Jiawei Luo

Certain materials, when placed in a strong magnetic field at low temperatures, exhibit a surprising property: their interior stops conducting, becoming an insulator, while along the very edge of the sample electricity runs like a river along a channel, always in one direction.

In the crystal HfTe5, electrons behave almost like light: their mass is so small that they obey the laws of relativity. If you place such a crystal in a strong magnetic field, the electrons start to circle in tiny orbits. Their energy then changes not smoothly but in steps, like floors in a building.