Novel X-ray methods for studying correlated quantum matter in the strong spin-orbit coupling limit

虽然它是材料最重要的特性之一，但对施加电压的响应可能是其最重要的特性之一。为什么有些材料是电导体，而另一些是绝缘体的最初见解来自量子力学的早期应用。在这种观点中，“简单”材料中的电子被视为独立的，固体根据填充量子态的电子数量进行分类：对于偶数，状态被填充，导致绝缘体，而对于奇数，状态被部分填充，允许电子导电。虽然这一经验法则适用于许多“简单”材料，包括例如铝和硅，我们目前的技术的很大一部分是基于这些材料的，但它对其他材料却非常失败。例如，过渡金属的简单氧化物以部分填充的电子状态存在。莫特首先提出，只有通过包括电子相互作用（在氧化物等材料中可能占主导地位），才能理解金属-绝缘体转变。哈伯德后来提出了一个看似简单的模型，只有两个参数，描述了电子局域化（绝缘行为）或离域（金属）的趋势。50多年来，Mott-Hubbard范式为合理化“复杂”量子固体的电子和磁性提供了持久的理论框架，这些量子固体被定义为表现出明确的集体量子效应，如高温超导性。最近，电子的内禀自旋与其轨道运动的相对论耦合-自旋轨道相互作用（SOI）-随着发现它可以导致新类型的电子态而成为焦点。已经表明，即使对于某些“简单”材料，SOI也会导致在体材料中绝缘的材料上的表面金属态。这些表面态是不平凡的，因为它们受到对称性或拓扑结构的保护，因此不能轻易被破坏。于是，问题自然出现了，在电子强烈相互作用的“复杂”量子材料中包含相对论效应的后果。答案需要开发一种新的范式--超越莫特-哈伯德范式--平等对待交互和SOI。这个提议是为了进行实验，这将是建立这种新范式的关键。这一新的前沿领域吸引了相当多的理论关注，人们对奇异的电子和磁性状态进行了大量的预测，其中一些从长远来看可能会导致新技术的出现。例子包括新型绝缘体、金属、超导体、量子自旋液体等，然而，历史表明，尽管理论提供了有用的指导，但它不能预测所有的可能性，许多令人兴奋的发现无疑将通过实验来实现。通过实验揭示复杂“量子物质”中电子和磁相关性的本质是非常具有挑战性的，需要具有极高灵敏度和特异性的技术。该提案的一个主要主题是开发新的X射线技术，这将为固体中的原子尺度秩序和激发提供前所未有的见解。这些技术将在国内和国际上的大型中央设施中开发，这些设施具有专用的粒子加速器，用于产生超强X射线束。最近出现的X射线激光源代表了这项技术的顶峰，它在飞秒脉冲（即光通过分子所需的时间）中提供比传统源高20个数量级的强度。这些源是转换的，使新的非平衡电子和磁性状态被创建和它们的演变进行实时研究。

Although it is one of the most prosaic properties of a material, the response to an applied electrical voltage can be one of its most profound. Initial insight into why some materials are electrical conductors while others are insulators came from the early application of quantum mechanics. In this view, electrons in "simple" materials are treated as independent, and solids are classified according to the number of electrons filling the quantum states: for an even number the states are filled, resulting in an insulator, whereas for an odd number the states are partly filled allowing the electrons to conduct. Although this rule of thumb works for many "simple" materials, including e.g. aluminum and silicon on which a large fraction of our current technologies are based, it fails spectacularly for others. Simple oxides of transition metals, for example, exist with partially filled electron states. Mott first proposed that it was only by including electron interactions, which in materials such as oxides can be dominant, that the metal-insulator transition can be understood. Hubbard later proposed a deceptively simple model with just two parameters, describing the tendency of electrons either to localize (insulating behaviour) or delocalize (metallic). For more than 50 years, the Mott-Hubbard paradigm has provided the abiding theoretical framework for rationalizing the electronic and magnetic properties of "complex" quantum solids defined as those that exhibit explicit collective quantum effects, such as high-temperature superconductivity. More recently, the relativistic coupling of an electron's intrinsic spin with its orbital motion - the spin-orbit interaction (SOI) - has come sharply into focus with the discovery that it can lead to qualitatively new types of electronic state. It has been shown that even for certain "simple" materials the SOI leads to surface metallic states on materials that in the bulk are insulating. These surface states are non-trivial, in that they are protected by symmetries - or topology - and therefore cannot be easily destroyed. The question then naturally arises as to the consequences of including relativistic effects in "complex" quantum materials in which the electrons interact strongly. The answer requires developing a new paradigm - beyond the Mott-Hubbard one - that treats interactions and the SOI on an equal footing. This proposal is to perform experiments that will be key to establishing this new paradigm. This new frontier has attracted considerable theoretical attention, and a plethora of predictions have been made for exotic electronic and magnetic states, some of which in the long run may lead to new technologies. Examples include novel types of insulators, metals, superconductors, quantum spin liquids, etc. However, history shows that although theory provides a useful guide, it cannot anticipate all possibilities, and many exciting discoveries will no doubt be made through experimentation. Revealing the nature of the electronic and magnetic correlations in complex "quantum matter" through experimentation is very challenging, requiring techniques with extremely high sensitivity and specificity. A major theme of this proposal is the development of novel X-ray techniques which will offer unprecedented insights into the atomic scale order and excitations in solids. The techniques will be developed at large-scale central facilities, both nationally and internationally, which have dedicated particle accelerators for producing ultra intense X-ray beams. The recent advent of X-ray laser sources represent the pinnacle of this technology which deliver 20 orders of magnitude higher intensity than conventional sources in femto-second pulses (i.e. the time taken for light to transit a molecule). These sources are transformational enabling novel non-equilibrium electronic and magnetic states to be created and their evolution to be studied in real-time.