Topological quantum phases in complex functional materials

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Our research program centres on understanding the basic physics of materials with complex structures and interesting functional properties. In particular, we will focus on finding novel topological quantum phases in materials containing second or third row transition metal elements such as iridium. These materials are characterized by their strong spin-orbit coupling, which arises from the relativistic interaction between the electron’s intrinsic magnetic moment and its motion. The best-known example of a spin-orbit coupling driven topological phase is a topological insulator. This material’s surface can conduct electricity while the interior of the sample remains insulating. Such electronic properties of topological phases are expected to be extremely robust against defects and disorder, making them useful for applications in spin-based electronics and fault-tolerant quantum computing. Even more exciting physics can be found when the spin-orbit coupling energy scale is balanced against the repulsive Coulomb energy between electrons. Iridium based materials turn out to be an excellent physical example in which these two energy scales compete. In fact, a plethora of exotic quantum phases, such as topological semimetals, superconductors, and quantum spin liquid phases with topological order have already been predicted to exist in iridium compounds. However, whether any of these theoretically predicted phases exist in real materials remains an open question. We propose to carry out a systematic experimental investigation of topological quantum phases in complex iridium oxides. Our experimental approach relies on exploratory materials synthesis efforts to find these exotic phases, through tuning materials parameters such as charge carrier doping, external hydrostatic pressure, epitaxial strain, and high magnetic fields. By varying these parameters, we will be able explore a large parameter space of several iridate materials with different structural motifs: honeycomb lattice, pyrochlore lattice, and square lattice. A particular attention will be paid to find topological quantum phases in epitaxial thin film samples. To detect topological order in iridium compounds, we will rely on state-of-the art x-ray and neutron spectroscopy methods to measure the full dynamic structure factor. In particular, the resonant inelastic x-ray scattering (RIXS) technique, which is rapidly developing into a powerful momentum dependent spectroscopy method, will play a significant role in our understanding of the physics of iridates. Novel functional properties found in these topological quantum phases could eventually be harnessed in future oxide-based electronics. Advances in epitaxial thin film growth technology in the last two decades have made it possible to envision electronics based on complex oxide materials. The exotic properties of topological quantum phases arising from the strong electron correlation and spin-orbit coupling could be crucial for the future application of oxide-based electronics. The proposed research will also make a significant contribution to the training of innovative materials researchers.