Kitaev magnetism of iridium oxides and halides

by Dr Alexander Tsirlin (University of Augsburg)

Monday 3 Dec 2018, 14:30 → 15:30 Europe/Berlin

HS 3 (Physics Department)

Kitaev spin liquid remains an intriguing and elusive state of matter. In this talk, I will review iridate compounds as material prototypes of the Kitaev spin model, and focus on the possibility of using hydrostatic pressure for tuning these materials toward the anticipated spin-liquid state.

The A$_2$IrO$_3$ iridates are proximate to a dimerization transition that shortens the Ir-Ir distances and suppresses local magnetism. Both $\alpha$- and $\beta$-Li$_2$IrO$_3$ undergo such a transition around 3.7 GPa, but the $\beta$-phase additionally shows a first-order transition around 1.4 GPa, where long-range magnetic order is suppressed without the loss of local magnetism. Using a combination of thermodynamic and local probes as well as band structure calculations, we suggest that above 1.4 GPa $\beta$-Li$_2$IrO$_3$ enters the classical spin liquid phase with part of the spins frozen below 15 K. This is accompanied by an increase in the off-diagonal anisotropy $\Gamma$ and a decrease in the Kitaev exchange $K$, in contrast to the original expectations of enhancing Kitaev magnetism under pressure.

I will further present recent results on the hexahalide Ir-based antifluorite compounds K$_2$IrCl$_6$ and K$_2$IrBr$_6$, where large single crystals can be grown from the solution. These compound entail the frustrated face-centered cubic arrangement of the Ir$^{4+}$ ions and may be proximate to the ideal $j_{\rm eff}=\frac12$ state, although we detect symmetry lowering in K$_2$IrBr$_6$ as well as a soft rotary mode in K$_2$IrCl$_6$, suggesting the proclivity of these compounds to structural distortions. Magnetic transitions are accompanied by structural anomalies, suggesting a strong coupling between magnetism and lattice. Moreover, large single crystals render neutron-scattering experiments more feasible than in any other iridium compounds studied to date.