Antiferromagnetic resonance and terahertz continuum in $\alpha-$RuCl$_3$

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We report measurements of optical absorption in the zig-zag antiferromagnet $\alpha$-RuCl$_3$ as a function of temperature, $T$, magnetic field, $B$, and photon energy, $\hbar\omega$ in the range $\sim$ 0.3 to 8.3 meV, using time-domain terahertz spectroscopy.

• Polarized measurements show that 3-fold rotational symmetry is broken in the honeycomb plane from 2 K to 300 K.
• We find a sharp absorption peak at 2.56 meV upon cooling below the N\'eel temperature of 7 K at $B=0$ that we identify as magnetic-dipole excitation of a zero-wavevector magnon, or antiferromagnetic resonance (AFMR).
• With application of $B$, the AFMR broadens and shifts to lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase.