Theory of magnon-polaritons in quantum Ising materials

Abstract

We present a theory of magnon-polaritons in quantum Ising materials, and develop a formalism describing the coupling between light and matter as an Ising system is tuned through its quantum critical point. The theory is applied to Ising materials having multilevel single-site Hamiltonians, in which multiple magnon modes are present, such as the insulating Ising magnet LiHoF${}_4$ . We find that the magnon-photon coupling strengths may be tuned by the applied transverse field, with the coupling between the soft mode present in the quantum Ising material and a photonic resonator mode diverging at the quantum critical point of the material. A fixed system of spins will not exhibit the diamagnetic response expected when light is coupled to mobile spins or atoms. Without the diamagnetic response, one expects a divergent magnon-photon coupling strength to lead to a superradiant quantum phase transition. However, this neglects the effects of damping and decoherence present in any real system. We show that damping and decoherence may block the superradiant quantum phase transition, and lead to weak coupling between the soft magnon mode and the resonator mode. The results of the theory are applied to experimental data on the model system LiHoF${}_4$ in a microwave resonator.

22 pages, 6 figures

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Cited by in corpus (6)

• Cavity magnonics with easy-axis ferromagnet: Critically enhanced magnon squeezing and light-matter interaction
• Linear response theory for cavity QED materials at arbitrary light-matter coupling strengths
• A Gallery of Soft Modes: Theory and Experiment at a Ferromagnetic Quantum Phase Transition
• Bound polariton states in the Dicke-Ising model
• Magnetostatic modes and criticality in quantum-Ising materials
• Amplified response of cavity-coupled quantum-critical systems