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: 2022
• 资助国家: 加拿大
• 起止时间: 2022-01-01 至 2023-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=763124
• 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可能适合应用的新材料的新材料对社会产生直接影响。在过去的几十年中，对只有一个或两个原子厚的二维（2D）材料的基础研究一直是具有改变社会潜力的材料来源。一些例子是石墨烯（石墨的一个原子层），高温超导体，原子上薄磁性膜进行数据存储以及拓扑绝缘子的2D表面特性，这些拓扑绝缘子是用于量子计算机中的候选者。这些是在3D世界中“生活”的真实2D系统。了解这些2D系统的特性涉及了解这些特性在相变时的变化（最著名的相变是水的冰过渡到水的液体过渡）。理论物理学中非常强大的定理表明，相位过渡的多种普遍类型，并且相同的定量理论描述了各种不同材料（或类）中各种不同材料的相变和性质。同样强大的定理表明，在统一的2D系统中不存在任何类别：这是由于不均匀性，即完全存在具有可靠材料属性的系统。这是一种理解为什么2D材料是如此有效的新型材料的一种方法 - 当它们以不可预见且不寻常的方式“使平衡”“给平衡点”时，它们会产生深远的影响，从而使相位过渡到具有稳定属性的状态这本身是不寻常的。 尽管对2D相变的理论和计算研究是良好的，但实际2D材料中相同过渡的定量实验研究不是。这些理论的许多详细预测是众所周知的，但尚未证明适用于真实材料。我提出了超薄2D磁性膜的实验研究，以进行这些定量比较。磁性材料的膜，例如铁，几个原子厚可以在非磁性底物晶体的表面生长，因此系统具有磁性2D。即使我仅研究磁性膜，也可以通过仔细选择膜和底物的组合来研究2D中几乎所有不同的通用相变类别 - 因此，结果比起初看起来更广泛地适用。具体而言，我建议研究三类2D转换：当膜的孤立部分部分覆盖基板一起连接在一起以形成连续膜时发生的三类；长距离相互作用破坏有序阶段并导致模式形成的一个；还有Kosterlitz-Thouless Trurnition，这是2016年诺贝尔物理学奖的主题。