Speaker
Description
The interplay between superconductivity and magnetism remains one of the central problems of condensed matter physics, particularly in nanoscale systems where strong electron correlations play a dominant role. A prominent example of interaction-driven magnetism is Nagaoka ferromagnetism, which arises purely from electron correlations in nearly half-filled systems [1,2], and which has recently been experimentally realized in quantum dot plaquettes [3]. When such systems are coupled to a superconducting reservoir, the ferromagnetic state competes with singlet-forming pairing correlations, leading to nontrivial many-body behavior [4].
In this work, we investigate a quadruple quantum dot array proximitized by an s-wave superconductor and coupled to a normal metallic lead, providing a minimal platform to study the competition between Kondo correlations, superconducting pairing, and Nagaoka ferromagnetism. Focusing on the subgap regime, we determine the phase diagram of the system as a function of the coupling to the superconducting substrate and show that the proximity-induced pairing modifies the ferromagnetic ground state, leading to its suppression beyond a critical coupling strength. The presence of the normal lead introduces an additional screening channel, resulting in a competition between Kondo correlations, superconductivity, and Nagaoka ferromagnetism, which gives rise to a new phase absent in isolated or weakly coupled systems. At the same time, high-spin states remain robust over a wide parameter range, indicating the persistence of Nagaoka-type ferromagnetism despite competing singlet correlations.
Our results demonstrate that hybrid quantum dot arrays provide a highly tunable platform for studying competing many-body phenomena and reveal how interaction-driven ferromagnetism evolves in the presence of superconducting correlations and coupling to external reservoirs.
References
[1] Nagaoka, Y. (1966). Ferromagnetism in a narrow, almost half-filled s band. Physical Review, 147(1), 392.
[2] Buterakos, D., & Sarma, S. D. (2019). Ferromagnetism in quantum dot plaquettes. Physical Review B, 100(22), 224421.
[3] Dehollain, J. P., Mukhopadhyay, U., Michal, V. P., Wang, Y., Wunsch, B., Reichl, C., ... & Vandersypen, L. M. (2020). Nagaoka ferromagnetism observed in a quantum dot plaquette. Nature, 579(7800), 528-533.
[4] Siuda, E., & Weymann, I. (2025). Competition between Nagaoka ferromagnetism and superconducting pairing in hybrid quantum dots. Sci. Rep., 15(25349), 25349.