  Understanding lattice dynamics is crucial for effective thermal management in
high-power electronic devices because phonons dominate thermal transport in
most semiconductors. This study utilizes complementary inelastic X-ray and
neutron scattering techniques and reports the temperature-dependent phonon
dynamics of alpha-GaN, one of the most important third-generation power
semiconductors. A prominent Matryoshka phonon dispersion is discovered with the
scattering tools and confirmed by the first-principles calculations. Such
Matryoshka twinning throughout the three-dimension reciprocal space is
demonstrated to amplify the anharmonicity of the related phonon modes through
creating abundant three-phonon scattering channels and cutting the phonon
lifetime of affected modes by more than 50%. Such phonon topology effectively
contributes to the reduction of the in-plane thermal transport, thus the
anisotropic thermal conductivity of alpha-GaN. The results not only have
significant implications for engineering the thermal performance and other
phonon-related properties of alpha-GaN, but also offer valuable insights on the
role of anomalous phonon topology in thermal transport of other technically
important semiconductors.
