黄铜矿CdGeAs $_{2}$中声子Weyl、节线和三重激发的共存及其热电性质
Coexistence of phononic Weyl, nodal line, and threefold excitations in chalcopyrite CdGeAs$_{2}$ and associated thermoelectric properties
摘要：
拓扑保护量子态的实现为基础科学和器件应用带来了前所未有的机会。在这里，我们展示了多个拓扑声子态的共存，并计算了黄铜矿材料CdGeAs $_2$使用第一性原理理论建模的相关热电性能。CdGeAs $_{2}$是直接带隙半导体，带隙为$0.65$ eV。通过分析声子谱和相关的对称性，我们表明存在近孤立的外尔，节线，和三倍带交叉在CdGeAs $_2$。具体地，在$k_{z}$轴上识别的两个三重简并点（TDP）由具有II型能量色散的光学声子带7、8和9形成。这些TDPs形成一个时间反转对，并通过零Berry相位的直节线连接。在带14、15和16之间形成的TDP呈现I型交叉，并且通过开放的直节点线连接。我们的运输计算表明，一个大的热电势超过$\sim$ 500和$200$ $\rm μV/K$的空穴和电子载流子，分别为500 K以上的载流子掺杂10 $^{18}$ cm $^{-3}$。$p$型CdGeAs $_{2}$中的大热电势是由于在$Γ$点处存在重空穴带而出现的尖锐态密度的结果。我们认为，声子带中的拓扑状态的存在下，可能会导致低的晶格热导率和驱动器的高图的优值在CdGeAs $_{2}$。
英文摘要：
Realization of topologically protected quantum states leads to unprecedented opportunities for fundamental science and device applications. Here, we demonstrate the coexistence of multiple topological phononic states and calculate the associated thermoelectric properties of a chalcopyrite material CdGeAs$_2$ using first-principles theoretical modeling. CdGeAs$_{2}$ is a direct bandgap semiconductor with a bandgap of $0.65$ eV. By analysing the phonon spectrum and associated symmetries, we show the presence of nearly isolated Weyl, nodal line, and threefold band crossings in CdGeAs$_2$. Specifically, the two triply degenerate points (TDPs) identified on the $k_{z}$ axis are formed by the optical phonons bands 7, 8, and 9 with type-II energy dispersion. These TDPs form a time reversal pair and are connected by a straight nodal line with zero Berry phase. The TDPs formed between bands 14, 15, and 16 exhibit type-I crossings and are connected through the open straight nodal line. Our transport calculations show a large thermopower exceeding $\sim$500 and $200$ $\rm μV/K$ for the hole and electron carriers, respectively, above 500 K with a carrier doping of 10$^{18}$ cm$^{-3}$. The large thermopower in $p$-type CdGeAs$_{2}$ is a consequence of the sharp density of states appear from the presence of a heavy hole band at the $Γ$ point. We argue that the presence of topological states in the phonon bands could lead to low lattice thermal conductivity and drive a high figure-of-merit in CdGeAs$_{2}$.
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