Novel spin-orbit coupled electronic states in Ir oxides

 

Jaejun Yu, Seoul National University

 

Recently we have witnessed evidences of strong correlation physics in 4d and 5d transition metal oxides (TMO). One of the examples is the manifestation of a novel jeff =1/2 Mott ground state in Sr2IrO4. The electron correlation combined with strong spin-orbit (SO) interactions under a large crystal field present in the 5d TMO is responsible for the observed peculiar electronic and magnetic properties. Based on LDA+SO+U calculations including both on-site Coulomb and spin-orbit interactions, we showed that an interesting interplay between these two interactions gives rise to an effective single and narrow band Hubbard model with jeff =1/2 at half-filling in Sr2IrO4, leading to a novel insulating ground state. Following up the extraordinary feature of Sr2IrO4, Na2IrO3 has been suggested to have possible quantum spin Hall effect arising from the novel jeff =1/2 state of 5d Ir atoms. Our tight-binding model analysis of the first-principles calculation results of Na2IrO3 reveals that the electronic states near the Fermi level have more of eg¡¯ character rather than that of jeff =1/2. The delocalized 5d orbitals lying in the edge sharing octahedron structure leads to (i) a significant direct hopping between neighboring Ir 5d states, (ii) a strong trigonal crystal field, and (iii) non-negligible next-nearest-neighbor and next-next-nearest-neighbor hopping. Based on a minimal model for the t2g orbital manifold integrated with spin space, we introduce a prototype model of spin-orbit integrated magnetism. The origin of anisotropic magnetic exchange interactions are discussed in connection with an extraordinary character of the ground state. The presence of spin-orbit integrated state with strong SO interactions can make 5d Ir-oxides a unique class of materials for the study of effective exchange interactions in the full spin-orbital Hilbert space.