压力下一锗化钴 CoGe 相的电学与动力学性质
Electronic and dynamical properties of cobalt monogermanide CoGe phases under pressure
AI总结：
基于第一性原理DFT与直接力常数法，研究六种CoGe相在压力下的电学与动力学性质，揭示软模相变路径、约3 GPa处立方P213相成为最稳定相，并发现kagome特征、Weyl费米子及压力诱导Lifshitz转变。
摘要：
我们给出了六种不同 CoGe 相在压力依赖下的电学与动力学性质：正交 Cmmm、六方 P6/mmm 和 P$\ar{6}$2m、单斜 C2/m、立方 P2$_{1}$3 以及正交 Pnma。利用第一性原理 DFT 计算和直接力常数法，我们研究了各个相在外部压力下的动力学稳定性。我们表明，正交 Cmmm 和六方 P6/mmm 结构在很宽的压力范围内是不稳定的，且两者中最显著的虚频声子软模都导致稳定的六方 P$\ar{6}$2m 结构，该结构在常压和低外部压力（约 3 GPa 以下）下具有所有研究相中最低的基态能量。在这些条件下，立方 P2$_{1}$3 相具有最高能量，但它与单斜 C2/m 和正交 Pnma 一样是动力学稳定的，这三种结构都可能作为亚稳相共存。在约 3 GPa 以上，立方 P2$_{1}$3 相成为能量上最有利的相。通过拟合 Birch–Murnaghan 状态方程，我们推导出所有上述相的体模量，表明 CoGe 具有相对较高的抗压缩性。这些结论得到了能带结构计算的证实。此外，我们表明六方 P$\ar{6}$2m 相的电子能带揭示了 kagome 状结构的特征，而在立方 P2$_{1}$3 相的能谱中，可以定位自旋-1 和双重 Weyl 费米子。在这两种情况下，外部压力都会诱导与费米面拓扑改变相关的 Lifshitz 转变。
英文摘要：
We present the pressure dependence of the electronic and dynamical properties of six different CoGe phases: orthorhombic Cmmm, hexagonal P6/mmm and P$\bar{6}$2m, monoclinic C2/m, cubic P2$_{1}$3, and orthorhombic Pnma. Using first-principles DFT calculations and the direct force-constants method, we study the dynamical stability of individual phases under external pressure. We show that the orthorombic Cmmm and hexagonal P6/mmm structures are unstable over a broad pressure range and most pronounced imaginary phonon soft mode in both cases leads to a stable hexagonal P$\bar{6}$2m structure of the lowest ground-state energy of all studied phases at ambient and low (below $\sim 3$ GPa) external pressure. Under these conditions, the cubic P2$_{1}$3 phase has the highest energy, however, together with monoclinic C2/m and orthorombic Pnma it is dynamically stable and all these three structures can potentially coexist as meta-stable phases. Above $\sim 3$ GPa, the cubic P2$_{1}$3 phase becomes the most energetically favorable. Fitting the Birch--Murnaghan equation of state we derive bulk modulus for all mentioned phases, which indicate relatively high resistance of CoGe to compression. Such conclusions are confirmed by band structure calculations. Additionally, we show that electronic bands of the hexagonal P$\bar{6}$2m phase reveal characteristic features of the kagome-like structure, while in the cubic P2$_{1}$3 phase spectrum, one can locate spin-1 and double Weyl fermions. In both cases, the external pressure induces the Lifshitz transition, related to the modification of the Fermi surface topology.
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