Touch a heated metal rod, and the warmth spreads evenly in all directions. This everyday intuition rests on the classical law of heat conduction formulated by Joseph Fourier in 1822: heat diffuses isotropically outward, following the temperature gradient. At the microscopic level, heat inside a solid is carried by phonons—quantized packets of energy arising from vibrations in the atomic lattice. In ordinary materials at room temperature, phonons collide with each other constantly. As a result, their wave-like character is instantly destroyed, and heat spreads through random diffusion, much like ink dispersing in water.
But if phonons inside a crystal could travel in straight lines without colliding, heat would converge along specific directions—much like light rays radiating from a star. This phenomenon, known as phonon focusing, has been recognized theoretically and experimentally in solid-state physics for more than half a century. The problem has always been that maintaining this orderly, wave-like behavior is fundamentally difficult at room temperature, where intense thermal vibrations cause frequent phonon scattering.
On July 23, 2026, a research team led by Yongjie Hu, professor of mechanical and aerospace engineering at the UCLA Samueli School of Engineering, reported in the journal Nature Physics that they had directly observed phonon focusing at room temperature (300 K) using the semiconductor material boron arsenide (). This study successfully drew out a wave-like heat transport phenomenon that had long been confined to extreme cryogenic regimes, adding new insight into the thermal physics of solids.
Bringing Cryogenic Physics to Room Temperature
Phonon focusing in solids was first experimentally captured between the 1960s and 1970s. At that time, researchers used high-purity crystals such as solid helium, sodium fluoride (), and germanium (), irradiating them with heat pulses under extreme cryogenic conditions ranging from below 1 K to a few kelvin. In environments close to absolute zero, atomic thermal motion is suppressed to the extreme. The mean free path—the distance a phonon can travel without colliding with another phonon—stretches to the millimeter scale, allowing phonons to travel ballistically through the crystal without scattering and converge along specific directions determined by the crystal's elastic anisotropy.
As temperature rises to room temperature, the energy density of lattice vibrations increases sharply. The scattering rate due to nonlinear interactions between phonons rises dramatically, and the mean free path shrinks rapidly to the nanometer scale. The phase relationships and directional propagation characteristic of waves are immediately destroyed, and heat transport becomes entirely governed by the classical diffusion equation. Until now, observing phonon focusing at room temperature has been regarded as one of the major barriers in thermal physics.
Professor Hu's research group challenged this conventional wisdom. Rather than relying on cryogenic cooling, the team sought to directly capture the wave-like propagation behavior of phonons at room temperature by selecting a material with a special crystal structure in which the probability of phonon-phonon collisions remains exceptionally low even at ambient conditions.
A Nanoscale Thermal Probe Captures Beam-Like Heat Flow
To demonstrate phonon focusing at room temperature, the research team built a custom nanoscale temperature mapping technique with extremely high spatial resolution. In the experiment, a fine gold nanoprobe was positioned on the sample surface. This probe served as a localized point heat source while simultaneously functioning as a highly precise sensor detecting how heat propagated outward.
The team first measured heat spreading using common semiconductor materials and control samples exhibiting isotropic thermal conduction. In these control experiments, heat released from the point source spread outward isotropically in concentric circular patterns exactly as theory predicts—clear evidence of classical diffusive conduction dominating the process.
By contrast, when localized heat was injected into a single crystal of boron arsenide held at room temperature (300 K, approximately 27°C), the observed temperature distribution took on an entirely different shape. Rather than spreading in a circle, the heat formed sharply defined beam-like patterns extending in specific directions—resembling the light rays radiating from a star.
This measured beam-like temperature profile and the dynamics of heat propagation matched precisely with simulation results based on the Boltzmann transport equation derived from first-principles calculations. As the paper in Nature Physics demonstrates, this asymmetric thermal dynamic was caused by the spatial redistribution within the crystal of non-equilibrium phonon waves possessing long propagation distances.
Crystal Orientation Shapes Symmetry and Micrometer-Scale Reach
In the experiments, the researchers cut different crystal facets of boron arsenide to examine how the geometric pattern of phonon focusing changed. They confirmed that distinctive heat flow patterns—exhibiting six-fold, eight-fold, and four-fold symmetry—emerged depending on the crystal's anisotropy. This suggests the possibility of controlling the direction of heat flow by selecting the orientation of the crystal's three-dimensional atomic arrangement.
| Item | Conventional Phonon Focusing Observations | This Study (Boron Arsenide / 300 K) | Typical Room-Temperature Solid Heat Conduction |
|---|---|---|---|
| Operating temperature range | Cryogenic () | Room temperature () | Room temperature () |
| Primary materials studied | Solid , , | Boron arsenide () | , , various metals |
| Mode of heat transfer | Ballistic transport / wave propagation | Wave-like phonon focusing | Classical diffusion (random scattering) |
| Temperature distribution pattern | Anisotropic (beam-like) | Anisotropic (four-fold, six-fold, eight-fold symmetry, etc.) | Isotropic (concentric circles) |
| Approximate thermal conductivity | Varies substantially by condition | Approximately | : approximately |
Important data was also obtained regarding the scale of phonon propagation observed in this study. While some general media reports suggested that phonon travel distances were limited to a mere 250 nanometers, according to UCLA's official announcement and the original paper, this ordered phonon behavior persists over distances exceeding 1 micrometer, and potentially extends into the range of tens of micrometers.
In modern miniaturized semiconductor integrated circuits, the channel length of transistors—which serve as localized heat sources—ranges from tens to hundreds of nanometers. The fact that directional heat propagation is maintained over distances exceeding 1 micrometer suggests there is substantial physical room to guide heat flow along specific pathways within fine-scale structures. The research team states in the paper that comprehensive verification and quantitative analysis of temperature-dependent experiments confirmed the reproducibility of their measurement technique and the robustness of its nanoscale spatial resolution.
The Unusual Lattice Properties of Boron Arsenide
Why was room-temperature phonon focusing—impossible in most other materials—observable only in boron arsenide? The answer lies in this material's extremely unusual phonon band structure.
Boron arsenide possesses a zinc-blende-type face-centered cubic lattice structure (space group ), with boron and arsenic atoms forming tetrahedral coordination. The combination of elements with an extreme mass difference—light boron atoms paired with heavy arsenic atoms—creates a large energy gap between the acoustic and optical phonon energy bands. This strongly suppresses three-phonon scattering, which is normally the dominant scattering pathway in typical crystals.
Professor Hu's team was a pioneer in this field, having experimentally established in a 2018 paper in Science that defect-free boron arsenide single crystals exhibit an extremely high thermal conductivity of approximately 1300 W/(m·K) at room temperature. This stands out remarkably compared to common silicon's thermal conductivity of about 150 W/(m·K) or copper's approximately 400 W/(m·K). Even when accounting for higher-order anharmonic effects such as four-phonon scattering, boron arsenide's acoustic phonons maintain an extraordinarily long mean free path. This exceptionally weak scattering property is the physical foundation that allows phonons to retain their wave-like coherence and converge along specific crystal axes even at a room temperature of 300 K.
Basic Research Achievements and Challenges Toward Practical Application
This achievement opens a new foundation for thermal design—one in which heat can be directed toward intended paths within microscopic regions. In their abstract, the research team notes that controlling the directionality of phonon waves, harnessing non-equilibrium phonon distributions, and controlling interactions with other energy carriers could open new frontiers for future thermal engineering and nanodevice design. Professor Hu also expressed hope that this discovery could serve as a solid foothold for quantum thermal engineering, helping to resolve heat management issues in microelectronics and high-power devices.
However, there remains a clear gap between academic progress and industrial application. While some media reports have implied a connection to the realization of room-temperature quantum computing, the paper itself does not demonstrate quantum computation—it reports basic physics elucidating the transport phenomena of phonons, the quanta of lattice vibration.
At present, this remains confined to laboratory observations of the highly specialized single crystal boron arsenide. Engineering challenges—including large-area synthesis of high-quality boron arsenide crystals, integration into existing silicon or gallium nitride () device fabrication processes, and reducing manufacturing costs—remain largely unaddressed. Can similar room-temperature focusing phenomena be drawn out in other material systems? And can heat flow be controlled without scattering within the complex layered structures of real devices? The quest to translate this wave-like heat behavior discovered in the laboratory into practical engineering technology has only just begun.