The striking in-plane anisotropy remains one of the most intriguing properties for the newly rediscovered black phosphorus (BP) 2D crystals. However, because of its rather low-energy band gap, the optical anisotropy of few-layer BP has been primarily investigated in the near-infrared (NIR) regime. Moreover, the essential physics that determine the intrinsic anisotropic optical property of few-layer BP, which is of great importance for practical applications in optical and optoelectronic devices, are still in the fancy of theory. Herein, we report the direct observation of the optical anisotropy of few-layer BP in the visible regime simply by using polarized optical microscopy. On the basis of the Fresnel equation, the intrinsic anisotropic complex refractive indices (n-iκ) in the visible regime (480-650 nm) were experimentally obtained for the first time using the anisotropic optical contrast spectra. Our findings not only provide a convenient approach to measure the optical constants of 2D layered materials but also suggest a possibility to design novel BP-based photonic devices such as atom-thick light modulators, including linear polarizer, phase plate, and optical compensator in a broad spectral range extending to the visible window.

Research in topological matter has expanded to include the Dirac and Weyl semimetals 1-10 , which feature three-dimensional Dirac states protected by symmetry. Zirconium pentatelluride has been of recent interest as a potential Dirac or Weyl semimetal material. Here, we report the results of experiments performed by in situ three-dimensional doubleaxis rotation to extract the full 4π solid angular dependence of the transport properties. A clear anomalous Hall effect is detected in every sample studied, with no magnetic ordering observed in the system to the experimental sensitivity of torque magnetometry. Large anomalous Hall signals develop when the magnetic field is rotated in the plane of the stacked quasi-two-dimensional layers, with the values vanishing above about 60 K, where the negative longitudinal magnetoresistance also disappears. This suggests a close relation in their origins, which we attribute to the Berry curvature generated by the Weyl nodes. Zirconium pentatelluride (ZrTe 5) has recently attracted considerable attention, following the observation of negative longitudinal magnetoresistance (LMR) 11. This negative LMR has been identified with the chiral anomaly 12-14 that is predicted to occur in Dirac and Weyl semimetals 1-10 and was recently observed in Na 3 Bi and GdPtBi 15,16. However, despite the observation of the negative LMR, there are no theoretical predictions showing that ZrTe 5 is a threedimensional (3D) Dirac or Weyl semimetal, in contrast to both Na 3 Bi (ref. 17) and Cd 3 As 2 (ref. 18). Furthermore, the results of angleresolved photoemission spectroscopy (ARPES) experiments 11,19-23 are not yet conclusive. It is therefore of interest to investigate other unusual transport properties of ZrTe 5 , especially the Hall response engendered by the Berry curvature. For Dirac and Weyl semimetals in an electric field E, a finite Berry curvature leads to an anomalous velocity Ω = × v E

Magnetic van der Waals (vdW) materials have been heavily pursued for fundamental physics as well as for device design. Despite the rapid advances, so far magnetic vdW materials are mainly insulating or semiconducting, and none of them possesses a high electronic mobilitya property that is rare in layered vdW materials in general. The realization of a magnetic high-mobility vdW material would open the possibility for novel magnetic twistronic or spintronic devices.Here we report very high carrier mobility in the layered vdW antiferromagnet GdTe 3. The electron mobility is beyond 60,000 cm 2 V -1 s -1 , which is the highest among all known layered magnetic materials, to the best of our knowledge. Among all known vdW materials, the mobility of bulk GdTe 3 is comparable to that of black phosphorus, and is only surpassed by graphite. By mechanical exfoliation, we further demonstrate that GdTe 3 can be exfoliated to ultrathin flakes of three monolayers, and that the magnetic order and relatively high mobility is retained in ~20-nm-thin flakes.VdW materials are the parent compounds of two-dimensional (2D) materials, which are currently actively studied for new device fabrications (1) involving the creation of heterostructure stacks (2) or twisted bilayers (3) of 2D building blocks. Magnetic vdW materials have recently led to the observation of intrinsic magnetic order in atomically thin layers (4-12), which was followed by exciting discoveries of giant tunneling magnetoresistance (13-16) and tunable magnetism (17)(18)(19) in such materials.So far, the known magnetic vdW materials (ferro-or antiferromagnetic) that can be exfoliated are limited to a few examples, such as: CrI3 (4), Cr2Ge2Te6 (5), FePS3 (6,7), CrBr3 (8, 9), CrCl3 (10-12), Fe3GeTe2 (17,20), and RuCl3 (21-23). Out of these, only Fe3GeTe2 is a metallic ferromagnet and there is no known vdW-based 2D antiferromagnetic metal. Moreover, no evidence of high carrier mobilities has been reported in any of these exfoliated thin materials or even in their bulk vdW crystals. In general, high mobility is limited to very few vdW materials, such as graphite (24) and black phosphorus (25). A material with high electronic mobility and a corresponding high mean-free-path (MFP), might be critical for potential magnetic "twistronic" devices (3) where a large MFP could enable interesting phenomena in a Moiré-supercell induced flat band. In addition, conducting antiferromagnetic materials are the prime candidates for high-speed antiferromagnetic spintronic devices (26). Here we report the realization of a very high electronic mobility in a vdW layered antiferromagnet, GdTe3, both in bulk and exfoliated thin flakes.We chose to study GdTe3, since rare-earth tritellurides (RTe3, R = La-Nd, Sm, and Gd-Tm) are structurally related to topological semimetal ZrSiS (27,28), while being known to exhibit an incommensurate charge density wave (CDW) (29-31), rich magnetic properties (32), and becoming superconducting under high-pressure (R = Gd, Tb and Dy) (33). Combined, these properties ...

In the Dirac/Weyl semimetal, the chiral anomaly appears as an "axial" current arising from charge-pumping between the lowest (chiral) Landau levels of the Weyl nodes, when an electric field is applied parallel to a magnetic field B. Evidence for the chiral anomaly was obtained from the longitudinal magnetoresistance (LMR) in Na3Bi and GdPtBi. However, current jetting effects (focussing of the current density J) have raised general concerns about LMR experiments. Here we implement a litmus test that allows the intrinsic LMR in Na3Bi and GdPtBi to be sharply distinguished from pure current jetting effects (in pure Bi). Current jetting enhances J along the mid-ridge (spine) of the sample while decreasing it at the edge. We measure the distortion by comparing the local voltage drop at the spine (expressed as the resistance Rspine) with that at the edge (R edge ). In Bi, Rspine sharply increases with B but R edge decreases (jetting effects are dominant). However, in Na3Bi and GdPtBi, both Rspine and R edge decrease (jetting effects are subdominant). A numerical simulation allows the jetting distortions to be removed entirely. We find that the intrinsic longitudinal resistivity ρxx(B) in Na3Bi decreases by a factor of 10.9 between B = 0 and 10 T. A second litmus test is obtained from the parametric plot of the planar angular magnetoresistance. These results strenghthen considerably the evidence for the intrinsic nature of the chiral-anomaly induced LMR. We briefly discuss how the squeeze test may be extended to test ZrTe5.arXiv:1802.01544v2 [cond-mat.str-el]

Dirac and Weyl semimetals display a host of novel properties. In Cd3As2, the Dirac nodes lead to a protection mechanism that strongly suppresses backscattering in zero magnetic field, resulting in ultrahigh mobility (∼ 10 7 cm 2 V −1 s −1 ). In applied magnetic field, an anomalous Nernst effect is predicted to arise from the Berry curvature associated with the Weyl nodes. We report observation of a large anomalous Nernst effect in Cd3As2. Both the anomalous Nernst signal and transport relaxation time τtr begin to increase rapidly at ∼ 50 K. This suggests a close relation between the protection mechanism and the anomalous Nernst effect. In a field, the quantum oscillations of bulk states display a beating effect, suggesting that the Dirac nodes split into Weyl states, allowing the Berry curvature to be observed as an anomalous Nernst effect.The field of topological quantum materials has recently expanded to include the Dirac (and Weyl) semimetals, which feature 3D bulk Dirac states with nodes that are protected by symmetry [1][2][3][4]. In Dirac semimetals, each Dirac cone is the superposition of two Weyl nodes which have opposite chiralities (χ = ±1). The Weyl nodes are prevented from hybridizing by the combination of point group symmetry, inversion symmetry and time-reversal symmetry (TRS) [4]. In the presence of a magnetic field B, the breaking of TRS leads to separation of the Weyl nodes and the appearance of a Berry curvature Ω(k).Because Ω(k) acts like an intense magnetic field, it exerts a strong force on charge carriers [5,6]. The first examples of Dirac semimetals, Na 3 Bi and Cd 3 As 2 , were identified by Wang et al. [7,8] Quite distinct from the chiral anomaly, the Berry curvature arising from separation of the Weyl nodes leads to other unusual transport effects, particularly the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) [24,25]. Unlike conventional system, no ferromagnetism is required for the AHE and ANE in Dirac semimetals because of the strong Berry curvature emanated by Weyl nodes. The anomalous Hall conductivity is expressed as [3,26],where ∆k i is the distance between the i th pair of Weyl nodes. The thermopower and Nernst effect in Weyl semimetals has been calculated in the Boltzmann equation approach [27][28][29][30].We report measurements of the thermoelectric tensor S ij of Cd 3 As 2 in two samples (A4, A5) in "set A" and two samples (B10, B20) in "set B" with the applied thermal gradient −∇T ||x and magnetic field B||ẑ (see Ref. [20] for details of the electrical transport measurements in set A and set B samples). We obtain S xx and S xy aswhere α ij is the thermoelectric linear response tensor, and ρ ij is the resistivity tensor (see Supplement for the details). In Dirac semimetals, the AHE and ANE arise because the Berry curvature Ω(k) imparts to the carriers an anomalous velocity v A = Ω(k) × k , i.e. Ω(k) acts like an effective magnetic field in k space (k is the rate of change of the wavevector k) [25]. Previously, the AHE was observed in Cd 3 As 2 as a wea...