Dr. Andrew Christianson from Oak Ridge National Laboratory will present on probing collective quantum effects of interacting spins on the honeycomb lattice.  In quantum magnetism, elegantly 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 experimentally by neutron scattering methods. The honeycomb lattice Heisenberg model is uncomplicated: only nearest neighbor Heisenberg interactions are considered; there is no frustration, and the ground state at T=0 is the Néel state. Inelastic neutron scattering measurements reveal that the low energy magnetic excitation spectrum consists of conventional spin waves and an unusually sharp feature within a broad continuum. The broad continuum corresponds to a two-magnon contribution while the sharp feature within this continuum is identified as a Van Hove singularity in the joint density of states. We discuss how quantum effects renormalize the single-magnon and multimagnon excitations and how this renormalization can be tuned and ultimately driven to the classical limit by applying a magnetic field. From a more general perspective, this work demonstrates that structures within magnetic continua can occur over a wide experimental parameter space and can be used as a reliable means of identifying quantum phenomena.