三元氮化物Li $_6$ WN $_4$中对称性增强的理想灯笼状声子
Symmetry-enforced ideal lantern-like phonons in ternary nitride Li$_6$WN$_4$
摘要：
凝聚态系统包含费米子和玻色子准粒子。由于费米能级的限制，具有高维简并流形（即，节线和节面）在电子系统中还没有发现。本文根据第一性原理计算和对称性分析，证明了实际三元氮化物Li $_6$ WN $_4$在声子谱中具有理想的（接近平坦的）表面和表面线结构。这些节点简并的形状像灯笼，它们的存在是由非对称对称性保证的。相应的拓扑声子表面态正好覆盖了表面布里渊区（BZ）的一半，因此可以与传统的半导体线和半导体表面半金属区分开来。我们的研究结果证实了理想灯笼状声子在现实材料中的存在，丰富了拓扑量子相的分类，为研究单一材料中的线-面声子相互作用提供了良好的基础.
英文摘要：
Condensed matter systems contain both fermionic and bosonic quasiparticles. Owing to the constraint imposed by the Fermi level, ideal material candidate for the emergent particles with higher-dimensional degeneracy manifold (i.e., nodal lines and nodal surfaces) has not been found in electronic systems. This paper demonstrates that according to the first-principle calculations and symmetry analysis, realistic ternary nitride Li$_6$WN$_4$ features ideal (nearly flat) nodal-surface and nodal-line structures in its phonon spectra. These nodal degeneracies are shaped like lanterns, and their existence is guaranteed by nonsymmorphic symmetry. The corresponding topological phonon surface state covers exactly half the surface Brillouin zone (BZ) and can thereby be distinguished from those of conventional nodal-line and nodal-surface semimetals. The results of our study demonstrate the existence of ideal lantern-like phonons in realistic materials, which enriches the classification of topological quantum phases and provides a good basis for investigating the interaction between nodal-line and nodal-surface phonons in a single material.
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