Transition metal oxides (TMO’s) containing a 5d element are increasingly attracting attention in the quest to discover and exploit novel electronic states. In the case of the 5d TMOs, these states arise from the strong spin-orbit interaction (SOI), which entangles spin and orbital moments. Iridium-based compounds have recently excited particular interest, including layered perovskite structures isostructural to the cuprate superconductors. Reporting in Physical Review Letters, an international team have presented the results of their investigation of the electronic and magnetic properties of single-layer perovskite 5d TMO’s. The experiments were performed on I16 at Diamond and P09 at Petra III, Desy, Germany, using X-ray resonant magnetic scattering.
Single-layer iridates are structurally similar to the high temperature superconductor La2CuO4, offering the enticing possibility of new families of superconductors. For existing high temperature superconductors, understanding the magnetic groundstate of the parent compound has proved pivotal.The prototypical example of iridium-based layered perovskite is Sr2IrO4, the electronic and structural features of which have been studied fairly comprehensively. X-ray resonant magnetic scattering (XRMS) on Sr2IrO4 has shown that the SOI leads to a Jeff = ½ ground state for the Ir4+ ions, and that a Mott-like insulator emerges as a result of relatively weak electronic correlations.
However Ba2IrO4 makes a more interesting subject for study as it is more structurally similar to La2CuO4 than Sr2IrO4. Nevertheless, there are a number of open questions about Ba2IrO4 including its detailed magnetic structure, and whether the ground state can be assigned to Jeff = ½.. The team used X-ray magnetic resonant scattering (XRMS) on I16 to probe the magnetic groundstate of Ba2IrO4. Similar experiments were carried out on Sr2IrO4 at Petra III.
The results revealed that Ba2IrO4 is a basal-plane commensurate antiferromagnet below TN =241K, with the moments ordered along the [1 1 0] direction. Comparison with Sr2IrO4 shows that it has a similar basal-plane antiferromagnetic structure, despite marked structural differences. The team also deduced that the electronic groundstate of Ba2IrO4 is Jeff = ½ and hence belongs to the same class of Mott insulators as Sr2IrO4. This shows that both the electronic and magnetic states of layered perovskites are remarkably robust to structural distortions, a fact that can be linked directly to the unique three-dimensional character of the electronic state produced by the strong SOI (see Figure) which renders them less sensitive to perturbations in local symmetry.
Figure 1. The proposed Jeff=1/2 electronic ground state wavefunction (in the strong spin-orbit coupling limit) of the 5d transition metal oxides such as Sr2IrO4 and Ba2IrO4 studied by Boseggia et al. The Jeff=1/2 state is itself formed from a coherent superposition of two entangled orbital-spin wavefunctions: |xy,↑> and |yz+i xz, ↓>. The extended, three-dimensional nature of the Jeff=1/2 state is believed to make it more robust to local structural distortions.
Des McMorrow from UCL was one of the co-authors on the study.
“You might say that X-ray resonant magnetic scattering was a technique looking for a real problem to solve, and that the 5d TMO’s represent an enormous opportunity for the technique and physics more generally.”
Des McMorrow, UCL
Robustness of Basal-Plane Antiferromagnetic Order and the Jeff=1/2 State in Single-Layer Iridate Spin-Orbit Mott Insulators, S. Boseggia, R. Springell, H. C. Walker, H. M. Rønnow, Ch. Rüegg, H. Okabe, M. Isobe, R. S. Perry, S. P. Collins, and D. F. McMorrow, Phys. Rev. Lett. 110, 117207 (2013)
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