High-resolution resonant inelastic x-ray scattering measurements (Delta E = 46 meV) have been performed on Cd2Os2O7 through the metal-to-insulator transition (MIT). A magnetic excitation at 125 meV evolves continuously through the MIT, in agreement with recent Raman-scattering results, and provides further confirmation for an all-in all-out magnetic ground state. Asymmetry of this feature is likely a result of coupling between the electronic and the magnetic degrees of freedom. We also observe a broad continuum of interband excitations centered at 0.3 eV energy loss. This is indicative of significant hybridization between Os 5d and O 2p states and the concurrent itinerant nature of the system. In turn, this suggests a possible breakdown of the free-ion model for Cd2Os2O7.

J. G. Vale ,1,2,* S. Calder,3,† N. A. Bogdanov ,4,5 C. Donnerer,1 M. Moretti Sala,6,‡ N. R. Davies,7 D. Mandrus,8,9 J. van den Brink,4 A. D. Christianson,3,9,10 and D. F. McMorrow1 1London Centre for Nanotechnology, University College London (UCL), Gower Street, London WC1E 6BT, United Kingdom 2Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne (EPFL) CH-1015, Switzerland 3Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 4Institute for Theoretical Solid State Physics, IFW Dresden, D01171 Dresden, Germany 5Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany 6European Synchrotron Radiation Facility (ESRF), CS 40220, F-38043 Grenoble Cedex, France 7Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom 8Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA 9Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 10Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA

We study the magnetic structure of the 'stuffed' (Tb-rich) pyrochlore iridate Tb2+x Ir2-x O7-y  (x ∼ 0.18), using resonant elastic x-ray scattering (REXS). In order to disentangle contributions from Tb and Ir magnetic sublattices, experiments were performed at the Ir L 3 and Tb M 5 edges, which provide selective sensitivity to Ir 5d and Tb 4f  magnetic moments, respectively. At the Ir L 3 edge, we found the onset of long-range [Formula: see text] magnetic order below [Formula: see text] K, consistent with the expected signal of all-in all-out (AIAO) magnetic order. Using a single-ion model to calculate REXS cross-sections, we estimate an ordered magnetic moment of [Formula: see text] at 5 K. At the Tb M 5 edge, long-range [Formula: see text] magnetic order appeared below  ∼[Formula: see text] K, also consistent with an AIAO magnetic structure on the Tb site. Additional insight into the magnetism of the Tb sublattice is gleaned from measurements at the M 5 edge in applied magnetic fields up to 6 T, which is found to completely suppress the Tb AIAO magnetic order. In zero applied field, the observed gradual onset of the Tb sublattice magnetisation with temperature suggests that it is induced by the magnetic order on the Ir site. The persistence of AIAO magnetic order, despite the greatly reduced ordering temperature and moment size compared to stoichiometric Tb2Ir2O7, for which [Formula: see text] K and [Formula: see text], indicates that stuffing could be a viable means of tuning the strength of electronic correlations, thereby potentially offering a new strategy to achieve topologically non-trivial band crossings in pyrochlore iridates.

J. G. Vale, 2, ∗ S. Calder, † C. Donnerer, D. Pincini, 4 Y. G. Shi, 6 Y. Tsujimoto, K. Yamaura, 7 M. Moretti Sala, J. van den Brink, A. D. Christianson, 10 and D. F. McMorrow London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, United Kingdom Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Switzerland Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Graduate School of Chemical Sciences and Engineering, Hokkaido University, North 10 West 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan ESRF, The European Synchrotron, 71 Avenue des Martyrs, 38043 Grenoble, France Institute for Theoretical Solid State Physics, IFW Dresden, D01171 Dresden, Germany Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA

C. Donnerer,1 M. Moretti Sala,2 S. Pascarelli,2 A. D. Rosa,2 S. N. Andreev,3 V. V. Mazurenko,3 T. Irifune,4 E. C. Hunter,5,* R. S. Perry,1 and D. F. McMorrow1 1London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom 2ESRF–The European Synchrotron, 71 Avenue des Martyrs, 38000 Grenoble, France 3Theoretical Physics and Applied Mathematics Department, Ural Federal University, 620002 Ekaterinburg, Russia 4Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan 5SUPA, School of Physics and Astronomy, and Centre for Science at Extreme Conditions, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, United Kingdom

The temperature dependence of the excitation spectrum in NaOsO_{3} through its metal-to-insulator transition (MIT) at 410 K has been investigated using resonant inelastic x-ray scattering at the Os L_{3} edge. High-resolution (ΔE∼56  meV) measurements show that the well-defined, low-energy magnons in the insulating state weaken and dampen upon approaching the metallic state. Concomitantly, a broad continuum of excitations develops which is well described by the magnetic fluctuations of a nearly antiferromagnetic Fermi liquid. By revealing the continuous evolution of the magnetic quasiparticle spectrum as it changes its character from itinerant to localized, our results provide unprecedented insight into the nature of the MIT in NaOsO_{3} [J. G. Vale, S. Calder, C. Donnerer, D. Pincini, Y. G. Shi, Y. Tsujimoto, K. Yamaura, M. M. Sala, J. van den Brink, A. D. Christianson, and D. F. McMorrow, Phys. Rev. B 97, 184429 (2018)PRBMDO2469-995010.1103/PhysRevB.97.184429].

High-resolution resonant inelastic x-ray scattering (RIXS) measurements (ΔE = 46 meV) have been performed on Cd_2Os_2O_7 through the metal-to-insulator transition (MIT). A magnetic excitation at 125 meV evolves continuously through the MIT, in agreement with recent Raman scattering results, and provides further confirmation for an all-in, all-out magnetic ground state. Asymmetry of this feature is likely a result of coupling between the electronic and magnetic degrees of freedom. We also observe a broad continuum of interband excitations centered at 0.3 eV energy loss. This is indicative of significant hybridization between Os 5d and O 2p states, and concurrent itinerant nature of the system. In turn, this suggests a possible break down of the free-ion model for Cd_2Os_2O_7.

High-pressure x-ray absorption spectroscopy was performed at the Ir L-3 and L-2 absorption edges of Sr3Ir2O7. The branching ratio of white-line intensities continuously decreases with pressure, reflecting a reduction in the angular part of the expectation value of the spin-orbit coupling operator, (L . S). Up to the high-pressure structural transition at 53 GPa, this behavior can be explained within a single-ion model, where pressure increases the strength of the cubic crystal field, which suppresses the spin-orbit induced hybridization of J(eff) = 3/2 and e(g) levels. We observe a further reduction of the branching ratio above the structural transition, which cannot be explained within a single-ion model of spin-orbit coupling and cubic crystal fields. This change in (L . S) in the high-pressure, metallic phase of Sr3Ir2O7 could arise from noncubic crystal fields or a bandwidth-driven hybridization of J(eff) = 1/2, 3/2 states and suggests that the electronic ground state significantly deviates from the J(eff) = 1/2 limit.

NaOsO3 undergoes a metal-insulator transition (MIT) at 410 K, concomitant with the onset of antiferromagnetic order. The excitation spectra have been investigated through the MIT by resonant inelastic x-ray scattering (RIXS) at the Os L-3 edge. Low resolution (Delta E similar to 300 meV) measurements over a wide range of energies reveal that local electronic excitations do not change appreciably through the MIT. This is consistent with a picture in which structural distortions do not drive the MIT. In contrast, high resolution (Delta E similar to 56 meV) measurements show that the well-defined, low-energy magnons in the insulating state weaken and dampen upon approaching the metallic state. Concomitantly, a broad continuum of excitations develops which is well described by the magnetic fluctuations of a nearly antiferromagnetic Fermi liquid. By revealing the continuous evolution of the magnetic quasiparticle spectrum as it changes its character from itinerant to localized, our results provide unprecedented insight into the nature of the MIT in NaOsO3. In particular, the presence of weak correlations in the paramagnetic phase implies a degree of departure from the ideal Slater limit.

Resonant elastic x-ray scattering (REXS) at the Eu M(5 )edge reveals an antiferromagnetic structure in layered EuCd(2)Sb(2 )at temperatures below T-N = 7.4 K with a magnetic propagation vector of (0, 0, 1/2) and spins in the basal plane. Magnetotransport and REXS measurements with an in-plane magnetic field show that features in the magnetoresistance are correlated with changes in the magnetic structure induced by the field. Ab initio electronic structure calculations predict that the observed spin structure gives rise to a gapped Dirac point close to the Fermi level with a gap of Delta E similar to 0.01 eV. The results of this study indicate that the Eu spins are coupled to conduction electron states near the Dirac point.

C. Donnerer, M. Moretti Sala, S. Pascarelli, A. D. Rosa, S.N. Andreev, V. V. Mazurenko, T. Irifune, E. C. Hunter, ∗ R. S. Perry, and D. F. McMorrow London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom ESRF The European Synchrotron, 71 Avenue des Martyrs, 38000 Grenoble, France Theoretical Physics and Applied Mathematics Department, Ural Federal University, 620002 Ekaterinburg, Russia Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan SUPA, School of Physics and Astronomy, and Centre for Science at Extreme Conditions, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, United Kingdom

NaOsO3 hosts a rare manifestation of a metal-insulator transition driven by magnetic correlations, placing the magnetic exchange interactions in a central role. We use resonant inelastic x-ray scattering to directly probe these magnetic exchange interactions. A dispersive and strongly gapped (58 meV) excitation is observed, indicating appreciable spin-orbit coupling in this 5d(3) system. The excitation is well described within a minimal model Hamiltonian with strong anisotropy and Heisenberg exchange (J(1) = J(2) = 13.9 meV). The observed behavior places NaOsO3 on the boundary between localized and itinerant magnetism.

The collective magnetic excitations in the spin-orbit Mott insulator (Sr1-x La-x)(2)IrO4 (x = 0, 0.01, 0.04, 0.1) were investigated by means of resonant inelastic x-ray scattering. We report significant magnon energy gaps at both the crystallographic and antiferromagnetic zone centers at all doping levels, along with a remarkably pronounced momentum-dependent lifetime broadening. The spin-wave gap is accounted for by a significant anisotropy in the interactions between J(eff) = 1/2 isospins, thus marking the departure of Sr2IrO4 from the essentially isotropic Heisenberg model appropriate for the superconducting cuprates.

We study the structural evolution of Sr3Ir2O7 as a function of pressure using x-ray diffraction. At a pressure of 54 GPa at room temperature, we observe a first-order structural phase transition, associated with a change from tetragonal to monoclinic symmetry and accompanied by a 4% volume collapse. Rietveld refinement of the high-pressure phase reveals a novel modification of the Ruddlesden-Popper structure, which adopts an altered stacking sequence of the perovskite bilayers. As the positions of the oxygen atoms could not be reliably refined from the data, we use density functional theory (local-density approximation+U+spin orbit) to optimize the crystal structure and to elucidate the electronic and magnetic properties of Sr3Ir2O7 at high pressure. In the low-pressure tetragonal phase, we find that the in-plane rotation of the IrO6 octahedra increases with pressure. The calculations further indicate that a bandwidth-driven insulator-metal transition occurs at similar to 20 GPa, along with a quenching of the magnetic moment. In the high-pressure monoclinic phase, structural optimization resulted in complex tilting and rotation of the oxygen octahedra and strongly overlapping t(2g) and e(g) bands. The t(2g) bandwidth renders both the spin-orbit coupling and electronic correlations ineffectual in opening an electronic gap, resulting in a robust metallic state for the high-pressure phase of Sr3Ir2O7.

C. Donnerer,1 Z. Feng,1 J. G. Vale,1,2 S. N. Andreev,3 I. V. Solovyev,3,4 E. C. Hunter,5 M. Hanfland,6 R. S. Perry,1 H. M. Rønnow,2 M. I. McMahon,5,7 V. V. Mazurenko,3 and D. F. McMorrow1 1London Centre for Nanotechnology, University College London, London WC1H 0AH, United Kingdom 2Laboratory for Quantum Magnetism, Ecole Polytechnique Federal de Lausanne, 1015 Lausanne, Switzerland 3Theoretical Physics and Applied Mathematics Department, Ural Federal University, 620002 Ekaterinburg, Russia 4Computational Materials Science Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 5SUPA, School of Physics and Astronomy, and Centre for Science at Extreme Conditions, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, United Kingdom 6European Synchrotron Radiation Facility, CS 40220, 38043 Grenoble Cedex 9, France 7Research Complex at Harwell, Didcot, Oxon OX11 0FA, United Kingdom (Received 8 July 2015; revised manuscript received 20 April 2016; published 31 May 2016)

Using resonant magnetic x-ray scattering we address the unresolved nature of the magnetic groundstate and the low-energy effective Hamiltonian of Sm$_2$Ir$_2$O$_7$, 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 Dzyaloshinskii-Moriya interaction ($D=4.9(3)$ meV), which provides a consistent description of the magnetic order and excitations. In establishing that Sm$_2$Ir$_2$O$_7$ has the requisite inversion symmetry preserving AIAO magnetic groundstate, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals.

Much consideration has been given to the role of spin-orbit coupling (SOC) in 5d oxides, particularly on the formation of novel electronic states and manifested metal-insulator transitions (MITs). SOC plays a dominant role in 5d(5) iridates (Ir4+), undergoing MITs both concurrent (pyrochlores) and separated (perovskites) from the onset of magnetic order. However, the role of SOC for other 5d configurations is less clear. For example, 5d(3) (Os5+) systems are expected to have an orbital singlet with reduced effective SOC. The pyrochlore Cd2Os2O7 nonetheless exhibits a MIT entwined with magnetic order phenomenologically similar to pyrochlore iridates. Here, we resolve the magnetic structure in Cd2Os2O7 with neutron diffraction and then via resonant inelastic X-ray scattering determine the salient electronic and magnetic energy scales controlling the MIT. In particular, SOC plays a subtle role in creating the electronic ground state but drives the magnetic order and emergence of a multiple spin-flip magnetic excitation.

High-pressure x-ray diffraction experiments reveal a first-order, tetragonal to monoclinic structural phase transition in the spin-orbit Mott insulator Sr3Ir2O7 at 54 GPa. The high-pressure phase adopts an altered stacking sequence of the two Ir-O bilayers of Sr3Ir2O7. Density functional calculations establish that the 4% volume collapse of the high-pressure structure is concomitant with a transition from insulating to metallic states. They further show that the orbital magnetic moment is quenched in the high-pressure phase with the consequence that the spin-orbit coupling is effectively switched off, and it no longer significantly alters the electronic structure