titus Neupert, professor in the Department of Physics at UZH. “You could think of the crystal edges as a kind of highway for electrons. They can’t simply make a U-turn.” Neupert and his team collaborated with B. Andrei Bernevig, professor of physics, and his team at Princeton University.

This property of dissipationless conductance, more commonly associated with superconductors at low temperatures, is not shared with the previously known topological insulator crystals that have conducting surfaces, but is specific to the higher-order topological crystals.

Further theoretical and experimental research needed

The physicists’ study still mostly relies on theoretical aspects. They have proposed tin telluride as the first compound to show these novel properties. “More material candidates have to be identified and probed in experiments,” says Neupert. The researchers hope that in the future, nanowires made of higher-order topological insulators may be used as conducting paths in electric circuits. These nanowires could be combined with magnetic and superconducting materials for use in building quantum computers.

Funding was provided by the U.S. Department of Energy (DE-SC0016239), a Simons Investigator Award, the David and Lucile Packard Foundation, and the Eric and Wendy Schmidt Transformative Technology Fund at Princeton University. The computational part of the Princeton work was performed with funding from the National Science Foundation’s Early-Concept Grants for Exploratory Research (DMR-1643312) and Materials Research Science and Engineering Centers (DMR-1420541), the Office of Naval Research (ONR-N00014-14-1-0330), and the Army Research Office (MURI W911NF-12-1-0461). Support was also provided by the Swiss National Science Foundation (grant number: 200021_169061), the European Union’s Horizon 2020 research and innovation program (ERC-StG-Neupert-757867-PARATOP), and the Spanish Ministry of Economy and Competitiveness (FIS2016-75862-P).

The study, “Higher-order topological insulators,” by Frank Schindler, Ashley M. Cook, Maia G. Vergniory, Zhijun Wang, Stuart S. P. Parkin, B. Andrei Bernevig, Titus Neupert was published online in Science Advances, on June 1st, 2018. DOI: 10.1126/sciadv.aat0346.