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Realization of a Harmonic Honeycomb Iridate

Realization of a Harmonic Honeycomb Iridate
(A-C) Crystal image and structural schematic of the new harmonic honeycomb structure. Lower panel shows anisotropic susceptibility vs temperature (inverse susceptibility in inset).

Scientific Achievement
Discovery of a new iridium-oxide based material, realization of a novel spin-anisotropic magnetic exchange mechanism, and prediction of a new materials family

Significance and Impact
Creation of a new class of materials that combine the physics of Mott insulators with strong spin-orbit coupling.

Summary
The physics of Mott insulators, which are metals that stop conducting at high temperature or low pressure despite the fact that classical theory predicts them to conduct, underlies diverse phenomena ranging from high temperature superconductivity to exotic magnetism. Although both the electron spin and the structure of the local orbitals play a key role in this physics, in most systems these are connected only indirectly. Iridium-based oxides (iridates) open a further dimension to this problem by introducing strong spin-orbit interactions, such that the Mott physics has a strong orbital character. In the layered honeycomb iridates, this is thought to generate highly spin-anisotropic interactions, coupling the spin orientation to a given spatial direction of exchange and leading to strongly frustrated magnetism.

The potential for new physics to emerge from such interactions has driven much scientific excitement, most recently in the search for a new quantum spin liquid. James Analytis of the MSD Quantum Materials program reports a new iridate structure that has the same local connectivity as the layered honeycomb, but in a three-dimensional framework. The temperature dependence of the magnetic susceptibility exhibits a striking reordering of the magnetic anisotropy, giving evidence for highly spin-anisotropic exchange interactions. Furthermore, the basic structural units of this material suggest the possibility of a new family of structures, the `harmonic honeycomb' iridates. This compound thus provides a unique and exciting glimpse into the physics of a new class of strongly spin-orbit coupled Mott insulators.

K. Modic et al, Realization of a three-dimensional spin-anisotropic harmonic honeycomb iridate. Nature Communications 5, 4203 (2014) doi:10.1038/ncomms5203.