Long coherence lifetime of optical phonons in enriched cubic boron arsenide

Cubic boron arsenide (BAs) has outstanding ambipolar mobility and thermal conductivity for potential next-generation electronics. Its high thermal conductivity is attributed to the suppression of three-phonon scattering process for the acoustic phonons, partly due to the large acoustic-optical phonon band gap arising from the large atomic mass contrast between boron and arsenic. However, it is challenging to precisely accounting for different high-order anharmonic scattering processes from both theory and experiment, so it is still under debate about the ultimate limit of phonon lifetime in BAs. Here we show that for zone-center optical phonons, the acoustic-optical phonon band gap nearly eliminates the three-phonon scattering in a wide temperature range. Below 100 K, the lack of scattering leads to a high quality factor above 3.7×10³, the record in inorganic crystals to our best knowledge. The coherence is only limited by the isotope purity for >98% enriched ¹¹BAs. From the temperature dependence of decoherence rate, we distinguish three decoherence mechanisms using high-resolution Raman and Fourier transform infrared spectroscopy. Interestingly, we find that defect scattering has negligible contributions to decoherence for the as-synthesized crystals. These results provide clean evidence of intrinsic and extrinsic scattering mechanisms of optical phonons in BAs and potentially help improving the anharmonicity calculations as well as realizing superior phonon transport.