RQC Seminar

45th RQC Seminar

• Speaker

  Prof. Oleksandr Dobrovolskiy　
  (Superconductivity and Spintronics Laboratory, Nanomagnetism and Magnonics, Faculty of Physics, University of Vienna, Vienna, Austria)

• Date

  13:30-14:30 (JST), May 11, 2023 (Thursday)

• Venue

  Hybrid (Zoom / Wako 2F Large Conference Room, Admin. Headquarters Bldg.)

• Title

  Coupled spin-wave and magnetic flux quanta in superconductor-based heterostructures

• Inquiries

  rqc_info[at]ml.riken.jp

Abstract
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 [4]. 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 [6] and enable the Cherenkov radiation of spin waves by fast-moving fluxons [7]. 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.

[1] Z.-L. Xiang, S. Ashhab, J. Q. You, F. Nori, Rev. Mod. Phys. 85 (2013) 623.
[2] D. Lachance-Quirion, S. Wolski, Y. Tabuchi, S. Kono, K. Usami, Y. Nakamura, Science 367 (2020) 425.
[3] D. Makarov, O. Volkov, A. Kákay, O. Pylypovskyi, B. Budinská, O. Dobrovolskiy, Adv. Mater. 34 (2022) 2101758.
[4] O. Dobrovolskiy, et. al., Nat. Phys. 15 (2019) 477.
[5] O. Dobrovolskiy, et al., Nat. Commun. 11 (2020) 3291.
[6] O. Dobrovolskiy, et al. arXiv:2103.10156.