Collective excitations in topological matter – Colector

COLECTOR project is organised around five Work Packages (WPs) to achieve its main objective. WP5 is dedicated to the project management including communication, dissemination and exploitation of the project's results. WPs 1-4 aim at investigating four specific types of collective excitations of interest in topological matter: intra-band plasmons (WP1), inter-band plasmons (WP2), plasma waves (WP3) and e-e interactions (WP4). Each of these four WPs is dedicated to achieve one of the four objectives defined in section I, all of them contributing to the final main objective of the project. Each scientific WP (1 to 4) corresponds to the study of a specific type of collective excitation, aiming at achieving its own objective as a contribution to the main goal of the project. However, all WPs share a same approach, leading to strongly interrelated WPs, in their transversal progression. For each collective excitation phenomena to be investigated, and thus for each WP, dedicated structures will be designed, grown and processed. Band structure calculations will be performed and collective modes theories will be developed integrating non-parabolicity and topological effects, supported by thorough experimental investigations.

Several notable articles were published during the period. The first appeared in Phys. Rev. B Rapid communications. This work clearly highlights the interest of considering multi-particle effects in optical transitions from zero-mode Landau levels in HgTe quantum wells. Another
important article was published in Phys. Rev. Lett., and discussed quantum plasmonics in semiconductors. This work investigates the frontier between classical and quantum plasmonics in highly doped semiconductor layers. In addition, significant work has been done to make HgTe's QW-based structures emit light in the THz frequency range by optimizing multi-quantum wells coupled to a plasmonic structure. This work was recently submitted to a high impact factor review.

The success of our study necessarily depends on a controlled technological process. The focus will therefore be on developing devices allowing for the study of plasma waves and magnetoplasmons in gated Hall bars and
transistors, but also to study electron-electron interactions by magneto-optics in devices with semi-transparent top gates.

1. S. Mantion et al., Phys. Rev. B 102, 075302 (2020)
2. L. Bovkun et al., JETP Letters 112, 508–512 (2020)
3. V. Aleshkin et al., J. Phys. Commun. 4 115012 (2020)
4. S. Krishtopenko et al. Phys. Rev. B 102, 041404 (2020)
5. A. Vasanelli et al. Phys. Rev. Lett. 125, 187401 (2020)
6. S. Krishtopenko et al. Phys. Rev. B 101, 205424 (2020)
7. K. Tikuišis et al., Phys. Rev. B 103, 155304 (2021)
8. V. Aleshkin et al., Journal of Optics Submitted (2021)

Collective excitations in topological matter represent a novel field of research focusing on joint effects of non-trivial topology of energy bands and electron-electron interactions. To date, there is little experimental work dealing with Dirac plasmons at the surface of topological insulators, and many other collective excitations, like spin-plasmons, plasmon-polaritons, or plasma-waves have never been studied. These collective modes should hold the non-trivial properties of topological materials with Dirac type band dispersion. Thus, topological collective excitations are expected to be even more useful for future applications than the well-known quantized conductance of topological states. The main objective of the project is therefore to evaluate theoretically and experimentally, the influence of non-parabolicity and band inversion on electron-electron interactions in topological and Dirac matter, as well as the potential of these new collective excitations to create ultra-efficient sources and detectors in the mid-infrared and terahertz domains. Different collective phenomena will therefore be studied in semiconductor heterostructures known for their topological properties and the relativistic kinetics of their charge carriers: (i) intra-band plasmons, (ii) inter-plasmons -bands, (iii) plasmonic instabilities, (iv) electron-electron interactions under magnetic field. The COLECTOR consortium gathers French and Russian teams having complementary and internationally recognized skills, both on the physics of electron-electron interactions and topological phases, but also on growth of Dirac materials. HgCdTe-based heterostructures will be manufactured in Russia by one of only three groups in the world with the know-how, while InAs/GaSb quantum wells will be grown in France by one of the world leaders in the field.