Interplay between spin-orbit coupling and correlation in transition metal materials

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Transition metal compounds may offer multiple routes to new devices in a wide range of green technologies such as solid oxide fuel cells and lossless power transmission. The promise for these opportunities come from a variety of coexisting or interconnected phenomena that these materials often possess. In particular, transition metal compounds exhibit novel phenomena including multiferroicity, high temperature superconductivity, antiferromagnetism, and ferroelectricity. Despite the potential usefulness and ubiquity, the fundamental mechanisms behind these novel properties are not well understood. Theoretical studies on these systems have so far concentrated on strong interactions between electrons due to the localized nature of d-orbitals of transition metal elements, but did not treat other degrees of freedom such as spin-orbit coupling on an equal footing. For instance, while spin-orbit coupling has been recognized as an essential ingredient in spintronics, especially in semi-conductor materials, the corresponding effect in transition metal compounds has been largely ignored or treated perturbatively. Even though this may be a reasonable practice for 3d-shell transition metal systems, as the mass of transition metal elements become heavier (for example, 5d systems such as Re-, Os-, and Ir-based oxides), the spin-orbit coupling strength increases drastically. Thus, the diversity of collective phenomena discovered in these systems calls for more systematic theoretical studies as to how the interplay between spin-orbit coupling and correlations plays a role in the emergence of novel phases and its more practical applications. The goal of this proposal is to achieve a theoretical understanding of how the combination of spin-orbit coupling and electron-electron interactions determines the collective properties of transition metal compounds. The proposed research will advance our knowledge of fundamental properties of quantum materials and suggest directions to the discovery of new phases of matter. It will also open a new avenue for designing and developing advanced materials to serve in a full array of device applications.