Bond Dependent Exchange Interactions in the Honeycomb Quantum Magnet ErCl3

ErCl3 hosts honeycomb layers of Er3+ with an effective spin ½ ground state and anisotropic exchange interactions. Previous magnetic susceptibility and powder/single crystal neutron diffraction experiments (performed before the advent of the celebrated Kitaev honeycomb model) revealed a transition into a 120° antiferromagnetic order at 350 mK. At the time of the initial determination of this magnetic structure, no proposal for the interactions that could lead to this state was made. Magnetic models back then could not account for this particular magnetic structure. However, within the past 5 years, it has been shown that this phase can be stabilized through modern quantum field theories, such as in the Kitaev or other, more general compass models. Furthermore, numerical analysis of the Kitaev-Gamma-Heisenberg (KGammaH) model suggests that, when Kitaev and Heisenberg interactions compete on the honeycomb lattice, this 120° phase borders a quantum spin liquid phase, which leaves open the possibility of experimentally accessing this highly sought-after state of matter. We are attempting to quantify the magnetic interactions that are present in ErCl3 via inelastic neutron scattering and thermodynamic experiments. By fitting our data with these modern models, we hope to show that these exotic new ideas in physics can be experimentally observed in this class of rare-earth materials, providing the scientific community with new routes to test their ever evolving theories.