Recently, semimetallic tungsten ditelluride (WTe₂) has been proposed as a natural material that supports hyperbolic plasmonic responses. In this paper, we have theoretically discovered that such hyperbolicity, which is due to strongly anisotropic in-plane interband transition of electrons, exists even in the monolayer and can become elliptic under proper doping. Using density functional theory, the permittivities include both the interband and intraband parts have been calculated, which are then used to derive the in-plane conductivities. Based on two-dimensional conductivity, the dispersion relations of the plasmonic modes in the extended monolayer have been analytically solved. It is surprising that monolayer WTe₂ supports both elliptic as well as hyperbolic plasmonic responses in the infrared. Edge-confined modes in the extended monolayer in the elliptic regime and waveguiding modes in nanoribbons in the hyperbolic regime have been numerically investigated. After being doped with electrons, the Fermi level is shifted; it is found that moderate electron doping can change the topology of the plasmonic responses from a hyperbolic to an elliptic one within some frequency range. The effects of band broadening are also discussed and the permittivities are calculated using optimal basis functions to further verify our main conclusions. Then, the states corresponding to large interband transition peaks are marked and the wavefunctions are used to explain the strong in-plane dipole. In the end, the permittivities of bulk WTe₂ have been investigated. Our investigations indicate that monolayer WTe₂ is a promising platform for plasmonic applications.

中文翻译：

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半金属单层WTe2中具有拓扑跃迁的等离子体响应

最近，半金属二碲化钨 (WTe ₂ ) 已被提议作为支持双曲等离子体响应的天然材料。在本文中，我们从理论上发现，由于电子的强烈各向异性面内带间跃迁，这种双曲性甚至存在于单层中，并且在适当的掺杂下可以变成椭圆形。使用密度泛函理论，计算了包括带间和带内部分的介电常数，然后将其用于推导面内电导率。基于二维电导率，已解析解决了扩展单层中等离子体模式的色散关系。令人惊讶的是，单层 WTe ₂支持红外线中的椭圆形和双曲形等离子体响应。已经对椭圆区域中扩展单层中的边缘限制模式和双曲线区域中纳米带中的波导模式进行了数值研究。掺杂电子后，费米能级发生位移；研究发现，适度的电子掺杂可以将等离子体响应的拓扑结构从某个频率范围内的双曲线改变为椭圆形。还讨论了频带展宽的影响，并使用最佳基函数计算了介电常数，以进一步验证我们的主要结论。然后，标记对应于大带间跃迁峰的状态，并使用波函数来解释强平面内偶极子。最后，散装 WTe ₂的介电常数已被调查。我们的研究表明单层WTe ₂是等离子体应用的有前途的平台。