The 4 d - and 5 d -transition metal oxides exhibit nearly every collective state known for solids. It is commonly expected that 4d -based ruthenates and 5d -based iridates are more metallic and less magnetic than their 3 d and 4 f counterparts because the extended nature of electron orbitals leads to a broad 4d - and 5d -bandwidth and a reduced Coulomb interaction such that the Stoner criterion anticipates a metallic, paramagnetic state. In marked contrast, many layered ruthenates and iridates are magnetic insulators with a wide range of exotic properties. This talk reviews underlying properties of these materials with an emphasis on a few intriguing issues: (1) an unusual colossal magnetoresistance and a spin-valve effect existing in bulk single crystals of bilayered Ca3Ru2O7 [1-3], and (2) a novel Jeff = ½ Mott insulator, Sr2IrO4, and its unstable ground state that readily swings between highly insulating and metallic state via only slight perturbations [4-7] . These novel phenomena bring to light the importance of spin-orbit coupling that rigorously competes with other energies, thus setting a new balance between various degrees of freedom in this class of materials.
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