Phase transitions in two-dimensional ultrathin magnetic films

二维超薄磁性薄膜中的相变

• 批准号: RGPIN-2019-06899
• 负责人: Venus, David
• 金额: $ 1.75万
• 依托单位: McMaster University
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2019
• 资助国家: 加拿大
• 起止时间: 2019-01-01 至 2020-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=679920
• 关键词: Phase; transitions; two; dimensional; ultrathin

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.*****

凝聚态物理学通过发现和理解具有可能适用于应用的新特性的新材料对社会产生直接影响。 在过去的几十年里，对只有一两个原子厚的二维（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膜的使用，以及拓扑绝缘体的2D表面特性，这些都是量子计算机的候选者。 这些是“生活”在3D世界中的真实的2D系统。 要理解这些二维系统的性质，就需要理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）。理论物理学中非常强大的定理表明，相变有少数几种普遍的类型，并且相同的定量理论描述了属于这些类型（或类别）的各种不同材料的相变和性质。 一个同样强大的定理表明，在均匀的2D系统中不存在类：正是由于不均匀性，具有可靠材料属性的有序系统才存在于2D中。 这是理解为什么2D材料是新材料的如此多产的来源的一种方式，当它们以不可预见和不寻常的方式“打破平衡”时，小的非均匀性可以产生深远的影响，这些方式允许相变到具有稳定性质的状态，这些状态本身就不寻常。* 虽然2D相变的理论和计算研究已经非常先进，但真实的2D材料中相同相变的定量实验研究还没有。 这些理论的许多详细预测都是众所周知的，但尚未被证明适用于真实的材料。 我建议的实验研究的2D磁性薄膜，使这些定量比较。 几个原子厚的磁性材料膜，例如铁，可以在非磁性衬底晶体的表面上生长，使得系统是磁性2D的。 尽管我只研究磁性薄膜，但通过仔细选择薄膜和基底的组合，几乎所有不同的二维相变都可以研究，因此结果比最初看起来更广泛适用。 具体来说，我建议研究三类2D转变：一种是当部分覆盖基底的膜的孤立部分连接在一起形成连续膜时发生的转变;一种是长程相互作用破坏有序相并导致图案形成的转变;以及2016年诺贝尔物理学奖获得者Kosterlitz-numerless转变。