Nature of Possible Magnetic Phases in Frustrated Hyperkagome Iridate

Based on Kitaev-Heisenberg model with Dzyaloshinskii-Moriya (DM) interactions, we studied nature of possible magnetic phases in frustrated hyperkagome iridate, Na$_4$Ir$_{3}$O$_8$ (Na-438). Using Monte-Carlo simulation, we showed that the phase diagram is mostly covered by two competing magnetic ordered phases; Z$_2$ symmetry breaking (SB) phase and Z$_6$ SB phase, latter of which is stabilized by the classical order by disorder. These two phases are separated by a first order phase transition line with Z$_8$-like symmetry. The critical nature at the Z$_6$ SB ordering temperature is characterized by the 3D XY universality class, below which U(1) to Z$_6$ crossover phenomena appears; the Z$_6$ spin anisotropy becomes irrelevant in a length scale shorter than a crossover length $\Lambda_*$ while becomes relevant otherwise. A possible phenomenology of polycrystalline Na-438 is discussed based on this crossover phenomena

Chiral Topological Excitons in a Chern Band Insulator

A family of semiconductors called as Chern band insulator are shown to host exciton bands with non-zero topological Chern integers and chiral exciton edge modes. Using a prototypical two-band Chern insulator model, we calculate a cross-correlation function to obtain the exciton bands and their Chern integers. The lowest exciton band acquires Chern integers such as $\pm 1$ and $\pm 2$ in electronic Chern insulator phase. The non-trivial topology can be experimentally observed both by non-local optoelectronic response of exciton edge modes and by a phase shift in the cross-correlation response due to the bulk mode. Our result suggests that magnetically doped HgTe, InAs/GaSb quantum wells and $\text{(Bi,Sb)}_{2} \text{Te}_{3}$ thin film are promising candidates for a platform of topological excitonics.Comment: 5+ pages, 3 figures with supplemental material

Magnetostatic wave analog of integer quantum Hall state in patterned magnetic films

A magnetostatic spin wave analog of integer quantum Hall (IQH) state is proposed in realistic patterned ferromagnetic thin films. Due to magnetic shape anisotropy, magnetic moments in a thin film lie within the plane, while all spin-wave excitations are fully gapped. Under an out-of-plane magnetic field, the film acquires a finite magnetization, where some of the gapped magnons become significantly softened near a saturation field. It is shown that, owing to a spin-orbit locking nature of the magnetic dipolar interaction, these soft spin-wave volume-mode bands become chiral volume-mode bands with finite topological Chern integers. A bulk-edge correspondence in IQH physics suggests that such volume-mode bands are accompanied by a chiral magnetostatic spin-wave edge mode. The existence of the edge mode is justified both by micromagnetic simulations and by band calculations based on a linearized Landau-Lifshitz equation. Employing intuitive physical arguments, we introduce proper tight-binding models for these soft volume-mode bands. Based on the tight-binding models, we further discuss possible applications to other systems such as magnetic ultrathin films with perpendicular magnetic anisotropy (PMA).Comment: 20 pages, 12 figure