Velocity and confinement of edge plasmons in HgTe-based two-dimensional topological insulators

High-frequency transport in the edge states of the quantum spin Hall (QSH) effect has rarely been explored, though it could cast light on the scattering mechanisms taking place therein. Here we report on the measurement of the plasmon velocity in topological HgTe quantum wells both in the QSH and quantum Hall (QH) regimes, using harmonic GHz excitations and phase-resolved detection. We observe low plasmon velocities in both regimes, with, in particular, large transverse widths in the QH regime despite a sharp edge confinement profile. We ascribe these observations to the prominent influence of charge puddles forming in the vicinity of edge channels. Together with other recent works, it suggests that puddles play an essential role in the edge state physics and probably constitute a main hurdle on the way to clean and robust edge transport.