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Jovian Delaforce presents

Quantum transport in novel superconducting hybrid junctions based on germanium nanowire heterostructures

Friday, February 4th 2022 at 9:30 am

Seminar room, Building A, Institut Néel: 60 people max

Link visio : https://www.twitch.tv/joviejove

The defence will be in English.



Superconductor-semiconductor-superconductor hybrid junctions have great potential for emerging high-performance nano-electronics and quantum devices. In addition to their potential applications, these devices are attractive for studies of quantum phenomena in low-dimensional hybrid systems. In particular, one can think to investigate the interplay between the electronic correlations and transport properties of the semiconductor and the superconducting leads. Crucial to their successful application is the quality and reproducibility of the semiconductor-superconductor interface.

Recently, a new method to obtain hybrid junctions has been established by our collaborators in Vienna. By employing a selective, thermally induced Al-Ge substitution process, monolithic Al-Ge-Al nanowire heterostructures can be achieved. These unique hybrid junctions feature crystalline quasi-one-dimensional Al leads which are connected to crystalline Ge segments by near atomically abrupt interfaces. To investigate their transport properties, the nanowire junctions are integrated into a back-gated field effect transistor architecture.

This thesis reports on the investigations of the transport properties of the Al-Ge-Al nanowire heterostructure devices at very low temperatures (400 mK). Two kinds of Ge nanowires have been used to build the junctions: intrinsic Ge nanowires passivated by a thin shell of Al2O3 and Ge/Si core/shell nanowires. Investigations of devices with Ge segment lengths of approximately 40 nm reveal gate tuneable Josephson current up to 10 nA. The high quality of the interface is confirmed by the observation of multiple Andreev reflections. Their analysis reveals that Al-Ge interface is up to 95 % transparent.

The junctions based on intrinsic germanium can be tuned from a completely insulating regime, through a low conductive regime that exhibits properties of a single-hole filling quantum dot to a supercurrent regime resembling a Josephson field effect transistor. This high tuneability is understood by modelling the Al-Ge-Al junction as two back-to-back Schottky barriers.

The junctions based on Ge/Si core/shell nanowires show quantification of the conductance versus the gate voltage, which is an experimental signature of a quasi-one-dimensional ballistic conductor. Using the Landauer formula, we estimate an average interface transparency of the Al-Ge/Si-Al core/shell nanowires, which is consistent with the transparency extracted from the multiple Andreev reflections.

The results presented in this thesis demonstrate the potential of these novel germanium nanowire based superconducting-semiconducting hybrid junctions as quantum devices

Thesis Supervisors:

Olivier Buisson et Cécile Naud
Institut Néel

Jury Members:

Madame Hélène Bouchiat

Laboratoire de Physique des Solides, Referee

Monsieur Georgios Katsaros

Institute of Science and Technology, Austria, Referee

Madame Julia Meyer

Université Grenoble Alpes, Examiner

Monsieur Jesper Nygård

University of Copenhagen, Examiner

Monsieur Jean-Damien Pillet

École Polytechnique, Examiner