Managing heat efficiently inside solid materials is essential for developing faster, more reliable electronics. Until now, a wave-based form of heat transport called phonon focusing had only been observed at extremely low cryogenic temperatures, which made it difficult to study and limited its potential applications.
Researchers at the UCLA Samueli School of Engineering have now shown that phonons, the atomic vibrations that carry heat and display quantum behavior, can move through a material in concentrated, ray-shaped paths at room temperature. Rather than dispersing evenly in every direction, heat can follow routes determined by the structure of a crystal. The finding could lead to new ways of controlling heat in advanced electronics and quantum technologies.
Guiding Heat Through Boron Arsenide
The study, published in Nature Physics, was led by Yongjie Hu, a professor of mechanical and aerospace engineering at UCLA Samueli. His team observed room-temperature phonon focusing in boron arsenide, a crystalline semiconductor known for its high thermal conductivity.
Materials such as boron arsenide may allow engineers to direct heat along planned routes with nanoscale accuracy instead of waiting for it to spread and then trying to remove it. The process is similar in principle to the way optical fibers channel light along a controlled path.
To observe this behavior, the researchers created a technique for mapping temperature at the nanoscale. In conventional materials, heat spread outward in circular patterns, which is consistent with normal diffusive heat conduction. Boron arsenide produced a very different result. The temperature maps revealed distinct ray-shaped patterns aligned with specific directions in the crystal.
Crystal Structure Shapes Heat Flow
The researchers also found that the pattern of heat flow changed in predictable ways when the orientation of the crystal changed. Different planes of boron arsenide produced sixfold, eightfold, and fourfold focusing patterns.
This quantum phonon behavior remained detectable across distances of one micrometer and may extend for tens of micrometers. That range is large enough to be useful in many modern electronic, photonic and quantum devices.
“This is a fundamental observation that enables us to think about thermal management in a new way,” said Hu, the study’s corresponding author and a member of the California NanoSystems Institute at UCLA. “By enabling heat to be guided, focused and redistributed with nanoscale precision at room temperature, the discovery establishes a foundation for quantum thermal engineering.”
Potential Benefits for AI and Quantum Devices
The ability to control heat at the atomic level could help address major limits in AI hardware, microelectronic devices, aerospace systems and other electronics. Overheating can reduce performance, reliability and scalability in all of these technologies.
Hu said the discovery may also make it possible to adjust how phonons interact with electrons and other energy carriers. That capability could support future advances in quantum information systems and sensing technologies.
Earlier observations of phonon focusing were mostly restricted to temperatures only a few degrees above absolute zero. Under those conditions, phonons can travel long distances without scattering very much. At room temperature, phonons generally scatter more frequently and quickly lose the coherence needed for wave-based transport, causing heat to spread through ordinary diffusion.
Why Boron Arsenide Is Different
The new findings build on Hu’s earlier work involving the experimental discovery of boron arsenide in 2018. Since then, his group has developed high-performance thermal interfaces and gallium nitride devices that incorporate boron arsenide for cooling. Those results have underscored the material’s potential for next-generation semiconductor systems.
Boron arsenide experiences unusually weak phonon scattering, which allows wave-based heat transport to continue even at room temperature.
The heat patterns observed in the experiments closely matched theoretical calculations. This agreement confirmed that phonons in the material can travel unusually long distances before scattering, which is a central reason the wave-based behavior can survive at room temperature.
Other authors of the study include Man Li, Huan Wu, Zihao Qin, Chuanjin Su and Huu Duy Nguyen, all current or former graduate students in Hu’s H Lab at UCLA Samueli.
Funding came from the U.S. Department of Energy, the National Science Foundation, the National Institute of General Medical Sciences and a gift fund from Parag and Falguni Patel. Computational resources were provided by the Research Technology Group at the UCLA Institute for Digital Research and Education and by Bridges 2 at the Pittsburgh Supercomputing Center.