A. Little, Liang Wu, P. Lampen-Kelley, A. Banerjee, S. Pantankar, D. Rees, C. A. Bridges, J. -Q. Yan, D. Mandrus, S. E. Nagler, and J. Orenstein
https://journals.aps.org/prl/accepted/78076Y70Qb613566849547557f2aaf480ab45f021
We report measurements of optical absorption in the zig-zag antiferromagnet \alpha-RuCl3 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\'{e}el 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.
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