Anomalous conductivity tensor in the Dirac semimetal Na_3Bi

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 $\rho_{xx}$ is B-linear up to 35 T, while the Hall angle exhibits an unusual profile approaching a step-function. The conductivities $\sigma_{xx}$ and $\sigma_{xy}$ 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. Comparison with Cd3As2 is made.Comment: 8 pages, 5 figure

Static and dynamic XY-like short-range order in a frustrated magnet with exchange disorder

A single crystal of the Co2+ based pyrochlore NaCaCo2F7 was studied by inelastic neutron scattering. This frustrated magnet with quenched exchange disorder remains in a strongly correlated paramagnetic state down to one 60th of the Curie-Weiss temperature. Below T_f = 2.4 K, diffuse elastic scattering develops and comprises 30 +/- 10% of the total magnetic scattering, as expected for J_{eff} = 1/2 moments frozen on a time scale that exceeds \hbar/\delta E=3.8 ps. The diffuse scattering is consistent with short range XY antiferromagnetism with a correlation length of 16 \AA. The momentum (Q) dependence of the inelastic intensity indicates relaxing XY-like antiferromagnetic clusters at energies below ~ 5.5 meV, and collinear antiferromagnetic fluctuations above this energy. The relevant XY configurations form a continuous manifold of symmetry-related states. Contrary to well-known models that produce this continuous manifold, order-by-disorder does not select an ordered state in NaCaCo2F7 despite evidence for weak (~12 %) exchange disorder. Instead, NaCaCo2F7 freezes into short range ordered clusters that span this manifold.Comment: 9 pages, 9 figures. This updated version features modified figures and some new discussio

Real-space investigation of short-range magnetic correlations in fluoride pyrochlores NaCaCo2_2F7_7 and NaSrCo2_2F7_7 with magnetic pair distribution function analysis

We present time-of-flight neutron total scattering and polarized neutron scattering measurements of the magnetically frustrated compounds NaCaCo$_2$F$_7$ and NaSrCo$_2$F$_7$, which belong to a class of recently discovered pyrochlore compounds based on transition metals and fluorine. The magnetic pair distribution function (mPDF) technique is used to analyze and model the total scattering data in real space. We find that a previously-proposed model of short-range XY-like correlations with a length scale of 10-15 \AA, combined with nearest-neighbor collinear antiferromagnetic correlations, accurately describes the mPDF data at low temperature, confirming the magnetic ground state in these materials. This model is further verified by the polarized neutron scattering data. From an analysis of the temperature dependence of the mPDF and polarized neutron scattering data, we find that short-range correlations persist on the nearest-neighbor length scale up to 200 K, approximately two orders of magnitude higher than the spin freezing temperatures of these compounds. These results highlight the opportunity presented by these new pyrochlore compounds to study the effects of geometric frustration at relatively high temperatures, while also advancing the mPDF technique and providing a novel opportunity to investigate a genuinely short-range-ordered magnetic ground state directly in real space

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 (STM) and angle-resolved photoemission spectroscopy (ARPES), allowing for the characterization of the three-dimensional (3D) Dirac semimetal (TDS) behavior and the observation of the topological surface states. Landau levels (LL) 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 (SdH) quantum oscillations are observed for the first time.Comment: To be published in a special issue of APL Material

A large new family of filled skutterudites stabilized by electron count

Based on the interplay of theory and experiment, a large new family of filled group 9 (Co, Rh and Ir) skutterudites is designed and synthesized. The new materials fill the empty cages in the structures of the known binary CoSb3, RhSb3 and IrSb3 skutterudites with alkaline, alkaline earth, and rare earth atoms to create compounds of the type AyB4X12; A atoms fill the cages to a fraction y, B are the group 9 transition metals, and X is a mixture of electronegative main group elements chosen to achieve chemical stability by adjusting the electron counts to electron-precise values. Forty-three new compounds are reported, antimony-tin and phosphorous-silicon based, with 63 compositional variations presented. The new family of compounds is large and general. The results described here can be extended to the synthesis of hundreds of new group 9 filled skutterudites.Comment: A revised version with the title"A large family of filled skutterudites stabilized by electron count"will appear in Nature Communication

Crystal Chemistry and Phonon Heat Capacity in Quaternary Honeycomb Delafossites: Cu[Li_(1/3)Sn_(2/3)]O)2 and Cu[Na_(1/3)Sn_(2/3)]O_2

This work presents an integrated approach to study the crystal chemistry and phonon heat capacity of complex layered oxides. Two quaternary delafossites are synthesized from ternary parent compounds and copper monohalides via a topochemical exchange reaction that preserves the honeycomb ordering of the parent structures. For each compound, Rietveld refinement of the powder X-ray diffraction patterns is examined in both monoclinic C2/c and rhombohedral R3̅m space groups. Honeycomb ordering occurs only in the monoclinic space group. Bragg peaks associated with honeycomb ordering acquire an asymmetric broadening known as the Warren line shape that is commonly observed in layered structures with stacking disorder. Detailed TEM analysis confirms honeycomb ordering within each layer in both title compounds and establishes a twinning between the adjacent layers instead of the more conventional shifting or skipping stacking faults. The structural model is then used to calculate phonon dispersions and heat capacity from first principles. In both compounds, the calculated heat capacity accurately describes the experimental data. The integrated approach presented here offers a platform to carefully analyze the phonon heat capacity in complex oxides where the crystal structure can produce magnetic frustration. Isolating phonon contribution from total heat capacity is a necessary and challenging step toward a quantitative study of spin liquid materials with exotic magnetic excitations such as spinons and Majorana fermions. A quantitative understanding of phonon density of states based on crystal chemistry as presented here also paves the way toward higher efficiency thermoelectric materials