Three Research Groups, Two Kinds of Electronic Properties, One Material

May 13, 2020

An outstanding collaboration combines materials science, experimental and theoretical physics. The work paves the way to new designed materials that combine in them multiple electrical functionalities.

This is the story of a unique material – made of a single compound, it conducts electrons in different ways on its different surfaces and doesn’t conduct at all in its middle. It is also the story of three research groups – two at the Weizmann Institute of Science and one in Germany, and the unique bond that has formed between them.
The material belongs to a group of materials discovered a decade and a half ago known as topological insulators. These materials are conducting on their surfaces and insulating in their inside “bulk.” But the two properties are inseparable: Cut the material, and the new surface will be conducting, the bulk will remain insulating.
Some five years ago, Dr. Nurit Avraham, was starting out as a staff scientist in the new group of Dr. Haim Beidenkopf of the Institute’s Condensed Matter Physics Department. Around that time, she and Beidenkopf met Prof. Binghai Yan when he had his first scientific visit to the Weizmann Institute. Back then Yan was working as a junior group leader in the group of Prof. Claudia Felser, a materials scientist who was developing new kinds of topological materials in her lab at the Max Planck Institute for Chemical Physics of Solids in Dresden. Beidenkopf and his group specialize in classifying and measuring these materials on the scale of single atoms and the paths of single electrons, while Yan was turning to theory – predicting how these materials should behave and working out the mathematical models that explain their unusual behavior.

The combined surface spectrum of a terrace on the (001) surface with 1D topological channel residing along the step-edge. It consists of six Dirac cones (located on the dashed circle) that represent the crystalline topological states coexisting with 1D linear spectrum (the X at the middle) that represents the helical metallic channel at the step edge.

Could this one material be not only at the same time insulating and conducting, but conduct in two different ways?

Finally, theory and experiment came together to show, in an article published in Nature Materials, that the material is, indeed, two different kinds of topological insulator in one. The exposed layers of the cleft, side surfaces create “step-edges” that channel the electrons into certain paths. While the sides are protected by both time reversal and translational symmetry, the tops and bottoms are protected by crystalline mirror symmetry, giving rise to a metal-like state in which the electrons can move.

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