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]

We present initial Raman spectroscopy experiments on exfoliated flakes of α-RuCl 3 , from tens of nm thick down to single layers. Besides unexpectedly finding this material to be air stable, in the thinnest layers we observe the appearance with decreasing temperature of a symmetry-forbidden mode in crossed polarization, along with an anomalous broadening of a mode at 164 cm −1 that is known to couple to a continuum of magnetic excitations. This may be due to an enhancement of magnetic fluctuations and evidence for a distorted honeycomb lattice in single-and bi-layer samples. arXiv:1709.00431v1 [cond-mat.str-el] 1 Sep 2017 the spin couple along different bonds (see Figure 1a). This model can be realized in materials under the right conditions of crystal electric field, spinorbit coupling and on-site Coulomb repulsion that produce an insulator with J ef f = 1/2 moments. In systems where the honeycomb lattice is formed by placing the magnetic atom inside edge-sharing octahedra, one can realize the necessary bond-dependent exchange due to the impact of strong spin-orbit coupling on the hopping (see Figure 1) [3,4,5]. A key difficulty with this proposal is that additional interaction terms may arise and produce long range order [6,7,8,9]. While some of these terms are enabled simply by symmetry, they are strongly enhanced by lattice distortions that mix the J ef f = 1/2 and J ef f = 3/2 states, altering the hopping terms. Recently, α-RuCl 3 has emerged as a potential candidate to realize a Kitaev quantum spin liquid state [10,11,2,12,13,14,15,16,17,18,19,20].IR, Raman and photo-emission spectroscopy combined with DFT calculations strongly suggest the system is close to the J ef f = 1/2 limit, with octahedra that are nearly undistorted at low temperatures. Perhaps due to the smaller spin-orbit coupling expected in a 4d system, α-RuCl 3 reveals an extremely narrow spin-orbit exciton (2 meV wide) well separated from charge excitations [14]. Thus the low-energy model of α-RuCl 3 does not contain any charge fluctuations, unlike the 5d Ir systems where the spin-orbit and onsite d−d excitations are overlapped in energy [9]. Perhaps most promising is the observed continuum of magnetic excitations, where the Raman temperature dependence and the excitation dispersion seen by neutrons is consistent with fractional particles expected from the pure Kitaev model [2,11,21,20].Despite its importance to the formation of an ordered state, the structure of α-RuCl 3 remains controversial. In particular, the exact structure appears to be sensitive to atomic disorder and stacking faults, which are not uncommon in van der Waals crystals such as α-RuCl 3 [15,22,23,21]. Surprisingly, the addition of stacking faults leads to an enhanced onset of antiferromagnetic order (higher T N ) [15]. This rather counterintuitive observation may result from additional tunneling pathways opened by the stacking disorder that boost the Heisenberg terms. If correct, this suggests that exfoliating α-RuCl 3 down to single layers could suppress the long range ord...

Magnetic van der Waals (vdW) materials are the centerpiece of atomically thin devices with spintronic and optoelectronic functions. Exploring new chemistry paths to tune their magnetic and optical properties enables significant progress in fabricating heterostructures and ultracompact devices by mechanical exfoliation. The key parameter to sustain ferromagnetism in 2D is magnetic anisotropy-a tendency of spins to align in a certain crystallographic direction known as easy-axis. In layered materials, two limits of easy-axis are in-plane (XY) and out-of-plane (Ising). Light polarization and the helicity of topological states can couple to magnetic anisotropy with promising photoluminescence or spin-orbitronic functions. Here, a unique experiment is designed to control the easy-axis, the magnetic transition temperature, and the optical gap simultaneously in a series of CrCl Br crystals between CrCl with XY and CrBr with Ising anisotropy. The easy-axis is controlled between the two limits by varying spin-orbit coupling with the Br content in CrCl Br . The optical gap, magnetic transition temperature, and interlayer spacing are all tuned linearly with x. This is the first report of controlling exchange anisotropy in a layered crystal and the first unveiling of mixed halide chemistry as a powerful technique to produce functional materials for spintronic devices.

Significant efforts have focused on the magnetic excitations of relativistic Mott insulators, predicted to realize the Kitaev quantum spin liquid (QSL). This exactly solvable model involves a highly entangled state resulting from bond-dependent Ising interactions that pro-1 arXiv:2003.09274v1 [cond-mat.str-el] 18 Mar 2020 duce excitations which are non-local in terms of spin flips. A key challenge in real materials is identifying the relative size of the non-Kitaev terms and their role in the emergence or suppression of fractional excitations. Here, we identify the energy and temperature boundaries of non-Kitaev interactions by direct comparison of the Raman susceptibility of α-RuCl 3 with quantum Monte Carlo (QMC) results for the Kitaev QSLs. Moreover, we further confirm the fractional nature of the magnetic excitations, which is given by creating a pair of fermionic quasiparticles. Interestingly, this fermionic response remains valid in the non-Kitaev range. Our results and focus on the use of the Raman susceptibility provide a stringent new test for future theoretical and experimental studies of QSLs. Exotic excitations with fractional quantum numbers are a key characteristic of QSLs 1-4 , which result from the long range entanglement of these non-trivial topological phases 5-7 . Originating from frustrated magnetic interactions, the fractional nature inspires an overarching goal of studying QSLs, realizing topological quantum computing immune to decoherence, with high operating temperatures from large exchange interactions 8, 9 . The last decade has seen great progress towards identifying the fractional excitations of QSLs 10-18 . Attention has focused on relativistic Mott insulators that are close to the exactly solvable Kitaev model with a QSL ground state. In materials such as A 2 IrO 3 (A = Cu, Li or Na) 4, 8, 19-23 and α-RuCl 3 24-27 , the large spin-orbit coupling and Coulomb repulsion result in j ef f = 1/2 moments on a honeycomb lattice 2, 9, 28-35 .According to the pure Kiteav model, in these materials spin flips could produce Z 2 gauge fluxes and dispersive Majorana fermions. 14, 36 .

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