Phase transitions in two-dimensional ultrathin magnetic films

凝聚态物理通过发现和理解可能适合应用的具有新特性的新材料，对社会产生直接影响。在过去的几十年里，对只有一到两个原子厚度的二维（2D）材料的基础研究，已经成为具有改变社会潜力的材料的一个非常富有成效的来源。一些例子是石墨烯（石墨的一个原子层），高温超导体，用于数据存储的原子薄磁性薄膜，以及用于量子计算机的候选拓扑绝缘体的二维表面特性。这些都是“活”在3D世界中的真实2D系统。要了解这些二维系统的性质，就需要了解这些性质在相变时是如何变化的（最著名的相变是水从冰到液体的转变）。理论物理学中非常有力的定理表明，存在少量的普遍类型的相变，并且相同的定量理论描述了属于每种类型（或类别）的大量不同材料的相变和性质。一个同样有力的定理表明，在均匀二维系统中不存在类：正是由于非均匀性，具有可靠材料性质的有序系统才在二维中存在。这是理解为什么二维材料是如此多产的新材料来源的一种方式-当它们以不可预见和不寻常的方式“打破平衡”时，微小的不均匀性可以产生深远的影响，这些不均匀性允许相变到具有稳定特性的状态，而这种状态本身就是不寻常的。虽然二维相变的理论和计算研究都很先进，但实际二维材料中相同相变的定量实验研究还不是。这些理论的许多详细预测是众所周知的，但尚未被证明适用于实际材料。我建议对超薄二维磁性薄膜进行实验研究，以进行这些定量比较。磁性材料，如铁，几个原子厚的薄膜可以生长在非磁性衬底晶体的表面上，从而使该系统具有磁性二维。尽管我将只研究磁性薄膜，但通过仔细选择薄膜和衬底的组合，几乎可以研究二维中所有不同的普遍类型的相变——因此，研究结果的适用性比最初看起来要广泛得多。具体来说，我建议研究三类二维过渡：一类是当部分覆盖衬底的薄膜的孤立部分连接在一起形成连续薄膜时发生的；一种是远程相互作用破坏有序相并导致模式形成；以及获得2016年诺贝尔物理学奖的Kosterlitz-Thouless跃迁。

Condensed matter physics makes a direct impact on society through the discovery and understanding of new materials with novel properties that might be suitable for applications. In the last couple of decades, fundamental research into two dimensional (2D) materials that are only one or two atoms thick, has been a very productive source of materials with the potential to change society. Some examples are graphene (one atomic layer of graphite), high temperature superconductors, the use of atomically thin magnetic films for data storage, and the 2D surface properties of topological insulators that are a candidate for use in quantum computers. These are real 2D systems "living" in the 3D world. Understanding the properties of these 2D systems involves understanding how these properties change at phase transitions (the most famous phase transition is the ice to liquid transition of water). Very powerful theorems in theoretical physics show that there are a small number of universal types of phase transition, and that the same quantitative theory describes the phase transitions and properties in the huge variety of different materials that fall within each of these types (or classes). An equally powerful theorem shows that no classes exists in uniform 2D systems: it is because of non-uniformities that ordered systems with reliable materials properties exist in 2D at all. This is one way of understanding why 2D materials are such a productive source of novel materials - small non-uniformities can have a profound effect when they "tip the balance" in unforeseen and unusual ways that allow a phase transition to a state with stable properties that are themselves unusual. Although theoretical and computational studies of 2D phase transitions are well-advanced, quantitative experimental studies of the same transitions in real 2D materials is not. Many detailed predictions of the theories are well-known but have not yet been shown to apply to real materials. I propose experimental studies of ultrathin 2D magnetic films to make these quantitative comparisons. Films of magnetic materials, such as iron, a few atoms thick can be grown on the surface of a non-magnetic substrate crystal, so that the system is magnetically 2D. Even though I will study only magnetic films, almost all of the different universal classes of phase transition in 2D can be studied by carefully choosing the combination of film and substrate - therefore the results are much more widely applicable than it might at first seem. Specifically, I propose to study three classes of 2D transitions: one that occurs when the isolated parts of a film that partially covers a substrate connect together to form a continuous film; one where long-range interactions disrupt the ordered phase and lead to pattern formation; and the Kosterlitz-Thouless transition, the subject for which the 2016 Nobel Prize in physics was awarded.