Topological Insulators Do the Twist

Topological insulators (TIs)—bulk insulators with electrically conducting surface states—don't behave like “normal” materials. For example, a TI in a magnetic field exhibits a quantized magneto-optical effect: when polarized light is transmitted through the material, it is rotated by an amount that is set by a fundamental constant. Most such effects have been predicted and measured for TIs that are at or near thermal equilibrium with their environment. Theorist Mohammad Maghrebi from Michigan State University, East Lansing, and colleagues have taken the challenging step of modeling a TI out of thermal equilibrium, and they found an unexpected mechanical effect. The researchers predict that thermal photons emitted by a TI hotter than its surroundings would impart a small—but potentially measurable—torque on the material.