Magnetoresistance is the change in a material's electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors, in magnetic memory, and in hard drives at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity.

Dirac semimetals and Weyl semimetals are 3D analogs of graphene in which crystalline symmetry protects the nodes against gap formation [1-3]. Na3Bi and Cd3As2 were predicted to be Dirac semimetals [4, 5], and recently confirmed to be so by photoemission [6-8]. Several novel transport properties in a magnetic field H have been proposed for Dirac semimetals [2, 10, 11, 16]. Here we report an interesting property in Cd3As2 that was unpredicted, namely a remarkable protection mechanism that strongly suppresses back-scattering in zero H. In single crystals, the protection results in ultrahigh mobility, 9 × 10 6 cm 2 /Vs at 5 K. Suppression of backscattering results in a transport lifetime 10 4 × longer than the quantum lifetime. The lifting of this protection by H leads to a very large magnetoresistance. We discuss how this may relate to changes to the Fermi surface induced by H.

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 conducted genome-wide association studies of three phenotypes: subjective well-being (N = 298,420), depressive symptoms (N = 161,460), and neuroticism (N = 170,910). We identified three variants associated with subjective well-being, two with depressive symptoms, and eleven with neuroticism, including two inversion polymorphisms. The two depressive symptoms loci replicate in an independent depression sample. Joint analyses that exploit the high genetic correlations between the phenotypes (|ρ̂| ≈ 0.8) strengthen the overall credibility of the findings, and allow us to identify additional variants. Across our phenotypes, loci regulating expression in central nervous system and adrenal/pancreas tissues are strongly enriched for association.

An ordered phase showing remarkable electronic anisotropy in proximity to the superconducting phase is now a hot issue in the field of high-transition-temperature superconductivity. As in the case of copper oxides, superconductivity in iron arsenides competes or coexists with such an ordered phase. Undoped and underdoped iron arsenides have a magnetostructural ordered phase exhibiting stripe-like antiferromagnetic spin order accompanied by an orthorhombic lattice distortion; both the spin order and lattice distortion break the tetragonal symmetry of crystals of these compounds. In this ordered state, anisotropy of in-plane electrical resistivity is anomalous and difficult to attribute simply to the spin order and/or the lattice distortion. Here, we present the anisotropic optical spectra measured on detwinned BaFe 2 As 2 crystals with light polarization parallel to the Fe planes. Pronounced anisotropy is observed in the spectra, persisting up to an unexpectedly high photon energy of about 2 eV. Such anisotropy arises from an anisotropic energy gap opening below and slightly above the onset of the order. Detailed analysis of the optical spectra reveals an unprecedented electronic state in the ordered phase.anisotropic electronic state | iron pnictide | optical spectrum H igh-transition-temperature (high-T c ) superconductivity realized in both copper oxides and iron arsenides shares common features, namely, the superconducting phase is in close proximity to a symmetry-breaking phase and these phases coexist under certain circumstances, but apparently compete with each other. The close proximity suggests that our understanding of high-T c superconductivity will greatly improve once the nature of this proximate phase is revealed. The parent compounds of iron-arsenide superconductors, with BaFe 2 As 2 as a representative example, are unique metals that undergo a tetragonal-toorthorhombic structural phase transition at temperature T s with a shorter b axis and a longer a axis in the orthorhombic phase always accompanied by antiferromagnetic (AF) spin order at temperature T N . T N is equal to T s in some compounds (1-3) and slightly lower than T s in others (4). BaFe 2 As 2 exhibits stripe-like AF order in which Fe spins align antiferromagnetically in the a-axis direction in the Fe plane and ferromagnetically in the b-axis direction. Anisotropic electronic properties have been experimentally examined by various methods, such as neutron scattering (5), scanning tunneling microscopy (STM) (6), and angleresolved photoemission spectroscopy (ARPES) (7,8). These experiments suggest strong anisotropy of spin excitation and of the shape of Fermi surfaces. However, most of the experiments were performed on twinned crystals with randomly oriented domains, which inhibit the observation of genuine anisotropy.Recently, anisotropic resistivity has been measured on detwinned crystals (9, 10). The anisotropy of resistivity is quite anomalous in that the resistivity along the spin-ferromagnetic (FM) direction with a shorter b axis is...