UCLA engineers have shown that heat can be guided like a beam of light at room temperature, not just in deep cryogenic labs. In a study published August 7, 2026, in Nature Physics, a team led by Professor Yongjie Hu at the UCLA Samueli School of Engineering demonstrated phonon focusing at room temperature in boron arsenide, a crystalline semiconductor with exceptionally high thermal conductivity. Instead of spreading in circles, heat traveled in sharp, ray-like paths determined by the crystal structure, opening a path to route heat around sensitive parts of chips and quantum devices with nanoscale precision.
Until now, this wave-based form of heat transport had only been seen within a few degrees of absolute zero, where atomic vibrations stay coherent. The new result shows it can survive at everyday temperatures because boron arsenide scatters phonons unusually weakly. The finding establishes a foundation for what the researchers call quantum thermal engineering, with direct relevance for AI hardware, power electronics, aerospace systems, and next-generation semiconductors.
What Is Phonon Focusing, and Why Does It Matter?
To understand the breakthrough, it helps to start with the carrier of heat itself.
A phonon is a quantum of atomic vibration. Like photons carry light, phonons carry heat through a solid as atoms jiggle and pass energy to neighbors. Phonons also display quantum behavior, showing both particle-like and wave-like properties.
There are two main types:
- Acoustic phonons: Low-frequency vibrations where neighboring atoms move together, similar to sound waves traveling through the material. They carry most heat in semiconductors.
- Optical phonons: Higher-frequency vibrations where neighboring atoms move opposite to each other. They interact strongly with light and affect thermal and electronic properties but carry less heat directly.
In most materials at room temperature, phonons scatter frequently off defects, other phonons, and crystal boundaries. After just a few collisions they lose coherence, and heat transport looks like diffusion. That is why heat from a hot spot normally spreads outward in a circular, uniform halo.
Phonon focusing is different. It is a wave-based regime where phonons propagate with long, coherent paths and their energy becomes concentrated along specific crystallographic directions. Instead of a diffuse circle, temperature maps show bright rays and star-like patterns. The effect depends on how fast phonons travel in different directions, which is set by the crystal’s symmetry and elastic constants. It is analogous to how an anisotropic lens can focus light.
Before the UCLA work, focusing was a low-temperature curiosity observed in ultrapure crystals cooled to cryogenic temperatures. At those temperatures phonons travel long distances without scattering, so wave effects survive. At room temperature, scattering usually wipes the waves out. That limitation made it difficult to study and impossible to use in real devices.
The UCLA Breakthrough: Heat Rays Observed at Room Temperature
The study, titled “Phonon focusing at room temperature,” was led by Yongjie Hu, professor of mechanical and aerospace engineering at UCLA Samueli and a member of the California NanoSystems Institute. First authors include Man Li, Huan Wu, Zihao Qin, Chuanjin Su, and Huu Duy Nguyen from Hu’s H Lab. The paper appeared in Nature Physics with DOI 10.1038/s41567-026-03335-y, and the university detailed the work in its UCLA Newsroom release.
The team demonstrated focusing in boron arsenide (BAs), a cubic semiconductor first experimentally realized with high quality by Hu’s lab in 2018. Boron arsenide is notable for its ultrahigh thermal conductivity, but it had never been shown to support quantum phonon wave propagation at room temperature.
To see it, the researchers built a new nanoscale temperature mapping technique. When they injected heat into conventional materials, the thermal image was a familiar circular diffusion pattern. In boron arsenide, the same experiment produced something strikingly different: distinct ray-shaped temperature patterns aligned with the crystal axes.
Key observations from the experiments:
- Heat did not spread uniformly. Instead, it formed narrow, beam-like channels that persisted over distances of at least one micrometer and potentially tens of micrometers, according to modeling.
- The measured ray patterns matched first-principles Boltzmann transport simulations with no adjustable parameters, confirming the waves were not an imaging artifact.
- The effect was reproducible across multiple samples and crystal planes.
“This is a fundamental observation that enables us to think about thermal management in a new way,” Hu said in the UCLA announcement. “By enabling heat to be guided, focused, and redistributed with nanoscale precision at room temperature, the discovery establishes a foundation for quantum thermal engineering.”
Why Boron Arsenide Makes Room-Temperature Quantum Heat Waves Possible
Room temperature is hostile to phonon waves because four-phonon and other scattering processes usually erase coherence quickly. Boron arsenide is an exception.
First-principles theory predicts cubic boron arsenide should have a room-temperature thermal conductivity around 1,300 to 2,200 W/m·K, comparable to diamond and graphite and far above silicon at about 150 W/m·K. Early synthetic crystals only reached about 190 W/m·K because of defects, but Hu’s group developed growth techniques that now routinely yield 900 to 1,300 W/m·K in high-quality single crystals. Isotope-enriched c-11BAs crystals have even been measured at about 1,500 W/m·K, as reported in recent peer-reviewed work on isotope-enriched boron arsenide.
This extreme conductivity reflects unusually weak phonon scattering. The paper attributes the survival of focusing to long phonon propagation lengths and a spatial redistribution of non-equilibrium phonon waves. In simpler terms, phonons in boron arsenide travel unusually far before colliding, so their wave character does not wash out. The thermal conductivity of boron arsenide places it among the highest of all bulk semiconductors and metals, which is why the material can host coherent transport without cryogenic cooling.
The same property underlies earlier advances from Hu’s lab, including highly conducting boron arsenide thermal interfaces and gallium nitride devices that integrate BAs for cooling. Funding for the new study 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, with computational support from UCLA’s Institute for Digital Research and Education and the Pittsburgh Supercomputing Center’s Bridges-2.
Crystal Orientation Controls the Pattern: Sixfold, Eightfold, and Fourfold Heat Rays
One of the most elegant parts of the result is its programmability. The direction of the heat rays is not random. It is set by how the crystal is cut and oriented.
The researchers mapped heat flow on different crystallographic planes of boron arsenide and found distinct symmetries:
- On one plane, heat formed a sixfold star with six dominant rays.
- On a second plane, an eightfold pattern appeared.
- On a third plane, a fourfold pattern emerged.
Each pattern matched the symmetry predicted by first-principles theory for phonon focusing along those directions. Changing the crystal orientation predictably switched the pattern, like rotating a kaleidoscope.
This orientation dependence matters for engineering. If heat rays follow crystal-defined routes, designers could in principle choose a wafer orientation to steer heat around a hotspot, toward a heat sink, or away from a quantum bit that decoheres when warm. The observed coherence length of about one micrometer, with modeling suggesting tens of micrometers, is already comparable to feature sizes in modern electronic, photonic and quantum devices.
The analogy the authors use is fiber optics for heat. Just as an optical fiber guides light along a controlled path by refractive index contrast, a boron arsenide structure could channel phonons along crystal-defined channels without needing external fields. Earlier cryogenic focusing experiments could not exploit this idea in practical hardware, because no device operates at a few kelvin without expensive cooling.
What This Means for AI Chips, Quantum Devices, and Smarter Cooling
Efficient heat removal is now the limiting factor for many technologies. In AI accelerators, power densities exceed hundreds of watts per square centimeter. In 3D chip stacks, hotspots are buried under layers of logic. In quantum processors, even milli-kelvin temperature fluctuations can destroy coherence.
Today’s cooling is largely diffusive and reactive: spread heat as fast as possible with copper spreaders, diamond heat sinks, or microfluidic cold plates, then pump it away. Guiding heat at the source would be more efficient. Potential applications highlighted by the authors and independent experts include:
- Directional heat routing in AI hardware: Channel waste heat along predefined rays to surface heat sinks, reducing thermal crosstalk between tightly packed chiplets.
- Quantum device stability: Route phonons away from qubits and sensitive sensors while preserving their quantum states. The team notes that tuning phonon-electron interactions could help control decoherence and enable new sensing modalities.
- Photonic and RF power electronics: Boron arsenide integration with gallium nitride has already shown better performance than GaN on silicon carbide or diamond. Adding focusing control could further improve the reliability of high-frequency and high-power devices.
- Aerospace and power systems: Lightweight, directionally cooled substrates that move heat tens of micrometers without extra energy input.
Beyond cooling, the work opens a research direction in thermal wave engineering. If phonons can be focused, they can in principle be interfered with, collimated, or even used to carry information, similar to how photonics uses light. The paper explicitly frames the result as a step toward controlling non-equilibrium phonon distributions and phonon-carrier interactions at room temperature.
There are still practical hurdles. Large, defect-free boron arsenide wafers remain expensive and hard to grow at scale, and integrating them with silicon CMOS processes will require advances in epitaxy and bonding. The current focusing distances, up to tens of micrometers in theory, cover chip-level distances but not board-level lengths. Yet the conceptual leap is significant: heat is no longer forced to diffuse. It can be steered.
Boron Arsenide vs. Diamond, Silicon, and Other Thermal Materials
How does boron arsenide compare to the materials engineers use today? Thermal conductivity alone does not tell the full story, since electronic bandgap, cost, and integration matter, but it is a useful starting point.
| Material | Room-Temp Thermal Conductivity (W/m·K) | Semiconductor? | Key Advantage / Limitation |
|---|---|---|---|
| Diamond | ~2,200 | Insulator (wide bandgap ~5.5 eV) | Highest conductivity, but expensive, hard to dope and integrate as active substrate |
| Cubic boron arsenide (c-BAs, defect-free) | ~1,300 measured, ~1,500 isotope-enriched | Yes (~1.8 eV bandgap) | Highest among semiconductors, dual function as active device and heat spreader, enables room-temp phonon focusing |
| Graphite (in-plane) | ~2,000 in-plane, ~10 cross-plane | Semi-metal | Highly anisotropic, not a semiconductor for logic |
| Silicon | ~150 | Yes (1.1 eV) | Industry standard, low conductivity limits power density |
| Silicon carbide (SiC) | ~300-490 | Yes | High-power standard, lower than BAs |
| Gallium arsenide (GaAs) | ~45 | Yes | Good optoelectronics, poor heat spreading |
| Copper | ~400 | Metal | Excellent passive spreader, but not semiconducting and heavier |
The table clarifies why boron arsenide is attracting attention. Unlike diamond or copper, it is a semiconductor that can host transistors while simultaneously acting as its own high-performance heat spreader. Isotope enrichment offers a further knob: c-11BAs has been measured at about 1,500 W/m·K, pushing conductivity even closer to diamond without losing semiconducting behavior.
For more on how thermal management shapes modern electronics, see our earlier coverage of hydrogen fuel cells for data center power and devices that channel heat into light for energy conversion.
Frequently Asked Questions
What is a phonon in simple terms?
Think of a crystal as atoms connected by springs. When one atom vibrates, the vibration travels through the springs. A phonon is the quantum packet of that vibration, just as a photon is the packet of light. Acoustic phonons are like sound waves carrying heat, while optical phonons involve opposite motion of neighboring atoms and couple more strongly to light.
Is a phonon a boson?
Yes. Phonons are bosons, meaning they follow Bose-Einstein statistics, and many can occupy the same quantum state. This property allows them to form coherent waves that can interfere and focus, which is central to the observed phonon focusing effect.
What is phonon scattering?
Phonon scattering is any collision that changes a phonon’s direction or energy, including collisions with crystal defects, boundaries, or other phonons. Frequent scattering breaks coherence and makes heat diffuse. Boron arsenide’s weak scattering lets phonons travel far enough to keep their wave-like focusing at room temperature.
What are the two main types of phonons?
Acoustic phonons and optical phonons. Acoustic phonons have atoms moving in phase and carry most heat. Optical phonons have atoms moving out of phase, sit at higher frequencies, and influence optical and electronic interactions.
How is phonon focusing different from normal heat conduction?
Normal conduction is diffusive: heat spreads in all directions and smooths out. Focusing is wavelike: heat concentrates into narrow rays set by crystal symmetry. The UCLA team saw circular halos in ordinary materials and sharp star patterns in boron arsenide under identical conditions.
Why does this work at room temperature only in boron arsenide?
Because boron arsenide hosts unusually long phonon propagation lengths even at 300 kelvin. Its crystal structure and bonding produce very weak anharmonic scattering, including suppressed four-phonon processes. That lets non-equilibrium phonon waves stay coherent over micrometers, preserving focus that would otherwise require cryogenic cooling.
Could this lead to thermal fibers that guide heat like optical fibers guide light?
That is the long-term vision. The authors compare the idea to optical fibers: engineered crystal orientations and heterostructures that define pathways for phonons. Early demonstrations are on bulk crystal planes, but future work aims at patterned boron arsenide channels that could route heat around chip components with nanoscale precision.
A New Way to Think About Heat
For decades, thermal engineers treated heat as something to spread and then remove. The UCLA demonstration suggests a more surgical approach: aim heat where you want it, using the crystal itself as the lens. By showing that phonon focusing survives at room temperature in boron arsenide, the team has moved a quantum transport phenomenon out of the cryostat and into the realm of practical electronics.
Many steps remain before thermal waveguides appear in phones or servers, from scalable crystal growth to heterogeneous integration with silicon and gallium nitride. But the physics foundation is now in place. If heat can be taught to travel in beams, the design rules for cooling AI accelerators, quantum processors, and high-power devices may soon be rewritten, turning one of engineering’s oldest problems into a controllable quantum resource.
