In a Dirac semimetal, each Dirac node is resolved into two Weyl nodes with opposite "handedness" or chirality. The two chiral populations do not mix. However, in parallel electric and magnetic fields ( E: || B: ), charge is predicted to flow between the Weyl nodes, leading to negative magnetoresistance. This "axial" current is the chiral (Adler-Bell-Jackiw) anomaly investigated in quantum field theory. We report the observation of a large, negative longitudinal magnetoresistance in the Dirac semimetal Na3Bi. The negative magnetoresistance is acutely sensitive to deviations of the direction of B: from E: and is incompatible with conventional transport. By rotating E: (as well as B: ), we show that it is consistent with the prediction of the chiral anomaly.

We report measurements of the surface Shubnikov de Haas oscillations (SdH) on crystals of the topological insulator Bi2Te2Se. In crystals with large bulk resistivity (∼4 Ωcm at 4 K), we observe ∼15 surface SdH oscillations (to the n = 1 Landau Level) in magnetic fields B up to 45 Tesla. Extrapolating to the limit 1/B → 0, we confirm the 1 2 -shift expected from a Dirac spectrum. The results are consistent with a very small surface Lande g-factor.

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 ...

Topological surface states protected by mirror symmetry are of interest for spintronic applications. Such states were predicted to exist in the rocksalt IV-VI semiconductors, and several groups have observed the surface states in (Pb,Sn)Te, (Pb,Sn)Se and SnTe using photoemission. An underlying assumption in the theory is that the surface states arise from bulk states describable as massive Dirac states, but this assumption is untested. Here we show that the thermoelectric response of the bulk states displays features specific to the Dirac spectrum. By relating the carrier density to the peaks in the quantum oscillations, we show that the first (N ¼ 0) Landau level is non-degenerate. This finding provides robust evidence that the bulk states are indeed massive Dirac states. In the lowest Landau level, S xx displays a striking linear increase versus magnetic field characteristic of massive Dirac fermions. In addition, the Nernst signal displays a sign anomaly in the gap-inverted phase at low temperatures.