Prof. Bruce Gaulin (Department of Physics \& Astronomy, McMaster University, Canada)
"Effective Spin 1/2 Hamiltonians in the Pyrochlore Magnets Er2Ti2O7 and Yb2Ti2O7"

New neutron scattering instrumentation offers unprecedented opportunities for mapping out the full dispersion and dynamic susceptibility of magnetic materials. In turn, these measurements can be exploited to determine their microscopic spin Hamiltonians in great detail. We've used these techniques to examine two pyrochlore magnets with unusual and exotic ground states, Er2Ti2O7 and Yb2Ti2O7. These materials are both known to display anisotropic g-tensors with XY anisotropy, yet their ground state properties are very different. Collaborative work with Lucile Savary and Leon Balents has modelled our spin wave data in terms of an anisotropic exchange Hamiltonian on the pyrochlore lattice. As a result we can understand Er2Ti2O7's ordered ground state on the basis of selection by an order-by-quantum-disorder mechanism[1], while Yb2Ti2O7's ground state is shown to be in reasonably close proximity to spin liquid and other exotic ground states[2].
[1] L. Savary, K.A. Ross, B.D. Gaulin, J.P.C. Ruff, and L. Balents, arXiv:1204.1320 and to appear, Phys. Rev. Lett.
[2] K.A. Ross, L. Savary, B.D. Gaulin and L. Balents, Phys. Rev X, 1, 021022, 2011.