In 2010, theoretical breakthroughs showed that combining superconductors and semiconductors could lead to the creation of Majorana zero modes (MZMs) in solid-state systems [1,2]. The first experimental signatures of MZMs were reported just a few years later, namely, zero bias peaks in tunneling spectroscopy experiments [3] and 2e-1e periodic changes in the Coulomb peak spacing of hybrid islands [4]. However, their existence is still under debate [5]. In the Nanoelectronics group, we aim to understand the physics of hybrid devices and look for robust experimental signatures of topological superconductivity.

Full-shell Al/InAs nanowires:

Al forms a cylinder around the nanowire and when a parallel magnetic field is applied, superconductivity is periodically modulated via the so-called Little-Parks effect. Theory predicts that if one flux quantum is applied, MZMs should appear [6,7]. A first signature of MZMs in this configuration has been already reported [6]. However, recent measurements performed in the Nanoelectronics group show that Yu-Shiba-Rusinov states can mimic Majorana features in tunneling spectroscopy experiments [8].

Germanium hybrid 2DHG:

Another interesting direction for the investigation of topological superconductivity are hybrid planar geometries [11,12]. In a preliminary work, we combined Al, which provides a good contact to the Ge 2DHG, and Niobium, which has a large superconducting gap and can withstand high magnetic fields, for forming Josephson junctions. The persistence of superconductivity beyond 1.5T in the reported devices, paves the way towards investigating topological superconductivity in such planar geometries [13].

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[1] Lutchyn, Roman M., Jay D. Sau, and S. Das Sarma. Physical review letters 105.7 (2010): 077001.

[ 2] Oreg, Yuval, Gil Refael, and Felix Von Oppen. Physical review letters 105.17 (2010): 177002.

[3] Mourik, Vincent, et al. Science 336.6084 (2012): 1003-1007.

[4] Albrecht, S. et al. Nature 531.7593 (2016): 206-209.

[5] Prada, E., et al. Nature Reviews Physics 2.10 (2020): 575-594.

[6] Vaitiekėnas, S., et al.  Science 367.6485 (2020).

[7] Peñaranda, Fernando, et al.  Physical Review Research 2.2 (2020): 023171.

[8] Valentini, Marco, et al. arXiv e-prints (2020): arXiv-2008.

[9] Krogstrup, P., et al. Nature materials 14.4 (2015): 400-406.

[10] Chang, W., et al. Nature nanotechnology 10.3 (2015): 232-236.

[11] Pientka, Falko, et al. Physical Review X 7.2 (2017): 021032.

[12] Ren, Hechen, et al.  Nature 569.7754 (2019): 93-98.

[13] Aggarwal, Kushagra, et al.  arXiv preprint arXiv:2012.00322 (2020).