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

Crystal Growth and Real Structure Effects of the First Weak 3D Stacked Topological Insulator Bi₁₄Rh₃I₉

A detailed account of the crystal-growth technique and real structure effects of the first 3D weak topological insulator Bi₁₄Rh₃I₉ = [(Bi₄Rh)₃I]­[BiI₄]₂ is given. As recently shown, this compound features decorated-honeycomb [(Bi₄Rh)₃I]²⁺ sheets with topologically protected electronic edge-states and thereby constitutes a new topological class. Meticulous optimization of the synthesis protocol, using thermochemical methods, yielded high-quality crystals of Bi₁₄Rh₃I₉ suitable for the experimental characterization of the structural as well as topological properties. Insightful information about the crystal structure, its pseudosymmetry, and the thereby caused stacking disorder and twinning phenomena, obtained by X-ray diffraction and TEM studies, is crucial for an adequate theoretical modeling of coupling between the topologically nontrivial sheets. As demonstrated here, Bi₁₄Rh₃I₉ is not an exotic anomaly, but a stable, structurally well-defined bulk material, which can be used for gaining experimental knowledge about the yet poorly investigated class of weak 3D topological insulators. It could equally foster the synthesis and understanding of related compounds with the bismuth-based decorated-honeycomb sheets