Plasmonic responses with topological transition in semimetallic monolayer WTe 2

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.