Phonons decay through scattering — colliding with other phonons, with defects, with isotopic mass variations. In most materials, three-phonon scattering dominates at room temperature: one phonon splits into two, or two merge into one. The rate of this process sets the phonon lifetime and limits thermal conductivity.
Isotopic enrichment of cubic boron arsenide nearly eliminates three-phonon scattering for zone-center optical phonons (arXiv:2603.11256). The effect is dramatic: quality factors exceed 3,700 in highly enriched samples below 100 K. By distinguishing three decoherence pathways through their temperature dependence, the researchers show that defect scattering contributes negligibly — the linewidth is dominated by isotope-related processes.
The mechanism is specific to boron arsenide's phonon dispersion. The large mass ratio between boron and arsenic creates a wide gap between acoustic and optical phonon branches. In natural-abundance material, the isotopic mass disorder of boron-10 and boron-11 enables scattering channels that bridge this gap. Remove the isotopic disorder by enrichment, and those channels close. The three-phonon processes that normally limit optical phonon lifetime are suppressed because the pathways that connect them require the isotopic disorder to satisfy energy and momentum conservation simultaneously.
The phonon doesn't decay because it has nowhere to go. The enrichment closes the decay channels, and the phonon rings with exceptional coherence — a vibration that persists because the exits have been sealed.