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Ultrastrong Magnon-Photon Coupling in Superconductor/Antiferromagnet/Superconductor Heterostructures at Terahertz Frequencies

V. M. Gordeeva, Yanmeng Lei, Xiyin Ye, G. A. Bobkov, A. M. Bobkov, Tao Yu, I. V. Bobkova

Abstract

We predict the realization of ultrastrong coupling between magnons of antiferromagnets and photons in superconductor/antiferromagnet/superconductor heterostructures at terahertz frequencies, from both quantum and classical perspectives. The hybridization of the two magnon modes with photons strongly depends on the applied magnetic field: at zero magnetic field, only a single antiferromagnetic mode with a lower frequency couples to the photon, forming a magnon-polariton, while using a magnetic field activates coupling for both antiferromagnetic modes. The coupling between magnon and photon is ultrastrong with the coupling constant $\sim$ 100 GHz exceeding 10% of the antiferromagnetic resonant frequency. The superconductor modulates the spin of the resulting magnon-polaritons and the group velocity, achieving values amounting to several tenths of the speed of light, which promises strong tunability of magnon transport in antiferromagnets by superconductors.

Ultrastrong Magnon-Photon Coupling in Superconductor/Antiferromagnet/Superconductor Heterostructures at Terahertz Frequencies

Abstract

We predict the realization of ultrastrong coupling between magnons of antiferromagnets and photons in superconductor/antiferromagnet/superconductor heterostructures at terahertz frequencies, from both quantum and classical perspectives. The hybridization of the two magnon modes with photons strongly depends on the applied magnetic field: at zero magnetic field, only a single antiferromagnetic mode with a lower frequency couples to the photon, forming a magnon-polariton, while using a magnetic field activates coupling for both antiferromagnetic modes. The coupling between magnon and photon is ultrastrong with the coupling constant 100 GHz exceeding 10% of the antiferromagnetic resonant frequency. The superconductor modulates the spin of the resulting magnon-polaritons and the group velocity, achieving values amounting to several tenths of the speed of light, which promises strong tunability of magnon transport in antiferromagnets by superconductors.

Paper Structure

This paper contains 6 sections, 84 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Model
  3. Dipolar fields
  4. Magnon-polaritons: quantum view
  5. Magnon-polaritons: Classical view
  6. Conclusions

Figures (3)

  • Figure 1: Sketch of the S/AF/S system. The equilibrium magnetizations of the two sublattices $\bm M_{1,2s}$ are oriented along the $z$ axis. The magnon with wavevector $\bm k$ can propagate at an angle $\theta$ with respect to $\bm M_{1s}$.
  • Figure 2: Dispersion of the eigenmodes in the S/AF/S structure in the cases of no external field [(a)], moderate external field $\mu_0H_0=0.05$ T [(b)], and strong external field $\mu_0H_0=0.5$ T [(c)]. The colors of the curves depict the value of the average spin $\langle S_z \rangle$ on the eigenmodes.
  • Figure 3: The magnetization configuration on the eigenmodes in the cases of zero external field [(a)], moderate external field $\mu_0H_0=0.05$ T [(b)], and strong external field $\mu_0H_0=0.5$ T [(c)]. The left side of each panel shows the ellipses of $\bm M_1$ precession at different points on the eigenmodes, plotted next to those points. The colors of the ellipses are the same as the colors of the corresponding modes. The right side shows the extended pictures, involving precession of the magnetizations at both sublattices, for some of the ellipses from the left side, marked by arrows. Parameters are the same as in Fig. \ref{['fig:eigenfrequencies_qv']}.