Xufeng Kou

A special material — a topological insulator — conducts current only on its surface, as if its interior is a perfect insulator. It hosts a quantum effect similar to the one discovered by Klaus von Klitzing, but mathematically it is equivalent to a four-dimensional world.

A neat stack of atomic layers lies on a substrate. An invisible beam of terahertz light shines down on it, with a magnetic field applied from the side. After reflection, the beam's polarization rotates slightly.