Interplay of spin, lattice and orbitals in the 5d oxides: microscopic insight from first principles approach

5d based transition metal based oxides where both correlation and spin-orbit coupling are competing, forms a new emerging field of research with huge promise for realizing different novel phase of matter such as Spin liquid, Topological phases etc. These various novel phenomena arise due to interplay of charge, lattice, orbital and spin degrees of freedom, which are governed by the “electrons”. Therefore, understanding of underlying electronic structure behavior from ab-initio perspective is indispensable for primarily to understand the ground state properties and then manipulating the properties in the known materials as well as for the search of new potential candidates. This presentation is devoted to the study of microscopic origin of many exciting and intriguing physical properties of different complex and novel oxides materials, employing density functional theory (DFT) based first principles (ab-initio) electronic structure calculations. This talk mainly focused on the Os and Ir based oxides. We have studied the complex low temperature magnetism of recently synthesized Os based double-perovskite. We found that at low temperature Sr2FeOsO6 and Sr2CoOsO6 show unconventional magnetic phase transitions. Our calculated magnetic exchange interactions [1,2] revealed that strong magnetic frustration causes lattice instability driven magnetic phase transition and site specific exotic spin dynamics at low temperature, as was observed in experiments. Using first principles calculations, we settle down the ambiguity of the non-monotonic trend of experimentally observed transition temperatures for the recently synthesized Os based double-perovskite [3]. Orbital ordering mainly found for the 3d systems. Recently we have predicted first example of orbital ordering in the Ir based double-perovskite by DFT calculations [4]. We found that Jahn-Teller structural distortion dominant over the other energy scales and drive orbital ordering, which is in accordance with the known experimental reports. We also explored the microscopic picture of the stability of only known example of square-planar coordination of Ir in Na4IrO4 with novel S=3/2 state for half-filled situation. Calculation explore that the structure is stabilized due to the competition between electronic correlation vs. crystal field splitting in the week correlation limit applicable for 5d-Ir. By tuning correlation we can drive whole phase diagram and predict unknown compounds of this series, which show combined magneto-structural phase transition [5]. Our investigation revels the fact that different energy scales in the solids, such as electronic correlation, spin-orbit coupling and crystal field splitting are competing and necessary to understand the physical properties of these 5d oxides in general. -------------------------------------------------------------------------------------------- [1] Phys. Rev. Lett. 112, 147202 (2014), [2] Phys. Rev. B 89, 214414 (2014), [3] Phys. Rev. B (under review) [4] Phys. Rev. Lett. (under review) [5] Angew. Chem. Int. Ed. 54, 5417 (2015).