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

尽管它是材料最平凡的特性之一，但它对外加电压的响应可能是它最深刻的特性之一。对为什么有些材料是电导体，而另一些材料是绝缘体的初步认识来自于量子力学的早期应用。在这种观点下，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，态被填充，从而产生绝缘体，而对于奇数，态被部分填充，从而允许电子传导。虽然这条经验法则适用于许多“简单”的材料，包括铝和硅，我们目前的技术有很大一部分是基于这些材料的，但对其他材料来说，它却失败得惊人。例如，过渡金属的简单氧化物以部分填充的电子态存在。莫特首先提出，只有通过包括电子相互作用，才能理解金属-绝缘体的转变。在氧化物等材料中，电子相互作用可能占主导地位。哈伯德后来提出了一个看似简单的模型，只有两个参数，描述了电子的定域化(绝缘行为)或离域化(金属)的趋势。50多年来，Mott-Hubbard范式提供了持久的理论框架，用于理顺“复杂”量子固体的电子和磁性，其定义是那些显示出明显的集体量子效应的量子固体，如高温超导。最近，电子的本征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性质上的新型电子态的发现而迅速成为人们关注的焦点。研究表明，即使对于某些“简单的”材料，SOI也会导致材料表面的金属状态，而这些材料基本上是绝缘的。这些表面态不是平凡的，因为它们受到对称性或拓扑学的保护，因此不容易被破坏。于是，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就自然而然地出现了。答案需要开发一种新的范式--超越莫特-哈伯德范式--在平等的基础上对待互动和SOI。这项提议是进行实验，这将是建立这一新范式的关键。这一新的前沿引起了相当大的理论关注，人们对奇异的电子和磁态做出了过多的预测，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，毫无疑问，许多令人兴奋的发现将通过实验获得。通过实验揭示复杂“量子物质”中电子和磁关联的本质是非常具有挑战性的，需要具有极高灵敏度和特异度的技术。这项提议的一个主要主题是发展新的X射线技术，这将提供对固体中原子尺度、有序和激发的前所未有的见解。这些技术将在国内和国际上的大型中央设施中开发，这些设施拥有专门的粒子加速器来产生超强X射线束。最近出现的X射线激光光源代表了这项技术的顶峰，它以飞秒脉冲(即光通过分子所需的时间)提供的强度比传统光源高20个数量级。这些源是变革性的，能够创建新的非平衡电子和磁态，并实时研究它们的演化。

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.