In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids. Often referred to as a quasiparticle, it represents an excited state in the quantum mechanical quantization of the modes of vibrations of elastic structures of interacting particles.
Phonons play a major role in many of the physical properties of condensed matter, such as thermal conductivity and electrical conductivity. The study of phonons is an important part of condensed matter physics.
The concept of phonons was introduced in 1932 by Russian physicist Igor Tamm. The name phonon comes from the Greek word φωνή (phonē), which translates as sound or voice because long-wavelength phonons give rise to sound. Wikipedia, Phonon
Sound, in the day-to-day classical world, can be understood as a density wave in a medium. However, from the perspective of quantum mechanics, sound can also be described as a particle: the phonon. This particle, the sound quantum, represents the smallest amount of energy that occurs as an acoustic wave at a certain frequency. In order to see and study a single quanta of sound, the number of phonons must be minimized. https://phys.org/news/2024-01-quantum-ground-state-waveguides-scientists.html
Phonon and Heat Transfer
Researchers have observed an unusual quantum phenomenon in which heat-carrying vibrations travel through a material in highly focused, ray-like paths at room temperature.
The study, published in Nature Physics, investigated phonon focusing in boron arsenide, a semiconductor with exceptionally weak phonon scattering. Phonons are quantized vibrations of a crystal's atomic lattice and are among the primary carriers of heat through solid materials.
Normally, heat spreads in many directions as phonons repeatedly scatter from atoms and imperfections. But boron arsenide has unusually long phonon mean free paths, allowing the vibrations to preserve their directional, wave-like behavior even at temperatures around 300 kelvin (27°C).
Using nanoscale thermal measurements, researchers observed distinctive patterns in the way heat traveled through the crystal. Instead of spreading smoothly, thermal energy concentrated along specific directions determined by the material's crystal structure.
The researchers observed several striking focusing patterns, including fourfold, sixfold and eightfold structures. The focused phonons could remain concentrated over distances of around a micrometer and potentially much farther under suitable conditions.
The discovery could provide a new way to control heat at extremely small scales. Precisely directing thermal energy could eventually benefit advanced electronics, photonic devices and quantum technologies, where unwanted heat can limit performance.
Importantly, these are not conventional heat waves traveling through space. The phenomenon arises from quantum mechanical lattice vibrations phonons whose propagation becomes strongly directional inside the crystal.
The work demonstrates that quantum effects in heat transport can remain observable at everyday temperatures, opening new possibilities for nanoscale thermal engineering.
[Source: Man Li et al., Nature Physics (2026), “Phonon focusing at room temperature”; UCLA Samueli School of Engineering.]