Exotic conductors from the lab and nature

They say that it’s what’s on the inside that counts. But that is not true for topological insulators — exotic materials that conduct electricity only along their surfaces.

Physicists have now shown this property in a naturally occurring mineral, and another group has synthesised the first two-dimensional topological insulator that conducts at room temperature.

Having more such materials could boost efforts to build spintronic devices — in which currents are driven by an intrinsic property of electrons called spin, rather than by voltages. Topological insulators that work at low temperatures were first synthesised from heavy elements in 2008. Their odd conducting abilities arise because each electron’s spin becomes coupled to its motion.

This compels each electron to circle around a specific spot, preventing them from moving through the bulk material, which means that they cannot conduct electricity. But at the material’s edge, the electrons do not have enough space for this circling motion; instead, they are forced to hop along the surface in semicircular jumps, enabling conduction. “Topological insulators raise the possibility of building spintronic devices that use electron spin, rather than charge,” says Pascal Gehring, a solid-state physicist at the Max Planck Institute for Solid State Research in Stuttgart, Germany, and a co-author of the study.

All about electron spins

Spins can be rotated quickly without expending much energy, so spintronic devices should be more efficient than their electronic counterparts, in which energy is required to change charges, he adds. In theory, it is difficult to corrupt spin values in a topological insulator. That is because, to flip the spin value accidentally, you would have to knock the system hard enough to cause the electron to make a U-turn. Gehring and his colleagues examined a natural sample of kawazulite, which contains bismuth, tellurium, selenium and sulphur, found at a former gold mine in the Czech Republic.

Lab-made samples of kawazulite have already been shown to be topological insulators, but no one had checked for the property in natural samples. The team cleaved off single crystalline sheets 0.7 mm wide and applied the standard test for a topological insulator: photoelectron spectroscopy. Their results confirm that the electrons’ energy and momentum distribution matches predictions for a topological insulator.

Feng Liu, a materials scientist at the University of Utah, notes that the team’s natural sample contains fewer structural defects than its lab-made counterparts. Jeroen van den Brink, a physicist at the Leibniz Institute for Solid State and Materials Research in Dresden, Germany, and his colleagues stacked bismuth-containing sheets with a honeycomb structure. The result is a bulk material that acts as topological insulator at room temperature. The next step should be to find organic materials that act as topological insulators, says Liu. His team recently proposed a design for such a compound, and says another group has synthesised a candidate structure.