arXiv subjects

Yun-Yun Bai

Publications and source records attributed to Yun-Yun Bai.

6 recordsLinked to original sources

arXiv2026

Symmetry-protected triplet Weyl complexes

The Nielsen-Ninomiya theorem dictates that Weyl nodes must appear in pairs of opposite chirality to preserve global charge neutrality. However, in crystals, specific crystalline symmetries can stabilize multi-Weyl nodes, circumventing this pairwise constraint and enabling compensated Weyl complexes with mixed chiral charges. The minimal configuration of this type is a triplet Weyl complex (TWC), comprising exactly three Weyl nodes. Here, we systematically investigate the symmetry conditions required to realize TWCs. By screening all 1651 magnetic space groups (MSGs) in both spinless and spinful systems, we establish that: (i) Only TWCs with charge magnitudes of $\{1,1,2\}$ and $\{1,2,3\}$ are permitted; (ii) the $\{1,1,2\}$ configuration can be realized in 166 spinless MSGs and 70 spinful MSGs; and (iii) the $\{1,2,3\}$-TWCs, which has not been reported before, can occur in 10 MSGs for both spinless and spinful cases. We explicitly demonstrate the existence of $\{1,2,3\}$-TWC in a tight-binding model. Furthermore, we present the first electronic realization of $\{1,1,2\}$-TWC topological semimetal state in the chiral carbon allotrope DZQH-C$_{36}$, in which the three Weyl nodes form a collinear configuration, leading to a characteristic ``S''-shaped surface Fermi arc pattern. Our findings uncover novel topological states featuring mixed chiral charges and provide guidance for exploring their physics in concrete material systems.

cond-mat.mtrl-sci↗

arXiv2026

Symmetry-protected four double-Weyl fermions and their topological phase transitions in nonmagnetic crystals

Realizing Weyl semimetals (WSMs) with the minimal number of Weyl points (WPs) fundamentally simplifies extracting intrinsic topological responses. While a minimum of four conventional ($|C|=1$) WPs in nonmagnetic crystals is well-established, the exact symmetry requirements and material realization for the unique configuration of four unconventional double-Weyl points (DWPs, $|C|=2$) remain unresolved. Here, we establish rigorous crystalline symmetry constraints restricting the existence of exactly four symmetry-protected DWPs to merely 28 space groups in both nonmagnetic spinless and spinful systems. Guided by this classification, we identify an $sp$$^2$--$sp$$^3$ hybridized chiral carbon allotrope, THRLN-C$_{32}$, as an ideal candidate hosting precisely this four-DWP configuration near the Fermi level. These $C_4$-protected DWPs project extended or closed-loop Fermi arcs onto the surface Brillouin zone, providing unambiguous spectroscopic signatures. Furthermore, external strain drives profound topological phase transitions encapsulated in a unified evolution landscape: the pristine four-DWP state dissociates into two exotic three-terminal Weyl complexes, degenerates into eight conventional $|C|=1$ WPs, or collapses into a trivial insulator. This work provides a definitive theoretical framework for minimal double-WSMs in nonmagnetic spinful systems and introduces an optimal material platform for investigating strain-tunable topological quantum phenomena.

cond-mat.mtrl-sci↗

arXiv2026

Single-pair charge-2 Weyl-Dirac composite semimetals

The Nielsen--Ninomiya theorem requires that the total topological chiral charges in a crystal vanish, a constraint typically satisfied by identical nodes like Weyl--Weyl pairs. Whether a minimal heterogeneous configuration -- comprising a single Weyl point (WP) and a single Dirac point (DP) -- can exist in an electronic system has remained unresolved. Here, by systematically classifying all 1651 magnetic space groups (MSGs), we reveal that only 14 MSGs without spin-orbit coupling (SOC) and 10 MSGs with SOC are compatible with this exotic state. Furthermore, for nonmagnetic crystals, this configuration is uniquely realized in the spinless limit of chiral space groups 92 and 96. Guided by this principle, we predict an ideal realization in chiral three-dimensional boron allotropes (SDHBN-B$_{28}$ enantiomers). First-principles calculations unveil a $|C|=2$ WP at the $Γ$ point and a $|C|=2$ DP at the $A$ point, which constitute the only fermions near the Fermi level within a large $2$ eV energy window. Strikingly, the structural chirality rigidly dictates the sign of the topological charges, yielding two ultralong Fermi arcs spanning the surface Brillouin zone. Our work provides a complete crystallographic classification and a definitive material platform for exploring minimal heterogeneous chiral fermions.

cond-mat.mtrl-sci↗

arXiv2026

Symmetry-Protected Minimum of Four Conventional Weyl Points in Nonmagnetic Crystals

Realizing nonmagnetic Weyl semimetals (WSMs) with the minimal number of conventional Weyl points (WPs) and a clean Fermi surface remains a central challenge. Here, combining symmetry analysis with first-principles calculations, we establish the definitive conditions under which a nonmagnetic crystal can host exactly four conventional ($C = \pm 1$) WPs, identifying 76 space groups in the spinless limit and 83 in the spinful case that allow this minimal configuration. Guided by this framework, we predict two previously unknown boron allotropes, P6-B$_{48}$ and TBIN-B$_{48}$, as ideal WSMs. Both exhibits precisely four isolated WPs near the Fermi level, with exceptionally clean electronic structures. Notably, the WPs in P6-B$_{48}$ are pinned to high-symmetry points, while those in TBIN-B$_{48}$ lie along high-symmetry lines, leading to distinct and experimentally accessible surface states, including single and double Fermi arcs. Our work provides a complete symmetry-based foundation and pristine material platforms for minimal Weyl physics.

cond-mat.mtrl-sci↗

arXiv2025

Ideal Weyl fermions and double Kagome bands in a series of distorted armchair-type all-$\emph{sp}^{2}$ carbon networks

The study of the Weyl fermions and Kagome bands has recently attracted significant attention in condensed matter physics. However, realizing of perfect Weyl semimetals and double Kagome bands remains challenging. Here, we report a new class of distorted armchair-type fully sp2-hybridized carbon networks, termed DACN-n. The DACN-n family is characterized by only six pairs of Weyl points on the $k_z=0$ plane near the Fermi level. We calculated the chirality of these Weyl points and found that each carries a topological charge of $\pm$1. Notably, DACN-5 exhibits a double Kagome band, where the Weyl points arise from the intersection between Dirac-type bands in the two sets of Kagome bands. The structural stability of DACN-n is confirmed by phonon spectra, ab initio molecular dynamics simulations, and elastic constant calculations. Additionally, we investigated the surface states of the (001) surface and found that its nontrivial topological properties are attributed to the Fermi arcs connecting a pair of Weyl points with opposite chirality. We also simulated x-ray diffraction patterns to guide experimental synthesis. Our findings not only present a new family of three-dimensional carbon allotropes, but also provide a unique opportunity for exploring ideal Weyl fermions and double Kagome bands.

cond-mat.mtrl-sci↗

arXiv2023

Realization of multiple topological states and topological phase transitions in (4,0) carbon nanotube derivatives

Exploring various topological states (TS) and topological phase transitions (TPT) has attracted great attention in condensed matter physics. However, so far, there is rarely a typical material system that can be used as a platform to study the TS and TPT as the system transforms from one-dimensional (1D) nanoribbons to two-dimensional (2D) sheet then to three-dimensional (3D) bulk. Here, we first propose that some typical TS in 1D, 2D, and 3D systems can be realized in a tight-binding (TB) model. Following the TB model and further based on first-principles electronic structure calculations, we demonstrate that the structurally stable (4,0) carbon nanotube derivatives are an ideal platform to explore the semiconductor/nodal-point semimetal states in 1D nanoribbons [1D-(4,0)-C16H4 and 1D-(4,0)-C32H4], nodal-ring semimetal state in 2D sheet [2D-(4,0)-C16], and nodal-cage semimetal state in 3D bulk [3D-(4,0)-C16]. Furthermore, we calculate the characteristic band structures and the edge/surface states of 2D-(4,0)-C16 and 3D-(4,0)-C16 to confirm their nontrivial topological properties. Our work not only provides new excellent 2D and 3D members for the topological carbon material family, but also serves as an ideal template for the study of TS and TPT with the change of system dimension.

cond-mat.mtrl-sci↗