Plasmons in Z2 Topological Insulators

TL;DR

This paper investigates edge and bulk plasmons in the Kane-Mele model of Z2 topological insulators, revealing spin-polarized, chiral edge plasmons and effects of magnetic fields, with implications for plasmonic and spintronic devices.

Contribution

It demonstrates the existence of gapless, spin-polarized edge plasmons in the quantum spin Hall phase of the Kane-Mele model and analyzes their behavior under magnetic fields.

Findings

Gapless spin-polarized edge plasmons exist in the QSH phase.

Magnetic fields can delocalize or selectively excite edge plasmons.

Low-energy circulating plasmons are observed on nanoflake boundaries.

Abstract

We study plasmonic excitations in the Kane-Mele model, a two-dimensional Z2 topological insulator on the honeycomb lattice, using the random phase approximation (RPA). In the topologically non-trivial phase, the model has conducting edge states that traverse the bulk energy gap and display spin-momentum-locking. Such a state of matter is called the quantum spin hall (QSH) phase, which is robust against time-reversal (TR) invariant perturbations. We find that in the QSH phase, gapless spin-polarized plasmons can be excited on the edges of the system. The propagation of these plasmons is chiral for each individual spin component and shows spin-momentum-locking for both spin components on the same edge. Moreover, we study the effect of external magnetic fields on the gapless edge plasmons. Specifically, out-of-plane magnetic fields delocalize edge plasmons propagating in one direction without affecting the other one, while an in-plane magnetic field can be applied to selectively excite a specific spin-plasmon branch with proper doping or gating to the system. Our findings may have potential applications in novel plasmonic and spintronic devices. We also investigate plasmons in the Kane-Mele model on a finite-sized diamond-shaped nanoflake and observe low-energy plasmons circulating the boundary of the material.