arXiv subjects

Ange Benise Niyikiza

Publications and source records attributed to Ange Benise Niyikiza.

4 recordsLinked to original sources

arXiv2026

Slow Thermalization and Long-Lived Coherence across Acoustic Phonon Branches in BAs

The exceptionally high thermal conductivity of cubic boron arsenide (BAs) arises from suppressed three-phonon scattering associated with its large acoustic--optical gap. In this regime, four-phonon processes become non-negligible, creating an unusual scattering hierarchy with unexplored consequences for nonequilibrium phonon dynamics. Here, time-resolved x-ray diffuse scattering reveals exceptionally slow, branch-dependent acoustic phonon thermalization and long-lived coherence. Exploiting branch sensitivity in one-phonon diffuse scattering, we resolve transverse acoustic (TA) and longitudinal acoustic (LA) thermalization times of 29.2 and 13.9~ps, respectively; even the LA timescale is at least three times that in common semiconductors. Phonon dispersion calculations assign coherent oscillations at 0.19 and 0.27~THz to the TA and LA modes, respectively; both persist with little decay over 50~ps, with comparable coherence found only in diamond. These findings provide a direct dynamical manifestation of the weak phonon scattering underlying the exceptional thermal transport of BAs.

cond-mat.mtrl-sci↗

arXiv2026

Exceptional Optical Phonon Coherence in Enriched Cubic Boron Arsenide via Suppression of Three-Phonon Scattering

Cubic boron arsenide (BAs) is a promising semiconductor for next-generation electronics due to its outstanding ambipolar mobility and thermal conductivity, the latter of which is attributed to the suppression of three-phonon scattering. However, precisely accounting for different high-order anharmonic scattering processes is challenging from both theory and experiment, so that questions remain open regarding the ultimate limit of phonon lifetime and thermal conductivity in BAs. Here we show that this gap nearly eliminates three-phonon scattering for zone-center optical phonons in a wide temperature range, leading to a record-high, isotope purity-limited phonon coherence with a quality factor above $3.7\times 10^3$ for >98% enriched $^{11}$BAs below 100 K. We discriminate three decoherence mechanisms by their temperature-dependent contribution to the damping rate using high-resolution Raman and Fourier transform infrared spectroscopy. For the as-synthesized crystals, we find that defect scattering has negligible contributions to the linewidth of optical phonons in comparison to isotope scattering. These results provide critical insights into the intrinsic and extrinsic scattering mechanisms of optical phonons in BAs, motivating further studies to quantify anharmonic effects and realize superior phonon transport.

cond-mat.mtrl-sci↗

arXiv2025

Thermal Conductivity above 2000 W/m.K in Boron Arsenide by Nanosecond Transducer-less Time-Domain Thermoreflectance

Cubic boron arsenide (c-BAs) has been theoretically predicted to exhibit thermal conductivity \k{appa} comparable to that of diamond, yet experimental measurements have plateaued at ~1300W/mK. We report room-temperature \k{appa} exceeding 2000W/mK in c-BAs, on par with single-crystal diamond. This finding is enabled by high-quality single crystals and a newly developed nanosecond, transducer-less time-domain thermoreflectance technique that allows spatial mapping of \k{appa} without metal transducers. Thermal conductivity correlates with crystal quality, as evidenced by stronger photoluminescence and longer photoluminescence lifetimes. However, the observed nanosecond lifetimes remain shorter than expected for an indirect bandgap semiconductor, suggesting room for further crystal quality improvement and higher \k{appa}. These results challenge current theoretical models and highlight c-BAs as a promising material for next-generation electronics.

physics.app-ph↗

arXiv2025

Thermal conductivity of boron arsenide above 2100 watts per meter per Kelvin at room temperature

Boron arsenide (BAs) single crystals had been previously reported to have thermal conductivity of 1500 W/mK at room temperature. Now we achieved thermal conductivity above 2100 W/mK at room temperature in BAs crystals due to much lower concentration of impurities Si, C, and O grown from purified arsenic. We also observed a T-1.8 dependence of the thermal conductivity, suggesting a more significant contribution from four-phonon scatterings than suggested by previous theory. We found that our experimental result can be fit with a modified theoretical calculation by tuning down the three-phonon scattering for phonons in the 4-8 THz range, although current phonon transport theory cannot provide a physical explanation. Such an advance will not only attract more effort on growing BAs single crystals and studying their practical applications but also stimulate theoretical work to predict more materials with possibly even higher thermal conductivities.

cond-mat.mtrl-sci↗