Phase transitions in two-dimensional ultrathin magnetic films

凝聚态物理学通过发现和理解可能适合应用的具有新性质的新材料，对社会产生了直接影响。在过去的几十年里，对只有一两个原子厚度的二维(2D)材料的基础研究，一直是具有改变社会潜力的材料的非常富有成效的来源。一些例子包括石墨烯(石墨的一个原子层)、高温超导体、使用原子薄的磁性薄膜进行数据存储，以及用于量子计算机的拓扑绝缘体的2D表面性质。这些都是真正的2D系统，“活”在3D世界中。了解这些2D系统的性质包括了解这些性质在相变时如何变化(最著名的相变是水的冰到液的转变)。理论物理中非常强大的定理表明，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一种类型(或类别)的各种不同材料中的相变和性质。一个同样强大的定理表明，在均匀的2D系统中不存在类：正是因为非一致性，具有可靠材料性质的有序系统才会在2D中存在。这是理解为什么2D材料是新材料如此丰富的来源的一种方式--当微小的不均匀以不可预见和不寻常的方式“打破平衡”时，它们可能会产生深远的影响，从而允许相变到具有稳定性质的状态，而这种状态本身就不寻常。虽然二维相变的理论和计算研究已经很先进，但在真实的二维材料中对相同相变的定量实验研究还不是很先进。这些理论的许多详细预测是众所周知的，但还没有被证明适用于真实的材料。我建议对超薄2D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，因此系统是磁性的2D。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2D中所有不同的普遍相变类别-因此，结果比乍看起来更广泛适用。具体地说，我建议研究三类2D转变：一类是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；一类是远程相互作用扰乱有序相并导致图案形成；以及2016年诺贝尔物理学奖获得者科斯特利茨-索利斯转变。

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.