Research on helical edge states in 2D topological insulators is motivated by exotic fundamental physics, prospects for robust topological quantum computation and novel spinorbitronics. However, topological transport is often visible only on short distances. On larger distances, microwave techniques offer powerful tools to investigate the origin of this fragility, as well as the possibility to generate and control excitations in the topological edge states.

In this talk, we report on first results on microwave transport in HgTe 2D topological insulators [1, 2, 3]. Via microwave capacitance spectroscopy [1], we highlight the response of the edges. We find that they have a rather large density of states but host mobile carriers, while bulk carriers are present as puddles but are drastically slowed down in the gap. This suggests that edge states can be selectively addressed on timescales over which bulk carriers are frozen. In a second study [3], we measure the velocity of edge states in both the quantum Hall and quantum spin Hall regime. We observe low plasmon velocities corresponding to large transverse widths, which we ascribe to the prominent influence of charge puddles forming in the vicinity of edge channels. Both works point towards the prominent role of charge puddles in the topological gap, which probably constitute a hurdle on the way to clean and robust edge transport.

• C. Dartiailh et al., Phys. Rev. Lett. 124, 076802 (2020)
• Gourmelon et al., Phys. Rev. Res. 2, 043383 (2020)
• Gourmelon et al., Phys. Rev. B 108, 035405 (2023)