Topological quantum phases in complex functional materials

我们的研究项目集中在理解具有复杂结构和有趣功能特性的材料的基本物理。特别是，我们将专注于在含有第二或第三行过渡金属元素（如铱）的材料中寻找新的拓扑量子相。这些材料的特点是它们强烈的自旋轨道耦合，这是由电子的本征磁矩与其运动之间的相对论相互作用产生的。最著名的自旋轨道耦合驱动拓扑相位的例子是拓扑绝缘体。这种材料的表面可以导电，而样品的内部保持绝缘。这种拓扑相的电子特性有望对缺陷和无序具有极强的鲁棒性，使其在基于自旋的电子学和容错量子计算中的应用非常有用。当自旋轨道耦合能量与电子之间的排斥库仑能量平衡时，可以发现更令人兴奋的物理现象。铱基材料是这两种能量尺度相互竞争的一个很好的物理例子。事实上，大量的奇异量子相，如拓扑半金属、超导体和具有拓扑顺序的量子自旋液相，已经被预测存在于铱化合物中。然而，这些理论上预测的相是否存在于实际材料中仍然是一个悬而未决的问题。

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.