Quantum Magnetism in the Honeycomb Lattice Material YbCl₃

In quantum magnetism, simple model systems exhibit rich behavior that enables the testing of fundamental ideas which in turn serve as the basis for understanding and identifying more complex behavior. This talk will focus on the physics of the honeycomb lattice Heisenberg model as probed by inelastic neutron scattering. The honeycomb lattice Heisenberg model is simple: only nearest neighbor interactions are considered; there is no frustration, and the ground state at T=0 is the Néel state. The model material discussed here is the rare earth halide YbCl₃. YbCl₃ exhibits a broad peak in the heat capacity at 1.8 K and very weak but sharper transition at 0.6 K corresponding to the onset of magnetic order. We have determined the crystal field Hamiltonian through simultaneous refinements of inelastic neutron scattering and magnetization data. The ground state crystal field doublet is well isolated and results in an effective spin-1/2 system. The low energy excitation spectrum consists of conventional spin waves and an unusually sharp feature within a broad continuum. By including both transverse and longitudinal channels of the neutron response, linear spin wave theory with a single Heisenberg interaction (J ~ 0.42 meV) on the honeycomb lattice reproduces all of the key features in the spectrum. In particular, the broad continuum corresponds to a two-magnon contribution from the longitudinal channel, while the sharp feature within this continuum is identified as a Van Hove singularity in the joint density of states. The experimental demonstration of a Van Hove singularity in a two-magnon continuum is important as a confirmation of basic notions of continua in quantum magnetism and additionally because analogous features in two-spinon continua could potentially be used to distinguish quantum spin liquids from merely disordered systems.