Exceptional points with coalescence of eigenvalues and eigenvectors are spectral singularities in the parameter space, achieving which often needs fine-tuning of parameters in quantum systems. We predict a persistent realization of nodal magnon-photon polariton, i.e., a polariton of long wavelength without any gap splitting in a thin ferromagnetic insulator film sandwiched by two normal metals, which persistently exists when the ferromagnet is sufficiently thick ∼100 nm due to the joint effect of dissipation and dissipative coupling. We perform the model calculation beyond the perturbation theory using a classical approach, develop a quantum scheme able to account for the Ohmic dissipation, and find ultrastrong coupling with coupling strength comparable to the bare magnon frequency. Via revealing a simple conversion relation, we extend this formalism to superconductors and predict the gap opened by the ultrastrong coupling strongly depends on the direction of polariton propagation. Our findings may help search for robust non-Hermitian topological phases in magnonic and spintronic devices.

中文翻译：

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铁磁异质结构中的持续节点磁振子-光子极化激元


特征值和特征向量合并的奇异点是参数空间中的光谱奇点，在量子系统中通常需要对参数进行微调才能实现。我们预测了节点磁振子-光子极化激元的持续实现，即在夹在两种正常金属之间的薄铁磁绝缘体薄膜中没有任何间隙分裂的长波长极化子，当铁磁体足够厚 ∼100 nm 时，由于耗散和耗散耦合的共同效应，它持续存在。我们使用经典方法在微扰理论之外进行模型计算，开发了一种能够解释欧姆耗散的量子方案，并发现了耦合强度与裸磁振子频率相当的超强耦合。通过揭示一个简单的转换关系，我们将这种形式扩展到超导体，并预测超强耦合打开的间隙在很大程度上取决于极化激元传播的方向。我们的发现可能有助于在 magnonic 和 spintronic 器件中寻找稳健的非 Hermitian 拓扑相。