The Dirac and Weyl semimetals are unusual materials in which the nodes of the bulk states are protected against gap formation by crystalline symmetry. The chiral anomaly, predicted to occur in both systems, was recently observed as a negative longitudinal magnetoresistance (LMR) in NaBi (ref. ) and in TaAs (ref. ). An important issue is whether Weyl physics appears in a broader class of materials. We report evidence for the chiral anomaly in the half-Heusler GdPtBi. In zero field, GdPtBi is a zero-gap semiconductor with quadratic bands. In a magnetic field, the Zeeman energy leads to Weyl nodes. We have observed a large negative LMR with the field-steering properties specific to the chiral anomaly. The chiral anomaly also induces strong suppression of the thermopower. We report a detailed study of the thermoelectric response function α of Weyl fermions. The scheme of creating Weyl nodes from quadratic bands suggests that the chiral anomaly may be observable in a broad class of semimetals.

We report that strong perpendicular magnetic anisotropy of the ferromagnetic layer in a W/CoFeB/MgO multilayer structure can be established by inserting a Hf layer as thin as 0.25 nm between the W and CoFeB layers. The Hf spacer also allows transmission of spin currents generated by an in-plane charge current in the W layer to apply strong spin torque on the CoFeB, thereby enabling current-driven magnetic switching. The antidamping-like and field-like components of the spin torque exerted on a 1 nm CoFeB layer are of comparable magnitudes in this geometry. Both components originate from the spin Hall effect in the underlying W layer.

The phase diagram of cuprate high-temperature superconductors features an enigmatic pseudogap region that is characterized by a partial suppression of low-energy electronic excitations 1 . Polarized neutron di raction [2][3][4] , Nernst e ect 5 , terahertz polarimetry 6 and ultrasound measurements 7 on YBa 2 Cu 3 O y suggest that the pseudogap onset below a temperature T * coincides with a bona fide thermodynamic phase transition that breaks time-reversal, four-fold rotation and mirror symmetries respectively. However, the full point group above and below T * has not been resolved and the fate of this transition as T * approaches the superconducting critical temperature T c is poorly understood. Here we reveal the point group of YBa 2 Cu 3 O y inside its pseudogap and neighbouring regions using high-sensitivity linear and second-harmonic optical anisotropy measurements. We show that spatial inversion and two-fold rotational symmetries are broken below T * while mirror symmetries perpendicular to the Cu-O plane are absent at all temperatures. This transition occurs over a wide doping range and persists inside the superconducting dome, with no detectable coupling to either charge ordering or superconductivity. These results suggest that the pseudogap region coincides with an odd-parity order that does not arise from a competing Fermi surface instability and exhibits a quantum phase transition inside the superconducting dome.The crystal system and point group of a material are encoded in the structure of its second-and higher-rank optical susceptibility tensors 8 , which can be determined from the anisotropy of its linear and nonlinear optical responses. The second-harmonic (SH) response is particularly sensitive to the presence of global inversion symmetry because unlike the linear electric-dipole susceptibility tensor χ ED ij , which is allowed in all crystal systems, the SH electricdipole susceptibility tensor χ ED ijk vanishes in centrosymmetric point groups, leaving typically the far weaker electric-quadrupole term χ EQ ijkl as the primary bulk radiation source 9,10 . For these reasons, it has been proposed that optical SH generation may be an effective probe of inversion symmetry breaking in the cuprates 11 . To fully resolve the spatial symmetries underlying the pseudogap and its neighbouring regions, we performed both linear and SH optical rotational anisotropy (RA) measurements on de-twinned single crystals of YBa 2 Cu 3 O y as a function of oxygen content and temperature. The RA measurements track variations in the intensities of light reflected at the fundamental (I ω ) and SH frequencies (I 2ω ) of an obliquely incident laser beam, which is resonant with the O 2p to Cu 3d charge transfer energy ( ω = 1.5 eV), as the scattering plane is rotated about the c axis (Fig. 1a). The use of a recently developed rotating optical grating-based technique 12 made it possible to perform full 360• sweeps of the scattering plane angle (ϕ) over large temperature ranges while keeping the incident beam spot (∼100 µm) fixe...

Abstract3D Dirac semi-metals (DSMs) are materials that have massless Dirac electrons and exhibit exotic physical properties. It has been suggested that structurally distorting a DSM can create a Topological Insulator but this has not yet been experimentally verified. Further, Majorana Fermions have been theoretically proposed to exist in materials that exhibit both superconductivity and topological surface states. Here we show that the cubic Laves phase Au 2 Pb has a bulk Dirac cone that is predicted to gap on cooling through a structural phase transition at 100 K. The low temperature phase can be assigned a Z 2 = -1 topological index, and this phase becomes superconducting below 1.2 K. These characteristics make Au 2 Pb a unique platform for studying the transition between bulk Dirac electrons and topological surface states as well as studying the interaction of superconductivity with topological surface states, combining many different properties of emergent materials -superconductivity, bulk Dirac electrons, and a topologically non trivial Z 2 invariant.

A complete structural solution of the bilayer iridate compound Sr3Ir2O7 presently remains outstanding. Previously reported structures for this compound vary and all fail to explain weak structural violations observed in neutron scattering measurements as well as the presence of a net ferromagnetic moment in the basal plane. In this paper, we present single crystal neutron diffraction and rotational anisotropy second harmonic generation measurements unveiling a lower, monoclinic symmetry inherent to Sr3Ir2O7. Combined with density functional theory, our measurements identify the correct structural space group as No. 15 (C2/c) and provide clarity regarding the local symmetry of Ir 4+ cations within this spin-orbit Mott material.