September 2020

Abstracts of the QSIT Lunch Seminar, Thursday, September 3, 2020

Compact SQUID realized in a double layer graphene heterostructure

David Indolese - Nanoelectronics (Schönenberger group), University of Basel

In todays condensed matter physics topological superconductivity is a leading topic. A promising route to realize this exotic state of matter is offered by engineered two-dimensional materials where braiding experiments could be realized in future complex device structures. Graphene is a promising candidate due to its high electronic quality, versatility in van der Waals heterostructures and its electron and hole-like degenerate 0th Landau level, which allows one to engineer a helical quantum Hall ground state [1,2,3]. Here, we report on the first step towards inducing superconductivity in a helical system, which can be seen as an engineered topological superconductor. The studied device consists of a six-layered heterostructure of graphite, hBN, graphene, hBN, graphene, hBN with one-dimensional superconducting side-contacts of MoRe. It forms a compact double layer superconducting quantum interference device, where the superconducting loop is reduced to the superconducting contacts. With two global gates the Fermi energy of both layers can be controlled separately, which leads to an individual tunability of the critical current in each Josephson junction. In the quantum Hall regime, the degeneracy of the 0th Landau level is fully lifted, and a conductance plateau of 2e2/h is observed at filling factors of ±1. This plateau indicates counter propagating edge channels in the two layers, realizing an engineered helical state. Further, we showed, thanks to the adjustable critical currents, that the current-phase relations of both graphene Josephson junctions are skewed, i.e. non-sinusoidal, indicating the presence of superconducting modes with high transparency [4]. Our work paves the way to investigate the interplay between helical quantum Hall edge states and superconducting contacts and to establish topological superconductivity in the proposed device structure.

References
[1] J. Sanches-Yamagishi et al., Nature Nanotechnology, 12 118 (2016)
[2] A. F. Young et al., Nature, 505 528 (2013)
[3] L. Veyrat et al., Science, 367 781 (2020)
[4] D. I. Indolese et al., arXiv:2006.05522 (2020)

Large twisting angles in Bilayer graphene (Moire) quantum dot structures (and bits of machine learning)

Frantisek Herman - Strongly Correlated Electrons (Sigrist group), ETH Zurich

After bits of machine learning, we focus on large twisting angles in bilayer quantum dots systems. Recent exploration of the commensurate structure in the turbostratic double layer graphene shows, that the large angle twisting can be treated as a perturbation of the effective velocity within the energy spectra of the single layer graphene. Within our work, we use this result as a starting point, aiming towards the understanding of the physics of the large angle twisted double layer graphene (i.e. Moire) quantum dot systems. It shows, that within the simple approach, using the language of the first quantization, yet another so far unrealized (not up to our knowledge), illustrative property of the commutation relation appears within the graphene physics. Large twisting angles show to be a suitable tunning knob of the position symmetry in the graphene systems. Complete overview of the large angle twisting on the considered dot systems should be provided as well.