For more than two centuries, the way we think about heat has been governed by a single elegant equation. Joseph Fourier’s law of heat conduction, written in 1822, treats heat as a diffuse fluid that spreads evenly in all directions, flowing from hot to cold like water soaking through sand. It works beautifully for a pot on a stove or a laptop warming your desk. But Fourier never met a phonon.
At the atomic scale, heat is not a fluid. It is a wave. The quantized vibrations of a crystal lattice, known as phonons, carry energy through solids with wavelike properties: they can interfere, refract, scatter, and, as it turns out, focus into directed beams, much like light through a lens. The catch has always been temperature. At room temperature, atomic vibrations are so chaotic that these wavelike effects were thought to be washed out entirely, observable only in crystals cooled to a few degrees above absolute zero.
That assumption has just collapsed. In a study published today in Nature Physics, a team led by Professor Yongjie Hu at the University of California, Los Angeles reports the first experimental demonstration of phonon focusing at room temperature. Working with cubic boron arsenide, a semiconductor with extraordinary thermal properties, the researchers used a custom-built nanoscale thermometer to directly image heat flowing along specific crystallographic directions, forming ray-like patterns that classical Fourier theory cannot explain. The finding does not merely extend a low-temperature curiosity to a warmer regime. It suggests that at everyday temperatures, inside the materials that will power the next generation of electronics, heat can be engineered, directed, and perhaps even switched, like light.
The Focusing Effect
Phonon focusing was first predicted in the 1960s by physicists studying how sound waves propagate through crystals. In an isotropic medium, vibrations travel outward uniformly. But in a crystal, the speed of a phonon depends on the direction it travels. Certain crystallographic directions act as highways, allowing phonons to propagate much farther; others act as dead ends. The result is a striking anisotropy: energy concentrates along preferred axes, producing patterns that resemble a starfish or a snowflake rather than a smooth spreading ring.
For decades, observing this effect required cryogenic temperatures below 10 Kelvin (-263 degrees Celsius). At such cold conditions, phonon scattering from thermal agitation is frozen out, and the wavelike character of heat transport dominates. At room temperature, conventional wisdom held, phonon-phonon collisions are so frequent and random that any directional preference should be lost in a fog of diffuse scattering. Heat becomes diffusive, just as Fourier said.
Why Boron Arsenide Breaks the Rules
The UCLA team’s breakthrough rests on a material that Hu’s group has been perfecting since 2018. Cubic boron arsenide (BAs) is a semiconductor with a simple zincblende structure, but its thermal properties are anything but simple. It possesses one of the highest thermal conductivities of any known semiconductor, reaching 1,300 watts per meter-kelvin at room temperature. That is more than three times the value of silicon carbide and competitive with diamond, yet in a material that can be integrated with existing semiconductor manufacturing.
The secret lies in boron arsenide’s phonon band structure. The material’s lattice vibrations have a large gap between acoustic and optical phonon branches, which suppresses the three-phonon scattering processes that normally drain energy. Instead, only the rarer four-phonon processes occur, giving phonons dramatically longer lifetimes and longer mean free paths. In this study, the researchers found that acoustic phonons in BAs have mean free paths exceeding those in diamond at equivalent frequencies. At room temperature, these phonons can travel hundreds of nanometers, even microns, without scattering. That is enough distance for wavelike effects to manifest.
A Nanoscale Thermometer
To see these effects, the team needed a measurement technique with extraordinary spatial resolution. They built a custom tip-enhanced Raman spectroscopy (TERS) system, combining a sharp gold-coated atomic force microscope tip with a Raman spectrometer. The tip concentrates laser light into a nanoscale hot spot roughly 10 nanometers across, small enough to probe local lattice temperatures with exquisite precision. By scanning the tip across the surface while heating a localized region with a separate laser, the researchers reconstructed two-dimensional temperature maps of the heat spreading through the crystal.
The maps were unambiguous. Instead of a smooth, symmetric temperature gradient predicted by Fourier’s equation, the heat flow concentrated along three directions, forming a pattern with three-fold rotational symmetry. The symmetry matched the cubic crystal structure of boron arsenide, confirming that the heat was channeled by the crystal lattice itself. The team validated their results with first-principles calculations, solving the full phonon Boltzmann transport equation including all three- and four-phonon scattering processes. The simulations reproduced the ray-like patterns quantitatively, confirming that the observed behavior arises from long-lived, wavelike phonon transport at room temperature.
What It Means for Technology
The practical implications ripple across several fields. In electronics cooling, the ability to direct heat along specific crystallographic axes offers a new design parameter. Instead of relying on complex nanostructuring to manage hot spots, engineers could simply align a chip’s crystallographic orientation to channel heat away from sensitive components. Boron arsenide, which has already been integrated into gallium nitride power transistors and shown to outperform diamond and silicon carbide in hot-spot cooling, becomes an even more attractive platform.
Beyond thermal management, the demonstration opens the door to phonon optics, an emerging field that treats phonon waves analogously to photons. If phonons can be focused, refracted, and guided at room temperature, it becomes possible to build phononic circuits that process information using lattice vibrations rather than electrons, with potentially lower energy dissipation. The long coherence of phonons in boron arsenide also makes it a candidate for quantum technologies, where phonons can serve as mediators between qubits or as carriers of quantum information in their own right.
“The observed heat dynamics originate from the spatial redistribution of non-equilibrium phonon waves with long propagation lengths,” the authors write. They further note that their first-principles theory identifies distinct transport symmetries governed by crystallographic orientation, validated across multiple samples.
The Road Ahead
What remains unknown is how to harness these phonon rays in practical devices. The focusing patterns are intrinsic to the crystal structure, but controlling them will require engineering: introducing interfaces, strain fields, or nanostructured geometries that can steer, collimate, or switch the phonon beams. The UCLA group has already demonstrated electrically gated thermal transistors capable of switching heat flow at megahertz frequencies, suggesting a possible path toward active control of phonon waves.
There is also the question of generality. Boron arsenide’s extreme properties make it the ideal platform for this first demonstration, but the principles should apply to other high-thermal-conductivity crystals. Identifying which materials support room-temperature phonon focusing, and designing new ones, will be a rich area for computational and experimental exploration.
For now, the result stands as a quiet revolution. Fourier’s law is not wrong. It remains an excellent approximation for macroscopic heat transport. But at the scale where phonons become individuals rather than a statistical ensemble, heat reveals its true nature: wave-like, directional, and far more interesting than a simple flow down a gradient. At room temperature, no less. The quantum world, it turns out, is not confined to the cryostat. It is running through the crystal beneath our fingertips.
Reference
Li, M., Wu, H., Qin, Z., Su, C., Nguyen, H. D., & Hu, Y. “Phonon focusing at room temperature.” Nature Physics (2026). DOI: 10.1038/s41567-026-03335-y
The authors are affiliated with the Department of Mechanical and Aerospace Engineering and the California NanoSystems Institute at the University of California, Los Angeles.