${\mathrm{Ca}}_2\mathrm{N}$ is a layered material that has been recently identified as a two-dimensional (2D) electride, an unusual ionic compound in which electrons serve as anions. The electronic properties of 2D electrides attract considerable interest as the anionic electrons, which form a 2D layer sandwiched between atomic planes, are highly mobile as they are not attached to any ion. Here, on the basis of first-principles time-dependent density-functional theory calculations, we investigate the collective excitations of the electrons—i.e., the plasmons—in ${\mathrm{Ca}}_2\mathrm{N}$ as a function of wave vector $\mathbf{q}$. Our calculations reveal an intrinsic negative in-plane dispersion of the anionic plasmon, in striking contrast with the homogeneous electron gas. Moreover, for wave vectors $\mathbf{q}$ normal to the planes, we find a long-lived plasmon that continues to exist well beyond the first Brillouin zone. This is a mark of the electronic inhomogeneities in the charge response that ${\mathrm{Ca}}_2\mathrm{N}$ shares with other layered materials like transition-metal dichalcogenides and ${\mathrm{MgB}}_2$. Finally, we compare the plasmon properties of ${\mathrm{Ca}}_2\mathrm{N}$ in its bulk and monolayer forms, which shows the effect of the different electronic structures and dimensionalities.

中文翻译：

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二维电子Ca2N的集体电荷激发

${钙}_{\mathrm{2个}}\mathrm{ñ}$是一种层状材料，最近被确定为二维（2D）驻极体，这是一种不寻常的离子化合物，其中电子充当阴离子。二维电子的电子性质引起了极大的兴趣，因为形成夹在原子平面之间的2D层的阴离子电子由于不与任何离子相连而具有很高的移动性。在此，根据第一原理的时变密度泛函理论计算，我们研究了电子（即等离激元）在电子中的集体激发。${钙}_{\mathrm{2个}}\mathrm{ñ}$ 作为波矢的函数 $\mathbf{q}$。我们的计算揭示了阴离子等离子体激元的固有负面内色散，与均质电子气形成鲜明对比。此外，对于波矢$\mathbf{q}$垂直于飞机，我们发现了一个长寿命的等离子体激元，该等离子体激元在第一个布里渊区以外的地方继续存在。这是电荷反应中电子不均匀性的标志，${钙}_{\mathrm{2个}}\mathrm{ñ}$ 与其他层状材料（例如过渡金属二卤化物和 ${\mathrm{镁硼}}_{\mathrm{2个}}$。最后，我们比较了${钙}_{\mathrm{2个}}\mathrm{ñ}$ 以其本体和单层形式显示了不同电子结构和尺寸的影响。