45th RQC Seminar
Prof. Oleksandr Dobrovolskiy
(Superconductivity and Spintronics Laboratory, Nanomagnetism and Magnonics, Faculty of Physics, University of Vienna, Vienna, Austria）
ハイブリッド（Zoom ・ 理研 和光事業所 本部棟2階 大会議室）
Coupled spin-wave and magnetic flux quanta in superconductor-based heterostructures
Ferromagnetism and superconductivity represent fundamental and antagonistic phenomena in condensed-matter physics. Recent discoveries in these topical areas enable the exploration of new physical phenomena at their interfaces. In this regard, interactions between the fundamental quasiparticles – magnetic flux quanta (fluxons) in superconductors and quanta of spin waves (magnons) in magnets – are especially fascinating. Nanoscale 2D and 3D hybrid systems offer unique platforms for the exploration of such interactions [1-3].
In my talk, after a brief introduction of fluxons and magnons, I will introduce a series of our experimental findings. First, magnon-fluxon interactions will be presented for ferromagnet/superconductor heterostructures . The lattice of vortices in the superconductor creates a periodic scattering potential for spin waves propagating in the adjacent ferromagnet and induces Bloch-like bandgaps in the spin-wave transmission spectra. A rather slow (~100 m/s) motion of vortices by the applied dc current results in Doppler shifts of the bandgap frequencies, suggesting that an increase of the vortex velocity to at least 1 km/s should lead to new types of magnon-fluxon interactions, because of the long-predicted generation of magnons via a Cherenkov-type mechanism. However, this regime has been not available experimentally so far, because of the escape of hot (unpaired) electrons from the vortex cores – the phenomenon of flux-flow instability. The modern techniques of focused ion and electron beam-induced deposition allow for the creation of direct-write superconductors with fast relaxation of heated electrons. In these, vortex velocities exceed 15 km/s  and enable the Cherenkov radiation of spin waves by fast-moving fluxons . The magnon Cherenkov radiation is monochromatic, unidirectional, and features sub-40 nm wavelengths determined by the period of the vortex lattice. The emission of spin waves is accompanied by a magnon Shapiro step in the current-voltage curve of the superconductor. Our findings present a novel approach for the generation of shortwave exchange magnons and the reduction of dissipation in superconductor-based hybrid systems.
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 D. Lachance-Quirion, S. Wolski, Y. Tabuchi, S. Kono, K. Usami, Y. Nakamura, Science 367 (2020) 425.
 D. Makarov, O. Volkov, A. Kákay, O. Pylypovskyi, B. Budinská, O. Dobrovolskiy, Adv. Mater. 34 (2022) 2101758.
 O. Dobrovolskiy, et. al., Nat. Phys. 15 (2019) 477.
 O. Dobrovolskiy, et al., Nat. Commun. 11 (2020) 3291.
 O. Dobrovolskiy, et al. arXiv:2103.10156.