Abstract
In this work, ultrastrong photon-to-magnon coupling is demonstrated for on-chip multilayered superconductor-ferromagnet-insulator hybrid thin-film structures reaching a coupling strength above 6 GHz, coupling ratio about 0.6, single-spin coupling strength about 350 Hz, and cooperativity about . Strong characteristics of coupling are achieved owing to a radical suppression of the photon phase velocity in an electromagnetic resonator. With achieved coupling, the spectrum reveals the inapplicability of the Dicke quantum model, and evidences a contribution of the diamagnetic interaction term in the Hamiltonian of the system, which satisfies the Thomas-Reiche-Kuhn sum rule. The contribution of the term denotes the validity of the Hopfield quantum model and manifests observation of a different hybrid polariton quasiparticle, namely the plasmon-magnon polariton.
- Received 24 May 2021
- Revised 7 July 2021
- Accepted 19 August 2021
DOI:https://doi.org/10.1103/PhysRevApplied.16.034029
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