Antiferromagnetic resonance and in-gap terahertz continuum in Kitaev Honeycone magnet α−RuCl3
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 my talk, I will present optical absorption measurements using time- domain terahertz spectroscopy in the range 0.3 to 10 meV that reveal several 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 reveal an 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 high- resolution measurements of the absolute spectral weight and lifetime of the q=0 magnon in α-RuCl3 . I will also discuss our recent effort of eliminating the antiferromagnetic order by applying an in-plane magnetic field to probe dynamical spin susceptibility, which adds constrains on our understanding of this material as a Kitaev spin liquid candidate.
. Little*, Wu*, et al. arXiv:1704.07357 (2017)