Spin controls the flow of heat

The quantum vacuum is not empty space. It’s full of electromagnetic fluctuations. And when two thin conducting plates are a few nanometers apart, attractive forces or torques between them can emerge from those fluctuations. In the phenomenon, known as the Casimir effect, the plates attract or twist because electric and magnetic fields vanish at their boundaries and change the free energy in the gap. (See the Quick Study by Jeremy Munday, Physics Today, October 2019, page 74 .) But the Casimir effect isn’t the only manifestation of vacuum fluctuations. When the plates’ separation is smaller than the wavelength of thermal radiation, the flow of heat between them can also change.

Whereas earlier scientists treated heat flow and Casimir interactions separately, Juan Deop-Ruano and Alejandro Manjavacas—both from the Institute of Optics in Madrid, Spain—now explore their interplay. In a new study, the theorists predict what happens when two nanostructure plates both rotate like a wheel with respect to each other and have different temperatures T₁ and T₂, as shown in the figure. In the absence of rotation, the heat flow depends only on the nanostructures’ temperature difference and is always directed from the hot nanostructure to the cold one. But when the nanostructures rotate relative to each other, the heat flow can be increased, decreased, or even reversed—simply by adjusting the rotation frequency.

The theorists derive closed analytical expressions that describe the torque and power transferred from one nanostructure to another. The sign of the torque is determined by the difference between the nanostructures’ rotational frequencies, with their relative temperature affecting only the magnitude of that torque. The heat flow, however, shows a very different behavior: Both its sign and magnitude depend on the rotation frequencies and temperatures.

Given nanostructures with the same temperature, heat flows from the nanostructure rotating faster to the one rotating slower. But when a difference in temperature also exists, a rich spectrum of behavior emerges. Although the theorists predicted those results for nanostructure plates, they expect them to hold for any nanoscale object having a dipolar resonance, such as a large molecule. What’s more, for temperatures on the order of 1 K, the results should be observable, so long as the relative rotation frequency is at least 100 GHz. That rotation frequency and temperature are within experimental reach. So the results of the new work could be used to control the heat transfer between nanoscale objects. (J. R. Deop-Ruano, A. Manjavacas, Phys. Rev. Lett. 130, 133605, 2023 .)