No abstract

We examine the role of spin-orbit coupling in the electronic structure of α-RuCl3, in which Ru ions in 4d 5 configuration form a honeycomb lattice. The measured optical spectra exhibit an optical gap of 220 meV and transitions within the t2g orbitals. The spectra can be described very well with firstprinciples electronic structure calculations obtained by taking into account both spin-orbit coupling and electron correlations. Furthermore, our x-ray absorption spectroscopy measurements at the Ru L edges exhibit distinct spectral features associated with the presence of substantial spin-orbit coupling, as well as an anomalously large branching ratio. We propose that α-RuCl3 is a spin-orbit assisted Mott insulator, and that the bond-dependent Kitaev interaction may be relevant for this compound.

The combination of electronic correlation and spin-orbit coupling is thought to precipitate a variety of highly unusual electronic phases in solids, including topological and quantum spin liquid states. We report a Raman scattering study that provides evidence for unconventional excitations in α-RuCl_{3}, a spin-orbit coupled Mott insulator on the honeycomb lattice. In particular, our measurements reveal unusual magnetic scattering, typified by a broad continuum. The temperature dependence of this continuum is evident over a large scale compared to the magnetic ordering temperature, suggestive of frustrated magnetic interactions. This is confirmed through an analysis of the phonon linewidths, which show a related anomaly due to spin-phonon coupling. These observations are in line with theoretical expectations for the Heisenberg-Kitaev model and suggest that α-RuCl_{3} may be close to a quantum spin liquid ground state.

Broadband, efficient and fast conversion of light to electricity is crucial for sensing and clean energy. Here we reveal the largest observed bulk photo-voltaic effect (BPVE), an intrinsic mechanism predicted to be ultrafast and exceed the Shockley-Quiesser limit. This discovery results from combining recent developments in the connection of BPVE to topology, Weyl semimetals and focused-ion beam fabrication. Our room temperature observation of the first BPVE in the mid-IR, is enabled by microscopic devices of the Weyl semimetal TaAs. Detailed symmetry analysis enables unambiguous separation of this response from competing photothermal effects. The size and wavelength range of the shift current offers new opportunities in optical detectors, clean energy, and topology, while directly demonstrating the utility of Weyl semimetals for applications.Converting light to electricity is crucial for clean energy, imaging, communications, chemical 1 arXiv:1712.04951v2 [cond-mat.mes-hall]