Abstract-Antiferromagnetic Resonance and Terahertz Continuum in α − RuCl 3

A. Little, Liang Wu, P. Lampen-Kelley, A. Banerjee, S. Patankar, D. Rees, C. A. Bridges, J.-Q. Yan, D. Mandrus, S. E. Nagler, and J. Orenstein

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.227201

We report measurements of optical absorption in the zigzag antiferromagnet $\alpha \text{−}{\mathrm{RuCl}}_3$ as a function of temperature $T$, magnetic field $B$, and photon energy $\hslash \omega$ in the range $\sim 0.3$–8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that threefold rotational symmetry is broken in the honeycomb plane from 2 to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the Néel temperature of 7 K at $B=0$ that we identify as the magnetic-dipole excitation of a zero-wave-vector magnon, or antiferromagnetic resonance (AFMR). With the application of $B$, the AFMR broadens and shifts to a lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase. From a direct, internally calibrated measurement of the AFMR spectral weight, we place an upper bound on the contribution to the dc susceptibility from a magnetic excitation continuum.

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