arXiv subjects

Daosheng Tang

Publications and source records attributed to Daosheng Tang.

3 recordsLinked to original sources

arXiv2024

Nonlocality and Strength of Interatomic Interactions Inducing the Topological Phonon Phase Transition

Understanding the phonon behavior in semiconductors from a topological physics perspective provides more opportunities to uncover extraordinary physics related to phonon transport and electron-phonon interactions. While various kinds of topological phonons have been reported in different crystalline solids, their microscopic origin has not been quantitatively uncovered. In this work, four typical analytical interatomic force constant (IFC) models are employed for wurtzite GaN and AlN to help establish the relationships between phonon topology and real-space IFCs. In particular, various nearest neighbor IFCs, i.e., different levels of nonlocality, and IFC strength controlled by characteristic coefficients, can be achieved in these models. The results demonstrate that changes in the strength of both the IFCs and nonlocal interactions can induce phonon phase transitions in GaN and AlN, leading to the disappearance of existing Weyl phonons and the appearance of new Weyl phonons. These new Weyl phonons are the result of a band reversal and have a Chern number of 1. Most of them are located in the kz=0 plane in pairs, while some of them are inside or at the boundary of the irreducible Brillouin zone. Among the various Weyl points observed, certain ones remain identical in both materials, while others exhibit variability depending on the particular case. Compared to the strength of the IFC, nonlocal interactions show much more significant effects in inducing the topological phonon phase transition, especially in cases modeled by the IFC model and SW potential. The larger number of 3NN atoms provides more space for variations in the topological phonon phase of wurtzite AlN than in GaN, resulting in a greater abundance of changes in AlN.

cond-mat.mtrl-sci↗

arXiv2023

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

The topological effects of phonons have been extensively studied in various materials, particularly in the wide-bandgap semiconductor GaN, which has the potential to improve heat dissipation in power electronics due to its intrinsic, topologically-protected, non-dissipative phonon surface states. Nevertheless, the phase transition of the Weyl phonons in nitrides and their composite alloys has yet to be elucidated. To unveil the microscale origin, topological phonon properties in AlGaN alloys are investigated using the virtual crystal approximation (VCA) and special quasi-random structure (SQS) approaches in this work. It is found that phase transitions in Weyl phonons are evidently present in AlGaN alloys and nitride single crystals. Under strain states, both GaN and AlN show a more prominent phase transition of Weyl phonons when subjected to biaxial compressive and uniaxial tensile strains. And it has been observed that the zz components in the self-term and the transverse 1NN force constants (FCs) are the most influential during the phase transition. The nonlinear Weyl phonon transition in AlGaN alloys, as modeled by the VCA, is reflected in the normalized self-term and first-nearest-neighbor (1NN) FCs, which vary in a nonlinear fashion with an increasing magnitude. This nonlinear phenomenon is also confirmed in the SQS modeling, where the unfolded phonon dispersions are consistent with those in the VCA modeling. With increased branches, hundreds of Weyl phonons are present accompanied by significant disorders in normalized FCs, which mainly occur for N atoms in self-terms and for all components in normalized 1NN FCs.

cond-mat.mtrl-sci↗

arXiv2023

Phonon modes and topological phonon properties in (GaN)x/(AlN)x and (AlGaN)x/(GaN)x superlattices

To effectively regulate thermal transport for the near-junction thermal management of GaN electronics, it is imperative to gain an understanding of the phonon characteristics of GaN nanostructures, particularly the topological phonon properties connected to low-dissipation surface phonon states. In this work, a comprehensive study on phonon modes and topological phonon properties is performed from first principles in (GaN)x/(AlN)x and (AlGaN)x/(GaN)x (x=1,2,3) superlattices. Phonon modes, including the dispersion relation, density of states, and participation ratio, were calculated for six GaN superlattices. The participation ratio results did not reveal the localized phonon mode. In topological phonon analyses, it is found that Weyl phonons with a Chern number of 1(-1) are present in all six GaN superlattices, consisting of trivial (GaN) and nontrivial (AlN and AlGaN) combinations. These phonons are located on either side of the kz = 0 plane symmetrically in the Brillouin zone. With the increase in the number of phonon branches in superlattices, the number of Weyl phonon points also increases from dozens to hundreds. One Weyl phonon with significant and clean surface states is selected and analyzed for each GaN superlattice. Among them, the Weyl phonon in (GaN)2/(AlN)2 superlattice mainly results from the lattice vibrations of Al and Ga atoms, while the Weyl phonons in other superlattices mainly result from the lattice vibrations of N atoms. The Weyl phonons at opposite kz planes form pairs in (GaN)2/(AlN)2, AlGaN/GaN, and (AlGaN)2/(GaN)2. Effects of strain including biaxial and uniaxial strain on Weyl phonons in GaN/AlN and AlGaN/GaN superlattices are investigated. Results indicate that Weyl phonons persist in large strain states, however, no monoclinic trend is observed due to the accidental degeneracy of these superlattices.

cond-mat.mtrl-sci↗