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.
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
Edge supercurrents in superconductors have long been an elusive target. Interest in them has reappeared in the context of topological superconductivity. We report evidence for the existence of a robust edge supercurrent in the Weyl superconductor molybdenum ditelluride (MoTe2). In a magnetic field B, fluxoid quantization generates a periodic modulation of the edge condensate observable as a “fast-mode” oscillation of the critical current Ic versus B. The fast-mode frequency is distinct from the conventional Fraunhofer oscillation displayed by the bulk supercurrent. We confirm that the fast-mode frequency increases with crystal area as expected for an edge supercurrent. In addition, weak excitation branches are resolved that display an unusual broken symmetry.
A long-standing issue in topological insulator research has been to find a bulk single crystal material that provides a high-quality platform for characterizing topological surface states without interference from bulk electronic states. This material would ideally be a bulk insulator, have a surface state Dirac point energy well isolated from the bulk valence and conduction bands, display quantum oscillations from the surface state electrons and be growable as large, high-quality bulk single crystals. Here we show that this material obstacle is overcome by bulk crystals of lightly Sn-doped Bi 1.1 Sb 0.9 Te 2 S grown by the vertical Bridgman method. We characterize Sn-BSTS via angle-resolved photoemission spectroscopy, scanning tunnelling microscopy, transport studies, X-ray diffraction and Raman scattering. We present this material as a high-quality topological insulator that can be reliably grown as bulk single crystals and thus studied by many researchers interested in topological surface states.
A quantum Hall insulator is observed to execute large jumps between metastable Chern states.
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