Title: Circuit-based cavity magnonics in the ultrastrong and deep-strong coupling regimes Show Chinese title

Title: 基于电路的腔体磁子学在超强耦合和深强耦合 regime 中

Abstract: We theoretically study nonperturbative strong-coupling phenomena in cavity magnonics systems in which the uniform magnetization dynamics (magnons) in a ferromagnet is coupled to the microwave magnetic field (photons) of a single LC resonator. Starting from an effective circuit model that accounts for the magnetization dynamics described by the Landau-Lifshitz-Gilbert equation, we show that a nontrivial frequency shift emerges in the ultrastrong and deep-strong coupling regimes, whose microscopic origin remains elusive within a purely classical framework. The circuit model is further quantized to derive a minimal quantum mechanical model for generic cavity magnonics, which corresponds to a two-mode version of the Hopfield Hamiltonian and explains the mechanism of the frequency shifts found in the {\it classical} circuit model. We also formulate the relation between the frequency shift and quantum quantities, such as the ground-state particle number, quantum fluctuations associated with the Heisenberg uncertainty principle, and entanglement entropy, providing a nondestructive means to experimentally access to these quantum resources. By utilizing soft magnons in an anisotropic ferromagnet, we further demonstrate that these quantum quantities diverge at the zeros of the magnon band edges as a function of the external magnetic field. This work paves the way for cavity magnonics beyond the conventional strong coupling regime. Show Chinese abstract

Abstract: 我们理论上研究了腔磁子系统中非微扰强耦合现象，其中铁磁体中的均匀磁化动力学（磁子）与单个LC谐振器的微波磁场（光子）耦合。 从一个考虑由Landau-Lifshitz-Gilbert方程描述的磁化动力学的有效电路模型出发，我们表明在超强耦合和深强耦合区域会出现一种非平凡的频率偏移，其微观起源在纯粹的经典框架内仍不明确。 该电路模型进一步被量子化，以推导出通用腔磁子系统的最小量子力学模型，该模型对应于Hopfield哈密顿量的两模版本，并解释了在{\it 经典}电路模型中发现的频率偏移机制。 我们还建立了频率偏移与量子量之间的关系，例如基态粒子数、与海森堡不确定性原理相关的量子涨落以及纠缠熵，提供了一种非破坏性的实验手段来获取这些量子资源。 通过利用各向异性铁磁体中的软磁子，我们进一步证明，这些量子量随着外加磁场的变化在磁子带边的零点处发散。 这项工作为超越传统强耦合区域的腔磁子学铺平了道路。