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

Abstract

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, $\hslash \omega$ 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 Néel 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. From 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.

5 pages, 3 figures in the main text. To appear in Phys. Rev. Lett., magnetic field data included. Supplementary information also included

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