arXiv subjects

Dongze Fan

Publications and source records attributed to Dongze Fan.

4 recordsLinked to original sources

arXiv2026

Catalog of phonon emergent particles and chiral phonons: Symmetry-based classification and materials database investigation

Chirality and topology are fundamental and ubiquitous in nature. Symmetry has proven to be a powerful tool for predicting topological phonons. However, to date, topological phonon emergent particles (EMPs) have not been systematically cataloged in material databases. Moreover, traditional symmetry methods are often inadequate for predicting chiral phonons, because realistic calculations can yield negative results even when symmetry analysis permits phonon chirality. Here, we first establish a complete symmetry-based classification: given any space group and Wyckoff positions (WYPOs) occupied by atoms, the number of occurrences of all (co-)irreducible representations ((co-)irreps) (that can host EMPs) can be unambiguously known without omission prior to expensive and parameter-dependent calculation. Moreover, whether a phonon mode (belonging to one (co-)irrep) is chiral can also be determined from the occupied WYPOs. We then perform a materials database investigation identifying over 25 million EMPs at high-symmetry points and along high-symmetry lines and computing the concrete value of phonon angular momentum for each mode. We demonstrate two main applications: identifying ideal materials with surface chirality momentum locking and identifying materials with giant phonon magnetic moment. All computational data are compiled into a website: http://phonon.nju.edu.cn, which is expected to stimulate future studies on topological and chiral phonons.

cond-mat.mtrl-sci↗

arXiv2024

Catalog of phonon emergent particles

The outcome of conventional topological materials prediction scheme could sensitively depend on first-principles calculations parameters. Symmetry, as a powerful tool, has been exploited to enhance the reliability of predictions. Here, we establish the relationship between the Wyckoff positions (WYPOs) and the phonon wavefunctions at each high-symmetry point (HSP) in all 230 space groups (SGs). Based on this, on one hand, we obtain a complete mapping from WYPO to the occurrence of emergent particles (EMPs) at each HSP in 230 SGs, and establish several rules of enforcing EMPs for phonons; on the other hand, we determine the contribution of the WYPO to the phonon angular momentum. Then we unambiguously identify 20,516,167 phonon EMPs in 111,872 materials in two databases. The purely symmetry-determined wavefunctions generalize the conventional Bloch theorem, could find a wide scope of application to physical properties related with basis functions of irreducible representations.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic interactions and possible structural distortion in kagome FeGe from first-principles study and symmetry analysis

Based on density functional theory and symmetry analysis, we present a comprehensive investigation of electronic structure, magnetic properties and possible structural distortion of magnetic kagome metal FeGe. We estimate the magnetic parameters including Heisenberg and Dzyaloshinskii-Moriya (DM) interactions, and find that the ferromagnetic nearest-neighbor $J_{1}$ dominates over the others, while the magnetic interactions between nearest kagome layers favors antiferromagnetic. The N\'{e}el temperature $T_{N}$ and Curie-Weiss temperature $\theta _{CW}$ are successfully reproduced, and the calculated magnetic anisotropy energy is also in consistent with the experiment. However, these reasonable Heisenberg interactions and magnetic anisotropy cannot explain the double cone magnetic transition, and the DM interactions, which even exist in the centrosymmetric materials, can result in this small magnetic cone angle. Unfortunately, due to the crystal symmetry of the high-temperature structure, the net contribution of DM interactions to double cone magnetic structure is absent. Based on the experimental $2\times 2\times 2$ supercell, we thus explore the subgroups of the parent phase. Group theoretical analysis reveals that there are 68 different distortions, and only four of them (space group $P622$ or $P6_{3}22$) without inversion and mirror symmetry thus can explain the low-temperature magnetic structure. Furthermore, we suggest that these four proposed CDW phases can be identified by using Raman spectroscopy. Since DM interactions are very sensitive to small atomic displacements and symmetry restrictions, we believe that symmetry analysis is an effective method to reveal the interplay of delicate structural distortions and complex magnetic configurations.

cond-mat.str-el↗

arXiv2022

All hourglass bosonic excitations in the 1651 magnetic space groups and 528 magnetic layer groups

The band connectivity as imposed by the compatibility relations between the irreducible representations of little groups can give rise to the exotic hourglass-like shape composed of four branches of bands and five band crossings (BCs). Such an hourglass band connectivity could enforce the emergence of nontrivial excitations like Weyl fermion, Dirac fermion or even beyond them. On the other hand, the bosons, like phonons, magnons, and photons, were also shown to possess nontrivial topology and a comprehensive symmetry classification of the hourglass bosonic excitations would be of great significance to both materials design and device applications. Here we firstly list all concrete positions and representations of little groups in the Brillouin zone (BZ) related with the hourglass bosonic excitations in all the 1651 magnetic space groups and 528 magnetic layer groups, applicable to three dimensional (3D) and two dimensional (2D) systems, respectively. 255 (42) MSGs (MLGs) are found to essentially host such hourglass BCs: Here ``essentially'' means that the bosonic hourglass BC exists definitely as long as the studied system is crystallized in the corresponding MSG/MLG. We also perform first-principles calculations on hundreds of 3D nonmagnetic materials essentially hosting hourglass phonons and propose that the 2D material AlI can host hourglass phonons. We choose AuX (X=Br and I) as illustrative examples to demonstrate that two essential hourglass band structures can coexist in the phonon spectra for both materials while for AuBr, an accidental band crossing sticking two hourglasses is found interestingly. Our results of symmetry conditions for hourglass bosonic excitations can provide a useful guide of designing artificial structures with hourglass bosonic excitations.

cond-mat.mtrl-sci↗