. This discovery not only confirms TaAs as a 3D TWS, but also provides an ideal platform for realizing exotic physical phenomena (for example, negative magnetoresistance, chiral magnetic e ects and the quantum anomalous Hall e ect) which may also lead to novel future applications.

We have performed powder inelastic neutron scattering measurements on the unconventional superconductor β-FeSe (T c 8 K). The spectra reveal highly dispersive paramagnetic fluctuations emerging from the squarelattice wave vector (π,0) extending beyond 80 meV in energy. Measurements as a function of temperature at an energy of ∼13 meV did not show any variation from T c to 104 K. The results show that FeSe is close to an instability towards (π,0) antiferromagnetism that is characteristic of the parent phases of the high-T c iron arsenide superconductors, and that the iron paramagnetic moment is neither affected by the orthorhombic-to-tetragonal structural transition at T s 90 K nor does it undergo a change in spin state over the temperature range studied.

We report theoretical and experimental evidence that EuCd2As2 in magnetic fields greater than 1.6 T applied along the c axis is a Weyl semimetal with a single pair of Weyl nodes. Ab initio electronic structure calculations, verified at zero field by angle-resolved photoemission spectra, predict Weyl nodes with wavevectors k = (0, 0, ±0.03) × 2π/c at the Fermi level when the Eu spins are fully aligned along the c axis. Shubnikov-de Haas oscillations measured in fields parallel to c reveal a cyclotron effective mass of m * c = 0.08 me and a Fermi surface of extremal area Aext = 0.24 nm −2 , corresponding to 0.1% of the area of the Brillouin zone. The small values of m * c and Aext are consistent with quasiparticles near a Weyl node. The identification of EuCd2As2 as a model Weyl semimetal opens the door to fundamental tests of Weyl physics.

We use resonant elastic x-ray scattering to determine the evolution of magnetic order in EuCd2As2 below TN = 9.5 K, as a function of temperature and applied magnetic field. We find an A-type antiferromagneticstructure with in-plane magnetic moments, and observe dramatic magnetoresistive effects associated with field-induced changes in the magnetic structure and domain populations. Our ab initio electronic structure calculations indicate that the Dirac dispersion found in the nonmagnetic Dirac semimetal Cd3As2 is also present in EuCd2As2, but is gapped for T < TN due to the breaking of C3 symmetry by the magnetic structure.

Using resonant magnetic x-ray scattering we address the unresolved nature of the magnetic groundstate and the low-energy effective Hamiltonian of Sm2Ir2O7, a prototypical pyrochlore iridate with a finite temperature metal-insulator transition. Through a combination of elastic and inelastic measurements, we show that the magnetic ground state is an all-in all-out (AIAO) antiferromagnet. The magnon dispersion indicates significant electronic correlations and can be well-described by a minimal Hamiltonian that includes Heisenberg exchange (J = 27.3(6) meV) and DzyaloshinskiiMoriya interaction (D = 4.9(3) meV), which provides a consistent description of the magnetic order and excitations. In establishing that Sm2Ir2O7 has the requisite inversion symmetry preserving AIAO magnetic groundstate, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals.The search for novel electronic and magnetic phenomena has recently been fruitful in the correlated, strong spin-orbit coupling regime [1][2][3][4]. The family of pyrochlore iridates, R 2 Ir 2 O 7 (where R is a rare-earth element), has received much interest since the prediction of topologically non-trivial states, most prominently the Weyl semimetal (WSM) [5][6][7][8]. This is motivated by the observation of metal-insulator transitions as a function of temperature and rare-earth ion radius that occur concomitantly with the onset of magnetic order [9][10][11][12]. As magnetic order breaks time-reversal symmetry, the WSM state in these correlated materials requires the preservation of inversion symmetry, a scenario distinct from the weakly correlated limit where the opposite is true. Theoretical proposals for the magnetic order with the required symmetries in pyrochlore iridates have focused on the antiferromagnetic all-in all-out (AIAO) structure, where the moments either all point towards or away from the center of the corner shared tetrahedra which form the iridium sublattice. The R 2 Ir 2 O 7 system thus offers an outstanding opportunity to study novel topological phases in the presence of electronic correlations.Despite substantial experimental effort, however, the nature of the magnetic order of the Ir ions and the effective spin Hamiltonian have remained elusive in pyrochlore iridates [13][14][15][16][17][18][19][20]. Resonant elastic x-ray scattering at the Ir L 3 edge of Eu 2 Ir 2 O 7 has found k = 0 magnetic order of undetermined type [17]. Due to the small magnetic moment of the Ir ion and its high neutron absorption, neutron diffraction has only been successful in studying the rare-earth sublattice. For R = Tb and Nd rare-earths, AIAO magnetic order was found, which has been argued to provide indirect evidence for identical ordering on the Ir lattice [14,19]. An upper limit on the size of the ordered Ir moment was placed at 0.2 µ B (Tb) [19] and 0.5 µ B (Y) [15].Here, we use resonant elastic and inelastic x-ray scattering (REXS and RIXS) at the Ir L 3 edge to reveal the nature of the magnetic order and excitations of the pyroch...