  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.
