UCLA Made Heat Travel in Straight Rays Through a Crystal at Room Temperature. It Had Only Ever Worked Near Absolute Zero.

Engineers at UCLA have watched heat move through a crystal in tight, ray-shaped beams at room temperature — a behavior that until now had been observed only within a few degrees of absolute zero, and one that could change how the semiconductor industry deals with its most stubborn problem.

Heat in a solid is carried by phonons, the collective vibrations of atoms in the material's lattice. Phonons obey quantum rules, and under the right conditions they behave less like a diffusing gas and more like a wave that can be steered. That steering — phonon focusing — has been known theoretically for decades, but it was always washed out at ordinary temperatures, because atoms jostle so vigorously that phonons scatter before they can travel in any organized direction. Studying it meant cooling samples to cryogenic temperatures, which made it a laboratory curiosity rather than an engineering tool.

The team, led by mechanical and aerospace engineering professor Yongjie Hu of the UCLA Samueli School of Engineering, worked with boron arsenide, a crystalline semiconductor already known for unusually high thermal conductivity and remarkably little phonon scattering. They built a nanoscale temperature-mapping technique capable of resolving where heat actually goes after it enters the crystal, rather than inferring it from bulk measurements.

What the maps showed were not the smooth circular halos textbooks predict. They showed distinct rays, aligned to specific crystallographic directions, with sixfold, eightfold and fourfold focusing patterns depending on how the crystal was oriented relative to the heat source. The effect held over distances of about one micrometer and, the researchers say, plausibly extends to tens of micrometers — the range that matters in real microelectronics.

The practical appeal is straightforward. Modern chips are limited less by how fast transistors can switch than by how quickly the resulting heat can be removed, and the standard approach is passive: let heat spread wherever it goes, then pull it out with a heat sink. Directional phonon transport implies a different strategy, in which heat is guided along planned routes with nanoscale precision, steered around temperature-sensitive components the way an optical fiber channels light around a corner instead of letting it scatter.

"It enables us to think about thermal management in a new way," Hu said, pointing to overheating as a limit on performance and reliability in dense computing hardware. The study, published in Nature Physics, was framed around three applications in particular: AI accelerators, where power density is now the binding constraint; quantum devices, which fail outright if stray heat reaches the wrong component; and next-generation semiconductor cooling generally.

Boron arsenide is not yet a mass-manufactured material, and no one has built a working heat-routing circuit. But the result moves phonon focusing out of the cryostat and into the temperature range where chips actually run.