PhD defense – Sarah Menouni

Sarah Menouni will defend her PhD, entitled “Improving high-Tc Josephson devices: higher reproducibility and a new pathway to tunability”, on June 19th 2025 at 10:00am, at the TRT auditorium.

Abstract:
The nanopatterning of superconducting materials enables the fabrication of Josephson junctions, which are the building blocks of devices relying on quantum interference for electronics and signal processing. These include SQIFs (Superconducting Quantum Interference Filters), which are series or parallel arrays of Josephson junctions. A Josephson junction is formed by two superconducting electrodes separated by a thin non-superconducting barrier. Devices based on Josephson junctions exploit their sensitivity to the phase difference between the superconducting wavefunctions of the two electrodes, which can be modulated by external stimuli, particularly magnetic fields.

This thesis focuses on the fabrication of Josephson junctions and SQIFs based on a high critical temperature material, YBa₂Cu₃O_7-x (YBCO). Unlike conventional Josephson junction devices made from low-temperature superconductors, which require cooling close to absolute zero, using YBCO significantly reduces energy consumption and system size, facilitating integration into embedded systems.

A primary objective of this work is the development of Josephson junction and SQIF devices by nanopatterning YBCO using ion irradiation. A major challenge addressed is improving the uniformity of the junctions, which is critical to SQIF performance. Modifications to the fabrication process are introduced and studied, with a particular focus on enhancing the evacuation of charges that can accumulate during different fabrication steps. The effect of a post-fabrication annealing step is also investigated. It is shown that the properties of Josephson junctions are altered by annealing, which can help to homogenize a series of junctions.

Finally, the thesis explores the development of a new type of Josephson junction based on a redox reaction between a metal and YBCO, leveraging YBCO’s sensitivity to its oxygen content. The oxidation of the metal leads to a local reduction of YBCO, forming a barrier. It is demonstrated that a Josephson effect is present in these junctions. These new “redox” junctions offer promising prospects for the development of tunable Josephson junctions, whose state could be modulated by external stimuli via the addition of an electric gate.