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arXiv · 1704.07357

Antiferromagnetic resonance and terahertz continuum in $\alpha-$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·J. Orenstein
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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, $\hbar\omega$ 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. 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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A. Little, Liang Wu, P. Lampen-Kelley, A. Banerjee, S. Patankar, D. Rees, C. A. Bridges, J. -Q. Yan, D. Mandrus, S. E. Nagler, J. Orenstein. 2017-04-24. Antiferromagnetic resonance and terahertz continuum in $\alpha-$RuCl$_3$. https://doi.org/10.1103/physrevlett.119.227201

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