University of Toronto
Physics Department
Condensed Matter and Quantum Optics
MONDAY SEMINAR

Abstract
 

The rare-earth titanate pyrochlores, RE₂Ti₂O₇ (RE=rare-earth), have been studied extensively for their unusual low temperature magnetic ground structures due to the geometrical frustration arising from the combination of the lattice geometry of corner-sharing tetrahedra and nearest-neighbor antiferromagnetic exchange interactions. In this talk, I will focus on the physics of the local <111> Ising pyrochlores Ho₂Ti₂O₇ and Dy₂Ti₂O₇, the spin ice materials, where long range dipole-dipole interactions and an effective ferromagnetic nearest neighbor interaction are crucial ingredients for any model to describe the static short range correlations and macroscopic degeneracy of the spin ice phase. Using local single-spin flip and non-local Monte Carlo algorithms, we constructed a generic phase diagram for the local <111> Ising pyrochlores. Our understanding of the spin ice materials is then contrasted with the puzzling behaviors presented by Tb₂Ti₂O₇. A long-range order is predicted by a <111> Ising dipolar model, but the experiments reveal a collective paramagnet down to 50mk. Crystal field calculations suggest Tb₂Ti₂O₇ is a <111> system with a first excited state of Delta approx 20K, while elastic and inelastic neutron scattering results suggest a more isotropic configuration. I will also comment on the recent observation of ordering in the pressure experiments on Tb₂Ti₂O₇.
 
 

Local Host: Hae-Young Kee  (416-978-5196) 

See   http://www.physics.utoronto.ca/~qocmp