A Systematic Transport and Thermodynamic Study of Heavy Transition Metal Oxides with Hexagonal Structure

There is no apparent, dominant interaction in heavy transition metal oxides (TMO), especially in 5d-TMO, where all relevant interactions are of comparable energy scales, and therefore strongly compete. In particular, the spin-orbit interaction (SOI) strongly competes with the electron-lattice and on-site Coulomb interaction (U). Therefore, any tool that allows one to tune the relative strengths of SOI and U is expected to offer an opportunity for the discovery and study of novel materials. BaIrO3 is a magnetic insulator driven by SOI whereas the isostructural BaRuO3 is a paramagnetic metal. The contrasting ground states have been shown to result from the critical role of the strong SOI in the iridate. This dissertation thoroughly examines a wide array of newly observed novel phenomena induced by adjusting the relative strengths of SOI and U via a systematic chemical substitution of the Ru4+(4d4) ions for Ir4+(5d5) ions in BaIrO3, i.e., in high quality single crystals of BaIr1-xRuxO3(0.0 \u3c x \u3c 1.0) . Our investigation of structural, magnetic, transport and thermal properties reveals that Ru substitution directly rebalances the competing energies so profoundly that it generates a rich phase diagram for BaIr1-xRuxO3 featuring two major effects: (1) Light Ru doping (0 \u3c x \u3c 0.15) prompts a simultaneous and precipitous drop in both the magnetic ordering temperature TC and the electrical resistivity, which exhibits metal-insulator transition at around TC. (2) Heavier Ru doping (0.41 \u3c x \u3c 0.82) induces a robust metallic and spin frustration state. For comparison and contrast, we also substituted Rh4+(4d5) ions for Ir4+(5d5) ions in BaIrO3, i.e. BaIr1-xRhxO3(0.0 \u3c x \u3c 0.10), where Rh only reduces the SOI, but without altering the band filling. Hence, this system remains tuned at the Mott instability and is very susceptible to disorder scattering which gives rise to Anderson localization

Ground-State Tuning of Metal-Insulator Transition by Compositional Variations in BaIr\u3csub\u3e1−\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3eRu\u3csub\u3e\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e (0 ≤ \u3cem\u3ex\u3c/em\u3e ≤ 1)

Hexagonal BaIrO3 is a magnetic insulator driven by the spin-orbit interaction (SOI), whereas BaRuO3 is an enhanced paramagnetic metal. Our investigation of structural, magnetic, transport, and thermal properties reveals that substitution of Ru4+ (4d4) ions for Ir4+ (5d5) ions in BaIrO3 reduces the magnitudes of the SOI and a monoclinic structural distortion and rebalances the competition between the SOI and the lattice degrees of freedom to render an evolution from a magnetic insulting state to a robust metallic state. The central findings of this paper are as follows: (1) light Ru doping (0 \u3c x ≤ 0.15) prompts simultaneous, precipitous drops in both the magnetic ordering temperature TN and the electrical resistivity, and (2) heavier Ru doping (0.41 ≤ x ≤ 0.9) induces a robust metallic state without any long-range magnetic order. All results suggest a critical role of the lattice degrees of freedom in determining the ground state in the heavy transition-metal oxides

Sr\u3csub\u3e2\u3c/sub\u3eIr\u3csub\u3e1−\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3eRh\u3csub\u3e\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e(x\u3c0.5): An Inhomogeneous \u3cem\u3ej\u3c/em\u3e\u3csub\u3eeff\u3c/sub\u3e=1/2 Hubbard system

In a combined experimental and theoretical study, we investigate the properties of Sr2Ir1−xRhxO4. From the branching ratios of the L-edge isotropic x-ray absorption spectra, we determine that the spin-orbit coupling is remarkably independent of x for both iridium and rhodium sites. DFT+U calculations show that the doping is close to isoelectronic and introduces impurity bands of predominantly rhodium character close to the lower Hubbard band. Overlap of these two bands leads to metallic behavior. Since the low-energy states for xjeff=1/2 character, we suggest that the electronic properties of this material can be described by an inhomogeneous Hubbard model, where the on-site energies change due to local variations in the spin-orbit interaction strength combined with additional changes in binding energy