Exploring Novel Quantum Materials with Neutrons and X-rays

这个研究项目的目标是研究一类新材料的结构、磁性和电子性质：自旋轨道驱动的量子材料。由于强烈的自旋-轨道耦合，即粒子的自旋和轨道运动之间的相互作用，这类材料显示出独特的物理性质。自旋-轨道耦合的强度随着原子质量的增加而增加，这意味着它在以重过渡金属元素为基础的材料中扮演着更重要的角色，如Ir。这种强烈的自旋-轨道耦合可以对重金属过渡金属化合物的物理产生深远的影响，从而产生一系列奇异的量子基态。它可以推动一种新的绝缘态--自旋轨道Mott绝缘体--在原本表现为金属的材料中形成。它还改变了磁矩、纠缠自旋和轨道矩的性质，形成了总有效角动量(杰夫)。在自旋轨道驱动的材料中可能出现一个特别有趣的现象：(A)表现为自旋轨道Mott绝缘体，(B)具有小的固有量子力学，Jeff=1/2磁矩，以及(C)显示基于边共享八面体的晶体结构。在这些条件下，传统的海森堡磁相互作用可以抵消，留下一种奇特的依赖于键的磁相互作用，称为基塔耶夫相互作用。最值得注意的是，在具有蜂窝晶格晶体结构的材料中，这些相互作用可以产生Kitaev量子自旋液体(QSL)，这是一种罕见的量子态，它将保持磁性无序，直到可能的最低温度。这种状态的激发显示出不同寻常的任意子统计，这表明Kitaev QSL可以为容错拓扑量子计算机提供基础。尽管已经确定了几个有希望的候选者(例如A_2IrO_3、RuCl3、H_3LiIr2O_6)，但寻找合适的Kitaev QSL态的实验实现仍然是一个持续的挑战。在这个研究计划中，我建议(A)合成新的自旋轨道驱动的量子材料和潜在的Kitaev QSL系统，(B)使用同步加速器X射线和中子散射技术来表征这些材料的性质，以及(C)通过化学掺杂、嵌入、外加压力和磁场来调整这些性质。X射线和中子是研究这些材料的理想工具，因为它们探测了这些奇异量子态的特征能量尺度和激发。除了探索传统蜂窝晶格上的Kitaev磁性外，该计划还将在基于不同晶体结构和键几何的新材料家族中搜索Kitaev磁性的特征。通过这种方式，我们希望更多地了解这些Kitaev材料的基本性质，并发现新的设计策略来生产用于量子设备和技术的材料。

The goal of this research program is to investigate the structural, magnetic, and electronic properties of a novel class of materials: spin-orbit-driven quantum materials. This family of materials displays unique physical properties due to strong spin-orbit coupling, i.e. the interaction between the spin of a particle and its orbital motion. The strength of spin-orbit coupling increases with atomic mass, meaning it plays a much more significant role in materials based on heavy transition metal elements such as iridium. This strong spin-orbit coupling can have a profound effect on the physics of heavy transition metal compounds, giving rise to a host of exotic quantum ground states. It can drive the formation of a novel insulating state, the spin-orbital Mott insulator, in materials which would otherwise behave like metals. It also changes the nature of magnetic moments, entangling spin and orbital moments to form a total effective angular momentum (Jeff). A particularly interesting phenomenon can arise in spin-orbit-driven materials which: (a) behave as spin-orbital Mott insulators, (b) possess small, inherently quantum mechanical, Jeff = 1/2 magnetic moments, and (c) display a crystal structure based on edge-sharing octahedra. Under these conditions, the conventional Heisenberg magnetic interaction can cancel out, leaving an exotic bond-dependent magnetic interaction known as the Kitaev interaction. Most notably, in materials with a honeycomb lattice crystal structure, these interactions can produce a Kitaev quantum spin liquid (QSL), a rare quantum state which remains magnetically disordered down to the lowest possible temperatures. The excitations of this state display unusual anyonic statistics, suggesting that the Kitaev QSL could provide the building blocks for a fault-tolerant topological quantum computer. Although several promising candidates have been identified (e.g. A2IrO3, RuCl3, H3LiIr2O6), identifying a suitable experimental realization of the Kitaev QSL state remains an ongoing challenge. In this research program, I propose to (a) synthesize new spin-orbit-driven quantum materials and potential Kitaev QSL systems, (b) characterize the properties of these materials using synchrotron x-ray and neutron scattering techniques, and (c) tune these properties via chemical doping, intercalation, applied pressure, and magnetic field. X-rays and neutrons are an ideal tool for studying these materials, as they probe the characteristic energy scales and excitations that are hallmarks of these exotic quantum states. In addition to exploring Kitaev magnetism on the traditional honeycomb lattice, this program will also search for signatures of Kitaev magnetism in new families of materials based on different crystal structures and bond geometries. In this way, we hope to learn more about the fundamental properties of these Kitaev materials, and discover new design strategies to produce materials for use in quantum devices and technology.