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Register a new userUltra-strong photon-to-magnon coupling in multilayered heterostructures involving superconducting coherence via ferromagnetic layers
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Igor A Golovchanskiy
Publication date
2020
fields
Physics
and research's language is
English
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The critical step for future quantum industry demands realization of efficient information exchange between different-platform hybrid systems, including photonic and magnonic systems, that can harvest advantages of distinct platforms. The major restraining factor for the progress in certain hybrid systems is the fundamentally weak coupling parameter between the elemental particles. This restriction impedes the entire field of hybrid magnonics by making realization of scalable on-chip hybrid magnonic systems unattainable. In this work, we propose a general flexible approach for realization of on-chip hybrid magnonic systems with unprecedentedly strong coupling parameters. The approach is based on multilayered micro-structures containing superconducting, insulating and ferromagnetic layers with modified both photon phase velocities and magnon eigen-frequencies. Phenomenologically, the enhanced coupling strength is provided by the radically reduced photon mode volume. The microscopic mechanism of the phonon-to-magnon coupling in studied systems evidences formation of the long-range superconducting coherence via thick strong ferromagnetic layers. This coherence is manifested by coherent superconducting screening of microwave fields by the superconductor/ferromagnet/superconductor three-layers in presence of magnetization precession. This discovery offers new opportunities in microwave superconducting spintronics for quantum technologies.
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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.
We analyze the influence of the surface passivation produced by oxides on the superconducting properties of $gamma$-Mo$_2$N ultra-thin films. The superconducting critical temperature of thin films grown directly on Si (100) with those using a buffer and a capping layer of AlN are compared. The results show that the cover layer avoids the presence of surface oxides, maximizing the superconducting critical temperature for films with thicknesses of a few nanometers. We characterize the flux-flow instability measuring current-voltage curves in a 6.4 nm thick Mo$_2$N film with a superconducting critical temperature of 6.4 K. The data is analyzed using the Larkin and Ovchinnikov model. Considering self-heating effects due to finite heat removal from the substrate, we determine a fast quasiparticle relaxation time $approx$ 45 ps. This value is promising for its applications in single-photon detectors.
Achieving quantum-level control over electromagnetic waves, magnetisation dynamics, vibrations and heat is invaluable for many practical application and possible by exploiting the strong radiation-matter coupling. Most of the modern strong microwave photon-magnon coupling developments rely on the integration of metal-based microwave resonators with a magnetic material. However, it has recently been realised that all-dielectric resonators made of or containing magneto-insulating materials can operate as a standalone strongly-coupled system characterised by low dissipation losses and strong local microwave field enhancement. Here, after a brief overview of recent developments in the field, I discuss examples of such dielectric resonant systems and demonstrate their ability to operate as multiresonant antennas for light, microwaves, magnons, sound, vibrations and heat. This multiphysics behaviour opens up novel opportunities for the realisation of multiresonant coupling such as, for example, photon-magnon-phonon coupling. I also propose several novel systems in which strong photon-magnon coupling in dielectric antennas and similar structures is expected to extend the capability of existing devices or may provide an entirely new functionality. Examples of such systems include novel magnetofluidic devices, high-power microwave power generators, and hybrid devices exploiting the unique properties of electrical solitons.
We experimentally study the magnon-photon coupling in a system consitsing of the compensating ferrimagnet gadolinium iron garnet (GdIG) and a three-dimensional microwave cavity. The temperature is varied in order to tune the GdIG magnetization and to observe the transition from the weak coupling regime to the strong coupling regime. By measuring and modelling the complex reflection parameter of the system the effective coupling rate g eff and the magnetization M eff of the sample are extracted. Comparing g eff with the magnon and the cavity decay rate we conclude that the strong coupling regime is easily accessible using GdIG. We show that the effective coupling strength follows the predicted square root dependence on the magnetization.
We present a generic theoretical framework to describe non-reciprocal microwave circulation in a multimode cavity magnonic system and assess the optimal performance of practical circulator devices. We show that high isolation (> 56 dB), extremely low insertion loss (< 0.05 dB), and flexible bandwidth control can be potentially realized in high-quality-factor superconducting cavity based magnonic platforms. These circulation characteristics are analyzed with materials of different spin densities. For high-spin-density materials such as yttrium iron garnet, strong coupling operation regime can be harnessed to obtain a broader circulation bandwidth. We also provide practical design principles for a highly integratible low-spin-density material (vanadium tetracyanoethylene) for narrow-band circulator operation, which could benefit noise-sensitive quantum microwave measurements. This theory can be extended to other coupled systems and provide design guidelines for achieving tunable microwave non-reciprocity for both classical and quantum applications.
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