Cu₂IrO₃: A New Magnetically Frustrated Honeycomb Iridate

We present the first copper iridium binary metal oxide with the chemical formula Cu₂IrO₃. The material is synthesized from the parent compound Na₂IrO₃ by a topotactic reaction where sodium is exchanged with copper under mild conditions. Cu₂IrO₃ has the same monoclinic space group (<i>C</i>2/<i>c</i>) as Na₂IrO₃ with a layered honeycomb structure. The parent compound Na₂IrO₃ is proposed to be relevant to the Kitaev spin liquid on the basis of having Ir⁴⁺ with an effective spin of 1/2 on a honeycomb lattice. Remarkably, whereas Na₂IrO₃ shows a long-range magnetic order at 15 K and fails to become a true spin liquid, Cu₂IrO₃ remains disordered until 2.7 K, at which point a short-range order develops. Rietveld analysis shows less distortions in the honeycomb structure of Cu₂IrO₃ with bond angles closer to 120° compared to Na₂IrO₃. Thus, the weak short-range magnetism combined with the nearly ideal honeycomb structure places Cu₂IrO₃ closer to a Kitaev spin liquid than its predecessors

Wedge Dyakonov Waves and Dyakonov Plasmons in Topological Insulator Bi₂Se₃ Probed by Electron Beams

Bi₂Se₃ has recently attracted a lot of attention because it has been reported to be a platform for the realization of three-dimensional topological insulators. Due to this exotic characteristic, it supports excitations of a two-dimensional electron gas at the surface and, hence, formation of Dirac-plasmons. In addition, at higher energies above its bandgap, Bi₂Se₃ is characterized by a naturally hyperbolic electromagnetic response, with an interesting interplay between type-I and type-II hyperbolic behaviors. However, still not all the optical modes of Bi₂Se₃ have been explored. Here, using mainly electron energy–loss spectroscopy and corresponding theoretical modeling we investigate the full photonic density of states that Bi₂Se₃ sustains, in the energy range of 0.8 eV–5 eV. We show that at energies below 1 eV, this material can also support wedge Dyakonov waves. Furthermore, at higher energies a huge photonic density of states is excited in structures such as waveguides and resonators made of Bi₂Se₃ due to the hyperbolic dispersion