Abstract-Antiferromagnetic resonance and terahertz conductivity in α−RuCl3

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

(Submitted on 24 Apr 2017)

https://arxiv.org/abs/1704.07357

Spin-1/2 moments in the antiferromagnetic Mott insulator α-RuCl3 are coupled by strongly anisotropic bond-dependent exchange interactions on a honeycomb lattice. Intense study of α-RuCl3 by inelastic scattering has been driven by the proposal that its low energy excitations may be adiabatically connected to the Majorana quasiparticles that emerge in the exact solution of the Kitaev spin liquid model. In this work, we report optical absorption measurements using time-domain terahertz spectroscopy in the range 0.8 to 7.5 meV that reveal new features of the low-energy spectrum of α-RuCl3. Above a sharp threshold photon energy of ∼1 meV, we observe a continuum of absorption that we assign to spin-driven electric dipole fluctuations. Polarized measurements demonstrate optical anisotropy, showing that 3-fold rotational symmetry is broken in α-RuCl3 at 300 K and down to low temperature. Below 7 K we observe the onset of a single, narrow antiferromagnetic resonance mode above the continuum, enabling the first measurements of the absolute spectral weight and lifetime of the q=0 magnon in α-RuCl3.