Do defect structures in the pyrochlore Tb2Ti2O7 control hidden order or the thermal Hall effect?

The rare earth pyrochlores have been at the forefront of research into exotic magnetic phenomena for some years. In general they are of interest because they are frustrated - the interactions between magnetic moments in this crystal structure cannot be completely satisfied leading to unusual states with large degeneracies or fluctuations. These states are often fragile, as the persistence of such degeneracies to very low temperatures is unfavourable thermodynamically, and so they are sensitive to very tiny levels of disorder in the crystal structure that perturb the competing interactions. Tb2Ti2O7 has various low temperature states, and a very exotic thermal Hall effect, which may depend on the type and concentration of defects. Using SXD, we will compare crystals with more or less possible disorder to examine its role in the low temperature state and thermal Hall effect.