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.
Maghrebi and colleagues imagine a 10-nm-thick TI film sandwiched between two ferromagnetic insulators. The magnetic layers constrain the TI’s surface electrons to be in a certain spin state, making all thermal photons radiated by the material circularly polarized in approximately the same direction. This sandwich structure is not new in itself, but the researchers uncovered a novel behavior when they considered the device to be hotter than its environment. Since a hot TI radiates more circularly polarized photons than it absorbs, the angular momentum carried by the outgoing photons adds up and imparts a net twisting force on the TI.
The researchers say the predicted magnetomechanical effect could be confirmed by analyzing the circularly polarized light emitted by the device. The torque might also be detected directly, as the calculated twisting force (approximately 1 pN) is above the sensitivity threshold of state-of-the-art force measurements.
This research is published in Physical Review Letters.
Marric Stephens is a freelance science writer based in Bristol, UK.