# Magnetically-defined topological edge plasmons in edgeless electron gas

## TL;DR

This paper introduces a new class of topological edge plasmons called domain-boundary magnetoplasmons in an edgeless 2DEG, which are unidirectional, defect-immune, and tunable, enabling reconfigurable topological devices.

## Contribution

It demonstrates the existence of domain-boundary magnetoplasmons in a uniform edgeless 2DEG, expanding topological plasmonics beyond physical edges and enabling new device applications.

## Key findings

- Domain-boundary magnetoplasmons are protected by gap Chern number differences.
- DBMPs propagate unidirectionally along domain boundaries.
- They are tunable via magnetic field or gate voltage.

## Abstract

Topological materials bear gapped excitations in bulk yet protected gapless excitations at boundaries. Magnetoplasmons (MPs), as high-frequency density excitations of two-dimensional electron gas (2DEG) in a perpendicular magnetic field, embody a prototype of band topology for bosons. The time-reversal-breaking magnetic field opens a topological gap for bulk MPs up to the cyclotron frequency; topologically-protected edge magnetoplasmons (EMPs) bridge the bulk gap and propagate unidirectionally along system's boundaries. However, all the EMPs known to date adhere to physical edges where the electron density terminates abruptly. This restriction has made device application extremely difficult. Here we demonstrate a new class of topological edge plasmons -- domain-boundary magnetoplasmons (DBMPs), within a uniform edgeless 2DEG. Such DBMPs arise at the domain boundaries of an engineered sign-changing magnetic field and are protected by the difference of gap Chern numbers (+/-1) across the magnetic domains. They propagate unidirectionally along the domain boundaries and are immune to domain defects. Moreover, they exhibit wide tunability in the microwave frequency range under an applied magnetic field or gate voltage. Our study opens a new direction to realize high-speed reconfigurable topological devices.