"Order by Disorder, Quenched Disorder, and Domain Selection in the S=1/2 Pyrochlore Magnet Er2Ti2O7"

 

 

Crystal field effects associated with Er3+ magnetic moments in Er2Ti2O7 give rise to local XY anisotropy and effective quantum S=1/2 spins which are antiferromagnetically coupled on this material’s cubic pyrochlore lattice. Er2Ti2O7 orders into a non-collinear antiferromagnetic Ψ2 state below ∼ 1.2 K, in zero magnetic field, but the mechanism for its ground state selection has been a puzzle for more than a decade. We have carried out inelastic neutron scattering measurements on single crystal samples of Er2Ti2O7 at low temperatures allowing us to determine the underlying spin Hamiltonian for this quantum antiferromagnet.  These results point to ground state selection via an order-by-quantum-disorder mechanism, and a concomitant order-by-disorder gap of ∼ 0.05 meV has been observed associated with the pseudo-Goldstone modes in the low field ordered state. In addition, I'll describe our work exploring magnetic field selection of Ψ2 and related Ψ3 domains in Er2Ti2O7 at low temperatures, as well as the sensitivity of the ground state selection to magnetic dilution in single crystals of Er2−xYxTi2O7. These studies are particularly topical in light of recent theoretical predictions related to domain stability in Er2Ti2O7.