The nature of the metal-insulator transition in 5d transition metal oxides

A number of 5d transition metal oxides (TMOs) either undergo, or lie proximate to, a metal-insulator transition (MIT). However these MITs frequently depart from a Mott-Hubbard picture, in which the interactions are dominated by the interplay between the on-site Coulomb repulsion and electronic bandwidth. In 5d TMOs the sizeable intrinsic spin-orbit coupling plays an important role, and gives rise to electronic and magnetic ground states -- at both sides of the MIT -- that cannot be adequately described within a purely L-S coupling scenario. In this thesis the aim is to understand the role of spin-orbit coupling in determining the electronic and magnetic properties of 5d TMOs. There has been a large amount of recent interest within this field (both experimentally and theoretically), however thus far has mostly been limited to the 5d5, j =1/2 limit. The perovskite iridates Sr2IrO4 and Sr3Ir2O7 lie within this limit. Theoretical predictions suggest a significant easy-plane anisotropy is present for the single layer Sr2IrO4. I show that this anisotropy can be observed and quantified, using magnetic critical scattering and previously published resonant inelastic X-ray scattering (RIXS) data. This differs from previous results that suggest purely 2D Heisenberg behaviour. Meanwhile the critical fluctuations in bilayer Sr3Ir2O7 have a three-dimensional nature, which can be directly related to the intra-bilayer coupling and significant anisotropy previously probed by RIXS. I also demonstrate that resonant X-ray scattering techniques can be successfully applied to other 5d systems, especially the d3 osmates. Both NaOsO3 and Cd2Os2O7 undergo MITs directly linked to the onset of antiferromagnetic order (Slater or Lifshitz mechanisms). The first ever high-resolution RIXS measurements at the Os L3 absorption edge reveal that there is a correlated evolution of the electronic and magnetic excitations through the respective MITs. The behaviour is consistent with a scenario in which the effect of spin-orbit coupling and electron correlations are reduced with respect to the iridates, yet still manifests through a strong spin wave anisotropy. Finally I show that the study of 5d TMOs can be extended into the time domain. Through the development of new instrumentation, the transient dynamics of photo-doped Sr2IrO4 were probed by time-resolved resonant (in)elastic X-ray scattering. The relevant time scales can be directly compared to the interaction strengths and anisotropies in the undoped state. Moreover, there seems to be an effective mapping of the transient behaviour in the photo-doped state to an equivalent level of bulk electron doping in (Sr_{1-x}La_x)2IrO4

Putative magnetic quantum criticality in (Sr1−xLax)3Ir2O7

(Sr1−x Lax)3Ir2O7 undergoes a bulk insulator-to-metal transition at x ≈ 0.04. Through careful analysis of previously published data (x = 0.053, 0.061, 0.076), we find an extended region below the Debye temperature in which the resistivity appears to scale linearly with temperature. Meanwhile, resonant (in)elastic x-ray scattering data (x = 0.065) suggest a possible crossover from quantum paramagnetic to quantum critical phenomenology between 100 and 200 K. We put this into context with other results, and propose a possible phase diagram as a function of doping

Towards an early warning system for Rhodesian sleeping sickness in savannah areas: man-like traps for tsetse flies

Background: In the savannahs of East and Southern Africa, tsetse flies (Glossina spp.) transmit Trypanosoma brucei rhodesiense which causes Rhodesian sleeping sickness, the zoonotic form of human African trypanosomiasis. The flies feed mainly on wild and domestic animals and are usually repelled by humans. However, this innate aversion to humans can be undermined by environmental stresses on tsetse populations, so increasing disease risk. To monitor changes in risk, we need traps designed specifically to quantify the responsiveness of savannah tsetse to humans, but the traps currently available are designed to simulate other hosts. Methodology/Principal Findings: In Zimbabwe, two approaches were made towards developing a man-like trap for savannah tsetse: either modifying an ox-like trap or creating new designs. Tsetse catches from a standard ox-like trap used with and without artificial ox odor were reduced by two men standing nearby, by an average of 34% for Glossina morsitans morsitans and 56% for G. pallidipes, thus giving catches more like those made by hand-nets from men. Sampling by electrocuting devices suggested that the men stopped flies arriving near the trap and discouraged trap-entering responses. Most of human repellence was olfactory, as evidenced by the reduction in catches when the trap was used with the odor of hidden men. Geranyl acetone, known to occur in human odor, and dispensed at 0.2 mg/h, was about as repellent as human odor but not as powerfully repellent as wood smoke. New traps looking and smelling like men gave catches like those from men. Conclusion/Significance: Catches from the completely new man-like traps seem too small to give reliable indices of human repellence. Better indications would be provided by comparing the catches of an ox-like trap either with or without artificial human odor. The chemistry and practical applications of the repellence of human odor and smoke deserve further study

All-in all-out magnetic order and propagating spin-waves in Sm2Ir2O7

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 Dzyaloshinskii- Moriya 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 preserv- ing AIAO magnetic groundstate, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals

Correlated electron metal properties of the honeycomb ruthenate Na₂RuO₃

We report the synthesis and characterization of polycrystalline Na_{2}RuO_{3}, a layered material in which the Ru^{4+} (4d^{4} configuration) form a honeycomb lattice. The optimal synthesis condition was found to produce a nearly ordered Na_{2}RuO_{3} (C2/c phase), as assessed from the refinement of the time-of-flight neutron powder diffraction. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetism [x_{0} ~ 1.42(2) x 10^{-3} emu/mol Oe] with no evidence of magnetic ordering down to 1.5 K, and with an absence of dynamic magnetic correlations, as evidenced by neutron scattering spectroscopy. The intrinsic susceptibility (x_{0}) together with the Sommerfeld coefficient of gamma = 11.7(2) mJ/Ru mol K^{2} estimated from heat capacity measurements gives an enhanced Wilson ratio of R_{w} ≈ 8.9(1), suggesting that magnetic correlations may be present in this material. While transport measurements on pressed pellets show nonmetallic behavior, photoemission spectroscopy indicates a small but finite density of states at the Fermi energy, suggesting that the bulk material is metallic. Except for resistivity measurements, which may have been compromised by near-surface and interface effects, all other probes indicate that Na_{2}RuO_{3} is a moderately correlated electron metal. Our results thus stand in contrast to earlier reports that Na_{2}RuO_{3} is an antiferromagnetic insulator at low temperatures

Critical fluctuations in the spin-orbit Mott Insulator Sr₃Ir₂O₇

X-ray magnetic critical scattering measurements and specific heat measurements were performed on the perovskite iridate Sr₃Ir₂O₇. We find that the magnetic interactions close to the N´eel temperature T_{N} = 283.4(2) K are threedimensional. This contrasts with previous studies which suggest two-dimensional behaviour like Sr₃IrO₄. Violation of the Harris criterion (dν > 2) means that weak disorder becomes relevant. This leads a rounding of the antiferromagnetic phase transition at T_{N}, and modifies the critical exponents relative to the clean system. Specifically, we determine that the critical behaviour of Sr₃Ir₂O₇ is representative of the diluted 3D Ising universality class

Magnetically induced metal-insulator transition in Pb2CaOsO6

We report on the structural, magnetic, and electronic properties of two new double-perovskites synthesized under high pressure, Pb2CaOsO6 and Pb2ZnOsO6. Upon cooling below 80 K, Pb2CaOsO6 simultaneously undergoes a metal-to-insulator transition and develops antiferromagnetic order. Pb2ZnOsO6, on the other hand, remains a paramagnetic metal down to 2 K. The key difference between the two compounds lies in their crystal structures. The Os atoms in Pb2ZnOsO6 are arranged on an approximately face-centered cubic lattice with strong antiferromagnetic nearest-neighbor exchange couplings. The geometrical frustration inherent to this lattice prevents magnetic order from forming down to the lowest temperatures. In contrast, the unit cell of Pb2CaOsO6 is heavily distorted up to at least 500 K including antiferroelectriclike displacements of the Pb and O atoms despite metallic conductivity above 80 K. This distortion relieves the magnetic frustration, facilitating magnetic order which, in turn, drives the metal-insulator transition. Our results suggest that the phase transition in Pb2CaOsO6 is spin driven and could be a rare example of a Slater transition

Strain control of a bandwidth-driven spin reorientation in Ca₃Ru₂O₇

The layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca3Ru2O7, which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a 90∘ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca3Ru2O7, using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO6 octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them

Left-handed color-sextet diquark in Kaon system

We investigate whether a color-sextet scalar diquark (${\bf H}_6$) coupling to the left-handed quarks contributes to the $\Delta S=2$ process. It is found that the box diagrams mediated by $W$ and ${\bf H}_6$ bosons have no contributions to $\Delta S=2$ when the limit of $m_t=0$ is used, and the flavor mixing matrices for diagonalizing quark mass matrices are introduced at the same time. When the heavy top-quark mass effects are taken into account, it is found that in addition to the $W-{\bf H}_6$ box diagrams significantly contributing to $\Delta S=2$, their effects can be as large as those from the ${\bf H}_6-{\bf H}_6$ box diagrams. Using the parameters that are constrained by the $K^0-\bar K^0$ mixing parameter $\Delta M_K$ and the Kaon indirect CP violation $\epsilon_K$, we find that the left-handed color-sextet diquark can lead to the Kaon direct CP violation being $Re(\epsilon'/\epsilon) \sim 0.4 \times 10^{-3}$. In the chosen scheme, although the diquark contribution to $K_L\to \pi^0 \nu \bar\nu$ is small, the branching ratio of $K^+ \to \pi^+ \nu \bar\nu$ can reach the current experimental upper bound.Comment: 22 pages, 6 figure

Probing electron-phonon interactions away from the Fermi level with resonant inelastic x-ray scattering

Interactions between electrons and lattice vibrations are responsible for a wide range of material properties and applications. Recently, there has been considerable interest in the development of resonant inelastic x-ray scattering (RIXS) as a tool for measuring electron-phonon ( e -ph) interactions. Here, we demonstrate the ability of RIXS to probe the interaction between phonons and specific electronic states both near to, and away from, the Fermi level. We perform carbon K -edge RIXS measurements on graphite, tuning the incident x-ray energy to separately probe the interactions of the π ∗ and σ ∗ electronic states. Our high-resolution data reveal detailed structure in the multiphonon RIXS features that directly encodes the momentum dependence of the e -ph interaction strength. We develop a Green’s-function method to model this structure, which naturally accounts for the phonon and interaction-strength dispersions, as well as the mixing of phonon momenta in the intermediate state. This model shows that the differences between the spectra can be fully explained by contrasting trends of the e -ph interaction through the Brillouin zone, being concentrated at the Γ and K points for the π ∗ states while being significant at all momenta for the σ ∗ states. Our results advance the interpretation of phonon excitations in RIXS and extend its applicability as a probe of e -ph interactions to a new range of out-of-equilibrium situations