Title: QuantumMagnetism in the Honeycomb Lattice Material YbC
Abstract: Inquantum magnetism, deceptively simple model systems exhibit rich behavior thatenables the testing of fundamental ideas which intern serve as the basis forunderstanding and identifying more complex behavior. In this talk, I will focus on the physics ofthe honeycomb lattice Heisenberg model as probed experimentally by inelasticneutron scattering. The honeycomblattice Heisenberg model is simple: onlynearest neighbor Heisenberg interactions are considered; there is nofrustration, 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 theheat capacity at 1.8 K and very weak but sharper transition at 0.6 Kcorresponding to the onset of magnetic order. We have determined the crystal field Hamiltonian through simultaneousrefinements of inelastic neutron scattering and magnetization data. The groundstate doublet of the crystal field Hamiltonian is well isolated and results inan effective spin-1/2 system. The low energy excitation spectrum consists of conventionalspin waves and an unusually sharp feature within a broad continuum. By including both transverse and longitudinalchannels of the neutron response, linear spin wave theory with a singleHeisenberg interaction (J ~ 0.42 meV) on the honeycomb lattice reproduces allof the key features in the spectrum. Inparticular, the broad continuum corresponds to a two-magnon contribution fromthe longitudinal channel, while the sharp feature within this continuum isidentified as a Van Hove singularity in the joint density of states. Theexperimental demonstration of a Van Hove singularity in a two-magnon continuumis important as a confirmation of basic notions of continua in quantummagnetism and additionally because analogous features in two-spinon continuacould potentially be used to distinguish quantum spin liquids from merelydisordered systems.
Host: N. Butch