层状拓扑半金属GaGeTe：具有非中心对称结构的新型多型体
Layered topological semimetal GaGeTe: new polytype with non-centrosymmetric structure
摘要：
GaGeTe是一种由锗烯和GaTe亚层组成的层状材料，最近被预测为基本的Z2拓扑半金属。迄今为止，只有一种GaGeTe的多型体已知具有三角中心对称结构（α相，空间群R-3 m，No.166）。在这里，我们表明，生长样品的GaGeTe显示的痕迹，至少另一个多型与六方非中心对称结构（β相，空间群P63 mc，第186号）。此外，我们认为，另一个散装六方多型体（γ相，空间群P-3 m1，编号164）也可以在接近室温条件下发现。α和β多型体已被确定和其特征在于通过X射线衍射和拉曼散射测量与从头计算的支持。我们提供了两种多型体的振动特性，并表明近40年前报道的GaGeTe的拉曼光谱归因于α相，事实上是次级β相。此外，我们表明，费米共振发生在非共振激发条件下的α-GaGeTe，但不是在共振激发条件下。理论计算表明，体β-GaGeTe是一种非中心对称的弱拓扑半金属，其晶格热导率比中心对称体α-GaGeTe还要小。从长远来看，我们的工作为GaGeTe多型体的控制和工程设计铺平了道路，并实现了复杂的范德华异质结构，该异质结构由中心对称和非中心对称层的组合形成，最多可在单一材料中形成三种不同的多型体，适用于许多基础研究和技术应用。
英文摘要：
GaGeTe is a layered material composed of germanene and GaTe sublayers that has been recently predicted to be a basic Z2 topological semimetal. To date, only one polytype of GaGeTe is known with trigonal centrosymmetric structure (alpha phase, space group R-3m, No. 166). Here we show that as-grown samples of GaGeTe show traces of at least another polytype with hexagonal non-centrosymmetric structure (beta phase, space group P63mc, No. 186). Moreover, we suggest that another bulk hexagonal polytype (gamma phase, space group P-3m1, No. 164) could also be found near room conditions. Both alpha and beta polytypes have been identified and characterized by means of X-ray diffraction and Raman scattering measurements with the support of ab initio calculations. We provide the vibrational properties of both polytypes and show that the Raman spectrum reported for GaGeTe almost forty years ago and attributed to the alpha phase, was, in fact, that of the secondary beta phase. Additionally, we show that a Fermi resonance occurs in alpha-GaGeTe under non-resonant excitation conditions, but not under resonant excitation conditions. Theoretical calculations show that bulk beta-GaGeTe is a non-centrosymmetric weak topological semimetal with even smaller lattice thermal conductivity than centrosymmetric bulk alpha-GaGeTe. In perspective, our work paves the way for the control and engineering of GaGeTe polytypes to design and implement complex van der Waals heterostructures formed by a combination of centrosymmetric and non-centrosymmetric layers of up to three different polytypes in a single material, suitable for a number of fundamental studies and technological applications.
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