A single material that can conduct electrons in two different ways on its different surfaces but not in its interior has been investigated by collaborators from the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany and the Weizmann Institute of Science in Rehovot, Israel. This topological insulator was discovered while the team was looking into layered forms of these materials. The team wanted to know how layering would affect the way that electrons were conducted over the surface of the material.

A topological insulator has conductivity across its surfaces but not within the bulk. Cut a piece of such material and the conductivity will be across the newly exposed surface but again not within the bulk. Theoretically, stacking layers of a two-dimensional topological insulator might allow materials scientists to construct a 3D topological insulator with particular properties, such as having some surfaces conductors and others as insulators. It was these kinds of materials on which the teams have joined forces.

The compound in question is one made from bismuth, tellurium and iodine. Its band structure involves "band inversion" which is what precludes electron flow within the bulk. The team used scanning tunneling microscopy, STM, to look at freshly cleaved surfaces and to track the electron density in different parts of the material. Theory had it that the surface measurements would reveal it to behave as a weak topological insulator - metallic along the edges and insulating on the upper and lower surfaces. Such characteristics had not been observed experimentally before. However, the experiments revealed something more intriguing - the material acts as a weak topological insulator on its cleft sides as predicted, but on the upper and lower surfaces the results indicated it to be a strong topological insulator, rather than an insulator. The team used new samples to double-check their findings.

The team has now brought theory and experiment together to explain how exposed layers of the cleft, side surfaces form "step-edges" that can channel the electrons along certain paths. The sides are "protected" by time reversal and translational symmetry and the upper and lower surfaces are protected by crystalline mirror symmetry. This gives rise to the various conducting and insulating phenomena the team observed. [Avraham, N. et al., Nature Mater., (2020); 19 (6): 610 DOI: 10.1038/s41563-020-0651-6]