Heat does not always diffuse uniformly in a material. Researchers at the University of California, Los Angeles have observed atomic vibrations propagating along directed paths, at room temperature, in a crystal of boron arsenide.
These vibrations, called phonons, carry thermal energy through solids. They also possess quantum properties. Until now, their focused propagation had mainly been observed at extremely low temperatures, near cryogenic conditions.
A dense black cluster is at the center, with narrow dark streaks in the form of rays extending outward in multiple directions against a mottled orange and yellow background, illustrating directional heat flow through the material.
Image UCLA
The team studied a crystal of boron arsenide heated to about 300 kelvins, i.e., nearly 27 °C. In this material, heat did not simply spread in all directions. It formed patterns resembling light rays.
This organization stems from the crystalline structure of boron arsenide. The vibrations follow certain preferred directions, imposed by the regular arrangement of atoms. The phenomenon is comparable, in some respects, to the way an optical fiber guides light.
To observe this movement, the researchers developed a method capable of mapping temperature at the nanoscale. In ordinary materials, the obtained maps showed circular diffusion. The studied crystal, on the contrary, produced elongated and oriented figures.
This discovery does not yet mean that electronic devices can immediately use thermal "highways." The experiments were conducted on a particular crystalline material and under controlled conditions. It will therefore be necessary to study its fabrication and integration into components.
The possibility of guiding heat could nevertheless be of interest to electronics and quantum technologies. Designers might seek to direct energy toward cooling zones, instead of waiting for it to spread throughout the component.