The main target of this ERC research program was to identify the key ingredient explaining the fragility of the Quantum Spin Hall (QSH) edge states, which despite the anticipated topological protection do not exhibit very robust transport signatures. While many mechanisms had been proposed at a theoretical level, no or few experimental works had at the start of the grant period offered significant insights into the scattering mechanisms in QSH edge states.
We have made significant achievement in this direction, and a consensus has emerged on the prominent role of the bulk, which is overall charge neutral, but exhibits residual charge puddles.
We have first studied the dynamics of charge carriers in the quantum spin Hall edges by the means of dynamical compressibility measurements. Our experimental results quantify the amount of charge puddles. Topological edge states can nonetheless be distinguished using their faster dynamics. It provides new methods for improving the sensitivity of transport measurements in topological phases of matter, relying on dynamical studies (M. Dartiailh et al., Phys. Rev. Letters 2020).
Measurements of the dynamics of transport in topological insulators have consequently been pursued. They have confirmed the difficulty to isolate the signal of QSH edge states. We have nonetheless achieved the measurements of excitations propagating along the edge channelsand illustrated the role of bulk charge puddles in the deteriorated transport of QSH edge states: the QSH edge plasmons are slow and attenuated, have a large transverse width, and host a large density of states. These observations have been published as Gourmelon et al., Phys. Rev. B 2023.
Altogether, this body of works has demonstrated the prominent role of puddles in the scattering and the alteration of the properties of QSH edge states, fulfilling the main target of the ERC grant proposal. It has taken advantage of microwave techniques to provide new insights hard to access with more conventional methods, and complements other recent works.
Additionally, we have started to investigate a robust alternative to the fragile QSH states, namely the quantum anomalous Hall (QAH) edge state. Though the experimental results need to be consolidated, we can already confirm that plasmons propagate on much longer distances in this material platform.
