Modern materials control, using molecular beam epitaxy (MBE), and modern lithographic technology, enable the construction of unique experimental systems in condensed matter physics. They allow quantum transport experiments in which the charge is carried by single modes, including cases where the direction of transport and spin are coupled (helical modes). One example is the discovery of the Quantum Hall effect (and its variants like fractional or anomalous QHE), where only a few edge modes govern the electron transport. A second step was quantum confinement, with quantized conductance, and quantum dots, which could be created by ‘shaping’ with electrostatic gates the local density of a 2-dimensional electron gas in an MBE grown sample. A third step is the creation of systems in which two different ground states can interplay, such as the edge states of two-dimensional topological insulators and superconductors, as in Erwann Bocquillon’s post-doctoral work.
Such hetero-systems have become an attractive playground for theoretical condensed matter physics, because of their intellectual fundamental richness, as well as their potential use for quantum computation. However, the experimental work is very challenging. For hetero-systems between superconductors and topological insulators, the lithographic structuring demands intermediate steps where sample is exposed to ambient. As a consequence the interfaces, crucial for the physical properties, are not very well defined, nor atomically aligned, and most likely polluted.
E. Bocquillon and his collaborators have made impressive progress in combining MBE-grown HgTe 2D topological insulators with a conventional superconductor (Al). This combination leads to a systematic and well-understood behavior of quantum transport, building confidence in the quality of the interfaces. Thanks to this progress in lithography, Erwann has demonstrated a range of new aspects of the Josephson weak links pointing to the existence of a “fractional” a.c. Josephson effect with halved frequency. This finding signals the presence of zero-gap Andreev states, which are considered to be the Majorana-Andreev bound states. The strength of Erwann’s work is, besides this undoubtedly important discovery, that he dealt with the uncertainty about the nature of interfaces by using electrical transport measurements as a source of information about the device itself, as well as for the new physics. Below are details of E. Bocquillon’s research.
1. Josephson radiation from gapless Andreev bound states in HgTe-based topological junctions
R.S. Deacon, J. Wiedenmann, E. Bocquillon, F. Dominguez, P. Leubner, T.M. Klapwijk, C. Brüne, E.M. Hankiewicz, S. Tarucha, K. Ishibashi, H. Buhmann, L.W. Molenkamp, submitted to PRX, ArXiv:1603.09611 (2016)
2. Gapless Andreev bound states in the quantum spin Hall insulator HgTe
E. Bocquillon, R.S. Deacon, J. Wiedenmann, P. Leubner, T.M. Klapwijk, C. Brüne, K. Ishibashi, H. Buhmann, L.W. Molenkamp, Nature Nanotechnology http://doi.org/10.1038/nnano.2016.159 (2016)
3. 4π-periodic Josephson supercurrent in HgTe-based topological Josephson junctions
J. Wiedenmann, E. Bocquillon, R.S. Deacon, S. Hartinger, O. Herrmann, T.M. Klapwijk, L. Maier, C. Ames, C. Brüne, C. Gould, A. Oiwa, K. Ishibashi, S. Tarucha, H. Buhmann, L.W. Molenkamp, Nature Communications 7, 10303 (2016)
In topological insulators, the topology of the band structure enforces the appearance of electronic conducting states at the boundary of the material while its bulk remains insulating. When coupled to superconductors, these states give rise to an unconventional p-type induced superconductivity. The latter results in the emergence of Majorana fermions, and open prospects for applications to fault-tolerant topological quantum computation. Despite the immense experimental activity on the topic, clear signatures of induced topological superconductivity were scarce.
During his post-doctoral research stay in the group of L.W. Molenkamp in Würzburg (Germany), Erwann led the research activities on the coupling of topological insulators to superconductors. He has in particular identified the fractional Josephson effect in HgTe-based Josephson junctions. The signatures strongly suggest the presence of gapless Andreev bound states, that are precursors for the long-awaited Majorana zero-energy states.
This set of three works has had a pioneering and strong impact on the ‘topological insulators’ community. They have brought the first robust signs of topological superconductivity, and already inspired new experimental and theoretical articles, to explore more in detail the topological superconductivity. This impact is not visible in bibliography indices yet, but has led to numerous invitations to give seminars in groups or conferences in the last year.
These articles demonstrate Erwann’s ability to conduct independent research activities, for which he supervised the work of J. Wiedenmann and coordinated a three-party collaboration between Delft (T.M. Klapwijk), Tokyo (S. Tarucha, K. Ishibashi, R.S. Deacon) and Würzburg. He participated in the measurements and in the lithography development. He also initiated and realized numerical simulations that capture the essential experimental observations and allowed for a good understanding of the underlying physics. He also wrote all three manuscripts with inputs from all other authors.
4. Coherence and undistinguishability of single electron wavepackets emitted by independent sources
E. Bocquillon, V. Freulon, J.-M. Berroir, P. Degiovanni, B. Plaçais, A. Cavanna, Y. Jin, G. Fève, Science 339, 1054 (2013)
The electronic transport in quantum Hall edge channels bears strong analogies with that of photons in the vacuum. Despite these deep and fertile analogies, inevitable differences subsist between photons and electrons, due to Coulomb interactions or the presence of the Fermi sea. During his PhD studies, Erwann implemented experiments with electrons inspired by quantum optics, for which the words “electron quantum optics” were coined. Using a single electron source developed within the group, Erwann demonstrated both experimentally and theoretically the relevance of this novel approach to study the dynamics and coherence of electronic states, even in the presence of strong interactions.