Jason W. Krizan

Evidence for the chiral anomaly in the Dirac semimetal Na3Bi

Breaking chiral symmetry in a solid Dirac semimetals have graphene-like electronic structure, albeit in three rather than two dimensions. In a magnetic field, their Dirac cones split into two halves, one supporting left-handed and the other right-handed fermions. If an electric field is applied parallel to the magnetic field, this “chiral” symmetry may break: a phenomenon called the chiral anomaly. Xiong et al. observed this anomaly in the Dirac semimetal Na3Bi (see the Perspective by Burkov). Transport measurements lead to the detection of the predicted large negative magnetoresistance, which appeared only when the two fields were nearly parallel to each other. Science, this issue p. 413, see also p. 378 Transport measurements in a magnetic field indicate the breaking of chiral symmetry. [Also see Perspective by Burkov] 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.

Observation of Fermi arc surface states in a topological metal

Nailing down the topology of a semimetal Topological insulators are exotic materials that have a conducting surface state that can withstand certain types of material imperfection. Theoreticians have predicted a different kind of surface state in related three-dimensional topological Dirac semimetals, which do not have an energy gap in the band structure of the bulk. Xu et al. used photoemission spectroscopy to map out the band structure of the material Na3Bi and detected the predicted surface state. Their results may lead to further insights into the physics of topological matter. Science, this issue p. 294 Angle-resolved photoemission spectroscopy is used to map out the band structure of Na3Bi and detect its exotic surface state. The topology of the electronic structure of a crystal is manifested in its surface states. Recently, a distinct topological state has been proposed in metals or semimetals whose spin-orbit band structure features three-dimensional Dirac quasiparticles. We used angle-resolved photoemission spectroscopy to experimentally observe a pair of spin-polarized Fermi arc surface states on the surface of the Dirac semimetal Na3Bi at its native chemical potential. Our systematic results collectively identify a topological phase in a gapless material. The observed Fermi arc surface states open research frontiers in fundamental physics and possibly in spintronics.

p-Type CuRhO2 as a Self-Healing Photoelectrode for Water Reduction under Visible Light

Polycrystalline CuRhO2 is investigated as a photocathode for the splitting of water under visible irradiation. The band edge positions of this material straddle the water oxidation and reduction redox potentials. Thus, photogenerated conduction band electrons are sufficiently energetic to reduce water, while the associated valence band holes are energetically able to oxidize water to O2. Under visible illumination, H2 production is observed with ~0.2 V underpotential in an air-saturated solution. In contrast, H2 production in an Ar-saturated solution was found to be unstable. This instability is associated with the reduction of the semiconductor forming Cu(s). However, in the presence of air or O2, bulk Cu(s) was not detected, implying that CuRhO2 is self-healing when air is present. This property allows for the stable formation of H2 with ca. 80% Faradaic efficiency.

Temperature−field phase diagram of extreme magnetoresistance

Significance Extreme magnetoresistance (XMR) has been recently discovered in a number of seemingly unrelated materials with diverse crystalline and electronic structures. In this work, we use lanthanum monopnictides, LaBi and LaSb, as simple platforms to reveal a common triangular Temperature–field phase diagram for XMR. Further, we show that, in the electronic structure of both materials, lanthanum d orbitals mix with the pnictogen p orbitals. Remarkably, we find that both the triangular phase diagram and the orbital texture exist in all families of semimetals with XMR. These results show that XMR is a ubiquitous phenomenon with a universal phase diagram that goes beyond certain material specifications; it is not a rare commodity of topological materials or noncentrosymmetric structures. The recent discovery of extreme magnetoresistance (XMR) in LaSb introduced lanthanum monopnictides as a new platform to study this effect in the absence of broken inversion symmetry or protected linear band crossing. In this work, we report XMR in LaBi. Through a comparative study of magnetotransport effects in LaBi and LaSb, we construct a temperature−field phase diagram with triangular shape that illustrates how a magnetic field tunes the electronic behavior in these materials. We show that the triangular phase diagram can be generalized to other topological semimetals with different crystal structures and different chemical compositions. By comparing our experimental results to band structure calculations, we suggest that XMR in LaBi and LaSb originates from a combination of compensated electron−hole pockets and a particular orbital texture on the electron pocket. Such orbital texture is likely to be a generic feature of various topological semimetals, giving rise to their small residual resistivity at zero field and subject to strong scattering induced by a magnetic field.

Anomalous conductivity tensor in the Dirac semimetal Na3Bi

Na3Bi is a Dirac semimetal with protected nodes that may be sensitive to the breaking of time-reversal invariance in a magnetic field B. We report experiments which reveal that both the conductivity and resistivity tensors exhibit robust anomalies in B. The resistivity is B-linear up to 35 T, while the Hall angle exhibits an unusual profile approaching a step function. The conductivities and share identical power-law dependences at large B. We propose that these significant deviations from conventional transport result from an unusual sensitivity of the transport lifetime to B. The transport features are compared with those in Cd3As2.

Polytypism, polymorphism, and superconductivity in TaSe2−xTex

Significance Although polymorphs of a substance can often have dramatically different physical properties, polytypes, which occur when the geometry of a structural layer is maintained but the number of layers in the layer-stacking sequence is changed, rarely do. Here we find, using random substitution of Te for some of the Se to induce structural changes in TaSe2, a classic layered dichalcogenide, so that the transition temperature to superconductivity (Tc) is significantly different for different polytypes and polymorphs and especially differs when going from one polytype to another. This observation implies either a surprising sensitivity of Tc to the layer-stacking sequence or a similarly surprising sensitivity of Tc to the small changes in layer geometry that accompany the change in polytype. Polymorphism in materials often leads to significantly different physical properties—the rutile and anatase polymorphs of TiO2 are a prime example. Polytypism is a special type of polymorphism, occurring in layered materials when the geometry of a repeating structural layer is maintained but the layer-stacking sequence of the overall crystal structure can be varied; SiC is an example of a material with many polytypes. Although polymorphs can have radically different physical properties, it is much rarer for polytypism to impact physical properties in a dramatic fashion. Here we study the effects of polytypism and polymorphism on the superconductivity of TaSe2, one of the archetypal members of the large family of layered dichalcogenides. We show that it is possible to access two stable polytypes and two stable polymorphs in the TaSe2−xTex solid solution and find that the 3R polytype shows a superconducting transition temperature that is between 6 and 17 times higher than that of the much more commonly found 2H polytype. The reason for this dramatic change is not apparent, but we propose that it arises either from a remarkable dependence of Tc on subtle differences in the characteristics of the single layers present or from a surprising effect of the layer-stacking sequence on electronic properties that are typically expected to be dominated by the properties of a single layer in materials of this kind.

Bulk crystal growth and electronic characterization of the 3D Dirac semimetal Na3Bi

High quality hexagon plate-like Na3Bi crystals with large (001) plane surfaces were grown from a molten Na flux. The freshly cleaved crystals were analyzed by low temperature scanning tunneling microscopy and angle-resolved photoemission spectroscopy, allowing for the characterization of the three-dimensional (3D) Dirac semimetal (TDS) behavior and the observation of the topological surface states. Landau levels were observed, and the energy-momentum relations exhibited a linear dispersion relationship, characteristic of the 3D TDS nature of Na3Bi. In transport measurements on Na3Bi crystals, the linear magnetoresistance and Shubnikov-de Haas quantum oscillations are observed for the first time.

Thermal Hall conductivity in the frustrated pyrochlore magnet Tb2Ti2O7

Probing the nature of an exotic magnet To minimize their energy, materials with magnetic interactions tend to become ordered at low temperatures. However, if the magnetism is frustrated (for example, if the geometry of the crystal lattice gets in the way of minimizing the energy), the material may not reach an ordered state even at very low temperatures. Hirschberger et al. studied the excitations of such a system—the pyrochlore compound Tb2Ti2O7—using thermal transport measurements. Thermal conductivity at very low temperatures resembled that of a disordered metal; a puzzling finding in an electrically insulating transparent material. Science, this issue p. 106 Thermal transport measurements reveal that low-temperature behavior of the insulator Tb2Ti2O7 resembles that of a disordered metal. In frustrated quantum magnets, long-range magnetic order fails to develop despite a large exchange coupling between the spins. In contrast to the magnons in conventional magnets, their spin excitations are poorly understood. Here, we show that the thermal Hall conductivity κxy provides a powerful probe of spin excitations in the “quantum spin ice” pyrochlore Tb2Ti2O7. The thermal Hall response is large, even though the material is transparent. The Hall response arises from spin excitations with specific characteristics that distinguish them from magnons. At low temperature (<1 kelvin), the thermal conductivity resembles that of a dirty metal. Using the Hall angle, we construct a phase diagram showing how the excitations are suppressed by a magnetic field.

Large thermal Hall conductivity of neutral spin excitations in a frustrated quantum magnet

Probing the nature of an exotic magnet To minimize their energy, materials with magnetic interactions tend to become ordered at low temperatures. However, if the magnetism is frustrated (for example, if the geometry of the crystal lattice gets in the way of minimizing the energy), the material may not reach an ordered state even at very low temperatures. Hirschberger et al. studied the excitations of such a system—the pyrochlore compound Tb2Ti2O7—using thermal transport measurements. Thermal conductivity at very low temperatures resembled that of a disordered metal; a puzzling finding in an electrically insulating transparent material. Science, this issue p. 106 Thermal transport measurements reveal that low-temperature behavior of the insulator Tb2Ti2O7 resembles that of a disordered metal. In frustrated quantum magnets, long-range magnetic order fails to develop despite a large exchange coupling between the spins. In contrast to the magnons in conventional magnets, their spin excitations are poorly understood. Here, we show that the thermal Hall conductivity κxy provides a powerful probe of spin excitations in the “quantum spin ice” pyrochlore Tb2Ti2O7. The thermal Hall response is large, even though the material is transparent. The Hall response arises from spin excitations with specific characteristics that distinguish them from magnons. At low temperature (<1 kelvin), the thermal conductivity resembles that of a dirty metal. Using the Hall angle, we construct a phase diagram showing how the excitations are suppressed by a magnetic field.

Superconducting properties of theKxWO3tetragonal tungsten bronze and the superconducting phase diagram of the tungsten bronze family

We report the superconducting properties of the K