Ultrastrong Magnon-Photon Coupling Achieved by Magnetic Films in Contact with Superconducting Resonators

TL;DR

This paper demonstrates ultrastrong magnon-photon coupling using YIG films on superconducting YBCO resonators, achieving a coupling strength exceeding 0.2 times the cavity frequency, with implications for quantum technologies.

Contribution

It introduces a novel contact-based architecture for achieving ultrastrong magnon-photon coupling with high cooperativity and provides an exact theoretical model for spectra analysis.

Findings

Coupling strength of approximately 2 GHz achieved

Cooperativity of 5 x 10^4 demonstrated

Magnon-photon coupling exceeds 0.2 times the cavity frequency

Abstract

Coherent coupling between spin wave excitations (magnons) and microwave photons in a cavity may disclose new paths to unconventional phenomena as well as for novel applications. Here, we present a systematic investigation on YIG (Yttrium Iron Garnet) films on top of coplanar waveguide resonators made of superconducting YBCO. We first show that spin wave excitations with frequency higher than the Kittel mode can be excited by putting in direct contact a 5~$\mu$m thick YIG film with the YBCO coplanar resonator (cavity frequency $\omega_c/2 \pi = 8.65$~GHz). With this configuration, we obtain very large values of the collective coupling strength $\lambda/2 \pi \approx 2$~GHz and cooperativity $C=5 \times 10^4$. Transmission spectra are analyzed by a modified Hopfield model for which we provide an exact solution that allows us to well reproduce spectra by introducing a limited number of free parameters. It turns out that the coupling of the dominant magnon mode with photons exceeds 0.2 times the cavity frequency, thus demonstrating the achievement of the ultrastrong coupling regime with this architecture. Our analysis also shows a vanishing contribution of the diamagnetic term which is a peculiarity of pure spin systems.