Heat normally spreads outward through a solid in every direction, which makes it notoriously hard to control once it starts moving. Now engineers at the UCLA Samueli School of Engineering have demonstrated a striking exception: in the right crystal, heat can travel along concentrated, ray-like paths at room temperature, behaving more like light in an optical fiber than heat in a frying pan.
The work, published in Nature Physics, was led by Yongjie Hu, a professor of mechanical and aerospace engineering at UCLA Samueli. His group observed room-temperature phonon focusing in boron arsenide, a crystalline semiconductor famous for its unusually high thermal conductivity. Phonons are quantum vibrations of the atoms in a crystal that carry heat, and until now their wavelike, focused transport had only been observed at cryogenic temperatures just a few degrees above absolute zero.
To make the invisible visible, the team developed a nanoscale temperature mapping technique. In ordinary materials, the maps showed the familiar circular pattern of heat diffusing outward. In boron arsenide, however, the temperature patterns formed distinct rays, revealing that heat was traveling preferentially along specific directions defined by the crystal structure. Rotating the crystal changed the pattern in predictable ways, producing sixfold, eightfold and fourfold arrangements of focused heat.
The quantum behavior persisted across distances of one micrometer and could potentially extend for tens of micrometers — large enough to matter for real electronic, photonic and quantum devices. "This is a fundamental observation that enables us to think about thermal management in a new way," Hu said. "By enabling heat to be guided, focused and redistributed with nanoscale precision at room temperature, the discovery establishes a foundation for quantum thermal engineering."
The discovery builds on Hu's earlier work, including the 2018 experimental discovery of boron arsenide's exceptional properties. Because the material scatters phonons only weakly, wavelike heat transport survives even at room temperature.
Guided heat could help solve one of the most stubborn problems in modern electronics: overheating. The ability to steer heat with nanoscale precision could improve thermal management in AI hardware, microelectronics and aerospace systems, and could allow researchers to tune interactions between phonons, electrons and other energy carriers for future quantum information and sensing technologies.
