Periodic Reporting for period 5 - CASTLES (Charge And Spin in TopologicaL Edge States)
Período documentado: 2022-05-01 hasta 2023-09-30
Topology provides mathematical tools to sort objects according to global properties regardless of local details, and manifests itself in various fields of physics. In solid-state physics, specific topological properties of the band structure, such as a band inversion, can for example robustly enforce the appearance of spin-polarized conducting states at the boundaries of the material, while its bulk remains insulating. The boundary states of these ‘topological insulators’ in fact provide a support system to encode information non-locally in ‘topological quantum bits’ robust to local perturbations. The emerging ‘topological quantum computation’ is as such an envisioned solution to decoherence problems in the realization of quantum computers. Despite immense theoretical and experimental efforts, the rise of these new materials has however been hampered by strong difficulties to observe robust and clear signatures of their predicted properties such as spin-polarization or perfect conductance.
These challenges strongly motivate my proposal to study two-dimensional topological insulators, and in particular explore the unknown dynamics of their topological edge states in normal and superconducting regimes. First it is possible to capture information both on charge and spin dynamics, and more clearly highlight the basic properties of topological edge states. Second, the dynamics reveals the effects of Coulomb interactions, an unexplored aspect that may explain the fragility of topological edge states. Finally, it enables the manipulation and characterization of quantum states on short time scales, relevant to quantum information processing. This project relies on the powerful toolbox offered by radiofrequency and current-correlations techniques and promises to open a new field of dynamical explorations of topological materials.
These challenges strongly motivate my proposal to study two-dimensional topological insulators, and in particular explore the unknown dynamics of their topological edge states in normal and superconducting regimes. First it is possible to capture information both on charge and spin dynamics, and more clearly highlight the basic properties of topological edge states. Second, the dynamics reveals the effects of Coulomb interactions, an unexplored aspect that may explain the fragility of topological edge states. Finally, it enables the manipulation and characterization of quantum states on short time scales, relevant to quantum information processing. This project relies on the powerful toolbox offered by radiofrequency and current-correlations techniques and promises to open a new field of dynamical explorations of topological materials.
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.
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.
Beyond the end of the project, we intend to further study QAH edge states as an alternative to the QSH edge states. The DC and RF transport in the edges already appear much more robust than in QSH edge states. However, most applications for topological quantum computing will require to proximitize the edge channel with superconductors in order to form Majorana excitations, and the induced proximity has been found to be very weak so far. We plan to continue researching both transport in the normal and proximitized edge channels.