Capecelatro, Roberto (2024) Transport and noise properties of Ferromagnetic Josephson junctions at the 0 − π transition. [Tesi di dottorato]

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Abstract

In recent years, the growing complexity of the barriers employed in Josephson junctions in terms of layout and materials has significantly increased the "parameters space" for a full understanding and control of their properties. In particular, junctions with ferromagnetic barriers, i.e. Superconductor - Ferromagnet - Superconductor (SFS) junctions, provide unique platforms for observing intriguing phenomena resulting from the coexistence and competition of superconducting and magnetic ordering such as 0 − π transitions and π states, spin-triplet supercurrents and hysteretic current vs. field, I(H), behavior. These effects, in turn, open up new scenarios for exploring different tuning mechanisms for junctions and incorporating smart functionalities in superconducting circuitry. In this framework, our research activity involves both transport and current-noise properties of magnetic junctions, with a focus on junctions with Ferromagnetic Insulator (FI) barriers, i.e. Superconductor - Ferromagnetic Insulator - Superconductor (SFIS), that would be more suitable for applications in superconducting quantum circuits, due to their lower dissipation with respect to junctions with normal-metal ferromagnets. We address both the study of 0 − π transitions and spin-triplet currents in SFIS junctions and the current noise characterization of magnetic junctions at the 0 − π transitions. Specifically, by using lattice Green’s Function (GF) techniques, we theoretically investigate the transport properties of SFIS junctions with particular attention to the temperature dependence of the critical current Ic(T), that might be used as a fingerprint of the junction, and to the spin-resolved pairing correlations. We study the realization of π states in SFIS junctions when spin-mixing and disorder effects are also considered. We explored the possibility of controlling and tuning temperature-induced 0 − π transitions in SFIS junctions by external control knobs, in view of engineering temperature stable π junctions. We theoretically modeled the transport in experimental NbN / GdN / NbN SFIS junctions displaying peculiar Ic(T) curves with non-monotonic trend, showing that this was due to an incipient 0 - π transition possibly resulting from the coexistence and competition of spin-singlet and spin-triplet currents. Further, we studied the current fluctuations, δI (φ), in a simple magnetic junction at the 0 − π transitions with the aim of understanding whether these phase switchings are accompanied by enhanced current noise.

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