We study a class of topological materials which in their momentum-space band structure exhibit threefold degeneracies known as triple points. Focusing specifically on $\mathcal{P}\mathcal{T}$-symmetric crystalline solids with negligible spin-orbit coupling, we find that such triple points can be stabilized by little groups containing a three-, four-, or sixfold rotation axis, and we develop a classification of all possible triple points as type A vs type B according to the absence vs presence of attached nodal-line arcs. Furthermore, by employing the recently discovered non-Abelian band topology, we argue that a rotation-symmetry-breaking strain transforms type-A triple points into multiband nodal links. Although multiband nodal-line compositions were previously theoretically conceived and related to topological monopole charges, a practical condensed-matter platform for their manipulation and inspection has hitherto been missing. By reviewing the known triple-point materials with weak spin-orbit coupling and by performing first-principles calculations to predict new ones, we identify suitable candidates for the realization of multiband nodal links in applied strain. In particular, we report that an ideal compound to study this phenomenon is ${\mathrm{Li}}_2\mathrm{Na}\mathrm{N}$, in which the conversion of triple points to multiband nodal links facilitates a largely tunable density of states and optical conductivity with doping and strain, respectively.

中文翻译：

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从三点材料到多频段节点链接

我们研究了一类拓扑材料，这些材料在其动量-空间带结构中表现出三重简并性，即三重点。专注于$\mathcal{P}\mathcal{Ť}$具有可忽略的自旋-轨道耦合的对称晶体固体，我们发现这样的三点可以通过包含三，四或六倍旋转轴的小群来稳定，并且我们将所有可能的三点分类为A型与根据是否存在附接的节点线弧，确定是否为B型。此外，通过采用最近发现的非阿贝尔频带拓扑，我们认为破坏旋转对称性的应变将A型三重点转换为多频带节点链接。尽管多频带节点线组合物以前在理论上已经被构思出来并且与拓扑单极电荷有关，但是迄今为止，仍然缺少用于其操纵和检查的实用的凝聚态平台。通过审查具有弱自旋轨道耦合的已知三点材料，并通过执行第一性原理计算来预测新材料，我们确定了在应用应变中实现多频带节点链接的合适候选物。特别是，我们报告说，研究此现象的理想化合物是${里}_{\mathrm{2个}}娜\mathrm{ñ}$，其中将三点转换为多频带节点链接有助于分别通过掺杂和应变极大地调节状态密度和光导率。