Year of Publication

2006

Degree Name

Doctor of Philosophy (PhD)

Document Type

Dissertation

College

Arts and Sciences

Department

Physics and Astronomy

First Advisor

Dr. Gang Cao

Abstract

In the 4d transition metal oxides, the extension of the 4d orbitals leads to comparable and thus competitive kinetic and coulomb energies. As a result, small perturbations can induce significant changes in their physical properties, giving rise to a class of exotic phenomena that are rarely found in other materials. The ruthenates materials with readily tunable parameters open an avenue to study the strong electronic correlation in the rarely explored territory: the 4d transition metal oxides.

The bilayered system, Ca3Ru2O7, belongs to the Ruddlesden-Popper series in which the physical properties are intimately linked to the lattice degrees of freedom. Ca3Ru2O7, with its quasi-2D and severe structure distortion, is believed to be placed in a unique position at which the role of orbital degrees of freedom is highlighted. The system displays strikingly different behaviors when the field is applied along different crystalline axes. A ferromagnetic (FM) state with full spin polarization is achieved for B||a-axis, but colossal magnetoresistance is realized only for B||b-axis by avoiding the ferromagnetic state. In addition, for B rotating within the ac-plane, slow and strong SdH oscillations periodic in 1/B are observed for T.1.5 K in the presence of metamagnetism. For B|| [110], oscillations are also observed but periodic in B (rather than 1/B) and persist up to 15 K. These properties together with highly unusual spin-charge-lattice coupling near the Mott transition (48 K) are driven by the orbital degrees of freedom.

Complex thermodynamic properties are also observed in the other ruthenates system such as Sr4Ru3O10 and Pr3RuO7. The Sr4Ru3O10 is a triple-layered system that shows a dedicate balance between fluctuations and order. Besides the anomaly at TC=102K, anomalous behavior at low temperatures are also observed in the thermal study, indicative of an unusual magnetic order in this material. The Pr3RuO7 shows one-dimensional structure with zig-zag chain of corner sharing RuO6 octahedra running in parallel with the rows of edge-shared PrO8 pseudo-cubes. Magnetic and thermal properties studies on its single crystals indicate that the exchange interaction is strongly anisotropic. A Schottky-type anomaly at low temperature suggests that the gorderedh chain Pr ions are still sensitive to a crystal field.

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