Abstract
Plasmons in two-dimensional (2D) Dirac materials feature an interesting regime with a tunable frequency, and long propagating length and lifetime, but are rarely achieved in the visible light regime. Using a tight-binding (TB) model in combination with first-principles calculations, we investigated plasmon modes in a 2D Lieb lattice with a Dirac nodal-line electronic structure. In contrast to conventional 2D plasmons, anomalous plasmons in the Lieb lattice exhibit the unique features of a carrier-density-independent frequency, being Landau-damping free in a wide-range of wave vectors, a high frequency, and high subwavelength confinement. Remarkably, by using first-principles calculations, we proposed a candidate material, 2D Be2C monolayer, to achieve these interesting plasmon properties. The plasmons in the Be2C monolayer can survive up to the visible frequency region and propagate to large momentum transfer that has rarely been reported. The anomalous plasmons revealed in the Lieb lattice offer a promising platform for the study of 2D plasmons as well as the design of 2D plasmonic materials.
Highlights
We demonstrated that the 2D Dirac nodal line (DNL) states in the Lieb lattice can lead to anomalous plasmons with a stable frequency independent of the carrier density n, in sharp contrast to normal plasmons in
We supposed that the DNL is formed from two crossing bands described by parabolic dispersion relations as follows: Ek;[1]
We revealed anomalous plasmons inherited in the Lieb lattice with DNL electronic structures using a TB model in combination with rst-principles calculations
Summary
As collective excitations of electrons, enable coupling between electromagnetic radiation and electrons in materials at subwavelength scales, and a wide-range of applications, such as photodetection, biosensing and nanophotonics.[1,2,3,4,5,6,7] The dimensionality of materials offers additional freedom to regulate plasmonic properties,[8,9,10,11,12] among which two-dimensional (2D) materials are of particular interest, due to their unique electronic structures and quantum-con nement effects.[13,14,15,16,17,18,19,20,21,22] For example, plasmons in graphene have been demonstrated to possess high tunability, large subwavelength con nement and a longer propagating length and lifetime both theoretically and experimentally.[2,18,19] These plasmonic properties are closely related to the unique linear energy–momentum dispersion relation in graphene, namely Dirac cones. The plasmon frequency (up) shows a special dependence on the carrier density (n), up f n1/4,26,27 which differs signi cantly from the up f n1/2 law in 3D metals, 2D electron gas[28] and bilayer graphene,[29] but similar to that of graphene.[18] The carrier-density-dependent plasmon frequency offers a simple strategy to regulate the plasmon properties, e.g. by doping or the gate voltage. It is unsuitable for applications where stable plasmons against environment perturbation are required. Anomalous plasmons with a density-independent frequency, intensity and damping were predicted in one-dimensional topological electrides with Dirac nodal-surface states,[30] paving a way for the design of plasmonic materials
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