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The topological effects of phonons have been extensively studied in variousmaterials, particularly in the wide-bandgap semiconductor GaN, which has thepotential to improve heat dissipation in power electronics due to itsintrinsic, topologically-protected, non-dissipative phonon surface states.Nevertheless, the phase transition of the Weyl phonons in nitrides and theircomposite alloys has yet to be elucidated. To unveil the microscale origin,topological phonon properties in AlGaN alloys are investigated using thevirtual crystal approximation (VCA) and special quasi-random structure (SQS)approaches in this work. It is found that phase transitions in Weyl phonons areevidently present in AlGaN alloys and nitride single crystals. Under strainstates, both GaN and AlN show a more prominent phase transition of Weyl phononswhen subjected to biaxial compressive and uniaxial tensile strains. And it hasbeen observed that the zz components in the self-term and the transverse 1NNforce constants (FCs) are the most influential during the phase transition. Thenonlinear Weyl phonon transition in AlGaN alloys, as modeled by the VCA, isreflected in the normalized self-term and first-nearest-neighbor (1NN) FCs,which vary in a nonlinear fashion with an increasing magnitude. This nonlinearphenomenon is also confirmed in the SQS modeling, where the unfolded phonondispersions are consistent with those in the VCA modeling. With increasedbranches, hundreds of Weyl phonons are present accompanied by significantdisorders in normalized FCs, which mainly occur for N atoms in self-terms andfor all components in normalized 1NN FCs.

Variations of Interatomic Force Constants in the Topological Phonon Phase Transition of AlGaN