In this work, the ultra-strong photon-to-magnon coupling is demonstrated for on-chip multilayered superconductor/ferromagnet/insulator hybrid thin film structures reaching the coupling strength above 6 GHz, the coupling ratio about 0.6, the single-spin coupling strength about 350 Hz, and cooperativity about 10^4. High characteristics of coupling are achieved owing to a radical suppression of the photon phase velocity in electromagnetic resonator. With achieved coupling the spectrum reveals inapplicability of the Dicke model, and evidences contribution of the diamagnetic A^2 interaction term in the Hamiltonian of the system, which satisfies the Thomas-Reiche-Kuhn sum rule. The contribution of the A^2 term denotes validity of the Hopfield quantum model and manifests observation of a different hybrid polariton quasi-particle, namely, the plasmon-magnon polariton.
@article{arxiv.2106.10450,
title = {Approaching to the deep-strong photon-to-magnon coupling},
author = {I. A. Golovchanskiy and N. N. Abramov and V. S. Stolyarov and A. A. Golubov and M. Yu. Kupriyanov and V. V. Ryazanov and A. V. Ustinov},
journal= {arXiv preprint arXiv:2106.10450},
year = {2021}
}