Quantum Nanoelectronics Seminar : Romain Danneau

Tuesday 19 December 2017 - 14h00 - Room « Remy Lemaire » (K223)

Tailoring supercurrent confinement in graphene bilayer weak links
Romain Danneau (Karlsruhe Institute of Technology Germany)

Graphene appears to be an ideal candidate for superconducting weak link [1-3] thanks to its low contact resistance, large mean free path and its two-dimensionality which allows device geometry flexibility. Designing nanostructures based on electrostatic gating has been at the heart of the research in mesoscopic physics for the last thirty years. While graphene undergoes Klein tunneling making it inappropriate for charge carrier confinement, it is possible to create nanostructures based on band gap engineering in bilayer graphene (BLG). By using edge connected hBN-BLG-hBN heterostructures, we have induced displacement fields between an overall back-gate and a local split-gate, i.e. in a quantum point contact-like structure, to confine the electrons and holes within a 1D constriction. Our superconducting leads allow measuring high supercurrent amplitudes and ballistic interferences. We have studied the confinement of the supercurrent by probing its magnitude and the variations of the magneto-interference patterns while the constriction is formed. We demonstrate that it is possible to fully gate-control both amplitude and density profile a supercurrent, making BLG a highly tunable superconducting weak link. Both analytical and numerical model support our findings. Our work opens up possibilities to create more complex circuits such as superconducting electronic interferometers or transition-edge sensors [3].


[1] V.E. Calado, et al., Nat. Nanotech. 10, 761-764 (2015).
[2] M. Ben Shalom, et al., Nat. Phys. 12, 318-322 (2016).
[3] R. Kraft, et al., arXiv: 1702.08773 (2017).

Contact : Romain Danneau

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Quantum Nanoelectronics Seminar : Erik Bakkers

Tuesday 6 March 2018 - 14h00 - Room « Remy Lemaire » (K223)

Bottom-up grown nanowire quantum devices
Erik Bakkers (Technische Universiteit Eindhoven)

InSb nanowires are used to detect first signatures of quasi particles called Majorana fermions. Recently, different schemes for performing braiding operations and uncovering the non-Abelian statistics of Majorana fermions are proposed. Such operations are fundamental for topological quantum computing. For such a universal computational architecture the realization of a near-perfect nanowire network assembly is needed in which Majorana states are coherently coupled.
Here, we demonstrate a generic process by which we can design any proposed braiding device by manipulating an InP substrate and thereby the nanowire growth position and orientation [1]. This approach combines recent advances in materials growth and theoretical proposals. Our method leads to highly controlled growth of InSb nanowire networks with single crystalline wire-wire junctions. Additionally, nanowire “hashtag” structures are grown with a high yield and contacted. In these devices, the Aharonov–Bohm (AB) effect is observed, demonstrating phase coherent transport. These measurements reveal the high quality of these structures. This generic platform will open new applications in quantum information processing. Furthermore, these structures are well suited for epitaxial shadow growth of a superconductor on the nanowire facets. We study the growth of superconductors on nanowires and reveal the electronic properties.

1. S. Gazibegovic et al. Nature 548 (2017), 434

Contact : Erik Bakkers

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