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...

The collective magnetic excitations in the spin-orbit Mott insulator (Sr 1−x La x ) 2 IrO 4 (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 Sr 2 IrO 4 from the essentially isotropic Heisenberg model appropriate for the superconducting cuprates.

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 (J1=J2=13.9 meV). The observed behavior places NaOsO3 on the boundary between localized and itinerant magnetism.The underlying mechanisms driving a metal-insulator transition (MIT) is an enduring focus of condensed matter physics [1]. Recent interest has extended investigations to 5d-based transition metal oxides that host new paradigms of competing interactions creating novel MITs [2]. For example spin-orbit coupling (SOC) in 5d 5 iridates dramatically influences the electronic ground state to allow even the presence of the reduced on-site Coulomb interaction (U ) to drive a relativistic Mott MIT [3]. Conversely, in 5d 3 osmium-based compounds MITs occur that cannot be reconciled with the reduced U in 5d systems, even when the large SOC is taken into account. These compounds therefore fall outside the Mott approximation. Of particular interest in this regard are the osmates NaOsO 3 and Cd 2 Os 2 O 7 which undergo a MIT that is continuous and coincident with the onset of magnetic order indicating the central role of magnetic interactions in the transition [4][5][6][7]. Accessing the collective role of the competing inter and intra-ion electron-electron interactions, SOC and magnetism in driving the MIT is required to gain an understanding of the novel MITs in these osmates.Outside of a Mott MIT several other mechanism exist to describe the transition, including an Anderson MIT driven by disorder [8] and a Peirels MIT driven by a structural distortion in a low dimensional system [9].Slater considered a route in which magnetism could drive a MIT with the central observation being that within a magnetically ordered system the potential created by an up-spin is different from that created by a down-spin [10]. By this definition three-dimensional magnetic ordering with oppositely aligned spins is a route to a MIT, and implicitly includes q=0 antiferromagnetic structures. NaOsO 3 exhibits several features consistent with Slater's general scenario [6,7,[11][12][13][14][15]. The MIT occurs concomitant with the onset of antiferromagnetic ordering (T N = T MIT = 410 K) that can create a periodic potential. Furthermore in NaOsO 3 the MIT is continuous and no structural symmetry change occurs. However several important questions have so far remained experimentally inaccessible hindering the development of further insight into the mechanism of this unusual MIT and prohibiting a quantitative description beyond the mean-field approach invoked by Slater for a magnetic MIT.Principally, since the MIT is driven by magnetic ordering the m...

The chemical and magnetic structures of the series of compounds Ca 2−x La x RuO 4 [x = 0, 0.05(1), 0.07(1), 0.12(1)] have been investigated using neutron diffraction and resonant elastic x-ray scattering. Upon La doping, the low-temperature S-P bca space group of the parent compound is retained in all insulating samples [x 0.07(1)], but with significant changes to the atomic positions within the unit cell. These changes can be characterized in terms of the local RuO 6 octahedral coordination: with increasing doping, the structure, crudely speaking, evolves from an orthorhombic unit cell with compressed octahedra to a quasitetragonal unit cell with elongated ones. The magnetic structure on the other hand, is found to be robust, with the basic k = (0,0,0), b-axis antiferromagnetic order of the parent compound preserved below the critical La doping concentration of x ≈ 0.11. The only effects of La doping on the magnetic structure are to suppress the A-centred mode, favoring the B mode instead, and to reduce the Néel temperature somewhat. Our results are discussed with reference to previous experimental reports on the effects of cation substitution on the d 4 Mott insulator Ca 2 RuO 4 , as well as with regard to theoretical studies on the evolution of its electronic and magnetic structure. In particular, our results rule out the presence of a proposed ferromagnetic phase, and suggest that the structural effects associated with La substitution play an important role in the physics of the system.

We observe and explain theoretically a dramatic evolution of the Dzyaloshinskii-Moriya interaction (DMI) in the series of isostructural weak ferromagnets, MnCO_{3}, FeBO_{3}, CoCO_{3}, and NiCO_{3}. The sign of the interaction is encoded in the phase of the x-ray magnetic diffraction amplitude, observed through interference with resonant quadrupole scattering. We find very good quantitative agreement with first-principles electronic structure calculations, reproducing both sign and magnitude through the series, and propose a simplified "toy model" to explain the change in sign with 3d shell filling. The model gives insight into the evolution of the DMI in Mott and charge transfer insulators.

NaOsO 3 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 ( E ∼ 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 ( E ∼ 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 . In particular, the presence of weak correlations in the paramagnetic phase implies a degree of departure from the ideal Slater limit.

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].

The ground-state orbital occupancy of the Ru 4+ ion in Ca2−xLaxRuO4 [x = 0, 0.05(1), 0.07(1) and 0.12(1)] was investigated by performing X-ray absorption spectroscopy (XAS) in the vicinity of the O K edge as a function of angle between the incident beam and the surface of the single-crystal samples. A minimal model of the hybridization between the O 2p states probed at the K edge and the Ru 4d orbitals was used to analyze the XAS data, allowing the ratio of hole occupancies nxy/nyz,zx to be determined as a function of doping and temperature. For the samples displaying a low-temperature insulating ground-state (x ≤ 0.07), nxy/nyz,zx is found to increase significantly with increasing doping, with increasing temperature acting to further enhance nxy/nyz,zx. For the x = 0.12 sample, which has a metallic ground-state, the XAS spectra are found to be independent of temperature, and not to be describable by the minimal hybridization model, while being qualitatively similar to the spectra displayed by the x ≤ 0.07 samples above their insulating to metallic transitions. To understand the origin of the evolution of the electronic structure of Ca2−xLaxRuO4 across its phase diagram, we have performed theoretical calculations based on a model Hamiltonian, comprising electron-electron correlations, crystal field (∆) and spin-orbit coupling (λ), of a Ru-O-Ru cluster, with realistic values used to parameterize the various interactions taken from the literature. Our calculations of the Ru hole occupancy as a function of ∆/λ provide an excellent description of the general trends displayed by the data. In particular they establish that the enhancement of nxy/nyz,zx is driven by significant modifications to the crystal field as the tetragonal distortion of the RuO6 octahedral changes from compressive to tensile with La doping. We have also used our model to show that the hole occupancy of the O 2p and Ru 4d orbitals display the same general trend as a function of ∆/λ, thus validating the minimal hybridization model used to analyze the data. In essence, our results suggest that the predominant mechanism driving the emergence of the low-temperature metallic phase in La doped Ca2RuO4 is the structurally induced redistribution of holes within the t2g orbitals, rather that the injection of free carriers.